The effect of catalyst preparation on the activity of MoO3-SiO2 catalyst in transesterification of diethyl oxalate

Abstract

Transesterification of diethyl oxalate (DEO) with phenol over MoO3-SiO2 catalysts prepared by the sol-gel technique (MoO3-SiO2 (SG)) and the impregnation method (MoO3-SiO2 (I)) was conducted to produce diphenyl oxalate (DPO), which can be used as a precursor for manufacturing diphenyl carbonate (DPC). The sample MoO3-SiO2 (SG) containing 12 wt % of MoO3 showed the best performance with 71.0% conversion of DEO and 32.0% selectivity to DPO. Compared to MoO3-SiO2 (I), improvements in the DEO conversion and DPO selectivity with MoO3-SiO2 (SG) were 16.1 and 7%, respectively. Crystal structure and phase composition of MoO3-SiO2 (I) and MoO3-SiO2 (SG) catalysts with varying MoO3 contents were investigated. The sample MoO3-SiO2 (SG) with a similar chemical composition to MoO3-SiO2 (I) has a larger specific surface area, indicating that the active component is well dispersed on the surface of the MoO3-SiO2 (SG) catalysts. Results of XRD and XPS measurements suggest a high degree of dispersion of MoO3-SiO2 (SG) catalysts that can account for an increase in DEO conversion and DPO selectivity. Coordinately unsaturated MoO3 species play a significant role in the catalytic performance of MoO3-SiO2 (SG) catalysts in transesterification of DEO with phenol. In addition, IR measurements of pyridine adsorption and NH3-TPD data indicate that the amount of acid sites on the surface of MoO3-SiO2 (SG) exceeds that found for the surface of MoO3-SiO2 (I). An enhanced concentration of surface MoO3 species in tetrahedral coordination coupled with the presence of weak Lewis acid sites appear to be the main reason why MoO3-SiO2 (SG) catalysts are superior to the MoO3-SiO2 (I) system.