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Bimodal porous Pd–silica for liquid-phase hydrogenation
Applied Catalysis A: General 284 (2005) 247–251 www.elsevier.com/locate/apcata
Bimodal porous Pd–silica for liquid-phase hydrogenation Satoshi Sato *, Ryoji Takahashi, Toshiaki Sodesawa, Masashi Koubata Department of Applied Chemistry, Faculty of Engineering, Chiba University, Yayoi, Inage, Chiba 263-8522, Japan Received 14 September 2004; received in revised form 17 January 2005; accepted 7 February 2005 Available online 3 March 2005
Abstract Palladium catalysts loaded on bimodal porous silica were examined in the liquid-phase hydrogenation of 2-butenal at 0 8C and a hydrogen pressure of 1.1 MPa. The sizes of mesopores and macropores of the silica support are controllable in the range of 4–23 nm and 0.5–25 mm, respectively. The macropores provide effective paths of mass transfer, and the mesopores present effective surfaces for dispersion of metals. The catalytic activity of Pd–silica with bimodal porous structure depends on the size of mesopores and macropores as well as on the particle size of the support silica. The most active Pd–silica catalyst, with mesopores of 12 nm and macropores of 2 mm, shows much higher activity than does a commercial palladium carbon catalyst without macropores. The results indicate that the diffusion process inside the catalyst particles dominantly determines the reaction rate in the hydrogenation. # 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogenation; Pd; Bimodal porous silica; Macropore; Mesopore; Diffusivity
1. Introduction The effectiveness of a solid catalyst decreases with increasing mass-transfer resistance [1,2]. The higher the catalytic activity, the more restricted the supply of reactants from bulk liquid to the catalyst surface is. In liquid-phase hydrogenation, even in powder catalysts, the rate of mass transfer is often small enough to compete with surface reaction [3]. In the previous reports, we examined the liquid-phase hydrogenation of ketones to the corresponding secondary alcohols catalyzed by porous Ni catalysts [4–7]. The hydrogenation rate of ketones observed over Raney nickel with small pores of 3.8 nm in diameter is lower than those of Ni–MgO catalysts with ‘‘lower Ni surface area’’ and with larger mesopores [4,5]. Diffusion of reactants in the small mesopores
* Corresponding author. Tel.: +81 43 290 3376; fax: +81 43 290 3401. E-mail address: [email protected] (S. Sato). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.02.003
mesopores, which is greatly restricted in small mesopores of ca. 4 nm [8]. One of the authors has developed a kind of amorphous silica with a distinct bimodal pore structure, which is prepared by sol–gel reactions of tetraethyl orthosilicate (TEOS) in the presence of organic polymers such as polyethylene oxide (PEO) [9]. Macropores in the silica have a sharp distribution in size, which can be controlled by the initial composition such as the water/PEO/TEOS ratio. The macropores are formed when transitional morphology of spinodal decomposition is fixed by sol–gel transition of the silicate solution, and the micrometer size varies with the timing of the onset of the spinodal decomposition and gelation. Mesopores are regulated from 2 to 20 nm by dissolution–precipitation in alkaline solution during the aging process of wet silica gel [10]. We have prepared an amorphous type of silica–alumina with macropores by sol–gel reactions of TEOS in the presence of aluminum nitrate and PEO [11,12]. The silica– alumina has macropores effective for the rapid molecular transportation and mesopores providing large specific surface areas of 600 m2 g 1. In addition to the macropores, the silica–alumina has a large number of Broensted acid
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sites, and shows excellent catalytic activities in the vaporphase cracking of cumene [11]. Group VIII metals such as Ru, Rh, Pd, Ir, and Pt have been used in catalytic hydrogenation of olefins and carbonyl compounds in organic syntheses [13]. In this work, we develop a novel Pd–silica catalyst for the liquid-phase hydrogenation instead of use of Pd–carbon, utilizing large macropores in the bimodal porous silica. We also discuss the efficiency of macroporous catalyst on the liquid-phase catalysis and clarify the size of the mesopores that are effective in the hydrogenation.
2. Experimental 2.1. Catalyst samples Silica with bimodal pore structure was prepared by fixing a transitional state of phase separation by gelation of silicate solution containing polyethylene oxide with average molecular weight of 100,000 (PEO, Aldrich) [9,14]. A 14-cm3 volume of TEOS (Shin-Etsu Chemical Co., Japan) was added into 17.6 cm3 of 1 mol dm 3 nitric acid aqueous solution containing PEO under vigorous agitation. The PEO content was varied from 1.0 to 1.8 g. After the mixture had become homogeneous, it was poured into a plastic container. The container was sealed and held at 50 8C for 20 h for gelation. The resulting wet gel plate was used for the preparation of samples with different mesopore sizes adopting the solventexchange method [10,14]. The resulting gel plate was dried at 50 8C for 2 weeks, and then heated at 600 8C for 2 h. The support silica was crushed and sieved at 250–650 mm in
particle size. As a reference, non-macroporous silica was prepared without using organic polymer in the same way as done in the above-mentioned procedure. Palladium was supported on the silica by impregnation using palladium acetate/acetone solution (1 wt.%), and calcined at 500 8C for 3 h to obtain a PdO–silica precursor. The PdO–silica sample was reduced in a glass tube vessel under hydrogen flow at room temperature for 1 h to provide Pd–silica. Prior to the catalytic tests and characterization, the Pd–silica was crushed and sieved at an appropriate particle size. Reference palladium catalysts such as palladium carbon and palladium alumina were purchased from Wako Pure Chemicals and Aldrich Chemicals, respectively. A scanning electron micrograph instrument, SEM, (SM200, Topcon), was used for the observation of the morphology of the samples. A mercury porosimeter, POREMASTER-60 (Quantachrome), was used for the measurement of a wide range of pore size distribution. The adsorption–desorption isotherm of N2 was obtained at 196 8C on a commercial gas-adsorption apparatus, OMNISORP 100CX (Coulter). Prior to the adsorption, the sample was out-gassed at 300 8C for 1 h. The mesopore size distribution was calculated using the method of Dollimore and Heal [15] from the desorption branch. The specific surface area was determined by the BET method using the nitrogen adsorption isotherm. 2.2. Catalytic reaction The commercial grade reagents, which were purchased from Wako Pure Chemical Industry, were used without further purification. Hydrogenation of 2-butenal was tested,
Fig. 1. (A) A SEM image of macroporous structure, (B) pore size distribution with wide range, (C) mesopore size distribution of bimodal porous silica.
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according to the procedure described previously [5]. The solution (10 cm3) containing a reactant olefin (0.96 mol dm 3) was reacted in a stirred pressure batch reactor TVS-1 (Taiatsu Glass Industry) under the following conditions: reaction temperature of 0 8C, agitation speed of 1350 rpm, and hydrogen pressure of 1.1 MPa. The reaction mixture filtered with a glass filter was analyzed by FID-GC with a fused silica capillary column of TC-WAX (30 m). Butanal was produced stoichiometrically, and the conversion of 2-butenal achieved 100% after 30– 50 min or at least after several hours. The conversion data were fitted on the integral form of first-order reaction rate, kt = ln [1/(1 X)], where k, t, and X are the first-order reaction rate constant, reaction time, and the 2-butenal conversion, respectively [5]. Then, the rate constant, k, was divided by the catalyst weight used in the reaction to obtain the first-order rate constants per unit weight of catalyst, kw.
3. Results and discussion Fig. 1A shows a typical SEM image of silica gel with the macropores in the size of 2 mm. The size of macropores in the silica can be regulated from 0.5 to 25 mm by preparation conditions. The silica has bimodal pore structure distributed in the regions of both macropore and mesopore (Fig. 1B). In addition, the narrow distribution of mesopores of the silicas is regulated at a specific diameter from 4 to 23 nm (Fig. 1C). It is also confirmed through the t-plot method that the silicas have no micropores. The silica with bimodal pore structure can be reproduced in the same way as in the literature [9,10,14]. Fig. 2 depicts XRD profiles of Pd–silica and commercial Pd–carbon. Pd–silica before reduction has PdO on its surface (Fig. 2a). After reduction, PdO is reduced to Pd in the Pd–silica (Fig. 2b), while the commercial Pd–carbon is available in the reduced form (Fig. 2c). Sizes of Pd metals are comparable with each other. We have examined the mass-transfer resistance with respect to the agitation speed in liquid-phase hydrogenation [5]. The gas–liquid mass-transfer coefficient simply
Fig. 2. XRD profiles of Pd–SiO2 and commercial Pd–carbon. (a) Before reduction, bimodal porous silica with macropores of 2 mm and mesopores of 12 nm; (b) sample (a) reduced by H2; (c) commercial Pd–carbon. Open circle, PdO; closed circle, Pd.
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Fig. 3. Hydrogenation of 2-butenal over Pd–silica with different particle size. (a) Bimodal porous silica with macropores of 25 mm and mesopores of 12 nm; (b) non-macroporous silica with mesopores of 10 nm; (c) commercial Pd carbon. Reacted in hexane at 0 8C, agitation speed of 1350 rpm, and H2 pressure of 1.1 MPa. Pd–silica samples with particle size of 250–650 mm were crushed and sieved through sizes of 45, 90, 180, 250, and 650 mm. Pd loading is 5 wt.%.
increases with increasing agitation speed. There is a critical value of agitation speed over which the reaction rate is not affected. Hydrogen transfer from the gas to the liquid phase and through the liquid phase depends not only on the agitation speed, but also on the reactor geometry (reactor shape and size, agitator shape and size, and presence of internal baffles etc.) and on the hydrogen solubility, which also depends on nature of the solvent, pressure, temperature and so on [6,16]. In the previous work of porous nickel catalysts [5], the critical speed over which the reaction rate is not affected was observed at ca. 1200 rpm at 0 8C and 1.1 MPa: the mass-transfer resistance of reactants in the bulk liquid and of gas-to-liquid hydrogen is negligible beyond these values. Fig. 3 shows the effect of particle size on the catalytic activity of Pd–silica for the reaction of 2-butenal at 0 8C, together with differences in catalytic activity of Pd–silica between macroporous and non-macroporous catalysts. We used silica with macropores of 25 mm for the measurement because it was not fragile during the reaction. The nonmacroporous silica is also hard. The catalytic activity of Pd– silica depends on the particle size: the catalytic activity increases with decreasing particle size. It is obvious that the bimodal porous Pd–silica is more active than the nonmacroporous Pd–silica. Commercial Pd–carbon even with small particle size has catalytic activity as low as the nonmacroporous Pd–silica. We examined the effect of macropore size of bimodal porous Pd–silica on the catalytic reaction of 2-butenal (Fig. 4). Pd–silica samples with macropores show high level of the rate constant kw, while the non-macroporous Pd–silica is much less active (at macropore of 0 mm). It depends on the macropore size: macropores as small as ca. 2 mm are effective for the liquid-phase hydrogenation. This is consistent with the effect of particle size mentioned above. The macropores are necessary for the improvement of catalytic activity because of the rapid molecular transportation inside the particles. It is reasonable that the macropores
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Fig. 4. Hydrogenation of 2-butenal over Pd–silica with different macropore sizes. The silicas have a particle size of 90–180 mm and mesopores of ca. 10 nm. Pd loading is 5 wt.%. Reacted in ethanol at 0 8C, agitation speed of 1350 rpm, and H2 pressure of 1.1 MPa.
play a key role in reducing the resistance of mass transfer from bulk to surface of solid catalyst. Table 1 summarizes the catalytic properties of several samples for the hydrogenation of 2-butenal. There is no clear correlation between the catalytic activity and specific surface area: a commercial Pd–carbon with small mesopores and large specific surface area is less active. However, the catalytic activity of Pd–silica with large mesopores of 12 nm is excellent. The catalytic activity linearly increases with Pd loading. Pd–silica with macropores is superior to Pd–carbon and Pd–alumina. In the liquid-phase hydrogenation of 2-butenal, moreover, the catalytic activity in ethanol solvent has similar trends to that in hexane. To optimize the size of mesopore in the liquid-phase hydrogenation, we examined the hydrogenation using Pd– silica catalysts with different mesopore sizes, which were prepared from bimodal porous silica displayed in Fig. 1C. Fig. 5 depicts the rate constant, kw, versus the mesopore size in the bimodal porous Pd–silica. In the hydrogenation of 2butenal, the kw is maximized at 12 nm in mesopore diameter. The most active Pd–silica catalyst is eight times as active as a commercial palladium carbon at the same Pd content.
Fig. 5. Hydrogenation of 2-butenal over Pd–silica with different mesopore size. The silica has a particle size of 90–180 mm and a macropore of 2 mm. Pd loading is 5 wt.%. The mesopore size is the peak of the pore size distribution as shown in Fig. 1C. Reaction conditions are the same as those in Fig. 3.
In heterogeneous reactions over porous catalysts, either the surface reaction including adsorption, surface reaction, and desorption or the diffusion within the pore system is considered to be the major process that determines the catalytic efficiency [1,2]. In the liquid-phase hydrogenation of methyl acetoacetate to methyl 3-hydroxybutyrate using supported nickel catalysts, Nitta et al. [17] confirmed that pore diffusion of the reactant was the rate-determining step in small pores of <5 nm. In Ni–MgO with large mesopores of 14 nm, the catalytic activity was determined entirely by surface reaction, being free from mass transfer resistance [6]. We have investigated the effects of the size of reactant ketones and solvents in the liquid-phase hydrogenation [6]. Reactivity of the ketones decreases linearly with increasing molecular size. For each ketone, reactivity decreases with increasing size of the alkane solvents, from n-pentane to n-tridecane. Strong resistance for the mass-transfer of reactants and/or solvents in smaller mesopores affects the overall kinetics in the specific liquid-phase reaction using mesoporous catalysts.
Table 1 Catalytic activity of various Pd catalysts Catalyst
Pd content (wt.%)
SAa (m2 g 1)
Mesopore sizeb (nm)
Pd–carbond Pd–aluminad Pd–silicae Pd–silicae Pd–silicae Pd–silicae Pd–silicaf
5 10 2 3 5 10 5
762 250 513 513 513 288 333
3.2 – 11.9 11.9 11.9 11.9 9.5
a b c d e f g
Specific surface area measured by N2 adsorption at 196 8C. Pore diameter: peak top of pore size distribution. Catalytic activity for hydrogenation of 2-butenal, 1st order rate constant. Particle diameter, 90–180 mm. Commercially available reagents, having no macropores. Pd–silica prepared in this study, having macropores with the size of ca. 2 mm. Pd–silica prepared in this study, having no macropores. Particle diameter, 45–90 mm.
kwc (h
1
g 1)
In hexane
In ethanol
105g 28 315 544 846 1590 142
228g 309 469 660 691 1390 145
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In our latest report, we estimated pore diffusion coefficient, Dp, of several ketones in various solvents within mesopores of transparent monolithic silica plate by monitoring change in the UV absorbance [8]. Monolithic plates of the silylized porous silica with different mesopore size at ca. 4 and 10 nm were used for the Dp measurement. In the plates, Dp of the ketones decreases with increasing the size of solvents, whereas it is independent of the size of solutes, namely reactants. In the small mesopores of ca. 4 nm, the diffusivity of ketones is greatly restricted. In large mesopores >10 nm, effectiveness of catalyst is promising because of minimized mass transfer resistance. The mesopore size of 12 nm, at which the largest rate constant (Fig. 5) is obtained, is consistent with the results reported previously [6,7]. These results indicate that the size of mesopores unaffected by mass transfer resistance is much larger than the size of the reactant.
4. Conclusions The liquid-phase hydrogenation of 2-butenal was examined at 0 8C and an H2 pressure of 1.1 MPa over Pd catalysts loaded on bimodal porous silica. The sizes of mesopores and macropores of the silica support have been controlled in the ranges of 4–23 nm and 0.5–25 mm, respectively. We developed a novel Pd–silica catalyst using the bimodal porous silica. The catalytic activity is dependent on the macropore size: small macropores are effective for the hydrogenation, while non-macroporous Pd–silica is inefficient. The most active Pd catalyst with mesopores of 12 nm and macropores of 2 mm is eight times as active as a commercial palladium carbon catalyst without macropores. The macropores provide effective paths of mass transfer from the bulk to the entrances of the mesopores. The silica with macropores used as a catalyst support improves the effectiveness of supported-metal catalyst. The catalytic activity of Pd loaded on the bimodal porous silica depends on the size of mesopores as well as
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on the particle size of the support silica. An optimum mesopore size is available in the hydrogenation: small mesopores at <10 nm restrict the mass diffusion, whereas the mesopores present large surface area for metal loading. We can conclude that the diffusion process of reactants dominantly determines the reaction rate in the liquid-phase hydrogenation.
References [1] J.M. Smith, Chemical Engineering Kinetics, third ed. McGraw-Hill, New York, 1981, 450. [2] R.I. Masel, Chemical Kinetics and Catalysis, John Wiley & Sons, New York, 2001, 738. [3] M. Boudart, W.-C. Cheng, J. Catal. 106 (1987) 134. [4] T. Nakayama, K. Yamashiro, S. Sato, F. Nozaki, Appl. Catal. A 151 (1997) 437. [5] S. Sato, R. Takahashi, T. Sodesawa, F. Nozaki, X.-Z. Jin, S. Suzuki, T. Nakayama, J. Catal. 191 (2000) 261. [6] H. Tsai, S. Sato, R. Takahashi, T. Sodesawa, S. Takenaka, Phys. Chem. Chem. Phys. 4 (2002) 3537. [7] S. Takenaka, R. Takahashi, S. Sato, T. Sodesawa, Phys. Chem. Chem. Phys. 5 (2003) 4968. [8] R. Takahashi, S. Sato, T. Sodesawa, T. Ikeda, Phys. Chem. Chem. Phys. 5 (2003) 2476. [9] K. Nakanishi, H. Komura, R. Takahashi, N. Soga, Bull. Chem. Soc. Jpn. 67 (1994) 1327. [10] R. Takahashi, S. Sato, T. Sodesawa, H. Nishida, Phys. Chem. Chem. Phys. 4 (2002) 3800. [11] R. Takahashi, S. Sato, T. Sodesawa, M. Yabuki, J. Catal. 200 (2001) 197. [12] M. Yabuki, R. Takahashi, S. Sato, T. Sodesawa, K. Ogura, Phys. Chem. Chem. Phys. 4 (2002) 4830. [13] P.N. Rylander, Catalytic Hydrogenation in Organic Syntheses, Academic Press, New York, 1979, 82. [14] R. Takahashi, S. Sato, T. Sodesawa, A. Haga, H. Nishino, J. Ceram. Soc. Jpn. 112 (2004) 99. [15] D. Dollimore, G.R. Heal, J. Appl. Chem. 14 (1964) 109. [16] C.V. Rode, M.J. Vaidya, R. Jaganathan, R.V. Chaudhari, Chem. Eng. Sci. 56 (2001) 1299. [17] Y. Nitta, O. Yamanishi, F. Sekine, T. Imanaka, S. Teranishi, J. Catal. 79 (1983) 475.
Bimodal porous Pd–silica for liquid-phase hydrogenation Satoshi Sato *, Ryoji Takahashi, Toshiaki Sodesawa, Masashi Koubata Department of Applied Chemistry, Faculty of Engineering, Chiba University, Yayoi, Inage, Chiba 263-8522, Japan Received 14 September 2004; received in revised form 17 January 2005; accepted 7 February 2005 Available online 3 March 2005
Abstract Palladium catalysts loaded on bimodal porous silica were examined in the liquid-phase hydrogenation of 2-butenal at 0 8C and a hydrogen pressure of 1.1 MPa. The sizes of mesopores and macropores of the silica support are controllable in the range of 4–23 nm and 0.5–25 mm, respectively. The macropores provide effective paths of mass transfer, and the mesopores present effective surfaces for dispersion of metals. The catalytic activity of Pd–silica with bimodal porous structure depends on the size of mesopores and macropores as well as on the particle size of the support silica. The most active Pd–silica catalyst, with mesopores of 12 nm and macropores of 2 mm, shows much higher activity than does a commercial palladium carbon catalyst without macropores. The results indicate that the diffusion process inside the catalyst particles dominantly determines the reaction rate in the hydrogenation. # 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogenation; Pd; Bimodal porous silica; Macropore; Mesopore; Diffusivity
1. Introduction The effectiveness of a solid catalyst decreases with increasing mass-transfer resistance [1,2]. The higher the catalytic activity, the more restricted the supply of reactants from bulk liquid to the catalyst surface is. In liquid-phase hydrogenation, even in powder catalysts, the rate of mass transfer is often small enough to compete with surface reaction [3]. In the previous reports, we examined the liquid-phase hydrogenation of ketones to the corresponding secondary alcohols catalyzed by porous Ni catalysts [4–7]. The hydrogenation rate of ketones observed over Raney nickel with small pores of 3.8 nm in diameter is lower than those of Ni–MgO catalysts with ‘‘lower Ni surface area’’ and with larger mesopores [4,5]. Diffusion of reactants in the small mesopores
* Corresponding author. Tel.: +81 43 290 3376; fax: +81 43 290 3401. E-mail address: [email protected] (S. Sato). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.02.003
mesopores, which is greatly restricted in small mesopores of ca. 4 nm [8]. One of the authors has developed a kind of amorphous silica with a distinct bimodal pore structure, which is prepared by sol–gel reactions of tetraethyl orthosilicate (TEOS) in the presence of organic polymers such as polyethylene oxide (PEO) [9]. Macropores in the silica have a sharp distribution in size, which can be controlled by the initial composition such as the water/PEO/TEOS ratio. The macropores are formed when transitional morphology of spinodal decomposition is fixed by sol–gel transition of the silicate solution, and the micrometer size varies with the timing of the onset of the spinodal decomposition and gelation. Mesopores are regulated from 2 to 20 nm by dissolution–precipitation in alkaline solution during the aging process of wet silica gel [10]. We have prepared an amorphous type of silica–alumina with macropores by sol–gel reactions of TEOS in the presence of aluminum nitrate and PEO [11,12]. The silica– alumina has macropores effective for the rapid molecular transportation and mesopores providing large specific surface areas of 600 m2 g 1. In addition to the macropores, the silica–alumina has a large number of Broensted acid
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sites, and shows excellent catalytic activities in the vaporphase cracking of cumene [11]. Group VIII metals such as Ru, Rh, Pd, Ir, and Pt have been used in catalytic hydrogenation of olefins and carbonyl compounds in organic syntheses [13]. In this work, we develop a novel Pd–silica catalyst for the liquid-phase hydrogenation instead of use of Pd–carbon, utilizing large macropores in the bimodal porous silica. We also discuss the efficiency of macroporous catalyst on the liquid-phase catalysis and clarify the size of the mesopores that are effective in the hydrogenation.
2. Experimental 2.1. Catalyst samples Silica with bimodal pore structure was prepared by fixing a transitional state of phase separation by gelation of silicate solution containing polyethylene oxide with average molecular weight of 100,000 (PEO, Aldrich) [9,14]. A 14-cm3 volume of TEOS (Shin-Etsu Chemical Co., Japan) was added into 17.6 cm3 of 1 mol dm 3 nitric acid aqueous solution containing PEO under vigorous agitation. The PEO content was varied from 1.0 to 1.8 g. After the mixture had become homogeneous, it was poured into a plastic container. The container was sealed and held at 50 8C for 20 h for gelation. The resulting wet gel plate was used for the preparation of samples with different mesopore sizes adopting the solventexchange method [10,14]. The resulting gel plate was dried at 50 8C for 2 weeks, and then heated at 600 8C for 2 h. The support silica was crushed and sieved at 250–650 mm in
particle size. As a reference, non-macroporous silica was prepared without using organic polymer in the same way as done in the above-mentioned procedure. Palladium was supported on the silica by impregnation using palladium acetate/acetone solution (1 wt.%), and calcined at 500 8C for 3 h to obtain a PdO–silica precursor. The PdO–silica sample was reduced in a glass tube vessel under hydrogen flow at room temperature for 1 h to provide Pd–silica. Prior to the catalytic tests and characterization, the Pd–silica was crushed and sieved at an appropriate particle size. Reference palladium catalysts such as palladium carbon and palladium alumina were purchased from Wako Pure Chemicals and Aldrich Chemicals, respectively. A scanning electron micrograph instrument, SEM, (SM200, Topcon), was used for the observation of the morphology of the samples. A mercury porosimeter, POREMASTER-60 (Quantachrome), was used for the measurement of a wide range of pore size distribution. The adsorption–desorption isotherm of N2 was obtained at 196 8C on a commercial gas-adsorption apparatus, OMNISORP 100CX (Coulter). Prior to the adsorption, the sample was out-gassed at 300 8C for 1 h. The mesopore size distribution was calculated using the method of Dollimore and Heal [15] from the desorption branch. The specific surface area was determined by the BET method using the nitrogen adsorption isotherm. 2.2. Catalytic reaction The commercial grade reagents, which were purchased from Wako Pure Chemical Industry, were used without further purification. Hydrogenation of 2-butenal was tested,
Fig. 1. (A) A SEM image of macroporous structure, (B) pore size distribution with wide range, (C) mesopore size distribution of bimodal porous silica.
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according to the procedure described previously [5]. The solution (10 cm3) containing a reactant olefin (0.96 mol dm 3) was reacted in a stirred pressure batch reactor TVS-1 (Taiatsu Glass Industry) under the following conditions: reaction temperature of 0 8C, agitation speed of 1350 rpm, and hydrogen pressure of 1.1 MPa. The reaction mixture filtered with a glass filter was analyzed by FID-GC with a fused silica capillary column of TC-WAX (30 m). Butanal was produced stoichiometrically, and the conversion of 2-butenal achieved 100% after 30– 50 min or at least after several hours. The conversion data were fitted on the integral form of first-order reaction rate, kt = ln [1/(1 X)], where k, t, and X are the first-order reaction rate constant, reaction time, and the 2-butenal conversion, respectively [5]. Then, the rate constant, k, was divided by the catalyst weight used in the reaction to obtain the first-order rate constants per unit weight of catalyst, kw.
3. Results and discussion Fig. 1A shows a typical SEM image of silica gel with the macropores in the size of 2 mm. The size of macropores in the silica can be regulated from 0.5 to 25 mm by preparation conditions. The silica has bimodal pore structure distributed in the regions of both macropore and mesopore (Fig. 1B). In addition, the narrow distribution of mesopores of the silicas is regulated at a specific diameter from 4 to 23 nm (Fig. 1C). It is also confirmed through the t-plot method that the silicas have no micropores. The silica with bimodal pore structure can be reproduced in the same way as in the literature [9,10,14]. Fig. 2 depicts XRD profiles of Pd–silica and commercial Pd–carbon. Pd–silica before reduction has PdO on its surface (Fig. 2a). After reduction, PdO is reduced to Pd in the Pd–silica (Fig. 2b), while the commercial Pd–carbon is available in the reduced form (Fig. 2c). Sizes of Pd metals are comparable with each other. We have examined the mass-transfer resistance with respect to the agitation speed in liquid-phase hydrogenation [5]. The gas–liquid mass-transfer coefficient simply
Fig. 2. XRD profiles of Pd–SiO2 and commercial Pd–carbon. (a) Before reduction, bimodal porous silica with macropores of 2 mm and mesopores of 12 nm; (b) sample (a) reduced by H2; (c) commercial Pd–carbon. Open circle, PdO; closed circle, Pd.
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Fig. 3. Hydrogenation of 2-butenal over Pd–silica with different particle size. (a) Bimodal porous silica with macropores of 25 mm and mesopores of 12 nm; (b) non-macroporous silica with mesopores of 10 nm; (c) commercial Pd carbon. Reacted in hexane at 0 8C, agitation speed of 1350 rpm, and H2 pressure of 1.1 MPa. Pd–silica samples with particle size of 250–650 mm were crushed and sieved through sizes of 45, 90, 180, 250, and 650 mm. Pd loading is 5 wt.%.
increases with increasing agitation speed. There is a critical value of agitation speed over which the reaction rate is not affected. Hydrogen transfer from the gas to the liquid phase and through the liquid phase depends not only on the agitation speed, but also on the reactor geometry (reactor shape and size, agitator shape and size, and presence of internal baffles etc.) and on the hydrogen solubility, which also depends on nature of the solvent, pressure, temperature and so on [6,16]. In the previous work of porous nickel catalysts [5], the critical speed over which the reaction rate is not affected was observed at ca. 1200 rpm at 0 8C and 1.1 MPa: the mass-transfer resistance of reactants in the bulk liquid and of gas-to-liquid hydrogen is negligible beyond these values. Fig. 3 shows the effect of particle size on the catalytic activity of Pd–silica for the reaction of 2-butenal at 0 8C, together with differences in catalytic activity of Pd–silica between macroporous and non-macroporous catalysts. We used silica with macropores of 25 mm for the measurement because it was not fragile during the reaction. The nonmacroporous silica is also hard. The catalytic activity of Pd– silica depends on the particle size: the catalytic activity increases with decreasing particle size. It is obvious that the bimodal porous Pd–silica is more active than the nonmacroporous Pd–silica. Commercial Pd–carbon even with small particle size has catalytic activity as low as the nonmacroporous Pd–silica. We examined the effect of macropore size of bimodal porous Pd–silica on the catalytic reaction of 2-butenal (Fig. 4). Pd–silica samples with macropores show high level of the rate constant kw, while the non-macroporous Pd–silica is much less active (at macropore of 0 mm). It depends on the macropore size: macropores as small as ca. 2 mm are effective for the liquid-phase hydrogenation. This is consistent with the effect of particle size mentioned above. The macropores are necessary for the improvement of catalytic activity because of the rapid molecular transportation inside the particles. It is reasonable that the macropores
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Fig. 4. Hydrogenation of 2-butenal over Pd–silica with different macropore sizes. The silicas have a particle size of 90–180 mm and mesopores of ca. 10 nm. Pd loading is 5 wt.%. Reacted in ethanol at 0 8C, agitation speed of 1350 rpm, and H2 pressure of 1.1 MPa.
play a key role in reducing the resistance of mass transfer from bulk to surface of solid catalyst. Table 1 summarizes the catalytic properties of several samples for the hydrogenation of 2-butenal. There is no clear correlation between the catalytic activity and specific surface area: a commercial Pd–carbon with small mesopores and large specific surface area is less active. However, the catalytic activity of Pd–silica with large mesopores of 12 nm is excellent. The catalytic activity linearly increases with Pd loading. Pd–silica with macropores is superior to Pd–carbon and Pd–alumina. In the liquid-phase hydrogenation of 2-butenal, moreover, the catalytic activity in ethanol solvent has similar trends to that in hexane. To optimize the size of mesopore in the liquid-phase hydrogenation, we examined the hydrogenation using Pd– silica catalysts with different mesopore sizes, which were prepared from bimodal porous silica displayed in Fig. 1C. Fig. 5 depicts the rate constant, kw, versus the mesopore size in the bimodal porous Pd–silica. In the hydrogenation of 2butenal, the kw is maximized at 12 nm in mesopore diameter. The most active Pd–silica catalyst is eight times as active as a commercial palladium carbon at the same Pd content.
Fig. 5. Hydrogenation of 2-butenal over Pd–silica with different mesopore size. The silica has a particle size of 90–180 mm and a macropore of 2 mm. Pd loading is 5 wt.%. The mesopore size is the peak of the pore size distribution as shown in Fig. 1C. Reaction conditions are the same as those in Fig. 3.
In heterogeneous reactions over porous catalysts, either the surface reaction including adsorption, surface reaction, and desorption or the diffusion within the pore system is considered to be the major process that determines the catalytic efficiency [1,2]. In the liquid-phase hydrogenation of methyl acetoacetate to methyl 3-hydroxybutyrate using supported nickel catalysts, Nitta et al. [17] confirmed that pore diffusion of the reactant was the rate-determining step in small pores of <5 nm. In Ni–MgO with large mesopores of 14 nm, the catalytic activity was determined entirely by surface reaction, being free from mass transfer resistance [6]. We have investigated the effects of the size of reactant ketones and solvents in the liquid-phase hydrogenation [6]. Reactivity of the ketones decreases linearly with increasing molecular size. For each ketone, reactivity decreases with increasing size of the alkane solvents, from n-pentane to n-tridecane. Strong resistance for the mass-transfer of reactants and/or solvents in smaller mesopores affects the overall kinetics in the specific liquid-phase reaction using mesoporous catalysts.
Table 1 Catalytic activity of various Pd catalysts Catalyst
Pd content (wt.%)
SAa (m2 g 1)
Mesopore sizeb (nm)
Pd–carbond Pd–aluminad Pd–silicae Pd–silicae Pd–silicae Pd–silicae Pd–silicaf
5 10 2 3 5 10 5
762 250 513 513 513 288 333
3.2 – 11.9 11.9 11.9 11.9 9.5
a b c d e f g
Specific surface area measured by N2 adsorption at 196 8C. Pore diameter: peak top of pore size distribution. Catalytic activity for hydrogenation of 2-butenal, 1st order rate constant. Particle diameter, 90–180 mm. Commercially available reagents, having no macropores. Pd–silica prepared in this study, having macropores with the size of ca. 2 mm. Pd–silica prepared in this study, having no macropores. Particle diameter, 45–90 mm.
kwc (h
1
g 1)
In hexane
In ethanol
105g 28 315 544 846 1590 142
228g 309 469 660 691 1390 145
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In our latest report, we estimated pore diffusion coefficient, Dp, of several ketones in various solvents within mesopores of transparent monolithic silica plate by monitoring change in the UV absorbance [8]. Monolithic plates of the silylized porous silica with different mesopore size at ca. 4 and 10 nm were used for the Dp measurement. In the plates, Dp of the ketones decreases with increasing the size of solvents, whereas it is independent of the size of solutes, namely reactants. In the small mesopores of ca. 4 nm, the diffusivity of ketones is greatly restricted. In large mesopores >10 nm, effectiveness of catalyst is promising because of minimized mass transfer resistance. The mesopore size of 12 nm, at which the largest rate constant (Fig. 5) is obtained, is consistent with the results reported previously [6,7]. These results indicate that the size of mesopores unaffected by mass transfer resistance is much larger than the size of the reactant.
4. Conclusions The liquid-phase hydrogenation of 2-butenal was examined at 0 8C and an H2 pressure of 1.1 MPa over Pd catalysts loaded on bimodal porous silica. The sizes of mesopores and macropores of the silica support have been controlled in the ranges of 4–23 nm and 0.5–25 mm, respectively. We developed a novel Pd–silica catalyst using the bimodal porous silica. The catalytic activity is dependent on the macropore size: small macropores are effective for the hydrogenation, while non-macroporous Pd–silica is inefficient. The most active Pd catalyst with mesopores of 12 nm and macropores of 2 mm is eight times as active as a commercial palladium carbon catalyst without macropores. The macropores provide effective paths of mass transfer from the bulk to the entrances of the mesopores. The silica with macropores used as a catalyst support improves the effectiveness of supported-metal catalyst. The catalytic activity of Pd loaded on the bimodal porous silica depends on the size of mesopores as well as
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on the particle size of the support silica. An optimum mesopore size is available in the hydrogenation: small mesopores at <10 nm restrict the mass diffusion, whereas the mesopores present large surface area for metal loading. We can conclude that the diffusion process of reactants dominantly determines the reaction rate in the liquid-phase hydrogenation.
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