Membrane-based Gas Separation Research Articles

Design of Enzyme Loaded W/O Emulsions by Direct Membrane Emulsification for CO2 Capture.

Membrane-based gas separation is a promising unit operation in a low-carbon economy due to its simplicity, ease of operation, reduced energy consumption and portability. A methodology is proposed to immobilise enzymes in stable water-in-oil (W/O) emulsions produced by direct membrane emulsification systems and thereafter impregnated them in the pores of a membrane producing emulsion-based supported liquid membranes. The selected case-study was for biogas (CO2 and CH4) purification. Upon initial CO2 sorption studies, corn oil was chosen as a low-cost and non-toxic bulk phase (oil phase). The emulsions were prepared with Nadir® UP150 P flat-sheet polymeric membranes. The optimised emulsions consisted of 2% Tween 80 (w/w) in corn oil as the continuous phase and 0.5 g.L−1 carbonic anhydrase enzyme with 5% PEG 300 (w/w) in aqueous solution as the dispersed phase. These emulsions were impregnated onto a porous hydrophobic PVDF membrane to prepare a supported liquid membrane for gas separation. Lastly, gas permeability studies indicated that the permeability of CO2 increased by ~15% and that of CH4 decreased by ~60% when compared to the membrane without carbonic anhydrase. Thus, a proof-of-concept for enhancement of CO2 capture using emulsion-based supported liquid membrane was established.

Cellulose acetate based sustainable nanostructured membranes for environmental remediation

Membrane-based gas separation has a great potential for reducing environmentally hazardous carbon dioxide (CO2) gas. The polymeric membranes developed for CO2 capturing have some limitations in their selectivity and permeability. There is a need to overcome these issues and developed such membranes having high-performance CO2 capture with cost-effectiveness. The present study aimed to synthesize mixed matrix membranes (MMMs) having improved properties CO2 adsorption performance and stability than that of pure polymer. Further, the effect on CO2 adsorption by increasing the filler concentration in MMMs was investigated. The MMMs were synthesized by incorporating (1–5 wt%) Cu-MOF-GO composites as filler into cellulose-acetate (CA) polymer matrix by adopting the solution casting method. The performance of MMMs was studied by changing the Cu-MOF-GO composite concentration (1–5 wt%) in the polymer matrix at 45 °C up to 15 bar. Morphological analysis by using SEM confirms that by increasing the concentration of Cu-MOF-GO more than 3% will result in their agglomeration in MMM. The successful incorporation of MOF within the polymer matrix of MMMs was confirmed through the presence of functional groups using FTIR and Raman spectroscopy. XRD analysis revealed that pure CA changes its semi-crystalline behaviour into crystalline by the addition of Cu-MOF-GO. The maximum tensile stress and strain rate of MMMs was 45.1 N/mm2 and 12.8%. In addition, with an increase in (4–5 wt%) Cu-MOF-GO concentration the hydrophilicity of MMMs decreases. The maximum uptake rate of CO2 was 1.79 mmol/g and 7.98 wt% at 15 bar. The adsorption results conclude that Cu-MOF-GO composite and CA-based MMM can be effective for CO2 capture.

Comparison of analytical film theory and a numerical model for predicting concentration polarisation in membrane gas separation

Accurate prediction of the concentration polarisation (CP) effect is very important in the design of an efficient membrane-based gas separation process. This study analyses the reliability of analytical film theory (FT) for evaluating the performance of gas separation membranes in terms of CP and flux. The analytical model is compared against a more rigorous numerical model developed by using Computational Fluid Dynamics (CFD) for various operating variables. The results show that the FT prediction is less accurate at high CP conditions when gas permeation through the membrane increases, due to higher permeance selectivity and pressure ratio. Hence, the results suggest that FT is not recommended for membranes with high permeance or high-pressure conditions. Given that the typical range of feed composition and temperature has little impact on fluid properties (i.e., gas diffusion coefficient, densities, and viscosities), the resulting CP does not vary much and hence both FT and CFD models predict a similar CP. The analysis also suggests that the FT model is more accurate in predicting CP in the region closer to the membrane entrance. Overall, the analytical film theory serves as a reliable approximation in membrane gas applications under low CP at high crossflow and low flux conditions.

Side-Chain Length and Dispersity in ROMP Polymers with Pore-Generating Side Chains for Gas Separations.

Bottlebrush polymers with flexible backbones and rigid side chains have shown ultrahigh CO2 permeability and plasticization resistance for membrane-based gas separations. To date, this class of polymers has only been studied with polydisperse side chains. Herein, we report gas transport properties of a methoxy (OMe) functionalized polymer synthesized via ring-opening metathesis polymerization (ROMP) with uniform side-chain lengths ranging from n = 2 to 5 repeat units to elucidate the role of both side-chain length and dispersity on gas transport properties and plasticization resistance. As side-chain length increased, both Brunauer–Emmett–Teller (BET) surface area and gas permeability increased with minimal losses in gas selectivity. Increased plasticization resistance was also observed with increasing side-chain length, which can be attributed to increased interchain rigidity from longer side chains. Controlling the side-chain length provides an effective strategy to rationally control and optimize the performance of ROMP polymers for CO2-based gas separations.

Challenges, Opportunities and Future Directions of Membrane Technology for Natural Gas Purification: A Critical Review.

Natural gas is an important and fast-growing energy resource in the world and its purification is important in order to reduce environmental hazards and to meet the required quality standards set down by notable pipeline transmission, as well as distribution companies. Therefore, membrane technology has received great attention as it is considered an attractive option for the purification of natural gas in order to remove impurities such as carbon dioxide (CO2) and hydrogen sulphide (H2S) to meet the usage and transportation requirements. It is also recognized as an appealing alternative to other natural gas purification technologies such as adsorption and cryogenic processes due to its low cost, low energy requirement, easy membrane fabrication process and less requirement for supervision. During the past few decades, membrane-based gas separation technology employing hollow fibers (HF) has emerged as a leading technology and underwent rapid growth. Moreover, hollow fiber (HF) membranes have many advantages including high specific surface area, fewer requirements for maintenance and pre-treatment. However, applications of hollow fiber membranes are sometimes restricted by problems related to their low tensile strength as they are likely to get damaged in high-pressure applications. In this context, braid reinforced hollow fiber membranes offer a solution to this problem and can enhance the mechanical strength and lifespan of hollow fiber membranes. The present review includes a discussion about different materials used to fabricate gas separation membranes such as inorganic, organic and mixed matrix membranes (MMM). This review also includes a discussion about braid reinforced hollow fiber (BRHF) membranes and their ability to be used in natural gas purification as they can tackle high feed pressure and aggressive feeds without getting damaged or broken. A BRHF membrane possesses high tensile strength as compared to a self-supported membrane and if there is good interfacial bonding between the braid and the separation layer, high tensile strength, i.e., upto 170Mpa can be achieved, and due to these factors, it is expected that BRHF membranes could give promising results when used for the purification of natural gas.

Interfacial design of polysulfone/Cu-BTC membrane using [Bmim][Tf2N] and [Dmim][Cl] RTILs for CO2 separation: Performance assessment for single and mixed gas separation

An upsurge in searching membranes with improved perm-selectivity and long-term stability is crucial in membrane-based gas separation. Herein, we engineered the interfacial characteristics of Cu-BTC MOFs with the polysulfone (PSf) matrix using ionic liquids (ILs) at the polymer-filler interface. Two different ILs, [Bmim][Tf2N] and [Dmim][Cl], were used to prepare composite PSf/Cu-BTC/[Bmim][Tf2N] and PSf/Cu-BTC/[Dmim][Cl] membranes. The membranes micro and chemical structures were characterized using XRD, FESEM, and XPS techniques. Results indicated that the multifunctional properties of ILs and Cu-BTC particles influenced the membrane formation mechanism. The uniform dispersion of Cu-BTC filler particles with the inclusion of [Bmim][Tf2N] and [Dmim][Cl] ILs on the membrane surface was evident from the morphological studies. The absence of a leaky interface by the ILs integration in the PSf/Cu-BTC/[Bmim][Tf2N] and PSf/Cu-BTC/[Dmim][Cl] membranes results in a pronounced increase in selectivities. For instance, PSf/Cu-BTC/[Dmim][Cl] membranes, the CO2 permeance was noticed to be 29.35 ± 0.35 GPU, and their corresponding gas pair selectivities were noted to be significantly higher than the neat and PSf/Cu-BTC membranes. Similarly, in PSf/Cu-BTC/[Bmim][Tf2N] membrane exhibited optimum CO2 permeance of 30.44 ± 0.49 GPU with the CO2/N2 and CO2/CH4 selectivities of 61 and 59.70%, respectively, higher than neat PSf membrane. Furthermore, these membranes showed better anti-aging resistance and long-term stability than the neat PSf membranes. The merits of a simultaneous increase in permeance and selectivities and the good stability and scalable properties make the proposed PSf/Cu-BTC/IL membranes of great interest for the industrial CO2 separation from the gas exhausts.

High-Throughput Screening of COF Membranes and COF/Polymer MMMs for Helium Separation and Hydrogen Purification.

Hundreds of covalent organic frameworks (COFs) have been synthesized, and thousands of them have been computationally designed. However, it is impractical to experimentally test each material as a membrane for gas separations. In this work, we focused on the membrane-based gas separation performances of experimentally synthesized COFs and hypothetical COFs (hypoCOFs). Gas permeabilities of COFs were computed by combining the results of grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, and many COF membranes were found to overcome the upper bound of polymeric membranes for He/H2, N2/CH4, H2/N2, He/CH4, H2/CH4, and He/N2 separations. We then examined the structure–permeability relations of the COF membranes that are above the upper bound for each of the six gas separations, and based on these relations, we proposed an efficient approach for the selection of the best hypoCOFs from a very large database. Molecular simulations showed that 120 hypoCOFs that we identified to be promising based on these structure–performance relations exceed the upper bound for He/CH4, He/N2, H2/CH4, and H2/N2 separations. Both real and hypothetical COFs were then studied as fillers in 25 different polymers, leading to a total of 29 020 COF/polymer and hypoCOF/polymer mixed matrix membranes (MMMs), representing the largest number of COF-based MMMs investigated to date. Permeabilities and selectivities of COF/polymer MMMs were computed for six different gas separations, and results revealed that 18 of the 25 polymers can be carried above the upper bound when COFs were used as fillers. The comprehensive analysis of COFs provided in this work will fully unlock the potential of COF membranes and COF/polymer MMMs for helium separation and hydrogen purification.

Enhanced gas separation by free volume tuning in a crown ether-containing polyimide membrane

Tuning coupled free volume and pore size distribution is of great significance for membrane-based gas separation. In the present work, a polyimide membrane containing bulky -CF3 group and crown ether ring structure was developed using 4, 4′ - (hexafluoroisopropylidene) diphthalic anhydride as dianhydride monomer, 2, 2′-bis [4-(4-aminophenoxy)phenyl] -hexafluoropropanane and di(aminobenze)-21-crown-7-ether as diamine monomers. Diamine monomer with large bulky -CF3 grant the polyimide membrane more free volume, which was helpful for gas permeability enhancement. Besides, crown ether could act as a pores size distribution tuning monomer, which endowed membranes with improved gas selectivity. Results showed that the introduction of crown ether into polyimide backbone exhibited a significant impact on free volume and pores size distribution. Typically, with the increasing crown ether content, the free volume decreased, whereas the relative available free volume ratio (FAVi/FAVj) achieved a remarkable increment. It’s also worth noting that the pores size distribution was changed by copolymerization with crown ether. The intensity of macro-pore gradually decreased accompanied with an increase in micro-pore structure. As a result, the gas selectivity significantly increased. Compared the membrane without crown ether, the maximum selectivity enhancement was achieved at 30 mol% di(aminobenze)-21-crown-7-ether content. It exhibited remarkable selectivity increments about 204.8%, 198.8%, 123.2%, and 115.4% for He/CH4, H2/CH4, CO2/CH4, and CO2/N2, respectively. These results took a huge step forward which nearly hit up to the 2008 Robeson’s upper bound, especially for CO2/CH4.

How to Get the Best Gas Separation Membranes from State-of-the-Art Glassy Polymers

Herein we focus on fundamental polymer science factors in membrane-based gas separation to open another chapter on this important topic. Realistically thinking about nanometer and angstrom scale matrix attributes that are translatable into higher performance thin glassy polymer asymmetric membrane layers in practical large-scale devices is crucial. Such thinking should be guided by expertise in both polymer and membrane communities to advance the state-of-the-art. Polymer structures, membrane morphologies, and effects of operating conditions are discussed here by using specific examples to illustrate key issues. Rubbery polymers are not the focus of this discussion since they lack diffusive discrimination for size-similar penetrants, which glassy polymers provide and make them the focus here. Four scalable subtopics guide necessary thinking: (i) plasticization, (ii) antiplasticization, (iii) dual-mode transport involving saturation of unrelaxed free volume, and (iv) nonuniform free volume and stress profiles in thin skin glassy polymer asymmetric or composite membranes. Using these subtopics in the context of macromolecular science, we discuss issues needed to expand the state-of-the-art in gas separation membranes.

Mixed-Matrix Ion Gel Membranes for Gas Separation

The potential of ionic liquid materials to improve membrane-based gas separation processes has been realized during the past few years, through the exploitation of diverse configurations, chemical functionalities, and properties. Nowadays, one of the most recent trends on this field is the development of mixed-matrix ion gels, an emerging family of ionic soft-solid materials, in which fillers are embedded into polymer/IL matrices. This review aims at presenting an overview on the latest developments and advances on ion gel membranes containing a third component, such as zeolites, metal organic frameworks, silica, graphene oxide, metal salts, and nanoparticles, for gas separation processes. We provide an analysis of this building system and highlight the diverse type of mixed-matrix ion gels hitherto reported, for which the impact of the different fillers on the ion gels’ overall properties is discussed in detail. The main challenges on their preparation, as well as their transport mechanism and how it can be influenced by the different fillers, are also addressed. Finally, technological aspects, perspectives, and opportunities to pursue this topic are discussed from a critical point of view.

An Aromatic Fluoropolymer for Hydrogen Separation from Hydrocarbons

Plasticization is a critical challenge in membrane-based gas separation. Here a novel fluoropolymer, poly(trifluoro styrene) (PTFS), is reported for hydrogen separation from hydrocarbons. The polymer structure is first characterized by different techniques such as nuclear magnetic resonance (NMR) and positron annihilation lifetime spectroscopy (PALS). Then, gas separation performances of the polymer are studied. The separation of H2 /CH4 is found to outperform most other fluorinated polymers and surpass the Robeson 1991 upper bound. Furthermore, the polymer demonstrates stable or increasing selectivity for hydrogen over hydrocarbons (e.g., CH4 , C2 H6 , and C3 H8 ) at higher pressure, suggesting excellent resistance to plasticization.

Gas separation performance of solid-state in-situ thermally crosslinked 6FDA-based polyimides

2,2-Bis(3,4-dicarboxyphenyl)-hexafluoropropane dianhydride (6FDA)-derived polyimides containing polar functional groups have been extensively studied for membrane-based gas separations and used as precursors for various solid-state thermal modifications such as thermal crosslinking of carboxyl-groups or thermal rearrangement of hydroxyl groups to form polybenzoxazoles. In this work, we studied four 6FDA-derived polyimides without polar functional groups to investigate in-situ thermal crosslinking involving partial decomposition of the hexafluoroisopropylidene (–C(CF3)2–) mainchain group. A thorough examination of the 6FDA-based polyimides via thermal gravimetric analysis coupled with mass spectroscopy (TGA-MS) and energy dispersive X-ray spectroscopy (EDX) under isothermal treatment at 450 °C for up to 3 h revealed that thermal treatment triggered continuous evolution of fluorine from (–C(CF3)2–) groups. The 6FDA-based polyimides were insoluble after thermal treatment for only 30 min and maintained good mechanical strength with favorable changes in gas separation properties. For example, after 3 h isothermal heat treatment, the CO2 permeability of crosslinked 6FDA-pPDA noticeably increased threefold from 39 to 115 barrer coupled with only a small drop in CO2/CH4 selectivity from 54 to 46. The crosslinked polyimides demonstrated excellent plasticization resistance when tested under high-pressure binary CO2/CH4 mixed-gas conditions.

Conjugate polymer-based membranes for gas separation applications: current status and future prospects

Conjugate polymers provide the possibility of exploiting both the chemical and physical attributes of the polymers for membrane-based gas separation. The presence of delocalized π electrons provides high chain stiffness with low packing density, thus making the membrane a rigid structure that favors facilitated transport. Historically, the polymeric membranes were constrained by the tradeoff relationship between gas permeability and gas selectivity. So, different methods were investigated to prepare the membranes that can overcome the limitation. In recent years, electroconductive polymeric membranes have gained attention with their enhanced transportation properties combining the separation behavior depending on both molecular size discrimination as well as the facilitated transport. They offer better selectivity toward polar gases such as CO2 because of the increased solubility. This review is aimed to provide a literature survey on gas separation using conjugate polymers such as polyaniline, polypyrrole, and some derivatives of polythiophenes. It contains various methods used by different researchers to enhance the gas separation properties of the membranes with improved mechanical and thermal stability such as changing the morphology and membrane preparation methods. In addition, it provides the pros and cons of various factors affecting the conjugate polymer membrane performance. The major challenges and future work that can be done in improving the transportation properties through the membrane to achieve viable membranes are also discussed so that they can be used for commercial and practical applications in the future.

Synthesis and gas separation properties of polyimide membranes derived from oxygencyclic pseudo-Tröger's base

Two neoteric oxygencyclic pseudo-Tröger's base (TB) -derived naphthalene diamines characterized by large π conjugated naphthalene rings and half-open cavity structure unit, 16H-8,16-methanodinaphtho [2,1-d:1′,2′-g] (Yampolskii, 2012; Maqsood et al., 2014) [1,3]dioxocine-3,13-diamine (OTB) and its furan-based bicyclic analogue, 7a, 14c-dihydronaphtho [( Xu et al., 2014; Yampolskii, 2012) 2,12,1-b]naphth [1′,2':4,5]furo [( Maqsood et al., 2014; Xu et al., 2014) 3,23,2-d] furan-3,12- diamine (FB), were successfully synthesized. Their 6FDA-based polyimides (6FDA-OTB and 6FDA-FB) were obtained for membrane-based gas separation study. 6FDA-OTB and 6FDA-FB exhibited good solubility and excellent thermal stability due to the presence of large π conjugated aromatic rings and half-open cavity structure unit. The two polymer membranes showed high gas pair selectivity and modest gas permeability as well as good physical aging properties. The 6FDA-FB membrane performed a high selectivity of H2/CH4 (104.0) and CO2/CH4 (49.1), which arises from the larger permeation activation energy difference of different gases. The performances of 100 days aged 6FDA-OTB and 6FDA-FB membranes are almost overlapped with the 1991 Robeson upper bounds for CO2/CH4 and H2/CH4. In short, introducing the novel rigid oxygencyclic pseudo-Tröger's base into the polyimide chains could make attractive membrane materials for various gas separation applications.

Significantly improved gas separation properties of sulfonated PIM-1 by direct sulfonation using SO3 solution

One of the biggest challenges in membrane-based gas separation application is how to obtain highly efficient membranes with both high permeability and selectivity. To achieve this target, we reported a novel simple method to modify polymer of intrinsic microporosity (PIM-1) membranes by direct sulfonation using sulfur trioxide (SO3)/dichloromethane solution to get a series of sulfonated PIM-1 (SPIM-1) membranes. The SO3H group was bonded to the main chain of the SPIM-1 and distributed homogeneously in the entire membrane that was confirmed by FTIR, XPS and SEM/EDS mapping. As the sulfonation time increased from 2 to 6 min, the concentration of sulfonic acid (SO3H) group in the repeat unit increased from 12.3% to 30.1%. The introduction of SO3H groups resulted in a decreased surface area and denser polymer chain packing. The resulting SPIM-1 membranes exhibited huge improved selectivity with separation performance much better than the pristine PIM-1. In which, the 6 min sulfonated PIM-1 membrane (SPIM-1-6) showed excellent gas separation properties with its performance approaches or even exceeds the latest trade-off curves for O2/N2, CO2/N2, H2/N2, and CO2/CH4. This is due to the SO3H group induced a compact packing of polymer main chain that remarkably enhanced the diffusion selectivity. The 60 days aged SPIM-1-6 demonstrated even higher gas pair selectivity, and the H2/N2 and O2/N2 selectivity reached as much as 125 and 8.43 coupled with H2 and O2 permeability of 1077 and 73.4 Barrer, respectively. Additionally, the SPIM-1-6 also showed excellent mixed-gas separation properties, with CO2/CH4 mixed-gas selectivity over 40 coupled with CO2 permeability of 296 Barrer even at the upstream pressure of 20 bar. These results suggested the great potential for this mild sulfonation method and sulfonated PIM-1 membranes in advanced membrane-based gas separation applications.

High-efficiency CO2 separation using hybrid LDH-polymer membranes

Membrane-based gas separation exhibits many advantages over other conventional techniques; however, the construction of membranes with simultaneous high selectivity and permeability remains a major challenge. Herein, (LDH/FAS)n-PDMS hybrid membranes, containing two-dimensional sub-nanometre channels were fabricated via self-assembly of unilamellar layered double hydroxide (LDH) nanosheets and formamidine sulfinic acid (FAS), followed by spray-coating with a poly(dimethylsiloxane) (PDMS) layer. A CO2 transmission rate for (LDH/FAS)25-PDMS of 7748 GPU together with CO2 selectivity factors (SF) for SF(CO2/H2), SF(CO2/N2) and SF(CO2/CH4) mixtures as high as 43, 86 and 62 respectively are observed. The CO2 permselectivity outperforms most reported systems and is higher than the Robeson or Freeman upper bound limits. These (LDH/FAS)n-PDMS membranes are both thermally and mechanically robust maintaining their highly selective CO2 separation performance during long-term operational testing. We believe this highly-efficient CO2 separation performance is based on the synergy of enhanced solubility, diffusivity and chemical affinity for CO2 in the sub-nanometre channels.

Recent advancements in molecular separation of gases using microporous membrane systems: A comprehensive review on the applied liquid absorbents

Industrial-based molecular separation of detrimental gaseous molecules (especially CO2) from various mixtures using different types of chemical absorbents has been recently of paramount attention all around the world to mitigate their severe impacts on ecosystem such as acid rain and global warming. This paper aims to present a state-of-the-art review about the recent advancements in the molecular separation of CO2 applying hollow fiber membrane contactors (HFMCs). Additionally, all employed membrane materials for separating CO2 contaminant including polymeric membranes (PMs), microporous organic polymers (MOPs), fixed-site-carrier membranes (FSCMs), inorganic membranes (IMs), mixed matrix membranes (MMMs), and carbon molecular sieve membranes (CMSMs) are comprehensively discussed. Discussion about the most important operating challenges towards the separation of CO2 inside the HFMCs (i.e., mass transfer resistance and membrane pores wettability) is considered as another objective of this paper. Another aim of this paper is to classify the prevalent types of liquid absorbents and critically discuss the advantages and disadvantages of each group in the field of membrane-based gas separation process.

Room temperature and ambient pressure deposition of Cu-BTC MOF on SBA-15 functionalized silica supports by simple spray layer-by-layer method

Metal-organic frameworks (MOFs) have received intense interest over the past decade due to its wide application such as catalysis, membrane-based gas separation, gas adsorption, sensing, and biomedical devices. Growth of MOFs on different solid surfaces allows improving their mechanical properties. Silica is a potential candidate. Due to the difficulty of rising Cu-BTC MOF on silica substrate our group was motivated to achieve a successful coating. This work outlines the functionalization of the gold surface through the MHDA self-assembled monolayer (SAM) and the construction of a MOF film (SURMOF) of Cu-BTC upward. The same MOF was also grown onto a modified mesoporous SBA-15 silica substrate. The first obtained silica particles were previously conformed and activated to receive the MOF. The layer-by-layer method was used to deposit MOF onto the substrates. The deposited MOFs films were characterized by X- ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed SBA-15 as a potential MOF support.

Performance of polysulfone hollow fiber membranes encompassing ZIF-8, SiO2/ZIF-8, and amine-modified SiO2/ZIF-8 nanofillers for CO2/CH4 and CO2/N2 gas separation

High tunable properties of metal–organic frameworks (MOFs) allows different modifications to be used for distinct applications. Zeolitic imidazolate frameworks-8 or ZIF-8 are hybrid organic–inorganic porous materials that have shown great potential in membrane-based gas separations. In our study, SiO2/ZIF-8 (S/ZIF-8) nanofiller was synthesized via a facile one-step strategy. Furthermore, the S/ZIF-8 nanofiller was subjected to amine modification using an ammonium hydroxide solution. X-ray photoelectron spectroscopy (XPS) analysis confirms the strong binding of amine and SiO2 on ZIF-8 nanofillers. The synthesized ZIF-8, S/ZIF-8, and amine-modified S/ZIF-8 (A@S/ZIF-8) of 0.5 wt% were incorporated into the polysulfone (PSf) matrix, and the hollow fiber mixed matrix membranes (MMMs) were fabricated using dry–wet phase inversion technique. The inclusion of an amine group in the S/ZIF-8 nanofillers enhances the interaction between the nanofillers and PSf matrices, thus improving the CO2/CH4 and CO2/N2 separation performances. The fabricated membranes were morphologically characterized by Scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET) surface area, and the pore size distribution. It is noteworthy that the BET surface area was enhanced with nanofillers addition and significantly increased gas separation performance. The fabricated asymmetric hollow fiber MMMs were further studied for gas permeation experiments for CO2, N2, and CH4. PSf with A@S/ZIF-8 MMMs showed the best performance as observed from the Robeson upper bound limit, CO2 permeability of 4.25 barrer with the improvements in CO2/N2 and CO2/CH4 selectivities of 89.75 and 61.46% respectively, compared with neat PSf membrane. The enhanced permselectivity of PSf–A@S/ZIF-8 membranes is explained by CO2-facilitated transport, where the high CO2 affinity of the amine group allows the selective transport of CO2 rather than CH4 and N2.

Evaluation of Flowsheet Design Approaches to Improve Energy Efficiency in Multistage Membrane Processes to Recover Helium

In pressure-driven membrane-based gas separation processes, the driving force-related cost (capital and operating costs) is one of the most significant cost components. To reduce this driving force...