STUDIES ON KINETICS OF DIMETHYL SYNTHESIS BY HOMOGENEOUS TRANSESTERIFICATION

Haznan Abimanyu Process Technology and Catalysis Division, Research Center for Chemistry, Indonesian Institute of Sciences (L1PI), Kawasan PUSPIPTEK Serpong, Tangerang - Banten E-mail:[email protected]

ABSTRACT chemical industry and in the motor fuel industry. It has been proposed as a methylating agent for A comprehensive kinetic study on dimethyl aromatic compounds, to replace methyl halides carbonate synthesis by transesterification of ethylene and , which are both toxic and carbonate with has been conducted. An alkali corrosive, and as an intermediate in the synthesize base metal (KOH) was used as catalyst in the synthesis polycarbonates and isocyanates, thereby avoiding of DMC and its catalytic ability was investigated in the need to use . DMC is also a candidate term of kinetics. The reaction orders, the activation for replacing tertiary butyl ether (MTBE) as an energy and the rate constants were determined for both oxygen-containing additive for gasoline because it forward and backward reactions. The reaction is reversible and the equilibrium rate constant was has high oxygen content, a good blending octane, estimated to be 135.77exp(-16542.4/RT). low toxicity, and quick biodegradation [1]. A number of DMC synthesis processes have Keywords : dimethyl carbonate synthesis, transesterification been suggested and some processes have already reaction, kinetic study on DMC synthesis been commercialized. The classic route to DMC involves the use of phosgene. This route is INTISARI disadvantaged by the safety problem of handling highly toxic phosgene and the environmental Telah dilakukan studi lengkap mengenai kinetik problem ofdisposing of sodium chloride. pembuatan dimetil karbonat (DMC) melalui COCI2 + CH,OH transesterifikasi etilen karbonat dengan methanol. Phosgene Sebuah alkali metal basa (KOH) digunakan sebagai katalis dalam pembuatan DMC ini dan kemampuan CH,OH + NO katalitiknya diselidiki dalam bentuk kinetiknya. Ordo o II ~0, reaksi, energi aktifasi dan konstanta laju reaksi O2 /C, -NO CH,OH + CO --- H,CO OCH, _-- CH,ONO + CO ditentukan baik untuk reaksi maju maupun reaksi balik. ICu] C>\1C Methyl Nitrite Reaksi ini bersifat reversibel dan konstanta laju reaksi kesetimbangannya diestimasi mempunyai nilai 135,77 o exp(-16542,4/RT). II + CH ,OH H2N-C-NH2 + CH ,OH Urea Kata Kunci: dimethyl carbonate synthesis, transesterification reaction, studi kinetik pembuatan dimetil t karbonai (DMC synthesis) l::, + CO 2

INTRODUCTION Figure 1. Routes to DMC ------Figure 1. shows three routes to DMC: Dimethyl carbonate (DMC)is a versatile and oxidative using copper chloride nontoxic chemical substance, which is an catalyst (actually a redox agent), transesterifi- important chemical material in the current cation between ethylene carbonate and methanol,

JKTI, VOL. 11, No.2, Desember 2009 15 and urea methanolysis. Indeed,the transesterifi- EXPERIMENTAL PROCEDURE cationmethod has been given increasing interest [2, 3]. All experiments were performed in a Overall transesterification reaction of stainless-steel autoclave (200 mL) in atmospheric ethylene carbonate (EC) with methanol (MeOH) pressure. For mixing in the reaction, magnetic can be written as follows: stirrer was used. Reaction temperature was maintained constantly using a temperature controller. A gas chromatograph (Gow-Mac SSOP) 2H9-~ °II H2C-C~ I Cr=O + 2CH,OH ~ /c...... + I I equipped with a TCD detector and a Carbowax 2HC-O/ 9 9 OH OH CH, CH, column (1/8 in. 2m, Chrom W-HP, 80/100) was used to analyze the products of reaction. Starting Schemel materials used in this work were methanol (purity 99.5%,) produced by Daejung Chemicals & Metals There are two kinds of transesterification for DMC Co., and ethylene carbonate ( 98% from Aldrich), synthesis: heterogeneous and homoge-neous. dimethyl carbonate ( 99%, Acros) and ethylene Tatsumi et.al. [4} introduced heterogeneous glycol (EG) ( 99%, Daejung) without further catalyst and suggested that a solid base catalyst purification. was excellent for the synthesis of DMC. Watanabe The molar ratio of the initial reactants for et.al. [5} reported that hydrotalcite-type catalysts forward reaction (MeOH:EC) was in the range of have good catalytic ability for DMC synthesis. 8:1 - 15:1, while for backward reaction the molar Knifton et.al, [6] suggested eight classes of catalysts ratio between DMC and EG was changed from for DMC synthesis through the transesterification 1:10 to 10:1. Potassium hydroxide, soluble in reaction. methanol and EG, was used as catalyst for both The homogeneous transesterification reactions. Catalyst concentration was maintained method is, however, featured by a large reaction around 0.2 wt%. The reaction temperature was rate and is most likely to be commercialized, varied from 30°Cto 60°Cand from 30°Cto 80°Cfor though it brings some difficulty in the catalyst forward and backward reaction, respectively. separation. A cheap alkali, which is soluble in the methanol and glycol solutions, can be used as RESULTS AND DISCUSSION homogeneous catalyst. Accordingly, a detail study on this method is imperative for an industrially The transesterification reaction as shown in acceptable DMC synthesis process. Scheme 1 composed of two reactions, that are In this work, the authors proposed kinetic forward and backward reaction. The rate of performance of KOH catalyst and estimated the reaction of ethylene carbonate (EC) can be rate constants and the reaction orders in DMC expressed as follows: synthesis, for both forward and backward reactions. KOH is a base alkali-metal catalyst, which has high activity for DMC synthesis. It has been found that KOH has a moderate reaction rate When only forward reaction is regarded, the among other base alkali-metal catalysts, and in the equation (1)can be rearranged to equation (2)since presence of KOH as catalyst, EC conversion in the the high concentration of MeOH was used in the DMC synthesis could be obtained around 42 % reactant for this reaction. and 5S % in reaction time of 1 and 2 hours, respectively [7]. Moreover, KOH is the cheapest - rEC =k forwardC;eOHC:c = k;orward c: (2) base alkali-metal catalyst.

JKTI. VOL. 11, No.2, Desember 2009 The reaction rate of EC with various initial The effect of reaction temperature on the concentrations is illustrated in Figure 2 at different reaction rate of EC was investigated by various reaction temperature. The molar ratio of MeOH to reaction temperature and the results are plotted in EC was maintained with higher than 8 and Fig. 2 and 3. As shown in Fig. 2., the reaction rate amount of catalyst was 0.2 wt%. It has been found increased with reaction temperature. The k values in the preliminary work [7J. that there is no were calculated at various temperatures were considerable effect of KOH more than 0.2 wt% to correlated by eq. (4) and shown in Fig. 3. The activation energy and rate constant were EC conversion, while below this amount the determined and illustrated in Table 1. catalyst concentration has considerable effect to EC conversion. So, the catalyst amount used in this work was maintained at 0.2 wt%. From the (4) data of Fig. 2, the reaction order was determined by regression based on equation (3) and presented Table 1.Estimated value of reaction orders and rate constants in Table 1. (3) loge-rEC ) = log klarward + b log CEC Reaction Reaction order Activation energy Rate constant (kJJmol) - 0,4 lZOO Kl - 0,45 Forward b=0.87 12.73 18.1e· / -,o'l - 0,5 ri W6 6 Kl 2457.5e· . / 3- 0,55 Backward c =1.25, d =0.9 29.28 1 ~ - 0,6 ~ ~- 0,65 ing backward reaction, the equation (1) can o'l be rearranged to equation (5). - 0,7

- 0,75 (5)

- 0,8 - 0,1 - 0,15 - 0,2 - 0,25 - 0,3 - 0,35 - 0,4 - 0,45 For equal concentrations of DMC and EG, Log (C",) (mol/L) equation (5)becomes Figure 2. The plot to determine the forward reaction order and constant in DMC synthesis k ,.,C d k c+d n (6) IEc = backwanl'-'DMCC EG = backwardC DMC = kbackwardC DMC -1

-1.5 and taking logs this gives

-2 (7) -2.5 log(rEc) = log kbackward + nlog CDMC .IIi C ri -3

-3.5 The overall order of backward reaction was

-4 determined by equation (7) as shown in Fig. 4. Knowing the overall order, the reaction order with -4.5 2.8 2.9 3 3.1 3.2 3.3 3.4 respect to each component was estimated with the l/T(xlO') following manipulation. Figure 3. Relationship of k with temperature

JKTI, VOL. 11, No.2, Oesember 2009 17 0.4 The reaction equilibrium constant can be d 0.3 calculated as follows: 'a -,..:II 0.2 .-I 0.1 K = kbackward (10) ! eq k i forward -~ -0.1 tII -0.2 Accordingly, with the values of rate constants of ..:I0 -0.3 forward and backward reactions illustrated in -0.4 Table 1, the equilibrium rate constant is calculated -0.5 tobe: -0.6 Keq = 135.77 e-l6542.4/RT. 0.5 0.6 0.7 0.8 0.9 Log (t;.,) (mol/L) Figure 4. The plot to determine the backward overall CONCLUSIONS reaction order and rate constant in DMC synthesis. The reaction kinetics and the experimental C C - t: C k C cn-c_ k: en (CDMC)C (8) rEC- "'hackwanl DMCc:EG - backward DMC EG - backwanl EG -C studies of transesterification reaction of BC with EG Taking logs and using the value of overall methanol are investigated. From the experimental and calculated results, it has been found that the order determined, the reaction order of individual reaction order for forward and backward reactions component was calculated with equation (9). In are 0.87 and 2.15, respectively. The activation Fig. 5., it was found that the correlation of energy is 12.73 and 29.28 kJ/mol, and the rate experiment data to determine the reaction order constant is 18.1 e-12734.2and/RT 2457.5 e-'192:J6.6/RT for was well fitted and the values calculated is forward and backward reactions, respectively. illustrated in The equilibrium rate constant of transesterification reaction w.as calculated to be -0.5 r------, 135.77e-165424/R. T

-1 •

•....•. REFERENCES ::: -1.5

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