Reactivity of CO 2 with aqueous choline-based ionic liquids probed by solid-state NMR spectroscopy Andrei Filippov a,b, ⁎, Oleg N. Antzutkin a,c , Faiz Ullah Shah a a Chemistry of Interfaces, Luleå University of Technology, SE-97187 Luleå, Sweden b Kazan State Medical University, 420012 Kazan, Russia c Department of Physics, Warwick University, Coventry CV4 7AL, UK abstract article info Article history: Received 25 March 2019 Accepted 3 May 2019 Available online 5 May 2019 CO 2 absorption in a series of choline-based ionic liquids is investigated using solid-state 13 C and 15 N MAS NMR spectroscopy. Natural abundance and 13 C enriched CO 2 gas was purged through 50 wt% aqueous solutions of alkyldimethyl(2-hydroxyethyl)ammonium threonine, [N 1,1,n,2OH ][Threo], (alkyl = butyl, pentyl and hexyl) and pentyldimethyl(2-hydroxyethyl)ammonium taurine [N 1,1,5,2OH ][Tau]. The process of CO 2 absorption results in precipitation of a solid sediment, which stays in equilibrium with the liquid phase. Upon degassing of the sample, the sediment is dissolved back into the IL-aqueous phase. Solid state 13 C and 15 N MAS NMR data suggest that the solid sediment is composed of neutral threonine (or taurine) in the zwitterionic forms and the liquid phase contained the products of reactions between the ionic liquids and CO 2 molecules. A plausible mechanism for for- mation of the solid sediments and the reaction products in liquid phases is suggested. © 2019 Elsevier B.V. All rights reserved. Keywords: CO 2 absorption; Aqueous choline-based ionic liquids Liquid-solid equilibrium 13 C and 15 N MAS and CP-MAS NMR 1. Introduction Ionic liquids (ILs) are typically low melting salts composed of or- ganic or inorganic cations and anions, which retain their liquid state at room temperature. Due to occurrence of bulky cations or/and anions in the liquid state, ILs have unique physicochemical properties such as high ionic conductivity, low vapor pressure, non-flammability and structural designability [1–4]. This makes them appropriate candidates for replacing conventional molecular liquids in many process technolo- gies. Often ILs are regarded as designer solvents due to their tunability of physical and chemical properties by the molecular design of cation and anion structures [4]. In the past years, ILs (and their mixtures with organic solvents and water) have emerged as potential candidates for CO 2 capture owing to their exceptional properties, such as a low volatility, designability, high CO 2 solubility and selectivity [5]. Applicability of ILs as CO 2 sorbents is very attractive for industrial processes, as compared with other operational sorbents, such as aqueous ethanol- amine solutions [5–24]. Moreover, amino acid and choline based ILs are nature-friendly. Amino acid based ILs with ammonium and phosphonium cations are the most effective ILs for CO 2 capture due to their fast reactivity towards CO 2 molecules and a low toxicity of ILs [7,16,25–27]. Recently, amino acid ILs with choline based cations have also been testified by many research groups as effective CO 2 sorbents in combination with ILs' low toxicity and biodegradability of both cations and anions [27]. Although highly efficient ILs have been developed for CO 2 capture, they have some drawbacks, such as their high viscosity and high costs as compared with molecular solvents [5]. Particularly, ILs' viscosity usu- ally is increasing significantly upon CO 2 absorption and thus the diffu- sivity of CO 2 molecules through the liquid phase is decreasing. This increase in viscosity of ILs' surface layers is adversely affecting the CO 2 absorption capacity of the whole bulk ILs. However, adding a small amount of water or organic solvents to ILs could greatly decrease viscos- ity and increase the CO 2 absorption ability of such prepared liquid mix- tures [5,11]. Impact of water on CO 2 capture by amino acid based ionic liquids in- cluding tetramethylammonium glycinate and tertaethylammonium prolinate has been studied by 1 H and 13 C liquid NMR [22]. It was found that CO 2 was initially captured in the form of a carbamate (i.e. co- valently bound CO 2 ) and CO 2 remaining in aqueous solutions of ILs stayed in the equilibrium mixture of carbonate and bicarbonate. Neutral amino acids have been detected by solution state 13 C NMR and X-ray diffraction [22]. Dupont and co-workers have recently reported that bi- carbonate anions were formed upon CO 2 capture by a number of amino acid based ionic liquids in the presence of water [23,24]. Recently we also have investigated the CO 2 capture by a series of ILs with different choline-based cations and amino acid based anions, mixed with water (50/50 wt%) [15]. The process of CO 2 absorption Journal of Molecular Liquids 286 (2019) 110918 ⁎ Corresponding author at: Chemistry of Interfaces, Luleå University of Technology, SE- 97187 Luleå, Sweden. E-mail addresses: andrei.filippov@ltu.se (A. Filippov), faiz.ullah@ltu.se (F.U. Shah). https://doi.org/10.1016/j.molliq.2019.110918 0167-7322/© 2019 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq