Hydrophobicity of Water-Tolerant Solid Acids Characterized by

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Langmuir 2000, 16, 2321-2325

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Hydrophobicity of Water-Tolerant Solid Acids Characterized by Adsorptions of Water and Benzene Takashi Yamada† and Toshio Okuhara* Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan Received May 18, 1999. In Final Form: November 10, 1999 Hydrophobicity of a “water-tolerant” solid acid, Cs2.5H0.5PW12O40, has been investigated by adsorptions of water and benzene. One water molecule was adsorbed irreversibly at 298 K per one proton of Cs2.5H0.5PW12O40 as well as H-ZSM-5 (Si/Al ) 40), while H3PW12O40 formed a stable hexahydrate. From the ratio of adsorption area of benzene to that of water, the hydrophobicity of the solid acid surface was estimated to be in the order H-ZSM-5 (Si/Al ) 628) > SiO2 > Cs3PW12O40 > H-ZSM-5 (Si/Al ) 40) = Cs2.5H0.5PW12O40 > SiO2-Al2O3 > Al2O3, where the adsorption area is defined as the product of the adsorption amount and molecular cross-sectional area. It is thus concluded that the surface of Cs2.5H0.5PW12O40 has a hydrophobic nature close to that of H-ZSM-5 (Si/Al ) 40). This hydrophobicity is responsible for the high catalytic activity of Cs2.5H0.5PW12O40 for hydrolysis of ester in excess water.

Introduction Heteropolyacids are oxide clusters which have a welldefined structure and are interesting materials as models of mixed-oxide catalysts.1 Strong acidity is one of the characteristics.2 Because to this, heteropolycompounds have high catalytic activities for a variety of acid-catalyzed reactions.3 An acidic cesium salt, Cs2.5H0.5PW12O40, also possesses a strong acidity,2b,4 a high surface area, and unique pores4 and catalyzes efficiently alkylation,5 ester decomposition,6 acylation,7 Diels-Alder-type reaction,8 skeletal isomerization of alkanes,9 etc. One of the recent topics in the catalysis of Cs2.5H0.5PW12O40 is “water-tolerant” catalysis. Izumi et al.10 reported that H3PW12O40 and Cs2.5H0.5PW12O40 immobilized on a SiO2 matrix were active for hydrolysis of ethyl acetate in excess water. Okuhara et al.11 found that Cs2.5H0.5PW12O40 was much more active for hydrolysis of * To whom correspondence should be addressed. Fax: 81-11757-5995. E-mail: [email protected]. † Present address: Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 6190292, Japan. (1) (a) Misono, M. Catal. Rev.-Sci. Eng. 1987, 29, 269. (b) Okuhara, T.; Mizuno, N.; Misono, M. Adv. Catal. 1996, 41, 113. (c) Pope, M. T.; Muller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34. (2) (a) Levefre, F.; Liu-Cai, F. X.; Auroux, A. J. Mater. Chem. 1994, 4, 125. (b) Okuhara, T.; Nishimura, T.; Watanabe, H.; Misono, M. J. Mol. Catal. 1992, 74, 247. (c) Jozefowicz, L. C.; Karge, H. G.; Vasiyeva, E.; Moffat, J. B. Microporous Mater. 1993, 1, 313. (3) (a) Kozhevnikov, I. V. Chem. Rev. 1998, 98, 171. (b) Izumi, Y.; Urabe, K.; Onaka, M. Zeolites, Clays, and Heteropoly Acid in Organic Reactions; Kodansha-VCH: Tokyo, 1992. (4) Okuhara, T.; Nishimura, T.; Misono, M. Stud. Surf. Sci. Catal. 1996, 101, 581. (5) (a) Nishimura, T.; Okuhara, T.; Misono, M. Appl. Catal. 1991, 73, L7. (b) Corma, A.; Martinez, A.; Martinez, C. J. Catal. 1996, 164, 142. (6) Okuhara, T.; Nishimura, T.; Ohashi, K.; Misono, M. Chem. Lett. 1990, 1201. (7) (a) Izumi, Y.; Ogawa, M.; Nohara, W.; Urabe, K. Chem. Lett. 1982, 1229. (b) Izumi, Y.; Ogawa, M.; Urabe, K. Appl. Catal. A 1995, 132, 127. (8) Meuzelaar, G. J.; Maat, L. M.; Sheldon, R. A.; Kozhevnikov, I. V. Catal. Lett. 1997, 45, 249. (9) (a) Na, K.; Okuhara, T.; Misono, M. J. Chem. Soc., Chem. Commun. 1993, 1442. (b) Okuhara, T.; Nishimura, T.; Watanabe, H.; Na, K.; Misono, M. Stud. Surf. Sci. Catal. 1994, 90, 419. (c) Essayem, N.; Kieger, S.; Coudurier, G.; Vedrine, J. C. Stud. Surf. Sci. Catal. 1996, 101, 591. (10) (a) Izumi, Y. Catal. Today 1997, 33, 371. (b) Izumi, Y.; Ono, M.; Ogawa, M.; Kitagawa, M.; Yoshida, M.; Urabe, K. Microporous Mater. 1995, 5, 255.

esters in water than H-ZSM-5 and SO42-/ZrO2, and the catalytic activity of Cs2.5H0.5PW12O40 for the ester hydrolysis was retained when it was used repeatedly without any treatment several times.12 Solid acids which are active and stable in large excesses of water are desirable for environmentally benign processes. It is essential for the development of “watertolerant” solid acids to endow the property depressing the inhibition by water, that is, hydrophobicity. The hydrophobicity of H-ZSM-5, which is an excellent “watertolerant” solid acid, has been proven.13 Olson et al.13b claimed that the hydrophobicity of H-ZSM-5 was enhanced as the Si/Al ratio increased. Nakamoto and Takahashi13c showed that the adsorption amounts of polar molecules such as water and methanol on H-ZSM-5 decreased drastically as the Si/Al ratio increased. Some researchers have reported the adsorptions of water on SiO2,14 H-mordenite,15 AlPO4-5,16 MCM-41,17 and FSM16.18 Inagaki and Fukushima18 measured the contact angle between the liquid water and FSM-16 surface on the basis of the Kelvin equation and showed that the surface hydrophobicity was affected by the irreversible adsorption of water on the surface. Because Cs2.5H0.5PW12O40 has both micropores and mesopores,4,5a the above method cannot be applicable to estimate the surface hydrophobicity. In the present study, we report the hydrophobic nature of the Cs2.5H0.5PW12O40 surface. The hydrophobicity has been estimated from the ratio of adsorption amount of the benzene to that of water. The data are compared with (11) (a) Okuhara, T.; Kimura, M.; Nakato, T. Appl. Catal. A 1997, 155, L9. (b) Kimura, M.; Nakato, T.; Okuhara, T. Appl. Catal. A 1997, 165, 227. (12) Nakato, T.; Kimura, M.; Nakata, S.; Okuhara, T. Langmuir 1998, 14, 319. (13) (a) Jentys, A.; Warecka, G.; Derewinski, M.; Lercher, J. A. J. Phys. Chem. 1989, 93, 4837. (b) Olson, D. H.; Haag, W. O.; Logo, R. M. J. Catal. 1980, 61, 390. (c) Nakamoto, H.; Takahashi, H. Zeolites 1982, 2, 67. (14) Young, G. J. J. Colloid Sci. 1958, 13, 67. (15) Newalker, B. L.; Jasra, R. V.; Kamath, V.; Bhat, S. G. T. Microporous Mesoporous Mater. 1998, 20, 129. (16) Chen, N. Y. J. Phys. Chem. 1976, 80, 60. (17) (a) Chen, C.-Y.; Li, H.-X.; Davis, M. E. Microporous Mater. 1993, 2, 17. (b) Llewellyn, P. L.; Schoth, F.; Grillet, Y.; Rouquerol, F.; Rouquerol, J.; Unger, K. K. Langmuir 1995, 11, 574. (18) Inagaki, S.; Fukushima, Y. Microporous Mesoporous Mater. 1998, 21, 667.

10.1021/la9906020 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/19/2000

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Yamada and Okuhara Table 1. BET Surface Area and Adsorption Amount catalysta

SN2b (m2 g-1)

Cs2.5 Cs3 H-ZSM-5(L) H-ZSM-5(H) SiO2 SiO2-Al2O3 Al2O3

135 149 410 387 188 539 166

adsorption amount (µmol g-1)c H2O 595 205 1261 113 48 2727 1160

(45) (15) (95) (8) (4) (205) (87)

benzene 419 432 915 736 146 846 352

(108) (112) (237) (191) (38) (219) (91)

r(B/W)d 2.4 7.2 2.5 22.5 10.6 1.1 1.0

a Cs2.5: Cs H PW O . Cs3: Cs PW O . b BET surface area 2.5 0.5 12 40 3 12 40 calculated from N2 adsorption isotherm. c Numbers in the parentheses show the adsorption area (m2 g-1) at p/p0 ) 0.04. d Ratio of the adsorption area of benzene to that of H2O at p/p0 ) 0.04.

Figure 1. H2O adsorption-desorption isotherm for H-ZSM5. A: the first run (O, b) for H-ZSM-5(H) (Si/Al ) 628); the first run (0, 9) and second run (4, 2) for H-ZSM-5(L) (Si/Al ) 40). B: difference in the adsorption isotherm between the first run and the second run for H-ZSM-5(L). Open and closed symbols are adsorption and desorption branches, respectively.

those of H-ZSM-5 and the other typical solid acids to elucidate the hydrophobicity of Cs2.5H0.5PW12O40 quantitatively. Experimental Section Materials. Cs2.5H0.5PW12O40 and Cs3PW12O40 (abbreviated as Cs2.5 and Cs3, respectively, hereafter) were prepared by a titration method described in a previous paper.4 To an aqueous solution of H3PW12O40 (Nippon Inorganic Color and Chemical Co.; 0.08 mol dm-3) was added dropwise with vigorous stirring at room temperature an aqueous solution of Cs2CO3 (Merck; extra pure, 0.10 mol dm-3). The obtained white milky substance was allowed to stand overnight at room temperature and then evaporated to dryness at 318 K. As reference, anhydrous H3PW12O40 obtained by evacuation at 473 K [BET surface area measured by N2 adsorption (denoted as SN2), 8 m2 g-1], H-ZSM-5 [Reference Catalyst of Catalysis Society of Japan, JRC-Z5-1000H (Si/Al ) 638); SN2 ) 386 m2 g-1, designated as H-ZSM-5(H) and JRC-Z5-70H (Si/Al ) 40); SN2 ) 418 m2 g-1, designated as H-ZSM-5(L)], SiO2 (Aerosil 200, SN2 ) 188 m2 g-1), SiO2-Al2O3 [JRC-SAL-2 (Si/Al ) 5.3), SN2 ) 539 m2 g-1], and Al2O3 (JRC-ALO-4, SN2 ) 166 m2 g-1) were used. Adsorption Isotherms of Water and Benzene. The adsorption-desorption isotherms of water and benzene were measured at 298 K by an automatic adsorption apparatus (BELSORP 18, BEL Japan Inc.). Before the measurements, H3PW12O40, Cs2.5, and Cs3 were preevacuated at 473 K for 4 h, and the other samples were pretreated by evacuation under the following conditions: H-ZSM-5, 673 K, 6 h; SiO2, 673 K, 24 h; SiO2-Al2O3 and Al2O3, 573 K, 6 h. Water (purified by Milli-Q, Millipore) and benzene (Wako Pure Chemical Industries Ltd.) were degassed by freeze-thaw cycles prior to use for adsorption measurements. For some cases, samples were evacuated after the measurement at room temperature for 24 h and were again used for repeated runs.

Results Water Adsorption-Desorption Isotherm. Figure 1 shows adsorption-desorption isotherms of water for H-ZSM-5(H) and -(L), together with the difference between the first run and the second run over H-ZSM5(L). The second run was carried out after the evacuation

Figure 2. H2O uptakes for H3PW12O40; the first run (O) and the second run (b).

of the sample used in the first run at room temperature for 24 h. The adsorption-desorption isotherm of water for H-ZSM-5(H) can be classified into type IV or type V.19 The isotherm of the second run for H-ZSM-5(H) (not shown) was very close to that of the first run. The adsorbed amounts of water are summarized in Table 1. It was very small on H-ZSM-5(H); the adsorption area at p/p0 ) 0.04 corresponded to only 8 m2 g-1 (2.1% of SN2). While the adsorption-desorption isotherm of water for H-ZSM-5(L) was also of type IV, the amount of water on H-ZSM-5(L) was about 12 times larger than that of H-ZSM-5(H) at p/p0 ) 0.04; the adsorption area was 95 m2 g-1 (23% of SN2). Figure 1B gives the difference between these runs, which means the irreversible amount of water adsorbed on H-ZSM-5(L) at 298 K. This amount was nearly constant in the range from 0.1 to 0.6 of p/p0 the adsorption area was about 10 cm3 g-1. Figure 2 provides uptake of water for H3PW12O40. Steep increases in the uptake were observed at p/p0 ) 0.01, 0.65, and 0.85. It is known that H3PW12O40 has the ability to sorb water molecules not only on the surface but in the bulk.1b The difference between the first and second runs is about 50 cm3 g-1 (corresponding to 6 H2O molecules (polyanion)-1) at the plateaus. Figure 2 suggests that there are several stable hydrates such as hexahydrate (p/p0 ) 0.01-0.65), octadecahydrate (p/p0 ) 0.65-0.85), and tetracosahydrate (p/p0 > 0.85). The hexahydrate was stable upon the evacuation at room temperature. (19) (a) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moucou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (b) Gregg, S. J.; Sing, K. S. W. Surface Area and Porosity; Academic Press: London, 1982.

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Figure 4. H2O adsorption-desorption isotherms for SiO2 (O, b), SiO2-Al2O3 (0, 9), and Al2O3 (4, 2). Open and closed symbols are adsorption and desorption branches, respectively. Figure 3. H2O adsorption-desorption isotherm for Cs2.5H0.5PW12O40 and Cs3PW12O40. A: the first run for Cs3PW12O40 (O, b) the first run (0, 9) and the second run (4, 2) for Cs2.5H0.5PW12O40. B: difference in the adsorption isotherm between the first run and the second run for Cs2.5H0.5PW12O40. Open and closed symbols are adsorption and desorption branches, respectively.

The adsorption-desorption isotherms of water on Cs2.5 and Cs3 are given in Figure 3. Contrary to H3PW12O40, there is no stable plateau in the isotherms of Cs2.5 and Cs3. The isotherm of water on Cs2.5 is classified as type IV, in which the sharp increase in the adsorption amount at low pressures was observed. The shape of the isotherm on Cs3 was close to that of Cs2.5. While SN2 for Cs3 (149 m2 g-1)20 was larger than that for Cs2.5 (135 m2 g-1), the adsorption amount on Cs3 was smaller than that for Cs2.5 in the wide range of the partial pressure. For example, at p/p0 ) 0.04, the adsorption on Cs3 was about 33% on that on Cs2.5 (Table 1). The adsorption areas of water for Cs3 and Cs2.5 were much less than SN2. The difference between the first and second runs for Cs2.5 was constant (4 ( 1 cm3 g-1) at p/p0 ) 0.1-0.7 (Figure 3B). Figure 4 shows the water adsorption-desorption isotherms for SiO2, SiO2-Al2O3, and Al2O3. In the case of SiO2, water was slightly adsorbed at the low-pressure ranges (adsorption area at p/p0 ) 0.04 was 4 m2 g-1, which is only 3% of SN2), but the adsorption amount increased steeply above p/p0 ) 0.75. This isotherm can be classified as type III, which is observed for nonporous materials having small interaction with adsorbate, and is typical for a water adsorption isotherm of active carbon and dehydroxylated silica.14,19b The desorption branches do not rejoin the adsorption branches, which is probably due to the hydroxylation of the SiO2 surface by water. On the other hand, SiO2-Al2O3 and Al2O3 gave type IV isotherms. The appreciable amounts of water were adsorbed on SiO2Al2O3 and Al2O3 from the low pressures, and the adsorption areas at p/p0 ) 0.04 were 205 (38% of SN2) and 87 m2 g-1 (52% of SN2), respectively. Benzene Adsorption-Desorption Isotherms. Benzene adsorption-desorption isotherms for H-ZSM-5(H) and -(L) are shown in Figure 5. Contrary to water, benzene was readily adsorbed onto H-ZSM-5 from the lower pressures. The amounts of benzene adsorbed are sum(20) Mizuno, M.; Misono, M. Chem. Lett. 1987, 967.

Figure 5. Benzene adsorption-desorption isotherm for H-ZSM-5(H) (Si/Al ) 628) (O, b) and H-ZSM-5(L) (Si/Al ) 40) (0, 9). Open and closed symbols are adsorption and desorption branches, respectively.

marized in Table 1. The adsorption area at p/p0 ) 0.04 reached 237 (58% of SN2) and 191 m2 g-1 (50% of SN2) for H-ZSM-5(L) and -(H), respectively. The isotherm of H-ZSM-5(L) has a clear hysteresis loop, and a considerable increase in the adsorbed amount is observed above p/p0 ) 0.4. It is noted that the adsorption amount of benzene was not influenced by the Si/Al ratio of H-ZSM-5. Figure 6 gives the benzene adsorption-desorption isotherms for Cs2.5 and Cs3. While the pressure at the hysteresis was different between the isotherms for Cs2.5 and Cs3, the difference of the adsorbed amount of benzene between them was small. As shown in Table 1, the adsorbed amount of benzene for Cs3 (corresponding to 112 m2 g-1 of the adsorption area) was larger than that for Cs2.5 (108 m2 g-1) at p/p0 ) 0.04. Thus, the amount of benzene adsorption on these heteropolycompounds was proportional to the surface area. As was not shown, the amount of benzene adsorption on H3PW12O40 was small and the isotherm exhibited a type III. The adsorption area of benzene at p/p0 ) 0.04 was less than the BET surface area with N2 (6 m2 g-1).

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Figure 6. Benzene adsorption-desorption isotherm for Cs3PW12O40 (O, b) and Cs2.5H0.5PW12O40 (0, 9). Open and closed symbols are adsorption and desorption branches, respectively.

Figure 7. Benzene adsorption-desorption isotherms for SiO2 (O, b), SiO2-Al2O3 (0, 9), and Al2O3 (4, 2). Open and closed symbols are adsorption and desorption branches, respectively.

Figure 7 shows the benzene adsorption-desorption isotherms of SiO2, SiO2-Al2O3, and Al2O3. SiO2 also appreciably adsorbed benzene from the lower pressure range, and the adsorption area at p/p0 ) 0.04 was 20 times larger than that of water (20 m2 g-1). The adsorptiondesorption isotherm of SiO2 was type II, which is commonly observed for physical adsorption of gases by nonporous or macroporous materials.19 In addition, SiO2-Al2O3 considerably adsorbed benzene from the lower pressure range. The isotherm of benzene on Al2O3 is classified as type IV. Discussion H2O Adsorption on Acid Sites of Solids. It is known that the SiO2 surface evacuated at elevated temperatures is highly hydrophobic.14 Young14 studied the interaction between the SiO2 surface and water vapor using thermogravimetry and IR and claimed that water molecules were adsorbed on the surface silanol groups (Si-O-H) at 298 K but not on Si-O-Si.14 It is reasonable that the dehydroxylated SiO2 surface is hydrophobic because of the nonpolarity of Si-O-Si. Because the pores of zeolites consisting mainly of silica are constructed with SiO4 tetrahedra, the ideal pores do not possess silanol groups

Yamada and Okuhara

and would be highly hydrophobic.16 If Al3+ was partly substituted by Si4+, a negative charge would remain on an oxygen atom of AlO4. Ordinarily, cations such as alkaline or alkaline-earth ions as well as protons compensate the charge and have adsorption abilities for water molecules. In fact, it was reported that the adsorption amount of water decreased as the Si/Al ratios increased;13b,c,16 zeolites with high Si/Al ratios are highly hydrophobic. Adsorption of water on protonic sites of H-ZSM-5 was investigated with IR and thermogravimetry. Jentys et al.13a showed that one water molecule was adsorbed on one proton of H-ZSM-5 (Si/Al ) 36) at a relative pressure of water of 10-6 at 308 K. Furthermore, Kondo et al.21 reported that when the relative pressure was below 10-2 at 423 K, hydrogen-bonded H2O was formed on the protonic site of H-ZSM-5 (Si/Al ) 50). They claimed that the monomer species was changed to protonated dimer (H5O2+) and polymeric water (H5O2+‚nH2O) when the relative pressure of water was raised. Sano et al.22 reported that the number of adsorbed water molecules per proton of H-ZSM-5 having different Si/Al ratios was close to 5 at saturated pressure of 298 K. Chen16 determined the number of adsorbed water molecules on a protonic site of H-mordenite (Si/Al ) 5.5) to be 4 at p/p0 ) 0.6 at 298 K. In the present study, hysteresis loops were observed in the water adsorption-desorption isotherms above p/p0 ) 0.3 (Figure 1A) for H-ZSM-5(H) and -(L). These hysteresis loops are considered to be arisen from the capillary condensation in the mesopores such as fissures. As a matter of fact, the presence of mesopores was confirmed by the analysis of N2 adsorption-desorption isotherms.23 From the difference in the amount of water adsorbed at p/p0 ) 0.2 in the micropores of H-ZSM-5(L) and -(H) (40 cm3 g-1), the number of water molecules per proton was estimated to be 4 for H-ZSM-5. The hydrophobicity of heteropolycompounds would be greatly influenced by the countercations. Among alkaline salts, Na+ salts are classified into group A and K+, Rb+, and Cs+ salts into group B.24 The group A salts are highly soluble in water and polar organic solvents, but the group B salts are insoluble.24 This difference can be explained by the difference in the hydration energy of the cations. The smaller hydration energy of Cs+ (264 kJ mol-1)25 than Na+ (406 kJ mol-1)25 is mainly responsible for the insolubility and hydrophobicity of Cs3, as will be discussed below. The interaction of water with protons of Cs2.5 has been examined. Okuhara et al.2,8b,25 showed that all protons in the bulk of Cs2.5 interact with water vapor. 31P NMR of dehydrated Cs2.5 gave four peaks assigned to polyanions having 0, 1, 2, and 3 protons. When Cs2.5 was exposed to water vapor at room temperature, the NMR spectrum changed to that having only the peak due to the polyanion having no protons. This was explained by the transfer of protons from the polyanion to water. If the difference in the adsorption of water of Cs2.5 and Cs3 (e.g., 10 cm3 g-1 at p/p0 ) 0.2) obtained in the present study is attributable to that in the amount of protons, the average number of water molecules per one proton of Cs2.5 becomes about 3 in the presence of water vapor. (21) Kondo, J. N.; Iizuka, M.; Domen, K.; Wakabayashi, F. Langmuir 1997, 13, 747. (22) Sano, T.; Kasuno, T.; Takeda, K.; Arazaki, S.; Kawakami, Y. Stud. Surf. Sci. Catal. 1997, 105, 1771. (23) Yamada, T.; Yoshinaga, Y.; Okuhara, T. Bull. Chem. Soc. Jpn. 1998, 71, 2727. (24) Niiyama, H.; Saito, Y.; Yoshida, S.; Echigoya, E. Nippon Kagaku Kaishi 1982, 569. (25) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.; John Wiley & Sons: New York, 1980.

Hydrophobicity of Water-Tolerant Solid Acids

Langmuir, Vol. 16, No. 5, 2000 2325 Table 2. Catalytic Activities for Hydrolysis of Ethyl Acetatea catalystb Cs2.5 Cs3 H-ZSM-5(L) H-ZSM-5(H) SiO2-Al2O3

Si/Al

SN2 (m2 g-1)

acid amount (mmol g-1)

40 628 5.3

128 146 418 386 560

0.15f 0 0.40f 0.03f 0.35

specific ratec,d r1

r2

244 0 86 7 0

1.57 0 0.22 0.27 0

a From reference 11b. b Cs2.5: Cs H PW O . Cs3: Cs PW O . 2.5 0.5 12 40 3 12 40 The reaction was carried out at 333 K using an aqueous solution (30 cm3) of 5 wt % ethyl acetate (16.9 mmol). d Initial rate, r1: mmol g-1 min-1. r2: mol (H+ mol)-1 min-1. e Unpublished data. f Calculated value based on chemical composition. c

Figure 8. Ratio of adsorption area of H2O to that of benzene for various solid acids: (O) Cs2.5H0.5PW12O40; (b) Cs3PW12O40; (0) H-ZSM-5(H); (9) H-ZSM-5(L); (4) SiO2; (2) Al2O3; (]) SiO2-Al2O3. SW and SB are adsorption areas of H2O and benzene, respectively (see text).

On H-ZSM-5, the number of water molecules adsorbed irreversibly at room temperature was near unity per one proton, according to Fukuya et al.26 This was confirmed in the present study over H-ZSM-5(L). On the other hand, the difference in the first and second runs for water adsorption on Cs2.5 (4 ( 1 cm3 g-1) indicates that one water molecule was irreversibly adsorbed per proton in the whole bulk of Cs2.5 at room temperature. Although these materials are different in chemical composition, a similar property in water adsorption was found, that is, hydrophobicity, as will be discussed below. Comparison of Water and Benzene Adsorption. It is useful to have a measure to estimate the surface hydrophobicity of various kinds of solid catalysts. Because hydrophobicity is the property of lacking affinity for water and is characteristic of oils, fats, waxes, etc.,27 it is considered that the ratio of the adsorption amount of water and that of a hydrocarbon would be a good measure for evaluation of “hydrophobicity” of the surface. In the present study, we used benzene as the the hydrocarbon, because it is a nonpolar molecule and its interaction with the solid surface is generally weak. Figure 8 shows the ratio of the adsorption area of benzene to that of water as a function of the relative pressure of the adsorbates. The molecular cross-sectional areas of water and benzene used in the present study are 0.125 and 0.430 nm,2,28 respectively. The values of SB/SW were 10.6 and 22.5 for SiO2 and H-ZSM-5(H) at p/p0 ) 0.04, respectively. SB/SW for H-ZSM-5(L) was smaller than those for SiO2 and H-ZSM-5(H), showing the low hydrophobicity of H-ZSM-5(L). The lower values for SiO2-Al2O3 and Al2O3 (1.1 and 1.0 at p/p0 ) 0.04) indicate that these are much less hydrophobic. Cs3 gave 7.2 for (26) Fukuya, S.; Kurita, S.; Wang, Z.-B.; Sano, T.; Soga, K. Preprints of 80th Annual Metting of Catalysis Society of Japan, 1997; 1P09. (27) Parker, S. P., et al., Eds. McGraw-Hill Dictionary of Chemistry, 3rd ed.; McGraw-Hill: New York, 1984. (28) McClellan, A. L.; Harnsberger, H. F. J. Colloid Surf. Sci. 1967, 23, 577.

SB/SW at p/p0 ) 0.04, which is higher than that of H-ZSM5(L). On the other hand, SB/SW for Cs2.5 was very close to that for H-ZSM-5(L). In conclusion, the order of the hydrophobicity is H-ZSM-5 (Si/Al ) 628) > SiO2 > Cs3PW12O40 > H-ZSM-5 (Si/Al ) 40) ) Cs2.5H0.5PW12O40 > SiO2-Al2O3 > Al2O3. Hydrophobicity and “Water-Tolerant” Catalysis. In “water-tolerant” solid catalysts, the active sites (e.g., protonic sites) must be protected from poisoning by water. There are some reports about water-tolerant solid acids for hydrolysis of ester10,11,29 and olefin hydration.30,31 H-ZSM-5 is the efficient water-tolerant solid acid, and its Si/Al ratio is critical.29 The Si/Al ratio around 50 in H-ZSM-5 is adequate, while the acid amount is not large. Catalytic activities for hydrolysis of ethyl acetate for Cs2.5, Cs3, H-ZSM-5, and SiO2-Al2O3 are summarized in Table 2. The activity (per unit of weight (r1)) of H-ZSM5(H) was lower than that of H-ZSM-5(L). This is due to the smaller amount of acid sites of H-ZSM-5(H). On the other hand, the specific activity (per unit of acid site (r2)) of H-ZSM-5(H) was somewhat higher than that of H-ZSM-5(L). This is probably due to the higher hydrophobicity of H-ZSM-5(H). It should be emphasized that Cs2.5 exhibited the highest activity among these solid acids; r1 and r2 were 2 and 7 times higher than those of H-ZSM-5(L). Considering the hydrophobicity of Cs2.5 similar to that of H-ZSM-5(L), the difference in the activity may be due to that of the acid strength, because the acid strength of Cs2.5 was reported to be higher than that of H-ZSM-5.9 Conclusion The surface hydrophobicity of various solid acids was estimated from the ratio of the adsorption area of benzene to that of water. The following order was obtained: H-ZSM-5 (Si/Al ) 628) > SiO2 > Cs3PW12O40 > H-ZSM-5 (Si/Al ) 40) ) Cs2.5H0.5PW12O40 > SiO2-Al2O3 > Al2O3. The catalytic results for hydrolysis of ethyl acetate in excess water indicate the importance of the surface hydrophobicity as well as the acid strength for watertolerant solid acid catalysts. LA9906020 (29) Namba, S.; Hosonuma, N.; Yashima, T. J. Catal. 1981, 72, 16. (30) Fauja, F.; Ibarra, R.; Figueras, F.; Gueguen, C. J. Catal. 1984, 89, 60. (31) Okuhara, T.; Kimura, M.; Nakato, T. Chem. Lett. 1997, 839.