Does the active hydrogen atom in the hydantoin anion affect the physical properties, CO₂ capture and conversion of ionic liquids?

Abstract

Compared to the effect of the active hydrogen atom in the cation in protic ionic liquids (ILs) on their properties and applications, there are very few reports on the role of the active hydrogen atom in the anion. In order to better understand the role of the active hydrogen atom in the anion, the physical properties, CO₂ capture and conversion of three hydantoin-based anion-functionalized ILs ([P₄₄₄₂][Hy], [P₄₄₄₂]₂[Hy], and [HDBU][Hy]) have been investigated via experiments, spectroscopy, and DFT calculations in this work. The results show that the active hydrogen atom in the anion can form anionic hydrogen bonding networks, which significantly increase the melting point and viscosity and decrease the basicity of the IL, thereby weakening its ability to capture and convert CO₂. Interestingly, [P₄₄₄₂][Hy] undergoes a solid/liquid two-phase transition during CO₂ absorption/desorption due to the formation of quasi-intramolecular hydrogen bonding between the active hydrogen atom and the O⁻ atom of the absorbed CO₂, suggesting that the presence of the active hydrogen atom gives [P₄₄₄₂][Hy] the potential to be an excellent molecular switch. As there is no active hydrogen atom in the anion of [P₄₄₄₂]₂[Hy], it shows excellent CO₂ capture and conversion performance through the double-site interaction. [HDBU][Hy] shows the weakest catalytic CO₂ conversion due to the presence of active hydrogen atoms on both its anion and cation. Therefore, the active hydrogen atom in the anion may play a more important role in the properties and potential applications of ILs than the active hydrogen atom in the cation.

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1. Introduction

Ionic liquids (ILs) are promising neoteric materials composed only of anions and cations with unique physical properties. They have a wide range of potential applications, and are classified as aprotic ILs (APILs) and protic ILs (PILs).^1–5 Among them, PILs are formed by the transfer of a proton from a Brønsted acid to a Brønsted base,¹ so they have an active hydrogen atom in the cation, which allows the formation of strong intermolecular hydrogen bonds with the anion, resulting in higher viscosity (η), lower molar volume, and lower enthalpy of vaporization than those of APILs.¹ Consequently, the effects of the active hydrogen atom of PILs on CO₂ capture and catalytic properties have attracted considerable attention. As reported by Wang et al.,^5,6 both the [HMTBD][Im] PIL and the [P₆₆₆₁₄][Im] APIL achieved equimolar CO₂ absorption through the chemical interaction of [Im]⁻ with CO₂, but the former had a relatively slower absorption rate and a significant increase in η after CO₂ absorption. Similar results were found for other azole-based anionic PILs and APILs, suggesting that the active hydrogen atom in the cation of PILs hardly affects the basicity, but their η generally increases with CO₂ uptake.^7–9 Subsequent theoretical studies have shown that the O⁻ in the [anion-CO₂]⁻ intermediate produced by PILs during CO₂ uptake can form intermolecular hydrogen bonds with the active hydrogen atom in the cation, leading to an increase in η and hindering CO₂ mass transfer.^10–13 Nevertheless, there is still some controversy about the role of the active hydrogen atom in the cation of PILs in catalyzing CO₂ conversion.^14–20 For example, Liu et al.¹⁴ and Mu et al.¹⁵ proposed that the catalytic performance of the [HDBU][TFE] PIL for the reaction of CO₂ with 2-aminobenzonitrile was superior to that of the [EtDBU][TFE] APIL, which was attributed to the activation of the –CN of the substrate by the active hydrogen atom in [HDBU]⁺ of [HDBU][TFE] through hydrogen bond interaction. However, Zhang et al.¹⁶ reported that the catalytic ability of [HDBU][Suc] and [HTMG][Suc] PILs was lower than that of the [BzTMA][Suc] APIL, which was attributed to the fact that the active hydrogen atom in the cation could enhance anion–cation interactions thereby weakening the CO₂ capture capacity of [Suc]⁻. Therefore, the active hydrogen atom in the cation of PILs is crucial for their physical properties and applications.

In fact, in addition to the presence of active hydrogen atoms in the cation of PILs, active hydrogen atoms may also be present in the anion of some APILs. For example, when a dibasic acid is neutralized with equimolar alkylphosphonium hydroxide, an active hydrogen atom is present in the anion of an APIL. As the position of the active hydrogen atom in the APIL is different from that of the PIL, the effects on the respective properties may be different. Compared with the study of the effect of the active hydrogen atom in the cation of a PIL on its properties and applications,^10–13 few reports have discussed the role of the active hydrogen atom in the anion of an APIL. Therefore, it is worthwhile to study the role of the active hydrogen atom in the anion and to reveal the similarities and differences between the active hydrogen atom in the anion of an APIL and that in the cation of a PIL.

To this end, a novel APIL with the active hydrogen atom in the anion, [P₄₄₄₂][Hy], is prepared via a neutralization reaction of [P₄₄₄₂][OH] with equimolar amounts of hydantoin (Scheme 1). For comparison, [P₄₄₄₂]₂[Hy] without the active hydrogen atom and [HDBU][Hy] with the active hydrogen atoms in both the anion and cation are also synthesized (Scheme 1). η and basicity of three hydantoin-based ILs are determined to study the effect of the active hydrogen atom in the anion on their physical properties. The CO₂ absorption behavior and absorption mechanism of [P₄₄₄₂][Hy] and [P₄₄₄₂]₂[Hy] are investigated to explore the role of the active hydrogen atom in the anion on CO₂ capture and activation. Moreover, the reaction of CO₂ with 2-aminobenzonitrile catalyzed by hydantoin-based ILs is used as an example to investigate the effect of the active hydrogen atom in the anion on the catalytic conversion of CO₂. Based on the physical properties, CO₂ absorption and catalytic reaction experiments, spectra, and DFT calculations, it is found that the active hydrogen atom in the hydantoin-based anion can form anionic hydrogen bonding networks, which significantly increase η and decrease the basicity of the IL thereby weakening its ability to capture and convert CO₂. Interestingly, the active hydrogen atom in the anion allows [P₄₄₄₂][Hy] to be a potential CO₂-response molecular switch. [P₄₄₄₂]₂[Hy] with no active hydrogen atom showing excellent CO₂ capture and conversion ability through the double-site interaction. The present work suggests that the active hydrogen atom in the anion may play a more important role in the properties and applications of the IL than the active hydrogen atom in the cation.

Scheme 1 Synthesis of [P₄₄₄₂][Hy], [P₄₄₄₂]₂[Hy], and [HDBU][Hy].

2. Experimental and theoretical

Materials

Hydantoin (Hy, >99%), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, >98%), tributylphosphine (P₄₄₄, >95%), bromoethane (C₂H₅Br, >99%), sodium hydroxide (NaOH, >99%), acetonitrile (CH₃CN, >98%), ethanol (>99%), 2-aminobenzonitrile (C₇H₆N₂, >99%), potassium hydrogen phthalate (C₈H₅O₄K, 99.99%), and a basic anion-exchange resin (Amberlite 717, AR) were purchased from Aladdin Reagent Co. Ltd, Shanghai, China. CO₂ (>99.9%) and N₂ (>99.999%) were purchased from Shaoxing Zhongxu Gas Factory. All chemicals were used as received and obtained in highest purity grade possible.

Synthesis and characterization of hydantoin-based ILs

The synthesis process of [P₄₄₄₂][Hy] and [P₄₄₄₂]₂[Hy] is similar to that in our previous work.^21,22 First, P₄₄₄ and C₂H₅Br were reacted under N₂ protection to give the tributylethylphosphonium bromide ([P₄₄₄₂][Br]). Second, [P₄₄₄₂][Br] was passed through the basic anion-exchange resin activated by NaOH to give the tributylethylphosphonium hydroxide ([P₄₄₄₂][OH]) ethanol solution, and its concentration was determined by titration of a potassium hydrogen phthalate solution. Finally, [P₄₄₄₂][Hy] was prepared by neutralization reaction of [P₄₄₄₂][OH] and Hy in a molar ratio of 1 : 1 for 48 h at room temperature. [P₄₄₄₂]₂[Hy] was synthesized similarly by neutralization of [P₄₄₄₂][OH] with Hy in a 2 : 1 molar ratio. A large amount of ethanol and a small amount of water were removed from the [P₄₄₄₂][Hy] and [P₄₄₄₂]₂[Hy] under vacuum at 343.15 K. Trace amounts of water were removed from [P₄₄₄₂][Hy] and [P₄₄₄₂]₂[Hy] with a freeze dryer for 72 h. [HDBU][Hy] was synthesized by the equimolar proton transfer reaction of Hy with DBU, similar to the preparation of [HDBU][Im] and [HTMG][Im].^7,8 In a typical synthesis of [HDBU][Hy], 0.25 mol Hy was divided into several parts and added into 0.25 mol DBU in batches under vigorous stirring at 353.15 K for 24 h. The obtained IL was then dried under vacuum at 353.15 K for at least 48 h to remove possible traces of water. The specific reaction equations are shown in Scheme 1.

The water content of [P₄₄₄₂][Hy], [P₄₄₄₂]₂[Hy], and [HDBU][Hy] was determined via Karl Fisher Coulometric Titration (Mettler Toledo C20S), and was found to be 820, 860, and 1000 ppm, respectively. The structure of [P₄₄₄₂][Hy], [P₄₄₄₂]₂[Hy], and [HDBU][Hy] was characterized by ¹H NMR spectroscopy using a Bruker AVANCE III 400 MHz spectrometer. The thermal stability of [P₄₄₄₂][Hy], [P₄₄₄₂]₂[Hy], [HDBU][Hy], [P₄₄₄₂][Hy–CO₂], and [P₄₄₄₂]₂[Hy–2CO₂] was determined by measuring their thermogravimetric analysis (TGA) curves under N₂ of 60 mL min⁻¹ at 10 °C min⁻¹. The melting point of [P₄₄₄₂][Hy] and [HDBU][Hy] was determined by differential scanning calorimetry (Mettler Toledo DSC1) at 15 °C min⁻¹.

Measurement of η of [P₄₄₄₂][Hy], [P₄₄₄₂][Hy–CO₂], and [P₄₄₄₂]₂[Hy]

η of [P₄₄₄₂]₂[Hy] was measured using an Anton Paar AMVn falling ball automated microviscometer. The measurement methods and procedures were described in our previous work.²¹ The temperature for η measurement was controlled with a built-in precise Peltier thermostat with an accuracy of T = ±0.05 K. η of [P₄₄₄₂][Hy] and [P₄₄₄₂][Hy–CO₂] was measured with a Brookfield DV2T viscometer,²³ and the temperature was controlled using a super-thermostatic water bath with an accuracy of T = ±0.05 K.

Measurement of the basicity of hydantoin-based ILs

The basicity of [P₄₄₄₂][Hy], [P₄₄₄₂]₂[Hy], and [HDBU][Hy] are characterized using pyrrole as a probe molecule.^24,25 FT-IR spectra of the mixtures of IL with equimolar pyrrole were measured by using a Nicolet 6700 Fourier transform infrared spectrometer equipped with a DTGS detector, KBr windows, and ATR accessory. Each ATR-IR spectrum was recorded with a resolution of 2 cm⁻¹ and 16 parallel scans, and the wavenumber was in the range from 4000 to 600 cm⁻¹. The basicity of the hydantoin-based ILs was characterized by comparing the changes of the N–H infrared stretching vibrations of pure pyrrole and pyrrole in the mixture.

CO₂ absorption/desorption and absorption mechanism by [P₄₄₄₂][Hy] and [P₄₄₄₂]₂[Hy]

The process of CO₂ absorption and desorption by [P₄₄₄₂][Hy] and [P₄₄₄₂]₂[Hy] was similar to that in our previous work.²¹ A typical process is as follows: 0.1 MPa CO₂ was bubbled through approximately 1.0 g IL in a glass container with an inner diameter of 10 mm, and the flow rate was controlled at about 60 mL min⁻¹. The glass container was placed in a thermostat at the desired temperature, and the amount of CO₂ absorbed was measured periodically using an analytical balance with an accuracy of ±0.0001 g until the weight was constant. Low partial pressure CO₂ was diluted with N₂ by controlling the flow rate ratio. Desorption was carried out at 343.15 K by bubbling N₂ at a flow rate of about 20 mL min⁻¹ through the CO₂-absorbed [P₄₄₄₂][Hy] and CO₂-absorbed [P₄₄₄₂]₂[Hy], and the obtained [P₄₄₄₂][Hy] and [P₄₄₄₂]₂[Hy] were used for the next capture of CO₂. The CO₂ absorption mechanism of [P₄₄₄₂][Hy] and [P₄₄₄₂]₂[Hy] was studied by comparing the FT-IR and ¹³C NMR spectra of CO₂-free and CO₂-absorbed ILs. FT-IR spectra were recorded on a Nicolet 6700 Fourier transform infrared spectrometer with an ATR accessory.

Reaction of CO₂ with 2-aminobenzonitrile to prepare quinazoline-2,4(1H,3H)-diones catalyzed by hydantoin-based ILs

The catalytic properties of [P₄₄₄₂][Hy], [P₄₄₄₂]₂[Hy], and [HDBU][Hy] for the reaction of CO₂ with 2-aminobenzonitrile to prepare quinazoline-2,4(1H,3H)-diones were investigated under optimal reaction conditions reported in our previous work.²² For example, 2-aminobenzonitrile (4 mmol) and [P₄₄₄₂]₂[Hy] (0.8 mmol) were added to a round bottom flask containing a magnet. CO₂ was passed through to remove the air in the flask. A CO₂ balloon was connected to the flask, and the CO₂ pressure was ensured to be 0.1 MPa and the mixture was stirred at 353.15 K. The progress of the reaction was monitored by HPLC (Zorbax RX-C8 column, the mobile phase was acetonitrile: water = 5 : 5 in volume ratio and the detection wavelength λ was 245 nm) and the content of quinazoline-2,4(1H,3H)-diones was determined using the external standard method to calculate the product yield.

Computation method

The DFT calculations were performed using the Gaussian 09 D.01 program,²⁶ with geometry optimizations and interaction energy calculations at the B3LYP-D3/6-31++G (d, p) basis set level.²⁷ The optimized geometry of [P₄₄₄₂][Hy] and [P₄₄₄₂][Hy–CO₂] ion pairs, [Hy]⁻, [Hy]²⁻, CO₂, [Hy–CO₂]⁻, [Hy–2CO₂]²⁻, chain-like intermolecular hydrogen bonding of [Hy]⁻, and six-membered cyclic quasi-intramolecular hydrogen bond formed by [Hy–CO₂]⁻ was considered to be a local minimum with no imaginary frequencies. The energies of chain-like intermolecular hydrogen bonding of [Hy]⁻ and [P₄₄₄₂][Hy] and [P₄₄₄₂][Hy–CO₂] ion pairs, the interactions between [Hy]⁻ and CO₂, six-membered cyclic quasi-intramolecular hydrogen bond formed by [Hy–CO₂]⁻, and the interactions between [Hy]²⁻ and CO₂, were calculated with the basis set superposition error (BSSE) corrections,^28,29 where the enthalpy (ΔH) and Gibbs free energy (ΔG) were calculated as the difference between the energy of the complex and the sum of the energies of the optimized monomers. Using the approaches described by Katsyuba and co-workers,³⁰ the energies of chain-like intermolecular hydrogen bonding of [Hy]⁻ were extracted and the method is described in the ESI.† The Cartesian coordinates of the optimized structures are shown in Fig. S1 (ESI†).

3. Results and discussion

Comparison of physical properties of hydantoin-based ILs

¹H NMR data of [P₄₄₄₂][Br] and hydantoin-based ILs are found to be in good agreement with their corresponding chemical structures (Fig. S2, ESI†). The TGA curves of the three hydantoin-based ILs show that they have good thermal stability, and the decomposition temperature is higher than 200 °C (Fig. S3, ESI†). Among them, [P₄₄₄₂][Hy] has the best thermal stability, while [HDBU][Hy] has relatively weak thermal stability.

At room temperature (∼298 K), [P₄₄₄₂]₂[Hy] is a liquid, [HDBU][Hy] is a gel, while [P₄₄₄₂][Hy] is a solid (Fig. S4, ESI†). As shown in Fig. S5 (ESI†), the melting points of [P₄₄₄₂][Hy] and [HDBU][Hy] are 325.20 and 358.39 K (onset temperature), respectively. As shown in Fig. S6 (ESI†), η of [P₄₄₄₂][Hy] is larger than that of [P₄₄₄₂]₂[Hy]. The differences in melting point and η of three hydantoin-based ILs may be attributed to the role of the active hydrogen atom in [Hy]⁻. To reveal the effect of the active hydrogen atom, the structure of [Hy]⁻ is optimized using DFT calculations (Fig. 1a), which shows that the active hydrogen atom in [Hy]⁻ can spontaneously form chain-like intermolecular hydrogen bonds with the N⁻ atom of another anion with ΔG = −56.48 kJ mol⁻¹, leading to an increase in the melting point and η.¹ In addition, the active hydrogen atom in [Hy]⁻ of [P₄₄₄₂][Hy] tends to form hydrogen-bonded aggregates of multiple anions, which is different from the formation of bimolecular hydrogen bonding between the active hydrogen in the cation and the anion of PILs.^5–7 This is the main reason why [P₄₄₄₂][Hy] with an active hydrogen atom in the anion is a solid, whereas most PILs with the active hydrogen atom in the cation are liquids at room temperature. When the active hydrogen atom in [Hy]⁻ of [P₄₄₄₂][Hy] is replaced by a methyl group, the obtained [P₄₄₄₂][1-MHy] is a liquid with η of 182.74 cp at 318.15 K,²¹ confirming that the active hydrogen atom in [Hy]⁻ leads to an increase in the melting point and η of [P₄₄₄₂][Hy] through the formation of chain-like intermolecular hydrogen bonding. Due to the presence of active hydrogen in both anions and cations, [HDBU][Hy] has a higher melting point than [P₄₄₄₂][Hy].

Fig. 1 The optimized geometry and the calculated energy. (a) Chain-like intermolecular hydrogen bonding of [Hy]⁻; (b) CO₂ absorption by [Hy]⁻ and quasi-intramolecular cyclic hydrogen bonds formed by [Hy–CO₂]⁻ and the active hydrogen atom; (c) double-site interaction between [Hy]⁻ and CO₂.

As shown in Fig. 2, the degree of red-shift of the wavenumber of N–H in the IL + pyrrole mixture indicates that the basicity of hydantoin-based ILs is as follows: [P₄₄₄₂]₂[Hy] > [P₄₄₄₂][Hy] > [HDBU][Hy], suggesting that the active hydrogen atom in the anion plays an important role in the basicity. The basicity of [P₄₄₄₂][Hy] is weaker than that of [P₄₄₄₂]₂[Hy], due to hydrogen bond interactions of the active hydrogen atom in [Hy]⁻ with the basicity center N⁻ atom of [Hy]⁻. Because the active hydrogen atoms in both the cation and anion can form hydrogen bond interactions with the N⁻ atom of [Hy]⁻, the basicity of [HDBU][Hy] is the weakest. By comparing the CO₂ capture capacity of [HMTBD][Im] and [P₆₆₆₁₄][Im], it is known that the active hydrogen atom in the cation has a weak effect on the basicity,^5,6 whereas the present results show that the active hydrogen atom in the anion can weaken the basicity of ILs, which seems to indicate that the basicity of ILs can be modulated by the position of the active hydrogen atom.

Fig. 2 FT-IR spectra of pure pyrrole and the IL + pyrrole mixture.

CO₂ absorption behaviour and the absorption mechanism of [P₄₄₄₂][Hy] and [P₄₄₄₂]₂[Hy]

As shown in Fig. 3, the CO₂ absorption capacity of [P₄₄₄₂][Hy] and [P₄₄₄₂]₂[Hy] decreases with increasing temperature, and the saturation capacity at 303 K is 0.84 and 1.90 mol CO₂ per mol IL, respectively. Interestingly, when CO₂ is poured into [P₄₄₄₂][Hy] at 303 K and stirred, [P₄₄₄₂][Hy] changes from a solid to a liquid with a gradual decrease in η (Fig. S4, ESI†), which is different from the increase in η of PILs with CO₂ absorption,⁷ while [HDBU][Hy] remains a gel in the same situation. The η of [P₄₄₄₂][Hy–CO₂] at saturation is 506.4 cp at 315.15 K, which is lower than the η of the pure [P₄₄₄₂][Hy]. The interaction energies of [P₄₄₄₂][Hy] and [P₄₄₄₂][Hy–CO₂] ion pairs (Fig. S7, ESI†) show a decrease in the interaction between the anion and cation after CO₂ absorption by [P₄₄₄₂][Hy], which may be a reason for the decrease in η of [P₄₄₄₂][Hy–CO₂]. Unlike [P₄₄₄₂]₂[Hy], the CO₂ capture capacity of [P₄₄₄₂][Hy] does not reach equimolar absorption. It is clear that the CO₂ absorption behaviour of [P₄₄₄₂][Hy] is different from that of [P₄₄₄₂]₂[Hy] and [HDBU][Hy], so it is worth investigating the CO₂ absorption mechanism of [P₄₄₄₂][Hy].

Fig. 3 CO₂ absorption behavior of [P₄₄₄₂][Hy] and [P₄₄₄₂]₂[Hy] at 0.1 MPa.

As shown in Fig. 4, a new peak at 1714 cm⁻¹ appears in FT-IR spectra of [P₄₄₄₂][Hy–CO₂], accompanied by a new resonance peak at 162.6 ppm in the ¹³C NMR spectrum, indicating the formation of a new carbonyl by chemical interaction of the N⁻ atom in [Hy]⁻ with CO₂.²¹ Furthermore, the broad peak at 3220 cm⁻¹ of [P₄₄₄₂][Hy] in the FT-IR spectra, which is attributed to the stretching vibration peak of N–H of the intermolecular hydrogen bond of [Hy]⁻, gradually disappears after CO₂ absorption, together with the weakening of the intensity of the original C=O of [Hy]⁻ at 1585 cm⁻¹, suggesting that N–H and C=O may have interacted simultaneously with the CO₂ absorbed by the anion. Correspondingly, the original C=O of [Hy]⁻ in the ¹³C NMR spectra at 191.2 and 175.0 ppm shifts to a lower field and appears at 179.0 and 160.1 ppm after CO₂ absorption. Therefore, considering the formation of a new C=O and the changes in the original N–H and C=O in [Hy]⁻ after CO₂ absorption, as well as the decrease in η of [P₄₄₄₂][Hy] after CO₂ absorption, the following CO₂ absorption mechanism is proposed (Scheme 2): The N⁻ atom of [Hy]⁻ first interacts with CO₂ to form carbamate carbonyl, and then a six-membered cyclic hydrogen bond is formed between the original C=O in [Hy]⁻ and the O⁻ atom in the carbamate carbonyl as well as the active hydrogen atom. To further confirm the mechanism, the formed intermediates and hydrogen bonds are optimized using DFT calculations. As shown in Fig. 1b, ΔH and ΔG for the interaction of the N⁻ atom in [Hy]⁻ with CO₂ are −17.35 and −20.17 kJ mol⁻¹, respectively. Interestingly, the [Hy–CO₂]⁻ intermediate can spontaneously form stable cyclic quasi-intramolecular bonds with ΔH and ΔG of −180.65 and −184.48 kJ mol⁻¹, respectively. Therefore, the combination of CO₂ absorption experiments, spectroscopy and DFT calculations suggests that the sub-equimolar CO₂ capture capacity of [P₄₄₄₂][Hy] is due to the N⁻ atom of [Hy]⁻ being partially occupied by the active hydrogen atom through the occurrence of chain-like hydrogen bond interactions (Fig. 1a). When CO₂ is absorbed, the original chain-like intermolecular hydrogen bonds are converted into cyclic quasi-intramolecular hydrogen bonds, resulting in a decrease in η.^31–33 As the original N–H and C=O groups of [Hy]⁻ are involved in the formation of the cyclic hydrogen bonds of [Hy–CO₂]⁻, their infrared absorption intensity is weakened and the chemical shift is shifted to higher fields.

Fig. 4 ¹³C NMR (a) and FT-IR (b) spectra of [P₄₄₄₂][Hy] before and after CO₂ capture.

Scheme 2 Possible mechanism of CO₂ absorption by [P₄₄₄₂][Hy] and [P₄₄₄₂]₂[Hy].

In addition, the CO₂ absorbed by [P₄₄₄₂][Hy] can be easily desorbed by heating, and then [P₄₄₄₂][Hy] is regenerated and returned to the solid state. As shown in Fig. 5a, five cycles of absorption/desorption indicate that [P₄₄₄₂][Hy] has good reusability. Therefore, considering the good reusability and the change of state during CO₂ absorption–desorption, [P₄₄₄₂][Hy] is a potential CO₂-responsive molecular switch,^34,35 resulting from the contribution of the active hydrogen atom in [Hy]⁻.

Fig. 5 CO₂ absorption/desorption by [P₄₄₄₂][Hy] (a) and [P₄₄₄₂]₂[Hy] (b) at 303 K and release at 343 K under N₂.

Although [P₄₄₄₂][Hy] has the potential to be a CO₂-response molecular switch, the hydrogen-bonded aggregates formed by the active hydrogen atoms are unfavorable for CO₂ capture. When the active hydrogen atom in [Hy]⁻ is neutralized to dual anion [Hy]²⁻, [P₄₄₄₂]₂[Hy] is not only less viscous (Fig. S6, ESI†), but also exhibits an excellent CO₂ absorption capacity (Fig. 3), which is larger than those of other dual anion ILs such as [P₄₄₄₂]₂[L-Ser] (1.10 mol CO₂ per mol IL at 298 K) and [P₄₄₄₂]₂[IDA] (1.69 mol CO₂ per mol IL at 313 K).^31,36 Therefore, the CO₂ absorption mechanism of [P₄₄₄₂]₂[Hy] is investigated. As shown in Fig. 6, the FT-IR spectra of CO₂-absorbed [P₄₄₄₂]₂[Hy] show two new peaks at 1716 and 1616 cm⁻¹, and its ¹³C NMR spectrum also shows two new peaks at 158.9 and 153.7 ppm, indicating that the two N⁻ atoms in [Hy]²⁻ can chemically interact with CO₂, thus realizing the efficient absorption of CO₂ through the double-site interaction. The TGA curve of [P₄₄₄₂]₂[Hy–CO₂] shows two weight loss peaks (Fig. S3b, ESI†), confirming that [P₄₄₄₂]₂[Hy] can absorb CO₂ through the double-site interaction. Accordingly, the CO₂ absorption mechanism of [P₄₄₄₂]₂[Hy] is proposed (Scheme 2), and the structure of [Hy–2CO₂]²⁻ is optimized by DFT calculations with ΔG of −97.57 kJ mol⁻¹ (Fig. 1c).

Fig. 6 ¹³C NMR (a) and FT-IR (b) of [P₄₄₄₂]₂[Hy] before and after CO₂ capture.

The effects of CO₂ partial pressure and temperature on the CO₂ absorption by [P₄₄₄₂]₂[Hy] are investigated and are shown in Fig. S8a (ESI†). As the CO₂ partial pressure decreases from 0.1 to 0.02 MPa, the saturation absorption capacity of [P₄₄₄₂]₂[Hy] at 303 K decreases from 1.90 to 1.55 mol CO₂ per mol IL, but it still shows a good absorption capacity, indicating that [P₄₄₄₂]₂[Hy] has the potential to absorb low concentrations of CO₂. The saturation absorption capacity of [P₄₄₄₂]₂[Hy] at different partial pressures and temperatures is plotted in Fig. S8b (ESI†), showing that the effect of temperature is greater than that of partial pressure. Based on the saturated absorption capacity at 303–333 K and 0.02 MPa, combined with the van der Hoff equation (Fig. S8c, ESI†), the absorption enthalpy of CO₂ of [P₄₄₄₂]₂[Hy] is obtained to be −58.62 kJ mol⁻¹, which is lower than that of [P₆₆₆₁₄]₂[Asp] (−223 kJ mol⁻¹) and [P₄₄₄₂]₂[IDA] (−141.8 kJ mol⁻¹).^36,37 Due to the lower absorption enthalpy, [P₄₄₄₂]₂[Hy] shows good absorption–desorption ability (Fig. 5b). Furthermore, the FT-IR spectra of pure and received [P₄₄₄₂]₂[Hy] compared in Fig. S9 (ESI†) show that [P₄₄₄₂]₂[Hy] has excellent reusability, suggesting that [P₄₄₄₂]₂[Hy] has the ability to activate and catalyze CO₂ conversion.^38–40

Comparison of the catalytic CO₂ conversion performance of hydantoin-based ILs

According to the literature,^41–43 when CO₂ is activated by the catalyst, the O–C–O angle decreases from 180° to about 130°, and the weaker the activation is when the O–C–O angle tends to be 180°. The optimized structures of the intermediates by DFT calculations show that the O–C–O angle of [Hy–2CO₂]²⁻ is close to 130° (Scheme 2), while that of [Hy–CO₂]⁻ is 170° (Scheme 2), resulting from the formation of the six-membered ring hydrogen bond. The difference in the O–C–O angle in [Hy–CO₂]⁻ and [Hy–2CO₂]²⁻ seems to indicate that the active hydrogen atom of [Hy]⁻ is unfavorable for CO₂ activation and subsequent conversion.

To confirm the above speculation, the reactions of CO₂ with 2-aminobenzonitrile catalyzed by [P₄₄₄₂][Hy], [P₄₄₄₂]₂[Hy], and [HDBU][Hy] under solvent-free conditions at 0.1 MPa CO₂ are studied (Fig. S10, ESI†). The optimal reaction conditions are listed in Table 1 and compared with the reported data, and the sequence of the catalytic properties of ILs over 24 h is as follows: [P₄₄₄₂]₂[Hy] > [P₄₄₄₂][Hy] > [HDBU][Hy], indicating that the active hydrogen atom in [Hy]⁻ may weaken the catalytic properties. From our previous work,²² it is known that the [Hy]²⁻ of [P₄₄₄₂]₂[Hy] simultaneously activates CO₂ and –NH₂ of 2-aminobenzonitrile through its basic center N⁻ atom to realize the efficient catalytic properties. In the case of [P₄₄₄₂][Hy], the anion basic center N⁻ atom of [Hy]⁻ is easily occupied by other active hydrogen atoms through intermolecular hydrogen bonding, which will reduce its ability to activate both CO₂ and –NH₂ and lead to the decrease of the catalytic performance. In addition, the absorbed CO₂ in [Hy–CO₂]⁻ forms cyclic quasi-intramolecular hydrogen bonds with the active hydrogen atom, which reduces η but does not favor CO₂ activation. Replacing the active hydrogen atom in [Hy]⁻ with a methyl group improves the catalytic performance of [P₄₄₄₂][1-MHy],²¹ confirming that the active hydrogen atom is detrimental to the catalytic performance of [P₄₄₄₂][Hy]. Moreover, comparison of entries 1–2 reveals that the presence of the active hydrogen atom in the anion reduces the product yield by 14%, while entries 8–9 show that the presence of the active hydrogen atom in the cation reduces the yield by 9.6%, indicating that the effects of the active hydrogen atom in the anion on the catalytic ability of ILs are slightly greater than those of the active hydrogen atom in the cation. Therefore, it is not difficult to understand why [HDBU][Hy] shows the worst catalytic performance given the presence of both active hydrogen atoms on its anion and cation. As can be seen from entries 1 and 10, the amount of [P₄₄₄₂]₂[Hy] is much smaller than that of [Ch][Im] at similar yields,³⁸i.e., the catalytic properties of the former are better than those of the latter, due to the fact that the absorbed CO₂ is more easily desorbed in the former than in the latter,³⁸ and also indicating that the ability to activate CO₂ is better for [P₄₄₄₂]₂[Hy] than for [Ch][Im]. Therefore, when designing ILs for CO₂ uptake and chemical conversion, it is necessary to consider not only the effect of the active hydrogen atom in the cation, but also the effect of the active hydrogen atom in the anion.

Table 1 The reaction of CO₂ with 2-aminobenzonitrile catalysed by ILs at 0.1 Mpa CO₂^a

Entry	ILs	Time (h)	yield (%)	Ref.
a Reaction conditions: 20 mol% of IL to 2-aminobenzonitrile, 353.15 K, without solvent, HPLC yield. b 100 mol% of IL to 2-aminobenzonitrile, 343.15 K, with DMSO. c 100 mol% of IL to 2-aminobenzonitrile.
1	[P4442]2[Hy]	24	98	This work
2	[P4442][Hy]	24	84	This work
3	[P4442][Hy]	46	94	This work
4	[HDBU][Hy]	24	72	This work
5	[HDBU][Hy]	43	73	This work
6	[P4442][1-MHy]	24	92	21
7	[P4442][1-MHy]	30	97	21
8	[HTMG][Suc]	10	89.2^b	16
9	[BzTMA][Suc]	10	98.8^b	16
10	[Ch][Im]	20	98^c	38

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Conclusions

The present study shows that the active hydrogen atom in [Hy]⁻ of [P₄₄₄₂][Hy] can form chain-like intermolecular hydrogen bonds with the N⁻ atom, which causes the basicity center of [Hy]⁻ to be occupied, leading to an increase in the melting point, a decrease in the basicity, and ultimately a decrease in CO₂ capture capacity and catalytic CO₂ conversion performance. Interestingly, [P₄₄₄₂][Hy] exhibits a solid/liquid two-phase transition during CO₂ absorption/desorption due to the formation of quasi-intramolecular hydrogen bonding between the active hydrogen atom and the O⁻ atom of the absorbed CO₂, suggesting that the presence of the active hydrogen atom gives [P₄₄₄₂][Hy] the potential to be an excellent molecular switch. Since there is no active hydrogen atom in the anion of [P₄₄₄₂]₂[Hy], it shows excellent carbon capture and conversion performance through the double-site interaction. The results suggest that the active hydrogen atom in the anion plays an important role in the properties and potential applications of ILs, which is different from the role of the active hydrogen atom in the cation.

Author contributions

Tingting Chen: conceptualization, investigation, and writing (original draft). Zhongyuan Sun: investigation and visualization. Yujun Guo: data curation, validation, and formal analysis. Yingjie Xu: supervision, writing (review & editing), funding acquisition, and resources.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21978172).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cp05965k

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