Catalysis of Organic Reactions

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ity of DMC, it is promising as a substitute for phosgene, dimethyl sulfate, or methyl iodide as well as a potential petrol octane enhancer. It is known that DMC.
58 Synthesis of Dimethyl Carbonate from Propylene Carbonate and Methanol over Mesoporous Solid Base Catalyst Tong Wei, Mouhua Wang, Wei Wei, Yuhan Sun, and Bing Zhong Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, P.R. China

ABSTRACT Mesoporous solid base catalyst was prepared by loading KI, K2CO3 and KOH on mesoporous active carbon and was used to catalyze dimethyl carbonate synthesis from propylene carbonate and methanol. The effect of preparation method, base strength, catalyst content, reaction time and temperature were investigated. INTRODUCTION Much attention has been paid to mesoporous materials in recent years (1). But little has been done to investigate them as catalysts, especially as solid base catalysts (2). Since most of organic reactions proceed in liquid phase, mesoporous catalysts are especially preferable in order to reduce inner diffusion resistance. Dimetyl carbonate is an important precursor of polycarbonate resins as well as a useful carbonylation and methylation agent (3,4). Because of the negligible toxicity of DMC, it is promising as a substitute for phosgene, dimethyl sulfate, or methyl iodide as well as a potential petrol octane enhancer. It is known that DMC can be synthesized concomitantly with propylene or ethylene glycol by the following liquid phase ester exchange reaction between propylene or ethylene carbonate

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and methanol (5,6), as shown in Scheme 1. For this reaction various base catalyst and group IV homogeneous catalyst are reported to be effective (6, 7). In the present work, KI, K2CO3 and KOH impregnated mesoporous active carbon was studied as solid base catalysts for the synthesis of dimethyl carbonate by transesterification of propylene carbonate with methanol. The effect of preparation method, strength of base site, catalyst content and reaction temperature were examined.

0

Cat

-CH 3 O - C —-OCH 3 OH

OH

= H, CH3

Scheme 1

EXPERIMENTAL Pore Size Distribution of Active Carbon Coconut mesoporous active carbon was provided by 702 group of Shanxi Institute of Coal Chemistry, Chinese Academic of Sciences. The pore size distribution of this active carbon was determined by BET method as illustrated in Figure 1. It can be seen that diameter of most of pores is between 2-10 nm. The average pore diameter is 3.65 nm. 0.35 0.30

°-25 0.20 015

0.10 0.05 0.00 100

Pore diameter (A)

Figure 1 Pore size distribution of active carbon.

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Dimethyl Carbonate from Propylene Carbonate

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Preparation of Catalyst Catalysts used in this paper were synthesized by two different methods. In first method (heat-treated method), mesoporous active carbon (20~40mesh) was heated from room temperature to 390K at a rate of 4 K/min and heated at 390K for 2 hr in N2 atmosphere. After being cooled to room temperature, 5 g mesorporous active carbon was impregnated in 10 mL 0.3M K2CO3 aqueous by incipient wetness, then after being laid for 24 hr, the particles were dried at 373K for 24 hr in N2 atmosphere, and subsequently heated from room temperature to 573 K at a rate of 4K min"1, and treated for lOhr at this temperature in flowing nitrogen. In second one (vacuum-treated method), mesoporous active carbon was heated at 353K for 4 hr at 0.02MPa. After being cooled to room temperature, 5 g active carbon was impregnated in lOmL 0.3M KI, K2CO3 and KOH aqueous respectively by incipient wetness and was laid at room temperature for 24 hr in air, and then dried at 353K for 8 hr at 0.02MPa. Catalytic Reaction Ester exchange reactions were carried out in a 75mL autoclave. After catalyst, propylene carbonate (4.73g) and methanol (5.99g) was put into autoclave, the reactor was heated at the rate of 5 K/min. When expected temperature was reached, violent magnetic stirring was turned on. Products were analyzed on GC 920 GasChromatograph with TCD using GDX-203 (3m) packed column after centrifugal separation of solid catalyst from liquid. RESULT AND DISCUSSION Effect of Preparation Method In order to investigate the effect of preparation method, K2CO3(0.6mmol/(g active carbon)) was loaded on mesorporous active carbon by two different methods (heat-treated method (HTM) and vacuum-treated method (VTM)). It can be seen from Table 1 that vacuum treated catalyst shows higher catalytic activity than that of heat-treated catalyst. In the process of evaporating water from the catalyst prepared by impregnation, the crystalline of active substance would grow with the increase of temperature. Therefore crystalline growth may be resisted in the process of vacuum desiccation because of the lower temperature, which would probaTable 1 The effect of preparation method on PC conversion and PMC yield3 Catalyst PC conversion % DMC yield % K2CO3/C( HTM) 2 8 6 2 5 A K2CO3/C(VTM) 1

30.2

27.7

Reaction condition catalyst 1.82 (wt)%; temperature: 393K; reaction time: 2 hr.

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bly result in the smaller K2CO3 particles and more active site on VTM catalyst than that on HTM catalyst. Effect of Base Strength The catalytic behavior of KI/C, K2CO3 1C, KOH/C with loading content 0.6 mmol/(g active carbon) is illustrated in Table 2. It can be seen that the sequence of PC conversion and DMC yield is KOH/O K2CO3/C» KI/C. As catalyzed by base, this transesterification reaction proceeds as shown in schem2 (6). As can be seen from the mechanism, the main function of base catalyst is to activate MeOH by hydrogen bound between hydrogen of MeOH and basic site on catalyst. While KI is a typical weak basic salt, K2CO3 and KOH is strong base with H being 18.4 and 15.0 respectively (8). Since the pKa of MeOH is 15.0 (9), both KOH/C and K2CO3/C can activate MeOH greatly and the activation capability goes up with the increase of base strength. Therefore the sequence of catalytic activity is KOH/O K2CO3/C» KI/C. Since potassium hydroxide has significant solubility in methanol (35 wt% at 30IK), in order to make a clear view of whether the catalytic ability comes from supported KOH or from soluble KOH, the concentration of K on catalyst was determined by ICPAES. It seems that only a little KOH dissolved in reaction liquid and most KOH was loaded on active carbon by strong van der walls force, since the concentration of K was 2.32 (wt)% before the reaction and was 2.28(wt)% after the reaction. B--H--OHC

B--H--OH 3 C

+

rf^

| CH2

9 CH

CH,—O—C—O —CH,— CH—CH

CH3

O CH3-0-C-O-CH2— CH—CH3 + B--H- OH3C

OH O

CH3-o-C-O-CH3 + CH 2 —CH—CH 3 + B I I OH OH

Scheme 2

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Table 2 The effect of base strength on PC conversion and DMC yield Catalyst

1

PC conversion%

DMC yield%

KI/C

2.7

2.6

K2CO3/C

30.2

27.7

KOH/C

32.4

29.1

reaction condition catalyst 1.82 (wt)%; temperature: 393K; reaction time: 2 hr.

The Effect of Catalyst Content and Reaction Time As said above, because of its highest catalytic activity in this three solid base catalyst, KOH/C (0.6mmol/(g active carbon)) was used as a catalyst in order to investigate the effect of catalyst content and reaction time as illustrated in Fig. 2 and Table 3. It can be seen that PC conversion and DMC yield increase greatly when catalyst content increase from 0.91% to 1.82%, but increase only a little when catalyst content increase from 1.82% to 4.70%. On the other hand, when reaction time attains 2hr, PC conversion and DMC yield almost keep constant despite the prolong of reaction time. Catalyst content mainly affects the activation of MeOH. The amount of activated MeOH would increase greatly with the increase of catalyst concentration at first. But after catalyst concentration reaches certain value, it would have only a little effect on activated MeOH concentration, and therefore the reaction rate almost keep constant with the continual increase of catalyst concentration. As for the effect of reaction time, this reaction almost attains equilibrium at 2hr 120D, so although the reaction time was extended, the reaction rate was so slow that PC conversion and DMC yield almost keep constant. IV

65 60

reaction time 2hr, reaction terrperature:393K

55 o5

— •— reconversion ••-• DMCyield

50

45

1

40 35

: ^r^ •g •x i a b

30

^~—""^«

:

• '"

25 20

15

10

5

0 05

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5

Catalyst (\M)%

Figure 2 Effect of catalyst content on PC conversion and DMC yield

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Table 3 The effect of reaction time on PC conversion and DMC yield Reaction time / hr

PC conversion%

DMC yield%

1

27.2

26.0

2

32.4

29.1

3

33.0

29.7

a: reaction condition catalyst: KOH /C; 1.82 (wt)%; temperature: 393K

Effect of Reaction Temperature Temperature is an important factor that could influence both reaction rate and reaction equilibrium. Figure 3 shows the effect of reaction temperature on PC conversion and DMC yield over KOH/C (0.6mmol/(g active carbon)). As can be seen from Figure 3, PC conversion and DMC yield increase greatly when temperature increase from 333K to 393KCUbut increase only a little when temperature continues going up. This reaction is exothermic with ArH=-7.092kJ/mol (10-13), which makes the reaction equilibrium being little affected by temperature. Therefore, The increase of temperature can elevate reaction rate greatly before the reach of reaction equilibrium. The more closely the reaction to equilibrium, the less the effect of temperature on reaction rate is. For reaction time is 2hr, the reaction is dynamically controlled when temperature is lower than393K and is thermally controlled when temperature is higher than 393K.

catalyst 1.82 (wt)%, reaction time : 2hr

-a—PC conversion o DMC yield

360

380

400

420

reaction temperature / K

Figure 3 Effect of temperature on PC conversion and DMC yield

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CONCLUSION In a word, loading K2CO3 and KOH on mesoporous active carbon by vacuum impregnation can prepare highly basic mesoporous solid base catalyst, which is very active for the synthesis of dimethyl carbonate from propylene carbonate and methanol. As for this reaction, the higher the strength of base site of catalyst, the higher the reaction rate is. Moreover, because this reaction is a little exothermic, temperature has little effect on reaction equilibrium.

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6 7 8 9 10 11 12 13

T. Kyotani, Control of pore structure in carbon. Carbon. 38: 269-286 2000. K R Kloetstra, H. V. Bekkum, Solid mesoporous base catalysts comprising of MCM-41 supported intraporous cesium oxide, Stud Surf Sci Cata 105: 431-438. 1997. W B Kim, J S Lee. Gas phase transesterification of dimethyl carbonate and phenol over surpported titanium dioxide. J Catal 185: 307-313, 1999. M A Pacheco, C.L. Marshall. Review of dimethyl carbonate (DMC) manufacture and its characteristics as fuel additive. Energy and Fuel 11: 2-29 1997. J A Cella, S W Bacon. Preparation of dialkyl carbonates via phase-transfercatalyzed alkylation of alkali metal carbonate and bicarbonate salts. J. Org. Chem 49: 1122-1125, 1984. J F Knifton, R G Duranleau. Ethylene glycol-dimethyl carbonate cogeneration. J Molecular Catal 67: 389-399 1991. Y Watanabe, T Tatsumi. Hydrotalcite-type materials as catalyst for the synthesis of dimethyl carbonate from ethylene carbonate and methanol. Microporous and mesoporous materials 22:399-407, 1998. J Zhu, Y Chun, Y Wang. Strong basic zeolite. Chinese Since Bulletin 44: 897903,1999. Organic Chemistry 2nd ed. Beijing: High Education press, 1992, pp88. Y Yin, Z Xi, D Li. Physical Chemistry. 3rd ed. Beijing: Higher Education Press, 1994,pp569. Z Dong . Synthesis of propylene carbonate by transesterification. Hangzhou Chemical Engineering, (3): 39-96,1979. R C Reid, J M Prausnitz, B E Poling. The properties of Gases and Liquids, 4th ed. New York: McGraw-Hill Company, 1987, pp 683. V Steele, R D Chirico, S E Knipmeyer, A Nguyen, N K Smith. Thermodynsmic properties and ideal-gas enthalpies of formation of dicyclohexyl sulfide, diethylenetriamine, di-n-octyl sulfide, dimethyl carbonate, piperazine, hexachloroprop-1ene, tetrakis (dimethylammo)ethylene, N,N' -bis(2-hydroxyethyl)ethylenediamine, and l,2,4-triazolo{ l,5-a}pyrimidine. J Chem Eng Data42: 1031-1052, 1997.

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