synthesis, characterization and ethanol partial oxidation studies of v 2o 5 catalysts supported on...

Journal of Molecular Catalysis A: Chemical 165 (2001) 199–209

Synthesis, characterization and ethanol partial oxidationstudies of V2O5 catalysts supported on TiO2–SiO2 and

TiO2–ZrO2 sol–gel mixed oxides

Jhansi L. Lakshmi, Nancy J. Ihasz, Jack M. Miller∗Department of Chemistry, Brock University, St. Catharines, Ont., Canada L2S 3A1

Received 24 April 2000; received in revised form 31 August 2000; accepted 31 August 2000

Abstract

Sol–gel derived titania based mixed oxides TiO2–SiO2 and TiO2–ZrO2 were used as supports for preparing a series ofcatalysts with vanadia contents varying between 1–25 wt.%. The supports and the catalysts were characterized by employing51V and 1H solid-state MAS NMR, diffuse reflectance FT-IR, and BET surface area measurements. The partial oxidationactivities of the catalysts were tested using ethanol oxidation as the model reaction.51V solid-state NMR studies of thecalcined catalysts showed the peaks corresponding to the presence of polymeric tetrahedral vanadia species at low vanadialoadings and51V chemical shifts corresponding to amorphous V2O5 like phases were observed in the catalysts with highV2O5 contents. After outgassing the catalysts, a broadening of the peaks was observed indicating a change in the coordinationenvironment around vanadium. DRIFTS studies of the catalysts indicated the vibrations corresponding to V=O bonds ofV2O5 agglomerates, in the catalysts with high vanadia loadings. Acetaldehyde was obtained as the major product with tracesof ethylene, ether, acetic acid, ethyl acetate, and COx during the ethanol partial oxidation activity studies of mixed oxidesupported vanadia catalysts. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Sol–gel mixed oxides; Vanadia catalysts; Solid-state NMR; DRIFT; Ethanol oxidation

1. Introduction

In our earlier investigations we reported the syn-thesis, characterization and activity studies of vanadiacatalysts supported on sol–gel derived mixed oxides[1–3]. Sol–gel derived materials are receiving in-creasing attention as the supports for various catalyticapplications as this method of synthesis allows theprecise control of the chemical composition of thegel and pore size distribution of the materials. Recentreviews on this topic describe the extensive appli-

∗ Corresponding author. Tel.:+1-905-688-5550/3789;fax: +1-905-684-2277.E-mail address:[email protected] (J.M. Miller).

cation of these materials [4,5]. Sol–gel synthesis ofthe mixed oxides and catalytic applications of thesematerials were studied by various research groups;the mixed oxides Al2O3–SiO2 [6,7], Al2O3–TiO2 [8],TiO2–SiO2 [9], TiO2–ZrO2 [10], ZrO2–SiO2 [11]commonly synthesized by sol–gel and co-precipitationmethods were used as supports and as catalysts forvarious reactions. The mixed oxides with M–O–M′bonds were found to be acidic in nature due to thecharge imbalance of the metal ions. Millar and Ko [11]have employed various characterization techniquessuch as X-ray diffraction, diffuse reflectance FT-IR,and solid-state NMR for the structural elucidationstudies of the sol–gel derived mixed oxides. Amor-phous Al2O3–SiO2 mixed oxide was employed for the

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200 J.L. Lakshmi et al. / Journal of Molecular Catalysis A: Chemical 165 (2001) 199–209

dehydration reactions of alcohols [6], titania–silicamixed oxides derived from sol–gel method were usedfor catalyzing epoxidation reactions [9].

Baiker et al. [12] have reported the sol–gel syn-thesis of vanadia–titania–silica mixed oxides forthe selective catalytic reduction of NOx . They havedemonstrated that sol–gel mixed oxides possess highsurface areas resulting in highly dispersed vanadiaspecies even at higher loadings. Vanadia catalystssupported on sol–gel derived Al2O3–SiO2 mixed ox-ide were investigated earlier by51V solid-state NMRand laser Raman spectroscopy [6,7]. The compositesupport materials — titania or zirconia coated on ahigh surface area, thermally stable support such asalumina/silica were studied in recent years to over-come the disadvantages of low surface area, low ther-mal stability and high surface acidity of the titania orzirconia carriers [13,14]. Vanadia catalysts supportedon modified and mixed oxides have been extensivelyemployed for catalyzing various reactions.

Mastikhin et al. [15,16], have employed51V and1H solid-state MAS NMR techniques to characterizethe vanadia catalysts supported on titania modifiedalumina, silica, and titania–zirconia mixed oxides.In the present investigation we report the synthesisand characterization of vanadia catalysts supportedon TiO2–SiO2 and TiO2–ZrO2 sol–gel derived mixedoxides using51V and 1H solid-state MAS NMR,diffuse reflectance FT-IR, and BET surface areameasurements. The characterization studies of theV2O5/TiO2–Al2O3 is reported in our earlier publi-cation [2] this series of catalysts were tested alongwith V2O5/TiO2–SiO2 and V2O5/TiO2–ZrO2 to studythe influence of the second component in the titaniabased sol–gel mixed oxides for the partial oxidationof ethanol.

2. Experimental section

Sol–gel synthesis and characterization of the mixedoxide supports is discussed in detail elsewhere [1]. Aseries of catalysts with calculated amounts of vana-dia were prepared by impregnating the mixed oxidesupport with a methanolic solution of vanadium (III)acetylacetonate (Gelest, Inc). Methanol was evapo-rated slowly to dryness in a rotary evaporator. Theresulting material was dried at 383 K overnight fol-

lowed by calcination at 773 K for 5 h. The vanadiacontents of the catalysts were estimated by induc-tively coupled plasma (ICP) analysis using a PerkinElmer Optima 3300 DV ICP-OES spectrometer. Aweighed sample was digested in hot concentrated ni-tric acid until the dissolution was complete and thenthe solution was diluted to∼2% (V/V HNO3) prior toanalysis. Vanadia catalysts supported on Al2O3 (BETsurface area 325 m2/g), SiO2 (Aesar, BET surface area215 m2/g), TiO2 (Degussa, BET surface area 48 m2/g)and ZrO2 (Aldrich, BET surface area 30 m2/g) sup-ports were also made following the above procedurefor comparing the ethanol partial oxidation activities.The experimental details for the measurement of BETsurface areas,51V and1H and solid-state MAS NMRexperiments, diffuse reflectance FT-IR spectroscopyand ethanol partial oxidation activity studies of thecatalysts were reported earlier [2,3].

3. Results and discussion

The BET surface areas and vanadia contents ofthe catalysts V2O5/TiO2–Al2O3, V2O5/TiO2–SiO2and V2O5/TiO2–ZrO2 are given in Table 1. It canbe seen from the table that there is a decrease in thesurface area of mixed oxide supports with the addi-tion of vanadia which may be due to the blockage ofthe pores of the supports with the active component.The surface areas of the three series of catalysts withhigh V2O5 contents are much lower in comparisonto the other lower loaded catalysts; this may be be-cause of the structural collapse of the sol–gel mixedoxide supports at higher vanadia loadings. Ocelliand Stencel [7] have noticed about 81% decrease insurface area of the aluminosilicate gel at a vanadialoading of 1.5 wt.%, upon steaming the catalyst at760◦C for 10 h, however, such a decrease in surfacearea was not noticed in the calcined catalysts bythese investigators. Vanadia supported on aluminosil-icate gels was found to be less resistant to steamingtreatment than V2O5/Al2O3 gels [7]. The vanadiacontents and surface areas of the V/Al, V/Si, V/Tiand V/Zr catalysts that were used for comparing theethanol partial oxidation activities are also includedin Table 1.

51V solid-state static NMR spectra of the cal-cined V/Ti–Si catalysts are shown in Fig. 1. Baseline

J.L. Lakshmi et al. / Journal of Molecular Catalysis A: Chemical 165 (2001) 199–209 201

Table 1Vanadia contents and BET surface areas of the V/Al–Ti, V/Ti–Siand V/Ti–Zr series of catalysts

Catalyst Catalyst code V2O5

(wt.%)aSurfacearea (m2/g)

V2O5/TiO2–Al2O3

Ti–Al – 3041 V/Ti–Al 1 1.3 2882 V/Ti–Al 2 4.2 2833 V/Ti–Al 3 7.9 2794 V/Ti–Al 4 11.4 2745 V/Ti–Al 5 14.1 1466 V/Ti–Al 6 24.4 126

V2O5/TiO2–SiO2

Ti–Si – 4261 V/Ti–Si 1 1.8 3762 V/Ti–Si 2 3.8 3583 V/Ti–Si 3 5.6 3334 V/Ti–Si 4 7.4 3025 V/Ti–Si 5 9.7 2746 V/Ti–Si 6 11.5 208

V2O5/TiO2–ZrO2

Ti–Zr – 1681 V/Ti–Zr 1 2.1 1422 V/Ti–Zr 2 4.2 1283 V/Ti–Zr 3 9.5 1184 V/Ti–Zr 4 11.6 1115 V/Ti–Zr 5 21.2 1046 V/Ti–Zr 6 27.6 80V2O5/Al2O3 V/Al 11.5 260V2O5/SiO2 V/Si 9.0 201V2O5/TiO2 V/Ti 8.0 42V2O5/ ZrO2 V/Zr 8.0 18

a Estimated from ICP analysis.

distortions were observed in all of the51V NMRspectra because of the sweep width limitation ofour NMR spectrometer. The peak at about−320 to−350 ppm (Type ‘A’ species) observed in the V/Ti–Sicatalysts can be attributed to the distorted octahedralvanadia species, and the peak at apprximately−500to −550 ppm (Type ‘B’ species) can be assigned tothe distorted tetrahedral species. With increase invanadia loading, a decrease in the intensity of Type‘B’ species and a corresponding increase in Type ‘A’species could be seen in the spectra, suggesting theformation of V2O5 microcrystallites. In the case ofV/Ti–Si 6 catalyst an additional peak at−1270 ppmcorresponding to the parallel component of the peak at−330 ppm could be seen. Fig. 2 shows the51V NMRspectra of the V/Ti–Si catalysts outgassed at 350◦C

Fig. 1. 78.9 MHz 51V solid-state NMR spectra of the calcinedV/Ti–Si catalysts.

for 30 min in a flow of He. A broad chemical shiftdistribution is seen in the evacuated catalysts result-ing in the complicated peak shapes. Type ‘A’ speciescorresponding to distorted OH species which wereprominent in the calcined samples became indistin-guishable in the V/Ti–Si 2, 3, and 4 catalysts. There is


Fig. 2. 78.9 MHz 51V solid-state NMR spectra of the V/Ti–Sicatalysts evacuated at 623 K for 30 min.

an increase in the intensity of the peak correspondingto Type ‘B’ species — distorted tetrahedral vanadiaspecies in these catalysts, suggesting the formation ofthese species by the removal of water molecules fromType ‘A’ species. A shoulder peak at about−680 ppmobserved in the V/Ti–Si 2 and 3 catalysts can be at-tributed to Type ‘C’ tetrahedral species which mighthave formed from either Type ‘B’ tetrahedral speciesby the removal of coordinated water molecules. Thatthere is not much difference in the spectra of theV/Ti–Si 5 and 6 calcined and outgassed catalystssuggests the presence of V2O5 agglomerates.

The 51V NMR spectra of the V/Ti–Zr series ofcalcined catalysts are shown in Fig. 3. It can be seenfrom the figure that the peaks are broad, due to widedistribution of the chemical shifts. Iyer et al. [6] intheir 51V NMR studies on aluminosilicate gel sup-ported vanadia catalysts have shown that spectral

Fig. 3. 78.9 MHz 51V solid-state NMR spectra of the calcinedV/Ti–Zr catalysts.


interpretation is complicated in these catalysts due tothe structural instability of the mixed oxide supportat higher vanadia loadings. The51V chemical shiftcentered at−500 to −516 ppm (Type ‘B’ species)can be attributed to distorted tetrahedral species. Ashoulder at about−650 ppm could be seen in theV/Ti–Zr 1 and 2 catalysts, corresponding to Type ‘C’tetrahedral vanadia species in these catalysts. Thepeak at−320 ppm observed in the V/Ti–Zr 5 and 6catalysts corresponds to the presence of V2O5 mi-crocrystallites. Fig. 4 shows the51V NMR spectraof the V/Ti–Zr catalysts outgassed at 350◦C. Clearresolution of the peaks corresponding to differentvanadia species could be seen in the evacuated sam-ples. The 51V chemical shift at about−690 ppmcorresponding to Type ‘C’ species, and the peak atapproximately−460 ppm corresponding to Type ‘D’distorted tetrahedral species were observed. Thesetetrahedral species are different and may correspondto the vanadia in interaction with both the titania andzirconia components of the mixed oxide support [17].

The51V MAS NMR spectra of the catalysts V/Ti–Siand V/Ti–Zr are shown in Figs. 5 and 6. There isnot much influence of magic angle spinning on thewidth of 51V signals in the catalysts with low vanadiacontents in both the series of catalysts. This may bedue to large quadrupole coupling constants and chem-ical shift parameters resulting in wide chemical shiftdistribution. The spinning side pattern in the case ofV/Ti–Si and V/Ti–Zr samples at high vanadia contentsis similar to that of V2O5 aggregates, indicating thepresence of V2O5 like clusters.

1H MAS NMR spectra of the Ti–Si support andV/Ti–Si catalysts are shown in Fig. 7. Fig. 8 shows the1H NMR spectra of the Ti–Zr support and the V/Ti–Zrcatalysts. In our earlier study [1], we reported decon-volution of the1H resonance signals of the Ti–Si andTi–Zr supports using Voigt lines shapes [18]. In thecase of Ti–Si support and V/Ti–Si catalysts a sharpresonance was observed at 1.5 ppm which may be as-signed to the hydroxyl groups of silica. In the caseof V/Ti–Zr catalysts a broad resonance centered at5.6 ppm could be seen. The deconvolution studies ofthe Ti–Zr mixed oxide indicated the1H resonance at0.03, 1.95, 4.5, 5.6 and 7.6 ppm. In our earlier1H MASNMR studies on Ti–Al mixed oxide and V/Ti–Al cat-alysts [2] we noticed the1H chemical shifts at−0.5,2.9, 5.1 and 7.5 ppm which were attributed to terminal

Fig. 4. 78.9 MHz 51V solid-state NMR spectra of the V/Ti–Zrcatalysts evacuated at 623 K for 30 min.

and bridged Al–OH and Ti–OH groups. We noticed apreferential reaction of vanadia with basic –OH groupswhich appear in the upfield region. Previous1H MASinvestigations on TiO2 [19] and ZrO2 [19,20] sup-ports, reported the peaks at 2.4 and 6.8 ppm for surfaceTi–OH groups and the1H chemical shifts at 1.6 and


Fig. 5. 78.9 MHz51V MAS NMR spectra of the V/Ti–Si catalystsevacuated at 623 K for 30 min (spin rate 10 kHz).

3.9 ppm for the Zr–OH groups. A broad resonance atabout 3.8 ppm in the catalysts V/Ti–Zr 3, 4, 5 and 6,may correspond to hydroxyl groups of vanadia [15].

Figs. 9 and 10 show the DRIFT spectra of the out-gassed V/Ti–Si and V/Ti–Zr catalysts in the skele-tal region 1400–400 cm−1. In the catalysts V/Ti–Si 1

Fig. 6. 78.9 MHz51V MAS NMR spectra of the V/Ti–Zr catalystsevacuated at 623 K for 30 min (spin rate 10 kHz).

and 2, the vibrations attributable to asymmetric andsymmetric stretching vibrations ofνasy of Si–O–Sibonds could be seen at approximately 1194, 1125 and952 cm−1. In the V/Ti–Si catalysts with high V2O5contents the support vibrations were less prominent.The vibrations at 1020 and 860 cm−1 attributable tostretching vibrations of V=O and V–O–V bonds wereobserved in the V/Ti–Zr catalysts [21]. The absenceof vibrations corresponding to surface vanadia speciesin the case of V/Ti–Si catalysts and V/Ti–Zr catalysts


Fig. 7. 1H MAS NMR spectra of the V/Ti–Si catalysts evacuatedat 623 K for 30 min (spin rate 10 kHz).

at lower vanadia loadings may be because of the over-lap of these vibrations with the broad IR bands of theTi–Si and Ti–Zr supports.

The information regarding the presence of mono-layers and multiple layers of vanadia on the supportsurface were obtained from IR overtone absorptions.Schraml-Marth et al. [22] in their FT-IR investiga-tions on V2O5/TiO2 catalysts have noticed overtonebands at 2040, 2020 and 1990 cm−1 at high vanadialoadings. At lower V2O5 loadings a single overtonevibration was noticed at 2040 cm−1 for the catalystin the oxidized and hydrated states. In the present

Fig. 8. 1H MAS NMR spectra of the V/Ti–Zr catalysts evacuatedat 623 K for 30 min (spin rate 10 kHz).

study, overtone vibrations for the V/Ti–Si catalystswere observed at 2005, and 1872 cm−1. In the caseof V/Ti–Zr 6 catalyst the overtones at 2010 and1972 cm−1 correspond to the V2O5 agglomerates [23].The DRIFT studies are in conformity with51V NMRstudies suggesting the presence of V2O5 clusters inthe catalysts V/Ti–Si and V/Ti–Zr with high vanadiacontents.

DRIFT spectrum of the evacuated Ti–Si mixedoxide in the hydroxyl region (4000–2000 cm−1showeda sharp spike at 3733 cm−1 that can be attributedto Si–OH vibrations [11]. In the V/Ti–Si catalysts,


Fig. 9. DRIFT spectra of the V/Ti–Si catalysts evacuated at 623 Kfor 30 min in the skeletal vibration region (1400–400 cm−1).

Fig. 10. DRIFT spectra of the V/Ti–Zr catalysts evacuated at 623 Kfor 30 min in the skeletal vibration region (1400–400 cm−1).

vibrations were observed at 3733, 3631 and 3147 cm−1.The Si–OH vibrations gradually decreased in intensitywith an increase in vanadia loading and disappearedin the catalysts V/Ti–Si 4, 5 and 6. In the case of


Ti–Zr support and V/Ti–Zr catalysts vibrations wereobserved at 3603, 3138, 2836 and 2336 cm−1. Inour earlier DRIFT studies on Al–Zr support [24] wenoticed similar IR bands which were attributed toZr–OH vibrations. We noticed a gradual decrease inthe intensity of the IR band at high frequency, cor-responding to the most basic hydroxyl groups of theAl–Zr support, with increase in vanadia loading. Thiswas due to the preferential reaction of active vanadia

Fig. 11. Ethanol partial oxidation rates of the V/Ti–Al, V/Ti–Si, and V/Ti–Zr series of catalysts as a function of reaction temperature(catalyst codes are given in Table 1).

species with the basic hydroxyl groups of the support.In the present investigation also, a gradual decrease inthe intensity of the IR vibration at 3603 cm−1 couldbe seen, with increase in V2O5 content suggestingconsumption of the most basic hydroxyl groups of theTi–Zr support with the active component. Eberhardtet al. [25] in their IR studies on V/Al2O3 catalystshave shown that the most basic surface hydroxylgroups exhibit high IR frequency and sequential con-


sumption of hydroxyl groups of the alumina supportwith vanadia starting with the most basic –OH groupsof the support.

The catalytic activities of the V/Ti–Al, V/Ti–Si andV/Ti–Zr series of catalysts and mixed oxide supportswere tested using ethanol oxidation as a probe reac-tion. The commercial oxide and sol–gel derived mixedoxide supports exhibited very low conversions underreaction conditions. The ethanol partial oxidation ratesof various catalysts are shown in Fig. 11 as a functionof reaction temperature. It can be seen from the fig-ure that the ethanol partial oxidation rates of the cata-lysts increased with increase in reaction temperature.There is an overlap of the reaction rates in each seriesof catalysts due to the structure insensitive nature ofthe ethanol partial oxidation reaction [26]. The ethanolpartial oxidation rates of the V/Ti–Si, V/Ti–Al, andV/Ti–Zr series of catalysts are not much different fromone another indicating the similar kind of reactivitiesin all the three series of catalysts. The ethanol oxida-tion rates of the V/Al–Ti 1, V/Ti–Si 1 and V/Ti–Zr1 catalysts were found to be lower in each series ofcatalysts possibly due to the low amount of the ac-tive V2O5 phase. In the case of V/Al–Ti series of cat-alysts (Fig. 11a) the activities of the catalysts are inthe order V/Ti > V/Al = V/Al–Ti, suggesting thealumina-like nature of the Al–Ti support in determin-ing the catalytic activity. The reactivities of V/Ti, V/Siand V/Ti–Si series of catalysts are in the order V/Ti >

V/Ti–Si > V/Si (Fig. 11b). The higher partial oxida-tion rates of the V/Ti–Si catalysts may be due to thepresence of vanadia species in interaction with TiO2aggregates in the Ti–Si mixed oxide [1]. Quaranta et al.[27] in their ethanol partial oxidation studies on vana-dia catalysts supported on titania modified silica, no-ticed an increase in the activity of the catalysts withincrease in the amount of titania modifier. In the caseof V/Ti–Zr series of catalysts, the reaction rates are inthe order of V/Ti > V/Zr = V/Ti–Zr (Fig. 11c).

Total selectivity to acetaldehyde was observed atlow reaction temperatures and selectivity to otherproducts such as ether, acetic acid, ethyl acetate andCOx was observed in the vanadia catalysts supportedon commercial oxides and sol–gel mixed oxide sup-ports. At total conversion levels, acetaldehyde wasobtained as the major product for all the catalysts.V/Al catalyst exhibited selectivity to acetic acid, ethylacetate, and COx as the minor products; V/Si catalyst

showed significant selectivity to ethylene (∼15%)with traces of ether, acetic acid, ethyl acetate andCOx ; V/Ti and V/Zr catalysts exhibited acetic acid,ethyl acetate and COx as the major byproducts. In themixed oxide catalysts total selectivity to acetaldehydewas obtained at low conversions and at total conver-sions the acetaldehyde selectivities of the V/Ti–Si andV/Ti–Zr catalysts were lower (∼45%) in comparisonto V/Al–Ti catalyst (∼55%).

4. Conclusions

Highly dispersed surface vanadia species were iden-tified using the51V solid state NMR technique. Diffusereflectance FT-IR and51V NMR studies of the mixedoxide catalysts with high vanadia contents indicatedthe presence of V2O5 microcrystallites. Ethanol par-tial oxidation rates of the mixed oxide catalysts withlow and high V2O5 content are lower in comparisonto other catalysts indicating certain optimum vanadialoading is required for the best catalytic activity. V2O5supported on commercial TiO2 was found to exhibithigher activity than vanadia on Al2O3, SiO2, and ZrO2supports. The ethanol partial oxidation activities of thevanadia catalysts supported on sol–gel derived mixedoxides were found to be lower than V2O5/TiO2, butthey were comparable to vanadia catalysts supportedon commercial oxides — Al2O3, SiO2, and ZrO2. Fur-ther investigations on the structure and reactivity ofthe vanadia catalysts added in situ during the sol–gelsynthesis of the mixed oxides are in progress.

Acknowledgements

We would like to thank NSERC, Canada for finan-cial support. Thanks are due to Prof. Ian Brindle forICP analysis and Tim Jones for his help during theNMR investigations.

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