Experimental and theoretical investigation of an ionic liquid-based biphasic solvent for post-combustion CO2 Capture: Breaking through the “Trade-off” effect of viscosity and loading

Abstract

To break through the “trade-off” effect of biphasic solvents between rich-phase viscosity and CO2 loading, we synthesized a novel ionic liquid (diethylenetriamine-3-hydroxypyridine, [DETA][3HPyr]) with multiple active sites and combined it with the organic solvent diethylene glycol monobutyl ether (DGME) and water to prepare a biphasic solvent for CO2 absorption, the best absorption condition was optimized. The results demonstrated that after absorption, the solvent transformed from homogeneous to biphasic with highly self-concentrated absorption products in the rich phase. The volume of the rich phase accounted for only 36.3 % of the total solvent, while its viscosity decreased significantly to 28.1 mPa·s, with a high blend absorption capacity of 2.67 mol·kg−1. The species of absorption product and absorption mechanism was investigated using 13C NMR. The stable presence of carbamic acid in the system was confirmed, contributing to enhanced absorption capacity. Density functional theory calculations revealed that DGME stabilized carbamic acid through intermolecular hydrogen bonding interactions, and highly polar ions generated during absorption and uneven charge distribution between ions dominated the phase-change process and increased the water content in the rich phase. Thermodynamic energy barrier calculation showed that the solvent effect of DGME reduced the Gibbs free energy barriers of proton transfer and facilitated reaction progress. 3IL4D3H exhibited excellent cyclic performance with low regeneration energy consumption at 1.77 GJ·t−1. This is the first work to revealing the intrinsic dynamics of carbamic acid formation, and provides a new perspective for the phase-change mechanism of ionic liquid-based biphasic solvent.