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Mesoporous silica containing sulfonic acid groups as catalysts for the alpha-pinene methoxylation
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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Mesoporous silica containing sulfonic acid groups as catalysts for the alpha-pinene methoxylation José E. Castanheiroa,b*, Liliana Guerreiroa, Isabel M. Fonsecaa, Ana M. Ramosa, Joaquim Vitala a
REQUIMTE, CQFB, Departamento de Química, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal b Centro de Química de Évora, Departamento de Química, Universidade de Évora,7000671 Évora, Portugal *Corresponding author. Tel.: +351 266745311; fax.: +351 266744971; E-mail address: [email protected]
Abstract The methoxylation of α-pinene was studied over sulfonic acid-functionalized mesoporous silica (MCM-41, PMO) at 60ºC. The support functionalization was achieved by the introduction of 3-(mercaptopropyl)trimethoxysilane onto the surface of these materials either by grafting or by co-condensation. The thiol groups were oxidized to SO3H by treatment with H2O2. All the catalysts were active in the studied reaction being the PMO-SO3H-g the best one. Good values of selectivity to α-terpinyl methyl ether were obtained with these catalysts. Catalytic stability of the PMO-SO3H-g was evaluated by performing consecutive batch runs with the same catalyst sample. After the third batch it was observed a stabilisation of the activity. Keywords: α-pinene; methoxylation, mesoporous silica; sulfonic acid groups.
1. Introduction α-Pinene is a renewable raw material usually obtained from pine gum or as a waste from the Kraft process. Its acid catalysed methoxylation yields a complex mixture of monoterpenic ethers, being α-terpinyl methyl ether the main product, which smells grapefruit-like and might be used as flavour and fragrance for perfume products. Beta zeolite [1] and ion exchange resins [2] have been used as catalyst for the α-pinene alkoxylation. Mesoporous materials, such as MCM-41 and PMO, have been used in heterogeneous catalysis as catalyst supports, due to a combination of high surface areas and controlled pore sizes. The functionalization of organic groups onto the surface of these materials could be by grafting on the surface or by co-condensation. These modified mesoporous silicas have been used as catalyst in a wide range of chemical reactions [3-5]. In this work we studied the α-pinene methoxylation over sulfonic-modified mesoporous silicas (PMOs and MCM-41). The support functionalization was achieved by the introduction of 3-(mercaptopropyl)trimethoxysilane onto the surface of these materials either by grafting or by co-condensation. The thiol groups were oxidized to SO3H by treatment with H2O2.
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2. Experimental 2.1. Catalysts preparation Two procedures were used to anchor sulfonic groups on the mesoporous silica surface: grafting and co-condensation. The introduction of sulfonic groups was made using 3mercaptopropyltrimethoxysilane (MPTMS) as the silylation reagent. Mercaptopropyl groups, anchored on the silica surface, were oxidized to the corresponding sulfonic groups by treatment with H2O2 (30 wt. %) at room temperature, for 24 h. PMO was prepared according to the literature [6] by using a triblock copolymer (Pluronic P123) as the template, while the MCM-41 was prepared as described by Corma et al. [7]. The introduction of SO3H groups by grafting was carried out according to the similar method described in the literature [8]. The samples were denoted as MCM-41-SO3H-g and PMO-SO3H-g. The introduction of SO3H groups by cocondensation was carried out according to S. Hamoudi et al. [9]. Sample was denoted as PMO-SO3H-d. 2.2. Catalysts characterization The textural characterization of the materials was based on the nitrogen adsorptiondesorption isotherms, determined at 77 K on a Micromeritics ASAP 2010 V1.01 B instrument. XRD analyses were recorded on a Brucker AXS-D8 Advance diffractometer with Cu Kalpha radiation. FTIR spectroscopy in KBr pellets was carried out on a Bio-Rad FTS 155 spectrometer. Sulphur content was determined on CHNS Elemental Analyser 1112 series Thermo Finnigan instrument. The acid capacities of the materials were determined by ion-exchange with sodium chloride, according to the literature [10]. 2.3. Catalytic experiments The catalytic experiments were carried out in a stirred batch reactor at 60ºC. In a typical experiment, the reactor was loaded with 50 mL of methanol and 0.2 g of sulfonic materials. Reactions were started by adding 3 mmol of α-pinene. Stability tests of PMO-SO3H-g catalyst were carried out by running four consecutive experiments, in the same reaction conditions. Between the catalytic experiments, the catalyst was separated from the reaction mixture by filtration, washed with methanol and dried at 100º C overnight. Samples were taken periodically and analysed by GC, using a KONIC HRGC-3000C instrument equipped with a 30 m x 0.25 mm DB-1 column.
3. Results and discussion 3.1. Catalyst characterization The textural properties of the supports and the functionalized samples are given in Table 1. We can observe that the introduction of sulfonic group decreases the surface area and pore volume, for the PMO materials, while for the MCM-41, it was observed only a slight decrease of BET area and pore volume. This result suggests that the organic molecules introduced in the mesostructure framework are occupying the surface of the pores and thus reduce the adsorption sites for the nitrogen molecules. Table 1 shows the sulphur content in the functionalised materials. The acid capacity data were below the sulphur content, indicating that the oxidation process using hydrogen peroxide is not completely efficient. It can be seen that the PMO-SO3H-g catalyst has the highest S incorporation. The XRD patterns for the modified materials are compared with the parent silicas, as shown in figure 1. We can observe that the anchoring of MPTMS by grafting on the
Mesoporous silica containing sulfonic acid groups as catalysts for the alpha-pinene methoxylation
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Table 1. Acid capacity and textural properties of the prepared materials. Vp Acid capacities SBET Titrationa S contentb S contentc S incorporation (%)d (m2/g) (cm3/g) PMO 1595 2.19 MCM-41 816 0.64 0.73 0.75 2.65 28 567 0.53 PMO-SO3H-d PMO-SO3H-g 1.43 1.48 2.15 69 764 0.80 MCM-41-SO3H-g 1.03 1.06 2.15 49 804 0.49 a Titration using NaCl 2M as ionic exchanger (mmol S/g SiO2) b Sulphur molar content determined by elemental analysis (mmol S/g SiO2) c Theoretical sulphur molar content (mmol S/g SiO2) d Incorporation of the functional groups based on elemental analysis an the theoretical values
Dp (Å) 52 34 41 48 35
Sample
MCM-41 material led to significant changes on the materials structure, while in the case of PMO no effects were observed. The collapse of the structure upon grafting of the function occurs before H2O2 treatment. It is possible to observe in Figure 1 that the peak at 2.5 º is maintained after the anchoring of MPTMS, but the two peaks in the range of 3.5-5.5 º disappear. This suggests that the final material exhibits some disorder on its structure, which is commonly observed in studies of the silylation of mesoporous silicas [11]. The PMO materials modified with sulfonic groups by co-condensation do not show any XRD peaks, suggesting a high degree of disorder. The FTIR spectra of the parent and modified silicas are in agreement with those reported Figure 1. XRD patterns of supports and modified materials. in the literature for mesoporous silicas. The incorporation of sulfonic groups is not clear by FTIR due to the low absorptivity of SH and SO3H groups [12]. MCM-41
Intensity (a.u.)
MCM-41-SO3H-g
1,5
2,5
3,5
4,5
5,5
6,5
7,5
2θ
PMO
Intensity (a.u.)
PMO-SO3H-g
1,5
2
2,5
3
3,5
4
4,5
5
2θ
5
Activity x 10 (mol/h.mmolSO3H)
5
Activity x 10 (mol/h.gcat)
12 8 3.2. Catalytic experiments The main product of α-pinene methoxylation 10 6 was α-terpinyl methyl ether being also 8 formed bornyl methyl ether, fenchyl methyl 6 4 ether, limonene and terpinolene, as byproducts. The figure 2 compares the initial 4 2 activity of mesoporous silica catalysts with 2 sulfonic acid groups on the methoxylation of 0 0 α-pinene with methanol. When the initial PMO-SO3H-g PMO-SO3H-d MCM-41-SO3H-g catalytic activity was reported to the catalyst Figure 2. Initial activity of the sulfonic acid-functionalized weight, it was observed that the PMO-SO3H- mesoporous silica in the α-pinene methoxylation. g showed a catalytic activity twice upper than the others catalysts. This result can be explained not only due to the higher porous volume of this sample and to the amount of sulfonic acid groups by gram of catalyst (table 1) but also, probably, due to the hydrophobic/hydrophilic balance of the surface of PMO-SO3H-g. On the other side, when the catalytic activity was reported to the amount of sulfonic acid groups, no significant differences were observed between PMO samples while the MCM-41-SO3H-g show the lower activity.
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Selectivity (%)
The dependence of selectivity to the α70 terpinyl methyl ether with the α-pinene 60 conversion, for the catalysts was shown on 50 figure 3. It was observed that all of the 40 catalysts tested on α-pinene methoxylation 30 exhibited good selectivity to α-terpinyl 20 methyl ether. The selectivity to ether does PMO-SO3H-d MCM-41-SO3H-g 10 not depend on the silica support. PMO-SO3H-g 0 The selectivity of the beta zeolite and ion 0 0,2 0,4 0,6 0,8 1 exchange resins to the α-terpinyl methyl Fractional conversion ether is about 36% and 45% at near Figure 3. Methoxylation of α-pinene over sulfonic complete conversion of the α-pinene, acid-functionalized mesoporous silica. Selectivity respectively, while with the PMO-SO3H-g to α-terpinyl methyl ether. catalyst is observed that the selectivity to ether is about 60 % at 90 % α-pinene conversion. However, beta zeolite and ion exchange resins show higher catalytic activity (1.8x10-3 mol/h.gcat [1] and 2.1x10-3 mol/h.gcat, respectively) than the PMO-SO3H-g catalyst. In order to study the catalytic stability of the PMO-SO3H-g, consecutive batch runs with the same catalyst sample were carried out. It was observed a decrease of the catalytic activity from the first to the third use. However, after the third batch it was observed a stabilisation of the activity. These results can be due to the leaching of sulfonic acid groups, which could only be adsorbed on mesoporous silica surface.
4. Conclusion Mesoporous catalysts consisting in sulfonic acid groups anchored on silica surface (MCM-41 and PMO) were prepared. The support fuctionalization was achieved by the introduction of 3-(mercaptopropyl)trimethoxysilane onto the surface of these materials either by grafting or by co-condensation. The thiol groups were oxidized to SO3H by treatment with H2O2. The PMO-SO3H-g sample shows the highest catalytic activity. All the catalysts tested exhibit good values of the selectivity to α-terpinyl methyl ether. The PMO-SO3H-g catalyst was evaluated by performing consecutive batch runs with the same sample. After the third batch it was observed a catalytic stabilisation of the activity. References [1] K. Hensen, C. Mahaim and W.F. Hölderich, Appl. Catal. A:Gen., 149 (1997) 311. [2] M. Yoshiharu and M. Masahiro, Jpn. Kokai, 75 131 948 (1976). [3] A.P.Wight and M.E. Davis, Chem. Rev. 102 (2002) 3589. [4] J.A. Melero, R. van Grieken and G. Morales, Chem. Rev. 106 (2006) 3790. [5] A. Taguchi and F. Schüth, Micropor. Mesopor. Mater. 77 (2005) 1. [6] X.Y. Bao, X.S. Zhao, X. Li, P.A. Chia and J. Li, J. Phys. Chem. B 108 (2004) 4684. [7] A. Corma, M.T. Navarro, J. Pérez-Pariente, F. Sánchez, Stud. Surf. Sci. Catal. 84 (1994) 69. [8] D. Das, J.-F. Lee and S. Cheng, J. Catal 223 (2004) 152. [9] S. Hamoudi, S. Royer and S. Kaliaguine, Micropor. Mesopor. Mater. 71 (2004) 17. [10] X. Wang, S. Cheng, J.C.C. Chan, J.C.H. Chao, Micropor. Mesopor. Mater. 96 (2006) 321. [11] Y. Yuan, W. Cao and W. Weng, J. Catal. 228 (2004) 311. [12] I. Díaz, C. Márquez-Alvarez, F. Mohino, J. Pérez-Pariente E. Sastre, J. Catal. 193 (2000) 283.
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Mesoporous silica containing sulfonic acid groups as catalysts for the alpha-pinene methoxylation José E. Castanheiroa,b*, Liliana Guerreiroa, Isabel M. Fonsecaa, Ana M. Ramosa, Joaquim Vitala a
REQUIMTE, CQFB, Departamento de Química, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal b Centro de Química de Évora, Departamento de Química, Universidade de Évora,7000671 Évora, Portugal *Corresponding author. Tel.: +351 266745311; fax.: +351 266744971; E-mail address: [email protected]
Abstract The methoxylation of α-pinene was studied over sulfonic acid-functionalized mesoporous silica (MCM-41, PMO) at 60ºC. The support functionalization was achieved by the introduction of 3-(mercaptopropyl)trimethoxysilane onto the surface of these materials either by grafting or by co-condensation. The thiol groups were oxidized to SO3H by treatment with H2O2. All the catalysts were active in the studied reaction being the PMO-SO3H-g the best one. Good values of selectivity to α-terpinyl methyl ether were obtained with these catalysts. Catalytic stability of the PMO-SO3H-g was evaluated by performing consecutive batch runs with the same catalyst sample. After the third batch it was observed a stabilisation of the activity. Keywords: α-pinene; methoxylation, mesoporous silica; sulfonic acid groups.
1. Introduction α-Pinene is a renewable raw material usually obtained from pine gum or as a waste from the Kraft process. Its acid catalysed methoxylation yields a complex mixture of monoterpenic ethers, being α-terpinyl methyl ether the main product, which smells grapefruit-like and might be used as flavour and fragrance for perfume products. Beta zeolite [1] and ion exchange resins [2] have been used as catalyst for the α-pinene alkoxylation. Mesoporous materials, such as MCM-41 and PMO, have been used in heterogeneous catalysis as catalyst supports, due to a combination of high surface areas and controlled pore sizes. The functionalization of organic groups onto the surface of these materials could be by grafting on the surface or by co-condensation. These modified mesoporous silicas have been used as catalyst in a wide range of chemical reactions [3-5]. In this work we studied the α-pinene methoxylation over sulfonic-modified mesoporous silicas (PMOs and MCM-41). The support functionalization was achieved by the introduction of 3-(mercaptopropyl)trimethoxysilane onto the surface of these materials either by grafting or by co-condensation. The thiol groups were oxidized to SO3H by treatment with H2O2.
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2. Experimental 2.1. Catalysts preparation Two procedures were used to anchor sulfonic groups on the mesoporous silica surface: grafting and co-condensation. The introduction of sulfonic groups was made using 3mercaptopropyltrimethoxysilane (MPTMS) as the silylation reagent. Mercaptopropyl groups, anchored on the silica surface, were oxidized to the corresponding sulfonic groups by treatment with H2O2 (30 wt. %) at room temperature, for 24 h. PMO was prepared according to the literature [6] by using a triblock copolymer (Pluronic P123) as the template, while the MCM-41 was prepared as described by Corma et al. [7]. The introduction of SO3H groups by grafting was carried out according to the similar method described in the literature [8]. The samples were denoted as MCM-41-SO3H-g and PMO-SO3H-g. The introduction of SO3H groups by cocondensation was carried out according to S. Hamoudi et al. [9]. Sample was denoted as PMO-SO3H-d. 2.2. Catalysts characterization The textural characterization of the materials was based on the nitrogen adsorptiondesorption isotherms, determined at 77 K on a Micromeritics ASAP 2010 V1.01 B instrument. XRD analyses were recorded on a Brucker AXS-D8 Advance diffractometer with Cu Kalpha radiation. FTIR spectroscopy in KBr pellets was carried out on a Bio-Rad FTS 155 spectrometer. Sulphur content was determined on CHNS Elemental Analyser 1112 series Thermo Finnigan instrument. The acid capacities of the materials were determined by ion-exchange with sodium chloride, according to the literature [10]. 2.3. Catalytic experiments The catalytic experiments were carried out in a stirred batch reactor at 60ºC. In a typical experiment, the reactor was loaded with 50 mL of methanol and 0.2 g of sulfonic materials. Reactions were started by adding 3 mmol of α-pinene. Stability tests of PMO-SO3H-g catalyst were carried out by running four consecutive experiments, in the same reaction conditions. Between the catalytic experiments, the catalyst was separated from the reaction mixture by filtration, washed with methanol and dried at 100º C overnight. Samples were taken periodically and analysed by GC, using a KONIC HRGC-3000C instrument equipped with a 30 m x 0.25 mm DB-1 column.
3. Results and discussion 3.1. Catalyst characterization The textural properties of the supports and the functionalized samples are given in Table 1. We can observe that the introduction of sulfonic group decreases the surface area and pore volume, for the PMO materials, while for the MCM-41, it was observed only a slight decrease of BET area and pore volume. This result suggests that the organic molecules introduced in the mesostructure framework are occupying the surface of the pores and thus reduce the adsorption sites for the nitrogen molecules. Table 1 shows the sulphur content in the functionalised materials. The acid capacity data were below the sulphur content, indicating that the oxidation process using hydrogen peroxide is not completely efficient. It can be seen that the PMO-SO3H-g catalyst has the highest S incorporation. The XRD patterns for the modified materials are compared with the parent silicas, as shown in figure 1. We can observe that the anchoring of MPTMS by grafting on the
Mesoporous silica containing sulfonic acid groups as catalysts for the alpha-pinene methoxylation
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Table 1. Acid capacity and textural properties of the prepared materials. Vp Acid capacities SBET Titrationa S contentb S contentc S incorporation (%)d (m2/g) (cm3/g) PMO 1595 2.19 MCM-41 816 0.64 0.73 0.75 2.65 28 567 0.53 PMO-SO3H-d PMO-SO3H-g 1.43 1.48 2.15 69 764 0.80 MCM-41-SO3H-g 1.03 1.06 2.15 49 804 0.49 a Titration using NaCl 2M as ionic exchanger (mmol S/g SiO2) b Sulphur molar content determined by elemental analysis (mmol S/g SiO2) c Theoretical sulphur molar content (mmol S/g SiO2) d Incorporation of the functional groups based on elemental analysis an the theoretical values
Dp (Å) 52 34 41 48 35
Sample
MCM-41 material led to significant changes on the materials structure, while in the case of PMO no effects were observed. The collapse of the structure upon grafting of the function occurs before H2O2 treatment. It is possible to observe in Figure 1 that the peak at 2.5 º is maintained after the anchoring of MPTMS, but the two peaks in the range of 3.5-5.5 º disappear. This suggests that the final material exhibits some disorder on its structure, which is commonly observed in studies of the silylation of mesoporous silicas [11]. The PMO materials modified with sulfonic groups by co-condensation do not show any XRD peaks, suggesting a high degree of disorder. The FTIR spectra of the parent and modified silicas are in agreement with those reported Figure 1. XRD patterns of supports and modified materials. in the literature for mesoporous silicas. The incorporation of sulfonic groups is not clear by FTIR due to the low absorptivity of SH and SO3H groups [12]. MCM-41
Intensity (a.u.)
MCM-41-SO3H-g
1,5
2,5
3,5
4,5
5,5
6,5
7,5
2θ
PMO
Intensity (a.u.)
PMO-SO3H-g
1,5
2
2,5
3
3,5
4
4,5
5
2θ
5
Activity x 10 (mol/h.mmolSO3H)
5
Activity x 10 (mol/h.gcat)
12 8 3.2. Catalytic experiments The main product of α-pinene methoxylation 10 6 was α-terpinyl methyl ether being also 8 formed bornyl methyl ether, fenchyl methyl 6 4 ether, limonene and terpinolene, as byproducts. The figure 2 compares the initial 4 2 activity of mesoporous silica catalysts with 2 sulfonic acid groups on the methoxylation of 0 0 α-pinene with methanol. When the initial PMO-SO3H-g PMO-SO3H-d MCM-41-SO3H-g catalytic activity was reported to the catalyst Figure 2. Initial activity of the sulfonic acid-functionalized weight, it was observed that the PMO-SO3H- mesoporous silica in the α-pinene methoxylation. g showed a catalytic activity twice upper than the others catalysts. This result can be explained not only due to the higher porous volume of this sample and to the amount of sulfonic acid groups by gram of catalyst (table 1) but also, probably, due to the hydrophobic/hydrophilic balance of the surface of PMO-SO3H-g. On the other side, when the catalytic activity was reported to the amount of sulfonic acid groups, no significant differences were observed between PMO samples while the MCM-41-SO3H-g show the lower activity.
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Selectivity (%)
The dependence of selectivity to the α70 terpinyl methyl ether with the α-pinene 60 conversion, for the catalysts was shown on 50 figure 3. It was observed that all of the 40 catalysts tested on α-pinene methoxylation 30 exhibited good selectivity to α-terpinyl 20 methyl ether. The selectivity to ether does PMO-SO3H-d MCM-41-SO3H-g 10 not depend on the silica support. PMO-SO3H-g 0 The selectivity of the beta zeolite and ion 0 0,2 0,4 0,6 0,8 1 exchange resins to the α-terpinyl methyl Fractional conversion ether is about 36% and 45% at near Figure 3. Methoxylation of α-pinene over sulfonic complete conversion of the α-pinene, acid-functionalized mesoporous silica. Selectivity respectively, while with the PMO-SO3H-g to α-terpinyl methyl ether. catalyst is observed that the selectivity to ether is about 60 % at 90 % α-pinene conversion. However, beta zeolite and ion exchange resins show higher catalytic activity (1.8x10-3 mol/h.gcat [1] and 2.1x10-3 mol/h.gcat, respectively) than the PMO-SO3H-g catalyst. In order to study the catalytic stability of the PMO-SO3H-g, consecutive batch runs with the same catalyst sample were carried out. It was observed a decrease of the catalytic activity from the first to the third use. However, after the third batch it was observed a stabilisation of the activity. These results can be due to the leaching of sulfonic acid groups, which could only be adsorbed on mesoporous silica surface.
4. Conclusion Mesoporous catalysts consisting in sulfonic acid groups anchored on silica surface (MCM-41 and PMO) were prepared. The support fuctionalization was achieved by the introduction of 3-(mercaptopropyl)trimethoxysilane onto the surface of these materials either by grafting or by co-condensation. The thiol groups were oxidized to SO3H by treatment with H2O2. The PMO-SO3H-g sample shows the highest catalytic activity. All the catalysts tested exhibit good values of the selectivity to α-terpinyl methyl ether. The PMO-SO3H-g catalyst was evaluated by performing consecutive batch runs with the same sample. After the third batch it was observed a catalytic stabilisation of the activity. References [1] K. Hensen, C. Mahaim and W.F. Hölderich, Appl. Catal. A:Gen., 149 (1997) 311. [2] M. Yoshiharu and M. Masahiro, Jpn. Kokai, 75 131 948 (1976). [3] A.P.Wight and M.E. Davis, Chem. Rev. 102 (2002) 3589. [4] J.A. Melero, R. van Grieken and G. Morales, Chem. Rev. 106 (2006) 3790. [5] A. Taguchi and F. Schüth, Micropor. Mesopor. Mater. 77 (2005) 1. [6] X.Y. Bao, X.S. Zhao, X. Li, P.A. Chia and J. Li, J. Phys. Chem. B 108 (2004) 4684. [7] A. Corma, M.T. Navarro, J. Pérez-Pariente, F. Sánchez, Stud. Surf. Sci. Catal. 84 (1994) 69. [8] D. Das, J.-F. Lee and S. Cheng, J. Catal 223 (2004) 152. [9] S. Hamoudi, S. Royer and S. Kaliaguine, Micropor. Mesopor. Mater. 71 (2004) 17. [10] X. Wang, S. Cheng, J.C.C. Chan, J.C.H. Chao, Micropor. Mesopor. Mater. 96 (2006) 321. [11] Y. Yuan, W. Cao and W. Weng, J. Catal. 228 (2004) 311. [12] I. Díaz, C. Márquez-Alvarez, F. Mohino, J. Pérez-Pariente E. Sastre, J. Catal. 193 (2000) 283.