Understanding the Competitive Gas Absorption of CO2 and SO2 in Superbase Ionic Liquids

During this work, an original study of the CO2 and SO2 competitive gas absorption in three superbase ionic liquids, namely trihexyltetradecylphosphonium 1,2,4-triazolide ([P66614][124Triz]) and trihexyltetradecylphoshonium benzimidazolide ([P66614][Benzim]), is reported for the first time. To initiate such a comprehensive study, the CO2 and SO2 mixed gas solubility in selected ILs was determined by using an original and accurate dynamic method coupled with a mass spectrometer after several absorption and desorption cycles. This method has been validated by comparing the gravimetric uptake of CO2 with the mass spectrometry data using trihexyltetradecylphosphonium benzotriazolide, 1,2,4-triazolide and benzimidazolide ionic liquids and shown to be consistent within 10 % in mole ratio units. Solubility results clearly show that the presence of SO2 in the gas stream decreases the CO2 capture capability of the investigated ILs. Furthermore, the viscosity, chemical analysis (water content and sulfur content) and spectroscopic data (1H-NMR, 13C-NMR, ATR-IR and XPS) changes before and after absorption/desorption of the gases were determined and depicted to truly understand the reaction mechanism which occurs in the liquid phase highlighting a clear competition between the SO2 vs. CO2 chemical reaction and selected superbase ILs.