DFT study on coupling reaction of carbon dioxide with ethylene oxide catalyzed by 1,4,6-triaza-bicyclo[3.3.0]oct-4-enium bromide (TBO.HBr)

ABSTRACT

Chemical fixation of CO₂ with epoxides catalyzed by organic-base salts were found to be efficient among the various catalysts tested due to synergetic effects of HBDs and halide-ions for ring-opening. In this study, 1,4,6-Triaza–bicyclo3.3.0]oct-4-enium bromide catalyzed conversion of CO₂ and epoxide into cyclic-carbonate has been studied by using DFT method to understand the reaction mechanism and the catalytic performance of TBO.HBr. Two hypothetical reaction mechanisms were proposed for the coupling reaction. Thermodynamic and kinetic parameters were computed for each steps to determine the more favorable route. Mechanism II is more favorable path whereby Br^- ion first interacts with epoxide to form bromo-alcohol, which directed to form carbonate-ion and finally ring-closure step yielded cyclic-carbonate with catalyst-regeneration. Cyclization step is rate-determining step with reaction barrier of 22.696?kcal/mol in gas phase. Ensuing the favorable mechanism, solvent-effects on the reaction barrier has been investigated using water and THF. Mechanism II is still more favorable reaction path in both THF and water. However, the rate-determining step is found to be ring-opening of the epoxide with reaction barrier of 22.658?kcal/mol (wate) and 21.969?kcal/mol (THF). In this study, TBO.HBr exhibited good catalytic activity for the title reaction investigated in both gas phase and solvents.