Transesterification of DMC with ethanol over K2CO3/Al2O3: The structure-performance relationship and catalytic mechanism

Abstract

The transesterification of dimethyl carbonate (DMC) with ethanol to ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) has attracted increasing attention, which could be greatly promoted by heterogeneous solid base catalysts. The K₂CO₃/Al₂O₃ solid base catalysts with different active sites such as K₂CO₃, KAl(OH)₂CO₃, and KAlO₂ were prepared for the transesterification reaction. The active sites of K₂CO₃ and KAl(OH)₂CO₃ mainly exist on the surface of KA-200 and KA-300, while K₂CO₃ and KAlO₂ were mainly present on KA-400 and KA-500. The effects of active sites and the phase of support were studied systematically. KAl(OH)₂CO₃ showed higher activity than K₂CO₃ and KAlO₂. In contrast, K₂CO₃ and KAlO₂ displayed higher stability, with the activity of KA-400 kept stable over 2000 h. The excessive OH group of γ-AlOOH and water in the reaction system have a negative effect on the activity of the K₂CO₃/Al₂O₃ catalysts. The catalytic characterizations, transesterification reactions, and DFT calculations suggested that K⁺ in the KAl(OH)₂CO₃ species, with higher electron cloud density than that in K₂CO₃/γ-Al₂O₃, could efficiently promote the dissociation of ethanol and subsequent replacement of methoxy with ethoxy. The rate-determining step for the transesterification reaction was suggested to be the dissociation of ethanol.