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Studies on immobilization of Co(II)-La(III) schiff base complex in MCM-41
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
311
Studies on immobilization of Co(II)-La(III) schifFbase complex in MCM-41 Binbin Fan^"^, Ruifeng Li^*, Zhihong Liu^, Jinghui Cao^, Bing Zhong^ ^Institute of special Chemicals, Taiyuan University of Technology, Taiyuan 030024, China ^State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, The Chinese Academy of Sciences, Taiyuan 030001, China Immobilization of heteronuclear Co(II)-La(III) salen (denoted as (CoLa)salen ) complex into the pores of MCM-41 matrix was studied and characterized by element analysis, TGDTA, FTIR and UV-vis. The results showed that the heteronuclear complex may undergo dissociation during immobilization step due to the strong guest/host interaction. Nevertheless, the obtained product(designated as (Co,La)salen/MCM-41) exhibits higher conversion than Cosalen/MCM-41 and Lasalen/MCM-41 and high stability for the oxidation of styrene. In addition, the effects of the reaction time, reaction temperature, and solvent on the catalytic properties of (Co,La)salen/MCM-41 in oxidation of styrene were also investigated. 1. INTRODUCTION Transition metal complexes encapsulated in the channel of zeolites have received a lot of attention, due to their high catalytic activity, selectivity and stability in field of oxidation reactions. Generally, transition metal complex have only been immobilized in the classical large porous zeolites, such as X, Y[l-4]. But the restricted sizes of the pores and cavities of the zeolites not only limit the maximum size of the complex which can be accommodated, but also impose resistance on the diffusion of substrates and products. Mesoporous molecular sieves, due to their high surface area and ordered pore structure, off*er the potentiality as a good host for inmiobilizing transition complexes[5-7]. The previous reports are mainly about molecular sieves encapsulated mononuclear metal complex, whereas the reports about inmiobilization of heteronuclear metal complex in the host material are few. Here, we try to prepare MCM-41 loaded with binuclear Co(II)-La(III) complex with bis-salicylaldehyde ethylenediamine schiff base. 2. EXPERIMENTAL 2.1 Preparation of (CoLa)salen (CoLa)salen was prepared as in Ref.[8]. To a 20ml isopropanol solution of Immol salen, a solution of 0.5mmol Co(N03)2 6H2O and 0.5mmol La(N03)3 in 5ml isopropanol was added. The mixture was stirred at room temperature for Ih under nitrogen atmosphere and a brown yellow precipitate was formed. After filtration the precipitate was washed for several times with isopropanol and ether, and ftirther dried under vacuum. Cosalen and Lasalen were prepared according to the method described in Ref.[8] and Ref [9] respectively.
312
2.2 Complex impregnation (Co,La)salen/MCM-41 were prepared by stirring 2g calcined MCM-41 and a solution of 0.36g (CoLa)salen in 30ml isopropanol at room temperature for 24h. The solid fraction was filtered, extracted with acetone, and finally dried at 333K for 5h. Cosalen/MCM-41 and Lasalen/MCM-41 were also prepared by impregnating 2g calcined MCM-41 with 30ml isopropanol solution of 0.36g Cosalen or Lasalen by using the similar method as preparation of(Co,La)/MCM-41. 2.3 Sample characterization Metal content was determined by a LABTAM 8401 inductively coupled plasma spectrometer. X-ray powder diffraction was carried out on a Rigaku 2304 diffractometer with CuK« radition(Ni filtered). IR and UV-vis spectra of the solid samples were recorded on a PE FTIR 1760 spectrometer and a PE Lambda Bio 40 instrument respectively. TG-DTA was performed on a CN8076E(Rigaku) thermal analysis instrument. 2.4 Catalytic measurement Styrene oxidation reactions were carried out in sealed batch reactors (100ml) at 333K for lOh. The reactant was composed of 3ml styrene, 2ml 30wt.% H2O2, 10ml acetone and O.lg catalyst. The products were analyzed on a GC-9A gaschromatography. 3. RESULTS AND DISCUSSION X-ray powder diffraction patterns of the calcined and immobilized (Co,La)salen MCM41 samples are presented in Fig.l. It can be seen that MCM-41 does not undergo structure change during the preparation of the catalyst and exhibits the hexagonal phase with at least two resolved peaks((100) and (110)) in the diffraction patterns. Upon immobilization and extraction, the obtained sample has light brown yellow color, suggesting that the complex be fixed in the MCM-41 host material. Elemental and thermogravimetric analyses were carried out to determine the complex content loaded on the MCM-41. Elemental analysis is given in table 1. In the isopropanol solution of (CoLa)salen the molar ratio of La/Co is 1.0, but fi-om table 1 it can be seen that the La/Co molar ratios are all less than 1.0 in the obtained products. With the increase of the Si02/Al203 ratio of the MCM-41, although the La content dramatically decreases, the Co content has not obvious change. According to the weight loss during 250-500°C, corresponding to the decomposition of salen ligand, and the metal content in the samples, it can be concluded that the molar ratios of (Co+La)/salen are nearly 1.0. These indicate that most of the (CoLa)salen complex may dissociate into mononuclear complex of Cosalen or Lasalen, possibly due to the strong interactions of (CoLa)salen complex and Si-OH in the wall of MCM-41 [7], and Cosalen is easier to be loaded on MCM41 than Lasalen. Table 1 Co and La content loaded on MCM-41 with different Si02/Al203 Si02/Al203 50 100 200
Co (wt.%) 0.9248 1.0915 0.8506
La (wt%) 0.6849 0.4933 0.1132
La/Co(mol.) 0.313 0.1918 0.0565
313
2000
1225 Wavenumbers, cm"
Figure 2, FTIR spectra of (Co, La) salen (a), (Co, La) salen/MCM-41 (b) and the calcined MCM-4I (c)
450
200
300
400
500
600
700
800
Wavelength, nm Figure 3. The diffuse UV-vis spectra of (Co,La)salen (a), Lasalen/MCM-41 (b), (Co,La)salen/MCM-41 (c) and Cosalen/MCM-41 (d)
The FTIR spectra of (Co, La)salen loaded/MCM-41, free (CoLa)salen and the parent MCM-41 material are depicted in Fig.2. From Fig.2, it can be seen that complex free MCM41 has no absorption bands in the wavenumber region between ca. 1300cm' and 1500cm . In the spectrum of the metal salen containing MCM-41, even if the (CoLa)salen may dissociate during the immobilization step, the characteristic adsorption bands of metal salen complexes are clearly visible, indicating that the metal salen complexes are indeed loaded on the mesoporous material and cannot be removed by acetone extraction. This can be confirmed by the reflectance spectra of different samples in Fig.3. From Fig.3, it can be seen that for the samples loaded metal salen an absorption band around 400nm ascribed to metal to ligand charge transfer[3], can be seen respectively. Generally, immobilization of metal complexes in the wall of mesorporous materials is attributed to the electrostatic interaction between anion oxygen sites on the channel walls of and positively charged complexes [5]. Comparing the IR spectra of MCM-41 loaded with (Co,La)salen and free (CoLa)salen, it can be seen that the band at 1380cm'' attributed to NO3' [8] is still remained, which indicates that the positive charge is balanced by NOs" instead of by the anionic host framework. Metal salen complexes are not held by ionic interaction between guest and the host framework, but by the strong guest/host interactions, mainly between the aromatic ring of the complex and the internal surface silanol groups of the walls of mesoporous[7].
314 The catalytic properties of the sample were tested with the oxidation of styrene. From table 2, it is obvious that (Co,La)salen/MCM-41 exhibits higher catalytic activity than Lasalen/MCM-41 and Cosalen/MCM-41 under the identical reaction conditions. This result suggests that there may be a synergism between two complexes containing different central ions, which makes it favorable for the oxidation of styrene. Table 2 Oxidation of styrene over different catalysts Conv.
Products PA BA % Cosalen/MCM-41 21.6 2.77 91.8 Lasalen/MCM-41 16.6 89.1 4.53 (Co,La)salen/MCM-41 30.7 1.55 92.5 BA = benzaldehyde; PA = phenlactaldehyde; SO = styrene oxide Catalyst
SO 5.48 6.33 5.93
Table 3 Recycling of (Co,La)salen/MCM-41 for styrene oxidation Oxidant
Number of
Conv.
H2O2
recycling 1st 2nd
% 30.7 27.8
H2O2
BA 92.5 88.74
Products PA 1.55 4.23
SO 5.93 7.03
Because (Co,La)salen is immobilized on the mesopore wall of MCM-41,it is anticipated that this catalyst would exhibit high stability for catalyst recycling. Therefore, the (Co.La)salen/MCM-41 sample was recovered by filtration and used again for oxidation of styrene. The activities for (Co,La)salen/MCM-41 for two successive oxidation of styrene are listed in table 3. The catalytic activity obtained for the second run is over 90% of that for the first run. This result indicates that (Co,La)salen is firmly immobilized in MCM-41 and has high stability in oxidation styrene. Figure 4 displays the catalytic properties of (Co,La)salen/MCM-41in the oxidation of styrene at room temperature. It shows that with the increase of reaction time, the conversion of styrene increases, the selectivity for benzaldehyde varies slightly, and the content of phenlacetaldehyde and styrene oxide in the products decrease and increase respectively, hi the experiment range, the (Co,La)salen exhibits high stability and no deactivation is observed. The influence of temperature on the conversion and product distribution in the oxidation of styrene is shown in table 4. At low reaction temperature, the conversion of styrene is very low with benzaldehyde and phenlacetaldehyde as the main products. With the increase of reaction temperature, the conversion of styrene significantly increase and styrene oxide is formed, at the same time the formation of phenlacetaldehyde is suppressed.
315
Figure 4. The catalytic activity and selectivity of (Co,La)salen/MCM-41 in the oxidation of styrene as a function of time Table 4 Influence of reaction temperature on styrene conversion over (Co,La)salen/MCM-41 Reaction temperature
Conv.
X
% 0.47 2.66 15.6 30.7
25 40 50 60
BA 85.4 91.8 89.1 92.5
Products PA 15.0 6.59 3.40 1.55
SO 6.12 6.56 5.93
Table 5 Effect of solvent on catalytic performance of (Co,La)salen/MCM-41 Solvents
Conv.
Products SO PA
Others BA % 9.23 Tetra-chloromethane 6.72 0.61 8.51 75.5 2.68 Acetonitrile 1.80 13.1 8.69 82.4 6.79 Water 4.14 3.98 4.61 84.5 Acetone 30.7 1.55 5.93 92.5 Others = benzene-1,2-ethanediol, benzoic acid and other unidentified high boilers The effects of solvent on the oxidation of styrene are illustrated in table 5. The conversion is highest when acetone was used as solvent. When acetonitril, water and tetrachloromethane were used as solvents respectively, not only is the conversion very low, but also in the products benzoic, benzene-1,2-ethane diol and other unidentified high boilors appear.
316 4. CONCLUSION Immobilization of heteronuclear Co(II)-La(III) salen complex on mesoporous materials was tried. Due to the strong interaction of CoLasalen and the mesoporous material, mainlybetween the aromatic rings of the complex and the internal surface silanol groups of the mesoporous, the heteronuclear complex may undergo dissociation during immobilization. Metal salen complexes are firmly loaded on the MCM-41 host material and exhibit high stability in the oxidation of styrene, no matter whether they exist as mononuclear or as heteronuclear. There may be a synergism between two different complexes, which makes (Co,La)salen/MCM-41 has higher conversion than Cosalen/MCM-41 and Lasalen/MCM-41 for the oxidation of styrene.
REFERENCES 1. K.J. Balkus Jr., M. Eissa, and R. Levado, J. Am. Chem. Soc, 117(1995)10753 2. S.B. Ogunwumi and T. Bein, Chem.Commun., (1997)901 3. C.R. Jacob, S.R Varkey and R Ratnasamy, Microporous and Mesoporous Mater., 22(1998)465 4. R Thibault-Starzyk, R.R Parton and R A. Jacobs, Stud. Surf. Sci. Catal. 84(1994)1419 5. S.S. Kim, W.Z. Zhang and T.J. Pinnavaia, Catal. Lett. 43(1997)149 6. M. Eswaramoorthy, Neeraj and C. N. R. Rao, Chem.Commun., (1998)615 7. L. Frunza, H. Kosslick, H. Landmesser, E. Hoft, and R. Fricke, J. Mol. Catal. A: Chemical, 123(1997)179 8. G. Lu, K. M. Yao and L.R Shen, Chinese Journal of Applied Chemistry, 15(1998)1 9. G. Lu, K.M. Yao, W.G. Chen, L.R Shen and H.Z. Yuan, Chinese Journal of Applied Chemistry, 15(1998)1
311
Studies on immobilization of Co(II)-La(III) schifFbase complex in MCM-41 Binbin Fan^"^, Ruifeng Li^*, Zhihong Liu^, Jinghui Cao^, Bing Zhong^ ^Institute of special Chemicals, Taiyuan University of Technology, Taiyuan 030024, China ^State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, The Chinese Academy of Sciences, Taiyuan 030001, China Immobilization of heteronuclear Co(II)-La(III) salen (denoted as (CoLa)salen ) complex into the pores of MCM-41 matrix was studied and characterized by element analysis, TGDTA, FTIR and UV-vis. The results showed that the heteronuclear complex may undergo dissociation during immobilization step due to the strong guest/host interaction. Nevertheless, the obtained product(designated as (Co,La)salen/MCM-41) exhibits higher conversion than Cosalen/MCM-41 and Lasalen/MCM-41 and high stability for the oxidation of styrene. In addition, the effects of the reaction time, reaction temperature, and solvent on the catalytic properties of (Co,La)salen/MCM-41 in oxidation of styrene were also investigated. 1. INTRODUCTION Transition metal complexes encapsulated in the channel of zeolites have received a lot of attention, due to their high catalytic activity, selectivity and stability in field of oxidation reactions. Generally, transition metal complex have only been immobilized in the classical large porous zeolites, such as X, Y[l-4]. But the restricted sizes of the pores and cavities of the zeolites not only limit the maximum size of the complex which can be accommodated, but also impose resistance on the diffusion of substrates and products. Mesoporous molecular sieves, due to their high surface area and ordered pore structure, off*er the potentiality as a good host for inmiobilizing transition complexes[5-7]. The previous reports are mainly about molecular sieves encapsulated mononuclear metal complex, whereas the reports about inmiobilization of heteronuclear metal complex in the host material are few. Here, we try to prepare MCM-41 loaded with binuclear Co(II)-La(III) complex with bis-salicylaldehyde ethylenediamine schiff base. 2. EXPERIMENTAL 2.1 Preparation of (CoLa)salen (CoLa)salen was prepared as in Ref.[8]. To a 20ml isopropanol solution of Immol salen, a solution of 0.5mmol Co(N03)2 6H2O and 0.5mmol La(N03)3 in 5ml isopropanol was added. The mixture was stirred at room temperature for Ih under nitrogen atmosphere and a brown yellow precipitate was formed. After filtration the precipitate was washed for several times with isopropanol and ether, and ftirther dried under vacuum. Cosalen and Lasalen were prepared according to the method described in Ref.[8] and Ref [9] respectively.
312
2.2 Complex impregnation (Co,La)salen/MCM-41 were prepared by stirring 2g calcined MCM-41 and a solution of 0.36g (CoLa)salen in 30ml isopropanol at room temperature for 24h. The solid fraction was filtered, extracted with acetone, and finally dried at 333K for 5h. Cosalen/MCM-41 and Lasalen/MCM-41 were also prepared by impregnating 2g calcined MCM-41 with 30ml isopropanol solution of 0.36g Cosalen or Lasalen by using the similar method as preparation of(Co,La)/MCM-41. 2.3 Sample characterization Metal content was determined by a LABTAM 8401 inductively coupled plasma spectrometer. X-ray powder diffraction was carried out on a Rigaku 2304 diffractometer with CuK« radition(Ni filtered). IR and UV-vis spectra of the solid samples were recorded on a PE FTIR 1760 spectrometer and a PE Lambda Bio 40 instrument respectively. TG-DTA was performed on a CN8076E(Rigaku) thermal analysis instrument. 2.4 Catalytic measurement Styrene oxidation reactions were carried out in sealed batch reactors (100ml) at 333K for lOh. The reactant was composed of 3ml styrene, 2ml 30wt.% H2O2, 10ml acetone and O.lg catalyst. The products were analyzed on a GC-9A gaschromatography. 3. RESULTS AND DISCUSSION X-ray powder diffraction patterns of the calcined and immobilized (Co,La)salen MCM41 samples are presented in Fig.l. It can be seen that MCM-41 does not undergo structure change during the preparation of the catalyst and exhibits the hexagonal phase with at least two resolved peaks((100) and (110)) in the diffraction patterns. Upon immobilization and extraction, the obtained sample has light brown yellow color, suggesting that the complex be fixed in the MCM-41 host material. Elemental and thermogravimetric analyses were carried out to determine the complex content loaded on the MCM-41. Elemental analysis is given in table 1. In the isopropanol solution of (CoLa)salen the molar ratio of La/Co is 1.0, but fi-om table 1 it can be seen that the La/Co molar ratios are all less than 1.0 in the obtained products. With the increase of the Si02/Al203 ratio of the MCM-41, although the La content dramatically decreases, the Co content has not obvious change. According to the weight loss during 250-500°C, corresponding to the decomposition of salen ligand, and the metal content in the samples, it can be concluded that the molar ratios of (Co+La)/salen are nearly 1.0. These indicate that most of the (CoLa)salen complex may dissociate into mononuclear complex of Cosalen or Lasalen, possibly due to the strong interactions of (CoLa)salen complex and Si-OH in the wall of MCM-41 [7], and Cosalen is easier to be loaded on MCM41 than Lasalen. Table 1 Co and La content loaded on MCM-41 with different Si02/Al203 Si02/Al203 50 100 200
Co (wt.%) 0.9248 1.0915 0.8506
La (wt%) 0.6849 0.4933 0.1132
La/Co(mol.) 0.313 0.1918 0.0565
313
2000
1225 Wavenumbers, cm"
Figure 2, FTIR spectra of (Co, La) salen (a), (Co, La) salen/MCM-41 (b) and the calcined MCM-4I (c)
450
200
300
400
500
600
700
800
Wavelength, nm Figure 3. The diffuse UV-vis spectra of (Co,La)salen (a), Lasalen/MCM-41 (b), (Co,La)salen/MCM-41 (c) and Cosalen/MCM-41 (d)
The FTIR spectra of (Co, La)salen loaded/MCM-41, free (CoLa)salen and the parent MCM-41 material are depicted in Fig.2. From Fig.2, it can be seen that complex free MCM41 has no absorption bands in the wavenumber region between ca. 1300cm' and 1500cm . In the spectrum of the metal salen containing MCM-41, even if the (CoLa)salen may dissociate during the immobilization step, the characteristic adsorption bands of metal salen complexes are clearly visible, indicating that the metal salen complexes are indeed loaded on the mesoporous material and cannot be removed by acetone extraction. This can be confirmed by the reflectance spectra of different samples in Fig.3. From Fig.3, it can be seen that for the samples loaded metal salen an absorption band around 400nm ascribed to metal to ligand charge transfer[3], can be seen respectively. Generally, immobilization of metal complexes in the wall of mesorporous materials is attributed to the electrostatic interaction between anion oxygen sites on the channel walls of and positively charged complexes [5]. Comparing the IR spectra of MCM-41 loaded with (Co,La)salen and free (CoLa)salen, it can be seen that the band at 1380cm'' attributed to NO3' [8] is still remained, which indicates that the positive charge is balanced by NOs" instead of by the anionic host framework. Metal salen complexes are not held by ionic interaction between guest and the host framework, but by the strong guest/host interactions, mainly between the aromatic ring of the complex and the internal surface silanol groups of the walls of mesoporous[7].
314 The catalytic properties of the sample were tested with the oxidation of styrene. From table 2, it is obvious that (Co,La)salen/MCM-41 exhibits higher catalytic activity than Lasalen/MCM-41 and Cosalen/MCM-41 under the identical reaction conditions. This result suggests that there may be a synergism between two complexes containing different central ions, which makes it favorable for the oxidation of styrene. Table 2 Oxidation of styrene over different catalysts Conv.
Products PA BA % Cosalen/MCM-41 21.6 2.77 91.8 Lasalen/MCM-41 16.6 89.1 4.53 (Co,La)salen/MCM-41 30.7 1.55 92.5 BA = benzaldehyde; PA = phenlactaldehyde; SO = styrene oxide Catalyst
SO 5.48 6.33 5.93
Table 3 Recycling of (Co,La)salen/MCM-41 for styrene oxidation Oxidant
Number of
Conv.
H2O2
recycling 1st 2nd
% 30.7 27.8
H2O2
BA 92.5 88.74
Products PA 1.55 4.23
SO 5.93 7.03
Because (Co,La)salen is immobilized on the mesopore wall of MCM-41,it is anticipated that this catalyst would exhibit high stability for catalyst recycling. Therefore, the (Co.La)salen/MCM-41 sample was recovered by filtration and used again for oxidation of styrene. The activities for (Co,La)salen/MCM-41 for two successive oxidation of styrene are listed in table 3. The catalytic activity obtained for the second run is over 90% of that for the first run. This result indicates that (Co,La)salen is firmly immobilized in MCM-41 and has high stability in oxidation styrene. Figure 4 displays the catalytic properties of (Co,La)salen/MCM-41in the oxidation of styrene at room temperature. It shows that with the increase of reaction time, the conversion of styrene increases, the selectivity for benzaldehyde varies slightly, and the content of phenlacetaldehyde and styrene oxide in the products decrease and increase respectively, hi the experiment range, the (Co,La)salen exhibits high stability and no deactivation is observed. The influence of temperature on the conversion and product distribution in the oxidation of styrene is shown in table 4. At low reaction temperature, the conversion of styrene is very low with benzaldehyde and phenlacetaldehyde as the main products. With the increase of reaction temperature, the conversion of styrene significantly increase and styrene oxide is formed, at the same time the formation of phenlacetaldehyde is suppressed.
315
Figure 4. The catalytic activity and selectivity of (Co,La)salen/MCM-41 in the oxidation of styrene as a function of time Table 4 Influence of reaction temperature on styrene conversion over (Co,La)salen/MCM-41 Reaction temperature
Conv.
X
% 0.47 2.66 15.6 30.7
25 40 50 60
BA 85.4 91.8 89.1 92.5
Products PA 15.0 6.59 3.40 1.55
SO 6.12 6.56 5.93
Table 5 Effect of solvent on catalytic performance of (Co,La)salen/MCM-41 Solvents
Conv.
Products SO PA
Others BA % 9.23 Tetra-chloromethane 6.72 0.61 8.51 75.5 2.68 Acetonitrile 1.80 13.1 8.69 82.4 6.79 Water 4.14 3.98 4.61 84.5 Acetone 30.7 1.55 5.93 92.5 Others = benzene-1,2-ethanediol, benzoic acid and other unidentified high boilers The effects of solvent on the oxidation of styrene are illustrated in table 5. The conversion is highest when acetone was used as solvent. When acetonitril, water and tetrachloromethane were used as solvents respectively, not only is the conversion very low, but also in the products benzoic, benzene-1,2-ethane diol and other unidentified high boilors appear.
316 4. CONCLUSION Immobilization of heteronuclear Co(II)-La(III) salen complex on mesoporous materials was tried. Due to the strong interaction of CoLasalen and the mesoporous material, mainlybetween the aromatic rings of the complex and the internal surface silanol groups of the mesoporous, the heteronuclear complex may undergo dissociation during immobilization. Metal salen complexes are firmly loaded on the MCM-41 host material and exhibit high stability in the oxidation of styrene, no matter whether they exist as mononuclear or as heteronuclear. There may be a synergism between two different complexes, which makes (Co,La)salen/MCM-41 has higher conversion than Cosalen/MCM-41 and Lasalen/MCM-41 for the oxidation of styrene.
REFERENCES 1. K.J. Balkus Jr., M. Eissa, and R. Levado, J. Am. Chem. Soc, 117(1995)10753 2. S.B. Ogunwumi and T. Bein, Chem.Commun., (1997)901 3. C.R. Jacob, S.R Varkey and R Ratnasamy, Microporous and Mesoporous Mater., 22(1998)465 4. R Thibault-Starzyk, R.R Parton and R A. Jacobs, Stud. Surf. Sci. Catal. 84(1994)1419 5. S.S. Kim, W.Z. Zhang and T.J. Pinnavaia, Catal. Lett. 43(1997)149 6. M. Eswaramoorthy, Neeraj and C. N. R. Rao, Chem.Commun., (1998)615 7. L. Frunza, H. Kosslick, H. Landmesser, E. Hoft, and R. Fricke, J. Mol. Catal. A: Chemical, 123(1997)179 8. G. Lu, K. M. Yao and L.R Shen, Chinese Journal of Applied Chemistry, 15(1998)1 9. G. Lu, K.M. Yao, W.G. Chen, L.R Shen and H.Z. Yuan, Chinese Journal of Applied Chemistry, 15(1998)1