Reac Kinet Mech Cat (2012) 105:469–481 DOI 10.1007/s11144-011-0383-3
V-Mn-MCM-41 catalyst for the vapor phase oxidation of o-xylene C. Mahendiran • T. Maiyalagan • P. Vijayan C. Suresh • K. Shanthi
Received: 4 May 2011 / Accepted: 1 October 2011 / Published online: 21 October 2011 Ó Akade´miai Kiado´, Budapest, Hungary 2011
Abstract The role of V and Mn incorporated mesoporous molecular sieves was investigated for the vapor phase oxidation of o-xylene. Mesoporous monometallic V-MCM-41 (Si/V = 25, 50, 75 and 100), Mn-MCM-41 (Si/Mn = 50) and bimetallic V-Mn-MCM-41 (Si/(V ? Mn) = 100) molecular sieves were synthesized by a direct hydrothermal (DHT) process and characterized by various techniques such as X-ray diffraction, DRUV-Vis spectroscopy, EPR, and transmission electron microscopy (TEM). From the DRUV-Vis and EPR spectral study, it was found that most of the V species are present as vanadyl ions (VO2?) in the as-synthesized catalysts and as highly dispersed V5? ions in tetrahedral coordination in the calcined catalysts. The activity of the catalysts was measured and compared with each other for the gas phase oxidation of o-xylene in the presence of atmospheric air as an oxidant at 573 K. Among the various catalysts, V-MCM-41 with Si/V = 50 exhibited high activity towards production of phthalic anhydride under the experimental condition. The correlation between the phthalic anhydride selectivity and the physico-chemical characteristics of the catalyst was found. It is concluded that V5? species present in the MCM-41 silica matrix are the active sites responsible for the selective formation of phthalic anhydride during the vapor phase oxidation of o-xylene.
C. Mahendiran (&) Department of Chemistry, Anna University of Technology Tirunelveli, University College of Engineering, Nagercoil Campus, Nagercoil 629004, India e-mail: email@example.com T. Maiyalagan School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 639798, Singapore P. Vijayan C. Suresh K. Shanthi Department of Chemistry, Anna University, Chennai 25, India
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Keywords V and Mn-MCM-41 Vapor phase Oxidation o-xylene Phthalic anhydride
Introduction Heterogeneous catalyzed gas phase oxidation plays a vital role in the chemical industry. In fact, selective oxidation is the simplest functionalization method; in particular, more than 60% of products synthesized by catalytic routes in the chemical industry are obtained by oxidation reactions . From the standpoint of environmental friendliness, much attention has been paid to the development of metal catalysts for the selective oxidation using molecular oxygen as an oxidant [2–5]. The oxidative product pthalic anhydride is a commercially important and versatile 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 and other minor uses in the production of alkyl resins used in paints and lacquers, certain dyes (anthraquinone, phthalein, rhodamine, phthalocyanine, fluorescein, and xanthene dyes), insect repellents, and urethane polyester polyols. It has also been used as a rubber scorch inhibitor . A method for converting naphthalene to phthalic anhydride using sulfuric acid as the oxidizing agent in the presence of mercury salt as the catalysts was discovered by 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 phthalic anhydride for use in the preparation of xanthene and the indigoid dyes led to research toward the discovery of cheaper processes for its manufacture . In this context, Dias et al.  found that V2O5 supported TiO2 as an efficient catalyst for phthalic anhydride production. However, due to its poor mechanical strength of V2O5/TiO2 and low surface area, attention has been focused on the development of catalysts with high mechanical strength and high surface area. In this context, mesoporous MCM-41 molecular sieves with high surface area and tunable pore size came into existence . The unique physical properties have made these materials highly desirable for catalytic applications [10, 11]. Isomorphous substitution of silicon with other elements is an excellent strategy in creating active sites and anchoring 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 the silica matrix of MCM-41 [12–15]. Molecular sieves containing redox active metals, like Ti, V, Cr, Fe, or Co, are increasingly used as heterogeneous catalysts for the selective oxidation of organic compounds. Among the metals, particularly vanadium and manganese were found to have remarkable catalytic activity for the selective oxidation of various organic molecules when incorporated into silicate molecular sieve [16–21]. In this paper, the vapor phase oxidation of o-xylene to phthalic anhydride over V-MCM-41, Mn-MCM-41 and V-Mn-MCM-41 catalysts has been investigated and correlated with the structural, electronic and surface results obtained.
Experimental Synthesis of V-MCM-41 V-MCM-41 (Si/V = 50) was synthesized by hydrothermal method reported elsewhere  using sodium metasilicate (CDH) as silica source, cetyl trimethyl ammonium bromide (CTAB, OTTO Chemie) as the structure-directing agent with the following molar gel composition SiO2:0.02 (VOS4H2O):0.2 CTAB:0.89 H2SO4:160 H2O. In a typical synthesis, 21.32 g of sodium metasilicate and 0.63 g of vanadyl sulfate monohydrate were dissolved in 60 g of water. The reaction mixture was stirred for 2 h. Meanwhile, CTAB (5.47 g) was dissolved in 20 g of water. Then, the resultant mixture of sodium metasilicate and vanadyl sulfate monohydrate was added dropwise into the CTAB solution. The final mixture was stirred for 1 h. The pH of the gel was adjusted to 10.5–11 using 2 M sulfuric acid followed by stirring for 3 h. The obtained gel was placed into an autoclave and heated to 413 K under static conditions for 12 h. The resultant precipitate was filtered, washed with deionized water and dried in air at 375 K and then finally calcined at 773 K for 1 h in N2 flow and for 12 h in CO2-free air flow. The catalysts V-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 the ratio of vanadyl sulfate monohydrate for vanadium source and manganese acetate for manganese source was adjusted. Characterization of the catalysts Inductively coupled plasma (ICP) optical emission spectroscopy was used for the determination of the metal content in each sample synthesized above. The measurements were performed with a Perkin-Elmer OPTIMA 3000 and the sample was dissolved in a mixture of HF and HNO3 before the measurements. XRD analysis was performed on Rigaku Miniflex X-ray diffractometer. A germanium solid 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 the counting time of 5 s at each point. N2 adsorption studies were carried out to examine the porous properties of each sample. The measurements were carried out on a Belsorpmini II (BEL Japan. Inc) instrument. All the samples were pre-treated in vacuum at 573 K for 12 h in flowing N2 at a flow rate of 60 mL/min. The surface area and pore size were obtained from these isotherms using the conventional BET and BJH equation. The coordination environment of vanadium and manganese containing MCM-41 catalysts was examined by diffuse reflectance UV-vis spectroscopy. The spectra were recorded between 200 and 800 nm on a Shimadzu UV-vis spectrophotometer (Model 2450) using BaSO4 as the reference. Furthermore, the coordination environment of vanadium and manganese was confirmed by EPR (Varian E112 spectrometer operating in the X-band 9.2 GHz frequency) at room temperature. Transmission electron microscopy (TEM) images were obtained by using a JEOL electron microscope with an acceleration voltage of 200 kV.
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Experimental procedure for the oxidation of o-xylene The oxidation of o-xylene was carried out in a fixed bed down flow quartz reactor at atmospheric pressure in the temperature range of 473–623 K with air flow of 0.02 mol h-1. Prior to the reaction, the reactor packed with 0.3 g of the catalyst was preheated in a tubular furnace equipped with a thermocouple. The reactant (oxylene) was fed into the reactor through a syringe infusion pump at a predetermined flow rate. The product mixture was collected at the time interval of 1 h and analyzed by a gas chromatograph (GC-17A, Shimadzu) equipped with a flame ionization detector. The gaseous products were analyzed by a TCD detector using an SE-30 column. 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 of various parameters, viz., temperature, weight hourly space velocity and time on stream was studied on the regenerated catalyst.
Results and discussion XRD
The 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 and its inset. A strong intense peak observed in the 2h range between 2 and 38 for all the samples is due to the reflection from (100) plane of MCM-41. Apart from this, low intensity peaks in the 2h range 3–5°, corresponding to the higher order reflections
a b 1
a b c d 0
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)
Table 1 Physico-chemical characteristics of the catalysts Pore diameter ˚) (A
Pore volume (cm3/g)
Wall thickness ˚) (A
V content (wt%)a
Mn content (wt%)a
dspacing ˚) (A
Unit cell parameter ˚) a0 (A
V-Mn-MCM-41 Si/(V ? Mn) = 100
Surface area (m2/g)
Results obtained from ICP-AES analysis
such as (110) and (200) planes, were also observed, which confirms the mesoporous nature of the samples. Higher angle XRD (not shown) does not show any peaks for extra framework vanadium oxide. The unit cell parameter (a0) calculated using the formula, a0 = 2d100/H3, and d spacing values obtained using the Bragg’s ˚ for Cu Ka radiation are presented in equation 2dsinh = nk, where k = 1.54 A Table 1. Upon introduction of V into the MCM-41, a slight decrease in the unit cell parameter value was observed. However, when the metal content was increased, the intensity of the diffraction peaks decreased, indicating that it may be due to structural irregularity of the mesopores at high metal content as reported in literature . Nitrogen adsorption–desorption isotherms The adsorption–desorption isotherms of the catalysts V, Mn and V-Mn-MCM-41 are illustrated in Fig. 2. A typical type IV isotherm as defined by IUPAC for mesoporous material was obtained. The adsorption isotherm exhibits a sharp increase in the P/Po range from 0.2 to 0.3 which is obviously characteristic of capillary condensation within mesopores . The P/Po position of the inflection points is clearly related to the diameter in the mesopore range, and the step indicates the mesopore size distribution. N2 adsorbed volumes at P/Po = 0.3, for Si/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 function of V, Mn and V-Mn content are shown in Table 1. The increase in the vanadium content slightly decreased the surface area, pore volume as well as pore diameter. From the results, the N2 adsorption studies clearly indicate the successful incorporation of V and Mn. When V and Mn are used together, partial amorphization is
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Fig. 2 N2 adsorptionâ€“desorption 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 partial collapse of pore structure. DR-UV-vis spectroscopy The DRUV-Vis spectra of V-MCM-41 (Si/V ratio = 25, 50, 75 and 100) catalysts showed the presence of two shoulder peaks at 260 and the other at 340 nm (Fig. 3aâ€“ d). These correspond to the tetrahedral V5? ions inside the wall and the tetrahedral V5? ions on the surface of the wall, respectively . The intensity ratio of these two peaks seems to be relatively high for a catalyst with high vanadium loading. It is also evident from the spectra that as the ratio of Si/V increased, there is a corresponding decrease in the intensity of the peaks due to decrease in the number of vanadium ions. These bands were attributed to the low-energy charge transfer transition between tetrahedral oxygen ligands and a central V5? ion [25, 26]. Such a tetrahedral environment was typical for silica matrix V5? ions. A typical spectrum is
b c d e 180
Wavelength (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 absorption bands and this indicates that V5? species are absent in spent catalyst. EPR The EPR spectra of the as-synthesized V-MCM-41 samples with varying values of Si/V atomic ratio (25, 50, 75, and 100) were recorded at room temperature and are shown in Fig. 4a窶電. The source for the synthesis of vanadium containing mesoporous materials was vanadyl sulfate (V4?, d1), and all the as-synthesized V-MCM-41 exhibits its characteristic EPR signal. In comparison, the EPR spectra of 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 for oxidation reaction. TEM TEM images of the calcined V-MCM-41 samples with Si/V atomic ratios of 100, 75, 50, and 25 are shown in Fig. 6a窶電. A highly ordered mesoporous framework with hexagonal arrays of cylindrical channels of the synthesized samples is confirmed by TEM images . These are virtually regular hexagonal arrays of fine pore arrangement existing in these samples. This ordered arrangement, typical for the MCM-41 materials, confirms the XRD data.
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Magnetic field strength (Gauss) Fig. 4 EPR spectra of as-synthesized catalysts (a) V-MCM-41 (25), (b) V-MCM-41 (50), (c) V-MCM41 (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) spent
Activity of V-MCM-41 catalyst The activity of V-MCM-41 (50) catalyst was studied for the vapor phase oxidation of o-xylene at 573 K with the flow rate of o-xylene 5.87 h-1 (WHSV) and CO2-free air 0.02 mol h-1 over a period of 7 h. The results are illustrated in Fig. 7. The percentage conversion and selectivity increased from 1 to 2 h and then decreased up to 5 h. Beyond 5 h, the catalyst attained steady state activity. The initial increase in conversion from 1 to 2 h is attributed to oxidation of V4? to V5?, which is necessary for oxidation. The decrease in trend up to 5 h may be due to some carbon deposition. Under this steady state reaction conditions, the activities of the catalysts with varying Si/V ratios are compared.
Fig. 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 find out the optimum vanadium content, the vapor phase oxidation of o-xylene was carried out at 573 K on V-MCM-41 catalyst with varying vanadium content (Si/V ratio 25, 50, 75 and 100) and the results are given in Fig. 8. It is observed that the conversion o-xylene increases with Si/V ratio till V-MCM-41 (50). Obviously, more vanadium loading can increase o-xylene conversion because of the increased amount of available active sites. This is revealed from the low intensity of DRUV-Vis spectral bands of V-MCM-41 (25) catalyst around 260 and 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 catalytic activity. However, beyond the Si/V ratio 50, there is a decrease in trend observed with respect to its conversion. This may be because of the lack of dispersion of vanadium even though available in large quantity. The decrease in conversion at high Si/V value may be attributed to the decrease in the concentration of V5? active sites as it is evident from the DRUV-Vis spectra where a decrease in the absorbance intensity is noticed with increase in Si/V ratio from 50 to 100. Hence, the high activity of V-MCM-41 (50) may be attributed to the availability of higher number of V5? in V-MCM-41 (50) than in V-MCM-41 (75) and V-MCM-41 (100).
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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 flow rate of air 0.02 mol h-1
The dispersion and amount of V5? become important in order to account for high conversion of o-xylene. The same trend was registered for the selectivity of phthalic anhydride. The selectivity of o-toluic acid (OTA) remained reverse trend to that of phthalic anhydride selectivity; hence it might be considered as the major intermediate for phthalic anhydride formation as showed in the reaction scheme. Based on the activity study and characteristics of catalysts, the vapor phase oxidation of o-xylene is proposed to take place as suggested in reaction scheme (Scheme 1). According to the scheme, molecular oxygen is activated by framework vanadium. The activated O2 is inserted between carbon and hydrogen bond of the methyl group in o-xylene. The resulting alcohol is rapidly oxidized to o-tolaldehyde which is also subsequently oxidized to o-toluic acid. The same process is also repeated on adjacent methyl group to yield phthalic acid. The product is subsequently oxidized to phthalic anhydride. Comparison of the catalyst supports The 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 to understand the influence of various metals on the oxidation reaction and presented in Fig. 9. Among the three catalysts, it is the V-MCM-41 (50) catalyst that exhibited maximum activity. The reason for the high activity of V-MCM-41 (50) may be due to the availability of silica matrix V5? in MCM-41which is evident from DRUV-Vis
Fig. 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 flow rate of air 0.02 mol h-1; reaction time = 120 min
V O Si
CH2 H CH2 OH
fast CHO CH3 O C
O Scheme 1 Vapor phase oxidation of o-xylene to phthalic anhydride
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% of Conversion & Selectivity
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 flow rate of air 0.02 mol h-1; reaction time = 120 min
spectra (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 the incorporated metal content into the silica matrix. Hence, it is concluded that silica matrix V5? was shown to be more active for the oxidation of o-xylene to phthalic anhydride . Manganese (Mn) incorporated into the MCM-41 is expected to support oxidative dehydrogenation of hydrocarbons because of the presence of successive acidic and redox sites. However, during the oxidative dehydrogenation of o-xylene, the strong aromaticity will be lost significantly. Hence, Mn incorporated MCM-41 does not support the oxidation reaction of o-xylene to phthalic anhydride under these experimental conditions. Finally, based on the literature, it can be understood that the poor activity of V-Mn-MCM-41 may be due to the presence of lower number of silica matrix V5? in V-Mn-MCM-41 catalyst.
Conclusions From 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 100 contains vanadyl ions (VO2?) in the as-synthesized form, whereas on calcination, vanadyl ions (VO2?) is converted into highly dispersed V5? species with tetrahedral coordination. Enhancement of the activity of MCM-41 for the vapor phase oxidation of oxylene is achieved by incorporating vanadium. The high activity of V-MCM-41 (50) for phthalic anhydride formation could be accounted due to the presence of large amount of well dispersed V5? on V-MCM-41. Both UV-Vis DRS and
EPR spectroscopies provide valuable information about the surface structure of V-MCM-41 catalysts. When the activity of vanadium loaded MCM-41 is compared with Mn and bimetal (V&Mn) loaded MCM-41, it is the vanadium that is the most preferred metal for oxidation reaction.
Acknowledgments The authors would like to thank the Defence Research and Development Organization (DRDO) of India for providing financial support.
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Published on Dec 7, 2013