A green strategy for CO2 cycloaddition: adjusting the charge distribution on MgO leads to mechanism reversal

Abstract

The CO₂ cycloaddition with ethylene oxide (EO) to generate ethylene carbonate (EC) is a green method for converting exhaust gas into fine chemicals. However, pure MgO exhibits low EC yield under halogen-free and mild reaction conditions. This study explores EC generation on single-atom catalysts of MgO(100) surfaces loaded with Cu, Ag, Au, Zn, Na, K, Ca, Sr, Ba, Al, Ga and In through density functional theory (DFT) calculation. The results show that the mechanism reversal observed in the reaction pathway for EC generation and the role of CO₂ are attributed to changes in the electrons number obtained by EO and CO₂ co-adsorbed on the MgO(100) surfaces loaded with metals. For EC generation, surfaces loaded with Cu, Ag, Au, and Zn show a synergistic route where EO and CO₂ obtained fewer than 0.50 |e|. However, other surfaces have a stepwise pathway when this number exceeds 0.50. The participation of CO₂'s frontier orbitals in the reaction depends on the electrons obtained from the catalyst. On the Cu, Ag, Au, Zn, Na, and K-MgO(100) surfaces, CO₂ obtains fewer than 0.50 |e| and contributes LUMO orbitals, whereas on other surfaces, it obtains >0.50 |e| and contributes HOMO orbitals. Notably, the Cu-MgO(100) surface exhibits over 98 % EC selectivity at 1 atm and 298 K due to its weak alkalinity. It provides an ideal microenvironment that activates EO and CO₂, replacing the role of halogens and offering a novel catalyst to significantly boost EC yield under halogen-free and mild reaction conditions.