Role of Hydrophilic Ammonium-Based Deep Eutectic Solvents in SO2 Absorption

Acid gases, especially sulfur dioxide (SO2), released from the burning of fossil fuels, have made a great contribution to environmental pollution. However, the technologies to remove the acid gases have high operation costs and energy requirements. A kind of hydrophilic deep eutectic solvents (DESs) were synthesized and applied to solve the problem of SO2 environmental pollution. The SO2 absorption conditions were optimized including the kind of DESs, viscosity, and temperature. The hydrophilic interaction in SO2 absorption was explored. The hydrophilic property of caprolactam/tetrabutylammonium bromide DESs (TBAB/CPL) greatly reduced the gas transfer barrier, thus accreting the SO2 absorption rate by enhancing the accessibility of the active sites on DESs and SO2. The surface tension decreased with the absorption of SO2, and the minimal surface tension of TBAB/CPL was 32 mN m–1 at 303.2 K (surface entropy, 0.0395 J K–1 m–2) when the concentration of TBAB/CPL aqueous solution was 2 mol L–1. The phase-transfer process for the SO2 absorption system was determined by nuclear magnetic resonance, Fourier transform infrared spectroscopy, and Raman spectroscopy. The mechanism of the hydrophilic absorption of SO2 in TBAB/CPL DESs was also developed.