CoZn-ZIF-derived carbon-supported Cu catalyst for methanol oxidative carbonylation to dimethyl carbonate

Abstract

The graphitization degree and N content in carbon materials significantly affect the reduction of Cu²⁺ to (Cu⁺ + Cu⁰), which are the active centers for methanol oxidative carbonylation to dimethyl carbonate (DMC). In this work, a series of N-doped carbon (NC-x) samples derived from CoZn-ZIFs-x materials with various molar percentages of Co were synthesized by carbonization. After nitric acid treatment, the prepared NC-x-H was used to support the Cu catalyst and were applied for DMC synthesis. Combined with XRD, TEM, N₂-physisorption, Raman, XPS, ICP-OES, and H₂-TPR characterizations, the effect of Co content on the structure, textural property, and valency of the Cu species was investigated. As the content of Co increased, the BET surface areas of ZIFs, NC-x-H, and Cu/NC-x-H all decreased, while the average pore size of NC-x-H and Cu/NC-x-H enlarged. Moreover, the graphitization degree gradually increased, accompanied with a decrease in the N content. The ratio of (pyridine-N (N1) + graphite-N (N3))/pyrrole-N (N2) firstly increased and then slightly decreased. The relationship between the graphitization degree, (N1 + N3)/N2 ratio, and (Cu⁺ + Cu⁰) content, as well as the relationship between the space-time yield (STY) of DMC and the (Cu⁺ + Cu⁰) content were discussed. Cu/NC-60-H catalyst with the highest (N1 + N3)/N2 ratio shows the highest content of (Cu⁺ + Cu⁰), which exhibits the best catalytic activity. After being reused 6 times, the STY of DMC of different Cu/NC-x-H catalysts were compared. The oxidation of Cu species is mainly responsible for the deactivation of the Cu/NC-60-H catalyst.