Silicotungstic Acid Impregnated MCM-41-like Mesoporous Solid Acid

May 13, 2008 - Chemical Engineering Department, Middle East Technical University, Ankara Turkey, and Chemical Engineering Department, Gazi University,...
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Ind. Eng. Chem. Res. 2008, 47, 4071–4076

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Silicotungstic Acid Impregnated MCM-41-like Mesoporous Solid Acid Catalysts for Dehydration of Ethanol Dilek Varisli,† Timur Dogu,*,† and Gulsen Dogu‡ Chemical Engineering Department, Middle East Technical UniVersity, Ankara Turkey, and Chemical Engineering Department, Gazi UniVersity, Ankara Turkey

In this study, mesoporous nanocomposite silicotungstic acid (STA) incorporated MCM-41 and mesoporous aluminosilicate catalysts with narrow pore size distributions, in the range of 2.5–3.5 nm, were successfully synthesized following different impregnation procedures. Results showed that the catalyst preparation procedure had significant influence on its activity as well as the product distribution in ethanol dehydration. STAMCM41C catalyst, which was prepared by the impregnation of STA into calcined MCM-41 containing a W/Si ratio of 0.24, and STAMAS catalyst, which was prepared by the impregnation of STA into mesoporous aluminosilicate, showed very high activities in dehydration of ethanol. Ethylene yield showed an increasing trend with temperature, reaching to about 100% above 250 °C. In contrast to ethylene, DEE was formed at lower temperatures, reaching to a yield value of about 70% at 180 °C with STAMCM41C. DEE formation at lower temperatures was concluded to be due to the presence of Bronsted acid sites of this catalyst. 1. Introduction Alcohols and their derivatives are considered as potential alternates to petroleum, to be used as transportation fuels and as feedstock for petrochemical industry.1 Ethanol, which can be produced by fermentation of crop and sugar wastes, is considered as an excellent alternative for gasoline. It has a high octane number (about 113), quite high oxygen content, and high latent heat of vaporization. However, due to low vapor pressure of ethanol, cold starting of ethanol fueled engines creates some problems. Diethylether (DEE), which is produced by a dehydration reaction of ethanol, can be mixed with ethanol to solve this problem.2 Besides being considered as a fuel blending oxygenate, DEE by itself can be substituted for gasoline, since its octane number was even higher than ethanol. With a cetane number (85–96) much higher than conventional diesel fuel (45–55), DEE can also be considered as a potential diesel fuel alternate. Ethylene, which is conventionally produced from petroleum, is an important feedstock for the petrochemical industry. Fast depletion of oil reserves3 initiated new research for the production of ethylene, following new pathways and using nonconventional feedstock.4,5 In our recent study,5 it was illustrated that ethylene could be produced by selective oxidation of ethanol over vanadium incorporated MCM-41 type mesoporous catalytic materials. Both ethylene and DEE can also be produced by dehydration reaction of ethanol over solid acid catalysts.6 Catalysts having acidic property, such as H-Mordenit,7 H-ZSM5, γ-alumina,8 and TiO2/alumina9 were tested in dehydration of ethanol. Mechanism of ethanol dehydration over γ-alumina catalyst was investigated by Golay et al.,8 by in situ characterization of surface intermediates. Due to their very strong acidity, heteropoly acids were also tested as catalysts in dehydration of alcohols.10–12 In our previous work,12 catalytic activities of silicotungstic acid (STA), tungstophosphoric acid (TPA), and molybdophosphoric acid (MPA) were tested in dehydration reaction of ethanol. Among them, STA showed the highest * To whom all correspondence should be addressed. E-maill: tdogu@ metu.edu.tr. † Middle East Technical University. ‡ Gazi University.

activity. This result was attributed to the higher number of protons and better high temperature stability of STA than the other heteropolyacid catalysts. Heteropolyacids show good catalytic activity, but they are nonporous and their surface area values are extremely low (less than 1 m2/g). Besides, they dissolve in polar solvents. Consequently, they are preferred to be used in vapor phase reactions. Catalytic performance of heteropolyacids can be significantly improved by the impregnation of these compounds into high surface area supports. Discovery of silicate structured high surface area mesoporous materials13 with ordered pores started a new pathway in catalysis research. Catalytic activities of such mesoporous materials were significantly improved by the incorporation of metals or metal oxides into their structure.14–17 In the present study, mesoporous solid acid catalysts were prepared by the impregnation of silicotungstic acid into the synthesized MCM-41 and mesoporous aluminosilicate, following different procedures. Activities of the synthesized catalysts were tested in dehydration of ethanol to produce ethylene and DEE. 2. Experimental Details 2.1. Preparation of the Catalysts. MCM-41 and mesoporous aluminosilicate were used as the support materials of the catalysts prepared in this work. Aluminosilicate was obtained from SIGMA; however, MCM-41 was synthesized following a hydrothermal synthesis procedure. Silicotungstic acid (SigmaAldrich) was impregnated either into calcined MCM-41 (STAMCM41C), uncalcined MCM-41 (STAMCM41U), or mesoporous aluminosilicate (STAMAS) in the preparation of the catalysts. Some details of catalyst synthesis procedures are given below. Preparation of MCM-41. MCM-41, which is the support of mesoporous nanocomposite silicotungstic acid H4SiW12O40 (STA) incorporated MCM-41 like catalysts, was prepared following a similar procedure described in our earlier publications.15,16 In this procedure, sodium silicate was used as the Si source and cetyltrimethylammonium bromide as the surfactant. The pH of the synthesis solution was adjusted to 11 and hydrothermal synthesis was carried out at 120 °C for 96 h in a Teflon-lined stainless-steel autoclave. After filtering, washing

10.1021/ie800192t CCC: $40.75  2008 American Chemical Society Published on Web 05/13/2008

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with deionized water and drying, MCM-41 sample was calcined at 550 °C for 8 h in a flow of dry air.18 Preparation of STAMCM41U. Silicotungstic acid was supported on uncalcined MCM-41 in the preparation of STAMCM41U. In this procedure, 1 g of the uncalcined MCM41 and 0.5 g of silicotungstic acid were dispersed in 12 mL of deionized water. STA was completely dissolved in this mixture. This mixture was stirred at room temperature for 51 h, which was supposed to be long enough to attain equilibrium of adsorption–desorption processes, as discussed by Vazquez et al.10 The resultant mixture was dried in an oven at 70 °C, for 24 h. Then, the sample was kept at 96 °C for 21 h and at 120 °C for 2 h, for complete dryness. It was finally calcined at 350 °C for 8 h in a flow of dry air. Preparation of STAMCM41C. In the case of preparation of STAMCM41C, MCM-41 was first calcined at 550 °C, as described above, and then impregnation of STA into the calcined MCM-41 was carried out. For this purpose, 0.5 g of STA was dissolved in deionized water and MCM-41 was added to this solution. This mixture was stirred at 30 °C for 65 h. The resultant mixture was dried at 70 °C for 48 h in order to evaporate the water and then the temperature of the oven was increased, first to 96 °C and then to 120 °C and kept at this temperature for 2 h. The catalyst was then heated up to 350 °C in the reaction system. Preparation of STAMAS. STAMAS was prepared by using mesoporous aluminosilicate as the support material. The Al/Si atomic ratio of this material was reported as 0.03 by the manufacturer. The procedure reported by Nandhini and coworkers19 was modified in the preparation of this catalyst. In this case, 1 g of the aluminosilicate and 0.5 g of silicotungstic acid were dispersed in 10 mL of ethanol, and the solution was stirred at 30 °C for 44 h. The resultant mixture was dried at 80 °C for 48 h in order to evaporate the alcohol. Finally, the sample was calcined at 200 °C for 8 h under a flow of dry air. The catalyst was then heated up to 350 °C in the reaction system. 2.2. Characterization of the Catalysts. In the characterization of the synthesized catalysts, X-ray diffraction (XRD), nitrogen adsorption, pyridine adsorption-DRIFTS, energy dispersive spectroscopy (EDS), and Fourier transform infrared (FTIR) techniques were used. The Rigaku D/MAX2200 diffractometer with a CuK radiation source was used for the XRD analysis. The scanning range of 2θ was set between 1° and 50° with a step size of 0.01°. The average pore size, pore volume, specific surface area, and the pore size distributions were determined by nitrogen adsorption experiments carried out at 77 K. Micromeritics-ASAP2000 and Quantachrome Autosorb equipment were used for this purpose. Chemical compositions of materials were identified by energy dispersive spectroscopy on a JSM-6400 (JEOL) equipped with the NORAN system. Samples were coated with gold for this analysis. FTIR of the synthesized materials were obtained by Bruker FTIR-IFS66/S instrument. In order to detect the presence of acid sites, diffuse reflectance FTIR (DRIFTS) analysis of the pyridine adsorbed samples was carried out by a Perkin-Elmer Spectrum1 FTIR instrument. Catalysts samples were dried at 120 °C before the pyridine adsorption step. DRIFT spectra of the pyridine adsorbed samples were obtained at room temperature and by using 0.08 g of catalyst sample (mixed with 5% KBr) placed into the sample pan of the DRIFTS cell. 2.3. Activity Tests for the Catalysts. Activities of the nanocomposite silicotungstic acid impregnated MCM-41-like mesoporous solid acid catalysts prepared in this work were tested in vapor phase dehydration reaction of ethanol. For this purpose,

Figure 1. XRD patterns of pure MCM-41 and supported catalysts STAMCM41C, STAMCM41U, and STAMAS. Table 1. Physical Properties of the Catalysts total pore lattice volume W/Si d100 parameter (cm3/g) BJH adsp a (nm) sample ID (atomic) (nm)

BET

BJH

MCM-41 STAMCM41C STAMCM41U aluminosilicate STAMAS

1038 310 271 903 690

340 288 1339 823

0.24 0.32 0.13

3.84 3.84 3.94 3.85 3.87

4.43 4.43 4.55 4.44 4.47

1.0 0.35 0.33 0.96 0.81

surface area (m2/g)

a stainless steel tubular reactor having an internal diameter of /4 in. and which was placed into a temperature programmed tubular furnace was used. In each experiment, 0.2 g of fresh catalyst was placed into the middle of this reactor. The catalyst was supported by quartz wool from both sides. The reaction temperature was changed from 180 to 350 °C. Liquid ethanol was pumped by a syringe pump at a flow rate of 2.9 mL/h to an evaporator, which was placed into an oven. To adjust the feed composition, helium was mixed with vapor phase ethanol within the evaporator. The total flow rate of the feed stream was kept at 44.2 mL/min. The products and the unreacted alcohol leaving the reactor were analyzed using a Varian CP 3800 gas chromatograph equipped with a Poropak T column and a thermal conductivity detector (TCD). 1

3. Results and Discussions 3.1. Characterization of Catalysts. The XRD patterns corresponding to the synthesized catalysts are presented in Figure 1. The sharp Bragg peaks observed at 2θ values of 2.24° and 2.30° for STAMCM41U and STAMCM41C, respectively, corresponds to the d100 of the characteristic MCM-41 structure. Two major reflections of these XRD peaks were observed at 2θ values of 3.78°, 4.34° and 3.88°, 4.44°, for STAMCM41U and STAMCM41C, respectively. The corresponding Bragg peaks of pure MCM-41 were observed at 2θ values of 2.30°, 3.90°, and 4.47°. The corresponding Bragg peaks of silicotungstic acid impregnated aluminosilicate catalyst (STAMAS) were observed at 2.28°, 3.96°, and 4.44°. Characteristic d100 values and the lattice parameter values (evaluated from16 a ) 2d100/ 3) of all these materials are given in Table 1. No sharp peaks corresponding to the impregnated silicotungstic acid (STA) were observed in the XRD patterns of STAMCM41C and STAMAS, indicating well dispersion of STA within the mesoporous supports. However, for the catalyst prepared by the impregnation of STA into uncalcined MCM41 (STAMCM41U), some of the peaks of STA were observed at 2θ ranges of 23.2°-24.44° and 33.34°-34.22° (Figure 1). Unlike the XRD pattern of pure STA, these peaks were quite

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Figure 4. Pore size distributions of STAMC41U, STAMCM41C, and STAMAS. Figure 2. FTIR analysis of STAMAS, STAMCM41C, and STAMCM41U.

broad and shifted to smaller angles in the XRD pattern of STAMCM41U. For pure STA, two main XRD peaks were expected at 2θ values of 25.5° and 34.74°. These results indicated that, for STAMCM41U, dispersion of STA within the mesoporous support is not as good as the other two catalysts and larger deformed STA structures were formed within this mesoporous material. EDS analysis of these samples showed that STA was successfully impregnated into the mesoporous supports used in this study. EDS results agreed well with the W/Si ratios in the synthesis solution of these catalysts. For instance, the W/Si ratio of the impregnation mixture and this ratio evaluated from EDS were both 0.24 for STAMCM41C. The W/Si atomic ratios of the catalysts prepared by the impregnation of STA into uncalcined MCM-41 (STAMCM41U) and aluminosilicate (STAMAS) were 0.32 and 0.13, respectively. FTIR analysis of the synthesized catalysts also showed that STA was successfully impregnated into the STAMCM41C and STAMAS (Figure 2). In this figure, FTIR spectrum of pure STA is also given. The characteristic IR bands observed between 800 and 1100 cm-1 (786 cm-1 WsOesW, 876 cm-1 WsOcsW, 921 cm-1 SisO, and 977 cm-1 WdO) correspond to the formation of Keggin structure24 of STA within the synthesized catalysts, especially within STAMCM41C and STAMAS. For STAMCM41C, all four of these characteristic peaks were observed. In the case of STAMAS, the band at 876 cm-1 is not seen. For STAMCM41U, only the bands at 792 and 917 cm-1 are observed with quite low intensity, indicating deformations in the Keggin structure. This result agrees with the conclusions obtained from the XRD

Figure 3. N2 adsorption isotherms for (a) STAMCM41C and (b)STAMCM41U.

analysis. The bands observed 1076 cm-1 with a shoulder at 1227 cm-1 in the FTIR spectra of the synthesized catalysts correspond to the SisOsSi stretching.23 Nitrogen adsorption–desorption isotherms of STAMCM-41C were typical type IV isotherms (Figure 3a), indicating a mesoporous structure. The isotherms of STAMCM41U were also quite close to type IV, with some deviations at relative pressures of nitrogen over 0.4 (Figure 3b). The pore size distributions of these catalysts also indicated a narrower distribution for STAMCM41C than STAMCM41U (Figure 4). For STAMCM41C, most of the pores lie between 2.5 and 3.0 nm and the average pore diameter is 2.76 nm. Although the pore volumes of these two catalysts were quite close (Table 1), the presence of some pores having diameters larger than 4 nm were also observed for STAMCM41U (Figure 4). By the impregnation of STA into the MCM-41 support, the pore volume of the material decreased from 1.0 to about 0.35 cm3/g. In parallel to the decrease of pore volume, surface area values of the materials were also significantly decreased (Table 1). However, the surface area values of the synthesized materials were still quite high (in the order of magnitude of 300 m2/g for the MCM-41 supported catalysts and 691 m2/g for STAMAS) for catalytic applications. Note that the amount of STA imregnated into aluminosilicate (STAMAS) was less than the amount impregnated into MCM-41 (STAMCM41C). Such decrease of pore volume and surface area values indicated plugging of some of the pores of the mesoporous supports by the impregnated STA. The nitrogen adsorption isotherms of pure aluminosilicate and STA impregnated aluminosilicate (STAMAS) were also type IV, indicating a mesoporous structure (Figure 5). The pore size

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Figure 5. Nitrogen adsorption isotherms of (a) pure aluminosilicate and (b) STA impregnated aluminosilicate (STAMAS).

Figure 6. DRIFT spectra of pyridine adsorption on STAMAS, STAMC41C, and STAMCM41U.

distribution of STAMAS (Figure 4) showed that most of the pores lie within the 2.5–3.5 nm range. Differences of DRIFTS results obtained with pyridine adsorbed samples and fresh catalysts (Figure 6) gave information about the Lewis and Bronsted acid sites of these materials. DRIFTS bands observed at 1447 and 1598 cm-1 correspond to the Lewis acid sites.20–22 On the other hand, the bands observed at 1540 and 1640 cm-1 are due to the pyridinium ion adsorbed on the Bronsted acid sites. The band observed at 1489 cm-1 was considered to be due to the contributions of Lewis and Bronsted acid sites. For STAMAS, the relative intensities of the bands corresponding to the Lewis acid sites (1447 and 1598 cm-1) are higher than the corresponding relative intensities of the other catalysts (STAMCM41C and STAMCM41U). This is due to the presence of Al in this catalyst. However, the relative intensity of the band of STAMAS at 1540 cm-1, which corresponds to the Bronsted acid sites, is less than the corresponding relative intensity observed with STAMCM41C. This is due to less amount of STA (less W/Si ratio) imregnated into this material than STAMCM41C. All these results indicated that Lewis acidity of STAMAS was higher than the Lewis acidity of STAMCM41C. However, as for the Bronsted acidity is concerned, acidity of STAMCM41C was higher. Comparison of the acidic characteristics of STAMCM41C and STAMCM41U showed that (Figure 6) STAMCM41U had negligibly low Bronsted acidity and lower Lewis acidity than STAMCM41C. This indicated loss of some of the protons of STA during the impregnation procedure of STAMCM41U. All these results clearly showed the importance of catalyst preparation procedure on its acidic and physical characteristics.

Figure 7. Ethanol conversion at different temperatures with STAMAS, STAMCM41C, and STAMCM41U catalysts (space time: 0.27 s · g · cm-3).

3.2. Results of Ethanol Dehydration Reaction Experiments. Vapor phase ethanol dehydration experiments carried out with a feed stream containing 48% ethanol (in He) and with a space time of 0.27 s · g · cm-3 (with 0.2 g of catalyst packed into the flow reactor) showed that, among the synthesized catalysts, STAMCM41C had very high catalytic activity for ethanol dehydration reaction, even at reaction temperatures as low as 180 °C (Figure 7). Each data point reported in Figure 7 corresponds to the average of at least three successive experiments. Ethanol conversion values reaching to 100% were obtained over 250 °C. The activity of this catalyst was also much higher than pure STA, as reported in our previous publication.12 This is attributed to the 2 orders of magnitude higher surface area of STAMCM41C than pure STA. In the case of STAMCM41U, conversion values were lower than that of obtained by using STAMCM41C (Figure 7). Especially at temperatures lower than 200 °C, very small ethanol conversion values were obtained with this catalyst. The fractional conversion values of ethanol observed at 180 °C were 0.8 and almost zero for STAMCM41C and STAMCM41U, respectively. These results showed that MCM-41 should be calcined before the STA impregnation step for better performance of STA impregnated MCM-41 catalysts. Also, the reaction experiments carried out in a duration of about 8 h showed no deactivation and quite stable activity of these catalysts during this period. The catalytic acitivity of STA impregnated aluminosilicate was also quite high at temperatures over 225 °C. For high temperatures, ethanol conversion values obtained with STAMCM41C and STAMAS catalysts were quite close to each other. However, at temperatures lower than 220 °C, a sharp decrease of activity of STAMAS was observed (Figure 7). As discussed in the catalyst

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Figure 8. Selectivity profiles of products at different temperatures using 0.2 g of STAMCM41C.

Figure 9. Yield of DEE at different temperatures using 0.2 g of STA impregnated MCM-41 catalysts.

characterization section, STAMAS had higher Lewis acidity and lower Bronsted acidity than STAMCM41C. Much higher activity of STAMCM41C than STAMAS observed at temperatures lower than 220 °C was attributed to the higher Bronsted acidity of STAMCM41C. Importance of Bronsted acidity of the solid acid catalysts on their dehydration activity was also reported by Takahara et al.7 As it was concluded in that work, increase of number of Bronsted acid sites of zeolite and silica–alumina catalysts caused an increse in their activity in ethanol dehydration reaction. Diethylether (DEE) and ethylene are the two main products of the ethanol dehydration reactions. 2CH3CH2OH f CH3CH2OCH2CH3 + H2O (ethanol)

Figure 10. Yield of ethylene at different temperatures using 0.2 g of STA impregnated MCM-41 catalysts.

(DEE)

CH3CH2OH f CH2 d CH2 + H2O (ethanol)

(ethylene)

The selectivities of these products are defined as the ratio of moles of ethanol converted to the specific product, to the total moles of converted ethanol. As seen in the selectivity profiles obtained with STAMCM41C (Figure 8), a sharp increase in ethylene selectivity with a corresponding decrease in DEE selectivity was observed with an increase in temperature. At temperatures higher than 250 °C, the main product of the dehydration reaction was ethylene. This figure also indicated that DEE formation started at much lower temperatures, with a quite high selectivity value of about 0.85 at 180 °C. This low temperature activity of STAMCM41C in DEE synthesis is attributed to its high Bronsted acidity. Ethanol and DEE yield values are defined as the ratio of moles of ethanol converted to the specific product to the moles of ethanol fed to the reactor. Yield of each product was calculated by multiplying its selectivity with the overall fractional conversion of ethanol. The variations of the yield values of DEE with reaction temperature are given Figure 9, for the STAMCM41C and STAMCM41U catalysts. Similar to the selectivity behavior, the yield of DEE decreased with an increase in temperature over the STAMCM41C catalyst. However, with STAMCM41U, DEE yield passed through a maximum at around 250 °C, indicating decomposition of formed DEE to ethylene at higher temperatures. Very high ethylene yield values reaching to 1.0 were obtained with the STAMCM41C catalyst at temperatures higher than 250 °C (Figure 10). STAMCM41U gave lower ethylene yield values than STAMCM41C. The maximum ethylene yield value was about 0.7 at 325 °C for STAMCM41U. As reported above, Lewis and especially the Bronsted acidities of STAMCM41U were much less than the corresponding values for STAMCM41C. Similar to the results obtained with STAMCM41C, STA impregnated aluminosilicate catalyst (STAMAS) gave very high ethylene yield values, reaching 1.0, over 260 °C (Figure 11).

Figure 11. Yield of products at different temperatures using 0.2 g of STA impregnated aluminosilicate catalyst (STAMAS).

However, DEE yield values passed through a maximum, as in the case of using STAMCM41U as the catalyst. In this case, the maximum is at a lower temperature (at around 220 °C). All these results indicated that DEE formation at low temperatures was majorly due to the Bronsted acid sites. Lewis acid sites are as effective as Bronsted acid sites in ethylene formation, especially at temperatures over 250 °C. As also mentioned by Golay et al.8 for the mechanism of ethanol dehydration over a γ-alumina catalyst, two different kinds of active sites were expected to be involved during the dehydration reaction of ethanol. At low temperatures, DEE and ethylene formation were probably taking place in parallel by the involvement of Bronsted and Lewis acid sites, respectively. However at higher temperatures, the main product is ethylene, which is also formed by the decomposition of DEE. 4. Conclusions Results of this work showed that very high ethylene yield values, approaching 100%, can be obtained by the dehydration reaction of ethanol at temperatures over 250 °C, using the

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silicotungstic acid impregnated MCM-41 (STAMCM41C) and aluminosilicate (STAMAS) type catalysts synthesized here. STAMCM41C showed very high activity even at temperatures as low as 180 °C, where the main product was DEE. Diethylether is an attractive transportation fuel alternate with very high cetane and octane numbers and clean burning characteristics. It was also shown that Bronsted acid sites are the main contributor of DEE formation at low temperatures, while the increased abondance of Lewis acid sites contribute to the dehydration of ethanol to ethylene. Acknowledgment The financial support of TUBITAK (Project No: 106M073) and DPT (Project No: BAP-03-04-DPT-2003 (06K120920-17)) through the Middle East Technical University Research Fund are gratefully acknowledged. The authors also thank METU Metallurgical Engineering Department for the XRD analysis and METU Central Laboratory for the FTIR analysis. Literature Cited (1) Dogu, T.; Varisli, D. Alcohols as Alternatives to Petroleum for Environmently Clean Fuels and Petrochemicals. Turk. J Chem. 2007, 31, 551. (2) Kito-Borsa, T.; Pacas, D. A.; Selim, S.; Cowley, S. W. Properties of an ethanol diethyl ether water fuel mixture for cold start assistance of an ethanol-fueled vehicle. Ind. Eng. Chem. Res. 1998, 37, 3366. (3) Bentley, R. D. Global oil & gas depletion: An overwiew. Energy Policy 2002, 30, 189. (4) Pereira, C. J. New avenues in ethylene synthesis. Science 1999, 285, 670. (5) Gucbilmez, Y.; Dogu, T.; Balci, S. Ethylene and Acetaldehyde Production by Selective Oxidation of Ethanol Using Mesoporous V-MCM41 Catalyst. Ind. Eng. Chem. Res. 2006, 45, 3496. (6) Mao, R., Le Van.; Nguyen, T. M. Superacidic Catalysts for Low Temperature ConVersion of Aqueous Ethanol to Ethylene. US Patent 4,847,223, July 1989. (7) Takahara, A.; Saito, M.; Inaba, M.; Murata, K. Dehydration of Ethanol into Ethylene over Solid Acid Catalysts. Catal. Lett. 2005, 105, 249. (8) Golay, S.; Doepper, R.; Renken, A. In-situ Characterisation of the Surface Intermediates for the Ethanol Dehydration Reaction over γ-alumina under Dynamic Conditions. Appl. Catal. A: Gen. 1998, 172, 97. (9) Chen, G.; Li S.; Jiao, F.; Yuan, Q. Catalytic Dehydration of Bioethanol to Ethylene over TiO2/γ-Al2O3 Catalysts in Microchannel Reactors. Catal. Today 2007, 125, 111. (10) Vázquez, P.; Pizzio, L.; Cáceres, C.; Blanco, M.; Thomas, H.; Alesso, E.; Finkielsztein, L.; Lantano, B.; Moltrasio, G.; Aguirre, J. Silica-

supported Heteropolyacids as Catalysts in Alcohol Dehydration Reactions. J. Molec. Catal. A: Chem. 2000, 161, 223. (11) Haber, J.; Pamin, K.; Matachowski, L.; Napruszewska, B.; Poltowicz, J. Potassium and Silver Salts of Tungstophosphoric Acid as Catalysts in Dehydration of Ethanol and Hydration of Ethylene. J. Catal. 2002, 207, 296. (12) Varisli, D.; Dogu, T.; Dogu, G. Ethylene and Diethyl-ether Production by Dehydration Reaction of Ethanol over Different Heteropolyacid Catalysts. Chem. Eng. Sci. 2007, 62, 5349. (13) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered Mesoporous Molecular Sieves Synthesized by a Liquid Crystal Template Mechanism. Nature 1992, 359, 710. (14) Pena, M. L.; Dejoz, A.; Fornes, V.; Rey, F.; Vazquez, M.; Nieto, J. M. L. V-Containing MCM-41 and MCM-48 for the Selective Oxidation of Propane in Gas Phase. Appl. Catal., A 2001, 209, 155. (15) Gucbilmez, Y.; Dogu, T.; Balci, S. Vanadium Incorporated High Surface Area MCM-41 Catalysts. Catal. Today 2005, 100, 473. (16) Sener, C.; Dogu, T.; Dogu, G. Effects of Synthesis Conditions on the Structure of Pd Incorporated MCM-41 Type Mesoporous Nanocomposite Catalytic Materials with High Pd/Si Ratios. Microporous Mesoporous Mater. 2006, 94, 89. (17) Lang, N.; Delichere, P.; Tuel, A. Post Synthesis Introduction of Transitions Metals in Surfactant-Containing MCM-41 Materials. Microporous Mesoporous Mater. 2002, 56, 203. (18) Varisli, D. Kinetic Studies for Dimethyl Ether and Diethyl Ether Production. PhD Thesis, METU, Ankara, 2007. (19) Nandhini,; K, U.; Arabindoo, B.; Palanichamy, M.; Murugesan, V. Al-MCM-41 supported phosphotungstic acid: Application to symmetrical and unsymetrical ring opening of succinic anhydride. J. Molec. Catal. A: Chem. 2006, 243, 183. (20) Damyanova, S.; Fierro, J. L. G.; Sobrados, I.; Sanz, J. Surface Behavior of Supported 12-Heteropoly Acid As Revealed by Nuclear Magnetic Resonance, X-ray PhotoelectronSpectroscopy and Fourier Transform Infrared Techniques. Langmuir 1999, 15, 469. (21) Damyanova, S.; Cubeiro, M. L.; Fierro, J. L. G. Acid Redox Properties of Titania- Supported 12-Molybdophosphates for Methanol Oxidation. J. Molec. Catal. A: Chem. 1999, 142, 85. (22) Lim, S.; Haller, G. L. Preparation of Highly Ordered VanadiumSubstituted MCM-41: Stability and Acidic Properties. J. Phys. Chem. 2002, 106, 8437. (23) Gomez-Cazalilla, M.; Merida-Robles, J. M.; Gurbani, A.; Rodrigues-Castellon, E.; Jimenez-Lopez, A. Characterization and Acidic Properties of Al-SBA-15 Materials Prepared by Post-Synthesis Alumination of a Low-Cost Ordered Mesoporous Silica. J. Solid State Chem. 2007, 180, 1130. (24) Parida, K. M.; Mallick, S. Silicotungstic Acid Supported Zirconia: An Effective Catalyst for Esterification Reaction. J. Molec. Catal. A: Chem. 2007, 275, 77.

ReceiVed for reView February 1, 2008 ReVised manuscript receiVed March 13, 2008 Accepted March 13, 2008 IE800192T