SO2 capture by guanidinium-based ionic liquids: a theoretical study.

Ionic liquids (ILs) show good performances in SO(2) separation science, e.g., SO(2) capture from high-temperature flue gas or separation from gas mixtures. In this work, the mechanism of capturing SO(2) by three guanidinium-based ILs, 1,1,3,3-tetramethylguanidinium lactate ([tmgHH][L]), 1,1,3,3-tetramethylguanidinium bis(trifluoromethylsulfonyl)imide ([tmgHH][Tf(2)N]), and 1,1,3,3-tetramethylguanidinium tetrafluoroborate ([tmgHH][BF(4)]), is investigated by using molecular dynamic simulation and ab initio calculation. The results of condensed phase molecular dynamic simulation for the mixtures of SO(2) and these three ILs indicate the similar SO(2) organization and interaction among them; SO(2) may organize around [tmgHH](+) while it favorably organizes around the anions through Lewis acid-base interaction. Gas phase ab initio calculations show that [tmgHH][L] chemically interacts with SO(2) while [tmgHH][Tf(2)N] and [tmgHH][BF(4)] do not, which is supported by the earlier FT-IR and (1)H NMR data and is also consistent with the experimental result of a much higher absorption capability of [tmgHH][L] for SO(2) than the latter two. The anion plays a key role in the chemical interaction between [tmgHH][L] and SO(2), the S atom is bonded to the N atom on -NH(2) of [tmgHH](+), and some products with aminosulfate or aminosulfinic acid fragment may be formed. This work shows that IL structures should be carefully tailored for their final application in SO(2) capture.