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H2O2 Catalytic System
CHINESE JOURNAL OF CATALYSIS Volume 29, Issue 2, February 2008 Online English edition of the Chinese language journal Cite this article as: Chin J Catal, 2008, 29(2): 102–104.
SHORT COMMUNICATION
Epoxidation of Various Functionalized Olefins by a Ti-MWW/H2O2 Catalytic System LI Ningning, LIU Yueming, WU Haihong, LI Xiaohong, XIE Wei, ZHAO Zhonglin, WU Peng*, HE Mingyuan Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
Abstract: The catalytic properties of Ti-MWW catalysts for the epoxidation of olefins with different functional groups using hydrogen peroxide as an oxidant were studied. Ti-MWW shows higher olefin conversion and epoxide selectivity than the conventional titanosilicate TS-1. Solvents influence the catalytic activity of Ti-MWW, and water is the best one in the epoxidation of ethyl acrylate and allyl acetate. The functional groups with high electrophilicity and adjacent to the C=C bonds retard the catalytic activity for olefin epoxidation. Key words: Ti-MWW; TS-1; functionalized olefin; epoxidation
Organic epoxides, a class of important intermediates in organic synthesis, are widely used in the organic chemistry and fine chemicals industries. Conventional methods for the production of epoxides are the chlorohydrin and co-oxidative routes, both of which suffer serious environmental pollution problems. The discovery of the titanosilicate/H2O2 catalytic system has given a new environmentally friendly process for the epoxidation of olefins. The first titanosilicate, TS-1, was reported to show high catalytic activity in the epoxidation of olefins [1–2]. However, TS-1 with the MFI topology has only a 10-membered ring (MR) channel system. Its pore entrance causes a serious steric restriction on the access of the substrate molecules to the Ti active species located inside the channels, which makes the absolute yield of epoxide products from TS-1 low. Moreover, the synthesis of a TS-1 catalyst with a high activity requires the use of expensive tetrapropylammonium hydroxide as a structure-directing agent, which leads to a high preparation cost for TS-1 and limits its application to some extent [3]. Recently, Ti-MWW, which is a new titanosilicate with the MWW topology, was developed. It possesses a unique pore structure consisting of two independent 10-MR channels, one of which contains a 12-MR supercage of 0.7 nm × 0.7 nm × 1.8
nm in dimension [4,5]. In addition, there are 12-MR side cups on the exterior crystal surface of the MWW zeolite. Thus, Ti-MWW is superior to other titanosilicates in the epoxidation of olefins [6–8]. It is necessary to study the catalytic properties of Ti-MWW in the epoxidation of various functionalized olefins with the purpose to produce the corresponding epoxides to provide the background for industrial applications. A series of Ti-MWW [9] and TS-1 [10] with different Si/Ti molar ratios were hydrothermally synthesized following previously reported methods. X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) images, and N2 adsorption measurements confirmed that they were highly crystalline materials. Both UV-Vis and IR spectra indicated that the Ti species mostly existed in the framework as a tetrahedrally coordinated state. The molar ratios of Si/Ti were analyzed by inductively coupled plasma (ICP). The epoxidation of olefins was carried out in a 50-ml flask reactor with magnetic stirring. The reaction products were analyzed using a Shimadzu 14B chromatograph equipped with a DB-1 capillary column and a FID detector. The remaining H2O2 in the reaction mixture was analyzed by titration with Ce(SO4)2 solution. The catalytic properties of Ti-MWW and TS-1 with various Ti contents in the epoxidation of allyl alcohol (AAL) were first
Received date: 2007-12-04. * Corresponding author. Tel/Fax: +86-21-62232292; E-mail: [email protected] Foundation item: Supported by the National Basic Research Program of China (973 Program, 2006CB202508), the National High Technology Research and Development Program of China (863 Program, 2007AA03Z342), the National Natural Science Foundation of China (20673038), the Science and Technology Commission of Shanghai Municipality (06SR07101, 07QA14017), and the Shanghai Leading Academic Discipline Project (B409). Copyright © 2008, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
LIU Ningning et al. / Chinese Journal of Catalysis, 2008, 29(2): 102–104
oxidation of allyl alcohol and diallyl ether where acetonitrile and acetone were shown to be the optimal solvent [11,12]. Here, water was the most suitable solvent in the epoxidation of ethyl acrylate and allyl acetate, and the conversion in acetone was higher than that in acetonitrile. This is probably because ethyl acrylate is soluble in water, while allyl acetate is only partially soluble in water, which leads to a decrease in the diffusion rate of the substrate in the order water > acetone > acetonitrile. On the other hand, the solubility of diallyl ether in water is much lower. Conversely, in the case of aprotic solvents (acetonitrile and acetone) and protic solvents (methanol and ethanol), the five-membered cyclic intermediate species are easily formed during reaction through the interaction of Ti active sites with the solvent, particularly with protic molecules [1]. These bulky intermediate species can cause a serious steric restriction on the substrate molecules, which then makes the conversion lower. Moreover, protic solvents may cause the solvolysis of epoxide products to etherification byproducts, which decreases the epoxide selectivity significantly. A detailed investigation on the epoxidation of allyl acetate showed that the hydrolysis of the epoxide product of glycidyl acetate proceeded gradually with prolonged reaction time and reached a diol selectivity of 14.7% at 2 h. In addition, the Ti-MWW catalyst without a de-boronation treatment catalyzed the hydrolysis of epoxide more easily than the de-boronated catalyst. This is because the former catalyst contained more Brönsted acid sites associated with the boron ions in the framework. When the used Ti-MWW catalyst was activated by washing with acetone and then calcination at 773 K in air for 5 h, it showed almost the same allyl acetate conversion and H2O2 utilization efficiency up to the fourth reuse. ICP analyses showed no obvious leaching of Ti species from the framework during the catalyst recycling. XRD patterns and UV-Vis spectra indicated that the crystalline structure and the coordination state of Ti species of the used catalyst were nearly the same as those of fresh Ti-MWW. These results indicated that Ti-MWW
Fig. 1 Comparison of allyl alcohol epoxidation catalyzed by Ti-MWW and TS-1 with different Ti contents (Reaction conditions: catalyst 50 mg, allyl alcohol 10 mmol, H2O2 10 mmol, solvent (acetonitrile for Ti-MWW and methanol for TS-1) 5 ml, T = 333 K, t = 0.5 h.)
investigated. The reactions were performed in acetonitrile for Ti-MWW and in methanol for TS-1 because they were suitable solvents for these two titanosilicates, respectively [11]. As shown in Fig. 1, at low Ti contents, both Ti-MWW and TS-1 showed low conversions of AAL. The ALL conversion on Ti-MWW increased rapidly with increasing Ti content and finally reached 82.4%, while the maximum ALL conversion on TS-1 was only 20.6% under the same conditions. In addition, the ring opening reaction of the glycidol product with methanol through solvolysis took place easily on TS-1 to produce a large amount of byproducts. Ti-MWW, with an inert solvent for solvolysis, showed glycidol selectivity as high as 99%, which was superior to TS-1. The solvent effect on the epoxidation of ethyl acrylate and allyl acetate with H2O2 over Ti-MWW was investigated. As shown in Table 1, the solvents influenced the catalytic activity of Ti-MWW. The solvent effect in ethyl acrylate and allyl acetate epoxidation was different to that observed in the ep-
Table 1 Effect of solvents on the epoxidation of ethyl acrylate and allyl acetate over Ti-MWW catalyst Substrate
Solvent
Ethyl acrylate
Allyl acetate
a
a
b
Conversion (%)
Selectivity (%) Epoxide
Others
c
H2O2 conversion
H2O2 efficiency
(%)
(%)
water
10.1
97.8
2.2
31.2
32
acetone
1.6
91.1
8.9
22.0
9
methyl cyanide
1.1
93.5
6.5
16.2
7
methanol
1.1
87.2
12.8
11.5
10
alcohol
1.0
49.2
50.8
19.6
6
water
51.9
91.0
9.0
64.0
77
acetone
48.3
95.5
4.5
62.7
75
methyl cyanide
34.0
94.6
5.4
40.2
76
methanol
13.8
85.3
4.7
16.3
80
alcohol
6.2
88.5
11.5
14.2
42
Catalyst 0.1 g, ethyl acrylate 10 mmol, H2O2 10 mmol, solvent 5 ml, T = 333 K, t = 2 h.
b
Catalyst 0.2 g, allyl acetate 10 mmol, H2O2 10 mmol, solvent 5 ml, T = 333 K, t = 30 min.
c
Mainly diols together with the products from the hydrolysis of esters.
LIU Ningning et al. / Chinese Journal of Catalysis, 2008, 29(2): 102–104
Table 2 Results of epoxidation of various functionalized olefins over Ti-MWW catalyst Catalyst amount
Time
Conversion
Epoxide selectivity
H2O2 conversion
H2O2 efficiency
(g)
(h)
(%)
(%)
(%)
(%)
1
0.1
2
< 0.2
trace
—
—
2a
0.1
0.5
10.1
97.8
31.2
32
3
0.1
1
95.2
99.4
96.7
98
4
0.1
2
83.4
99.9
89.5
96
0.1
0.5
8.1
41.5
26.6
13
0.1
3
38.6
20.4
88.6
9
0.1
0.5
28.7
94.1
38.0
72
0.1
3
42.7
85.5
64.2
64
7
0.2
1
11.6
96.4
17.1
57
8
0.2
1
81.2
91.9
95.7
87
Entry
5 6
Substrate
Reaction conditions: substrate 10 mmol, H2O2 10 mmol, solvent methyl cyanide 5 ml, T = 333 K. a Using water as solvent.
was a reusable and highly efficient catalyst for the epoxidation of unsaturated hydrocarbons. Table 2 compares the results obtained from the epoxidation of various functionalized olefins over the Ti-MWW catalyst. Since these substrates are all linear olefins, they can adsorb and diffuse freely into the channels of Ti-MWW without shape selective limitation. The conversion and product selectivity of allyl acetate epoxidation (entry 6) were higher than that of vinyl acetate epoxidation (entry 5), whereas the conversion and product selectivity for ethyl allyl ether (entry 8) were higher than that for ethyl vinyl ether (entry 7). This is because the C=C bond in the vinyl group is conjugated with the C=O bond or adjacent to the C–O bond, which makes it electronically more deficient than the C=O bond in the allyl group. The catalytic activity of the other olefins decreased in the order allyl alcohol > allyl chloride > vinyl acetate > ethyl acrylate >> acrylic acid. These results indicate that the epoxidation reaction occurred more easily when the C=C bond is connected to the groups having a higher electron-donating ability. In contrast, electron-drawing groups made the epoxidation of C=C bonds more difficult. Since the alcoholic hydroxyl group is more nucleophilic than the chloride group, allyl alcohol was much easily epoxided to form glycidol than allyl chloride. The C=C bond in allyl acetate is relatively far from the C=O bond, which leads to a higher epoxidation activity than ethyl acrylate. When the C=C and C=O bonds are next to each other, the conjugating effect makes the epoxidation of the C=C bond almost impossible.
The present study verified that Ti-MWW is a suitable catalyst for the epoxidation of functionalized olefins with H2O2. Ti-MWW showed good catalytic activity and selectivity for a variety of olefins, and also good reusability. It is a promising catalyst for developing environmentally friendly processes.
References [1] Cleric M G, Bellussi G, Romano U. J Catal, 1991, 129(1): 159 [2] Sheldon R A, Dakka J. Catal Today, 1994, 19(2): 215 [3] Jung K T, Shul Y G. Microporous Mesoporous Mater, 1998, 21(4–6): 281 [4] Leonowicz M E, Lawton J A, Lawton S L, Rubin M K. Science, 1994, 264(5167): 1910 [5] Lawton S L, Leonowicz M E, Partridge R D, Chu P, Rubin M K. Microporous Mesoporous Mater, 1998, 23(1–2): 109 [6] Wu H H, Liu Y M, Wang L L, Zhang H J, He M Y, Wu P. Appl Catal A, 2007, 320: 173 [7] Wang L L, Liu Y M, Xie W, Zhang H J, Wu H H, Jiang Y W, He M Y, Wu P. J Catal, 2007, 246(1): 205 [8] Wang L L, Liu Y M, Zhang H J, Wu H H, Jiang Y W, Wu P, He M Y. Chin J Catal, 2006, 27(8): 656 [9] Wu P, Tatsumi T, Komatsu T, Yashima T. J Phys Chem B, 2001, 105(15): 2897 [10] Thangaraj A, Eapen M J, Sivasanker S, Ratnasamy P. Zeolites, 1992, 12(8): 943 [11] Wu P, Tatsumi T. J Catal, 2003, 214(2): 317 [12] Wu P, Liu Y M, He M Y, Tatsumi T. J Catal, 2004, 228(1): 183
SHORT COMMUNICATION
Epoxidation of Various Functionalized Olefins by a Ti-MWW/H2O2 Catalytic System LI Ningning, LIU Yueming, WU Haihong, LI Xiaohong, XIE Wei, ZHAO Zhonglin, WU Peng*, HE Mingyuan Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
Abstract: The catalytic properties of Ti-MWW catalysts for the epoxidation of olefins with different functional groups using hydrogen peroxide as an oxidant were studied. Ti-MWW shows higher olefin conversion and epoxide selectivity than the conventional titanosilicate TS-1. Solvents influence the catalytic activity of Ti-MWW, and water is the best one in the epoxidation of ethyl acrylate and allyl acetate. The functional groups with high electrophilicity and adjacent to the C=C bonds retard the catalytic activity for olefin epoxidation. Key words: Ti-MWW; TS-1; functionalized olefin; epoxidation
Organic epoxides, a class of important intermediates in organic synthesis, are widely used in the organic chemistry and fine chemicals industries. Conventional methods for the production of epoxides are the chlorohydrin and co-oxidative routes, both of which suffer serious environmental pollution problems. The discovery of the titanosilicate/H2O2 catalytic system has given a new environmentally friendly process for the epoxidation of olefins. The first titanosilicate, TS-1, was reported to show high catalytic activity in the epoxidation of olefins [1–2]. However, TS-1 with the MFI topology has only a 10-membered ring (MR) channel system. Its pore entrance causes a serious steric restriction on the access of the substrate molecules to the Ti active species located inside the channels, which makes the absolute yield of epoxide products from TS-1 low. Moreover, the synthesis of a TS-1 catalyst with a high activity requires the use of expensive tetrapropylammonium hydroxide as a structure-directing agent, which leads to a high preparation cost for TS-1 and limits its application to some extent [3]. Recently, Ti-MWW, which is a new titanosilicate with the MWW topology, was developed. It possesses a unique pore structure consisting of two independent 10-MR channels, one of which contains a 12-MR supercage of 0.7 nm × 0.7 nm × 1.8
nm in dimension [4,5]. In addition, there are 12-MR side cups on the exterior crystal surface of the MWW zeolite. Thus, Ti-MWW is superior to other titanosilicates in the epoxidation of olefins [6–8]. It is necessary to study the catalytic properties of Ti-MWW in the epoxidation of various functionalized olefins with the purpose to produce the corresponding epoxides to provide the background for industrial applications. A series of Ti-MWW [9] and TS-1 [10] with different Si/Ti molar ratios were hydrothermally synthesized following previously reported methods. X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) images, and N2 adsorption measurements confirmed that they were highly crystalline materials. Both UV-Vis and IR spectra indicated that the Ti species mostly existed in the framework as a tetrahedrally coordinated state. The molar ratios of Si/Ti were analyzed by inductively coupled plasma (ICP). The epoxidation of olefins was carried out in a 50-ml flask reactor with magnetic stirring. The reaction products were analyzed using a Shimadzu 14B chromatograph equipped with a DB-1 capillary column and a FID detector. The remaining H2O2 in the reaction mixture was analyzed by titration with Ce(SO4)2 solution. The catalytic properties of Ti-MWW and TS-1 with various Ti contents in the epoxidation of allyl alcohol (AAL) were first
Received date: 2007-12-04. * Corresponding author. Tel/Fax: +86-21-62232292; E-mail: [email protected] Foundation item: Supported by the National Basic Research Program of China (973 Program, 2006CB202508), the National High Technology Research and Development Program of China (863 Program, 2007AA03Z342), the National Natural Science Foundation of China (20673038), the Science and Technology Commission of Shanghai Municipality (06SR07101, 07QA14017), and the Shanghai Leading Academic Discipline Project (B409). Copyright © 2008, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
LIU Ningning et al. / Chinese Journal of Catalysis, 2008, 29(2): 102–104
oxidation of allyl alcohol and diallyl ether where acetonitrile and acetone were shown to be the optimal solvent [11,12]. Here, water was the most suitable solvent in the epoxidation of ethyl acrylate and allyl acetate, and the conversion in acetone was higher than that in acetonitrile. This is probably because ethyl acrylate is soluble in water, while allyl acetate is only partially soluble in water, which leads to a decrease in the diffusion rate of the substrate in the order water > acetone > acetonitrile. On the other hand, the solubility of diallyl ether in water is much lower. Conversely, in the case of aprotic solvents (acetonitrile and acetone) and protic solvents (methanol and ethanol), the five-membered cyclic intermediate species are easily formed during reaction through the interaction of Ti active sites with the solvent, particularly with protic molecules [1]. These bulky intermediate species can cause a serious steric restriction on the substrate molecules, which then makes the conversion lower. Moreover, protic solvents may cause the solvolysis of epoxide products to etherification byproducts, which decreases the epoxide selectivity significantly. A detailed investigation on the epoxidation of allyl acetate showed that the hydrolysis of the epoxide product of glycidyl acetate proceeded gradually with prolonged reaction time and reached a diol selectivity of 14.7% at 2 h. In addition, the Ti-MWW catalyst without a de-boronation treatment catalyzed the hydrolysis of epoxide more easily than the de-boronated catalyst. This is because the former catalyst contained more Brönsted acid sites associated with the boron ions in the framework. When the used Ti-MWW catalyst was activated by washing with acetone and then calcination at 773 K in air for 5 h, it showed almost the same allyl acetate conversion and H2O2 utilization efficiency up to the fourth reuse. ICP analyses showed no obvious leaching of Ti species from the framework during the catalyst recycling. XRD patterns and UV-Vis spectra indicated that the crystalline structure and the coordination state of Ti species of the used catalyst were nearly the same as those of fresh Ti-MWW. These results indicated that Ti-MWW
Fig. 1 Comparison of allyl alcohol epoxidation catalyzed by Ti-MWW and TS-1 with different Ti contents (Reaction conditions: catalyst 50 mg, allyl alcohol 10 mmol, H2O2 10 mmol, solvent (acetonitrile for Ti-MWW and methanol for TS-1) 5 ml, T = 333 K, t = 0.5 h.)
investigated. The reactions were performed in acetonitrile for Ti-MWW and in methanol for TS-1 because they were suitable solvents for these two titanosilicates, respectively [11]. As shown in Fig. 1, at low Ti contents, both Ti-MWW and TS-1 showed low conversions of AAL. The ALL conversion on Ti-MWW increased rapidly with increasing Ti content and finally reached 82.4%, while the maximum ALL conversion on TS-1 was only 20.6% under the same conditions. In addition, the ring opening reaction of the glycidol product with methanol through solvolysis took place easily on TS-1 to produce a large amount of byproducts. Ti-MWW, with an inert solvent for solvolysis, showed glycidol selectivity as high as 99%, which was superior to TS-1. The solvent effect on the epoxidation of ethyl acrylate and allyl acetate with H2O2 over Ti-MWW was investigated. As shown in Table 1, the solvents influenced the catalytic activity of Ti-MWW. The solvent effect in ethyl acrylate and allyl acetate epoxidation was different to that observed in the ep-
Table 1 Effect of solvents on the epoxidation of ethyl acrylate and allyl acetate over Ti-MWW catalyst Substrate
Solvent
Ethyl acrylate
Allyl acetate
a
a
b
Conversion (%)
Selectivity (%) Epoxide
Others
c
H2O2 conversion
H2O2 efficiency
(%)
(%)
water
10.1
97.8
2.2
31.2
32
acetone
1.6
91.1
8.9
22.0
9
methyl cyanide
1.1
93.5
6.5
16.2
7
methanol
1.1
87.2
12.8
11.5
10
alcohol
1.0
49.2
50.8
19.6
6
water
51.9
91.0
9.0
64.0
77
acetone
48.3
95.5
4.5
62.7
75
methyl cyanide
34.0
94.6
5.4
40.2
76
methanol
13.8
85.3
4.7
16.3
80
alcohol
6.2
88.5
11.5
14.2
42
Catalyst 0.1 g, ethyl acrylate 10 mmol, H2O2 10 mmol, solvent 5 ml, T = 333 K, t = 2 h.
b
Catalyst 0.2 g, allyl acetate 10 mmol, H2O2 10 mmol, solvent 5 ml, T = 333 K, t = 30 min.
c
Mainly diols together with the products from the hydrolysis of esters.
LIU Ningning et al. / Chinese Journal of Catalysis, 2008, 29(2): 102–104
Table 2 Results of epoxidation of various functionalized olefins over Ti-MWW catalyst Catalyst amount
Time
Conversion
Epoxide selectivity
H2O2 conversion
H2O2 efficiency
(g)
(h)
(%)
(%)
(%)
(%)
1
0.1
2
< 0.2
trace
—
—
2a
0.1
0.5
10.1
97.8
31.2
32
3
0.1
1
95.2
99.4
96.7
98
4
0.1
2
83.4
99.9
89.5
96
0.1
0.5
8.1
41.5
26.6
13
0.1
3
38.6
20.4
88.6
9
0.1
0.5
28.7
94.1
38.0
72
0.1
3
42.7
85.5
64.2
64
7
0.2
1
11.6
96.4
17.1
57
8
0.2
1
81.2
91.9
95.7
87
Entry
5 6
Substrate
Reaction conditions: substrate 10 mmol, H2O2 10 mmol, solvent methyl cyanide 5 ml, T = 333 K. a Using water as solvent.
was a reusable and highly efficient catalyst for the epoxidation of unsaturated hydrocarbons. Table 2 compares the results obtained from the epoxidation of various functionalized olefins over the Ti-MWW catalyst. Since these substrates are all linear olefins, they can adsorb and diffuse freely into the channels of Ti-MWW without shape selective limitation. The conversion and product selectivity of allyl acetate epoxidation (entry 6) were higher than that of vinyl acetate epoxidation (entry 5), whereas the conversion and product selectivity for ethyl allyl ether (entry 8) were higher than that for ethyl vinyl ether (entry 7). This is because the C=C bond in the vinyl group is conjugated with the C=O bond or adjacent to the C–O bond, which makes it electronically more deficient than the C=O bond in the allyl group. The catalytic activity of the other olefins decreased in the order allyl alcohol > allyl chloride > vinyl acetate > ethyl acrylate >> acrylic acid. These results indicate that the epoxidation reaction occurred more easily when the C=C bond is connected to the groups having a higher electron-donating ability. In contrast, electron-drawing groups made the epoxidation of C=C bonds more difficult. Since the alcoholic hydroxyl group is more nucleophilic than the chloride group, allyl alcohol was much easily epoxided to form glycidol than allyl chloride. The C=C bond in allyl acetate is relatively far from the C=O bond, which leads to a higher epoxidation activity than ethyl acrylate. When the C=C and C=O bonds are next to each other, the conjugating effect makes the epoxidation of the C=C bond almost impossible.
The present study verified that Ti-MWW is a suitable catalyst for the epoxidation of functionalized olefins with H2O2. Ti-MWW showed good catalytic activity and selectivity for a variety of olefins, and also good reusability. It is a promising catalyst for developing environmentally friendly processes.
References [1] Cleric M G, Bellussi G, Romano U. J Catal, 1991, 129(1): 159 [2] Sheldon R A, Dakka J. Catal Today, 1994, 19(2): 215 [3] Jung K T, Shul Y G. Microporous Mesoporous Mater, 1998, 21(4–6): 281 [4] Leonowicz M E, Lawton J A, Lawton S L, Rubin M K. Science, 1994, 264(5167): 1910 [5] Lawton S L, Leonowicz M E, Partridge R D, Chu P, Rubin M K. Microporous Mesoporous Mater, 1998, 23(1–2): 109 [6] Wu H H, Liu Y M, Wang L L, Zhang H J, He M Y, Wu P. Appl Catal A, 2007, 320: 173 [7] Wang L L, Liu Y M, Xie W, Zhang H J, Wu H H, Jiang Y W, He M Y, Wu P. J Catal, 2007, 246(1): 205 [8] Wang L L, Liu Y M, Zhang H J, Wu H H, Jiang Y W, Wu P, He M Y. Chin J Catal, 2006, 27(8): 656 [9] Wu P, Tatsumi T, Komatsu T, Yashima T. J Phys Chem B, 2001, 105(15): 2897 [10] Thangaraj A, Eapen M J, Sivasanker S, Ratnasamy P. Zeolites, 1992, 12(8): 943 [11] Wu P, Tatsumi T. J Catal, 2003, 214(2): 317 [12] Wu P, Liu Y M, He M Y, Tatsumi T. J Catal, 2004, 228(1): 183