Elucidation of the reaction mechanism of indirect oxidative carbonylation of methanol to dimethyl carbonate on Pd/NaY catalyst: Direct identification of reaction intermediates

Abstract

Gas-phase indirect oxidative carbonylation of methanol to dimethyl carbonate (DMC) has been industrialized, but the reaction mechanism is still ambiguous. In this work, the reaction mechanism of DMC synthesis using a NaY zeolite catalyst doped with 1.0 wt% Pd was revealed by combining in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results with density functional theory (DFT) calculations. Two key reaction intermediates CO* (*, a surface site) and COOCH3* were identified through the adsorption of a probe molecule of methyl chloroformate (CH3OCOCl), and characterized by steady-state, dynamic-pulse and time-resolved transient DRIFTS experiments. The CO* intermediate is predominant on the catalyst surface when the reaction reached steady-state. The DFT results also showed that the inclusion of OCH3* into CO* had the highest energy barrier of 150.1 kJ mol−1. This verified that the formation of COOCH3* is the rate-determining step for the DMC synthesis. A Langmuir-Hinshelwood mechanism including the fast formation of CO*, and rate-determining insertion of OCH3* into CO* toward generation of COOCH3* to yield DMC was proposed.