Novel Efficient Mesoporous Solid Acid Catalyst UDCaT-4 - American

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Novel Efficient Mesoporous Solid Acid Catalyst UDCaT-4: Dehydration of 2-Propanol and Alkylation of Mesitylene Ganapati D.Yadav* and Ambareesh D. Murkute Department of Chemical Engineering, University Institute of Chemical Technology (UICT), University of Mumbai, Matunga, Mumbai - 400 019, India Received February 17, 2004. In Final Form: June 25, 2004 A novel mesoporous solid acid catalyst named UDCaT-4 was synthesized by incorporating superacidic centers of persulfated alumina and zirconia into highly ordered and well-defined hexagonal mesoporous silica. The catalyst is well characterized, and its properties are compared with those of bulk persulfated alumina and zirconia (PAZ) by FTIR spectroscopy, X-ray diffraction, Brunauer-Emmett-Teller surface area, pore size analysis, scanning electron microscopy, energy dispersive X-ray spectroscopy, and ammonia temperature-programmed desorption. UDCaT-4 is more acidic than PAZ. The dehydration processes of 2-propanol, diisopropyl ether, and n-propanol were studied independently including a mixture of n-propanol and 2-propanol to throw light on kinetics and mechanism. For alkylation of mesitylene with 2-propanol, UDCaT-4 exhibits superior catalytic activity in comparison with PAZ and also it shows remarkable stability toward coke formation. Kinetic interpretations of the observed rate data are presented for all reactions, and mechanistic models are developed. The results are novel.

1. Introduction Friedel-Crafts alkylation and acylation reactions, practiced in a number of industries, are catalyzed by a variety of acid catalysts. Many of these well-established processes still employ homogeneous acid catalysts such as AlCl3, BF3, TiCl4, H2SO4, and HF in batch reactors using large excess of substrate and hazardous solvents such as nitrobenzene, CS2, halohydrocarbons, HF (both as catalyst and solvent), and so forth, causing problems of corrosion, pollution, loss of selectivity of the desired product, and so on. A relatively high catalyst concentration is needed; often the amounts are more than stoichiometric, making the reactions inherently polluting.1-5 Stringent environmental norms provide an impetus to develop either new synthetic routes with 100% atom economy or novel catalytic materials. Solid catalysts can be tailor-made to give a desired level of activity and be easily removed from the reaction mixture with no residual inorganic contamination of the organic products. Solid acids obviously offer several advantages over existing homogeneous catalysts and could be employed for developing cleaner FriedelCrafts processes. Several catalysts such as clays, zeolites, acid-treated inorganic oxides, cation-exchange resins, and so forth have been employed as possible replacements, among which zeolites have emerged as an alternative. Unfortunately, slow diffusion of bulky reactants through the microporous structure of zeolites makes them relatively poor catalysts and also the stability of zeolites due to coke formation at high temperatures is susceptible.6-12 Thus, to surmount the limitations of zeolites, there is still a tremendous scope for synthesizing catalysts with high * To whom correspondence should be addressed. Tel: +91-222410 2121; 2414 5616 ext 291. Fax: +91-22-2410 2121; 2414 5614. E-mail: [email protected], [email protected]. (1) Olah, G. A. Friedel-Craft and Related Reactions; Wiley Interscience: New York, 1963-1964; Vols. 1-4. Olah, G. A.; Krishanamuri, R.; Surya Prakash, G. K. In Comprehensive Organic Synthesis; Trost, B. M., Ed.; Pergamon: Oxford, 1991; Vol. 3, Chapter 1.8. (2) Clark, J. H.; Macquarrie, D. J. Org. Proc. Res. Develop. 1997, 1, 149. (3) Streekumar, R.; Padamakumar, R. Synth. Commun. 1997, 27, 781. (4) Croft, M. T.; Murphy, E. J.; Wells, R. J. Anal. Chem. 1994, 66, 4459. (5) Gao, Y.; Zhu, Z. N.; Yuan, W. K. Prog. Nat. Sci. 1996, 6, 625.

surface area, uniform porosity in the meso range, and higher acidity for acylation and alkylation reactions of bulky molecules. Among many solid acids other than zeolites, incorporation of superacidity into metal oxide has received considerable attention. Especially sulfated zirconia is the most extensively studied catalyst due to its superacidity.13 However, some of the major problems associated with sulfated zirconia in its bulk form are its low efficiency due to low surface area (∼100 m2/g), rapid deactivation, and relatively poor stability in reactions where water is generated as a coproduct. Incorporation of different transition metals has been attempted to improve the acidity of zirconia,14 but still it suffers from deactivation at high temperatures and there is an inherent lack of ordered mesoporosity. The activity and stability of sulfated zirconia can be improved by incorporating alumina and treatment with ammonium persulfate (e.g., isomerization of n-butane).15 However, the surface area of persulfated alumina and zirconia (PAZ) is lower than that of sulfated zirconia. Thus, new catalysts with superacidity, greater stability, high surface area, and ordered mesoporosity need to be designed to minimize the diffusion resistance for the reaction of bulky molecules. Considerable progress has (6) Cybluski, A.; Moulijn, J.; Sharma, M. M.; Sheldon, R. A. In Fine Chemicals Manufacture Technology and Engineering Book, 1st ed.; Elsevier: Amsterdam, 2001. (7) Pe´rot, G.; Guisnet, M. In International Conference on Precision Process Technology, Perspective for Pollution Prevention; Weijnen, M. P. C., Drinkenburg, A. A. H., Eds.; Kluwer Academic: Delft, 1993; p 157. (8) Spagnol, M.; Gilbert, L.; Alby, D. Ind. Chem. Libr. 1996, 8, 29. (9) Spagnol, M.; Gilbert, L.; Jacquot, R.; Guillot, H.; Tirel, P. J.; LeGovic, M. A. In Proceedings of the Fourth International Symposium on Heterogeneous Catalysis and Fine Chemicals, 8-12 Sept, 1996, Basel; p 92. (10) Metivier, P. In Fine Chemicals through Heterogeneous Catalysis; Sheldon, R. A., van Bekkum, H., Eds.; Wiley: Weinheim, 2001; p 161. (11) Moreau, P.; Finiels, A.; Meric, P. J. Mol. Catal. 2000, 154, 185. (12) Fan, L.; Nakamura, I.; Ishida, S.; Fujimoto, K. Ind. Eng. Chem. Res. 1997, 36, 1458. (13) Yadav, G. D.; Nair, J. J. Microporous Mesoporous Mater. 1999, 33, 1. (14) Hsu, C. R.; Gates, B. C. J. Chem. Soc., Chem. Commun. 1992, 1645. (15) Xia, Y. D.; Hua, W. M.; Tang Y.; Gao, Z. Chem. Commun. 1999, 1899.

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been made in designing new solid acid catalysts in the UDCaT series in our laboratory.16-30 Hexagonal mesoporous silica (HMS) is an excellent mesoporous support,31 and thus it was identified as a support for persulfated alumina and zirconia in order to synthesize a new catalyst called UDCaT-4 possessing high surface area and ordered mesoporosity. To test its activity, alkylation of mesitylene with 2-propanol was chosen as a reaction which generates water as a coproduct and thus, the stability of the catalyst in the presence of water can be really tested. The alkylated product with mesitylene is a promising precursor for a number of industrial chemicals. There is practically no literature on the alkylation of mesitylene with 2-propanol by using solid acids. Supercritical alkylation of mesitylene with propylene is reported and environmentally acceptable,32 but it requires high pressure and appropriate costly instruments and is uneconomical. Hence, it was thought desirable to bring out the novelties of UDCaT-4 in the alkylation of mesitylene by using 2-propanol, and the current paper embraces both the catalyst development and application including kinetics of reactions. It was also pertinent to study independently the kinetics of dehydration of 2-propanol, diisopropyl ether, n-propanol, and also a mixture of n-propanol and 2-propanol to throw light on reaction mechanism and selectivity. 2. Experimental Section 2.1. Chemicals. Zirconium oxychloride, aluminum nitrate (AR grade), aqueous ammonia solution, ammonium persulfate (AR grade), isopropyl alcohol (IPA), toluene, benzyl chloride, and commercial grade (95%) ethanol were procured from M/s. s. d. Fine Chemicals Ltd., Mumbai, India. Mesitylene was procured from Merk, Germany. Tetraethyl orthosilicate (TEOS) (Fluka) was taken as the neutral silica source, and dodecylamine and hexadecylamine (Spectrochem Ltd., Mumbai) as the neutral amine for templating. 2.2. Catalyst Preparation. Persulfated modified alumina and zirconia (PAZ) was prepared according to a method described elsewhere.15 Aqueous ammonia was added dropwise to a mixed solution of ZrOCl2‚8H2O and Al(NO3)3‚9H2O until the pH was in the range of 9-10. Mixed hydroxides of zirconia and alumina were washed with deionized water until a neutral filtrate and the absence of chlorine ion was detected by phenolphthalein and AgNO3 tests. The mixture of zirconium hydroxide and aluminum hydroxide was dried in an oven for at 110 °C for 24 h and crushed to 100 mesh size. It was immersed in 0.5 M ammonium persulfate solution for 30 min to get persulfated Al(OH)3-Zr(OH)4. It was (16) Yadav, G. D.; Kirthivasan, N. J. Chem. Soc., Chem. Commun. 1995, 203. (17) Yadav, G. D.; Doshi, N. S. Appl. Catal., A 2002, 236, 129. (18) Yadav, G. D.; Asthana, N. S.; Kamble, V. S. J. Catal. 2003, 217, 88-99. (19) Yadav, G. D.; Krishnan, M. S. Ind. Eng. Chem. Res. 1998, 27, 3358. (20) Yadav, G. D.; Nair, J. J. J. Chem. Soc., Chem. Commun. 1998, 2369. (21) Yadav, G. D.; Pujari, A. A. Green Chem. 1999, 1, 6. (22) Yadav, G. D.; Sengupta, S. Org. Proc. Res. Dev. 2002, 6, 256. (23) Yadav, G. D.; Thorat, T. S. Ind. Eng. Chem. Res. 1996, 35, 721. (24) Yadav, G. D.; Thorat, T. S. Tetrahedron Lett. 1996, 37, 5405. (25) Yadav, V. M.; Kumbhar, P. S.; Yadav, G. D. In Chemically modified oxide surfaces; Layden, D. E., Ed.; Gordon and Breach: New York, 1989. (26) Yadav, G. D.; Krishnan, M. S.; Pujari, A. A.; Doshi, N. S.; Mujeebur Rahuman M. S. M. Ger. Offen. DE 19,857,314, 2000. (27) Yadav, G. D.; Krishnan, M. S.; Pujari, A. A.; Doshi, N. S.; Mujeebur Rahuman M. S. M. U.S. Patent 6,204,424 B1, 2001. (28) Yadav, G. D.; Goel, P. K.; Joshi, A. V. Green Chem. 2001, 3, 92. (29) Yadav, G. D.; Doshi, N. S. Green Chem. 2002, 4, 528. (30) Yadav, G. D.; Murkute, A. D. J. Catal. 2004, 224, 218-223. (31) 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.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (32) Hitzler, M. G.; Smail, F. R.; Ross, S. K.; Poliakoff, M. Chem. Commun. 1998, 359.

Yadav and Murkute filtered off, dried at 110 °C for 24 h, and thereafter calcined at 650 °C for 3 h to get the catalyst PAZ with 0.6% w/w of alumina (3 mol %). The ordered HMS was prepared according to our earlier work.27 Desired quantities of zirconium oxychloride (2.39 g) and aluminum nitrate (0.11 g) were dissolved in an aqueous solution and added to 5 g of precalcined HMS by the incipient wetness technique. After addition, the solid was dried in an oven at 110 °C for 3 h. The dried material was hydrolyzed by ammonia gas and washed with deionized water until a neutral filtrate was obtained and the absence of chlorine ion in the filtrate was detected by phenolphthalein and silver nitrate tests. It was then dried in an oven for 24 h at 110 °C. Persulfation was carried out by immersing the above solid material in a 0.5 M aqueous solution of ammonium persulfate for 30 min. It was dried at 110 °C for 24 h and calcined at 650 °C for 3 h to get the catalyst called UDCaT-4 with 0.6% w/w of alumina. 2.3. Characterization of Catalyst. Infrared spectra of the samples pressed in KBr pellets were obtained at a resolution of 2 cm-1 between 4000 and 350 cm-1. The spectra were collected with a Perkin-Elmer instrument, and in each case the sample was referenced against a blank KBr pellet. Powder X-ray diffraction (XRD) patterns were obtained using Cu KR radiation (λ ) 1.540 562). Samples were step-scanned from 1 to 40 in 0.045 steps with a stepping time of 0.5 s. Surface area measurements and pore size distribution analysis were done by nitrogen adsorption on a Micromeritics ASAP 2010 instrument at an adsorption temperature of 77 K, after pretreating the sample under a high vacuum at 300 °C for 4 h. Ammonia temperature-programmed desorption (TPD) (Micromeritics Autochem 2920) was used to determine the acid strength of the catalysts. A 0.5 g quantity of catalyst was heated to 600 °C using helium at a flow rate of 30 mL min-1 to remove any adsorbed components. After cooling the sample to room temperature under helium flow, a mixture of predried ammonia and helium was passed over it. Then the reversibly adsorbed ammonia was desorbed in a stream of helium. After ensuring complete desorption of physisorbed ammonia at 100 °C, the temperature was ramped up to 600 °C at a rate of 10 °C/min. A TCD detector was used to measure the ammonia desorption profile. The elemental composition of HMS and UDCaT-4 was obtained by energy dispersive X-ray spectroscopy (EDXS) on a KEVEX X-ray spectrometer. Scanning electron micrographs of HMS and UDCaT-4 were taken on a Cameca SU 30 microscope. The dried samples were mounted on specimen studs and sputter coated with a thin film of gold to prevent charging. The gold-coated surface was then scanned at various magnifications on the scanning electron microscope. 2.4. Reaction Procedure and Analysis. Vapor phase alkylation of mesitylene was conducted in a down flow fixed bed haste alloy HC-276 reactor of 25.4 mm i.d. and 300 mm length at atmospheric pressure, equipped with an upstream vaporizer and a downstream condenser (Chemito, India). Liquid feed containing mesitylene and 2-propanol in the proper proportion was fed by a double piston (Well Chrom HPLC-pump K-120) pump to the vaporizer then into the reactor by using N2 as a carrier gas at a weight hourly space velocity (WHSV) of 20 h-1. For dehydration of 2-propanol, diisopropyl ether, n-propanol, and also a mixture of n-propanol and 2-propanol over UDCaT-4, the effluent was analyzed online by a gas chromatograph (GC1000 Chemito) equipped with a capillary column of 0.22 mm × 25 m and a FID detector. For alkylation of mesitylene with 2-propanol, the effluent was analyzed by a gas chromatograph (GC1000 Chemito) equipped with a stainless steel column (3.8 mm × 4 m) packed with 10% SE-30 on chromosorb WHP and a FID detector (Figure 1). In a typical run, 1 g of catalyst was charged to the reactor and stacked between glass beads and ceramic wool. The reactor was maintained under isothermal conditions during all runs, and the effects of various parameters such as mole ratio, WHSV, reaction temperatures, and W/FA0 (weight of catalyst per unit molar flow rate of reactant) were studied for all reactions. The WHSV here is based on the total flow of gas including carrier

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Figure 1. Schematic diagram of fixed bed catalytic reactor. gas, whereas space time is defined on the basis of the reactant mesitylene alone (W/FAo).

3. Results and Discussion 3.1. Characterization. 3.1.1. Brunauer-EmmettTeller (BET) Surface Area and Pore Size Analysis. The textural characterization of HMS and UDCaT-4 was determined by nitrogen BET surface area and pore size analysis (Table 1). Both HMS and UDCaT-4 display Table 1. Textural Characteristics of HMS, PAZ, and UDCaT-4 Langmuir average pore BET surface surface area pore diameter volume (m2 g-1) (Å) (cm3 g-1) catalysts area (m2 g-1) HMS PAZ UDCaT-4

833 81 233

865 90 364

36 20 30

0.78 0.11 0.21

characteristic type IV adsorption isotherms (Figure 2A,B) with well-defined steps in the N2 adsorption and desorption isotherm at p/p0 ∼ 0.45-0.8 and hysteresis in the desorption isotherm over the same relative pressure range. It reflects uniformity of the pore size distribution, representing spontaneous filling of the mesopores due to capillary condensation in both HMS and UDCaT-4. HMS retains its mesoporosity even after converting it to UDCaT4, and this result is in accordance with IUPAC classification.33 Furthermore, UDCaT-4 displays fairly uniform pore

Figure 2. N2 adsorption isotherm of (A) HMS and (B) UDCaT4.

Figure 3. X-ray diffraction pattern of HMS and UDCaT-4.

size distribution centered at 30 Å and the average pore diameter of UDCaT-4 is reduced marginally to 30 Å from 36 Å for HMS (Table 1), but there was no broadening of the size distribution, indicating PAZ is uniformly dispersed in mesopores of HMS. The reduction in the BET surface area and in the pore volume of UDCaT-4 is much more remarkable than the reduction of the pore size of HMS, indicating in situ formation of nanoparticles of persulfated alumina and zirconia and that large particles can block a few pore junctions thereby reducing accessibility of those channels. Thus, there is a reduction in surface area and pore volume of UDCaT-4. These results are in consonance with earlier literature reports.34 It is obvious that pore volume, pore size, and surface area of UDCaT-4 are far greater than those of unsupported PAZ. Therefore, UDCaT-4 was expected to exhibit greater activity than PAZ. 3.1.2. X-ray Diffraction. The structural integrity of HMS as well as UDCaT-4 was determined with X-ray diffraction. One diffraction peak in the low-angle region (2θ ) 1-10°) is visible, indicating that HMS has a longrange hexagonal ordering (Figure 3A). The structural integrity of HMS is retained even after converting it into UDCaT-4 (Figure 3A). Unsupported bulk PAZ shows the pure tetragonal phase of zirconia in the ordinary region (2θ ) 30° and 50°). However, UDCaT-4 shows reflections of zirconia at 2θ ) 30° and 2θ ) 50° having very low and broad intensities (Figure 3B). These diffraction peaks indicate the presence of tetragonal crystalline zirconia rather than monoclinic zirconia phase in the pores of HMS. The introduction of a small amount of alumina and sulfate ions must have stabilized the tetragonal phase of the zirconia, which is an ideal phase conducive for superacidity in sulfated zirconia.13,30 FTIR spectroscopy and energy dispersive analysis of X-rays (EDAX) further support the previous inference about the introduction of sulfate ions on UDCaT-4. XRD, BET surface area, and pore size analysis provide an explanation for entrapment of PAZ in mesoporous HMS. During the synthesis of UDCaT-4, growth of crystalline zirconia and alumina is very unlikely because XRD and BET surface area have shown that the structure of HMS and mesoporosity are not perturbed after its conversion to UDCaT-4. Further, this also confirms the framework stability. Any crystal growth inside HMS would have resulted in extensive damage to the framework and (33) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Mouscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (34) Xia, Q. H.; Hidajat, K.; Kawi, S. J. Catal. 2002, 205, 318-331.

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also in broadening of the pore size distribution, which was not observed. A number of experiments have been done by others to evaluate the sensitivity of scattering intensities to the materials inside the pores of mesoporous media. Anderson et al.35 found that the removal of surfactant by calcination was accompanied by a very large increase (by a factor of almost 5) in the intensity of the most intense Bragg X-ray peak in comparison to assynthesized material (without calcination). Marler et al. have provided experimental evidence for a decrease in X-ray Bragg intensities of calcined boron-substituted MCM-41 silica by filling the pores with various organic liquids. Pore filling by 1,2-dibromoethane, bromoform, and diiodomethane almost completely eliminated the (100) Bragg reflection at low angle.36 Recalcination to remove the organic adsorptive restored the (100) reflection to nearly its original intensity. Glinka et al.37 have carried out neutron diffraction contrast studies on MCM-41 and observed that Bragg diffraction disappeared when the pores were filled with D2O/H2O mixtures that have the same scattering of cross section as SiO2. Further, Edler and White38 reported large changes in Bragg intensities when MCM-41 silicas were dried for long periods. Introduction of scattering material in the pores of HMS leads to an increased phase cancellation between scattering from the wall and the pore regions, and therefore it reduces scattering intensities for the Bragg reflections at low angle (2θ ) 1-7°). The degree of cancellation is mainly determined by the scattering contrast between the framework walls and the pores.39 Figure 3A shows the reduction of the typical (100) Bragg reflections of UDCaT-4 vis-a`-vis HMS. Our results are in consonance with all previous literature reports, which led us to corroborate that nanoparticles of PAZ (400 °C). PAZ exhibits three peaks, at 180, 450, and 550 °C. The first and second peaks suggest that PAZ possesses a large number of acid sites with intermediate and strong acid strength (Figure 5). The peak at 550 °C corresponding to very strong acid strengths imparts superacidic centers to zirconia-based catalysts.30,41 On the contrary, UDCaT-4 (40) Ward, D. A.; Ko, E. I. J. Catal. 1994, 150, 18.

Efficient Mesoporous Solid Acid Catalyst UDCaT-4 Table 2. Acidity and Activity of HMS, PAZ, and UDCaT-4 catalysts parameters

HMS

PAZ

UDCaT-4

total acidity by NH3-TPD (mmol g-1) TONa of alkylation of mesitylene selectivity toward monoalkylation of mesitylene

0 0 0

0.09 20 98

0.56 123 98

a TON (turnover number) ) moles of product/moles of zirconia in the catalyst.

exhibits only one peak at 180 °C and another at 225 °C which corresponds to intermediate and strong acid strength. Catalytic active centers are generated in UDCaT-4 by embedding 20% of PAZ into otherwise inactive HMS. Thus due to low content of PAZ in UDCaT-4 (that is, 20% w/w PAZ in UDCaT-4 as compared to 100% PAZ), it does not exhibit strong acid strength peaks at 450 and 550 °C. Although UDCaT-4 possesses medium and strong acid strength, the total acid sites of UDCaT-4 (0.56 mmol g-1) are greater than those of PAZ (0.09 mmol g-1) and hence UDCaT-4 shows more activity per unit mass of PAZ in comparison with bulk PAZ (Table 2). 3.1.5. Scanning Electron Microscopy (SEM) and EDXS. Figure 6 shows SEM of HMS and UDCaT-4. It reveals that HMS is made up of submicrometer-sized freestanding or aggregated sphere-shaped particles.42 A similar type of morphology is observed in the case of UDCaT-4. The EDXS analysis shows the incorporation of zirconia and sulfur in HMS. Sulfur Ka1 and zirconium La1 distribution spectra determined by SEM-EDXS analysis have clearly shown homogeneous distribution of

Figure 6. SEM and EDAX of HMS and UDCaT-4.

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S and Zr atoms in UDCaT-4. However, EDXS analysis did not detect the presence of Al in UDCaT-4, possibly due to low alumina content (0.6 w/w %) in UDCaT-4 which is lower than the determination limitation of EDXS analysis. These results are in accordance with earlier literature reports.34 SEM and EDXS analyses further support the argument that active centers of the PAZ are successfully embedded in HMS and the structural integrity of HMS is not altered even after it is converted to UDCaT4. 3.2. Catalytic Activity of UDCaT-4. Since the new catalyst UDCaT-4 is a mesoporous material, it was thought worthwhile to evaluate its activity vis-a`-vis PAZ for vaporphase alkylation of mesitylene with IPA (Table 2). In this system, dehydration of IPA leads to propylene and other likely products such as diisopropyl ether, and it was necessary to study cracking of diisopropyl ether independently. Furthermore, to understand the mechanism and kinetics, it was decided to study dehydration of 2-propanol, diisopropyl ether, n-propanol, and also a mixture of n-propanol and 2-propanol. 3.3. Dehydration of 2-Propanol. The use of IPA as an alkylating agent was expected to reduce coke formation, which usually occurs in vapor-phase reactions with alkenes at higher temperatures. In the presence of an acid catalyst, 2-propanol is dehydrated into propylene and water. It is known that the presence of water in the reaction could have a detrimental effect on the active sites of the solid acid catalysts. Thus it was thought worthwhile to study the dehydration of 2-propanol over UDCaT-4 independently. The dehydration of 2-propanol was studied

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Yadav and Murkute

Figure 7. Effect of temperature and space time on dehydration of 2-propanol.

Figure 8. Effect of space time and temperature on dehydration of 2-propanol.

in two sets of temperatures at 110-150 and 180-220 °C by keeping WHSV and W/FA0 constant under otherwise similar conditions. The reasons for choosing two temperature ranges were (i) temperature has a strong effect on the type of species generated in situ via cracking of IPA and (ii) mesitylene alkylation was significant only in the vapor phase above 180 °C, its boiling point being 164 °C at 1 atm. The conversion of 2-propanol to propylene is remarkably dependent on the temperature and space time. It is found that the conversion of IPA increases substantially with increasing temperature at high space time (W/FA0, g-cat h mol-1) (Figures 7 and 8). It was observed that no diisopropyl ether was formed above 150 °C. Thus, to elucidate the effect of the temperature and W/FA0 on selectivity toward propylene and diisopropyl ether (DIPE) a temperature range of 110-150 °C was selected. DIPE was formed in significant quantities in this range. Selectivity toward propylene was also found be dependent (41) Corma, A.; Forne`s, V.; Juan-Rajadell, M. I.; Lo´pez Nieto, J. M. Appl. Catal., A 1994, 116, 151-163. (42) Mokaya, R.; Zhou, W.; Jones, W. J. Mater. Chem. 2000, 10, 11391145.

on temperature and W/FA0, and it increases with increasing temperature and W/FA0 (Figure 9). There was no coking in the temperature range of 110-220 °C covered in this work. 3.4. Cracking of Diisopropyl Ether. The foregoing results suggested that the dehydration of 2-propanol to propylene and water must proceed via formation of diisopropyl ether. To understand the exact mechanism of dehydration, the cracking of diisopropyl ether was also studied independently under conditions that were otherwise similar to those of the dehydration of 2-propanol in the range of 110-150 °C. The conversion of DIPE and selectivity toward propylene and 2-propanol were found to depend on the temperature and space time. Both the conversion of DIPE and selectivity toward propylene increase substantially with increasing temperature and W/FA0 (Figures 10 and 11). This further corroborates that dehydration of 2-propanol proceeds via formation of diisopropyl ether which is instantaneously cracked into propylene and water. 3.5. Dehydration of n-Propanol and a Mixture of n-Propanol and 2-Propanol (50:50 vol/vol). To throw light on the mechanism and to develop a true kinetic model

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Figure 9. Effect of temperature and space time on selectivity toward propylene and diisopropyl ether in dehydration of 2-propanol.

Figure 10. Effect of temperature and space time on cracking of DIPE.

for the dehydration of 2-propanol, the dehydration of n-propanol and a 50:50 mixture of n-propanol and 2-propanol was studied independently in the temperature range 180-220 °C under conditions that were otherwise similar to those of the dehydration of 2-propanol. It was observed that conversion of n-propanol increases with increasing temperature and W/FA0, and propylene and di-n-propyl ether were found to be the major products (Figure 12). A similar trend was observed in the case of dehydration of a mixture of n-propanol and 2-propanol. (Figure 13). 3.6. Effect of the Mole Ratio of Mesitylene to IPA. The effect of the mole ratio of mesitylene to 2-propanol on the conversion of mesitylene and selectivity toward monoalkylation was studied in the range of 1:1-1:7 (Figure 14). In all these experiments, the total molar flow of the gas phase was maintained constant. Monoalkylated

product was obtained as the major product (98%+) along with minor amounts of dialkylated product (