1. Reac Kinet Mech Cat (2012) 105:469481DOI 10.1007/s11144-011-0383-3V-Mn-MCM-41 catalyst for the vapor phase oxidationof o-xyleneC. Mahendiran T. Maiyalagan P. VijayanC. Suresh K. ShanthiReceived: 4 May 2011 / Accepted: 1 October 2011 / Published online: 21 October 2011 Akademiai Kiado, Budapest, Hungary 2011 Abstract The role of V and Mn incorporated mesoporous molecular sieves wasinvestigated for the vapor phase oxidation of o-xylene. Mesoporous monometallicV-MCM-41 (Si/V = 25, 50, 75 and 100), Mn-MCM-41 (Si/Mn = 50) and bime-tallic V-Mn-MCM-41 (Si/(V ? Mn) = 100) molecular sieves were synthesized bya direct hydrothermal (DHT) process and characterized by various techniques suchas X-ray diffraction, DRUV-Vis spectroscopy, EPR, and transmission electronmicroscopy (TEM). From the DRUV-Vis and EPR spectral study, it was found thatmost of the V species are present as vanadyl ions (VO2?) in the as-synthesizedcatalysts and as highly dispersed V5? ions in tetrahedral coordination in the calcinedcatalysts. The activity of the catalysts was measured and compared with each otherfor the gas phase oxidation of o-xylene in the presence of atmospheric air as anoxidant at 573 K. Among the various catalysts, V-MCM-41 with Si/V = 50exhibited high activity towards production of phthalic anhydride under the exper-imental condition. The correlation between the phthalic anhydride selectivity andthe physico-chemical characteristics of the catalyst was found. It is concluded thatV5? species present in the MCM-41 silica matrix are the active sites responsible forthe selective formation of phthalic anhydride during the vapor phase oxidation ofo-xylene.C. Mahendiran (&)Department of Chemistry, Anna University of Technology Tirunelveli,University College of Engineering, Nagercoil Campus, Nagercoil 629004, Indiae-mail: cmmagi@gmail.comT. MaiyalaganSchool of Chemical and Biomedical Engineering, Nanyang Technological University,Singapore 639798, SingaporeP. Vijayan C. Suresh K. ShanthiDepartment of Chemistry, Anna University, Chennai 25, India123

2. 470C. Mahendiran et al.Keywords V and Mn-MCM-41 Vapor phase Oxidation o-xylene Phthalic anhydrideIntroductionHeterogeneous catalyzed gas phase oxidation plays a vital role in the chemicalindustry. In fact, selective oxidation is the simplest functionalization method; inparticular, more than 60% of products synthesized by catalytic routes in thechemical industry are obtained by oxidation reactions [1]. From the standpoint ofenvironmental friendliness, much attention has been paid to the development ofmetal catalysts for the selective oxidation using molecular oxygen as an oxidant[25]. The oxidative product pthalic anhydride is a commercially important andversatile intermediate in organic chemistry. The primary use of phthalic anhydride(PA) is as a chemical intermediate in the production of plastics from vinyl chloride.Phthalate esters, which function as plasticizers, are derived from phthalic anhydride.Phthalic anhydride has another major use in the production of polyester resins andother minor uses in the production of alkyl resins used in paints and lacquers, certaindyes (anthraquinone, phthalein, rhodamine, phthalocyanine, uorescein, andxanthene dyes), insect repellents, and urethane polyester polyols. It has also beenused as a rubber scorch inhibitor [6].A method for converting naphthalene to phthalic anhydride using sulfuric acid asthe oxidizing agent in the presence of mercury salt as the catalysts was discoveredby E. Sapper, and was patented by the Badische Anilin and Soda Fabrik in 1896.During the last decade of the nineteenth century, the growing demand for phthalicanhydride for use in the preparation of xanthene and the indigoid dyes led toresearch toward the discovery of cheaper processes for its manufacture [7]. In thiscontext, Dias et al. [8] found that V2O5 supported TiO2 as an efcient catalyst forphthalic anhydride production. However, due to its poor mechanical strength ofV2O5/TiO2 and low surface area, attention has been focused on the development ofcatalysts with high mechanical strength and high surface area. In this context,mesoporous MCM-41 molecular sieves with high surface area and tunable pore sizecame into existence [9]. The unique physical properties have made these materialshighly desirable for catalytic applications [10, 11]. Isomorphous substitution ofsilicon with other elements is an excellent strategy in creating active sites andanchoring sites for active molecules in the design of new heterogeneous catalyst.Many metals, e.g., Al, Ti, Mn, Fe, B, Ni and V, have been incorporated into thesilica matrix of MCM-41 [1215]. Molecular sieves containing redox active metals,like Ti, V, Cr, Fe, or Co, are increasingly used as heterogeneous catalysts for theselective oxidation of organic compounds. Among the metals, particularlyvanadium and manganese were found to have remarkable catalytic activity forthe selective oxidation of various organic molecules when incorporated into silicatemolecular sieve [1621]. In this paper, the vapor phase oxidation of o-xylene tophthalic anhydride over V-MCM-41, Mn-MCM-41 and V-Mn-MCM-41 catalystshas been investigated and correlated with the structural, electronic and surfaceresults obtained.123 3. V-Mn-MCM-41 catalyst 471ExperimentalSynthesis of V-MCM-41V-MCM-41 (Si/V = 50) was synthesized by hydrothermal method reportedelsewhere [21] using sodium metasilicate (CDH) as silica source, cetyl trimethylammonium bromide (CTAB, OTTO Chemie) as the structure-directing agent withthe following molar gel composition SiO2:0.02 (VOS4H2O):0.2 CTAB:0.89H2SO4:160 H2O. In a typical synthesis, 21.32 g of sodium metasilicate and0.63 g of vanadyl sulfate monohydrate were dissolved in 60 g of water. Thereaction mixture was stirred for 2 h. Meanwhile, CTAB (5.47 g) was dissolved in20 g of water. Then, the resultant mixture of sodium metasilicate and vanadylsulfate monohydrate was added dropwise into the CTAB solution. The nal mixturewas stirred for 1 h. The pH of the gel was adjusted to 10.511 using 2 M sulfuricacid followed by stirring for 3 h. The obtained gel was placed into an autoclave andheated to 413 K under static conditions for 12 h. The resultant precipitate wasltered, washed with deionized water and dried in air at 375 K and then nallycalcined at 773 K for 1 h in N2 ow and for 12 h in CO2-free air ow. The catalystsV-MCM-41 (Si/V = 25, 75,100), Mn-MCM-41 (Si/Mn = 50) and V-Mn-MCM-41(Si/(V ? Mn) = 100) were also synthesized in a similar manner wherein only theratio of vanadyl sulfate monohydrate for vanadium source and manganese acetatefor manganese source was adjusted.Characterization of the catalystsInductively coupled plasma (ICP) optical emission spectroscopy was used for thedetermination of the metal content in each sample synthesized above. Themeasurements were performed with a Perkin-Elmer OPTIMA 3000 and the samplewas dissolved in a mixture of HF and HNO3 before the measurements. XRDanalysis was performed on Rigaku Miniex X-ray diffractometer. A germaniumsolid state detector cooled in liquid nitrogen with Cu Ka radiation source was used.The samples were scanned between 0.5 and 8.5 (2h) in steps of 0.02 with thecounting time of 5 s at each point. N2 adsorption studies were carried out toexamine the porous properties of each sample. The measurements were carried outon a Belsorpmini II (BEL Japan. Inc) instrument. All the samples were pre-treatedin vacuum at 573 K for 12 h in owing N2 at a ow rate of 60 mL/min. The surfacearea and pore size were obtained from these isotherms using the conventional BETand BJH equation. The coordination environment of vanadium and manganesecontaining MCM-41 catalysts was examined by diffuse reectance UV-visspectroscopy. The spectra were recorded between 200 and 800 nm on a ShimadzuUV-vis spectrophotometer (Model 2450) using BaSO4 as the reference. Further-more, the coordination environment of vanadium and manganese was conrmed byEPR (Varian E112 spectrometer operating in the X-band 9.2 GHz frequency) atroom temperature. Transmission electron microscopy (TEM) images were obtainedby using a JEOL electron microscope with an acceleration voltage of 200 kV.123 4. 472C. Mahendiran et al.Experimental procedure for the oxidation of o-xyleneThe oxidation of o-xylene was carried out in a xed bed down ow quartz reactor atatmospheric pressure in the temperature range of 473623 K with air ow of0.02 mol h-1. Prior to the reaction, the reactor packed with 0.3 g of the catalyst waspreheated in a tubular furnace equipped with a thermocouple. The reactant (o-xylene) was fed into the reactor through a syringe infusion pump at a predeterminedow rate. The product mixture was collected at the time interval of 1 h and analyzedby a gas chromatograph (GC-17A, Shimadzu) equipped with a ame ionizationdetector. The gaseous products were analyzed by a TCD detector using an SE-30column. After every run, the catalyst was regenerated to remove the coke deposit,by passing a stream of pure dry air at a temperature of 773 K for 6 h. The effect ofvarious parameters, viz., temperature, weight hourly space velocity and time onstream was studied on the regenerated catalyst.Results and discussionXRDThe XRD patterns of calcined V-MCM-41 materials with an atomic ratio of(Si/V = 100, 75, 50, and 25), Mn-MCM-41 (50), and V-Mn-MCM-41(Si/(V ? Mn) = 100) recorded at low diffraction angles are shown in Fig. 1 andits inset. A strong intense peak observed in the 2h range between 2 and 38 for all thesamples is due to the reection from (100) plane of MCM-41. Apart from this, lowintensity peaks in the 2h range 35, corresponding to the higher order reections Intensity (a.u.)Intensity (a.u.)ab1 3 5 79112 (Deg.) a b c d 0 2 46 810 2 (Deg.)Fig. 1 X-ray diffraction patterns of (a) V-MCM-41 (100), (b) V-MCM-41 (75), (c) V-MCM-41 (50) and(d) V-MCM-41 (25) catalysts. Inset (a) Mn-MCM-41 (50), (b) V-Mn-MCM-41 (Si/(V ? Mn) = 100)123 5. V-Mn-MCM-41 catalyst473Table 1 Physico-chemical characteristics of the catalystsCatalystV Mnd-Unit cell Surface Pore PoreWallcontent content spacing parameter areadiameter volumethickness(wt%)a(wt%)a (A) a0 (A)(m2/g) (A)(cm3/g) (A)V-MCM-41 (25) 1.14 40.01 46.2081428.100.7618.10V-MCM-41 (50) 0.58 39.60 45.7389328.400.7917.33V-MCM-41 (75) 0.38 39.50 45.6197628.600.8117.00V-MCM-410.24 39.30 45.38 1,013 28.700.8216.68 (100)V-Mn-MCM-41 0.570.8641.30 47.6986328.480.8019.21 Si/(V ? Mn) = 100Mn-MCM-41 0.8840.75 47.0690427.750.8119.31 (50)aResults obtained from ICP-AES analysissuch as (110) and (200) planes, were also observed, which conrms the mesoporousnature of the samples. Higher angle XRD (not shown) does not show any peaks forextra framework vanadium oxide. The unit cell parameter (a0) calculated using theformula, a0 = 2d100/H3, and d spacing values obtained using the Braggsequation 2dsinh = nk, where k = 1.54 A for Cu Ka radiation are presented inTable 1. Upon introduction of V into the MCM-41, a slight decrease in the unit cellparameter value was observed. However, when the metal content was increased,the intensity of the diffraction peaks decreased, indicating that it may be due tostructural irregularity of the mesopores at high metal content as reported inliterature [22].Nitrogen adsorptiondesorption isothermsThe adsorptiondesorption isotherms of the catalysts V, Mn and V-Mn-MCM-41are illustrated in Fig. 2. A typical type IV isotherm as dened by IUPAC formesoporous material was obtained. The adsorption isotherm exhibits a sharpincrease in the P/Po range from 0.2 to 0.3 which is obviously characteristic ofcapillary condensation within mesopores [23]. The P/Po position of the inectionpoints is clearly related to the diameter in the mesopore range, and the stepindicates the mesopore size distribution. N2 adsorbed volumes at P/Po = 0.3, forSi/V = 100, 75, 50, 25, Si/Mn = 50 and Si/Mn ? V = 100 are 350, 330, 315, 295,275 and 250. The BET surface area, pore volume, and pore diameter, as a functionof V, Mn and V-Mn content are shown in Table 1. The increase in the vanadiumcontent slightly decreased the surface area, pore volume as well as pore diameter.From the results, the N2 adsorption studies clearly indicate the successful incor-poration of V and Mn. When V and Mn are used together, partial amorphization is123 6. 474 C. Mahendiran et al.Fig. 2 N2 adsorptiondesorption isotherms of catalysts (a) V-MCM-41 (100), (b) V-MCM-41 (75),(c) V-MCM-41 (50) and (d) V-MCM-41 (25) Inset (a) Mn-MCM-41 (50), (b) V-Mn-MCM-41 (Si/(V ? Mn) = 100)occurs. It may be due to metal oxides blocking the molecular sieves pore or partialcollapse of pore structure.DR-UV-vis spectroscopyThe DRUV-Vis spectra of V-MCM-41 (Si/V ratio = 25, 50, 75 and 100) catalystsshowed the presence of two shoulder peaks at 260 and the other at 340 nm (Fig. 3ad). These correspond to the tetrahedral V5? ions inside the wall and the tetrahedralV5? ions on the surface of the wall, respectively [24]. The intensity ratio of thesetwo peaks seems to be relatively high for a catalyst with high vanadium loading. It isalso evident from the spectra that as the ratio of Si/V increased, there is acorresponding decrease in the intensity of the peaks due to decrease in the numberof vanadium ions. These bands were attributed to the low-energy charge transfertransition between tetrahedral oxygen ligands and a central V5? ion [25, 26]. Such atetrahedral environment was typical for silica matrix V5? ions. A typical spectrum is123 7. V-Mn-MCM-41 catalyst475 Absorbance (a.u)abcde180 280380480580680Wavelength (nm)Fig. 3 DR-UV-vis spectra of catalysts (a) V-MCM-41 (25), (b) V-MCM-41 (50), (c) V-MCM-41 (75),(d) V-MCM-41 (100) and (e) spent V-MCM-41 (50)recorded for the spent V-MCM-41 (50) catalyst (Fig. 3e). There are no absorptionbands and this indicates that V5? species are absent in spent catalyst.EPRThe EPR spectra of the as-synthesized V-MCM-41 samples with varying valuesof Si/V atomic ratio (25, 50, 75, and 100) were recorded at room temperature andare shown in Fig. 4ad. The source for the synthesis of vanadium containingmesoporous materials was vanadyl sulfate (V4?, d1), and all the as-synthesizedV-MCM-41 exhibits its characteristic EPR signal. In comparison, the EPR spectraof as-synthesized and spent V-MCM-41 (Si/V = 50) are also shown in Fig. 5a, b.It is interesting to note the absence of the EPR signal in the spent catalyst (Fig. 5b)which may be the indication for the complete utilization of the V5? species foroxidation reaction.TEMTEM images of the calcined V-MCM-41 samples with Si/V atomic ratios of 100,75, 50, and 25 are shown in Fig. 6ad. A highly ordered mesoporous frameworkwith hexagonal arrays of cylindrical channels of the synthesized samples isconrmed by TEM images [27]. These are virtually regular hexagonal arrays of nepore arrangement existing in these samples. This ordered arrangement, typical forthe MCM-41 materials, conrms the XRD data.123 8. 476 C. Mahendiran et al.abcd 0 2000 4000 6000 8000 Magnetic field strength (Gauss)Fig. 4 EPR spectra of as-synthesized catalysts (a) V-MCM-41 (25), (b) V-MCM-41 (50), (c) V-MCM-41 (75), (d) V-MCM-41 (100)Fig. 5 EPR spectra of catalysts (a) as-synthesized V-MCM-41 (50) and (b) V-MCM-41 (50) spentActivity of V-MCM-41 catalystThe activity of V-MCM-41 (50) catalyst was studied for the vapor phase oxidationof o-xylene at 573 K with the ow rate of o-xylene 5.87 h-1 (WHSV) and CO2-freeair 0.02 mol h-1 over a period of 7 h. The results are illustrated in Fig. 7. Thepercentage conversion and selectivity increased from 1 to 2 h and then decreased upto 5 h. Beyond 5 h, the catalyst attained steady state activity. The initial increase inconversion from 1 to 2 h is attributed to oxidation of V4? to V5?, which is necessaryfor oxidation. The decrease in trend up to 5 h may be due to some carbondeposition. Under this steady state reaction conditions, the activities of the catalystswith varying Si/V ratios are compared.123 9. V-Mn-MCM-41 catalyst477Fig. 6 TEM pictures of a V-MCM-41 (100), b V-MCM-41 (75), c V-MCM-41 (50), d V-MCM-41 (25) In order to nd out the optimum vanadium content, the vapor phase oxidation ofo-xylene was carried out at 573 K on V-MCM-41 catalyst with varying vanadiumcontent (Si/V ratio 25, 50, 75 and 100) and the results are given in Fig. 8. It isobserved that the conversion o-xylene increases with Si/V ratio till V-MCM-41(50). Obviously, more vanadium loading can increase o-xylene conversion becauseof the increased amount of available active sites. This is revealed from the lowintensity of DRUV-Vis spectral bands of V-MCM-41 (25) catalyst around 260and 340 nm corresponding to V5? (Fig. 3a, b) compared to that of V-MCM-41 (50).The optimum ratio is around 50. V-MCM-41 (50) exhibited the maximum catalyticactivity. However, beyond the Si/V ratio 50, there is a decrease in trend observedwith respect to its conversion. This may be because of the lack of dispersion ofvanadium even though available in large quantity. The decrease in conversion athigh Si/V value may be attributed to the decrease in the concentration of V5? activesites as it is evident from the DRUV-Vis spectra where a decrease in the absorbanceintensity is noticed with increase in Si/V ratio from 50 to 100. Hence, the highactivity of V-MCM-41 (50) may be attributed to the availability of higher numberof V5? in V-MCM-41 (50) than in V-MCM-41 (75) and V-MCM-41 (100). 123 10. 478C. Mahendiran et al.Fig. 7 Effect of reaction time on the oxidation of o-xylene. Reaction conditions: temperature = 573 K,weight of V-MCM-41 (Si/V = 50) = 0.3 g, WHSV = 5.87 h-1 and ow rate of air 0.02 mol h-1The dispersion and amount of V5? become important in order to account for highconversion of o-xylene. The same trend was registered for the selectivity of phthalicanhydride. The selectivity of o-toluic acid (OTA) remained reverse trend to thatof phthalic anhydride selectivity; hence it might be considered as the majorintermediate for phthalic anhydride formation as showed in the reaction scheme.Based on the activity study and characteristics of catalysts, the vapor phaseoxidation of o-xylene is proposed to take place as suggested in reaction scheme(Scheme 1). According to the scheme, molecular oxygen is activated by frameworkvanadium. The activated O2 is inserted between carbon and hydrogen bond of themethyl group in o-xylene. The resulting alcohol is rapidly oxidized to o-tolaldehydewhich is also subsequently oxidized to o-toluic acid. The same process is alsorepeated on adjacent methyl group to yield phthalic acid. The product issubsequently oxidized to phthalic anhydride.Comparison of the catalyst supportsThe activity of V-MCM-41 (50), Mn-MCM-41 (50) and V-Mn-MCM-41(V:Mn = 50:50), was measured at 573 K with the WHSV of o-xylene 5.87 h-1(WHSV). The results are compared under the optimized reaction conditions tounderstand the inuence of various metals on the oxidation reaction and presentedin Fig. 9. Among the three catalysts, it is the V-MCM-41 (50) catalyst that exhibitedmaximum activity. The reason for the high activity of V-MCM-41 (50) may be dueto the availability of silica matrix V5? in MCM-41which is evident from DRUV-Vis123 11. V-Mn-MCM-41 catalyst 479Fig. 8 Effect of Si/V ratio on the oxidation of o-xylene over V-MCM-41. Reaction conditions:temperature = 573 K, catalyst weight = 0.3 g, WHSV = 5.87 h-1 and ow rate of air 0.02 mol h-1;reaction time = 120 minO O . CH2 HOOV O2V CH2 OH OO O O CH3O OSiSiSiSiCH3SiSi fast CHO CH3OC COOHCOOHRepeated O fast CCOOHCH3 OScheme 1 Vapor phase oxidation of o-xylene to phthalic anhydride 123 12. 480 C. Mahendiran et al.100% of Conversion & Selectivity80 60Conversion 40Selectivity 200V-MCM-41- Mn-MCM-41- V-Mn-MCM-41-(50+50)Fig. 9 Comparison of activity of the catalysts for the oxidation of o-xylene. Reaction conditions:temperature = 573 K, weight of the catalyst = 0.3 g, WHSV = 5.87 h-1 and ow rate of air0.02 mol h-1; reaction time = 120 minspectra (Fig. 3). Further evidence of the elemental analysis results also (Table 1,ICP-AES) reveals that decrease the Si/V ratios (from 100 to 25) there is increase theincorporated metal content into the silica matrix. Hence, it is concluded that silicamatrix V5? was shown to be more active for the oxidation of o-xylene to phthalicanhydride [28]. Manganese (Mn) incorporated into the MCM-41 is expected tosupport oxidative dehydrogenation of hydrocarbons because of the presence ofsuccessive acidic and redox sites. However, during the oxidative dehydrogenationof o-xylene, the strong aromaticity will be lost signicantly. Hence, Mnincorporated MCM-41 does not support the oxidation reaction of o-xylene tophthalic anhydride under these experimental conditions. Finally, based on theliterature, it can be understood that the poor activity of V-Mn-MCM-41 may be dueto the presence of lower number of silica matrix V5? in V-Mn-MCM-41 catalyst.ConclusionsFrom the scrutiny of the above work, the following conclusions can be drawn:1.Mesoporous V-MCM-41 molecular sieves with Si/V ratio 25, 50, 75 and 100contains vanadyl ions (VO2?) in the as-synthesized form, whereas oncalcination, vanadyl ions (VO2?) is converted into highly dispersed V5?species with tetrahedral coordination.2.Enhancement of the activity of MCM-41 for the vapor phase oxidation of o-xylene is achieved by incorporating vanadium. The high activity of V-MCM-41(50) for phthalic anhydride formation could be accounted due to the presence oflarge amount of well dispersed V5? on V-MCM-41. Both UV-Vis DRS and123 13. V-Mn-MCM-41 catalyst 481 EPR spectroscopies provide valuable information about the surface structure of V-MCM-41 catalysts.3. 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