Comparative study on heterogeneous CeO2 and homogeneous 1,8-diazabicyclo[5.4.0]-7-undecene catalysts for conversion of CO2 and monoethanolamine/ethylenediamine into cyclic compounds

Abstract

Heterogeneous CeO₂ and homogeneous 1,8-diazabicyclo[5.4.0]-7-undecene (DBU) catalysts have been employed for non-reductive conversion of CO₂; however, their activity and catalysis have not yet been clarified clearly in the identical reactions or conditions. Here, we conducted the kinetic study on the reactions between CO₂ and monoethanolamine (MEA) (or ethylenediamine (EDA)) into 2oxazolidinone (or 2-imidazolidinone) with the two catalysts under high CO₂-pressure conditions. The conversion of MEA with CeO₂ proceeded faster than that with DBU under all the conditions tested in this study owing to the better affinity between CeO₂ and possible intermediate of CO₂-captured MEA. In contrast, the conversion of EDA over CeO₂ became slower at higher CO₂ pressure, while the same reaction with DBU was accelerated and became faster than that with CeO₂ upon the increase of CO₂ pressure. These contrasting behaviors of CeO₂- and DBU-catalyzed reactions against CO₂ pressure were rationalized by the different effects of gaseous CO₂ on the possible intermediate species. In the case of CeO₂, excess CO₂ capped the reactive amino group of EDA-derived intermediate chemisorbed on the CeO₂ surfaces, decreasing the nucleophilicity of this functional group for the cyclization reaction and lessening the reaction rate. Meanwhile, the DBU-catalyzed reactions under high pressure CO₂ enabled to form the zwitterionic pair of CO₂-captured DBU and CO₂-captured EDA species, and such strong electrostatic interaction between these species led to the better reaction progress compared to low CO₂-pressure conditions.