· 1 effect of additives to vo x/oxide support catalysts on their physicochemical and catalytic...
TRANSCRIPT
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Effect of additives to VOx/oxide support catalysts
on their physicochemical and catalytic properties
in oxidative dehydrogenation of lower alkanes
Wpływ domieszek do układów VOx/nośnik tlenkowy
na ich właściwości fizykochemiczne i katalityczne
w reakcjach utleniającego odwodornienia
niższych alkanów
A thesis presented by
Anna Klisińska – Kopacz, M.Eng.
for the degree of
Doctor of Chemistry
Supervisor: Prof. dr hab. Barbara Grzybowska-Świerkosz
Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences
Kraków, 2005
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To my Family
3
ACKNOWLEDGEMENTS I am most grateful to my supervisor Professor Barbara Grzybowska-Świerkosz for
her scientific guidance, help, inspiration and sharing with me her research knowledge
during my work at the Institute of Catalysis and Surface Chemistry, PAS.
I also sincerely thank:
Ms. Zofia Czuła for performing the BET measurements, Ms. Irena Gressel for the
technical assistance in the tests of decomposition of isopropanol, and
Dr. Jerzy Stoch and Mr. Maciej Mikołajczyk (M.Sc) for performing and interpreting the
XPS measurements, and Ms Katarzyna Samson (M.Eng) for introducing me into the
activities of the oxidation research group.
I feel very much indebted to my colleagues from the Institute of Catalysis and
Surface Chemistry, PAS for their advices, encouragements and stimulating working
atmosphere.
Physico-chemical characterization (XRD, Raman, IR, UV-VIS, TPR, SEM, TEM, 51V
NMR) has been performed in the Institut de Recherches sur la Catalyse C.N.R.S in
Villeurbanne under supervision of Dr Stephane Loridant within the framework of
ECC, Marie Curie Fellowship. The financial support of EC to my stay in IRC C.N.R.S
is gratefully acknowledged. I would like to thank Dr Jean-Claude Volta of this Institute
for help in arranging this fellowship and to Dr Stephane Loridant for his guidance.
Financial support to this work given by the Polish State Committee for Scientific
Research, KBN grant No PBZ/KBN/018/T09/99/4b is gratefully acknowledged.
Finally, I am greatly indebted to my parents and my husband for their love,
considerable support and encouragement throughout the years.
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CONTENTS
ACKNOWLEDGEMENTS 3
LIST OF TABLES 8
LIST OF FIGURES 10
Chapter 1 INTRODUCTION 15
1.1. Selective oxidation reactions 16
1.1.1. Oxidative dehydrogenation of lower alkanes 18
1.1.1.1. Generalities 18
1.1.1.2. ODH reactions in light of general concepts on selective oxidation
of hydrocarbons 20
1.1.2. Catalysts for oxidative dehydrogenation reactions 26
1.1.2.1. Ethane 26
1.1.2.2. Propane 28
1.1.2.3. n-Butane 30
1.1.2.4. iso-Butane 32
1.2. Vanadium oxide catalysts in selective oxidation reactions 35
1.2.1. Unsupported vanadium oxide 36
1.2.2. Supported vanadium oxide 37
1.2.3. Alkaline vanadates 42
1.3. Effect of additives on the physicochemical and catalytic properties of
oxide catalysts in oxidation reactions 44
1.3.1. Effect of additives on structural properties 44
1.3.2. Effect of additives on activity 46
1.3.3. Effect of additives on selectivity 47
Chapter 2 AIM AND SCOPE OF THE WORK 49
2. Aim and scope of the work 50
Chapter 3 EXPERIMENTAL PART 52
3.1. Materials and reagents 53
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3.2. Preparation of catalysts 55
3.3. Physicochemical characterization 58
3.3.A. Structure and texture of the catalysts 58
3.3.1. Determination of specific surface area (BET) 58
3.3.2. X-ray diffraction analysis 58
3.3.3. X-ray photoelectron spectroscopy (ESCA – XPS) 59
3.3.4. Infrared spectroscopy (IR) 59
3.3.5. Laser Raman spectroscopy (LRS) 60
3.3.6. Ultraviolet visible spectroscopy (UV-VIS) 60
3.3.7. Solid-state 51V nuclear magnetic resonance spectroscopy (NMR) 60
3.3.8. Electron microscopy measurements 61
3.3.8.1. Scanning Electron Microscope (SEM) 61
3.3.8.2. Transmission Electron Microscopy (TEM) 61
3.3.B. Determination of physicochemical properties 62
3.3.9. Temperature-programmed reduction techniques 62
3.3.9.1. Temperature-programmed reduction (TPR – H2) 62
3.3.9.2. Temperature-programmed desorption (propane TPD-MS) 62
3.3.10. Determination of acido-basic properties 63
3.3.10.1. Decomposition of isopropanol 63
3.3.10.2. Adsorption of pyridine as a probe molecule 63
3.4. Catalytic measurements 65
3.4.1. Description of apparatus, conditions of measurements, and
analysis of reaction products 65
3.4.2. Presentation of the catalytic data 67
Chapter 4 Results of studies and discussion 69
4.1. Physicochemical properties of studied systems 70
4.1.1. Structure and texture of catalysts 70
4.1.1.1. VOx / SiO2 catalysts 70
4.1.1.1.1. VOx/SiO2 catalysts with additives of main group elements
(K+, P5+) and of transition metal ions (Ni2+, Cr3+, Nb5+, Mo6+) 70
4.1.1.1.2. VOx/SiO2 catalysts with additives of different alkali metal ions
(Li+, Na+, K+, Rb+) and of alkaline earth metal ions (Ca2+,
Mg2+) 89
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4.1.1.2. VOx / MgO catalysts 92
4.1.1.2.1. VOx/MgO catalysts with additives of main group elements
(K+, P5+) and of transition metal ions (Ni2+, Cr3+, Nb5+, Mo6+) 92
4.1.1.2.2. VOx/MgO catalysts with additives of different alkali metal
ions (Li+, Na+, K+, Rb+) and of alkaline earth metal ions
(Ca2+) 104
4.1.2. Reducibility of the catalysts 107
4.1.2.1. VOx / SiO2 catalysts 107
4.1.2.2. VOx / MgO catalysts 110
4.1.3. Acido-basic properties of VOx/SiO2 and VOx/MgO catalysts 112
4.1.3.1. The catalysts with additives of main group elements (K+, P5+) and
of transition metal ions (Ni2+, Cr3+, Nb5+, Mo6+) 112
4.1.3.1.1. Adsorption of pyridine as a probe molecule 112
4.1.3.1.2. Decomposition of isopropanol 115
4.1.3.2. The catalysts with additives of different alkali metal ions (Li+, Na+,
K+, Rb+) and of alkaline earth metal ions (Ca2+, Mg2+) 119
4.1.4. Summary of physicochemical characterization of VOx/SiO2
and VOx/MgO catalysts 122
4.1.4.1. VOx/SiO2 catalysts 122
4.1.4.2. VOx/MgO catalysts 123
4.2. Catalytic properties of studied systems in oxidative dehydrogenation of
propane and ethane 124
4.2.1. VOx / SiO2 catalysts 124
4.2.1.1. VOx/SiO2 catalysts with additives of main group elements (K+,
P5+) and of transition metal ions (Ni2+, Cr3+, Nb5+, Mo6+) 124
4.2.1.2. VOx/SiO2 catalysts with additives of alkali metal (Li+, Na+, K+, Rb+)
and of alkaline earth metal ions (Mg2+, Ca2+) 132
4.2.2. VOx / MgO catalysts 138
4.2.2.1. VOx/MgO catalysts with main group elements (K+, P5+) and of
transition metal ions (Ni2+, Cr3+, Nb5+, Mo6+) 138
4.2.2.2. VOx/MgO catalysts with additives of alkali metal ions (Li+, Na+, K+,
Rb+) and of alkaline earth metal ions (Ca2+) 146
4.2.3. Summary of the catalytic studies 151
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Chapter 5 GENERAL DISCUSSION 153
5. Physicochemical properties of VOx/SiO2 and VOx/MgO catalysts vs
catalytic performance in ODH of ethane and propane 154
5.1. VOx/SiO2 catalysts 154
5.2. VOx/MgO catalysts 164
5.3. Comparison of properties of VOx/SiO2 and VOx/MgO catalysts 168
Chapter 6 FINAL CONCLUSIONS 173
6. Final conclusions 174
BIBLIOGRAPHY 177 ANNEXES
Annexe I Effect of potassium promoter content in VOx/SiO2 catalysts 190
Annexe II Determination of specific surface area (BET) 195
Annexe III X-ray diffracion analysis 196
Annexe IV X-ray photoelectron spectroscopy (ESCA – XPS) 198
Annexe V Infrared spectroscopy (IR) 200
Annexe VI Laser Raman spectroscopy (LRS) 201
Annexe VII Ultraviolet visible spectroscopy (UV-VIS) 203
Annexe VIII Solid-state 51V nuclear magnetic resonance spectroscopy (NMR) 204
Annexe IX Electron microscopy measurements 206
Annexe X Temperature-programmed reaction techniques 209
Annexe XI Decomposition of isopropanol 211
Annexe XII Adsorption of pyridine as a probe molecule 213
Annexe XIII Description of catalytic measurements 217
Annexes – Bibligraphy 219
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LIST OF TABLES
Table 1-1. Comparison of the reactions of classical and oxidative
dehydrogenation 18
Table 1-2. Thermodynamic data for dehydrogenation (DH) and oxidative
dehydrogenation (ODH) reactions of lower alkanes [6]. 19
Table 1-3. Bond energies in hydrocarbons [4] 21
Table 1-4. Catalysts for ODH of ethane. 27
Table 1-5. Catalysts for ODH of propane 29
Table 1-6. Catalysts for ODH of n-butane. 31
Table 1-7. Catalysts for ODH of iso-butane. 33
Table 3-1. List of reagents used in experimental part 53
Table 3-2. The list of the prepared catalysts 57
Table 3-3. Conditions of analysis 65
Table 4-1. List and composition of samples of VOx/SiO2 catalysts 70
Table 4-2. The results of XPS measurements of VOx/SiO2 catalysts 71
Table 4-3. Raman band assignments for vanadium oxide species. 74
Table 4-4. Raman band assignments for Ni and Cr compounds 76
Table 4-5. Infrared spectra of V2O5 and silica oxide 78
Table 4-6. 51 V NMR paramerters of V-containing compounds. 82
Table 4-7. Band maxima of the DRS spectra of some reference V-
compounds 84
Table 4-8. Band maxima of the DRS spectra of some reference Cr-
compounds 86
Table 4-9. Band maxima of the DRS spectra of some reference Ni-
compounds 87
Table 4-10. Band maxima of the DRS spectra of some reference Mo-
compounds 88
Table 4-11. List of samples of VOx/SiO2 catalysts with alkali and alkaline earth
metal ions. 89
Table 4-12. The results of XPS measurements of VOx/SiO2 catalysts with
alkali and alkaline earth metal ions. 90
Table 4-13. List of samples of VOx/MgO catalysts 92
Table 4-14. The results of XPS measurements of VOx/MgO catalysts 93
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Table 4-15. Raman spectra of magnesium vanadates. 95
Table 4-16. Infrared spectra of magnesium vanadates. 99
Table 4-17. List of samples of VOx/MgO catalysts with alkali and alkaline earth
metal ions. 104
Table 4-18. The results of XPS of VOx/MgO catalysts with alkali and alkaline
earth metal ions. 105
Table 4-19. Reducibility of the VOx/SiO2 catalysts as revealed by the H2-TPR 107
Table 4-20. Propane TPR/TPD of VOx/SiO2 catalysts 109
Table 4-21. Reducibility of the VOx/MgO catalysts as revealed by the H2-TPR measurements 111
Table 4-22. Propane TPR/TPD of VOx/MgO catalysts 111
Table 4-23. Number of LAS and BAS on VOx/SiO2 catalysts derived from IR
spectra of adsorbed pyridine. 112
Table 4-24. Number of LAS and BAS on VOx/MgO catalysts derived from IR
spectra of adsorbed pyridine. 113
Table 4-25. Decomposition of isopropanol on VOx/SiO2 catalysts at 200 oC 115
Table 4-26. Decomposition of isopropanol on VOx/MgO catalysts at 200 oC 116
Table 4-27. Decomposition of isopropanol on VOx/SiO2 catalysts with alkali
and alkaline earth metal ions at 200 oC 119
Table 4-28. Decomposition of isopropanol on VOx/MgO catalysts with alkali
and alkaline earth metal ions at 200 oC 120
Table 4-29. The total activity of VOx/SiO2 catalysts in ODH of propane and
Ethane 127
Table 4-30. The total activity of VOx/SiO2 catalysts with alkali and alkaline
earth metal ions in ODH of propane at 450 oC 135
Table 4-31. The total activity of VOx/MgO catalysts in ODH of propane and
ethane 141
Table 4-32. The total activity of VOx/MgO catalysts with alkali and alkaline
earth metal ions in ODH of propane 149
Table 5-1. Values of potassium desorption energy by SR-TAD method 171
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LIST OF FIGURES
Fig.1-1. Catalytic oxidation reactions of hydrocarbons 17
Fig.1-2. The “rake” mechanism of oxidation reactions [3] 18
Fig.1-3. A parallel-consecutive way of oxidation reaction 20
Fig.1-4. Rates of ODH reactions of propane on VOx/TiO2 [9] and of isobutene on CrOx/CeO2 [10] 20
Fig.1-5. Two steps of Mars and Van Krevelen mechanism: a) reduction, b) reoxidation. 22
Fig.1-6. Scheme of reoxidation of active sites a) by gaseous oxygen and b) by lattice diffusion of O2- oxygen adsorbed on vacancy distant from the active site 22
Fig.1-7. Steps of reoxidation of catalyst 24
Fig.1-8. Schematic view of V2O5: a) oxygen coordination around vanadium atom, b) crystal structure [93] 36
Fig.1-9. Selectivity of unsupported V2O5 and V-Mg-O system in oxidative dehydrogenation of alkanes. Reaction temperatures 500-550 oC, conversion ~10% [94, 95]. 37
Fig.1-10. Different [VOx]n species on oxide support. 38
Fig.1-11. Effect of VOx surface density on supported vanadium oxide catalysts on a) ODH rates (per V-atom) b) initial propene selectivities [75]. 41
Fig.1-12. Scheme of active sites a) VO4 in Mg3V2O8 and b) V2O7 in Mg2V2O7.[80] 43
Fig.1-13. Scheme of the locations of additives, A, in oxide catalysts: A and A’’, in substitutional and interstitial lattice positions, respectively; □, oxygen vacancy (if valency of A<Me); A’, on the surface; AO, bidimensional surface compound. 45
Fig.3-1. Scheme of preparation of supported vanadia catalysts. 56
Fig.4-1. X-ray diffraction patterns for VOx/SiO2 catalysts and observed phases: (a) VSi, (b) VSiK, (c) VSiNi, (d) VSiCr, (e) VSiNb, (f) VSiP, (g) VSiMo, ■ V2O5; ● K2V18O45 , K0.23V2O5; ● Ni2V2O7; ● Cr2V4O13; ● NbVO5; ● V3.6Mo2.4O16 72
Fig.4-2. Raman spectra of the VOx/SiO2 catalysts: (a) VSi; (b) VSiK; (c) VSiNi; (d) VSiCr (e) VSiNb; (f) VSiP; (g) VSiMo 73
Fig.4-3. IR spectra of VOx/SiO2 catalysts: (a) SiO2 support; (b) VSi; (c) VSiK; (d) VSiCr; (e) VSiNi; (f) VSiNb; (g) VSiP; (h) VSiMo 79
Fig.4-4. IR spectra of VOx/SiO2 catalysts: (a) SiO2 support; (b) VSi; (c) VSiK; (d) VSiCr; (e) VSiNi; (f) VSiNb; (g) VSiP; (h) VSiMo 80
Fig.4-5. 51V MAS NMR spectra of VOx/SiO2 catalysts: (a) VSi, (b) VSiK 81
11
Fig.4-6. UV-vis spectra of VOx/SiO2 catalysts: (a) VSi; (b) VSiK; (c) VSiNi; (d) VSiCr; (e) VSiNb; (f) VSiP; (g) VSiMo 83
Fig.4-7. UV-vis spectra of VOx/SiO2 catalysts refered to VSi: (a) VSiMo; (b) VSiP; (c) VSiNb; (d) VSiCr; (e) VSiNi; (f) VSiK 86
Fig.4-8. X-ray diffraction patterns for VOx/SiO2 catalysts and observed phases: (a) VSiLi, (b) VSiRb, (c) VSiMg, (d) VSiCa; ● V2O5; ■ LiVO3, ● LiV3O8 91
Fig.4-9. X-ray diffraction patterns for VOx/MgO catalysts and observed phases: a) MgO (b) VMg, (c) VMgK, (d) VMgNi, (e) VMgCr, (f) VMgNb, (g) VMgP, (h) VMgMo, ● MgO; ■ MgCrO4 94
Fig.4-10. Raman spectra of VOx/MgO catalysts: a) VMgCr, b) VMgMo, c) VMgP, d) VMgNb, e) VMgNi, f) VMgK, g) VMg 96
Fig.4-11. 51V MAS NMR spectra of VOx/MgO catalysts: (a) VMg, (b) VMgK 98
Fig.4-12. IR spectra of VOx/MgO catalysts: (a) VMgMo; (b) VMgP; (c) VMgNb; (d) VMgCr; (e) VMgNi; (f) VMgK; (g) VMg; (h) MgO support 100
Fig.4-13. UV-VIS spectra of VOx/MgO catalysts: (a) VMgMo; (b) VMgP; (c) VMgNb; (d) VMgCr; (e) VMgNi; (f) VMgK; (g) VMg 101
Fig.4-14. UV-vis spectra of VOx/MgO catalysts refered to the VMg sample: (a) VMgCr; (b) VMgNi; (c) VMgK; (d) VMgNb; (e) VMgP; (f) VMgMo 102
Fig.4-15. Scaning electron microscopy of (a), (b) MgO support; (c), (d), (e) VMg and transmission electron microscopy of (f) VMg 103
Fig.4-16. X-ray diffraction patterns for VOx/MgO catalysts and observed phases: (a) VMgLi, (b) VMgNa, (c) VMgRb, (d) VMgCa ● MgO 106
Fig.4-17. TPR profiles of VOx/SiO2 catalysts: : (a) VSi; (b) VSiK; (c) VSiCr; (d) VSiNi; (e) VSiNb; (f) VSiP; (g) VSiMo 108
Fig.4-18. TPR profiles of VOx/MgO catalysts: (a) VMg; (b) VMgK; (c) VMgNi; (d) VMgCr; (e) VMgNb; (f) VMgP; (g) VMgMo 110
Fig.4-19. Dependence of the acid strength on electronegativity of additives for VOx/SiO2 and VOx/MgO catalysts. 114
Fig.4-20. Dependence of the propene formation in isopropanol decomposition on the number of LAS and BAS for VOx/SiO2 catalysts. 117
Fig.4-21. Dependence of the propene formation in isopropanol decomposition on the electronegativity of additive ions for VOx/SiO2 catalysts. 117
Fig.4-22. Dependence of the propene formation in isopropanol decomposition on the electronegativity of additive ions for VOx/SiO2 catalysts. 121
Fig.4-23. Dependence of the acetone formation in isopropanol decomposition on the electronegativity of additive ions for VOx/SiO2 catalysts. 121
12
Fig.4-24. Changes of the propane conversion (at τ = 0.5 s) with the reaction temperature for VOx/SiO2 catalysts. 125
Fig.4-25. Changes of the selectivity to C3H6 in propane-oxygen reactions (at τ = 0.5 s) with the reaction temperature for VOx/SiO2 catalysts. 125
Fig.4-26. Changes of the selectivity to CO and CO2 in propane-oxygen reactions (at τ = 0.5 s) with the reaction temperature for VOx/SiO2 catalysts; CO _____, CO2 - - - - - . 126
Fig.4-27. Changes of the selectivity to C3H6, CO and CO2 in propane-oxygen reactions at 420 oC with the total propane conversion for VOx/SiO2 catalyst. 126
Fig.4-28. Changes of the activity ODH reactions of ethane at 500 oC and propane at 450 oC with the electronegativity of the additives’ ions for VOx/SiO2 catalysts. 128
Fig.4-29. Changes of the activation energy for ODH of propane with the electronegativity of the additives’ ions for VOx/SiO2 catalysts. 129
Fig.4-30. Selectivity to various products at 10 % (± 2) propane conversion for VOx/SiO2 catalysts. Reaction temperature: 450oC, * 470oC. 130
Fig.4-31. Selectivity to various products at 10 % (± 2) ethane conversion for VOx/SiO2 catalysts. Reaction temperature: 500oC. 131
Fig.4-32. Changes of the selectivity to C2H4 and C3H6 with the electronegaticvity of the additives’ ions for ODH of ethane and propane on VOx/SiO2 catalysts. 132
Fig.4-33. Changes of the propane conversion (at τ = 0.5 s) with the reaction temperature for alkali-doped VOx/SiO2 catalysts. 133
Fig. 4-34. Changes of the selectivity to C3H6 in propane-oxygen reactions (at τ = 0.5 s) with the reaction temperature for alkali-doped VOx/SiO2 catalysts. 133
Fig.4-35. Changes of the selectivity to CO and CO2 in propane-oxygen reactions (at τ = 0.5 s) with the reaction temperature for alkali-doped VOx/SiO2 catalysts; CO _____, CO2 - - - - - . 134
Fig.4-36. Changes of the selectivity to C3H6, CO and CO2 in propane-oxygen reactions with the canversion at 500 oC for VOx/SiO2 with Li catalyst. 134
Fig.4-37. Changes of the activity in reaction of propane at 450 oC with the electronegativity of additives’ alkali and alkaline earth metal ions for VOx/SiO2 catalysts. 136
Fig.4-38. Selectivity to various products at 10 % (± 2) propane conversion for alkali-doped VOx/SiO2 catalysts. Reaction temperature: 470oC, * 450oC. 136
Fig.4-39. Changes of the selectivity to C3H6 with the electronegativity of alkali and alkaline earth metal ions for ODH of propane on VOx/SiO2 catalysts. 137
13
Fig.4-40. Changes of the propane conversion (at τ = 0.5 s) with the reaction temperature for VOx/MgO catalysts. 139
Fig.4-41. Changes of the selectivity to C3H6 in propane-oxygen reactions (at τ = 0.5 s) with the reaction temperature for VOx/MgO catalysts. 139
Fig.4-42. Changes of the selectivity to CO and CO2 in propane-oxygen reactions (at τ = 0.5 s) with the reaction temperature for VOx/MgO catalysts; CO _____, CO2 - - - - - . 140
Fig.4-43. Changes of the selectivity to C3H6, CO and CO2 in propane-oxygen reactions with the conversion at 450 oC for VOx/MgO catalyst. 141
Fig.4-44. Changes of the activation energy for ODH of propane with the electronegativity of ions for VOx/MgO catalysts. 142
Fig.4-45. Selectivity to various products at 10 % (± 2) propane conversion for VOx/MgO catalysts. Reaction temperature: 450oC. 143
Fig.4-46. Selectivity to various products at 10 % (± 2) ethane conversion for VOx/MgO catalysts. Reaction temperature: 500oC. 145
Fig.4-47. Changes of the selectivity to C2H4 and C3H6 with the electronegativity of ions for ODH of ethane and propane on VOx/MgO catalysts. 145
Fig.4-48. Changes of the propane conversion (at τ = 0.5 s) with the reaction temperature for alkaliand alkaline earth -doped VOx/MgO catalysts. 147
Fig.4-49. Changes of the selectivity to C3H6 in propane-oxygen reactions (at τ = 0.5 s) with the reaction temperature for alkali and alkaline earth-doped VOx/MgO catalysts. 147
Fig.4-50. Changes of the selectivity to CO and CO2 in propane-oxygen reactions (at τ = 0.5 s) with the reaction temperature for alkali and alkaline earth-doped VOx/MgO catalysts; CO _____, CO2 - - - - - . 148
Fig.4-51. Changes of the selectivity to C3H6, CO and CO2 in propane-oxygen reactions with the conversion at 470 oC for VOx/MgO with Na catalyst. 148
Fig.4-52. Selectivity to various products at 10 % (± 2) propane conversion for alkali and alkaline earth-doped VOx/MgO catalysts. Reaction temperature: 470oC. 150
Fig.5-1. Dependence of the activity of VOx/SiO2 catalysts in ODH of ethane (500 oC) and propane (450 oC) on the strength of BAS. 154
Fig.5-2. Dependence of the activity of VOx/SiO2 catalysts in ODH of ethane (500 oC) and propane (450 oC) on the strength of LAS. 155
Fig.5-3. Dependence of the activity of VOx/SiO2 catalysts in ODH of ethane (500 oC) on the number of LAS and BAS. 155
Fig.5-4. Dependence of the activity of VOx/SiO2 catalysts in ODH of propane (450 oC) on the number of LAS and BAS. 156
14
Fig.5-5. Dependence of the selectivity to propene at 10% conversion in ODH of propane at 450 oC on the strength of LAS and BAS for VOx/SiO2 catalysts. 157
Fig.5-6. Dependence of the selectivity to propene 10% conversion in ODH of propane at 450 oC on the number of LAS and BAS for VOx/SiO2 catalysts. 158
Fig.5-7. Dependence of the selectivity to ethane at 10% conversion and propene at 10% conversion in ODH of ethane (500 oC) and propane (450 oC), on the acidic properties (rate of formation of C3H6 in isopropanol decomposition) for VOx/SiO2 catalysts. 158
Fig.5-8. Dependence of the selectivity to ethane at 10% conversion and propene at 10% conversion in ODH of ethane (500 oC) and propane (450 oC), on the basic properties (rate of formation of C3H6O in isopropanol decomposition) for VOx/SiO2 catalysts. 159
Fig. 5-9. Dependance of the selectivity to propene in ODH of propane at 450 oC, on the maximum reduction temperature (Tmax) in propane TPD measurements for VOx/SiO2 catalysts. 161
Fig.5-10. Dependence of the selectivity to ethene at 10% conversion in ODH of ethane (500 oC) on the strength of LAS and BAS for VOx/SiO2 catalysts. 162
Fig.5-11. Dependence of the selectivity to ethene at 10% conversion in ODH of ethane (500 oC) on the number of LAS and BAS for VOx/SiO2 catalysts. 162
Fig.5-12. Dependence of the activity of VOx/MgO catalysts in ODH of ethane (500 oC) and propane (450 oC) on the strength of LAS. 164
Fig.5-13. Dependence of the activity of VOx/MgO catalysts in ODH of ethane (500 oC) and propane (450 oC) on the number of LAS. 165
Fig.5-14. Dependence of the activity of VOx/MgO catalysts in ODH of ethane (500 oC) and propane (450 oC) on the acidic properties (rate of formation of C3H6 in isopropanol decomposition). 165
Fig.5-15. Dependence of the activity of VOx/MgO catalysts in ODH of ethane (500 oC) and propane (450 oC) on the basic properties (rate of formation of C3H6O in isopropanol decomposition). 166
Fig.5-16. Dependence of the selectivity to propene at 10% conversion and ethane at 10% conversion in ODH of propane (450 oC) and ethane (500 oC) on the number of LAS for VOx/MgO catalysts. 166
Fig.5-17. Dependence of the selectivity to propene at 10% conversion and ethane at 10% conversion in ODH of propane (450 oC) and ethane (500 oC) on the strength of LAS for VOx/MgO catalysts. 167
Fig. 5-18. Dependance of the selectivity to propene at 10%conversion in ODH of propane at 450 oC, on the maximum reduction temperature (Tmax) in propane TPD measurements for VOx/MgO catalysts. 167
Chapter 1
INTRODUCTION
Introduction
16
1.1. Selective oxidation reactions
The rapid development of industry and new technologies in the world
surrounding us requires the increasing supply of raw materials for production of new
synthetic fabrics. Current areas of interest and activities in catalysis involve
production and transformation of fuel materials, large-scale synthesis of inorganic
and organic products, production of fine chemicals, and protection of the
environment. The general trends are to use cheaper feedstocks, such as natural gas
and lower alkanes for the synthesis of chemicals and power production, to minimize
the waste products and to conduct catalytic processes under milder conditions.
Catalytic selective oxidation processes are one of the major areas of modern
chemical industrial production [1, 2]. One of the most important applications of
selective oxidation catalysis is the functionalization of hydrocarbons. The
petrochemical industry mainly is based mainly on the conversion of olefins. Olefins
can be easily and economically obtained from petroleum or natural gas and they are
easily functionalized. The demand of olefins is increasing extensively and the present
industrial capacity of olefins obtained from steam or catalytic cracking of naphta and
from fluid catalytic cracking (FCC) of vacuum gas oil is insufficient. In view of the
gradual exhaustion of the reserves of oil, its fuller utilization (alkanes on the one side
and heavy fractions on the other) for the production of energy media and in chemical
synthesis has been proposed and the use of natural gas as a raw material has been
suggested. Consequently, the petrochemical industry is moving toward the direct use
of alkanes, which are more economical than the corresponding olefins, and can be
obtained from both petroleum and natural gas.
The recent trends and developments in industrial selective oxidation
technology are: a) the use of new raw materials : alkanes are increasingly replacing
aromatics and alkenes as raw materials and b) the development of new catalytic
systems and processes : heterogenous rather then homogenous catalysts are being
used, and oxidative dehydrogenation is gradually replacing simple dehydrogenation.
The most research studies on oxidation reactions have been concerned with
oxidation in the gas phase of lower alkanes. The reactions comprise oxidative
dehydrogenation, oxidation with incorporation of oxygen into organic molecule and
Introduction
17
ammoxidation. The subject is still at the stage of data accumulation and of the search
for effective catalysts.
Oxidative dehydrogenation, ODH is the first step in the chain of transformation
of alkanes in the presence of oxygen, leading to the products of higher and higher
oxygen content, and ending in the production of the most favorable
thermodynamically carbon oxides. The reaction chain for lower alkanes C2-C4 is
shown in Fig.1-1.
C2H2n+2 →→→→ C2H2n →→→→ C2H2n-2O →→→→ C2H2n-2O2 →→→→ CO2
ALKANES ALKENES ALDEHYDES ACIDS
oxidative dehydrogenation (ODH)
ethane →→→→ ethene →→→→ acetic acid
propane →→→→ propene →→→→ acrolein →→→→ acrylic acid
n-butane →→→→ butene →→→→ butadiene →→→→ maleic anhydride
isobutane →→→→ isobutene →→→→ methacrolein →→→→ methacrylic acid
Fig.1-1. Catalytic oxidation reactions of hydrocarbons.
To obtain selectively the intermediate (thermodynamically less stable) products, such
as, successively, olefins, alcohols, aldehydes and acids, the kinetic factor plays the
decisive role. The kinetic factors in catalytic oxidation can be modified by an
appropriate choice of the catalysts.
The mechanism of the consecutive route for oxidation reactions so called
“rake mechanism” was proposed by Montarnal and Germain [3]. It is presented in
Fig.1-2, where R’ and RO’i denote surface complexes of substrate and intermediate
products, ROi are products of different number of oxygen atoms, i, in a molecule, ri
are rates of surface reactions and r’i and r’-i are the rates of adsorption and
desorption of intermediate products on or from the surface, respectively.
+ O2
- H2O
+ O2
- H2O
+ O2
- H2O
+ O2
- H2O
Introduction
18
In terms of the rake mechanism, the selectivity to a product ROi depends on
the relative rate of its desorption to the rate of the surface reaction leading to ROi+1.
The high rate of desorption requires a low heat of bonding of the product to the
catalyst surface.
Fig.1-2. The “rake” mechanism of oxidation reactions [3].
1.1.1. Oxidative dehydrogenation of lower alkanes
1.1.1.1. Generalities
Oxidative dehydrogenation (ODH) of lower alkanes has been considered as
an alternative to the classical dehydrogenation (DH) for production of olefins [4, 5],
which are necessary for several industrial processes such as polymerization,
selective oxidation, ammoxidation and production of tertbutyl methyl ether, MTBE –
(an additive to Pb-free petrol) from isobutene. Comparison of general features of the
two reactions is given in Table 1-1.
Table 1-1. Comparison of the reactions of classical and oxidative dehydrogenation
Dehydrogenation (DH) Oxidative dehydrogenation (ODH)
CnH2n+2 ↔ CnH2n + H2 CnH2n+2 + 1/2O2 → CnH2n + H2O
endothermic reaction ∆H0 > 0 exothermic reaction ∆H0 < 0
higher reaction temperatures
(550 – 700 oC)
lower reaction temperatures
(350 – 550 oC)
low equilibrium constants high equilibrium constants
formation of a coke deposit
(necessity of catalyst regeneration)
no carbon deposit
Introduction
19
The dehydrogenation reactions (DH) have several disadvantages. One of the
essential shortcomings is that the alkane dehydrogenation is an endothermic
reaction, which is limited by chemical equilibrium. According to Le Chatelier’s
principle, to obtain higher conversions higher temperatures are required. The
temperatures of a 50% conversion of alkane to corresponding alkene range from
about 720 oC for ethane to 500-600 oC for propane and butane. High-temperature
processes have several disadvantages including formation of undesirable cracking
products, and high rates of catalyst deactivation by coking, the latter necessitating
frequent catalyst regeneration. Finally, as the dehydrogenation is highly endothermic
heat must be added to sustain the reaction, which adds considerably to the cost of
the process.
These drawbacks can be avoided in the case of exothermic oxidative
dehydrogenation (ODH) reactions in the presence of oxygen in the reaction mixture.
The formation of water, which is a very stable product, makes this reaction
thermodynamically favourable. The abstracted hydrogen is oxidized, with evolution of
the heat of reaction, and conversion becomes significant at a much lower reaction
temperature. The comparison of thermodynamic data for dehydrogenation and
oxidative dehydrogenation of propane, n- and iso- butane is given in Table 1-2.
The ODH reactions have, however, also some disadvantages: like in all
hydrocarbon oxidation reactions, the formation of the desired partial oxidation
product is accompanied in this case by total combustion to thermodynamically most
favourable carbon oxides, which lowers the yields and the selectivity to alkenes and
leads to the waste of some of the substrate alkane.
Table 1-2. Thermodynamic data for dehydrogenation (DH) and oxidative dehydrogenation (ODH) reactions of lower alkanes [6].
DH ODH Alkane
∆∆∆∆H0 (kcal/mol)
Keq 1200 K
∆∆∆∆H0 (kcal/mol)
Keq 1200 K
C3H8 124 44 -118 3.5 · 109
n-C4H10 126 43 -116 3.5 · 109
iso-C4H10 139 140 -103 1.1 · 1010
Introduction
20
1.1.1.2. ODH reactions in light of general concepts on selective
oxidation of hydrocarbons
The basic facts and concepts on the mechanism of selective oxidation
reactions, described in [1, 7, 8], can be summarized as follows:
1. The oxidation reactions proceed by a parallel-consecutive reaction
network shown in Fig. 1-3:
Fig. 1-3. A parallel-consecutive way of oxidation reaction.
In terms of parallel-consecutive reaction network, the formation of the desired
partial oxidation product is accompanied by total combustion to thermodynamically
most favourable carbon oxides. Carbon oxides are formed either by consecutive
overoxidation of a selective oxidation product – olefin or oxygenated product (k2),
formed in the first step (k1), or by a parallel direct oxidation of a substrate – alkane
(k3).
To illustrate the extent of reactions 1, 2 and 3 in the case of the ODH reactions
the kinetic data for oxidative dehydrogenation of propane and isobutane are
presented after [9, 10] in Fig. 1-4.
Fig.1-4. Rates of ODH reactions of propane on VOx/TiO2 [9] and of isobutane on CrOx/CeO2 [10].
In contrast to oxidation of hydrocarbons to oxygenated compounds (aldehydes, acids
or anhydrides) the products of ODH – olefins are more easily oxidized than a
substrate (alkane), the rate constants k2 having much higher values than k3. This in
, oxygenated product
Introduction
21
turn is a consequence of lower bond energies of C-H bonds in olefins, e.g. allylic in
C3 and C4 alkanes, as compared with the C-H bond energies in respective alkanes.
The only exception is ethane-ethene reaction, in which a vinylic C-H bond in ethene
is stronger than the C-H bonds in an ethane molecule. Table 1-3 shows the bond
energies in alkanes and alkenes.
Table 1-3. Bond energies in hydrocarbons [4]
Bond type Energy (kJ mol-1)
C – C 376
C – H primary 420
C – H secondary 401
C – H tertiary 390
C – H allylic 361
C – H vinylic 445
Thus kinetic studies show that most of the the undesirable COx in ODH
originates from consecutive reaction of the olefin formed in step 1. It is then difficult to
stop the alkane/oxygen reaction at the stage of formation of an olefin and to avoid its
further nonselective oxidation under the conditions at which the reactant is oxidized.
The selectivity to olefin is hence controlled mainly by the rate of latter process. In
terms of the rake mechanism to ensure higher selectivity to olefin the easy
desorption from the catalyst surface and low readsorption of an olefin is then
necessary.
2. Reactions proceed by a redox, Mars and Van Krevelen mechanism in
two steps:
a) reduction of the catalyst by a hydrocarbon molecule with extraction of the
catalyst oxygen and incorporation it into the reaction products :
RH + KO →→→→ RO + K� + H2O
were KO and K� denote oxidized and reduced catalyst respectively, � is an oxygen
vacancy, RH is a molecule of a hydrocarbon, RO - reaction oxygenated product.
b) reoxidation of a reduced catalyst by gaseous oxygen :
K� + 1/2O2 →→→→ KO
Introduction
22
According to this mechanism the oxygen species introduced into the substrate
stems from the lattice. The role of gaseous O2 is to regenerate or to maintain the
oxidized state of catalyst. The mechanism involves the presence of two types of
active sites: an active double site MOx (where M is a metal cation) for oxidation of a
substrate and another site (oxygen vacancy) active for gaseous O2 reduction, which
should be separated from the reduced centres (Fig. 1-6). The scheme of Mars and
Van Krevelen mechanism and mechanism of reoxidation of oxide catalysts are
presented in Fig. 1-5 and 1-6, respectively.
a)
b)
Fig.1-5. Two steps of Mars and Van Krevelen mechanism: a) reduction, b) reoxidation.
Fig.1-6. Scheme of reoxidation of active sites a) by gaseous oxygen and b) by lattice diffusion of O2- oxygen adsorbed on vacancy distant from the active site.
Introduction
23
Redox mechanism has been proved for oxidation of olefins and
alkyloaromatics to oxygenated products and has been reported also for some ODH
reactions. It has been shown in several works that oxidative dehydrogenation of
alkanes can proceed in the absence of oxygen in the reaction mixture at the expense
of the oxygen of a catalyst. Usually the pulses of an alkane/inert gas mixture were
introduced on an oxidized catalyst surface and the changes in the conversion and
amounts of the products with the number of pulses (the extent of the catalyst
reduction) were recorded. Thus formation of the ODH products was observed when
pulses of : a) propane/inert gas mixture were injected on the V-Mg-O [11],
magnesium molybdate [12], cobalt molybdate [13] or Mg-V-Sb-O system [14] and b)
butane/He mixture were introduced on the VOx/Al2O3 catalysts [15], or V-Mg-O
catalysts [16]. In most cases the conversion of alkane decreased, whereas the
selectivity to alkene increased with the number of pulses (i.e. with the extent of
reduction of the catalyst).
The initial activity and selectivity were restored after oxidizing the catalyst with
gaseous oxygen. The formation of products in the absence of oxygen in the reaction
mixture indicates, that the redox, Mars and Van Krevelen mechanism, involving the
participation of the catalyst oxygen in the reaction, may operate also in the ODH
reactions.
3. The activation of the weakest C-H bond in a reacting RH molecule is
usually the rate determining step, r.d.s of the reaction and occurs on a centre Mn+-
O2- often considered as an acid–base couple. Transition metal cations, not fully
coordinated on the surface, can be considered as Lewis acid centres, whereas oxide
ions O2- as Lewis bases. Mn+–O2- couple is estimated as an active centre for the
activation of a C–H bond. Two possibilities can be envisaged assuming heterolytic
splitting on such a couple [17]:
a) abstraction of a hydrogen atom from the weakest C-H bond in a
hydrocarbon molecule on a basic O2- centre, formation of a proton and a carboanion
which by donation of electrons to a cation (a Lewis site) is transferred into a
carbocation (the mechanism proposed for oxidation of olefins and alkyloaromatics for
which the activity has been found to increase with the increase in the O2- basicity),
b) abstraction of a hydride ion H- with the formation of a Mn+–H bond, and
a carbocation which would require the presence of very strong acid sites and suggest
Introduction
24
a correlation between the activity and the acidity. Such mechanism was discussed
but not proved convincible for alkane activation.
In the case of alkane activation the possibility of breaking of a C-H bond by a
reaction with adsorbed oxygen species O- or O2- has been also envisaged. Such the
mechanism was proved for methane activation [18] and it cannot be excluded for
higher alkanes.
4. Selectivity to the partial oxidation products depends on:
a) the nature and properties of oxygen species present on the catalyst
surface, their concentration and the strength of the bonding in a catalyst ( the M-O
bond energy, EM-O) :
- chemisorbed, weakly bound electrophilic species e.g. O2-, O-, on the catalyst
surface lead to total combustion of a hydrocarbon molecule (shown for some olefin
and alkyloaromatics [19a] oxidation to oxygenated products and also for ODH of
propane on V2O5/TiO2 and MoO3/TiO2 [19b].
- lattice oxygen O2- nucleophilic species lead to selective oxidation, provided
the EM-O is relatively high.
At low EM-O and a high number of oxygen atoms around the cation involved in
the hydrocarbon activation centre and/or at high rate of oxygen diffusion in a catalyst,
the lattice oxygen may also lead to total oxidation products.
The centres of reduction and reoxidation of the catalyst should be separated
(Fig. 1.6.b) to avoid the presence in a vicinity of a reacting RH molecule of
nonselective electrophilic species, formed during the reoxidation stage in the reaction
(Fig. 1-7):
O2 + 4e- →→→→ 2O2-
1 2 3
O2(ads) + e- →→→→ O2-(ads) + e- →→→→ 2O-
(ads) + 2e- →→→→ 2O2-(lattice)
↓↓↓↓1’ ↓↓↓↓2’ ↓↓↓↓3’
RH RH RH
↓↓↓↓ ↓↓↓↓ ↓↓↓↓
COx COx R’
Fig. 1-7. Steps of reoxidation of catalyst.
Introduction
25
If those reoxidation steps are slow, a hydrocarbon molecule may be a subject
of an attack of nonselective oxygen species. To accelerate the reoxidation step the
concentration of the electrons in the solid should be high and the surface potential
(the energy barrier which the electrons must overcome to pass from the solid to the
chemisorbed oxygen molecule) should be low.
b) acid–base properties which may control, on one hand, the rate of
desorption of intermediate selective oxidation products in the chain of consecutive
reactions, and, on the other hand, may determine adsorption of reactants, leading-
when it is strong- to parallel total combustion.
The notation of acido-basicity is often used as equivalent to electrophilicity-
nucleophilicity, acid and basic centres being considered as electron-acceptor and
electron-donor centres, respectively. The electrophilicity is related to electronegativity
of component cations, whereas nucleophilicity of oxygen in an oxide can be
evaluated from the binding energy of the O 1s level in XPS spectra.
A basic (electron-donating) molecule such as an olefin would be weakly bound
to centres of low electron-accepting power (low electrophilicity, low acidity), and high
nucleophilicity (basicity), which facilitates its desorption, preventing further
overoxidation. Conversely, an acidic (electron accepting) molecule is less strongly
bound to centers of high electrophilicity (acidity).
The presence of Brönsted acidic sites may lead to the side reactions of the
hydrocarbon cracking: the addition of protons derived from the Brönsted centres to a
hydrocarbon molecule lead to creation of carbocations which are further easily
cracked. The broken fragments of the initial molecules are oxidized in the oxygen
presence to carbon oxides.
5. The activity depends on the M-O bond energy, the rate of oxygen
diffusion and electron transfer towards the reduced site.
Introduction
26
1.1.2. Catalysts for oxidative dehydrogenation reactions
1.1.2.1. Ethane
The catalysts active in oxidative dehydrogenation of ethane can be classify
into three groups:
a) systems based on ions and oxides of group I A and II A metals dispersed on
inactive supports( e.g. Li, Na/MgO or Li/metal chlorides),
b) rare earth metals oxides ( La2O3, Sm2O3, CeO2 ),
c) catalysts based on transition metal oxides, containing Mo and V.
The catalytic performances of various systems in ODH of ethane are presented in
Table 1-4. The catalysts based on ions and oxides of group I A and II A metals are
usually active at temperatures higher than 600 oC. They were found earlier as active
for the reaction of oxidative coupling of methane, OCM [18]. The mechanism of
ethane molecule activation on these catalysts is similar to that for methane activation.
The activation consists in abstraction of a hydrogen atom from an ethane molecule
by an anion radical O- generated on a catalyst surface on a Li+-O- centre. The formed
ethyl radical reacts further in the gas phase. Thus, the catalyst chiefly participates in
initiating the gas-phase reaction. The activity of the catalysts based on Group IA and
IIA metals can be significantly increased by the addition of small amounts of chlorine
or chlorine-containing compounds (e.g. tetrachloromethane or HCl) in the feed, or
halides directly added to the catalyst in the preparation stage. It is suggested that
chlorine radicals present in the gas phase facilitate the decomposition of ethyl radical
to ethene. The most successful of these catalysts is the Cl - Li/MgO. The ethene
yields as high as 30 - 40 % and selectivities 70 - 80 % have been found at
conversions ~ 40 - 60 % [20, 21].
The second group of catalysts for oxidative dehydrogenation of ethane
comprises rare earth metals oxides (La2O3, Sm2O3, CeO2, Pr6O11). The rare earth
metals oxides are active at the range 500 - 700 oC, exhibiting a very high stability
even at high temperature. These systems are believed to operate with a mechanism
similar to that of catalysts based on Group IA and IIA metals. The good catalytic
performances in ODH of ethane have been obtained with La2O3 and Sm2O3, however
the best system contained La2O3 in a mixture with Fe2O3 supported on α-Al2O3. The
Introduction
27
yields 40 - 80 % with selectivities between 70 - 90 % at conversions were reported
[22-26].
The third class of ethane ODH catalysts includes systems based on transition
metal oxides, containing in particular Mo and V. They can activate ethane at
temperatures usually 400-550 oC and operate most probably by a redox mechanism.
Among then, Mo-V-O system promoted with Nb, Sb, active at relatively low
temperatures ~ 400oC gives yields ~ 50 % with selectivities to ethene ~ 70 % [28,
29]. Vanadium oxides dispersed on different supports, magnesium vanadates or
molybdates, which are active in ODH of higher alkanes, show relatively poor
performance in the reaction of ethane (ethane yields usually below 10 %) [30, 31].
Table 1-4. Catalysts for ODH of ethane.
Conversion C2H6
Selectivity C2H4
Yield C2H4
Catalyst Temp. [°C]
[%]
Ref.
Li-Mg 580 38 80 30.4 [20]
Li-Na-Mg-O 650 38 85 32.3 [20]
Li-Cl-Mg 620 75 77 58.0 [21]
BaF2-LaOF 660 55 74 40.7 [22]
BaF2/Sm2O3-LaF3 680 42 84 35.3 [23]
BaCl2/Ho2O3 640 57 68 38.8 [24]
Fe2O3-α Al2O3- La2O3 550 87 96 83.5 [25]
La-Ba-Mn-Cu-O-F 680 38 49 18.6 [26]
La-Ba-Mn-Cu-O-Cl 680 63 63 39.7 [26]
α-NiMoO4 560 11 65 7.2 [27]
Mo-O-V-Nb-Sb-Ca 400 73 71 51.8 [28]
Mo-O-V-Nb-Sb-Mg 400 67 74 49.6 [28]
Sn-V-Sb/Al2O3 500 24 44 10.6 [29]
V-Mg-O 560 21 22 4.6 [11]
V2O5/Al2O3 530 25 54 13.5 [30]
K-V2O5/Al2O3 530 6 39 2.3 [30]
V2O5/SiO2 540 30 22 6.6 [31]
the catalysts containing vanadium oxide are marked in blue
Introduction
28
1.1.2.2. Propane
The catalysts for oxidative dehydrogenation of propane are mainly based on
transition metals oxides of V or Mo, in the mixture with other oxides: either of metals
of the group II (Mg), III A (Al) and IV A (Si), or of other transition metals (Co, Ni, Nb).
The oxides of transition metals may be dispersed on inactive (e.g. Al2O3, SiO2, TiO2),
or slightly active (Nb2O3) support, or it may form definite compounds such as
vanadates and molybdates. These systems are active in the temperature range 350-
500 oC and the reaction proceeds by the redox mechanism on the catalyst surface.
A general feature of most of these catalysts is that the selectivity to propene is
decreased with the increase of propane conversion due to consecutive reaction of
the propene total oxidation.
The catalysts for oxidative dehydrogenation of propane may be roughly
classified into three groups:
a) vanadium containing oxide systems,
b) molybdenum containing systems,
c) rare earth oxides supported on oxides or fluorides.
The first group of compounds are vanadium containing oxide systems,
including V-Mg-O catalysts, orthovanadates of Zn, Mg, rare earth elements, and
supported vanadia. Within this group the best results are obtained for Zn
orthovanadate, for which the maximum yields of propene about 20 % have been
noticed [41]. The similar yields of propene (about 17%) are obtained for vanadium
oxide dispersed on alumina support [43]. Among the supported vanadium oxide
systems the VOx/TiO2 promoted with alkali metals ( K, Rb ) gives yields of about 8 %
with the selectivities of about 60 % at 10 % conversion at temperatures < 400 oC [42].
For V-Mg-O system the maximum yields of 10-15 % of propene, with selectivities 50-
60 % at conversions 20-30 % have been obtained at temperatures > 500 oC [41].
Second group are molybdenum containing systems including molybdates of
different metals and molybdenum oxide dispersed on different supports. Among the
molybdenum-based systems, nickel and cobalt molybdates give the best selectivities
(65-80% at 10% conversion) and yields (8-12%) of olefins in ODH of propane at
temperatures > 500oC [32-34]. Good performance of Mg molybdates in ODH of
propane was also reported, though the activity in this system has been ascribed to a
Introduction
29
layer of excess MoOx present on the molybdates surface [35,]. The excess of Mo in
Ni-Mo-O catalysts has enhanced activity and selectivity in the propene ODH [34].
Molybdena dispersed on oxide supports such as TiO2 [19b, 37,45], Nb2O5, ZrO2,
SiO2, Al2O3 and MgO [38], and ZrO2 [39] has been also tested in ODH of propane,
giving the results comparable to those obtained for the molybdates. Among them
titania anatase was found to be the most effective support. Small amounts of alkali
metal additives improve the selectivity to propene in the ODH of propane on
MoO3/TiO2 [19b, 37] and MoO3/MgO- γ-Al2O3 [40] catalysts.
The third group of catalysts are rare earth oxides supported on oxides or
fluorides. Those supported on oxides react at higher temperatures > 550oC (usually
around 700 oC) with rather low yields (max. 10 %) and low selectivities ( 20 % at 40-
50 % conversion) [48]. However, the systems dispersed on fluorides have high yields
to propene. Particularly good results are reported for cerium fluoride supported
oxides of Sm, Ce, Nd, and Y. The best catalytic performances were obtained for
CeO2/CeF3 (yields to propene 34 %) [49].The reaction temperatures for these
systems are around 500oC.
Table 1-5 presents the catalytic performances of different systems in ODH of
propane.
Table 1-5. Catalysts for ODH of propane
Conversion C3H8
Selectivity C3H6
Yield C3H6
Catalyst Temp. [°C]
[%]
Ref.
V-Mg-O 540 25 48 12.0 [41]
Zn3V2O8 580 41 51 20.9 [41]
V2O5 /TiO2 360 10 38 3.8 [42]
K- V2O5/TiO2 360 10 57 5.7 [42]
Rb- V2O5/TiO2 400 10 55 5.5 [42]
V2O5/Al2O3 610 71 25 17.8 [43]
K-V2O5/Al2O3 585 41 39 16.0 [43]
V2O5/ZrO2 370 5 53 2.7 [44]
MoO3/Al2O3 360 5 59 3.0 [45]
Mo/Al2O3 500 13 33 4.3 [40]
Li-Mo/Al2O3 550 12 52 6.2 [40]
Introduction
30
MoO3/TiO2 460 5 57 2.9 [45]
MoO3/SiO2 520 5 70 3.5 [45]
Ni-M`o-O 560 17 64 10.9 [33]
Ni-Co-Mo-O 560 24 76 18.2 [33]
Mo-Mg-O 480 12 81 9.7 [46]
β-NiMoO4 530 13 72 9.4 [47]
La2O3/SrO 680 54 20 10.8 [48]
CeO2/CeF3 500 41 81 33.2 [49]
Nd2O3/CeF3 520 33 71 23.4 [49]
Y2O3/CeF3 520 33 65 21.5 [49]
the catalysts containing vanadium oxide are marked in blue
1.1.2.3. n-Butane
The catalysts active in oxidative dehydrogenation of n-butane contain mainly
the transition metal oxides and can be classified into three groups:
a) catalysts based on vanadium,
b) molybdenum based systems
c) phosphates of transition metal oxides.
The catalytic performances of various systems in ODH of n-butane are
presented in Table 1-6. The reaction products comprise usually product of deeper
dehydrogenation (butadiene), in addition to different n-butene isomers.
In V containing catalysts the best results so far have been obtained on a
VOx/MgO system, which appears to be the most promising, although the results
reported by different authors vary considerably. The best yields of oxidative
dehydrogenation products (sum of n-butenes and butadiene) are about 15 %, the
selectivities of about 50-60 % at conversions of 35-25 % were found [50-52].
Recently, several metal oxides, i.e. Mo, W, Cr and Fe, were studied as promoters of
vanadium-magnesium based catalysts for the ODH of n-butane [35]. Only the Mo-
doped catalysts showed better selectivity to C4-olefins than the undoped ones.
The promoter effect of Mo, B, Al, Ga, Sb and SbP in the catalytic behaviour of
Mg2V2MOx has been also studied. Sb- and B-containing catalysts were the most
Introduction
31
selective ones, while Ga-containing catalyst presented the best yield of
butene/butadiene [54].
Molybdenum containing catalysts include Ni molybdate and MoO3/MgO
system. The introduction of alkali additives (Li, Na, K, Cs) to NiMoO4 increased the
selectivity to n-butene. On the whole the Mo-based catalysts give lower yields of
butenes than V-based systems [55].
The different group of catalysts for ODH of n-butane consists of transition
metal oxide pyrophosphates. Cd-Sn-P-O mixed phosphate catalyst was found to
convert n-butane to butadiene at about 500 oC with 56% selectivity for 8% conversion
[12]. The best two catalysts were Sn and TiP2O7, but the first one gave mainly
butenes whereas the second produced larger amounts of butadiene [56]. Titanium
pyrophosphate with a maximum conversion of 25% and selectivity in
dehydrogenation products of 56%, it is an almost as efficient catalyst as the VMgO
catalyst for that reaction, although it produced less butadiene compared to butenes.
The reaction temperatures for these catalysts are relatively high 500-550oC.
Very high yields (54 %) of the ODH products (mixture of butenes, butadiene
and propene) with selectivities of about 80 % at conversions of 70 % were reported
for CoO-ZrO2 system [57].
Table 1-6. Catalysts for ODH of n-butane.
Conversion C4H10
Selectivity C4H8
Yield C4H8
Catalyst Temp. [°C]
[%]
Ref.
V-Mg-O 550 35 60 21.0 [50]
V-Mg-O 550 30 61 18.3 [50]
V-Mg-O 550 54 27 14.6 [51]
V-Mg-O 500 28 41 11.5 [52]
VOx/Al2O3 450 23 67 15.4 [53]
VOx/Al2O3-MgO 450 28 53 14.8 [53]
Mo-V-Mg 500 21 53 11.1 [52]
MoO3/MgO 500 10 64 6.4 [52]
Mg2V2Al2Ox 500 7 69 4.8 [54]
Mg2V2GaOx 500 8 66 5.3 [54]
Mg2V2W2Ox 500 5 53 2.7 [54]
Introduction
32
NiMoO4 525 39 10 3.9 [55]
K-NiMoO4 525 15 48 7.2 [55]
Cs-NiMoO4 525 12 68 8.2 [55]
MoO3/MgO 500 10 64 6.4 [52]
CeP2O7 530 19 56 10.6 [56]
SnP2O7 530 19 63 12.0 [56]
TiP2O7 530 25 56 14.0 [56]
CoO-ZrO2 665 68 80 54.4 [57]
the catalysts containing vanadium oxide are marked in blue
1.1.2.4. iso-Butane
The catalysts reported for the isobutane ODH can be classified into :
a) phosphates of transition metal oxides
b) chromium oxide based catalysts
c) vanadium oxide based systems
d) Wells-Dawson-type heteropolymetallates
e) miscellaneous catalysts
Phosphates of transition metal oxides (Mg, Cr, Co, Mn, Sn, Ag, Zn, Ni, Ce, Zr,
Ba and V) were the first catalysts reported for ODH of isobutene out of which Ni
pyrophosphate (Ni2P2O7) seemed to be the most promising (yields of isobutane of
9 %, selectivities of ~ 83 % at conversion 11 %) [70]. Ag, Zn, Mg pyrophosphates
presented a good selectivity for the isobutene formation but with very low conversion
rates [5]. The reaction temperatures for these catalysts are relatively high 500-550oC.
Chromium oxide catalysts dispersed on different supports such as Al2O3, TiO2,
CeO2, SiO2 and La2(CO3)3 have been found most promising catalysts at relatively low
temperatures (around 200-300 oC) [58-63]. The isobutane yields for the best of them
(CrOx/Al2O3, CrOx/TiO2, CrOx/CeO2) of 7-12 % with the selectivities of 50-60 % at
conversions 10-15 % were reported.
In the midst of vanadium oxide based systems only V-Mg-O catalysts have
been reported in this reaction [11], with max. yields of about 6 % and selectivities of
Introduction
33
53 % at 12 % conversion. V2O5/Al2O3 catalysts exhibited a selectivity of less than
15%, at 7% of isobutane conversion [64].
Another group of catalysts are Dawson-type heteropolyoxoanions with
tungsten [65], of the general formula KxP2WyOz (were x = 6,7, or 10, y = 18 or 17, z =
62 or 61 ), in which 1 atom of tungsten may be substituted by Fe, Mn, Co or Cu. They
were found to be effective in ODH of isobutene at 430 oC. The isobutene yields of
about 10 % with selectivities of 60-66 % at conversions of about 15 % have been
reported, the best systems being those containing Fe and Cu, and unsubstituted
compound.
The last group of catalysts for oxidative dehydrogenation of isobutane contains
miscellaneous system such as:
- nickel oxide on different supports; for NiOx/CeO2 system the maximum yields
of isobutene of about 5 % with rather low selectivities (about 30 %) at 15 %
conversion was noticed [66],
- rare earth oxides or salts: the selectivities to isobutene up to 70% at 500oC
were obtained for a number of rare earth catalysts promoted by cerium fluoride e.g.
for Y2O3/CeFe3 catalyst 9 % yield of isobutene with 75 % selectivity at 12 %
conversion were reported [49],
- zinc titanate (ZnO-TiO2) for which very high yields (~53 %) and high
selectivities ~ 65 % at about 80 % conversion have been reported [66],
- Pt-coated ceramic foam monolith [6], operating at very high temperatures
(800-900 oC) and very short contact time (ca. 5 ms) have been shown to catalyze the
reaction at the equimolar mixture of isobutene and propene with 25-30 % isobutene
yield.
The catalytic performances of various systems in ODH of isobutane are
presented in Table 1-7.
Table 1-7. Catalysts for ODH of iso-butane.
Conversion i-C4H10
Selectivity i-C4H8
Yield i-C4H8
Catalyst Temp. [°C]
[%]
Ref.
V-Mg-O 500 12 53 6.4 [11]
V2O5/Al2O3 350 6 53 3.2 [69]
Ni2P2O7 550 11 83 9.8 [70]
Introduction
34
CrOx/CeO2 270 10 57 5.7 [60]
CrOx/Al2O3 250 10 56 5.6 [71]
K-CrOx/Al2O3 260 20 45 9.0 [58]
CrOx/TiO2 265 20 43 8.6 [58]
K-CrOx/TiO2 325 20 45 9.0 [58]
NiOx/CeO2 350 18 27 4.9 [66]
Y2O3/CeFe3 480 12 75 9.0 [49]
LaBaSm 600 29 27 7.8 [68]
Zn2TiO4 570 81 65 52.6 [67]
Pt/Al2O3 900 75 33 24.7 [7]
the catalysts containing vanadium oxide are marked in blue
The survey presented above indicated that the maximum yields and
selectivities to olefins decrease with the increase in the number of C atoms in an
alkane molecule, which could have been expected on the basis of the decreasing
values of C-H bond energies in the series.
Moreover, no common catalyst effective for the ODH of all of C2-C4 alkanes
has been so far proposed. The elemental composition of the optimum catalyst for
each of the alkane in the C2-C4 series is different. Still vanadia based catalysts are
most widely represented in oxidative dehydrogenation of ethane, propane and n-
butane and efforts at optimization of their composition including introduction of
additives seem to be worth-while.
Introduction
35
1.2. Vanadium oxide catalysts in selective oxidation reactions
The catalysts based on vanadium oxide are used in several industrial
processes for production of important chemicals such as sulphuric acid, maleic and
phthalic anhydride, and for the reduction of environmental pollution e.g. nitrogen
oxides from flue gas of power plants [72-74]. Vanadium oxide catalysts have been
tested in a number of heterogenous catalytic oxidation reactions such as:
a) selective oxidation of alkanes to olefins (oxidative dehydrogenation reaction)
[11, 16, 19, 28, 37, 75-77, 106] and to aldehydes and acids [78-80].
b) selective oxidation of olefins to unsaturated aldehydes and acids, nitriles,
dienes, and organic acids [81-83].
c) selective oxidation of alkyloaromatics to aldehydes, anhydrides and acids [84-
87].
d) selective oxidation of aromatics [88, 89].
Out of these reactions those applied on an industrial scale include: synthesis of
maleic anhydride, originally from benzene (V-Mo-O system) and recently from n-
butane (V-P-O catalyst), selective oxidation of o-xylene to phtalic anhydride
(V2O5/TiO2) and synthesis of acrylic acid from acrolein (V-Mo-O system) [72, 90, 91].
The vanadia containing systems appear also promising catalysts for oxidation of
methanol to formaldehyde (V-Ti-O) [92] and oxidative dehydrogenation of lower
alkanes (V-Mg-O, K-V2O5/TiO2) [11, 16, 28,19b, 37].
Vanadium oxide based catalysts include unsupported V2O5 or mixed oxide
systems. The mode of mutual arrangement of the components in the mixed oxide
systems can be different, ranging from oxides dispersed to different extent on
inactive supports as VOx on TiO2, Al2O3, SiO2, ZrO2, to well defined compounds such
as vanadates (e.g. magnesium vanadates) or solid solutions of the active component
in an inactive matrix e.g. V4+ in SnO2, V4+ in TiO2.
Introduction
36
1.2.1. Unsupported vanadium oxide
The stereochemistry of vanadium ions in V2O5 can be considered as distorted
octahedrons. The structure of V2O5 is often shown by zigzag ribbons of tetragonal
pyramids of VO5. Each vanadium atom and its five nearest oxygen neighbors create
VO5-pyramids, which share their corners, building double chains along the b-
direction. The chains are connected by their edges and the resulting layers are
stacked along the c-direction. As a result, a vanadium oxide consists of an
octahedrally coordinated (distorted from regular shape) VO6 units with three distinct
vanadium–oxygen bond distances: V-O(1)=1.58 Å (strong and short bond of vanadyl
oxygen along the c-direction); V-O(2)=1.77–2.02 Å (bridging oxygens in the basal
plane); V-O(3)=2.79 Å (weak bonds in between the layers) (Fig. 1-8a). As seen from
Fig. 1-8b the crystal structure of V2O5 possesses channels and permit thus the facile
diffusion of oxygen.
a) b)
Fig.1-8. Schematic view of V2O5: a) oxygen coordination around vanadium atom, b) crystal structure [93]
Pure vanadium pentoxide is active in most selective oxidation reaction though
the selectivity to the desired products of partial oxidation is usually low. Addition of
other elements improves the selectivity. For example the selectivities to phthalic
anhydride in selective oxidation of o-xylene are higher for the mixed V-Ti catalysts
(~ 73% at conversion 99% and reaction temperature 350 oC) as compared with pure
V2O5 (~55 % at conversion 99% and reaction temperature 370 oC) [97].
Figure 1-9 shows the differences in selectivity to olefins in oxidative
dehydrogenation of lower alkanes for pure V2O5 and V2O5 in mixed oxide compound,
V-Mg-O. It can be noticed that the selectivities to propene and butene for V-Mg-O
Introduction
37
system are much higher. The modifications in selectivity come from the different
structure and properties of dispersed vanadia phase in the V-Mg-O catalysts.
Fig. 1-9. Selectivity of unsupported V2O5 and V-Mg-O system in oxidative dehydrogenation of alkanes. Reaction temperatures 500-550 oC, conversion ~10% [94, 95].
1.2.2. Supported vanadium oxide
The most of vanadia based catalysts consist of a vanadium oxide phase
deposited on the surface or in pores of an oxide support. The following oxides were
used as the supports:
a) metal oxides such as SiO2, Al2O3, TiO2, MgO and ZrO2,
b) mixed metal oxides i.e. TiO2/SiO2, TiO2/Al2O3, SiO2/Al2O3, calcined
hydrotalcite (Mg-Al-O mixed metal oxides), sepiolite (Mg-Si-O), AlNbO4,
c) other type of materials i.e. MgF2, amorphous or microporous
aluminophosphates (AlPO4)
ethane propane butane0
10
20
30
40
50
60
70
Sel
ectiv
ity [%
] pure V
2O
5
V-Mg-O
Introduction
38
Initially, the support was considered as inert substance that provided a high
surface area to carry the active metal oxide component or to improve the mechanical
strength of the catalyst material. Deposition of a metal oxide on the surface of
another oxide was proposed to improve the catalytic activity of the active metal oxide
phase due to a gain in surface area and mechanical strength. However, numerous
studies in the last decade indicated that the activity and selectivity of supported metal
oxide catalysts are significantly affected by the properties of the support oxide
material, and that the dispersed phase has a particular structure and properties
different from that of the bulk oxide.
Much effort has been made to characterize the structure of dispersed oxides
by various techniques, such as Raman, IR, XPS, 51V NMR, UV-VIS, EXAFS, EPR
etc. The results of these studies, which are summarized in several reviews [74, 96,
97], show the presence of different types of VOx species on the support surface. The
evidenced oxide species vary by their molecular structure and the strength of
bonding to the support.
Fig. 1-10. Different [VOx]n species on oxide support.
Three main types of vanadium-oxo species, presented in Fig. 1-10, are
generally observed:
a) isolated, monomeric, tetrahedral VOx groups, strongly bound to the support at
low loadings of the vanadia phase (1-2 V at/nm2),
b) two-dimensional chains of polyvanadates [VOx]n, the density of which
increases with the loading up to a monolayer coverage (7-10 VOx/nm2),
c) crystalline V2O5, observed clearly at higher loadings.
Theoretical monolayer surface coverage is defined as the maximum amount of
amorphous or two-dimensional vanadia in contact with the oxide support. Monolayer
Introduction
39
surface coverage of surface vanadia overlayer has been estimated from V-O bond
length of crystalline V2O5 and has been assumed as 10 VOx per nm2 of a support.
Usually at very low vanadia loading, corresponding to about several % of a
monolayer, only isolated monomeric forms with tetrahedral coordination of vanadium
(VO4) are observed. With the increase of the loading the amount of monomers
increases, and the appearance of polymeric forms is observed. Their amount and
size – number of V atoms in species – increase with vanadia content.
At high loading tridimensional V2O5 in amorphous or crystalline forms are
observed. In some cases e.g. vanadia on TiO2 anatase calcined at high
temperatures, incorporation of vanadium into TiO2 with formation of solid solution
(V4+/TiO2) is noticed, this process being accompanied by polymorphic transformation
anatase – rutile.
The structure of the two-dimensional clusters or a monolayer of polyvanadates
is not clear. Beside the V-O-V chains and V-O-S bonds, where S is an atom of the
support surface (Al, Ti, Si etc.), the presence of vanadyl terminal [V5+=O] groups, and
both tetra- and octahedral coordination of vanadium have been observed by Raman
spectroscopy. The problem which of these bonds in supported vanadium oxide is
responsible for the oxidation activity in various catalytic oxidation reactions, is under
discussion. Wachs et al [74] suggested that the oxygen in V-O-S bond can be
involved in a catalytic reaction. According to them the strength of formed V-O-S bond
is a function of electronegativity of an oxide support cations. The bridging oxygens in
V-O-S that are more electronegative (basic), corresponding to oxide support cations
with a lower electronegativity, are associated with the critical oxygen required for
hydrocarbon oxidation reactions over supported vanadia catalysts.
The concentration of different species, their structure and vanadia loading at
which they appear, depend on the type of support and for the same support on the
degree of its hydroxylation, presence of impurities, morphology or method of
deposition of vanadia phase. For Al2O3, TiO2, ZrO2, Nb2O5 and CeO2 the essentially
identical molecular structure has been claimed. Isolated VO4 tetrahedra were
observed at very low vanadium coverage, while with the increasing of V content the
polyvanadates containing both octa- and tetrahedral species are observed.
In contrast, vanadia species supported on SiO2 consist exclusively of isolated
tetrahedral VOx species at very low loading and three-dimensional agglomerates of
Introduction
40
crystalline V2O5 observed well below monolayer coverage. Polymeric [VOx]n species
were not observed for this system.
The important factor which influences on the distribution of vanadium oxide is
different acido-basic character of various supports [74]. The acidity of metal oxides is
related to the pH at which the surface possesses zero surface charge (point of zero
charge, pzc). The acid character of different metal oxides changes in order: V2O5 >
SiO2 > TiO2 = ZrO2 > Al2O3 > MgO. Agglomeration of vanadium oxide to form
crystallites of V2O5 is favoured by the acid character of the support (SiO2). On less
acidic and/or amphoteric oxides (TiO2, ZrO2, Al2O3) the dispersed mono- ,
polyvanadates are formed, while metal vanadates are formed at high vanadium
coverage on basic oxide supports as MgO. These differences result from the acid
character of vanadium oxide which can easily interact with the basic oxies, whereas
a weaker interaction with an acidic oxide will favour the agglomeration of VOx species
to crystalline V2O5.
The number of VOx units in a cluster required to obtain the optimum catalytic
performance in oxidative dehydrogenation reactions and the coordination of
vanadium ions, as well as their valence, are a subject of a discussion. The isolated
tetrahedral V5+ species have been considered as active and selective in the ODH of
propane and butane on vanadium supported on MgO, magnesium silicates, SiO2,
Al2O3, TiO2, and vanadium aluminophosphate catalysts [94, 98-100]. Octahedral
vanadium species, associated with the presence of crystalline V2O5, were found less
selective.
On the other hand, the polymeric forms of VOx species have been claimed as
required for good catalytic performance in the ODH reactions. Bridging V-O-V groups
from the two-dimensional array of VO4 tetrahedra in VOx/Al2O3 or AlNbO4 has been
proposed to be active phase in the propane ODH [101, 102]. Monomeric vanadium
species have been found less active in ODH of propane than vanadium clusters
containing pairs of vanadium atoms or ensembles of three vanadium atoms on
vanadium-niobium oxide catalysts [103].
The recent studies by Bell et al. [75] of ODH of propane on vanadium oxide
supported over the wide range of surface densities (0.5 - 15 VOx nm-2) on Al2O3,
SiO2, HfO2, TiO2 and ZrO2 have shown, that monomeric vanadium species are less
active and less selective than polyvanadates. Oxidative dehydrogenation rates (per
V-atom) on supported vanadium oxide depend on the support composition and on
Introduction
41
VOx surface density, the reaction rates increase as VOx surface density increases for
all supports (Fig. 1-11a). Also selectivities to propene extrapolated to zero conversion
increase with the increasing VOx surface density for all catalysts (Fig. 1-11b),
suggesting that polyvanadate domains of intermediate size at vanadium surface
density 4-7 V/nm-2 are more selective than monovanadate species. Lower
selectivities at low vanadia loading (monomeric VOx species) could be understood in
terms of the theory of selective oxidation [7, 8] which claims separation of the
reduction and reoxidation sites as one of requirements for high selectivity: If the
reaction takes place on a single MOx site, hydrocarbon and nonselective O2-, O-
species are in vicinity (cf Fig.1-6a)
a) b)
Fig. 1-11. Effect of VOx surface density on supported vanadium oxide catalysts on
a) ODH rates (per V-atom) b) initial propene selectivities [75].
The low selectivities to olefins observed in several cases at low loading of the
deposited phase may also be due to secondary reactions of the olefin intermediate
on the uncovered support surface, especially if this surface exposes acidic centres
capable of strong sorption of a hydrocarbon. In the presence of oxygen the strongly
bound olefin may undergo total oxidation to unfavourable carbon oxides.
Introduction
42
1.2.3. Alkaline vanadates
In the case of catalyst where vanadium oxide is dispersed on basic metal
oxide supports i.e. MgO, La2O3, Sm2O3, Bi2O3, the acid active phase could readily
interact with the basic support and the corresponding metal-vanadate is formed [94].
Particular attention has been given to vanadium magnesium catalysts since
they have been found as promising in oxidative dehydrogenation of lower alkanes
[11]. Depending on the V/Mg ratio, magnesium orthovanadate Mg3V2O8, magnesium
pyrovanadate Mg2V2O7 and magnesium metavanadate MgV2O6 have been observed
as crystalline phases. The structure of Mg3V2O8 is made up of chains of edge-sharing
MgO6 units linked together by isolated VO4 tetrahedra. Magnesium pyrovanadate is
built of corner-sharing VO4 tetrahedra that form V2O7 units, whereas the MgV2O6 is
characterized by metavanadate chains of edge-sharing VO5 units [16].
Structural investigations of V-Mg-O catalysts show no evidence for either
isolated vanadyl or polymeric vanadate species, or crystallites V2O5. The absence of
the V2O5 phase supported on MgO is confirmed by XRD and IR and Raman
spectroscopies [28, 104, 105].
The active species in the vanadium-magnesium oxide catalysts are a subject
of a discussion [4, 16, 106-108]. Crystalline magnesium vanadates, which may exist
in the optimum range of the vanadium content (20-60 wt. % V2O5) are claimed to be
active phases in the V-Mg-O systems, however the researchers are not in agreement
as to the composition of the most active and selective vanadate.
Kung and al. [4, 15, 16] attributed the active phase in propane and n-butane
ODH to magnesium orthovanadate Mg3V2O8. They explained that isolated vanadium
tetrahedron VO4, existing in its structure is playing the role of an active site (Fig. 1-
12). According to them higher selectivity of these sites in comparison to dimeric V2O7
unit in magnesium pyrovanadate is the result of presence of Mg-O-V units less easily
reducible than V-O-V units in pyrovanadate. They concluded that the easier it is to
reduce magnesium vanadate the more likely the vanadate will react with the
hydrocarbon to form carbon oxides.
In contrast Volta et al. [28, 107] and Delmon et al. [108] claim magnesium
pyrovanadate Mg2V2O7 more active and selective in comparison with other vanadates
in the ODH of propane, while Mg3V2O8 was recognized as responsible for total
Introduction
43
oxidation. The higher selectivity for magnesium pyrovanadate has been related by
these authors to its ability to stabilize V4+ ions associated with oxygen vacancies.
According to this approach it has been proposed that bridging oxygen atoms in the V-
O-V bond for the magnesium pyrovanadate could be removed during the reaction
generating two V4+ cations. The rearrangement of local structure transforms V2O7
unit to two two-edge sharing square-based VO3 units. This conclusion was supported
by the easier reducibility of magnesium pyrovanadate phase.
Fig. 1-12. Scheme of active sites a) VO4 in Mg3V2O8 and b) V2O7 in Mg2V2O7.[80]
Moreover, it has been shown that some synergy effect existing between the
phases may lead to the increase in the selectivity. Mirodatos et al. [229] attributed
high catalytic performaces to the presence of significant amounts of MgO and distinct
magnesium vanadates. The best propene yields in oxidative dehydrogenation of
propane were obtained for V-Mg-O system containing 14 wt% of vanadium after
calcination at temperature of 800 oC, where magnesium orthovanadate phase with
MgO were coexisted. Thus the selectivity of Mg3V2O8 increased in the presence of
the Mg2V2O7 or MgO phases in intimate contact [109], although this phenomenon is
not understood up to now.
Reasuming, it should be pointed out that V-Mg-O catalysts are active and
selective in oxidative dehydrogenation of propane and butane, though no clear
conclusion about the role of each vanadates in catalytic performances can be
reached.
The catalytic behaviour of separate phases in V-Mg-O system depends also
on the nature of alkane. The requirements for the size of a cluster may be different
for alkanes of different chain length. Kung [4, 15] argues that in Mg3V2O8, the
adsorbed C2, C3, C4 species can interact with only one surface VO4 unit, separated
from another by MgO6 octahedra, forming on the surface chains Mg-O-V-O-Mg. The
close presence of two VO4 units in Mg2V2O7 leading to the appearance of Mg-O-V-O-
V-O-Mg groups, makes it possible for larger molecules (e.g. n-butane to interact with
Introduction
44
several oxygen atoms surrounding V). This would lead to the lower olefin selectivity
in ODH of n-butane as compared with that in the propane ODH on Mg
pyrophosphate. More detailed studies on characterization of the catalysts, in
particular of the catalyst surface, in the optimal composition range, are necessary to
decide between these possibilities.
1.3. Effect of additives on the physicochemical and catalytic
properties of oxide catalysts in oxidation reactions
Beside the main components constituting an active phase the catalysts can
contain small amounts of various additives (usually below 2 wt%). The main group
elements (e.g. alkali metals, alkaline earths, phosphorus) or transition metals (e.g.
Cu, Ti, Fe, Co, Te, Mn, Ni), often quoted in the patent literature, have effect on the
catalytic properties and lifetime of catalytic systems. Practically no systematic studies
on effect of additives of different nature on structure and the catalysts’ properties
have been reported. Scarce fundamental studies on the effect of additives to
oxidation catalysts on their performance in oxidation reactions have been concerned
mainly with the effect of alkalis [19, 37, 42, 94, 110].
1.3.1. Effect of additives on structural properties
The small amounts of additives introduced into the bulk of an oxide, may
account for point defects in both cationic and anionic sublattices, including interstitial
or substitutional cations, oxygen, and cationic vacancies.
At low content of additives (<0.01 at%, so called “doping” in older literature)
the electronic properties of oxides such as electrical conductivity or electron work
function are generally changed, whereas at higher concentrations both the structure
and structure related properties such as length and energetics of bonds may be
modified. The solid solutions of additives with the main oxide phase, e.g. MoO3 in
V2O5 and definite compounds of additives with the main phase e.g. potassium
vanadates and bronzes in the V2O5 + K system may be formed [110].
Introduction
45
On the other hand the additives may modify local structure and properties of
active centers on the surface of catalyst. The two-dimensional oxide compounds can
be formed, or some sites can be eliminated, e.g. Brönsted acid centres by
substitution of a proton in an acidic OH group by atoms of alkali metals.
The schematic model of the different location of additives in an oxide system is
presented after [110] in figure 1-13.
Fig.1-13. Scheme of the locations of additives, A, in oxide catalysts: A and A’’, in substitutional and interstitial lattice positions, respectively; □, oxygen vacancy (if valency of A<Me); A’, on the surface; AO, bidimensional surface compound.
For example, introduction of P5+, W6+, Mo6+, and Sn4+ ions into V2O5 increases
the number of V=O species on planes other than (010) [111], while the presence of
potassium during the preparation of V-Mg-O system leads to the formation of both
magnesium ortho- and pyro- vanadates, whereas without potassium only magnesium
orthovanadate is formed [112].
Additives may also change the surface composition due to segregation effects.
For instance introduction of alkali and alkaline earth elements into the bulk of
(VO)2P2O7 systems resulted in the increase in the surface P/V ratio [113].
The studies concerning effect of additives to vanadia-based catalysts have
been in most cases performed for systems based on supported vanadia with addition
of K or other alkalis. At low content of vanadium oxide, below the monolayer, the
potassium ions increase the number of monomeric species, decreasing the amount
of polymeric forms [114, 115], and influence the bond length and coordination of
terminal V5+=O groups [116–118]. Potassium ions can also change the strength of
the bonding of vanadia species to the support decreasing the electron density on V
atoms [114]. At high content of vanadia, at which the tridimensional species are
present, the introduction of alkali metals in VOx/TiO2 catalysts leads to a higher
dispersion of vanadia on the surface [19, 37], spreading (wetting) of vanadia on the
support and its amorphization [119]. The presence of potassium vanadates has been
Introduction
46
also observed both in the monolayer content when the K/V ratio is high [114], and
above the monolayer loading [119].
The effect of other additives to VOx/TiO2 catalysts on the catalyst surface was
considered in several papers [74, 116, 120, 121]. Wachs [116] considers two types of
additives: a) non-interacting (W, Nb, S, Si, M, Ni, Co, and Fe), which coordinate to
the support and change only the ratio of isolated monomeric to polymerized vanadia
species, not affecting the bond length of vanadia species; and b) interacting (alkali
and alkaline earth elements, P), which interact with vanadia species, modifying the
length of the V=O terminal and probably V-O-support bonds, or forming the surface
compounds, e.g. phosphates.
Further studies of the same group on VOx/TiO2 catalysts with different
additives shown that the presence of W6+, Zn2+, Nb5+, Ga3+, Ge4+, Sn4+ or Ce4+ ions
does not appear to affect the structure of the surface vanadia species, which in the
promoted samples remain primarily in the isolated monomeric form, however the
addition of Mo6+, Mn4+ or Fe3+ ions appears to enhance the formation of the
polymeric vanadium species [121].
1.3.2. Effect of additives on activity
The additives may affect the activity in oxidation reactions. The alkali metals
usually exert the poisoning effect. For example the decrease in activity has been
observed in various reactions such as the oxidation of toluene on vanadia–titania
catalysts with K additives [122], ODH of propane on the same type of catalysts with
Li, K, and Rb additives [37] and oxidation of butane on vanadyl pyrophosphate
modified on the surface with K [123]. The kinetic studies of propane ODH reaction
implied that addition of alkali metals led to a decrease of the rate constants of the
propene formation and a simultaneous increase of the activation energy of the
subsequent oxidation of propene, which resulted in the decrease of activity [37, 124,
241, 242].
The decrease of the activity for the alkali doped samples observed for most of
the vanadia-based catalysts was ascribed, in the first place, to geometrical blocking
of active centres on the vanadia surface. Observed effect is more evident when the
Introduction
47
ionic radius of the promoter increases. For a series of alkali metals the sequence of
the decreasing activities corresponds to the increasing radius of the alkali metal: Li
(0.68 Å)>K (1.33 Å)>Rb (1.48 Å) [37].
Beside geometric effects, additives may affect the activity by modifying the
reducibility of catalysts (the M-O bond strength). For several system the correlations
between the reducibility, i.e. the ease of the removal of the catalyst oxygen, taken as
a measure of Me-O bond energy, and the catalytic activity has been reported. The
decrease in reducibility, (evaluated usually from H2-TPR data), on addition of alkali
metals to supported vanadia catalysts has been reported [48]. An increase in the
activity with a decrease in the reducibility has been reported for the ODH of propane
on V2O5/Al2O3 catalysts with K, Bi, P, and Mo additives [50] and for n-butane
oxidation on vanadyl pyrophosphate catalysts, containing in their structure additives
of alkali and alkaline earth elements [113].
1.3.3. Effect of additives on selectivity
The additives may affect the catalysts selectivity in two parallel ways: a) by
modification of acido-basic properties and b) by modification of metal-oxygen bond
energy and type of the oxygen species.
It has been reported in several recent works that introduction of alkali metal
additives to oxide systems has a beneficial effect on the selectivity to olefins in the
ODH of lower alkanes. The positive effect of alkali metals on the selectivity to olefins
has been observed for the ODH of propane on VOx/TiO2 and MoOx/TiO2 catalysts
with Li, K, and Rb [37] and on MoOx/ZrO2 with Li, K, and Cs [124], and of n-butane on
nickel molybdate with Li, Na, K, and Cs [125]. The increase in the selectivity to
propene for the alkali-doped VOx/TiO2 catalysts has been correlated with the
decrease of the catalyst acidity and increase in the basicity [19, 37], the acid-base
properties being measured by a probe reaction of the isopropanol decomposition. It
was argued that propene (a base) is less strongly held on the less acidic and more
basic surface and is desorbed before reacting further to carbon oxides. This
hypothesis is confirmed by the decrease of heat of propene adsorption for VOx/TiO2
catalysts with addition of alkali metals [128], and the decrease in the amount of
Introduction
48
propene sorbed on VOx/TiO2 catalysts with Na additive [129]. The effect of alkalis has
been found to depend on the nature of alkane.
The addition of phosphorus to VOx/TiO2 catalyst increases the selectivities to
acidic products such as maleic anhydride in butadiene oxidation [130], benzoic acid
in toluene oxidation [131, 132], and phthalic anhydride in o-xylene oxidation at high
conversions [86], whereas it decreases the selectivity to the basic product as
butadiene in butene oxidation [130]. This is in agreement with the presumption that
acidic products are less strongly held on the more acidic surface, and undergo easy
desorption before they can be overoxidized to carbon oxides, whereas the inverse
effect operates for basic products.
Beside acido-basic properties the metal-oxygen bond energy and oxygen
forms present on the surface which decide about selectivity may be also affected by
the additives.
A decrease in the selectivity to isobutene in the ODH of isobutane was
observed for CrOx/Al2O3 catalyst with additives of both lower (K) and higher (Mo)
than Cr ions electronegativities. The introduction of P to that system had practically
no effect on selectivity. It has been found that the K and Mo ions increase the rate of
reduction of chromia-alumina catalyst, whereas P has no effect. The lower selectivity
to isobutene was correlated with the weakening of the Cr-O bond by the additives
[110].
The additives can modified the surface potential (work function) of catalysts.
The beneficial effect of potassium on selectivity to olefin in the ODH of propane on
VOx/TiO2 catalysts was ascribed to the considerable decrease of the surface
potential, observed on doping of the system with potassium [19]. The correlation
between the selectivity to propene and the work function was also observed for the
same system doped with Cr, Al, Fe and W [126].
Measurements of surface potential have shown, moreover, that the addition of
K ions to VOx/TiO2 and MoOx/TiO2 hinders the formation of electrophilic O- species
and facilitates the electron transfer leading to formation of nucleophilic O2- species
[19]. Higher selectivity of K-containing catalysts in ODH of propane can be then partly
related to the absence of electrophilic oxygen forms on the surface. Beside the
modification of the acid–base properties this would be another reason for the higher
selectivity of the catalysts in the ODH of propane.
Chapter 2
AIM AND SCOPE OF THE WORK
Aim and scope of the work
50
2. Aim and scope of the work
As discussed in the preceding paragraph, catalytic properties of oxide catalysts
can be improved by introduction of additives. The patent literature provides ample
examples of additives of various chemical nature introduced to industrial catalysts of
oxidation, the fundamental studies on mechanism of their action are, however,
scarce. The brief review of academic research on the problem [110] demonstrates
the variety of effects of the additives on structure, physicochemical and catalytic
properties of oxide catalysts in selective oxidation reaction, and shows a need for
systematic studies on the problem.
No systematic studies on effect of additives to oxide catalysts on their
performance in ODH reactions of lower alkanes have been reported so far, to date
research being limited mostly to the alkali metal additives (c.f. Introduction).
The aim of the present work has been to determine the effect of a wide
range of additives of various nature on physicochemical properties and
catalytic performance in oxidative dehydrogenation of ethane and propane on
vanadia-based catalysts (promising, as shown in the Introduction, in the ODH
reactions of lower alkanes). Furthermore, an attempt has been made to
correlate the acido-basic and redox properties claimed as essential in
oxidation reactions (c.f. Introduction) with the catalytic data obtained in
oxidative dehydrogenation reactions of ethane and propane. Such correlations
could be useful for description of the mechanism of action of oxide catalysts in
the ODH reactions of lower alkanes. The use of C3H8 and C2H6 in catalytic tests
allowed to determine the effect of the alkane nature on the catalytic
performance on catalysts of different properties.
Two series of the catalysts have been selected, containing vanadia on supports
of different nature: a) SiO2, acidic, weakly interacting with the dispersed vanadia
phase and b) MgO, basic which can react with vanadia.
Aim and scope of the work
51
The additives represent both main group elements (K, P) and transition metal
ions of different valency with respect to the formal valency of vanadium ions V5+. The
latter include Ni2+, Cr3+, Nb5+ and Mo6+ ions.
In selection of the second group of additives, a simple hypothesis was adopted: it
has been assumed that cationic additives of lower electronegativity than that of the
active cation (V5+) in a V-O couple (an active centre), shift the electrons towards the
active centre, rendering it less acidic (less electrophilic) and more basic
(nucleophilic), whereas those of higher electronegativity should withdraw the
electrons from an active centre, rendering it more acidic and less basic. The change
of electron density over V5+-O centre induced by the additives should be reflected in
the changes of catalytic properties, in particular in the selectivity to olefins, as well as
in other physico-chemical properties.
In view of promising effect of K on the selectivity to propene observed in the
preliminary studies, the effect of other alkali (Li, Na, Rb) and alkaline earth (Ca, Mg)
metals on catalytic properties was also studied.
The scope of the work comprised:
a) synthesis of catalysts,
b) characterisation of their composition and structure with XRD, XPS, 51V NMR,
Raman, UV-VIS and IR spectroscopies, and texture (BET, and EM for some of the
catalysts),
c) determination of their acido-basic properties by IR of adsorbed pyridine and a
probe reaction decomposition of isopropanol, IPrOH,
d) determination of catalysts’ reducibility by H2-TPR and TPD of adsorbed
propane,
e) catalytic tests in the reaction of oxidative dehydrogenation, ODH of propane
and ethane.
Chapter 3
EXPERIMENTAL PART
Experimental part
53
3.1. Materials and reagents
The materials and reagents used for preparation and characterization of
catalysts under study are listed in Table 3-1, which gives the formula, name and
molecular weight of a reagent, its purity and name of a producer.
Table 3-1. List of reagents used in experimental part.
Formula Name Molecular
weight [g/mol]
Purity Producer
MgO magnesium oxide 40.30 ≥99% Ubichem
SiO2 silicon dioxide,
Aerosil®200
60.08 >99.8% Degussa
NH4VO3 ammonium
(meta)vanadate
116.98 p.p.a. Fluka
KNO3 potassium nitrate 101.10 p.p.a. POCha)
Ni(NO3)2 · 6H2O nickel (II) nitrate
heksahydrate
290.79 p.p.a. POCh
Cr(NO3)3 · 9H2O chromium (III)
nitrate nonahydrate
400.15 p.p.a. POCh
Nb2O5 niobium (V) oxide 265.81 p.p.a. CBMMb)
H3PO4 ortho-phosphoric
acid
98.00 p.p.a. POCh
(NH4)6Mo7O24 ·
4H2O
ammonium
(para)molybdate
tetrahydrate
1235.86 p.p.a. POCh
LiNO3 · 3H2O lithium nitrate 122.99 p.p.a. POCh
NaOH sodium hydroxide 40.00 p.p.a. POCh
RbNO3 rubidium nitrate 147.48 p.p.a. Fluka
Ca(NO3)2 · 4H2O calcium nitrate 164.09 p.p.a. POCh
Mg(NO3)2 · 6H2O magnesium nitrate 256.41 p.p.a. POCh
C2H2O4 · H2O oxalic acid
dihydrate
126.07 p.p.a. Chempur
Experimental part
54
C5H5N pyridinum 79.1 p.p.a. Aldrich
KBr potassium bromide 119.00 p.p.a. POCh
C2H6 ethane 30.07 ≥99% Linde Gas
C3H8 propane 44.10 ≥99% Linde Gas
He helium 4.00 ≥99.999% Linde Gas
Ar argon 39.95 ≥99.998% Linde Gas
H2 hydrogen 2.02 ≥99.999% Messer
a) Polskie Odczynniki Chemiczne (Polish Chemical Reagents) b) Companhia Brasileira de Metalurgia e Mineração p.p.a. – pure pro analysis
Experimental part
55
3.2. Preparation of catalysts
The VOx/SiO2 (VSi) catalysts were prepared using 100 cm3 of 0.22 M aqueous
solution of ammonium metavanadate (NH4VO3) and a commercial silica support
(Aerosil 200). Before impregnation the support was pre-treated in water at room
temperature and dried at 90 oC for 12h: its specific surface area after such treatment
was 175 m2/g. The impregnation at 90 oC for 4 h followed by evaporation of the
solute at 100 oC for 1 h, under stirring drying for 5h at 120 oC and calcination under a
flow of air for 5h at 500 oC.
The additives (A) were introduced by adding appropriate amounts of their
soluble salts or acids [KNO3, LiNO3, RbNO3, Ca(NO3)2, Mg(NO3)2, Ni(NO3)2,
Cr(NO3)3, H3PO4, NaOH, ammonium paramolybdate, and niobia dissolved in oxalic
acid] to the metavanadate solution. All the reactants used were of p.p.a. grade. The
atomic ratio A/V in the calcined catalysts was 0.2 with the exception of alkali and
alkaline earth metal ions, for which A/V was 0.1. The A/V ratio of 0.1 was shown in
previous studies as optimal for VOx/TiO2 catalysts with potassium additive [42]. In a
special study on catalysts containing K, varying between 0.05 and 0.2 the A/V ratio of
0.1 was also found optimal for VSi catalysts.
The VOx/MgO (VMg) catalysts were prepared in the same way using 100 cm3
0.09 M aqueous solution of ammonium metavanadate (NH4VO3) and a commercial
MgO (Ubichem, 72 m2/g).
The content of vanadia in all the samples corresponded to 1.5 monolayers of
V2O5. In the present work the term “a monolayer” signifies a theoretical value
calculated from the crystalographic data. From the V-O bond length of crystalline
V2O5 it can be estimated that an isolated monomeric vanadium oxide layer and a
two-dimensional polyvanadate oxide layer correspond to a surface density of 2.5 and
10 V atoms per 1 nm2, respectively [93]. The vanadium oxide content in VOx/SiO2
(VSi) and VOx/MgO (VMg) was calculated with the assumption that one monolayer
contains 10 V atoms per 1 nm2 of the support surface.
The symbols of the samples, adopted further in the text, are VSiA or VMgA for
respectively vanadia-silica and vanadia-magnesia catalysts, where A is the additive.
Experimental part
56
The procedure of synthesis of supported vanadium oxide catalysts is shown fig. 3-1.
Table 3-2 gives a list of prepared catalysts.
Fig. 3-1. Scheme of preparation of supported vanadia catalysts.
500 oC , 5 hours
120 oC , 5 hours
Impregnation
Drying
Calcination
NH4VO3
70 oC
Solution of vanadium oxide
precursor
H2O
Solution of additive
precursor
Support
H2O Additive
salts
VOx / oxide support + additive
catalyst
Evaporation 100 oC , 1 hour
90 oC , 4 hours
Experimental part
57
Table 3-2. The list of the prepared catalysts*.
Catalysts A / V ratio wt % V2O5 wt % additive Symbol
VOx/SiO2 - 28.39 - VSi
VOx/SiO2 + K 0.02 28.29 0.24 VSiK0.02
VOx/SiO2 + K 0.05 28.18 0.61 VSiK0.05
VOx/SiO2 + K 0.1 28.05 1.21 VSiK
VOx/SiO2 + K 0.2 27.68 2.39 VSiK0.2
VOx/SiO2 + Li 0.1 27.93 1.48 VSiLi
VOx/SiO2 + Na 0.1 28.15 0.71 VSiNa
VOx/SiO2 + Rb 0.1 27.56 2.80 VSiRb
VOx/SiO2 + Ca 0.1 28.00 1.24 VSiCa
VOx/SiO2 + Mg 0.1 28.14 0.75 VSiMg
VOx/SiO2 + Ni 0.2 27.39 3.54 VSiNi
VOx/SiO2 + Cr 0.2 27.50 3.15 VSiCr
VOx/SiO2 + Nb 0.2 26.84 5.48 VSiNb
VOx/SiO2 + P 0.2 27.85 1.90 VSiP
VOx/SiO2 + Mo 0.2 26.79 5.65 VSiMo
VOx/MgO - 14.09 - VMg
VOx/MgO + K 0.1 14.01 0.60 VMgK
VOx/MgO + Li 0.1 13.99 0.74 VMgLi
VOx/MgO + Na 0.1 14.04 0.35 VMgNa
VOx/MgO + Rb 0.1 13.91 1.31 VMgRb
VOx/MgO + Ca 0.1 14.00 0.62 VMgCa
VOx/MgO + Ni 0.2 13.84 1.79 VMgNi
VOx/SiO2 + Cr 0.2 13.87 1.59 VMgCr
VOx/SiO2 + Nb 0.2 13.70 2.80 VMgNb
VOx/SiO2 + P 0.2 13.76 0.95 VMgP
VOx/SiO2 + Mo 0.2 13.69 2.89 VMgMo
*) in Annex I, in which the optimalization of K amount is described, the symbol of
VOx/SiO2 + K (K/V=0.1) sample is VSiK0.1
Experimental part
58
3.3. Physicochemical characterisation
Various techniques were used for characterization of structure and of some
essential properties of the studied catalysts. XRD, IR, Raman, NMR and UV-VIS
spectroscopies were used for determination of phase composition and of structure of
vanadia species dispersed on the supports. The surface composition was determined
by ESCA-XPS technique. Electron microscopy (SEM, TEM) techniques were applied
for textural studies. Characterization of physical properties included measurements of
reducibility by H2-TPR technique, interaction of propane with the catalysts (propane
TPD-MS method) and measurements of acid-base properties. The latter were
determined by registration of the IR spectra of adsorbed pyridine and by a probe
reaction of the isopropanol decomposition.
3.3.A. Structure and texture of the catalysts
3.3.1. Determination of specific surface area (BET)
The specific surface area of the samples was determined by BET method of
nitrogen adsorption at -196 oC, after degassing for about 16 hours at 200 oC with a
Quantachrome Autosorb-1 apparatus. The principles of the method are given in
Annex II.
3.3.2. X-ray diffraction analysis
XRD patterns were recorded on a Bruker D 5005 diffractometer between 10
and 80º (2θ) with a step 0.02º at 50kV and 35 mA, using Cu Kα radiation (λ =
0.015418 nm). Diffraction patterns were assigned to a given compound using the
PDF database supplied by the International Centre for Diffraction Data (PDF2 –
Diffraction Database File). XRD resources of IRC, CNRS in Lyon were used. The
principles of the X-ray diffraction technique are described in Annex III.
Experimental part
59
3.3.3. X-ray photoelectron spectroscopy (ESCA – XPS)
The XPS spectra were recorded with a VG Scientific ESCA-3 spectrometer
using Al Kα1.2 radiation (1486.6 eV) from an X-ray source operating at 12 kV and 20
mA. The working pressure was better than 2 x 10-8 Torr (1 Torr = 133.3 Pa). All
spectra were recorded at a photoelectron take-off angle of 45. Usually 20 – 30 scans
were accumulated for a band. The binding energies (BE’s) were referenced to the C
(1s) peak from the carbon surface deposit at 284.8 eV. Data processing consisted of
the basic operations calibration of the peak position against the C (1s) line,
background and Kα3.4 peaks removal, together with routines for the analysis of
composite spectra by fitting with single peaks or doublets. In the case well resolved
peaks, the B.E. was determined within ± 0.1 eV. Spectra were decomposed into
symmetric gaussian-20% lorenzian peaks. The relative element content (NA) was
calculated from the formula [133]:
NA = F · IA · Ee0.25 · σA
-1 exp(dC · λCA-1)
where NA is the relative element content,
F contains all instrumental factors and is assumed to be constant
for the measurements,
IA is the intensity of the line A measured,
Ee is the kinetic energy of electrons,
σA is the elemental cross section for photoionization [134],
λA is the A-level electrons’ inelastic mean free path in the
adventitious carbon deposit [135]
dC is the thickness of the carbon deposit [135].
The relative dispersion of the surface vanadium oxide species was estimated
by the XPS vanadium to the support cation intensity ratio (IV/IS, S = support cation).
The principles of the X-ray photoelectron spectroscopy are given in Annex IV.
3.3.4. Infrared spectroscopy (IR)
The IR spectra in the range 400 – 4000 cm-1 of the catalysts were registered at
room temperature with a Perkin Elmer 580. For the IR study the samples were
compacted with KBr as wafers. In the KBr pellet method, the solid sample is finely
Experimental part
60
pulverized with pure, dry KBr, the mixture is pressed in a hydraulic press to form a
transparent pellet, and the spectrum of the pellet is measured. A major advantage of
this method is that KBr has no absorptions in the IR above 250 cm-1, so that an
unimpeded spectrum of the compound is obtained. The principles of the IR
spectroscopy are described in Annex V.
3.3.5. Laser Raman spectroscopy (LRS)
The Raman spectra in the range 610 – 1180 cm-1 characteristic for M-O
vibration in oxides and in the range 1230 - 1760 cm-1 for eventual carbon deposition
were obtained on a DILOR XY spectrometer equipped with a CCD detector. The
emission line at 514.5 nm from an Ar+ ion laser (Spectra Physics) was focused on the
samples under the microscope; the analysed spot was about 1 µm. The power of the
incident beam on the sample was 100 mW. The time of the acquisition was adjusted
according to the intensity of the Raman scattering. The scheme of Optical setup of
the DILOR XY spectrometer and the bases of the technique are given in the Annex
VI.
3.3.6. Ultraviolet visible spectroscopy (UV-VIS)
UV-VIS spectra were obtained in the range 200 – 800 nm with a Perkin Elmer
Lambda 9 spectrometer equipped with a reflectance accessory and a homemade
sample holder containing 0.2 g solid powder. BaSO4 was used as a reference in the
measurements. The principles of the UV-VIS spectroscopy are described in Annex
VII.
3.3.7. Solid-state 51V nuclear magnetic resonance spectroscopy
(NMR)
Solid state 51 V NMR spectra were recorded at ambient conditions with a
Brucker DSX 400 (9.4 T) NMR spectrometer operating at 105.2 MHz conditions. The 51 V chemical shift were calibrated against liquid VOCl3, using a 0.16 M NaVO3
Experimental part
61
aqueous solution (chemical shift at -574.3 ppm) as a secondary reference. The
bases of the technique are given in Annex VIII.
3.3.8. Electron microscopy measurements
The principles of the Scanning Electron Microscopy measurements are
described in Annex IXa and of the Transmission Electron Microscopy measurements
in Annex IXb.
3.3.8.1. Scanning Electron Microscope (SEM)
The Scanning Electron Microscope (SEM) was used to carry out the
morphological study of the samples. Scanning electron micrographs were recorded
using a JSM-840A JOEL scanning electron microscopy operated at 15 kV.
3.3.8.2. Transmission Electron Microscopy (TEM)
Electron micrographs were taken on a JEOL JEM 2010 electron microscope,
with a point to point resolution of 0.19 nm (coefficient of spherical aberration Cs = 0.5
mm). The voltage used was 200 kV.
Experimental part
62
3.3.B. Determination of physicochemical properties
3.3.9. Temperature-programmed reduction techniques
The principles of the H2 temperature-programmed reduction (TPR – H2) are
described in Annex Xa and of the propane temperature-programmed desorption
(propane TPD-MS) in Annex Xb.
3.3.9.1. Temperature-programmed reduction (TPR – H2)
The H2-TPR measurements were carried out using a U-type quartz
microreactor. The samples were about 20 mg for VSiA and 40 mg for VMgA series.
The catalysts were first pretreated 3 h at 400 oC in the flow of argon, then cooled
down, and afterwards contacted with an 1 % volume ratio H2 : Ar mixture (total flow
1.3 l/h). Next, the samples were heated at the rate 3 oC/min to the final temperature
750 oC. The hydrogen consumption was monitored using TCD of type Delsi Nermag
(DN) 11.
3.3.9.2. Temperature-programmed desorption (propane TPD-MS)
The interaction of propane with catalysts surface have been examined with the
help of the TPD technique. The measurements were carried out in a microreactor
coupled to a mass spectrometer VG/Fisons Quartz-200D. About 0.3 g of a sample
was heated in a stream of a dried air at 400 oC for 1 hour and cooled down in the flow
of air to the room temperature. After pretreatment the sample was flushed with He for
20 minutes, and a stream of mixture of propane (5%) in He was introduced at 200 oC
for 0.5 hour. The system was then cooled down to room temperature, and again
flushed with He for several minutes. The desorption was performed from 25 oC to
600 oC in a stream of He (20 ml/min at 10 oC/min). The mass peaks registered were
as follows: CO2 = 44, CO = 28, H2O = 18, propane = 29, propene = 41, 39.
Experimental part
63
3.3.10. Determination of acido-basic properties
3.3.10.1. Decomposition of isopropanol
The decomposition of isopropanol to propene, diisopropyl ether (DIE) and
acetone was studied at 200 oC by the pulse method with GC analysis. Dried helium
as a carrier gas and 0.1 ml of the sample and 0.5 µl of isopropanol for VMgA and 1 µl
for VSiA were used. The total flow rate of helium was 50 ml min-1 for VMgA series
and 100 ml min-1 for VSiA catalysts. Prior to the test the samples were treated in situ
in a stream of dried helium for 1 h at 200 oC to purify them from the adsorbed water
and physisorbed oxygen. The isopropanol pulses were injected successively until
constant values of conversion and yield of products were attained. The conversion
and the yield of products decreased slightly with the number of pulses up to 3-5
pulses and then remained constant. The principles of the decomposition of
isopropanol are given in Annex XI.
3.3.10.2. Adsorption of pyridine as a probe molecule
Surface acidic properties of catalysts were determined from infrared spectra of
adsorbed pyridine. A self-supporting disc (10-30 mg, 14 mm diameter) was initially
activated at 450 oC in flow of oxygen for 1 h, then evacuated at 400 oC for 2 h and
subsequently exposed to pyridine at 100 oC. The desorption of pyridine was
performed for 5 min at 100 and then for 30 min at 100, 150, 200, 250 and 300 oC.
The IR spectra of the adsorbed pyridine were recorded with a Perkin Elmer 580
spectrometer at room temperature after adsorption and desorption at each
temperature. The number of Lewis acid sites (LAS) and Brönsted acid sites (BAS)
was estimated from the intensity of IR bands at 1450 and 1540 cm-1, respectively. A
quantitative comparison of the acid sites number of the samples was made by
calculation the area under the LPy and BPy peaks, after desorption at 150 oC for
LyPy and 100 oC for BPy, using a molar extinction coefficient ε value of 1.5 and 1.8
(cm/µmol) for coordinated and pyridinium species, respectively [136-139]. The
concentration (number) of BAS and LAS referenced to 1 m2 surface area of sample
Experimental part
64
has been obtained by calculating the area under Bpy and LPy peaks, using the
equation:
AI Π R2
c =
w єI SSA
where R (cm) is a radius of the catalyst wafer and w (g) is a weight of the dry sample
and SSA is specific surface area (m2/g) [140]. The strength of Lewis or Brönsted acid
sites were determined from the A150/A100 ratios, where A100 and A150 are the areas of
LPy or BPy bands after the pyridine desorption at 100 and 150 oC. The principles of
the method are described in Annex XII.
Experimental part
65
3.4. Catalytic measurements
3.4.1. Description of apparatus, conditions of measurements,
and analysis of reaction products
The activity of the catalysts in oxidative dehydrogenation of propane and
ethane was measured in a fixed bed flow apparatus in the temperature range 360-
520 oC. A stainless steel reactor (120mm long, internal diameter 13mm) was coupled
directly by a set of manifold valves to a of gas chromatograph, the thermocouple for
the temperature measurements being placed coaxially in the catalyst bed. Analysis of
products and unreacted propane and ethane was performed by on-line gas
chromatography using Hewlett-Packard 5890 (ODH of propane) and SRI 8619C
(ODH of ethane) chromatographs with catharometric detection.
The details concerning conditions of the ODH reaction are given in table 3-3.
Table 3-3. Conditions of GC analysis
ODH of ethane ODH of propane
apparatus SRI 8619C Hewlett-Packard 5890
detector TCD TCD
columns type Molecular
sieves
Hayesep “C” Molecular
sieves 13 X
Durapac
columns length ~2 m ~2 m ~2 m ~2 m
analyzed
products
O2 ; N2 ; CO CH4 ; CO2 ;
C2H4 ; C2H6
O2 ; N2 ; CO CO2 ; C3H8 ;
C3H6
retention times
[min]
N2 : 1.20
O2 : 1.60
CH4 : 2.30
CO : 2.70
CO2 : 5.00
C2H4 : 7.90
C2H6 : 9.70
N2 : 1.20
O2 : 1.65
CO2 : 2.50
C3H8 : 3.42
CO : 3.75
C3H6 : 4.46
Experimental part
66
The reaction mixture contained 7.1 vol% of propane and 17 vol% of ethane in
air from the literature. The composition of the reaction mixture and the constant flow
of the reactants was maintained by mass flow controllers: Brooks SL 5850D and
ERG 020. 0.5 ml (for propane) and 1 ml (for ethane) of a catalyst sample of grain
size 0.63-1mm, diluted with acid-washed quartz beads in order to avoid temperature
and concentration gradients was used . Analysis of the products and unreacted
alkane was started after 1 hour of stabilization in the reaction mixture.
The blank tests with an empty reactor or reactor filled quartz beads show no
C3H8 or C2H6 conversion up to 520 oC. Both supports SiO2 and MgO were also found
inactive up to this temperature. For each temperature and flow rate the analysis was
performed three fold – the values reported further in the text are the average of the
three measurements.
Propene, or ethene and carbon oxides (CO and CO2) were found to be main
reaction products. The amounts of oxygenates (acrolein and acrylic acids) for both
reactions, and of the degradation, C2 products for the reaction of propane were below
1% of the total amount of products.
Since the selectivity to olefins decreased with the increasing conversion (a
typical behaviour for the ODH reactions on oxide catalysts [4, 5, 9, 94, 106], cf.
Chapter 4.2), the selectivities at the same conversions (iso-conversions) were
compared. To obtain the same conversions for a series of catalysts of different
activities, the total flow of the reaction mixture at a given reaction temperature varied
between 20-120 ml/min, which corresponded to the contact time τ = 0.2-1.5 s. On the
other hand the flow rate (contact time) had to be adjusted for some catalysts to keep
the conversions below ~20%: above these values the conversions were limited by
the high consumption of oxygen. The carbon balance for conversions higher than
about 10% was better than 97 ± 2%, at lower conversions the balance was poorer.
The oxygen conversion was below 60 % in all cases. The schemes of a set-up and of
the reactor are given in Annex XIII.
Experimental part
67
3.4.2. Presentation of the catalytic data
The data obtained from GC measurements were calculated from the peak
area of the chromatograms taking into account calibration coefficient for each of the
analysed compounds. The latter was derived from calibration measurements in which
the calibration coefficient of a given compound varied 10 x. The total conversion (Xi)
was calculated as:
ci0- ci
Xi (%) = x 100 ci
0
where ci0 and ci are the concentrations of ethane or propane at the inlet and outlet of
the reactor respectively.
The selectivity (Sx) to a given product was calculated from the number of
moles of product divided by the total number of moles of products in the product
mixture using the general formula:
vn-1 cn
Sn(%) = x 100
ΣΣΣΣ vn-1 cn
where cn are concentrations of products n and vn the stoichiometric coefficient of the
reaction leading to n-th product.
The yields (Y) of ethene and propene were calculated as:
XC2H6 ⋅⋅⋅⋅ SC2H4
YC2H4 (%) = 100
XC3H8 ⋅⋅⋅⋅ SC3H6
YC3H6 (%) = 100
The product balance (B) was checked by comparing the conversion to
products (Xp) to the total conversion (Xi)
Xp
B = x 100 Xi
Experimental part
68
where conversion to products (Xp) was calculated as:
ΣΣΣΣ vn cn
Xp = x 100
ΣΣΣΣ vn cn + ci
The contact time τ (in seconds) of substrate with catalyst bed was calculated
using the formula:
Vcat
ττττ [s] = flowi
where Vcat [cm3] is the volume of catalyst,
flowi [ml/s] is a flow of ethane or propane at the inlet of the reactor,
The total areal (specific) activity (A) of catalysts was calculated as:
flowi x Xi
A [µµµµmol m-2 min-1] = 100 x SSA x mcat x Vm
where flowi [ml/min] is a flow of ethane or propane at the inlet of the reactor,
Xi [%] is the total conversion,
mcat [g] is the weight of sample,
SSA [m2/g] is the specific surface area of catalyst and
Vm [m3/mol] is the volume occupied by one mole of gas; Vm = 0.0224.
The activation energy the alkane disappearance of the total reaction of
propane was estimated from the Arrhenius equation by plotting ln A versus 1/T.
Chapter 4
RESULTS OF STUDIES
AND DISCUSSION
Results of studies and discussion
70
4.1. Physicochemical properties of studied systems
4.1.1. Structure and texture of catalysts
4.1.1.1. VOx / SiO2 catalysts
4.1.1.1.1. VOx/SiO2 catalysts with additives of main group elements (K+, P5+)
and of transition metal ions (Ni2+, Cr3+, Nb5+, Mo6+)
Bulk composition and surface area
The basic data of the support and vanadium oxide supported on silica
catalysts are listed in Table 4-1, which gives composition of the samples, their
specific surface area, obtained with the BET method and electronegativity of the
introduced additive ions. For the VSiA series the specific surface area of the
catalysts, SSA was lower than that of the pure SiO2 support (175 m2g-1). The
presence of the additives led to the further decrease of the specific surface area with
respect to the undoped VSi catalyst, the effect being the biggest for the P, Mo and K
additives. For VSiP and VSiMo samples the decrease by ~ 40-45% of the value for
VSi, and for VSiK the decrease by ~20% were observed. The decrease in the
specific surface area of the support on deposition of the vanadia phase has been
frequently observed in supported vanadia catalysts [37] and can be due to sintering
of the support in the presence of deposited phase.
Table 4-1. List and composition of samples of VOx/SiO2 catalysts
List of catalysts SSA [m2/g] wt % V2O5 wt % additive χχχχi*
SiO2 support 175.0 - - -
VSi 116.6 28.39 - V5+ 17.93
VSiK 92.2 28.05 1.21 K+ 2.46
VSiNi 110.8 27.39 3.54 Ni2+ 9.55
VSiCr 109.1 27.50 3.15 Cr3+ 11.62
VSiNb 104.2 26.84 5.48 Nb5+ 17.60
VSiP 70.2 27.85 1.90 P5+ 24.09
VSiMo 76.8 26.79 5.65 Mo6+ 19.44
χχχχi* electronegativity of introduced ions – calculated from χMez+ = (1 + 2z) χMe
where χMe is an electronegativity of an atom and z its valency
Results of studies and discussion
71
X-ray photoelectron spectroscopy (XPS)
The results of the XPS measurements are shown in Table 4-2. The values of
binding energies BE indicate that the additives are present in the form corresponding
to: P5+, Nb5+, Cr3+, Ni2+ and K+ ions in oxides. The spectra were taken for fresh
catalysts and for the VSiMo catalyst also after the reaction. The Mo-doped fresh
catalyst contains fraction of Mo4+ ions (42%) beside Mo6+ ions. After the catalytic
reaction the main Mo ion on the surface was Mo6+ (86%). The BE value of V2p3/2
were between 516.6 (VSiNi) and 517.4 eV (VSiP) which suggested pentavalent
oxidation state of vanadium. According to the literature BE of V5+ in different systems
is contained in the range 516.6 – 517.3 eV [141]. Within the studied series the value
of BE of 516.6 eV (VSiNi) indicates higher electron density around V ion in this
sample as compared with VSi, whereas that observed for VSiP (517.4 eV) suggests
lower electron density i.e. higher acidity of the sample. The additives may influence
then the electron density around V, though no clear correlation between the values of
BE and electronegativity of ions χi is observed.
Table 4-2. The results of XPS measurements of VOx/SiO2 catalysts.
XPS binding energy [eV] XPS atomic ratio Catalysts
V 2p3/2 O 1s OI OII
A
Detected additive
ions V/Si A/V OI / OII
VSi 516.8 530.3 533.0 - - 0.069 - 0.064
VSiK 516.9 529.8 533.0 292.6 (K2p)
K+ 0.075 0.217 0.077
VSiNi 516.6 530.4 532.5 856.5 (Ni2p3/2)
Ni2+ 0.044 0.133 0.076
VSiCr 517.1 530.4 533.2 577.1 (Cr2p3/2)
Cr3+ 0.164 0.333 0.203
VSiNb 516.8 530.3 532.6 206.7 (Nb3d)
Nb5+ 0.074 0.273 0.152
VSiP 517.4 530.4 532.8 133.7 (P2p)
P5+ 0.083 0.437 0.071
VSiMo 516.9 530.3 532.0 230.4
233.4 (Mo3d)
Mo4+
Mo6+
0.058 0.167 0.048
Two values of O1s BE were observed. The first at ~ 530 eV (OI) is
characteristic of oxygen species in transition metal oxides, whereas that at ~533
Results of studies and discussion
72
eV(OII) corresponds to oxygen ions in SiO2 [142]. The OI/OII atomic ratio is ~ 0.1 for
most of the samples. It is noticeably higher for VSiCr and VSiNb samples, which
suggests better coverage of the SiO2 support with the active phase, and decreases
for the VSiMo sample.
The A/V surface ratios, calculated from the XPS data, show considerable
enrichment of the surface with respect to the nominal values in the case of P, Nb, Cr
and K additives and slight impoverishment in the case of the Ni additive. The
additives do not affect markedly the V/Si ratio i.e. the dispersion of vanadium on the
silica surface, with the exception of the Cr additive for which the increase of this ratio
is observed. The latter result is in keeping with the considerable increase in OI/OII
ratio for VSiCr sample.
X-ray diffraction (XRD)
The X-ray diffraction patterns of VSiA catalysts are presented in Fig. 4-1.
Fig. 4-1 X-ray diffraction patterns for VOx/SiO2 catalysts and observed phases: (a) VSi, (b) VSiK, (c) VSiNi, (d) VSiCr, (e) VSiNb, (f) VSiP, (g) VSiMo, ■ V2O5; ● K2V18O45 , K0.23V2O5; ● Ni2V2O7; ● Cr2V4O13; ● NbVO5; ● V3.6Mo2.4O16
Results of studies and discussion
73
Pure SiO2 support did not give an X-ray pattern, which indicated its
amorphicity. All samples exhibited the presence of V2O5 (ICDD 09-0387) as the
main crystalline phase.
For VSi and VSiP catalysts V2O5 was the only vanadium containing phase
detected. For other catalysts the mixed vanadium-additive oxide compounds were
observed in addition to V2O5. For VSiK and VSiNi catalysts, beside V2O5, small
amounts of mixed phases K2V18O45 (ICDD 24-0907), K0.23V2O5 (ICDD 37-0070) and
Ni2V2O7 (ICDD 29-0945) were evidenced respectively. For VSiCr, VSiNb and VSiMo
samples, V2O5 and the considerable amounts of mixed phases Cr2V4O13 (ICDD 51-
0445), NbVO5 (ICDD 46-0046) and V3.6Mo2.4O16 (ICDD 84-1952) were detected.
Raman spectroscopy
Raman spectra of fresh SiO2-supported V2O5 catalysts are shown in Fig 4-2.
Fig. 4-2 Raman spectra of the VOx/SiO2 catalysts: (a) VSi; (b) VSiK; (c) VSiNi; (d) VSiCr (e) VSiNb; (f) VSiP; (g) VSiMo
1100 1000 900 800 700
933
898
966
857
925
700
994
g
f
e
d
c
b
a
Inte
nsity
(a.
u.)
R am an Shift (cm -1)
Results of studies and discussion
74
All spectra performed for VSiA catalysts exhibited sharp peaks at 994 cm-1 and
bands at 700 cm-1, with the exception of VSiCr sample for which these peaks were
less intense. Additional peaks were observed at 770 (vw) and 870 (vw) cm-1 for VSiK,
at 857 (w) and 925 (w) cm-1 for VSiNi, and at 898 (w), 933 (ms) and 966 (s) cm-1 for
VSiCr.
The literature data concerning the Raman spectra of VOx / oxide support
(SiO2, Al2O3, TiO2, ZrO2, HfO2, Nb2O5, CeO2) catalysts are presented in Table 4-3,
which shows the positions of the bands associated with various VOx species. Raman
spectroscopy reveals the presence of the following species on supported V2O5:
monomeric vanadyls, one- and two-dimensional polyvanadates, and crystalline
V2O5.The narrow band at 1020–1040 cm-1 is assigned to V=O stretching modes
within isolated monovanadate species. The broad bands in the 600–1000 cm-1 region
can be ascribed to V-O-V (600–800 cm-1) and V=O (800–1000 cm-1) stretching
modes in polyvanadate species. The frequency of these bands is found to increase
with the increase in the V2O5 loading. At higher loadings, crystalline V2O5 is detected.
Bulk V2O5 can be identified by bands at about 143, 282, 305, 405, 485, 525, 700, and
997 cm-1 [143]. The small shifts in the frequency values are due to the different
support ligands and surface coverage effects. The type and structure of the VOx
species depends on both support nature and the loading of V2O5.
In order to determine the structure of vanadia and/or additive species
dispersed on the silica support, the results obtained for VSiA catalysts under study
were compared with the literature data.
Table 4-3. Raman band assignments for vanadium oxide species.
Sample (VOx/nm2)
Monovanadate (V=O)
Polyvanadate (V=O & V-O-V)
V2O5 Support Ref.
0.3 % V2O5/SiO2 (0.21)
1042 none none - [144]
7 % V2O5/SiO2 (1.22)
1038 none none - [145]
10 % V2O5/SiO2 (2.42)
1036 none 143, 280, 486, 695, 995
- [75]
15 % V2O5/SiO2 (3.95)
1035 none 143, 282, 406, 489,
524, 694, 993
- [75]
9.8 % V2O5/SiO2 (8.07 )
1042 none 283, 305, 405, 483,
527, 703, 997
- [144]
Results of studies and discussion
75
17 % V2O5/Al2O3 (6.44)
1028 920, 800, 600, 550, 350, 240
none - [145]
10 % V2O5/Al2O3 (7.69)
1025 none 140, 282, 406, 495,
521, 696, 993
[75]
1 % V2O5/TiO2 (1.20)
1026 920 none [145]
3.5 % V2O5/TiO2 (3.33)
1022 750-950 none 148, 395,
513,636 anatase
[75]
5 % V2O5/TiO2 (5.70)
1022 750-950 (810) none 150, 393,
511, 633 anatase
[75]
5 % V2O5/TiO2 (6.02)
1032 940 none [145]
6.1 % V2O5/TiO2 (9.2)
1032 850, 940, 960 none [146]
9.8 % V2O5/TiO2 (23.2)
1028 830, 955, 960 997 [146]
4 % V2O5/ZrO2 (2.14)
1022 690-970 none 178, 330, 377, 475,
562, 626
[75]
4 % V2O5/ZrO2 (6.80)
1032 820, 920, 950 992 [145]
15 % V2O5/ZrO2 (6.20)
1028 700-980 143, 996 313, 477, 640
[75]
3 % V2O5/HfO2 (2.51)
1021 750-980 none 500 [75]
9.4 % V2O5/HfO2 (5.41)
1027 820-980 (939) 143, 282, 408, 489, 524, 993
[75]
6 % V2O5/Nb2O5 (7.22)
1032 920, 960 none 980 [145]
3 % V2O5/CeO2 (5.50)
1027 920 none [145]
For all the VSiA catalysts a narrow peak at 994 cm-1 can be assigned to
the symetric stretch of V=O groups, characteristic of crystalline V2O5 [147].
Additional band presents at 700 cm-1 is due to the stretching vibrations of V-O
in the square octahedron of V2O5 [143, 148]. This band was observed in spectra
of V2O5 / SiO2, Al2O3, ZrO2, HfO2 at higher loading of V2O5 and ascribed to
crystalline V2O5 [75, 144, 145].
Results of studies and discussion
76
No bands at ~1040 cm-1 due to monovanadate species [75, 144, 145] were
observed for the VSiA catalysts.
Raman spectra of nickel and chromium compounds are given in Table 4-4.
Table 4-4. Raman band assignments for Ni and Cr compounds.
Sample Raman bands (cm-1) Ref.
β-Ni(OH)2 309, 446, 510 [149]
305, 379, 461, 524 [149] α-Ni(OH)2
462, 530 [150]
HOFN* 470, 550 [151]
364, 499, [149]
400, 510 [152]
NiO
497, 540, 1065 [153]
320, 364, 821, 856, 880, 925 [154] Ni3(VO4)2
324, 409, 574, 609, 790, 830, 850, 861, 927, 960 [155]
Cr2O3 300, 345, 550, 600 [156]
Cr3O8 186, 206, 217, 228, 250, 268, 314, 343, 377, 401, 478, 554,
733, 828, 886, 902, 968, 980, 1001
[157]
Cr2O5 170, 191, 218, 271, 316, 328, 354, 373, 386, 411, 426, 448,
488, 693, 743, 839, 869, 884, 933, 985, 995
[157]
CrO2Cl2 211, 357, 984, 994 [158]
K2Cr2O7 217, 370, 560, 770, 902, 945 [158]
CrO3 208, 338, 375, 404, 497, 563, 975, 1001 [158]
454, 923 [154] CrVO4
660, 773, 889, 924 [155]
*Higher oxide forms of nickel such as NiOOH, Ni2O3, Ni3O4 and NiO2.
The VSiNi and VSiCr catalysts showed several small peaks in the range
between 800 and 960 cm-1. A detailed interpretation of the Raman spectra obtained
for these samples is difficult because different types of vibrations exist in this spectral
domain, involving both V-O and A-O vibrations. We could observe either the bending
vibrations of VO4 tetrahedron existing in vanadium-additive mixed phases or the
vibrations involving the octahedral NiO6 and CrO6 species. Thus for VSiNi sample the
Results of studies and discussion
77
peaks at 925 cm-1 and at 857 cm-1 can be due to V-O vibrations in polymeric [VOx]
species. Similar peaks were also observed for Ni3(VO4)2 [154, 155].
The Raman spectrum of VSiCr catalyst shows the presence of sharp feature
at 966 cm-1 and a broad Raman bands at 933 cm-1 and at 898 cm-1. These bands
could be assigned to the Cr=O stretching vibrations [147] and also to V=O vibrations.
Similar peaks were also observed in another chromium compounds such as Cr3O8,
K2Cr2O7, Cs2Cr4O13 and CrVO4. After the catalytic reaction the ratio of the bands
corresponding to chromium compound to those characteristic of V2O5 decreases and
band assigned to potassium vanadate in VSiK disappears.
The peaks characteristic for NiO and Cr2O3 were not observed in VSiNi and
VSiCr, respectively.
The Raman spectra performed for VSiA series confirmed then the
presence of crystalline V2O5 and small amounts (except VSiCr) of mixed
phases in these samples.
After oxidative dehydrogenation of propane at 450 oC the VSiA catalysts were
also checked for the carbon deposition on catalysts’ surface. No peaks at 1360 cm-1
and at 1580 cm-1, characteristic of carbonaceous materials [159-163] were detected
for the VSiA series, which indicated the absence of coke.
Infrared spectroscopy
Figure 4-3 presents the infrared adsorption spectra of VSiA catalysts, in the
range from 400 to 1500 cm-1 and fig. 4-4 in the range from 1500 to 4000 cm-1. All
VSiA samples had the intensive adsorption bands with their maxima at about 480,
810, 1105 and 1220 cm-1. For VSiNi and VSiCr additional minor peaks at 580, 650,
710, 880 and 950 cm-1 were observed.
Some previously published data on the vibrational spectra of vanadia and
silica are presented in Table 4-5. It is known from the literature [164] that pure V2O5
shows IR absorption bands at respectively 515, 603, 827, 950, 1019, 1600, 3666
cm−1. The absorption bands between 550 and 670 cm−1 are attributed to the rocking
modes of the V–O–V bonds, while the bands between 670 and 770 cm−1 correspond
to the stretching of the same bonds. The absorption band at 980 cm−1 is usually
attributed to the symmetric stretching of V=O bond in amorphous V2O5, while the
band at 1020 cm−1 corresponds to the vibration of the same bond and is
Results of studies and discussion
78
characteristic of the crystalline V2O5. At last, the band at 3670 cm−1 is associated to
the vibration of the V–OH bond. At low loading of V2O5 on supports, the band at 1020
cm−1 disappears and that at 980 cm-1 is enhanced. It is then possible to identify the
crystalline V2O5 in a catalyst.
Table 4-5. Infrared spectra of V2O5 and silica oxide.
Type of vibration IR bands (cm-1) Ref.
800, 810, 1080, 1095, 1080-1105, 1180, 1200, 1220 [164]
450, 800, 1070 [165]
460, 800, 1080 [166]
Si-O-Si
793, 1080, 1210 [167]
980 [164]
950 [165]
950 [166]
Si-OH
950 [167]
950-960 [164] Si-O-V
955 [167]
975.5, 980.5, 982 [169]
984, 1016, 1035 [170]
V-OA
926, 990, 1027, 1030, 1040 [171]
796, 975 [168]
477, 830 [171]
V-OB-V
473, 501, 795, 895 [36]
412, 532, 602, 865 [168]
411, 506.5, 586, 842.5 [169]
3V-Oc
503, 578-612, 895 [170]
A. Surca and B. Orel [170] made the analysis of the vibrational spectra of
V2O5. Authors claimed that the group of bands appearing below 600 cm-1
corresponds to the edge-shearing 3V-Oc stretching and the bridging V-OB-V
deformations. The bands between 700 and 900 cm-1 were attributed to the bridging
V-OB-V deformations. The bands present between 950 and 1020 cm-1 are ascribed to
the V-OA (vanadyl) stretching modes.
Results of studies and discussion
79
Fig. 4-3 IR spectra of VOx/SiO2 catalysts: (a) SiO2 support; (b) VSi; (c) VSiK; (d) VSiCr; (e) VSiNi; (f) VSiNb; (g) VSiP; (h) VSiMo
It has been well demonstrated that the employment of infrared spectroscopy
provides information on the structural properties of silica [172]. The highest frequency
and the most intense band near 1070 cm−1, corresponds to the asymmetric stretching
vibration of the bonds Si–O–Si for the tetrahedric SiO4 units. Other bands which are
observable at 1200 cm−1 can be assigned to the asymmetric stretching vibration of
the Si–O−Si bonds, and at 800 cm−1 can be attributed to the symmetric stretching
vibration of the same bond. The peak at 980 cm-1 is assigned to Si-OH vibrations.
The SiO2 support shows an intense band of absorption at 3747 cm−1 corresponding
to isolated silanol groups Si–OH.
1500 1000 500
810
1220
1105
480
710
950
880
650
580
h
g
f
e
d
c
b
a
Abs
orba
nce
(a.u
.)
Wavenumber (cm-1)
Results of studies and discussion
80
The IR spectra of vanadia-silica mixed oxides were characterized by a band
which can be attributed to an asymmetric stretching mode of SiO4 tetrahedrons
connected to V-ions. This band was found at about 950 cm-1 for vanadia-silica
xerogels [173] and at about 960 cm-1 for V-silicalites [174], so the band at 950-960
cm-1 is characteristic for Si-O-V vibrations [164].
Fig. 4-4 IR spectra of VOx/SiO2 catalysts: (a) SiO2 support; (b) VSi; (c) VSiK; (d) VSiCr; (e) VSiNi; (f) VSiNb; (g) VSiP; (h) VSiMo
Comparison of the spectra obtained with the literature data shows that
the VSiA catalysts under study exhibit mainly the peaks of silica: the
adsorption bands with their maxima at about 480, 810, 1105 and 1220 cm-1
could be assigned to the stretching vibrations of Si-O bond in silica support
[165, 175-179].
4000 3500 3000 2500 2000 1500
1620
3430
h
g
f
e
d
c
b
a
Abs
orba
nce
(a.u
.)
W avenum ber (cm -1)
Results of studies and discussion
81
1000 500 0 -500 -1000 -1500 -2000 -2500
*
*
ppm from VOCl3
Weak bands at 580, 650, 710, 880 cm-1 observed only for VSiNi and VSiCr
samples correspond to crystalline V2O5 [165, 168-171]. The band at 950 cm-1
could be assigned to the Si-O-V vibrations, indicating the formation of a bond
between the deposited phase and the support.
The broad weak bands at 1620 and 3430 cm-1 (fig. 4-4) can be due to
adsorption of molecular water [171].
Solid state 51V NMR spectroscopy
The 51V NMR analyses were performed for some of the VSiA samples (VSi
and VSiK) and the spectra are shown if fig. 4-5. For both VSi and VSiK samples the
intense peak at about -350 ppm and minor peaks at about -900 ppm and -1100 ppm
were observed.
Fig. 4-5 51V MAS NMR spectra of VOx/SiO2 catalysts: (a) VSi, (b) VSiK
The main spectral parameters of model compounds related to VSiA catalysts
or used in the interpretation of NMR spectra are summarized in Table 4-6.
Results of studies and discussion
82
In general, lower vanadia concentration results in broad often multicomponent 51V NMR lines in the -300 to -800 ppm range, assignable to tetrahedral environment
of vanadium. In contrast, higher vanadium concentrations lead to the occurrence of a
line with an axial anisotropy of the chemical shift tensor with a major peak near -300
ppm and a minor peak in the -900 to -1000 ppm range; V2O5 itself has -280 and -
1250 ppm peaks.
A shift in the peak position upon evacuating the catalysts, indicate the
presence of water molecules in the coordination sphere of vanadia [105, 109, 180,
181].
Table 4-6. 51 V NMR paramerters of V-containing compounds.
∆δδδδ δδδδizo δδδδ11 δδδδ22 δδδδ33 Compound Coordination
[ppm]
Ref.
Mg3V2O8 isolated tetrahedral 40 -554 - - -
Mg2V2O7 104 -551 -495 -561 -599
NH4VO3
distorted tetrahedral
460 -570 -370 -530 -830
MgV2O6 537 -548 -353 -444 -890
PbV2O6
distorted octahedral
690 -533 -310 -320 -1000
V2O5 square pyramidal 970 -609 -280 -280 -1250
[182]
In the literature [101, 182] the peaks of NMR spectra have been attributed to
different vanadia symmetries. Eckert and Wachs [182] assigned to dimeric form of
vanadium in tetrahedral symmetry the peak around −550 ± 30 ppm on V2O5/Al2O3.
The peak at −300 ppm corresponded to vanadium species in distorted octahedral
structure, similar to bulk V2O5, or superficial polymeric species. Eon et al. [48]
observed the presence of two peaks at −310 and −540 ppm on the NMR spectra of
V2O5/Al2O3 samples. The authors suggested that they were due to octahedral and
tetrahedral species, respectively. Also, O.B. Lapina and co-workers [183], who
performed 51V NMR spectroscopy for V2O5/Al2O3 catalysts, observed at -350 ppm
and at -1270 ppm the peaks of vanadium in distorted octahedral coordination.
The studies of V2O5/SiO2 catalysts exhibited that for low vanadium oxide
loading (1.6 wt% V2O5), a peak corresponding to tetrahedral vanadium species was
centered at around −625 ppm and shifted to lower values with the increase of
vanadium oxide amount (−580 ppm for 4.0 and 8.2 wt % V2O5) and finally
Results of studies and discussion
83
disappeared in favour of the octahedral signal at −300 ppm when vanadia content
increased to 25 wt% [184].
On the basis of the above given facts the major peak at about -350 ppm
and small peaks at about -900 ppm and -1100 ppm observed in the present
studies for both VSi and VSiK samples can be then assigned to the surface
vanadium-oxygen structures containing a distorted octahedron of oxygen
atoms around the V atom. No signals characteristic of the tetrahedral vanadium
were observed.
UV-VIS spectroscopy
The diffuse reflectance UV-VIS spectra of VSiA catalysts are shown in fig 4-6.
The broad band between 280 and 500 nm was observed in all the samples.
Fig. 4-6 UV-vis spectra of VOx/SiO2 catalysts: (a) VSi; (b) VSiK; (c) VSiNi; (d)
VSiCr; (e) VSiNb; (f) VSiP; (g) VSiMo
200 400 600 800
340
280
500
g
f
e
d
c
b
a
Inte
nsity
(a.
u.)
W ave length (nm )
Results of studies and discussion
84
UV-VIS spectroscopy can give information about coordination and valency of
V species. The lower energy charge transfer bands for vanadium ion, which are
related to charge transfer between vanadium and oxygen, are dependent on the
number of oxygen atoms surrounding the central vanadium ion. The charge transfer
energy is strongly influenced by the coordination and/or the oxidation state of
vanadium [185-199]. The most important bands assignment from V - containing
compounds are listed below in table 4-7.
Table 4-7. Band maxima of the DRS spectra of some reference V-compounds Nature of species Band max.
[nm]
Remarks Ref.
Tetrahedral monomeric
(isolated VO4)
240 – 290
280
240
253 , 294
Compound: Na3VO4
(ortho-vanadate)
[185]
[186]
[187]
[188]
Tetrahedral 1D chains
(polymerized VO4)
270 – 290
280 , 340
288, 363
281 , 353
Compounds: NH4VO3 ; NaVO3
(meta-vanadate)
[185]
[189]
[190]
[191]
Square pyramidal 410 Compound: α-VPO5 [185]
Octahedral multilayer
(polymerized VO5/VO6)
470
330 – 500
480
330 , 460
334 , 481
Compound: V2O5
[185]
[186]
[187]
[189]
[192]
VIV d-d transition 980
1052
[186]
[187]
Thus, the adsorption band for vanadium (V4+) ions is observed in 240-280 nm
region. The band corresponding to the vanadium (V5+) ions with tetrahedral
coordination appears in the 280-340 nm range, while the adsorption band for
vanadium (V5+) ions with octahedral coordination is noticed in 400-500 nm region. In
addition to these bands, d-d transition of V4+ (d1) may be observed in the range of
550-850 nm [185-199].
Results of studies and discussion
85
The influence of vanadium oxide loading on surface structures of V2O5/SiO2
catalysts using diffuse reflectance UV-VIS spectroscopy were studied by X. Gao
[197] and by F. Arena [198]. They observed that, at low V2O5 loading (1-2 wt. %
V2O5), the spectrum was consisted of a main absorption band at ca. 250 nm
associated with the charge-transfer (CT) process in the terminal vanadyl V=O bond
VV=O-II + hv → VIV=O-I (CT absorption)
That indicated, that vanadia existed mainly in the form of a monomeric tetrahedrally
coordinated species. The spectrum of 5 wt. % V2O5/SiO2 sample was shifted to the
maximum absorption band at ca. 290 nm indicating that the vanadium oxide species
were well dispersed, however suggesting that the concentration of the oligomeric
structures had increased with loading, although monomeric forms were still present.
A further increase of the vanadia loading (10-12 wt. % V2O5/SiO2 system) produced a
spectrum with predominant band at ca. 310 nm, characteristic of more complex
structures with a higher degree of “nuclearity” though the tetrahedral coordination of
V ions was still preserved. Finally, the bands at ca. 400 nm and 470 nm, were
observed for highly loaded 15-50 wt. % V2O5/SiO2 catalysts. These spectral features
were attributed to 2D patches of pentacoordinated vanadium ions and 3D V2O5
crystallites with V5+ in octahedral coordination respectively, and indicated a deep
change in the surface structures of V2O5/SiO2 catalysts at high V2O5 loading.
The spectra presented in Fig. 4-6 indicate then the presence of both
tetra- and octahedral coordination of V5+ ions. The bands at 240-280 nm and
above 550 nm characteristic for vanadium (V4+) ions [185-199] were not
observed.
UV-VIS measurements, performed with reference to VSi catalyst, were applied
to determine the oxidation state and coordination of the ions in VSiA catalyst (Fig. 4-
7).
For VSiCr sample band between ~520 nm to ~750 nm with the maximum
around 690 nm was observed. The DRS spectra of VSiMo sample showed a board
band between 500 and 800 nm. For VSiNi catalyst a broad, small band around 740
nm was noticed.
Band maxima of the DRS spectra of some reference chromium, nickel and
molybdenum compounds are given in Table 4-8, 4-9, 4-10 respectively.
Results of studies and discussion
86
Fig. 4-7 UV-vis spectra of VOx/SiO2 catalysts refered to VSi: (a) VSiMo; (b) VSiP; (c) VSiNb; (d) VSiCr; (e) VSiNi; (f) VSiK
Table 4-8. Band maxima of the DRS spectra of some reference Cr-compounds
Nature of species Band max. [nm] Remarks Ref.
Cr6+
tetrahedral coordination
275 ; 370
229 ; 265 ; 340 ; 459
275 ; 322 ; 445
261 ; 365 ; 435
229 ; 262 ; 332 ; 526
278 ; 363
Compounds:
K2CrO4 ;
K2Cr2O7
CrO3
[200]
[201]
[200]
[202]
[201]
[202]
Cr3+
distorted octahedral
coordination
263 ; 304 ; 410 ; 575
295 ; 465 ; 625
274 ; 351 ; 461 ; 595 ; 645 ; 714
272 ; 355 ; 467 ; 610
Compounds:
Cr(NO3)3ּ9H2O
Cr2O3
[201]
[200]
[201]
[202]
2 0 0 4 0 0 6 0 0 8 0 0
690
740
520
f
e
d
c
b
a
Inte
nsity
(a.
u.)
W a v e le n g th (n m )
Results of studies and discussion
87
Cr2+
distorted tetrahedral
coordination
1000
1430
Compound:
K2CrCl4 ;
[200]
[201]
Cr2+
octahedral coordination
769
800
Compound:
Cr(H2O)62+
[201]
[200]
The number, wavelength and intensity of the d-d bands depend on the
oxidation state (Cr2+ ; Cr3+) and coordination environment (octahedral or tetrahedral).
The five different theoretical components may be labeled as: (1) chromate (main
absorptions at 275 and 370 nm), (2) dichromate (main absorptions at 275, 350, and
445 nm), (3) pseudo-octahedral Cr3+ (absorption at 625 nm), (4) pseudo-octahedral
Cr2+ (absorption at 800 nm) [200, 203, 204].
For nickel compounds cited in table 4-9, the more intense band for the purely
octahedral nickel is ca. 725 nm and for tetrahedral nickel is between 540 and 640
nm.
Table 4-9. Band maxima of the DRS spectra of some reference Ni-compounds
Nature of species Band max. [nm] Remarks Ref.
Ni2+
tetrahedral coordination
600 – 640
580 ; 620
630
540-625
[205]
[206]
[207]
[208]
Ni2+
octahedral coordination
710
725
400 ; 660 ; 730
Compound:
crystalline NiO
[205]
[207]
[208]
From literature (Table 4-10), it is established that the Mo6+ in tetrahedral
coordination exhibits electronic absorptions bands at 220–270 nm. For the samples
having Mo6+ in octahedral coordination the bands at higher wavelengths 270–290 nm
and 310–350 nm are observed [209-212]. The absorption of wavelength between
400 and 800 nm is due to reduced Mo ions such as Mo5+ and/or Mo4+. For the broad
band at a longer wavelength than 450 nm, deeply reduced Mo ions such as MoO2
Results of studies and discussion
88
species are likely. For MoO2, which has slightly distorted octahedral symmetry
around the Mo4+ ion, the absorption is maximized at 480 nm [213].
Table 4-10. Band maxima of the DRS spectra of some reference Mo-compounds
Nature of species Band max. [nm] Remarks Ref.
Mo6+
tetrahedral coordination
256
220 – 260
Compounds: MgMoO4 ;
Na2MoO4
[213]
[214]
Mo6+
octahedral coordination
315
220 , 270-290, 310-350
348
220 , 260, 280-350
Compounds: AHM
[(NH4)6Mo7O24ּ4H2O] ;
MoO3 ;
MoO3 / γ-Al2O3
[213]
[214]
[213]
[215]
Mo5+ and/or Mo4+ 400 – 800 [213]
Mo4+ in octahedral
coordination
480 Compound: MoO2 [213]
Bands between ~520 nm to ~750 nm, which are observed in the VSiCr
sample, are assigned to Cr3+ in octahedral ligand field in accordance with the
literature data [59, 214, 216]. For the Cr2O3/Al2O3 catalysts G. Deo et al. [217] and
L.C. Dieguez [218] reported the appearance of shoulders at around 570-600 nm
which were ascribed to Cr3+ species in octahedral coordination. Also for Cr2O3/ZrO2
catalyst the bands at 440 and 600 nm were ascribed to transitions of Cr3+ in the
distorted octahedral coordination [203].
For VSiMo sample a broad band between 500 and 800 nm is attributed to
reduced Mo ions Mo5+ and/or Mo4+ [213]. M.C. Abello et al. observed for MoO3 / γ-
Al2O3 catalysts two absorption bands at 220 and 260 nm and a broad band in the
280–350 nm region, suggesting that Mo6+octahedral species are mainly present
[215]. For VSiMo the band around 300 nm attributed to Mo6+ was not observed.
The Ni2+ ions, in VSiNi sample (Fig. 4-7e), were located in oxygen
octahedra. According to the literature data a band around 740 nm indicates the
appearance of Ni2+ ions in octahedral coordination.
Several papers describe the nickel catalysts supported on niobia, titania,
magnesia and alumina which were characterized by UV-VIS spectroscopy. The
appearance of a band around 660-670 nm noticed for NiO/TiO2 [219] and NiO/ MgO
Results of studies and discussion
89
[220] catalysts was attributed to the dispersed octahedrally coordinated Ni2+ surface
species. Similar results were obtained by Y. Chen et al. (~665 nm) [207] and M.A.
Zanjanchi (~660 and 730 nm) [208]. For NiO/Nb2O5 well defined bands attributed to
octahedrally coordinated Ni2+ were observed at ca. 840 nm [205].
4.1.1.1.2. VOx/SiO2 catalysts with additives of different alkali metal ions (Li+,
Na+, K+, Rb+) and of alkaline earth metal ions (Ca2+, Mg2+)
Bulk composition and surface area
The bulk composition of supported vanadium oxide catalysts with addition of
alkali and alkaline earth metal ions is given in Table 4-11, which presents the
contents of vanadium and the additives (in wt. %), the electronegativity of introduced
ions and the BET specific surface area. The A/V ratio in all the samples of this series
was 0.1.
Table 4-11. List of samples of VOx/SiO2 catalysts with alkali and alkaline earth metal
ions.
List of catalysts SSA [m2/g] wt % V2O5 wt % additive χχχχi*
SiO2 support 175.0 - -
VSi 116.6 28.39 - V5+ 17.93
VSiLi 68.42 27.93 1.48 Li+ 3.00
VSiNa 78.29 28.15 0.71 Na+ 2.70
VSiK 92.2 28.05 1.21 K+ 2.70
VSiRb 65.09 27.56 2.80 Rb+ 2.40
VSiMg 109.36 28.14 0.75 Mg2+ 6.00
VSiCa 107.99 28.00 1.24 Ca2+ 5.00
χχχχi* electronegativity of introduced ions
The introduction of the additives leads to the decrease of the specific surface
area with respect to the undoped VSi catalyst. The effect is stronger for the group IA
additives (the decrease by ~ 30-40%) and much smaller for the group IIA additives (~
8 %).
Results of studies and discussion
90
X-ray photoelectron spectroscopy (XPS)
Table 4-12 exhibits the XPS data for the catalysts under study.
Table 4-12. The results of XPS measurements of VOx/SiO2 catalysts with alkali and
alkaline earth metal ions.
XPS binding energy [eV] XPS atomic ratio Catalysts
V 2p3/2 O 1s OI OII
A
Detected additive
ions V/Si A/V OI / OII
VSi 516.8 530.3 533.0 - - 0.069 - 0.064
VSiLi 517.0 530.1 532.6 no data no data 0.093 no
data
0.076
VSiNa 516.9 530.4 532.9 494.2 (Na1s)
Na+ 0.109 0.194 0.083
VSiK 516.9 529.8 533.0 292.6 (K2p)
K+ 0.075 0.217 0.077
VSiRb 516.9 529.9 533.2 237.7 (Rb3p3/2)
Rb+ 0.136 0.016 0.109
VSiMg 517.1 530.0 532.8 305.9 (MgKLL)
Mg2+ 0.119 0.021 0.094
VSiCa 517.0 530.0 532.8 347.3 (Ca2p)
Ca2+ 0.131 0.008 0.122
The BE of V2p3/2 is ~516.9 eV, the value typical of the V5+ ions [141]. The
small peak for O1s at ~530.0 eV corresponds to oxygen in transition metal oxides,
whereas major peak at higher BE at ~533 eV corresponds to oxygen ions in SiO2
[132]. The values of the BE of the dopant elements indicate the presence of Na+, K+,
Rb+, Ca2+, Mg2+ ions in studied samples. For VSiLi sample the detection of Li+ ion
was not possible due to the low photoionization cross section of Li1s peak. The A/V
ratios, calculated from the XPS data, show impoverishment of the surface with
respect to the nominal values in the case of Rb, Mg and Ca additives and enrichment
in the case of the Na and K additives. The atomic ratio OI/OII for all VSiA samples is
around 0.1. The surface atomic ratio V/Si increases for the samples with alkali metal
ions, which indicates a better dispersion of the vanadia phase at the alkali metal
addition.
Results of studies and discussion
91
X-ray diffraction (XRD)
The X-ray diffraction patterns of the VSiA catalysts of this series are presented
in Fig. 4-8. The XRD measurements revealed the presence of V2O5 (ICDD 09-0387)
in all catalysts and a small amount of vanadium-additive mixed phases in VSiK and
VSiLi samples. As it was shown previously (see 4.1.1.A) for the VSiK the K2V18O45
and K0.23V2O5 phases were observed. The diffractogram of VSiLi revealed additional
the presence of LiVO3 (ICDD 70-1545) and LiV3O8 (ICDD 72-1193) phases.
Fig. 4-8 X-ray diffraction patterns for VOx/SiO2 catalysts and observed phases: (a) VSiLi, (b) VSiRb, (c) VSiMg, (d) VSiCa; ● V2O5; ■ LiVO3, ● LiV3O8
Results of studies and discussion
92
4.1.1.2. VOx / MgO catalysts
4.1.1.2.1. VOx/MgO catalysts with additives of main group elements (K+, P5+)
and of transition metal ions (Ni2+, Cr3+, Nb5+, Mo6+)
The composition of the samples and their specific surface area
The content of vanadium and additives, the electronegativity of introduced ions
and the BET specific surface area of VMgA catalysts are presented in Table 4-13.
For the VMgA series the specific surface area was, with the exception of
VMgCr, higher than that of the MgO support (72 m2g-1). The presence of the
additives (in particular of P, Mo, Nb and K) led to a considerable increase in SSA.
Similar increase of the specific surface area was observed in some cases for vanadia
[221-223] and chromia [58] deposited on MgO. B. Grzybowska and co-workers [58]
observed that the treatment of magnesia with water, followed by evaporation and
drying in the same conditions as those in the preparation of the catalysts, led to the
formation of microcrystalline Mg(OH)2, as verified by XRD : on calcination the
magnesium hydroxide decomposed into MgO which had a higher specific surface
area than the original support. No explanation can be, however, offered at this time
for the effect of the additives on SSA.
Table 4-13. List of samples of VOx/MgO catalysts
List of catalysts SSA [m2/g] wt % V2O5 wt % additive χχχχi*
MgO support 72.4 - - -
VMg 79.7 14.09 - V5+ 17.93
VMgK 135.6 14.01 0.60 K+ 2.46
VMgNi 80.3 13.84 1.79 Ni2+ 9.55
VMgCr 63.4 13.87 1.59 Cr3+ 11.62
VMgNb 164.4 13.70 2.80 Nb5+ 17.60
VMgP 175.1 13.76 0.95 P5+ 24.09
VMgMo 155.7 13.69 2.89 Mo6+ 19.44
χχχχi* electronegativity of introduced ions
Results of studies and discussion
93
X-ray photoelectron spectroscopy (XPS)
The results of the XPS measurements are shown in Table 4-14. The values of
BE in XPS measurements confirmed that the additives are present in VMgA samples
in the form corresponding to: Mo6+, Nb5+, P5+, Ni2+ and K+ ions. The Cr-doped catalyst
contained fraction of Cr6+ ions (31 %) beside Cr3+ ions. The BE value of V2p3/2 were
between 517.1 and 517.4 eV, which suggested pentavalent oxidation state of
vanadium [141].
Table 4-14. The results of XPS measurements of VOx/MgO catalysts
XPS binding energy [eV] XPS atomic ratio Catalysts
V 2p3/2 O 1s OI OII
A
Detected additive
ions V/Mg A/V OI / OII
VMg 517.4 529.9 531.8 - - 0.099 - 1.028
VMgK 517.4 530.0 531.9 293.0 (K2p)
K+ 0.106 0.083 1.289
VMgNi 517.2 530.0 532.0 855.6 (Ni2p3/2)
Ni2+ 0.140 0.115 1.670
VMgCr 517.1 530.1 532.1 577.2
579.8 (Cr2p3/2)
Cr3+
Cr6+
0.153 0.277 2.245
VMgNb 517.5 530.2 532.1 206.7 (Nb3d)
Nb5+ 0.129 0.409 1.192
VMgP 517.1 529.8 531.8 134.3 (P2p)
P5+ 0.106 0.366 2.284
VMgMo 517.4 530.0 532.0 232.7 (Mo3d)
Mo6+ 0.139 0.118 2.107
Two BE of O1s oxygen were observed. The BE value of the main O1s peak at
~530 eV (OI ) could correspond to Mg3V2O8 phase or magnesium oxide , while a
peak at ~532 eV (OII) can be attributed to the OH groups on the surface, or to oxygen
species in carbonates, found on the samples of this series by Raman and IR
spectroscopy.
The V/Mg ratio was higher for the samples with the additives (except K and P)
in comparison to the VMg sample.
The A/V surface ratios, calculated from the XPS data, show enrichment of the
surface with respect to the nominal values for P, Nb, and Cr additives. In the case of
the Ni and K additives the slight impoverishment of the surface was observed.
Results of studies and discussion
94
X-ray diffraction (XRD)
X-ray diffraction results for VMgA catalysts are shown in Fig. 4-9. All the
catalysts of the series, showed the peaks at 2Θ = 43 and 62o (at Cu Kα) characteristic
of MgO (JCPDS 4-829). This suggests that if vanadium-containing phases are
present, they are in small quantities and/or poorly crystalline. Only VMgCr sample
exhibited beside MgO also traces of the MgCrO4 phase (ICDD 21-1255). The
presence of MgCrO4 mixed phase was confirmed by XPS measurements, which
detected Cr6+ ions beside Cr3+. This phase disappeared, however, in the sample after
the catalytic test.
Fig. 4-9 X-ray diffraction patterns for VOx/MgO catalysts and observed phases: a) MgO (b) VMg, (c) VMgK, (d) VMgNi, (e) VMgCr, (f) VMgNb, (g) VMgP, (h) VMgMo, ● MgO; ■ MgCrO4
This results are in agreement with J.C. Volta and co-workers [28] who studied
for V-Mg-O catalysts the effect of different vanadium content from 5 up to 82 wt.% in
the composition range of the different magnesium vanadates [4, 28, 109]. At low
vanadium content (up to ~30%) only the diffraction lines of MgO were observed.
Chaar [180] and Pak [181] did not detect either clear vanadium containing
phases for the V2O5 loading below 30 wt.%.
Results of studies and discussion
95
Raman spectroscopy
Raman spectra of the VMgA catalysts are presented in Fig. 4-10. The spectra
of all samples have three peaks: a relatively narrow peak at around 870 cm-1 with a
shoulder at 825 cm-1 and a broad peak at 1090 cm-1. The literature data concerning
the position of the vibrational bands for the magnesium vanadates are given in
table 4-15.
Table 4-15. Raman spectra of magnesium vanadates.
Sample vV=O vVO4 or vVO6 vV-O-V Ref.
V2O5 1023,
998, 923
846, 788, 465, 412, 371, 324 729, 670, 603, 492 [105]
V-Mg-O 940 865, 799, 448, 412, 353, 301 768, 645, 580, 532,
480
[105]
1008 897, 881, 861, 410, 382, 340,
309, 284, 246
846, 785, 526 [105] Mg2V2O7
- 948, 902, 873, 845, 410, 403,
377, 354, 335, 305, 282, 243
630, 625, 570 [100]
- 868, 833, 828, 462, 448, 420,
387, 344, 296, 279
541, 524, 484 [105]
- 858, 821, 721, 468, 450, 413 - [154]
Mg3V2O8
- 862, 827, 724, 690, 473, 448,
411, 391, 351, 344, 330, 290
- [224]
932 846, 797, 450, 383, 310, 274 745, 530, 495 [105] MgV2O6
923 440, 332,309, 268, 204, 174 836, 731, 523 [224]
The position and relative intensities of the main peaks registered for
VMgA catalysts agree well with those reported for Mg3V2O8, the spectrum of
which shows strong bands at 827 (strong) and 862 cm-1 (very strong).
Therefore, the Raman results indicate the presence of magnesium
orthovanadate phase in all VMgA catalysts. The magnesium orthovanadate
phase could be present in an amorphous form, or as a thin layer on the MgO
surface as it was not detected by XRD. In the samples with the additives, the
Results of studies and discussion
96
bands were much broadened, which may indicate a higher distortion of the
crystalline network.
Previous studies have shown that peaks at 825 and 860 cm-1 can be assigned
to V–O vibrations of tetrahedrally coordinated VO4 species. The bands between 820
and 920 cm-1, with maximum at about 870 cm-1, corresponded to vV-O stretching
modes of disordered Mg3V2O8 [222, 225, 226]. The characteristic bands of pyro-
Mg2V2O7 (bands at 947, 901, 630 and 620 cm−1) and meta- MgV2O6 (920 and 731
cm−1) vanadates were not observed. There is also no peak near 1030 cm-1
corresponding to the stretching vibrations of V=O bonds of isolated monovanadate
species which has been observed for vanadia dispersed on Al2O3, SiO2, TiO2, and
ZrO2 (see table 4.3).
Fig. 4-10. Raman spectra of VOx/MgO catalysts: a) VMgCr, b) VMgMo, c) VMgP, d) VMgNb, e) VMgNi, f) VMgK, g) VMg
1100 1000 900 800 700
870
962 93
3
g
f
edcb
a
820
1090
Inte
nsity
(a.
u.)
R am an S h ift (cm -1)
Results of studies and discussion
97
J.M. Lopez Nieto [226] suggested that the broad band centered at around 870
cm-1, could be due to the presence of non-crystalline Mg3V2O8 with V5+ ions in a
distorted tetrahedral coordination. This fact could be explained by the relation
between the symmetry and position of laser Raman spectra - the higher symmetry of
tetrahedral vanadium species the lower the wave number at which its laser Raman
bands appear.
Thus, it can be proposed for VMgA catalyst under study, that species
responsible for the laser Raman feature at 870 cm-1 correspond to distorted VO4
tetrahedra occurring in Mg3V2O8.
Two additional weak peaks at 933 and 962 cm-1 were noticed in VMgCr
sample. Comparing the Raman spectra of VMgCr sample with those of chromium
oxide based reference compounds (see table 4.8) we may observe that these peaks
were also observed in other chromium compounds such as Cr3O8, K2Cr2O7,
Cs2Cr4O13 and CrVO4. Bands observed at 933 and 962 cm-1 for VMgCr sample
are due to the Cr-O stretching vibrations and can be assigned to MgCrO4
phase. They disappeared in the sample after the catalytic test.
The small band at 1090 cm-1, observed in all the samples, corresponded to
carbonate species on the surface. Several authors have observed the carbonate
vibration in the range 1060 to 1120 cm-1 [96]. For Co-Al and Ni-Al hydrotalcites the
bands at 1059 and 1062 cm-1 were assigned to the symmetric vibrations of carbonate
of high symmetry [227].
The VMgA catalysts were also checked for the casual presence of the carbon
deposition on the catalyst surface after the oxidative dehydrogenation of propane at
450 oC. No peaks at 1360 cm-1 and at 1580 cm-1, characteristic of carbonaceous
materials [159-163] were detected for the VMgA series.
51V Nuclear magnetic resonance
Figure 4-11 shows the spectra performed for VMg and VMgK samples. Only
one band centered at -553 ppm was observed.
J.C. Volta et al. investigated magnesium ortho-, phyro- and metha- vanadate
by 51 V MAS NMR. For Mg3V2O8 very narrow signal at -550 ppm was observed. The
spectrum of Mg2V2O7 presented two signals at -615 and -550 ppm with a higher
density for the second one. The spectrum of MgV2O6 was different, with signals at -
378, -478, -547 and above –800 ppm [28].
Results of studies and discussion
98
-2000 -1000 0 1000
b)
a)
ppm from VOCl3
A.T. Bell [181] who studied V-Mg-O catalysts with different vanadia loading
(10-30 wt.% of V2O5) observed broad band centered at ca. -560 ppm. The chemical
shift at that peak was very similar for that noticed for pure Mg3V2O8.
Fig. 4-11 51V MAS NMR spectra of VOx/MgO catalysts: (a) VMg, (b) VMgK
The spectra obtained for VMg and VMgK catalysts confirmed the
presence of magnesium orthovanadate phase. The peaks characteristic of
other vanadates (Mg2V2O7 and MgV2O6) were not observed (see table 4-6).
Infrared spectroscopy
Infrared spectra of MgO support and VMgA catalysts in the vibrational region
from 400 to 4000 cm-1 are depicted in Fig. 4-12. All VMgA spectra appeared similar
and consisted of bands at 430, 700, 860, 1450, 1650 and 3500 cm-1. The observed
bands were compared with the spectra of reference phases as crystalline V2O5,
Mg3V4O13 and ortho-, metha- and pyro- vanadates presented in table 4-16.
Results of studies and discussion
99
Table 4-16. Infrared spectra of magnesium vanadates.
Sample vV=O vVO4 or vVO6 vV-O-V Ref
V2O5 1020 851; 825; 795; 440; 382; 298; 265 597; 503; 497 [105]
VMg 955; 913 870; 820; 763; 460; 420; 349; 277 685; 590; 507 [105]
Mg3V4O13 975; 963;
915
879; 849; 816; 456; 436; 401; 384;
377; 359; 336; 317; 297; 282; 272;
246
795; 669; 662;
571; 550;
[105]
986; 926 873; 856; 815; 472; 440; 400; 390;
381; 334; 305; 281; 260; 246
715; 535 [105]
977, 965,
919
821, 456, 440, 395, 360, 320 692, 668, 575, [28]
972 820, 444, 402 666, 574 [221]
Mg2V2O7
992, 925 878, 856, 440, 400, 390,381, 334 715, 535 [106]
962; 915 863; 827; 464; 395; 369; 341; 334;
316; 288; 258; 248
695; 477 [105]
916 865, 833, 373, 337, 318 715,696, 485 [28]
Mg3V2O8
- 859, 834, 463, 405 700 [221]
950 883; 428; 406; 373; 341; 300; 290;
284; 255; 240
740; 530 [105] MgV2O6
960 888, 425, 375 740, 535 [28]
The presence of intense band at 430 cm-1, associated to the presence of MgO
[225], is observed in all the samples. The bands characteristic for bulk V2O5 (at 600
and 1020 cm-1) were not observed. The appearance of the bands around 700 and
860 cm-1 indicates the presence of weakly crystallized Mg3V2O8. The bands at
860 cm-1 are due to the antysymmetric stretch of (VO4)3- [28, 105, 109, 221, 222,
224].
The bands between 1400 and 1500 cm-1 confirmed the presence of carbonate
species on the surface of VMgA catalysts [107, 227-229].
Infrared spectrum in the hydroxyl region of VMgA samples shows an
absorption band of free hydroxyl groups at 1650 cm-1 and at the broad region
between 2800 and 3800 cm-1, which is typical for Mg-OH surface group. The
corresponding HOH bending vibration of physically adsorbed water is located at 1650
Results of studies and discussion
100
cm-1 [227], whereas previous studies on Mg compounds shown that the bands near
3700 cm-1 are characteristic for the v OH vibrations [224, 228]. The latter bands are
representative for Mg-OH groups on surface and are in excellent agreement with the
results obtained for Mg(OH)2 [230, 231].
Fig. 4-12 IR spectra of VOx/MgO catalysts: (a) VMgMo; (b) VMgP; (c) VMgNb; (d) VMgCr; (e) VMgNi; (f) VMgK; (g) VMg; (h) MgO support
UV-VIS spectroscopy
The diffuse reflectance spectra of the samples under study are shown in Fig.
4-13. According to the literature data presented in table 4-7, the adsorption band
between 280 and 390 nm was assigned to vanadium (V5+) ions in distorted
tetrahedra. This is in agreement with the crystal structure of Mg3V2O8. The absence
of the adsorption bands at 400 – 500 nm range indicated that vanadium (V5+) ions
4000 3500 3000 2500 2000 1500 1000 500
430
700
1650
3800
2800
860
1450
h
g
f
e
d
c
b
a A
bsor
banc
e (a
.u.)
W avenum ber (cm -1)
Results of studies and discussion
101
with octahedral coordination didn’t occur. The bands at 240-280 nm and above 550
nm, characteristic for vanadium V4+ ions were neither observed.
Fig. 4-13 UV-VIS spectra of VOx/MgO catalysts: (a) VMgMo; (b) VMgP; (c) VMgNb; (d) VMgCr; (e) VMgNi; (f) VMgK; (g) VMg
Figure 4-14 presents the UV-VIS spectra of the VMgA samples with reference
to VMg catalyst.
For VMgCr catalyst a band in the range between 380 and 430 nm was
observed. From the literature data summarized in table 4-8 this band can be
assigned to Cr6+ in tetrahedral ligand field.
The spectra of VMgNi sample exhibited a broad band in the region between
380-600 nm with maximum at 480 and 510 nm. According to the literature data
presented in table 4-9., band around 500 nm indicates the appearance of Ni2+ ions in
tetrahedral coordination. The bands at higher wavenumber - around 700 nm
characteristic for Ni2+ ions in octahedral coordination were not observed.
200 400 600 800
380
280
g
f
e
d
c
b
aIn
tens
ity (
a.u.
)
W avelength (nm)
Results of studies and discussion
102
The VMgMo sample revealed the presence of two weak peaks at 230 and 310
nm. It has been reported in the literature that Mo6+ ions in tetrahedral symmetry
exhibited bands in the range 220-260 nm, while in the octahedral coordination
additional bands at higher wavelengths (270-290 and 310-350 nm) were also
observed [209, 214, 215]. The bands obtained for VMgMo sample indicated then the
presence of Mo6+ ions in octahedral symmetry.
Fig. 4-14 UV-vis spectra of VOx/MgO catalysts refered to the VMg sample: (a) VMgCr; (b) VMgNi; (c) VMgK; (d) VMgNb; (e) VMgP; (f) VMgMo
Electron microscopy
The SEM and TEM pictures of the MgO support and the VMg sample are
shown in Fig. 4-15. SEM images show MgO particles in the forms of the platelets.
TEM analysis showed steps at the surface of the particles and evidenced no
separated particles of Mg3V2O8. The Mg3V2O8 phase probably is present as a thin
layer on MgO platelets in VMgA sample and the additives are located on or in this
layer.
2 0 0 4 0 0 6 0 0 8 0 0
560
310
430
380
f
e
d
c
b
a
Inte
nsity
(a.
u.)
W a v e le n g th (n m )
Results of studies and discussion
103
a) b)
c) d)
e) f)
Fig. 4-15 Scaning electron microscopy of (a), (b) MgO support; (c), (d), (e) VMg and transmission electron microscopy of (f) VMg
Results of studies and discussion
104
4.1.1.2.2. VOx/MgO catalysts with additives of different alkali metal ions (Li+,
Na+, K+, Rb+) and of alkaline earth metal ions (Ca2+)
Bulk composition and surface area
The bulk composition of magnesia supported vanadium oxide catalysts with
addition of alkali and alkaline earth metal ions are listed in Table 4-17, which
presents the vanadium and additives contents, the electronegativity of introduced
ions and the BET specific surface area.
The introduction of the additives leads to the increase of the specific surface
area with respect to the undoped VMg catalyst. The effect is the strongest for VMgNa
and VMgRb samples - the increase by ~ 100% of the value for VMg. As it was shown
previously for VOx/MgO catalysts with addition of transition metal ions (see 4.1.1.2.1)
similar increase of the specific surface area was observed.
Table 4-17. List of samples of VOx/MgO catalysts with alkali and alkaline earth
metal ions.
List of catalysts SSA [m2/g] wt % V2O5 wt % additive χχχχi*
MgO support 72.4 - -
VMg 79.7 14.09 - V5+ 17.93
VMgLi 80.30 13.99 0.74 Li+ 3.00
VMgNa 164.51 14.04 0.35 Na+ 2.70
VMgK 135.6 14.01 0.60 K+ 2.70
VMgRb 165.03 13.91 1.31 Rb+ 2.40
VMgCa 130.85 14.00 0.62 Ca2+ 5.00
χχχχi* electronegativity of introduced ions
X-ray photoelectron spectroscopy (XPS)
The results form XPS are shown in Table 4-18. The BE value of V2p3/2 are for
most of the samples between 517.1 and 517.4 eV, which suggests pentavalent
oxidation state of vanadium [141]. For VMgLi the BE value is higher, but similar effect
is observed for BE’s of O1s. The shift can be due to the charge effect for this sample
and not to the change of electron density around V. The values of BE in XPS
measurements confirmed that the additives are present in VMgA samples in the form
Results of studies and discussion
105
corresponding to: Na+, Rb+ and K+ ions. For VMgCa and VMgLi samples the
detection of Ca2+ and Li+ ion was not possible due to the overlapping of the line of Ca
with the line of Mg and low photoionization cross section of Li1s peak, respectively.
Two BE of O1s oxygen were observed. The BE value of the main O1s peak at ~530
eV (OI ) could correspond to Mg3V2O8 phase or magnesium oxide , while a peak at
~532 eV (OII) can be attributed to the OH groups on the surface, or to oxygen
species in carbonates.
The values of the A/V atomic ratios are lower then the nominal, introduced
amount (A/V = 0.1), and indicate considerable loss of additive ions from the catalyst
surface. The V/Mg ratio was higher for the samples with additives in comparison to
VMg pure sample.
Table 4-18. The results of XPS of VOx/MgO catalysts with alkali and alkaline earth
metal ions.
XPS binding energy [eV] XPS atomic ratio Catalysts
V 2p3/2 O 1s OI OII
A
Detected additive
ions V/Mg A/V OI / OII
VMg 517.4 529.9 531.8 - - 0.099 - 1.028
VMgLi 518.0 530.9 532.4 no data no data 0.160 no data 1.042
VMgNa 517.5 530.0 531.9 1072.3 (Na1s)
Na+ 0.116 0.003 1.243
VMgK 517.4 530.0 531.9 293.0 (K1s)
K+ 0.106 0.083 1.289
VMgRb 517.3 529.9 531.9 238.8 (Rb3p3/2)
Rb+ 0.110 0.021 1.804
VMgCa 517.4 530.0 531.9 no data no data 0.129 no data 2.106
X-ray diffraction (XRD)
The diffraction patterns of VMgA catalysts with addition of alkali metal ions are
presented in Fig. 4-16.
All the catalysts of the series showed lines characteristic for magnesium oxide.
The apperance of peaks at 2Θ = 43 and 62o (at Cu Kα) indicates the presence of
MgO phase (JCPDS 4-829). This suggests that if vanadium-containing phases are
present, they are small and poorly crystalline.
Results of studies and discussion
106
Fig. 4-16 X-ray diffraction patterns for VOx/MgO catalysts and observed phases: (a) VMgLi, (b) VMgNa, (c) VMgRb, (d) VMgCa ● MgO
Results of studies and discussion
107
4.1.2. Reducibility of the catalysts
4.1.2.1. VOx / SiO2 catalysts
Table 4-19 gives the temperatures of the maximum of reduction (Tmax), in H2-
TPR measurements, the molar ratio of consumed dihydrogen to V and the average
oxidation state (AOS) of vanadium after reduction for VSiA catalysts at 750oC,
calculated from the amount of the consumed hydrogen. The values of AOS, close
to 3 for the VSiA series indicate that the reduction of the vanadia phase
proceeds to V2O3. Similar value of AOS was observed in earlier studies on the
catalysts containing vanadia phase [232]. The additives do not affect markedly this
value.
Table 4-19. Reducibility of the VOx/SiO2 catalysts as revealed by the H2-TPR.
Catalyst Tmax [oC] H2/V AOS of V
VSi 635 0.94 3.1
VSiK 624; 645; 708 1.05 2.9
VSiNi 640 1.08 2.8
VSiCr 624 1.06 2.9
VSiNb 646; 705 1.04 2.9
VSiP 522, 631; 721 1.01 3.0
VSiMo 661 1.09 2.8
The TPR profiles for VSiA catalysts are presented in Fig. 4-17. All the
samples exhibit a peak at 624~- 660 oC. K, Nb and Mo slightly increase Tmax, as
compared with that of VSi, Cr and P slightly drcrease it. In catalysts with K and
Nb the additional peaks of smaller intensity were observed at temperatures
around 700 oC and for VSiK sample the peak at ~ 630 oC was split into two of
comparable intensity. The peak for VSiP is broad: the existence of several
poorly resolved reduction peaks within it can be suspected.
H2-TPR studies on pure vanadia or supported catalysts containing V2O5,
reported in literature, show one single reduction peak at ~ 600 oC [233] or multiple
major reduction peaks located at 461, 661, 698 and 860 oC [184, 234]. The presence
Results of studies and discussion
108
0 200 400 600 800
g
f
e
d
c
b
a
H2 c
onsu
mpt
ion
(a.u
.)
Tem perature (°C)
of several peaks was ascribed to succesive steps of the reduction to V2O3 via V6O13
and V2O4, or/and to heterogeneity of V-O centres which are reduced. TPR data were
shown to depend on size and morphology of vanadia grains. The appearance of
additional peaks in some of the VSiA catalysts with the additives indicate then the
presence of several M-O reduction centres, which may be due to either different
structure and/or morphology of the vanadia phase, modified by the presence of the
additives, or to the presence of mixed additive-vanadia compounds.
Fig. 4-17 TPR profiles of VOx/SiO2 catalysts: : (a) VSi; (b) VSiK; (c) VSiCr; (d) VSiNi; (e) VSiNb; (f) VSiP; (g) VSiMo
Table 4-20 gives temperature of maximum, (Tmax) of evolution of
reduction products in propane TPD measurements for VSiA catalysts. The
products (only CO2 and H2O, with the exception of VSi catalyst for which some
amounts of propene at low temperature of 140 oC were observed) are formed at the
expense of the lattice oxygen, so the temperature of their appearance can be taken
Results of studies and discussion
109
as a measure of reducibility with propane. As seen K, Ni and Cr additives decrease
the reducibility with propane (increase Tmax) of VSiA. The highest effect is
observed in the case of potassium ions.
Table 4-20. Propane TPR/TPD of VOx/SiO2 catalysts.
Catalyst Tmax* [oC]
VSi 290
VSiK 510
VSiNi 350
VSiCr 300
* Temperature of appearance of CO2
The decrease in the reducibility implies the increase in the strength of M-
O bonds in catalysts. Such the effect may lead to the increase in selectivity to
propene in ODH of propane.
Results of studies and discussion
110
0 200 400 600 800
g
f
e
d
c
b
aH
2 con
sum
ptio
n (a
.u.)
Temperature (°C)
4.1.2.2. VOx / MgO catalysts
Fig. 4-18 presents the TPR profiles for VMgA catalysts.
Fig. 4-18. TPR profiles of VOx/MgO catalysts: (a) VMg; (b) VMgK; (c) VMgNi; (d) VMgCr; (e) VMgNb; (f) VMgP; (g) VMgMo
Only one peak of reduction at Tmax of ~ 640 oC was obtained for all the
catalysts, with the exception of VMgCr sample for which an additional peak at
498 oC was observed. The latter peak could correspond to the reduction of
MgCrO4. All the additives slightly increase Tmax except Ni for which a distinct
decrease was observed. Comparison of these data with the results of Blasco et al.
[100] obtained for VMgO catalyst (of a V content similar to ours), indicate that the
peak with Tmax ~ 640 oC can be attributed to magnesium orthovanadate. No low
temperature peak observed in [100] at 370 oC and ascribed to tetrahedral vanadium
species dispersed on the surface of the catalyst was observed in our studies.
Results of studies and discussion
111
Table 4-21 gives the temperatures of reduction maxima (Tmax), the molar ratio
of consumed dihydrogen and the average oxidation state (AOS) of vanadium after
reduction for VMgA catalysts. For VMg catalysts the additives decrease the H2/V
ratio and increase the AOS value: for pure VMg it is again close to 3 (2.8),
whereas it varies from 3.2 -3.7 for the VMg with the additives. The higher than 3
value of AOS suggests the presence in the reduction products of other than
V2O3 vanadium oxides of higher than V3+ oxidation state. Thus the introduction
of the additives lowers the extent of the reduction of VMg, the smallest effect
being observed for VMgP.
Table 4-21. Reducibility of the VOx/MgO catalysts as revealed by the H2-TPR measurements.
Catalyst Tmax [oC] H2/V AOS of V
VMg 632 1.08 2.8
VMgK 645 0.68 3.6
VMgNi 610 0.75 3.5
VMgCr 498; 643 0.77 3.5
VMgNb 637 0.63 3.7
VMgP 636 0.91 3.2
VMgMo 646 0.79 3.4
The results of propane TPR/TPD measurements performed for some catalysts
of VMgA series are shown in Table 4-22. TPD of adsorbed propane yielded CO2 and
H2O as the only reduction products. The potassium decreases slightly the
reducibility, whereas nickel and chromium increase it.
Table 4-22. Propane TPR/TPD of VOx/MgO catalysts.
Catalyst Tmax* [oC]
VMg 480
VMgK 470
VMgNi 510
VMgCr 550
* Temperature of appearance of CO2
Results of studies and discussion
112
4.1.3. Acido-basic properties of VO x/SiO 2 and VO x/MgO
catalysts
4.1.3.1. The catalysts with additives of main group elements (K+,
P5+) and of transition metal ions (Ni2+, Cr3+, Nb5+, Mo6+)
4.1.3.1.1. Adsorption of pyridine as a probe molecule
The data obtained in IR studies of adsorbed pyridine on VSiA and VMgA
catalysts are given in Table 4-23 and 4-24, respectively.
The IR data of the adsorbed pyridine show no Brönsted (BAS) and Lewis
(LAS) acid sites on the SiO2 support and some Lewis acid sites on MgO. The
catalysts containing vanadium show the presence of Lewis acid sites (VSiA
and VMgA) and Brönsted acid sites (VSiA), their number and strength
depending on the additive nature.
Table 4-23. Number of LAS and BAS on VOx/SiO2 catalysts derived from IR spectra
of adsorbed pyridine.
Number of acid sites per m2 x 1016 Strength of acid sites Catalyst
Lewis Brönsted Lewis Brönsted
SiO2 0 0 0 0
VSi 18.2 39.6 0.65 0.30
VSiK 2.9 18.6 0.56 0.27
VSiNi 16.6 36.4 0.50 0.30
VSiCr 16.4 27.7 0.56 0.29
VSiNb 16.6 29.8 0.54 0.31
VSiP 19.0 53.1 0.67 0.48
VSiMo 19.0 44.4 0.63 0.34
The number of LAS calculated per square meter of VSiA catalysts surface increases
in the sequence:
VSiK << VSiCr ≈ VSiNb ≈ VSiNi < VSi < VSiP ≈ VSiMo,
Results of studies and discussion
113
whereas the number of BAS calculated per square meter of VSiA follows the
sequence:
VSiK << VSiCr ≈ VSiNb < VSiNi ≈ VSi < VSiMo < VSiP.
The strength of LAS calculated for VSiA series increases in the order:
VSiNi < VSiNb ≈ VSiK = VSiCr << VSi ≈ VSiMo < VSiP
whereas that of BAS for VSiA catalysts can be written in the following sequence:
VSiK < VSiCr ≈ VSiNi = VSi ≈ VSiNb < VSiMo << VSiP.
Table 4-24. Number of LAS and BAS on VOx/MgO catalysts derived from IR spectra
of adsorbed pyridine.
Catalyst Number of Lewis acid sites
per m2 x 1016
Strength of Lewis acid sites
MgO 2.7 0.45
VMg 10.0 0.59
VMgK 4.0 0.47
VMgNi 7.6 0.49
VMgCr 8.9 0.46
VMgNb 16.5 0.49
VMgP 15.1 0.61
VMgMo 12.5 0.55
The number of LAS calculated per square meter of VMgA catalysts surface increases
in the sequence:
VMgK << VMgNi ≈ VMgCr ≈ VMg < VMgMo < VMgP < VMgNb,
whereas the strength of LAS calculated for VMgA series increases in the order:
VMgCr ≈ VMgK < VMgNi = VMgNb << VMgMo << VMg < VMgP.
The number of LAS is generally lower for VMgA series as compared with
VSiA. Their strength does not differ considerably, but on the whole the VMgA
catalysts exhibit slightly weaker LAS. The K additive decreases considerably the
number of both LAS and BAS, and, to the smaller extent, also the strength of them.
The modification of the number of acid sites by other additives is smaller, but distinct:
Results of studies and discussion
114
0 5 10 15 20 250,25
0,30
0,35
0,40
0,45
0,50
0,55
0,60
0,65
0,70
Str
engt
h of
aci
d si
tes
Electronegativity of ions χi
Lewis VSiA Bronsted VSiA Lewis VMgA
the Ni and Cr additives lead to the decrease, and P and Mo to the increase in the
number of sites for both VSiA and VMgA series.
For both VSiA and VMgA catalysts with the Ni, Cr and Nb additives the
strength of LAS is slightly lower than that for the catalysts without additives, whereas
it is practically not changed for the P and Mo additives. On the other hand P and Mo
increase the strength of the BAS in the VSiA series.
Fig. 4-19 shows the dependence of the strength of acid sites on
electronegativity of the additive ion for the two series of catalysts. For both series the
strength of acid sites increases with the electronegativity of introduced ions, although
the points are rather scattered.
Fig. 4-19. Dependence of the acid strength on electronegativity of additives for VOx/SiO2 and VOx/MgO catalysts.
The pyridine sorption measurements show then that the two series differ
in the type of the acid sites, the VMgA series exhibiting less and slightly
weaker Lewis acid sites as compared with VSiA, and no Brönsted sites,
whereas both Lewis and Brönsted sites are observed for VSi.
Results of studies and discussion
115
4.1.3.1.2. Decomposition of isopropanol
The data of the isopropanol decomposition for vanadia supported on SiO2 and
on MgO catalysts with different additives are presented in Tables 4-25 and 4-26,
respectively. The activity of the both supports in the isopropanol decomposition is
negligible in comparison with that of the VSiA and VMgA catalysts, so the data
pertain to vanadia phase and its modification.
The main products of the decomposition include propene and acetone, small
amounts of diisopropyl ether (DIE) were also observed for VSiA catalysts.
Table 4-25. Decomposition of isopropanol on VOx/SiO2 catalysts at 200 oC.
C3H6 ether C3H6O Catalyst
x 10-6 [mol/m2 s]
C3H6O/C3H6
SiO2 0.025 - 0.05 2.00
VSi 30.1 - 1.79 0.06
VSiK 10.0 0.78 3.75 0.38
VSiNi 24.2 0.65 2.89 0.12
VSiCr 36.6 0.04 1.56 0.04
VSiNb 33.1 - 2.92 0.09
VSiP 42.5 0.64 1.92 0.05
VSiMo 51.1 0.05 2.92 0.06
For VSiA catalysts the sequence of the increasing rate of the propene
formation (a measure of the acidity) is:
VSiK << VSiNi< VSi ≈ VSiNb ≈ VSiCr < VSiP << VSiMo,
The rate of the acetone formation for the VSiA series follows the sequence:
VSiCr ≈ VSi ≈ VSiP << VSiNi ≈ VSiMo = VSiNb << VSiK,
The acetone/propene ratio, which according to Ai [235] can be taken as a measure of
the basicity for VSiA series is:
VSiCr ≈ VSiP ≈ VSiMo = VSi < VSiNb < VSiNi << VSiK
Results of studies and discussion
116
Table 4-26. Decomposition of isopropanol on VOx/MgO catalysts at 200 oC.
C3H6 ether C3H6O Catalyst
x 10-6 [mol/m2 s]
C3H6O/C3H6
MgO 0.005 - 0.02 4.00
VMg 0.16 0.02 0.81 5.06
VMgK 0.09 - 0.45 5.00
VMgNi 0.08 - 2.50 31.25
VMgCr 0.03 - 3.66 122.0
VMgNb 0.003 - 0.28 93.33
VMgP 0.01 - 0.65 65.00
VMgMo 0.004 - 1.04 260.0
For VMgA series the sequence of the propene formation is:
VMgNb ≈ VMgMo < VMgP < VMgCr << VMgNi ≈ VMgK << VMg,
The rate of the acetone formation for the VMgA catalysts follows the sequence:
VMgNb ≈ VMgK < VMgP < VMg < VMgMo << VMgNi << VMgCr,
The sequence of acetone/propene ratio is:
VMgK ≈ VMg << VMgNi << VMgP < VMgNb < VMgCr << VMgMo
As seen, considerable differences in the behaviour of the two series in the
decomposition of isopropanol are observed. For the VSiA series the main
decomposition product is propene, a product of dehydration, the acetone/propene
ratio varying between 0.06 and 0.4, depending on the type of the additive. The K ions
decrease the formation of propene significantly. The lower amount of propene as
compared to VSi without additives was also observed for the VSiNi sample. In
contrast, P and Mo additives increase considerably the propene formation.
The prevalence of propene over acetone formation, which indicates the
prevalence of acid over basic (redox) centres, was observed previously in the studies
of the isopropanol decomposition over pure vanadia [236]: the similar results for VSiA
catalysts are then in keeping with the present characterization data, which showed
V2O5 as the main active phase for this series. The amount of propene in the VSiA
series roughly correlate with the number of BAS determined in the pyridine sorption
measurements (Fig. 4-20) and with the additive electronegativity (Fig. 4-21), the
correlation with the number of LAS is less evident. No distinct correlation is observed
Results of studies and discussion
117
0
10
20
30
40
50
0 10 20 30 40 50 60
Number of acid sites / m2 x 10 16
C3H
6 x
10-6 [m
ol /
m2 x
s]
Lewis Bronsted
0 5 10 15 20 25
10
20
30
40
50
C3H
6 x
10-6 [m
ol /
m2 x
s]
Electronegativity of ions χi
between the amount of acetone (a product of dehydrogenation, occurring on basic
centres) and electronegativity of added ions.
Fig. 4-20. Dependence of the propene formation in isopropanol decomposition on the number of LAS and BAS for VOx/SiO2 catalysts.
Fig. 4-21. Dependence of the propene formation in isopropanol decomposition on the electronegativity of additive ions for VOx/SiO2 catalysts.
Results of studies and discussion
118
In contrast, for the VMgA catalysts only very small amounts of propene
are observed, the dehydrogenation to acetone being the prevailing path in the
decomposition. The acetone/propene ratio varies between 4.6 and 260.0, being
then several orders of magnitude higher than for VSiA series. The basic (redox)
centres prevail then in this series over acid ones. Similarly as in the case of VSiK
sample the K ions decrease the propene formation. In view of the considerable error
in determination of the small amounts of propene, comparison of the propene data
with those of the pyridine sorption, or with the electronegativity seems however rather
risky.
The characterization of acido-basic properties of the VSiA and VMgA series
indicated the striking differences in the rate of isopropanol dehydration, usually taken
as a measure of acidity, with smaller differences in the number and strength of their
LAS and BAS, obtained in the IR pyridine sorption studies. This may suggest a
different mechanism of the isopropanol decomposition on the catalysts of the two
series.
Several mechanisms of the isopropanol decomposition have been proposed
[237-239], involving both the LAS and BAS and basic lattice oxygen centres. In the
light of these mechanisms we may argue, that in the dehydration of isopropanol on
VSiA series the Brönsted centres may participate with the addition of proton to an
isopropanol molecule, and formation of a carbocation, accounting for the high rate of
propene formation. Propene can also be formed by E-1 mechanism on Lewis acid
sites provided by the acidic V ions. On the VMgA series, for which BAS were not
observed, the concerted E-2 mechanism on an acid-base Lewis M-O (V-O) couple
typical for catalysts with not very strong either acid or basic Lewis sites may be
proposed.
In this case the rate of the propene formation is not directly related to the
sample acidity. The E-1 mechanism on basic centres (typical for pure MgO) cannot
be also excluded.
In view of only the small differences between the VSiA and VMgA series in the
number and strength of LAS, and the presence of BAS only on a VSiA catalysts, one
could suggest that the former centres (BAS) are responsible for high propene
formation in isopropanol decomposition on the latter catalysts.
Results of studies and discussion
119
4.1.3.2. The catalysts with additives of different alkali metal ions
(Li+, Na+, K+, Rb+) and of alkaline earth metal ions (Ca2+,
Mg2+)
Decomposition of isopropanol
Tables 4-27 and 4-28 summarize the data obtained in isopropanol
decomposition for VSiA and VMgA catalysts with additives of different alkali metal
ions (Li+, Na+, K+, Rb+) and of alkaline earth metal ions (Ca2+, Mg2+), respectively.
Table 4-27. Decomposition of isopropanol on VOx/SiO2 catalysts with alkali and
alkaline earth metal ions at 200 oC.
C3H6 ether C3H6O Catalyst
x 10-6 [mol/m2 s]
C3H6O/C3H6
SiO2 0.025 - 0.05 2.00
VSi 30.13 - 1.79 0.06
VSiK 10.05 0.78 3.75 0.37
VSiLi 7.55 0.02 6.63 0.88
VSiNa 22.08 0.48 10.74 0.49
VSiRb 10.61 0.32 6.38 0.60
VSiCa 8.68 0.13 2.81 0.32
VSiMg 14.57 0.12 1.85 0.13
The sequence of the increasing rate of the propene formation for VSiA
catalysts is:
VSiLi < VSiK ≈ VSiRb << VSiNa << VSi,
VSiCa < VSiMg << VSi
The rate of the acetone formation the sequence for VSiA follows:
VSi < VSiK < VSiRb ≈ VSiLi << VSiNa,
VSi ≈ VSiMg << VSiCa
The acetone/propene ratio is:
VSi << VSiK < VSiNa < VSiRb < VSiLi,
VSi << VSiMg << VSiCa
Results of studies and discussion
120
Table 4-28. Decomposition of isopropanol on VOx/MgO catalysts with alkali and
alkaline earth metal ions at 200 oC.
C3H6 ether C3H6O Catalyst
x 10-6 [mol/m2 s]
C3H6O/C3H6
MgO 0.005 - 0.02 4.00
VMg 0.16 0.02 0.81 5.06
VMgK 0.09 - 0.45 5.00
VMgLi 0.05 - 0.43 8.60
VMgNa 0.004 - 0.84 210.0
VMgRb 0.02 - 0.21 10.5
VMgCa 0.03 - 1.62 54.0
For VMgA series the sequence of the propene formation is:
VMgNa << VMgRb < VMgLi < VMgK << VMg,
VMgCa << VMg
The sequence of the rate of the acetone formation follows:
VMgRb << VMgLi ≈ VMgK << VMg ≈ VMgNa,
VMg << VMgCa
The acetone/propene ratio sequence is:
VMgK ≈ VMg < VMgLi < VMgRb << VMgNa,
VMg << VMgCa
All the alkali and alkaline earth metals decrease the amount of propene
(the acidity) of both the VSiA and VMgA catalysts and increase the basicity
(acetone/propene ratio).
For the VSiA catalysts with additives of different alkali metal ions (Li+,
Na+, K+, Rb+) and with alkali earth ions (Ca2+, Mg2+) the main decomposition
product is again propene. For this series the changes in the amount of
products of propane decomposition roughly correlate with the
electronegativity of additives. The amount of propene increases (Fig. 4-22),
while the amount of acetone decreases (Fig. 4-23) with the increasing
electronegativity of the additives. The marked deviation from this correlation
can be observed for the Na ion, VSiNa sample showing relatively high acidity in
spite of low electronegativity.
Results of studies and discussion
121
2 4 6 8 10 12 14 16 18 205
10
15
20
25
30
C3H
6 x 1
0-6 [m
ol /
m2 x
s]
Electronegativity of ions χi
2 4 6 8 10 12 14 16 18 20
2
4
6
8
10
12
C3H
6O x
10-6
[mol
/ m
2 x s
]
Electronegativity of ions χi
For VMgA series the main reaction product is acetone and the
correlations between amounts of propene or acetone and electronegativity of
introduced ions were not observed.
Fig. 4-22. Dependence of the propene formation in isopropanol decomposition on the electronegativity of additive ions for VOx/SiO2 catalysts.
Fig. 4-23. Dependence of the acetone formation in isopropanol decomposition on the electronegativity of additive ions for VOx/SiO2 catalysts.
Results of studies and discussion
122
4.1.4. Summary of physicochemical characterization of
VO x/SiO 2 and VO x/MgO catalysts
4.1.4.1. VOx/SiO2 catalysts
1. The VOx/SiO2 catalysts contain crystalline V2O5 and small amounts of
vanadium-additive mixed phases as evidenced by XRD, IR and Raman
spectroscopy. Octahedral coordination of vanadium was confirmed by NMR and UV-
VIS spectroscopy. The XPS measurements indicate that vanadium and the additives
are present in the form corresponding to: V5+, P5+, Nb5+, Cr3+, Ni2+, Mg2+, Ca2+, Na+,
Rb+ and K+ ions in oxides. The Mo-doped fresh catalyst contains a fraction of Mo4+
ions beside Mo6+ ions which was confirmed by UV-VIS spectroscopy. The fraction of
reduced Mo ions decreases in the catalysts after the propane ODH reaction. The
additives do not affect coordination state of vanadium but affect the dispersion of
vanadium on the SiO2 phase: in particular the alkali ions increase the coverage of the
SiO2 surface, as can be inferred from the V/Si values in XPS measurements.
2. Introduction of the additives affects only slightly the reducibility of V2O5 phase
with H2, and to a higher extent the reducibility with propane. The decrease in the
reducibility is observed for K (strong effect), Mo and Nb additives.
3. The VOx/SiO2 catalysts exhibit Lewis and Brönsted acidity of medium strength.
The acidic properties prevail over basic (redox) properties as can be inferred from a
probe reaction of isopropanol decomposition. The acido-basic properties are
modified by the additives. The acidity (number and strength of sites) increases with
the electronegativity of the additives. In particular considerable decrease of acidity is
observed for alkali and alkaline earth doped catalysts.
Results of studies and discussion
123
4.1.4.2. VOx/MgO catalysts
1. The VOx/MgO catalysts contain a disordered magnesium orthovanadate
(Mg3V2O8) phase with tetrahedral coordination of vanadium as evidenced by Raman
IR and 51V NMR spectroscopies. This phase is most probably present in an
amorphous form, or as a thin layer on the MgO surface, as it was not detected by
XRD.
The additives do not influence the structure of the catalysts. Only for fresh
VMgCr small amounts of MgCrO4 disappearing during the catalytic reaction were
also observed.
The XPS measurements confirmed that the additives are present in VMgA
samples in the form corresponding to: V5+, Mo6+, Nb5+, P5+, Ni2+, Cr3+, Na+, Rb+ and
K+ ions. Beside Cr3+ the Cr6+ ions (~ 30% of all amount of Cr) were also detected and
their presence was confirmed by UV-VIS spectroscopy.
2. Effect of additives on the reducibility of VOx/MgO catalysts is very small.
Reduction by propane indicated that the K additive decreases slightly the reducibility,
whereas Ni and Cr increase it.
3. The VOx/MgO catalysts exhibit less and slightly weaker Lewis acid sites as
compared with VSiA, and no Brönsted sites. The strength and number of Lewis acid
sites depend on the type of the additive, the strength increasing with the
electronegativity of introduced ions. The catalysts show high dehydrogenation and
low dehydration rates in isopropanol decomposition. The basic (redox) centres
predominate then over acidic ones. All the additives: transition, alkali and alkaline
earth metal ions decrease the acidity of VOx/MgO catalysts.
Results of studies and discussion
124
4.2. Catalytic properties of studied systems in oxidative
dehydrogenation of propane and ethane
4.2.1. VO x / SiO 2 catalysts
4.2.1.1. VOx/SiO2 catalysts with additives of main group elements
(K+, P5+) and of transition metal ions (Ni2+, Cr3+, Nb5+,
Mo6+)
Fig. 4-24 presents the variations of the propane conversion with the reaction
temperature in the range 380-520 oC for VSiA catalysts with addition of main group
elements and of transition metal ions. The changes of the selectivity to propene and
carbon oxides as a function of the temperature are shown in Figs. 4-25 and 4-26.
The increase of the conversion with the temperature is accompanied by the
decrease of the selectivity to propene and the increase of the selectivity to CO. In
most cases the selectivity to CO2 varies only slightly with the reaction temperature
(the propane conversion) with the exception of the VSiNi catalyst. The course of the
changes in the selectivity with the temperature (corresponding to different
conversion) suggests that CO is mainly formed by consecutive overoxidation of
propene, formed in the first step of the reaction, whereas the main source of CO2 is a
parallel oxidation of propane. The selectivities to CO are much higher than those to
CO2. Such behavior is characteristic of reactions of oxidative dehydrogenation of
alkanes on supported vanadia catalysts. Similar course was observed previously for
Mo-V-Nb catalysts in ODH of ethane [240] and vanadia-titania catalysts in ODH of
propane [37]. The detailed kinetic studies performed by R. Grabowski et al. [ 241,
242] have shown, that the propane-oxygen reaction in the ODH of propane on
V2O5/TiO2 and V2O5/SiO2 (the VSi sample) catalysts could be described adequately
by a parallel consecutive kinetic network in which the rates of overoxidation of
propene are much higher than those of parallel oxidation to COx.
The direct proof of the parallel-consecutive reaction network for VSiA catalysts
is given in Fig 4-27, which shows as an example the plot of selectivities to different
products vs total conversion of propane at constant temperature of 420 oC for the VSi
sample. The different conversions were obtained by modifying the contact time τ. As
Results of studies and discussion
125
380 400 420 440 460 480 500 520
10
20
30
40
50
60
70
80
90
Sel
ectiv
ity, C
3H6 [%
]
Temperature [OC]
VSi VSiK VSiNi VSiCr VSiNb VSiP VSiMo
can be seen, for the same reaction temperature the selectivity to propene
decreases with the increasing conversion. The selectivity to carbon monoxide
increases with the increasing conversion of propane, whereas the selectivity to
carbon dioxide (considerably lower than that to CO) changes only slightly.
Similar curves were obtained for other catalysts of both VSiA and VMgA series.
Fig. 4-24. Changes of the propane conversion (at τ = 0.5 s) with the reaction temperature for VOx/SiO2 catalysts.
Fig. 4-25. Changes of the selectivity to C3H6 in propane-oxygen reactions (at τ = 0.5 s) with the reaction temperature for VOx/SiO2 catalysts.
380 400 420 440 460 480 500 5200
5
10
15
20
25
30C
onve
rsio
n [%
]
Temperature [OC]
VSi VSiK VSiNi VSiCr VSiNb VSiP VSiMo
Results of studies and discussion
126
380 400 420 440 460 480 500 5200
10
20
30
40
50
60
70
80
Sel
ectiv
ity, C
O, C
O2 [%
]
Temperature [OC]
VSi VSiK VSiNi VSiCr VSiNb VSiP VSiMo
4 6 8 10 12 14 16 18
10
20
30
40
50
60
70
Sel
ectiv
ity [%
]
Conversion [%]
C3H
6
CO CO
2
Fig.4-26. Changes of the selectivity to CO and CO2 in propane-oxygen reactions
(at τ = 0.5 s) with the reaction temperature for VOx/SiO2 catalysts; CO _____, CO2 - - - - - .
Fig.4-27. Changes of the selectivity to C3H6, CO and CO2 in propane-oxygen reactions at 420 oC with the total propane conversion for VOx/SiO2 catalyst.
Results of studies and discussion
127
Table 4-29 gives the specific total activity at 450 oC in the ODH of propane and
at 500 oC in the ODH of ethane and apparent activation energies for the total reaction
of propane on the studied samples.
Table 4-29. The total activity of VOx/SiO2 catalysts in ODH of propane and ethane
Specific activity (µmol m-2 min-1)
Activation energy c) (kJ / mol)
Catalyst symbol
propane a) ethane b) propane VSi 0.70 0.85 85
VSiK 0.09 0.17 149
VSiNi 0.71 0.99 66
VSiCr 0.74 1.15 74
VSiNb 0.92 1.08 84
VSiP 3.57 5.61 130
VSiMo 1.61 1.88 105 a) 450 oC, b) 500 oC, c) apparent activation energies determined from the Arrhenius plot
In view of the lower activity of the catalysts in the ethane ODH the catalysts
were compared only at higher temperature of 500 oC. Lower conversions of ethane
as compared with those of propane can be due to the higher C-H bond energy in the
ethane molecule as compared with propane, the breaking of this bond being the rate
determining step in the oxidation reactions of hydrocarbons.
The specific activity of ethane was referred to the same alkane partial
pressure as in the ODH of propane (7kPa), assuming a first-order dependence of the
rate of the ethane ODH on the ethane partial pressure.
For the reaction of propane the total specific activity follows the sequence:
VSiK << VSi < VSiNi = VSiCr < VSiNb < VSiMo << VSiP
For the reaction of ethane the total specific activity decreases in the sequence:
VSiK < VSi < VSiNi = VSiNb = VSiCr < VSiMo << VSiP.
Since the loading with vanadium is the same for all the catalysts and the
support was found inactive in the conditions of the catalytic test, the data reflect the
activity of the vanadia phase. Thus all the additives except potassium increase
the total activity of the VSi catalysts. The activity of the samples with the P and
Mo additives is significantly higher than that of the undoped sample, whereas
Results of studies and discussion
128
0 5 10 15 20 25
0
1
2
3
4
5
6
Act
ivity
[ µm
ol m
-2 m
in-1 ]
Electronegativity of ions χi
C2H
4
C3H
6
the effect of Cr, Nb and Ni additives is smaller. K decreases conciderably the
activity. It can be observed that the previous studies on vanadia-based catalysts
showed that the addition of the alkali metals generally lowers the total activity of the
catalysts [37, 58, ]. The effect was ascribed in the first place to blocking of the active
V-O centres on the surface by alkalis.
For the reaction of ethane the sequence of the activities is the same as
that for the ODH of propane, though the extent of changes is different e.g.
potassium decreases the activity to a smaller extent .
Fig. 4-28 presents the variations of the specific activity in the reaction of
ethane and propane with the electronegativity of introduced ions for VSiA catalysts.
For both reactions the activity of the samples increases with the increasing
electronegativity of the additives.
Fig.4-28. Changes of the activity ODH reactions of ethane at 500 oC and propane at 450 oC with the electronegativity of the additives’ ions for VOx/SiO2 catalysts.
The apparent activation energies, obtained from the Arrhenius plots, are
lower for samples doped with Ni and Cr ions, practically unchanged for sample
with Nb, and higher for the samples with K, Mo and P additives. Considerable
increase of the activation energy in systems with K additive was observed also in
kinetic studies [241, 242]. High activity of VSiP and VSiMo in spite of high activation
energy values suggests the increase of the pre-expotential factor A (number of
Results of studies and discussion
129
0 5 10 15 20 25
60
80
100
120
140
160A
ctiv
atio
n en
ergy
[ kJ
/mol
]
Electronegativity of ions χi
centres ) in this case. Low activity of VSiK catalyst may be due to both high value of
activation energy and low value of number of the active centers. Changes of the
activation energy for ODH of propane with the electronegativity of introduced ions are
presented in Fig. 4-29. The activation energy increases with the increasing
electronegativity of additives with the exception of the VSiK sample.
Fig.4-29. Changes of the activation energy for ODH of propane with the electronegativity of the additives’ ions for VOx/SiO2 catalysts.
Fig. 4-30 compares selectivities to various products at ~10% conversion of propane
and at the reaction temperature of 450 oC. In view of dependence of selectivity on the
conversion, resulting from the parallel – consecutive mechanism of oxidative
dehydrogenation reaction, the selectivities to olefins are compared for the same
value of the total conversion (isoconversion).
The predominance of CO formation over CO2 in the total oxidation products on
VSi series is clearly seen. The selectivities to CO2 (10-15%) are lower than the
selectivities to CO (40-60%).
For the VSiK sample, for which conversions higher than 1-3% could not be
obtained at this temperature, the data at 470 oC are given.
The selectivities to different products change on introduction of the additives
as compared with undoped catalysts. The sequence of the increasing selectivities for
the VSi series is:
Results of studies and discussion
130
VSiP < VSi < VSiNb < VSiNi ≈ VSiMo < VSiCr << VSiK
As seen, the K additive increases considerably the selectivity to propene
from about 25% for undoped sample to 65 - 75%. The distinct increase in the
selectivity (though much smaller than for the K -doped sample) is observed
also for the samples with the Ni, Cr and Mo additives. A small decrease is
observed for the P additive. The propene selectivities (26% at 450 oC), for VSi
catalyst without additives at 10% conversion are close to those reported by López
Nieto [98] for similar loading of vanadia. Other literature data [243, 244] on selectivity
to propene in oxidative dehydroganation of propane on V2O5/SiO2 catalysts without
additives show higher values of selectivity at ~10% conversion (about 50-55%). The
loading of V2O5, however was in these studies much lower than for our sample and
the reaction conditions were different.
For VSiA catalysts the best yields of propene were obtained for the VSiK
sample. The observed yield to propene at 10% isoconversion was 5.0 %. The
highest yields of propene (10.3 %) observed for VSiK sample at conversion
22.1% at 520 oC, compare well with those reported for supported vanadia
catalysts [5, 106].
Fig.4-30. Selectivity to various products at 10 % (± 2) propane conversion for VOx/SiO2 catalysts. Reaction temperature: 450oC, * 470oC.
VSi VSiK VSiNi VSiCr VSiNb VSiP VSiMo0
10
20
30
40
50
60
70
*
Sel
ectiv
ity [%
]
C3H
6
CO CO
2
Results of studies and discussion
131
VSi VSiK VSiNi VSiCr VSiNb VSiP VSiMo0
10
20
30
40
50
60
Sel
ectiv
ity [%
]
C2H
4
CO CO
2
Fig. 4-31 gives selectivities to different products at ~10% of ethane conversion
at 500 oC for VSiA series. It can be observed that the VSiA catalysts are slightly
more selective for ethene than for propene formation (with except of VSiK
sample). The most striking difference in the selectivities to olefins for the two
alkanes is practically no effect of K on the selectivity to ethene on VSiK
catalyst (one can observe even a slight decrease) and very significant effect
(increase) for propene.
Fig.4-31. Selectivity to various products at 10 % (± 2) ethane conversion for VOx/SiO2 catalysts. Reaction temperature: 500oC.
On the whole, the effect of the additives on the selectivity to ethene is less
significant than in the case of the propane ODH. The sequence of the selectivities is:
VSiP ≈ VSiCr < VSiK < VSi ≈ VSiNi < VSiMo < VSiNb
The selectivity to ethene of about 40 % observed for the VSi sample is similar to
these reported earlier by other authors for this catalyst [245]. The highest yields of
ethene obtained for this series are ~4%. The best selectivity to ethene is obtained for
VSiNb sample, however, the increase of selectivity with respect VSi sample is small
(by ~ 6%).
Fig. 4-32 presents the changes of the selectivity to ethene and propene in the
reaction of ethane and propane with the electronegativity of introduced ions for VSiA
Results of studies and discussion
132
0 5 10 15 20 2520
30
40
50
60
70S
elec
tivity
[%]
Electronegativity of ions χi
C2H
4
C3H
6
catalysts. For the reaction of propane the order of the decreasing selectivity to
propene follows the order of the increasing electronegativity of additives, while
for the ethane reaction the selectivity to ethene practically does not depend on
the electronegativity of the introduced ions.
Fig.4-32. Changes of the selectivity to C2H4 and C3H6 with the electronegaticvity of the additives’ ions for ODH of ethane and propane on VOx/SiO2 catalysts.
4.2.1.2. VOx/SiO2 catalysts with additives of alkali metal (Li+, Na+,
K+, Rb+) and of alkaline earth metal ions (Mg2+, Ca2+)
The changes of the propane conversion as a function of the temperature for
VSiA catalysts with addition of alkali and alkaline earth metal ions are presented in
Fig. 4-33.
Figs. 4-34 and 4-35 show the variations of the selectivity to propene and
carbon oxides as a function of the temperature, and Fig. 4-36 presents the changes
of the selectivity to products with the conversion. Similarly like in the case of other
VSiA catalysts, for all VSiA alkali-doped catalysts the selectivity to C3H6 decreases
and that to CO increases with the increasing conversion (temperature), while that to
Results of studies and discussion
133
CO2, (present in lower amounts), shows a weak dependence on it, confirming parallel
consecutive mechanism of the reaction.
Fig.4-33. Changes of the propane conversion (at τ = 0.5 s) with the reaction temperature for alkali-doped VOx/SiO2 catalysts.
Fig. 4-34. Changes of the selectivity to C3H6 in propane-oxygen reactions (at τ = 0.5 s) with the reaction temperature for alkali-doped VOx/SiO2 catalysts.
380 400 420 440 460 480 500 5200
5
10
15
20
25
30
35
40
45
50
Con
vers
ion
[%]
Temperature [oC]
VSiLi VSiNa VSiK VSiRb VSiMg VSiCa VSi
380 400 420 440 460 480 500 520
10
20
30
40
50
60
70
80
Sel
ectiv
ity, C
3H6 [%
]
Temperature [oC]
VSiLi VSiNa VSiK VSiRb VSiMg VSiCa VSi
Results of studies and discussion
134
2 4 6 8 10 12
10
20
30
40
50
60
70
Sel
ectiv
ity [%
]
Conversion [%]
C3H
6
CO CO
2
Fig.4-35. Changes of the selectivity to CO and CO2 in propane-oxygen reactions
(at τ = 0.5 s) with the reaction temperature for alkali-doped VOx/SiO2 catalysts; CO _____, CO2 - - - - - .
Fig.4-36. Changes of the selectivity to C3H6, CO and CO2 in propane-oxygen reactions with the canversion at 500 oC for VOx/SiO2 with Li catalyst.
380 400 420 440 460 480 500 5200
10
20
30
40
50
60
70
80
Sel
ectiv
ity, C
O,C
O2 [%
]
Temperature [oC]
VSiLi VSiNa VSiK VSiRb VSiMg VSiCa VSi
Results of studies and discussion
135
The specific total activity at 450 oC in the ODH of propane for alkali and
alkaline earth-doped VSiA catalysts is presented in table 4-30. For all VSiA
catalysts with alkali and alkaline earth metal ions with the exception of Na
additive, the decrease of total specific activity was observed. The decrease in
activity in ODH of propane for VSiA catalysts with addition of alkali metal ions may
indicate a poisoning effect of the alkaline cations on the centres for hydrocarbon
activation, though no simple correlation between the activity and ionic radius is
observed. This is in contrast with the results obtained for VOx/TiO2 [128] and
V2O5/Al2O3 [95] with addition of alkaline cations, where the poisoning effect
depended on the size of blocking cation: the decrease in the activity was correlated
with the increasing ionic radius of the alkali metal promoters.
Table 4-30. The total activity of VOx/SiO2 catalysts with alkali and alkaline earth metal ions catalysts in ODH of propane at 450 oC
Catalyst symbol Specific activity
(µmol C3H8 m-2 min-1)
Activation energy a) (kJ / mol)
VSi 0.70 85
VSiLi 0.14 57
VSiNa 0.61 76
VSiK 0.09 149
VSiRb 0.15 119
VSiMg 0.43 71
VSiCa 0.33 85 a) apparent activation energies determined from the Arrhenius plot
The order of decrease of specific total activity for VSiA catalysts with addition
of alkali and alkaline earth metal ions is the following:
VSiK < VSiLi = VSiRb < VSiNa < VSi
VSiCa < VSiMg < VSi
Fig. 4-37 shows changes of the activity with the electronegativity of additives.
The activity in the reaction of propane of the VSiA samples increases with the
increasing electronegativity of introduced ions, Na ion being an exception.
The apparent activation energies for ODH of propane, obtained from Arrhenius
plot, are slightly lower for samples with Na and Mg additives, unchanged for sample
Results of studies and discussion
136
2 4 6 8 10 12 14 16 18 200,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
Act
ivity
[µm
ol m
-2 m
in-1]
Electronegativity of ions χi
with Ca, and higher for the catalyst with Rb additives. The significant increase in the
activation energy was observed for the sample doped with K and Rb ions.
Fig.4-37. Changes of the activity in reaction of propane at 450 oC with the electronegativity of additives’ alkali and alkaline earth metal ions for VOx/SiO2 catalysts.
Fig.4-38. Selectivity to various products at 10 % (± 2) propane conversion for VOx/SiO2 catalysts with alkali and alkaline earth metal ions. Reaction temperature: 470oC, * 450oC.
VSi VSiLi VSiNa VSiK VSiRb VSiMg VSiCa0
10
20
30
40
50
60
70
*
Se
lect
ivity
[%]
C3H
6
CO CO
2
Results of studies and discussion
137
2 4 6 8 10 12 14 16 18 2020
30
40
50
60
70
Sel
ectiv
ity to
C3H
6 [%
]
Electronegativity of ions χi
The selectivities to various products at isoconversion (~10%) of propane at the
reaction temperature of 470 oC for VSiA with addition of alkali metal and alkaline
earth metal ions are presented in Fig. 4-38. The sequence of the increasing
selectivities is:
VSiNa < VSi < VSiLi < VSiRb = VSiK
VSi < VSiCa < VSiMg.
All additives with the exception of Na increase the selectivity to propene,
however the highest changes are observed in the case of K and Rb additives.
The best yields of propene were obtained for the VSiK sample (10.3 % at
conversion 22.1 % at 520 oC), though VSiRb catalyst also exhibited high yields
of propene (9.4 % at conversion 19.7 % at 520 oC).
It is worth observing that the increase in the selectivity to propene is
mainly due to the decrease in the selectivity to CO: the selectivity to CO2
increases for most of the catalysts with the alkali additives.
Fig. 4-39 presents the changes of the selectivity to propene in the reaction of
propane with the electronegativity of introduced ions for VSiA catalysts. Generally
selectivity to propene decreases with the increase of electronegativity of introduced
ions with exception of VSiNa sample, however the points are rather scattered.
Fig.4-39. Changes of the selectivity to C3H6 with the electronegativity of alkali and
alkaline earth metal ions for ODH of propane on VOx/SiO2 catalysts.
Results of studies and discussion
138
4.2.2. VO x / MgO catalysts
4.2.2.1. VOx/MgO catalysts with main group elements (K+, P5+)
and of transition metal ions (Ni2+, Cr3+, Nb5+, Mo6+)
The changes of the propane conversion with the reaction temperature for
VMgA catalysts with addition of main group ions and transition metal ions are shown
in Fig. 4-40. The variations of the selectivity to propene and carbon oxides with the
temperature for the VMgA samples are presented in Figs. 4-41 and 4-42,
respectively. The selectivities to propene and COx change with the reaction
temperature in a complex way: for most cases the selectivity to propene increases
first with the temperature and then decreases. The increase in the selectivity to
propene is accompanied by the decrease in the selectivity to CO2 (the predominant
product of total combustion), whereas decrease of the selectivity to propene at higher
temperatures is followed by the increase in the CO selectivity like it was observed for
VSiA catalysts.
The lower selectivity to partial oxidation products and high to CO2 at lower
reaction temperatures was observed earlier in o-xylene oxidation on V2O5/TiO2 [97]
and was ascribed to the presence of nonselective, electrophilic oxygen species at
low temperatures, which can react with the hydrocarbon by an associative
mechanism [8]. At higher temperatures, at which the lattice oxygen becomes mobile,
the reaction takes place by Mars and van Krevelen mechanism with participation of
selective, nucleophilic O2- species.
It can be suggested that similar phenomena occur for VMgA catalysts: the low
reducibility with propane (lower than that of VSiA series) would imply that the redox
mechanism starts to operate at relatively higher temperatures. At low temperatures
the presence of electrophilic species and the associative mechanism of oxidation are
possible.
Results of studies and discussion
139
400 420 440 460 480 500 5200
5
10
15
20
25
30
Con
vers
ion
[%]
Temperature [OC]
VMg VMgK VMgNi VMgCr VMgNb VMgP VMgMo
400 420 440 460 480 500 520
25
30
35
40
45
50
Sel
ectiv
ity, C
3H6
[%]
Temperature [OC]
VMg VMgK VMgNi VMgCr VMgNb VMgP VMgMo
Fig.4-40. Changes of the propane conversion (at τ = 0.5 s) with the reaction temperature for VOx/MgO catalysts.
Fig.4-41. Changes of the selectivity to C3H6 in propane-oxygen reactions (at τ = 0.5 s) with the reaction temperature for VOx/MgO catalysts.
Results of studies and discussion
140
400 420 440 460 480 500 5200
10
20
50
60
70
80
Sel
ectiv
ity, C
O, C
O2
[%]
Temperature [OC]
VMg VMgK VMgNi VMgCr VMgNb VMgP VMgMo
Fig.4-42. Changes of the selectivity to CO and CO2 in propane-oxygen reactions
(at τ = 0.5 s) with the reaction temperature for VOx/MgO catalysts; CO _____, CO2 - - - - - .
Fig. 4-43 shows the changes of the selectivity to the reaction products with the
increase of conversion at 450 oC for VMg catalyst. Similarly like in the case of VSiA
catalysts selectivity to propene decreases with the increasing of conversion of
propane, which is accompanied by an increase in the CO2 selectivity and slight
increase in the CO selectivity at the higher conversion. In contrast, however, to the
VSiA samples, the changes of the selectivity with the conversion are small and the
overabundance of CO2 over CO in total combustion products can be observed .
These facts may suggest a higher contribution of the parallel way of the oxidative
dehydrogenation reaction for VMgA catalysts. The excess of CO2 over CO in total
combustion products was also observed in other works (on V-Mg-O catalysts) [112,
246], though not commented. The difference between V-Mg-O catalysts and vanadia
supported on other oxides in distribution of the total oxidation products implies
different type or population of the active oxygen species on catalysts of the both
types.
Results of studies and discussion
141
10 12 14 16 18 20 2210
20
30
40
50
60
Sel
ectiv
ity [%
]
Conversion [%]
C3H
6
CO CO
2
Fig.4-43. Changes of the selectivity to C3H6, CO and CO2 in propane-oxygen reactions with the conversion at 450 oC for VOx/MgO catalyst.
The Table 4-31 gives the specific total activity in the ODH of propane at
450 oC and in the ODH of ethane at 500 oC of the studied samples.
Table 4-31. The total activity of VOx/MgO catalysts in ODH of propane and ethane. Specific activity
(µmol m-2 min-1) Activation energy c)
(kJ / mol) Catalyst symbol
propane a) ethane b) propane
VMg 0.87 1.85 110
VMgK 0.24 1.11 110
VMgNi 0.66 1.81 120
VMgCr 0.67 2.14 116
VMgNb 0.40 1.86 95
VMgP 0.45 1.17 92
VMgMo 0.40 1.38 98 a) 450 oC, b) 500 oC,
c) apparent activation energies determined from the Arrhenius plot
Results of studies and discussion
142
0 5 10 15 20 25
90
95
100
105
110
115
120
Act
iva
tion
ene
rgy
[kJ/
mo
l]
Electronegativity of ions χi
The total specific activity in the ODH of propane at temperature 450 oC follows
the sequence:
VMgK < VMgMo = VMgNb < VMgP < VMgNi = VMgCr < VMg
Thus all the additives decrease the total activity of the VMgA catalysts in the
reaction of propane.
For the reaction of ethane at 500 oC the total specific activity decreases in the
sequence:
VMgK < VMgP < VMgMo < VMgNi < VMg = VMgNb < VMgCr
The sequence of the activities for the VMgA catalysts for ODH of ethane is
different from that for the ODH of propane. The Cr and Nb ions increase the
activity, while for the reaction of propane they decrease it. However, for both
alkanes the potassium additive decreases the activity, though to the smaller
extent for the ethane reaction.
Fig. 4-44 presents variations of the activation energy for ODH of propane with
the electronegativity of the introduced ions. For all samples with the exception of
VMgK the activation energy decreases with the increase of electronegativity of
additives. The dependence is inverse to that found for VSiA series, which suggests
that the mechanism of the reaction on catalysts of two series may be different.
Fig.4-44. Changes of the activation energy for ODH of propane with the electronegativity of ions for VOx/MgO catalysts.
Results of studies and discussion
143
VMg VMgK VMgNi VMgCr VMgNb VMgP VMgMo0
10
20
30
40
50
60
Se
lect
ivity
[%
]
C3H
6
CO CO
2
In contrast to the VSiA series the correlations between the specific activity in
the reaction of ethane and propane and the electronegativity of introduced ions for
VMgA catalysts were not found.
Fig 4-45 compares the selectivities to various products at isoconversion
(~10%) of propane for the reaction temperature of 450oC.
Fig.4-45. Selectivity to various products at 10 % (± 2) propane conversion for VOx/MgO catalysts. Reaction temperature: 450oC.
The predominance of CO2 over CO in the total oxidation products on VMg
catalysts is again clearly observed. The selectivities to CO2 (50-60%) are higher than
selectivities to CO (15-20%).
The selectivities to different products change on introduction of the additives
as compared with undoped catalyst. The sequence of the increasing selectivities for
the VMgA catalysts with transition metal ions is:
VMgK < VMgP < VMg < VMgMo ≈ VMgNb < VMgCr < VMgNi
For VMg catalyst without additives the values of selectivity to propene (32% at
450 oC), at 10% isoconversion are close to those reported by López Nieto [223] for V-
Mg-O catalyst with comparable loading of vanadia.
Results of studies and discussion
144
For VMgA series the Ni, Cr, Nb and Mo additives increase the selectivity
to propene, whereas P slightly decreases it. Positive effect of Cr [246] and Mo
[246, 247] on activity and selectivity in ODH of n-butane was reported in the
literature.
The striking difference between the VSiA and VMgA series can be observed in
the effect of K. The potassium ions decrease the selectivity to propene for the
VMgK sample. The different for VSi and VMg catalysts effect of K on the selectivity
to propene suggests that the mechanism of action of this additive is different in the
both cases. A detrimental effect of the presence of potassium in magnesium
vanadates on the selectivity to olefins in the oxidative dehydrogenation of propane
and n-butane was reported by H.H Kung and M.C. Kung [112] and on selectivity to
propene in the reaction of propane was also noticed by R.X. Valenzuela et al. [248].
For VMgA catalysts the best yields of propene were obtained for the
VMgNi sample. The yield to propene at 10% isoconversion was 3.8 %. The
highest yields of propene (7.3 %) were observed for VMgNi sample at
conversion 19.6% at 470 oC.
Fig. 4-46 gives selectivities to different products at ~10% of ethane conversion
for VMgA series. The sequence of the selectivites for VMgA catalysts is:
VMg < VMgP < VMgK = VMgCr < VMgNb < VMgMo < VMgNi
Similarly to the propane ODH the VMgA catalysts yield more CO2 than
CO. In contrast to VSiA, for the VMgA system the selectivity to ethene is
considerably lower than that to propene in the propane ODH. All the additives
increase the selectivity to ethene, however the changes are rather small.
Practically no marked effect of K on the selectivity to ethene on VMgK catalyst
was observed.
The latter result is in agreement with the data reported earlier by Kung et al
[16], who reported that in general, for oxidative dehydrogenation reactions of
propane, butane, 2-metylpropane and cyclohexane on V-Mg-O catalysts the
dehydrogenation products were the dominant products, especially at low
conversions. The exception was the oxidation of ethane, where the combustion
products dominated.
The yields to ethene are ~2 %, the highes value was obtained for VMgNi
sample (2.3 % at conversion 11.0 % and at temperature 500 oC).
Results of studies and discussion
145
0 5 10 15 20 25
10
20
30
40
Sel
ect
ivity
[%
]
Electronegativity of ions χi
C2H
4
C3H
6
Fig.4-46. Selectivity to various products at 10 % (± 2) ethane conversion for VOx/MgO catalysts. Reaction temperature: 500oC.
Fig.4-47. Changes of the selectivity to C2H4 and C3H6 with the electronegativity of ions for ODH of ethane and propane on VOx/MgO catalysts.
VMg VMgK VMgNi VMgCr VMgNb VMgP VMgMo0
10
20
30
40
50
60
70
Sel
ectiv
ity [%
]
C2H
4
CO CO
2
Results of studies and discussion
146
Fig. 4-47 presents the changes of the selectivity to ethene and propene in the
reaction of ethane and propane with the electronegativity of introduced ions. For the
reaction of propane, for all the samples with the exception of VMgK, the selectivity to
propene decreases the increase of electronegativity of additives. Similarly like in the
case of VSiA series, the selectivity to ethene does not depend on the
electronegativity of the introduced ions.
4.2.2.2. VOx/MgO catalysts with additives of alkali metal ions (Li+,
Na+, K+, Rb+) and of alkaline earth metal ions (Ca2+)
The changes of the propane conversion, selectivity to propene and to carbon
oxides with the temperature for the VMgA catalysts with addition of alkali metal ions
and of alkaline earth metal ions are presented in Figs. 4-48, 4-49 and 4-50,
respectively. Similarly like in the case of other VMgA catalysts the selectivity to
propene and carbon oxides change with the reaction temperature in a complex way
and the predominance of CO2 formation over CO in the total oxidation products is
observed. The changes of the selectivity to the reaction products with the increase of
conversion at constant temperature (presented in Fig. 4-51) are the same as for
VMgA samples with addition of transition metal ions: the decrease in the selectivity to
propene is accompanied by the increase in the selectivity to CO2, whereas that to CO
does not change with the conversion.
Results of studies and discussion
147
Fig.4-48. Changes of the propane conversion (at τ = 0.5 s) with the reaction temperature for alkaliand alkaline earth -doped VOx/MgO catalysts.
Fig. 4-49. Changes of the selectivity to C3H6 in propane-oxygen reactions (at τ = 0.5 s) with the reaction temperature for alkali and alkaline earth-doped VOx/MgO catalysts.
400 420 440 460 480 500 520
0
10
20
30
40
50
60
70
Con
vers
ion
[%]
Temperature [0C]
VMg VMgLi VMgNa VMgK VMgRb VMgCa
400 420 440 460 480 500 5200
10
20
30
40
50
60
70
Sel
ectiv
ity C
3H6
[%]
Temperature [0C]
VMg VMgLi VMgNa VMgK VMgRb VMgCa
Results of studies and discussion
148
10 12 14 16 18 2015
20
25
30
35
40
45
50
55
60
Sel
ectiv
ity [%
]
Conversion [%]
C3H
6
CO CO
2
Fig.4-50. Changes of the selectivity to CO and CO2 in propane-oxygen reactions
(at τ = 0.5 s) with the reaction temperature for alkali and alkaline earth-doped VOx/MgO catalysts; CO _____, CO2 - - - - - .
Fig.4-51. Changes of the selectivity to C3H6, CO and CO2 in propane-oxygen reactions with the conversion at 470 oC for VOx/MgO with Na catalyst.
400 420 440 460 480 500 520
0
10
20
30
40
50
60
70
Sel
ectiv
ity C
O, C
O2
[%]
Temperature [oC]
VMg VMgLi VMgNa VMgK VMgRb VMgCa
Results of studies and discussion
149
The specific total activity at 450 oC in the ODH of propane, and the apparent
activation energy for alkali and alkaline earth-doped VMgA catalysts are presented in
table 4-32.
Table 4-32. The total activity of VOx/MgO catalysts with alkali and alkaline earth
metal ions in ODH of propane
Catalyst symbol Specific activity (µmol C3H8 m
-2 min-1) Activation energy a)
(kJ / mol)
VMg 0.87 110
VMgLi 0.33 103
VMgNa 0.32 126
VMgK 0.24 110
VMgRb 0.20 135
VMgCa 0.39 133 a) apparent activation energies determined from the Arrhenius plot
For VMgA catalysts alkali metal ions and alkali earth metal ions decrease
the total specific activity, the activity order is:
VMgRb < VMgK < VMgNa ≈ VMgLi < VMg,
VMgCa < VMg
The similar decrease of the activity in the oxidative dehydrogenation of propane for
the V-Mg-O catalyst with K promoter was earlier noticed in the literature [16, 246].
The apparent activation energies obtained from Arrhenius plots for VMgA
catalysts with alkali and alkaline earth metal ions are higher than for undoped VMg
sample, with the exceptions of VMgK (activation energy doesn’t change) and VMgLi
(activation energy slightly decreases).
The selectivities to various products at isoconversion (~10%) of propane at the
reaction temperature of 470 oC for VMgA catalysts with addition of alkali and alkaline
earth metal ions are presented in Fig. 4-52. The sequence of the increasing
selectivities is:
VMgRb < VMgK < VMgLi < VMgNa < VMg,
VMg < VMgCa
Results of studies and discussion
150
Fig.4-52. Selectivity to various products at 10 % (± 2) propane conversion for alkali and alkaline earth-doped VOx/MgO catalysts. Reaction temperature: 470oC.
All the alkali metal additives introduced to VMgA catalysts decrease the
selectivity to propene in ODH of propane. The highest decrease was observed
for the samples with K and Rb ions. The alkaline earth metal ions (Ca) added to
VMg catalyst slightly increase the selectivity to propene. The best yield to
propene (~10.7%) was noticed for VMgCa sample at conversion 59 %, at
temperature 520 oC.
VMg VMgLi VMgNa VMgK VMgRb VMgCa0
10
20
30
40
50
60
70
Sel
ectiv
ity [%
]
C3H
6
CO CO
2
Results of studies and discussion
151
4.2.3. Summary of the catalytic studies
For all the studied catalysts in ODH of propane, the decrease of the selectivity
to propene with the increasing total conversion of propane has been observed. Such
the course of the selectivity changes indicates the parallel - consecutive reaction
network.
The effect of additives, introduced to vanadia-based catalysts VOx/SiO2 and
VOx/MgO on their catalytic performance in oxidative dehydrogenation of alkanes
(ethane and propane, depends on the type of a catalyst and on the oxidized alkane.
1. VOx/SiO2 catalysts:
a) all the transition metal ion additives and P increase the total specific activity in
both ODH of propane and ethane, the presence of the K and other alkali and alkaline
earth metal ions leads to the decrease in the activity,
b) all the additives with exception of P increase the selectivity to propene. The
highest effect is exerted by alkali metals, in particular potassium and rubidium, Na
decreases the selectivity to propene.
c) practically no effect of additives on the selectivity to ethene in the ODH of
ethane was observed.
d) the selectivities to ethene are generally higher than those to propene.
e) CO is the main product of total combustion and is formed in a consecutive
reaction of propene oxidation, CO2 is a minor product and is mainly formed in a
parallel reaction : alkali metal additives increase the selectivity to CO2 and decrease
that to CO.
f) the highest selectivity to propene at conversion ~10% at 450 oC was obtained
for the VSiK sample (67 %), for which the maximum yield to propene (10.3 %) at
conversion 22.1% at 520 oC was noticed. The best selectivity to ethene at conversion
~10% at 500 oC was obtained for VSiNb sample (47 %) with the maximum yield to
ethene (4.9 %)
Results of studies and discussion
152
2. VOx/MgO catalysts:
a) all the additives decrease the specific activity in the propane ODH. For the
ODH of ethane Cr and Nb lead to the increase in the activity, whereas Ni, Mo, P and
K decrease it,
b) additives of alkali metal ions (K, Li, Na, Rb) decrease the selectivity to
propene, whereas transition metal ions (Ni, Cr, Nb, Mo) and alkaline earth metal ions
(Ca) increase it or have no effect (P).
c) in ethane ODH all transition metal ions increase slightly the selectivity to
ethane, whereas alkali metal ions (K) have no effect,
d) the selectivities to ethene are generally lower than those to propene,
e) CO2 is the main product of total combustion, CO is formed in minor amounts,
f) the highest selectivity to propene at conversion ~10% at 450 oC was obtained
for the VMgNi sample (43 %), for which the maximum yield to propene (7.3 %) at
conversion 19.6% at 470 oC was noticed. The highest yield to propene (~10.7%) was
noticed for VMgCa sample at conversion 59 %, at temperature 520 oC.
The best selectivity to ethene at conversion ~10% at 500 oC was obtained also
for VMgNi sample (22 %) with the maximum yield to ethene (2.3 %).
Chapter 5
GENERAL DISCUSSION
General discussion
154
0,25 0,30 0,35 0,40 0,45 0,50
0
1
2
3
4
5
6
Act
ivity
[µm
ol m
-2 m
in-1]
Strength of BAS
C2H
6
C3H
8
5. Physicochemical properties of VOx/SiO2 and VOx/MgO
catalysts vs catalytic performance in ODH of ethane and
propane
5.1. VO x/SiO 2 catalysts
The dependence of the activity in the reaction of ethane at 500 oC and
propane at 450 oC for VSiA catalysts, on the strength of Brönsted (BAS) and Lewis
(LAS) acid sites are presented in Figs. 5-1 and 5-2. For both reactions, the activities
increased with an increase in the strength of the acidic sites, though the dependence
on the strength of the Lewis sites is less clear.
Fig. 5-3 and 5-4 present the changes of the activity in ODH of ethane and
propane,respectively, with the number of LAS and BAS. The activity of VSiA
catalysts in both reactions increases with an increase in the number of BAS, the
correlation with the number of LAS is again less evident.
For the isopropanol decomposition no clear correlation can be observed
between the activity in ODH of ethane and propane and the amounts of propene.
Fig.5-1. Dependence of the activity of VOx/SiO2 catalysts in ODH of ethane (500 oC) and propane (450 oC) on the strength of BAS.
General discussion
155
0,50 0,55 0,60 0,65 0,700
1
2
3
4
5
6
Act
ivity
[µm
ol m
-2 m
in-1]
Strength of LAS
C2H
6
C3H
8
5 10 15 200
1
2
3
4
5
6 Lewis
Act
ivity
[µm
ol m
-2 m
in-1]
Number of LAS / m2 x 10 16
15 20 25 30 35 40 45 50 55
Bronsted
Number of BAS / m2 x 10 16
Fig.5-2. Dependence of the activity of VOx/SiO2 catalysts in ODH of ethane (500 oC) and propane (450 oC) on the strength of LAS.
Fig.5-3. Dependence of the activity of VOx/SiO2 catalysts in ODH of ethane (500 oC) on the number of LAS and BAS.
General discussion
156
5 10 15 200
1
2
3
4 Lewis
Act
ivity
[µm
ol m
-2 m
in-1]
Number of LAS / m2 x 10 16
15 20 25 30 35 40 45 50 55
Bronsted
Number of BAS / m2 x 10 16
Fig.5-4. Dependence of the activity of VOx/SiO2 catalysts in ODH of propane (450 oC) on the number of LAS and BAS.
The observed correlations between the activity and the strength of acid sites
imply that acidic centres are involved in an activation of the C-H bond in an alkane
molecule (the r.d.s. of the reaction). A mechanism of the C-H bond activation in
alkanes has been a subject of a debate [8, 17], two possibilities of splitting on a M-O
centre having been envisaged. Different groups of authors claim either high basicity
of oxygen (abstracting a proton) or strong acidity of the oxide cation (abstracting H
atom in a form of a hydride ion) as a factor controlling the activity.
The latter mechanism has been proposed for V-P-O catalysts for n-butane
oxidation, in which the presence of very strong LAS has been claimed. The rough
dependence of the activity on the strength of LAS observed for VSiA catalysts, which
seems to confirm this mechanism, is difficult to understand since in VSiA series the
LAS are rather of an intermediate strength. No clear explanation can be provided
either for the dependence of the activity on the strength of BAS: the transfer of H+ to
an alkane molecule being possible only for the superacids.
General discussion
157
0,3 0,4 0,5 0,620
30
40
50
60
70
Lewis
Sel
ectiv
ity [
%]
Strength of LAS
0,5 0,6 0,7 0,8
Bronsted
Strength of BAS
The correlations of the catalytic activity in the ODH of propane with the number
of acid sites observed also in the literature [35] cannot constitute a definite proof for
the direct participation of acid centers in the activation of propane and ethane
molecules: they reflect only the fact that the transition metal cation (a Lewis acid) can
be involved in an active site for both the oxidation and the sorption of bases.
The changes of the selectivity to propene at 10% conversion in ODH of
propane at 450 oC with the strength and number of acid sites are shown in Figs. 5-5
and 5-6, respectively. Correlation between the selectivity to propene and the results
from isopropanol decomposition: rate of formation of propene and acetone are
presented in Figs. 5-7 and 5-8.
Fig.5-5. Dependence of the selectivity to propene at 10% conversion in ODH of propane at 450 oC on the strength of LAS and BAS for VOx/SiO2 catalysts.
General discussion
158
0 5 15 2020
30
40
50
60
70 Lewis
Sel
ectiv
ity [
%]
Number of LAS / m2 x 10 16
15 20 25 30 35 40 45 50 55
Bronsted
Number of BAS / m2 x 10 16
Fig.5-6. Dependence of the selectivity to propene 10% conversion in ODH of propane at 450 oC on the number of LAS and BAS for VOx/SiO2 catalysts.
Fig.5-7. Dependence of the selectivity to ethane at 10% conversion and propene at 10% conversion in ODH of ethane (500 oC) and propane (450 oC), on the acidic properties (rate of formation of C3H6 in isopropanol decomposition) for VOx/SiO2 catalysts.
0 10 20 30 40 5020
30
40
50
60
70 C
3H
6
C2H
4
Sel
ectiv
ity [
%]
C3H
6 x 10-6 [mol / m2 x s]
General discussion
159
1,5 2,0 2,5 3,0 3,5 4,020
30
40
50
60
70
Sel
ectiv
ity [%
]
C3H
6O x 10-6 [mol / m2 x s]
C2H
4
C3H
6
Fig.5-8. Dependence of the selectivity to ethane at 10% conversion and propene at 10% conversion in ODH of ethane (500 oC) and propane (450 oC), on the basic properties (rate of formation of C3H6O in isopropanol decomposition) for VOx/SiO2 catalysts.
For VSiA catalysts the selectivity to propene decreases with the increase in the
number of LAS and BAS, the increase in the strength of LAS and with the increase in
propene formation rate and decrease in acetone formation rate in isopropanol
decomposition. The decrease in the selectivity to propene with the increase in acidity
and decrease in the basicity, has been reported earlier for V2O5/TiO2 catalysts with
alkali metal additives [37], and V2O5/Al2O3 with the K additive [94, 249]. The present
results show that this effect is also observed for VOx/SiO2 catalysts and is not limited
to alkali metals additives, but also (though to a smaller extent) to transition metals.
The decrease in the selectivity with the increase in acidity can be ascribed to the
stronger bonding of a propene molecule (a base) on more acid centres and its further
oxidation in consecutive reactions to carbon oxides.
Confirmation of the role of Lewis acido-basic properties in controlling the
selectivity in the propene ODH was provided by quantum chemical calculations
performed by DFT method for a model O=V-O-V* cluster in which V* atom is replaced
by an atom of A [250]. From the experimental data the selectivities to propene can be
written in the following sequence:
General discussion
160
VSiK(67) > VSiCr(38) > VSiMo = VSiNi (32) > VSiP(28) ~ VSi(27) (1)
where values in brackets correspond to selectivities (in percent). This sequence,
corresponds quite well to sequence (2) of the decreasing charge transfer towards the
O=V-O centre, obtained from the results of quantum chemical calculations:
K(0.74e) > Cr(0.54e) > Mo(0.47e) > Ni(0.35e) > V(0.27e) > P(0.16e) (2)
where numbers in brackets correspond to the number of transferred electrons.
Comparison of these results indicates, that the selectivity to propene increases
with the extent of the electron transfer from the additive atom A to the VOx group (i.e.
with the nucleophilicity, or in other words, with higher basicity and lower acidity of this
group).
Both experimental and theoretical data underline a specific effect of potassium
as an additive; experiment indicates the highest selectivity to propene for a catalyst
containing this additive, whereas theory points out the fact that out of all the additives
K is capable to transfer its electrons not only to the nearest oxygen, but also to
vanadyl group leading to the reduction of vanadium. In addition K-O bond seems to
result from electrostatic interaction rather than from chemical attraction. Results of
other calculations indicate that potassium tends to interact with many oxygen atoms
[251]. The results of the calculations confirm then and explain on a molecular scale
the hypothesis about the role of acid-base properties in controlling the selectivity in
ODH reactions on some vanadia-based catalysts [110].
On the other hand, the increase in the selectivity to propene with the Tmax of
appearance of CO2 in the propane TPD was observed (Figs. 5-9), suggesting a
correlation between the selectivity to propene and the M-O bond energy.
General discussion
161
250 300 350 400 450 500 55020
30
40
50
60
70
VSi
VSiCr VSiNi
VSiK
Sel
ectiv
ity [
%]
Temperature maximum [ oC ]
Fig. 5-9. Dependance of the selectivity to propene in ODH of propane at 450 oC,
on the maximum reduction temperature (Tmax) in propane TPD measurements for VOx/SiO2 catalysts.
The correlations between the reducibility (the ease of the removal of the
catalyst oxygen, taken as a measure of the M-O bond energy) and the selectivity in
the ODH reactions were reported for several vanadia based systems. For vanadia
on alumina catalysts of different reduction degree, Kung et al [16] have found that
the selectivity to butenes in the butane ODH increased with the increase of the V-O
bond energy, evaluated from the heats of the catalyst reoxidation. Similar correlation
has been observed for a V2O5/SiO2 catalyst in ODH of ethane [252]: the samples,
whose reducibility was low, had good performances in the selective oxidation of
ethane.
Figs. 5-10 and 5-11 present the changes of selectivity to ethene in ODH of
ethane with the strength and number of LAS and BAS. In contrast to the propene, the
selectivity to ethene in the ODH of ethane does not depend on the acidic properties,
indicating that the selectivity/acidity correlations depend on the type of the oxidized
hydrocarbon molecule and of the products obtained.
General discussion
162
0,3 0,4 0,520
30
40
50
60
70
Lewis
Sel
ectiv
ity [
%]
Strength of LAS
0,5 0,6 0,7
Bronsted
Strength of BAS
0 5 15 2020
30
40
50
60
70 Lewis
Sel
ectiv
ity [
%]
Number of LAS / m2 x 10 16
15 20 25 30 35 40 45 50 55
Bronsted
Number of BAS / m2 x 10 16
Fig.5-10. Dependence of the selectivity to ethene at 10% conversion in ODH of ethane (500 oC) on the strength of LAS and BAS for VOx/SiO2 catalysts.
Fig.5-11. Dependence of the selectivity to ethene at 10% conversion in ODH of ethane (500 oC) on the number of LAS and BAS for VOx/SiO2 catalysts.
General discussion
163
The differences in the effect of additives on the selectivity to olefins in ODH of
ethane and propane have been observed previously for K-doped VOx/Al2O3 catalysts
[30, 94] and for VPO catalysts with Bi, La, Mo and Zr additives [253]. These data
show that ethane and propane have different requirements towards the catalyst
active centre, putting some doubt on the possibility of finding a common catalyst for
the ODH reactions of these two alkanes.
Following the concept of Ai [130], mentioned already (which claimed that a
basic [electron-donating] molecule such as olefin should be weakly bound to centres
of low electron-accepting power [low electrophilicity, acidity], and high nucleophilicity
[basicity]),. López Nieto et al [30, 94] point out that the differences in the propene and
ethene selectivity may be due to different basicity of the two olefins. The sorption
energy of ethene molecule (less basic as compared with propene) is less affected by
the acidity of the catalyst surface. In the case of a propene molecule, in which the
hyperconjugation of methyl group with the double bonds makes it more basic, the
molecule is more strongly adsorbed on the acidic centres, and on the whole more
sensitive to the modifications in the acidic properties. The decrease in the heat of
adsorption of propene, observed for K and Rb-doped V2O5/TiO2 [128], confirmed that
the increase in the selectivity is due to weaker adsorption of the olefin on the less
acidic surface.
It cannot be forgotten either that, out of all the olefins, an ethene molecule has
C-H bonds stronger (less easily activated) than the parent alkane .It can be thus
further oxidized by a different mechanism than other olefins, not involving necessarily
the initial activation of a C-H bond, but, for instance, the addition of oxygen across
the double bond.
General discussion
164
0,45 0,50 0,55 0,60 0,650,0
0,5
1,0
1,5
2,0
2,5
Act
ivity
[µm
ol m
-2 m
in-1]
Strength of LAS
C2H
6
C3H
8
5.2. VO x/MgO catalysts
The changes of the activity in the reaction of ethane and propane for VMgA
catalysts, with the strength and number of acid sites are shown in Figs. 5-12 and 5-
13, respectively. The dependences of the activity in both reactions on the propene
and acetone formation from isopropanol decomposition are presented in Fig. 5-14
and 5-15.
As seen for VMgA series no distinct correlations between activity and the acido-
basic properties are observed for either of the reactions.
The selectivities to propene and ethene do not depend on the number of Lewis
acid sites (Fig. 5-16), neither on the rate of propene and acetone formation. On the
other hand a trend of the decreasing selectivity to propene with the decreasing acid
strength has been observed for most of the catalysts of this series except VMgK,
which exhibited considerably low selectivity in spite of the low acidity (Fig. 5-17). For
ethene no such trend was observed and K additive did not affect the selectivity.
Fig.5-12. Dependence of the activity of VOx/MgO catalysts in ODH of ethane (500 oC) and propane (450 oC) on the strength of LAS.
General discussion
165
0,00 0,05 0,10 0,15 0,200,0
0,5
1,0
1,5
2,0
2,5
Act
ivity
[µm
ol m
-2 m
in-1]
C3H
6 x 10-6 [mol / m2 x s]
C2H
6
C3H
8
2 4 6 8 10 12 14 16 180,0
0,5
1,0
1,5
2,0
2,5
Act
ivity
[µm
ol m
-2 m
in-1]
Number of LAS / m2 x 10 16
C2H
6
C3H
8
Fig.5-13. Dependence of the activity of VOx/MgO catalysts in ODH of ethane (500 oC) and propane (450 oC) on the number of LAS.
Fig.5-14. Dependence of the activity of VOx/MgO catalysts in ODH of ethane (500 oC) and propane (450 oC) on the acidic properties (rate of formation of C3H6 in isopropanol decomposition).
General discussion
166
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,00,0
0,5
1,0
1,5
2,0
Act
ivity
[µm
ol m
-2 m
in-1]
C3H
6O x 10-6 [mol / m2 x s]
C2H
6
C3H
8
2 4 6 8 10 12 14 16 185
10
15
20
25
30
35
40
45
Sel
ectiv
ity [%
]
Number of LAS / m2 x 10 16
C2H
4
C3H
6
Fig.5-15. Dependence of the activity of VOx/MgO catalysts in ODH of ethane (500 oC) and propane (450 oC) on the basic properties (rate of formation of C3H6O in isopropanol decomposition).
Fig.5-16. Dependence of the selectivity to propene at 10% conversion and ethane at 10% conversion in ODH of propane (450 oC) and ethane (500 oC) on the number of LAS for VOx/MgO catalysts.
General discussion
167
460 480 500 520 540 56020
25
30
35
40
45
VMgCrVMgNi
VMg
VMgK
Sel
ectiv
ity [%
]
Temperature maximum [oC]
0,45 0,50 0,55 0,60 0,6510
15
20
25
30
35
40
45
Sel
ectiv
ity [%
]
Strength of LAS
C2H
4
C3H
6
Fig.5-17. Dependence of the selectivity to propene at 10% conversion and ethane at 10% conversion in ODH of propane (450 oC) and ethane (500 oC) on the strength of LAS for VOx/MgO catalysts.
Fig. 5-18. Dependance of the selectivity to propene at 10%conversion in ODH of propane at 450 oC, on the maximum reduction temperature (Tmax) in propane TPD measurements for VOx/MgO catalysts.
General discussion
168
For VMgA series the correlations between selectivity and acid-base properties
are not clear. The decrease in the selectivity to propene in ODH of propane,
observed for K-doped VMg catalyst, in spite of the decrease in the acidity, indicates
that other factors , control the selectivity in the ODH reactions. Such factors may
include both local and long-range properties of catalysts including type and bond
strength of oxygen species, reducibility and reoxidation rate.
Fig 5-18 presents the dependence of the selectivity to propene on the Tmax of
appearance of CO2 in the propane TPD. The observed curve shows an increase in
the selectivity with the increase in Tmax ( i.e. the increase in the M-O bond energy).
The lower Tmax for VMgK as compared with VMg catalyst (cf chapter 4.1.2.2. p.111)
could then account for the lower selectivity to propene of the former catalyst
5.3. Comparison of properties of VO x/SiO 2 and VO x/MgO catalysts
The characterization of structure of VSiA and VMgA catalysts of the same
vanadium loading (1.5 mnl of vanadia) and doped with additives of main group
elements and of transition metal ions confirmed that the acidic support SiO2 weakly
interacts with the dispersed vanadia phase, the crystalline V2O5 being the main
component of VSiA catalysts, while basic support MgO reacts with vanadia forming
magnesium orthovanadate Mg3V2O8, which is an active phase in VMgA catalysts.
Different surroundings of vanadium ions in the VSiA (octahedral) and VMgA (VO4
tetrahedra) lead to different charge on V and O atoms. Quantum chemical
calculations for V2O5 and Mg3V2O8, have shown [254] the higher charge on V atoms
(1.59) i.e. their higher acidity for V2O5, as compared with the V charge in magnesium
ortovanadate (1.26). On the other hand the charge on O atoms (basicity) is higher for
Mg3V2O8 (varying from -0.42 to -0.97, depending on the oxygen coordination) as
compared with the O charge in vanadia (from -0.33 to -0.87).
The differences in electronic properties are reflected in different
physicochemical properties and catalytic performance, and in some cases in different
effect of the additives on the properties of both systems.
General discussion
169
Thus:
••• VSiA catalysts exhibit high number of acid sites (both Lewis and Brönsted) of
medium strength, which prevail over basic (dehydrogenating) sites, whereas for
VMgA catalysts less numerous Lewis acid sites have slightly lower acid
strength, and the basic dehydrogenating sites predominate.
••• Reducibility of VMgA catalysts is lower than that of VSiA catalysts.
� The introduction of the additives affects the acido-basic properties in the same
way: the acidities increase with the electronegativity of additives for both VSiA
and VMgA catalysts.
� The K additive has opposite effect on reducibility in VSiK (decrease) and VMgK
(slight increase) samples.
The main differences between the two series of the catalysts in the catalytic
performance in oxidative dehydrogenation, ODH of propane and ethane consist in:
� The predominance of CO2 over CO in the total oxidation products on VMgA
catalysts and CO over CO2 for VSiA series, clearly seen in both reactions: ODH
of ethane and propane. Moreover, CO2 is formed on VMgA series in both
parallel and consecutive oxidation of C3H8, whereas the consecutive oxidation
on VSiA catalysts leads mainly to CO. These differences imply different type
and/or different population of the active oxygen species on catalysts of the both
types, and may indicate a different mechanism of the ODH reactions on VSiA
and VMgA catalysts
� increase in the specific activity in the ODH reactions on introduction of the
additives (except K) for VSiA and decrease for VMgA catalysts
� increase in the activation energy of the overall reaction of propane with the inc
rease in the electronegativity of the additives for VSiA series and decrease for
VMgA, which could suggest different mechanism of a C-H bond activation,
� considerable increase in the selectivity to propene in ODH of propane for K-
doped VSi catalyst and decrease for K-doped VMg catalyst, whereas all
transition metal ions increase the selectivity to propene for both series. No
marked effect of the additives, in particular of K, on the selectivity to ethene on
VSiA and VMgA catalysts is however observed,
� higher selectivities to ethene as compared with those to propene for VSiA and
lower to ethene than to propene for VMgA catalysts, which could be understood
General discussion
170
in view of their different acido-basic properties. Propene (a base) is more
strongly adsorbed on acidic VSiA catalysts and thus undergoes further oxidation
to COx, whereas ethene (less basic) is less strongly held. On more basic VMgA
catalysts ethene (more acidic) is more strongly bond then propene,
� correlations between catalytic performance and acido-basic properties in the
case of VSiA catalysts, and absence of such correlations for VMgA catalysts.
The most intriguing difference between the VSiA and VMgA series is the opposite
effect of K on the selectivity to propene (positive for VSiA and negative for VMgA
catalysts).
For VSi catalyst of high acidity, the K additive decreases strongly the acidity,
increasing at the same time basicity. The decrease in acidity and increase in basicity
may facilitate the desorption of propene molecule from the catalyst surface, before it
reacts further to CO2. The easy desorption of propene leads then to the increase in
the selectivity.
In the case of the much more basic VMg catalyst, potassium modifies acido-
basic properties to a much smaller extent, still it decreases distinctly the acidity. This
decrease – following the above discussion – should lead to the increase in the
selectivity. The explanation of negative effect of K should be then sought for in other
beside acido-basic properties of VMgK. The decrease of M-O bond energy (increase
in the reducibility) observed for VMgK as compared with VMg catalyst without K (in
contrast to VSiK), could explain its lower selectivity.
Beside the differences in the effect of K on the acido-basic properties and
reducibility, the opposite catalytic effects of K in these two types of catalysts can be
due to different oxygen species. The differences can arise also from the different
surface states of potassium in the two series.
The studies of the state and binding energy of potassium in the VSiK and
VMgK catalysts were performed by the characterization of desorption energies of
potassium atoms (Ea) and ions (Ei) using the SR-TAD (Species Resolved Thermal
Alkali Desorption) method [255, 256]. The results of these measurements performed
on VSiK and VMgK samples by A. Kotarba [257] are shown in Table 1. The values of
Ea and Ei are related to work function eφ (from the Schottky energy cycle eφ = Ea – Ei
+ IPK, where IPK is ionization potential of potassium).
General discussion
171
Table 5-1. Values of potassium desorption energy by SR-TAD method.
Samples Desorption energy of K
atoms /eV
Desorption energy of
K+ cations /eV
2.37 ± 0.03 3.90 ± 0.04 K-Mg
1.48 ± 0.04 1.66 ± 0.03
3.33 ± 0.02 3.59 ± 0.02 K-VMg
1.43 ± 0.05 2.02 ± 0.05
K-Si 3.36 ± 0.04 3.34 ± 0.01
K-VSi 1.25 ± 0.02
0.85 ± 0.02
2.08 ± 0.02
While the K+ ions desorb from both catalysts with the same energy, the
desorption energies for K atoms are lower for VSiK as compared with VMgK system.
The lower desorption energy of K atoms indicates lower work function the catalyst
surface, which can affect the reoxidation rate of the catalysts and the type of oxygen
species during the catalytic reaction.
In the oxidation processes the transfer of electrons from a catalyst to the
reacting molecules (e.g. O2) is involved in the reoxidation step described by the
overall reaction: O2 + 4e → 2O2-. The latter reactions proceeds most probably
through successive steps of formation of electrophilic O2-, and atomic O- species:
O2(ads) + e- → O2-(ads) + e- → 2O-
(ads) + 2e- → 2O2-(lattice) (1)
The electrons necessary for this steps have to surmount an energy barrier
equal to the work function [8]. The rate of the reoxidation ro can be expressed by:
ro = k [e-] exp –eφ/kT (2)
where φ is the surface potential, eφ is energy barrier for electron transfer from the
catalyst to O2 molecule (work function), [e-] is the concentration of electrons in the
solid.
To accelerate the formation of selective form O2-latt, the concentrations of
electrons [e-] in the solid should be high, and the work function (eφ) should be low.
The lower energy barrier for the electron transfer would facilitate reactions (1) and
decrease the possibility of the presence on the catalyst surface in the stationary state
General discussion
172
of the reaction of molecular or atomic electrophilic oxygen forms, which take part in
non-selective total oxidation of hydrocarbons. The increase in the selectivity to
propene with the decrease of work function was indeed observed in ODH of popane
on V2O5/TiO2 catalysts doped with different ions [126].
For VSiK catalyst the work function is lower than in VMgK and (in view of
higher reducibility) concentration of electrons and vacancies is high enough to ensure
fast reoxidation and accelerate the transformation of the non-selective into selective
oxygen species. This (in addition to lowering the acidity) could account for the
positive influence of K on the selectivity to propene observed for this system. Similar
effect of the K additive (increase in the selectivity to propene with the decrease in the
work function) was found previously for vanadia-titania and molybdena-titania
catalysts [19].
In the case of less reducible VMgK catalysts the concentration of electrons is
expected to be lower, and the work function is found higher, thus reoxidation steps
are slower. This would give rise to higher coverage of the surface with electrophilic,
nonselective oxygen species.
The above reasoning could provide a tentative explanation of the negative
effect of the K additive on the selectivity to propene observed for this system.
Moreover high coverage of the surface with electrophilic species would account for
the higher amounts of CO2 observed in the reaction products for VMgA system.
It cannot be either excluded that electrophilic oxygen species on VMgA
catalysts can activate an alkane molecule: high activity of O- in methane and ethane
reactions on nontransition metal oxides has been indeed shown [18, 258].
On the whole, catalytic performance of VSiA catalysts is influenced mainly by
acido-basic properties, whereas for VMgA catalysts the properties of the catalysts’
oxygen seem to be more important.
Chapter 6
FINAL CONCLUSIONS
Final conclusions
174
6. Final conclusions
I. The additives (A) of main group elements (P, alkali metal ions {K, Rb, Li, Na} and
alkaline earth metal ions {Ca, Mg}), and transition metal ions (Ni, Cr, Nb, Mo)
introduced to VOx/SiO2 (VSiA) and VOx/MgO (VMgA) catalysts do not affect
markedly the catalysts’ structure, but modify their acido-basic properties,
reducibility, and catalytic performance in oxidative dehydrogenation of propane
and ethane.
The character and magnitude of the effect of the additives depend on the type of
the catalyst and on the alkane nature.
1. For VSiA catalysts which:
• contain V2O5 as a main component,
• exhibit both Lewis and Brönsted acid sites,
• show prevalence of acid sites over basic sites
a)
� all the transition metal ion additives increase the total specific activity in both ODH
of propane and ethane, the presence of the K and other alkali and alkaline earth
metal ions leads to the decrease in the activity,. The activity increases with the
electronegativity of the additives’ ions
� the activity in both reactions increases with the concentration of acid sites and the
increase in the strength of Brönsted acid sites,
b)
� all the additives with the exception of P increase the selectivity to propene. The
highest effect is exerted by alkali metals, in particular potassium and rubidium, Na
decreases the selectivity to propene,
� selectivity to propene in ODH of propane increases:
�� with the decrease in the strength and number of Lewis and Brönsted
acid sites (decrease in electronegativity of the additives ions)
�� with the decrease in reducibility
� practically no effect of additives on the selectivity to ethene in the ODH of ethane
was observed.
Final conclusions
175
� the selectivities to ethene are generally higher than those to propene
2. For VMgA catalysts which:
• contain magnesium orthovanadate Mg3V2O8 as an active phase,
• exhibit Lewis acid sites,
• show prevalence of dehydrogenation (basic) sites over acidic sites
a)
� all the additives decrease the specific activity in the propane ODH.
� for the ODH of ethane Cr and Nb lead to the increase in the activity, whereas Ni,
Mo, P and K decrease it
� no correlations between activity and acido-basic properties are observed
b)
� additives of alkali metal ions (K, Li, Na, Rb) decrease the selectivity to propene,
whereas transition metal ions (Ni, Cr, Nb, Mo) and alkaline earth metal ions (Ca)
increase it or have no effect (P).
� in ethane ODH all transition metal ions increase slightly the selectivity to ethane,
� the selectivities to ethene are generally lower than those to propene
� no clear correlations between selectivity and acido-basic properties are observed,
selectivity increases however with the decrease in the reducibility
3. For both VSiA and VMgA catalysts the strength of the acidic sites increases with
the electronegativity of the additive.
4. The opposite effect of K on selectivity to propene observed for VSiK (increase)
and VMgK (decrease) catalysts, in spite of the decrease in the acidity in both
catalysts, indicate, that different catalyst properties control the selectivity:
� acido-basic in the case of VSiK
� properties of catalyst oxygen including reducibility, and probably the type of
oxygen species (more of electrophilic forms) in the case of VMgK
5. The small effect of the additives, in particular of K, on the selectivity to ethene as
compared with propene can be due to the different nucleophilicity (basicity) of the
two alkanes: less basic ethene is less strongly adsorbed on acidic sites and
Final conclusions
176
desorbed more easily then propene, without undergoing the consecutive oxidation
to carbon oxides.
Most of the results obtained in this work, concerning oxidative
dehydrogenation of lower alkanes, can be explained in terms of the concepts
proposed for selective oxidation of hydrocarbons (olefins and aromatics). In particular
the factors controlling the selectivity to desirable products (olefins) i.e acido-basic
and/or redox properties appear to be the same as in oxidation of other hydrocarbons
to oxygenated products.
Some results and their explanation remain however disputable. In particular
the importance of the catalyst’ acidity (acid strength) in controllling the activity (the
activation of a C-H bond) in ODH reactions on relatively acidic catalysts (e.g. VSiA
series), remains to be solved. Moreover, further studies are required to furnish a
direct proof for the proposed participation of electrophilic oxygen species in the ODH
reactions on less reducible and more basic catalysts (e.g. VMg series). Determination
of their role, both in the initial step (activation of a C-H bond in alkanes), and in
consecutive steps of overoxidation of the intermediate products, would contribute to
extending the concepts of the selective oxidation reactions to oxidative
dehydrogenation of lower alkanes.
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[249] G. Garcia Cortez, J.L.G. Fierro, M.A. Bañares, Catal. Today, 78 (2003)
219
[250] A. Klisińska, A. Haras, K. Samson, M. Witko, B. Grzybowska, J. Molec. Catal.
A: Chem., 210 (2004) 87
[251] R. Gryboś, PhD thesis, Kraków 2005
[252] Z. Zhao, Y. Yamada, A. Ueda, H. Sakurai, T. Kobayashi, Catal. Today,
93–95 (2004) 163
[253] M. Gąsior, I. Gressel, V.A. Zazhigalov, B. Grzybowska, Polish J. Chem. 77
(2003) 909
[254] R.Tokarz, M.Witko, in preparation
[255] A. Kotarba, J. Dmytrzyk, W. Raróg-Pilecka, Z. Kowalczyk, Appl. Surf.
Science, 207 (2003) 327
[256] A.Kotarba, I.Kruk, Z. Sojka, J.Catal., 211 (2002) 265
[257] A. Kotarba, B. Grzybowska, A. Klisinska, in preparation
[258] M. Che, A.J. Tench, Adv. Catal., 31 (1982) 77, ibid. 32 (1983) 1, and
references therein
Annexes
Annexe
190
Annexe I
Effect of potassium promoter content in VOx/SiO2 catalysts
In order to optimize the content of potassium promoter in the VOx/SiO2
catalysts on their catalytic properties in oxidative dehydrogenation reactions, the
samples containing different content of potassium in the range of the K/V ratio
varying from 0.02 to 0.2 were synthesized and tested in the oxidative
dehydrogenation of propane.
For all the studied catalysts in ODH of propane, the decrease of the selectivity
to propene with the increasing total conversion of propane has been observed, as
illustrated in Fig. A-1. for VSiK0.05 sample. Such the course of the selectivity
changes indicates the parallel - consecutive reaction network. According to this
network undesirable, but thermodynamically favored, carbon oxides can be formed
either by sequential overoxidation of a propene formed in the first step, or by total
oxidation of a propane molecule, parallel to the olefin formation.
Table A-1 gives the specific total activity of studied samples at 450 and 470 oC. The activity decreases sharply after introducing small amounts of potassium. The
further decrease of activity with the increase of potassium content is rather
insensibly. The decrease of the total activity indicates that potassium block the
centres of the propane reaction with oxygen. The changes of total activity with the
K/V ratio for studied catalysts are presented in Fig. A-2.
Table A-1. The total activity of VSiK catalysts in ODH of propane at 450 and 470oC.
Specific activity (µmol C3H8 m-2 min-1) Catalyst symbol 450 oC 470 oC
VSi 0.70 0.94
VSiK0.02 0.13 0.28
VSiK0.05 0.11 0.25
VSiK0.1 0.09 0.17
VSiK0.2 0.04 0.08
The selectivities to propen, CO and CO2 were compared at isoconversion of
propane and at the different reaction temperatures. The catalytic results at 450, 470,
500 and 520 oC are shown in Fig. A-3 to A-6, respectively.
Annexe
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0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16 0,18 0,20
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Tot
al a
ctiv
ity [
µm
ole
C3H
8 m-2 m
im-1 ]
K/V atomic ratio
450 oC 470 oC
2 4 6 8 10 12 14 16 180
10
20
30
40
50
60
70
80
90
100
Sel
ectiv
ity [%
]
Conversion [%]
C3H
6
CO CO
2
Fig. A-1 Selectivities to different reaction products as a function of the propane conversion for VSiK0.1 catalyst, at 500 oC.
Fig. A-2 Rate of the propane consumption as a function of the potassium content.
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Fig. A-3 Selectivity to various products at 4 % propane conversion for VOx/SiO2 + K catalysts. Reaction temperature: 450oC (* 470 oC).
Fig. A-4 Selectivity to various products at 8 % propane conversion for VOx/SiO2 + K catalysts. Reaction temperature: 470oC.
VSiK0.02 VSiK0.05 VSiK0.1 VSiK0.20
10
20
30
40
50
60
70
Sel
ectiv
ity [%
]
C3H
6
CO CO
2
VSi VSiK0.02 VSiK0.05 VSiK0.1 VSiK0.20
10
20
30
40
50
60
70
80
*
Sel
ectiv
ity [%
]
C3H
6
CO CO
2
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193
VSiK0.02 VSiK0.05 VSiK0.1 VSiK0.20
10
20
30
40
50
60
70
80
Sel
ectiv
ity [%
]
C3H
6
CO CO
2
Fig.A-5 Selectivity to various products at 6 % propane conversion for VOx/SiO2 + K catalysts. Reaction temperature: 500oC.
Fig. A-6 Selectivity to various products at 10 % propane conversion for VOx/SiO2 + K catalysts. Reaction temperature: 520oC.
VSiK0.02 VSiK0.05 VSiK0.1 VSiK0.20
10
20
30
40
50
60
70
80
Sel
ectiv
ity [%
]
C3H
6
CO CO
2
Annexe
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It follows from the data presented above that the extent of effect of K on the
selectivity to propene depends on the K content and on the reaction temperature.
The sequences of the selectivities to propene at comparable propane
conversions are:
at 450 oC (at 4 % of propane conversion)
VSi (40) << VSiK0,05 (69) < VSiK0,02 (76) < VSiK0,1 (78)
and at 470 oC (at 8 % of propane conversion)
VSiK0,2 (62) = VSiK0,02 (63) = VSiK0,05 (63) < VSiK0,1 (67)
The effect of the potassium amount is clearly seen, especially at higher
temperatures, the selectivities to propene increase in the sequences:
at 500 oC (at 6 % of propane conversion)
VSiK0,2 (67) < VSiK0,1 (78) = VSiK0,02 (79) = VSiK0,05 (79)
and at at 520 oC (at 10 % of propane conversion)
VSiK0,2 (65) < VSiK0,02 (72) = VSiK0,05 (73) = VSiK0,1 (73)
The numbers in brackets correspond to the selectivity to propene.
The optimal amount of potassium for the VOx/SiO2 catalysts appears in the
range of K/V ratio between 0.02 and 0.1. At higher content (K/V = 0.2) the selectivity
to propene decrease significantly.
Obtained results are according with the previous studies performed for
V2O5/TiO2 catalysts by B. Grzybowska and co-workers [1]. The vanadia-titania
systems with the potassium/vanadium (K/V) ratio ranging from 0.01 to 0.2 were
tested in oxidative dehydrogenation of propane. It has been found that the total rate
of the propane consumption decreases considerably after addition of small amounts
of potassium (K/V = 0.01) to V2O5/TiO2 catalyst, and only slightly at higher potassium
content. The activity has been behaved practically constant beginning from the K/V
atomic ratio equal 0.05. Addition of K to vanadia-titania catalysts led to the increase
in the selectivity to propene. The ratio K/V in the range between 0.05 and 0.1 has
been appeared then optimal for the high selectivity to propene.
Based on the exhibited results the potassium to vanadium atomic ratio
equals 0.1 (K/V = 0.1) as the best for oxidative dehydrogenation of propane on
supported vanadium oxide catalysts was chosen for the farther studies.
Additive to vanadia atiomic ratio equals 0.1 was used in all alkali metal ions
doped samples.
Annexe
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Annexe II
Determination of specific surface area (BET)
The specific surface area of a catalyst is determined by physical adsorption of
a gas on the surface of the solid and by measuring the amount of adsorbate gas
corresponding to a monomolecular layer on the surface.
Before performing the gas sorption experiments, catalyst surface must be
degassed to remove adsorbed contaminants acquired from atmospheric exposure,
for example water. Surface cleaning is carried out by placing a sample of the solid in
a cell and heating it under vacuum or under a steam of a flowing gas.
After pretreatment, the sample is cooled, under vacuum conditions to a
constant, usually to cryogenic temperature by means of an external bath. Then, small
amounts of the adsorptive (typically nitrogen) are admitted to the sample in controlled
increments. After each dose of adsorptive, the pressure within sample chamber is
allowed to equilibrate and the quantity of gas adsorbed is calculated. The gas volume
adsorbed at each pressure (at one constant temperature) defines an adsorption
isotherm, from which the quantity of gas required to form a monolayer over the
external surface of the solid and its pores is determined. Based on the Brunauer,
Emmett and Teller (BET) theory, one can estimate the number of molecules required
to cover the adsorbent surface with a monolayer of adsorbed molecules, Nm).
Multiplying Nm by the cross-sectional area of an adsorptive molecule yields the
sample’s surface area.
The BET equation is applicable at low p/p0 range and it is written in the linear
form [2]:
p / na (p0 – p) = 1 / nam C + (C – 1 / na
m C) · p / p0
where p is the pressure of gas adsorbed,
p0 is the saturation vapour pressure,
na is the amount of gas adsorbed at the relative pressure p/p0
nam is the monolayer capacity
C the so-called BET constant.
Annexe
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Annexe III
X-ray diffracion analysis
X-ray diffraction is used to identify the crystalline bulk phases present in the
sample and to determine size of crystallites. The method is limited to crystalline
materials and its sensitivity is poor (element content more than 2-5 % can be
detected). A collimated beam of X-rays is diffracted by the crystalline phases in the
sample according to Bragg’s Law [3]:
n λx = 2 d sin Θ
where λx is the wavelength of the X-rays,
d is the distance between two atomic planes in the crystalline
phase (d-spacing),
n is an integer,
Θ the incoming diffraction angle
The maxima of the pattern are related to the distance between the crystal
planes. The crystal plane separations, d give a characteristic pattern of lines that can
be compared with data in standard files and used to identify compounds and their
morphology. By measuring the degree of broadening of the peaks, an estimate of the
mean crystallite size is can be calculated.
A detailed analysis of the line width gives the information about the size of the
material particles. The line width of diffraction peaks is related to the diameter of the
crystallites by Scherrer’s formula:
D = λx / (β cos Θ)
where D is the diameter of the crystallites
λx is a wavelength of the X rays,
β is the line width,
Θ the incoming diffraction angle
The powder X-ray diffractometer consists of an X-ray source, a movable
sample platform, an X-ray detector, and associated computer-controlled electronics.
The sample is packed into a shallow cup-shaped holder and the sample holder spins
slowly during the experiment to reduce sample heating. The X-ray source is Cu. The
X-ray beam is fixed and the sample platform rotates with respect to the beam by an
Annexe
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angle theta. The detector rotates at twice the rate of the sample and is at an angle of
2-theta with respect to the incoming x-ray beam. Scheme of an X-ray diffractometer
is given in fig. A-7.
Fig.A-7. Scheme of an X-ray diffractometer
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Annexe IV
X-ray photoelectron spectroscopy (ESCA – XPS)
While XRD gives information on the bulk phases present in the sample, X-ray
photoelectron spectroscopy (XPS) can be used to study the sample surface. This
technique provides information about atomic composition of the solid surface (both
qualitatively and quantitatively), the oxidation state of atoms and sometimes also
about the chemical environment of a given atom due to shifts in the binding energies
[4–12]. The relative coverage of support and dispersion in the surface region of the
catalysts (2 – 20 atomic layers) can be also obtained using this technique.
X-ray photoelectron spectroscopy (XPS, branch of ESCA) is an electron
spectroscopic method that uses x-rays to knock electrons out of inner-shell orbitals.
When X-ray photons of energy hv hit a surface, photoelectrons are ejected having a
kinetic energy depending on the wavelength of the incident photon and on the energy
which binds the electrons to the nucleus. The binding energy, which is the ionization
potential of the electron, is calculated from the equation:
B.E. = Ep – Ee-Φ
Where B.E. is the binding energy, for an electron formula given electron level
in an atom,
Ep is the energy of the incident photon,
Ee is the kinetic energy of the ejected electron
Φ is a factor which includes the work function of the surface [13].
Ejection of electrons may occur from any orbital for which the binding energy
is less than the energy of the incident photon [14]. The oxidation state, and the
chemical environment of a given atom can be determine from the binding energy
(B.E.) shift with respect to the B.E. of a pure element [14], while a peak intensity,
modified by the photoionization cross section and the escape depth factor, reflects
the content of a given element on the surface [15].
The molecular structural information cannot be however, obtained using XPS,
since it cannot discriminate between different VOx structures possessing the same
oxidation state, i.e. between VO6, VO5 and VO4 species. Other disadvantages are
that XPS requires high vacuum conditions for its operation, which may result in the
reduction of surface vanadium species in the measurement chamber [16].
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The detection of photoelectrons requires the sample is placed in a high
vacuum chamber. Since the photoelectron energy depends on the X-ray energy, the
excitation source must be monochromatic. The energy of the photoelectrons is
analyzed by an electrostatic analyzer and the photoelectrons are detected by an
electron multiplier tube or a multichannel detector such as a microchanel plate.
Schematic of a X-ray photoelectron spectrometer is shown in fig. A-8.
Fig. A-8. Scheme of X-ray photoelectron spectrometer.
Annexe
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Annexe V
Infrared spectroscopy (IR)
Infrared spectroscopy (IR) is one type of vibrational spectroscopy, where
molecular vibrations are analyzed. An IR spectrum is produced by absorption of
energy (4000-400 cm-1) due to vibrations of polar covalent bonds.
IR spectroscopy consist in the measurement of the wavelength and intensity of
the absorption of infrared light by a sample. Infrared light is energetic enough to
excite molecular vibrations to higher energy levels. The wavelengths of IR absorption
bands are characteristic of specific types of chemical bonds and hence IR
spectroscopy giving information about functional groups in a molecule finds its
application for identification of molecules. In the case of solids IR spectroscopy may
supplement X-ray diffraction method in identification of compounds present in the
solid, and/or allows to identify the species dispersed on a support. An IR spectrum is
generally displayed as a plot of the energy of the infrared radiation (expressed either
in microns or wavenumbers) versus the percent of light transmitted by the compound.
Figure A-9 presents a scheme of an infrared spectrometer.
Fig. A-9. Scheme of infrared spectrometer.
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Annexe VI
Laser Raman spectroscopy (LRS)
Raman spectroscopy is the measurement of the wavelength and intensity of
inelastically scattered light from molecules. The Raman scattered light occurs at
wavelengths that are shifted from the incident light by the energies of molecular
vibrations. The mechanism of Raman scattering is different from that of infrared
absorption, and Raman and IR spectra provide complementary information [17].
Raman spectroscopy can discriminate between the different molecular states
of supported metal oxides, because each state possesses a unique vibrational
spectrum that is related to its structure.
When light is scattered from a molecule most photons are elastically scattered.
The scattered photons have the same frequency and therefore, wavelength as the
incident photons. However, a small fraction of light (approximately 1 in 107 photons)
is scattered at optical frequencies different from, and usually lower than, the
frequency of the incident photons. The process leading to this inelastic scatter is the
termed the Raman effect. During the scattering event, the molecule takes the energy
of the incoming photon, and can add or subtract from the photon energy by the
energy level differences within the molecule. The energy of the incoming photon is
not required to be in resonance with an energy level difference in the molecule as
there is only a "virtual state" at the energy of the incoming photon. Raman scattering
can occur with a change in vibrational, rotational or electronic energy of a molecule.
This scattering process yields a resulting photon whose energy differs from that of
the initial photon by the energy differences within the molecule. The difference in
energy between the incident photon and the Raman scattered photon is equal to the
energy of a vibration of the scattering molecule. A plot of intensity of scattered light
versus energy difference is a Raman spectrum. The energy difference between the
incident and scattered photons is represented by the arrows of different lengths in fig.
A-10a. The scheme of Optical setup of the DILOR XY spectrometer is given in fig. A-
11.
Annexe
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Fig. A-10. The virtual state description of scattering.
Fig. A-11. Optical setup of the DILOR XY spectrometer.
Annexe
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Annexe VII
Ultraviolet visible spectroscopy (UV-VIS)
Ultraviolet- visible (UV-VIS) spectroscopy involves the absorption of ultrafiolet-
visible light wavelengths, λ, between 190 and 800 nm) by a molecule causing the
promotion of an electron from a ground electronic state to an excited electronic state.
There are three types of electronic transition which can be considered:
1. Transitions involving π, σ, and n electrons,
2. Transitions involving charge-transfer electrons,
3. Transitions involving d and f electrons
UV-VIS spectroscopy provides insight on the different electronic transitions of
metal ions that depend on the symmetry and environment. Numerous experiments
using this technique to study the different oxidation states and coordination
geometries of supported vanadium oxide catalysts have been performed [18-22]. UV-
VIS spectroscopy probes d-d transitions of vanadium ions at the catalyst surface and
is quantitative for vanadium oxide loadings, however, has some disadvantages. The
UV-VIS bands are usually broad and overlap with each other, which decreasing the
sensitivity for given species. Additionally, the origin of specific electronic transition is
sometimes difficult to isolate due to its dependence on various parameters, which
were classified as follow: local symmetry < overall symmetry < condensation degree
and dispersion on a support [23].
Figure A-12 presents schematic of a dual-beam UV-VIS spectrophotometer.
The light source is usually a hydrogen or deuterium lamp for UV measurements and
a tungsten lamp for visible measurements. The wavelengths of these continuous light
sources are selected with a wavelength separator such as a prism or grating
monochromator. Spectra are obtained by scanning the wavelength separator and
quantitative measurements can be made from a spectrum or at a single wavelength.
Fig. A-12. Schematic of a dual-beam UV-VIS spectrophotometer.
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Annexe VIII
Solid-state 51V nuclear magnetic resonance spectroscopy
(NMR)
Nuclear magnetic resonance spectroscopy (NMR) is the absorption of radio
frequency electromagnetic radiation (EM) by a nucleus in a strong magnetic field.
Absorption of the radiation causes the nuclear spin to realign or flip from the low
energy spin aligned state to the higher energy spin opposed state. After absorbing
energy the nuclei will reemit EM radiation and return to the lower-energy state.
The energy of a NMR transition depends on the magnetic-field strength and a
proportionality factor for each nucleus called the magnetogyric ratio. The local
environment around a given nucleus in a molecule will slightly perturb the local
magnetic field exerted on that nucleus and affect its exact energy of transition. With
no applied scope, there is no difference in energy of the spin states, but as the field
increases so does the separation of energies of the spin states and therefore so does
the frequency required to cause the spin-flip, referred to as resonance. This
dependence of the transition energy on the position of a particular atom in a molecule
makes NMR spectroscopy extremely useful for determining the structure of
molecules.
The basic arrangement of an NMR spectrometer is shown in the fig. A-13. The
sample is positioned in the magnetic field and excited via pulsations in the radio
frequency input circuit. The realigned magnetic fields induce a radio signal in the
output circuit which is used to generate the output signal. Fourier analysis of the
complex output produces the actual spectrum. The pulse is repeated as many times
as necessary to allow the signals to be identified from the background noise.
The frequency of a signal is known as its chemical shift. The chemical shift in
absolute terms is defined by the frequency of the resonance expressed with
reference to a standard compound which is defined to be at 0 ppm. The scale is
made more manageable by expressing it in parts per million (ppm) and is
independent of the spectrometer frequency.
(frequency of signal) – (frequency of reference)
spectrometer frequency
x 106 Chemical shift (δ) =
Annexe
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Fig. A-13. The basic arrangement of an NMR spectrometer.
Annexe
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Annexe IX
Electron microscopy measurements
Electron microscopy is a technique that use a beam of highly energetic
electrons to examine the catalysts. This method can yield the following information:
a) topography: the surface features of the sample and its texture,
b) morphology: the shape and size of the particles making up the sample,
c) composition: the elements and compounds that the catalyst is composed of
and the relative amounts of them,
d) crystallographic Information: how the atoms are arranged in the catalyst.
Annex IXa
Scanning Electron Microscope (SEM)
The Scanning Electron Microscope (SEM) is a microscope that uses electrons
reflected from a sample to form an image. The use of electron beams requires that
the sample be placed in a vacuum chamber for analysis. A beam of electrons is
produced at the top of the microscope by heating of a metallic filament. The electron
beam follows a vertical path through the column of the microscope. It makes its way
through electromagnetic lenses which focus and direct the beam down towards the
sample. Once it hits the sample, backscattered (secondary) electrons are ejected
from the sample. Detectors collect and convert them to a signal that is sent to a
viewing screen. SEM images are useful for studying surface morphology or
measuring particle sizes [24]. Figure A-14 shows the schematic of a scanning
electron microscope.
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Fig. A-14. Schematic of a scanning electron microscope.
Annex IXb
Transmission Electron Microscopy (TEM)
The Transmission Electron Microscope (TEM) allows determining the internal
structure of materials. TEM images the electrons that pass through a sample. Since
electrons interact strongly with matter, electrons are attenuated as they pass through
a solid requiring the samples to be prepared thickness, which allow electrons to
transmit through the sample. The basic arrangement of transmission electron
microscope is presented in figure A-15.
The ray of electrons is produced by a pin-shaped cathode heated up by
current. The electrons are vacuumed up by a high voltage at the anode. The
acceleration voltage is between 50 and 200 kV. The accelerated ray of electrons
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passes a drill-hole at the bottom of the anode. Its following way is analogous to that
of a ray of light in a light microscope. The lens-systems consist of electronic coils
generating an electromagnetic field. The ray is first focused by a condenser. It then
passes through the object, where it is partially deflected. The degree of deflection
depends on the electron density of the object. The greater the mass of the atoms, the
greater is the degree of deflection. After passing the object the scattered electrons
are collected by an objective. Thereby an image is formed, that is subsequently
enlarged by an additional lens-system (called projective with electron microscopes).
The thus formed black and white image is made visible on a screen [25].
Fig. A-15. The basic arrangement of transmission electron microscope.
Annexe
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Annexe X
Temperature-programmed reaction techniques
Temperature-programmed reduction (TPR) and desorption (TPD) are common
thermoanalytical techniques for characterizing interactions between gas and solid
substance. The experimental data collected by these techniques, e.i. the
consumption of reactants or the evolution of products as a function of temperature,
are commonly interpreted on qualitative basis and generally termed a thermogram.
Many authors reviewed the application of temperature-programmed techniques
presenting the information on the experimental and theoretical aspects of the
analysis [26-30].
For performing TPR/TPD measurements a small catalyst sample (typically 10-
500 mg) is placed in a reactor system equipped with a programmable furnace. A
scheme of the reactor system for TPR/TPD measurements is presented in Fig. A-16.
Fig. A-16. A scheme of TPR/TPD setup.
The reactor is a quartz tube fixed bed. In the pretreatment stage sample is
exposed to continuous flow of inert or reactive gas mixture, while the temperature is
raised according to a predetermined program. The sample temperature and the
outlet gas composition are continuously monitored. Typical detectors are the thermal
conductivity detector (TCD) and mass spectrometer (MS).
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Annexe Xa
Temperature-programmed reduction (TPR – H2)
This technique provides information about the ease of the reduction of a solid
(it’s reducibility) and is often related to the strength of M-O bond in a solid and
correlated with activity and/or selectivity of a catalytic oxides.
TPR has been applied to study the influence of support materials, preparation
and pre-treatment procedures, and the influence of additives on the reduction
behaviour of a catalytic material. In the TPR technique an oxidized catalyst precursor
is submitted to a programmed temperature rise, while a reducing gas mixture is
flowed over it (usually, hydrogen diluted in some inert gas like argon). The reduction
rates are continuously measured by monitoring the change in composition of the
reactive mixture after the reactor. The decrease in H2 concentration in the effluent
gas with respect to the initial percentage monitors the reaction progress. The
temperatures of the peak maxima (Tmax) are characteristic of the reduction processes
and may be used for "finger-print" identification. The amount of reducible species in
the catalyst and their degree of reduction can be derived from the integrated
hydrogen consumption. Characterization of redox properties of materials,
temperature range of consumption of reducing agent, total consumption of reducing
agent, valence states of metal atoms in metal oxides can be obtained by used TPR
technique.
Annex Xb
Temperature-programmed desorption (TPD) is one of the most useful
temperature-programmed methods for characterisation of solid catalysts. The first
step is the reactants adsorption, which precedes the desorption of products of
reaction following on the catalytic surface.
The TPD technique provides information about absorptive properties of materials and
temperature range of adsorbate release, which are relevant to catalytic properties.
TPD data help in to clarifying the complexity of interactions between solid and gas.
Several authors reviewed the theory and application of TPD [31-35].
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Annexe XI
Decomposition of isopropanol
The chemisorption of gaseous probe molecules is a well known method to
characterize the acidity or basicity of catalyst surface. The decomposition of alcohols
such as isopropanol [36, 37], 2-methyl-3-butyn-2-ol [38, 39], and cyclohexanol [40]
have been widely reported in the literature.
The decomposition of isopropanol was used as model reaction to investigate
the acido–basic properties of the catalysts. Isopropanol undergoes dehydration and
dehydrogenation that can proceed through different mechanisms depending on the
acido–basic nature of the catalysts. The dehydration leads to olefin and ether, in
particular propene and di-isopropyl ether, respectively, whereas the dehydrogenation
leads to ketones, in particular acetone. The different mechanisms of decomposition
of isopropanol, which can occur on acid and basic solids are shown in fig. A-17.
Fig.A-17. Reaction mechanisms of IPA decomposition on acid and basic catalysts leading to dehydration products:DIE, di-isopropyl ether, and P, propene, and to dehydrogenation product: A, acetone. (H+, Brönsted acid site;
Aδ+, Lewis acid site; :B, basic site; m, s, w indicate medium, strong, and weak sites) from [41]
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It is generally accepted that acidic solids such as silica-aluminas or zeolites
dehydrate isopropanol to the olefin in an E1 mechanism in which only very strong
acidic sites take part. Amphoteric oxides such as alumina leads reaction to ether and,
in a concerted E2 mechanism occurring on both acid and basic sites, to olefin.
However, isopropanol dehydration can proceed also through the E1b mechanism on
strongly basic catalysts containing acid and basic sites with imbalanced strength. On
the other hand, the dehydrogenation reaction proceeds in relevant extent only on
strongly basic catalysts. The different mechanisms of decomposition of isopropanol,
which can occur on acid and basic solids are shown in fig. A-17.
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Annexe XII
Adsorption of pyridine as a probe molecule
Adsorption of probe molecules on ionic oxide surface mainly concerns acid-
base interactions. The detection of surface acidic and basic centres using IR or
Raman spectroscopy is often applied technique and based on observation of the
vibrational perturbation undergone by probe molecules when they adsorb on them.
The presence of the acidic and basic centres is usually determined by measuring the
amount of adsorbed bases, e.g., ammonia, pyridine for acidic centres, or of acidic
compounds as CO2, SO2 for basic centres. Pyridine is the most largely used basic
probe molecule for surface acidity characterization. The data on pyridine as a probe
molecule are reported in table A-2 and the interactions between pyridine and metal
oxide are shown in table A-3.
Table A-2. The data on pyridine [42]
Symbol C5N5N
Conjugated acid C5N5NH+
pKa = 5.2 Basic strength
PA* = 912
Mode Position (cm-1)
Sensitive bands (base)
Lewis acidity
v8a
v19b
v1 (ring)
1632 – 1580
1455 – 1438
1020 - 990
Diagnostic band (acid)
Br¢nsted acidity
v8a
v19b
~1640
~1540
* PA = proton affinity (kJ mol-1)
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Table A-3. Adsorption modes of a pyridine on a metal oxide surface [43].
Adsorption mode Type of acid site Remarks
Lewis
This type of species depicts a typical
Lewis interaction, where the base
coordinates as such on Lewis acid
sites located at the solid’s surface.
Brönsted
This type of species occurs when the
acid-base Brönsted-type interaction is
very weak, and only results in
hydrogen bonding.
Brönsted
This type of species is an intermediate
case that gives rise to partial proton
transfer and the formation of a
“symmetric hydrogen bond” with the
proton nearly half way between the
base and the acid’s conjugated base.
Brönsted
This type of species depicts the true
acid-base interaction, with a total
transfer of the Brönsted proton from
the Brönsted acidic OH to the base,
resulting in its protonation and in the
formation of its conjugated acid.
To record the infrared spectra of adsorbed molecules, special vacuum cells
are used. These cells have to possess the following parameters: can be heated up to
1000 oC, cooled to the temperature of liquid nitrogen, to adsorb/desorb both gases
and vapors at different temperatures, be able to maintain a high vacuum, and to
record the spectra of adsorbed molecules without exposing the pellet (disk) to the air.
The cell windows (plates) should be transparent in the IR and hermetic sealed.
Frequently, such a sealing is made by the use of different cements and glues with
low vapor pressures, or alternatively by using O-rings and flanges [44]. Various
Annexe
215
optical materials used for lenses and windows in infrared studies are presented in
table A-4.
Table A-4. Parameters of optical materials [44].
Material Low-energy cutoff (cm-1)
Comments
Quartz >2500 Good for high-temperature work and in the
overtone region; insoluble; easy to work in fused
form
LiF >1200 Good dispersion in the near-IR region;
easily scratches
MgF2 1400 Strong; chemically durable
CaF2 1200 Inert to most chemicals; tends to be costly;
slightly soluble; good from –200oC to 100oC
Silicon 1100 Inert; insoluble; connected with glass; not
transparent at high temperatures
NaF 1000 Slightly hydroscopic
BaF2 900 Hard; expensive
ZnS 714 Good up to 1000 oC; strong; chemically durable
KBr 350 Hydroscopic; easily scratched; used as
powder for pressed-disk technique
CsBr 250 Very hard; expensive
The cell used for recording infrared spectra under high vacuum was made by CsBr,
large in length, which gives the possibility of separating a heater from the region of
the sealed windows. The recording of spectra and the heating of the sample were
carried out in different parts of the cell.
Fig. A-18. The cell used for recording
infrared spectra.
Annexe
216
The pyridine adsorption was carried out in a high vacuum equipment, which
consist of four main part:
- high vacuum system,
- gas feed and storage system,
- measuring cell,
- temperature measuring and control.
A diagram of the equipment is shown in fig. A-19.
Fig. A-19. The scheme of pyridine adsorption high vacuum equipment.
Annexe
217
Annexe XIII
Description of catalytic measurements
Figure A-20 shows schematically the arrangement of main parts of catalytic
set-up such as dose (mass flow controllers), reactionary (reactor) and analytical (gas
chromatograph) units. The constant flow of the reactants was maintained by mass
flow controllers. The reactor was made of stainless steel, the thermocouple for the
temperature measurements being placed coaxially in the catalyst bed. Analysis of
products and unreacted substrates was performed by on-line gas chromatography.
The scheme of reactor is shown in fig. A-21.
Fig.A-20. Scheme of set-up for catalytic measurements.
Annexe
218
Fig.A-21. Scheme of reactor.
Annexes - Bibligraphy
219
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