Catalytic function of CeO₂ in non-reductive conversion of CO₂ with alcohols

CeO₂ shows unique catalytic properties by an acid–base bifunctionality as well as redox properties. The acid–base bifunctional properties of CeO₂ have been applied to the non-reductive CO₂ conversion with alcohols such as dimethyl carbonate (DMC) synthesis from CO₂ and methanol. CeO₂ shows very high selectivity to DMC; however, the yield of DMC is strongly limited by the equilibrium. The combination of the synthesis of organic carbonates from CO₂ and corresponding alcohols with suitable H₂O removal methods can enhance the yield of the organic carbonates beyond the equilibrium limitation. Many CeO₂-based materials have been prepared and tested in the reaction of CO₂ with alcohols. As is known, what catalysts can do is to make the reactions progress to the equilibrium level in reversible reactions and the catalysts do not influence the equilibrium level and thermodynamics of the reactions. In order to evaluate the catalytic properties of CeO₂-based materials in this reaction, we should pay more attention to the catalytic activity. In addition, the catalytic activity should be measured under the reaction conditions at a sufficiently lower conversion level than that at the equilibrium level, because the effect of the reverse reaction can be neglected. The catalytic performance of the CeO₂-based materials is compared considering the equilibrium level under the reaction conditions. Morphology, oxide ion defects (or oxygen vacancies), as well as acid–base properties on the catalytic performance can influence the catalytic activity; however, it seems to be difficult to elucidate crucial factors. Therefore, development of CeO₂-based materials with much higher catalytic activities than reported results is necessary, which will contribute to the determination of the most crucial factor.