Catalytic conversion of carbon dioxide into dimethyl carbonate using reduced copper-cerium oxide catalysts as low as 353 K and 1.3 MPa and the reaction mechanism

Synthesis of dimethyl carbonate (DMC) from CO(2) and methanol under milder reaction conditions was performed using reduced cerium oxide catalysts and reduced copper-promoted Ce oxide catalysts. Although the conversion of methanol was low (0.005–0.11%) for 2 h of reaction, DMC was synthesized as low as 353 K and at total pressure of as low as 1.3 MPa using reduced Cu–CeO(2) catalyst (0.5 wt% of Cu). The apparent activation energy was 120 kJ mol(−1) and the DMC synthesis rates were proportional to the partial pressure of CO(2). An optimum amount of Cu addition to CeO(2) was 0.1 wt% for DMC synthesis under the conditions at 393 K and total pressure of 1.3 MPa for 2 h (conversion of methanol: 0.15%) due to the compromise of two effects of Cu: the activation of H(2) during reduction prior to the kinetic tests and the block (cover) of the surface active site. The reduction effects in H(2) were monitored through the reduction of Ce(4+) sites to Ce(3+) based on the shoulder peak intensity at 5727 eV in the Ce L(3)-edge X-ray absorption near-edge structure (XANES). The Ce(3+) content was 10% for reduced CeO(2) catalyst whereas it increased to 15% for reduced Cu–CeO(2) catalyst (0.5 wt% of Cu). Moreover, the content of reduced Ce(3+) sites (10%) associated with the surface O vacancy (defect sites) decreased to 5% under CO(2) at 290 K for reduced Cu–CeO(2) catalyst (0.1 wt% of Cu). The adsorption step of CO(2) on the defect sites might be the key step in DMC synthesis and thus the DMC synthesis rate dependence on the partial pressure of CO(2) was proportional. Subsequent H atom subtraction steps from methanol at the neighboring surface Lewis base sites should combine two methoxy species to the adsorbed CO(2) to form DMC, water, and restore the surface O vacancy.