Synthesis of MIL-88B(Fe)/Matrimid mixed-matrix membranes with high hydrogen permselectivity

Abstract

A cheap and biocompatible metal–organic framework MIL-88B(Fe) (MIL = Material Institut Lavoisier) was synthesized and incorporated into Matrimid® 5218 to fabricate mixed-matrix membranes (MMMs) for gas separation. Separation performances of the MIL-88B(Fe)/Matrimid MMMs were tested for the single gas permeation of H₂ and CH₄ as well as the mixture gas separation of an equimolar binary H₂–CH₄ mixture. Due to the molecular sieving effect, the incorporation of MIL-88B(Fe) can enhance H₂ permeability but hinder the transport of CH₄ in MIL-88B(Fe)/Matrimid MMMs, thus resulting in enhanced separation selectivity of H₂–CH₄. At 298 K and ΔP = 3.0 bar, compared with those of a pure polymeric membrane, the H₂ permeability and H₂–CH₄ mixture separation factor of MMMs with a MIL-88B(Fe) loading of 10% increased by 16% and 66%, respectively. In addition, the operation temperature has a significantly positive effect on the separation of a H₂–CH₄ mixture.

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1. Introduction

Driven by concerns for energy security, there is an increasing demand for clean hydrogen in various fields such as fuel cells, semiconductor processing and the petrochemical industry.^1,2 The widely used industrial process for hydrogen production is steam-methane reformation (SMR).³ To achieve high purity hydrogen, it is of great importance to separate hydrogen from methane. Because of lower energy costs and fewer environmental impacts, membrane technology has been recognized as a commercially feasible way for gas separations.^4–9 Currently, the easy processing and mechanical strength of polymeric membranes make them more attractive for industrial applications in gas separation and therefore dominating the market share.^10,11 However, these conventional polymeric membranes suffer from a trade-off between permeability and separation selectivity, as demonstrated by Robeson.¹² In addition, another drawback of polymeric membranes is the plasticization caused by the presence of plasticizers (e.g. CO₂ and moisture), which will severely degrade the membrane separation performances.^13,14 To tackle the key challenges, an alternative solution is the incorporation of inorganic or hybrid fillers such as zeolites,¹⁵ carbon nanotubes¹⁶ and metal–organic frameworks (MOFs)^17,18 into polymeric material to fabricate mixed-matrix membranes.

MOFs are a relatively new family of nanoporous materials, which were built up of metal-containing nodes bridged by organic linkers through coordination bonds.^19–22 Due to their intrinsic organic nature, MOFs can exhibit excellent adhesion to the polymer matrices. Recently, a great number of research efforts have been devoted to the preparation of MOF-based mixed matrix membranes for gas separations.^{17,18,23–30} Zhang et al. fabricated the Cu–BPY–HFS/Matrimid MMMs and found that the incorporation of Cu–BPY–HFS increased the gas permeability but decreased the ideal separation factor of H₂–CH₄ mixture.¹⁷ Bae et al. prepared high-performance ZIF-90/6FDA-DAM MMMs containing submicrometer-sized crystals for CO₂–CH₄ separation.²⁴ Zornoza et al. reported the combination of MOFs and zeolites for MMMs and their use in gas separations.²⁵ In the case of H₂–CH₄ mixture, ZIF-8/PSF MMMs showed the best separation factor-permeability results due to the increased free volume and molecular sieving effect of ZIF-8.²⁵ Abedini et al. fabricated the highly permeable PMP/NH₂-MIL-53(Al) (PMP = poly(4-methyl-1-pentyne)) mixed-matrix membranes for CO₂–CH₄ separation, and they observed that incorporation of NH₂-MIL-53(Al) in PMP phase enhances the gas permeability (especially for CO₂) and CO₂–CH₄ selectivity.²⁹ Recently, Nordin et al. investigated the effect of particle size and heat treatment of ZIF-8 on CO₂–CH₄ separation of asymmetric mixed matrix membranes. They demonstrated that incorporation of only the heat-treated ZIF-8 of the smallest size increases the CO₂–CH₄ selectivity, from 19.4 of the neat PSF membrane to 28.5.³⁰ For as-prepared MMMs, the desired improvement in separation properties is to enhance the permeability and separation selectivity simultaneously for the target gas. Therefore, the selection of appropriate MOF particles as additives is very crucial to fabricate high-performance MMMs.

MIL-88B(Fe) is a type of cheap, biocompatible, and biodegradable MOF, constructed from trimers of oxocentered iron(III) octahedra and 1,4-terephthalate,^31–33 as shown in Fig. 1. Upon thermal activation, the framework of MIL-88B(Fe) can maintain its closed form with free aperture around 3 Å,^32,33 between the kinetic diameter of H₂ (2.89 Å) and that of CH₄ (3.76 Å). Hence, it can be expected that this material can show molecular sieving effect and thus greatly enhances the performance of MIL-88B(Fe)/polymer membrane for separating H₂–CH₄ mixture. With this in mind, a commonly investigated and commercially available polymer Matrimid® 5218 is chosen as matrix to prepare the MIL-88B(Fe)-based mixed-matrix membrane. Matrimid® 5218 is a glassy polymer, which provides high separation selectivity but relatively low gas permeability.³⁴ To the best of our knowledge, there is no any study in recent literature of MMMs containing MIL-88B(Fe) for gas separation. Therefore, we reported for the first time the preparation of MIL-88B(Fe)/Matrimid membranes for H₂ purification from H₂–CH₄ mixture.

Fig. 1 MIL-88B(Fe) structure viewed along a axis (top) and c axis (bottom): Fe, yellow polyhedra; O, red; C, gray; H, white.

2. Experimental

2.1. Materials

Matrimid® 5218 polymer was purchased from VWR International GmbH, Darmstadt. Iron chloride hexahydrate (FeCl₃·6H₂O), terephthalic acid (H₂BDC), and N,N′-dimethylformamide (DMF) were obtained from Aladdin Chemistry Co., Ltd. Chloroform and ethanol were obtained from Sinopharm Chemical Reagent Co., Ltd. All chemical were used as received without further purification.

2.2. Synthesis of MIL-88B(Fe)

In a typical synthesis, 1.023 g FeCl₃·6H₂O and 0.630 g H₂BDC were dissolved in 80 mL DMF at room temperature to form a clear solution with Fe³⁺/H₂BDC/DMF molar ratio of 1 : 1 : 275. The mixture was then introduced in a 100 mL Teflon-lined steel autoclave. The system was placed in a conventional oven and maintained at 383 K for 24 hours. After the reaction, the autoclave was naturally cooled down to room temperature. The resulting yellow powder was filtered off and washed with DMF several times. In order to remove the unreacted H₂BDC and DMF molecules in the channels, the powder was re-dispersed in ethanol for 10 hours at room temperature and then filtered off. Finally, the sample was dried at 373 K overnight.

2.3. Fabrication of MIL-88B(Fe)/Matrimid MMMs

MIL-88B(Fe) based MMMs were prepared according to the literature protocol with some modifications.^25,35 Matrimid® 5218 polymer was dried at 453 K overnight before use to remove the adsorbed water. For the pure membrane, 0.4 g of Matrimid was dissolved in 3.6 g chloroform and stirred for 2 days, leading to a viscous solution. To fabricate the MMMs, the required amount of MIL-88B(Fe) to get a given weight loading was previously dispersed in the solvent under sonication for 30 min. During the preparation procedure, the percentage of solvent maintained constant, i.e. a weight proportion solvent/MOF-polymer of 90/10 was used in all cases. Matrimid was then added and the whole mixture was stirred for 2 days. Before the membrane casting, three intervals of sonication for 15 min were carried out to ensure a well-dispersed solution as well as remove the trapped air bubbles. Subsequently, the homogenous solutions were cast on a flat glass plate with a doctor blade. Once surface dried within several minutes, the membranes were immediately peeled off and annealed in a conventional oven at 453 K for 16 hours to remove the remaining solvent. These membranes obtained had a thickness of 20(±2) μm, measured using a digital micrometer (accuracy of ±0.001 mm, Guilin Guanglu Measuring Instrument Co., Ltd.).

2.4. Characterizations

The qualities of as-synthesized MIL-88B(Fe) as well as MIL-88B(Fe)/Matrimid MMMs were characterized by X-ray diffraction (XRD). The XRD patterns were recorded at room temperature under ambient with a Bruker D8 ADVANCE X-ray diffractometer with Cu Kα radiation. The morphology of MIL-88B(Fe) and MMMs were characterized by field emission scanning electron microscopy (FESEM) operated on an S-4800 (Hitachi). The cross-sections were prepared by freeze-fracturing the membranes after several minutes of immersion in liquid nitrogen. Additionally, the nitrogen adsorption of MIL-88B(Fe) sample at 77 K and ambient temperature were measured by using an Autosorb-iQ-MP (Quantachrome Instruments) automated gas sorption analyzer. Before measurements, the sample was degassed at 423 K overnight.

2.5. Permeability analysis

The pure membrane and MMMs were evaluated by single gas permeation and mixed gas separation. For the permeations studies, these membranes were sealed in a permeation module with silicone O-rings. The N₂ sweep gas was fed on the permeate side to keep the concentration of permeating gas as low as possible thus providing a driving force for permeation. The permeation of single gases H₂ and CH₄ as well as the separation of equimolar binary mixtures of H₂ with CH₄ were studied using the Wicke–Kallenbach technique.⁹ The fluxes of feed and sweep gases were determined using mass flow controllers, and a calibrated gas chromatograph (Echrom A90) was used to measure the gas concentrations. The separation factor of binary mixture permeation is defined as the quotient of the molar ratio of H₂ and CH₄ in the permeate side, divided by the quotient of the molar ratio of H₂ and CH₄ in the retentate side, as given by:

3. Results and discussion

3.1. Characterization of MIL-88B(Fe) and MMMs

MIL-88B(Fe) materials were synthesized using the most commonly solvothermal method without assistance of alkali³¹ and microwave.³⁶ The negligible BET surface area and N₂ adsorption at ambient temperature of MIL-88B(Fe) shows that this material is not accessible to N₂ molecules due to the closed pore upon thermal activation (see Fig. S1 in the ESI†), which is similar to the previous report.³¹ Fig. 2a compares the powder XRD pattern of as-synthesized MIL-88B(Fe) with that published in the literature,³⁶ suggesting that the pattern obtained in this work is in good agreement with the reported one. In order to confirm the crystal structure of MIL-88B(Fe) particles in MIL-88B(Fe)/Matrimid MMMs, XRD measurements were also performed for pure polymeric membrane and MMMs. Fig. 2b shows the XRD patterns of MMMs with different loadings, together with that of pure MIL-88B(Fe) for comparison. For the pure Matrimid membrane, a broad peak between 10° and 25° can be observed, which is characteristic of an amorphous structure. In the case of MMMs, both the intense crystalline peaks and broad amorphous peak exist, indicating the presence of MIL-88B(Fe) and Matrimid phases, respectively. Furthermore, the intensity of crystalline peaks of MIL-88B(Fe) increases with increasing MIL-88B(Fe) loading, whereas the broad amorphous peak decreases.

Fig. 2 (a) The powder XRD pattern of MIL-88B(Fe) synthesized in this work. The vertical red bars present the 2θ positions taken from the reported literature.³⁶ (b) Normalized XRD patterns of MIL-88B(Fe)/Matrimid MMMs at different loadings.

As presented in Fig. 3, the FESEM image shows that MIL-88B(Fe) particles have hexagonally bipyramid-shaped morphology with a uniform size of ca. 1.6 μm in length and 1.0 μm in diameter, without significant agglomeration which is desirable for MMMs fabrication. Fig. 4 reveals the cross-sections of MIL-88B(Fe)/Matrimid MMMs with different MIL-88B(Fe) loadings. It can be seen that the MIL-88B(Fe) particles can be well dispersed in the polymeric phase. Moreover, no obvious phase separation can be observed even at a high MIL-88B(Fe) loading of 20%, indicating a good compatibility and adhesion between MIL-88B(Fe) particles and Matrimid polymer.

Fig. 3 FESEM image of as-synthesized MIL-88B(Fe) powder.

Fig. 4 FESEM images of (a) pure Matrimid membrane, (b) 5%-MIL-88B(Fe)/Matrimid, (c) 10%-MIL-88B(Fe)/Matrimid, and (d) 20%-MIL-88B(Fe)/Matrimid.

3.2. Gas permeation behavior of MIL-88B(Fe)/Matrimid MMMs

3.2.1. Single-gas permeation. During the MMMs fabrication, a crucial step is to anneal these membranes at high temperature. As demonstrated by Song et al., the annealing temperature has a significant effect on the permeability and separation factor of MMMs prepared by Matrimid polymer, and the appropriate temperature ranges from 453 to 573 K.²⁶ Considering the thermal stability of MIL-88B(Fe) (up to 493 K),³¹ an annealing temperature of 453 K was selected for all cases in the present work. Fig. 5 shows the permeability of H₂ and CH₄ through the annealed MMMs as a function of the trans-membrane pressure difference (ΔP). For the pure Matrimid membrane, the H₂ permeability drops by 20% with increasing ΔP from 1.5 to 3.0 bar. This decreasing trend along with ΔP can be explained by the correction of gas permeability by trans-membrane pressure difference³⁷ as well as the decrease of gas solubility with increase of pressure.³⁸ On the contrary, a distinctly different phenomenon can be observed for CH₄, i.e. the larger ΔP, the higher permeability of CH₄, which may originate from the intrinsic property of such type of polymer. As illustrated by Basu et al., the pristine Matrimid membrane possesses a d-spacing value at 5.47 Å, and its chain mobility facilitates diffusion of the gases.³⁹ Therefore, the strong driving force derived from high ΔP probably enhances the better retained CH₄ molecule diffusion between the polymer chains. Gascon and co-workers studied the structure–performance relationships in CO₂–CH₄ separation over NH₂-MIL-53(Al)/Matrimid mixed-matrix membranes. They also found that the CH₄ permeability increases with increasing ΔP for the pure Matrimid membrane below 9.0 bar.³⁷ In addition, compared to the reported Matrimid membranes,^23,26 the pure polymeric membrane obtained in this work exhibits a much higher H₂ and CH₄ permeabilities. This might be attributed to the presence of some defective voids, likely caused by the rapid evaporation of chloroform during the annealing process^28,40 and/or trapped air bubbles.¹⁷ Cao et al. reported a highly permeable MMM incorporated with NH₂-functionalized MOF CAU-1 for H₂–CO₂ separation.⁴¹ In their work, a low concentration of PMMA solution was used to fabricate CAU-1-based MMMs, resulting in the presence of some non-selective voids. Consequently, the as-prepared thin PMMA membrane exhibits a three order of magnitude higher permeability of H₂ than that of the reported PMMA membrane.⁴¹ Very recently, Coronas and co-workers prepared mixed-matrix membranes containing MOFs and porous silicate filler through spin-coating technique. They stated that this faster solvent evaporation could produce more defective and less stable membrane compared to the controlled release of solvent in the traditional casting process, thus leading to the increase in gas permeability.²⁸ These results demonstrate that the introduction of some defects into polymeric membrane can be helpful to enhance gas permeation.

Fig. 5 Permeability of single gas H₂ (a) and CH₄ (b) in pure Matrimid membrane and MMMs with different MIL-88B(Fe) loading at 298 K, as a function of the trans-membrane pressure difference.

In the MMMs, both H₂ and CH₄ permeabilities decrease with increasing ΔP from 1.5 to 3.0 bar. However, it is found that that MIL-88B(Fe) has a different influence on the transport of H₂ and CH₄ in the MMMs. Remarkably, the incorporation of MIL-88B(Fe) can enhance the H₂ permeability, and the improvement in permeability becomes more pronounced with increasing the MIL-88B(Fe) loading with respect to the pure membrane. It should be noted that the pore size of MIL-88B(Fe) is larger than the kinetic diameter of H₂, allowing for the direct diffusion of H₂ through pores of the MIL-88B(Fe), thus contributing significantly to the increase in permeability (see Fig. 6). Galve et al. fabricated the copolyimide-based MMMs with oriented microporous JDF-L1 sheet particles. JDF-L1 is a layered titanosilicate with the pore size across the layers of about 3 Å. It was found that the H₂–CH₄ separation factor could enhance, but H₂ permeability decreased in the MMMs containing JDF-L1 sheet.⁴² For the case of CH₄, the permeability decreases greatly after the incorporation of MIL-88B(Fe). Despite of the flexible feature of MIL-88 series, it has been well established that CH₄ is difficult to open the closed pores of MIL-88 series due to the weak interactions with the framework walls.³³ During the permeation of CH₄ in the MMMs containing MIL-88B(Fe), CH₄ has to bypass the MOF fillers, as illustrated schematically in Fig. 6. That's to say, MIL-88B(Fe) can show an efficient molecular sieving effect and thus hinder the transport of CH₄ in the MMMs, i.e. decrease CH₄ permeability. Recently, Song et al. reported a molecular sieving ZIF-8 based Matrimid nanocomposite membranes for gas separation.²⁶ The crystallographic pore aperture of ZIF-8 crystals is around 3.4 Å, and ideally it could block the large molecules including N₂ (3.64 Å) or CH₄ (3.76 Å). However, it was found that both solubility and diffusion coefficient of CH₄ increases accompanied with an increase of permeability in MMMs, which was attributed to the flexible pore structure of ZIF-8 (via gate-open).²⁶ In addition, a slightly larger permeability at higher loading can be attributed to the agglomeration of MIL-88B(Fe) particles, which is a common issue, also observed by other researchers.^43–45

Fig. 6 Schematic illustration of the transport of H₂ and CH₄ through the MIL-88B(Fe)/Matrimid MMMs. The dash and solid line represents the diffusion path of H₂ and CH₄, respectively.

In addition to permeability, the separation factor of gas pair also is a key parameter in separation process. Fig. 7 shows the ideal separation factor of H₂–CH₄ in MMMs as a function of the MIL-88B(Fe) loading, by taking ΔP = 3.0 bar as an example. As expected, the ideal separation factor of H₂–CH₄ in the pristine membrane is about 37, lower than the reported one (∼113).²³ At the MIL-88B(Fe) loading of 10%, the ideal separation factor reaches maximum of 80, suggesting that the incorporation of MIL-88B(Fe) can greatly enhance the performance of MMMs for separation of H₂–CH₄ due to the molecular sieving effect. We are aware that in term of separation factor, the as-synthesized pristine membrane and MMM containing 10% MIL-88B(Fe) in this work show lower performance than the reported ones. However, besides the strategy to effectively enhance the separation factor via incorporating molecular sieving MOFs into polymer, we also aim to fabricate the industrially attractive membranes with high permeability and good separation factor.

Fig. 7 Ideal separation factor of H₂–CH₄ in MIL-88B(Fe)/Matrimid MMMs as function of the MIL-88B(Fe) loading at 298 K and ΔP = 3.0 bar.

3.2.2. Binary-gas separation. The molecular sieving performances of the MIL-88B(Fe)/Matrimid MMMs were also confirmed by the separation of equimolar mixture at 298 K and ΔP = 3.0 bar, as shown in Fig. 8. Comparing the H₂ and CH₄ permeabilities in the mixture with single gases ones, only a slight difference is found for H₂, whereas CH₄ permeability decreases significantly. This indicates that competitive transport between these two species can occur in the MIL-88B(Fe)/Matrimid MMMs. At the MIL-88B(Fe) loading of 10%, the mixture separation factor of H₂–CH₄ is 96, increased by 66% with respect to the pristine membrane. The separation properties of some MMMs containing different fillers in the literature were summarized in Table S1 in the ESI.† Clearly, H₂ permeability obtained in this work is about one order of magnitude higher than those reported previously with comparable H₂–CH₄ selectivity. As pointed out recently, ultrahigh membrane separation factor is not necessarily needed for large-scale gas separation due to the fact that high separation factor is usually gained at the expense of permeability.^41,46 Considering that industrially attractive membranes should require high permeability and good separation factor,²⁴ MIL-88B(Fe)/Matrimid MMMs developed in the present work exhibit excellent separation performance for H₂–CH₄ mixture.

Fig. 8 The permeability of H₂ and CH₄ as well as mixture separation factor of H₂–CH₄ in MIL-88(Fe)/Matrimid MMMs, as function of the MIL-88B(Fe) loading at 298 K and ΔP = 3.0 bar.

3.2.3. Influence of the operation temperature. The effect of the operation temperature on the separation performance was evaluated for the pure Matrimid membrane and MMM containing 10% MIL-88B(Fe). As shown in Fig. 9, increasing the operation temperature from 298 to 373 K, the H₂ and CH₄ permeabilities as well as mixture separation factor of H₂–CH₄ simultaneously increase for both the pure membrane and 10%-MIL-88B(Fe)/Matrimid MMM. This phenomenon can be explained by a solution–diffusion model.⁴⁷ As demonstrated by Ruthven⁴⁸ and Zhao et al.,⁴⁹ the permeability can either increase or decrease with increasing temperature, depending on the difference between the enthalpy of solution and the activation energy of diffusion. For the H₂–CH₄ mixture, the separation mechanism is mainly based on diffusion and not on the solution difference in the Matrimid-based membranes.^15,25 Increasing temperature can facilitate the diffusion of gases,⁴⁸ and therefore increase both the permeability. At the same time, CH₄ dissolution is largely restrained at higher temperatures and thus more H₂ can diffuse in the resulting free volume, leading to more significant enhancement of H₂ permeability. Since the SMR reaction is generally carried out at elevated temperatures,³ it is favourable for membranes with temperature-enhanced separation performance in the H₂ purification application.

Fig. 9 The permeability of H₂ and CH₄ as well as mixture separation factor of H₂–CH₄ in pure membrane (a) and 10%-MIL-88B(Fe)/Matrimid MMM (b) at ΔP = 3.0 bar, as a function of operation temperature.

4. Conclusions

In this work, a cheap and biocompatible MOF, MIL-88B(Fe), was synthesized and used as additive into Matrimid polymer to prepare MIL-88B(Fe)/Matrimid MMMs for H₂–CH₄ separation. Due to the efficient molecular sieving effect of MIL-88B(Fe) (with free aperture around 3 Å, between the kinetic diameter of H₂ (2.89 Å) and CH₄ (3.76 Å)), the incorporation of this type of MOF can simultaneously enhance H₂ permeability and separation factor of H₂–CH₄. At 298 K and ΔP = 3.0 bar, with respect to those of pure polymeric membrane, H₂ permeability and H₂–CH₄ mixture separation factor of MMM with a MIL-88B(Fe) loading of 10% increased by 16% and 66%, respectively. In addition, operation temperature has a significant effect on improving the separation performance for H₂–CH₄ mixture. As MOFs have superior advantages of fine-tunable pore size and controllable design over conventional porous materials, MOFs can be considered as very promising fillers for MMMs for this separation target.

Acknowledgements

This work was supported by the National Key Basic Research Program of China (“973”) (2013CB733503) and the Natural Science Foundation of China (nos 21136001, 21322603, and 51402320).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: N₂ adsorption isotherm in MIL-88B(Fe) at 298 K. Comparison of H₂–CH₄ separation performance of MMMs in this work and literature. See DOI: 10.1039/c4ra13727b

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