A DFT study of dimethyl carbonate synthesis from methanol and CO2 on zirconia: Effect of crystalline phases

Abstract

The reaction mechanism of dimethyl carbonate (DMC) synthesis from methanol and CO2 over cubic (c), tetragonal (t), and monoclinic (m) ZrO2 phases have been investigated by density functional theory (DFT) calculations. The two possible reaction routes have been examined on the three most stable surfaces of zirconia, namely the dry perfect c-ZrO2(1 1 1), t-ZrO2(1 0 1), and m-ZrO2(1¯ 1 1). The Mulliken charge analysis suggested that methanol and CO2 were activated by the Lewis basic sites. Our DFT results showed that the reaction took place through the same route on all three surfaces: 2CH3OH + CO2 → 2CH3O + 2H + CO2 → CH3OCOO + 2H + CH3O → CH3OCO + O + CH3O + 2H → DMC + H2O. In the process, CH3OCOO → CH3OCO + O was the rate-controlling step on the c-ZrO2(1 1 1) and m-ZrO2(1¯ 1 1) surfaces, while CH3OCO + CH3O → DMC was rate-controlling step on the t-ZrO2(1 0 1) surface. The activation free barriers were evaluated on the basis of the rate-controlling steps. The lowerst value was found for the reaction on the t-ZrO2(1 0 1) surface and was equal to 196.4 kJ/mol, while the highest activation free barrier was 274.1 kJ/mol, relative to the reaction on c-ZrO2(1 1 1). Therefore, the results showed that the order of catalytic activity of the dry catalysts was as follows: t-ZrO2 > m-ZrO2 > c-ZrO2. In addition, the catalytic activity of ZrO2 could be determined by the acid-baisic properties of surface and electronic structures. The present work therefore provides a theoretical guidance for designing the hydrated and composite catalysts for DMC synthesis from methanol and CO2.