Mechanical pressure-mediated Pd active sites formation in NaY zeolite catalysts for indirect oxidative carbonylation of methanol to dimethyl carbonate

Abstract

The structural transformation of metal-containing zeolite catalysts subjected to mechanical compression is often disregarded in the preparation of catalysts. Herein, the impact of the mechanical compression on the catalytic active sites of Pd/NaY catalysts responsible for the indirect oxidative carbonylation of methanol to dimethyl carbonate (DMC) has been disclosed. The DMC selectivity of the catalysts was found to strongly depend on the mechanical-pressure applied, as the mechanical pressure controlled the ratio of Pd(2+δ)+ and Pdδ+ species (0 < δ ≤ 2) in the catalysts. The mono-dispersed Pd clusters (1.3 nm) in the Pd/NaY catalyst were obtained under mechanical treatment of 300 MPa. This catalyst showed high CO conversion of 89% and DMC selectivity of 83%, that maintained for at least 150 h. Combining experimental and density functional theory studies, we revealed that the Pdδ+ rather than the Pd(2+δ)+ species enhanced the adsorption of CO and CH3ONO reactants and inhibited the decomposition of CH3ONO reactant into byproducts, and thus enhanced the DMC selectivity. The mechanical pressure applied had a noticeable effect on the structural features of the metal-containing zeolite catalysts, but despite its importance, this aspect has been poorly considered in the field of heterogenous catalysis.