Intermolecular interactions upon carbon dioxide capture in deep-eutectic solvents

Herein we report the CO2 uptake in potential deep-eutectic solvents (DESs) formed between hydrogen bond acceptors (HBAs) such as monoethanolammonium chloride ([MEA Cl]), 1-methylimidazolium chloride ([HMIM Cl]) and tetra-n-butylammonium bromide ([TBAB]) and hydrogen bond donors (HBDs) like ethylenediamine ([EDA]), diethylenetriamine ([DETA]), tetraethylenepentamine ([TEPA]), pentaethylenehexamine ([PEHA]), 3-amino-1-propanol ([AP]) and aminomethoxypropanol ([AMP]) and analyzed the outcome in terms of the specific polarity parameters. Among various combinations of HBAs and HBDs, [MEA Cl][EDA]-, [MEA Cl][AP]-, [HMIM Cl][EDA]and [HMIM Cl][AP] showed excellent CO2 uptake which was further improved upon increasing the mole ratio of HBA : HBD from 1 : 1 to 1 : 4. The lowest CO2 uptake in [MEA Cl][PEHA] (12.7 wt%) and [HMIM Cl][PEHA] (8.4 wt%) despite the highest basicity of [PEHA] infers that the basicity is not the sole criteria for guiding the CO2 uptake but, in reality, CO2 capture in a DES relies on the interplay of H-bonding interactions between each HBA and HBD. The role of HBAs in guiding CO2 uptake was more prominent with weak HBDs such as [TEPA] and [PEHA]. The speciation of absorbed CO2 into carbamate, carbonate, and bicarbonate was favorable in DES characterized by comparable hydrogen bond donor acidity (a) and hydrogen bond acceptor basicity (b) values, whereas the conversion of carbamate to carbonate/bicarbonate was observed to depend on a. The addition of water in DES resulted in lower CO2 uptake due to the decreased basicity (b).


Introduction
Carbon dioxide (CO 2 ) emissions due to anthropogenic activities have been constantly increasing over the past century and gaining worldwide attention due to the deleterious action of this notorious greenhouse gas. Subsequently, to develop materials that selectively, efficiently and inexpensively capture CO 2 is of great importance. [1][2][3] Among various methods proposed to reduce anthropogenic CO 2 emissions, a carbon dioxide capture and sequestration (CCS) process that incorporates both physical and chemical adsorptions and absorption rely heavily on the concept of using alkanolamine-based solvents. Typical solutions involved include monoethanolamine (MEA) and methyldiethylamine (MDEA) and their aqueous solutions. 4 Despite their wide acceptability, several disadvantages linger with MEA-and MDEA-based technologies such as their volatility, high cost, substantial energy consumption and corrosiveness. These shortcomings pose a potential threat to human health and critical mutilations to the environment. 5 As a result, the design of alternative materials for CO 2 capture is of high importance and has garnered much attention in the last decade. 6 Among the many possibilities, ionic liquids (ILs) have been extensively investigated as promising candidates because of their several advantageous properties to substitute the traditional scrubbing agents for reversible CO 2 capture. 7 In this regard, a variety of ILs containing pyridinium, imidazolium, guanidinium, amine-functionalized cation and inorganic/organic anion have been examined and the basicity of the anion has been identified to play a major role upon CO 2 capture 8-14 following semimolar or multi-molar mechanism. 15,16 However, the multi-molar CO 2 absorption in ILs became less attractive when converted to the gravimetric uptake values. 13,16,17 For example, dual aminofunctionalized cation-tethered IL, capable of absorbing an equimolar amount of CO 2 exhibited only 18.5 wt% capacity on the gravimetric scale. 18 Moreover, high viscosities, unfavorable energy consumption and the high cost of starting materials still hint that ILs are further inapt for large scale applications. 19,20 In quest of the sustainable solvents, deep-eutectic solvents (DESs) has been extensively explored in the last decade. DESs are eutectic mixtures of Lewis or Brønsted acids also called as HBA and bases/HBDs and can contain a variety of anionic and/or cationic species and therefore exhibit properties similar to ILs. 21 DES can be prepared by a single-step atom-economic reaction under solvent-free conditions and does not require further purification thereby simplifying the scale up for various processes. 22 The hydrogen bonding strength via charge delocalization between the HBA and HBD component is responsible for the relative difference in the melting point of the eutectic mixture and thus, a variety of potential DES can be easily prepared by varying the strength and molar ratios of the HBD and HBA substituents. 23,24 DES's efficacy in gas absorption was recognized after the pioneering work of Zhu et al., who employed choline chloride (ChCl) + urea as a heterogeneous sorbent and catalyst upon chemical fixation of CO 2 to cyclic carbonates. 25 Li et al. measured the CO 2 solubility at various temperatures and pressures in ChCl + urea-based DESs, at different mole ratios. 26 Later, different combinations of HBD (urea, lactic acid, glycerol etc.) and HBA (tetraalkylammonium/phosphonium halides, choline chloride etc.) components were tried to optimize the CO 2 uptake capacity in DESs but the molar capability of all the tested DESs remained in the range of 0.02-0.14 mole per mole of DES with the highest value equivalent to 7 wt%. [27][28][29][30] Mirza et al. measured CO 2 solubility in three DESs reline, ethaline and malinine in the temperature range 309-329 K and at pressure up to 160 kPa. 31 The modified Peng-Robinson equation of state was used to correlate the experimental data. The thermodynamic parameters derived from the equation suggested CO 2 absorption as a nonspontaneous exothermic process. 31 32 The large CO 2 uptake in [MEAÁCl][EDA]-based DESs was assumed to depend on the change in polarity and basicity, although a direct correlation between the actual polarity parameters and CO 2 uptake was absent. 32 To the best of our knowledge, so far no report discussing the role of intermolecular interactions on CO 2 capture and speciation of absorbed CO 2 in ILs and DESs can be found in open literature. The solvatochromic polarity parameters include electronic transition energy (E T (30)), hydrogen bond donor acidity (a), hydrogen bond acceptor basicity (b) and, dipolarity/polarizability (p*). [33][34][35][36] The E T (30) measures various possible directional and nondirectional solute-solvent interactions between Reichardt's dye (30) and solvent molecules and can be determined by using the absorption maxima of Reichardt's dye 30. The dipolarity/ polarizability (p*) parameter denotes the electrolytic strength of the DES and relies on the interaction of probe molecule with its cybotactic environment and can be determined from the spectroscopic shift of N,N-diethyl-4-nitroaniline. The hydrogen bond acceptor tendency (b) exhibits the basicity of the hydrogen bond donor (HBD) component of DES and can be determined by using the spectroscopic shift of 4-nitroaniline with respect to N,N-diethyl-4-nitroaniline. The hydrogen bond donor acidity (a) denotes the donating ability of the hydrogen bond acceptor (HBA) component of DES and can be obtained by employing E T (30) and p*. The empirical equations to obtain various polarity parameters are given in Table 2. These polarity parameters (E T (30), a, b and p*) can be employed as a tool for accounting the influence of intermolecular solute-solvent interactions upon various physical and chemical changes. 37 Recently, Shukla and Kumar have used solvatochromic polarity parameters during the Hammett acidity (H 0 ) measurements to discuss the role of intermolecular interactions on the dissociation of aqueous carboxylic acids in protic ionic liquids. 38 The acidity-polarity correlation was further supported by thermodynamic parameters (DH 0 and DS 0 ). 38 Liu et al. discussed the role of intermolecular interaction upon CO 2 capture (0.7551 mol CO 2 per mol DES) in the 1,3-butanediol + 1,3-ethanediamine system in terms of excess molar volume (V E M ), viscosity deviations (DZ) and apparent molar volumes (V j,1 and V j,2 ), from density and viscosity values measured at different temperatures and compositions. 39 Contrary to the excess properties, polarity parameters have the advantage in terms of predicting the nature and specificity of the interactions.
In the present work, we have attempted to investigate the role of intermolecular interactions on CO 2 capture and its speciation into the carbamate, carbonate, and bicarbonate in potential DESs. For this purpose, the gravimetric CO 2 uptakes in these DESs are analyzed in terms of the E T (30), a, b, and p* values. The adverse effect of viscosity on CO 2 absorption is also taken into account while correlating the polarity parameters with CO 2 wt%. The structure of HBD and HBA components of DESs are shown in Table 1. The structure of different solvatochromic dyes involved in the polarity determinations is shown in Fig. 1.

Determination of polarity parameters
All solvatochromic dyes were dissolved in methanol to prepare a stock solution (10 À2 M) prior to the polarity measurements. The stock solution was taken in a glass vial first and methanol was removed by blowing nitrogen gas into the vial followed by the addition of 1 ml of DESs. Consequently, the resultant solution was transferred into a 1.3 ml quartz cuvette under nitrogen atmosphere and sealed with a septum. The wavelength of maximum absorption (l max ) was recorded at room temperature using a UV-visible spectrophotometer. Different polarity parameters were derived by employing the empirical equations given in Table 2.

Viscosity measurement
The viscosities (Z) of different DES systems were measured at room temperature by using a Brookfield rotating viscometer (RVDV1) with a cone and plate arrangement. The viscosity (Z) values of DESs were obtained according to the given equation: where, TK (0.09373), RPM and SMC (0.327) are the viscometer torque constant, speed and spindle multiplier constant, respectively.

CO 2 absorption experiment
In the present study, CO 2 absorption measurements were carried out by bubbling CO 2 gas, with a flow rate of 50 ml min À1 into a vial containing B3 g of DESs. The vial was weighed at regular intervals to record the differential weight of absorbed CO 2 and, consequently, the weight percent of absorbed CO 2 in the respective DESs. The electronic balance used for weighing had an accuracy of AE0.1 mg. Desorption experiments were also performed under inert atmosphere by weighing the vial at regular time intervals. The water content in DESs as measured by the Karl-Fisher Coulometer was 430 ppm.

Speciation of absorbed CO 2 in carbamate, carbonate and bicarbonate
The 13 C NMR spectroscopy was employed for the speciation of absorbed CO 2 in DESs. Speciation of CO 2 was based on the calibration of the 13 C NMR spectra of CO 2 -treated samples with those of the reference spectrum of K 2 CO 3 and KHCO 3 , as shown by Mani et al. 40 The 13 C NMR spectrum of 1 M K 2 CO 3 and KHCO 3 and the weighted mixture of two salts were prepared in D 2 O. The speciation results of absorbed CO 2 into carbamate, carbonate and bicarbonate were obtained based on the linear relation between the change of chemical shifts in K 2 CO 3 , KHCO 3 and their mixtures against the molar compositions as shown in Fig. S1 (ESI †).      (Fig. 2(a)). The lower increment in CO 2  [EDA]class of DESs exhibited lower CO 2 uptake during optimization from 1 : 1 to 1 : 4 with identical HBD component ( Fig. 2(b) and therefore coordinates strongly with [EDA] at all mole ratios and brings early dissipation upon CO 2 uptake. In [MEAÁCl][AP]based DESs CO 2 uptake was found to increase with the mole ratio of [AP] from 1 : 1 to 1 : 2 but decreased beyond 1 : 2 (Fig. 2(c)). Surprisingly, in case of the [HMIMÁCl][AP]-based DESs, a significant improvement in the CO 2 uptake was observed upon increasing [AP] at all mole ratios (Fig. 2(d) in CO 2 uptake because of the increased basicity.
The TBAB-based DESs demonstrated a significant improvement in the CO 2 uptake upon optimization (Fig. S2(a), ESI †). Surprisingly, the CO 2 absorption kinetics in case of [TBAB][AMP], at a molar ratio of 1 : 3, was faster than that at 1 : 4 in the initial phase but improved later and went through a crossover point (Fig. S2(b) In-depth analysis of the CO 2 uptake values in DESs in terms of the change in intermolecular interactions is presented by employing the E T (30) and Kamlet-Taft parameters (a, b and p*).

For [MEAÁCl]
[EDA]-based DESs an inverse relation (r 2 = 0.99325) between E T (30) values and CO 2 wt% was obtained ( Fig. 3(a)) suggesting involvement of non-polar interactions during CO 2 capture. The polarity index value p*, which reflects the electrolytic strength of a medium, was found decreasing during optimization in [MEAÁCl][EDA] (Fig. 3(b)). The polarity (E T (30)) of a medium depends on the relative magnitude of a and b. In general, the polarity of a medium decreases with the increasing value of b over a and vice versa. 41    [AP]-based DESs showed high CO 2 uptake capacity which indicates that the system acidity (a) plays a crucial role in CO 2 capture. In [AP], the -OH group at a-position to the -NH 2 seems stabilizing CO 2 at the carbamic acid state and, thus facilitate CO 2 absorption in DESs. A pictorial representation of the involvement of intermolecular interactions in the CO 2 uptake is shown as Scheme 2. Similar behavior was observed in ILs where an acidic group (-OH, -COOH) in the close vicinity of the amine is attributed for the multi-molar CO 2 absorption. 42  Based on the above correlation it can be inferred that the CO 2 uptake in DESs does not solely depend on the basicity of the HBD component but principally on the strength of the intermolecular H-bonding interactions between the HBD and HBA components as suggested by the equilibrium between a and b values. Higher CO 2 uptake in all classes of DESs is also favored by a drop in the viscosity (Z) during optimization. At lower viscosity, the CO 2 molecules diffuse faster in the DES continuum and therefore, improves the probability of chemisorption. It is noted that a large difference in the a and b values  -based DESs might arise due to the higher viscosity of former than later. The high viscosity retards the diffusion of CO 2 molecules towards the active sites and, consequently, impede the rapid CO 2 uptake (Fig. S3, ESI †).

Effect of HBA on CO 2 capture
Similar to HBDs, the acidity of the HBA components also affects the CO 2 uptake significantly. It is evident from Fig. 5 44,45 In general, if a has an edge over b, it gives acidic or polar characteristic to the active site, whereas in reverse scenario a stable chemical environment is formed for the CO 2 capture due to the abundance of the apolar sites in the medium. Therefore, smaller differences in the a-b value indicate the prevalence of stable interacting sites in DES for CO 2 stabilization. This view is further supported by the viscosity data as [MEAÁCl]-based DESs have higher CO 2 uptake than [HMIMÁCl]-class of DESs despite their higher viscosities. Thus, the most proficient DES possess nearly similar a and b values. The equilibration between donor and acceptor sites lowers polarity and thereby increase non-directional forces in the CO 2 capture.

Speciation of absorbed CO 2 in DESs
CO 2 absorption in different DESs was also monitored by recording the FT-IR and 1 H-and 13 C-NMR spectra before and after the experiments. FTIR spectra of DESs showed a broad peak at B2900 cm À1 due to N-H-mediated H-bonding (Fig. S4, ESI †). The presence of carbamate species was revealed by the absorption bands at B1559, 1293 and 851 cm À1 , respectively, owing to the asymmetric, symmetric and bending vibrations. In [AP]-based DESs, the additional absorption band at B1350-1500 cm À1 indicates the presence of HCO 3 À /CO 3

2À
. The enhanced peaks at B1098 and 1190 cm À1 correspond to the stretching modes of C-N and C-O groups, respectively, and fortify CO 2 absorption. CO 2 uptake in solvent results in carbamate (NH 2 COO À ), carbonate (CO 3 2À ) and bicarbonate (HCO 3 À ) depending on the strength of basicity and acidity. Under the acidic condition, NH 2 COO À hydrolyzes to CO 3 2À /HCO 3 À . 46,47 The 1 H NMR spectrum of CO 2 treated DESs show carbamate peak at 3.13 ppm, along with the downfield shifting of other peaks due to the conversion of -NH 2 to -NH (Fig. 6). On 13 C NMR scale, NH 2 COO À appears at 4164 ppm whereas CO 3 2À /HCO 3 À resonate below 162 ppm as exhibited in Fig. 7. The fast equilibration of a proton between HCO 3 À and CO 3 2À results in a single peak in 13 Fig. 9(a). The speciation outcome of CO 2 in different DESs are enlisted in Table 4.
The molar uptake plot ( Fig. 9(b)) suggest semimolar mechanism, where two moles of DES interacts one mole of CO 2 (2 : 1), was operative in all DESs as shown in Table 3. The semimolar CO 2 uptake in DESs can be explained by the steps shown in Scheme 3.

Effect of water
With the aim of simulating CO 2 uptake under more realistic conditions, uptake kinetics was measured by mixing water in DESs at 1 : 4. The inferior uptake kinetics was noted with the increasing wt% of water in DES. These observations are in contradiction to the previous reports where improved CO 2 uptake is reported due to the reduced intermolecular interactions in the presence of water. 32 In contrast to the 39 wt% of CO 2 by pure [MEAÁCl][EDA] at 1 : 4, 35.7 wt%, 35 wt% and 34.4 wt% of CO 2 was obtained in [MEAÁCl][EDA] at 1 : 4 containing 10%, 20% and 30% of water, respectively ( Fig. 10(a)). This deterioration in CO 2 uptake entails that water competes with CO 2 molecule for the active sites in DES. This contest increases

Recyclability experiment
The feasibility of recycling DESs was monitored by performing the sequential absorption-desorption cycles with [MEAÁCl][EDA] at 1 : 4 ( Fig. 10(b)). A 30 wt% of ethylene glycol in [MEAÁCl][EDA] is taken as a reference solution for recyclability test and the cut-off limit was set around 18 wt%. CO 2 desorption in each step was achieved by heating the solution at 70 1C for 2 h under inert environment until complete desorption. 49,50 Contrary to the report where loss in mass of task-specific DES was noted, the initial mass of solvent was preserved after each desorption cycle in our study. 30 The stability of the [MEAÁCl][EDA] post consecutive absorption-desorption cycles was verified by 13 C NMR analysis (Fig. S8, ESI †). The recycling experiments are consistent with the other studies performed with ILs and DESs. 51

Conclusions
In conclusion, CO 2 uptake in potential DESs is observed to depend on the nature of HBD substituents and their mole ratio relative to the HBA. Both these changes reduce polarity and thereby affect the strength of intermolecular interactions and consequently regulate CO 2 uptake. In the light of Kamlet-Taft parameters, it is observed that the CO 2 uptake capacity of DESs does not depend solely on the basicity of the HBD but is rather controlled by the strength of the intermolecular interactions between the components. The strong H-bonds in DESs result in high viscosity because of the large difference between a and b value of constituents. In the most effective DESs, HBA and HBD were found least associated by the H-bonding as suggested by the equilibrating a and b values.

Conflicts of interest
There are no conflicts to declare.