A comprehensive review on zinc-based mixed metal oxide catalysts for dimethyl carbonate synthesis via urea alcoholysis process

Urea alcoholysis is one of the environmental-friendly and economic methods for dimethyl carbonate (DMC) production. Various catalysts such as bases, organic tin, acids, ionic liquids and metal oxides have been used for synthesis of DMC through urea alcoholysis process. Based on literature survey, catalysts containing both acidic and basic sites, are more suited for catalyzing the reaction; however, the basicity plays more conspicuous role. Zinc-based mixed metal oxides (MMOs) have emerged as potential catalysts for this purpose mainly due to the amphoteric nature of ZnO as well as basic/acidic sites abundance. This article reviews zinc-based MMO catalysts focusing on catalytic performances and mechanisms, synthesizing methods, characterizations and operating conditions of the reactors during a 20-years period. Literature survey uncovered that pure ZnO catalyst could exhibit a maximum of 37% DMC yield under the utmost operating conditions. However, zinc-based MMO catalysts (binary or ternary) showed superior performance in DMC conversion through the urea alcoholysis process, usually under moderate operating conditions, mainly due to synergetic effects among metal oxide phases. Among MMO catalysts, ZnO/CaO binary metal oxide and Zn/Ce/La ternary metal oxide catalysts tabulated the highest DMC production yields of 41.2% and 50.4% respectively. Moreover, two different catalyst-test modes including batch and continuous, and three catalyst synthesizing methods including sol–gel, co-precipitation and urea precipitation (UPM) have been spotlighted in this review.