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Glucose hydrogenation over Ru nanoparticles embedded in templated porous carbon
Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
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Glucose hydrogenation over Ru nanoparticles embedded in templated porous carbon Jiajia Liu, X. S. Zhao* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore
Abstract In this study, the template synthesis method was used to prepare ruthenium nanoparticles sandwiched in the pore walls of porous carbons. The materials were used as a catalyst for glucose hydrogenation. Compared with a commercial nickel catalyst and ruthenium catalysts prepared using other methods, the Ru nanoparticles sandwiched in the carbon pore walls displayed a remarkably high catalytic activity and stability in hydrogenation of glucose because of the unique surface contact between the Ru nanoparticles and the carbon supports, together with the unblocked pores, firm immobilization and high dispersion of Ru nanoparticles in the carbon matrix. Moreover, the pore structure had a significant influence on the catalytic results. Keywords: ruthenium nanoparticles, porous carbon, glucose hydrogenation
1. Introduction The hydrogenation of glucose to sorbitol is of great industrial importance because sorbitol is a common product used in food, pharmaceutical and chemical industries as well as an additive in many end-products [1]. The most common catalyst in glucose hydrogenation is nickel promoted by electropositive metals, such as molybdenum and chromium [2]. However, due to the risk of nickel and metallic promoters leached into the final catalytic product, the nickel-based catalyst will soon be replaced by rutheniumbased catalyst. However, it has been reported that the deactivation of Ru-based catalyst is a critical issue [3]. Therefore, catalytically high active and stable Ru catalyst is greatly desirable. In this work, the template-synthesis method was used to prepare porous carbons with Ru nanoparticles embedded in the carbon walls. The catalytic property of the Ru catalysts thus prepared was evaluated using hydrogenation of glucose, and compared with that of a commercial catalyst and Ru catalysts prepared using traditional methods.
2. Experimental 2.1. Catalysts preparation The preparation of Ru nanoparticles sandwiched in the templated porous carbons was detailed in the previous paper [4]. Briefly, a dried hard template (either HY or SBA-15 silica) was impregnated with a ruthenium chloride hydrate solution. Then the dried Ruimpregnated solid was infiltrated of carbon using the chemical vapor deposition method. After removing the templates the Ru catalyst obtained using HY and SBA-15 as hard template are designed as RuC(HY), RuC(SBA15), respectively. Templated porous carbon and silica were impregnated with a RuCl3 solution, followed by hydrogen
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reduction to obtain four supported Ru catalysts, designed as Ru/C-HY-H (HY as template), Ru/C-SBA15-H (SBA-15 as template), Ru/HY-H, and Ru/SBA15-H, in which H denotes hydrogen reduction. The commercial nickel catalyst Ni65 (nickel, ~65wt% on silica/alumina) and RuC (Ruthenium on activated charcoal) were used as delivered. 2.2. Catalyst Characterization Characterization of the catalysts will be presented below as obtained from transmission electron microscope (TEM), and Brunauer, Emmett and Teller (BET) surface area analysis. 2.3. Measurement of catalytic activity The evaluation of the catalytic properties of the catalysts for hydrogenation of glucose was performed in a Parr batch reactor (300 mL). About 0.05g of a solid catalyst and 30 mL of 40wt% glucose solution were placed in the reactor. Subsequently, the reactor was purged with highly pure H2 three times. Then, the reaction conditions were: temperature T=100 oC, total pressure P=80 bar, stirring rate =300 rpm, reaction time t=3h. The reactant and products were analyzed using an isocratic high-performance liquid chromatograph system. The concentration of glucose was quantitatively determined based on the standard calibration curve obtained prior to every analysis using and eluent of 5 mM H2SO4 at a flow rate of 0.05 mL/min under isobaric conditions. The turnover frequency (TOF) was calculated based on the yield of moles of sorbitol per mole of metal catalyst per second.
3. Results and Discussion
(a)
(c)
(b)
(d)
Figure 1. TEM images of catalysts (a, b) RuC(HY), (c, d) RuC(SBA15) Figure 1 shows the TEM images of RuC(HY) and RuC(SBA15). Ru nanoparticles are the dark dots on the grey carbon background, which were uniformly dispersed within
Glucose hydrogenation over Ru nanoparticles embedded in templated porous carbon
1317
the carbon framework. Using zeolite HY as template, Ru particles in RuC(HY) is around 1-2 nm which is comparable to that of pore channel or cage of HY(1-2nm). While using SBA-15 as template, Ru nanoparticles in RuC(SBA15) were around 7-8 nm, corresponding to the pore channel size of SBA-15 (7-8 nm). It is also clearly seen in Figure 1d that the Ru particles were studded inside the pore walls of replicated carbon and did not block the arrayed pore channels. Thus, Ru particle size could be controlled by the template pores. The pore structure properties of RuC(HY) and RuC(SBA15) were determined by using nitrogen sorption measurement. The pore size distribution (PSD) curve of RuC(HY) was calculated using the density-functional theory (DFT) method while the PSD curve of RuC(SBA) was derived from the Barett-Joyner-Halenda (BJH) method using the adsorption branch. Figure 2 shows the adsorption-desorption isotherms, together with pore size distribution curves. It can be seen from Figure 1a that the isotherms are between type I and type IV, indicating the sample has both micropores and mesopores. In addition to a peak centered at around 1.9 nm, another peak at about 3.7 nm can be seen. The presence of mesopores is probably due to incomplete infiltration of the zeolite pores. It is seen from Figure 2b that sample RuC(SBA15) displays a type IV isotherm with an H2 hysteresis loop, indicating a mesoporous material. The BET surface area and the pore structure parameters determined from the adsorption data are summarized in Table 1. (a)
(b)
600
3
600
Volume ads. (cm /g, STP)
800
3
Volume ads. (cm /g, STP)
800
1.9nm
400 3.7nm
200 0
2
4
6
Pore size (nm ) 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/P0)
400
3.1nm
200 0 10
100
Pore size (nm)
0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/P0)
Figure 2. N2 adsorption-desorption isotherms and PSD curves (the inset) of catalysts: (a) RuC(HY), (b) RuC(SBA15) The catalytic properties of the catalysts prepared in this work in liquid-phase glucose hydrogenation were measured and compared with that of two commercial catalysts, and are summarized in Table 1. The turnover frequency (TOF) (defined as mol sorbitol yielded per mol of ruthenium per minute) values of the catalysts follow a sequence of RuC(SBA-15) > RuC(HY) > RuC > Ru/C-SBA-15-H > Ru/C-HY-H > Ni65. All Rubased catalysts are at least 20-time more active than the Ni-based catalysts. Among all of the Ru-based catalysts, RuC(SBA-15) showed the highest activity. In addition, HPLC analysis data showed that sorbitol was the only product when RuC(SBA15) was used as the catalyst. Several factors could account for the highly TOF value observed. First, the pore structure of the catalyst plays a critical role in the transport of the reactant and product. The lower catalytic activity of Ru nanoparticles supported on the microporous
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carbon (catalysts RuC(HY), Ru/C-HY-H, Ru/C) than that of Ru nanoparticle supported on the mesoporous materials RuC(SBA15) is due to the stronger mass transfer resistance of the formers to the transport of glucose and sorbitol [5]. The exception of Ru/C-SBA15-H is due to the presence of residual chlorine species when using a Ru chlorine slat as a catalyst precursor. Removal of such species during hydrogen reduction seems difficult [6]. Second, during the CVD process, Ru particles were embedded in graphitic carbon matrix, enhancing the Ru-carbon contact. This contact may stimulate the hybridization of the pz orbital of graphene (-bonded states) and the d orbital of Ru, resulting in electron transfer from Ru to the graphene of the carbon substrate. As a result, the Ru metal particles were less easily oxidized by oxygen than those nanoparticles lying on a planar surface [7]. Such a unique contact between Ru nanoparticles and carbon support may also favor hydrogen spillover [8]. Thus, hydrogen was more easily adsorbed dissociatively on the carbon surface, enhancing the hydrogenation activity of catalysts RuC(SBA15). Table 1 The physicochemical properties and catalytic activities of the catalysts. Average Glucose TOF Sample Metal Surface Pore volume pore conversion (mol/mol content area (cm3/g) diameter (%) ·s) (wt%) (m2/g) (nm) RuC(HY) 6.1 951 1.14 1.9 35 0.08 RuC(SBA15) 6.3 834 1.10 3.1 56 0.13 Ru/C-HY-H 6.1 1234 1.41 1.9 16 0.03 Ru/C-SBA15-H 5.9 389 0.56 3.2 20 0.05 Ru/HY-H 5.4 626 0.46 1.1 8 0.02 Ru/SBA15-H 5.7 616 0.90 7.4 15 0.04 Ru/C 5 690 0.48 3.7 40 0.09 Ni65 65 129 0.30 4.5 4 0.001
4. Conclusion Ru nanoparticles sandwiched in the pore walls of templated carbons exhibit a higher catalytic activity and stability in hydrogenation of glucose because of the enhanced contact between the Ru nanoparticles and the carbon matrix, together with unblocked pores of the catalyst. Moreover, the pore structure had a significant influence on the catalytic results.
References [1] K. Van Gorp, E. Boerman, C. V. Cavenaghi, P. H. Berben, Catalysis Today, 52 (1999) 349 [2] P. Gallezot, P. J. Cerino, B. Blanc, G. Fleche and P. Fuertes, J. Catal., 146 (1994) 93 [3] P. Gallezot, N. Nicolaus, G. Fleche, P. Fuertes and A. Perrard, J. Catal., 180 (1998) 51 [4] F. Su, L. Lv, F. Y. Lee, J. Liu, Tian XN, Zhao XS, Adv. Funct. Mat., 17 (2007) 1926 [5] B. Kusserow, S. Schimpf and P. Claus, Advanced Synthesis & Catalysis, 345 (2003) 289 [6] X. Wu, B. C. Gerstein and T. S. King, J. Catal., 135 (1992) 68 [7] F. Su, L. Lv, F. Y. Lee, T. Liu, A. I. Cooper and X. S. Zhao, J. Am. Chem. Soc., 129 (2007) 14213 [8] W. C. Conner and J. L. Falconer, Chemical Reviews, 95 (1995) 759
1315
Glucose hydrogenation over Ru nanoparticles embedded in templated porous carbon Jiajia Liu, X. S. Zhao* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore
Abstract In this study, the template synthesis method was used to prepare ruthenium nanoparticles sandwiched in the pore walls of porous carbons. The materials were used as a catalyst for glucose hydrogenation. Compared with a commercial nickel catalyst and ruthenium catalysts prepared using other methods, the Ru nanoparticles sandwiched in the carbon pore walls displayed a remarkably high catalytic activity and stability in hydrogenation of glucose because of the unique surface contact between the Ru nanoparticles and the carbon supports, together with the unblocked pores, firm immobilization and high dispersion of Ru nanoparticles in the carbon matrix. Moreover, the pore structure had a significant influence on the catalytic results. Keywords: ruthenium nanoparticles, porous carbon, glucose hydrogenation
1. Introduction The hydrogenation of glucose to sorbitol is of great industrial importance because sorbitol is a common product used in food, pharmaceutical and chemical industries as well as an additive in many end-products [1]. The most common catalyst in glucose hydrogenation is nickel promoted by electropositive metals, such as molybdenum and chromium [2]. However, due to the risk of nickel and metallic promoters leached into the final catalytic product, the nickel-based catalyst will soon be replaced by rutheniumbased catalyst. However, it has been reported that the deactivation of Ru-based catalyst is a critical issue [3]. Therefore, catalytically high active and stable Ru catalyst is greatly desirable. In this work, the template-synthesis method was used to prepare porous carbons with Ru nanoparticles embedded in the carbon walls. The catalytic property of the Ru catalysts thus prepared was evaluated using hydrogenation of glucose, and compared with that of a commercial catalyst and Ru catalysts prepared using traditional methods.
2. Experimental 2.1. Catalysts preparation The preparation of Ru nanoparticles sandwiched in the templated porous carbons was detailed in the previous paper [4]. Briefly, a dried hard template (either HY or SBA-15 silica) was impregnated with a ruthenium chloride hydrate solution. Then the dried Ruimpregnated solid was infiltrated of carbon using the chemical vapor deposition method. After removing the templates the Ru catalyst obtained using HY and SBA-15 as hard template are designed as RuC(HY), RuC(SBA15), respectively. Templated porous carbon and silica were impregnated with a RuCl3 solution, followed by hydrogen
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reduction to obtain four supported Ru catalysts, designed as Ru/C-HY-H (HY as template), Ru/C-SBA15-H (SBA-15 as template), Ru/HY-H, and Ru/SBA15-H, in which H denotes hydrogen reduction. The commercial nickel catalyst Ni65 (nickel, ~65wt% on silica/alumina) and RuC (Ruthenium on activated charcoal) were used as delivered. 2.2. Catalyst Characterization Characterization of the catalysts will be presented below as obtained from transmission electron microscope (TEM), and Brunauer, Emmett and Teller (BET) surface area analysis. 2.3. Measurement of catalytic activity The evaluation of the catalytic properties of the catalysts for hydrogenation of glucose was performed in a Parr batch reactor (300 mL). About 0.05g of a solid catalyst and 30 mL of 40wt% glucose solution were placed in the reactor. Subsequently, the reactor was purged with highly pure H2 three times. Then, the reaction conditions were: temperature T=100 oC, total pressure P=80 bar, stirring rate =300 rpm, reaction time t=3h. The reactant and products were analyzed using an isocratic high-performance liquid chromatograph system. The concentration of glucose was quantitatively determined based on the standard calibration curve obtained prior to every analysis using and eluent of 5 mM H2SO4 at a flow rate of 0.05 mL/min under isobaric conditions. The turnover frequency (TOF) was calculated based on the yield of moles of sorbitol per mole of metal catalyst per second.
3. Results and Discussion
(a)
(c)
(b)
(d)
Figure 1. TEM images of catalysts (a, b) RuC(HY), (c, d) RuC(SBA15) Figure 1 shows the TEM images of RuC(HY) and RuC(SBA15). Ru nanoparticles are the dark dots on the grey carbon background, which were uniformly dispersed within
Glucose hydrogenation over Ru nanoparticles embedded in templated porous carbon
1317
the carbon framework. Using zeolite HY as template, Ru particles in RuC(HY) is around 1-2 nm which is comparable to that of pore channel or cage of HY(1-2nm). While using SBA-15 as template, Ru nanoparticles in RuC(SBA15) were around 7-8 nm, corresponding to the pore channel size of SBA-15 (7-8 nm). It is also clearly seen in Figure 1d that the Ru particles were studded inside the pore walls of replicated carbon and did not block the arrayed pore channels. Thus, Ru particle size could be controlled by the template pores. The pore structure properties of RuC(HY) and RuC(SBA15) were determined by using nitrogen sorption measurement. The pore size distribution (PSD) curve of RuC(HY) was calculated using the density-functional theory (DFT) method while the PSD curve of RuC(SBA) was derived from the Barett-Joyner-Halenda (BJH) method using the adsorption branch. Figure 2 shows the adsorption-desorption isotherms, together with pore size distribution curves. It can be seen from Figure 1a that the isotherms are between type I and type IV, indicating the sample has both micropores and mesopores. In addition to a peak centered at around 1.9 nm, another peak at about 3.7 nm can be seen. The presence of mesopores is probably due to incomplete infiltration of the zeolite pores. It is seen from Figure 2b that sample RuC(SBA15) displays a type IV isotherm with an H2 hysteresis loop, indicating a mesoporous material. The BET surface area and the pore structure parameters determined from the adsorption data are summarized in Table 1. (a)
(b)
600
3
600
Volume ads. (cm /g, STP)
800
3
Volume ads. (cm /g, STP)
800
1.9nm
400 3.7nm
200 0
2
4
6
Pore size (nm ) 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/P0)
400
3.1nm
200 0 10
100
Pore size (nm)
0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/P0)
Figure 2. N2 adsorption-desorption isotherms and PSD curves (the inset) of catalysts: (a) RuC(HY), (b) RuC(SBA15) The catalytic properties of the catalysts prepared in this work in liquid-phase glucose hydrogenation were measured and compared with that of two commercial catalysts, and are summarized in Table 1. The turnover frequency (TOF) (defined as mol sorbitol yielded per mol of ruthenium per minute) values of the catalysts follow a sequence of RuC(SBA-15) > RuC(HY) > RuC > Ru/C-SBA-15-H > Ru/C-HY-H > Ni65. All Rubased catalysts are at least 20-time more active than the Ni-based catalysts. Among all of the Ru-based catalysts, RuC(SBA-15) showed the highest activity. In addition, HPLC analysis data showed that sorbitol was the only product when RuC(SBA15) was used as the catalyst. Several factors could account for the highly TOF value observed. First, the pore structure of the catalyst plays a critical role in the transport of the reactant and product. The lower catalytic activity of Ru nanoparticles supported on the microporous
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carbon (catalysts RuC(HY), Ru/C-HY-H, Ru/C) than that of Ru nanoparticle supported on the mesoporous materials RuC(SBA15) is due to the stronger mass transfer resistance of the formers to the transport of glucose and sorbitol [5]. The exception of Ru/C-SBA15-H is due to the presence of residual chlorine species when using a Ru chlorine slat as a catalyst precursor. Removal of such species during hydrogen reduction seems difficult [6]. Second, during the CVD process, Ru particles were embedded in graphitic carbon matrix, enhancing the Ru-carbon contact. This contact may stimulate the hybridization of the pz orbital of graphene (-bonded states) and the d orbital of Ru, resulting in electron transfer from Ru to the graphene of the carbon substrate. As a result, the Ru metal particles were less easily oxidized by oxygen than those nanoparticles lying on a planar surface [7]. Such a unique contact between Ru nanoparticles and carbon support may also favor hydrogen spillover [8]. Thus, hydrogen was more easily adsorbed dissociatively on the carbon surface, enhancing the hydrogenation activity of catalysts RuC(SBA15). Table 1 The physicochemical properties and catalytic activities of the catalysts. Average Glucose TOF Sample Metal Surface Pore volume pore conversion (mol/mol content area (cm3/g) diameter (%) ·s) (wt%) (m2/g) (nm) RuC(HY) 6.1 951 1.14 1.9 35 0.08 RuC(SBA15) 6.3 834 1.10 3.1 56 0.13 Ru/C-HY-H 6.1 1234 1.41 1.9 16 0.03 Ru/C-SBA15-H 5.9 389 0.56 3.2 20 0.05 Ru/HY-H 5.4 626 0.46 1.1 8 0.02 Ru/SBA15-H 5.7 616 0.90 7.4 15 0.04 Ru/C 5 690 0.48 3.7 40 0.09 Ni65 65 129 0.30 4.5 4 0.001
4. Conclusion Ru nanoparticles sandwiched in the pore walls of templated carbons exhibit a higher catalytic activity and stability in hydrogenation of glucose because of the enhanced contact between the Ru nanoparticles and the carbon matrix, together with unblocked pores of the catalyst. Moreover, the pore structure had a significant influence on the catalytic results.
References [1] K. Van Gorp, E. Boerman, C. V. Cavenaghi, P. H. Berben, Catalysis Today, 52 (1999) 349 [2] P. Gallezot, P. J. Cerino, B. Blanc, G. Fleche and P. Fuertes, J. Catal., 146 (1994) 93 [3] P. Gallezot, N. Nicolaus, G. Fleche, P. Fuertes and A. Perrard, J. Catal., 180 (1998) 51 [4] F. Su, L. Lv, F. Y. Lee, J. Liu, Tian XN, Zhao XS, Adv. Funct. Mat., 17 (2007) 1926 [5] B. Kusserow, S. Schimpf and P. Claus, Advanced Synthesis & Catalysis, 345 (2003) 289 [6] X. Wu, B. C. Gerstein and T. S. King, J. Catal., 135 (1992) 68 [7] F. Su, L. Lv, F. Y. Lee, T. Liu, A. I. Cooper and X. S. Zhao, J. Am. Chem. Soc., 129 (2007) 14213 [8] W. C. Conner and J. L. Falconer, Chemical Reviews, 95 (1995) 759