Thin supported MOF based mixed matrix membranes of Pebax® 1657 for biogas upgrade

Abstract

This work shows the preparation of thin mixed matrix membranes (MMMs) with a 2–3 μm thick Pebax® 1657 layer on two different supports: a porous asymmetric polyimide P84® and dense polytrimethylsilylpropyne (PTMSP). Nanoparticles of metal–organic frameworks (MOFs) ZIF-8, MIL-101(Cr), UiO-66 and ZIF-7/8 core–shells were selected as fillers for the Pebax® 1657 based MMMs, all of them being MOFs with high CO₂ adsorption capacity but different pore size distribution. All the membranes were characterized by SEM, FTIR, Raman, TGA and XRD analyses, showing in all cases a perfect compatibility of the Pebax® layer with both supports and also a good dispersion of the fillers in the polymeric matrix. These membranes were applied for the separation of equimolar CO₂/CH₄ mixtures at 35 °C under feed pressures between 3 and 5 bar, where an improvement in the gas separation performance with increasing pressure was noticed, thanks to the favored solubility of CO₂. The synergistic compatibility between Pebax® 1657 and P84® gave rise to a 470% enhancement in CO₂/CH₄ selectivity, reaching a maximum value of 114 while the CO₂ permeance increased by 40% up to 7.5 GPU. The addition of fillers in the Pebax® polymeric phase produced an improvement in the gas separation performance of the membranes, especially in terms of permeance, where the MMMs containing a 10 wt% loading of UiO-66 reached the optimum value of 11.5 GPU of CO₂ (together with a CO₂/CH₄ selectivity of 55.6).

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1. Introduction

Biogas production from renewable sources (e.g., from agriculture, landfills, or sewage plants) is one of the fields where membrane technology can show its greatest potential.^1,2 The main components of biogas are methane (CH₄, the combustible component) and carbon dioxide (CO₂, the non-combustible component), although it also typically contains traces of H₂O, N₂, H₂S and other organic aromatics.³ The high concentration of CO₂ and CH₄ in the mixture, basically in the same proportion, makes biogas upgrading appropriate to be carried out with polymeric membranes, a technology that offers advantages such as low energy costs and environmental benignity,⁴ and that can be an alternative to other existing approaches, such as cryogenic upgrading or liquefaction.⁵ For example, PVAm/PVA blends have shown a CH₄ recovery of 99% at a low running cost in a 2-stage recycled process.⁶ Besides purifying the CH₄ flow, the captured CO₂ is also suitable for its conversion into high value-added products, such as MeOH.⁷

The major materials used for membranes are polyimides and fluoropolymers.⁸ To obtain membranes with a good gas separation performance (i.e. high CO₂ permeation flux and CO₂/CH₄ selectivity), materials with intrinsic separation capacity for the target mixture are necessary. Poly(ether-block-amide), best known under the trademark Pebax®, constitutes a family of polymers that possess these advantageous properties. These polymers combine linear chains of rigid polyamide with flexible, CO₂-philic polyether segments, building crystalline/amorphous structures that show the properties of both thermoplastics and rubbers. It is believed that the hard amide block provides mechanical strength, whereas gas selective transport occurs primarily through the soft ether block.⁹ The polyamide/polyetheroxide proportion in the blend determines the Pebax® grade. The membranes in this work were prepared with Pebax® 1657, consisting of 40 wt% polyamide.¹⁰

Membranes with high permeance are essential for large-scale applications, such as biogas upgrading.¹¹ This variable is not only related to the membrane permeability but also to the thickness of the membrane, and membranes consisting of a very thin selective layer are necessary to achieve this goal. Such high performance membranes can be prepared as composite materials, where the selective layer is deposited on a highly porous support that provides mechanical stability.¹² Pebax® 1657 can be found in the literature in the form of thin film composites on several polymeric supports, such as polyvinylidene fluoride (PVDF),^13,14 polyacrylonitrile (PAN)^15,16 and polysulfones.^17,18 The CO₂ permeances vary from 13 to 670 GPU according to the membrane morphology and the CO₂/CH₄ selectivities are found to be between 13.6 and 18.0. These works also provide CO₂/N₂ selectivities, which show highly dispersed values (between 32 and 70).

In general, the gas separation performances of polymeric membranes can be enhanced through the concept of mixed matrix membranes (MMMs), consisting of the dispersion of inorganic fillers within a polymeric matrix so that either or both permeability and selectivity of the membrane can be improved through the synergistic combination of the two components.¹⁹ Metal–organic frameworks (MOFs) are materials that have been widely used as fillers in MMMs. In the case of the Pebax® 1657 polymer, ZIF-8 has been used as the filler by Xu et al.²⁰ and Zheng et al.²¹ The former found an increase in the CO₂ permeability from 79.2 to 156 Barrer as the ZIF-8 loading increased from 0 to 20 wt%, but the CO₂/N₂ selectivity decreased until 40.5. The latter showed fluctuating CO₂ permeabilities between 55.8 and 179 Barrer and practically constant CO₂/N₂ and CO₂/CH₄ selectivities. Within the ZIF family, ZIF-7 has also been used as the filler in Pebax® 1657 membranes. Li et al.²² prepared thin Pebax® 1657 based MMMs supported on PAN that showed the best performance results at 34 wt% loading with a CO₂ permeance of 39 GPU, and CO₂/N₂ and CO₂/CH₄ selectivities of 105 and 44, respectively. ZIF-7 has also been used as a filler by Sutrisna et al.¹³ who prepared MMMs on PVDF hollow fibers with optimum values of 300 GPU of CO₂, and with CO₂/N₂ and CO₂/CH₄ selectivities of 47.5 and 17.0, respectively. Other MOF-Pebax® 1657 combinations for dense MMMs included MOFs ZIF-94, NH₂-MIL-53(Al), MIL-69(Al) and MIL-96(Al), with the latter giving rise to the best CO₂/N₂ performance: permeability and selectivity were enhanced by 25 and 18%, respectively, as compared to the pure polymer.²³ Interestingly, the effect of the MOF functionalization (comparing the use of MIL-53(Al) and NH₂-MIL-53(Al) with better CO₂ permeability and CO₂/CH₄ selectivity values for the latter) has been recently studied on dense MMMs with Pebax® 1657.²⁴ There is no doubt that 1657 is the most used Pebax® code in the MMM field.

This work shows the preparation of thin film composite membranes with a thin mixed-matrix selective top layer of MOF/polymer Pebax® 1657 for biogas upgrade. The membranes have been prepared on different polymeric supports and the influence of the feed pressure on the gas separation performance has been studied. Different MOFs (ZIF-8, ZIF-7/8 core–shells, UiO-66 and MIL-101(Cr)) have been embedded in Pebax® 1657, dissolved in a water–ethanol mixture,²⁵ as fillers to obtain thin supported MMMs. Materials with a high CO₂ uptake (see Table 1) have been selected to favor the solubility of this gas in the membrane composite and thus enhance its CO₂/CH₄ separation performance.

Table 1 CO₂ adsorption capacities of different MOFs used in this work

MOF	Adsorption conditions	CO2 uptake, (mmol g^−1)	Pore aperture (nm)	Cavity (nm)	Ref.
ZIF-8	273 K, 1 bar	1.3	0.34	1.16	26,27
	298 K, 30 bar	35
UiO-66	273 K, 1 bar	2.4	0.80	2.1	28–30
	300 K, 35 bar	7.0
MIL-101(Cr)	303 K, 1 bar	1.6	1.2–1.6	2.9–3.4	31–33
	304 K, 50 bar	40
ZIF-7/8 core–shells	273 K, 1 bar	2.5	0.29–0.34	0.43–1.16	26

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2. Experimental section

2.1 Synthesis of MOF nanoparticles

Four different MOFs were synthesized to be used as fillers in the MMMs of this work. The synthesis of ZIF-8 was performed following a method based on a MeOH–water mixture as the solvent.³⁴ UiO-66 was synthesized solvothermally in N,N-dimethylformamide (DMF, 99.8%, Sigma-Aldrich).³⁵ The synthesis of MIL-101(Cr) was microwave assisted, with DI water as the solvent for the metal source and the ligand.³⁶ And finally, the ZIF-7/8 core–shells were prepared via post-synthetic modification of the previously described explained ZIF-8 nanoparticles.²⁶ The experimental details are described in the ESI.†

2.2 Membrane preparation

P84® asymmetric supports. Flat asymmetric porous P84® supports were prepared following the phase inversion method.³⁷ A 23 wt% doped solution of P84® (HP polymer GmbH) was prepared by dissolving the corresponding amount of powder in N,N-dimethylacetamide (DMAc, >99,8% Sigma Aldrich). The polymer solution was cast on a glass plate using the Elcometer 4340 Automatic Film Applicator placed in a fume hood and set at a thickness of 250 μm. Immediately afterwards the resultant polymer sheets were immersed into a tap water bath at 25 °C for 10 min. After precipitation, the membranes were kept in a deionized (DI) water bath overnight and then rinsed with IPA to remove the remaining DMAc. The films were dried at 100 °C one day prior to use.

Before testing the gas separation performance, several membranes were treated with PDMS (Sylgard® 184, Dow Corning) by dip coating. The coating solution was prepared by mixing the PDMS polymer base and the hardener (dimethyl, methylhydrogen siloxane) provided with the Sylgard® kit with a weight ratio of 10 : 1. The mixture was added to n-hexane to obtain a 3 wt% solution. The membranes were immersed in the coating solution for 5 min, and then allowed to evaporate at room temperature for 2 h. Finally, the membranes were cured in an oven at 100 °C for 18 h.

Dense PTMSP supports. For the preparation of dense PTMSP supports, the polymer was first dissolved at room temperature in hexane at 5 wt% concentration. The solution was then cast on a glass petri dish and allowed to dry at room temperature for 24 h. The obtained film was immersed in MeOH for another 24 h to remove traces of solvent and dried afterwards at 100 °C for 24 h more. The resulting films had a thickness of around 80 μm.

Pebax® 1657 membranes. Thin films of Pebax® 1657 were prepared on the two previously described supports (asymmetric porous P84® and dense PTMSP) following a solution-casting procedure. Pellets of Pebax® (kindly provided by Arkema) were dissolved in a 70/30 (v/v) EtOH/H₂O mixture by refluxing at 90 °C for 1 h. The polymer solution was then cast on the corresponding support using the same film applicator as before. Afterwards, the membrane was kept at room temperature for 1 day for complete solvent evaporation. In the case of the Pebax® MMMs, the corresponding amount of MOF (ZIF-8, UiO-66, MIL-101(Cr) or ZIF-7/8 core–shells) was dispersed in the EtOH/H₂O mixture. Afterwards, the pellets of Pebax® were added and the suspension was heated at 90 °C until full dissolution of the polymer after ca. 1 h. Then the suspension was cast on the polymeric supports and allowed to dry, as explained above. The casting solution had to be used within a few hours because long storage times lead to defective films. Self-supported membranes of the bare polymer Pebax®, with an approximate thickness of 80 μm, were also prepared for comparison purposes. In this case the polymer solution was poured into a petri dish and the solvent was allowed to evaporate slowly for 72 h at room temperature.

2.3 Membrane characterization

Thermogravimetric analyses (TGA) were carried out using a Mettler Toledo TGA/STDA 851e. Samples (10 mg) placed in 70 μL alumina pans were heated under a 40 cm³(STP) min⁻¹ air flow from 25 to 900 °C at a heating rate of 10 °C min⁻¹. Differential scanning calorimetry (DSC) analyses were performed on a Mettler Toledo DSC822e. Samples (10 mg) placed in 70 μL aluminum pans were heated under a 40 cm³(STP) min⁻¹ of nitrogen flow from 25 to 500 °C at a heating rate of 10 °C min⁻¹. The scanning electron microscopy (SEM) images of the MOFs and membranes were obtained using a FEI Inspect F50 model SEM, operated at 20 kV. Cross-sections of the membranes were prepared by freeze-fracturing after immersion in liquid N₂ and subsequently coated with Pt. Fourier transform infrared spectroscopy (FTIR) was performed for the MOF powders and for the different membrane samples, using a Bruker Vertex 70 FTIR spectrometer equipped with a DTGS detector and a Golden Gate diamond ATR accessory. The spectra were recorded on the Pebax® side by averaging 40 scans in the 4000–600 cm⁻¹ wavenumber range at a resolution of 4 cm⁻¹. Membranes were also characterized by Raman spectroscopy using a WiTec Alpha300 Confocal Raman Microscope, with a 785 nm laser excitation beam. X-ray diffraction (XRD) patterns of the MOFs and MMMs were obtained using Panalytical Empyrean equipment, using CuK_α radiation (λ = 1.540 Å), taking data from 2θ = 2.5° to 40° at a scan rate of 0.03° s⁻¹.

2.4 Gas separation analysis

The membrane samples were placed in a module consisting of two stainless steel pieces and a 316LSS macroporous disk support of 3.14 cm² (from Mott Co.) with a 20 μm nominal pore size, and gripped inside with silicon O-rings. The permeation module was placed in a UNE 200 Memmert oven to control the temperature of the experiments. Gas separation measurements were carried out by feeding a CO₂/CH₄ equimolar mixture (25/25 cm³(STP) min⁻¹) at 3–5 bar to the feed side by means of two mass-flow controllers (Alicat Scientific, MC-100CCM-D), while the permeate side of the membrane was swept with a 1 cm³(STP) min⁻¹ mass-flow controlled stream of He at 1 bar (Alicat Scientific, MC-5CCM-D). The concentrations of CO₂ and CH₄ in the outgoing streams were analyzed using an Agilent 3000A online gas microchromatograph equipped with a thermal conductivity detector. Permeances were calculated in GPU (10⁻⁶ cm³(STP) cm⁻² s⁻¹ cmHg⁻¹) once the steady-state of the membrane module exit stream was reached (for at least 3 h), and the separation selectivity was calculated as the ratio of permeances. At least 2–3 membrane samples of each type were fabricated and measured to provide the corresponding error estimations.

3. Results and discussion

3.1 Membrane characterization

Fig. 1 shows the cross-sections of three different membranes based on Pebax® 1657: a self-supported dense Pebax® 1657 membrane of around 80 μm thickness (Fig. 1a) and two supported Pebax® 1657 membranes prepared on dense PTMSP and asymmetric P84® supports (Fig. 1b and c, respectively). The cross-section of the Pebax® 1657/P84® composite shows a thickness of 120 μm for the P84® support, of which 15 μm corresponds to the denser top layer. Moreover, it can be observed in the inset at a higher magnification that the Pebax® 1657 layer is approximately 3 μm thick and shows a good adhesion to the polyimide support. A good compatibility can also be observed in the composite membrane prepared on PTMSP, the Pebax® 1657 layer in this case being 2 μm thick.

Fig. 1d shows the Raman spectra of the cross-section of the Pebax® 1657/P84® membrane. Two different points on the zones corresponding to the Pebax® 1657 layer and the P84® support were measured. Although the Pebax® 1657 Raman spectrum shows weak signals owing to its fluorescence, three peaks can be distinguished at 1133, 1305 and 1454 cm⁻¹ related to the C–O and C=O vibration modes.³⁸ Signals in the 1300–1800 cm⁻¹ range can be seen in the P84® spectrum. The signals at 1376 and 1435 cm⁻¹ correspond to the C=O in-phase stretching mode. The band at 1613 cm⁻¹ is related to the aromatic ring stretching mode, and that at 1780 cm⁻¹ corresponds to the aromatic C–N stretching.³⁹

Fig. 1 SEM images with higher magnification insets of the cross-sections of: Pebax® 1657 self-supported dense membrane (a), Pebax® 1657 supported on PTMPS (b), and Pebax® 1657 supported on asymmetric P84® (c). The Raman spectra corresponding to the latter are also provided (d).

Pebax® 1657 MMMs are shown in Fig. 2. Membranes containing a 10 wt% loading of ZIF-8, UiO-66, MIL-101(Cr) and ZIF-7/8 core–shell particles can be seen at three different magnifications. By visual inspection a good dispersion of the different fillers in the Pebax® thin layer can be observed, resulting in homogeneous membranes where a good filler–polymer adhesion is noticeable. The SEM images of the fillers are also provided (see Fig. S1 from the ESI†), from which the cumulative and differential particle size distributions were obtained using the ImageJ 1.49b software, together with median particle sizes of 150, 25, 33 and 124 nm for ZIF-8, UiO-66, MIL-101(Cr) and ZIF-7/8 core–shell particles, respectively (see Fig. S2 and Table S1 in the ESI†).

Fig. 2 SEM images of the cross-section of Pebax® supported (on asymmetric P84®) membranes containing the 10 wt% loading of ZIF-8 (a–c), MIL-101(Cr) (d–f), UiO-66 (g–i) and ZIF-7/8 core–shell particles (j–l).

Fig. 3 shows the XRD patterns of the different membranes, MOFs and of the pure polymeric Pebax® 1657 membrane for comparison. Pristine Pebax® 1657 is a semicrystalline copolymer which consists of both crystalline and amorphous PEO and PA6 phases, showing characteristic peaks at 2θ = 5.8°, 12.6° and 24.4°.⁴⁰ These signals are also noticeable in the patterns of the MMMs, although with a lower intensity due to the higher crystallinity of the fillers. It is also clear that ZIF-8 and UiO-66 maintain their crystallinity in the polymer matrix since their XRD reflections dominate over the amorphous band of the polymer. In the case of the other two MOFs, the peaks are not so well defined. This is due to the lower crystallinity of MIL-101(Cr) and the fact that the ZIF-7/8 core–shells are not as crystalline as the original ZIF-8 from which they are synthesized, according to our previous study.²⁶ Besides, after the incorporation of the MOFs, the peak positions of Pebax® 1657 remained almost unaltered, proving that there were no changes in the d-spacing of the polymer.

Fig. 3 XRD spectra of bare Pebax® 1657, MMMs and MOF powders.

The FTIR spectra were recorded to further characterize and analyze the Pebax® 1657 MMMs (see Fig. S3 in the ESI†). The observed peak at 1094 cm⁻¹ is attributed to the stretching vibration of the C–O–C group of the soft segment part of PEO.⁴⁰ Regarding the hard segment of PA chains, the peak corresponding to the –N–H– linkages is found at 3298 cm⁻¹ and the characteristic peak at 1636 cm⁻¹ is assigned to the H–N–C=O group.⁴¹ The most intense signals of each MOF can be found in the corresponding MMM spectrum. However, none of the membranes show new absorbance peaks, suggesting weak chemical interactions between the filler nanoparticles and the polymer chains or that the filler loading is too low for their visualization.

Thermogravimetric analyses (TGA) in flowing air were used to elucidate the thermal stability of the different membranes prepared in this work. As seen in Fig. S4 in the ESI,† while the P84® support shows an onset temperature of 592 °C, Pebax® 1657 was less stable since it started to degrade at around 400 °C. This is consistent with a slightly reduced thermal stability of the supported Pebax® 1657/P84® composite. Regarding the MMMs, the thermograms indicate that all the MOFs started to decompose over 300 °C. Besides, these TGA analyses helped to verify that the actual MOF content in the mixed matrix thin layer (12.5 wt% for ZIF-8, 8.2 wt% for MIL-101(Cr), 10.9 wt% for UiO-66 and 13.4 wt% for ZIF-7/8 MMMs) fits with the nominal (10 wt%). The thermal properties of Pebax® 1657 were further investigated by DSC (see Fig. S5 in the ESI†). Pristine Pebax® 1657 shows two endothermic peaks whose maxima occur approximately at 40 and 130 °C. These can be attributed to the fusion of the crystalline fraction of the blocks of poly(ethylene oxide) and polyamide, and limit the operating temperature of the membranes.⁴²

3.2 Gas separation performance

The different membranes prepared were tested for the separation of the CO₂/CH₄ equimolar mixtures at 35 °C and under different feed pressures ranging from 3 to 5 bar.

Fig. 4 depicts the gas separation performance of pristine Pebax® 1657 membranes. Three different types of membranes were studied, self-supported Pebax® 1657 membranes and supported Pebax® 1657 using supports of two different polymers: a dense PTMSP and an asymmetric porous P84®. Thick self-supported Pebax® 1657 and thin Pebax® 1657 supported on PTMSP showed similar CO₂/CH₄ selectivities, with values around 20. However, the difference in CO₂ permeance was much more noticeable since the former showed only 1.5 GPU while that of the latter increased up to 64 GPU. This is consistent with the difference in thickness between both membranes: 80 μm of the self-supported membrane vs. 2 μm of the supported membrane. Taking into account the corresponding value of this parameter for each membrane, the calculated CO₂ permeability would be around 120 Barrer in both cases. This highlights the reliability of the membrane permeation characterization system.

Fig. 4 Gas separation performance of pristine Pebax® 1657 membranes at 35 °C and under different feed pressures: self-supported and supported on PTMSP and P84®. Bars stand for CO₂ permeance and scatters for CO₂/CH₄ selectivity.

When testing the Pebax® 1657 supported on P84® also at 3 bar, the CO₂ permeance was 6.0 GPU, the flow increase being smaller than for the previous PTMSP supported membrane. Nevertheless, the CO₂/CH₄ selectivity increased considerably, reaching a value of 79.2, four-fold higher than that of the self-supported membrane. This behavior indicates that the P84® support affects the gas separation performance of the composites, increasing the membrane selectivity and simultaneously decreasing the gas permeability. For a better understanding of the role that the P84® support was playing in the gas separation, the support itself was tested for the CO₂/CH₄ separation (see Table S2 from the ESI†). The results showed that the P84® support exhibited a CO₂ permeance of 270 GPU but showed no CO₂/CH₄ selectivity. When the P84® was coated with PDMS the permselectivity was enhanced by defect healing, but only the inherent CO₂/CH₄ selectivity of PDMS was noticeable (5.5), along with its CO₂ permeance (55.1 GPU).⁴³ This fact means that P84® and Pebax® 1657 possess a specific compatibility, giving a composite whose gas separation performance is much better than that of the bare polymers. Besides, coating the polyimide P84® support with a more selective polymer such as Pebax® 1657 may lead to a healing effect and the selectivity of the polyimide would approach values found in the literature for this polymer (CO₂/CH₄ selectivity of 33.4).⁴⁴

The effect of the feed pressure on the gas separation performance of the CO₂/CH₄ mixture was also studied. As seen in Fig. 4, the supported Pebax® 1657/P84® membranes were tested from 3 to 5 bar, showing that the increase in pressure implied an augment in both the CO₂ permeance and the CO₂/CH₄ selectivity, reaching optimum values at 5 bar with 7.5 GPU and 114, respectively. The higher permeance of CO₂ results from its smaller molecular diameter in combination with its enhanced solubility due to its high quadrupole moment (4.30 D Å for CO₂vs. 0.02 D Å for CH₄), which enables strong specific interactions with the polar polyether groups in Pebax®.¹⁰ Moreover, the CH₄ permeance showed a contrary tendency, decreasing at the higher feed pressures tested. A similar reduction of permeation flux resulting from compression has been reported for N₂ and CH₄ in rubbery polymers such as PDMS and poly(octylmethylsiloxane) (POMS).^45,46 Besides, as seen in Fig. S6 in the ESI,† both CO₂ and CH₄ permeances follow an exponential tendency as a function of feed pressure as described by Stern et al.⁴⁷ (eqn (S1)†), with beta (the constant characteristic of the penetrant–membrane system at the testing temperature, 35 °C in this case) values positive for CO₂ (0.11 bar⁻¹) and negative for CH₄ (−0.18 bar⁻¹).

Membranes based on Pebax® 1657 using ZIF-8, UiO-66, MIL-101(Cr) and ZIF-7/8 core–shell particles as fillers have been prepared on P84® supports, forming thin supported MMMs. These MOFs have been selected because of their high CO₂ uptake (1.3–2.5 mmol g⁻¹ at 1 bar, see Table 1) in order to favor the solubility of this gas over CH₄ in the Pebax® 1657 based MMMs. Only MIL-101(Cr) has cavities in the mesoporous range, while other MOFs are microporous materials (see Table 1). Fig. 5a shows the gas separation performance of these MMMs at 35 °C. Two different feed pressures of 3 and 5 bar were tested showing that, as in the previous separation with pristine Pebax® 1657 (see Fig. 4), both the CO₂ permeance and the CO₂/CH₄ selectivity enhanced with increasing pressure. In terms of CO₂ permeance, the gas separation performance of the membranes improved with the incorporation of MOFs into the polymeric matrix. MMMs showed an average increase in CO₂ permeance of 6%, except for the UiO-66 MMMs, which showed a much greater improvement with a maximum value of 11.5 GPU at 5 bar, almost twice that of pristine Pebax® 1657 at the same feed pressure. Regarding the CO₂/CH₄ selectivity, its value decreased to one half when any of the fillers were incorporated into Pebax® 1657. Nevertheless, CO₂/CH₄ selectivities remained high with values between 50 and 60 for different MMMs, making them still very attractive. The best value was obtained for the ZIF-8 MMMs, with a CO₂/CH₄ selectivity of 65.1 (with 7.7 GPU of CO₂) at 5 bar. This result is logical since ZIF-8, besides having a moderate CO₂ adsorption, is the MOF with the narrowest pore access (0.34 nm), between the kinetic diameters of CO₂ and CH₄ (0.33 and 0.36 nm, respectively). The narrowest porosity of the ZIF-7/8 material (see Table 1), which is the worst performer in terms of CO₂/CH₄ selectivity, may hinder the transport of CO₂ in comparison to other MOFs.

Fig. 5 Comparison of the gas separation performance of pristine Pebax® 1657 and the different supported on P84® MMMs in the form of histogram (a) and upper bound type graph (b).

Considering separately the effect of the diffusivity and selectivity of the MOFs in the gas separation performance of the membranes, ZIF-8 and ZIF-7/8 core–shells are expected to have a greater effect on the diffusivity thanks to their narrower pore distribution (see Table 1). In contrast, UiO-66 and MIL-101(Cr) may have a greater effect on the contribution of the solubility due to their higher CO₂ uptake (see Table 1).

The gas separation performances of all MMMs were plotted on a selectivity–permeance graph (Fig. 5b). Since the Robeson's upper bound was originally defined in Barrer⁴⁸(see the values in Table S3 from the ESI†), a new upper bound was calculated in GPU to obtain a more accurate comparison (Fig. S7 from the ESI†). The Robeson's upper bound, revisited in 2008⁴⁸ was defined from the pure component permeability data of dense membranes, allowing the determination of the state-of-the-art limits for gas separation with polymeric membranes. The upper bound relationship is expressed by P_i = k·αⁿ_ij, where P_i is the permeability of the more permeable gas, α is the separation factor (P_i/P_j) and n is the slope of the log–log limit. It was observed that the representation of −1/n vs. d_ij (where d_ij is the difference between the gas molecular diameters (d_j − d_i)) yielded a straight line relationship. Since the gas permeability was defined for the explained purpose in Barrer, a new CO₂/CH₄ upper bound relationship in GPU has been calculated here. This used the values from the literature that defined the original upper bound but changing permeabilities in Barrer by permeances in GPU (see Table S3 from the ESI†), as done in a previous work for H₂/CO₂ mixtures.³⁷ The thicknesses used are those reported in the publications cited in Table S3 in the ESI,† although possible inaccuracies in the ex situ measurement of this length, such as experimental errors or membrane swelling, might affect such values. These values are represented in Fig. S7 in the ESI† and fitted to a logarithmic equation, resulting in the following upper bound relationship: . A factor k of 8175 GPU was obtained and the slope n of −2.086 was not far from the value found in the original publication (−2.636). Fig. 5b shows that all the membranes prepared in this work clearly surpassed the new calculated upper bound, reaching the so-called commercially attractive region. UiO-66 MMMs exhibited the highest CO₂ permeances, followed by MIL-101(Cr) MMMs, thanks to their wide porosity (see Table 1). In contrast, ZIF-7/8 MMMs are the least permeable and they also contain fillers with the narrowest pore distribution. ZIF-8 MMMs are the best balanced membranes, showing a great CO₂/CH₄ selectivity with high CO₂ permeance.

4. Conclusions

Thin membranes of Pebax® 1657 have been successfully prepared on dense PTMSP and asymmetric porous P84® supports. The obtained supported Pebax® 1657 membranes, with a thickness ranging from 2–3 μm, have been found to exhibit a good compatibility and adhesion between the support and the selective layer. The membranes were tested for CO₂/CH₄ separation at 35 °C and different feed pressures (3–5 bar), showing an improvement in both the CO₂ permeance and the CO₂/CH₄ selectivity with increasing pressures, thanks to the favored CO₂ solubility. While the performance of Pebax® 1657/PTMSP membranes was similar to those of self-supported dense Pebax®, the Pebax® 1657/P84® composites showed a great enhancement in the CO₂/CH₄ selectivity thanks to the synergistic compatibility between the two polymers. Thin MMMs of Pebax® 1657 containing 10 wt% of ZIF-8, MIL-101(Cr), UiO-66 and ZIF-7/8 core–shell nanoparticles were also prepared supported on P84®. The incorporation of MOFs enhanced the CO₂ permeance of the membranes on average 6%, but especially embedding UiO-66, which allowed doubling the permeance of pristine Pebax® 1657 membranes. ZIF-8 MMMs are the best performing composites, maintaining a high CO₂ permeance with a good CO₂/CH₄ selectivity. In any event, it has been demonstrated that the good physicochemical interaction between polymer Pebax® 1657 and P84® support allowed an enhancement in the CO₂/CH₄ separation. The highest CO₂/CH₄ selectivity obtained along the work was that of the membrane made of bare Pebax® 1657 on P84®, with a value of 114 (at 7.5 GPU of CO₂).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the Spanish MINECO and FEDER (MAT2016-77290-R), the Aragón Government (T43-17R) and the ESF is gratefully acknowledged. J. S.-L. thanks the Spanish Education Ministry Program FPU2014 for his PhD grant. All the microscopy work were carried out in the Laboratorio de Microscopías Avanzadas at the Instituto de Nanociencia de Aragón (LMA-INA). Finally, the authors would like to acknowledge the use of the Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza.

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Footnote

† Electronic supplementary information (ESI) available: MOF synthesis, membrane characterization and gas separation performance. See DOI: 10.1039/c8nj04769c

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