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Carboxyl-Appended Ionic Liquids
CHINESE JOURNAL OF CATALYSIS Volume 27, Issue 11, November 2006 Online English edition of the Chinese language journal Cite this article as: Chin J Catal, 2006, 27(11): 943–945.
SHORT COMMUNICATION
Selective Oxidation of Styrene Catalyzed by Pd/CarboxylAppended Ionic Liquids LI Xuehui*, GENG Weiguo, WANG Furong, WANG Lefu Guangdong Provincial Laboratory of Green Chemical Technology, Department of Chemical Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China
Abstract: A novel catalytic system, PdCl2/TSILs, for the catalytic oxidation of styrene to acetophenone with hydrogen peroxide was designed through the combination of PdCl2 and task-specific ionic liquids (TSILs) of N-carboxyl-appended imidazolium cations with various anions. The PdCl2/TSILs catalytic system displayed excellent catalytic properties for the target reaction. The yield and selectivity of acetophenone significantly depended on the structure of TSILs with different combinations of carboxyl groups and counter anions. The catalytic activity of PdCl2/TSILs was enhanced with the increase in the number of the carboxyl groups and the degree of asymmetry of the cations. The activity of PdCl2/TSILs with the same cation also increased with the assorted anion sequence of PF6− < H2PO4− < Cl− < BF4−, which is in contrast to their acid strengths. Among all of these PdCl2/TSILs, TSILs with three carboxyl groups showed the best activity with a complete conversion of styrene at 55oC. The measured turnover frequency and the selectivity for acetophenone were as high as 125 h−1 and 91%, respectively. Key Words: carboxyl; task-specific ionic liquid; palladium; styrene; selective catalytic oxidation; acetophenone; imidazolium cation
Acetophenone is an important stock chemical for the preparation of a variety of pharmaceuticals. However, traditional routes for the synthesis of acetophenone result in environmental pollution and have the disadvantage of low reaction selectivity [1]. Although selective catalytic oxidation of styrene can partly overcome these disadvantages by the use of aqueous hydrogen peroxide as oxidant, the biphasic system with substrate and oxidation products partitioned into different phases is harmful to the environment because large quantities of volatile organic solvents, such as acetone, tetrahydrofuran, methanol, acetonitrile, etc., have been added as cosolvent to improve the miscibility of the reaction system [2]. This violates the basic principle of green chemistry. On the other hand, the selective catalytic oxidation of styrene can be achieved over Pd(II) to yield products including acetophenone, phenyl aldehyde, and phentylformic acid [3]. However, although Pd catalysts have been widely used in chemical industry, the global supply of Pd is rather limited. So, it is useful to develop an environmentally friendly resource-efficient catalytic system for the selective oxidation of
styrene to acetophenone. Recently, ionic liquids (ILs) have been developed as novel reaction media. ILs are regarded as green solvents and have the potentials to replace traditional organic solvents for their special physical and chemical properties. Studies on the applications of ILs in the field of catalysis and separation have shown attractive prospects. Many reactions can be carried out in novel task-specified ionic liquids (TSILs) synthesized to adjust their physicochemical properties through molecular designing [4, 5]. The application of ILs in catalysis is becoming a hot spot [5–12]. For example, PdCl2 can react with [C3CNmim]BF4 to generate [Pd(NCC3dimim)2Cl2](BF4)2, which is capable of efficiently catalyzing the hydrogenation of 1,3-cyclohexadiene [13]. In this paper, novel catalytic systems for the selective catalytic oxidation of styrene to acetophenone with hydrogen peroxide were designed through the combination of PdCl2 with a series of N-carboxyl-appended imidazolium ionic liquids. The relationships between the compositions and the structure of TSILs and their influence on the catalytic proper-
Received date: 2006-05-07. * Corresponding author. Tel: +86-20-87114707; Fax: +86-20-87114707; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (20206010), the Key Technologies Research and Development Program of Guangdong Province of China (2006B13801002), and the Key Technologies Research and Development Program of Guangzhou City of China (2006Z3-E0671). Copyright © 2006, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
LI Xuehui et al. / Chinese Journal of Catalysis, 2006, 27(11): 943–945
ties were investigated. Scheme 1 shows the structure of the cations of carboxyl-appended imidazolium ionic liquids. The synthetic routes for the preparation of multicarboxyl-appended TSILs used in this paper have been described in the authors' previous articles [14, 15]. These TSILs were composed of 3-(1-carboxylic)-propylcarboxylic-1-ethylcarboxylic imidazolium
([CCPCEIm]+), 3-(1-carboxylic)-propylcarboxylic-1-ethylimidazolium ([CCPEIm]+), and 3-propylcarboxylic-1-ethylcarboxylic imidazolium ([CPCEIm]+) cations and charged with counter anions Cl−, BF4−, PF6−, and H2PO4−. Monocarboxyl-appended ionic liquids composed of 1-ethylcarboxylic-3-methylimidazolium ([CEMIm]+) cation and the same anions as above were synthesized according to Ref. [16].
Scheme 1 Cations of carboxyl-appended imidazolium ionic liquids
In a typical reaction, palladium chloride (0.25 mmol), N-carboxyl-appended ionic liquids (0.5 mmol), styrene (250 mmol), and H2O2 (30%, 287.5 mmol) were mixed and stirred at 55oC in a round-bottomed flask fitted with a condenser. GC–MS (Shimadzu GCMS-QP 2010) was used to analyze the composition of the reaction mixture during the progress of the reaction. After the completion of the reaction, the organic compounds in the mixture were removed by distillation under reduced pressure. The residue was first washed with ethyl ether (3 × 20 ml) and then evaporated in a rotary evaporator for 4 h at 100oC under a reduced pressure of 600 Pa to recover the PdCl2/TSILs for further recycling. The catalytic properties of PdCl2 and PdCl2/TSILs for the oxidation of styrene with H2O2 are summarized in Table 1. In the absence of palladium chloride, the oxidation of styrene proceeded quite slowly. The selectivity for acetophenone was very poor and the main product was phenyl aldehyde. PdCl2 showed better catalytic properties for the selective oxidation of styrene. The total conversion of styrene reached 78% and the selectivity for acetophenone was as high as 93%, but the activity of PdCl2 was relatively low as the TOF value was only 24 h−1. Without PdCl2, only phenyl aldehyde was produced and the reaction proceeded very slowly when only tricarboxyl-appended ILs, [CCPCEIm]PF6, was used. This indicates that [CCPCEIm]PF6 has no catalytic activity for the production of acetophenone. On the other hand, PdCl2/TSILs can effectively catalyze the oxidation of styrene and has an excellent selectivity for acetophenone. No organic cosolvent was required in this system. It is therefore concluded that the catalytic activity of PdCl2 for the selective oxidation of styrene can be improved by the addition of TSILs. Among all these PdCl2/TSILs, PdCl2/[CCPCEIm]BF4 showed the best catalytic properties and the amount of PdCl2 was significantly reduced as compared with other studies [1, 2]. The catalytic properties of the catalyst system obviously depend on the structure and the compositions of the TSILs.
For example, the catalytic properties were closely related to the nature of the counter anions when the same imidazolium cation was used, as the TOF of these catalysts markedly increased in the sequence of PF6− < H2PO4− < Cl− < BF4−, which Table 1 Selective oxidation of styrene catalyzed by Pd/carboxylappended task-specific ionic liquids Catalyst —a [CCPCEIm]PF6
a
PdCl2
TOF
X(styrene)
S(acetophenone)
/ h−1
/%
/%
—
8
7
—
10
0
24
78
93
PdCl2/CH3COOH
40
77
73
PdCl2/HOOCCH2CH2COOH
28
53
81
PdCl2/[CCPCEIm]PF6
49
83
84
PdCl2/[CCPCEIm]H2PO4
57
87
80
PdCl2/[CCPCEIm]Cl
72
93
94
PdCl2/[CCPCEIm]BF4
125
100
91
PdCl2/[CCPCEIm]BF4b
120
100
90
PdCl2/[CCPEIm]PF6
40
73
76
PdCl2/[CCPEIm]H2PO4
49
78
79
PdCl2/[CCPEIm]Cl
60
91
92
PdCl2/[CCPEIm]BF4
65
86
88
PdCl2/[CPCEIm]PF6
29
63
67
PdCl2/[CPCEIm]H2PO4
30
74
73
PdCl2/[CPCEIm]Cl
55
88
89
PdCl2/[CPCEIm]BF4
46
74
67
PdCl2/[CEMIm]PF6
27
60
63
PdCl2/[CEMIm]H2PO4
26
74
60
PdCl2/[CEMIm]Cl
40
81
82
PdCl2/[CEMIm]BF4
41
72
61
Reaction conditions: styrene 250 mmol, PdCl2 0.25 mmol, 55oC, ionic liquids 0.5 mmol, H2O2 287.5 mmol. a
After 15 h of reaction. b Results of 5 cycles. TOF—Turnover frequency.
LI Xuehui et al. / Chinese Journal of Catalysis, 2006, 27(11): 943–945
appeared to be inverse to the acidic strength [15]. The TOF values obtained using PdCl2/acetic acid and PdCl2/succinic acid were all increased to some extent although their selectivity for acetophenone and the conversion of styrene were lower than that of PdCl2. This demonstrates that the activity of PdCl2 can be improved through a coupling with the organic acid. Succinic acid (pKa1 = 4.21) has a stronger acidity than acetic acid (pKa = 4.76), but the TOF value of PdCl2/acetic acid is larger than that of PdCl2/succinic acid, which is consistent with the relationship between the acidity of TSILs and the TOF value discussed above. This shows that the weaker the acidity of the ILs coupled with PdCl2, the higher the activity of the PdCl2/TSILs. It can be concluded that the activity of PdCl2/TSILs is related to the acidity of TSILs. Phenylformic acid, a usual by-product of the selective oxidation of styrene, cannot be detected with all the PdCl2/TSILs, which implied that PdCl2/TSILs can inhibit the deep oxidation of styrene. An explanation is that the carboxyl groups in PdCl2/TSILs prevent the transformation of reaction intermediates into carboxyl groups and hence suppress the formation of deep oxidation products. For TSILs with the same anion, when more of the carboxyl groups were appended, better selectivity for acetophenone and catalytic properties were obtained. Furthermore, the high degree of asymmetry of the TSILs containing the same number of carboxyl groups led to high activity. This means that the properties of this type of catalyst can be finely tuned through molecular design of TSILs and changes in the counter anions. Moreover, the ionic liquid effect exerted by these novel TSILs cannot be overlooked. Further investigation on the catalytic mechanism is underway. In conclusion, the catalytic oxidation of styrene to acetophenone with hydrogen peroxide was achieved using a novel PdCl2/TSILs catalyst system. This catalyst was designed through the combination of PdCl2 with specified ionic liquids of N-carboxyl-appended imidazolium cations with different counter anions. The yield and the selectivity for acetophenone
were varied by TSILs with different structure, different number of carboxyl groups, and different counter anions. The catalytic properties of PdCl2/TSILs were enhanced by increasing the number of carboxyl groups and the asymmetry degree of the cations.
References [1] Namboodiri V V, Varma R S, Sahle-Demessie E, Pillai U R. Green Chem, 2002, 4(2): 170 [2] Jiang H, Gong H, Qiao Q D. Petrochem Technol, 1999, 28(8): 509 [3] Freitag J, Nüchter M, Ondruschka B. Green Chem, 2003, 5(3): 291 [4] Welton T. Chem Rev, 1999, 99(8): 2071 [5] Li X H, Zhao D B, Fei Zh F, Wang L F. Sci China (Ser B), 2006, 36(3): 181 [6] Mu X D, Meng J Q, Li Z C, Kou Y. J Am Chem Soc, 2005, 127(27): 9694 [7] Antonietti M, Kuang D, Smarsly B, Zhou Y. Angew Chem, Int Ed, 2004, 43(38), 4988 [8] Tao G H, Chen Zh Y, He L, Kou Y. Chin J Catal, 2005, 26(3): 253 [9] Qiao K, Deng Y Q. Chin J Catal, 2002, 23(6): 559 [10] Li X H, Zheng B G, Zhao J G. Chin J Catal, 2006, 27(2): 106 [11] Zhang W Ch, Zhao W J, Zhuo G L, Jiang X Zh. Chin J Catal, 2006, 27(1): 36 [12] Fei Zh F, Geldbach T J, Zhao D B, Dyson P J. Chem Eur J, 2006, 12(8): 2123 [13] Zhao D B, Fei Zh F, Scopelliti R, Dyson P J. Inorg Chem, 2004, 43(6): 2197 [14] Geng W G, Li X H, Wang L F, Duan H L, Pan W P. Chin Chem Lett, 2006, 17(2): 169 [15] Geng W G, Li X H, Wang L F, Duan H L, Pan W P. Acta Phys-Chim Sin, 2006, 22(2): 230 [16] Fei Zh F, Zhao D B, Geldbach T J, Scopelliti R, Dyson P J. Chem Eur J, 2004, 10(19): 4886
SHORT COMMUNICATION
Selective Oxidation of Styrene Catalyzed by Pd/CarboxylAppended Ionic Liquids LI Xuehui*, GENG Weiguo, WANG Furong, WANG Lefu Guangdong Provincial Laboratory of Green Chemical Technology, Department of Chemical Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China
Abstract: A novel catalytic system, PdCl2/TSILs, for the catalytic oxidation of styrene to acetophenone with hydrogen peroxide was designed through the combination of PdCl2 and task-specific ionic liquids (TSILs) of N-carboxyl-appended imidazolium cations with various anions. The PdCl2/TSILs catalytic system displayed excellent catalytic properties for the target reaction. The yield and selectivity of acetophenone significantly depended on the structure of TSILs with different combinations of carboxyl groups and counter anions. The catalytic activity of PdCl2/TSILs was enhanced with the increase in the number of the carboxyl groups and the degree of asymmetry of the cations. The activity of PdCl2/TSILs with the same cation also increased with the assorted anion sequence of PF6− < H2PO4− < Cl− < BF4−, which is in contrast to their acid strengths. Among all of these PdCl2/TSILs, TSILs with three carboxyl groups showed the best activity with a complete conversion of styrene at 55oC. The measured turnover frequency and the selectivity for acetophenone were as high as 125 h−1 and 91%, respectively. Key Words: carboxyl; task-specific ionic liquid; palladium; styrene; selective catalytic oxidation; acetophenone; imidazolium cation
Acetophenone is an important stock chemical for the preparation of a variety of pharmaceuticals. However, traditional routes for the synthesis of acetophenone result in environmental pollution and have the disadvantage of low reaction selectivity [1]. Although selective catalytic oxidation of styrene can partly overcome these disadvantages by the use of aqueous hydrogen peroxide as oxidant, the biphasic system with substrate and oxidation products partitioned into different phases is harmful to the environment because large quantities of volatile organic solvents, such as acetone, tetrahydrofuran, methanol, acetonitrile, etc., have been added as cosolvent to improve the miscibility of the reaction system [2]. This violates the basic principle of green chemistry. On the other hand, the selective catalytic oxidation of styrene can be achieved over Pd(II) to yield products including acetophenone, phenyl aldehyde, and phentylformic acid [3]. However, although Pd catalysts have been widely used in chemical industry, the global supply of Pd is rather limited. So, it is useful to develop an environmentally friendly resource-efficient catalytic system for the selective oxidation of
styrene to acetophenone. Recently, ionic liquids (ILs) have been developed as novel reaction media. ILs are regarded as green solvents and have the potentials to replace traditional organic solvents for their special physical and chemical properties. Studies on the applications of ILs in the field of catalysis and separation have shown attractive prospects. Many reactions can be carried out in novel task-specified ionic liquids (TSILs) synthesized to adjust their physicochemical properties through molecular designing [4, 5]. The application of ILs in catalysis is becoming a hot spot [5–12]. For example, PdCl2 can react with [C3CNmim]BF4 to generate [Pd(NCC3dimim)2Cl2](BF4)2, which is capable of efficiently catalyzing the hydrogenation of 1,3-cyclohexadiene [13]. In this paper, novel catalytic systems for the selective catalytic oxidation of styrene to acetophenone with hydrogen peroxide were designed through the combination of PdCl2 with a series of N-carboxyl-appended imidazolium ionic liquids. The relationships between the compositions and the structure of TSILs and their influence on the catalytic proper-
Received date: 2006-05-07. * Corresponding author. Tel: +86-20-87114707; Fax: +86-20-87114707; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (20206010), the Key Technologies Research and Development Program of Guangdong Province of China (2006B13801002), and the Key Technologies Research and Development Program of Guangzhou City of China (2006Z3-E0671). Copyright © 2006, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
LI Xuehui et al. / Chinese Journal of Catalysis, 2006, 27(11): 943–945
ties were investigated. Scheme 1 shows the structure of the cations of carboxyl-appended imidazolium ionic liquids. The synthetic routes for the preparation of multicarboxyl-appended TSILs used in this paper have been described in the authors' previous articles [14, 15]. These TSILs were composed of 3-(1-carboxylic)-propylcarboxylic-1-ethylcarboxylic imidazolium
([CCPCEIm]+), 3-(1-carboxylic)-propylcarboxylic-1-ethylimidazolium ([CCPEIm]+), and 3-propylcarboxylic-1-ethylcarboxylic imidazolium ([CPCEIm]+) cations and charged with counter anions Cl−, BF4−, PF6−, and H2PO4−. Monocarboxyl-appended ionic liquids composed of 1-ethylcarboxylic-3-methylimidazolium ([CEMIm]+) cation and the same anions as above were synthesized according to Ref. [16].
Scheme 1 Cations of carboxyl-appended imidazolium ionic liquids
In a typical reaction, palladium chloride (0.25 mmol), N-carboxyl-appended ionic liquids (0.5 mmol), styrene (250 mmol), and H2O2 (30%, 287.5 mmol) were mixed and stirred at 55oC in a round-bottomed flask fitted with a condenser. GC–MS (Shimadzu GCMS-QP 2010) was used to analyze the composition of the reaction mixture during the progress of the reaction. After the completion of the reaction, the organic compounds in the mixture were removed by distillation under reduced pressure. The residue was first washed with ethyl ether (3 × 20 ml) and then evaporated in a rotary evaporator for 4 h at 100oC under a reduced pressure of 600 Pa to recover the PdCl2/TSILs for further recycling. The catalytic properties of PdCl2 and PdCl2/TSILs for the oxidation of styrene with H2O2 are summarized in Table 1. In the absence of palladium chloride, the oxidation of styrene proceeded quite slowly. The selectivity for acetophenone was very poor and the main product was phenyl aldehyde. PdCl2 showed better catalytic properties for the selective oxidation of styrene. The total conversion of styrene reached 78% and the selectivity for acetophenone was as high as 93%, but the activity of PdCl2 was relatively low as the TOF value was only 24 h−1. Without PdCl2, only phenyl aldehyde was produced and the reaction proceeded very slowly when only tricarboxyl-appended ILs, [CCPCEIm]PF6, was used. This indicates that [CCPCEIm]PF6 has no catalytic activity for the production of acetophenone. On the other hand, PdCl2/TSILs can effectively catalyze the oxidation of styrene and has an excellent selectivity for acetophenone. No organic cosolvent was required in this system. It is therefore concluded that the catalytic activity of PdCl2 for the selective oxidation of styrene can be improved by the addition of TSILs. Among all these PdCl2/TSILs, PdCl2/[CCPCEIm]BF4 showed the best catalytic properties and the amount of PdCl2 was significantly reduced as compared with other studies [1, 2]. The catalytic properties of the catalyst system obviously depend on the structure and the compositions of the TSILs.
For example, the catalytic properties were closely related to the nature of the counter anions when the same imidazolium cation was used, as the TOF of these catalysts markedly increased in the sequence of PF6− < H2PO4− < Cl− < BF4−, which Table 1 Selective oxidation of styrene catalyzed by Pd/carboxylappended task-specific ionic liquids Catalyst —a [CCPCEIm]PF6
a
PdCl2
TOF
X(styrene)
S(acetophenone)
/ h−1
/%
/%
—
8
7
—
10
0
24
78
93
PdCl2/CH3COOH
40
77
73
PdCl2/HOOCCH2CH2COOH
28
53
81
PdCl2/[CCPCEIm]PF6
49
83
84
PdCl2/[CCPCEIm]H2PO4
57
87
80
PdCl2/[CCPCEIm]Cl
72
93
94
PdCl2/[CCPCEIm]BF4
125
100
91
PdCl2/[CCPCEIm]BF4b
120
100
90
PdCl2/[CCPEIm]PF6
40
73
76
PdCl2/[CCPEIm]H2PO4
49
78
79
PdCl2/[CCPEIm]Cl
60
91
92
PdCl2/[CCPEIm]BF4
65
86
88
PdCl2/[CPCEIm]PF6
29
63
67
PdCl2/[CPCEIm]H2PO4
30
74
73
PdCl2/[CPCEIm]Cl
55
88
89
PdCl2/[CPCEIm]BF4
46
74
67
PdCl2/[CEMIm]PF6
27
60
63
PdCl2/[CEMIm]H2PO4
26
74
60
PdCl2/[CEMIm]Cl
40
81
82
PdCl2/[CEMIm]BF4
41
72
61
Reaction conditions: styrene 250 mmol, PdCl2 0.25 mmol, 55oC, ionic liquids 0.5 mmol, H2O2 287.5 mmol. a
After 15 h of reaction. b Results of 5 cycles. TOF—Turnover frequency.
LI Xuehui et al. / Chinese Journal of Catalysis, 2006, 27(11): 943–945
appeared to be inverse to the acidic strength [15]. The TOF values obtained using PdCl2/acetic acid and PdCl2/succinic acid were all increased to some extent although their selectivity for acetophenone and the conversion of styrene were lower than that of PdCl2. This demonstrates that the activity of PdCl2 can be improved through a coupling with the organic acid. Succinic acid (pKa1 = 4.21) has a stronger acidity than acetic acid (pKa = 4.76), but the TOF value of PdCl2/acetic acid is larger than that of PdCl2/succinic acid, which is consistent with the relationship between the acidity of TSILs and the TOF value discussed above. This shows that the weaker the acidity of the ILs coupled with PdCl2, the higher the activity of the PdCl2/TSILs. It can be concluded that the activity of PdCl2/TSILs is related to the acidity of TSILs. Phenylformic acid, a usual by-product of the selective oxidation of styrene, cannot be detected with all the PdCl2/TSILs, which implied that PdCl2/TSILs can inhibit the deep oxidation of styrene. An explanation is that the carboxyl groups in PdCl2/TSILs prevent the transformation of reaction intermediates into carboxyl groups and hence suppress the formation of deep oxidation products. For TSILs with the same anion, when more of the carboxyl groups were appended, better selectivity for acetophenone and catalytic properties were obtained. Furthermore, the high degree of asymmetry of the TSILs containing the same number of carboxyl groups led to high activity. This means that the properties of this type of catalyst can be finely tuned through molecular design of TSILs and changes in the counter anions. Moreover, the ionic liquid effect exerted by these novel TSILs cannot be overlooked. Further investigation on the catalytic mechanism is underway. In conclusion, the catalytic oxidation of styrene to acetophenone with hydrogen peroxide was achieved using a novel PdCl2/TSILs catalyst system. This catalyst was designed through the combination of PdCl2 with specified ionic liquids of N-carboxyl-appended imidazolium cations with different counter anions. The yield and the selectivity for acetophenone
were varied by TSILs with different structure, different number of carboxyl groups, and different counter anions. The catalytic properties of PdCl2/TSILs were enhanced by increasing the number of carboxyl groups and the asymmetry degree of the cations.
References [1] Namboodiri V V, Varma R S, Sahle-Demessie E, Pillai U R. Green Chem, 2002, 4(2): 170 [2] Jiang H, Gong H, Qiao Q D. Petrochem Technol, 1999, 28(8): 509 [3] Freitag J, Nüchter M, Ondruschka B. Green Chem, 2003, 5(3): 291 [4] Welton T. Chem Rev, 1999, 99(8): 2071 [5] Li X H, Zhao D B, Fei Zh F, Wang L F. Sci China (Ser B), 2006, 36(3): 181 [6] Mu X D, Meng J Q, Li Z C, Kou Y. J Am Chem Soc, 2005, 127(27): 9694 [7] Antonietti M, Kuang D, Smarsly B, Zhou Y. Angew Chem, Int Ed, 2004, 43(38), 4988 [8] Tao G H, Chen Zh Y, He L, Kou Y. Chin J Catal, 2005, 26(3): 253 [9] Qiao K, Deng Y Q. Chin J Catal, 2002, 23(6): 559 [10] Li X H, Zheng B G, Zhao J G. Chin J Catal, 2006, 27(2): 106 [11] Zhang W Ch, Zhao W J, Zhuo G L, Jiang X Zh. Chin J Catal, 2006, 27(1): 36 [12] Fei Zh F, Geldbach T J, Zhao D B, Dyson P J. Chem Eur J, 2006, 12(8): 2123 [13] Zhao D B, Fei Zh F, Scopelliti R, Dyson P J. Inorg Chem, 2004, 43(6): 2197 [14] Geng W G, Li X H, Wang L F, Duan H L, Pan W P. Chin Chem Lett, 2006, 17(2): 169 [15] Geng W G, Li X H, Wang L F, Duan H L, Pan W P. Acta Phys-Chim Sin, 2006, 22(2): 230 [16] Fei Zh F, Zhao D B, Geldbach T J, Scopelliti R, Dyson P J. Chem Eur J, 2004, 10(19): 4886