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

Adam J Greer,Nora H De Leeuw,C Richard A Catlow,S F Rebecca Taylor,Matthew G Quesne,Christopher Hardacre,Helen Daly,Johan Jacquemin

doi:10.1021/acssuschemeng.1c01451

Abstract

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.

Highlights

Post-combustion CO2 capture is an important requirement of many industrial processes
NOx is known to have a significant impact on health and the environment, causing the formation of atmospheric ozone and acid (40 μragi·nm.4−3Itaisytehaer)re, floeraedivnitgaltothtahteNfiOttxinegmoisfsNioOnsx are regulated scrubbers to power stations, comprising oxidizing and reducing agents responsible for the conversion of NOx to N2.5 Aqueous alkanolamines have been employed as CO2 capture sorbents, but the presence of NOx was found to result in the irreversible formation of carcinogenic nitrosamines and a capture efficiency.[6−8] Ionic liquids (ILs) have decrease in CO2 been widely investigated for the capture of CO2 as a non-volatile alternative to toxic alkanolamines
Superbase ILs (SBILs) containing an aprotic heterocyclic anion (AHA) were developed to minimize the increase in viscosity observed in amine-functionalized ILs, and they can reversibly capture a greater than equimolar amount of CO2.15−18 Extensive studies into the absorption of other acidic gases such as SO2 and NO by SBILs have found that irreversible absorption was observed in many cases, often on multiple active sites within the IL, affecting the recyclability of the system.[19−25]

Summary

■ INTRODUCTION

Post-combustion CO2 capture is an important requirement of many industrial processes. The competitive absorption of CO2 with industrially relevant concentrations of H2O, SO2, or NO, independently, has been investigated previously in [P66614][Benzim], and this IL was selected for the current study to gain a comprehensive insight into more complex, multi-component feeds.[18,26,27] The use of a recently developed analytical method utilizing mass spectrometry allows the study of this superbase IL under realistic and dry flue gas conditions, with a feed containing 14% CO2 and 0.2% NO2.26 Further molecular-level information was provided by density functional theory (DFT). The gas absorption measurement techniques used in this work were reported in detail previously, and the same protocol was followed in this work.[26,27] To briefly summarize this, the uptake of a single component gas feed (1% NO2 in argon) by [P66614][Benzim] was studied gravimetrically at 22 ± 0.5 °C.

■ RESULTS AND DISCUSSION

■ ACKNOWLEDGMENTS

■ REFERENCES