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 CO 2 over cubic ( c ), tetragonal ( t ), and monoclinic ( m ) ZrO 2 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 -ZrO 2 (1 1 1), t -ZrO 2 (1 0 1), and m -ZrO 2 ( 1 ¯  1 1). The Mulliken charge analysis suggested that methanol and CO 2 were activated by the Lewis basic sites. Our DFT results showed that the reaction took place through the same route on all three surfaces: 2CH 3 OH + CO 2  → 2CH 3 O + 2H + CO 2  → CH 3 OCOO + 2H + CH 3 O → CH 3 OCO + O + CH 3 O + 2H → DMC + H 2 O. In the process, CH 3 OCOO → CH 3 OCO + O was the rate-controlling step on the c -ZrO 2 (1 1 1) and m -ZrO 2 ( 1 ¯  1 1) surfaces, while CH 3 OCO + CH 3 O → DMC was rate-controlling step on the t -ZrO 2 (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 -ZrO 2 (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 -ZrO 2 (1 1 1). Therefore, the results showed that the order of catalytic activity of the dry catalysts was as follows: t -ZrO 2  >  m -ZrO 2  >  c -ZrO 2 . In addition, the catalytic activity of ZrO 2 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 CO 2 .