Deep eutectic solvents consisting of EmimCl and amides: Highly efficient SO2 absorption and conversion

There has been a surge in interest and research in the arena of utilizing deep eutectic solvents (DESs) as green solvents for removal of SO2 in recent years. In this work, an effective strategy for SO2 reversible capture and fixation by four amides based DESs was demonstrated. These DESs are composed of 1-ethyl-3-methylimidazolium chloride (EmimCl) and dimethylurea (DMU), N-methylurea (MU), thioacetamide (SAA), or caprolactam (CLAA) in a 2:1 M ratio, which achieve both high gravimetric and molar absorption capacity of SO2 at 20 °C and 1.0 bar (up to 1.26 g per gram of DES, or 8.00 mol per mole of DES). Results of the NMR and FTIR spectra reveal that there are physical interactions between SO2 and DESs. Furthermore, absorbed SO2 can be desorbed and no significant drop in SO2 absorption was observed after five successive absorption–desorption cycles. Particularly, the absorbed SO2 was quickly converted in situ into the charge-transfer complex of bis(sulfur dioxide)-1,4-diazabicyclo[2.2.2]octane adduct (DABCO·2SO2) as a high value-added chemical. The SO2-lean phase was directly reused for SO2 capture, thus greatly reducing energy requirements and providing a promising method SO2 resource utilization.