Towards Biohydrogen Separation Using Poly(Ionic Liquid)/Ionic Liquid Composite Membranes

Abstract

Considering the high potential of hydrogen (H₂) as a clean energy carrier, the implementation of high performance and cost-effective biohydrogen (bioH₂) purification techniques is of vital importance, particularly in fuel cell applications. As membrane technology is a potentially energy-saving solution to obtain high-quality biohydrogen, the most promising poly(ionic liquid) (PIL)–ionic liquid (IL) composite membranes that had previously been studied by our group for CO₂/N₂ separation, containing pyrrolidinium-based PILs with fluorinated or cyano-functionalized anions, were chosen as the starting point to explore the potential of PIL–IL membranes for CO₂/H₂ separation. The CO₂ and H₂ permeation properties at the typical conditions of biohydrogen production (T = 308 K and 100 kPa of feed pressure) were measured and discussed. PIL–IL composites prepared with the [C(CN)₃]⁻ anion showed higher CO₂/H₂ selectivity than those containing the [NTf₂]⁻ anion. All the membranes revealed CO₂/H₂ separation performances above the upper bound for this specific separation, highlighting the composite incorporating 60 wt % of [C₂mim][C(CN)₃] IL.

1. Introduction

Due to its outstanding intrinsic features, hydrogen (H₂) is considered to be the energy carrier of the future. Besides being the simplest element in the universe, the H₂ molecule has the highest energy content per unit weight of any known fuel. However, H₂ is not a primary fuel source, which means that it is not available in nature and thus needs to be produced [1]. Hydrogen has been produced mainly on an industrial scale from fossil fuels, through natural gas reforming or coal gasification, and from water, using electrolysis in which water (H₂O) can be split into hydrogen and oxygen (O₂) [2]. Although water splitting is an ecologically clean process compared to the previously mentioned H₂ production processes, it is a highly energy-demanding technology [3].

Hydrogen production using biological processes has been attracting growing attention since it is more environmentally friendly and less energy intensive than the other described H₂ production systems because its conditions are close to room temperature (303–313 K) and ambient pressure (100 kPa) [2]. BioH₂ production can be divided into two main categories: light-dependent (direct or indirect biophotolysis and photo fermentation) and -independent (dark fermentation) methodologies [4,5,6]. The dark fermentation process has several advantages compared to the other biological processes namely, its potential for cost-effective hydrogen production, the high rate of cell growth, and the non-requirement of light energy [6]. In spite of the recognized potential of biohydrogen for sustainable development, there are still issues regarding its production and purification [7], such as the elimination of CO₂, N₂, and other impurities (H₂O and H₂S), so that an enriched H₂ stream can be obtained for efficient energy generation, mostly through fuel cells [8]. Among the several methodologies for separating hydrogen, such as pressure swing adsorption (PSA), cryogenic distillation, and membrane separation, the first two are designed mainly for large-scale hydrogen production and cannot be used without modification for an upgrade of relatively small amounts of biohydrogen [9]. Thus, membrane technology has been reported as an attractive alternative for biohydrogen separation and purification [10] since it can be introduced easily into hydrogen-producing bioreactors, leading to an integrated process of bioH₂ production and purification [11,12], not omitting its important engineering and economic advantages. Particularly, polymeric membranes, such as polysulfone (PSF) and polyimide (PI) [13], have been considered a suitable choice for biohydrogen separation as they can not only be used at the bioreactors’ operating conditions but they also have low cost, high energy efficiency, and a smaller ecological footprint than conventional separation processes [14,15,16].

Few articles have been published using membranes for bioH₂ separation. Among them, the combination of different polymers and ionic liquids (ILs) to prepare polymer/IL composite membranes is one of the most promising approaches, which takes into account the inherent CO₂-philicity of ILs [16]. For instance, Kanehashi et al. [17] prepared different membranes based on a fluorine-containing polyimide and different amounts of [C₄mim][NTf₂] IL up to 81 wt %, and studied their gas separation performance at 308 K and 100 kPa of feed pressure. The highest CO₂/H₂ permselectivity (6.6) was obtained for the membrane that contained the maximum amount of IL (81 wt %) [17]. Moreover, Friess et al. [18] studied the gas permeation properties at 298 K and 100 kPa of feed pressure through membranes composed of poly(vinylidene fluoride-co-hexafluoropropylene) and 20 to 80 wt % of [C₂mim][NTf₂] IL. Again, the largest CO₂ permeability (533 Barrer) and CO₂/H₂ permselectivity (12.1) were obtained when the highest amount of IL (80 wt %) was used [18].

With the aim of designing efficient CO₂/H₂ separation membranes, this work explores the use of poly(ionic liquid)s (PILs), which are well recognized by their high CO₂ affinity and designer nature [19]. Different approaches have been proposed to produce PIL-based CO₂ separation membranes, such as neat PIL membranes [20,21,22,23], PIL copolymer membranes [19,24], and the incorporation of ILs alone or together with nanoporous materials as fillers, including zeolites or metal-organic frameworks (MOFs), to form PIL/IL/filler mixed matrix membranes (MMMs) [25,26,27]. Among all these approaches, the blend of PILs and ILs to produce homogeneous PIL–IL composite membranes is the most promising due to their high CO₂ separation performance, as well as the good mechanical stability of the membranes [28]. Notwithstanding the potential of PILs for CO₂ separation [28,29,30,31,32], only a reduced number of works concerning CO₂/H₂ separation have been reported in the literature. For instance, Carlisle et al. [33] explored the CO₂/H₂ separation through imidazolium PIL–IL gel membranes produced by UV polymerization. The time-lag experiments performed at room temperature and 200 kPa of feed pressure showed ideal CO₂/H₂ selectivities ranging from 6.6 to 12 for membranes prepared with 5 to 100 mol% of a cross-linking monomer and different amounts of free IL and IL monomer. Their best result (CO₂ permeability of 540 Barrer and CO₂/H₂ selectivity of 12) was achieved for the composite containing 100 mol% of cross-linking monomer and 75 wt % of IL [33]. Moreover, Kharul et al. [34] studied the CO₂ and H₂ separation properties of polybenzimidazole-based PILs. The different synthesized polybenzimidazole-based PILs exhibited very similar CO₂ and H₂ permeabilities (<30 Barrer) and, consequently, CO₂/H₂ selectivities approximately equal to 1 [34].

Amongst the PIL–IL membranes developed so far for CO₂/N₂ separation, the most widely explored are those composed of imidazolium-based PILs with fluorinated or cyano-functionalized anions [35,36,37]. However, our group reported PIL–IL membranes made of pyrrolidinium-based PILs combining the same anions, which are particularly simple to prepare through a metathesis reaction of a commercially available polyelectrolyte. The PIL–IL membranes displayed CO₂/N₂ separation performances near or even above the Robeson upper bound [38,39,40,41]. In fact, the CO₂-phylic behavior of the [NTf₂]⁻ anion and the CO₂ separation efficiency of the [C(CN)₃]⁻ anion [42] motivated us to explore the most promising pyrrolidinium-based PIL–IL composites based on these two anions for CO₂/N₂ separation, now for CO₂/H₂ separation.

In this work, solvent casting method was used to prepare composite membranes composed of two pyrrolidinium-based PILs: poly([Pyr₁₁][NTf₂]) and poly([Pyr₁₁][C(CN)₃]). The poly([Pyr₁₁][NTf₂]) was blended with 40 and 60 wt % of the structurally similar [C₄mpyr][NTf₂] IL and also with 40 wt % of an imidazolium-based IL ([C₂mim][NTf₂]), while poly([Pyr₁₁][C(CN)₃]) was mixed with 40 and 60 wt % of [C₂mim][C(CN)₃] IL. The chemical structures of the PILs and ILs used are shown in Figure 1. The CO₂ and H₂ permeation properties (permeability, diffusivity and solubility) were determined at two different temperatures (T = 293 K and T = 308 K) under a transmembrane pressure differential of 100 kPa. A temperature of 293 K was used first so that the results obtained herein could be compared to those previously reported by our group, while T = 308 K was chosen to reproduce the hydrogen bioproduction conditions [13].

2. Experimental Section

2.1. Materials

Poly(diallyldimethylammonium) chloride solution (average M_w 400,000–500,000, 20 wt % in water), acetone (99.8%), and acetonitrile (99.8%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Lithium bis(trifluoromethylsulfonyl)imide (LiNTf₂, 99 wt % pure) and sodium tricyanomethanide (NaC(CN)₃, 98 wt % pure) were supplied by IoLiTec GmbH (Heilbronn, Germany). The PILs used were previously synthesized by anion metathesis reactions, as described in previous studies [39,43]. All the starting materials used for PIL syntheses, as well as the organic solvents, were used as received. The water was double distilled. The ILs, 1-ethyl-3-methylimidazolium tricyanomethanide ([C₂mim][C(CN)₃], >98 wt % pure), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂mim][NTf₂], >99 wt % pure), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C₄mpyr][NTf₂], >99 wt % pure) were provided by IoLiTec GmbH. Carbon dioxide (CO₂) and hydrogen (H₂) were supplied by Air Liquide with a minimum purity of 99.99%. Gases were used with no further purification.

2.2. Preparation of PIL–IL Membranes

Several free-standing membranes composed of the synthesized PILs and specific quantities of different ILs containing the same anions were produced by solvent casting. The first step was the preparation of 6 (w/v)% and 12 (w/v)% solutions of poly([Pyr₁₁][C(CN)₃]) and poly([Pyr₁₁][NTf₂]), respectively, in the most suitable solvents and the addition of the respective IL amounts (

Table 1. Description of the poly(ionic liquid)–ionic liquid (PIL–IL) membrane’s composition and experimental preparation conditions of the solvent casting procedure.

PIL–IL Membrane	Polymer (PIL)	wt % of IL	Solvent	T (K)	Evaporation Time (Days)
PIL C(CN)3–40 [C2mim][C(CN)3]	Poly([Pyr11][C(CN)3])	40	Acetonitrile	298	3
PIL C(CN)3–60 [C2mim][C(CN)3]		60
PIL NTf2–40 [C4mpyr][NTf2]	Poly([Pyr11][NTf2])	40	Acetone	298	2
PIL NTf2–60 [C4mpyr][NTf2]		60
PIL NTf2–40 [C2mim][NTf2]	Poly([Pyr11][NTf2])	40	Acetone	298	2

). The solutions were mixed using a magnetic stirrer until complete dissolution of the PIL and IL components and then poured into Petri dishes for slow evaporation of the solvent at room temperature. With the aim of obtaining homogeneous membranes, the solvent evaporation took place slowly, for 2/3 days, depending on the solvent used (Table 1), and in a saturated solvent environment. The thicknesses of the prepared membranes (70–210 μm) were measured using a digital micrometer (Mitutoyo, model MDE-25PJ, Kanagawa, Japan). Average thickness was calculated from six measurements taken at different locations of each PIL–IL membrane. All the PIL–IL membranes studied were considered to have good stability since they were malleable and flexible enough to be used in the gas permeation measurements, even for the composites with 60 wt % of IL. Moreover, the evaluation of the mechanical stability of the PIL–IL composite membranes having the [C(CN)₃]⁻ anion was recently reported by Tomé et al. [44] (Young’s modulus (PIL–40IL) ~14 MPa; Young’s modulus (PIL–60IL) ~5).

2.3. Gas Permeation Experiments

A time lag equipment described in detail elsewhere [38] was used to measure and determine the ideal CO₂ and H₂ permeabilities and diffusivities through the prepared PIL–IL composites. Initially, each membrane was degassed under vacuum inside the permeation cell for at least 12 h before testing. The gas permeation experiments were performed at 293 K and 308 K with an upstream pressure of 100 kPa (feed) and vacuum (<0.1 kPa) as the initial downstream pressure (permeate). Three separate CO₂ and H₂ experiments on a single composite membrane were measured. Between each run, the permeation cell and lines were evacuated until the pressure was below 0.1 kPa.

The gas transport through the PIL–IL membranes was assumed to occur according to the solution-diffusion mass transfer mechanism [45]. Thus, the permeability (P) is related to diffusivity (D) and solubility (S) as follows:

$$P=D\times S$$

(1)

The permeate flux of each studied gas (J_i) was experimentally determined using Equation (2) and assuming an ideal gas behavior and a homogeneous membrane [46]:

$$J_i=\frac{V_p\Delta p_d}{AtRT}$$

(2)

where $V_p$ is the permeate volume, $\Delta p_d$ is the variation of downstream pressure, A is the effective membrane surface area, t is the experimental time, R is the gas constant, and T is the temperature. Equation (3) was then used to calculate the ideal gas permeability (P_i) from the pressure driving force ($\Delta p_i$) and membrane thickness ($\ell$):

$$P_i=\frac{J_i}{\Delta p_i/\ell }$$

(3)

Gas diffusivity (D_i) was determined according to Equation (4). The time-lag parameter (θ) was deduced by extrapolating the slope of the linear portion of the p_d vs. t curve back to the time axis, where the intercept is equal to θ [47]:

$$D_i=\frac{{\ell }^2}{6\theta }$$

(4)

After defining both P_i and D_i, the gas solubility (S_i) was also calculated using Equation (1).

The ideal permeability selectivity (or permselectivity), ${\alpha }_{i/j}$, which can also be expressed as the product of the diffusivity selectivity and the solubility selectivity, was obtained by dividing the permeability of the more permeable species i to the permeability of the less permeable species j, as expressed in Equation (5):

$${\alpha }_{i/j}=\frac{P_i}{P_j}=\left(\frac{D_i}{D_j}\right)\times \left(\frac{S_i}{S_j}\right)$$

(5)

3. Results and Discussion

3.1. CO₂ and H₂ Permeation Properties

3.1.1. Gas Permeability (P)

The CO₂ and H₂ permeabilities through the PIL–IL composite membranes that were studied are presented in Figure 2. The CO₂ permeability was always higher than that of H₂ and both permeabilities increased with increasing temperature, although the increment was not the same between the studied composites, varying from 15 to 50% for CO₂ permeability values and from 39 to 77% for H₂ permeabilities. The CO₂ permeabilities at 293 K for all the membranes discussed here are in good agreement with those already reported [38,39,41], which emphasizes the high reproducibility of the method used. As expected, the incorporation of high amounts of IL led to enhanced CO₂ and H₂ permeabilities. Additionally, at 308 K, the temperature of bioH₂ purification, the PIL NTf₂–40 [C₂mim][NTf₂] membrane presented similar CO₂ and H₂ permeability values to those of PIL NTf₂–60 [C₄mpyr][NTf₂]. This means that despite the higher structural compatibility of [C₄mpyr][NTf₂] with the pyrrolidinium-based PIL, the imidazolium-based cation of the IL is determinant in promoting increased gas permeabilities. However, and as already reported by our group [41], the use of [C₂mim][NTf₂] instead of [C₄mpyr][NTf₂] only allowed the incorporation of free IL up to 40 wt % so that a mechanically stable and homogeneous membrane could be obtained. Considering the PIL–IL membranes that comprise the [C(CN)₃]⁻ anion in both PIL and IL, the PIL C(CN)₃–60 [C₂mim][C(CN)₃] composite showed the highest CO₂ permeability (505 Barrer) at 308 K but not the highest H₂ permeability (40.3 Barrer), which was obtained for the PIL NTf₂–60 [C₄mpyr][NTf₂] composite membrane (46.0 Barrer). Moreover, CO₂ permeability increased about 42% while H₂ permeability increased approximately 57% when the IL content in the PIL C(CN)₃–[C₂mim][C(CN)₃] composite varied from 40 to 60 wt %. A significant difference in CO₂ permeability (76%) was obtained for the PIL–IL membranes that contained [C₄mpyr][NTf₂] IL when the IL amount increased from 40 to 60 wt %, while the increment in H₂ permeability was only around 34%.

3.1.2. Gas Diffusivity (D)

The experimental gas diffusivity results at 293 K and 308 K through the prepared membranes are listed in

Table 2. Experimental gas diffusivities (D) through the studied PIL–IL membranes at T = 293 K and T = 308 K.

PIL–IL Membrane	Gas Diffusivity (×1012) (m2 s−1) (T = 293 K)		Gas Diffusivity (×1012) (m2 s−1) (T = 308 K)
	D CO2 ± σ	D H2 ± σ	D CO2 ± σ	D H2 ± σ
PIL C(CN)3–40 [C2mim][C(CN)3]	64 ± 1.0	970 ± 36.2	112 ± 2.5	1146 ± 34.0
PIL C(CN)3–60 [C2mim][C(CN)3]	127 ± 1.1	1130 ± 5.70	181 ± 0.6	1211 ± 3.2
PIL NTf2–40 [C4mpyr][NTf2]	34 ± 0.1	484 ± 18.5	62 ± 1.8	546 ± 20.6
PIL NTf2–60 [C4mpyr][NTf2]	44 ± 0.7	610 ± 6.30	76 ± 0.5	673 ± 16.9
PIL NTf2–40 [C2mim][NTf2]	61 ± 0.4	722 ± 1.80	106 ± 1.5	792 ± 3.70

. A high difference (one or, in some cases, two orders of magnitude) between CO₂ and H₂ diffusivity values, which corresponds to CO₂/H₂ diffusivity selectivities around 0.1, was observed. This difference in gas diffusivities was mainly due to the smaller size of H₂ (2.89 Å) compared to CO₂ kinetic diameter (3.30 Å) [48]. Moreover, both CO₂ and H₂ diffusivity increased with increasing temperature and with increasing IL content in the PIL–IL composite (Table 2). The same behavior was also found for CO₂ and H₂ permeabilities (Figure 2). From Table 2, it can also be seen that CO₂ and H₂ diffusivities through the prepared membranes can be ordered as follows: PIL NTf₂–40 [C₄mpyr][NTf₂] < PIL NTf₂–60 [C₄mpyr][NTf₂] < PIL NTf₂–40 [C₂mim][NTf₂] < PIL C(CN)₃–40 [C₂mim][C(CN)₃] < PIL C(CN)₃–60 [C₂mim][C(CN)₃], which means that the presence of the [C(CN)₃]⁻ anion in the composites, either in the PIL or IL’s structure, leads to higher CO₂ and H₂ diffusivities compared to the [NTf₂]⁻ anion. The same trend was also observed for N₂ diffusivities [38,39,41]. Thus, and as expected, it can be concluded that gas diffusivities follow the order of the kinetic diameters CO₂ < N₂ < H₂. It can also be noted that the presence of imidazolium-based cation ([C₂mim]⁺) in the ILs led to superior gas diffusivities compared to the pyrrolidinium-based cation ([C₄mpyr]⁺).

Another interesting point is the comparison between gas permeability and diffusivity behaviors. Regardless of the anion, although the composite that comprised 60 wt % of IL had the highest H₂ diffusivities (>1200 m² s⁻¹ at 308 K), it did not present the highest H₂ permeabilities (Figure 2). An equivalent behavior was also obtained for the PIL NTf₂–40 [C₄mpyr][NTf₂] composite membrane, which displayed the lowest H₂ diffusivities (546 m² s⁻¹ at 308 K) but not the lowest H₂ permeabilities. In the case of CO₂, it can be seen from Table 2 and Figure 2 that CO₂ permeability followed the same trend as CO₂ diffusivity, with the exception of the PIL NTf₂–60 [C₄mpyr][NTf₂] and PIL C(CN)₃–40 [C₂mim][C(CN)₃] membranes. This behavior led us to conclude that the very high difference (three or, in some cases, four orders of magnitude) among H₂ diffusivities is somehow compensated by a reverse behavior in H₂ solubilities (as will be discussed in the next section), which has a significant impact on the H₂ permeability results.

3.1.3. Gas Solubility (S)

Gas solubility (S) values were calculated using Equation (1) at temperatures of 293 K and 308 K and are displayed in Figure 3. The CO₂ solubility decreased with increasing temperature while H₂ solubility increased with increasing temperature for all the PIL–IL membranes studied. Analogous reverse H₂ solubility behavior with temperature was also found and discussed by Raeissi et al. [49] in imidazolium-based ILs, such as [C₄mim][NTf₂], which means that hydrogen dissolves better at higher than at lower temperatures. This trend seems to be the general rule for H₂ solubility in ILs [49,50,51,52,53] and has been attributed to the extreme lightness and small intermolecular forces of hydrogen molecules [49].

It can also be observed that, as expected, both CO₂ and H₂ solubilities were enhanced with the incorporation of high amounts of IL in the composite. For instance, when the amount of [C₂mim][C(CN)₃] increased from 40 to 60 wt %, the CO₂ and H₂ solubilities at 293 K increased almost 50% and 15%, respectively, while at 308 K the increment in CO₂ and H₂ solubilities was around 39% and 5%, respectively. Similar behavior was found for the PIL–IL composites comprising the [C₄mpyr][NTf₂] IL. Moreover, the large differences between CO₂ and H₂ solubilities can be explained by the high CO₂ critical temperature (CO₂, 31 °C; H₂, −240 °C), corresponding to the superior condensability of CO₂ (T_ε/k = 195.2 K) compared to H₂ (T_ε/k = 59.7 K) [48,54]. The fact that H₂ can almost be considered an ideal gas due to its small kinetic diameter and non-interacting nature, while CO₂ displays a higher kinetic diameter and a quadrupole moment, also plays a role in the difference in solubilities of the two gases. For the PIL–IL composites studied in this work at 308 K, the CO₂ solubility ranged from 14 to 28.5 (×10⁻⁶) m³_(STP)·m⁻³·Pa⁻¹ whereas the H₂ solubility values were two orders of magnitude lower, varying from 0.17 to 0.51 (×10⁻⁶) m³_(STP)·m⁻³·Pa⁻¹. Among all the tested membranes, the PIL NTf₂–60 [C₄mpyr][NTf₂] composite presented the highest CO₂ and H₂ solubilities at both 293 and 308 K. Regarding the influence of the anion’s structure and considering the same amount of free IL in the composite, it can be concluded that the presence of the [NTf₂]⁻ anion in the PIL–IL membranes leads to higher CO₂ and H₂ solubilities compared to those membranes comprising the [C(CN)₃]⁻ anion. As mentioned before, this behavior masks the higher H₂ diffusivities of composites with a cyano-functionalized anion, somehow explaining the low influence of H₂ diffusivity on the H₂ permeability results.

3.2. CO₂/H₂ Separation Performance

The CO₂ and H₂ permeabilities and the ideal CO₂/H₂ permselectivities determined at 293 K and 308 K are summarized in

Table 3. Single gas permeabilities (P) ^a and ideal permselectivities (α) of the PIL–IL membranes studied ^b.

PIL–IL Membrane	Gas Permeability (Barrer) (T = 293 K)			Gas Permeability (Barrer) (T = 308 K)
	P CO2 ± σ	P H2 ± σ	α CO2/H2	P CO2 ± σ	P H2 ± σ	α CO2/H2
PIL C(CN)3–40 [C2mim][C(CN)3]	139 ± 0.5	14.5 ± 0.2	9.6 ± 0.2	209 ± 0.9	25.7 ± 0.1	8.1 ± 0.1
PIL C(CN)3–60 [C2mim][C(CN)3]	438 ± 2.1	29.1 ± 0.4	15.1 ± 0.3	505 ± 0.3	40.3 ± 1.1	12.5 ± 0.3
PIL NTf2–40 [C4mpyr][NTf2]	119 ± 0.2	21.9 ± 0.1	5.4 ± 0.1	164 ± 1.6	34.4 ± 0.3	4.8 ± 0.1
PIL NTf2–60 [C4mpyr][NTf2]	232 ± 2.2	29.8 ± 0.1	7.8 ± 0.1	288 ± 1.6	46.0 ± 0.1	6.3 ± 0.1
PIL NTf2–40 [C2mim][NTf2]	214 ± 0.6	26.2 ± 0.1	8.2 ± 0.1	287 ± 2.4	43.8 ± 0.2	6.5 ± 0.1

^a Barrer (1 Barrer = 10⁻¹⁰ cm_(STP)³·cm·cm⁻²·s⁻¹·cm·Hg⁻¹). ^b The listed uncertainties represent the standard deviations (σ) based on three experiments.

. Amongst the PIL−IL membranes studied, those bearing the [C(CN)₃]⁻ anion revealed slightly higher CO₂/H₂ permselectivities than those containing the [NTf₂]⁻ anion. This behavior was also observed in our previous works concerning the use of PIL–IL membranes for CO₂/N₂ and CO₂/CH₄ separation [38,39,41]. Moreover, from Table 3, it can be seen that CO₂/H₂ permselectivities decreased with increasing temperature. This result can be explained by the variations in CO₂/H₂ solubility selectivity with temperature, which leads to a decrease in CO₂/H₂ permselectivity as the temperature increases [55]. In fact, CO₂/H₂ solubility selectivity through the studied composite membranes decreased from 78–145 at 293 K to 42–84 at 308 K. It can also be emphasized that CO₂/H₂ permselectivity seems to be controlled by a solubility mechanism, since CO₂/H₂ diffusivity selectivity (D CO₂/H₂) values were approximately equal to 0.1 at both temperatures while solubility selectivity (S CO₂/H₂) values ranged from 78–145 at 293 K and 42–84 at 308 K.

The gas separation performance of the studied PIL–IL membranes is shown in Figure 4, where the CO₂/H₂ permselectivity is plotted against the permeability of the more permeable gas species (CO₂). This graph displays a tradeoff (black line) between gas permeability and permselectivity. These upper bound limits for several gas pairs were first developed by Robeson [56] who correlated data obtained from measurements carried out with polymeric membranes at low temperatures (298–308 K). Later, Rowe et al. [55] studied the influence of temperature on the tradeoff between gas permeability and permselectivity for different gas pairs. Thus, the upper bound at 300 K developed by Rowe et al. [55] for the CO₂/H₂ gas pair is represented in Figure 4 and was used to evaluate the performance of the studied PIL–IL membranes for biohydrogen purification (T = 308 K and 100 kPa).

Figure 4 clearly shows that all the PIL–IL membranes that were studied displayed CO₂/H₂ separation performances above the upper bound. The best CO₂/H₂ separation performance was obtained for the membrane composed of poly([Pyr₁₁][C(CN)₃]) and 60 wt % of [C₂mim][C(CN)₃] IL, which is in agreement with what has been observed in our recent works regarding the use of PIL–IL composites for CO₂/N₂ separation [39]. Literature data points for other reported PIL–IL membranes are also plotted in Figure 4 for comparison. The gas permeation measurements reported by Carlisle et al. [33] were performed at room temperature with a transmembrane pressure differential of 200 kPa. Also, their PIL–IL membranes were not prepared by the solvent casting method but through UV polymerization by mixing different percentages of imidazolium-based IL monomers, a cross-linking monomer, and free IL [33]. From Figure 4, it can be seen that the PIL–IL membranes reported in the literature also present CO₂/H₂ separation performances above the upper bound for the CO₂/H₂ gas pair at 300 K, but the PIL C(CN)₃–60 [C₂mim][C(CN)₃] membrane studied in this work still revealed superior CO₂/H₂ separation performance.

4. Conclusions

In this work, dense composite membranes made of pyrrolidinium-based PILs with [C(CN)₃]⁻ or [NTf₂]⁻ anions and different amounts of free IL ([C₂mim][C(CN)₃], [C₄mpyr][NTf₂] or [C₂mim][NTf₂]) incorporated were prepared by the solvent casting method. Their CO₂ and H₂ permeation properties (permeability, diffusivity, and solubility) were determined at biohydrogen production conditions (T = 308 K and 100 kPa of feed pressure) and discussed. The CO₂ and H₂ permeation properties were measured at T = 293 K and the effect of temperature on gas separation performance was evaluated.

The PIL–IL membranes containing the [NTf₂]⁻ anion presented the highest H₂ permeability and solubility values, while the PIL–IL composites having the [C(CN)₃]⁻ anion showed the highest H₂ diffusivities and CO₂/H₂ permselectivities. As previously reported, increments in gas permeabilities, diffusivities, and solubilities were observed with increasing temperature and amounts of IL, with the exception of H₂ solubility that showed the opposite behavior with temperature compared to what occurred in CO₂ solubility. Overall, all the PIL–IL membranes studied revealed similar or superior CO₂/H₂ separation performance compared to the few PIL–IL composites reported so far in the literature. Particularly, at 308 K, the best result was obtained through the PIL C(CN)₃–60 IL C(CN)₃ composite membrane (CO₂ permeability of 505 Barrer and CO₂/H₂ selectivity of 12.5), which, as shown in our previous work, also presented remarkable results for CO₂/N₂ separation.