CO 2 Absorption by Solvents Consisting of TMG Protic Ionic Liquids and Ethylene Glycol: The Influence of Hydrogen Bonds

: Herein, the absorption of CO 2 by the TMG-based (TMG: 1,1,3,3-tetramethylguanidine) ionic liquids (ILs) and the absorbents formed by TMG ILs and ethylene glycol (EG) is studied. The TMG-based ILs used are formed by TMG and 4-fluorophenol (4-F-PhOH) or carvacrol (Car), and their viscosities are low at 25 ◦ C. The CO 2 uptake capacities of [TMGH][4-F-PhO] and [TMGH][Car] are low (~0.09 mol CO 2 /mol IL) at 25 ◦ C and 1.0 atm. However, the mixtures [TMGH][4-F-PhO]-EG and [TMGH][Car]-EG show much higher capacities (~1.0 mol CO 2 /mol IL) than those of parent ILs, which is unexpected because of the low CO 2 capacity of EG (0.01 mol CO 2 /mol EG) in the same conditions. NMR spectra and theoretical calculations are used to determine the reason for these unexpected absorption behaviors. The spectra and theoretical results show that the strong hydrogen bonds between the [TMGH] + cation and the phenolate anions make the used TMG-based ILs unreactive to CO 2 , resulting in the low CO 2 capacity. In the Ils-EG mixtures, the hydrogen bonds formed between EG and phenolate anions can weaken the [TMGH] + –anion hydrogen bond strength, so ILs-EG mixtures can react with CO 2 and present high CO 2 capacities.


Introduction
The increasing anthropogenic CO 2 emissions are regarded as the main root of global warming and climate change, which poses a great threat to the environment.Cutting CO 2 emissions has become an urgent issue today and has received substantial attention.A lot of technologies have been designed and applied to reduce CO 2 emissions [1,2].Among them, carbon capture technology plays a crucial role in curbing carbon emissions.Currently, alkanolamine-based aqueous solutions are widely used as CO 2 capture absorbents to absorb CO 2 in industrial gas streams, which can form carbamate species in the absorption process.Nonetheless, these alkanolamine-based methods suffer serious shortcomings, such as solvent degradation and their high energy cost to regenerate absorbents [3,4].Therefore, it is also important to develop effective and efficient CO 2 capture methods.
Ionic liquids (ILs) have gained considerable attention because of their fascinating properties, such as extremely low vapor pressure, low flammability, high ionic conductivity, and high thermal stability [5,6].ILs are usually obtained by combining organic cations like ammonium, imidazolium, and phosphonium; inorganic or organic anions like chloride; and dicyanamide hexafluorophosphate.The characteristics of ILs can be tuned by changing the structures of the component ions and the cation-anion combinations to meet specific applications [7,8], which make them promising alternatives to traditional organic solvents and volatile molecular solvents in many areas of applications.
ILs have also been used as absorbents to capture CO 2 and exhibit promising performance.Functionalized ILs which can chemically capture CO 2 have received a great deal of interest owing to their high CO 2 capture capacities [9][10][11].Amino-functionalized ILs, which have amino groups on cations or anions, could absorb CO 2 through the reaction between CO 2 and amino groups by forming carbamate species [12].Carboxylate-anion-functionalized ILs are also found to be reactive to CO 2 .For instance, the carboxylate-based IL 1-ethyl-3methylimidazolium acetate ([Emim][Ac]) could chemically capture CO 2 through the interactions between CO 2 and N-heterocyclic carbene intermediate [13].Moreover, azolide-based functionalized ILs containing aprotic heterocyclic anions (AHAs) [14][15][16], such as imidazolide, triazolide, 2-cyano-pyrrolide, and benzimidazolide, have also been developed to capture CO 2 .The basicity of azolide anions impacts both CO 2 capacities and absorption enthalpies.Functionalized ILs with phenolic anions and hydroxypyridine anions are also used to absorb CO 2 .CO 2 can react with the O atom of phenolic anions by forming carbonate species [17], while CO 2 reacts with both the O and N atoms of hydroxypyridine anions to form carbonate species and carbamate species [18], respectively.In addition, ILs with imide anions are also used for CO 2 capture, and the structures of imide anions greatly affect the CO 2 chemisorption behaviors of ILs [19].
Protic ILs (PILs) have been developed as well for CO 2 capture because the method for synthesizing them is relatively simple.They can be easily obtained just by mixing organic bases with organic/inorganic acids.The organic bases used to prepare PILs are usually superbases, such as 1,8-diazabicyclo- [5,4,
FTIR spectra were acquired on a Perkin-Elmer frontier spectrometer (PerkinElmer Corp., Waltham, MA, USA) equipped with an attenuated total reflection (ATR) accessory. 1H NMR (600 MHz) and 13 C NMR (151 MHz) spectra were obtained on a Bruker spectrometer (Bruker Biospin, Karlsruhe, Germany), using DMSO-d 6 as the internal reference.The viscosities of solvents were determined on an Anton Paar MCR92 (Anton Paar, Graz, Austria) viscometer at 25 • C. The melting points of ILs and IL-EG mixtures were measured by differential scanning calorimetry (DSC) using a Mettler Toledo DSC1 (Mettler Toledo, Greifensee, Switzerland) instrument at a heating rate of 10 • C/min under N 2 atmosphere.

Synthesis of ILs and IL-EG Mixtures
In a round-bottom flask (10 mL), 4-F-PhOH or Car was mixed with TMG at an equimolar ratio.Then, the solution was stirred for about 2 h at 40 • C to obtain ILs.
In a round-bottom flask (25 mL), the IL ([TMGH][4-F-PhO] or [TMGH][Car]) was mixed with EG at desired molar ratios, and the IL-EG absorbents were obtained by stirring the mixtures for about 2 h at room temperature.

Absorption of CO 2
The procedures of CO 2 absorption can be found in our previous work [27].

Computational Methodology
All calculations were carried out using the Gaussian 09 suite of programs (version E.01) [32].The geometries of all studied complexes were optimized using the M06-2x/augcc-pVDZ theoretical level [33,34], with frequency calculations conducted at the same level.Furthermore, the atoms-in-molecules (AIM) analysis [35] and atomic-dipole-momentcorrected Hirshfeld (ADCH) charge [36] calculations were performed using the Multiwfn program (version 3.8) [37], which were then visualized using the VMD package [38].-4) ppm can be found after capture in the 13 C NMR spectra (Figure 2b).The above-mentioned new peaks suggest that CO 2 reacts with EG in the solvent, forming EG-derived carbonate [39,40].The peak at 157.3 (C-4) ppm is attributed to the carbonyl carbon of the carbonate species produced from the reaction between CO 2 and EG [41].For the [TMGH][Car]:EG (1:3) system, similar new hydrogen and carbon peaks can be detected as well in the NMR spectra after capture (Figure S3).2a), and three new carbon sig at 61.1 (C-2), 66.0 (C-3), and 157.3 (C-4) ppm can be found after capture in the 13 C N spectra (Figure 2b).The above-mentioned new peaks suggest that CO2 reacts with E the solvent, forming EG-derived carbonate [39,40].The peak at 157.3 (C-4) ppm is tributed to the carbonyl carbon of the carbonate species produced from the reaction tween CO2 and EG [41].For the [TMGH][Car]:EG (1:3) system, similar new hydrogen carbon peaks can be detected as well in the NMR spectra after capture (Figure S3).The interactions between solvents and CO 2 are also disclosed using FTIR spectra.As shown in Figure 4a, there are two new peaks located at 1634 and 1289 cm −1 after CO 2 uptake for the [TMGH][4-F-PhO]:EG (1:3) absorbent.The band at 1634 cm −1 is assigned to the C=O stretching mode of OCOO − , and the band centered at 1289 cm −1 can be due to the O-C-O stretching mode [39].The FTIR results suggest the formation of carbonate species after absorption.Similar bands can be found as well in the FTIR spectra of  The interactions between solvents and CO2 are also disclosed using FTIR spectra.As shown in Figure 4a, there are two new peaks located at 1634 and 1289 cm −1 after CO2 uptake for the [TMGH][4-F-PhO]:EG (1:3) absorbent.The band at 1634 cm −1 is assigned to the C=O stretching mode of OCOO − , and the band centered at 1289 cm −1 can be due to the O-C-O stretching mode [39].The FTIR results suggest the formation of carbonate species after absorption.Similar bands can be found as well in the FTIR spectra of    The interactions between solvents and CO2 are also disclosed using FTIR spectra.As shown in Figure 4a, there are two new peaks located at 1634 and 1289 cm −1 after CO2 uptake for the [TMGH][4-F-PhO]:EG (1:3) absorbent.The band at 1634 cm −1 is assigned to the C=O stretching mode of OCOO − , and the band centered at 1289 cm −1 can be due to the O-C-O stretching mode [39].The FTIR results suggest the formation of carbonate species after absorption.Similar bands can be found as well in the FTIR spectra of system, no new bands can be observed as well after capture (Figure S5b).Considering the above-mentioned findings, the possible reason for the unusual CO 2 capture behaviors can be obtained.The pK a values of EG, [TMGH] + , and 4-F-PhOH are 15.1 [42], 13.0 [43], and 9.9 [44], respectively.[TMGH] + is therefore a stronger acid and a stronger hydrogen bond donor in comparison to EG.The strength of ionic hydrogen bonds (IHBs) can be estimated using ∆pK a , which is the difference between the pK a values of hydrogen bond donors and the conjugated acid of bond acceptors.The hydrogen bond strength will increase with decreasing ∆pK a values [45,46].Therefore, the hydrogen bond strength between [TMGH] + and [4-F-PhO] − (∆pK a = 3.1) is stronger than that between EG and the anion Considering the above-mentioned findings, the possible reason for the unusual CO2 capture behaviors can be obtained.The pKa values of EG, [TMGH] + , and 4-F-PhOH are 15.1 [42], 13.0 [43], and 9.9 [44], respectively.[TMGH] + is therefore a stronger acid and a stronger hydrogen bond donor in comparison to EG.The strength of ionic hydrogen bonds (IHBs) can be estimated using ΔpKa, which is the difference between the pKa values of hydrogen bond donors and the conjugated acid of bond acceptors.The hydrogen bond strength will increase with decreasing ΔpKa values [45,46].Therefore, the hydrogen bond strength between   Given that the dominant impetus behind the formation of attractive forces between these systems arises from their electrostatic interactions, this approach offers profound insights.Consequently, we mapped the ESP of the cation, anion, and EG on the van der Waals surface, as shown in Figure 6.

Results and Discussion
bonds between EG and [TMGH][4-F-PhO].The electrostatic potential (ESP) profiles of the anion [4-F-PhO] − , the cation [TMGH] + , and EG molecules upon their molecular surfaces are computed and depicted.Given that the dominant impetus behind the formation of attractive forces between these systems arises from their electrostatic interactions, this approach offers profound insights.Consequently, we mapped the ESP of the cation, anion, and EG on the van der Waals surface, as shown in Figure 6.In the case of the anion [4-F-PhO] − , the region of most negativity (−13.31kcal/mol) manifests itself proximal to the oxygen atom, which can attributed to the presence of lone pair electrons.For the cation [TMGH] + , the regions of highest positivity are situated at the periphery of the hydrogen atoms within the amino group.This observation indicates a propensity for the establishment of robust hydrogen bonds, potentially involving the N-H covalent bond.As for the EG molecule, the oxygen atoms of hydroxyl groups exhibit the most substantial ESP values (−2.97 kcal/mol) due to the existence of lone pair electrons.Consequently, these O atoms are appropriate candidates for hydrogen bond acceptors.Conversely, the hydrogen atoms of hydroxyl groups show the most positive ESP value (+4.28 kcal/mol), rendering them appropriate candidates for hydrogen bond donors.
The atoms-in-molecules (AIM) analysis employed to study hydrogen bond interactions.The AIM analysis, which relies on the identification of critical points (CPs) situated between neighboring atoms, effectively discerns and confirms the presence of the crucial interactions.Notably, an AIM analysis of all hydrogen bonds in the investigated systems results in the identification of CPs between the atoms involved, as illustrated in Figure 7 and Table 1.The corresponding hydrogen bond energies (EHB) can be obtained from the potential energy density (V) at the CPs [47].In the case of the anion [4-F-PhO] − , the region of most negativity (−13.31kcal/mol) manifests itself proximal to the oxygen atom, which can attributed to the presence of lone pair electrons.For the cation [TMGH] + , the regions of highest positivity are situated at the periphery of the hydrogen atoms within the amino group.This observation indicates a propensity for the establishment of robust hydrogen bonds, potentially involving the N-H covalent bond.As for the EG molecule, the oxygen atoms of hydroxyl groups exhibit the most substantial ESP values (−2.97 kcal/mol) due to the existence of lone pair electrons.Consequently, these O atoms are appropriate candidates for hydrogen bond acceptors.Conversely, the hydrogen atoms of hydroxyl groups show the most positive ESP value (+4.28 kcal/mol), rendering them appropriate candidates for hydrogen bond donors.
The atoms-in-molecules (AIM) analysis is employed to study hydrogen bond interactions.The AIM analysis, which relies on the identification of critical points (CPs) situated between neighboring atoms, effectively discerns and confirms the presence of the crucial interactions.Notably, an AIM analysis of all hydrogen bonds in the investigated systems results in the identification of CPs between the atoms involved, as illustrated in Figure 7 and Table 1.The corresponding hydrogen bond energies (E HB ) can be obtained from the potential energy density (V) at the CPs [47].[39], suggesting that the solvents formed by EG and protic ionic liquid [TMGH][4-F-PhO] can be classified as deep eutectic solvents [28,39].

In summary, TMG-based ILs ([TMGH][4-F-PhO] and [TMGH][Car]
) and Ils-EG mixtures are studied for CO 2 capture.The NMR and FTIR results indicate that the TMG-based ILs studied in this work are chemically inert to CO 2 .Nevertheless, after mixing ILs with EG, the formed ILs-EG mixtures can chemically capture CO 2 , thus resulting in the higher CO 2 capacities of ILs-EG absorbents than those of ILs.The NMR and theoretical calculation studies provide insights into the CO 2 absorption behaviors, and the hydrogen bonds play an important role in tuning the CO 2 absorption process.The strong hydrogen bonds between [TMGH] + and phenolate anions are responsible for the ILs' nonreactivity to CO 2 .The hydrogen bonds from EG weaken the bond between [TMGH] + and phenolate anions, which causes ILs-EG solvents to be able to chemically capture CO 2 .
For [TMGH][4-F-PhO] IL, the N-H•••O and C-H•••O hydrogen bonds are formed between [TMGH] + and [4-F-PhO] − .The hydrogen bond energy of the N-H•••O hydrogen bond (−21.0 kcal/mol) is far larger (absolute value) than that of the C-H•••O bond (−3.3 kcal/mol), indicating that the hydrogen bond between the amine proton on the cation and the O atom on the anion is much stronger than that between the methyl proton on the cation and the O atom on the anion in [TMGH][4-F-PhO] [48].As expected, the N-H•••O bond length (1.576Å) is much shorter than that of the C-H•••O bond (2.385 Å).The strong N-H•••O hydrogen bond may make [TMGH][4-F-PhO] unreactive to CO2.

Table 1 .
The geometric and energetic data for the hydrogen bonds studied.

Table 1 .
The geometric and energetic data for the hydrogen bonds studied.critical points of hydrogen bonds; b bond length in Å; c bond angles in degrees; d bond energies in kcal/mol. a