Study of the mechanisms of dialkyl carbonates directly formed from carbon dioxide and alcohols: New insights from kinetic and thermodynamic processes

Abstract The direct conversion of CO2 by alcohols has attracted wide attention as it offers a new green method of capturing CO2 with high selectivity. However, its biggest drawback is that its yield is extremely poor. In order to understand and address this issue, it is necessary to study the kinetic and thermodynamic processes of dialkyl carbonate formation, and develop effective catalysts for the rate-determining step. Here, we applied the density function theory to determine its molecular mechanism. The results showed that CO2 reacted with alcohols to first form monoalkyl carbonates, and then the monoalkyl carbonates further reacted with alcohols to generate dialkyl carbonates. In terms of kinetics, three alcohol molecules could directly activate CO2, and dialkyl carbonates formation was the rate-determining step, which conformed to the characteristics of an esterification reaction. In terms of thermodynamics, the formation of monoalkyl carbonates limited the conversion of the whole reaction. Pressuring CO2 and lowering the temperature contributed to its conversion. However, high temperature was conducive to increase the conversion of dialkyl carbonates. Ultimately, we proposed the suitable experimental conditions in which the temperature range was from 237 to 252 K and the CO2 pressure should be higher than 1 MPa.