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Increasing demand to strengthen energy security has increased the importance of natural gas sweetening and biogas upgrading processes. Membrane-based separation of carbon dioxide (CO2) and methane (CH4) is a relatively newer technology, which offers several competitive advantages, such as higher energy-efficiency and cost-effectiveness, over conventional technologies. Recently, the use of graphene-based materials to elevate the performance of polymeric membranes have attracted immense attention. Herein, we do not seek to provide the reader with a comprehensive review of this topic but rather highlight the key challenges and our perspectives going ahead. We approach the topic by evaluating three mainstream membrane designs using graphene-based materials: (1) nanoporous single-layer graphene, (2) few- to multi-layered graphene-based stacked laminates, and (3) mixed-matrix membranes. At present, each design faces different challenges, including low scalability, high production cost, limited performance enhancement, and the lack of robust techno-economic review and systematic membrane design optimization. To help address these challenges, we have mapped out a technology landscape of the current graphene-based membrane research based on the separation performance enhancement, commercial viability, and production cost. Accordingly, we contend that future efforts devoted to advancing graphene-based membranes must be matched by progress in these strategic areas so as to realize practical and commercially relevant graphene-based membranes for CO2/CH4 separation and beyond. [ABSTRACT FROM AUTHOR]

[Display omitted] • Conditions and parameters controlled in the fabrication of carbon molecular sieve membranes (CMSMs) are summarized. • Mass transfer properties of CMSMs are discussed. • Gas separation properties of CMSMs are summarized. • Current challenges in and future directions for practical application of CMSMs are surveyed. Carbon molecular sieve membranes (CMSMs) have drawn substantial research attention in recent years due to their ability to overcome the trade-off limitation between permeability and selectivity that is commonly observed in polymeric membranes used for gas separation. The performance of CMSMs is governed by various factors, such as the choice of polymeric precursors and their pre-treatment, pyrolysis, and post-treatment conditions. This review examines the critical aspects in the process of developing CMSMs based on polyimide precursors for gas separation. In addition, the mass transfer mechanism and characterization methods of CMSMs are discussed. Then, the performances of various CMSMs developed so far are examined against the Robeson upper bound limit, and pilot-scale applications and an economic analysis of CMSM-based gas separation are provided. Finally, the challenges and perspective are presented as the concluding remarks. [ABSTRACT FROM AUTHOR]

To improve the CO2/N2 separation performance, mixed-matrix carbon molecular sieve membranes (mixed-matrix CMSMs) were fabricated and tested. Two carbon-based fillers, graphene oxide (GO) and activated carbon (YP-50F), were separately incorporated into two polymer precursors (Matrimid® 5218 and ODPA-TMPDA), and the resulting CMSMs demonstrated improved CO2 permeability. The improvement afforded by YP-50F was more substantial due to its higher accessible surface area. Based on the gas permeation data and the Robeson plot for CO2/N2 separation, the performances of the CMSMs containing 15 wt % YP-50F and 15 wt % GO in the mixed polymer matrix surpassed the 2008 Robeson upper bound of polymeric membranes. Hence, this study demonstrates the feasibility of such membranes in improving the CO2/N2 separation performance through the appropriate choice of carbon-based filler materials in polymer matrices. [ABSTRACT FROM AUTHOR]

Hydrogen (H2) is an industrial gas that has showcased its importance in several well-known processes such as ammonia, methanol and steel productions, as well as in petrochemical industries. Besides, there is a growing interest in H2 production and purification owing to the global efforts to minimize the emission of greenhouse gases. Nevertheless, H2 which is produced synthetically is expected to contain other impurities and unreacted substituents (e.g., carbon dioxide, CO2; nitrogen, N2 and methane, CH4), such that subsequent purification steps are typically required for practical applications. In this context, membrane-based separation has attracted a vast amount of interest due to its desirable advantages over conventional separation processes, such as the ease of operation, low energy consumption and small plant footprint. Efforts have also been made for the development of high-performance membranes that can overcome the limitations of conventional polymer membranes. In particular, the studies on graphene-based membranes have been actively conducted most recently, showcasing outstanding H2-separation performances. This review focuses on the recent progress and potential challenges in graphene-based membranes for H2 purification. [ABSTRACT FROM AUTHOR]

Nanocrystalline UiO-66 and its derivatives (containing -NH2, -Br, -(OH)2) were developed via pre-synthetic functionalization and incorporated into a polyimide membrane to develop a mixed-matrix membrane (MMM) for CO2/N2 separation. Incorporation of the non-functionalized UiO-66 nanocrystals into the polyimide membrane successfully improved CO2 permeability, with a slight decrease in CO2/N2 selectivity, owing to its large accessible surface area. The addition of other functional groups further improved the CO2/N2 selectivity of the polymeric membrane, with UiO-66-NH2, UiO-66-Br, and UiO-66-(OH)2 demonstrating improvements of 12%, 4%, and 17%, respectively. Further evaluation by solubility–diffusivity analysis revealed that the functionalized UiO-66 in MMMs can effectively increase CO2 diffusivity while suppressing N2 sorption, thus, resulting in improved CO2/N2 selectivity. Such results imply that the structural tuning of UiO-66 by the incorporation of various functional groups is an effective strategy to improve the CO2 separation performance of MMMs. [ABSTRACT FROM AUTHOR]

Membrane-based air separation (O2/N2) is of great importance owing to its energy efficiency as compared to conventional processes. Currently, dense polymeric membranes serve as the main pillar of industrial processes used for the generation of O2- and N2-enriched gas. However, conventional polymeric membranes often fail to meet the selectivity needs owing to the similarity in the effective diameters of O2 and N2 gases. Meanwhile, mixed-matrix membranes (MMMs) are convenient to produce high-performance membranes while keeping the advantages of polymeric materials. Here, we propose a novel MMM for O2/N2 separation, which is composed of Matrimid® 5218 (Matrimid) as the matrix, cobalt(II) phthalocyanine microparticles (CoPCMPs) as the filler, and Pluronic® F-127 (Pluronic) as the compatibilizer. By the incorporation of CoPCMPs to Matrimid, without Pluronic, interfacial defects were formed. Pluronic-treated CoPCMPs, on the other hand, enhanced O2 permeability and O2/N2 selectivity by 64% and 34%, respectively. We explain the enhancement achieved with the increase of both O2 diffusivity and O2/N2 solubility selectivity. [ABSTRACT FROM AUTHOR]

The energy-intensive ethylene/ethane separation process is a key challenge to the petrochemical industry. HKUST-1, a metal–organic framework (MOF) which possesses high accessible surface area and porosity, is utilized in mixed-matrix membrane fabrication to investigate its potential for improving the performance for C2H4/C2H6 separation. Prior to membrane fabrication and gas permeation analysis, nanocrystal HKUST-1 was first synthesized. This step is critical in order to ensure that defect-free mixed-matrix membranes can be formed. Then, polyimide-based polymers, ODPA-TMPDA and 6FDA-TMPDA, were chosen as the matrices. Our findings revealed that 20 wt% loading of HKUST-1 was capable of improving C2H4 permeability (155% for ODPA-TMPDA and 69% for 6FDA-TMPDA) without excessively sacrificing the C2H4/C2H6 selectivity. The C2H4 and C2H6 diffusivity, as well as solubility, were also improved substantially as compared to the pure polymeric membranes. Overall, our results edge near the upper bound, confirming the effectiveness of leveraging nanocrystal HKUST-1 filler for performance enhancements in mixed-matrix membranes for C2H4/C2H6 separation. [ABSTRACT FROM AUTHOR]

Abstract The incorporation of solid fillers into mixed matrix membranes (MMMs) is a promising approach to overcome the permeability-selectivity trade-off characteristic of polymeric membranes. However, MMMs that contain conventional fillers (e.g., zeolites, silicas, and porous carbons) usually exhibit defects due to incompatible interfaces between fillers and glassy polymers. High porosity and good robustness are also desirable properties of filler materials that enhance the gas separation performance of resulting MMMs. Herein, amine-functionalized organic porous polymers (PP-DETA and PP-menm) were synthesized and used as filler materials to yield mechanically stable MMMs that possess good CO 2 /CH 4 separation performance. We found that in comparison with the MMMs loaded with bare porous polymers (PP), composite membranes that contain amine-functionalized fillers exhibited both superior mechanical strength and more effective adhesion to glassy Matrimid® Polyimide (PI) and polysulfone (PSf) polymers. In particular, the tensile strength of 10 wt%_PP-menm@PI was measured to be 69 MPa, which was higher than 26 MPa and 33 MPa for 10 wt%_PP@PI and pure PI membrane, respectively. More importantly, both the CO 2 permeability and CO 2 /CH 4 selectivity of MMMs improved significantly after the introduction of amine-functionalized fillers. These results imply that organic porous polymers, particularly those that contain amine-functional groups, are outstanding filler materials for the fabrication of defect-free MMMs with enhanced gas separation performance. Graphical abstract fx1 Highlights • Porous polymer (PP) and amine-appended PP were synthesized and incorporated into polyimide and polysulfone membranes. • The filler-polymer adhesion was improved for amine-appended PP. • Amine-appended PP improved the mechanical strength of mixed-matrix membranes. • Incorporation of amine-appended PP significantly increased both CO 2 permeability and CO 2 /CH 4 selectivity of resulting membranes. [ABSTRACT FROM AUTHOR]

In this study, polymer-fluorinated silica composite hollow fiber membranes were fabricated and applied to a membrane contactor system for the recovery of methane dissolved in the anaerobic effluent. Such composite membranes allowed us to tailor the physical property such as porosity and mechanical strength and the surface hydrophobicity in separated processes. To develop the composite membranes, porous hollow fiber substrates were first fabricated with Matrimid ® , a commercial polyimide. Subsequently, fluorinated silica particles were synthesized and anchored on the substrates via a strong covalent bonding. Due to the high porosity as well as the high hydrophobicity, our membrane showed an outstanding performance for the recovery of CH 4 in the membrane contactor, such that the CH 4 flux reached 2900 mg CH 4 /m 2 –h at the liquid velocity of 0.42 m/s at which the liquid phase still controlled the overall mass transfer. The composite membrane prepared in this work also showed a much better performance in the CH 4 recovery than a commercial polypropylene membrane made for degasification of water. In addition, a long-term test with tap water saturated with the model biogas made up of 60:40 CH 4 /CO 2 mixture demonstrated that our membrane can be stably operated for more than 300 h without experiencing pore wetting problem. [ABSTRACT FROM AUTHOR]

A series of porous organic polymers (denoted as PBP- x) possessing various functional groups was synthesized via Friedel–Crafts alkylation followed by post-synthetic functionalization using amine and sulfonic groups. These functionalized porous polymers were incorporated into an in-house polyimide to fabricate mixed-matrix membranes for CO 2 /N 2 separation. Gas permeation testing revealed that these functionalized porous fillers substantially improved the CO 2 separation performance. Moreover, optimizing both the functionality and pore structure of the porous fillers was found to drastically improve the CO 2 /N 2 separation performance. In particular, the addition of 10 wt% PBP-menm (menm = 1,2-methylethylenediamine) induced an ultrahigh CO 2 permeability of 2988 barrers (158% higher than that of a pure polymer membrane), thus realizing excellent performance that surpasses the Robeson upper bound for CO 2 /N 2 separation. [Display omitted] • Porous organic polymers possessing various functional groups were synthesized. • Defect-free mixed-matrix membranes comprising these functionalized porous fillers were successfully fabricated. • Optimizing both functionality and pore structure of porous fillers led to enhanced CO 2 transport. • The performance of the membrane with the optimum filler surpassed the Robeson upper bound for CO 2 /N 2 separation. [ABSTRACT FROM AUTHOR]

Conventional mixed-matrix membranes (MMMs) possess a dense structure with a filler material uniformly dispersed within the polymer matrix to engineer the transport properties and achieve enhanced gas separation performances with respect to pure polymeric membranes. However, a dense membrane structure increases the transport resistance and undesirably requires a high filler loading to see a substantial enhancement in separation performances. To address this problem, we fabricated asymmetric MMMs that have a thin-selective layer of around 0.5 μm and a low loading (0.03–0.10 wt%) of nitrogen-doped graphene (N-G) nanosheets. The presence of the nitrogen- and oxygen-containing functional groups on the N-G nanosheets ensured good compatibility between filler and polymer matrix, which resulted in strong polymer/filler interfacial adhesions. The N-G nanosheets were also found to be capable of migrating to the top of the membranes during phase inversion. Hence, despite the low filler loadings used in this work, the capacity of the dense selective layers was greatly enhanced. Based on our experimental results, 0.07 wt% and 0.10 wt% loading of N-G nanosheets can improve both O 2 /N 2 (126.9%) and CO 2 /N 2 (45.8%) selectivities with respect to pure polymeric membranes, resulting in O 2 /N 2 separation performance surpassing the Robeson upper bound. Image 1 • Asymmetric mixed matrix membranes (MMMs) comprising nitrogen-doped graphene (N-G) were fabricated. • Asymmetric MMMs have a thin-selective layer and a very low lading of N-G (0.03–0.10 wt%). • Most N-G fillers were migrated to the dense (top) layer to reduce interface energy during the phase inversion. • The O 2 /N 2 and CO 2 /N 2 selectivities of MMMs increased by 126.9% and 45.8%, respectively. • The selectivity improvement is due to the formation of more selective tortuous pathway by N-G fillers. [ABSTRACT FROM AUTHOR]

• Interlayer spacing of GO membranes was tuned by intercalation of cations. • A continuous selective layer of GO membrane was developed. • The effects of intercalated ions on gas separation performance were studied. • GO-La3+ membranes exhibited the best H 2 /CO 2 separation performance. Graphene oxide (GO) laminate membranes of controlled interlayer spacing were synthesized for the investigation in H 2 /CO 2 separation. The interlayer distance of GO flakes was tuned by crosslinking and intercalation of La3+ or Co2+ cations between GO layers. Investigation via field-emission scanning electron microscope showed that a continuous selective layer was present in the GO-La3+, GO-Co2+ and GO membranes. Gas permeation results revealed that the GO-La3+ membrane had better H 2 permeability and H 2 /CO 2 selectivity than the other membranes, due to an increase in d -spacing between the GO layers and restricted defect formation in the GO laminate membranes. In addition, the H 2 /CO 2 separation performance of GO-La3+ membrane was found to be close to the upper bound limit of conventional polymer membranes established in 2008. [ABSTRACT FROM AUTHOR]

HKUST-1 nanocrystals with large accessible surface areas and high porosity were functionalized with amine groups and incorporated into polymer matrices to form mixed-matrix membranes for application in CO 2 capture. This gas permeation study reveals that pristine HKUST-1 nanocrystals are able to increase the gas permeabilities of polymer membranes with no noticeable effect on CO 2 /N 2 selectivity as a result of their high porosity and large pore dimensions. Contrarily, 20 wt% amine-functionalized HKUST-1 can improve the CO 2 /N 2 selectivity of polymer membranes by as much as 38%. As shown in a detailed analysis of the diffusivity-solubility, the amine-functionalized HKUST-1 suppressed N 2 solubility and diffusivity but enhanced CO 2 diffusivity. This resulted in an increased CO 2 /N 2 perm-selectivity. Meanwhile, the pristine HKUST-1 nanocrystals increased the diffusivity and solubility of both species in a non-selective manner. These results indicate the feasibility of tuning the CO 2 separation performance of mixed-matrix membranes by introducing amine functionality into the filler. Image 1 • HKUST-1 and amine-functionalized HKUST-1 nanocrystals were successfully synthesized. • HKUST-1 nanocrystals improved CO 2 permeability with no effect on CO 2 selectivity. • Amine-functionalized HKUST-1 enhanced CO 2 /N 2 selectivity and CO 2 permeability. • Amine-functionalized HKUST-1 suppressed N 2 solubility and diffusivity substantially. [ABSTRACT FROM AUTHOR]