Efficient SO2 capture and conversion to cyclic sulfites by protic ionic liquid-based deep eutectic solvents under mild conditions

[Display omitted] •PIL-based DESs were designed for SO2 absorption and conversion to cyclic sulfites.•SO2 solubility of 1.45 g/g (20 °C, 1 bar) and 0.56 g/g (20 °C, 0.6 vol%) were achieved.•Moderate to excellent yields were obtained under mild (30 °C), solvent-free, and additive-free conditions. SO2 capture has been a research hotspot targeting mitigation of SO2 emission, and the subsequent conversion of SO2 is an ingenious protocol, which can avoid regeneration of absorbents and realize utilization of SO2 simultaneously. To this end, a series of protic ionic liquids (PILs)-based deep eutectic solvents (DESs) composed of N-alkylimidazole hydrochloride-based PILs and azoles were designed for the capture and conversion of sulfur dioxide. Among them, [MImH]Cl/2-MIm (1:1) exhibited SO2 loading capacity as high as 1.45 g/g at 1.0 bar and 20 °C, and remarkable SO2 solubility up to 0.56 g/g were achieved at low concentrations (0.6 vol%, 20 °C). In particular, the absorbed SO2 can be directly converted to cyclic sulfites through cycloaddition reaction catalyzed by these DESs under very mild condition (30 °C and 1.0 bar of SO2) without any solvent of additive. By using gas chromatograph-mass spectrometer (GC-MS), yields of cyclic sulfites were determined to be excellent for styrene oxide (98 %) and 1,2-epoxybutane (97 %), and moderate for the sterically hindered epoxides (56 %–76 %). Recycling experiments proved that these DESs maintained good absorption and catalytic performance after five cycles. Furthermore, the absorption and catalytic mechanism were studied by Fourier transform infrared (FT-IR), nuclear magnetic resonance (NMR) spectroscopies, and theoretical calculations, revealing the multi-site absorption mechanism and synergic catalysis in PIL-based DES.