Decarbonisation Technology - February 2023

than 100⁰C (Ohno, 2005), (Mohammad and Inamuddin, 2012). Over the past decade, ILs have been extensively studied as scrubbers of greenhouse gases (Zheng, et al., 2017). Many studies used imidazolium-based ILs for CO2 capture (Wang, et al., 2019), (Nguyen and Zondervan, 2018). Due to the low stability, high volatility, and corrosiveness of materials used in conventional CO2 capture processes, such as amine-based absorption, water and organic liquid scrubbing, and hollow fibre membranes, ILs potentially infract green chemistry principles. (Zheng, et al., 2017), (Daryayesalameh, Nabavi and Vaferi, 2021), (Lu, et al ., 2018). Advantages of ILs include good dissolution properties, low volatility, high decomposition temperature, energy, and cost-efficient separation of CO2 from post-combustion flue gases. These solvents can be readily removed and recycled. Their properties enable their use as room temperature ionic liquids (RTIL) to give a wider acceptance as ‘green’ solvents, replacing classical volatile organic solvents in a variety of processes, including industrial chemical processes (Mukherjee, 2021). One class of ILs, the cation ILs, consists of imidazolium, pyridinium, and amines. The anion class consists of organic/inorganic ILs, (carboxylate, azolate, phenoxide) (Shukla, et al ., 2019). When compared with cation ILs, the basicity of the anion ILs was noted to increase

CO2 uptake. Process simulations showed that substituting MDEA (methyl diethanolamine) with ILs can reduce the total electrical and thermal energy by 42.8% and 66.04%, respectively (Liu, et al., 2016). The regeneration energy demand also decreased by 15% when IL (1-butylpyridinium tetrafluoroborate) was used in place of MEA (Mumford, et al., 2015). Numerous IL structures have been developed during the past 20 years. Pure IL configurations, typically referred to as RTILs, retain CO2 by a physio-sorption mechanism, but the CO2 absorption performance is not competitive with conventional amine-based solvents. Amine- functionalised ILs, also called task-specific ILs, react with CO2 by chemi-sorption to improve CO2 capture performance over the physio- sorption-based ILs, in both biogas/natural gas treatment and CCS (Vega, et al. n.d.). The major drawback of ILs as a scrubbing agent arises from their high viscosity, which further increases upon CO2 capture. However, the high viscosity of ILs can be lowered by incorporating ether functionality, the addition of molecular solvents, and incorporating them on polymer membranes. IL-mixed solvents are another approach to overcome the known limitations of ILs. Mixtures of ILs with water or organic solvents have been proposed as new absorbents for CO2 separation (Anderson, et al., 2015). At present, research on IL mixed solvents for CO2

Carbon capture u tili s ation and storage

Green solvents

Ionic liquid mixed solvents

Ionic liquids

Deep eutectic solvents

Amino acid-based ILs

Immobilisation on ordered mesophorous silica

Gas-liquid membrane contactor

Microencapsulation

Figure 1 Schematic representation of carbon adsorption methods Source: Nematollahi and Carvalho, 2019