Combined Superbase Ionic Liquid Approach to Separate CO₂ from Flue Gas

Abstract

Superbase ionic liquids (ILs) with a trihexyltetradecylphosphonium cation and a benzimidazolide ([P 66614 ][Benzim]) or tetrazolide ([P 66614 ][Tetz]) anion were investigated in a dual-IL system allowing the selective capture and separation of CO 2 and SO 2 , respectively, under realistic gas concentrations. The results show that [P 66614 ][Tetz] is capable of efficiently capturing SO 2 in preference to CO 2 and thus, in a stepwise separation process, protects [P 66614 ][Benzim] from the negative effects of the highly acidic contaminant. This results in [P 66614 ][Benzim] maintaining >53% of its original CO 2 uptake capacity after 30 absorption/desorption cycles in comparison to the 89% decrease observed after 11 cycles when [P 66614 ][Tetz] was not present. Characterization of the ILs post exposure revealed that small amounts of SO 2 were irreversibly absorbed to the [Benzim] − anion responsible for the decrease in CO 2 capacity. While optimization of this dual-IL system is required, this feasibility study demonstrates that [P 66614 ][Tetz] is a suitable sorbent for reversibly capturing SO 2 and significantly extending the lifetime of [P 66614 ][Benzim] for CO 2 uptake.