Enhancement Of Dimethyl Carbonate Synthesis From Methanol And CO₂ By In Situ Hydrolysis

Dimethyl carbonate（DMC）has attracted much attention in the chemical field as an environmentally benign and biodegradable chemical for recent years. The applications of DMC have covered a variety of fields. The direct synthesis of DMC from methanol and CO₂ is highly favorable since such a method is environmentally benign by nature. However, the DMC yield is far from satisfactory because of the thermodynamic limitation of the direct synthesis of DMC reaction. In situ removal of water coproduced in the reaction system is an important strategy to effectively improve the yield of DMC. Therefore, enhancement of the direct synthesis of DMC from methanol and CO₂ by addition of dehydration agent was investigated in this paper.The influence of the addition of 2,2-dimethoxy propane（DMP） on the DMC formation was investigated over CeO₂ catalysts. The yield of DMC greatly improved up to 52.0 mmol·gcat-1 by in situ removal of water from the reaction system through the addition of DMP. The thermodynamic calculations showed that the Gibbs energy of the coupling reaction system decreased by 4.7 kJ·mol-1 compared to that of the direct synthesis of DMC. This indicated that the addition of the DMP could effectively improve DMC yield by in situ removal of water.This paper also describes the effect of the in situ hydrolysis of 2-cyanopyridine and its derivatives on the synthesis of DMC from CO₂ and methanol over CeO₂. DFT calculations showed that 2-cyanopyridine had the highest electronic charge number of the carbon in the cyanogroup and the relative high negative charge of the N in the pyriding ring, which is beneficial to the strong adsorption of the intermediate Cen+— OHδ-. Therefore, 2-cyanopyridine was the most effective dehydrant to accelerate the direct synthesis of DMC by in situ hydrolysis.Rod-CeO₂, octahedron-CeO₂ and cube-CeO₂ were synthesized through hydrothermal method. The HRTEM showed that rod-CeO₂ exposure the most（110） planes and slight（111） planes. The main crystal faces of the octahedron-CeO₂ is（111）, and cube-CeO₂ mainly obtained the（100） crystal faces.The hydrolysis of 2-cyanopyridine with three different morphologies CeO₂ catalysts showed that the conversion of 2-cyanopyridine reached 93.7% over rod-CeO₂ with the most（110） crystal faces, meaning that（110） crystal faces were the most active for the hydrolysis of 2-cyanopyridine. When the synthesis of DMC was coupled with the hydrosis of 2-cyanopyridine over rod-CeO₂（110）, the DMC yield was improved drastically up to 378.5 mmol·gcat-1 from 12.8 mmol·gcat-1, which is much higher than that obtained over octahedron-CeO₂（111） and cube-CeO₂（100）. This indicats that（110） planes were active factors for the hydrolysis of 2-cyanopyridine, further enhancing the DMC formation by in situ removal of water.