Combined Superbase Ionic Liquid Approach to Separate CO from Flue Gas.

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