Synergism of potassium iodide and ascorbic acid in promoting fixation of CO2 with propylene oxide: A DFT study

Abstract

Chemical fixation of CO₂ into useful chemicals has dual significance in view of both environmental protection and sustainable chemistry. The study computationally examined the fixation of CO₂ into propylene carbonate catalyzed by a binary system consisting of potassium iodide (KI) and five hydrogen bond donors, including ascorbic acid (AA), ethylene glycol, 2,6-bis(trifluoromethyl)phenylboronic acid, 2,6-dimethylphenylboronic acid, and H₂O. Density functional theory (DFT) calculations were performed using M06-2X, B3PW91, B3LYP-D3, 6–31+G(d,p) and 6–311++G(d,p) basis sets for non-iodine atoms, and LANL2DZ for iodine. The mechanistic detail for the catalytic cycloaddition was elucidated. According to the calculated barrier for the rate-determining step (RDS), KI/AA promoted the CO₂/propylene oxide (PO) cycloaddition reaction best. The analyses from Quantum Theory of Atoms in Molecules, Reduced Density Gradient scatter and Non-Covalent Interactions plots all confirmed that the ring-opening of PO is facilitated by activating the epoxide and stabilizing the transition state through hydrogen bonding interactions. The ring-opening step promoted by KI/AA is an exothermic, spontaneous process, resulting from increased stability of the alkoxide intermediate due to the migration of a proton from AA to the O atom of PO. The barrier calculated for the RSD in the gas phase ranged from 13.04 to 19.74 kcal/mol, with a slight increase observed in water. Although the barrier for the RDS is slightly higher in water than in the gas phase, DFT calculations show that water, a green solvent with the highest dielectric constant, is a suitable solvent candidate for the studied reaction.