1. Introduction
Modern society is moving towards the transition to the sixth technological paradigm, sustainable technologies, and ESG (Environmental, Social and Corporate Governance) [
1]. In this regard, chemical engineering is actively searching for economically beneficial and environmentally friendly approaches to meet the current needs of holistic chemical and petroleum production. Membrane technologies are in a large part considered the main alternative to traditional methods of purification and separation of substances due to their high energy efficiency. This is because they enable continuous processes without the need to replace or regenerate the chemical sorbent [
2]. Moreover, membrane processes’ hardware is compact, and easy to operate, manage, and scale when compared to sorption separation of substances. These advantages have caused the rapid development of membrane technologies since the 1960s [
3]. Since then, the variety of membranes and membrane systems has grown more than a hundredfold, and the annual production of membrane materials exceeds several tens of millions of square meters. Industrial applications of membrane processes fall into six main groups: reverse osmosis, ultrafiltration, microfiltration, electrodialysis, gas separation, and pervaporation [
4]. Baromembrane processes, which include the first three groups, as well as the use of electrodialysis for the desalination of brackish groundwater, are the most commercially developed areas of membrane technology [
5]. However, the commercial application of diffusion processes—gas separation and pervaporation—is also growing, with nearly twenty companies worldwide implementing membrane gas separation and pervaporation systems. The potential of membrane applications for obtaining high-purity gases for microelectronics, the production of liquid nitrogen, hydrogen purification for ammonia production, the drying of organic solvents, and especially acid gas and other impurities separation from natural gas has led to a scientific trend in the study of diffusion processes [
6].
Currently, the leading industrial application of diffusion processes is natural gas pre-pipeline treatment [
7]. Although the composition of natural gas varies depending on the certain source, the main impurities, such as water, carbon dioxide, nitrogen, and hydrogen sulfide, are always present in the stream. Membrane gas separation techniques are mostly focused on removing acid gases from light alkanes, particularly methane. Carbon dioxide is the main target for membrane separation due to its significant presence in the vast majority of natural gas sources. The presence of CO
2 can cause undesirable issues, such as corrosion and a reduction in the energy impact of the fuel [
8]. Besides the natural gas treatment, membrane technology is also applicable to CO
2 separation from flue gas and biogas.
Polymeric materials currently dominate the market in membrane-based gas separations [
6], driving further development of polymer membrane science. Several types of polymer materials have been extensively studied for CO
2 separation and capture, including polymeric ionic liquids, rubbery polymers with a high content of ether oxygen, perfluoropolymers, and polymers of intrinsic microporosity, among others [
9]. In this study, we focus on polymeric ionic liquids as a promising type of polymer matrix for membranes, providing high CO
2/CH
4 or CO
2/N
2 selectivity.
Ionic liquids (ILs) have been attracting much attention in the field of membrane gas separation [
10] since the first report of CO
2 capture by this class of salts [
11]. Liquid at ambient conditions ILs, also known as room temperature ionic liquids (RTILs) [
12], have been widely studied as liquid phase absorbent of CO
2 in supported liquid membranes (SILMs) [
13,
14,
15,
16,
17,
18], mixed matrix membranes (MMMs) [
19] and in advanced materials based on inorganic oxides for CO
2 capture [
20,
21]. SILMs are obtained by impregnating RTILs into porous polymer supports, and MMMs are synthesized by mixing RTILs with polymer solution before membrane casting. While ILs-based membranes have shown great potential for CO
2/N
2 and CO
2/CH
4 separations [
13,
14,
15], they cannot withstand high transmembrane pressure, as RTILs tend to leak out from a porous support [
22]. To solve this problem, their polymerized analogues—polymeric ionic liquids (PILs)—have been implemented instead of low molecular weight substances, allowing the combination of macromolecular features with the unique properties of ILs [
23,
24,
25].
PILs are formally composed of “IL’s moieties” covalently linked into a polymer backbone. Two main strategies are used for PILs synthesis: (1) the polymerization of the corresponding monomeric IL [
26,
27,
28,
29] or (2) the modification a polymer precursor [
30,
31,
32,
33,
34]. Although the first route allows precise control of polymer composition and structure, the polymerization process is complicated by the redistribution of electron density caused by ionic groups, which limits the achievement of high degrees of conversion during polymerization and obtaining high molecular weight polymers [
28]. The second approach allows the synthesis of high molecular weight PILs, but the composition of the polymer may not be entirely uniform due to the less than 100% degree of modification caused by steric hindrance of the polymer chains [
34]. Since the pioneering work was published by Nobel’s group [
35], PILs have been extensively studied as a membrane material in a gas separation field [
10,
36,
37,
38,
39]. Although some PILs in combination with ILs exhibit extraordinary selectivity for CO
2/CH
4 and CO
2/N
2 gas pairs and overcome the Robeson upper bound [
40,
41], they also have some disadvantages as a membrane material. Most PILs proposed as membrane matrix for CO
2 separation form brittle films incapable of withstanding the pressure drop of the gas flow. Another problem is the drastic decrease of CO
2 permeability in the direct ratio to polymer molecular weight [
42]. However, a wide range of possibilities for polymer design through a reasonable choice of monomeric units, as well as polymer structure, may significantly improve a polymer’s performance as a membrane material for CO
2 separation.
In the case of PILs, both the type of ions and the nature of the polymer backbone play a significant role in membrane separation performance [
28,
43,
44,
45]. Recently, polymerizable ILs based on vinylbenzyl chloride (VBCl) modified with ammonium moieties have been proposed and investigated as CO
2 adsorbents and gas separation membrane matrices [
27,
44,
46,
47,
48,
49,
50]. Noble’s group has shown that styrene-based PILs are more CO2-selective than vinyl-based PILs [
48]. Polystyrene is indeed a good choice for a PIL backbone due to its exceptional mechanical properties and relatively high T
g. Incorporating ionic substitutes undoubtedly influences the overall properties of PILs, however, styrene and VBCl are cheap, available, and easy-to-modify starting materials. In
Table 1, we have summarized the gas separation properties of membranes synthesized from VBCl homopolymer derivatives that have been previously reported in the literature.
Although some advantages of applying styrene-based PILs in membrane design have been demonstrated, there remains a need for a deeper understanding of how ionic composition affects CO2 separation performance for this class of polyelectrolytes. This work focuses on the synthesis and characterization of PILs with polystyrene as the polymer backbone, utilizing either methylimidazolium or pyridinium cationic substitutes and a range of counter-anions: chloride [Cl], tetrafluoroborate [BF4], hexafluorophosphate [PF6], and bis(trifluoromethylsulfonyl)imide [Tf2N]. Composite gas separation membranes based on the synthesized PILs were prepared and tested for individual gases (CO2, CH4, and N2) permeation to investigate the influence of polycation functionality and anion type on the CO2 separation performance of styrene-based PILs.
2. Materials and Methods
2.1. Materials
The following reagents were used for PILs synthesis and membrane preparation: 4-vinylbenzyl chloride (VBCl, 90%, Sigma Aldrich, Darmstadt, Germany) was used after purification by vacuum distillation, azobisisobutyronitrile (98%, Chemical line) was used after purification by recrystallization from a dry ethanol solution. Reagents: pyridine (99%), 1-methylimidazole (99%); salts: sodium tetrafluoroborate (NaBF4, 98%), potassium hexafluorophosphate (KPF6, >99%), lithium bis(trifluoromethylsulfonyl)imide (LiTf2N, >99%) were purchased from Sigma-Aldrich; organic solvents: toluene (≥99.5%), chloroform (≥99.5%), isopropyl alcohol (≥98%), diethyl ether (≥99%), dimethylsulfoxide (DMSO, ≥99%) were procured from Chimreaktiv and used without additional purification. Water was purified by double distillation. A commercial microfiltration fluoroplastic membrane (MFFK-hydrophilic, pore size 0.15 µm by Vladipor, Vladimir, Russian Federation) was used as a support for composite membrane preparation.
2.2. Poly(vinylbenzyl chloride) (pVBCl) Synthesis
pVBCl was synthesized via free radical polymerization in mass according to a well-established technique. The procedure was as follows: initiator AIBN (0.05 g) was dissolved in VBCl (20 g) and placed into an ampule. The ampule was frozen in liquid nitrogen, exposed to vacuum and then defrosted. The freezing-vacuuming treatment was repeated three times. Then, the ampule was sealed under vacuum. Polymerization was carried out for 12 h at 70 °C in an oil bath. The product of polymerization was purified by dissolution in chloroform followed by precipitation with isopropanol. Dissolving-precipitation treatment was repeated three times. The resulting pVBCl was dried under vacuum at 60 °C to constant weight. The yield of the product was 91%.
2.3. Poly(vinylbenzylpyridinium chloride) (pVBPyCl) and Poly(vinylbenzylmethylimidazolium chloride) (pVBmimCl) Synthesis
The obtained pVBCl was used as a precursor for pVBPyCl, pVBmimCl synthesis by Menshutkin reaction. For pVBPyCl synthesis, pVBCl (4 g) was dissolved in pyridine (10 mL). For pVBmimCl synthesis, pVBCl (4 g) was dissolved in toluene (10 mL) and a 10 mol% excess of 1-methylimidazole was added under constant stirring. The reaction was carried out for 12 h at 60 °C with reflux in an oil bath. The products were purified by decantation followed by threefold washing of the precipitate in diethyl ether. The resulting products were dried under vacuum at 50 °C to constant weight. The yield of pVBPyCl and pVBmimCl was 96% and 92%, respectively. Conductometric titration was performed to determine the functionalization degree (FD) as it is described in
Section 2.5.
2.4. Anion Exchange Reaction
A synthetic route for PIL synthesis is demonstrated in
Scheme 1. A series of PILs (
pVBPyBF4,
pVBPyPF6,
pVBPyTf2N,
pVBmimBF4,
pVBmimPF6,
pVBmimTf2N) with various counterions was synthesized from pVBPyCl and pVBmimCl by ion exchange reactions with one of the following salts: NaBF
4, KPF
6, or LiTf
2N. The procedure for all PILs was as follows: a weighted amount of polyelectrolyte with Cl-anion was dissolved in water in a flask, and a 10 mol% excess of salt solution in water was gradually added under constant stirring. The reaction products were insoluble in water and precipitated immediately. Due to the polymeric nature of cations, the reaction was carried out for 24 h at 25 °C with stirring to ensure the maximum possible exchange. The product was isolated by filtering using a Buchner filter funnel with fine frit (G3) and purified by washing three times in distilled water. Following, drying under vacuum at 40 °C to constant weight was performed. Conductometric titration was performed to determine the anion exchange degree (ExD) as it is described in
Section 2.5.
2.5. Conductometric Titration
Conductometric titration was used to confirm and quantify pVBCl functionalization as a functionalization degree (FD), the ratio of the experimentally determined amount of substituent groups to the calculated amount expressed in percentage, and the anion exchange degree (ExD), the ratio of the experimentally determined amount of exchanged anions to the calculated one expressed in percentage. The measurements were performed using a Aquasearcher benchtop meter (OHAUS, NJ, USA) equipped with STCON3 conductivity electrodes (OHAUS, NJ, USA). FD was determined by pVBPyCl or pVBmimCl solution in water (0.0001 M, 100 mL) titration with AgNO3 0.0001 M solution. The measurements were carried out in intervals of 1 mL and 0.2 mL near the equivalence point. Titration was performed 3 times; the average value was calculated. To evaluate ExD, solutions of Cl-containing inorganic salts, formed as a result of an ion exchange reaction, were titrated with AgNO3 0.0001 M solution in water. The measurements were carried out in intervals of 1 mL and 0.2 mL near the equivalence point. Titration was performed 3 times; the average value was calculated.
2.6. Nuclear Magnetic Resonance Spectroscopy (NMR)
The 1H NMR spectra were recorded in DMSO-d6 solution (99.9% atom D, Sigma-Aldrich) on a DD2 400NB NMR spectrometer (Agilent, CA, USA) at 400 MHz. The residual solvent peak was used as an internal standard. The chemical shifts (δ) are reported in parts per million (ppm); J values are given in hertz (Hz).
2.7. Gel Permeation Chromatography (GPC)
Molecular weight (number-average molecular weight (Mn) and weight-average molecular weight (Mw)), as well as polydispersity (Mw/Mn) of pVBCl were determined by means of GPC on Prominence LC-20VP (Shimadzu) equipped with a RI-detector. The analyses were performed at a flow rate of 0.7 mL·min− 1 and at 40 °C using THF (≥99.9%, HPLC quality, Sigma-Aldrich) as an eluent. The GPC system was calibrated using the narrow-dispersed polystyrene (PS) ranging (Fluka).
2.8. Attenuated Total Reflectance Fourier Transforms Infrared Spectroscopy (ATR-FTIR)
IR spectra of the samples were recorded using an FTIR spectrophotometer IRTracer-100 (Shimadzu, Kyoto, Japan) equipped with modified HATR accessory with a ZnSe crystal plate (PIKE, NC, USA) in transmittance mode at ambient temperature. The mirror system of the accessory was constantly exposed to a N
2 flow in order to exclude atmospheric CO
2 from registered spectra. As it was previously reported [
52], HATR accessory was modified with inlet and outlet gas valves in the top cover enabling in situ experiments of gas (CO
2) sorption by the sample in the cell. The sample of polymer for FTIR analysis was prepared by polymer solution (1 mass% in DMSO) casting on a ZnSe crystal plate followed by solvent evaporation under vacuum at 40 °C. Three types of spectra were recorded for all polymer samples: (1) pure polymer, (2) in situ CO
2 sorption, (3) polymer after CO
2 desorption. A minimum of 30 scans was signal-averaged with a resolution of 4 cm
−1 within the 4000–500 cm
−1 range.
2.9. Density of Polymers
The density of the polymer films obtained by polymer solution (2 mass% in DMSO) casting on inert glass support was measured by implementing a flotation method [
53,
54]. To conduct an experiment, a graduated cylinder (50 cm
3) with a ground-in glass stopper was filled to half with a mixture of two miscible liquids with different densities at a 1:1 volume ratio. Particular liquids were chosen based on the following requirements: (1) liquids should be chemically inert to the polymer material, (2) liquids should not cause swelling or solvation of the polymer, (3) the mixture should cover the supposed density range for the polymer. Mixtures of liquid used for different PILs are listed in
Table S1. A sample of the polymer film was immersed in the mixture of liquids and brought to an equilibrium middle position, according to the graduation, by adding one of the liquids dropwise. Then, the density of the obtained mixture of liquids was measured using a pycnometer (10 cm
3). Density was measured three times on each membrane to obtain the average density values of the polymers. The experiments were conducted at 20 °C.
2.10. Membrane Preparation
Composite polymer membranes with a selective layer based on synthesized PILs were obtained by casting corresponding solutions (2% in DMSO) onto a commercial microfiltration fluoropolymer membrane MFFK using an automatic casting knife MemcastTM Plus (POROMETER, Nazareth, Belgium) with a 100 μm blade followed by solvent evaporation in enclosed area under ambient temperature. After major solvent evaporation membranes were dried under vacuum to a constant weight.
2.11. Scanning Electron Microscopy (SEM)
Topography and morphological characteristics of the obtained composite membranes were studied by scanning electron microscopy (SEM) using an JSM-IT300LV electron microscope (JEOL, Peabody, MA, USA) with an electron probe diameter of about 5 nm and a probe current of less than 0.5 nA (operating voltage 20 kV). SEM scanning was performed using low-energy secondary electrons and backscattered electrons under a low vacuum to eliminate the charge.
Supplementary Information for SEM images is listed in
Table S3.
2.12. Wettability Measurements and Surface Free Energy Calculation
Wettability tests and surface energy calculation were performed according to a previously published technique [
55]. The contact angle of wetting (θ) with three test liquids with different surface tensions (
Table 2): water, glycerol, and diiodomethane was measured at an equilibrium state in a closed beaker and calculated using ImageJ software with a contact angle plugin based on experimental data.
The measurements were conducted at 293 K. The results were collected for a series of five drops with contact angle deviations that did not exceed ± 1°. Total surface free energy and its components were calculated using the Owens-endt method. According to the Owens-Wendt method, the surface free energy
can be calculated as a sum of its dispersive (
) and polar (
) components:
These values can be graphically obtained from the Owens-Wendt equation using the results of surface wettability measurements with three different testing liquids obtained previously:
Here, is the surface tension of the wetting liquid (mJ·m−2), is a dispersive component of the surface tension of the wetting liquid (mJ·m−2), and is a polar component of the surface tension of the wetting liquid (mJ·m−2).
2.13. Gas Permeation Tests
For the gas permeation test, three individual gases, nitrogen (N
2), carbon dioxide (CO
2), and methane (CH
4), were chosen to characterize the separation performance of the obtained membranes in natural gas and flue gas treatment. The presence of water influences the performance of ILs and related substances as well as the mechanism of CO
2 solubility and transport [
56,
57,
58,
59]. In this regard, the presence of water in membranes was measured using infrared moisture determination balance FD-610 (Kett, Tokyo, Japan). The membrane sample (5.5 g) was placed in the balance and dried at 120 °C to a constant weight. The presence of water in all membranes was less than 0.1%. All gases used were 99.9% purity with content of water in CO
2 at 0.001%; in N
2, 0.007%; and in CH
4, 0.0001%. The pure gas permeabilities of N
2, CO
2, and CH
4 through the polymeric membranes were measured by an experimental setup (
Figure 1) supported by an automatic computing system based on a software-logic controller (Unitronix, Israel) at the initial transmembrane pressure of 130 kPa and ambient temperature (293 K) in a constant volume mode [
15].
Each single-gas test was repeated at least three times. The selected experimental curve of CO
2 pressure time course in a permeation cell obtained for a membrane with a selective layer based on pVBmimTf
2N is represented in
Figure S7. Permeability coefficients were calculated according to the following formula [
60]:
where
β is a geometric setup parameter (m
−1),
is the pressure in a high-pressure compartment (Pa),
is the pressure in a low-pressure compartment (Pa),
P is the permeability coefficient (Barrer),
t is time (s), and
l is the selective layer thickness in a composite membrane (m). The ideal selectivity of the polymeric membranes was calculated as the ratio of the single gas permeability coefficients (gases A and B):
The ideal selectivity gives some idea of the membrane separation properties. However, this value is based on a simplified model of gas adsorption and diffusion in the mass-transfer process. In this regard, a true selectivity might differ when comes to a gas mixture.