Theoretical and experimental study on the interaction between 1-butyl-3-methylimidazolium acetate and CO2

Recently, CO2 had been applied for regeneration of biomass polymers from acetate based ionic liquids (AcILs). To explore the interaction mechanisms between the AcILs and CO2, the reaction of [CO2]n (n = 1-4) with 1-butyl-3-methylimidazolium acetate ([Bmim]OAc) has been investigated using the B3LYP hybrid density functional methods at 6-311++G(d,p) level. The complexes formed by [Bmim]OAc and CO2 were optimized and the most stable structure of the complexes was chosen to form [Bmim]OAc + 2CO2, [Bmim]OAc + 3CO2 and [Bmim]OAc + 4CO2 systems. Results show that strong hydrogen bonds exist between the O atom in CO2 and the H atom in [Bmim]+ in each complex. Along with the increase of the number of CO2, the ability of [Bmim]OAc interacts with CO2 and the strength of the hydrogen bonds between cation and anion can be reduced continuously. Compared with other complexes, the interaction energy of [Bmim]OAc + 4CO2 system is so small that this complex is easy to dissociate into [Bmim]OAc + 3CO2 system and CO2. In addition, the vibrational frequencies of the complexes are analyzed. Owing to the interaction between CO2 and [Bmim]OAc, the OC-O asymmetrical stretching frequency appears an obviously red shift in the complex. Furthermore, for natural bond orbital (NBO) analysis, the second order perturbation stabilization energies (E(2)) that denotes the intensity of the interactions between [Bmim]OAc and CO2 indicates the interaction of [Bmim]OAc with CO2 plays an important role in the H-bonding properties of [Bmim]OAc. Atoms in molecules (AIM) analysis shows the bonding properties of these complexes. These results show that the strength of hydrogen bonds between [Bmim]OAc and CO2 is strong, which means that the interactions between [Bmim]OAc and CO2 affect the initial hydrogen bonds in IL due to the formation of new hydrogen bonds in the complexes. All the calculation data provide the interaction mechanism on the dissolution of CO2, and help to develop the precipitation of biomass from ionic liquid solutions by compressed CO2.