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.