Emerging co-synthesis of dimethyl oxalate and dimethyl carbonate using Pd/silicalite-1 catalyst with synergistic interactions of Pd and silanols

Abstract

An emerging synthetic route for the co-synthesis of dimethyl oxalate (DMO) and dimethyl carbonate (DMC) via the indirect oxidative carbonylation of CH3OH is presented. The promising co-synthesis process combines the advantages of two separate routes of DMO and DMC syntheses. Herein, palladium containing silicalite-1 zeolite (Pd/S-1) catalyst was first found to be highly active and stable for the co-synthesis of DMO and DMC. The catalytic performance strongly depended on the catalyst activation, as the heat treatment could tune the silanols around the Pd species on the silicalite-1 zeolite. The Pd/S-1 catalyst with abundant silanols exhibited a higher turnover frequency (TOF) of 0.18 s−1 and a lower apparent activation energy of 51 kJ mol−1 in comparison to the catalyst with low silanol sites (0.06 s−1 and 101 kJ mol−1). The best performing catalyst had an average Pd nanoparticle size of 3.2 nm and Pd loading of 0.48 wt%. This catalyst showed a high CO conversion of 78% and DMO + DMC selectivity of 96%, maintaining high stability for at least 200 h. Furthermore, the synergistic interactions of silanols of silicalite-1 zeolite and Pd were confirmed by a series of in situ infrared experiments, and were found to be responsible for the enhanced activity. The silanols of S-1 zeolite played an essential role in forming the Pd active sites and had a very pronounced effect on the adsorption of CO reactant on the Pd species. Understanding the interactions between metal species and silanols of zeolite is critical for the development of high-performance zeolite catalysts.