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

Abstract

A superbase ionic liquid (IL), trihexyltetradecylphosphonium benzimidazolide ([P 66614 ][Benzim]), is investigated for the capture of CO 2 in the presence of NO 2 impurities. The effect of the waste gas stream contaminant on the ability of the IL to absorb simultaneously CO 2 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 NO 2 in a combined feed with CO 2 has the effect of reducing the capacity of the IL to absorb CO 2 efficiently by ∼60% after 10 absorption–desorption cycles. This finding is supported by physical property analysis (viscosity, 1 H and 13 C 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-NO 2 complex. The results are presented in comparison with another flue gas component, NO, demonstrating that the absorption of NO 2 is more favorable, thereby hindering the ability of the IL to absorb CO 2 . Significantly, this work aids understanding of the effects that individual components of flue gas have on CO 2 capture sorbents, through studying a contaminant that has received limited interest previously.

Combined Superbase Ionic Liquid Approach to Separate CO2 from Flue Gas

“…Previous work with NO x and SO 2 demonstrated that acidic gases can physically and chemically interact with both N-sites on the [Benzim] − anion, suggesting that under these conditions SO 2 could feasibly be strongly absorbed to one N-site, while CO 2 is more weakly absorbed on the second N-site and competes with the lower SO 2 concentration in the feed. 33 , 34 It is important to note that similar studies have shown that in the presence of the [Tetz] − anion, SO 2 can interact on multiple N-sites. In this case, the absorption enthalpies decrease as subsequent absorption sites are filled.…”

“… 36 , 47 − 51 However, it is essential to study conditions that are representative of real-world environments to assess the impact of realistic amounts of flue gas contaminants on the ability of IL sorbents to reversibly capture CO 2 . 32 − 34 Gas capture in thin films of ILs, such as [P 66614 ][Benzim], is also being investigated as studies have shown that their behavior differs in comparison to bulk solution. 52 …”

“…[P 66614 ][Benzim] has been used in a series of studies investigating the effect of flue gas contaminants on CO 2 uptake and was also shown to capture significantly more CO 2 per mole of IL in comparison to [P 66614 ][Tetz] (1.20 vs 0.08 n CO 2 / n IL) due to a higher absorption enthalpy (−52.1 vs −19.1 kJ·mol –1 ). 20 , 27 , 31 − 34 Importantly, when SO 2 capture is considered, both ILs can absorb large amounts. However, [P 66614 ][Benzim] does so irreversibly, which has been shown to have a negative effect on its ability to simultaneously absorb CO 2 .…”

“…The effect of the anion on the capture of CO 2 and SO 2 is displayed in Table for two SBILs: trihexyltetradecylphosphonium benzimidazolide, [P 66614 ]­[Benzim], and trihexyltetradecylphosphonium tetrazolide, [P 66614 ]­[Tetz]. [P 66614 ]­[Benzim] has been used in a series of studies investigating the effect of flue gas contaminants on CO 2 uptake and was also shown to capture significantly more CO 2 per mole of IL in comparison to [P 66614 ]­[Tetz] (1.20 vs 0.08 n CO 2 / n IL) due to a higher absorption enthalpy (−52.1 vs −19.1 kJ·mol –1 ). ,,− Importantly, when SO 2 capture is considered, both ILs can absorb large amounts. However, [P 66614 ]­[Benzim] does so irreversibly, which has been shown to have a negative effect on its ability to simultaneously absorb CO 2 . , In contrast, a small residue following desorption (0.06 n SO 2 / n IL) was reported for [P 66614 ]­[Tetz] after exposure to 0.2 vol % SO 2 which may be due to a weaker binding affinity when compared to [P 66614 ]­[Benzim] (−89.4 vs −123.9 kJ·mol –1 ). − However, a study of the recyclability of [P 4442 ]­[Tetz] showed that the working absorption capacity remained constant …”

“…Herein, the feasibility of using [P 66614 ]­[Tetz] with [P 66614 ]­[Benzim] in a dual-IL system has been investigated to assess the effect on the lifetime and recyclability of the [P 66614 ]­[Benzim] sorbent for CO 2 uptake. This study has utilized the previously developed mass spectrometry-based gas absorption rig, – with the ILs after exposure characterized using NMR and elemental analyses to determine their recyclability. This paper is a feasibility study for the use of ILs in a stepwise separation process and shows [P 66614 ]­[Tetz] to increase the lifetime of [P 66614 ]­[Benzim] as a CO 2 capture sorbent.…”