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Ind. Eng. Chem. Res. 2005, 44, 692-701
Supported Metal Oxide Catalysts: Their Activity to Vapor Phase Oxidation of Ethylbenzene S. Vetrivel and A. Pandurangan* Department of Chemistry, Anna University, Chennai 600 025, India
Mesoporous MCM-41 and Al-MCM-41 (Si/Al ) 99 and 158) molecular sieves were synthesized hydrothermally and characterized by low-angle X-ray powder diffraction (XRD), nitrogen adsorption isotherms, FT-IR, and TG-DTA techniques. Monometallic manganese and cobalt oxides and also their bimetallic forms impregnated Si-MCM-41 and Al-MCM-41 catalysts were prepared by wet method and characterized by XRD, AAS, DRS UV-vis, and ESR techniques. The presence of Mn2+ and Co3+ in the mono- and bimetal impregnated catalysts was evident through DRS studies. The analysis with DRS-UV-vis spectroscopy showed independent existence of manganese and cobalt oxides in the bimetallic catalysts. AAS analysis showed higher loading of manganese than cobalt, and the same difference was also observed for bimetal impregnated catalysts. The catalytic activity of the impregnated catalysts was studied for the vapor phase oxidation of ethylbenzene with air (CO2-free) at 250, 300, 350, 375, and 400 °C. The major products were found to be acetophenone, benzaldehyde, and styrene. The ethylbenzene conversion was found to be higher over Mn-MCM-41 than Co-MCM-41. The results with bimetallic catalysts showed the absence of any new chemical compound between manganese and cobalt. The selectivity to acetophenone, which was the principally aimed product in this study, was found to be higher than that of the others. The activities of catalysts follow the order Mn,Co-MCM-41 > Mn,Co-AlMCM-41 (99) > Mn,Co-AlMCM-41 (158) > Mn-MCM-41 > Mn-AlMCM-41 (99) > Mn-AlMCM-41 (158) > Co-MCM-41 > Co-AlMCM-41 (99) > Co-AlMCM-41 (158). The effect of weight hourly space velocity and time on stream was also studied on conversion and products selectivity, and the results are discussed. Introduction Acetophenone is one of the key product in the industries. It is used as a component of perfumes and as an intermediate for the manufacture of pharmaceuticals, resins, alcohols, and tear gas (chloroacetophenone). Benzaldehyde, one of the key products in the oxidation of ethylbenzene is also used as a solvent for cellulose, ethers, and esters, and as a drug to induce sleep. Oxidation of ethylbenzene with hydrogen peroxide (H2O2) or tert-butyl hydroperoxide (TBHP) over bis(acetylacetonate) nickel(II) and µ-oxo dimeric metalloporphyrine catalyst under mild condition has been reported.1-3 However, these catalysts give rise to many problems such as handling, catalyst recovery, and recycling. The classical synthetic laboratory procedures preferably use stoichiometric oxidants such as permanganate or dichromate, which are hazardous. In this context, oxidation of alkyl aromatic compounds over heterogeneous catalysts using cleaner peroxide oxidants is an especially attractive goal. Titanium-substituted silicates usually were thought to catalyze ring hydroxylation of arenes with H2O2 in the liquid phase, but vanadium,4-8 tin,7-10 and chromium10 in a variety of zeolites and aluminophosphate molecular sieves have led to favored oxidation at the side-chain. The transition metal-containing molecular sieves show many interesting properties for various oxidation and ammoxidation reactions.11 With the chromium-substituted aluminophosphate catalysts, formation of ketones from alkyl arenes with * To whom correspondence should be addressed. Tel.: +9144-22203158. Fax: +91-44-2220660. E-mail: pandurangan_a@ yahoo.com.
TBHP as an oxidant in moderate yield with selectivity generally greater than 90%12 was observed. Manganese analogues of these systems have also been shown to catalyze oxidation of alkanes using TBHP.13-15 Most of the oxidations of alkyl aromatics were carried out in the slurry phase using sacrificial oxidants such as H2O2 and TBHP. Manganese anchored on siliceous MCM-41 at a very high coverage was shown to have high activity for propane oxidation.16,17 Niwal Kishor and Ramaswamy have used Ti-, V-, and Si-containing silicates and obtained 62% product distribution of acetophenone in the low-temperature range of 30-50 °C under liquidphase reactions.18 Vetrivel and Pandurangan have used Mn-MCM-41 with Si/Mn ) 29, 56, 73, and 104 for 1043% selectivity to acetophenone prepared in the temperature range of 60-80 °C with tert-butyl hydroperoxide as an oxidant.19 Developing heterogeneous catalysts for vapor phase oxidation of alkyl aromatics using molecular oxygen as an oxidant may be more advantageous, as the process is made continuous. In addition, reaction parameters need to be controlled to enhance conversion and products selectivity. On the basis of these reports, in the present investigation, we studied the vapor phase oxidation of ethylbenzene with air as an oxidant over manganese and cobalt oxide impregnated MCM-41 catalysts. Controlled experiments were also carried out to allow comparison of the results. Experimental Section Materials. The synthesis of MCM-41 material was carried out by the hydrothermal method using sodium metasilicate (Na2SiO3‚5H2O), cetyltrimethylammonium bromide (C16H33(CH3)3N+Br-), aluminum sulfate (Al2-
10.1021/ie049565k CCC: $30.25 © 2005 American Chemical Society Published on Web 01/12/2005
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(SO4)‚18H2O), sulfuric acid (H2SO4), manganese acetate (C4H6MnO4‚4H2O), and cobalt nitrate (Co(NO3)2‚6H2O). To study the oxidation of ethylbenzene to acetophenone, the reagent ethylbenzene was used. The AR grade chemicals were purchased from E-Merck & Co. Synthesis of MCM-41 Catalysts. Al-MCM-41 (Si/ Al ) 99 and 158) samples were synthesized hydrothermally20 using a gel composition of SiO2:XAl2O3:0.2CTAB: 0.89H2SO4:120H2O (X varies with Si/Al ratio). In a typical synthesis, 21.2 g of sodium metasilicate dissolved in 80 mL of deionized water was mixed with an appropriate amount of aluminum sulfate (dissolved in 25 mL of deionized water). This mixture was stirred for 30 min using a mechanical stirrer at a speed of about 250 rpm, and the pH of the solution was adjusted to 10.5 with constant stirring for another 30 min to form a gel. After that, 7.2 g of cetyltrimethylammonium bromide was added drop by drop (30 mL/h) through the syringe infusion pump so that the gel was changed into a suspension. The suspension was transferred into a Teflon-lined steel autoclave and heated to 160 °C for 48 h. After being cooled to room temperature, the product formed was filtered, washed with deionized water, and finally calcined at 550 °C for 6 h in flowing air. Pure MCM-41 was also synthesized in a similar manner without addition of aluminum sulfate. Preparation of Mn and Co Impregnated Catalysts. 50 mL of 0.3 M manganese acetate/cobalt nitrate solution was mixed with 3 g of Si-MCM-41 or Al-MCM41 to prepare the impregnated catalysts with constant stirring. The residue was filtered and gently washed with deionized water to remove metal ions adsorbed on the external surface. The filtrate was dried under reduced pressure and finally calcined in air at 550 °C for 6 h. Physicochemical Characterization. The catalysts were characterized by different instrumental methods to establish the mesostructure and high surface area of the support, and the manganese and cobalt content and oxidation state of mono- and bimetallic catalysts. Elemental Analysis. The aluminum content in AlMCM-41 (Si/Al ) 99 and 158) was determined using ICP-AES with allied analytical ICAP 9000. The Mn and Co content in Si-MCM-41 and Al-MCM-41 was estimated using AAS (GBC 932 plus) by flowing acetylene and air at a rate of 1.85 and 13.1 L/min, respectively. X-ray Diffraction. XRD analysis was performed on a Siemens D5005 diffractometer equipped with liquid nitrogen-cooled germanium solid-state detector using Cu KR radiation. The samples were scanned between 0.5° and 8.5° (2θ) in steps 0.02° with the counting time of 5 s at each point. X-ray tube voltage and current were set at 40 kV and 30 mA, respectively. To protect the detector from the high energy of the incident and diffracted beams, slits were used in this work. Nitrogen Physisorption Isotherms. ASAP-2010 volumetric adsorption analyzer manufactured by the Micromeritics Corp. (Norcross, GA) was used to determine the specific surface area of the catalytic samples at liquid nitrogen temperature. Before the measurement, each sample was degassed at 623 K at 10-5 Torr overnight in an outgassing station of the adsorption apparatus. The specific surface areas of the samples were determined from the linear part of the BET plots. The pore size distribution and wall thickness were calculated from the nitrogen adsorption-desorption
isotherms using the BJH algorithm (ASAP 2010 built in software from Micromeritics). FT-IR Spectroscopy Studies. The catalysts were characterized by FT-IR spectroscopy in a Bruler IFS 66 v spectrometer using the potassium bromide (KBr) pellet technique. Potassium bromide powder containing 1 wt % sample was pressed at 500 kg cm-2 into a thin wafer with an effective thickness of 90 mg cm-2. The spectra were recorded at room temperature in the region of 4000-400 cm-1 TG-DTA Studies. Thermogravimetric-differential thermal analysis of the as-synthesized samples was examined on a Rheometric scientific (STA 15 H+) thermobalance. 10-15 mg of as-synthesized MCM-41 sample was placed in a platinum pan and heated from room temperature to 1000 °C at a heating rate of 20 K min-1 in air with a flow rate of 50 mL/min. The data were collected at 30 s intervals using an on-line PC. DRS UV-Vis Spectroscopy. The coordination environment of manganese- and cobalt-containing MCM41 samples was examined by diffuse reflectance UVvis spectroscopy. The spectra were recorded between 200 and 800 nm on a Shimadzu UV-vis spectrophotometer (model 2101 PC) using BaSO4 as the reference. The spectra were recorded in air at room temperature. The sample was pressed at 100 kg cm-2 into a thick wafer with an effective thickness of about 1 cm. ESR Spectroscopy. The manganese-containing MCM-41 catalysts were analyzed by the ESR technique (Varian E112 spectrometer operating in the X-band 9-GHz region). DPPH was used as the reference to mark the g value. The relative ESR intensities were calculated by double integration of the recorded ESR signal. Experimental Procedure for Oxidation of Ethylbenzene. The oxidation of ethylbenzene with air was carried out in a fixed bed downflow quartz reactor under atmospheric pressure in the temperature range of 250400 °C. Prior to the reaction, the reactor packed with 0.3 g of the catalyst sample was preheated in a tubular furnace equipped with a thermocouple. The ethylbenzene was fed into the reactor using a syringe infusion pump at a predetermined flow rate. The oxidation of ethylbenzene was carried out, and the products mixture was collected for a time interval of 1 h. The products were analyzed by gas chromatography (Hewlett-Packard 5890A) equipped with a flame ionization detector and PONA column. The identification of products was also performed on a Shimadzu GC-MS-QP1000EX gas chromatograph-mass spectrometer. No significant ethylbenzene conversion was observed when the reaction was carried out without catalyst, indicating that there is no thermal effect on conversion. All of the catalysts were regenerated by burning away the coke deposit formed on them from the previous reaction temperature by passing a stream of pure dry air at a temperature of 500 °C for 6 h. The catalysts were used continuously to study the effect of various parameters, temperature, weight hourly space velocity, and time on stream. Result and Discussion Elemental Analysis. Table 1 gives the ICP-AES values obtained for the Al-MCM-41 catalysts. It can be seen that the Si/Al molar ratio of the sample is almost the same as that of the gel composition. The results of AAS analysis to determine the manganese and cobalt content of the materials are given in the same table.
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Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 Table 2. Structure Parameters for the Al-, Mn-, and Co-Containing MCM-41-type Materialsa unit-cell parameter (Å)
d100 spacing value (Å) catalysts
A
B
C
A
B
C
Si-MCM-41 40.16 36.21 46.37 41.81 Al-MCM-41 (99) 38.75 36.82 44.74 42.52 Al-MCM-41 (158) 38.08 37.44 43.97 43.23 Mn-MCM-41 43.06 49.72 Mn-AlMCM-41 (99) 48.12 55.50 Mn-AlMCM-41 (158) 47.21 54.50 Co-MCM-41 42.89 49.53 Co-AlMCM-41 (99) 36.82 42.52 Co-AlMCM-41 (158) 38.42 44.36 Mn,Co-MCM-41 61.30 70.84 Mn,Co-AlMCM-41 (99) 50.08 57.83 Mn,Co-AlMCM-41 (158) 51.67 59.66 a
Figure 1. XRD of (A) as-synthesized, (B) calcined (a) Si-MCM41, (b) Al-MCM-41 (99), and (c) Al-MCM-41 (158), and (C) after impregnation (a) Co-MCM-41, (b) Co-AlMCM-41 (99), (c) Co-AlMCM-41 (158), (d) Mn-MCM-41, (e) Mn-AlMCM-41 (99), (f) Mn-AlMCM-41 (158), (g) Mn,Co-MCM-41, (h) Mn, Co-AlMCM-41 (99), and (i) Mn, Co-AlMCM-41 (158). Table 1. Elemental Analysis of Al-, Mn-, and Co-Containing MCM-41 Catalysts AAS-manganese content catalysts Si-MCM-41 Al-MCM-41 (99) Al-MCM-41 (158) Mn-MCM-41 Mn-AlMCM-41 (99) Mn-AlMCM-41 (158) Co-MCM-41 Co-AlMCM-41 (99) Co-AlMCM-41 (158) Mn,Co-MCM-41 Mn,Co-AlMCM-41 (99) Mn,Co-AlMCM-41 (158)
theor. exp. (ppm) g value Si/Al ICP (ppm) Mn Co of ESR 100 150
99 158 5.67 5.67 5.67 5.67 5.67 5.67 11.45 11.45 11.45
5.12 5.00 4.98
2.0018 2.0005 2.0018
5.06 4.57 4.39 5.92 4.97 6.28 4.33 3.95 6.55
X-ray Diffraction Study. Figure 1A and B illustrates the X-ray powder diffraction patterns of assynthesized and calcined Si-MCM-41 and Al-MCM-41, respectively. The X-ray diffraction spectrum of MCM41 samples contains a sharp d100 reflection line in the 2θ range 1.9-2.4°. Additionally, two broad peaks at 2θ range 3.6°-4.5° are obtained. These peaks have been attributed to the broadening effect of higher reflection lines due to small size.21 The physicochemical properties of these mesoporous materials are summarized in Table 2. The hexagonal unit cell parameter (ao) was calculated using the formula ao ) 2d100/x3, which was obtained from the peak in the XRD pattern by Bragg’s equation (2d sin θ ) λ, where λ ) 1.54 Å for the Cu KR radiation). The value of ao was equal to the internal pore diameter plus one pore wall thickness. The existence of the same peaks in the Al-MCM-41 catalysts suggests that the long-range order is sustained even after the incorporation of metal. These peaks was broadened and shifted
A ) as-synthesized, B ) calcined, and C ) after impregnation.
slightly to higher angle with increasing metal content, although the hexagonal structure still remained intact. These results suggest that the regularity of the mesoporous structure decreased and the pore size became slightly narrower with the introduction of metals.22 After calcination (Figure 1B), the 100 reflection shifts to a higher value, indicating a contraction of the lattice caused by template removal and subsequent condensation of silanol groups. Figure 1C shows the XRD patterns of the manganese and cobalt impregnated catalysts. The intensity of the patterns due to the 100 plane decreased, and that of the 110 and 200 planes disappeared because of radiation diffusion. This can be attributed to the nanosize of the impregnated particles present in the pores. Textural Property. Nitrogen adsorption-desorption isotherms for the calcined samples and their corresponding pore size distribution calculated using the BJH method based on the adsorption isotherms are presented in Figure 2A-C. It is observed that there are three distinct well-defined stages in the isotherm. The initial increase in nitrogen uptake at low P/Po may be due to monolayer-multilayer adsorption on the pore walls, a sharp steep at intermediate P/Po may indicate the capillary condensation in the mesopores, and a plateau portion at high P/Po may be associated with multilayer adsorption on the external surface of the catalysts. All of the catalysts show a characteristic step around P/Po ≈ 0.3, indicating the mesoporous nature of the materials.23 The sharpness and height of the capillary condensation step are the indications of pore size uniformity. Deviations from the sharp and well-defined pore filling step are the indications of increase in pore size heterogeneity. A steep rise in the adsorbed amount was observed at relative pressures in the range 0.240.32 Pa, being caused by capillary condensation of nitrogen in the mesopores. This rise became gentler and was shifted to lower relative pressure with increasing metal content, which suggests that the pore size was narrowed and distributed. The specific surface area of samples determined by the BET surface area lies in the range of 949-1041 m2/g for Si-MCM-41 and Al-MCM41. The surface area, pore size, pore volume, and wall thickness are given in Table 3. It is observed that as the silicon-to-metal ratio increased for Al-MCM-41, the surface area, pore diameter, and pore volume decreased while the wall thickness increased. FT-IR Spectroscopy. The FT-IR spectral analysis of the catalysts was carried out from 4000 to 400 cm-1
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Figure 3. FT-IR spectra of as-synthesized (a) Si-MCM-41, (b) AlMCM-41 (99), and (c) Al-MCM-41 (158) and calcined (d) Si-MCM41, (e) Al-MCM-41 (99), and (f) Al-MCM- 41 (158).
Figure 2. N2 adsorption isotherm of calcined (A) Si-MCM-41, (B) Al-MCM-41 (99), and (C) Al-MCM-41 (158).
covering the entire middle infrared region, and the spectra are shown in the Figure 3. The as-synthesized samples exhibit adsorption bands around 2930 and 2860 cm-1 corresponding to C-H vibrations of the surfactant molecules, but in the case of calcined samples, the peak disappeared due to removal of the template. The broad band around 3500 cm-1 may be attributed to surface silanols and adsorbed water molecules. The OH2 bends are observed between 1490 and 1650 cm-1. The adsorption bands at 1070 and 1230 cm-1 are due to asymmetric
Figure 4. TG-DTA curve of as-synthesized (a) Si-MCM-41, (b) Al-MCM-41 (99), and (c) Al-MCM- 41 (158).
stretching vibrations of Si-O-Si bridges, while those between 820 and 987 cm-1 are due to defective sites.24 Thermal Analysis. Thermogravimetric analysis of the catalysts shows distinct weight losses that depend on framework composition. Figure 4 exhibits three distinct stages of weight loss. The first weight loss due
Table 3. Physical Property and Thermal Analysis of MCM-41 and Al-MCM-41 Catalysts N2 adsorption isotherm
catalysts
BET surface area (m2 g-1)
pore size (Å)
pore volume (cm3/g)
wall thickness (Å)
total
50-150 °C
150-350 °C
350-550 °C
Si-MCM-41 Al-MCM-41 (99) Al-MCM-41 (158)
949 978 1041
30.1 32.7 33.9
0.95 0.97 0.99
16.27 12.04 10.07
52.36 51.05 52.57
11.43 11.52 11.61
32.49 27.60 29.47
8.44 11.93 11.47
TG-DTA weight loss (wt %)
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Figure 5. DR UV-vis spectra of calcined (a) Mn-MCM-41, (b) Mn-AlMCM-41 (99), (c) Co-AlMCM-41 (99), and (d) Mn,CoAlMCM-41 (99).
to desorption of water ∼11.43-11.61% was observed between 50 and 150 °C. The second stage between 150 and 350 °C, corresponding to a weight loss of ∼32.4929.47%, is due to the decomposition of the surfactant species. Finally, the weight loss of ∼8.44-11.47% from 350 to 550 °C is assigned to condensation of adjacent silanol groups to form a siloxane bond.25 The total weight loss up to 1000 °C of the Si-MCM-41 and AlMCM-41 samples is in the range 52.36-52.57%. However, the distribution of successive weight losses depends on the framework or substituted silicon-to-metal ratio.26 Thus, weight loss was higher in the low metal content of MCM-41 materials than in the high metal content of MCM-41 materials (Table 3). The DTA of the as-synthesized samples was carried out between 30 and 1000 °C at a heating rate of 20 K min-1 in air. The DTA trace of Si-MCM-41 shows two broad exotherms: the first at 230 °C and the second at 450 °C. They are assigned to decomposition of template and condensation of defective sites, respectively. The DTA trace due to Al-MCM-41 shows two exothermic peaks between 200 and 400 °C. The intense first exotherm is due to loss of template bonded to silicon sites, and the second minor exotherm is due to template bonded to aluminum sites. This is the evidence for condensation of defective sites above 400 °C, but it is not as high as Si-MCM41. From this study, it can be revealed that the presence of defective sites is more for Si-MCM-41 than Al-MCM-41. DRS UV-Vis Spectroscopy. The DRS UV-vis analysis of Mn and Co impregnated MCM-41 and AlMCM-41 catalysts was carried out between 200 and 800 nm covering the entire ultra violet and visible region. The spectra are presented in Figure 5. Mn-MCM-41 and Mn-AlMCM-41 produce a less resolved absorption band with maxima at 270 and 500 nm. These absorption maxima coincide with the reports in the literature.27 As Mn is in the nonframework, it is to have an octahedral environment of oxygen. As Mn is in the +2 oxidation state, it is to have an 6S ground term. As 6S does not have crystal field components, the electronic excitations are forbidden, but there may be spin-orbit interactions, as reported in the literature28 with which some transitions may have allowance. In conformation of this, in the DRS UV spectra of Mn-MCM-41 and Mn-AlMCM41, the absorption bands are absorbed. Yet they cannot be due to 6A1g f 4T2g and charge-transfer transition as reported in the literature.29,30 The DRS UV spectrum of Co-AlMCM-41 is shown in the same figure. There are
Figure 6. ESR spectra of calcined (a) Mn-MCM-41, (b) MnAlMCM-41 (99), and (c) Mn-AlMCM-41 (158).
two absorption maxima, one at 280 and the other at 580 nm corresponding to cobalt in the octahedral environment. The latter one is assigned to the 5T2g f 5Eg transition, and the former is assigned to charge transition. The absorption shoulder between 300 and 400 nm is assigned to the electronic transition of Co3+ in a disordered tetrahedral environment. The DRS UV spectrum of Mn,Co-AlMCM-41, shown in the same figure, exhibits absorption maxima, which are the combination of absorptions due to Mn-Al-MCM41 and Co-AlMCM-41. On the basis of this observation, it could be concluded that both Mn and Co oxides are of separate independent moieties without forming any new chemical compound. ESR Spectroscopy. Figure 6 shows X-band ESR spectra of calcined samples at liquid nitrogen temperature. There are six hyperfine lines centered around g ) 2.00 (Table 1) corresponding to Mn2+ in octahedral environment. Similar observations were noted for MnMCM-4131 and MnAPO-532 with the Mn species located at nonframework positions. The splitting of the sextet increases from 3010 to 3550 G. Meanwhile, the peakto-peak line-width increases and the line height decreases. These observations indicate that the Mn2+ ions are strongly interacting with their environment in the octahedral coordination, which is consistent with nonframework position.33 Catalytic Oxidation of Ethylbenzene. The vapor phase oxidation of ethylbenzene with air was carried out at 250, 300, 350, 375, and 400 °C over Mnimpregnated siliceous MCM-41, Al-MCM-41 (99), and Al-MCM- 41 (158), and the major products were found to be acetophenone, benzaldehyde, and styrene. Ethylbenzene conversion increased with an increase in temperature from 250 to 350 °C but decreased above 350 °C (Figure 7A). The decrease above 350 °C is due to blocking of active sites by coke. The selectivity to acetophenone showed a trend similar to that of conversion (Figure 7B). The selectivity to benzaldehyde decreased from 250 to 350 °C, but above 350 °C benzaldehyde was not observed. Styrene was observed only above 350 °C, and the selectivity increased with an increase in temperature. The increase in selectivity for acetophenone and the decrease of the same for benz-
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Figure 7. Effect of temperature on the (A) conversion of ethylbenzene, (B) selectivity of acetophenone, (C) selectivity of benzaldehyde, and (D) selectivity of styrene over Mn-MCM-41, Mn-AlMCM-41 (99), Mn-AlMCM-41 (158), Co-MCM-41, Co-AlMCM-41 (99), Co-AlMCM41 (158), Mn,Co-MCM-41, Mn,Co-AlMCM-41 (99), and Mn,Co-AlMCM-41 (158).
aldehyde from 250 to 350 °C illustrate the presence of the same intermediate to produce these products. The intermediates to yield acetophenone and benzaldehyde are indicated in the following reaction scheme:
Oxygen is chemisorbed on the metal ion to produce metal peroxide. It abstracts hydrogenatom from ethylbenzene to yield an alkyl free radical, which in turn reacts with free molecular oxygen to produce peroxide radical. The peroxide radical abstracts hydrogen from metal hydroperoxide or ethylbenzene to yield alkyl peroxide. The alkyl peroxide rapidly decomposes to yield benzaldehyde and methanol by route (I), and acetophenone and water by route (II). Formation of styrene can be explained through dehydrogenation of ethylbenzene over the metal oxides. As it is an endothermic reaction, it is more favored at higher temperatures (Figure 7D) and is illustrated in the scheme (b). Ethylbenzene gets chemisorbed on the surface of metal oxides and is dehydrogenated to styrene. To verify this direct dehydrogenation of ethylbenzene to styrene, the reaction was also studied with an air-free feed of ethylbenzene at 400 °C with the flow rate of 1.5 mL/h. The product mixture contains 18.6% styrene, supporting the direct dehydrogenation of ethylbenzene to styrene over the metal oxide. Absence of styrene at 250 and 300 °C confirms the activation requirement of the reaction. A drastic decrease in selectivity to benzaldehyde is observed with an increase in temperature over all of the catalysts (Figure 7C). Formation of benzaldehyde would increase with increasing temperature because it is formed through self-decomposition of alkyl hydroperoxide [D]. The decrease in selectivity with increase in temperature therefore suggests the influence of catalyst
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Table 4. Oxidation of Ethylbenzene: Variation with Different Catalystsa
catalysts
conversion of ethylbenzene (wt %)
acetophenone
Mn-MCM-41 Mn-AlMCM-41 (99) Mn-AlMCM-41 (158) Co-MCM-41 Co-AlMCM-41 (99) Co-AlMCM-41 (158) Mn,Co-MCM-41 Mn,Co-AlMCM-41 (99) Mn,Co-AlMCM-41 (158)
67.8 52.8 44.7 59.9 50.0 47.9 78.3 73.8 65.8
93.6 91.2 89.9 85.8 82.4 78.5 94.5 92.4 89.7
products selectivity (%) benzaldehyde styrene 3.9 6.1 7.0 7.1 8.5 10.1 2.9 3.9 4.2
2.4 2.7 3.1 5.2 7.0 8.1 2.6 3.4 6.1
others 0.1 0 0 1.9 2.1 3.3 0 0.2 0
a Reaction conditions: 0.3 g of catalyst, reaction temperature 350 °C, TOS (time on stream) ) 1 h, WHSV ) 5.7 h-1 for reactant and 0.021 mol h-1 of molecular oxygen from CO2 free air.
Figure 8. Effect of WHSV on the (A) conversion of ethylbenzene, (B) selectivity of acetophenone, (C) selectivity of benzaldehyde, and (D) selectivity of styrene over Mn-MCM-41, Mn-AlMCM-41 (99), Mn-AlMCM-41 (158), Co-MCM-41, Co-AlMCM-41 (99), Co-AlMCM-41 (158), Mn,Co-MCM-41, Mn,Co-AlMCM-41 (99), and Mn,Co-AlMCM-41 (158).
on the decomposition of alkyl hydroperoxide to benzaldehyde. Formation of benzaldehyde and acetophenone on the catalyst surface may be understood by the following reaction scheme:
Formation of acetophenone requires chemisorption of alkyl hydroperoxide on the Lewis acidic and basic sites of the catalyst, as shown in scheme (c). Appropriate cleavage of bonds gives acetophenone and water. Formation of benzaldehyde requires chemisorption of alkyl
hydroperoxide on Lewis acid sites alone, as shown in scheme (d). On the basis of this requirement, one may infer that formation of acetophenone could occur on the plain surface and benzaldehyde on the edges and corners where the Lewis acid sites might be left with more coordinated unsaturation. The results obtained over Mn-AlMCM-41 (99) and Mn-AlMCM-41 (158) show a similar trend for conversion and products selectivity. Although the conversion trend over these two catalysts is similar to that of Mn-MCM41, the magnitude is slightly less particularly at 350 °C. The slight decrease in conversion is attributed to the low content of manganese. The selectivity to acetophenone is not significantly affected over these catalysts, but the selectivity to benzaldehyde is increased at 250 °C. The magnitude of selectivity to styrene at various reaction temperatures remains al-
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Figure 9. Effect of time on stream on the (A) conversion of ethylbenzene, (B) selectivity of acetophenone, (C) selectivity of benzaldehyde, and (D) selectivity of styrene over Mn,Co-MCM-41, Mn,Co-AlMCM-41 (99), and Mn,Co-AlMCM-41 (158).
most the same except at 400 °C. From these results, it can be concluded that the nature of the support has a negligible effect on ethylbenzene conversion and products selectivity. Under the same conditions, the reaction was also studied over Co-MCM-41, Co-AlMCM-41 (99), and CoAlMCM-41 (158). The ethylbenzene conversion and products selectivity are illustrated in Figure 7A-D. The conversion exhibits a trend similar to that observed with Mn catalyst: the conversion increases from 200 to 350 °C but decreases thereafter. As compared to Mn catalysts, the Co catalyst exhibits slightly low conversion, which may be due to either less loading or the large size of cobalt oxide as compared to Mn catalysts. AAS analysis of these catalysts shows a level of loading (Table 1) almost similar to that of Mn; therefore, a small decrease in conversion might be attributed to the large size of cobalt oxides. Yet this proposed increment in size might not be detrimental to the reaction, as the conversion is not greatly decreased. The selectivity to the products also exhibits a variation with increase in temperature similar to that of Mn catalyst. Hence, the mechanism dealt for Mn catalysts (scheme a) might be similarly operating over Co catalysts. The reaction was also studied with binary Mn and Co oxide impregnated catalysts, Mn,Co-MCM-41, Mn,Co-AlMCM-41 (99), and Mn,Co-AlMCM-41 (158), and the results are illustrated in Figures 7A-D. The conversion and products selectivity are higher (Table 4) than the mono metal oxide, Mn or Co oxide, at each temperature. It is attributed to higher loading of metal
oxides. Hence, it can be suggested that the metal oxide particles are to be still in a state of fine dispersion to exhibit activity similar to that of either Mn or Co oxide impregnated catalysts. The selectivity to acetophenone, benzaldehyde, and styrene appears to have a trend similar to that obtained with mono metal oxide catalysts. Hence, the mechanism of the reaction still remains the same with these catalysts too. Therefore, the active sites of the catalyst may not be different from the mono metal oxide catalysts, suggesting the absence of any new chemical compound formation in bimetallic oxide catalysts. It is also evident in the DRS-UV-visible spectra analysis. The metal oxide particles therefore may retain their identity in the range of temperatures studied. Effect of WHSV. The effect of WHSV on conversion and products selectivity was studied at 350 °C over all of the catalysts at 3.8, 5.77, 7.69, and 9.6 h-1. The conversion over Mn-MCM-41 increased when WHSV was increased from 3.8 to 5.77 h-1, but above 5.77 h-1 it decreased. The increase in conversion from 3.8 to 5.77 h-1 (Figure 8A) is due to enhanced adsorption of ethylbenzene and oxygen on the active surface. The selectivity to acetophenone shows a high value at 5.77 h-1 (Figure 8B), but in other WHSVs the value is less. The decrease at high WHSV is due to an increase in the rate of diffusion of ethylbenzene at 3.8 h-1, but the selectivity is less, thus contradicting our previous view. Therefore, more transport of ethylbenzene yields styrene, and more transport to edges yields benzaldehyde. The selectivity to benzaldehyde decreased at 5.77 h-1, but above this it increased (Figure 8C). This might
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be due to more transport of ethylbenzene for hydrogen abstraction, and subsequent peroxide formation and decomposition at the edges and corners as compared to a plain surface. The selectivity to styrene decreased with an increase in WHSV (Figure 8D), due to the fast rate of diffusion of ethylbenzene through the pores. The results obtained over Mn-AlMCM-41 (99) also indicate similar trends. Conversion is found to be more at 5.77 h-1 than at other WHSV. Selectivity to acetophenone is high at 5.77 h-1 as compared to other WHSV. The selectivity to acetophenone, benzaldehyde, and styrene exhibits a similar trend. The selectivity to acetophenone is more at 5.77 h-1, but that of benzaldehyde decreased from 29.6 to 6.1 and then increased. The selectivity to styrene shows a decrease with increase in WHSV. Although conversion is less over Mn-AlMCM-41 (158) than Mn-MCM-41, a higher selectivity to styrene is observed. Because at WHSV 5.77 h-1 conversion is maximum with a high yield of acetophenone, this value is kept as the optimum WHSV for this reaction. The effect of WHSV on conversion and products selectivity was investigated over Co-MCM-41, CoAlMCM-41 (99), and Co-AlMCM-41 (158). The results are illustrated in Figures 8A-D. The conversion at 3.85 h-1 is low over all of the catalysts, but at 5.77 h-1 it is high. It is followed by lower values at 7.67 and 9.61 h-1. The selectivity to acetophenone, benzaldehyde, and styrene exhibits a trend similar to that of Mn catalyst. The results of the effect of WHSV on conversion studied over Mn,Co-MCM-41, Mn,Co-AlMCM-41 (99), and Mn,Co-AlMCM-41 (158) are also illustrated in the same figures. The conversion is higher for each WHSV than the mono metal oxide catalysts, so their cumulative effect due to their independent existence on conversion is clearly evident. The selectivity to the products exhibits a variation similar to that of single metal oxide catalysts with an increase in WHSV. Time on Stream. The effect of time on stream on conversion and products selectivity was studied for 5 h. The study was carried out at 350 °C with the flow rate of ethylbenzene 1.5 mL/h and 0.021 mol h-1 of molecular oxygen from CO2-free air. The catalyst exhibits a decrease in conversion with stream (Figure 9A). Nearly a 50% decrease is observed for each catalyst. The selectivity to acetophenone decreases gradually with increase in stream (Figure 9B), but the selectivity to benzaldehyde and styrene increases with increase in stream (Figure 9C and D). Hence, the active sites that produce these products are suggested to be not deactivated by coke deposition. Therefore, the deactivation of catalyst by coke might be better pronounced over the plain surface than the edges and corners of the particles. This is to be true as the coke precursors with their larger size prepare to rest on a plain surface as it provides more surface area than the edges or corners. Conclusions This study illustrates that the manganese- and cobalt oxides-supported MCM-41 and Al-MCM-41 catalysts are active to fuctionalization of ethylbenzene in the vapor phase using air as the oxidant. This study is more advantageous than liquid-phase reactions that use peroxide oxidants, is continuous, and is very simple to carry out. The major product is acetophenone, and benzaldehyde and styrene are the side products. The study with bimetallic oxide catalyst, manganese and
cobalt oxide, illustrated that the metal oxides are to have independent existence without forming any new chemical compound. The reaction was repeated thrice at 350 °C to examine whether there was sintering of metal oxides, but the conversion remained the same for the three experiments, illustrating avoidance of sintering. As MCM-41-supported metal oxide catalysts give more conversion than Al-MCM-41-supported catalysts, the former may be suggested to be a better support than the latter. It is the more hydrophobic force of Si-MCM41 that enhances metal oxide dispersion rather than oglomerization. Literature Cited (1) Alcantara, R.; Canoiva, L.; Guilharme-Joao, P.; Maria Santos, J.; Vazquaz, I. Ethylbenzene oxidation with air catalysed by bis(acetylacetonate)nickel (II) and tetra-n-butylammonium tetrafluoroborate. Appl. Catal., A 2000, 203, 259. (2) Alcantara, R.; Canoiva, L.; Guilharme-Joao, P.; Pedro PerezMendo, J. Air oxidation of ethylbenzene catalysed by bis(acetylacetonate)nickel (II) and 1-n-butyl-3-methylimidazolium hexafluorophosphate. Appl. Catal., A 2001, 218, 269. (3) Gue, C.; Peng, Q.; Liu, Q.; Jiang, G. Selective oxidation of ethylbenzene with air catalyzed by simple µ-oxo dimeric metalloporphyrins under mild conditions in the absence of additives. J. Mol. Catal. A: Chem. 2003, 192, 295. (4) Reddy, K. R.; Ramaswamy, A. V.; Ratnasamy, P. Studies on Crystalline Microporous Vanadium Silicates: IV. Synthesis, Characterization, and Catalytic Properties of V-NCL-1, a LargePore Molecular-Sieve. J. Catal. 1993, 143, 275. (5) Sen, T.; Chatterjee, M.; Sivasankar, S. Novel large-pore vanadium alumino- and boro-silicates with BEA structure. J. Chem. Soc., Chem. Commun. 1995, 207. (6) Bhanumik, A.; Dongare, M. K.; Kumar, R. Synthesis of MTW-type microporous material and its vanadium-silicate analogue using a new diquaternary ammonium cation as a template. Microporous Mater. 1995, 5, 173. (7) Selvam, T.; Singh, A. P. Single step selective oxidation of para-chlorotoluene to para-chlorobenzaldehyde over vanadium silicate molecular sieves. J. Chem. Soc., Chem. Commun. 1995, 883. (8) Reddy, K. R.; Ramaswamy, A. V.; Ratnasamy, P. Synthesis and characterization of a large pore vanadium-containing molecular sieve, V-NCL-1. J. Chem. Soc., Chem. Commun. 1992, 1613. (9) Mal, N. K.; Veda R.; Gonaphathy, S.; Ramaswamy, A. V. Synthesis of tin-silicalite molecular sieves with MEL structure and their catalytic activity in oxidation reactions. Appl. Catal. 1995, 125, 223. (10) Chen, J. D.; Sheldon, R. A. Selective Oxidation of Hydrocarbons with O2 over Chromium Aluminophosphate-5 MolecularSieve. J. Catal. 1995, 153, 1. (11) Singh, P. S.; Kosuge, K.; Ramaswamy, V.; Rao, B. S. Characterization of MeAPO-11s synthesized conventionally and in the presence of fluoride ions and their catalytic properties in the oxidation of ethylbenzene. Appl. Catal., A 1999, 177, 149. (12) Rogovin, M.; Neumann, R. Silicate xerogels containing cobalt as heterogeneous catalysts for the side-chain oxidation of alkyl aromatic compounds with tert-butyl hydroperoxide. J. Mol. Catal. A: Chem. 1999, 138, 315. (13) Tetrad, D.; Rabion, A.; Verlhac, J. B.; Guilhem, J. Alkane hydroxylation by a manganese analogue of the iron core from methane monooxygenase. J. Chem. Soc., Chem. Commun. 1995, 531. (14) Menage, S.; Collomb-Dunand-Sauthier, M. N.; Lambeaux, C.; Fontecave, M. Manganese (II) based oxidation of alkanes: generation of a high valent binuclear catalyst in situ. J. Chem. Soc., Chem. Commun. 1994, 1885. (15) Vincent, J. M.; Menage, S.; Lambeaux, C.; Fontecave, M. Oxidation of alkanes catalyzed by binuclear metal complexes: Control by the coordination sphere. Tetrahedron Lett. 1994, 35, 6287. (16) Burch, R.; Cruise, N. A.; Gleeson, D.; Tsang, S. C. Surfacegrafted manganese-oxo species on the walls of MCM-41 channels-a novel oxidation catalyst. J. Chem. Soc., Chem. Commun. 1996, 8, 951.
Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 701 (17) Burch, R.; Cruise, N. A.; Gleeson, D.; Tsang, S. C. Extended X-ray absorption fine structure study of manganese-oxo species and related compounds on the surface of MCM-41 channels. J. Mater. Chem. 1998, 8, 227. (18) Nawal Kishor, M.; Ramaswamy, A. V. Oxidation of ethylbenzene over Ti-, V- and Sn-containing silicalites with MFI structure. Appl. Catal., A 1996, 143, 75. (19) Vetrivel, S.; Pandurangan, A. Side-chain oxidation of ethylbenzene with tert-butylhydroperoxide over mesoporous MnMCM-41 molecular sieves. J. Mol. Catal. A: Chem. 2004, 217, 165. (20) Selvaraj, M.; Pandurangan, A.; Seshadri, K. S.; Sinha, P. K.; Krishnasamy, V.; Lal, K. B. Comparison of mesoporous AlMCM-41 molecular sieves in the production of p-cymene for isopropylation of toluene. J. Mol. Catal. A: Chem. 2002, 186, 173. (21) Tanev, P. T.; Pinnavaia, T. J. A Neutral Templating Route to Mesoporous Molecular Sieves. Science 1995, 267, 865. (22) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; Mccullen, S. B.; Higgins, J. B.; Schlinker, J. A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates. C. J. Am. Chem. Soc. 1992, 114, 10834. (23) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: New York, 1982. (24) Kiseler, A. V.; Lygin, V. I. Infrared Spectrum of Surface Compounds and Adsorbed Substances; Nauka: Moscow, 1992 (in Russian). (25) Occelli, M. L.; Biz, S.; Auroux, A.; Ray, G. J. Effect of the nature of the aluminium source on the acidic properties of some mesostructure materials. Microporous Mesoporous Mater. 1998, 26, 193.
(26) Chen, C.-Y.; Li, H.-X.; Davis, M. E. Studies on Mesoporous Molecular Materials. 1. Synthesis and Characterization of MCM41. Microporous Mater. 1993, 2, 17. (27) Zhang, Q.; Wang, Y.; Itsuki, S.; Shishido, T.; Takehira, K. Manganese-containing MCM-41 for epoxidation of styrene and stilbene. J. Mol. Catal. A: Chem. 2002, 188, 189. (28) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley Eastern Private Limited: New Delhi, 1966. (29) Velu, S.; Shah, N.; Jyothi, T. M.; Sivasankar, S. Effect of manganese substitution on the physicochemical properties and catalytic toluene oxidation activities of Mg-Al layered double hydroxides. Microporous Mesoporous Mater. 1999, 33, 61. (30) Molella, F.; Gallardo-Amores, J. M.; Baldi, M.; Busca, G. A study of Mn-Ti oxide powders and their behaviour in propane oxidation catalysis. J. Mater. Chem. 1998, 8, 2525. (31) Xu, J.; Luan, A.; Wasowiscz, T.; Kevan, L. ESR and ESEM studies of Mn-containing MCM-41 materials. Microporous Mesoporous Mater. 1998, 22, 179. (32) Levi, Z.; Raitsimring, A. M.; Goldfrab, D. ESR and electron spin-echo studies of MnAPO5. J. Phys. Chem. 1991, 95, 7830. (33) Vetrivel, S.; Pandurangan, A. Vapour phase oxidation of ethylbenzene with air over Mn-containing MCM-41 mesoporous molecular sieves. Appl. Catal., A 2004, 264, 243.
Received for review May 20, 2004 Revised manuscript received October 1, 2004 Accepted November 12, 2004 IE049565K
