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Chinese Science Bulletin © 2007
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Comparative research on the catalytic activities of different molecular sieves for acetalization and ketalization LIANG XueZheng, GAO Shan, WANG WenJuan, CHENG WenPing & YANG JianGuo† Shanghai Key Laboratory of Green Chemistry and Chemical Process, East China Normal University, Shanghai 200062, China
Molecular sieves with different acidities and pore-sizes have been applied to catalyze the acetalization and ketalization of carbonyl compounds with glycol. A comparative research on the catalytic activities of the catalysts for certain reactions has been carried out to figure out the relationship between the characters of the catalysts and the activities. The results show that the pore-size of the molecular sieves is one of the most important factors that determine the catalytic activity. Appropriate pore-size is needed for the certain reaction. The acidity is another factor that affects the activities. They also have great effects on the recycled activities of the catalysts.
Acetalization is commonly utilized as a protecting method for carbonyl groups in the presence of other functional groups when processing multifunctional organic molecules, because 1, 3-dioxolanes are stable un- der neutral and basic condition[1 3]. Besides the interest in acetals as protecting groups, many of them have found direct applications as fragrances in cosmetics, as food and beverage additives in pharmaceuticals, deter- gents, and lacquer industries[4 6]. Also, acetals are used for solvents or intermediates in organic reactions[7,8]. Acetals and ketals are generally prepared from carbonyl compounds with alcohols or diols in the presence of acid catalysts. The commonly used catalysts are protonic acid, Lewis acid, transitional metal complexes including Rh, - Pd and Pt[9 17]. However, many of the methods mentioned above present limitations due to the use of expensive reagents, the tedious work-up procedure and the necessity of neutralization of the strong acid media producing undesired wastes (with the exception of solid catalysts). Molecular sieves appear to be the promising catalysts with obvious advantages of easy separation, controlled acidity, shape selectivity and reusability. www.scichina.com
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Thus many molecular sieves are applied to the reac- tions[18 23]. But most attention was paid to the activities of the catalysts and little to the differences between them. The present work reported a series of molecular sieves with different acidities and pore-sizes used as catalysts for the synthesis of acetals (ketals) from carbonyl compounds and glycol. After optimizing the reaction condition, the catalytic activities of different catalysts were compared to find out the relationship between the catalytic activities and the characters of the catalysts with the purpose of obtaining determining-factor of the catalysts for these reactions.
1 Experiment 1.1 Reagents and equipment All organic reagents were commercial products of highReceived September 20, 2006; accepted December 19, 2006 doi: 10.1007/s11434-007-0261-6 † Corresponding author (email:
[email protected]) Supported by the National Key Project of Scientific and Technical Supporting Programs Funded by the Ministry of Science & Technology of China (Grant No. 2006BAE03B06)
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molecular sieves, comparison, catalytic activities, acetal, ketal
1.2 Synthesis of catalysts Pure molecular sieves were synthesized in our laboratory according to the methods in refs. [24-27]. Then they were submitted to 1 mol/L NH4NO3 with stirring at 70℃ for 4 h. The mixture was filtrated and washed free of Na+ using de-ionized water. They were dried at 110℃ over night in an air-oven and calcined at 500℃ in a maffle furnace for 3 h. After repeating the procedure twice, we obtained the catalysts. 1.3 Preparation of acetals and ketals A mixture of carbonyl compounds (about 0.1 mol), ethylene glycol, 10 mL cyclohexane and a certain amount of catalyst were mixed together in a three-necked round bottom flask equipped with magnetic stirrer, thermometer, and we used a Dean-Stark apparatus to remove the water continuously from the reaction mixture to improve the yield of the product. The mixture was refluxed for the specified periods. The process of the reaction was monitored by GC analysis of the small aliquots withdraw at half an hour intervals. On competition, the catalyst was recycled by filtration and dried in oven at 453 K for about 1 h. The qualitative analysis of the liquid reaction mixture was carried out on the GC-MS referred to above with HP-5 column using helium as carrier gas. The column temperature was raised from 40 to 260℃ at a heating rate of 10℃ min−1. The quantitative analysis of the reaction mixture was carried out on a temperature-programmed SHIMADZU (GC-14B) gas chromatograph. The concentration of the reactants and products was directly given by the system of GC chemstation according to the area of each chromatograph peak.
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2 Results and discussion 2.1 Characterization of the catalysts The crystalline nature of the molecular sieves was revealed by the X-ray diffraction (XRD) studies using CuKα radiation. The BET surface area and pore-size distribution measurements were performed by Quancachrome BET 02108-KR-1 equipment using nitrogen adsorption-desorption isotherms at the temperature of liquid nitrogen. The acidity of the catalysts was determined by NH3-TPD. The characters of the molecular sieves are presented in Tables 1 and 2. For each catalyst, the XRD patterns match well with the literature reported. The acid structural properties of the samples were determined by thermodesorption of chemisorbed ammonia (NH3-TPD). This method provides general information on the number and distribution of acid strength of the active sites. The amounts of ammonia desorbed were formally divided into three temperature ranges to denote three types of acidities: 1) weak acid sites, ranging from 100 to 200℃; 2) moderate acid strength, ranging from 200 to 400℃; 3) high acid strength, ranging from 400 to 600℃ as shown in Table 2. The overall amount of ammonia desorbed enables one to evaluate the concentration of accessible acid sites. For different molecular sieves, the acidity values follow the order: HAlMSU-Y > Hβ> HMCM-22 > HAlMCM-41. 2.2 Catalytic acetalization of carbonyl compounds 2.2.1 Optimization of reaction conditions. HMCM22 was used first to catalyze the ketalization of cyclohexanone and ethylene glycol. The influences of different amounts of catalyst, mole ratios of reactants, and reaction time on the yield of product were investigated (Table 3). Results in Table 3 indicated that the optimal reaction condition was the molar ratio of cyclohexanone to ethylene glycol, 1:1.4; amount of catalyst 0.2 g and reaction time 1.5 h (entry 6). Under that condition, the conversion achieved 99% with the complete selectivity. The reason for that may be the low steric hinderance of the carbonyl due to the fixation of the 6 MR of cyclohexanone. When different molecular sieves were applied to the reaction (Figure 1), the results showed that all the molecular sieves could catalyze the reaction effectively with high yield (over 98%). It could also be deduced that Hβ was the most active catalyst for the reaction with the
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est purity (>98%) and used for the reaction without further purification. Cyclohexanone, butanone, propionaldehyde, n-butyraldehyde, diphenyl ketone, benzaldehyde, and glycol were purchased from Shanghai Chemicals Co., China. The crystalline nature of the molecular sieves was revealed by X-ray diffraction studies performed using a Bruker D8 Advance X-ray diffractometer from Germany. BET surface area and pore volume measurements were performed using a Quancachrome 02108-KR-1 surface area analyzer from America. GC measurements were taken on SHIMADZU (GC-14B) gas chromatograph. GC-MS measurements were performed on an American Agilent 6890/5973N.
Table 1
The characteristics of the catalysts
Molecular sieve Hβ HMCM-22 HAlMCM-41 HAlMSU-Y Table 2
Si/Al 24 40 20 9
SBET (m2·g−1) 430 400 943 989
Pore size (nm) 0.66×0.67 0.45×0.51/0.71×0.71×1.82 2.24 3.74
Pore volume (cm3·g−1) 0.38 0.33 0.56 0.60
TPD of ammonia from different molecular sieves Amount of ammonia desorbed within certain temperature range (mmol/g ) 100-200℃ (w) 200-400℃ (m) 400-600℃ (s) 0.49 0.51 0.43 0.44 0.26 0.12 0.26 0.24 0.10 0.22 0.76 0.69
Molecular sieve Hβ HMCM-22 HAlMCM-41 HAlMSU-Y
total acidity 1.43 0.82 0.67 1.67
Table 3 Ketalization of cyclohexanone and glycol catalyzed by HMCM-22a),b) Entry 1 2 3 4 5 6 7 8 9
Reaction time (h) 0.5 1.0 1.5 2.0 1.5 1.5 1.5 1.5 1.5
Amount of catalyst (g) 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.3
Mole ratio c) 1:1.2 1:1.2 1:1.2 1:1.2 1:1.0 1:1.4 1:1.5 1:1.4 1:1.4
Conversion (%) 63.4 93.9 98.4 98.5 96.8 99.6 99.5 97.8 99.4
Selectivity (%) 100 100 100 100 100 100 100 100 100
a) All reactions were carried out under Dean-Stark conditions. b) The conversion and selectivity were determined by GC using an internal standard method based on cyclohexanone. In all cases, the corresponding products were exclusively obtained. c) Mole ratio referred to n(cyclohexanone)/n(glycol).
Figure 1
The catalytic activities of different catalysts.
highest reaction rate. Because the spectra of pore topologies and dimensions explored are broad enough for the reaction. But the mesoporosity is too large for the small molecular cyclohexanone, so the stabilization effect could not be embodied while the microporous catalysts showed higher activities for the reaction. Furthermore, the acidity attached much importance to the reaction as to the certain pore-size catalysts. 2.2.2 Recovery and reuse of molecular sieves. One property of these molecular sieves is immiscibility with 1782
organic compounds or solvents. Thus, recovery of catalysts is very important in molecular sieves-catalyzed reactions. After reactions, by cooling the reaction mixture to room temperature, the catalysts were recovered by filtration. To rule out the differences between them, the activities of the recovered catalysts in each reaction were investigated carefully. The experimental results revealed that each molecular sieve could be recycled with high activity (over 95%). But the differences between these catalysts were obvious. The activities of microporous molecular sieves (β and MCM-22) drop more quickly than mesporous ones (MCM-41 and MSU-Y). There might be two reasons. First, the microporosity has stronger acidity which makes the reactants and products attracted stronger with the catalysts. Second, the mesoporosity has the diffusing preponderance (Figure 2). 2.2.3 Acetalization of other aldehydes or ketones and glycol. Different aldehydes and ketones were selected to compare the catalytic activities of different molecular sieves (Table 4). It can be deduced from Table 4 that acetals and ketals formation was strongly affected by electronic and steric factors. It is generally accepted that the rate-determining step of acetaliztion is the formation
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Acetalization of other aldehydes or ketones and glycola),b)
Entry Catalyst Aldehydes or ketones Conversion (%) Selectivity (%) 1 propionaldehyde 88.2 98.4 Hβ 2 HMCM-22 propionaldehyde 85.5 98.3 3 HAlMCM-41 propionaldehyde 75.8 99.3 4 HAlMSU-Y propionaldehyde 77.1 98.0 5 butanone 88.8 98.5 Hβ 6 HMCM-22 butanone 86.6 96.5 7 HAlMCM-41 butanone 83.1 92.8 8 HAlMSU-Y butanone 80.5 99.1 9 n-butyraldehyde 97.3 99.5 Hβ 10 HMCM-22 n-butyraldehyde 96.8 98.7 11 HAlMCM-41 n-butyraldehyde 93.1 97.3 12 HAlMSU-Y n-butyraldehyde 93.9 95.0 13 benzaldehyde 92.5 99.5 Hβ 14 HMCM-22 benzaldehyde 94.6 99.5 15 HAlMCM-41 benzaldehyde 97.5 99.3 16 HAlMSU-Y benzaldehyde 97.2 99.5 17 diphenyl ketone 14.1 99.5 Hβ 18 HMCM-22 diphenyl ketone 13.4 99.6 19 HAlMCM-41 diphenyl ketone 63.5 99.7 20 HAlMSU-Y diphenyl ketone 74.8 99.5 a) All reactions were carried out under Dean-Stark conditions: carbonyl compounds 0.1 mol, glycol 0.14, catalyst 0.2 g, cyclohexane 10 mL, and reaction time 1.5 h. b) The conversion and selectivity were determined by GC using an internal standard method based on carbonyl compounds. In all cases, the corresponding acetals were exclusively obtained.
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Table 4
termining factors of the catalytic activities are the pore-size and the acidity. The pore-size becomes the chief factor for the reactions because it could stabilize intermediates of the reaction. Also appropriate pore-size should be chose for the certain reaction because too large or small pore-size would decrease the catalytic activities.
Figure 2 Influence of recycled times on yield.
of a cation from the protonated hemiacetal. In order to compensate the low rate of hemiacetal formation, the catalyst must be sufficiently acidic to promote effective protonation of any hemiacetal formed and polar enough to stabilize the cationic intermediate. The results in Table 4 clearly demonstrate that the catalytic activities of these microporous molecular sieves for small molecules such as propionaldehyde and butanone were much higher. And the order is consistent with the acidity for the certain molecular sieves. When the reactant molecules become larger, mesporous catalysts (HAlMCM-41, HAlMSU-Y) showed especially high activities for them such as diphenyl ketone. So we can deduce that the de-
A series of molecular sieves with different acidities and pore-sizes herein are efficient and selective heterogeneous catalysts for the acetalization and ketalization between carbonyl compounds and diols. After optimizing reaction conditions, carbonyl compounds with different sizes were successfully converted to corresponding products. An especial attention was paid to the distinction of the catalytic activities between the catalysts. The results show that the mesoporous molecular sieves are very effective for large molecules, while the microporosity decreases remarkably for the pore and channel not broad enough for the big molecules accessing the acidic sites available. The determining factors of the catalytic activities are the pore-size and the acidity. The appropriate pore-size becomes the chief factor for the certain reaction.
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