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

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.