Acidic and basic sites on the surface of sodium montmorillonite active for catalytic transesterification of glycerol to glycerol carbonate

Abstract

Montmorillonite (Mt) as a solid acid catalyst or support has been widely used in catalytic reactions. However, the existence and catalytic activities of its surface basic sites have rarely been revealed. Here, the surface and structure of Na-Mt are modulated by thermal treatment, providing both acidic and basic sites for the transesterification of glycerol with dimethyl carbonate (DMC) to glycerol carbonate (GLC). The experimental results showed that the thermally activated Na-Mt exhibited bifunctional catalytic properties in glycerol transesterification. The Na-Mt calcined at 400 °C had a basic site density of 1.38 mmol/g and led to a glycerol conversion of 96.8% and a GLC yield of 94.5%. Edge surfaces of Na-Mt provided MII (MIII) atoms as Lewis acidic sites for facilitating the generation of glyceroxide anions from activated glycerol and -M-OH groups as Brønsted and Lewis basic sites for enhancing the carbonyl activation of DMC. This work revealed the co-existence of acidic and basic sites over thermally activated Na-Mt for synergetic catalysis in the transesterification of glycerol to GLC, making the development of Mt-based materials as bifunctional catalysts for one-pot acid-base catalytic processes possible.