Cooperative CO2 absorption by amino acid-based ionic liquids with balanced dual sites

In this study, a variety of functionalized ILs with dual sites including amino acid group (AA) and basic anion (R) were synthesized to investigate the suppression and cooperation between the sites in CO2 absorption. The basic anions selected in this study with different basicity include sulfonate (Su), carboxylate (Ac), imidazolium (Im), and indolium (Ind). These ILs ([P66614]2[AA–R]) were applied to CO2 absorption. The results present that CO2 capacity increases first and then decreases later with the continuous increase in the activity of the anion site. Combined with CO2 absorption experiments, IR and NMR spectroscopic analyses and DFT calculation demonstrate that the ability of one site to capture CO2 would be suppressed when the activity of another site is much stronger. Thus, the cooperation of dual site-functionalized ILs and high CO2 capacity might be achieved through balancing the two sites to be equivalent. Based on this point, [P66614]2[5Am–iPA] was further synthesized by taking the advantage of the conjugated benzene ring. As expected, [P66614]2[5Am–iPA] showed capacity as high as 2.38 mol CO2 per mol IL at 30 °C and 1 bar without capacity decrease even after 10 times recycling performance of CO2 absorption and desorption.


Introduction
Rapid anthropogenic climate change caused by a large number of greenhouse gases is one of the most signicant environmental problems in the world today. 1 The development of sustainable and environmentally friendly technology to reduce greenhouse gas emissions, particularly carbon dioxide (CO 2 ), is the focus of attention in many countries. Unique physicochemical properties of ionic liquids (ILs) such as high thermal stability, low vapor pressure, and tunable properties make ILs suitable for CO 2 absorption. 2,3 Functionalized ILs, such as amine, 4,5 azolium, 6 phenolate, 7 and carbene-based ILs 8 were reported as potential CO 2 absorption solvents owing to their electron-rich property, or the basicity of ILs. High CO 2 absorption capacity is one of the performance evaluations of ILs. In this regard, a series of strategies, including tuning the basicity of the active sites, [9][10][11] changing the steric hindrance of the ILs, 12,13 utilization of entropic effects 14,15 and hydrogen bond formation, 16,17 were developed to enhance the CO 2 absorption capacity. However, the CO 2 capacity of ILs with single site was low compared to those multiple-sites, which have attracted attention to enhance the CO 2 capacity.
Designing ILs with cooperative sites was considered as attractive to enhance the CO 2 capacity as well as gas adsorption materials. Vaidhyanathan and Woo 18 reported CO 2 capture by an amine-functionalized nanoporous solid with cooperative sites for the low-pressure binding and large uptake of CO 2 .
McDonald 19 reported an energy-saving CO 2 separation by small temperature or pressure swings via the cooperative insertion of CO 2 in diamine-appended MOFs such as mmen-Mg 2 (dobpdc). It has been found that cooperative CO 2 capture is a much feasible way to achieve high capacity and reversibility. 20,21 Recently, cooperative sites were considered in designing ILs. Wang et al. 22 synthesized hydroxyl-pyridinium based ILs with dual cooperative sites to x CO 2 for the delocalized p electrons, which enhanced by 85% of the capacity to 1.58 mol mol À1 IL. Similarly, the cooperation of Lewis acid-basic reaction and the hydrogen bond interactions of IL with CO 2 were put to good use to improve CO 2 capture with imidazolium ILs. 16 31 with a dual amine group in cation were reported as a CO 2 absorbent but have capacity just up to equimolar similar to the ILs with a single site, which means one site might be suppressed or inactive. Mu 27 thought that there is interplay of the dual amine in ILs, which might restrict their ability. As can be seen, cooperation is the key to the high capacity of ILs with dual sites, 32,33 but the depression of one site is a common problem making the active site to be suppressed and have low CO 2 capacity.
A dual site-functionalized IL consists of three parts including cation, site A and site B in anion, as shown in Fig. 1. Two possible causes inuence the activity of sites, including the interactions between a cation and anion, and the interactions between site A and site B. In our previous study, the effects of cation was investigated and the results indicated that strong interactions between cation and anion would deactivate one site. 34 In this study, the interplay between dual sites was investigated; amino acids with binary acids are considered as an anion precursor to investigate the depression effects of CO 2 absorption sites. Phosphonium ions [P 66614 ] and [P 4442 ] are selected as the cations. The structure of the used ILs is presented in Chart 1. There are two potential sites including amino acid (AA) and anion site (R) in [AA-R]. Thereinto, it has been reported that [P 66614 ][AA] could capture CO 2 efficiently via the reaction of an amine group with CO 2 to carbamic acid. Indolium (Ind) and imidazolium (Im) ions are also good choices for CO 2 absorption with high capacity, while carboxylate (Ac) anion prefers to react with CO 2 and should be active to x CO 2 efficiently, sulfonate (Su) ion would not react with CO 2 . The results in this study indicate that the dual sites in ILs could cooperate and do their best in the CO 2 capture if two sites have quite an activity; otherwise, the less active site would be suppressed by another site. Furthermore, [P 66614 ] 2 [Am-iPA] was synthesized with equivalent dual sites to cooperative CO 2 absorption, and the results showed that it presented high capacity as 2.38 mol CO 2 per mol IL at 30 C and 1 bar.

Properties of ILs
Some properties of these synthesized ILs such as their thermal property and viscosity were detected, and are shown in Table 2 [Am-PA] with a benzene substituent was stable until the temperature reached above 300 C, and they are thermally stable enough for the application as a CO 2 absorbent. The viscosity of these ILs are thousands of cPa and it is considered that the high viscosity was derived from the hydrogen bond formation in aminebased ILs.

CO 2 absorption
These ILs were applied to CO 2 absorption under 1 bar CO 2 pressure at 30 C (Fig. 2 2 [AA-Im] with azolium ions were applied to CO 2 absorption. However, their CO 2 absorption capacities are 1.45 and 1.55 mol mol À1 IL, respectively, which are lower than that of [P 66614 ] 2 [AA-Ac]. This means the activity of the CO 2 absorption site AA or azolium ions might be suppressed.

The possible mechanism for CO 2 absorption
The possible interaction process controlled by enthalpy was speculated via theoretical calculations performed using the Gaussian 03 program at the B3LYP/6-31G++(d,p) level, the optimized structures of the [AA-R], and its CO 2 complexes are listed in Fig. S1 † and the enthalpies are listed in Table 2. It is reported that CO 2 could be chemisorbed when the reaction enthalpy is less than about À50 kJ mol À1 . 6 As seen from Table 2, the amine groups are all active for CO 2 absorption according to the reaction enthalpy DH(AA-CO 2 ). The reaction enthalpy of CO 2 with an Ac anion in [AA-Ac] is À61.08 kJ mol À1 , which means that Ac could react with CO 2    including anion and amine, where the amine site competes with anion in CO 2 reaction. The CO 2 capacity was associated with the relative activation of two sites, which was considered as the value of DH(R-CO 2 ) divided by DH(AA-CO 2 ). The results in Fig. 3 indicate that there is a high CO 2 capacity when the activation of two sites are almost equivalent, whereas the CO 2 capacity is much lower.
The CO 2 absorption with these ILs was investigated via IR and 13 C NMR spectroscopy, as shown in Fig. 4. There are 2 new peaks at 160.4 ppm and 158.2 ppm in the 13 C NMR spectra compared with CO 2 saturated [P 66614 ] 2 [AA-Ac] and its fresh state, while the chemical shi of the CH (marked with green circle) and CH 2 (marked with red square) groups have a few ppm changes, which indicates that CO 2 is xed in two forms. As seen from the IR spectra of [P 66614 ] 2 [AA-Ac] in Fig. 4(b), the vibration absorption of the carboxylate anion at 1585 cm À1 shi to 1610 cm À1 and the IR absorption intensity of the captured CO 2 between 1630-1760 cm À1 increases with a gradual increase in the CO 2 content, which indicates that the carboxylate anion assists in CO 2 absorption.
The 2D IR spectroscopy is a usual method to study the dynamics of interactions. 36,37 Thereinto, the IR spectra of ILs associated with the CO 2 content in 2D correction forms are shown in Fig. 5. Compared with the synchronous and asynchronous correction of [P 66614 ] 2 [AA-Ac], it is interesting that the absorption between 1630-1780 cm À1 consists of several peaks. There are cross-correlation peaks marked with the elliptical line that appeared at (1715 cm À1 , 1585 cm À1 ), where 1585 cm À1 belongs to the carboxylate anion and 1715 cm À1 belongs to the xed CO 2 . The J(1715 cm À1 , 1585 cm À1 ) in Fig. 5(a) is opposite in signs to F(1715 cm À1 , 1585 cm À1 ) in Fig. 5(c), which indicates that the change of 1585 cm À1 precedes 1715 cm À1 . Similarly, with the analysis of the correlation among the peaks of [P 66614 ] 2 [AA-Ac] at 1585, 1660 and 1715 cm À1 , it indicates that the absorption at 1660 cm À1 belongs to the xed CO 2 with the amine group in [AA-Ac] also follows 1585 cm À1 , while 1715 cm À1 and 1660 cm À1 appear simultaneously for no crosscorrelation peak at F(1713 cm À1 , 1660 cm À1 ), which is marked with dotted square in Fig. 5(c). Thus, the amine site of [AA-Ac] reacts with one CO 2 , then another CO 2 xed by an anion site, dual sites of ILs such as [P 66614 ] 2 [AA-Ac] could be listed as path (a) in Fig. 6, it presents high capacity of up to 1.97 mol CO 2 per mol IL owing to the cooperation of amine group and the carboxylate anion.
However, what happens on using [P 66614 ] 2 [AA-Im] as a CO 2 absorption agent?
As can be seen from Fig. S3 † that the partial IR spectra of [P 66614 ] 2 [AA-Im] varied with the CO 2 content, the IR a Viscosity date were obtained using a Bookeld DV-II+ Pro viscometer at 30 C. b Decomposition temperature was measured by DTG with temperature increase from 30 C to 600 C at a rate of 10 C min À1 under an argon gas ow.  Paper absorption of the carboxylate anion at 1578 cm À1 gets blue-shi with the uptake of CO 2 , and the absorption of the xed CO 2 with the Im and amine group appear at 1713 and 1664 cm À1 , respectively. Compared with the synchronous and asynchronous 2D correction IR spectra of [P 66614 ] 2 [AA-Im] associated with the CO 2 content, the sign of J(1713 cm À1 , 1578 cm À1 ) in Fig. 5(b) and F(1713 cm À1 , 1578 cm À1 ) in Fig. 5

Application of cooperation
Based on this opinion that the sites with equivalent ability are more likely to form cooperative CO 2 absorption, we know that the valence electron delocalizes in the conjugated plane, so that the charge can be dispersed. Thereinto, [P 66614 ] 2 [Am-iPA] was synthesized by taking advantage of a benzene ring, which is one of the most common conjugated planes. The reaction enthalpy of CO 2 with the amine group and carboxylate anion     were calculated to predict the possibility of the cooperative CO 2 absorption of dual sites.  Fig. 8(c). In the carbon capture utilization, moisture may be one of the strongest competitors to CO 2 in the absorption with ILs; therefore, the humid CO 2 absorption performance with [P 66614 ] 2 [Am-iPA] was tested at 30 C (Fig. S5 †). The results indicated that [P 66614 ] 2 [Am-iPA] was diluted with 2.5 wt% water, and the CO 2 absorption capacity of [P 66614 ] 2 [Am-iPA] with the copresence of water remained at 8.56 wt%. Compared with the dry CO 2 absorption capacity of 9.14 wt% (2.38 mol CO 2 per mol IL), it was demonstrated that a small amount of water in IL did not signicantly reduce the CO 2 capture capability. 33,34 The inuence of temperature and pressure were investigated. The results in Fig. 8(a and b) indicate that the CO 2 capacity decreases to 1.19 mol mol À1 when temperature increase to 70 C under 1.0 bar, or decreases to 1.79 mol mol À1 when the CO 2 partial pressure decreases to 0.1 bar at 30 C, which means that the captured CO 2 could be desorbed by increasing the temperature and decreasing the CO 2 partial pressure. The thermal stability of [P 66614 ] 2 [Am-iPA] was characterized via TGA measurement with its decomposition temperature set as 322 C. Ten consecutive absorption cycles of CO 2 by [P 66614 ] 2 [Am-iPA] are displayed in Fig. 8(d) and exhibit reversibility with the captured CO 2 being desorbed at 80 C and 1 kPa vacuum for 1 hour. With the cooperation of dual sites, higher CO 2 capacity as well as weaker interactions between CO 2 and IL could be achieved.

Synthetic procedures
ILs were prepared via synthesizing halide ILs, anion exchange, and neutralization with amino acids. Taking

Characterization
The synthesized ILs were conrmed via NMR and FTIR spectroscopies (see the NMR and IR absorption data in ESI †), and FTIR spectra were recorded using a Nicolet iS50 FT-IR spectrometer. 1 H and 13 C NMR spectra were recorded on a 500 MHz Bruker Avance III spectrometer in a deuterated reagent using tetramethylsilane as the standard, and the purities of ILs were calculated based on the NMR spectra. Moreover, the content of the ILs used in this work in higher than 95%. Furthermore, the water content of these ILs was determined by Karl Fischer titration, which was lower than 0.5 wt%. The viscosity of ILs was detected on a Brookeld DV-II+ Pro viscometer at 30 C. Their thermal stability was analysed via TGA on Shimadzu DTG-60H with an increase in temperature from 25 C to 600 C with an increased ratio of 10 C min À1 under N 2 gas protection.

CO 2 absorption
CO 2 absorption experiments with the IL were carried out according to our previous report. CO 2  was according to the so-called Noda's rule. The intensity of the synchronous 2D correlation spectrum F(n 1 , n 2 ) represents the simultaneous or coincidental changes in two spectral intensity variations measured at n 1 and n 2 during the measurement interval of the NH 3 content, while the intensity of an asynchronous spectrum J(n 1 , n 2 ) represents sequential or successive but not coincidental changes in spectrum measured separately at n 1 and n 2 .

Conclusions
In summary, a variety of dual site-functionalized ILs [P 66614 ] 2 [AA-R] were synthesized with an amino acid group and basic anion including sulfonate, carboxylate, imidazolium, and indolium to investigate the suppression and cooperation between each site in CO 2 absorption. Combined with CO 2 absorption experiments, IR and NMR spectroscopic analyses, and DFT calculations, CO 2 absorption results indicated that the CO 2 capacity increases rst but decreases later with the continuous increase in the CO 2 absorption ability of R. The ability of the amine group to capture CO 2 would be suppressed when the interactions of another site with CO 2 is stronger. Thus, the dual site-functionalized ILs might be cooperative to achieve high CO 2 capacity by balancing two sites to be equivalent. Based on this point, [P 66614 ] 2 [Am-iPA] was further synthesized by taking the advantage of a conjugated benzene ring. As expected, [P 66614 ] 2 [Am-iPA] showed capacity as high as 2.38 mol CO 2 per mol IL and without capacity decrease within 10 times recycle performance of CO 2 absorption and desorption. Cooperation exists widely in a variety of elds as well as gas absorption, and the investigation of the knowledge of suppression would be better to achieve cooperation.

Conflicts of interest
There are no conicts to declare.