Synthesis Of Diethyl Carbonate By Ethanolysis Of Urea Over Metal Oxide Catalysts

As an important organic intermediate, diethyl carbonate (DEC) has a wide application prospect in many industrial fields. In this paper, the synthesis of DEC by ethanolysis of urea was investigated over metal oxide catalysts.Firstly, the catalysis of lead oxide for the reaction of ethyl carbamate (EC) and ethanol to DEC, the controlling step in ethanolysis of urea to DEC, was studied. Lead oxide catalyst exhibited an excellent catalytic stability; it could be reused five times without a significant loss in its catalytic activity. XRD analysis showed that the recovered catalyst was a mixture of cubic metal Pb and orthorhombic PbO₂, which was proved to be the real active composition for the synthesis of DEC from EC and ethanol.Secondly, a series of double metal oxides containing lead oxide were prepared in order to increase the catalytic performance of the catalyst. The activity evaluation results revealed that ZnO-PbO presented the highest catalytic activity. The effect of preparation conditions on the catalytic performance of ZnO-PbO was studied and the suitable preparation conditions were as follows: Zn₂(OH)₂CO₃ and PbCO₃ as the precursors, lead oxide weight percentage 10% and calcination temperature 500℃. The influe nce of reaction parameters on ZnO-PbO catalytic performance was also investigated. Under the suitable reaction conditions of molar ratio of ethanol to EC 7:1, catalyst weight percentage 1%, reaction temperature 190 and reaction time 7 h, the conversion of EC, the yield and selectivity of DEC were 50.2%, 20.6% and 41.0%, respectively. Furthermore, ZnO-PbO catalyst could be reused five times and no significant loss in the catalytic activity was observed. According to the results of XRD measurement and IR analysis, the recovered catalyst was found to be a mixture of Zn(NCO)₂(NH₃)₂, cubic metal Pb and orthorhombic PbO₂, indicating that this mixture may be the real active composition for DEC synthesis. Furthermore the results of a series of verification experiment showed that the reaction between DEC and PbO was the main reason for the reduction of PbO to metal Pb.Finally, MgO catalyst prepared by calcination and then treated with H₂/N₂ was evaluated in the reaction between urea and ethanol to DEC. The suitable preparation conditions for MgO catalyst were as follows: Mg(OH)₂ as the precursor, calcination temperature 60,0 calcination time 4 h, H₂/N₂ treatment temperature 400℃and H₂/N₂ treatment time 7 h. Under the reaction conditions of molar ratio of ethanol to urea 10:1, weight percentage of catalyst 2%, reaction temperature 180℃, and reaction time 7 h, the yield of DEC was 11.8% . According to the results of activity evaluation and XPS analysis, the increase of oxygen vacancies on the MgO surface may be the main reason for the increase in the catalytic activity of MgO after H₂/N₂ treatment.