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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]

Abstract: For the first time, we report the C4s’ transport properties: solubility, diffusivity, permeability in 6FDA–DAM polymer, one of most permeable glassy polymers with significant nC4 vs. iC4 selectivity. An nC4 permeability of 3.7Barrer, and nC4/iC4 ideal selectivity of 21 was found in pure 6FDA–DAM polymer membrane. Mixed matrix films were successfully fabricated using a 6FDA–DAM as the matrix, with up to 35wt% loading of MFI, modified by a two-step Grignard treatment (GT) that produced Mg(OH)2 whiskers on the surface of the MFI particles. The permeability of nC4 more than doubled; however, the selectivity for the C4s remained the same. Permeation of mixed matrix films with impermeable GT-uncalcined-MFI agree with Maxwell modeling of films with an impermeable solid loading, thereby supporting the existence of a defect-free interface between MFI particles and the polymer matrix. This indicates that the MFI is too permeable to optimally match the properties of the 6FDA–DAM, which is one of the most permeable selective matrix polymers available. It appears unlikely that any currently known, adequately selective glassy polymer can match the high permeability of nC4 in MFI to enable development of promising composite membrane for the C4s separation based on MFI. Therefore a smaller pore size zeolite is required for a better match. [Copyright &y& Elsevier]

A membrane separation process was coupled to a sequencing batch reactor (SBR) for biological nutrient removal (BNR) processes and a combined system was named a membrane sequencing batch reactor (MSBR). MSBR was used for the treatment of dairy industry wastewater and optimized to increase the treatment efficiency. Since a diffuser-attached module design, subcritical flux operation, and intermittent suction method were adapted to the system, long-term operation was possible, i.e. the system could be operated for more than 110 days with only one membrane washing. BOD removal was high (97–98%) and stable. SS (suspended solid)-free effluent was obtained by membrane separation. Since nitrogen was mainly consumed as nutrient for synthesis of new cells due to the low influent concentration, the removal rate reached 96%. Phosphorus removal was relatively low because of the limit of the biological process, i.e. removal efficiency ultimately depends on the amount of excess sludge wasting. A removal rate of 80% was reached after system optimization. [Copyright &y& Elsevier]

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]

Graphene-based laminate membranes have recently demonstrated unique advantages over the traditional separation membranes made of polymers or ceramics. For both gas and liquid separations, various designs of crosslinker-stabilized graphene oxide (GO) membranes have been reported. However, the preparation methods used for those are poorly scalable for industrial applications. Herein, we report the fabrication of large-area (1333 cm2) GO-based membranes via doctor-blade casting of gel-like slurries prepared by incorporating calcium ions (Ca2+), ferric ions (Fe3+), polyethylene oxide (PEO), or polyethyleneimine (PEI) as crosslinkers. We found that all crosslinkers tested are suitable for the gelation of dilute GO dispersions (1–5 mg/mL) for doctor-blade casting. Besides, all crosslinked-GO membranes demonstrated outstanding stability under sonication compared to a GO-only membrane prepared via vacuum-assisted filtration. In aqueous nanofiltration tests, Fe3+-crosslinked GO membranes achieved virtually complete rejection of different organic dyes. PEO-crosslinked GO membranes, on the other hand, exhibited an outstanding performance for the separation of carbon dioxide (CO 2) and nitrogen (N 2) gases with a CO 2 /N 2 selectivity of 52. Given the scaleup potential of doctor-blade casting and the practicality of crosslinking-based gelation approach, the proposed approach is promising to increase the industrial relevance of GO-based laminate membranes. Image 1 [ABSTRACT FROM AUTHOR]

Abstract The physicochemical properties of filler materials are critical considerations influencing the separation performances of mixed-matrix membranes (MMMs). Herein, a three-dimensional covalent organic framework (3D-COF) with a secondary amine-containing backbone was designed to offer large surface area, high porosity and good affinity toward CO 2 molecules. Membranes prepared from this 3D-COF filler and a 6FDA-DAM polyimide matrix demonstrated a more significant enhancement in the CO 2 /CH 4 separation performance, which was unattainable by its 2D-COF analogue. Specifically, with 10 and 15 wt% loadings of 3D-COF fillers, the MMMs membranes were able to enhance the CO 2 permeability by ~57% and 140%, respectively, at a comparable, if not better, CO 2 /CH 4 selectivity than the unfilled membrane. Furthermore, glassy polymers of high fractional free volume such as 6FDA-DAM tend to suffer from a ubiquitous loss in performance over time due to a physical aging effect. In this regard, the 3D-COF was effective in slowing down the aging process by capitalizing on its high surface area and amine functional groups to immobilize and rigidify the 6FDA-DAM polymer chains, preventing the collapse of the free volume. This allows 97% of the initial membrane performances to be effectively retained after 240 days of aging. Our findings suggest the potential of morphologically-tuned COFs to develop high-performance MMMs with strong practical relevance. Graphical abstract fx1 Highlights • A 3D covalent organic framework filler was synthesized using a Schiff-base reaction. • High morphologically induced surface area, porosity and affinity toward CO 2 molecules. • The 3D-COF was paired with 6FDA-DAM polymer to give mixed-matrix membranes. • Mixed-matrix membranes showed performances beyond the 2008 Robeson upper bound. • The 3D-COF retarded aging by retaining 97% of the initial performance after 240 days. [ABSTRACT FROM AUTHOR]

An emerging class of carbon nanomaterials (CNMs), in particular graphene-based materials and carbon nanotubes, has been extensively studied for its extraordinary water transport and sieving properties. However, harnessing the full potential of these CNMs as separation membranes for water treatment and desalination is a challenge. As such, we identify five strategic areas of focus, namely, concentration polarization, fouling, stability, scalability and cost to help path future research directions on CNM-based membranes. We reason that concentration polarization and fouling are limitations which compromise the performance of CNM-based membranes while mechanical stability of the membranes towards cross-flow hydrodynamics is an important prerequisite to tackle these Achilles heels. Such challenges, together with the safety profile of CNMs, membrane scalability and production costs, are potential barriers that require urgent attention and necessitate a shift in research efforts away from the pursuit of enhanced performances alone. This article therefore puts into perspective the relevance of these areas of focus and elaborates how carbon science can be leveraged as an interdisciplinary approach to develop innovative solutions essential for realizing the revolutionary promise that CNMs hold for water treatment and desalination. [ABSTRACT FROM AUTHOR]

Organic solvent nanofiltration (OSN) often needs to be conducted under extreme conditions in the chemical and food industries. In this work, we fabricated polybenzimidazole (PBI)-based OSN membranes that possess excellent stability under strongly basic conditions at pH 13. Two crosslinked PBI membranes were prepared by treating pristine PBI membranes with two epoxy-containing crosslinkers. Various permeation and characterization tests were performed to investigate the resistance of the membranes to solvents and basic environments. Both crosslinked membranes displayed reasonable permeance in pure solvents at room temperature under 10 bar operating pressure, along with a sharp molecular discrimination performance in ethanol environments. The crosslinked PBI membranes were found to be stable and did not completely lose their separation properties at high pH, although the effective pore size of the membranes increased presumably due to swelling. The separation performance of the membranes was found to be reversible between filtrations of neutral and basic aqueous solutions. Therefore, our crosslinked PBI membranes, which display exceptional stability, hold great potential for applications in OSN under basic conditions. [Display omitted] • Polybenzimidazole (PBI) OSN membranes were fabricated using crosslinkers containing epoxy groups at both ends. • The OSN performance of PBI membrane was found to be controlled by the structures of the crosslinkers. • The crosslinked PBI membranes exhibited a sharp molecular separation performance in organic solvents. • The crosslinked PBI membranes showed an outstanding chemical resistance under strongly basic conditions. [ABSTRACT FROM AUTHOR]

Nano-sized PS-MFI, ETS-10 and SAPO-34 zeolites were utilized as CO 2 -selective fillers to fabricate high-performance mixed-matrix carbon molecular sieve membranes (CMSMs) for application in the CO 2 /N 2 separation process. In this work, an in-house polyimide, ODPA-TMPDA, was used as a polymer precursor for CMSMs, due to its higher intrinsic gas permeability compared with commercial polyimides. Zeolite-filled CMSMs were then successfully fabricated without any appreciable defects at the filler/matrix interfaces. Gas permeation testing revealed that both CO 2 permeability and CO 2 /N 2 selectivity can be significantly improved upon incorporation of such zeolite fillers, enabling selective adsorption and transport of CO 2. In particular, SAPO-34 yielded the best results among the zeolite fillers tested, resulting in an excellent performance far beyond the Robeson upper bound limit for CO 2 /N 2 separation. Image 1 • PS-MFI, ETS-10 and SAPO-34 nanoparticles were synthesized. • ODPA-TMPDA polyimide was used as a precursor for the porous carbon matrix. • Various zeolite nanoparticles were incorporated in the porous carbon matrix. • Mixed-matrix carbon molecular sieve membranes showed outstanding CO 2 /N 2 separation performance. [ABSTRACT FROM AUTHOR]

Membrane process is a robust technology for CO 2 /CH 4 separation. To achieve the energy-efficiency necessary for a cost-effective membrane-based CO 2 /CH 4 separation, high-performance membranes are often desirable. There are several approaches that can be undertaken to realize such membranes. In this study, we amalgamated three strategies: 1) filler design optimization by synthesizing hierarchical zeolite 5A comprising micro and mesoporous domains to strengthen the second strategy; 2) mixed-matrix approach by incorporating zeolite fillers to facilitate CO 2 diffusion and reduce transport resistance through a Matrimid® matrix; and 3) membrane carbonization to induce thermal rearrangement, and create free volumes and pore apertures for a faster CO 2 transport. These efforts afforded mixed-matrix carbon molecular sieve membranes with results confirming that our membrane at 30 wt% loading of hierarchical zeolite 5A was able to surpass the 2008 Robeson upper bound limit given a performance of 2450 barrers CO 2 permeability and 19.3 CO 2 /CH 4 selectivity. Notably, at this loading, the negative effect brought by the interfacial nanogaps between the filler and carbon matrix diminished, unleashing the potential of the mesopores in facilitating CO 2 diffusion, which led to a two orders of magnitude enhancement in the CO 2 permeability relative to the unfilled carbon molecular sieve membrane. Despite a decrease in the CO 2 /CH 4 selectivity, the hierarchical zeolite 5A is effective in alleviating the intensity of the permeability-selectivity trade-off. Also, benchmarking of the filler effectiveness is first demonstrated using a new filler enhancement index, F index. The largest F index value in this study was 1.97. This attaches a label of "competent" to our hierarchical zeolite 5A filler at 30 wt% loading, demonstrating the success of our strategies. Image 1 • H-zeolite 5A possessing both microporous and mesoporous domains was synthesized. • H-zeolite 5A was incorporated into carbon molecular sieve membranes to form MMMs. • H-zeolite 5A dramatically improved CO 2 permeability and thereby CO 2 /CH 4 separation performance of the CMSMs. • The effectiveness of H-zeolite 5A as the functional filler was further validated by the F index. [ABSTRACT FROM AUTHOR]