Understanding the Competitive Gas Absorption of CO2 and SO2 in Superbase Ionic Liquids

Combined Superbase Ionic Liquid Approach to Separate CO2 from Flue Gas

Superbase ionic liquids (ILs) with a trihexyltetradecylphosphonium cation and a benzimidazolide ([P₆₆₆₁₄][Benzim]) or tetrazolide ([P₆₆₆₁₄][Tetz]) anion were investigated in a dual-IL system allowing the selective capture and separation of CO₂ and SO₂, respectively, under realistic gas concentrations. The results show that [P₆₆₆₁₄][Tetz] is capable of efficiently capturing SO₂ in preference to CO₂ and thus, in a stepwise separation process, protects [P₆₆₆₁₄][Benzim] from the negative effects of the highly acidic contaminant. This results in [P₆₆₆₁₄][Benzim] maintaining >53% of its original CO₂ uptake capacity after 30 absorption/desorption cycles in comparison to the 89% decrease observed after 11 cycles when [P₆₆₆₁₄][Tetz] was not present. Characterization of the ILs post exposure revealed that small amounts of SO₂ were irreversibly absorbed to the [Benzim]^- anion responsible for the decrease in CO₂ capacity. While optimization of this dual-IL system is required, this feasibility study demonstrates that [P₆₆₆₁₄][Tetz] is a suitable sorbent for reversibly capturing SO₂ and significantly extending the lifetime of [P₆₆₆₁₄][Benzim] for CO₂ uptake.

Combined Experimental and Theoretical Study of the Competitive Absorption of CO2 and NO2 by a Superbase Ionic Liquid

A superbase ionic liquid (IL), trihexyltetradecylphosphonium benzimidazolide ([P66614][Benzim]), is investigated for the capture of CO2 in the presence of NO2 impurities. The effect of the waste gas stream contaminant on the ability of the IL to absorb simultaneously CO2 is demonstrated using novel measurement techniques, including a mass spectrometry breakthrough method and in situ infrared spectroscopy. The findings show that the presence of an industrially relevant concentration of NO2 in a combined feed with CO2 has the effect of reducing the capacity of the IL to absorb CO2 efficiently by ∼60% after 10 absorption-desorption cycles. This finding is supported by physical property analysis (viscosity, 1H and 13C NMR, and X-ray photoelectron spectroscopy) and spectroscopic infrared characterization, in addition to density functional theory (DFT) calculations, to determine the structure of the IL-NO2 complex. The results are presented in comparison with another flue gas component, NO, demonstrating that the absorption of NO2 is more favorable, thereby hindering the ability of the IL to absorb CO2. Significantly, this work aids understanding of the effects that individual components of flue gas have on CO2 capture sorbents, through studying a contaminant that has received limited interest previously.

Charge-Tagged N-Heterocyclic Carbenes (NHCs): Revealing the Hidden Side of NHC-Catalysed Reactions through Electrospray Ionization Mass Spectrometry

N-heterocyclic carbenes (NHCs) are key intermediates in a variety of chemical reactions. Owing to their transient nature, the interception and characterization of these reactive species have always been challenging. Similarly, the study of reaction mechanisms in which carbenes act as catalysts is still an active research field. This Minireview describes the contribution of electrospray ionization mass spectrometry (ESI-MS) to the detection of charge-tagged NHCs resulting from the insertion of an ionic group into the molecular scaffold. The use of different mass spectrometric techniques, combined with the charge-tagging strategy, allowed clarification of the involvement of NHCs in archetypal reactions and the study of their intrinsic chemistry.

Industrial Applications of Ionic Liquids

Since their conception, ionic liquids (ILs) have been investigated for an extensive range of applications including in solvent chemistry, catalysis, and electrochemistry. This is due to their designation as designer solvents, whereby the physiochemical properties of an IL can be tuned for specific applications. This has led to significant research activity both by academia and industry from the 1990s, accelerating research in many fields and leading to the filing of numerous patents. However, while ILs have received great interest in the patent literature, only a limited number of processes are known to have been commercialised. This review aims to provide a perspective on the successful commercialisation of IL-based processes, to date, and the advantages and disadvantages associated with the use of ILs in industry.