Theoretical study on the efficacy of DBN/LiBr system as a catalyst for fixation of CO2 with propylene oxide: Solvent effects on activation barriers

Abstract

The fixation of carbon dioxide with propylene oxide (PO) catalyzed by DBN/ LiBr system to produce a five-membered cyclic carbonate, has been extensively studied employing DFT method in gas phase and in solvents. The solvent effects were considered by means of a PCM model. The title reaction was investigated in the absence of catalyst, in the presence of LiBr alone and DBN/LiBr mediated pathways. Two hypothetical reaction pathways were proposed for DBN/LiBr catalyzed reaction and their corresponding energetics are explored to demonstrate the favorable route. DFT results indicated that mechanism I is more favorable pathway kinetically and thermodynamically than mechanism II. The more favorable pathway comprises three main steps: ring-opening of the epoxide by Br- anion, carbon dioxide insertion, and ring-closure process. The free energy of activation computed at the B3LYP/6-311++G(d,p)//B3LYP/6-31+G(d,p) level reveals that the rate-determining step is the ring-opening of epoxide; which demanded 34.95, 22.55, 19.83 and 19.75 kcal/mol in gas phase, diethyle ether, ethanol and water, respectively. The carbon dioxide insertion step was determined to be barrierless. Although the ring opening step is endothermic, the overall reaction is exothermic. Finally, the catalytic activity of other catalysts reported for the title reaction somewhere else was compared with the activity of DBN/LiBr system.