Protic ionic liquids with low viscosity for efficient and reversible capture of carbon dioxide

Protic ionic liquids (PILs) are considered as potential solvents for CO₂ capture due to their simple synthetic routes and unique properties. In this work, three low viscous PILs, tetramethylgunidinium imidazole ([TMGH][Im]), tetramethylgunidinium pyrrole ([TMGH][Pyrr]) and tetramethylgunidinium phenol ([TMGH][PhO]) were synthesized and the effect of anions, temperature, CO₂ partial pressure and water content on CO₂ absorption performance of PILs was also systematically studied. It was found that the PILs with larger basicity show higher CO₂ absorption capacity, and [TMGH][Im] simultaneously shows relatively high absorption rate and CO₂ absorption capacity of 0.154 g CO₂/g IL at 40 °C, 1 bar. The addition of H₂O has a positive effect on gravimetric absorption capacity of CO₂ at the range of 0–20 wt% H₂O, and the highest capacity of 0.186 g CO₂/g IL was achieved as the water content was 7 wt%. In situ FTIR, 13C NMR and theoretical calculations verified that more stable bicarbonate are produced during CO₂ absorption by [TMGH][Im]-H2O system. However, neat [TMGH][Im] can react with CO₂ to form the reversible carbamate, leading to excellent recyclability after four absorption-desorption cycles. The results implied that neat [TMGH][Im] shows great potentials in CO₂ absorption applications.