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Catalytic Activity of Co-HMS Modified by Organic Groups for Cyclohexane Oxidation
CHINESE JOURNAL OF CATALYSIS Volume 29, Issue 1, January 2008 Online English edition of the Chinese language journal Cite this article as: Chin J Catal, 2008, 29(1): 4–6.
SHORT COMMUNICATION
Catalytic Activity of Co-HMS Modified by Organic Groups for Cyclohexane Oxidation CHEN Chen1,2, ZHANG Qiaohong1,2, MA Hong1, GAO Jin1, SUN Zhiqiang1, XU Jie1,* 1
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
2
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: By a one-step co-condensation route using hexadecylamine as the template, various organic groups were successfully immobilized on the surface of hexagonal mesoporous silicas (HMS) along with Co2+ ions incorporated into the framework. The obtained bifunctionalized materials showed high activity in the selective oxidation of cyclohexane. The Co2+ cations were active for oxidation while the organic groups played the key roles in modifying the surface properties. During the oxidation reaction, a phenyl group was found to be better than methyl and propyl groups. Key words: organic modification; mesoporous silica; cyclohexane; selective oxidation
Hexagonal mesoporous silica (HMS) can be synthesized by the assembly pathway using hydrogen-bonding interactions between neutral primary alkylamine and neutral inorganic precursors at room temperature. Compared with the M41S materials, HMS has better thermal, hydrothermal, and mechanical stability [1,2] and are attractive for catalytic applications. The incorporation of redox metal ions into the framework of HMS is an important method to give it catalytic activity [3,4]. Another method is to modify the surface of HMS using organic group immobilization [5]. By combining these two methods, we have obtained a bifunctionalized material CoPh-HMS, which showed excellent activity in our catalytic application [6]. The organic group played a key role in the modification of the catalyst surface and changed the surface from hydrophilic to hydrophobic. In this paper, various organic groups were used to study their effect on catalyst performance. The catalyst was synthesized by the S0I0 assembly pathway at room temperature using hexadecylamine (HDA) as template, tetraethyl orthosilicate (TEOS) and silane as silicon sources, Co(OAc)2·4H2O as Co source, and mesitylene as swelling agent. The molar composition of the gel was 0.02 Co : 0.8 TEOS : 0.2 silane : 0.27 HDA : 0.27 mesitylene : 9 EtOH : 72 H2O. The silanes were methyltriethoxysilane, propyltrimethoxysilane, and phenyltriethoxysilane. The materials obtained
are designated CoMe-HMS, CoPr-HMS, and CoPh-HMS, respectively. Non-organofunctionalized Co-HMS was synthesized without silane and mesitylene. The catalytic properties of these materials for the liquid-phase oxidation of cyclohexane with molecular oxygen in the absence of solvent were studied. The reactions were performed in a 100 ml autoclave reactor with a Teflon inside. The analytic method was the same as described in the literature [7]. Table 1 compares the results over the different catalysts at 388 K for 6 h. It can be seen that compared with a non-organofunctionalized Co-HMS and homogeneous catalyst Co(OAc)2, the catalysts modified by organic groups showed higher activity. In particular, the selectivity for the target product K-A oil (cyclohexanol and cyclohexanone) was also improved. Among the catalysts, CoPh-HMS showed the best activity with 6.7% cyclohexane conversion and 80.5% selectivity for K-A oil. The activity of CoMe-HMS was a little less than that of CoPh-HMS, but compared with that of Co-HMS, it still showed an increase of 20.8% relatively. However, when CoPr-HMS was used as the catalyst, only the selectivity to K-A oil increased by 2.8%, and no evident increase in cyclohexane conversion was observed. This may be caused by the poor mesoporous structure of CoPr-HMS, which is not conducive for the effective diffusion of the reactant and products (Fig. 1).
Received date: 2007-11-01. * Corresponding author. Tel/Fax: +86-411-84379245; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (20672111, 20736010) and the Knowledge Innovation Program of the Chinese Academy of Sciences (K2007D6). Copyright © 2008, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
CHEN Chen et al. / Chinese Journal of Catalysis, 2008, 29(1): 4–6
Table 1 Catalytic activity of various catalysts for oxidation of cyclohexane Catalyst
Product distribution (%)
Conversion (%)
K-A oil
Acid
Ester
Co(OAc)2
3.3
78.2
15.0
2.5
Co-HMS
4.8
76.9
15.6
7.1
CoMe-HMS
5.8
78.8
14.3
6.5
CoPr-HMS
4.9
79.7
15.5
4.5
CoPh-HMS
6.7
80.5
15.4
3.8
Reaction conditions: catalyst 0.12 g, tert-butyl hydroperoxide 0.12 g, cyclohexane 15 g, 388 K, 6 h, 1.0 MPa O2. K-A oil, cyclohexanol and cyclohexanone; Acid, mainly adipic acid; Ester, mainly dicyclohexyl adipate; the rest is cyclohexyl hydroperoxide.
The infrared spectra of the samples are shown in Fig. 2. Typical bands of the corresponding organic groups can be seen in their spectra. For example, in the spectrum of CoMe-HMS, the absorption bands at 2856 and 2928 cm–1 are the symmetric and asymmetric C–H stretching vibrations of the methyl groups, and the absorption band at 1276 cm–1 is due to Si–C vibration [9]. In the spectrum of CoPr-HMS, besides the adsorption bands associated with methyl groups, adsorption bands at 2875 and 1460 cm–1 that are assigned to C–H stretching and bending vibrations of methylene groups can also be observed. Typical bands of the phenyl groups and Si–Ph vibrations were detected in the spectrum of CoPh-HMS [6]. The presence of these adsorption bands is a proof of the successful immobilization of these organic groups.
Generally, the immobilization of organic groups on the surface of a catalyst modifies the surface to become more hydrophobic and facilitates the adsorption of low-polar cyclohexane and desorption of the polar products. Thus, high cyclohexane conversion and K-A oil selectivity were obtained. Due to their weakest polarity, phenyl groups can facilitate the above diffusion more and have the highest activity.
Fig. 2 FT-IR spectra of various catalysts (1) Co-HMS, (2) CoMe-HMS, (3) CoPr-HMS, (4) CoPh-HMS
Fig. 1 Small-angle XRD patterns of various catalysts (1) CoMe-HMS, (2) CoPr-HMS, (3) CoPh-HMS
The small-angle X-ray diffraction patterns of CoMe-HMS, CoPr-HMS, and CoPh-HMS are shown in Fig. 1. Each sample exhibited a diffraction peak corresponding to the (100) plane at 2θ equal 1°–4°, which is characteristic of HMS materials [1–4]. From the TEM and N2 adsorption-desorption measurements, it can also be seen that all these materials possess a mesoporous structure. However, compared with CoMe-HMS and CoPh-HMS, the peak intensity of CoPr-HMS is much lower, implying that the immobilization of propyl groups decreased the crystallinity of the material. A similar phenomenon was observed by Mercier et al. [8] when they synthesized Pr-HMS. During the synthesis process, due to its large bulk, propyl groups can cause the curvature of the micelle assembly to increase and thus decrease the degree of order.
In conclusion, using the incorporation of Co2+ in the framework and immobilization of organic groups on the surface of HMS, efficient bifunctionalized catalysts for the selective oxidation of cyclohexane were obtained. On comparing the modification effects by the immobilization of various organic groups, it was found that the least polar phenyl group gave the best activity.
References [1] Bagshaw S A, Prouzet E, Pinnavaia T J. Science, 1995, 269(5228): 1242 [2] Tanev P T, Pinnavaia T J. Chem Mater, 1996, 8(8): 2068 [3] Tanev P T, Chibwe M, Pinnavaia T J. Nature, 1994, 368(6469): 321 [4] Liu H, Lu G Z, Guo Y L, Guo Y, Wang J S. Nanotechnology, 2006, 17(4): 997 [5] Mori Y, Pinnavaia T J. Chem Mater, 2001, 13(6): 2173 [6] Chen C, Zhou L P, Zhang Q H, Ma H, Miao H, Xu J. Nanotech-
CHEN Chen et al. / Chinese Journal of Catalysis, 2008, 29(1): 4–6
nology, 2007, 18(21): 215603 [7] Zhou L P, Xu J, Miao H, Wang F, Li X Q. Appl Catal A, 2005, 292: 223
[8] Mercier L, Pinnavaia T J. Chem Mater, 2000, 12(1): 188 [9] Hasegawa I, Imamura W, Takayama T. Inorg Chem Commun, 2004, 7(4): 513
SHORT COMMUNICATION
Catalytic Activity of Co-HMS Modified by Organic Groups for Cyclohexane Oxidation CHEN Chen1,2, ZHANG Qiaohong1,2, MA Hong1, GAO Jin1, SUN Zhiqiang1, XU Jie1,* 1
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
2
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: By a one-step co-condensation route using hexadecylamine as the template, various organic groups were successfully immobilized on the surface of hexagonal mesoporous silicas (HMS) along with Co2+ ions incorporated into the framework. The obtained bifunctionalized materials showed high activity in the selective oxidation of cyclohexane. The Co2+ cations were active for oxidation while the organic groups played the key roles in modifying the surface properties. During the oxidation reaction, a phenyl group was found to be better than methyl and propyl groups. Key words: organic modification; mesoporous silica; cyclohexane; selective oxidation
Hexagonal mesoporous silica (HMS) can be synthesized by the assembly pathway using hydrogen-bonding interactions between neutral primary alkylamine and neutral inorganic precursors at room temperature. Compared with the M41S materials, HMS has better thermal, hydrothermal, and mechanical stability [1,2] and are attractive for catalytic applications. The incorporation of redox metal ions into the framework of HMS is an important method to give it catalytic activity [3,4]. Another method is to modify the surface of HMS using organic group immobilization [5]. By combining these two methods, we have obtained a bifunctionalized material CoPh-HMS, which showed excellent activity in our catalytic application [6]. The organic group played a key role in the modification of the catalyst surface and changed the surface from hydrophilic to hydrophobic. In this paper, various organic groups were used to study their effect on catalyst performance. The catalyst was synthesized by the S0I0 assembly pathway at room temperature using hexadecylamine (HDA) as template, tetraethyl orthosilicate (TEOS) and silane as silicon sources, Co(OAc)2·4H2O as Co source, and mesitylene as swelling agent. The molar composition of the gel was 0.02 Co : 0.8 TEOS : 0.2 silane : 0.27 HDA : 0.27 mesitylene : 9 EtOH : 72 H2O. The silanes were methyltriethoxysilane, propyltrimethoxysilane, and phenyltriethoxysilane. The materials obtained
are designated CoMe-HMS, CoPr-HMS, and CoPh-HMS, respectively. Non-organofunctionalized Co-HMS was synthesized without silane and mesitylene. The catalytic properties of these materials for the liquid-phase oxidation of cyclohexane with molecular oxygen in the absence of solvent were studied. The reactions were performed in a 100 ml autoclave reactor with a Teflon inside. The analytic method was the same as described in the literature [7]. Table 1 compares the results over the different catalysts at 388 K for 6 h. It can be seen that compared with a non-organofunctionalized Co-HMS and homogeneous catalyst Co(OAc)2, the catalysts modified by organic groups showed higher activity. In particular, the selectivity for the target product K-A oil (cyclohexanol and cyclohexanone) was also improved. Among the catalysts, CoPh-HMS showed the best activity with 6.7% cyclohexane conversion and 80.5% selectivity for K-A oil. The activity of CoMe-HMS was a little less than that of CoPh-HMS, but compared with that of Co-HMS, it still showed an increase of 20.8% relatively. However, when CoPr-HMS was used as the catalyst, only the selectivity to K-A oil increased by 2.8%, and no evident increase in cyclohexane conversion was observed. This may be caused by the poor mesoporous structure of CoPr-HMS, which is not conducive for the effective diffusion of the reactant and products (Fig. 1).
Received date: 2007-11-01. * Corresponding author. Tel/Fax: +86-411-84379245; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (20672111, 20736010) and the Knowledge Innovation Program of the Chinese Academy of Sciences (K2007D6). Copyright © 2008, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
CHEN Chen et al. / Chinese Journal of Catalysis, 2008, 29(1): 4–6
Table 1 Catalytic activity of various catalysts for oxidation of cyclohexane Catalyst
Product distribution (%)
Conversion (%)
K-A oil
Acid
Ester
Co(OAc)2
3.3
78.2
15.0
2.5
Co-HMS
4.8
76.9
15.6
7.1
CoMe-HMS
5.8
78.8
14.3
6.5
CoPr-HMS
4.9
79.7
15.5
4.5
CoPh-HMS
6.7
80.5
15.4
3.8
Reaction conditions: catalyst 0.12 g, tert-butyl hydroperoxide 0.12 g, cyclohexane 15 g, 388 K, 6 h, 1.0 MPa O2. K-A oil, cyclohexanol and cyclohexanone; Acid, mainly adipic acid; Ester, mainly dicyclohexyl adipate; the rest is cyclohexyl hydroperoxide.
The infrared spectra of the samples are shown in Fig. 2. Typical bands of the corresponding organic groups can be seen in their spectra. For example, in the spectrum of CoMe-HMS, the absorption bands at 2856 and 2928 cm–1 are the symmetric and asymmetric C–H stretching vibrations of the methyl groups, and the absorption band at 1276 cm–1 is due to Si–C vibration [9]. In the spectrum of CoPr-HMS, besides the adsorption bands associated with methyl groups, adsorption bands at 2875 and 1460 cm–1 that are assigned to C–H stretching and bending vibrations of methylene groups can also be observed. Typical bands of the phenyl groups and Si–Ph vibrations were detected in the spectrum of CoPh-HMS [6]. The presence of these adsorption bands is a proof of the successful immobilization of these organic groups.
Generally, the immobilization of organic groups on the surface of a catalyst modifies the surface to become more hydrophobic and facilitates the adsorption of low-polar cyclohexane and desorption of the polar products. Thus, high cyclohexane conversion and K-A oil selectivity were obtained. Due to their weakest polarity, phenyl groups can facilitate the above diffusion more and have the highest activity.
Fig. 2 FT-IR spectra of various catalysts (1) Co-HMS, (2) CoMe-HMS, (3) CoPr-HMS, (4) CoPh-HMS
Fig. 1 Small-angle XRD patterns of various catalysts (1) CoMe-HMS, (2) CoPr-HMS, (3) CoPh-HMS
The small-angle X-ray diffraction patterns of CoMe-HMS, CoPr-HMS, and CoPh-HMS are shown in Fig. 1. Each sample exhibited a diffraction peak corresponding to the (100) plane at 2θ equal 1°–4°, which is characteristic of HMS materials [1–4]. From the TEM and N2 adsorption-desorption measurements, it can also be seen that all these materials possess a mesoporous structure. However, compared with CoMe-HMS and CoPh-HMS, the peak intensity of CoPr-HMS is much lower, implying that the immobilization of propyl groups decreased the crystallinity of the material. A similar phenomenon was observed by Mercier et al. [8] when they synthesized Pr-HMS. During the synthesis process, due to its large bulk, propyl groups can cause the curvature of the micelle assembly to increase and thus decrease the degree of order.
In conclusion, using the incorporation of Co2+ in the framework and immobilization of organic groups on the surface of HMS, efficient bifunctionalized catalysts for the selective oxidation of cyclohexane were obtained. On comparing the modification effects by the immobilization of various organic groups, it was found that the least polar phenyl group gave the best activity.
References [1] Bagshaw S A, Prouzet E, Pinnavaia T J. Science, 1995, 269(5228): 1242 [2] Tanev P T, Pinnavaia T J. Chem Mater, 1996, 8(8): 2068 [3] Tanev P T, Chibwe M, Pinnavaia T J. Nature, 1994, 368(6469): 321 [4] Liu H, Lu G Z, Guo Y L, Guo Y, Wang J S. Nanotechnology, 2006, 17(4): 997 [5] Mori Y, Pinnavaia T J. Chem Mater, 2001, 13(6): 2173 [6] Chen C, Zhou L P, Zhang Q H, Ma H, Miao H, Xu J. Nanotech-
CHEN Chen et al. / Chinese Journal of Catalysis, 2008, 29(1): 4–6
nology, 2007, 18(21): 215603 [7] Zhou L P, Xu J, Miao H, Wang F, Li X Q. Appl Catal A, 2005, 292: 223
[8] Mercier L, Pinnavaia T J. Chem Mater, 2000, 12(1): 188 [9] Hasegawa I, Imamura W, Takayama T. Inorg Chem Commun, 2004, 7(4): 513