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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.

frontiers in chemistry

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