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Energy & Fuels 2001, 15, 666-670
Si-MCM41 Supported Sulfated Zirconia and Nafion for Ether Production Shaobin Wang* and James A. Guin Department of Chemical Engineering, 230 Ross Hall, Auburn University, Alabama 36849 Received September 20, 2000. Revised Manuscript Received January 26, 2001
Various unsupported and supported sulfated zirconia and Nafion catalysts were prepared and investigated for etherification of C6 olefins with methanol. Supported sulfated zirconia and Nafion generally show higher activity than unsupported sulfated zirconia and Nafion. The nature of the support and catalyst preparation techniques were found to affect the catalytic activity. Mesoporous Si-MCM41 with high surface area was the best support for the acid catalysts. Si-MCM41 supported sulfated zirconia and Nafion catalysts produced higher ether yields than other supports and in some cases exhibited a better catalytic activity than the reference resin catalyst, Amberlyst 15. The wt % loading of acidic component and reaction temperature were important factors influencing the catalytic behavior.
1. Introduction Ethers have been utilized for some time as oxygenated additives for gasoline and diesel since they have high octane numbers, and burn cleaner (reduced CO and hydrocarbon emissions).1 The most widely used oxygenate additive at this time is methyl tert-butyl ether (MTBE) produced commercially from the reaction of methanol and isobutylene. This compound is under environmental pressures due to its low odor and taste threshold and its appearance in groundwaters. The heavier tertiary olefins, e.g., isoamylenes, tert-hexenes, and heptenes, which are present in light FCC gasoline fractions, also have good potential to react with methanol to yield higher ethers with perhaps somewhat more desirable properties than the currently used MTBE.1 For example, the heavier C5 and C6 ethers have lower vapor pressures than MTBE and the olefins which they replace. These properties could be valuable in meeting amended tighter limitations on the Reid vapor pressure for reformulated gasoline.2 Other properties of the higher ethers are also favorable, e.g., higher energy density than MTBE. These factors, i.e., lower vapor pressure and higher energy density, as well as lower water solubility, make higher molecular weight ethers potentially desirable as oxygenated transportation fuel additives.3 Ion-exchange resins such as Amberlyst 15 are currently the dominant catalysts for ether production. Several other acid solids such as zeolites, sulfated zirconia, heteropolyoxoanions (HPA), and supported HPA catalysts have been also tested for the production of MTBE.4-7 Nafion resin, a perfluorosulfonic acid resin, which is a copolymer of tetrafluoroethene and a per* Corresponding author. E-mail:
[email protected]. (1) Ignatius, J.; Harvelin, H.; Lindqvist, P. Hydrocarbon Processing 1995, (2), 51. (2) Linnekoski, J. A.; Krause, A. O. I.; Rihko-Struckmann, L. K. Ind. Eng. Chem. Res. 1997, 36, 310. (3) Wender, I. Fuel Processing Technol. 1996, 48, 189.
fluorosulfonyl ether derivative with a backbone similar to Teflon, is a superacid solid. This material has been studied in various acid-catalyzed reactions.8,9 However, due to the limited surface area of the pure resin and the very limited accessibility to the acid sites, the activity in less polar solvents or in gas-phase reactions has been reduced. To circumvent these difficulties, a new class of silica-supported Nafion resin nanocomposites has been developed recently and shows a higher activity for certain reactions.10,11 Sulfated zirconia is believed to be an important superacid solid which has been found active for various reactions.12,13 However, as was the case with the pure Nafion, the number of active sites of sulfated zirconia is limited by the available specific surface area. Moreover, the conventional preparation method of hydrolyzing zirconium salts is costly and the particles of unsupported sulfated metal oxides are generally very small, a fact which makes operations difficult. For these reasons, development of dispersed supported sulfated zirconias with high surface areas is important for acidcatalyzed reactions. Along these lines, several researchers have reported the applications of supported sulfated zirconia for isomerization.14-16 We also have found that (4) Cillgnon, F.; Mariani, M.; Moreno, S.; Remy, M.; Poncelet, G. J. Catal. 1997, 166, 53. (5) Cillgnon, F.; Loenders, R.; Martens, J. A.; Jacobs, P. A.; Poncelet, G. J. Catal. 1999, 182, 302. (6) Baronetti, G.; Briand, L.; Sedran, U.; Thomas, H. Appl. Catal. A 1998, 172, 265. (7) Quiroga, M. E.; Figoli, N. S.; Sedran, U. A. React. Kinet. Catal. Lett. 1998, 63, 75. (8) Olah, G. A.; Malhotra, R.; Narang, S. C.; Olah, J. A. Synthesis 1978, 672. (9) Waller, F. J.; van Scoyoc, R. W. CHEMTEC 1987, 17, 438. (10) Harmer, M. A.; Farneth, W. E.; Sun, Q. J. Am. Chem. Soc. 1996, 118, 7708. (11) Heidekum, A.; Harmer, M. A.; Hoelderich, W. F. J. Catal. 1998, 176, 260. (12) Song, X.; Sayari, A. Catal. Rev. Sci. Eng. 1996, 38, 3. (13) Yadav, G. D.; Nair, J. J. Microporous Mesoporous Mater. 1999, 33, 1. (14) Ishida, T.; Yamaguchi, T.; Tanabe, K. Chem. Lett. 1998, 1869.
10.1021/ef000205t CCC: $20.00 © 2001 American Chemical Society Published on Web 03/09/2001
Supported Sulfated Zirconia and Nafion for Ether Production
silica-supported sulfated zirconia catalysts are effective for etherification reactions.17 Mesoporous molecular sieves, M41S, are a new family of synthetic porous silicates, which have uniform mesopores of 16-100 Å in diameter, implying that the materials may be developed as powerful shape-selective catalysts, sorbents, and ion exchangers for large molecules used in many technologically important processes. These materials have a high thermal stability, large surface areas, and large adsorption capacities for organic molecules. Therefore, it may be reasonably assumed that these materials can function as good catalyst supports. Kozhevikov et al. have employed MCM-41 to prepare supported HPA catalysts for liquidphase alkylations.18 Verhoef et al.19 also reported liquidand gas-phase esterifications on MCM-41 supported HPA. However, to our knowledge, no one except for the recent communication by Xia et al.20 has thus far reported the preparation and application of Si-MCM41 supported sulfated zirconias or Nafion for acid-catalyzed reactions. Xia et al. synthesized a SO42-/ZO2/MCM-41 superacid catalyst and found that it exhibited more activity than SO42-/ZO2 for MTBE synthesis and npentane isomerization. Thus, in this paper, we report our investigation on preparation of several Si-MCM41 supported sulfated zirconia and Nafion catalysts and their catalytic behavior in etherification of certain C6 olefins with methanol. These particular C6 olefins were chosen on the basis of their potential for making oxygenated transportation fuel additives.21 2. Experimental Section 2.1. Catalyst Preparation. Several commercial solids, including alumina (SBET ) 150 m2/g), silica gel (SBET ) 675 m2/g), fumed silica (SBET ) 380 m2/g), bentonite (BT, SBET ) 80 m2/g), and mortmorillinite K-10 (MT, SBET ) 250 m2/g), were obtained from Aldrich. All other reagents were obtained from Aldrich or as otherwise specified. Si-MCM41 samples were prepared using a method similar to that reported by Chen et al.22 In a typical preparation for Si-MCM41, 40.4 g of water, 6.2 g of cetyltrimethylammonium bromide (CTMABr), and 10 g of 20 wt % tetraethylammonium hydroxide (TEAOH) solution were mixed and stirred at room temperature until all the CTMABr was dissolved. Then, to this mixture, 4.1 g of fumed silica was added. The mixture was first stirred at 70 °C for 2 h and aged at room temperature for 24 h. Then it was transferred into an autoclave and synthesized at 100 °C for 48 h under autogenous pressure. After the autoclave was cooled to room temperature, the as-synthesized MCM-41 material was filtered, washed, and air-dried at 100 °C overnight. Then the sample was heated in air at 1 °C/min to 550 °C and calcined at 550 °C for 9 h. The various supported sulfated zirconia catalysts were prepared by impregnation of zirconium sulfate (Aldrich, 99.99%) on the above commercial and prepared supports. (15) Grau, J. M.; Vera, C. R.; Parera, J. M. Appl. Catal. A 1998, 172, 311. (16) Lei, T.; Xu, J. S.; Tang, Y.; Hua, W. M.; Gao, Z. Appl. Catal. A 2000, 192, 181. (17) Wang, S.; Guin, J. A. Chem. Commun. 2000, 2499. (18) Kozheunikov, I. V.; Sinnema, A.; Jansen, R. J. J.; Pamin, K.; van Bekkum, H. Catal. Lett. 1995, 30, 241. (19) Verhoef, M. J.; Kooyman, P. J.; Peters, J. A.; van Bekkum, H. Microporous Mesoporous Mater. 1999, 27, 365. (20) Xia, Q. H.; Hidajat, K.; Kawi, S. Chem. Commun. 2000, 2229. (21) Hendriksen, D. E. U.S. Patent No. 5,752,992, Exxon Chemical Patents Inc, May 19, 1998. (22) Chen, L.; Horiuchi, T.; Mori, T.; Maeda, K. J. Phys. Chem. B. 1999, 103, 1216.
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Figure 1. Reaction network for the synthesis of ether from 23DM1B with methanol. Typically, 3 g of zirconium sulfate was dissolved in 25 mL water and mixed with 3 g of the support samples, stirred, and evaporated at 80 °C and followed by calcination at 600 °C for 2 h. One sample, referred to as SZ/SiO2-II, was prepared using another zirconium precursor compound, ZrO(NO3)2, as a source of zirconia. In this preparation, 2.45 g of ZrO(NO3)2 was dissolved in 25 mL 0.5 M H2SO4 solution and mixed with 3 g of fumed silica, stirred, and evaporated at 80 °C and then calcined at 600 °C for 2 h. An unsupported sulfated zirconia sample (SZ) was prepared by sulfuric acid treatment of Zr(OH)4, obtained by precipitation of ZrO(NO3)2 solution with NH3‚H2O at pH ) 10, and calcination at 600 °C for 2 h. The supported Nafion catalysts were prepared by impregnating Nafion (5 wt % Nafion in water-alcohol solution) on the various supports, stirring at 60 °C for 2 h, and then evaporating the water and alcohols. After that, the samples were further dried under high vacuum at 60 °C overnight. Two commercial Nafion samples, Nafion NR50 and a Nafion/silica composite SAC-13 with a Nafion content of 13 wt % prepared by a sol-gel method, were obtained from Fluka and Aldrich, respectively. A commercial ion-exchange resin, Amberlyst 15, was used as a reference catalyst. 2.2. Catalytic Evaluation. The reactions were carried out at 80 °C for 2 h in 25 mL stainless steel batch reactors under a pressure of 250 psi of dried helium atmosphere with constant agitation unless otherwise indicated. Catalyst loadings were 0.5 g in all cases. The reactants, 0.19 g of methanol and 0.5 g of 2,3-dimethyl-1-butene (23DM1B) or 2,3-dimethyl-2-butene (23DM2B) at a methanol/olefin molar ratio of ca. 1:1, were mixed with 4 g of heptane as a solvent. The products were determined by a Varian GC equipped with a capillary column and a FID. The conversion, product selectivity, and product yield were calculated as follows:
total conversion (%) )
() ()
product selectivity (%) )
Cn × 100% C0 Ci × 100% Cn
product yield (%) ) [product selectivity (%) × total conversion (%)]/100 where Cn, Ci, and C0 are moles of reactant converted to all products, moles of reactant converted to a particular product, and moles of reactant in feed, respectively.
3. Results and Discussion 3.1. Effect of Support on Catalytic Activity. For the C6 olefins used in this investigation, the reaction network of etherification with methanol is shown in Figure 1. In the process of etherification, each olefin also
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Figure 2. Catalytic activity of various sulfated zirconia catalysts for etherification of 23DM1B with methanol.
Figure 3. Catalytic activity of various Nafion catalysts for etherification of 23DM1B with methanol.
undergoes R T β isomerization on the catalyst. 23DM1B can be isomerized to 23DM2B which further reacts with alcohol to produce ether, although generally at a slower rate. Even though both olefins are eventually converted to the same ether compound, for a nonequilibrium reaction, it is desirable to have high selectivity and yield to the ether, rather than to the olefin isomer. Figure 2 shows the catalytic behavior of the prepared sulfated zirconia and various supported sulfated zirconia catalysts. In the reaction with the R-olefin 23DM1B and methanol, the β-olefin isomer 23DM2B and the ether are the major products. A small amount of 2,3-dimethyl2-butanol was also detected, probably due to the reaction between olefins and a trace amount of water from catalysts and reactant solution. Unsupported sulfated zirconia shows quite a low activity, giving a conversion of only 4% and 20% selectivity to isomerization. Other supported SZ catalysts give higher activity except for SZ/Al2O3, which shows no activity in this reaction. The catalytic activity of the supported SZ catalysts varies depending on the support employed. For the two claysupported SZ systems, SZ/MT exhibits higher conversion than SZ/BT. The selectivities to 23DM2B and ether are similar. Silica gel and fumed silica-supported SZ show a similar 23DM1B conversion but the selectivity to 23DM2B is different. The selectivity to isomerization is higher on the SZ/SiO2(gel), resulting in a lower ether yield. The conversion on SZ/Si-MCM41 is quite high, ca. 75%, and the selectivity to 23DM2B is about 60%, close to that on the SZ/SiO2(gel) thus resulting in an ether yield of about 30%. SZ/SiO2-II shows a slightly higher conversion but lower ether selectivity than SZ/SiO2(fumed); however, in terms of ether yield, these two catalysts exhibit similar activity. In general, the catalytic activity of supported SZ catalysts is dependent on the surface area and acidbase properties of the catalysts. For the clay supports, the commercial MT has a higher surface area and ionexchange capacity. Investigations on the catalytic activity of MT and BT in this reaction have indicated that the MT alone shows some activity while the BT has no activity. The inactivity of SZ/Al2O3 was unexpected. Several researchers have reported that SZ/Al2O3 is active for isomerization of n-butane.16 The difference may be due to the different Al2O3 samples employed and reactions. The higher activity of silica-supported SZ, no matter what silica precursors were employed, can be
attributed to their higher surface areas, especially for Si-MCM41, which has the highest surface area around 900 m2/g.22 SZ/SiO2-II and SZ/SiO2(fumed) were both prepared on the same support, but using a different zirconium precursor compound. The results shown in Figure 2 indicate that the effect of precursor on catalytic activity of silica-supported SZ is not significant in this case. Figure 3 presents the catalytic activity of the various Nafion catalysts. The loading of Nafion on the supported catalysts prepared is about 11 wt %. It is seen that pure Nafion is active for etherification, giving 30% 23DM1B conversion and 55% ether selectivity. The commercial Nafion/silica composite SAC-13 shows little activity, giving only about 4% conversion, similar to the unsupported SZ. Nafion/MT shows about 20% conversion while exhibiting the highest ether selectivity. Other silica-supported Nafion catalysts exhibit varying activity but all these catalysts evidence higher activity and selectivity to ether than the commercial Nafion/silica composite SAC-13. Nafion/Si-MCM41 produces the highest conversion and ether yield among the three catalysts. Comparing with the conversion with the pure Nafion, it is seen that the supported Nafion catalysts exhibit lower activity. This is believed to be due to the lower amount of actual Nafion resin present with a resulting lower activity. Additional data regarding the effect of Nafion loading on catalytic activity will be shown later in this paper. Several investigations have found that Nafion/silica SAC-13 has improved the catalytic activity compared to the original Nafion resin in Friedel-Crafts alkylation and rearrangement reactions.10,11 Botella et al.23 investigated the influence of textural and compositional characteristic of Nafion/silica composites on liquid-phase alkylation of isobutane with 2-butene and found that a silica-supported Nafion catalyst showed an activity as good as that of Nafion/ silica prepared by sol-gel technique. However, Torok et al.24 investigated the catalytic activity of Nafion/silica composites and a silica-supported Nafion sample for Fries rearrangement and found that the impregnated sample exhibited much lower activity. Those results are (23) Botella, P.; Corma, A.; Lopez-Nieto, J. M. J. Catal. 1999, 185, 371. (24) Torok, B.; Kiricsi, I.; Molnar, A.; Olah, G. A. J. Catal. 2000, 193, 132.
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Figure 4. Effect of Zr(SO4)2 loading on catalytic activity in SZ/Si-MCM41 catalysts.
Figure 6. Catalytic activity of 50 wt % Zr(SO4)2/Si-MCM41 at different temperatures.
Figure 5. Effect of Nafion loading on catalytic activity in Nafion/Si-MCM41 catalysts.
Figure 7. Catalytic activity of 28 wt % Nafion/Si-MCM41 at different temperatures.
different from our results in this investigation, probably due to the different reactions and accompanying reaction mechanisms. Similar to the behavior of the Al2O3supported SZ, Nafion/Al2O3 also gives no activity in this reaction. 3.2. Effect of Loading of Zr(SO4)2 and Nafion on Si-MCM41. Figure 4 presents the catalytic activity of SZ/Si-MCM41 with a variation in Zr(SO4)2 loading. It is seen that conversion and selectivity to isomerization increase with increasing Zr(SO4)2 loading and reach the highest values at a Zr(SO4)2 loading of 50 wt %. The ether yield also shows an increasing trend as the loading increases. The highest ether yield of ca. 30% is achieved at a loading of 40-50 wt %. Higher Zr(SO4)2 loadings on SiO2 result in a slight decrease in ether yield. The catalytic activity of the Nafion/Si-MCM41 catalysts with various Nafion loadings was also investigated and the results are shown in Figure 5. As seen, conversion and isomerization activity increase with the Nafion content up to 28 wt % and then decrease as the Nafion content is further increased. The maximum value of
ether yield appears at a Nafion loading of 20 wt %. From Figure 2 and Figure 5, it is seen that Nafion/Si-MCM41 catalysts will exhibit higher activity than that of the pure Nafion when the Nafion loading is higher than 15 wt %. 3.3. Catalytic Activity of SZ/Si-MCM41 and Nafion/Si-MCM41 at Different Reaction Temperatures. Figure 6 depicts a relationship between catalytic activity and reaction temperature for 50 wt % Zr(SO4)2/Si-MCM41. Conversion and isomerization selectivity show an increasing trend with the increasing temperature while ether selectivity gives a maximum of 60% at 70 °C. For the ether yield, the maximum value is achieved at 80 °C. Higher reaction temperatures will result in a decrease in ether yield. Temperature effects on the catalytic behavior of 28 wt % Nafion/Si-MCM41 are illustrated in Figure 7. This catalyst shows a similar behavior to the SZ/Si-MCM41. Although the conversion reaches a high level of 80% at 90 °C, the maximum values of ether selectivity and ether yield are obtained at 70 °C and 80 °C, respectively.
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Figure 8. Catalytic activity of etherification of 23DM1B with methanol on commercial and prepared supported sulfated zirconia and Nafion catalysts.
Figure 9. Catalytic activity of etherification of 23DM2B with methanol on commercial and prepared supported sulfated zirconia and Nafion catalysts.
3.4. Comparison of Catalytic Activity of Amberlyst 15, SZ/Si-MCM41, and Nafion/Si-MCM41 for Etherfication. The catalytic activity of the commercial resin Amberlyst 15 and the above SZ/Si-MCM41 at 50 wt % Zr(SO4)2 and 28 wt % Nafion/MCM41 were compared for the reaction at 80 °C for 2 h (Figure 8). The Amberlyst 15 exhibits the best performance converting 23DM1B to 23DM2B and ether at an overall rate of 90%; however, this catalyst is nonselective for etherification, having instead a high (undesirable) isomerization selectivity at 80% and only 19% ether yield. SZ/Si-MCM41 and Nafion/Si-MCM41 exhibit a comparable overall conversion of 75% and lower isomerization selectivity of 60%, resulting in an ether yield around 30%, higher than that of the Amberlyst 15. The catalytic activity of the above three catalysts for etherification of the β-olefin, 23DM2B, with methanol was also investigated and the conversion and product selectivity are presented in Figure 9. As shown, the SZ/ Si-MCM41 and Nafion/Si-MCM41 exhibit lower 23DM2B conversion than that of the Amberlyst 15, which is similar to the behavior found in the reaction of 23DM1B with methanol, while they all show higher selectivity to ether. The ether yield in this case is somewhat less than that of the Amberlyst 15. In comparison to the results for etherification of 23DM1B, it is seen that conversion of 23DM2B is lower on all catalysts than 23DM1B etherification. In general it has been observed that the β-olefins react at a slower rate than the R-olefins. Our prior kinetic studies on Amberlyst 15 indicate that the methanol etherification rate of 23DM1B
is much higher than that of 23DM2B at the same temperature.25 Rihko and Krause 26 investigated the etherification of some C5-C7 olefins with methanol on resin catalysts and found that equilibrium constant for 23DM1B etherification to be much higher than that for 23DM2B reaction with methanol. 4. Conclusions MCM-41 supported sulfated zirconia and Nafion are active catalysts for etherification of the C6 olefins with methanol. Their catalytic activity is dependent on the nature of support and preparation method. Catalysts prepared by impregnation techniques show a better activity than those prepared by the sol-gel method. The loadings of sulfated zirconia and Nafion and reaction temperature also affect the catalytic activity. SZ/SiMCM41 and Nafion/Si-MCM41 exhibit a comparable and in some cases higher ether yield than that of the commercial resin catalysts such as Amberlyst 15 and thereby offer potential as alternative acid catalysts. Acknowledgment. This research was supported by the U.S. Department of Energy under contract no. DEFC26-99FT40540 as part of the research program of the Consortium for Fossil Fuel Liquefaction Science. EF000205T (25) Liu, J.; Wang, S.; Guin, J. A. Fuel Process Technol. 2000, 69, 205. (26) Rihko, L. K.; Krause, A. O. I. Ind. Eng. Chem. Res. 1996, 35, 4563.
