Mechanistic insights into CeO2-catalyzed direct synthesis of diethyl carbonate from CO2 and ethanol assisted by zeolite and 2,2-diethoxypropane

Abstract

Direct and non-reductive conversion of CO₂ and ethanol to form diethyl carbonate (DEC) is limited by its severe equilibrium and typically necessitates a dehydrant to shift the equilibrium toward the product side. We previously found the combination of two additives, 2,2-diethoxypropane (DEP) and H-FAU, to function as an effective dehydrating system and accelerate the DEC synthesis, although their actual function has remained unclear. In this study, the roles of DEP and H-FAU and the whole catalytic mechanism were investigated by a detailed kinetic study and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Our results have suggested that an ethyl carbonate species is formed on the CeO₂ surface from CO₂ and either ethanol or DEP. Meanwhile, DEP is not involved in the rate-determining step and instead readily adsorbed and activated on the H-FAU surface. An ethanol molecule behaves as another ethoxy source in the DEC synthesis. Altogether, the ethyl carbonate adspecies on CeO₂, the activated DEP species on H-FAU, and the ethanol molecule have been suggested to participate in the rate-determining step of the DEC synthesis.