Revealing the roles of hydrogen bond and Al2Cl7 anion species and kinetics for ionic liquid catalyzed transesterification of ethylene carbonate with methanol

The co-production of dimethyl carbonate (DMC) and ethylene glycol (EG) from the transesterification of ethylene carbonate (EC), which is synthesized from CO2 and ethylene oxide, with methanol has received researchers' attention. Herein, this transformative process catalyzed with metal chloride-based ionic liquids (ILs) was investigated by employing spectroscopic characterizations in combination with kinetic studies. The interaction between cation and methanol via hydrogen bond and activation of carbonyl group in EC (or 2-hydroxyethyl methyl carbonate, HEMC) by metal chloride-contained anion were identified to show synergistic effects on catalytic transesterification. The chain length of alkyl group substituted at the 1-position N atom in imidazole ring affected the reaction rates due to the steric hindrance effect. Not AlCl4 but Al2Cl7 anion species in AlCl3-based IL exhibited activity in this transesterification, and the content of Al2Cl7 species was observed to show linear relationship with the transformative rates of both EC and HEMC. The kinetic studies also elucidated that the rates of both EC (or HEMC) and methanol are first-order dependent on their concentrations, and the formation of HEMC intermediate was the kinetic-relevant step with the activation barrier of 92 kJ/mol. As a result, the yield and selectivity of DMC (or EG) could reach up to around 95 % and 97 % with catalysis of [MDMIM]Cl/ AlCl3 with AlCl3 molar fraction of 0.67 after 12 h reaction at 140 degrees C, which showed higher performance than any other tested metal chloride-based IL. Besides, no decay in catalytic activity and selectivity of this IL was observed after 10 times' recycling.

Automated summary

The co-production of dimethyl carbonate (DMC) and ethylene glycol (EG) via transesterification of ethylene carbonate (EC) has been studied using metal chloride-based ionic liquids (ILs) as catalysts. The interaction between cation and methanol, as well as the activation of carbonyl group in EC by metal chloride-contained anion, showed synergistic effects on catalytic transesterification. The reaction rates were affected by the steric hindrance effect of the alkyl group substituted at the N atom in the imidazole ring. The Al2Cl7 species in AlCl3-based IL exhibited activity in the transesterification, and its content showed a linear relationship with the transformative rates of both EC and HEMC. The kinetic studies revealed that the reaction rates were first-order dependent on the concentrations of EC (or HEMC) and methanol, and the formation of HEMC intermediate was the kinetic-relevant step. The catalysis of [MDMIM]Cl/AlCl3 with an AlCl3 molar fraction of 0.67 showed higher performance compared to other metal chloride-based ILs, with a yield and selectivity of DMC (or EG) reaching up to 95% and 97%, respectively, after 12 hours of reaction at 140 degrees C. The IL also demonstrated good recyclability without decay in catalytic activity and selectivity after 10 times' recycling.