Investigation of a deep eutectic solvent formed by levulinic acid with quaternary ammonium salt as an efficient SO2 absorbent

Efficient and reversible absorption of SO2 in six new deep eutectic solvents (DESs) composed of levulinic acid (LA) and quaternary ammonium salts (choline chloride, CC; choline acetyl chloride, CAC; tetraethylammonium chloride, TEAC; tetraethylammonium bromide, TEAB; tetrabutylammonium chloride, TBAC; tetrabutylammonium bromide, TBAB) was systematically investigated. The molar ratio of LA to quaternary ammonium salt was fixed at 3:1. The results showed that all DESs possessed satisfactory performance for SO2 absorption, with the maximum absorption capacity of 0.625 g SO2 per g DES of LA-TEAC at 293.15 K under ambient pressure. The absorbed SO2 was easily released by bubbling N-2 at 323.15 K and the regenerated DES was recycled five times without obvious loss of absorption performance. The solubility of SO2 in DES of LA-CC at T = 293.15, 303.15, 313.15, and 323.15 K under subatmospheric pressures was also determined. The relative thermodynamic parameters, i.e., absorption enthalpy, absorption entropy, and absorption Gibbs free energy, were further calculated, giving results with small negative values. The influence of water content on the SO2 absorption was also investigated. According to NMR and FTIR analyses, the absorption of SO2 in DES was a physical process. Moreover, all the DESs exhibited high selectivity for SO2/CO2. The present DESs are promising absorbents for SO2 because of their high absorption capacity and good reversibility.