Efficient SO2 Capture and Fixation to Cyclic Sulfites by Dual Ether-Functionalized Protic Ionic Liquids without Any Additives

The capture of SO2 with ionic liquids (ILs) has attracted much attention in recent years; however, the examples involving SO2 capture and utilization (SCU) in the same medium are scarce. Here, we demonstrated an innovative strategy for SO2 capture and fixation to cyclic sulfites in dual ether-functionalized protic ionic liquids (PILs) for the first time. These dual ether-functionalized PILs exhibited low viscosities and remarkable SO2 loading capacities (up to 6.12 mol of SO2 per mol of IL and 1.34 g of SO2 per g of IL at 1.0 bar) that is conducive to conversion of SO2 absorbed in situ. The mechanism of absorption was proposed which includes both chemical and physical absorptions from the spectral results and theoretical calculations. Particularly, the SO2 absorbed in the PILs was directly transformed into cyclic sulfites without any additives; meanwhile, these PILs were also used as efficient catalysts for the synthesis of a series of cyclic sulphites using equimolar SO2 and epoxides. Good to excellent yields of cyclic sulfites were obtained for varied substrates. The dual roles of PILs as both absorbents and catalysts as well as the recyclability of the PILs are examined in detail in this paper. This innovative strategy not only eliminated the traditional intensive energy input for SO2 desorption but also enabled the production of value-added cyclic sulfites.