Improving SO₂ capture by basic ionic liquids in an acid gas mixture (10% vol SO₂) through tethering a formyl group to the anions

Abstract

Low partial pressure SO₂ gas can be efficiently absorbed by ionic liquids (ILs), especially by basic ILs, through strong chemical interaction. However, it is still a challenge to achieve a high absorption capacity under these SO₂ conditions. In this work, a new class of formyl-containing aprotic task-specific ILs, including [P₆₆₆₁₄][3-CHO-Indo], [P₆₆₆₁₄][2-CHO-Pyro], [P₆₆₆₁₄][4-CHO-PhCOO], and [P₆₆₆₁₄][4-CHO-PhO], were designed and prepared through an acid–base neutralization reaction. The absorption of SO₂ in simulated acid gas (SO₂/N₂ = 10/90 vol) by these functional ILs was investigated. Compared with the formyl-free ILs, up to 100% increased capacity could be achieved by introducing a formyl group into the basic anions. In addition, the interactions between formyl-containing task-specific ILs and SO₂ were investigated through a combination of multiple techniques, including FT-IR, NMR, and quantum chemical calculations. It was found that the remarkable enhancement of SO₂ absorption capacity under low partial pressure was due to the C=O⋯S interaction and C–H⋯O hydrogen bonding. Furthermore, the SO₂ absorption process by the ILs could be repeated several times without loss of absorption capability.

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Introduction

In the past few decades, ionic liquids (ILs) have found wide spread applications as a kind of “green media” in chemical synthesis, biomass dissolution, energy production, separation science, and gas capture. They were highly emphasized by researchers because of their unique properties, such as ultra-low vapor pressure, wide liquid temperature range, non-flammability, chemical stability, and tunable structure and properties.^1–7 ILs have already been widely used in the separation of acidic gases such as SO₂,^8,9 CO₂,^10–15 H₂S,^16–19 BF₃ (ref. 20) and NO_x.^21,22 In these acidic gases, SO₂ is a significant source of atmospheric pollution that directly threatens the environment and human health, thus minimizing the emissions of SO₂ is highly important. Herein, we developed a new strategy for the highly efficient capture of SO₂ by basic ILs in an acid gas mixture (a mixture of N₂ and 10% volume content of SO₂) through introducing a formyl group into the basic anions.

Several conventional removal processes, such as limestone scrubbing and ammonia scrubbing, have been developed for flue gas desulfurization (FGD), and been widely used in industry, where the concentration of SO₂ in flue gas was up to thousands of ppm.^23,24 Nevertheless, the inherent disadvantages of these technologies should not be ignored, including the production of large quantities of wastewater and useless by-products.^25–28 Compared with conventional removal process, the SO₂ capture through ILs will not produce useless by-products and the captured SO₂ also can be collected for directly use in Claus reaction.²⁹ In addition, ILs can be easily recycled through heating and bubbling with N₂ without vast mass loss or quality change. Thus, SO₂ capture by ILs is more consist with the concept of sustainability.

High-concentration SO₂ has a high solubility in some ILs through physical interaction, especially in ether-functionalized ILs.³⁰ Thereby, effective capture of SO₂ from flue gas requires strong interaction between IL and SO₂ because of the relatively low SO₂ partial pressure in this stream. Han et al.³¹ reported the first example for chemical absorption of SO₂ by 1,1,3,3-tetramethylguanidine lactate ([TMG][L]), the absorption capacity of which was about 1.0 mole SO₂ per mole of IL at 1 bar with 8% SO₂ in a gas mixture of SO₂ and N₂. Since then, the SO₂ capture by several other kinds of ILs had been studied. For example, Zhang et al.³² reported a kind of hydroxyl ammonium ILs which can uptake SO₂ through chemical interaction. Elliott et al.³³ investigated the solubility of SO₂ in imidazolium-based ILs through theoretical studies using Monte Carlo simulation. Wang et al.³⁴ designed a series of halogenated ILs to improve SO₂ capture through tuning the anion of ILs. Zhang et al.³⁵ studied SO₂ absorption by several TMG-based ILs. Wu et al.³⁶ reported a kind of porous polymer for the SO₂ adsorption. Other investigations using supported ionic liquid membranes (SILMs) for SO₂ gas separation had also been reported.^37,38

In the past few years, Wang et al.³⁹ reported a kind of phosphonium IL trihexyl(tetradecyl) phosphonium tetrazolate ([P₆₆₆₁₄][Tetz]) which could capture 1.50 mole SO₂ per mole of IL under 10% SO₂, while Han et al.⁴⁰ reported a kind of IL 1-(2-diethylaminoethyl)-3-methylimidazolium tetrazolate ([Et₂NEMim][Tetz]) that could achieve 1.85 mole SO₂ per mole of IL under same SO₂ partial pressure through a combination of amine–SO₂ and tetrazolate–SO₂ interaction (Scheme 1). As the anion of ILs plays a key role in SO₂ capture, Wang et al.⁴¹ designed another kind of anion-functionalized IL 1-ethyl-3-methylimidazolium thiocyanate ([Emim][SCN]) that could absorb 0.98 mole SO₂ per mole of IL under 10% SO₂ through S(anion)–S(SO₂) sulfur bridge (Scheme 2). A similar result was also reported by Deng et al.⁴² using other [SCN]-based ILs. Kirchner et al.⁴³ carried out an ab initio molecular dynamics study on the SO₂ solvation in [Emim][SCN]. However, these functionalized ILs were based on the single-site absorption mechanism under low concentration of SO₂. That is, single-site interaction can reach the 1 : 1 (SO₂: anion or cation) stoichiometry for SO₂ absorption.

Scheme 1 The reaction between [Et₂NEMim][Tetz] and SO₂.⁴⁰

Scheme 2 Optimized structure showing the interaction between the anion [SCN] and SO₂.⁴¹

Hence, how to increase the absorption site of the anion and enhance the interaction between anion with SO₂ under low concentration of SO₂ are essential for a breakthrough in absorption techniques. Up to now, many works of SO₂ capture by ILs were mainly focused on the improvement of capacity under high SO₂ concentration. Thus, development of alternative anion-functionalized ILs that are able to achieve high capacity under low SO₂ partial pressure is always highly desired.

In this work, we described a new strategy for significant improving SO₂ capture by basic ILs under low partial pressure (10% vol SO₂ mixed with 90% vol N₂) through introducing a formyl group into the basic anions. For this purpose, several kinds of formyl-containing ILs as well as formyl-free counterparts were prepared and applied to the capture of SO₂ (see Scheme 3 for structures of the formyl-containing ILs). Indeed, these formyl-containing basic ILs were found to exhibit a high capacity up to 1.92 mole SO₂ per mole of IL, compared with their formyl-free counterparts. The promoting role of formyl group tethered to the basic anions of ILs for the enhancement of SO₂ capture under low partial pressure was studied through a combination of spectroscopic investigations and quantum chemical calculations, and the importance of the C=O⋯S interaction and C–H⋯O hydrogen bonding between formyl-based anion and SO₂ was emphasized.

Scheme 3 Structures of cation and anions of the formyl-containing ILs studied in this work.

Experimental

Materials

Several kinds of different formyl-containing basic compounds, such as indole-3-carboxaldehyde (3-CHO-Indo), pyrrole-2-carboxaldehyde (2-CHO-Pyro), 4-formylbenzoic acid (4-CHO-PhCOOH) and p-hydroxybenzaldehyde (4-CHO-PhOH), were purchased from Sigma-Aldrich as the proton donors. Trihexyl(tetradecyl) phosphonium bromide ([P₆₆₆₁₄][Br]) was purchased from Strem Chemicals. A series of gases with different SO₂ partial pressure were prepared by mixing SO₂ (99.95%) and N₂ (99.9993%) which were obtained from Beijing Oxygen Plant Specialty Gases Institute Co., Ltd. An anion-exchange resin (Amersep 900 OH) was obtained from Alfa Aesar. All the chemicals were in the highest purity grade possible, and were used as received unless otherwise stated.

Preparation of ILs

In a typical synthesis of a formyl-containing IL such as [P₆₆₆₁₄][3-CHO-Indo], equimolar 3-CHO-Indo was added to an ethanol solution of phosphonium hydroxide ([P₆₆₆₁₄][OH]), which was prepared from [P₆₆₆₁₄][Br] by the anion-exchange method.^15,44,45 The mixture was stirred at room temperature for 24 h. Then, ethanol and water were distilled off at 60 °C under reduced pressure. All the ILs obtained were dried with P₂O₅ under vacuum at 60 °C for 24 h to reduce possible traces of water.

Characterizations

¹H NMR and ¹³C NMR spectra were recorded on a Bruker spectrometer (400 MHz) in DMSO-d₆ or CDCl₃ with tetramethylsilane (TMS) as the standard. FT-IR spectra were recorded on a Nicolet 4700 FT-IR spectrometer. The structures of these ILs were confirmed by NMR and FT-IR spectra measurements, and no impurities were found by NMR spectra. NMR and FT-IR spectra measurements were also performed to track SO₂ binding and release. The water content in these ILs was determined with a Karl Fisher titrator (Mettler Toledo DL32, Switzerland) and found to be less than 0.1 wt%. The bromide content in these ionic liquids was determined by means of a Br⁻ selective electrode (Shanghai Precision & Scientific Instrument Co. Ltd) coupled with a saturated calomel electrode (Shanghai Precision & Scientific Instrument Co. Ltd), which showed that bromide content was lower than 0.009 mole per kilogram.

SO₂ capture and release

In the measurements of SO₂ absorption under low partial pressure (0.1 bar), SO₂ diluted with N₂ (SO₂/N₂ = 10/90 vol) was bubbled through about 1.0 g IL in a glass container with an inner diameter of 10 mm, and the flow rate was about 40 ml min⁻¹. The glass container was partly immersed in a circulation water bath controlled at desirable temperature. The amount of SO₂ absorbed was determined at regular intervals by the electronic balance with an accuracy of ±0.1 mg until the weight remained constant. The amount of SO₂ absorbed could be calculated by subtracting the amount of IL. The standard deviation of the absorption loadings under 10% SO₂ was 0.05 mole of SO₂ per mole of IL. The simulated acid gases with different SO₂ partial pressure were achieved by changing the flow rate ratio of SO₂ and N₂. After capture, the ILs were regenerated by heating or bubbling N₂ through the SO₂-saturated ILs. In a typical desorption of SO₂, N₂ at atmospheric pressure was bubbled through about 1.0 g SO₂-saturated IL in a glass container, which was partly immersed in a circulation oil bath at 80 °C, and the flow rate was about 40 ml min⁻¹. The release of SO₂ was determined at regular intervals by gravimetric method.

Results and discussion

Absorption of SO₂

To study the SO₂ capture by basic ILs, several kinds of different formyl-containing proton donors (3-CHO-Indo, 2-CHO-Pyro, 4-CHO-PhCOOH, and 4-CHO-PhOH) were selected. These ILs were easily prepared by the acid–base neutralization between each weak proton donor and a solution of [P₆₆₆₁₄][OH] in ethanol, which was prepared by anion-exchange method. The structures of these ILs were verified by NMR and IR spectroscopy (see ESI†).

The effect of different formyl-containing ILs on the absorption of SO₂ was investigated, and the results were shown in Table 1. It can be seen that the mole ratio of SO₂ to [P₆₆₆₁₄][3-CHO-Indo] was 1.92 at 20 °C and 1 bar (SO₂/N₂ = 10/90 vol), which is close to twice of that by [P₆₆₆₁₄][Indo]. Besides, the SO₂ absorption capacities by [P₆₆₆₁₄][2-CHO-Pyro] and [P₆₆₆₁₄][Pyro] were 1.87 and 0.92 mole of SO₂ per mole of IL under same conditions. Thus, the results exhibit and indicate an extremely high increase of SO₂ absorption capacity by the addition of a formyl group in the anion. Additionally, the capacities under the same conditions by [P₆₆₆₁₄][4-CHO-PhO] and [P₆₆₆₁₄][4-CHO-PhCOO] were 1.61 and 1.59 mole SO₂ per mole of IL, whereas that by [P₆₆₆₁₄][PhO] and [P₆₆₆₁₄][PhCOO] were only 1.25 and 1.51 mole of SO₂ per mole of IL, respectively. Clearly, these formyl-containing ILs exhibits a higher SO₂ absorption capacity than that by formyl-free counterparts. Additionally, introducing the formyl group into ILs can also improve the solubility of other acid gases such as CO₂.⁴⁶

Table 1 The effect of different formyl-containing ILs on SO₂ absorption^a

IL	Capacity^b		Data reference
	10% SO2	100% SO2
a SO2 capture at 20 °C and 1 bar until reach the equilibrium. b Mole of SO2 per mole of IL.
[P66614][3-CHO-Indo]	1.92	4.24	This work
[P66614][Indo]	1.00	2.92	47
[P66614][2-CHO-Pyro]	1.87	4.15	This work
[P66614][Pyro]	0.92	2.51	48
[P66614][4-CHO-PhO]	1.61	3.73	This work
[P66614][PhO]	1.25	3.02	34
[P66614][4-CHO-PhCOO]	1.59	3.59	This work
[P66614][PhCOO]	1.51	3.74	34

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It is also clear from Table 1 that the performance of formyl group was greatly affected by the basicity of the ILs. There is no doubt that the formyl group substituent can decrease the basicity of such weak acids as indole, pyrrole, benzoic acid, and phenol. However, the charge on these anions could flow to the formyl groups and make them as an efficient interaction site. For such ILs with strongly basic anions as [P₆₆₆₁₄][Indo] (pK_a = 16.2,⁴⁹ in water) and [P₆₆₆₁₄][Pyro] (pK_a = 16.5,⁵⁰ in water), when formyl group was added into the anions, the increments under 10% SO₂ were 0.92 and 0.95 mole of SO₂ per mole of IL for [P₆₆₆₁₄][4-CHO-Indo] and [P₆₆₆₁₄][4-CHO-Pyro], respectively. While, when formyl group was added into such ILs with weakly basic anions as [P₆₆₆₁₄][PhO] (pK_a = 9.99,⁵¹ in water) and [P₆₆₆₁₄][PhCOO] (pK_a = 4.2,⁵¹ in water), the increments under 10% SO₂ were only 0.36 and 0.08 mole of SO₂ per mole of IL for [P₆₆₆₁₄][4-CHO-PhO] and [P₆₆₆₁₄][4-CHO-PhCOO], respectively.

The increased absorption capacities could be a result of the interactions between formyl group on the anion and SO₂. For [Indo] and [Pyro], their strong basicity results in the strong C=O⋯S interaction and C–H⋯O hydrogen bonding between –CHO and SO₂, thus significantly increased SO₂ absorption capacities by [P₆₆₆₁₄][3-CHO-Indo] and [P₆₆₆₁₄][2-CHO-Pyro]. However, for [PhO] and [PhCOO] with low basicity, the effect of formyl group on the absorption capacity by [P₆₆₆₁₄][4-CHO-PhCOO] is low, because the C=O⋯S interaction and C–H⋯O hydrogen bonding were weak. The results indicated that the effect of formyl group on the anion was strongly affected by the basicity of the anion. Thus, the SO₂ absorption can be facilely improved by formyl-containing ILs through tuning the basicity of the ILs.

Effect of temperature and pressure on SO₂ absorption

The effect of temperature and partial pressure on the SO₂ absorption by [P₆₆₆₁₄][3-CHO-Indo] was investigated as an example. Fig. 1a shows the effect of SO₂ partial pressure on its absorption by [P₆₆₆₁₄][3-CHO-Indo] at 20 °C. It was shown that as the pressure decreased from 1.0 to 0.1 bar, the molar ratio of SO₂ to [P₆₆₆₁₄][3-CHO-Indo] dropped from 4.24 to 1.92. Fig. 1b shows the temperature dependence of the SO₂ absorption at 1.0 bar. It can be seen that when the temperature was increased from 20 °C to 80 °C, SO₂ absorption capacity by [P₆₆₆₁₄][3-CHO-Indo] decreased from 4.24 to 1.70 mole of SO₂ per mole of IL. It is indicated that the captured SO₂ by [P₆₆₆₁₄][3-CHO-Indo] could be facilely stripped by heating or bubbling N₂ through the IL.

Fig. 1 Effect of SO₂ partial pressure (at 20 °C, a) and temperature (at 1.0 bar, b) on SO₂ absorption performance by [P₆₆₆₁₄][3-CHO-Indo].

In addition, multiple SO₂ absorption cycles were investigated by using [P₆₆₆₁₄][3-CHO-Indo] as an example (Fig. 2). It is evident that the SO₂ absorption process could be recycled for more than six times without a loss of absorption capability, indicating that SO₂ absorption process by [P₆₆₆₁₄][3-CHO-Indo] was highly reversible.

Fig. 2 Six consecutive cycles of SO₂ absorption and release by [P₆₆₆₁₄][3-CHO-Indo]. SO₂ absorption was carried out at 20 °C and 1 bar under 100% SO₂ (40 ml min⁻¹), and desorption was performed at 80 °C and 1 bar under 100% N₂ (40 ml min⁻¹).

Quantum chemical calculations

To investigate the role of the two interaction sites on the anion [3-CHO-Indo] in SO₂ absorption, we calculated the charge distribution of N and O atoms on the anion from Natural bond orbital (NBO)⁵² analysis using the Gaussian 09 program⁵³ at B3LYP/6-31++G(p,d) level.^54–56 It was shown in Fig. 3a that the Mulliken atomic charge of N and O atom in the anion [3-CHO-Indo] were −0.552 and −0.676, respectively, while that of N atom in the formyl-free anion [Indo] was −0.594.⁴⁷ Compared with in [Indo], the NBO atomic charge of N atom in [3-CHO-Indo] decreased because the formyl group is a kind of electron withdrawing group. On the other hand, formyl group shared the negative charge of N atom in the anion, which enhanced interactions between formyl group and SO₂, leading to the increase of absorption capacity.

Fig. 3 Optimized structures of [3-CHO-Indo] (a) and [3-CHO-Indo]–SO₂ complexes (b-d) at B3LYP/6-31++G(d,p) level. (b), O atom in [3-CHO-Indo] with the closest SO₂ molecule, [3-CHO-Indo]·SO₂, ΔH = −97.1 kJ mol⁻¹; (c–d), multiple-site chemical interactions between [3-CHO-Indo] and SO₂ molecule: (c), N atom in [3-CHO-Indo] with the closest SO₂ molecule, [3-CHO-Indo]–SO₂, ΔH₁ = −115.3 kJ mol⁻¹; (d), [3-CHO-Indo]–2SO₂, ΔH₂ = −76.3 kJ mol⁻¹. Note that van der Waals radii (in Å) are 1.70 (C), 1.20 (H), 1.52 (O), 1.55 (N), and 1.80 (S), respectively.⁵⁷ O, red; S, yellow; N, blue; C, grey.

To further investigate the interactions between the formyl-containing anion [3-CHO-Indo] and SO₂, we calculated the geometry and energy optimizations for the free anion, the free SO₂ molecule, and each anion–SO₂ complex by dispersion corrected density functional theory (DFT-D3(BJ)) at the B3LYP/6-31++G(p,d) level. The optimized structures reflecting the interactions between the anions and SO₂ were shown in Fig. 3 and S1.† It can be seen from Fig. 3b and c that the intermolecular distance between O or N atoms in the anion [3-CHO-Indo] and S atom in SO₂ was predicted to be 2.194 Å and 2.037 Å, which corresponds to a reduction of approximately 33.9% and 39.2% of the sum of the van der Waals radii of the two interacting atoms, respectively, indicating that the interaction of CHO–SO₂ was weaker than N–SO₂. The calculated SO₂ absorption enthalpies for each site of [3-CHO-Indo] were −115.3 kJ mol⁻¹ for N–SO₂ and −97.1 kJ mol⁻¹ for O–SO₂, respectively. These data suggest that the interactions of each site with SO₂ were both in the chemical regime due to the absorption enthalpies were larger than −50 kJ mol⁻¹. It should be noted from Fig. 3b that hydrogen bond was formed simultaneously between H of –CHO and O of SO₂, and the bond distance was predicted to be 2.252 Å.

In addition, the calculated absorption enthalpies for [3-CHO-Indo]–SO₂ and [3-CHO-Indo]–2SO₂ complexes were −115.3 and −76.3 kJ mol⁻¹, respectively (Fig. 3c and d). Therefore, 1 mole of [3-CHO-Indo]-based IL could absorb about 2 mole of SO₂ under low partial pressure, and formyl-containing ILs can be regarded as the most potential absorbent for SO₂ absorption under 10% SO₂.

Spectroscopic investigations

The interactions of formyl-containing ILs with SO₂ were further investigated by FT-IR and NMR spectroscopy in order to provide new proof for the results obtained from experimental and theoretical calculations. Here, [P₆₆₆₁₄][3-CHO-Indo] was selected as a typical case for such a study.

FT-IR spectrum of [P₆₆₆₁₄][3-CHO-Indo] before and after reaction with SO₂ was shown in Fig. 4. It is clear that new peaks at 1325 cm⁻¹ and 1143 cm⁻¹ appeared after the absorption of SO₂ by [P₆₆₆₁₄][3-CHO-Indo], which could be assigned to asymmetric stretching and symmetric vibration of S=O bonds. On the other hand, the new peak at 947 cm⁻¹ was attributed to S–O stretching,^31,39 indicating SO₂ chemisorption. Another new peak at 1045 cm⁻¹ could be assigned to the vibration of O–S stretching mode of –OSO₂⁻,⁵⁸ which indicates the strong C=O⋯S interactions between the formyl group on the anion and SO₂. Furthermore, a new peak at 2460 cm⁻¹ should be assigned to the C–H⋯O=S hydrogen bonding interaction between the –CHO and SO₂.^47,58 Additionally, a stretching vibration peak of C–H in –CHO shifted from 2700 cm⁻¹ to 2738 cm⁻¹, also suggesting the C–H⋯O hydrogen bonding.^59,60

Fig. 4 The FT-IR spectrum of the fresh IL [P₆₆₆₁₄][3-CHO-Indo] (grey area) and [P₆₆₆₁₄][3-CHO-Indo] after the absorption of SO₂ (red line).

¹H NMR spectra of [P₆₆₆₁₄][3-CHO-Indo] before and after reaction with SO₂ was shown in Fig. 5. In comparison with the ¹H NMR spectra of the fresh [P₆₆₆₁₄][3-CHO-Indo], the typical peak of formyl group in [3-CHO-Indo] moved downfield from 9.61 ppm to 9.93 ppm after reaction with SO₂. Similarly, the typical peak of formyl group in the ¹³C NMR spectrum moved downfield from 179.9 to 185.4, also indicating the formation of C=O⋯S interaction and C–H⋯O hydrogen bonding between formyl group and SO₂. Additionally, the peaks of two C atoms adjacent to the N atom on the anion [3-CHO-Indo] in the ¹³C NMR spectra shifted from 150.4 and 149.5 ppm to 139.0 and 137.6 ppm, respectively, which could be attributed to the strong N⋯S interaction.

Fig. 5 ¹H NMR (a) and ¹³C NMR (b) spectra of the fresh IL [P₆₆₆₁₄][3-CHO-Indo] (blue) and [P₆₆₆₁₄][3-CHO-Indo] after the absorption of SO₂ (red).

Conclusions

In summary, a new strategy for improving SO₂ uptake capacity under low partial pressure was developed through introducing a formyl group into the basic anions of ILs. Compared with the formyl-free counterparts, up to 100% increased capacity could be achieved by formyl-containing ILs. The effect of formyl group on the anion was strongly affected by the basicity of the anion. Quantum chemical calculations and spectroscopic investigations indicate that the absorption capacity increased when formyl group was introduced on the anion because of the additional C=O⋯S interaction and C–H⋯O hydrogen bonding between –CHO and SO₂. Thus, this method provides a potential alternative for acid gas capture and separation such as SO₂, CO₂, and H₂S.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21403059, 21133009), the Natural Science Foundation of Henan Province (No. 142300413213), the S&T Research Foundation of Education Department of Henan Province (No. 14A150031), and the Doctoral Scientific Research Foundation of Henan Normal University (No. qd13007).

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Footnote

† Electronic supplementary information (ESI) available: NMR and IR data of the formyl-containing anion-functionalized ILs. See DOI: 10.1039/c6ra18589d

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