Film forming polymeric ionic liquids (PILs) based on polybenzimidazoles for CO₂ separation

Abstract

PILs are emerging as promising materials for CO₂ capture. Film formation, which is a requisite for membrane formation, is induced in a family of PILs by N-substituting a rigid thermo-mechanically stable polybenzimidazole, followed by metathesis. This provided two IL groups per repeat unit of the PIL and enhanced the CO₂ separation characteristics.

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The continual rise in the CO₂ concentration in the atmosphere is a major concern as it may lead to environmental and economic threats caused by climate change. One of the major sources of CO₂ emissions is from the combustion of fossil fuels.^1–7 CO₂ capture by various techniques has been an area of extensive research in recent years. The most common CO₂ capture technologies include absorption (using an amine or alkaline solution), pressure swing adsorption and membrane based separation.^5–8 Absorption technologies are energy intensive, need solvent regeneration steps and encounter solvent loss and corrosion.^2,7 Adsorption technologies are cost intensive and inadequate for CO₂ capture from flue gas.⁸ Membrane based separations require lower capital costs, have operational simplicity, are energy efficient, are modular in nature (easy to scale up) and most importantly, are environmentally benign (physical nature of separation). Considerable research efforts are devoted worldwide to make techniques techno-economically feasible for CO₂ separtion.^1,6–10

A growing interest in using ionic liquids (ILs) as CO₂ separation media stems from their exceptional properties, e.g. their negligible vapour pressure, high thermal stability and tunable physico-chemical properties, which improve the CO₂ uptake.^2,3,7,11 Although IL impregnated membranes look attractive, they require a thicker support, have poor mechanical stability, have inadequate long-term sustainability and their operation is limited to low pressures.^5,6,12 As an alternative, the polymeric counterpart of ILs, viz. ‘polymeric ionic liquids’ (PILs), are emerging as options for membrane materials. They exhibit the combined properties of ILs as well as polymeric materials, showing higher CO₂ sorption as well as faster adsorption–desorption kinetics compared to ILs. They also have an appreciable thermal stability, which is required for CO₂ separation at high temperatures.^2,3,13,14 However, most of the present PILs are unable to be processed into a film form due to their brittle nature and cannot be used as gas separation membranes.^15–17 They require crosslinking or copolymerization methodologies even for their effective testing in a flat film form.¹⁶ Hu et al. have demonstrated a film forming PIL by grafting polyethylene glycol (PEG) onto the glassy P[VBTMA][BF₄] and P[MATMA][BF₄].¹⁵ However, the higher amount of PEG needed reduced the IL character in the PIL–copolymer membrane. Although coating an IL on a porous support followed by UV polymerization provided a film, its fragile nature restricted the application pressure to ∼1–2 bar.^9,16 Li et al. have demonstrated a crosslinking methodology to obtain a PIL film,⁶ which is known to hamper the CO₂ separation performance of PILs.^13,16 Thus, a radically different approach is required to obtain film forming PILs for effective CO₂ separation.

Interestingly, many of the known ILs and PILs contain the imidazolium cation.^{3,11,12,14,18} With this base, we developed a new, versatile class of PILs, where a fully aromatic polybenzimidazole (PBI, possessing two imidazole groups per repeat unit, excellent thermo-mechanical stability and film forming ability)^19,20 is transformed into a PIL via an imidazole N-quaternization reaction by an alkyl group. This approach offered two IL groups per repeat unit of the PIL, while allowing structure–property tunability by varying the PBI and substituent alkyl group. A metathesis reaction (anion exchange reaction) using anions of interest further widens the possibility of PIL structure variation. To demonstrate the feasibility of this approach, we chose a PBI possessing a tert-butyl group (PBI-BuI, known for its higher gas permeability).²⁰ The imidazole N-quaternization reaction was performed using methyl iodide (Scheme 1), followed by metathesis with anions that are known to show significant CO₂ sorption in ILs, viz. BF₄, Tf₂N and acetate^7,11,12 (procedure given in ESI†).

Scheme 1 Synthesis of PIL based on PBI-BuI.

The degree of N-quaternization (DQ) in the resulting [TMPBI-BuI][I] was assessed by ¹H-NMR spectroscopy and was found to be 96%, (ESI, Fig. S1†). Three PILs, viz. [TMPBI-BuI][BF₄], [TMPBI-BuI][Tf₂N] and [TMPBI-BuI][Ac], were obtained by metathesis of [TMPBI-BuI][I] using AgBF₄, LiTf₂N and CH₃COOAg, respectively. In the case of two PILs, viz. [TMPBI-BuI][BF₄] and [TMPBI-BuI][Ac], quantitative anion exchange (confirmed by Volhard's method,²¹ as given in the ESI†) was achieved by using the Ag salt of the anion during metathesis, which was irreversible in nature due to precipitate formation of AgI. Analysis of [TMPBI-BuI][Tf₂N] for the remaining iodide in it showed the anion exchange was 86%. This incomplete exchange is attributable to the reversible nature of the metathesis (between iodide and Tf₂N), since both the formed PIL, [TMPBI-BuI][Tf₂N], and the by-product, LiI, are soluble in the solvent used (DMF). FTIR analysis showed the appearance of new peaks attributable to the respective anion, as shown in Fig. S2, ESI.†

PIL membranes prepared by the solution casting method (see ESI†) showed exciting results. Fig. 1 shows that the [TMPBI-BuI][BF₄] and [TMPBI-BuI][Tf₂N] based membranes (thickness 40 μm) were mechanically strong and could easily sustain an applied load of 50 g to a 1 cm wide film, without any breaks or tears upon hanging for 24 h. Further support of their pressure-withstanding ability of a minimum of 20 atm was seen during the permeability investigations.

Fig. 1 PIL films (a: [TMPBI-BuI][BF₄]; b: [TMPBI-BuI][Tf₂N]; c: [TMPBI-BuI][Ac]) and d: a representation of a PIL matrix containing IL as well as polymeric characteristics.

The [TMPBI-BuI][BF₄] membrane was amber colored, while the [TMPBI-BuI][Tf₂N] membrane was pale yellow. Surprisingly, the [TMPBI-BuI][Ac] membrane could not form a strong film. Thus, a mere variation of the anion in the PIL caused a significant effect on their appearance and stability, endorsing the role of the anion in dictating the physical properties of the formed PIL, even when the cation remained the same.

An excellent performance of these PILs towards CO₂ separation was seen during the gas sorption and permeation analysis using N₂, CH₄ and CO₂ (see ESI†).

Sorption isotherms (Fig. 2) conveyed that their sorption at all pressures was 2–3 times higher than that of conventionally used membrane materials, such as polysulfone (PSF) and polycarbonate (PC).²² The CO₂ solubility coefficients [S(CO₂)] and sorption selectivities (Table 1) for these PILs possessing a common cation, clearly revealed that the anion had a profound effect on governing their CO₂ sorption properties. The S(CO₂) value increased with the anions in the following order: Tf₂N < BF₄ < Ac. The order of the pK_a value of their conjugate acids ([Tf₂N] = −4 < [BF₄] = −0.44 < [Ac] = 4.75) (ref. 23) conveyed that the CO₂ sorption of these PILs followed the order of increasing anion basicity. Variations in CO₂ sorption by altering the anion in brittle PILs based on aliphatic backbones is known.¹⁷ It appears that not only the nature of the anion or cation, but also the properties related to the polymeric nature of the PIL, could play a role in governing the CO₂ sorption ability.

Fig. 2 Sorption isotherms for (a) CO₂, (b) N₂ and CH₄ at 35 °C.

Table 1 CO₂ solubility coefficient [S(CO₂)] and solubility selectivity of the PILs at 10 atm

PIL	S(CO2)	S(CO2)/S(N2)	S(CO2)/S(CH4)
[TMPBI-BuI][BF4]	3.83	9.52	4.89
[TMPBI-BuI][Tf2N]	2.18	7.13	5.21
[TMPBI-BuI][Ac]	4.59	19.60	13.08

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A comparison of the S(CO₂) values in the present PILs with those of promising ones from the literature is given in Fig. 3. Most of the literature PILs are based on either aliphatic backbones^{5,6,13,15–17,24–31} or are copolymers, where the IL character may not be necessarily available on every repeat unit of the PIL.^10,32 A wide range of pressure (1 to 20 atm) was covered in this comparison, in view of the practical applicability of PILs as a separation media (as a membrane or as an absorbent).

Fig. 3 Comparison of S(CO₂) values of the present PILs possessing an aromatic backbone with those of reported ones.

The significance of the present PILs can be seen by their appearance in the upper region, exhibiting a higher sorption (the band marked in blue). Although some of the literature PILs also showed their presence in this region, they were brittle in nature. Fig. 3 also shows that an increase in pressure led to a decrease in the S(CO₂) value, which is a typical behaviour for glassy polymers.²² The S(CO₂) value for P[VBTMA][BF₄] investigated at 1 atm and 22 °C¹³ was similar to that for the present PILs, viz. [TMPBI-BuI][BF₄] and [TMPBI-BuI][Ac], however its sorption at higher pressure (10, 15 and 20 atm)^17,24,25 was lower than that of the present PILs. This conveyed the significance of the aromatic backbone in the present PILs. In addition to their film forming nature, these PILs also offered a high CO₂ sorption at elevated pressures, which is highly advantageous for gas separation purposes.

The film forming ability and the attractive CO₂ sorption selectivities of the two present PILs, viz. [TMPBI-BuI][BF₄] and [TMPBI-BuI][TF₂N], prompted us to investigate their gas permeation performance. During this analysis, the PIL membranes were stable at the high operating pressure of 20 atm, which was highly encouraging. Gas separation using membranes is typically performed at elevated pressures (e.g. natural gas separation: 30–60 bar,¹² precombustion CO₂ separation: ∼30 bar,¹ etc.). The fact that the present membranes could withstand 20 atm offers confidence towards their application possibilities for real-life gas separation.

As shown in Fig. 4a, [TMPBI-BuI][Tf₂N] displayed a higher CO₂ permeability than [TMPBI-BuI][BF₄]. The permeability is governed by gas sorption as well as diffusion. The higher permeability of [TMPBI-BuI][Tf₂N] compared to [TMPBI-BuI][BF₄] stems from a higher gas diffusion, possible due to its looser chain packing. This is evidenced by the higher average d-spacing of [TMPBI-BuI][Tf₂N] (6.86 Å) compared to [TMPBI-BuI][BF₄] (4.27 Å) (X-ray spectra are given in Fig. S3, ESI†).

Fig. 4 (a) CO₂ permeability and CO₂ based permselectivity at 20 atm for PILs, PSF, and PC and (b) TGA spectra of the PILs (i: [TMPBI-BuI][BF₄]; ii: [TMPBI-BuI][Tf₂N]; iii: [TMPBI-BuI][Ac]).

Synergistic effects of IL character in the present PILs are further conveyed by their higher gas permeability compared to common gas separation membrane materials, viz. PSF³³ and PC.³⁴ As seen from Fig. 4a, [TMPBI-BuI][Tf₂N] exhibited ∼3 times higher CO₂ permeability and at least double the CO₂/CH₄ selectivity compared to PSF or PC. The CO₂/N₂ permselectivity of the present PILs also increased marginally over PSF or PC. This reflects the affirmative outcome of introducing IL character into PILs: the favourable interactions with CO₂ led to enhanced sorption and permeability properties, and improved permselectivity, conveying their promise in practical applicability.

The major application areas of membranes for CO₂ separation are in natural gas treatment, biogas sweetening, enhanced oil recovery, flue gas separation and water–gas shift reaction.^1,35 Although a large proportion of industrial scale gas separations need membranes to be operated at ambient temperature, some of the operations needing elevated temperatures (e.g. flue gas: 50–75 °C, water–gas shift reaction: 250–450 °C, etc.)¹ can also be met by the present PILs. This is conveyed by their high thermal stability (Fig. 4b). The initial decomposition temperature (IDT) of [TMPBI-BuI][Tf₂N] was 465 °C, while [TMPBI-BuI][BF₄] exhibited an IDT of 374 °C. These values are close to the IDTs of commonly used membrane materials such as PSF (448 °C),³⁶ PC (478 °C)³⁷ and matrimid (510 °C).³⁸

Conclusions

This work reports a new class of film forming thermally stable PILs possessing a high IL density (2 IL groups per repeat unit), while adopting an altogether different approach of converting a thermo-mechanically stable PBI into a PIL. The PILs not only withstand a high operating pressure (20 atm), but also exhibit higher CO₂ sorption and permeability coupled with better permselectivity compared to the established gas separation membrane materials and known PILs. Their fully aromatic backbone (based on PBI) seems to be responsible for inducing these highly favourable properties. The agile PIL synthetic methodology allows for structure–property tunability, via (i) backbone variation (through PBI structural variations), (ii) choosing different substituents for the N-quaternization reaction and (iii) selection of appropriate anions. This means that further improvements in the permeation properties can be obtained. Their film forming ability, high thermal stability and pressure sustainability is applicable not only as a membrane material for CO₂ separation from flue gas or natural gas, but also in other upcoming areas such as fuel cell membranes, solar cells, Li-ion batteries, opto-electronics, etc.

Acknowledgements

We acknowledge funding from the Council of Scientific and Industrial Research, Govt. of India (Grant no. CSC 0122 and research fellowships to S. Kumbharkar and R. Bhavsar), and the Department of Science and Technology, Govt. of India (Grant no. SR/S3/CE/069/2009).

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Footnote

† Electronic supplementary information (ESI) available: Materials for supporting the discussions presented in the main text, including the experimental methods and characterization. See DOI: 10.1039/c3ra44632h

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