Research On Synthesis Of Dimethyl Carbonate By Transesterification Incorporated With Reactive Extraction

Zinc-lead double oxide (ZnO-PbO ) was prepared and its catalytic performance on synthesis of dimethyl carbonate (DMC) by transesterification of propylene carbonate (PC) and methanol was studied. And then reactive extraction technique was incorporated in the reaction of synthesizing DMC by transesterification of PC and methanol.ZnO-PbO was prepared by coprecipitation method firstly. The dependence of its properties on the preparation conditions, such as precursors of Zn and Pb, precipitators, weight ratio of Zn to Pb, and calcination temperature, was investigated. The optimal preparation conditions are as follows: Zn(NO3)2-6H2O and Pb(OAc)2-3H2O as the precursors, (NaOH:Na2CO3 = 3:l) as the precipitator, weight ratio of Zn to Pb being 3.5, and calcination temperature of 500°C. Then the effect of reaction conditions on the performance of ZnO-PbO catalyst in the transesterification reaction was studied. Under the following reaction conditions: reaction temperature of 110℃, reaction time of 2h, molar ratio of methanol to PC being 8.4, and catalyst amount of 3.0% (wt), the conversion of PC reached 63.8%, and the yield and the selectivity of DMC were 62.4% and 97.8%, respectively. XRD analysis result shows that the synergetic effect of ZnO, PbO, and suitable amount of Pb3O4 promotes the increase of the catalytic activity of ZnO-PbO. CO2-TPD measurement shows that the catalytic activity of ZnO-PbO can relate with the basic strength and the alkali amount of the alkali sites on the surface of the sample. In addition, the reuse of the catalyst was examined and the reason for deactivation was also analyzed.Incorporation of the transesterification of PC and methanol with reactive extraction technique can not only increase the equilibrium conversion but also avoid the formation of DMC-methanol azeotrope.At first, a series of alkanes were tested as the extractants in the homogeneous reaction system catalyzed by KOH. The result shows that n-octane is a feasible extractant and di-n-octyl phthalate (DOP) is the accessory extractant. The volumetric ratio of extract phase to reaction phase is 2.8 and the volumetric ratio of the main extratant and the accessory extractant is 0.7. Under the conditions of reaction temperature 100℃, reaction time 3h, and the weight percentageof the catalyst in the reaction system 3.0%, the distribution coefficient of DMC is 1.07, DMC yield is 64.8 %, and the selectivity is 76.0%. Comparing with the reaction without extractant, DMC yield increases by 47.6% and the selectivity increases by 26.7%.Reactive extraction technique has been incorporated in the heterogeneous reaction system catalyzed by ZnO-PbO. The result shows that n-dodecane is a feasible extractant and o-xylene is the accessory extractant. The optimal volumetric ratio of the accessory extractant and the main extratant is 0.4 and the volumetric ratio of extract phase to reaction phase is 0.3. Under the conditions of reaction temperature 100℃, reaction time 2h, and the weight percentage of the catalyst in the reaction system 2.7%, DMC yield is 66.0%, and the selectivity is 97.5%. Comparing with the reaction without extractant, DMC yield increases by 17.0% and the selectivity keeps almost constant.It is obvious that the reaction of synthesizing DMC by transesterification of PC and methanol can be promoted preferably by the reactive extraction technique.