Structure and SO2 Absorption Properties of Guanidinium-Based Dicarboxylic Acid Ionic Liquids

A new series of functionalized ionic liquids (ILs) with 1,1,3,3-tetramethylguanidine ([TMG]+) cations as well as dicarboxylic acid anions {[−OOC–(CH2–CH2)n–COO–], where n = 1, 3, and 5} was synthesized, and their SO2 absorption properties were investigated. A high SO2 absorption capacity of the prepared ILs was achieved. The molar ratios of SO2 to [TMG][succinic acid] {([TMG][SUC]) (n = 1)}, [TMG][suberic acid] {([TMG][SUB]) (n = 3)}, and [TMG][dodecanedioic acid] {([TMG][DOD]) (n = 5)} were 4.76, 5.96, and 5.96, respectively. The evidence of spectroscopic measurements and quantum chemical calculations confirmed that SO2 could interact with the carboxyl and adjacent methylene groups on the anion as well as the amino group on the cation. The SO2 absorption capacity of these ILs was strongly influenced by their asymmetry and space resistance. The high symmetry and large steric hindrance could reduce the SO2 absorption capacity of ILs.