- Email: [email protected]
Efficient synthesis of benzoin methyl ether catalyzed by hydrotalcite containing cobalt
Available online at www.sciencedirect.com
Catalysis Communications 9 (2008) 1128–1131 www.elsevier.com/locate/catcom
Efficient synthesis of benzoin methyl ether catalyzed by hydrotalcite containing cobalt Xianmei Xie *, Kai Yan, Jingpin Li, Zhizhong Wang College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China Received 15 August 2007; received in revised form 27 September 2007; accepted 24 October 2007 Available online 30 October 2007
Abstract An efficient synthesis of benzoin methyl ether with benzaldehyde and methanol in the presence of MgCoAl–hydrotalcite catalyst has been developed. By this new method, not only the cyanide poisoning is avoided, but also the synthesis of benzoin methyl ether can be completed in one step instead of the traditional two steps with both condensation and etherification. A new way for major industrial processes to synthesize clean benzoin methyl ether was explored. 2007 Elsevier B.V. All rights reserved. Keywords: MgCoAl–hydrotalcite; Benzoin methyl ether; Catalysis; Conversion; Selectivity
1. Introduction Benzoin methyl ether is a kind of practical value photoinitiators [1]; it is generally synthesized by a two-step process [2] from benzaldehyde and methanol. First, benzaldehyde is condensated into benzoin in which cyanide is used as the catalyst. Second, methanol and benzoin dehydrate and etherify to yield benzoin methyl ether on the condition of acidity. The key of the whole synthetic process was the successful synthesis of benzoin. The traditional method has inevitable defects in catalyst themselves; where cyanide contains virulent poison, thus the defect greatly limits its application in major industrial processes. Therefore it has been recognized that developing clean benzoin methyl ether is one of the most important challenges in green chemistry. The use of heterogenous catalysts offers the advantages such as ease of work up, recyclability and development of environmentally benign synthetic procedures [3]. To our knowledge, there is no information available using hydrotalcite (abbreviated as HT) as catalysts for *
Corresponding author. E-mail address: [email protected] (X. Xie).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.10.020
the synthesis of clean benzoin methyl ether (Scheme 1). MgCoAl–hydrotalcite is firstly introduced into the synthesis of clean benzoin methyl ether, in which the conversion of benzaldehyde is up to 77.49% and the selectivity of benzoin methyl ether is nearly 100%, besides, the catalyst can be recycled in the process of synthesis. This environmentally friendly way for the synthesis of benzoin methyl ether has been communicated herein. 2. Experimental 2.1. Preparation and characterization of sample This paper adopted the method of coprecipitation to synthesize MgCoAl–hydrotalcite (MgCoAl–HT). The typical preparation process was as following [3–5]: exactly measured different volumes of 1.0 mol/l Mg(NO3)2Æ6H2O, 0.5 mol/l Al(NO3)3Æ9H2O, 1.0 mol/l Co(NO3)2Æ6H2O to mix into M2+/M3+ = 2.0, different Mg2+/Co2+ mixed nitrate solution at room temperature, the mixed solution was acutely stirred. At the same time, 1.0 mol/l NaOH solution and 0.5 mol/l Na2CO3 solution were titrated at a constant rate of 1.0 ml/min into the mixed solution. NaOH and Na2CO3 solution were continuingly dropped in, while
O OCH 3
009
H
110
O
C C
018
+ CH3 OH
015
MgCoAl-hydrotalcite
006
C H
1129
003
X. Xie et al. / Catalysis Communications 9 (2008) 1128–1131
(g)
Scheme 1.
(f)
maintaining a constant pH of 8.3 ± 0.2, for which well crystallinity could be obtained. The slurry was stirred for additional 0.5 h, then parked the slurry into stainless steel reactor, hydrothermal treatment at 383 K for 6 h. After hydrothermal treatment, the slurry was filtered and washed with a large amount of distilled water to remove nitrate ions until the pH value equaled 7. The filter cake was dried at 353 K in an air oven overnight to remove adsorbed water and samples were obtained, then reserved in desiccators before measurement. The samples were characterized for the crystalline phase(s) and spacing between hydroxide layers by XRD (using Rigaku D/max-2500 instrument (40 kV, 100 mA) and Cu Ka radiations at a scanning speed of 8 deg min1, with the scanning range of 5–85). 2.2. Typical procedure for synthesis of benzoin isopropyl ether The 250 ml three-neck flask installed condensator and thermometer was fixed in DF-101S compositive magnetic force stirrer, then definite dosage of benzaldehyde, methanol and MgCoAl–hydrotalcite catalyst (pretreatment under the condition of 353 K with N2 puffing for 3 h) were added, the reactant reacted at a constant temperature and routine pressure, the reaction mixture was analyzed by American HP (C6890A/5973MSD) gas chromatography equipped with a DV-101 column (0.2 mm · 50 m) and FID detector. 3. Results and discussion 3.1. The catalytic activity of hydrotalcite containing different Mg2+/Co2+/Al3+ ratios Seven kinds of samples with different Mg2+/Co2+/Al3+ ratios were prepared by the method of coprecipitation [6– 9]. Coprecipitation is probably the best technique for the synthesis of hydrotalcites, as it allows homogeneous precursors as starting materials. In this study we used the high supersaturation of coprecipitation for the preparation of 3+ hydrotalcite with Mg2+, C2þ in a O combined with Al 2+ 3+ 1 molar ratio M /M of 2.0 and NO3 as charge balancing anion. The structure patterns were recorded on XRD (Fig. 1). Fig. 1 shows the XRD patterns of MgCoAl–samples. The typical reflections (0 0 3), (0 0 6), (0 0 9) and (1 1 0) were easily recognized. Furthermore, the (0 0 l) reflections were characterized by high intensities combined with broad line shapes indicating that the hydrotalcites were of relatively
(e) (d) (c) (b) (a)
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
2θ / °
Fig. 1. XRD spectra of MgCoAl–hydrotalcite samples with different combined cations: (a) 1.2:0.8:1.0, (b) 1.0:1.0:1.0, (c) 0.8:1.2:1.0, (d) 0.6:1.4:1.0, (e) 0.2:1.8:1.0, (f) 0:2.0:1.0, (g) 0.4:1.6:1.0.
high crystallinity. No other crystalline phases could be detected in the XRD patterns, indicating that all seven synthetic samples were successful. Further analysis of the observed XRD patterns revealed small difference in the spectrum as a function of the composition, which can be explained by the substitution of layer Mg2+ ions by larger Co2+ ion [10]. Based on the above research, MgCoAl–hydrotalcite was acquired on the proper conditions and applied in the reaction of benzoin methyl ether to study their catalytic performances. Seven kinds of MgCoAl–hydrotalcites catalysts having effect on conversion of benzaldehyde and selectivity of benzoin methyl ether were reviewed (Table 1). Table 1 shows that MgCoAl–hydrotalcite with different 3þ Mg2þ =C2þ ratios had a stable catalytic activity on O =Al benzoin methyl ether. MgCoAl–hydrotalcite ðMg2þ =C2þ O = Al3þ ¼ 0:4 : 1:6 : 1:0Þ catalyst manifested the optimally catalytic activity, the highest conversion of benzaldehyde could reach 77.49%, the selectivity of benzoin methyl ether were nearly up to 100%. As a general trend, the presence of Co2+ and Mg2+ in the MgCoAl–hydrotalcite seems to have synergetic effect on the catalytic reaction.
Table 1 Catalytic performance of different MgCoAl–hydrotalcite catalysts Catalyst number
Synthetic Mg2+/Co2+/Al3+
X/%
S/%
1 2 3 4 5 6 7
2:0:1 (MgAl–HT) 1.0:1.0:1.0 0.8:1.2:1.0 0.6:1.4:1.0 0.4:1.6:1.0 0.2:1.8:1.0 0:2:1 (CoAl–HT)
0.00 20.56 51.24 62.95 77.49 74.72 54.05
0.00 98.64 99.17 98.86 99.89 99.59 99.53
X: conversion of benzaldehyde S: selectivity of benzoin methyl ether. Reaction conditions: MgCoAl–hydotalcite catalyst 0.1 g, benzaldehyde 3 ml, methanol 50 ml, reaction temperature 333 K, reaction time 140 min.
1130
X. Xie et al. / Catalysis Communications 9 (2008) 1128–1131
3.2. The effect of structure diversion of catalyst on the reaction
Table 2 Catalytic performance of MgCoAl–HT after different calcined temperature for 3 h
The mixed oxides derived from hydrotalcite exhibit characteristic structure and surface properties. Consequently, they have been utilized in catalytic reactions such as isomerization, dehydration, alkylation and aldol condensation [11–13]. The structure of catalysts uncalcined and calcined was firstly researched by XRD (Fig. 2). Hydrotalcie (HT) is a kind of double hydroxyl hydroxide with low thermogravimetric stability. According to Fig. 2, the structure of HT was encountered to demolish around 523 K and came into being complex oxides. Besides, the spectra of XRD detected the diffractive spinel peak of CoAl2O4, Co3O4, the diffractive peak of MgO in all samples. When the temperature was over 623 K, the diffractive peak of Al2O3 disappeared, the result presented that Al2O3 existed in unformed modality. When the calcined temperature gradually rised to 923 K, no new phase appeared and only the intensity of the peak had slowly diversification, it presented that the complex oxides which were derived from HT had better stability. Catalytic activity of complex oxides derived from hydrotalcite was assessed in the synthesis of benzoin methyl ether (Table 2). Table 2 presents that the structure of MgCoAl–hydrotalcite plays an important role in the catalytic results. Comparing to fresh catalyst (Fig. 2), the layered structure of MgCoAl–hydrotalcite catalyst was demolished after high temperature calcination (T > 523 K) and the formed complex oxides did not fit for the reaction. Comparing the calcined temperature of 423 K with 523 K, the conversion of benzaldehyde sharply reduced from 77.49% to 43.28%. This result presented that the layered structure was important for the catalytic activity of MgCoAl–hydrotalcite.
Calicinated temperature (K)
Benzaldehyde conversion (%)
Benzoin methyl ether selectivity (%)
423 523 723 923
76.31 43.28 15.21 0.00
99.82 98.67 98.90 0.00
Reaction condition Al3þ ¼ 0:4 : 1:6 : 1:0Þ.
as
Table
1,
MgCoAl–HT
ðMg2þ =Co2þ =
Besides, with the calcined temperature increasing from 523 K to 923 K, the conversion of benzaldehyde gradually reduced from 43.28% to 0.00%, this may be due to the sintering on the surface of the mixed metal hydroxides. 3.3. The effect of reaction temperature and time on the reaction The effect of reaction temperature on the conversion of benzaldehyde was studied by varying the temperature from 303 K to 343 K as shown in Fig. 3. The conversion of benzaldehyde was found to increase steadily with the temperature increasing from 303 K to 333 K. When the temperature increased to 343 K, the conversion of benzaldehyde was sharply reduced. This is due to the volatilization of methanol (boiling point: 338 K), while the temperature had little effect on the selectivity with nearly 100% for benzoin methyl ether during the process. Fig. 3 also shows the change in the conversion with time. The conversion was found to attain steady state values after 140 min or so. The catalyst if, therefore, quite stable at the reaction conditions. 3.4. Stability of hydrotalcite structure during the reaction The stability of hydrotalcite was shown in Fig. 4. After the reaction, the catalyst was separated from the reactant 0.8
(e)
0.7
(d) (c) (b)
0.6
X/%
0.5
(a)
0.4 0.3 0.2 0.1
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
2θ / °
0.0 0
20
40
60
80
100
120
140
160
Time / min Fig. 2. XRD patterns of MgCoAl–HT samples after different calcined temperature for 3 h: (a) 423 K, (b) 523 K, (c) 623 K, (d) 723 K, (e) 823 K, (f) 923 K [O: Typical diffraction peak of MgCoAl–HT, (r) MgO, (*) Co3O4, (h) CoAl2O4, (h) Al2O3].
Fig. 3. Effect of different temperatures on the conversion of benzaldehyde with reaction time changing [(a) 303 K, (b) 313 K, (c) 323 K, (d) 343 K, (e) 333 K].
110
015 018
009
006
003
X. Xie et al. / Catalysis Communications 9 (2008) 1128–1131
(c) NO.3
(b) NO.2
(a)NO.1
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
2θ / °
Fig. 4. The stability of MgCoAl–hydrotalcite after reaction.
mixture through centrifugal method, then the catalyst was washed with deionized water and dried at 353 K in an air oven overnight. Fig. 4 shows the stable structure of hydrotalcite after reaction. When the catalyst used in the reaction (entry No. 1) was recycled, the conversion of benzaldehyde in the second (No. 2) and third (No. 3) reuse of the catalyst for the same reaction was 76.53% and 75.89%, respectively. 4. Conclusion 3þ Hydrotalcite with different Mg2þ =C2þ molar ratios O =Al were prepared by coprecipitation method. These samples are transformed into mixed oxides when calcined over
1131
523 K, the layered structure being completely destroyed. The layered structure was the key for the catalytic activity of MgCoAl–hydrotalcite. The synthesized MgCoAl– 3þ ¼ 0:4 : 1:6 : 1:0Þ presented a hydrotalcite ðMg2þ =C2þ O =Al high conversion with 77.49% and selectivity with nearly 100% for benzoin methyl ether. The presence of Co2+ and Mg2+ in the MgCoAl–hydrotalcite seems to have synergetic effect on the catalytic reaction. It is easily removed from the product and can be reused. The hydrotalcite can be used as a good catalyst for the synthesis of clean benzoin methyl ether. References [1] S.P. Pappas, Prog. Org. Coat. 2 (4) (1973/1974) 333. [2] Fabian Kai, Enke Steffen, Tilly Herbert. Merck Patent Gmb H. US Patent 6849762, 2005. [3] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173. [4] X.M. Xie, J.X. Liu, J.L. Song, Z.Z.H. Wang, Chin. J. Catal. 24 (2003) 569. [5] S. Kannan, A. Dubey, S. Velu, J. Catal. 31 (2005) 381. [6] V.R. Choudhary, D.K. Dumbre, V.S. Narkhede, Catal. Lett. 86 (2003) 229. [7] Y.Z. Chen, C.M. Hwang, C.W. Liaw, Appl. Catal. A 169 (1998) 207. [8] Katsutoshi Nagaoka, Andreas Jentys, Johannes A. Lercher, J. Catal. 229 (2005) 85. [9] A.E. Palomares, J.M. Lo´pez-Nieto, F.J. La´zaro, Appl.Catal. B 20 (1999) 257. [10] S. Brindely, N. Kikkava, Am. Miner. 64 (1979) 836. [11] S. Gusi, F. Trifiro, A. Vaccari, J. Catal. 94 (1985) 120. [12] Emil Dumitriu, Vasile Hulea, Carmen Chelaru, Appl. Catal. A 178 (1999) 78. [13] U.R. Pillai, E. Sahle-Demessie, J. Mol. Catal. A 191 (2003) 93.
Catalysis Communications 9 (2008) 1128–1131 www.elsevier.com/locate/catcom
Efficient synthesis of benzoin methyl ether catalyzed by hydrotalcite containing cobalt Xianmei Xie *, Kai Yan, Jingpin Li, Zhizhong Wang College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China Received 15 August 2007; received in revised form 27 September 2007; accepted 24 October 2007 Available online 30 October 2007
Abstract An efficient synthesis of benzoin methyl ether with benzaldehyde and methanol in the presence of MgCoAl–hydrotalcite catalyst has been developed. By this new method, not only the cyanide poisoning is avoided, but also the synthesis of benzoin methyl ether can be completed in one step instead of the traditional two steps with both condensation and etherification. A new way for major industrial processes to synthesize clean benzoin methyl ether was explored. 2007 Elsevier B.V. All rights reserved. Keywords: MgCoAl–hydrotalcite; Benzoin methyl ether; Catalysis; Conversion; Selectivity
1. Introduction Benzoin methyl ether is a kind of practical value photoinitiators [1]; it is generally synthesized by a two-step process [2] from benzaldehyde and methanol. First, benzaldehyde is condensated into benzoin in which cyanide is used as the catalyst. Second, methanol and benzoin dehydrate and etherify to yield benzoin methyl ether on the condition of acidity. The key of the whole synthetic process was the successful synthesis of benzoin. The traditional method has inevitable defects in catalyst themselves; where cyanide contains virulent poison, thus the defect greatly limits its application in major industrial processes. Therefore it has been recognized that developing clean benzoin methyl ether is one of the most important challenges in green chemistry. The use of heterogenous catalysts offers the advantages such as ease of work up, recyclability and development of environmentally benign synthetic procedures [3]. To our knowledge, there is no information available using hydrotalcite (abbreviated as HT) as catalysts for *
Corresponding author. E-mail address: [email protected] (X. Xie).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.10.020
the synthesis of clean benzoin methyl ether (Scheme 1). MgCoAl–hydrotalcite is firstly introduced into the synthesis of clean benzoin methyl ether, in which the conversion of benzaldehyde is up to 77.49% and the selectivity of benzoin methyl ether is nearly 100%, besides, the catalyst can be recycled in the process of synthesis. This environmentally friendly way for the synthesis of benzoin methyl ether has been communicated herein. 2. Experimental 2.1. Preparation and characterization of sample This paper adopted the method of coprecipitation to synthesize MgCoAl–hydrotalcite (MgCoAl–HT). The typical preparation process was as following [3–5]: exactly measured different volumes of 1.0 mol/l Mg(NO3)2Æ6H2O, 0.5 mol/l Al(NO3)3Æ9H2O, 1.0 mol/l Co(NO3)2Æ6H2O to mix into M2+/M3+ = 2.0, different Mg2+/Co2+ mixed nitrate solution at room temperature, the mixed solution was acutely stirred. At the same time, 1.0 mol/l NaOH solution and 0.5 mol/l Na2CO3 solution were titrated at a constant rate of 1.0 ml/min into the mixed solution. NaOH and Na2CO3 solution were continuingly dropped in, while
O OCH 3
009
H
110
O
C C
018
+ CH3 OH
015
MgCoAl-hydrotalcite
006
C H
1129
003
X. Xie et al. / Catalysis Communications 9 (2008) 1128–1131
(g)
Scheme 1.
(f)
maintaining a constant pH of 8.3 ± 0.2, for which well crystallinity could be obtained. The slurry was stirred for additional 0.5 h, then parked the slurry into stainless steel reactor, hydrothermal treatment at 383 K for 6 h. After hydrothermal treatment, the slurry was filtered and washed with a large amount of distilled water to remove nitrate ions until the pH value equaled 7. The filter cake was dried at 353 K in an air oven overnight to remove adsorbed water and samples were obtained, then reserved in desiccators before measurement. The samples were characterized for the crystalline phase(s) and spacing between hydroxide layers by XRD (using Rigaku D/max-2500 instrument (40 kV, 100 mA) and Cu Ka radiations at a scanning speed of 8 deg min1, with the scanning range of 5–85). 2.2. Typical procedure for synthesis of benzoin isopropyl ether The 250 ml three-neck flask installed condensator and thermometer was fixed in DF-101S compositive magnetic force stirrer, then definite dosage of benzaldehyde, methanol and MgCoAl–hydrotalcite catalyst (pretreatment under the condition of 353 K with N2 puffing for 3 h) were added, the reactant reacted at a constant temperature and routine pressure, the reaction mixture was analyzed by American HP (C6890A/5973MSD) gas chromatography equipped with a DV-101 column (0.2 mm · 50 m) and FID detector. 3. Results and discussion 3.1. The catalytic activity of hydrotalcite containing different Mg2+/Co2+/Al3+ ratios Seven kinds of samples with different Mg2+/Co2+/Al3+ ratios were prepared by the method of coprecipitation [6– 9]. Coprecipitation is probably the best technique for the synthesis of hydrotalcites, as it allows homogeneous precursors as starting materials. In this study we used the high supersaturation of coprecipitation for the preparation of 3+ hydrotalcite with Mg2+, C2þ in a O combined with Al 2+ 3+ 1 molar ratio M /M of 2.0 and NO3 as charge balancing anion. The structure patterns were recorded on XRD (Fig. 1). Fig. 1 shows the XRD patterns of MgCoAl–samples. The typical reflections (0 0 3), (0 0 6), (0 0 9) and (1 1 0) were easily recognized. Furthermore, the (0 0 l) reflections were characterized by high intensities combined with broad line shapes indicating that the hydrotalcites were of relatively
(e) (d) (c) (b) (a)
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
2θ / °
Fig. 1. XRD spectra of MgCoAl–hydrotalcite samples with different combined cations: (a) 1.2:0.8:1.0, (b) 1.0:1.0:1.0, (c) 0.8:1.2:1.0, (d) 0.6:1.4:1.0, (e) 0.2:1.8:1.0, (f) 0:2.0:1.0, (g) 0.4:1.6:1.0.
high crystallinity. No other crystalline phases could be detected in the XRD patterns, indicating that all seven synthetic samples were successful. Further analysis of the observed XRD patterns revealed small difference in the spectrum as a function of the composition, which can be explained by the substitution of layer Mg2+ ions by larger Co2+ ion [10]. Based on the above research, MgCoAl–hydrotalcite was acquired on the proper conditions and applied in the reaction of benzoin methyl ether to study their catalytic performances. Seven kinds of MgCoAl–hydrotalcites catalysts having effect on conversion of benzaldehyde and selectivity of benzoin methyl ether were reviewed (Table 1). Table 1 shows that MgCoAl–hydrotalcite with different 3þ Mg2þ =C2þ ratios had a stable catalytic activity on O =Al benzoin methyl ether. MgCoAl–hydrotalcite ðMg2þ =C2þ O = Al3þ ¼ 0:4 : 1:6 : 1:0Þ catalyst manifested the optimally catalytic activity, the highest conversion of benzaldehyde could reach 77.49%, the selectivity of benzoin methyl ether were nearly up to 100%. As a general trend, the presence of Co2+ and Mg2+ in the MgCoAl–hydrotalcite seems to have synergetic effect on the catalytic reaction.
Table 1 Catalytic performance of different MgCoAl–hydrotalcite catalysts Catalyst number
Synthetic Mg2+/Co2+/Al3+
X/%
S/%
1 2 3 4 5 6 7
2:0:1 (MgAl–HT) 1.0:1.0:1.0 0.8:1.2:1.0 0.6:1.4:1.0 0.4:1.6:1.0 0.2:1.8:1.0 0:2:1 (CoAl–HT)
0.00 20.56 51.24 62.95 77.49 74.72 54.05
0.00 98.64 99.17 98.86 99.89 99.59 99.53
X: conversion of benzaldehyde S: selectivity of benzoin methyl ether. Reaction conditions: MgCoAl–hydotalcite catalyst 0.1 g, benzaldehyde 3 ml, methanol 50 ml, reaction temperature 333 K, reaction time 140 min.
1130
X. Xie et al. / Catalysis Communications 9 (2008) 1128–1131
3.2. The effect of structure diversion of catalyst on the reaction
Table 2 Catalytic performance of MgCoAl–HT after different calcined temperature for 3 h
The mixed oxides derived from hydrotalcite exhibit characteristic structure and surface properties. Consequently, they have been utilized in catalytic reactions such as isomerization, dehydration, alkylation and aldol condensation [11–13]. The structure of catalysts uncalcined and calcined was firstly researched by XRD (Fig. 2). Hydrotalcie (HT) is a kind of double hydroxyl hydroxide with low thermogravimetric stability. According to Fig. 2, the structure of HT was encountered to demolish around 523 K and came into being complex oxides. Besides, the spectra of XRD detected the diffractive spinel peak of CoAl2O4, Co3O4, the diffractive peak of MgO in all samples. When the temperature was over 623 K, the diffractive peak of Al2O3 disappeared, the result presented that Al2O3 existed in unformed modality. When the calcined temperature gradually rised to 923 K, no new phase appeared and only the intensity of the peak had slowly diversification, it presented that the complex oxides which were derived from HT had better stability. Catalytic activity of complex oxides derived from hydrotalcite was assessed in the synthesis of benzoin methyl ether (Table 2). Table 2 presents that the structure of MgCoAl–hydrotalcite plays an important role in the catalytic results. Comparing to fresh catalyst (Fig. 2), the layered structure of MgCoAl–hydrotalcite catalyst was demolished after high temperature calcination (T > 523 K) and the formed complex oxides did not fit for the reaction. Comparing the calcined temperature of 423 K with 523 K, the conversion of benzaldehyde sharply reduced from 77.49% to 43.28%. This result presented that the layered structure was important for the catalytic activity of MgCoAl–hydrotalcite.
Calicinated temperature (K)
Benzaldehyde conversion (%)
Benzoin methyl ether selectivity (%)
423 523 723 923
76.31 43.28 15.21 0.00
99.82 98.67 98.90 0.00
Reaction condition Al3þ ¼ 0:4 : 1:6 : 1:0Þ.
as
Table
1,
MgCoAl–HT
ðMg2þ =Co2þ =
Besides, with the calcined temperature increasing from 523 K to 923 K, the conversion of benzaldehyde gradually reduced from 43.28% to 0.00%, this may be due to the sintering on the surface of the mixed metal hydroxides. 3.3. The effect of reaction temperature and time on the reaction The effect of reaction temperature on the conversion of benzaldehyde was studied by varying the temperature from 303 K to 343 K as shown in Fig. 3. The conversion of benzaldehyde was found to increase steadily with the temperature increasing from 303 K to 333 K. When the temperature increased to 343 K, the conversion of benzaldehyde was sharply reduced. This is due to the volatilization of methanol (boiling point: 338 K), while the temperature had little effect on the selectivity with nearly 100% for benzoin methyl ether during the process. Fig. 3 also shows the change in the conversion with time. The conversion was found to attain steady state values after 140 min or so. The catalyst if, therefore, quite stable at the reaction conditions. 3.4. Stability of hydrotalcite structure during the reaction The stability of hydrotalcite was shown in Fig. 4. After the reaction, the catalyst was separated from the reactant 0.8
(e)
0.7
(d) (c) (b)
0.6
X/%
0.5
(a)
0.4 0.3 0.2 0.1
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
2θ / °
0.0 0
20
40
60
80
100
120
140
160
Time / min Fig. 2. XRD patterns of MgCoAl–HT samples after different calcined temperature for 3 h: (a) 423 K, (b) 523 K, (c) 623 K, (d) 723 K, (e) 823 K, (f) 923 K [O: Typical diffraction peak of MgCoAl–HT, (r) MgO, (*) Co3O4, (h) CoAl2O4, (h) Al2O3].
Fig. 3. Effect of different temperatures on the conversion of benzaldehyde with reaction time changing [(a) 303 K, (b) 313 K, (c) 323 K, (d) 343 K, (e) 333 K].
110
015 018
009
006
003
X. Xie et al. / Catalysis Communications 9 (2008) 1128–1131
(c) NO.3
(b) NO.2
(a)NO.1
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
2θ / °
Fig. 4. The stability of MgCoAl–hydrotalcite after reaction.
mixture through centrifugal method, then the catalyst was washed with deionized water and dried at 353 K in an air oven overnight. Fig. 4 shows the stable structure of hydrotalcite after reaction. When the catalyst used in the reaction (entry No. 1) was recycled, the conversion of benzaldehyde in the second (No. 2) and third (No. 3) reuse of the catalyst for the same reaction was 76.53% and 75.89%, respectively. 4. Conclusion 3þ Hydrotalcite with different Mg2þ =C2þ molar ratios O =Al were prepared by coprecipitation method. These samples are transformed into mixed oxides when calcined over
1131
523 K, the layered structure being completely destroyed. The layered structure was the key for the catalytic activity of MgCoAl–hydrotalcite. The synthesized MgCoAl– 3þ ¼ 0:4 : 1:6 : 1:0Þ presented a hydrotalcite ðMg2þ =C2þ O =Al high conversion with 77.49% and selectivity with nearly 100% for benzoin methyl ether. The presence of Co2+ and Mg2+ in the MgCoAl–hydrotalcite seems to have synergetic effect on the catalytic reaction. It is easily removed from the product and can be reused. The hydrotalcite can be used as a good catalyst for the synthesis of clean benzoin methyl ether. References [1] S.P. Pappas, Prog. Org. Coat. 2 (4) (1973/1974) 333. [2] Fabian Kai, Enke Steffen, Tilly Herbert. Merck Patent Gmb H. US Patent 6849762, 2005. [3] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173. [4] X.M. Xie, J.X. Liu, J.L. Song, Z.Z.H. Wang, Chin. J. Catal. 24 (2003) 569. [5] S. Kannan, A. Dubey, S. Velu, J. Catal. 31 (2005) 381. [6] V.R. Choudhary, D.K. Dumbre, V.S. Narkhede, Catal. Lett. 86 (2003) 229. [7] Y.Z. Chen, C.M. Hwang, C.W. Liaw, Appl. Catal. A 169 (1998) 207. [8] Katsutoshi Nagaoka, Andreas Jentys, Johannes A. Lercher, J. Catal. 229 (2005) 85. [9] A.E. Palomares, J.M. Lo´pez-Nieto, F.J. La´zaro, Appl.Catal. B 20 (1999) 257. [10] S. Brindely, N. Kikkava, Am. Miner. 64 (1979) 836. [11] S. Gusi, F. Trifiro, A. Vaccari, J. Catal. 94 (1985) 120. [12] Emil Dumitriu, Vasile Hulea, Carmen Chelaru, Appl. Catal. A 178 (1999) 78. [13] U.R. Pillai, E. Sahle-Demessie, J. Mol. Catal. A 191 (2003) 93.