Energy & Fuels 2006, 20, 17-20
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Production of Dimethyl Carbonate via Alkylene Carbonate Transesterification Catalyzed by Basic Salts Paolo De Filippis,* Marco Scarsella, Carlo Borgianni, and Fausto Pochetti UniVersity of Rome “La Sapienza”, Department of Chemical Engineering, Via Eudossiana 18, 00184 Rome, Italy ReceiVed May 12, 2005. ReVised Manuscript ReceiVed September 22, 2005
Dimethyl carbonate (DMC) is actually recognized as an environmentally benign chemical because of its negligible ecotoxicity and low bioaccumulation and persistence. The diffusion of DMC as safe chemical necessarily implies a clean and safe route for its production, such as the base-catalyzed transesterification reaction with methanol of alkylene carbonates. Until now few bases were reported to be effective as heterogeneous catalysts under mild conditions (low temperature and atmospheric pressure). Therefore inorganic salts, namely, Na3PO4 and Na2CO3, were selected because of their basic properties and their negligible ecotoxicity. Using Na3PO4 as catalyst, the equilibrium yield for the reaction ethylene carbonate f DMC is reached in about 1 h at atmospheric pressure and at a temperature above 50 °C, while in the case of the propylene carbonate f DMC reaction, the equilibrium yield is difficult to reach, because the kinetics of this reaction are 5 times slower than those of the EC f DMC reaction. Both reactions can be well represented with first-order kinetics and present a similar activation energy, suggesting a diffusional rate-determining step. The quite different frequency factors suggest that the PC f DMC reaction is slow because of steric factors.
Introduction The drastic decrease of the impact of the chemical industry on the environment and human health is probably one of the main challenges of this century. Clean routes for the manufacture of the most important chemicals are required, and in this perspective, the development of chemical processes that can represent a more environmentally friendly alternative to the current methods is a priority. Dimethyl carbonate (DMC) is actually recognized as an environmentally benign chemical because of its negligible ecotoxicity and low bioaccumulation and persistence.1-2 An increased use of DMC as a safe substitute for hazardous chemicals can be predicted: principally as methylating agent in place of dimethyl sulfate and methyl halides and as a carbonylation agent in place of phosgene for the production of polycarbonates and polyurethanes.3-5 Furthermore, as nonaqueous electrolyte component, it has found application in lithium rechargeable batteries.6 Additionally, its high oxygen content makes it a promising candidate to be used as an oxygenated fuel additive,7-8 for instance to replace the methyl-tert-butyl ether (MTBE). It is obvious that the potential diffusion of a safe chemical necessarily implies a clean and safe route for its production. * To whom correspondence should be addressed. Fax: ++39 06 4458 5416. E-mail:
[email protected] (1) Rivetti F. In Green Chemistry: Challenging PerspectiVes; Tundo, P., Anastas, P., Eds.; Oxford University Press: Oxford, 2001; pp 201219. (2) Cassar, L. Chim. Ind. 1990, 72, 18. (3) Ono, Y. Appl. Catal. A 1997, 155, 133. (4) Tundo, P.; Selva, M. Chem. Technol. 1995, 25, 31. (5) Ono, Y. Catal. Today 1997, 35, 15. (6) Scrosati, B. Chim. Ind. 1997, 79, 463. (7) Pacheco, M. A.; Marshall, C. L. Energy Fuels 1997, 11, 2. (8) Ikeda, Y.; Sakaihori, T.; Tomishige, K.; Fujimoto, K. Catal. Lett. 2000, 66, 59.
Among the few still-existing manufacturing processes of DMC considered environmentally compatible, the most interesting are the catalytic oxidative carbonylation of methanol9 and, particularly, the transesterification with methanol of alkylene carbonates, specifically, ethylene or propylene carbonate (EC or PC) obtained by direct synthesis from epoxides and carbon dioxide (CO2). Several papers were written to describe such synthesis processes, and different catalysts were studied.10-12 The transesterification reaction has several advantages. First of all, the mixture of methanol, DMC, alkylene carbonate, and alkylene glycol that is obtained under the equilibrium conditions is neither dangerous nor corrosive. Furthermore, by distillation, it is possible to recover from this blend the DMC-methanol azeotrope, whose composition fits its use as a gasoline or diesel fuel additive. Glycol and alkylene carbonate can be successively recovered and alkylene carbonate recycled, while glycol can be sold as anti-freeze. The transesterification process is then potentially safe and wasteless, and furthermore it implies the use of carbon dioxide as a starting material. For this reaction, both base and acid catalysts were reported to be effective, but the base catalyst appeared to be more favorable.10 Unfortunately, the inorganic and organic bases proposed as catalysts have the major drawback of their solubility in the reaction mixture,12-14 with the consequent difficulties of their separation and recovery. Until now few solid bases were reported to be effective as heterogeneous catalysts under mild (9) Delledonne, D.; Rivetti, F.; Romano, U. Appl. Catal. A 2001, 221, 241. (10) Knifton, J. F.; Duranleau, R. G. J. Mol. Catal. 1991, 67, 389. (11) Tatsumi, T.; Watanabe, Y.; Koyano, K. A. Chem. Commun. 1996, 2281. (12) Buysch, H. J.; Krimm, H.; Rudolph, H. US Patent 4181676, 1980. (13) Frevel, L. K.; Gilpin, J. A. US Patent 3642858, 1972. (14) Buysch, H. J.; Klausener, A.; Langer, R.; Mais, J. Ger. Offen. DE 4129316, 1991.
10.1021/ef050142k CCC: $33.50 © 2006 American Chemical Society Published on Web 10/22/2005
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conditions (low temperature and atmospheric pressure) to reach thermodynamic DMC yield.10,15-18 The aim of this paper is to investigate the efficiency of nontoxic salts as base catalysts in DMC production via the transesterification reaction. On the basis of their negligible ecotoxicity and basic properties, Na3PO4 and Na2CO3 were selected. A comparison with CH3COOTl, reported in the patent literature as a very efficient and selective base catalyst under the mild conditions for this reaction,19 was also made. Experimental Section Materials. Ethylene carbonate and propylene carbonate 99.0%, used for the transesterification reactions, were supplied by Aldrich. Methanol 99.5% anhydrous (H2O < 0.01%; Fluka) was used without further purification. Trisodium phosphate 96%, sodium carbonate 99.9%, and thallium acetate 99.99% (Aldrich) were used as catalysts. Dimethyl carbonate (Fluka), ethylene glycol, and propylene glycol (Carlo Erba Reagents) were used as reference standards. Apparatus and Procedure. Transesterification reactions were carried out in isothermal conditions with temperature ranging from 20 to 68 °C. The experimental procedure was as follows: Methanol and alkylene carbonates (molar ratio 4:1) were put into a 250 mL glass reactor, equipped with a magnetic stirrer, thermometer, and reflux condenser with a silica gel trap on the top. The system was then heated in a thermostatic bath with stirring at about 750 rpm. When the mixture reached the selected reaction temperature, the catalyst was added, and the reaction started. The amount of catalyst relative to the reaction mixture was 0.25 wt % for the sodium salts and 0.6 wt % for the thallium acetate, for the latter, the procedure described in the patent was followed.19 To determine the DMC concentration, approximately 0.5 mL aliquots of liquid samples were withdrawn from the reactor at fixed time intervals and analyzed by gas chromatography. GLC analyses were performed with an HP 5890 GLC gas chromatograph equipped with a flame-ionization detector, using a 25 m, i.d. 0.32 mm, HP-5 column. The main parameters were the follows: carrier gas, helium with a flow of 2 mL/min; injector temperature, 250 °C; detector temperature, 250 °C; split ratio, 1/100; temperature program, 40 °C for the first 5 min, 40-100 °C at 10 °C/min, 100 °C for 2 min, 100-200 °C at 15 °C/min, 200 °C for 5 min, 200-250 °C at 10 °C/min, 250 °C for 15 min.
Results and Discussion All the transesterification reactions were carried out using a methanol/alkylene carbonate molar ratio equal to 4:1. This large excess of methanol was chosen to reduce the influence of its consumption during the reaction progress. Preliminary runs using ethylene carbonate were conducted at the highest temperature in the chosen range (68 °C) to test the catalytic performance of the selected salts. In all the runs no byproducts were detected, indicating a high selectivity toward the transesterification reaction. The results of the runs are summarized in Figure 1. As reported, trisodium phosphate showed the highest activity, and sodium carbonate showed about half the activity of (15) Watanabe, Y.; Tatsumi, T. Micropor. Mesopor. Mater. 1998, 22, 399. (16) Bhanage, B. M.; Fujita, S.; Ikushima, Y.; Arai, M. Appl. Catal. A 2001, 219, 259. (17) Bhanage, B. M.; Fujita, S.; He, Y.; Ikushima, Y.; Shirai, M.; Torii, K.; Arai, M. Catal. Lett. 2002, 83, 137. (18) Wei, T.; Wang, M.; Wei, W.; Sun, Y.; Zhong, B. Green Chem. 2003, 5, 343. (19) Krimm, H.; Buysch, H. J.; Rudolph, H. US Patent 4307032, 1981.
Figure 1. Effect of different catalysts on the EC f DMC transesterification reaction (T ) 68 °C).
Figure 2. Temperature effect on the EC f DMC transesterification reaction with the Na3PO4 catalyst.
trisodium phosphate, while thallium acetate had the lowest activity. The apparent equilibrium concentration of DMC was, in fact, reached in 60, 120, and 1440 min, respectively. When base catalysts, both homogeneous and heterogeneous, are used, the methanolysis will proceed toward the formation of an active species, namely the methoxide ion generated by the reversible acid-base reaction of methanol with the catalyst. For this reason, salts whose hydrolysis reaction is basic will potentially work as catalysts, and it could be expected that catalytic efficiency will increase with basicity. Because the degree of hydrolysis is the result of the relative strengths of the conjugated acid and base of the ions constituting it, the experimental data agree with the nature of the tested salts. As a consequence of these preliminary runs, only Na3PO4 was used in the following experiment. The effects of temperature on the EC f DMC and PC f DMC reactions are shown in Figures 2 and 3, respectively. Data in Figure 3 show that the transesterification rate of propylene carbonate is slower: runs at 68 °C (where the reaction PC f DMC is near its equilibrium value before the run is stopped) show that the final yield in DMC is about three times lower than that measured for the EC f DMC reaction (22% and 65%, respectively). Despite the starting concentration, the considerable consumption of methanol during the reaction leaves some doubts about its influence on the reaction rate. As a consequence, in the kinetic analysis of the experimental data, the influence of both the alkylene carbonate and methanol have to be considered. With this assumption, the reaction kinetics will be described as first order with respect to concentration of both CH3OH and alkylene carbonate, Ca and Cc, respectively.
Production of Dimethyl Carbonate
Energy & Fuels, Vol. 20, No. 1, 2006 19
Figure 3. Temperature effect on the PC f DMC transesterification reaction with the Na3PO4 catalyst.
Figure 4. Comparison between EC f DMC reactions (Na3PO4 catalyst).
Considering the reaction stechiometry (2 mol of methanol for 1 mol of alkylene carbonate), the actual concentration of methanol, Ca, can be expressed as
Table 1. First- and Second-Order Rate Constants Calculated for the EC f DMC Reaction, with the Na3PO4 Catalyst, as a Function of Temperature
Ca ) Ca0 - 2(Cc0 - Cc)
(1)
where Ca0 is the initial concentration of methanol, Cc0 is the initial concentration of alkylene carbonate, and Cc is the actual concentration of alkylene carbonate. The kinetic equation can be written as
-dCc/dt ) k(Cc - Cce)(Ca0 - 2(Cc0 - Cc) - Cae)
(2)
where Cce is the equilibrium concentration of alkylene carbonate, Cae is the equilibrium concentration of methanol, t is the running time, and k is the specific kinetic constant. The term (Ca0 2Cc0 - Cce) being constant, eq 2 can be rewritten as
2dCc/(D + 2Cc) - dCc/(Cc - Cce) ) k′dt
(3)
where
D ) (Ca0 - 2Cc0 - Cce) k′ ) k(D + 2Cce) The integration of eq 3 and the substitution of D in accordance with its definition give
(Ca0 - Cae - 2Cc0 + 2Cc)(Cc0 - Cce)
ln
(Ca0 - Cae)(Cc - Cce)
) k′t
(4)
If the methanol concentration exceeds that of the alkylene carbonate, namely, Ca0 is far larger than 2(Cc0-Cc), the Ca variation is very small and can be neglected. Then eq 4 can be simplified as follows
ln((Cc0 - Cce)/(Cc - Cce)) ) k′′t
(5)
where k′′ is the specific rate of the reaction. Equation 5 is the pseudo-first-order reaction when the rate determining step depends on the variation of concentration of alkylene carbonate only. The values of the rate constants for the EC f DMC reaction calculated with both the eqs 4 and 5 are reported in Table 1 as a function of the temperature. For the EC f DMC reaction, Figure 4 shows a comparison between the experimental data and the values calculated using
T (°C)
k′′ (min-1)
k′ (min-1)
20 30 40 50 68
0.013 ( 0.003 0.023 ( 0.005 0.032 ( 0.004 0.049 ( 0.004 0.07 ( 0.01
0.011 ( 0.003 0.017 ( 0.005 0.05 ( 0.01
Table 2. Pseudo-First-Order Rate Constants Calculated for the PC f DMC Reaction, with the Na3PO4 Catalyst, as a Function of Temperature T (°C)
k (min-1)
20 30 40 50 68
0.0037 ( 0.0003 0.0048 ( 0.0005 0.0077 ( 0.0005 0.0087 ( 0.0005 0.0178 ( 0.0005
eqs 4 and 5 (dotted and continuous lines, respectively). Both equations fit the experimental data rather well and hence both could be adopted, meaning that the effect of the methanol concentration on the data is negligible under the selected experimental conditions. Furthermore, as reported in Table 1, although k′ is systematically lower than k′′, within the experimental error k′ could be considered equal to k′′ and the simplest equation (eq 5) could be used to analyze all of the data. For this reason, the continuous lines shown in Figure 2 were calculated using eq 5. Figure 3 shows experimental and calculated data for the PC f DMC reaction, considering a pseudo-first-order kinetics (eq 5). As reported in Table 2, the kinetic constants, appear to be about 5 times lower than those of EC f DMC reaction. The calculated values of both Figures 2 and 3 show that for the EC f DMC reaction the equilibrium value increases with temperature, indicating that this reaction is endothermic. On the contrary, from the data relative to the PC f DMC reaction (Figure 3), an exothermicity could be hypothesized, in agreement with the NIST data.20,21 The Arrhenius plot, Figure 5, shows an activation energy, Ea, equal to 29 ( 2 kJ/mol for both reactions, while the frequency factors, calculated as the intercepts, are quite different. Data from the Arrhenius plot could reasonably indicate a common rate-determining step for both of the transesterification reaction substrates, while the different reaction rates could be (20) NIST Standard Reference Database Number 69, March 2003, http:// webbook.nist.gov/chemistry. (21) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1998, 17 (Suppl. 1).
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Figure 5. Arrhenius plots for both reactions
attributed mainly to steric factors because of the chemical structures of the different alkylene carbonates. Provided that the rate-determining step is common for both the reactions, it could be reasonably identified with a diffusion step. This agrees with the energy bonds involved in the systems considered, which range between 89 and 113 kJ/mol22 and are higher than the observed activation energy, Ea. Conclusions It is necessary to consider both the thermodynamic and kinetic aspects of the transesterification reactions of alkylene carbonates
De Filippis et al.
to produce DMC equilibrium reactions to evaluate their potential industrial applications. In this context, the production of dimethyl carbonate from ethylene carbonate appears to be very promising, showing the EC f DMC reaction to have satisfactory equilibrium yield (about 64%), whereas for the PC f DMC reaction the yield is about 3 times lower. The basic salt Na3PO4 has shown good catalytic performances for the EC f DMC reaction, allowing it to reach the equilibrium yield in very mild conditions and short times. On the contrary, for the PC f DMC reaction, the activity of this catalyst does not seem to be adequate because the equilibrium yield is reached very slowly. From a kinetic analysis, given a sufficiently high methanol concentration, the reactions can be considered to be first order with respect to the alkylene carbonate concentration. For both reactions, the calculated activation energy is the same, suggesting a rate-determining step that could be identified with a diffusional one. The different frequency factors suggest that the PC f DMC reaction is slower because of steric factors. EF050142K (22) Glaude, P. A.; Pitz, W. J.; Thomson, M. J. Proc. Combust. Inst. 2005, 30, 1111.
