Tuning the CO2 absorption and physicochemical properties of K+ chelated dual functional ionic liquids by changing the structure of primary alkanolamine ligands

Abstract

To reveal the effects of the alkyl substitutions at alpha (α) or beta (β) position adjacent to amino group of primary alkanolamine ligands on CO2 absorption and physicochemical properties of metal chelated dual functional ionic liquids (DFILs), four DFILs were prepared by reacting potassium imidazole salt (KIm) with alkanolamine ligands, including 2-amino-2-methyl-1-propanol (AMP), 2-amino-1-butanol (AMB), DL-1-amino-2-propanol (DLAMP), and monoethanolamine (MEA). CO2 absorption behavior and mechanism of DFILs were studied, and density (ρ), speed of sound (u), and viscosity (η) of DFILs were measured to correlate with the CO2 absorption performance. DFT calculations were employed to explore the influence of the interactions between K+ and alkanolamine on the CO2 absorption and physicochemical properties of DFILs. The results show that CO2 capacity at 333.2 K is as follows: [K(AMP)2][Im] > [K(AMB)2][Im] > [K(DLAMP)2][Im] > [K(MEA)2][Im], indicating that the introduction of alkyl group at α or β position of alkanolamines can enhance CO2 capacity, and the effect of the α position is more significant than the β position. Both ρ and u follow the order: [K(AMP)2][Im] < [K(AMB)2][Im] < [K(DLAMP)2][Im] < [K(MEA)2][Im], while η is in the reverse order. Moreover, the saturated CO2 capacity of DFILs has an approximately linear relation with the thermal expansion coefficient. DFT calculations show that the presence of the alkyl group at α or β position of alkanolamines reduces the Mulliken charge of N and O atom, thereby weakening the cation coordination interaction between K+ with ligand, resulting in the decrease in ρ and u of DFILs. Moreover, the decrease in the Mulliken charge of N and O atom in alkanolamine leads to the increase of chelated cation-[Im]− interaction, thereby increasing η of DFILs. Consequently, [K(AMP)2][Im] exhibits higher CO2 capacity and good reversibility, due to the fact that CO2 can react with the chelated cation and [Im]– simultaneously. The present study provides a new method for effectively regulating the performance of DFILs.