DMC Synthesis by Transesterification of Propylene Carbonate and Methanol over CaO near Room Temperature

Dimethyl carbonate (DMC) is an important precursor of polycarbonate resins as well as a useful carbonylation and methylation agent. Because of the moderate reaction conditions and the use of CO_2 as raw material, preparation of DMC by transesterification of cyclic carbonate with methanol has gained increasing attention in recent years. In the present paper, CaO , which shows excellent catalytic activity, was used as a solid base catalyst for the rudimental investigation of its catalytic behavior for synthesis of DMC from propylene carbonate (PC) and methanol and the mechanism of this reaction over CaO. CaO was prepared from CaCO_3 calcined at 900 ℃ in N_2 atmosphere. The reaction was carried out in a flask (250 ml) equipped with a reflux condenser, water bath and magnetic stirring. The reaction products were analyzed by GC after centrifugal separation of the solid catalyst from the solution. On the basis of the assumption that hydroxylpropyl methyl carbonate (HPMC) was the intermediate of this reaction, the charge distribution of PC, methanol and HPMC was calculated and the possible reactions were proposed. There were both consecutive reactions and competitive reactions. The first step of consecutive reaction, the transesterification of PC with methanol, was slow; whereas the second step, the transesterification of HPMC with methanol, was fast, with a slow side reaction of PC polymerization competing with it. The result of the effect of temperature on DMC selectivity was consistent with the assumption of HPMC as an intermediate and the inference of possible reaction. Further investigation of the effect of addition sequence on reaction rate elucidates that the main function of the catalyst is to activate MeOH other than PC and the H-bond between PC and MeOH can activate PC to some extent. From the result of the effect of temperature on reaction rate, it is found that the reaction rate is controlled by both the activation of methanol and its reaction with PC. Based on the results illustrated above, the mechanism of synthesis of DMC from PC and MeOH over CaO is proposed.