Crystal facet-dependent CO2 cycloaddition to epoxides over ZnO catalysts

Abstract

With regard to green chemistry and sustainable development, the fixation of CO₂ into epoxides to form cyclic carbonates is an attractive and promising pathway for CO₂ utilization. Metal oxides, renowned as promising eco-friendly catalysts for industrial production, are often undervalued in terms of their impact on the CO₂ addition reaction. In this work, we successfully developed ZnO nanoplates with (002) surfaces and ZnO nanorods with (100) surfaces via morphology-oriented regulation to explore the effect of crystal faces on CO₂ cycloaddition. The quantitative data obtained from electron paramagnetic resonance spectroscopy indicated that the concentration of oxygen vacancies on the ZnO nanoplate surfaces was more than twice that on the ZnO nanorod surfaces. Density functional theory calculations suggested that the (002) surfaces have lower adsorption energies for CO₂ and epichlorohydrin than the (100) surfaces. As a result, the yield of cyclochloropropene carbonate on the ZnO nanoplates (64.7%) was much greater than that on the ZnO nanorods (42.3%). Further evaluation of the reused catalysts revealed that the decrease in the oxygen vacancy concentration was the primary factor contributing to the decrease in catalytic performance. Based on these findings, a possible catalytic mechanism for CO₂ cycloaddition with epichlorohydrin was proposed. This work provides a new idea for the controllable preparation of high-performance ZnO catalysts for the synthesis of cyclic carbonates from CO₂ and epoxides.