Xing
Liu
abc,
Zhenyuan
Li
b,
Zanchun
Du
b,
Can
Wang
b,
Haitao
Zhang
b,
Shuangjiang
Luo
*ab and
Suojiang
Zhang
*b
aShanxi-Zheda Institute of Advanced Materials and Chemical Engineering, Taiyuan, 030000, China. E-mail: sjluo@ipe.ac.cn
bCAS Key Laboratory of Green Process and Engineering, Beijing Key Laboratory of Solid State Battery and Energy Storage Process, State Key Laboratory of Mesoscience and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. E-mail: sjzhang@ipe.ac.cn
cUniversity of Chinese Academy of Sciences, Beijing 101408, China
First published on 14th August 2025
Polymeric hollow fiber membranes (HFMs) have emerged as a transformative technology for sustainable natural gas valorization, offering energy-efficient solutions for acid gas (CO2 and H2S) removal and helium (He) recovery. This review highlights recent advancements in material design, fabrication strategies, and performance optimization of HFMs tailored for aggressive natural gas feeds. Key innovations include the engineering of polymer structures, coupled with asymmetric architectures (dual-layer, thin-film composites), which overcome the intrinsic permeance-selectivity trade-off, achieving industrially relevant permeance and selectivities. Advanced mitigation strategies for membrane aging and plasticization, including crosslinking and siloxane hybridization, are elucidated at the molecular level, demonstrating enhanced stability under high-pressure conditions. Furthermore, breakthroughs in processing techniques (e.g., co-extrusion spinning and interfacial polymerization) enable the fabrication of ultra-thin selective layers with defect-free interfaces. Emerging tools like machine learning and green solvents are also discussed as enablers of scalable and eco-friendly manufacturing. This work provides a comprehensive roadmap for next-generation HFMs, bridging molecular design with industrial deployment to advance decarbonization in energy systems. By addressing critical challenges and future opportunities, this review aims to inspire further research in high-performance membrane materials for sustainable natural gas processing.
Current separation technologies face sustainability trade-offs. Solvent absorption, though effective for bulk CO2 and H2S removal, consumes 30–40% of processing energy for solvent regeneration and generates corrosive waste.10 Cryogenic distillation, while achieving high helium purity (99.999%), incurs prohibitive costs for lean reserves (<0.1% He) due to energy-intensive liquefaction.11 These limitations underscore the urgency for energy-efficient alternatives. Membrane-based separation has gained traction as a low-carbon solution, offering continuous operation without chemical additives and modular scalability suited to decentralized gas fields.12,13 Among membrane configurations, hollow fiber modules dominate industrial adoption due to their intrinsic self-supporting architecture, unmatched packing densities (up to 104 m2 m−3), ease of scale-up, and superior mechanical resilience compared to spiral-wound and plate-and-frame modules.14
The evolution of polymeric hollow fiber membranes (HFMs) for natural gas treatment traces back to the 1970s with asymmetric cellulose acetate HFMs, achieving a CO2/CH4 selectivity of 12–15 but succumbing to plasticization at CO2 partial pressures >5 bar (Fig. 1).10,15,16 A paradigm shift occurred in 1994 when polyimide-based HFMs (Medal, Air Liquide) doubled CO2/CH4 selectivity to 20–25 while resisting hydrocarbon-induced plasticization.16 By 2020, HFM systems accounted for 18% of global natural gas processing capacity, with over 500 installations treating >200 billion m3 annually. These industrial deployments now demonstrate high-efficiency decarbonization, resource valorization, and extreme-condition resilience. For instance, ADNOC's Habshan facility achieves 99% CO2 removal from 10 MMSCFD streams with <2% CH4 loss, Evonik's helium-selective HFMs (He/CH4 selectivity >150) enable economic extraction from sources containing <2% He, Air Products’ methane recovery modules, and UBE's H2S-tolerant polyimide HFMs handle >30000 ppm H2S streams without performance decay. These field-proven capabilities establish HFMs as essential components for achieving net-zero emissions targets, specifically in carbon capture applications and helium recovery from unconventional resources. However, persistent challenges associated with plasticization and physical aging phenomena necessitate fundamental innovations in advanced membrane materials.
Despite these advances, existing reviews inadequately address the material–process–performance interplay specific to natural gas treatment scenarios. Prior studies predominantly focused on generic fabrication techniques or broad gas separation applications.17–23 For instance, Chung et al. (2012) detailed phase-inversion principles for HFM formation, with applications spanning CO2 capture, H2 production, and desalination,24 while Othman et al. (2018) pioneered the analysis of dual-layer HFMs, emphasizing coextrusion fabrication and comparative advantages over single-layer designs.25 Kujawski et al. (2020) systematically categorized asymmetric, thin-film composites and mixed-matrix HFMs, correlating transport mechanisms with performance,17 and Yong et al. (2021) advanced theoretical frameworks linking spinning parameters to membrane morphology and gas separation efficiency in applications like air separation and propylene/propane splitting.26 Recent studies, such as Imtiaz et al. (2022) on braid-reinforced HFMs and Lau et al. (2024) on surface modification techniques, further expanded the technical repertoire.15,27 However, these studies lack dedicated focus on natural gas treatment, particularly in addressing two critical challenges: (1) the intrinsic relationship between the hierarchical structure of HFMs and their gas separation performance remains unclear, particularly regarding natural gas sweetening under complex gas source conditions and helium extraction from low-concentration sources, and (2) the microscopic mechanisms of aging in polymeric HFMs, along with their plasticization behaviors and anti-plasticization strategies under high CO2 and light/heavy hydrocarbon exposure, require further elucidation.
This review bridges these gaps by providing a comprehensive analysis of HFMs tailored for natural gas treatment. We begin by elucidating the configuration of asymmetric HFMs and the fundamental mechanism governing gas transport, followed by a critical evaluation of fabrication methods (e.g., co-extrusion, dip-coating, and interfacial polymerization) and post-modification strategies. Subsequently, we critically analyze advanced polymer materials, including cellulose acetate, polysulfone, polyimides, thermally rearranged polymers, and carbon molecular sieves, specifically engineered for acid gas removal and helium recovery. Challenges such as nanoporous transition layer collapse, physical aging, and plasticization are examined alongside emerging solutions like crosslinking and hybrid matrix designs. Finally, we propose a sustainability-driven roadmap for future research, emphasizing scalability, lifecycle assessment, and alignment with decarbonization goals (Fig. 2). By integrating material innovation with industrial practicality, this work advances HFM technology toward sustainable natural gas valorization.
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Fig. 2 Schematic diagram of emerging innovations in polymeric hollow fiber membranes for sustainable natural gas valorization. |
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Fig. 3 Schematic for gas transport mechanisms of (a) solution diffusion and (b) molecular sieving, surface diffusion, and Knudsen diffusion. Reproduced with permission from ref. 28 and Copyright 2024, Elsevier. |
An integrally asymmetric HFM achieves optimal separation performance through a carefully designed multilayer structure, as shown in Fig. 4a. The membrane consists of three fundamental layers: (1) a macroporous support layer (100–300 μm thick, 60–80% porosity) that provides mechanical stability while contributing less than 5% of total transport resistance; (2) a mesoporous transition layer (1–10 μm thick, 1–50 nm pore size) that serves two critical functions which gradually bridges the large pores of the support with the dense selective layer to prevent delamination and regulates solvent exchange kinetics during phase inversion for facilitating formation of defect-free selective layers; (3) an ultrathin dense selective skin layer (10–1000 nm thick) where molecular sieving primarily occurs. Some advanced configurations may include additional protective coatings (e.g., 10–1000 nm silicone rubber) to heal surface defects, particularly for membranes operating under harsh conditions.13 This hierarchical architecture enables more than 95% of the separation to occur at the selective layer while maintaining excellent mechanical properties.30
Gas permeation through the dense selective layer of polymeric HFMs normally follows the solution-diffusion mechanism. In this process, gas molecules first adsorb onto the membrane high-pressure upstream surface, then diffuse through the polymer matrix via thermally activated jumps between transient free-volume elements driven by a concentration gradient and finally desorb into the low-pressure downstream side as permeate. The steady-state gas flux (Qi) for component i is expressed as:31
Permeability (P) is the product of the solubility coefficient (S, cm3 (STP) cmHg−1) and diffusivity coefficient (D, cm2 s−1):
P = S × D |
For HFMs with ultrathin skin layers (L < 1 μm), permeance (P/L) is preferred and expressed in gas permeation units (GPU, 1 GPU = 1 × 10−6 cm3 (STP) cm−2 s−1 cmHg−1).32,33 Solubility favors gases with high condensability and strong polymer–penetrant interactions (e.g., quadrupole interactions in polar polymers).34 Diffusivity dominates in glassy polymers, where rigid chains create narrow free-volume distributions, preferentially passing smaller molecules.35 Enhancing diffusivity requires smaller penetrant size, larger/more frequent free-volume elements, and increased polymer chain mobility.34 The ideal selectivity of two gases is defined as follows:36
Polymeric HFMs exhibit an intrinsic permeability–selectivity trade-off.37 Modifying polymers to enlarge free-volume elements often disproportionately benefits larger molecules (e.g., CH4 over CO2), decreasing selectivity.38 This limitation arises from the inability to simultaneously increase free-volume size and narrow size distribution.35 The empirical upper-bound relationship quantifies this trade-off:35
Pi = kαijn |
The coefficient k incorporates solubility and polymer free-volume distribution:40
Molecular sieving in membrane-based gas separation operates through a well-defined activated diffusion mechanism that integrates two synergistic governing principles.41 First, steric exclusion effects arise from the precise size matching between gas molecules and membrane pore apertures, creating molecular-level filtration. Second, adsorption potential develops through specific interactions between gas molecules and pore walls, including van der Waals forces, electrostatic interactions, and surface chemistry effects.41 This dual-control mechanism establishes a distinct size-dependent transport behavior with three characteristic regimes. In the preferential transport regime, where gas molecules possess kinetic diameters substantially smaller than the pore dimensions, van der Waals interactions dominate, leading to enhanced surface adsorption and subsequent surface diffusion.42 The transition regime occurs when molecular dimensions approach the pore size, where steric repulsion creates substantial energy barriers that must be overcome by thermal activation.42 Finally, the exclusion regime emerges when molecular sizes exceed pore dimensions, resulting in near-complete rejection with selectivity factors exceeding 103.41 These fundamental transport principles remain universally applicable across molecular sieve materials, with particular relevance to polymer-derived CMS hollow fiber membranes. Through pyrolysis at controlled temperatures, these membranes develop precisely tuned ultramicroporous networks that exhibit exceptional molecular discrimination. The pyrolysis process carefully balances carbonization and activation to optimize pore size distribution while maintaining structural integrity, enabling targeted separation of gas pairs such as CO2/CH4 and He/CH4 with industrial-scale efficiency.
Surface diffusion achieves gas separation through two coupled processes: selective adsorption of components with higher affinity onto pore surfaces and surface migration via hopping between adsorption sites.43 This mechanism dominates CO2 separation in microporous membranes under low-temperature/high-pressure conditions, where enhanced adsorption selectivity drives performance.43 Membranes leveraging surface diffusion contain slightly larger micropores than molecular sieves. The surface diffusion coefficient (Ds) follows Gilliland's model:44
Ea = a × q |
The governing flow regime in porous membranes is determined by the Knudsen number (Kn = λ/r), where λ is the mean free path and r is the pore radius. Poiseuille flow dominates when Kn ≪ 1 (r > 10λ), described by:45
pavg = (pf + pp)/2 |
The solution-diffusion mechanism governs gas transport in dense polymeric layers, providing high selectivity while being fundamentally constrained by the permeability–selectivity trade-off. Molecular sieving (e.g., in carbon molecular sieves) enables precise size discrimination, although it requires sub-nanometer pore-size control (<0.7 nm) and exhibits vulnerability to aging-induced structural degradation. Surface diffusion enhances selectivity for condensable species (e.g., CO2), but experiences diffusivity–selectivity compromises due to strong sorption hysteresis. Knudsen and Poiseuille flows yield separation factors below 5 (α < 5), rendering them inadequate for molecular-level separation. Consequently, advanced membrane architectures integrate complementary mechanisms: ultrathin solution-diffusion skins enabling high-flux permeation, combined with molecular-sieving domains or surface-functionalized nanopores to achieve selectivity enhancement under aggressive industrial feed gas.
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Fig. 5 Schematic ternary phase diagram of polymer–solvent–nonsolvent systems. Reprinted with permission from ref. 23, Copyright 2022, the Royal Society of Chemistry. |
Li et al.57 developed defect-free 6FDA-mPDA0.9-TFMB0.1 copolyimide HFMs for He enrichment by optimizing polymer concentration and phase separation kinetics. At <25 wt% polymer concentrations, porous substructures achieved a He permeance of 72.1 GPU, while >30 wt% concentrations produced slightly thick skin layers (200–300 nm), reducing permeance by 35%. By tuning air gap height (5 cm) and coagulation bath composition (water/ethanol = 70:
30), the resultant HFMs exhibited a burst pressure of 10.5 MPa and He/CH4 selectivity of 178. Remarkably, these membranes retained structural integrity under extreme conditions (750 PSIA mixed-gas feeds, −40 °C), showing a He permeance of 46 GPU and He/CH4 selectivity of 429.58 Similarly, Kosuri et al.59 engineered a Torlon® HFM for high-pressure CO2 separation by suppressing macrovoids through an increased polymer concentration (28 wt%) and elevated dope temperatures (60 °C). The optimized membranes achieved a CO2/CH4 selectivity of 39.6 under 1100 PSIA feed pressure at 35 °C, demonstrating suitability for industrial high-pressure natural gas upgrading.
Polymer blending offers a cost-efficient approach to enhance hollow fiber membrane (HFM) performance by combining complementary properties of base polymers and additives. Yong et al.60 demonstrated this strategy by incorporating 5–15 wt% PIM-1 into Matrimid® dopes, achieving ultrathin selective layers (30–70 nm) (Fig. 6a). The blend HFMs exhibited CO2 permeance increases of 78% (153.4 GPU) and 146% (212.4 GPU) at 5 wt% and 10 wt% PIM-1 loadings, respectively, while maintaining CO2/CH4 selectivity >26. Further optimization with 15 wt% PIM-1, 95:
5 NMP
:
H2O bore fluid, and silicone rubber coating improved CO2 permeance to 243.2 GPU and CO2/CH4 selectivity to 34 by sealing surface defects (Fig. 6b and c). However, PIM-1's limited solubility constrained the blending ratio. To address this critical limitation, Jue et al.61 developed a triple-orifice spinneret for coextruding the PIM-1 dope with a protective sheath layer of 1-butanol/THF (Fig. 6d). This approach reduced volatile solvent THF evaporation relative to the polymer dope directly exposed to air, yielding defect-free asymmetric HFMs with a 3–6 μm skin layer. These HFMs initially achieved a CO2 permeance of 540 GPU and selectivity of 25, stabilizing at 360 GPU, and CO2/CH4 selectivity of 23 after two-month aging. In a complementary additive approach, Hu et al.62 incorporated 0–12 wt% polyethylene glycol (PEG) as a CO2-philic additive into dope solution for Matrimid® 5218 HF spinning. Increasing the PEG concentration accelerated phase separation during membrane formation, producing hollow fibers with higher porosity and thinner selective skins and elevating CO2 permeance from 21 GPU (neat Matrimid®) to 37 GPU (12 wt% PEG).
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Fig. 6 (a) Selective layer thickness versus PIM-1 content in PIM-1/Matrimid® blend HFMs. (b) Inner surface morphology of PIM-1/Matrimid® (15![]() ![]() |
Surface modification and crosslinking strategies significantly enhance separation performance of DLHFMs. Li et al.68 fabricated DLHFMs using coextrusion technology where both inner and outer layers consisted of polyethersulfone (PES) with different polymer concentrations of 23 wt% and 35 wt%, respectively. These HFMs subsequently underwent surface modification through a three-step chemical treatment: chloromethylation, sulfonation, and silver ion exchange. The incorporated silver ions selectively enhanced CO2 transport via quadrupole interactions, achieving an exceptional CO2/CH4 selectivity of 118 with a CO2 permeance of 25.1 GPU. Liu et al.69 chemically crosslinked the outer 6FDA-durene-mPDA layer of 6FDA-durene-mPDA/PES DLHFMs with p-xylylenediamine, forming rigid networks that boosted CO2/CH4 selectivity to 101 despite reduced CO2 permeance (28.3 GPU). Ma et al.70 engineered ester-cross-linkable Torlon®/6FDA-DAM:
DABA (3
:
2) DLHFMs, where optimized quench bath temperature (50 °C), sheath/core flow ratios (1
:
3), and an air gap residence time of 0.04 s yielded defect-free fibers (Fig. 7a–c). Crosslinked hollow fibers demonstrated stable CO2 permeance (40 GPU) and CO2/CH4 selectivity (39) under 100 PSIA feeds (CO2/CH4, 50/50 v/v) containing 1000 ppm toluene, highlighting industrial viability.
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Fig. 7 SEM images of PDMC/Torlon® composite hollow fibers under varied processing conditions: (a) quench bath temperature, (b) sheath/core flow ratio, and (c) air gap residence time. Reprinted with permission from ref. 69, Copyright 2013, American Chemical Society. |
Filler incorporation into DLHFMs enhances performance through complementary mechanisms: molecular sieving via filler nanopores improved gas affinity through functional groups (e.g., amine–CO2 interactions) and tuned polymer chain packing that suppresses non-selective pathways. However, filler loading exceeding 5 wt% can create interfacial defects, necessitating surface modifications for uniform dispersion.71 For instance, Khan et al.72 incorporated polyethyleneimine (PEI)-functionalized bimetallic MOF (PEI@HKUST-1(Cu, Mg)) exclusively into the outer layer dope with polysulfone (PSf) utilized as a polymer in both inner and outer dopes for DLHFM spinning. The amine groups from PEI-MOFs enhanced CO2 affinity while open metal sites combined with high porosity provided additional CO2 binding sites and transport channels. The synergistic design promoted rapid CO2 permeation, yielding 75% higher CO2 permeance (28 GPU) and 85% improved CO2/CH4 selectivity (51) compared to unmodified PSf HFMs.
Material innovations in TFC HFMs now strategically address two critical industry barriers: reducing the environmental footprint of membrane manufacturing and overcoming permeance-selectivity trade-offs. Theodorakopoulos et al.84 demonstrated these by engineering P84 co-polyimide/Pebax-1657 composite HFMs. They replaced toxic N-methyl-2-pyrrolidone (NMP) with greener γ-butyrolactone (GBL) in support HF fabrication, reducing solvent toxicity while maintaining spinnability. Simultaneously, graphene nanoplatelets (GNPs) incorporated into both support and selective layers enhanced separation performance. The resulting (GGreen/Pebax + GNPs)/P84 composite HFMs achieved a CO2/CH4 selectivity of 82 and He/N2 selectivity of 20, with CO2 and He permeances of 3GPU and 2.7GPU respectively under binary gas testing (1.3 bar, 25 °C). Complementing solvent and nanofiller innovations, novel polymer architecture could also enhance TFC HFM separation performance. Gutiérrez-Hernández et al.85 coated PIM-1 with 11% side-chain substitution as a selective layer on PP-HF impregnated PDMS. The B-PIM-1/PDMS/PP HFMs achieved a CO2 permeance of 650 GPU and CO2/CH4 selectivity of 14.
Interfacial polymerization (IP) enables ultra-thin (<500 nm), chemically tailored selective layers through controlled monomer diffusion.59,86 Jo et al.87 polymerized trimesoyl chloride (TMC) and 1,3-cyclohexanebis-methylamine (CHMA) on PSf HFMs, where the CHMA concentration governed crosslinking density. At 1.0% CHMA/0.5% TMC, the membrane achieved a CO2 permeance of 25 GPU and CO2/CH4 selectivity of 28. Choi et al.88 fabricated a He-selective membrane via m-phenylenediamine (m-PDA)/TMC IP on PAN supports, followed by PTMSP gutter layer deposition. The resultant membrane exhibited a He permeance of 46.2 GPU and He/CH4 selectivity of 11.9. Future directions integrate machine learning (ML) to optimize curing kinetics and monomer ratios. Neural networks trained on experimental datasets could predict optimal dip-coating parameters for target selectivity.89,90 ML models can bridge molecular design with process engineering, accelerating the development of TFC HFMs for extreme industrial conditions (e.g., high-pressure natural gas feeds).
Emerging synergies bridge different membrane architectural paradigms through cross-technology integration. Electrospinning also offers significant potential for fabricating HFMs with tunable nanofibrous architectures and enhanced surface functionalities, enabling novel transport pathways for gas separation. Recent advances in coaxial spinning techniques demonstrate promising routes toward scalable hollow fiber production, complementing conventional fabrication methods.91 Hybrid fabrication enables co-extruded dual-layer supports to host ultrathin selective skins via interfacial polymerization, eliminating delamination while enhancing molecular sieving. Concurrently, machine learning-guided architecture selection dynamically matches membrane types to feed conditions: thin-film composite HFMs for CO2-dominant streams and asymmetric HFMs for helium enrichment scenarios. These fabrication innovations directly enable the advanced polymers discussed in Section 4, where hierarchical structures (e.g., DLHFMs) maximize the performance of emerging materials like TR polymers and CMSs. Collectively, these pathways advance hollow fiber membranes toward adaptive, economically viable gas separation systems that maintain industrial robustness while addressing evolving sustainability imperatives.
Incorporating nanoparticles into cellulose acetate matrices produces mixed matrix hollow fiber membranes (MMHFMs) with enhanced CO2/CH4 separation performance. Strategic manipulation of spinning parameters minimizes interfacial voids in these MMHFMs, further improving their gas separation efficiency. For instance, Mubashir et al.105 fabricated NH2-MIL-53(Al)/CA MMHFMs containing 15 wt% MOF by optimizing take-up speed and air-gap distance. They observed that increasing take-up speed improved ideal selectivity by 40% due to shear-induced molecular alignment, while air-gap variations produced distinct “V” and “Λ” patterns. Under optimal conditions (12.2 m min−1 take-up speed, 5.0 cm air gap), the membrane achieved a CO2/CH4 ideal selectivity of 16.0 with a CO2 permeance of 6.7 GPU at 25 °C and 3 bar feed pressure. In a follow-up study using design of experiments, they demonstrate that increasing permeance testing temperature from 30 °C to 50 °C reduced the CO2/CH4 separation factor of MMHFMs due to accelerated polymer chain relaxation, whereas elevating feed pressure from 3 to 15 bar and CO2 feed composition from 15 to 42.5 vol% enhanced separation performance. Feed pressure emerged as the dominant factor, reducing CH4 permeance by 32% while marginally lowering CO2 permeance, yielding a maximum CO2/CH4 separation factor of 14.4 at 15 bar, 30 °C, and 15 vol% CO2.106 Similarly, Sunder et al.102 fabricated NH2-MIL-125(Ti)/CTA MMHFMs via a 1 cm air-gap process, where dual PDMS coating reduced surface defects but introduced permeance loss (35%) due to thicker coating. The modified membranes exhibited a CO2/CH4 ideal selectivity of 6.87 and CO2 permeance of 26.46 GPU.
The gas separation performance of PS HFMs is highly sensitive to spinning parameters and post-treatment conditions. Lower dope solution temperatures (Td) and coagulation bath temperatures (Tc) slow solvent-non-solvent exchange rates, promoting denser skin layers with reduced macrovoid formation.66 Liu et al.113 demonstrated that reducing Td from 35 °C to 3 °C and Tc from 25 °C to 7 °C minimized finger-like macro-voids, yielding membranes with a CO2 permeance of 53.5 GPU. However, unmodified PS HFMs exhibit low selectivity (CO2/CH4 ≈ 1.24), necessitating post-coating treatments. Silicone rubber (PDMS) coating (3 wt%) on optimized PS HFMs enhanced selectivity to 39.4 while maintaining CO2 permeance at 31.9 GPU. Roslan et al.114 further showed that increasing the polysulfone polymer concentration (from 15 to 35 wt%) in spin dopes reduced gas permeance by 45% but improved CO2/CH4 selectivity by 150% due to denser membrane structures. Multilayer coating (3 wt% PDMS + 3 wt% Pebax® 1657) on PS HFMs achieved optimal performance (CO2 permeance of 21.12 GPU and a CO2/CH4 selectivity of 34.28), outperforming single-layer PDMS by 87% in permeance.
PS serves as an ideal matrix for mixed-matrix and thin-film composite HFMs. Khan et al.115 incorporated 0.5 wt% ZIF-8 into PSf HFMs, leveraging molecular sieving and CO2-philic open metal sites to achieve a CO2 permeance of 47.75 GPU and CO2/CH4 selectivity of 25.7. Sasikumar et al.116 engineered amine-modified SiO2 and ZIF-8 MM-HFMs with Pebax®-1657 coating, where amine groups enhanced CO2 affinity, yielding a CO2 permeance of 41.15 GPU and CO2/CH4 selectivity of 22.25. Sharif et al.117 developed PES-based nanocomposite HFMs with PVP/PDMS/SiO2 skin layers, achieving a CO2/CH4 selectivity of 45, CO2 permeance of 102 GPU, and tensile strength of 58 MPa, a 40% improvement over pristine PS.
Soluble homopolyimides, such as 6FDA/DAM and Matrimid®, are commonly used due to their ability to form workable dopes.55 The rigid backbone architecture, characterized by kinked structures (e.g., the phenylindane moiety in Matrimid®) and bulky substituents (e.g., hexafluoroisopropylidene groups in 6FDA), generates a tunable free-volume fraction (15–25%) that directly governs gas transport.92,120,121 For instance, Matrimid® 5218 (BTDA-DAPI) achieves a CO2 permeance of 17.7 GPU and CO2/CH4 selectivity of 24.3 at 2 bar, surpassing polyethersulfone HFMs by 50% in CO2/CH4 selectivity under comparable conditions.121,122 Cao et al.123 developed 6FDA-2,6-DAT HFMs via NMP-based spinning, showing an initial CO2 permeance of 300 GPU. Physical aging reduced permeance to 76 GPU over 185 days due to free-volume collapse, but mixed-gas tests (CO2/CH4 = 50/50) showed stabilized permeance at 59 GPU with a CO2/CH4 selectivity of 40, highlighting industrial viability. Gutiérrez-Hernández et al.124 fabricated 6FDA-TMPD TFC HFMs using polypropylene hollow fibers as porous supports and highly permeable PDMS as a blocking layer to mitigate the intrusion of 6FDA-TMPD. The membrane achieved a CO2 permeance of 54.5 GPU and CO2/CH4 selectivity of 13.9 in CO2/CH4 (50:
50 vol%) gas mixtures, demonstrating stable performance up to 30 bar.
Spinning parameter optimization (e.g., dope solvent selection and polyimide concentration) critically impacts the polyimide HFM microstructure and gas separation performance. Hachisuka et al.125 demonstrated that diethylene glycol dimethyl ether (DGDE) as the good solvent produces defect-free 6FDA-BAAF HFMs with ultrathin selective layer, achieving a CO2 permeance of 550 GPU and CO2/CH4 selectivity of 27. In contrast, NMP generates finger-like macrovoids due to rapid phase separation, reducing selectivity by 35%. Qin et al.126 optimized shear rates during 6FDA-durene/mPDA fiber spinning, yielding defect-free structures with a CO2 permeance of 53.3 GPU and CO2/CH4 selectivity of 42.9 after post-PDMS coating, outperforming dense films by 29% in selectivity. Besides, lower molecular weight PIs require adjusting dope formations to maintain spinnability. Xu et al.127 increased the polymer concentration and reduced ethanol/LiNO3 additives to suppress phase separation, achieving defect-free HFMs with a CO2/CH4 selectivity of 22.7 (29% higher than dense films). This highlights the inverse relationship between molecular weight and required polymer concentration for optimal fiber formation.
Kim et al.134 minimized substructure collapse by first conducting thermal imidization at 300 °C on hydroxyl poly(amic acid) precursors, stabilizing the fiber morphology, before thermal rearrangement at 450 °C for 60 min. The resulting TR-PBO HFMs achieved a CO2 permeance of 1938 GPU and CO2/CH4 selectivity of 14 with a 2 μm selective layer, retaining 60% higher permeance than conventional TR-HFMs. To enhance selectivity while preserving permeance, Fan et al.135 designed phenolphthalein-modified 6FDA-DAP:
DAM (1
:
2) precursors. Sub-Tg crosslinking (via lactone ring opening) locked the polymer network, preventing pore coalescence during pyrolysis. Subsequent thermal rearrangement at 400 °C generated slit-like ultramicropores, yielding a CO2 permeance of 436.22 GPU and CO2/CH4 selectivity of 30.63, a 120% selectivity jump over non-crosslinked TR analogs without permeance sacrifice. These collective advances demonstrate that TR polymer performance hinges critically on preserving pore architecture integrity during rearrangement, a challenge now addressable via molecular-scale precursor control.
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Fig. 9 (a) Structural evolution of carbon/carbon mixed-matrix membranes with pyrolysis temperature (750–900 °C). (b) Pore-size distribution, (c) WAXRD, and (d) Raman spectra of CCMS membranes. Reprinted with permission from ref. 141, Copyright 2017, John Wiley and Sons. |
Precursor-derived chemistry and post-synthesis functionalization fine-tune microporosity and gas separation performance of HFMs. Kamath et al.144 retained 18.5% fluorine (–CF3 groups) in 6FDA-DETDA:
DABE CMS HFMs via a two-stage pyrolysis protocol: crosslinking at 370 °C followed by carbonization at 500 °C under argon, which expanded free volume while narrowing pore size distribution. The resultant membranes exhibited a CO2 permeance of 552 ± 28 GPU, with a CO2/CH4 selectivity of 37.0 ± 4. Wu et al.145 fluorinated partially carbonized 6FDA0.9-ODPA0.1-mPDA HFMs, polarizing the pore wall through C–F bonds. This strengthened H2 interaction and reduced effective pore size, boosting He/H2 selectivity from 3.1 to 9.7, while maintaining He permeance >100 GPU (α(He/CH4) = 2193).
CMS membranes demonstrate unparalleled robustness under extreme conditions. Zhang et al.146 reported 6FDA/BPDA (1:
1)-DAM CMS HFMs maintaining CO2/CH4 selectivity ∼60 under supercritical natural gas feeds (1800 PSIA, 50% CO2, and 500 ppm C7 hydrocarbons) across 200 h and −50 to 100 °C thermal cycles. Lei et al.147 developed cellulose-derived CMS HFMs via ionic liquid spinning, where the precursor dried at 140 °C reduced average pore size from 6 to 4.9 Å. Despite a 22% decrease in CO2 diffusivity, the narrowed pores amplified size exclusion, achieving a CO2/CH4 ideal selectivity of 917 (2 bar) and mixed-gas separation factor of 131 (50 bar, 60 °C).
Composite CMS membranes represent the future of membrane technology by enabling tunable separation properties and substantial cost reductions. By integrating economically engineered supports (e.g., P84) with high-performance polymers, precursor hollow fibers with dense skin layers achieve a 25-fold material cost reduction compared to monolithic or ceramic supports. Cao et al.148 demonstrated this through multi-layer asymmetric CMS HFMs fabricated by coating P84 supports with 6FDA:
BPDA-DAM polyimide followed by controlled pyrolysis. Optimizing the polymer solution concentration, coating humidity, and pyrolysis temperature yielded CMS-675 membranes with a CO2/CH4 selectivity of 58.8 and CO2 permeance of 310 GPU under CO2/CH4 (50
:
50) mixed-gas feed at 35 °C. Building on this, Cao et al.149 developed co-extruded composite CMS membranes using 6FDA-DAM (sheath) and Matrimid® (core), eliminating post-carbonization defect repairs. Pyrolyzed at 675 °C, these membranes achieved an enhanced CO2/CH4 selectivity of 64.3 and CO2 permeance of 232 GPU, outperforming their single-layer counterparts by 11% in selectivity while maintaining industrial-grade permeance.
These inherent limitations now drive three transformative innovations: computational materials design integrating multiscale modeling approaches to optimize the permselectivity–stability–manufacturability trade-offs; bio-hybrid systems integrating enzymatic recycling pathways for closed-loop polymer recovery; and stimuli-responsive membranes featuring dynamically tunable free-volume architectures. This convergence positions next-generation HFMs as adaptive platforms for precision gas separation, where programmable molecular architectures enable autonomous optimization of gas transport across fluctuating natural gas compositions.
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Fig. 11 Schematic illustrating plasticization mechanisms for glassy polymers, demonstrating four different permeability trends based on varying penetrant behaviors. (a) Non-plasticizing with low sorption, (b) non-plasticizing with moderate sorption, (c) highly sorbing, where the plasticization pressure marks the point at which increased diffusion overcomes the reduction in sorption, and (d) highly plasticizing with very high sorption. Reprinted with permission from ref. 154, Copyright 2024, Royal Society of Chemistry. |
Chemical crosslinking has emerged as a principal strategy for enhancing plasticization resistance (Fig. 14). Five main approaches have been systematically investigated: thermally induced decarboxylation crosslinking, esterification crosslinking, metal ion coordination, amine-mediated nucleophilic crosslinking, and halogen induced radical crosslinking.166,167 Decarboxylation crosslinking forms stable C–C covalent bonds via decarboxylation-induced free radical reactions that restrict polymer chain mobility while maintaining structural integrity under high feed pressures. Li et al.168 demonstrated this using 6FDA-mPDA0.65-DABA0.3-TFMB0.05 copolyimide HFMs (Fig. 12a–f). Under ternary He/CO2/CH4 (0.3/49.5/49.2, v/v/v) feed at 100–600 PSIA, crosslinked HFMs showed minimal permeance variation: He permeance decreased from 13.4 to 9.5 GPU, CH4 permeance remained stable (0.07–0.08 GPU), and He/CH4 selectivity declined from 186 to 119. When challenged with hydrocarbon contaminants (propane/heptane), selectivity reductions were limited to 8.7/11.4% at 600 PSIA (Fig. 12g and h), respectively, attributed to the crosslinked polymer structure for preventing polymer chain swelling and alleviating competitive adsorption. Wang et al.169 developed dual thermally crosslinked asymmetric HFMs using 4,4′-diamino-2,2′-biphenyldicarboxylic acid (DCB)-containing copolyimides. The dual crosslinking mechanism combined stable C–C bonds with bulky CF3 groups from TFMB diamine restricting imide bond rotation, inhibiting transition layer collapse and achieving a skin layer thickness of 1.2 μm. The optimized PI-TFMB-HF@400 membrane demonstrated a He permeance of 25 GPU and He/CH4 selectivity of 269. Under aggressive ternary feed (He/CO2/CH4, 0.3/49.7/50, v/v/v), CO2/CH4 selectivity decreased by only 24%, while mixed-gas [He/(CO2 + CH4)] selectivity increased by 80%, showcasing exceptional pressure stability.
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Fig. 12 Structural and performance characterization of crosslinked HFMs. (a–f) SEM images showing preserved substrate porosity and dense skin layer post-crosslinking. Effect of feed pressure on the mixed-gas He and CH4 permeance and He/CH4 selectivity of pristine and crosslinked HFMs in ternary (g) He/C3H8/CH4 (0.3/3.0/96.7, v/v/v) and (h) He/n-C7H16/CH4 (0.3/0.4/99.3, v/v/v). Reprinted with permission from ref. 166, Copyright 2023, Elsevier. |
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Fig. 13 (a–f) SEM images illustrating CF3-modified fiber morphology; (g–i) gas separation performance under hydrocarbon-contaminated high-pressure feeds. Reprinted with permission from ref. 171, Copyright 2016, John Wiley and Sons. |
Esterification crosslinking creates robust interchain bridges through transesterification, effectively restricting chain mobility and free volume swelling under high pressure conditions. This significantly enhances plasticization resistance while maintaining size-sieving capabilities. Wallace et al.170 engineered ester-crosslinked polyimide (6FDA-DAM:
DABA 3
:
2) HFMs, demonstrating that crosslinking temperature more effectively improved plasticization resistance than prolonged treatment duration. The optimized HFMs maintained a stable CO2/CH4 selectivity of 28 at 1000 PSIA over 100 hours, with full performance recovery at 200 PSIA, indicating reversible swelling without permanent plasticization. Babu et al.171 extended this approach to TEGMC (Triethylene Glycol Monoesterified Crosslinkable) HFMs, achieving a H2S/CH4 selectivity of 22 under extreme conditions (20% H2S, 500 PSIA).
To address the risk of transition layer collapse caused by reduced gas permeance in HFMs with ultrathin skin layers, particularly those weakened by thermal crosslinking, Li et al.172 developed polyimide–cerium (PI–Ce) complex HFMs with a 300 nm ultrathin skin layer made from 6FDA-mPDA0.65-DABA0.3-TFMB0.05 copolyimide via metal ion coordination. This approach enhances plasticization resistance, particularly for helium separation. In a ternary gas mixture (He/CO2/CH4, 0.3/49.4/50.3, v/v/v) at 35 °C and feed pressure ranging from 100 to 600 PSIA, the He permeance of the PI–Ce HFM decreased marginally from 29 to 27 GPU, while CH4 permeance rose slightly from 0.122 to 0.127 GPU. This led to a 10.8% decline in He/CH4 selectivity (239 to 213) as pressure increased from 100 to 400 PSIA. When heavy hydrocarbons (n-C7H16) were introduced, He permeance dropped by 26.3% (19.8 to 16.5 GPU) and He/CH4 selectivity decreased by 14.6% (229 to 194) between 100 and 300 PSIA. The robust plasticization resistance of PI–Ce HFMs highlights their potential for helium recovery in harsh natural gas environments. Liu et al.173 incorporated CF3 groups into polyimide backbones to enhance chain rigidity, producing TFM-PI-CF3 fibers without skin layer thickening after pyrolysis (200 °C, 2 h; Fig. 13a–f). In contrast, the cross-linked TFM-PI-0 fiber showed a significantly reduced CO2 permeance of 46 GPU, 2.5 times lower than its pristine non-cross-linked fiber's permeance of ∼115 GPU (Fig. 13g), which is consistent with its increased skin thickness (Fig. 13c and d). Under 55 bar feed gas pressure with 50/50 CO2/CH4, these CF3-modified membranes maintained stable CO2 permeance (∼90 GPU) and selectivity (∼43). Even when exposed to 250 ppm hydrocarbon contaminants (heptane/toluene), they retained selectivity > 30 at 55 bar, demonstrating exceptional plasticization resistance (Fig. 13h–i).
For amine-mediated nucleophilic crosslinking, amine-rich agents open imide rings via carbonyl addition, forming amide linkages. Cao et al.174 demonstrated that immersing 6FDA-2,6-DAT HFMs in p-xylenediamine (PXDA)/methanol solution triggers nucleophilic attack on carbonyl groups, generating bis-amide crosslinks. This raised plasticization pressure from 100 PSIA (unmodified and lightly cross-linked) to >200 PSIA, while stabilizing CO2 permeance at ∼20 GPU. Similarly, Ren et al.175 demonstrated that PXDA treatment of 6FDA-ODA/NDA HFMs with immersion times exceeding 1.5 min could achieve plasticization resistance up to 550 PSIA (37.4 atm), as confirmed by FTIR analysis showing conversion of imides to amides. This performance marked a substantial enhancement compared to non-crosslinked membranes that plasticized at 75 PSIA.
Thermal annealing, a cost-effective post-treatment, improves membrane stability by increasing packing density, reducing free volume, and elevating glass transition temperature (Tg).176,177 Chung et al.178 thermally treated 6FDA-2,6 DAT HFMs at 250 °C for 5 min, minimizing skin layer thickening while suppressing CO2-induced plasticization. Aged for 60 days, these membranes withstood feed pressures exceeding 500 PSIA, maintaining a stable CO2 permeance of ∼34 GPU. This enhanced stability was attributed to tighter chain packing and a denser skin layer. Dong et al.162 corroborated this by annealing Matrimid® HFMs at 250 °C for 30 min, which retained CO2 permeance near 20 GPU and a selectivity of ∼28 under CO2/CH4 (20/80) mixed-gas at 10–35 bar, showing no plasticization.
Hydrogen bonding has emerged as another strategy to restrict chain mobility and counteract plasticization.71 Kosuri et al.59 fabricated Torlon® HFMs with inter- and intra-chain hydrogen bonds, achieving a CO2/CH4 selectivity of 44 (85% of its dense film value of 52). Compared to Ultem® (400 PSIA), Matrimid® (174 PSIA), and 6FDA-DAM:
DABA 2
:
1 (PSIA), the Torlon® HFM exhibited superior stability, maintaining constant CO2 permeance up to 1220 PSIA and a selectivity of 39.6 under supercritical 90% CO2/10% CH4 feed. While plasticization typically degrades membrane performance, Liu et al.165 revealed its benefits for CTA HFMs in aggressive natural gas feeds containing H2S (20 mol%), light hydrocarbons (C2H6: 3 mol%; C3H8: 3 mol%), and toluene (300 ppm) at 31.3 bar and 35 °C. Plasticization-induced chain mobility elevated CO2 and H2S diffusivity (>110 GPU) while maintaining size-sieving selectivity (22–28 over CH4). This counterintuitive behavior stems from enhanced condensable gas transport through transient free-volume elements without compromising rigid matrix integrity.
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Fig. 15 (a) Specific volume vs. temperature for an amorphous polymer. The extrapolated equilibrium specific volume (dashed line) above Tg diverges from the non-equilibrium state (solid line) below Tg, where excess free volume (green area) becomes kinetically trapped. Physical aging (represented by the dark blue arrow) indicates the slow relaxation toward equilibrium. Reprinted with permission from ref. 180, Copyright 2020, Elsevier. (b) Schematic of free volume diffusion and lattice contraction during aging. Reprinted with permission from ref. 155, copyright 2009, Elsevier. |
For instance, Ma et al.193 mitigated physical aging of PDMC hollow fiber membranes through CO2 conditioning and continuous CO2/CH4 feed tests. It was found that periodical 15 PSIG CO2 conditioning reduced the CO2 permeance loss of cross-linked HFMs by ∼50%, compared to unconditioned ones during a 2300 h aging test. However, the HFM exposed to 200 PSIA 50/50 CO2/CH4 continuous feed showed a CO2 permeance loss of only up to 3%, compared to the 25% loss for samples without continuous mixed gas feed during a 400 h aging test. These results suggest that physical aging may be essentially quenched as long as the membrane remains in contact with a high CO2 partial pressure feed typical of actual aggressive feeds for which the membrane would be used. Beyond conditioning strategies, nano-fillers mitigate physical aging by restricting polymer chain relaxation through geometric confinement effects. When incorporated into high-free-volume polymers, the porous/lamellar fillers create tortuous diffusion paths that delay free-volume collapse.194,195 Sutrisna et al.196 stabilized poly(trimethylsilyl)-1-propyne] (PTMSP) gutter layers using ZIF-8/Pebax-1657 composite coatings. Hydrogen bonding between ZIF-8 ligands and polyamide chains enhanced chain rigidity, while interpenetration of Pebax into PTMSP reduced segmental mobility. The composite membrane maintained stable CO2 permeance (220 ∼ 350 GPU) over 15 days at 2 bar, contrasting with bare PTMSP layers, which lost >22% permeance.
Notably, physical aging also impacts CMS HFMs, where the microporous structure undergoes rearrangement over time after pyrolysis.197 This structural evolution reduces pore accessibility and constricts critical diffusion pathways, leading to substantial declines in gas permeance. Hybridization and surface engineering effectively address this. Shin et al.198 incorporated ladder-structured poly(phenyl-co-pyridylethyl)silsesquioxane (LPPyr64) into a 6FDA-DAM:
DABA (3
:
2) polyimide before pyrolysis. The rigid siloxane framework preserved the porous microstructure during pyrolysis, yielding CMS fibers with a CO2 permeance of 956 GPU, a 546% increase over precursor fibers (148 GPU). However, untreated CMS PI-LPSQ20 fibers exhibited rapid aging, with CO2 permeance dropping from 956 to 406.9 GPU within 24 hours post-pyrolysis. PDMS-coated CMS PI-LPSQ20 membranes retained a CO2 permeance of 354 GPU and CO2/CH4 selectivity of 56 after 72 days, with no plasticization observed at 13.2 bar CO2 partial pressure. Kamath et al.199 eliminated aging in 6FDA-DETDA
:
DABA derived CMSs by esterifying DABA to DABE and pretreating it with 10% vinyltrimethoxysilane (VTMS). This suppressed residual stress-driven rearrangements, stabilizing CO2 permeance > 1000 GPU for 7 days with a CO2/CH4 selectivity of 25 and 780 GPU (selectivity: 48) after 72 days under 21 bar (50 mol% CO2/CH4). Synergistic approaches-combining rigid frameworks (LPSQs and MOFs) with polymer backbone engineering and protective coatings (e.g., PDMS)-demonstrate exceptional aging resistance in both polymeric and CMS HFMs.
For CMS HFMs, sub-1 μm skin layers are highly susceptible to transition layer collapse. During pyrolysis, precursor hollow fiber substrates often collapse, resulting in CMS membranes with thick skin layers (15–50 μm).201 Pre-treating precursors with silane (e.g., vinyltrimethoxysilane, VTMS) prior to pyrolysis mitigates substrate collapse, reducing skin thickness to ∼3–6 μm, though still thicker than precursor fibers (Fig. 16b and c).200 To achieve ultrathin skins (<1 μm), Zhang et al.202 developed dual-layer HFMs combined with silane treatment and in situ hybridization (Fig. 16a). After hybridization, dual layer ULT CMS-MA1 membranes with a 1 μm skin layer achieved a CO2/CH4 separation factor of 7.9 ∼ 36.7 and CO2 permeance of ∼1177 GPU, demonstrating effective defect repair. Further optimization reduced the skin layer to 0.5 μm (Fig. 16d and e), yielding a CO2 permeance of 1452 GPU and CO2/CH4 separation factor of ∼18 under an equimolar CO2/CH4 feed at 100 PSIA and 35 °C. Emerging strategies such as hierarchical pore structuring, selective silica deposition via silane optimization, and hybrid precursor designs are critical for scaling these innovations to industrial applications.
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Fig. 16 (a) Schematic of CMS HFM defect repair substrate collapse suppression via silane treatment and in situ hybridization. (b and c) SEM images of monolithic Matrimid® precursor HF and derived CMS HF with silane treatment (6 μm skin). (d) and (e) SEM images of dual-layer Matrimid®/Matrimid® precursor HF with silane and hybridization treatments, yielding ultrathin (1 μm skin) CMS HF (ULT CMS MA1). Reprinted with permission from ref. 200, Copyright 2019, John Wiley and Sons. |
Integrated stabilization strategies empower polymeric hollow fiber membranes to successfully overcome the aforementioned tripartite challenges. Specifically, covalent crosslinking reinforces polymeric networks through three-dimensional bonding, effectively immobilizing polymer chains to suppress swelling-induced plasticization under CO2 partial pressures exceeding 500 PSIA. Concurrently, nanoconfined architectures incorporating porous fillers mitigate physical aging by sterically stabilizing interconnected free-volume elements, reducing permeance decay through restricted chain segment relaxation. Complementarily, transition-layer integrity is preserved via optimized solvent-exchange sequences and silane-based interfacial modifications, enabling ultrathin selective skins (<1 μm) with sustained helium permeance. Strategic implementation necessitates balancing chemical resilience against aggressive feed stream, scalable manufacturing viability, and validated operational stability.
The data supporting this review article are derived from published studies cited throughout the manuscript. All referenced datasets and experimental results are available in the original publications, as indicated in the reference list.
Supplementary information is available. See DOI: https://doi.org/10.1039/d5ta04814a.
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