Improved Reversible and Selective SO2 Absorption by a Stable Phosphonium Carboxylate Ionic Liquid

The ionic liquid tetrabutylphosphonium 1H-tetrazole-1-acetate, [P-4,P-4,P-4,P-4][TetrazC(1)COO], appears as promising for the selective and reversible absorption of SO2 at low pressures and in the presence of CO2. The ionic liquid reversibly reacts with SO2 and to a minor extent with CO2 as shown by high-pressure NMR analysis. High absorption capacities were measured for SO2 at pressures below 1 bar using an isochoric saturation technique, the CO2 being measured in a gravimetric microbalance. We could calculate one of the highest SO2/CO2 selectivity reported so far. Contrary to previously reported ionic liquids, the solutions have lower viscosities, as inferred by the diffusivities measured by NMR, than that of the pure absorbent & horbar;a clear advantage for practical usages. Molecular dynamics simulations using validated polarizable force fields allow for a molecular understanding of the physical absorption of both acidic gases and how they impact of the microscopic structure of the liquid. Ab initio calculations provide estimates of the energetics of chemical absorption in line with the experiments confirming that the regeneration of the absorbent can be done at mild conditions of temperature and pressure.