Tuning the structure of pyridinolate-based functional ionic liquids for highly efficient SO2 absorption

Abstract

A series of pyridinolate anion-functional ionic liquids (ILs) containing different kinds of cations were designed and prepared, including tri-n-hexyltetradecylphosphonium [P66614], tri-n-butylethylphosphonium [P4442], and 1,1,3,3-tetramethylguandinium [TMG] cations and pyridinolate [PyO] anions with different N positions. Densities and viscosities at different temperatures were measured because of their importance for the absorption of SO2. Effect of different structures on the absorption of SO2 was studied, including the type and the alkyl chain length of cations, and the N positions of [PyO] anions (2-, 3-, and 4-). Additionally, effects of SO2 concentration and temperature for absorption were investigated. Especially, up to 6.00 and 2.53 mol SO2 per mole IL could be obtained by [P66614][4-PyO] at 1 bar and 0.1 bar of SO2, respectively, when absorptions performed at 20 °C. The results indicate that (1) the aprotic cations weakened the cation⋯anion interaction and resulted in the released active sites on the anions, and (2) N position has an influence on charge distribution of [PyO] anions and resulted in the tunable absorption of SO2. Reversible SO2 absorption and [P66614][4-PyO] regeneration could be performed several times without losing efficiency. Furthermore, the mechanism of absorption was studied through FT-IR and NMR methods and the results showed that the absorption of SO2 by [P66614][4-PyO] includes physisorption and chemisorption.