Theoretical investigations on the reaction mechanisms of amine-functionalized ionic liquid [aEMMIM][BF4] and CO2

•DFT calculations show that [aEMMIM][BF4] capture CO2 with a 1:1mol stoichiometry.•The energy barriers of [aEMMIM]+CO2 and [aEMMIM][BF4]+CO2 were calculated.•[BF4] confers on [aEMMIM][BF4] a favorable CO2 absorption.•Calculated and experimental frequencies of [aEMMIM][BF4]+CO2 are consistent. In this study, an amine-functionalized ionic liquid 1,2-dimethyl-(3-aminoethyl) imidazolium tetrafluoroborate [aEMMIM][BF4] was designed and synthesized for better performance of CO2 capture based on previous reports. Quantum chemical calculations had been used to investigate the interactions between CO2 molecules and the as-synthesized ionic liquids. The molecular structures of the most stable conformation of [aEMMIM] cation and [aEMMIM][BF4] were optimized at B3LYP/6-311++G (d, p) level. At the same time, we had performed CO2 capture by [aEMMIM] cation. Compared with [aEMMIM][BF4], the performance of [aEMMIM] cation on CO2 capture was not as good as supposed. It was found that the anionic part of [aEMMIM][BF4] played a non-neglectful role in CO2 capture [aEMMIM][BF4] was found to capture CO2 with a 1:1mol stoichiometry by quantum calculations due to the steric effect of the methyl group on C2 position of imidazolium ring. It was consistent with the experimental results. In order to understand the reaction mechanisms, we calculated the configuration variations on the reactant, intermediate, transition state and product, as well as energy barrier and vibration frequency changes in gas phase and using the conductor-like polarizable continuum model (CPCM) in water solution. The barrier height for [aEMMIM] and ([aEMMIM][BF4]) capturing CO2 is 47.25 (38.25)kcal/mol in gas phase and 38.92 (20.02)kcal/mol in water solution. These results indicate that the polarity of the solvent has played an important role on the reaction mechanisms. Frequency analysis indicates that the experimental results of the vibration frequencies are in better agreement with the scaled calculated values.