Study On Synthesis Of Dimethyl Carbonate By Heterogeneously Catalyzed Reaction Of Urea With Methanol

The reaction of urea and methanol catalyzed heterogeneously for synthesis of dimethyl carbonate (DMC) has been systematically studied in this thesis.The reaction system of synthesizing DMC from urea and metlianol has been analyzed thermodynamically. The conclusion is as follows: firstly, the main reaction proceeds in two steps: urea reacts with methanol to give methyl carbomate (MC), and then MC is converted to DMC by reaction with methanol. The second step can not happen until the reaction temperature rises above 337K, and the equilibrium constant of this step is very small. Secondly, the most important side reaction is methylation of MC with DMC to N-methyl methyl carbomate. Thirdly, elevating reaction temperature is in favor of both the main reaction and the side reaction. On the other hand, removing DMC out of the reaction zone in time can not only promote the reaction of MC to DMC, but also suppress the side reaction effectively.ZnO, PbO and LaiOs as well as ZnO-PbO, ZnO-LaiOa nano-meter metal oxide catalysts have been prepared by precipitation and sol-gel process in combination with supercritical drying technique. The crystal structure, surface morphology and specific surface area of the catalysts have been characterized by means of XRD, TEM and BET, and the catalytic activities have been evaluated in batch reactor system. It can be found that nm-ZnO particles can be prepared through both precipitation and sol-gel process. PbO particles can be made in one-dimensional nano-meter range through sol-gel process. Also, nm-I^Os particles can be obtained using sol-gel process in combination with supercritical drying technique. Nm-ZnO-PbO binary metal oxide catalysts are available from both methods. As far as PbO catalyst is concerned, it can be deactivated during the reaction process due to transition of crystal structure and reduction reaction. The catalytic activity of ZnO can be improved after PbO or I^Os is added.For the first time, Zn and Pb have been developed as catalysts in the reaction for DMC synthesis from urea and methanol, and their supporting methods have been studied emphatically. After the reduction temperature has been determined by TPR technique, the effects of support and amount of supported metal atoms on the activities of catalysts have been evaluated. And the catalytic properties of Zn-Pb binary metal catalyst have also been investigated. Finally, on the basis of characterization by XRD arid XPS, the interaction of support with the active component has been analyzed theoretically.In order to improve DMC yield, a set of apparatus for continuously producing DMC has been built, and its operational suitability test has also been performed. On this basis, the operation conditions over ZnO catalyst have been optimized, and each influential factor has been discussed in the mean time. Under the same conditions, the catalytic properties of differentiicatalysts have been evaluated. The results show that DMC yield can be greatly improved under continuous operation conditions; the highest DMC yield reaches 46.5%.