Preparation and performance of a novel mixed-matrix membrane supported on porous substrate for CH 4 /N 2 separation

Abstract

Coalbed methane enrichment is crucial for improving energy efficiency and reducing greenhouse gas emissions. Membrane separation technology shows significant potential for efficient CH₄/N₂ separation, making membrane material design a key research focus. Metal-organic framework-based mixed-matrix membranes have attracted much attention due to its good comprehensive performances and superior development potential. However, the balance between gas permeability and selectivity in membranes still needs further improvement. In this study, a novel supported mixed-matrix membrane with Ni-MOF-74 as the functional fillers and SBS polymer as the matrix material was fabricated by a simple dip-coating method, in which Al₂O₃ porous tube was used as the substrate. This strategy can significantly enhance CH₄ permeability while maintaining or improving CH₄/N₂ permeation selectivity. Results show that porous substrate exhibits good compatibility with the polymer-filler composite separation layer. The ratio of SBS polymer to Ni-MOF-74 particles has a significant impact on the comprehensive performances of the membrane. The as-fabricated SBS/Ni-MOF-74 membrane with 10% Ni-MOF-74 loading displays superior morphology and excellent CH₄ permeation performance. Pure CH₄ and N₂ permeation tests indicate that the optimized membrane exhibits a high CH₄ permeability of 66.4 Barrer, which is about 1.5 times higher than that of pure SBS membrane. It has an ideal CH₄/N₂ selectivity of 3.5, greatly exceeding the 1.6 selectivity of SBS membrane. This improvement can be attributed to the differences in diffusion and solubility coefficients of CH₄ and N₂ in the membrane. Moreover, this membrane exhibits relatively stable gas permeability and selectivity under different pressure differentials. Therefore, the membrane developed in this study exhibits excellent mechanical and gas permeation properties, showing potential applications in methane enrichment.

Keywords

mixed-matrix membrane metal-organic framework substrate-supported

Introduction

In the contemporary context, the escalating energy crisis and environmental degradation have emerged as critical global challenges demanding urgent resolution. The transition towards a low-carbon economy is increasingly perceived as an imperative, representing a paradigm shift of significance paralleling the industrial revolution.^1,2 The crux of this transition lies in harnessing clean energy while optimizing the use of conventional energy resources.³ In this regard, coalbed methane, distinguished by its environmental compatibility and abundant availability, emerges as a promising alternative to traditional fossil fuels. Its strategic development and utilization are thus of paramount importance.^4,5

The efficiency of coalbed methane usage hinges critically on the enrichment of methane.⁶ Currently, several industrial methods for CH₄/N₂ separation are prevalent, including pressure swing adsorption, cryogenic separation, and adsorption techniques.^7–9 Despite their maturity, these methods are encumbered by limitations such as bulky separation apparatus and elevated costs, particularly pronounced in scenarios involving low CH₄ concentration coalbed gases where the energy expenditure for enrichment is disproportionately high.¹⁰ Membrane separation processes offer a compelling alternative, characterized by their precision, efficiency, minimal energy requirements, and environmentally benign nature.¹¹ Consequently, there is a growing research impetus towards developing cost-effective and high-performance membrane materials, particularly mixed-matrix membranes (MMMs) which synergize the benefits of both inorganic and organic constituents.^12–15

MMMs are engineered by integrating inorganic particles into polymer matrices, thus amalgamating the superior attributes of polymers with the gas separation efficacy of fillers.^16–18 This approach aims to transcend the limitations inherent to purely organic or inorganic membranes. Typically, these membranes comprise a polymer film base (such as PEBA, SBS, etc.) interspersed with inorganic particles like zeolite, activated carbon, metal-organic frameworks (MOFs), and graphite oxide.^19–23 Recent years have witnessed a growing interest in MOFs, attributable to their design flexibility, ease of preparation, high porosity, substantial surface area, tunable pore dimensions, and gas affinity.^24,25 Their compatibility with polymer matrices renders MOFs particularly suitable as functional fillers in MMMs for CH₄ separation.²⁶

Notable studies in this field include Zhang et al.,²⁷ who incorporated Cu BPY-HFS fillers into Matrimid^®5218 polymer, enhancing the CH₄ solubility in the membrane due to strong Cu BPY-HFS and MOF interactions, thereby elevating the CH₄/N₂ permeation factor. Similarly, Perez et al.²⁸ utilized MOF-18 as fillers in Matrix^® 5218 to fabricate MMMs. This not only augmented CH₄ molecule affinity but also increased N₂ molecular diffusion resistance, thereby doubling CH₄ permeability compared to the pure Matrix^® 5218 membrane and enhancing CH₄/N₂ selectivity. Furthermore, Wang et al.²⁹ used MOF-74 (Ni) as fillers significantly improved CH₄/N₂ separation performance of MMMs. The above-mentioned works collectively underscore the efficacy of integrating MOFs with high CH₄ affinity into polymer matrices for enhanced separation performance. Although, these studies have made positive progress in improving CH₄ separation efficiency, but still face many issues, such as complex fabrication processes and low gas permeability.^14,26 As is well known, traditional MMMs feature a self-supporting structure. While aiming for thinner membrane layers to meet performance demands, this approach often compromises the mechanical strength of MMMs, posing potential stability risks during operation.²⁷ In contrast, increasing the thickness of the membrane to enhance its mechanical property often results in a significant increase in gas transmission resistance, leading to inadequate gas permeability performance. Therefore, achieving a balance between membrane structure and gas permeability through the design of novel membrane is crucial for enhancing membrane performance.³⁰

This study innovates upon traditional supported membrane and MMM methodologies by introducing a novel porous tubular substrate-supported MMM. This design effectively minimizes separation layer thickness while maintaining mechanical robustness, thus improving methane gas permeation selectivity. The study encompasses the synthesis of Ni-MOF-74 particles, fabrication of MMMs with varied filler loadings, and a comprehensive analysis of their structural characteristics and gas permeation efficacy.

Experimental

Materials

Poly(styrene-b-butadiene-b-styrene) (SBS, PS 30 wt%, PB 70 wt%) triblock copolymer was purchased from Sigma Aldrich; nickel acetate tetrahydrate and 2, 5-dihydroxyterephthalic acid were purchased from Shanghai Bide Pharmaceutical technology Co., Ltd; tetrahydrofuran (THF) and methanol were purchased from Shanghai Titan technology Co., Ltd; acetone, hydrochloric acid and nitric acid were purchased from purchased from Sinopharm Chemical Reagent Co., Ltd; Nitrogen (N₂, ≥ 99.99%) and methane (CH₄, ≥ 99.99%) were supplied by Dalian special gases Co., Ltd; Al₂O₃ tubular substrate (OD 12 mm, ID 8 mm, average pore size 0.2 μm) was supplied by Foshan ceramic research institute, Guangdong province; deionized water was home-made. All reagents were of analytical grade and used as received.

Synthesis of Ni-MOF-74

Ni-MOF-74 was synthesized employing a standard hydrothermal method, as delineated in existing literature.²⁹ This synthesis involved initially mixing 350 mL of deionized water with an equal volume of tetrahydrofuran, stirred for 15 min to create a homogenized solvent. To this mixture, 13.06 g of nickel acetate tetrahydrate and 5.22 g of 2, 5-dihydroxyterephthalic acid were added. Continuous stirring for two hours resulted in a clear, stable precursor solution. This solution, approximately 60 ml, was then transferred into a Teflon-lined stainless steel autoclave, sealed, and subjected to a temperature of 140°C for 2.5 h. Then, the autoclave was allowed to cool to room temperature. The resultant yellowish product was centrifuged and repeatedly washed with deionized water. Finally, the synthesized Ni-MOF-74 particles were dried in a vacuum oven at 100°C to yield a yellow crystalline powder, earmarked for use as fillers in MMMs.

Fabrication of substrate-supported MMMs

The fabrication process commenced with dissolving 10 g of SBS polymer in 100 ml of tetrahydrofuran (THF), under continuous stirring for two hours, to produce a clear polymer solution. A quantified amount of Ni-MOF-74 particles was then introduced into this solution, followed by vigorous stirring for six hours at room temperature to ensure a uniform coating solution. This solution was subsequently subjected to ultrasonic treatment for five minutes to eliminate any air bubbles, resulting in a homogenized coating mixture. The next step involved immersing an Al₂O₃ porous tube, sealed at both ends, into the coating solution for 15 min. After immersion, the coated tubes were extracted and dried in a vacuum oven at 80°C for 12 h to completely evaporate the solvent. The resulting MMMs were designated as MS-0, MS-5, MS-10, MS-20, and MS-40, corresponding to the mass ratio of Ni-MOF-74 to SBS polymer. Furthermore, an evaluation of the composite separation layer thickness was conducted, based on the diameter variation of the substrate before and after the coating process.

Characterization

The X-ray diffraction (XRD) patterns of the Ni-MOF-74 particles were recorded using a Rigaku RINT-2000 diffractometer (Cu Kα radiation, 1.5406 Å). The XRD scans spanned an angular range from 5 to 60°. For the analysis of nitrogen adsorption and desorption isotherms, a BELSORP-max instrument (BEL, Japan) was utilized. Prior to these measurements, the samples underwent an activation process at 50°C for 12 h under vacuum conditions. The morphological characteristics of the samples were thoroughly examined via a field-emission scanning electron microscope (SEM, model JSM-6701F), with each sample being gold-coated in preparation for SEM imaging.

Permeation tests for pure gases, specifically CH₄ and N₂, were conducted employing a constant pressure method as detailed in the reference¹⁷ and illustrated in Figure 1. These tests were pivotal in assessing the permeability and selectivity of the various membrane samples. To facilitate this, each membrane was carefully mounted within a stainless steel module and securely sealed at both ends using silicone O-rings. Temperature control during the tests was achieved by housing the membrane module within a temperature-regulated oven. The permeation performance of the membranes, in relation to the driving force, was investigated by adjusting the feed pressure while maintaining a constant pressure. In this study, a bubble flow meter was employed to accurately record the permeance of the different gases. The permeance, measured in gas permeation units GPU (1 GPU = 10⁻⁶ cm³ cm⁻² s⁻¹ cmHg⁻¹), was subsequently converted to permeability expressed in Barrer units, to enable direct comparison with the performance metrics of MMMs reported in the literature. This conversion was based on the specific thickness of the MS-X membranes under study. The gas permeability of these membranes was then calculated using the following equation:

P = \frac{Q}{A t} \cdot \frac{l}{Δ p}

(1)

Where P is the gas permeability, Q is the gas volume (cm³), A is the membrane area (cm²), l is the thickness of the membrane (cm), ΔP is the pressure difference between the upstream and downstream sides (cmHg).

The CH₄/N₂ separation selectivity is calculated from the following equation:

α_{CH 4 / N 2} = \frac{P_{CH 4}}{P_{N 2}}

(2)

Where α_CH4/N2 represents the ideal selectivity of the membrane for CH₄ and N₂; P_CH4 and P_N2 represent the permeability of CH₄ and N₂, respectively.

The diffusion coefficient was obtained from the time-lag (θ) value according to the following equation:

D = \frac{l^{2}}{6 θ}

(3)

Where D denotes the diffusion coefficient (m²/s), l represents the membrane thickness (m), and θ refers to the time lag (s), defined as the intercept on the time axis of the asymptotic line from the time-pressure curve.

The value S, representing the solubility coefficient, was derived from the diffusivity and permeability using the following calculation:

S = \frac{P}{D}

(4)

Results and discussion

Morphology and structure of Ni-MOF-74

The microstructural characteristics of MOFs are crucial in enhancing the permeation performance of substrate-supported MMMs. Figure 2(A) illustrates the morphology of the as-synthesized MOFs, fabricated using a hydrothermal method in this study. Notably, the Ni-MOF-74 particles exhibit a relatively uniform size, predominantly ranging from 500 to 800 nm in diameter. Additionally, the crystal morphology of these particles is consistent with the descriptions reported in prior literature^.²⁴

To further elucidate the crystalline structure of Ni-MOF-74, XRD analysis was conducted. The XRD pattern, presented in Figure 2(B), identifies distinct peaks at 6.84° and 11.88°, which correspond to the (120) and (030) crystallographic planes, respectively. A comparison with both simulated patterns and previously reported data for Ni-MOF-74 crystallinity reveals that our as-prepared sample exhibits the characteristic crystal structure of Ni-MOF-74, predominantly evidenced by its prominent peaks.²⁹ This finding unequivocally confirms the successful synthesis of the intended Ni-MOF-74 structure. Furthermore, the absence of any impurity-related peaks in the XRD patterns substantiates the high purity of the synthesized Ni-MOF-74.

N₂ adsorption-desorption analysis was conducted to ascertain the porosity of the synthesized Ni-MOF-74 crystals, with the findings illustrated in Figure 3. As depicted in Figure 3(A), the isotherms demonstrate a characteristic Type-I behavior, devoid of any hysteresis loop, indicative of a pronounced microporous structure. Notably, a steep increase in adsorption at the low-pressure region (P/P0 < 0.1) was observed, suggesting that the open metal sites within the framework are readily accessible to gas molecules. The BET specific surface area, pore volume, and average pore size of the Ni-MOF-74 crystals were determined to be 1265 m² g⁻¹, 0.48 cm³ g⁻¹, and 0.8 nm, respectively. These parameters are in remarkable concordance with previously reported values for Ni-MOF-74.²⁹

Figure 3(B) presents the adsorption isotherms of Ni-MOF-74 for CH₄ and N₂. At ambient temperature and a pressure of 0.5 MPa, the CH₄ adsorption capacity of Ni-MOF-74 was found to be 1.82 mmol·g⁻¹, aligning closely with existing literature. Intriguingly, its adsorption capacity for N₂ was approximately half that for CH₄. This enhanced selectivity for CH₄ is attributed to the favorable interaction between polarized CH₄ molecules and the organic framework of Ni-MOF-74. This characteristic suggests that substrate-supported MMMs incorporating Ni-MOF-74 could exhibit high CH₄ permeation selectivity, leveraging this molecular affinity.

Morphology and structure of supported MMMs

In this study, MMMs with varying Ni-MOF-74 loadings were synthesized to evaluate the impact of fillers on the performance of MMMs. Figure 4 shows the surface morphology of Al₂O₃ substrate and the different MMMs (MS-0, MS-5, MS-10, MS-20, and MS-40). As observed in Figure 4(A), the Al₂O₃ tubular substrate exhibits a relatively smooth and porous surface structure, comprised of small alumina particles. According to the supplier's specifications, the average pore size and porosity of the substrate are 0.2 μm and 55%, respectively, aligning with the general morphology and structural characteristics commonly reported in studies of supported membranes.¹²

The supported MMMs displayed distinct surface morphologies contingent on the Ni-MOF-74 content within the coating layer. MS-0, devoid of functional fillers and covered by the SBS polymer, revealed a much smoother and denser surface morphology, as illustrated in Figure 4(B). Notably, the surface of MS-0 lacked visible Al₂O₃ particles or apparent defects, indicating the effective film-forming properties of the SBS polymer on the substrate. With increasing Ni-MOF-74 content in the coating slurry, the surfaces of the MMMs became progressively rougher, and the morphological traits of Ni-MOF-74 particles began to manifest. Figure 4(E) shows that MS-20 exhibited a unique surface morphology characterized by Ni-MOF-74 particles enveloped in SBS polymer. This indicates a uniform distribution of Ni-MOF-74 particles, each encased within the SBS matrix, on the surface of MS-20, suggesting excellent compatibility between the fillers and SBS. This compatibility is likely to endow the MMMs with enhanced permeability and selectivity. However, at a filler content of 40 wt%, the membrane demonstrated notable particle aggregation and the presence of some defects, as depicted in Figure 4(F). A large number of Ni-MOF-74 particles were exposed on the surface of MS-40, implying a compromised integration of the benefits of SBS polymer matrix and MOF fillers. It should be noted that the thicknesses of MS-0, MS-5, MS-10, MS-20 and MS-40 are about 35, 40, 48, 65 and 89 μm, respectively, as determined by the changes in membrane tube diameter. From the XRD patterns in Figure 5, it can be observed that the composite membrane contains characteristic peaks of both the Al₂O₃ tube and the Ni-MOF-74 filler. This indicates good compatibility between the different materials, leading to the formation of a dense, defect-free membrane layer with well-preserved filler properties.

Permeation performance of different MMMs

The gas permeation efficiency of MMMs is intricately associated with the chemical and physical properties of both the polymer matrix and the embedded fillers. Figure 6 illustrates the impact of Ni-MOF-74 loadings on the permeation performance of MMMs, evaluated under conditions of 0.1 MPa and 25°C. Notably, the incorporation of varying amounts of Ni-MOF-74 within the separation layer induces distinct permeability trends for CH₄ and N₂ gases. The data indicate that the presence of Ni-MOF-74 significantly influences gas permeability, which exhibits an increase-decrease pattern across the filler loading range. At a Ni-MOF-74 filler-to-SBS resin mass ratio of 20%, MS-20 membrane demonstrated the highest gas permeation flux, with CH₄ and N₂ permeability values reaching 68.1 and 24.2 Barrer, respectively. This suggests that the structure of MS-20 offers minimal resistance to gas transmission. Two primary factors could be used to explain this enhancement in gas permeability. First, the strong interaction between CH₄ and the Ni-MOF-74 filler surface, along with the intrinsic affinity of Ni-MOF-74 for CH₄ over N₂, contributes to increased CH₄ permeability.⁷ Second, the formation of possible interconnected porous structures within MMMs provides dual-mode transport pathways for both CH₄ and N₂, accommodating their kinetic diameters.²⁰ Additionally, due to the differential changes in permeability between the two gases, CH₄ selectivity peaks in the MS-10 membrane, which exhibited a 151% increase in CH₄/N₂ selectivity compared to pure SBS membrane. Generally, increasing MOF loading in MMMs enhances the selectivity of gas pairs, leading to a considerable expense in permeability. However, the fabricated MMMs with Ni-MOF-74 displayed significantly higher CH₄ permeability and improved CH₄/N₂ selectivity. This observation aligns with the findings of Su et al.,³¹ where the MMM achieved the highest CH₄ permeability at a filler-to-resin mass ratio of 15% within the 0–20% MOF loading range.

The permeability behaviors observed are closely linked to the materials and structure of MMMs. In this work, CH₄ permeability increased with the addition of Ni-MOF-74 fillers, confirming an intrinsic affinity between Ni-MOF-74 and CH₄ as mentioned in Figure 3(B). However, the permeability declined when Ni-MOF-74 loading exceeded 20 wt%. For MS-40 membrane, its CH₄ permeability was 43.3 Barrer, about 90% less than that of MS-20. This decrease can be attributed to filler agglomeration, which hinders gas permeability. Moreover, the increased incorporation of Ni-MOF-74 particles in SBS matrix enhances polymer rigidity and reduces free volume, leading to a further decrease in permeability.¹⁶ Although exhibiting a similar trend, N₂ permeability among different MMMs fluctuated less due to its smaller kinetic diameter and weaker gas molecule-filler interactions. Compared to previous studies, Buonomenna et al.²⁸ developed a SBS-based MMM exhibiting a very high CH₄/N₂ selectivity of 7.2. This was achieved through the integration of amine-functionalized SBA-15 fillers with smaller effective channels (0.65 nm). However, its permeability was limited to 24 Barrer, mainly due to the significant transport resistance of CH₄ molecules through the 100 µm thick membrane layer. Therefore, in this work the MMM supported by cost-effective, porous Al₂O₃ tubes exhibits relatively balanced CH₄ permeability and selectivity.

The solution-diffusion mechanism does not only apply to gas transmission through dense polymer membrane but can also be used to explain the diffusion of gases in MMMs.¹¹ The roles of diffusivity (D) and solubility (S) were considered based on equation (2) and (3) at 298 K and 0.1 MPa. As shown in Figure 7(A), the D values of CH₄ and N₂ increased with Ni-MOF-74 loading raised to 20 wt%, resulting in improved gas permeability. For CH₄ and N₂, the maximum diffusion coefficients were 161.5 × 10⁻⁸ cm² s⁻¹ and 105.2 × 10⁻⁸ cm² s⁻¹, respectively, due to the porosity introduced by Ni-MOF-74 and the specific interaction between Ni-MOF-74 and CH₄ molecules.²⁹ However, at 40% filler loading, the diffusivities of both gases decreased due to longer gas residence times in MMMs, mainly caused by filler particle agglomeration. These changes resulted in the CH₄/N₂ diffusion selectivity decreasing from 2.3 to 1.1. This phenomenon was also observed in Shin's work,¹³ which involved MMMs containing SIFSIX-3-Ni to improve CO₂/CH₄ separation performance. Based on Figure 7(A) and (B), it can be seen that increasing Ni-MOF-74 content had a more pronounced effect on enhancing the diffusivity coefficient compared to reducing the solubility coefficient.

It can be concluded that the improvement in CH₄ permeability is primarily due to the dual-mode transport pathways in MMMs containing the optimal ratio of SBS matrix to Ni-MOF-74 fillers. The enhanced CH₄/N₂ selectivity results from the combined effects of gas diffusion selectivity and solubility selectivity.

Figure 8 shows the changes in gas permeability and selectivity for different membranes as a function of feed pressure from 0.1 to 0.5 MPa. As depicted in Figure 8(A) and (B), the increase in feed pressure results in similar impacts on CH₄ and N₂ permeability. For MS-0 membrane, CH₄ permeability showed an upward trend, increasing from 25.1 to 27.3 Barrer within the feed pressure range, while N₂ permeability rose from 16.5 to 18.0 Barrer. This led to a slightly reduced CH₄/N₂ selectivity, which remained around 1.5. In the case of MS-10 membrane, there was a 2.1% increase in CH₄ permeability and a 3.5% increase in N₂ permeability, with the CH₄/N₂ selectivity ranging from 3.2 to 3.3 as shown in Figure 7(C).

Generally, increased feed pressure tends to compress the membrane layer, resulting in the densification of the polymer segments. This densification increases the resistance to gas transport through the membrane.³² However, the MMMs fabricated in this study feature relatively thin separation layers with lower SBS polymer content. The incorporated Ni-MOF-74 particles effectively restrict the densification of SBS polymer chains, stabilizing the resin material and reducing gas transport resistance. Additionally, high pressure promotes the dissolution and diffusion of CH₄ and N₂ molecules within the membrane. For the MMMs designed in this study, maintaining the feed pressure within an optimal range can achieve good CH₄ permeability and selectivity. In summary, the Al₂O₃ tubular substrate-supported MMM with optimized Ni-MOF-74 loading demonstrates superior CH₄ separation performance, marking a significant advancement in CH₄ purification technology for coalbed methane.

A comparative analysis of CH₄/N₂ separation performance between Al₂O₃ supported MMMs and other MMMs in literatures is presented in Table 1. The table clearly shows that the as-fabricated Ni-MOF-74/SBS MMM (MS-10 and MS-20) exhibited superior CH₄ separation performance. Notably, the CH₄ permeability and selectivity of MS-10 reached 66.4 Barrer and 3.5, respectively, surpassing most reported results. Meanwhile, an SBS/SBA-15 MMM exhibits an extremely high CH₄/N₂ selectivity of 7.2 and a relatively low CH₄ permeability of 24 Barrer.²⁸ This is attributed to the incorporation of amine-functionalized fillers with smaller channels and a thicker membrane layer. A ZIF-8-based MMM demonstrates a very high CH₄ permeability of 566.2 Barrer and a moderate CH₄/N₂ selectivity of 2.2. It should be noted that the separation performance of this MMM is derived from a polyacrylic acid/ZIF-8 separation layer with a thickness of only about 400 nm. This extremely thin separation layer is supported on a highly hydrophilic and permeable interface layer, achieved through a series of intricate, multi-step fabrication processes.³³ Figure 9 compares CH₄/N₂ separation performance of various polymer membranes and MMMs.^{28,29,37–45} The results indicate that the supported SBS/Ni-MOF-74 membrane developed in this study demonstrates superior separation performance in terms of both CH₄ permeability and selectivity. The use of cost-effective Al₂O₃ tubes as supports highlights the economic and technical feasibility of this approach. Additionally, this MMM was produced via a simple dip-coating process. From both technical and economic perspectives, the MMMs developed in this study show exceptional potential for efficient CH₄ separation.

Table 1.

Gas permeation performance of different mixed-matrix membranes.

Polymer	Filler	loading (wt.%)	P_CH4(Barrer)	α_CH4/N2	Ref.
SBS	SBA-15	10	24.0	7.2	28
SBS	Ni-MOF-74	15	83.5	2.9	29
PSF	MCM-48	20	0.8	1.0	34
PeBax-1657	zeolite-4A	30	19.7	1.6	35
Matrimid^®	MOF-5	30	0.5	0.8	36
Polyacrylic acid	ZIF-8	60	566.2	2.2	33
SBS	Ni-MOF-74	10	66.4	3.5	This work
SBS	Ni-MOF-74	20	68.1	2.8	This work

Abbreviations: PSF, Polysulfone; SBA, Santa Barbara Amorphous; ZIF, Zeolitic Imidazolate Framework.

Gas permeation mechanism of different membranes

The permeation performance test indicates the high potential of the supported SBS/Ni-MOF-74 membrane in the purification of CH₄. It is necessary to analyze and compare the transport mechanisms of CH₄ and N₂ molecules in different membranes. Typically, self-supported MMMs are considerably thick, often spanning several tens to hundreds of micrometers, to satisfy mechanical performance criteria during application. As previously discussed and represented in Figure 10, such thickness inevitably heightens the transport resistance encountered by gas molecules, resulting in diminished gas permeability.

In contrast, the novel SBS/Ni-MOF-74 membrane exhibits a markedly thinner profile, only a few micrometers thick, primarily due to the structural reinforcement provided by the porous Al₂O₃ tube serving as the membrane substrate. This unique architectural design substantially shortens the transport pathways for gas molecules. Methane, characterized by its higher solubility and polarizability, encounters relatively lower transport resistance through this thinner membrane. Conversely, as depicted in Figure 7, the solubility of N₂ in the SBS polymer remains low, even in the context of a thin membrane. Additionally, the microporous structure and surface polarity of Ni-MOF-74 act as further impediments to the diffusion of N₂ molecules through the membrane. Collectively, these multifaceted effects bestow upon the novel supported membrane its impressive CH₄ permeation performance, distinguishing it as a promising candidate for methane purification applications.

Conclusion

In this study, a straightforward dip-coating method was employed to fabricate tubular substrate-supported MOF-based MMMs, specifically Ni-MOF-74/SBS, designed for CH₄/N₂ separation. This innovative structural approach notably diminishes the thickness of the separation layer, effectively harmonizing the permeability and selectivity of the MMMs. Additionally, the porous nature of the support provides the MMMs with requisite mechanical strength. A critical finding of this research is the influential role of Ni-MOF-74 particle loading in tailoring the membrane's performance. This loading optimizes the microstructure of both the polymer and filler phases within the membrane. Particularly, at a Ni-MOF-74 particle content of 10 wt%, the membrane surface reveals a unique composite structure, where the particles are enveloped by a thin polymer layer. This configuration is instrumental in refining the gas transport channels throughout the membrane, culminating in enhanced separation efficiency. At an operational temperature of 25°C and a pressure of 0.1 MPa, the optimized membrane demonstrates a relatively high CH₄ permeability of 66.4 Barrer and a selectivity of 3.5. These findings underscore the substantial potential of novel MMMs in advancing CH₄/N₂ separation technologies.

Footnotes

Acknowledgements

The authors are grateful for financial support from the National Natural Science Foundation of China (Grant Nos. 52174229 and 52174230), the Natural Science Foundation of Liaoning Province (Grant No. 2022-KF-13-05), Fushun Revitalization Talents Program (Grant No. FSYC202107010).

Credit authorship contribution statement

Yuntao Liang: Conceptualization, Supervision, Funding acquisition. Yongjing Wang: Writing-Review & Editing, Wenbin Feng: Investigation, Visualization, Writing- original draft. Jingkai Xv: Investigation, Formal analysis, Resources. Wei Xiao: Methodology, Validation.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Fushun Revitalization Talents Program, Liaoning Province Education Administration, Natural Science Foundation of Liaoning Province, National Natural Science Foundation of China, (grant number FSYC202107010, LJKZ0411, 2022-KF-13-05, 52174229, 52174230).

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