Efficient CO₂ capture by tertiary amine-functionalized ionic liquids through Li⁺-stabilized zwitterionic adduct formation

Introduction

Carbon capture and sequestration (CCS) from flue gas formed by combustion of fossil fuel is a critical part of efforts directed towards the stabilization of atmospheric greenhouse gas levels [1]. In recent years, there has been intense research worldwide aimed at the development of various processes and technologies for efficient CO₂ capture. These efforts include the development of liquid and solid absorbents and membranes [2-7].

Ionic liquids (ILs), which have attractive properties such as negligible vapor pressure, a wide liquid temperature ranges, good thermal stability, high ionic conductivity, and versatile solvation properties [8-11], can be designed for task-specific applications through the smart choice of the respective cations and/or anions. Application fields include green solvents for synthesis [9,12-15], efficient catalysts in organic synthesis [2,16,17], media for advanced separation [18,19], novel electrolytes for energy applications [20,21], and efficient absorbents for gas separation [2,22-24]. In particular, amino-functionalized IL [APBIm][BF₄] (1-aminopropyl-3-butylimidazolium tetrafluoroborate) and ILs being composed of amino acid (AA) anions and phosphonium or ammonium cations were developed for efficient CO₂ chemisorption [23,25-30]. Binary absorbents derived from superbases together with various non-volatile weak proton donors such as hydroxy-functionalized ILs, imidazolium ILs, fluorinated alcohol, imidazole and phenol, were also found to be efficient liquid absorbents allowing for reversible CO₂ chemisorption [31-35]. In general, two absorbent molecules are involved to react with one CO₂ molecule generating ammonium carbamate (Scheme 1a) or ammonium alkyl formate (Scheme 1b). Hence, increasing the 1:2 (CO₂:absorbent molecule) stoichiometry for the CO₂ capacity to 1:1 is an essential prerequisite for a breakthrough in absorption techniques [23]. In this respect, task-specifically designed absorbents have been successfully synthesized from AAs and applied for 1:1 CO₂ capture through a carbamic acid formation pathway (Scheme 1a, step 1). Notably, equimolar CO₂ absorption was obtained using task-specific ionic liquids (TSILs) with the phosphonium cation containing long alkyl chains and anions derived from AAs (prolinate and methioninate) [36], or AA salts with bulky N-substituents in polyethylene glycol (PEG) solution [37]. However, procedures for the preparation of ILs usually include complicated purification procedures or the use of volatile organic solvents (e.g., toluene, acetonitrile). Recently, Wang et al. developed novel alkanolamine-based ILs through multi-dentate cation coordination between alkanolamine and Li⁺ for reversible CO₂ capture, by simple mixing of equimolar amounts of alkanolamines with LiNTf₂ [38]. The strong complexation of alkali metal cations by crown ethers could be used to achieve equimolar CO₂ absorption in systems containing crown ethers and easily available alkali metal salts of amino acids, resulting in the respective carbamates (onium salts) [39].

As previously reported, strong amidine and guanidine bases such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) can form the base-CO₂ zwitterionic adduct in a 1:1 manner under strictly anhydrous conditions (Scheme 1c) [40,41]. Herein, we present such a method combining the formation of ILs and coordination to achieve equimolar CO₂ capture through zwitterionic adduct formation. The essence of our strategy is to make use of the multisite coordination interaction between Li⁺ and organic bases or PEG-functionalized organic bases. The readily prepared ILs were reacted then with CO₂ to form intramolecular zwitterionic adducts.

Results and Discussion

Taking PEG₁₅₀MeTMG as a neutral ligand (Table 1), coordination effects of various lithium salts were investigated (Table S1, Supporting Information File 1). Only LiSO₃CF₃ and LiNTf₂ formed complexes with PEG₁₅₀MeTMG through multisite coordination. Subsequently, different neutral ligands with alkyl chains or PEG chain were selected to evaluate the effect of chelating with LiNTf₂ on the physicochemical properties of the resulting ILs as well as the CO₂ capacities (Table 1).

Typical optimized structures of cations derived from chelation between neutral ligands and Li⁺ are shown in Figure 1. The geometry optimizations were carried out by performing DFT calculations. Four O atoms in PEG₁₅₀Me chelate with Li⁺ in a quasi-crown ether manner. For neutral ligands with PEG chain (PEG₁₅₀MeIm, PEG₁₅₀MeNH₂, PEG₁₅₀MeTMG and PEG₁₅₀MeBu₂N), all the O and N atoms coordinate with Li⁺ in a quasi-aza-crown ether fashion. OctIm, OctTMG, TMG, DBU and DBN generate complexes whereby Li⁺ is bound only to N atoms. In contrast, the coordination ability of the N atom in OctBu₂N is not strong enough to form a homogeneous chelated IL with LiNTf₂.

Calculation of the energy of the gas phase reaction between neutral ligands and Li⁺ gave a value for the enthalpy change in the range of −41.59 to −106.56 kcal mol⁻¹ (Table 1, entries 1–11), indicating that the formation of chelated ILs is feasible.

In addition, PEG-functionalization of the organic base enhanced the complexation ability. For example, ΔH_f decreased from −56.49 kal mol⁻¹ (OctIm) to −91.17 kcal mol⁻¹ (PEG₁₅₀MeIm) for imidazole (Table 1, entry 2 vs 3), from −47.79 kcal mol⁻¹ (OctTMG) to −106.56 kcal mol⁻¹ (PEG₁₅₀MeTMG) for guanidine (Table 1, entry 6 vs 7), and from no complexation (OctBu₂N) to −96.26 kal mol⁻¹ (PEG₁₅₀MeBu₂N) for tertiary amine (Table 1, entry 8 vs 9).

The structures of the chelated ILs were confirmed by thermogravimetric analysis (TGA), NMR (see Supporting Information File 1, Figure S2), in situ FTIR under CO₂ pressure and mass spectrometry. The thermal stability of the chelated ILs was strongly increased compared to the corresponding neutral ligands. The decomposition temperature increased by 90 °C and 60 °C for PEG₁₅₀MeTMG and PEG₁₅₀MeBu₂N, respectively, after coordination with LiNTf₂ (Figure 2a).

In the ¹H NMR spectrum, the proton signals of the four methyl groups in guanidine of PEG₁₅₀MeTMG shifted from 2.63–2.72 ppm to 2.96 ppm after reacting with equimolar amounts of LiNTf₂ (Figure 2b). The corresponding anion CF₃SO₃⁻, the twelve protons moving only from 2.63–2.72 ppm to 2.71–2.75 ppm, indicate that there is only little influence of the anion on the nature of the coordinative bond. In the case of tertiary amines (e.g., PEG₁₅₀MeBu₂N), all the protons shifted downfield after forming the chelated IL with LiNTf₂. Changing the anion to CF₃SO₃⁻ had negligible influence on the coordination ability. All the neutral ligands shown in Table 1 gave a downfield shift in the ¹H NMR spectrum after chelating with Li⁺, in accordance with the electron density decreasing through coordination. Furthermore, the formation of chelated ILs was also verified by ESI-MS (m/z = 268.27 for [PEG₁₅₀MeTMGLi]⁺ and m/z = 282.32 for [PEG₁₅₀MeBu₂NLi]⁺) (Figure S3, see Supporting Information File 1). In the in situ FTIR spectrum, the C=N absorption band of the neutral ligand PEG₁₅₀MeTMG was shifted from 1616 cm⁻¹ to 1592 cm⁻¹ after reacting with equimolar amounts of LiNTf₂ as depicted in Figure 3a.

The effect of various chelated ILs on CO₂ absorption was subsequently examined using LiNTf₂ as coordinating reagent. The CO₂ absorption capacity, defined as mol of CO₂ captured per mol of IL, was estimated from the weight increase of the reaction mixture. As shown in Table 1, ILs of weak basicity, such as [PEG₁₅₀MeLi][NTf₂], [OctImLi][NTf₂] and [PEG₁₅₀MeImLi][NTf₂] showed a poor CO₂ sorption capacity, implying that only physical interactions with CO₂ were present (Table 1, entries 1–3). The primary amine-functionalized IL [PEG₁₅₀MeNH₂Li][NTf₂] gave rise to CO₂ uptake approaching 1:2 stoichiometry (Table 1, entry 4) as expected from the proposed mechanism for the formation of ammonium carbamate as shown in Scheme 1a [38]. Notably, the CO₂ capacity of guanidine-functionalized ILs increased in the order of [TMGLi][NTf₂] (0.65) < [OctTMGLi][NTf₂] (0.80) < [PEG₁₅₀MeTMGLi][NTf₂] (0.89 mol CO₂ absorbed per mol of base) (Table 1, entries 5–7), indicating that the CO₂-philic nature of the PEG chain facilitates CO₂ sorption. Generally, anhydrous tertiary amines absorb CO₂ only under high CO₂ pressures to form instable zwitterionic alkylcarbonate salts (Table 1, entry 9) [42]. [PEG₁₅₀MeBu₂NLi][NTf₂] and [PEG₁₅₀MeBu₂NLi][SO₃CF₃] were able to rapidly reach 0.66 and 0.61 CO₂ capacity, respectively. Thus, tertiary amino-functionalized ILs with multidentate cation coordination have a much better performance probably due to the formation of a zwitterionic adduct being stabilized by Li⁺ (Table 1, entries 10 and 11). Indeed, the CO₂ absorption capacity of [PEG₁₅₀MeBu₂NLi][NTf₂] increased steadily from 0.10 to 0.66 mol CO₂ absorbed per mol of base when the molar ratio of LiNTf₂/PEG₁₅₀MeBu₂N was varied from 0 to 1. When the molar ratio was increased to 1.5, no further promotion of the CO₂ capacity was observed (Figure 4). In contrast, when PEG₁₅₀MeTMG was employed as the neutral ligand, the CO₂ capacity decreased from 1.66 to 0.89 mol CO₂ absorbed per mol of base as the molar ratio of LiNTf₂/PEG₁₅₀MeTMG was increased from 0 to 1, probably due to decreased basicity of guanidine after coordinating with Li⁺. At last, ILs [DBULi][NTf₂] (0.50) and [DBNLi][NTf2] (0.75 mol CO₂ absorbed per mol of base) had a CO₂ capacity below 1:1 stoichiometry expected from the proposed mechanism (Scheme 1c), owing to highly increased viscosity after CO₂ absorption (Table 1, entries 12 and 13). Compared on a weight basis, a CO₂ capacity of 5.0 wt % to 8.0 wt % was obtained with ILs from neutral ligands/LiNTf₂ (Table 1, entries 5–7 and 10–13). This is much higher than for the conventional IL 1-hexyl-3-methylimidazolium hexafluorophosphate (0.0881 wt %) [43], and comparable to amino-functionalized imidazolium-based IL (7.4 wt %) [23] and ILs derived from amino acids [24]. Hence, our IL system has the potential to be utilized for industrialized CO₂ absorption processes.

The in situ FTIR spectrum of [PEG₁₅₀MeTMGLi][NTf₂] and [PEG₁₅₀MeBu₂NLi][NTf₂] before and after reaction with CO₂ are shown in Figure 3. For [PEG₁₅₀MeTMGLi][NTf₂] as absorbent, the C=N stretching band at 1592 cm⁻¹ shifted to 1622 cm⁻¹ after reaction with CO₂. A characteristic peak centered at 1685 cm⁻¹ is assigned to the stretching vibration of the carbonyl group in the zwitterionic alkyl carbamate, which is quite different from that in HCO₃⁻ (1588 cm⁻¹). For the reaction of [PEG₁₅₀MeBu₂NLi][NTf₂] with CO₂, a new band at 1635 cm⁻¹ was assigned to the stretching vibration of the C=O bond in the zwitterionic product. In addition, a distinct broad band at around 2118 cm⁻¹ corresponding to the ammonium cation was observed in the presence of water.

To gain deeper insight into the reaction mechanism of CO₂ absorption with chelated ILs, DFT calculations were carried out. We performed geometry and energy optimizations for the free [BaseLi]⁺ cation, free CO₂ and the [BaseLi]⁺ + CO₂ complex. As shown in Figure 5, the chelated ILs react with CO₂ through nucleophilic attack by the N atom, to form the zwitterionic adducts, which are stabilized through coordination with Li⁺. In addition, formation of the zwitterionic alkyl carbamate is calculated to be associated with an enthalpy changes of −102.36 kcal mol⁻¹ and −89.50 kcal mol⁻¹ for [PEG₁₅₀MeTMGLi]⁺/CO₂ and [PEG₁₅₀MeBu₂NLi]⁺/CO₂, respectively, indicating that the absorption process is thermodynamically favourable.

Conclusion

In summary, efficient CO₂ capture was achieved through formation of zwitterionic adducts with readily synthesized chelated ILs. Multisite coordination interaction between the Li⁺ cation and organic bases or PEG-functionalized organic bases is thought to be responsible for forming the amidine, guanidine or tertiary amine-functionalized ILs, after reaction with CO₂ to form zwitterionic adducts in a 1:1 manner. Coordination effects between various lithium salts and neutral ligands, and the CO₂ capacity of the obtained chelated ILs were investigated. Indeed, the thermal stability and the CO₂ capacity of the neutral ligands (e.g., PEG₁₅₀MeBu₂N) was highly increased after coordination with lithium salts to form chelated ILs (e.g., [PEG₁₅₀MeBu₂NLi][NTf₂]).

Experimental

Materials

All reagents used in this work were purchased from Sigma-Aldrich and used without further purification. CO₂ with a purity of 99.999% was obtained commercially.

Experimental methods

¹H NMR spectra were recorded on a Brucker 400 spectrometer in CDCl₃. Residual CHCl₃ (7.26 ppm) was used as internal reference. ¹³C NMR spectra were recorded at 100.6 MHz in CDCl₃. Residual CHCl₃ (77.0 ppm) was used as internal reference. In situ FTIR spectra were collected on a Mettler Toledo React IR ic10, which was equipped with a diamond ATR probe, using an ic IR analysis system. The probe was placed into the absorption mixture. Spectra were collected in situ during CO₂ absorption, while the mixture was stirred continuously using a magnetic stir bar. ESI-MS spectra were recorded on a Thermo Finnigan LCQ Advantage spectrometer in ESI mode at a spray voltage of 4.8 kV.

General procedure for CO₂ absorption

The CO₂ absorption procedure was analogous to the CO₂/SO₂ absorption procedure we had reported before [35,44]. In a typical procedure, the CO₂ capture was carried out in a 10 mL Schlenk flask. The absorbents were charged into the reactor at room temperature. Then, the air in the flask was replaced by passing CO₂ through a needle, which was inserted into the bottom of the flask. The absorption was conducted at 25 °C with a CO₂ flow rate of 0.1 L/min. The amount of CO₂ absorbed was determined by following the weight of the mixture with an Analytical Balance. Data points were taken with an accuracy of ±0.0001 g every five minutes. Absorption/desorption was determined for at least three cycles.