Interaction between CO₂ and ionic liquids confined in the nanopores of SAPO-11

Abstract

A series of ionic liquids (P_4,4,4,6BF₄, APMIMBF₄, P_4,4,4,6Triz, as well as the newly prepared anion dual-functionalized amino-triz IL P_4,4,4,6ATriz, etc.) supported on glass powder and SAPO-11 were prepared, and their interaction with CO₂ was investigated by “limited” temperature-programmed desorption. The results showed that the strength of the interaction of CO₂ with IL/glass powder (11 wt%) followed the order P_4,4,4,6BF₄ < APMIMBF₄ < P_4,4,4,6Triz < P_4,4,4,6ATriz. Two desorption peaks were observed for P_4,4,4,6Triz, probably attributed to the two sites of interaction between Triz and CO₂, and the Gaussian 03 program was employed to obtain the optimized structures, revealing the interaction between P_4,4,4,6Triz and CO₂. When ILs were confined in the nanopores of SAPO-11, the desorption capacity and interaction strength increased because of the nano-confinement effect. Meanwhile, the loading, as well as the structure of ILs and Na₂CO₃-modified SAPO-11, significantly affected the interaction between CO₂ and ILs. Besides, P_4,4,4,6ATriz/SAPO-11 (modified with Na₂CO₃, 30 wt%) with a maximum CO₂ desorption capacity of 1.55 mole_CO2/mole_ILs could reversibly adsorb CO₂ 15 times without any apparent reduction in the desorption capacity.

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1. Introduction

Carbon dioxide (CO₂) is considered to be the main greenhouse gas, and it is highly desirable to develop a method to efficiently capture CO₂. In the past decades, ionic liquids (ILs) have tremendously attracted attention for use in the capture of acid gases, especially CO₂, caused by their unique properties; they are also good alternatives to traditional aqueous amine solvents.^1–6 To increase the absorption capacity of CO₂, Davis et al. have reported the binding of CO₂ with pioneering amino-functionalized ILs by physical and chemical absorption, with an absorption capacity of approximately 0.5 mole_CO2/mole_ILs.⁷ Subsequently, some anion-functionalized ILs, including amino-acid-based ILs, acetate ILs, azolate ILs and pyridine ILs, have been developed for the chemisorption of CO₂ and exhibited improved absorption capacity.^8–13 Compared to lithium-based ceramic adsorbents (CO₂ capture at high temperatures: 400–700 °C),^14–18 the process of capturing CO₂ by ILs (at room temperature) obviously leads to more energy savings. In addition, the captured CO₂ can easily undergo catalytic conversion into valuable compounds such as CO, CH₄ and epoxide, particularly in the case of electrocatalysis conducted at room temperature because ILs exhibit conductivity higher than that of metal organic frameworks.^19–24

However, currently, the conventional method for measuring the CO₂ absorption capacity predominantly involves weighing the quality changes of bulk ILs before and after the absorption of CO₂, which provides marginal information about CO₂ absorption; however, detailed desorption information, such as the number and types of desorption sites and temperature of the maximum and complete desorption, is not obtained. Typically, temperature-programmed desorption (TPD) analyses are employed for understanding the properties of the active sites on a catalyst, which in turn can provide information about the number and types of surface sites as well as their reactivity and sensitivity to adsorbate coverage.^25,26 When TPD employs to investigate the interaction between ILs and CO₂, information such as desorption capacity, different absorption sites, temperatures of the maximum and complete desorption and basicity of ILs can be obtained.

In this study, we first employed the “limited” TPD technology to explore the interaction between ILs and CO₂ to obtain detailed information of desorption. We used glass powder as the support as it has almost no pores and does not have the ability to capture CO₂; hence, we can observe actual interactions between ILs and CO₂. Silicoaluminophosphate molecular sieve SAPO-11 (mean mesopore size: 7 nm; pore volume: 0.45 cm³ g⁻¹) has excellent mean nanopores and mild acidity^27,28 such that bare SAPO-11 exhibits a small desorption capacity; hence, we used it as the support to determine the interaction between CO₂ and ILs confined in nanoscale pores. Herein, a series of ILs supported on glass powder and SAPO-11 were prepared, and their interaction with CO₂ was investigated by TPD. Moreover, the effects of the loadings of ILs, as well as the structure of ILs and Na₂CO₃-modified SAPO-11, on the interaction between CO₂ and ILs were also investigated. The newly prepared IL P_4,4,4,6ATriz supported on glass powder with a loading of 11 wt% exhibited a higher desorption capacity (0.79 mol_CO2/mol_ILs) and the maximum desorption temperature (80 °C, 136 °C) caused by the introduction of electron-donating ammonium groups. The desorption capacity further increased to 0.97 mol_CO2/mol_ILs when confined in the nanopores of SAPO-11 with a loading of 30 wt%.

2. Experimental

2.1 Chemicals

All reagents and solvents of pure analytical grade were purchased from commercial sources and were used without further purification, if not stated otherwise.

2.2 Preparation of ILs

1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄), 1-aminopropyl-3-methylimidazolium tetrafluoroborate (APMIMBF₄), 1-ethyl-3-methylimidazolium acetate (EMIMOAc), 1-ethyl-2,3-dimethylimidazolium acetate (EMMIMOAc), alkyltributylphosphonium tetrafluoroborate (P_4,4,4,nBF₄ n = 2, 4, 6, 8, 10), aminopropyltributylphosphonium tetrafluoroborate (P_4,4,4,APBF₄), hexyltributylphosphonium X (P_4,4,4,6X X = bromide (Br), hexafluorophosphate (PF₆) and bis((trifluoromethyl)sulfonyl)imide (TFSI) investigated in this study (Scheme 1) were synthesized and purified according to well-established procedures²⁹ and dried under high vacuum at 80 °C for 8 h before use.

Scheme 1 Structures and abbreviations of the cations and anions used in this study.

Hexyltributylphosphonium triazole (P_4,4,4,6Triz) and hexyltributylphosphonium aminotriazole (P_4,4,4,6ATriz) were prepared by the neutralization of hexyltributylphosphonium hydroxide (P_4,4,4,6OH) and 1,2,4-triazole or 3-amino-1,2,4-triazole according to literature methods.^30,31 Typically, a solution of P_4,4,4,6OH in ethanol was first prepared from P_4,4,4,6Br using the anion-exchange resin method. Second, equimolar 1,2,4-triazole or 3-amino-1,2,4-triazole was added to the P_4,4,4,6OH solution in ethanol. Next, the mixture was stirred at room temperature for 12 h. Subsequently, ethanol and water were removed by distillation at 60 °C under reduced pressure. The obtained ILs were dried under high vacuum at 80 °C for 8 h. The structures of these ILs were confirmed by NMR spectroscopy. ¹H NMR and ¹³C NMR spectra were recorded on a Bruker AMX FT 400 MHz NMR spectrometer. Chemical shifts were reported downfield in parts per million (ppm, δ) using tetramethylsilane as the reference.

P_4,4,4,6Triz ¹H NMR (CDCl₃) 0.95 (m, 12H, CH₃), 1.30–1.52 (m, 20H, CH₂), 2.29–2.38 (m, 8H, PCH₂), 8.07 ppm (s, 2H, Triz C₂ and C₅); ¹³C NMR (CDCl₃) 13.3, 13.8, 18.5, 19.0, 21.7, 22.2, 23.7, 23.9, 30.3, 30.9, 148.8, 152.5 ppm.

P_4,4,4,6ATriz ¹H NMR (CDCl₃) 0.94 (m, 12H, CH₃), 1.29–1.50 (m, 20H, CH₂), 2.21–2.29 (m, 8H, PCH₂), 7.34 (s, 2H, Triz-NH₂), 7.49 ppm (s, H, Triz C5); ¹³C NMR (CDCl₃) 13.4, 13.8, 18.5, 19.1, 21.7, 22.2, 23.6, 23.9, 30.4, 30.9, 148.6, 158.9 ppm.

The water content in these ILs was determined by Karl–Fischer titration; it was found to be less than 0.05 wt%. The residual bromide content in these ILs was determined using a Mettler–Toledo Seven Multi pH meter with a Br-selective electrode; the bromide content was lower than 0.1 wt%. The thermal stability of the ILs (Fig. S1†) was determined on a TA Instruments Q500 series thermal gravimetric analyzer with samples held in platinum pans under a continuous flow of nitrogen. Samples were heated at a constant rate of 5 °C min⁻¹ during all TGA experiments.

2.3 Preparation of supported ILs

We describe the preparation of P_4,4,4,6BF₄/SAPO-11 (30 wt%) as an example; it was prepared in a manner similar to a previous procedure in a filling experiment.³² First, 1.2 g SAPO-11 was placed in a two-neck flask (one of the necks was sealed by a rubber stopper). The flask was maintained at 100 °C for 4 h under vacuum to remove the gas inside SAPO-11. Second, 360 mg P_4,4,4,6BF₄ dissolved in methanol was transferred into the flask using a syringe, and the mixture was ultrasonically vibrated for 1 h to fill the porous SAPO-11 with ILs. Next, methanol was removed by distillation at 80 °C under reduced pressure. Finally, P_4,4,4,6BF₄/SAPO-11 (30 wt%) thus obtained was dried under high vacuum at 80 °C for 8 h.

2.4 Preparation of Na₂CO₃-modified SAPO-11

First, 5 g SAPO-11 was added to 200 mL of a Na₂CO₃ solution (0.02 mol L⁻¹) at room temperature and then the mixture was stirred vigorously for 12 h. Next, the mixture was filtered and washed with water till a pH of 7 was attained in the washing solution. Finally, the resultant precipitate was dried at 120 °C for 12 h and further calcined at 300 °C for 5 h to yield Na₂CO₃-modified SAPO-11.

2.5 Temperature-programmed desorption

Temperature-programmed desorption (TPD) was performed using a TPD flow system (Fig. 1) equipped with a thermal conductivity detector (TCD). In a typical experiment, the supported IL sample (P_4,4,4,6BF₄/glass powder, 1.11 g, 11 wt%) was first pretreated at 180 °C for 1 h under Ar flow (65 mL min⁻¹) and then cooled to 10 °C. Second, the sample was subsequently exposed to a stream of CO₂ (65 mL min⁻¹) at 10 °C for 1 h and flushed again with Ar (65 mL min⁻¹) for 1 h to remove any weakly adsorbed CO₂. Next, the desorption profile was recorded at a heating rate of 15 °C min⁻¹ from 10 to 180 °C, and 180 °C was maintained until the TCD signal of CO₂ returned to the baseline. The peak area was quantitatively analysed by the standard peak area of one quantitative ring of CO₂ (0.1727 mL) and was confirmed by a titrimetric method of the CO₂ collected using a Ba(OH)₂/BaCl₂ standard solution.

Fig. 1 Schematic diagram of a temperature-programmed desorption instrument.

3. Results and discussion

3.1 Interaction between CO₂ and ILs supported on glass powder and SAPO-11

Fig. 2A shows the desorption curves of P_4,4,4,6BF₄, APMIMBF₄, P_4,4,4,6Triz and P_4,4,4,6ATriz (11 wt%) supported on glass powder; desorption capacities of 0, 0.1, 0.4 and 0.79 mole_CO2/mole_ILs were observed (Table 1), respectively. No peak was observed in the desorption curve of P_4,4,4,6BF₄ as TPD only exhibited a relatively strong interaction between CO₂ and ILs. This result indicates that the interaction between CO₂ and P_4,4,4,6BF₄ is very weak, probably only physical interaction, and the adsorbed CO₂ is completely flushed out by Ar in the blowing process. On the other hand, for APMIMBF₄, one peak was observed, and the maximum and complete desorption was observed at 53 °C and 120 °C, respectively. The relative high desorption capacity was attributed to the formation of an ammonium carbamate salt by CO₂ and ammonium ILs. Moreover, the desorption capacity of APMIMBF₄/glass powder was lower than those of bulk ILs reported by Davis et al.⁷ The supported ILs are speculated to have been adsorbed by the membrane, with a majority of the weakly adsorbed CO₂ being removed in the blowing process. On the other hand, for P_4,4,4,6Triz, two desorption peaks were observed at 60 °C and 119 °C, attributed to two absorption sites corresponding to the P_4,4,4,6Triz anion (Scheme 2), although only one new peak was observed at 1745 cm⁻¹ in the FTIR spectrum after CO₂ absorption, attributed to the C=O stretch of carbamate (Fig. 3A). According to Scheme 2, the optimized structures reflecting the interaction between the Triz anion and free CO₂ molecules was obtained by theoretical calculations using the Gaussian 03 program at the B3LYP/6-31++(d, p) level of theory^33,34 (Fig. 3B). In the curve of P_4,4,4,6Triz, the first peak was observed at 60 °C, possibly attributed to the interaction site of the low atomic charge 1 N of 1,2,4-triazole (Mulliken charges: −0.301), while the second peak was observed at 119 °C, attributed to the high atomic charge of 4 N of 1,2,4-triazole (Mulliken charges: −0.419). This new proposed mechanism possibly urges us to reconsider the interaction between CO₂ and triz-based ILs. When the temperature increased to 180 °C, approximately 30% of the adsorbed CO₂ was still not desorbed, and subsequently we maintained the temperature at 180 °C until the TCD signal returned to the baseline to ensure complete desorption of CO₂. This indicated that the diffusion and desorption of CO₂ in the ILs are slow.

Fig. 2 (A) TPD-CO₂ profiles of different ILs supported on glass powder; (B) TPD-CO₂ profiles of different ILs confined in SAPO-11 (30 wt%).

Table 1 CO₂ desorption of different ILs/support

IL/support	Desorption capacity/moleCO2/moleILs	Maximum desorption temperature/°C	Complete desorption temperature/°C
P4,4,4,6BF4/glass powder (11 wt%)	0	0	0
APMIMBF4/glass powder (11 wt%)	0.1	53	120
P4,4,4,6Triz/glass powder (11 wt%)	0.4	60, 119	—
P4,4,4,6ATriz/glass powder (11 wt%)	0.79	80, 136	—
P4,4,4,6BF4/SAPO-11 (30 wt%)	0.14	77	110
APMIMBF4/SAPO-11 (30 wt%)	0.47	87	—
P4,4,4,6Triz/SAPO-11 (30 wt%)	0.16	35, 75	120
P4,4,4,6ATriz/SAPO-11 (30 wt%)	0.97	82, 132	—

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Scheme 2 Proposed reaction mechanism between P_4,4,4,6Triz and CO₂.

Fig. 3 (A) FT-IR spectra of P_4,4,4,6Triz before and after absorption of CO₂; optimized structures of anion-2CO₂ complexes at the B3LYP/6-31++G (d, p) level. (B) Triz-2CO₂; (C) ATriz-2CO₂.

To increase the desorption capacity and interaction strength between ILs and CO₂, we described a new strategy for significantly increasing the capture of CO₂ by the introduction of an ammonium group on the triz-based anion. The desorption capacity of this anion dual-functionalized IL supported on glass powder (11 wt%) reached up to 0.79 mole_CO2/mole_ILs, and the maximum desorption temperature increased to 80 °C and 136 °C. The increase in the desorption capacity of this new dual amino-triz IL could be explained by the synergistic effect of the ammonium group and the N atoms of the triazole ring. Simultaneously, the electron-donating ammonium group caused the increase of the atomic charges of 1 N (Mulliken charges: −0.381) and 4 N (Mulliken charges: −0.502) of triazole; consequently, the interaction between P_4,4,4,6ATriz and CO₂ clearly enhanced. The desorption peaks of the ammonium group and 1 N of the triazole ring may overlap with each other to form one peak; hence, two peaks were observed in the desorption curve of P_4,4,4,6ATriz/SAPO-11.

Then, the desorption performance of P_4,4,4,6BF₄, APMIMBF₄, P_4,4,4,6Triz and P_4,4,4,6Atriz confined in the nanopores of molecular sieve SAPO-11 was investigated. Fig. 2B shows the desorption curves of ILs/SAPO-11 (30 wt%); bare SAPO-11 exhibited a low desorption capacity with a relatively low maximum and complete desorption temperature of 31 °C and 68 °C, respectively. Compared to those of ILs supported on glass powder, the desorption capacities of P_4,4,4,6BF₄, APMIMBF₄ and P_4,4,4,6Atriz confined in SAPO-11 increased to 0.14, 0.47 and 0.97 mole_CO2/mole_ILs (Table 1), respectively, with the corresponding temperature of desorption peaks increasing to 77 °C, 87 °C as well as 82 °C and 132 °C. On the contrary, the desorption capacity of P_4,4,4,6Triz obviously decreased to 0.16 mole_CO2/mole_ILs. The increase is probably attributed to the nano-confinement effect of SAPO-11. When the ILs are confined in the pores of SAPO-11, the higher dispersion of ILs by SAPO-11 could enhance the absorption of CO₂ caused by the more efficient contact of CO₂ with the ILs confined in the matrices of SAPO-11 as compared to that in case of bulk ILs. The ILs containing an ammonium group may exhibit a favourable interaction with the inner surface of SAPO-11, although SAPO-11 exhibits mild acidity, which probably decreases the whole basicity of ILs/SAPO-11. The obvious decrease in the desorption capacity and temperatures of maximum and complete desorption of P_4,4,4,6Triz suggested that the interaction between the support's inner surface and ILs is very complicated, and P_4,4,4,6Triz does not exhibit excellent interaction with SAPO-11 probably because the mild acidity of SAPO-11 significantly influences P_4,4,4,6Triz.

The reusability of the supported IL is a critical factor for gas absorption, which directly impacts cost because it determines the frequency at which the supported IL is replaced. Fig. 4 shows the results after 15 adsorption–desorption cycles of P_4,4,4,6Atriz/SAPO-11 (30 wt%). The high desorption capacity was well maintained during 15 cycles, indicating that CO₂ absorption by P_4,4,4,6Atriz/SAPO-11 (30 wt%) is highly reversible.

Fig. 4 CO₂ desorption capacity of P_4,4,4,6ATriz/SAPO-11 (30 wt%) for 15 cycles.

3.2 Effect of IL loading

Furthermore, the effect of IL loading on the desorption performance of ILs/SAPO-11 was investigated. For 10 wt% loading of APMIMBF₄ confined in SAPO-11 (Fig. 5A), the amounts of desorbed CO₂ increased to 0.63 mole_CO2/mole_ILs. However, the temperature for maximum desorption exhibited almost no change, indicating that the loading of APMIMBF₄ marginally affects the strength of interaction between CO₂ and APMIMBF₄. On the other hand, at a 50 wt% loading of APMIMBF₄, the amounts of desorbed CO₂ reduced to 0.25 mole_CO2/mole_ILs (Table 2), probably caused by the blocking of pores by the excess ILs. Moreover, the temperature for the maximum desorption slightly changed, although there was an additional shoulder peak (relatively weak absorption) at 45 °C, attributed to the interaction of CO₂ with ILs accumulated inside the pores of SAPO-11. Similar changing trends were observed from the TPD-CO₂ profiles of P_4,4,4,6Triz (Fig. 5B) and P_4,4,4,6ATriz (Fig. S2†). Very interestingly, when the loading of P_4,4,4,6Triz decreased from 30 wt% to 20 wt%, the desorption capacity (0.84 mole_CO2/mole_ILs) sharply increased (Table 2), and the temperature of the maximum desorption peaks (88 °C, 141 °C), while only small changes were observed for P_4,4,4,6ATriz. Such an obvious enlargement is possibly attributed to the fact that the interaction between SAPO-11 and P_4,4,4,6Triz becomes benign at low loading of P_4,4,4,6Triz.

Fig. 5 (A) TPD-CO₂ profiles of APMIMBF₄ confined in SAPO-11 at different loading values; (B) TPD-CO₂ profiles of P_4,4,4,6Triz confined in SAPO-11 at different loading values.

Table 2 CO₂ desorption of ILs confined in SAPO-11 at different loading

IL/SAPO-11 (wt%)	Desorption capacity/moleCO2/moleILs	Maximum desorption temperature/°C	Complete desorption temperature/°C
APMIMBF4 (10%)	0.63	88	138
APMIMBF4 (30%)	0.47	87	—
APMIMBF4 (50%)	0.25	96	—
P4,4,4,6Triz (11%)	0.94	85	—
P4,4,4,6Triz (20%)	0.84	88, 141	—
P4,4,4,6Triz (30%)	0.16	35, 75	120

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3.3 Effect of the structure of ILs

The effect of the structure of ILs on the interaction between CO₂ and supported ILs was also investigated. In comparison with that of P_4,4,4,6Triz/glass powder (11 wt%), the desorption capacity of EMIMTriz/glass powder (11 wt%) decreased to 0.15 mole_CO2/mole_ILs (Table 3), while the temperature of the desorption peaks increased to 79 °C and 151 °C (Fig. 6A). This result suggests that, as compared to P_4,4,4,6Triz, EMIMTriz has lesser total basic sites, albeit with stronger basic strength. Similar results were observed for P_4,4,4,APBF₄/glass powder (11 wt%), as compared to APMIMBF₄/glass powder (11 wt%), where the desorption capacity increased to 0.39 mole_CO2/mole_ILs, and the temperature of maximum and complete desorption increased to 82 °C and 150 °C, respectively, according to the increase in both basic sites and basic strength. For EMIMOAc/glass powder (11 wt%), the chemisorption of CO₂ onto 1,3-dialkylimidazolium acetate ILs was confirmed by direct experimental evidence obtained from TPD. The desorption capacity of EMIMOAc was 0.13 mole_CO2/mole_ILs with a desorption peak at 49 °C, while a desorption peak was not observed for EMMIMOAc/glass powder (11 wt%). This chemisorption of EMIMOAc could be attributed to the interaction between CO₂ and the C(2) of the imidazolium ring to form imidazolium carboxylate.¹¹ When the C(2)-H proton was occupied by the methyl group, no chemisorption of CO₂ occurred for EMMIMOAc.

Table 3 CO₂ desorption of ILs with different cations or anions

Supported ILs	Desorption capacity/moleCO2/moleILs	Maximum desorption temperature/°C	Complete desorption temperature/°C
EMIMTriz/glass powder (11 wt%)	0.15	79, 151	—
P4,4,4,6Triz/glass powder (11 wt%)	0.4	60, 119	—
APMIMBF4/glass powder (11 wt%)	0.1	53	120
P4,4,4,APBF4/glass powder (11 wt%)	0.39	82	150
EMIMOAc/glass powder (11 wt%)	0	0	0
EMMIMOAc/glass powder (11 wt%)	0.13	49	84

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Fig. 6 (A) TPD-CO₂ profiles of ILs with different cations; (B) TPD-CO₂ profiles of different alkyl chain lengths of P_4,4,4,nBF₄/SAPO-11 (30 wt%) (n = 2, 4, 6, 8, 10); (C) TPD-CO₂ profiles of different anions of P_4,4,4,6X/SAPO-11 (30 wt%) (X = BF₄, Br, PF₆, TFSI).

Subsequently, we examined the effect of the alkyl chain length of the cation on the interaction between CO₂ and P_4,4,4,nBF₄/SAPO-11 (30 wt%) (Fig. 6B). The result showed that P_4,4,4,6BF₄/SAPO-11 (30 wt%) exhibited the maximum desorption capacity (Fig. S3†). Meanwhile, the desorption capacity of P_4,4,4,nBF₄/SAPO-11 (30 wt%) followed the order of C6 > C10 > C8 > C2 > C4 (Table S2†), indicating that the total basic sites of P_4,4,4,nBF₄ follow this order. However, the basic strength (maximum desorption temperature) of P_4,4,4,nBF₄/SAPO-11 (30 wt%) almost gradually weakened with the increasing alkyl chain length of P_4,4,4,nBF₄, except in the case of P_4,4,4,4BF₄, which has the best structure symmetry on the cation of IL. The effect of the anion on the desorption performance of P_4,4,4,6X/SAPO-11 (X = BF₄, Br, PF₆, TFSI; 30 wt%) was also investigated. As shown in Fig. 6C, the desorption capacity (basic sites) followed the order of P_4,4,4,6BF₄ > P_4,4,4,6PF₆ > P_4,4,4,6TFSI > P_4,4,4,6Br (Table S3†), while the maximum desorption temperatures (basic strength) followed the order of P_4,4,4,6BF₄ > P_4,4,4,6PF₆ > P_4,4,4,6Br > P_4,4,4,6TFSI.

3.4 Effect of modification of SAPO-11 by Na₂CO₃

As SAPO-11 exhibits mild acidity, which will decrease the basicity of the total supported ILs, we modified SAPO-11 using Na₂CO₃ (0.02 mol L⁻¹) to yield ILs/SAPO-11 (30 wt%, modified with Na₂CO₃); the CO₂ desorption performance of these ILs/SAPO-11 exhibited a significant boost (Fig. 7). Table 4 shows the amounts of CO₂ desorbed when the ILs confined in Na₂CO₃-modified SAPO: the desorption capacities of P_4,4,4,6BF₄, APMIMBF₄, P_4,4,4,6Triz and P_4,4,4,6ATriz increased to 0.31, 0.70, 0.32 and 1.55 mole_CO2/mole_ILs, respectively. For P_4,4,4,6Triz/SAPO-11 (30 wt%, modified with Na₂CO₃), the temperature of the second desorption peak obviously increased to 180 °C. However, the peaks of the other desorption curves only exhibited marginal changes. P_4,4,4,6ATriz/SAPO-11 (30 wt%, modified with Na₂CO₃) with an improved desorption capacity could also reversibly adsorb CO₂ 15 times without apparent reduction in the desorption capacity (Fig. S4†).

Fig. 7 TPD-CO₂ profiles of ILs confined in Na₂CO₃-modified SAPO-11 (30 wt%).

Table 4 CO₂ desorption of ILs confined in Na₂CO₃-modified SAPO-11 (30 wt%)

ILs	Desorption capacity/moleCO2/moleILs	Maximum desorption temperature/°C	Complete desorption temperature/°C
P4,4,4,6BF4	0.31	78	154
APMIMBF4	0.70	88	—
P4,4,4,6Triz	0.32	42, 180	—
P4,4,4,6ATriz	1.55	120	—

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4. Conclusions

In summary, we developed a “limited” temperature-programmed desorption method for investigating the interaction between CO₂ and ILs. The results showed that the strength of interaction between CO₂ and ILs/glass powder (11 wt%) followed the order P_4,4,4,6BF₄ < APMIMBF₄ < P_4,4,4,6Triz < P_4,4,4,6ATriz. The desorption capacity and strength of the interaction of most ILs increased owing to the nano-confinement effect when they are confined in SAPO-11 as compared to glass powder. Furthermore, the loading, as well as the structure of ILs and Na₂CO₃-modified SAPO-11, significantly affect the interaction between CO₂ and supported ILs. P_4,4,4,6ATriz/SAPO-11 (modified with Na₂CO₃) with an IL loading of 30 wt% exhibited the maximum CO₂ desorption capacity of 1.55 mole_CO2/mole_ILs, and reversible absorption of CO₂ was possible for 15 times without apparent reduction. Further investigating on the interaction between ILs and other waste gases (such as SO₂ and NH₃) by TPD and conversion the captured CO₂ are currently underway.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (no. 21173240).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08624h

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