Metal-Organic Framework-Enabled Mixed Matrix Membranes: Recent Breakthroughs, Limitations, and Interfacial Engineering for Gas Separation

Metal-organic framework (MOF)-based mixed matrix membranes (MMMs) have shown great promises to overcome the performance upper limit of polymeric membranes for various gas separation processes. However, the gas separation properties of the MMMs largely depend on the MOF-polymer interfacial compatibility which is a metric difficult to quantify. In most cases, whether a MOF filler and a polymer matrix make a good pair is not revealed until the gas transport experiments are performed. This is because there is a lack of characterization techniques to directly probe the MOF-polymer interfacial compatibility. In this work, we demonstrate a self-sorting method to rank the interface compatibility among several MOF-polymer pairs. By mixing one MOF with two polymers in an MMM, the demixing of two polymers will form two polymer domains. The MOF particles will preferably partition into the "preferred" polymer domain due to their higher interfacial affinity. By scanning different polymer pairs, a rank of MOF-polymer interfacial compatibility from high to low can be obtained. Moreover, based on this ranking, it was also found that a highly compatible MOF-polymer pair suggested by this method also corresponds to a more predictable MMM gas separation performance.

Defect-engineered metal-organic frameworks (MOFs) with outstanding structural and chemical features have become excellent candidates for specific separation applications. The introduction of structural defects in MOFs as an efficient approach to manipulate their functionality provides excellent opportunities for the preparation of MOF-based mixed matrix membranes (MMMs). However, the use of this strategy to adjust the properties and develop the separation performance of gas separation membranes is still in its early stages. Here, a novel defect-engineered MOF (quasi ZrFum or Q-ZrFum) was synthesized via a controlled thermal deligandation process and incorporated into a CO2-philic 6FDA-durene polyimide (PI) matrix to form Q-ZrFum loaded MMMs. Defect-engineered MOFs and fabricated MMMs were investigated regarding their characteristic properties and separation performance. The incorporation of defects into the MOF structure increases the pore size and provides unsaturated active metal sites that positively affect CO2 molecule transport. The interfacial compatibility between the Q-ZrFum particles and the PI matrix increases via the deligandation process, which improves the mechanical strength of Q-ZrFum loaded membranes. MMM containing 5 wt.% of defect-engineered Q-ZrFum exhibits excellent CO2 permeability of 1308 Barrer, which increased by 99 % compared to the pure PI membrane (656 Barrer) at a feed pressure of 2 bar. CO2/CH4 and CO2/N2 selectivity reached 44 and 26.6 which increased by about 70 and 16 %, respectively. This study emphasizes that defect-engineered MOFs can be promising candidates for use as fillers in the preparation of MMMs for the future development of membrane-based gas separation applications.

The efficient separation of gases is a subject of considerable interest due to economic and environmental threats associated with air pollution, and is an imperative to meet energy demands of the world. Membrane based gas separation is considered as an efficient, productive, readily scalable, and environmentally friendly process that can operate in a continuous fashion. The recent advances have shifted towards the development of mixed matrix membranes (MMM), due to the challenges with the current spectrum of polymeric and inorganic membranes. MMMs have been commonly prepared by incorporating inorganic fillers such as zeolites or metal organic frameworks in a continuous polymer matrix. However, the success of MMMs depends greatly on the screening and selection of suitable polymer matrix, inorganic filler and interaction between them. Though the past decade has witnessed substantial progress in both the fundamental and application aspects of MMMs in gas separation, interface problems such as the formation of non-selective voids, rigidified polymer and pore blockage due to poor interaction between the polymer and inorganic filler are still challenging. Hence, understanding and minimising interfacial barriers between the polymer and the inorganic filler are critical to the design and optimisation of MMMs; however, trial and error experimentation is required to address these non-ideal interface issues. On the other hand, atomistic simulations have become an important tool in the screening and selection of suitable materials in MMMs. The present thesis aims to develop a fundamental knowledge of the polymer structure near a surface, and thus facilitate the design of MMMs, especially for gas-separation.Firstly, the morphology of the polyimide (PI) polymer membrane is characterized by exploring the volume-temperature relations, distribution of free volume elements in the polymer and available free volume analysis. Then, the separation performance of a PI membrane in pure gas conditions is investigated. Gas sorption isotherms were extracted via a two-step methodology considering the dynamics and structural transitions in the polymer matrix upon gas sorption using a combination of equilibrium molecular dynamics (EMD) in the constant pressure ensemble and grand canonical Monte Carlo simulations. The gas transport behavior in the polymer membrane is evaluated by extracting Maxwell-Stefan diffusivities, and found to be consistent with experimental evidence.Further, the separation performance of a polymer membrane in mixed gas conditions is investigated by considering an equimolar mixture of CO2 and CH4 in a fluorinated PI polymer membrane. Significant swelling of the polymer in the presence of CO2 is found, as a result of which the predictions of traditional models such as ideal adsorption solution theory and dual mode sorption for mixed gases in mixed gas conditions are inaccurate, particularly for CH4. The Onsager coefficients indicate that in mixed gas conditions finite correlations exist between the diffusing species in the polymer membrane. Further, the swollen membrane is diffusive selective for CH4 at high pressures in mixtures due to availability of large pores, in contrast to pure gas conditions where the membrane is diffusive selective for CO2 over CH4 at all pressures. Analysis of membrane behavior under practical conditions using EMD-based transport coefficients shows that while the CO2/CH4 perm-selectivity increases with increase in pressure based on pure component data, the trend is opposite for mixture data. Thus, the commonly used approach of screening membrane materials based on pure component data can be misleading, as it overlooks the correlation effects arising from the presence of other species in the mixture.Subsequently, the structure of the PI in the vicinity MFI-zeolite, and its CO2/CH4 transport properties is investigated. It was found that incorporation of MFI zeolite into PI results in the formation of a densified polymer layer near the surface, having thickness around 1.2 nm, contradicting empirical suggestions of an approximately 1-micron thick interface between the polymer and filler. This interfacial region offers extra resistance to gas diffusion, which increases with kinetic diameter. Consequently, significant increase in CO2/CH4 selectivity as well as gas permeability is observed in the PI-MFI composite membrane compared to the pure PI polymer membrane, which is correlated with the high selectivity of the rigidified interfacial layer in the polymer. Thus, while enhancing transport resistance, the rigidified layer can be beneficial to membrane selectivity.Finally, the structure of a PI in the vicinity of the ZIF-8 surface is investigated. It is seen that incorporation of ZIF-8 into PI results in formation of sub-nanometer voids as defects near the polymer-filler interface. We then identified an ionic liquid (BMIM-BF4) which has favorable interactions with both ZIF-8 as well as polymer to achieve a defect-free interface, thus exhibiting superior gas separation performance compare to the pure polymer membrane.In summary, this thesis has developed a nanoscale understanding of polymer structure near a surface for the information necessary to design MMMs. This investigation also includes strategies to minimise the interfacial defects such as nano-scale voids to achieve separation performances surpassing the Robeson upper bound limit in MMM membranes.

MOF-based MMMs breaking the upper bounds of polymers for a large variety of gas separations

Metal-organic frameworks (MOFs) are promising candidates for membrane gas separation. MOF-based membranes include pure MOF membranes and MOF-based mixed matrix membranes (MMMs). This Perspective discusses the challenges for the next stage of the development of MOF-based membranes based on research conducted in the past decade. We focused on three major issues associated with pure MOF membranes. First, some MOF compounds have been overstudied, despite the availability of numerous MOFs. Second, gas adsorption and diffusion in MOFs are often independently investigated. The correlation between adsorption and diffusion has seldom been discussed. Third, we identify the importance of characterizing the gas distribution in MOFs to understand the structure-property relationships for gas adsorption and diffusion in MOF membranes. For MOF-based MMMs, engineering the MOF-polymer interface is essential for achieving the desired separation performance. Various approaches to modify the MOF surface or polymer molecular structure have been proposed to improve the MOF-polymer interface. Herein, we present defect engineering as a facile and efficient approach for engineering the MOF-polymer interfacial morphology and its extended application for various gas separations.

Influence of polymer modification on intra-MOF self-diffusion in MOF-based mixed matrix membranes

Herein, we report a facile approach to engineer a metal–organic framework (MOF) structure and polymer–MOF interface in a mixed-matrix membrane (MMM) to achieve high gas separation performance and plasticization resistance. Hierarchical ZIF-8-NH2 nanoparticles with a relatively small concentration of amine functionality (∼5 mol %) were prepared. The hierarchical MOF structure provides fast molecular transport pathways because of the MOF–MOF percolated network and high MOF packing density. Moreover, the interfacial interaction between the 6FDA-DAM:DABA(3:2) polyimide and the amine-functionalized MOF enhances the chemical stability and polymer chain rigidity. The hierarchical ZIF-8-NH2 MMMs exhibit a significantly improved gas permeability because of the accelerated molecular diffusion through the direction-oriented MOF pathway. For example, the ZIF-8-NH2 30 wt % MMM showed ∼6- and ∼4-fold enhanced H2 (761.7 Barrer) and CO2 (552.4 Barrer) permeabilities when compared to those of a pure polyimide film, with an H2/CH4 selectivity of 35.7 and CO2/CH4 selectivity of 25.9. Additionally, the MMM exhibits an improved plasticization resistance due to the connected MOF structure and MOF–polymer interaction. This strategy provides remarkable insight into the rational design of the polymer and MOF constituents in the MMM system.

Membrane separation is an energy efficient technology with a small physical footprint in which the membrane is the core of process. Membranes need to be further developed to be specifically applied in the field of gas separation. The most challenging target in designing membranes is to improve the permeation and selectivity, simultaneously. This goal cannot be achieved without acquiring the knowledge of material science to tune the membrane material properties. This PhD thesis focusses on designing mixed matrix membranes (MMMs) by using a new class of crystalline materials known as metal organic frameworks (MOFs) as filler. In combination with polymers as continuous phase it was expected to improve both the processability and separation performance of this composite material in comparison with the polymer only. This work has been performed in the framework of the FP7-EU project M4CO2 ('MOF-based Mixed Matrix Membranes for energy efficient CO2 capture', grant agreement n° 608490). Therefore the focus in this thesis was on, but not limited to, membranes for the separation of CO2 from N2, as a model for stack gases in coal combustion ('post-combustion separation'). To this aim, the overall concept of this thesis is divided into three parts in which the most relevant aspects of design in mixed matrix membranes are carefully studied. Part I (Chapter 2) elucidated the influence of MOF pore structure and topology on the MMMs separation performance. In part II (Chapter 3 and 4) the effect of MOF morphology and polymer free volume is studied. Finally, part III (Chapter 5) reports a study on free-standing and thin supported MOF nanosheet based membranes by using industrially viable methods. The summary of each Chapter in this thesis is presented as follows...

Energy-efficient separation of gases has attracted intensive attentions both in research and in industry.nThe separation of gases by membranes is more effective and energy saving with lower productionnand equipment cost than some traditional gas separation methods, such as adsorption or distillation.nHowever, most of the polymeric membranes suffer from the trade-off between mass transport ratesnand separation efficiency. Polymeric membranes show high gas permeation flux but low selectivity,nand vice versa. To overcome such weakness, mixed matrix membranes (MMMs) can providenpromising potentials in high performance gas separation, by combining high separation properties ofnthe inorganic filler with low cost and flexible of the polymers. For filler selection, metal-organicnframeworks (MOFs) are promising adsorbents for gas storage and separation due to their high surfacenarea and porosity, adjustable pore sizes and controllable surface functionality.This thesis is focused on developing novel MOFs-based MMMs for gas separation with highnpermeability and selectivity, as well as good thermal and chemical stability. The studies includenfabrication and optimization of MOFs-based MMMs, designing MMMs with good filler/polymerninteraction and good interfacial morphology, as well as evaluating the permeation and selectivitynperformance of all the prepared membranes. It aims to establish the guidance for the MOF/polymernpair selection and interface manipulation in the fabrication of MMMs to greatly increase thenmembrane permeability and selectivity.In the first part of the experimental chapters, novel MMM with dispersed MOF into polyimide matrixnwere fabricated and the derived membranes are employed for gas separation. MMMs werensynthesized from 2,2-bis(3,4-anhydrodicarboxyphenyl) hexafluoropropane (6FDA) and 4,4(-diaminodiphenyl ether (oxydianiline, ODA) into which were incorporated Cd-6F MOF filler. Cd-6Fnwas synthesized by using 4,4p-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) as the organicnlinker which is also one of the monomers in the 6FDA-ODA synthesis. A specific interfacialninteraction between MOF crystals and polymer chains was innovatively targeted with this specificnMOF/polymer pair by controlling the in-situ 6FDA-ODA polymerization procedure with thenexistence of MOF particles. The enhanced adhesion between filler particles and polymer phase wasnachieved and the improved interfacial interaction between MOF and polymeric matrix was confirmednby FTIR, NMR and XPS. Moreover, it was found that the MOF/polymer interfacial morphologynstrongly affects the gas permeability and selectivity of the membrane. The MMM prepared by in-situnpolymerization displays excellent interfaces between micron-ized Cd-6F crystals and polymernmatrix as well as increased permeability and selectivity compared to pure 6FDA-ODA polymericmembrane. The interaction between MOF crystals and polymeric matrix can be controlled for theninterfacial voids elimination and optimal membrane transport properties.The second part of the experimental chapters focuses on MMMs with MOF/CNT composite. NovelnMMM filler CNT-MIL was synthesized by in-situ growth of NH2-MIL-101(Al) on the externalnsurface of carbon nanotubes (CNT) and applied to fabricate MMMs for CO2/CH4 separation. Extranamino groups and active sites were introduced to the surface of CNT. Compared to pure polymericnmembrane, MMMs containing synthesized MOF/CNT composite displayed significantly increasednCO2 permeance (up to 150%) and selectivity (up to 37.5%). The separation performance of derivednMMMs clearly transcends the 1991 upper bound and being on the 2008 upper bound for polymericnmembrane performance. MMMs for efficient C3H6/C3H8 separation were fabricated by embeddingnZIF-8/CNT composite into 6FDA-durene polymer matrix. The volume of filler and voids in thenpolymeric matrix as well as the distribution of the fillers and their contact with the polymeric matrixnwere quantitatively evaluated by using the tomographic focused ion beam scanning electronnmicroscopy (FIB-SEM). The dispersion of ZIF-8 in 6FDA-durene polymer was enhanced by growthnof ZIF-8 on the CNT surfaces. Meanwhile, good adhesion between the synthesized MOF/CNT fillersnand polymer matrix was observed, only 0.086% voids volume fraction is determined, even at a highnfiller loading. The improved ZIF-8 dispersion and filler/polymer interface lead to the efficient C3H6separation in MMMs. MMMs containing synthesized MOF/CNT composite displayed significantlynincreased C3H6 permeability (up to 105%) and selectivity (up to 96%) compare to pure 6FDA-durenenmembrane.The third part of the experimental chapters focuses on elimination of interfacial voids bynincorporating ionic liquid (IL) as the MOFs/polymer interfacial binder into MMM. Thin layer of ILnhas been fabricated on CuBTC fillers for MMMs fabrication. With the aid of ionic liquid, theninterfacial voids of the CuBTC-IL MMM was significantly reduced due to the favourable MOF/ILnand IL/polymer adhesion, thus improving CO2 selectivity compare to pure polymer membrane andnMMMs with untreated CuBTC. The performance of 10% CuBTC-IL/6FDA-durene MMM clearlyntranscends the 2008 upper bound for polymer membrane. The strategy of IL decoration method cannprovide an effective way to eliminate interfacial voids and enhance CO2 selectivity and can be appliednin large sized fillers with better gas separation properties.n

Mixed-matrix membranes (MMMs) have emerged as a promising approach for developing new, stable, and highly effective gas and liquid separation materials. MMMs combine porous crystalline framework materials, such as Metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and hydrogen-bonded organic frameworks (HOFs), as fillers incorporated in a polymer matrix. This article comprehensively reviews MMM research, discussing the structure and properties of MOFs, COFs, and HOFs and their attractiveness for use in MMMs. The article also reviews the use of mixed matrix filtration membranes with MOFs, COFs, and HOFs for various water treatment and gas separation applications. The potential of MMMs for meeting the needs of different industries is demonstrated through the discussion of specific examples. Overall, this article highlights the significant potential of MMM technology for developing next-generation separation materials and attempts to cover the most recent progress in the design and deployment of MOFs, COFs and HOFs-based MMMs, as are the remaining obstacles and prospects. This work also highlights the enormous potential of these materials for separation applications and raises attention toward the economic aspect and market diffusion of such MMMs.

The membrane process stands as a promising and transformative technology for efficient gas separation due to its high energy efficiency, operational simplicity, low environmental impact, and easy up-and-down scaling. Metal-organic framework (MOF)-polymer mixed matrix membranes (MMMs) combine MOFs' superior gas-separation performance with polymers' processing versatility, offering the opportunity to address the limitations of pure polymer or inorganic membranes for large-scale integration. However, the incompatibility between the rigid MOFs and flexible polymer chains poses a challenge in MOF MMM fabrication, which can cause issues such as MOF agglomeration, sedimentation, and interfacial defects, substantially weakening membrane separation efficiency and mechanical properties, particularly gas separation. This review focuses on engineering MMMs' interfaces, detailing recent strategies for reducing interfacial defects, improving MOF dispersion, and enhancing MOF loading. Advanced characterisation techniques for understanding membrane properties, specifically the MOF-polymer interface, are outlined. Lastly, it explores the remaining challenges in MMM research and outlines potential future research directions.

Gas separation and purification using polymeric membranes is a promising technology that constitutes an energy-efficient and eco-friendly process for large scale integration. However, pristine polymeric membranes typically suffer from the trade-off between permeability and selectivity represented by the Robeson's upper bound. Mixed matrix membranes (MMMs) synthesized by the addition of porous nano-fillers into polymer matrices, can enable a simultaneous increase in selectivity and permeability. Among the various porous fillers, metal-organic frameworks (MOFs) are recognized in recent days as a promising filler material for the fabrication of MMMs. In this article, we review representative examples of MMMs prepared by dispersion of MOFs into polymer matrices or by deposition on the surface of polymeric membranes. Addition of MOFs into other continuous phases, such as ionic liquids, are also included. CO2 separation from hydrocarbons, H2, N2, and the like is emphasized. Hybrid fillers based on composites of MOFs with other nanomaterials, e.g., of MOF/GO, MOF/CNTs, and functionalized MOFs, are also presented and discussed. Synergetic effects and the result of interactions between filler/matrix and filler/filler are reviewed, and the impact of filler and matrix types and compositions, filler loading, surface area, porosity, pore sizes, and surface functionalities on tuning permeability are discoursed. Finally, selectivity, thermal, chemical, and mechanical stability of the resulting MMMs are analyzed. The review concludes with a perspective of up-scaling of such systems for CO2 separation, including an overview of the most promising MMM systems.

MOF-74 (metal-organic framework) is utilized as a filler in mixed-matrix membranes (MMMs) to improve gas selectivity due to its unique one-dimensional hexagonal channels and high-density open metal sites (OMSs), which exhibit a strong affinity for CO2 molecules. Reducing the agglomeration of nanoparticles and improving the compatibility with the matrix can effectively avoid the existence of non-selective voids to improve the gas separation efficiency. We propose a novel, layer-by-layer modification strategy for MOF-74 with graphene oxide. Two-dimensional graphene oxide nanosheets as a supporting skeleton creatively improve the dispersion uniformity of MOFs in MMMs, enhance their interfacial compatibility, and thus optimize the selective gas permeability. Additionally, they extended the gas diffusion paths, thereby augmenting the dissolution selectivity. Compared with doping with a single component, the use of a GO skeleton to disperse MOF-74 into Pebax®1657 (Polyether Block Amide) achieved a significant improvement in terms of the gas separation effect. The CO2/N2 selectivity of Pebax®1657-MOF-74 (Ni)@GO membrane with a filler concentration of 10 wt% was 76.96, 197.2% higher than the pristine commercial membrane Pebax®1657. Our results highlight an effective way to improve the selective gas separation performance of MMMs by functionalizing the MOF supported by layered GO. As an efficient strategy for developing porous MOF-based gas separation membranes, this method holds particular promise for manufacturing advanced carbon dioxide separation membranes and also concentrates on improving CO2 capture with new membrane technologies, a key step in reducing greenhouse gas emissions through carbon capture and storage.

Selection of good filler-polymer pair results in improved gas separation performances. Present study deals with fabrication of Matrimid based hybrid or mixed matrix membranes (MMMs) by using three weight loadings (10, 20 and 30 wt. %) of 1D rod shaped nanoporous [Pr(BTC)(H2O)6] metal organic framework (MOF). The fourier transform infrared (FTIR) spectrophotometric analysis indicated the bonding of lanthanide metal ion with the linker and thus formation of the MOF, and further revealed that the structure of MOF remained intact after incorporation into the polymer matrix. Powder X-ray (PXRD) analysis revealed purity and crystallinity of the MOF while the PXRD results for the MMMs indicated that crystallinity of the MOF remained unaffected after fabrication of the MMMs. The Scanning electron microscopy (SEM) results gave an indication of the formation of the MOF in same morphology as it is previously reported. Additionally, nice distribution and well adherence of the additive particles throughout the polymer matrix was observed while the interfacial voids were visibly absent. The MOF and the MMMs were thermally stable and crystallinity of MOF particles remained intact after dispersion into polymer matrix. The Brunauer Emmett Teller (BET) surface area of the MOF was also obtained that indicated its permanent porosity. The prepared MMMs were flexible enough to handle for all analysis. These findings led to conclude that a proper polymer-filler pair was made that produced defect free MMMs. The prepared MMMs can be evaluated in future for their performance towards separation of different gases after modification of the pore of the filler

Due to the enormous increase in the number of metal-organic frameworks (MOFs), combining molecular simulations with machine learning (ML) would be a very useful approach for the accurate and rapid assessment of the separation performances of thousands of materials. In this work, we combined these two powerful approaches, molecular simulations and ML, to evaluate MOF membranes and MOF/polymer mixed matrix membranes (MMMs) for six different gas separations: He/H2, He/N2, He/CH4, H2/N2, H2/CH4, and N2/CH4. Single-component gas uptakes and diffusivities were computed by grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, respectively, and these simulation results were used to assess gas permeabilities and selectivities of MOF membranes. Physical, chemical, and energetic features of MOFs were used as descriptors, and eight different ML models were developed to predict gas adsorption and diffusion properties of MOFs. Gas permeabilities and membrane selectivities of 5249 MOFs and 31,494 MOF/polymer MMMs were predicted using these ML models. To examine the transferability of the ML models, we also focused on computer-generated, hypothetical MOFs (hMOFs) and predicted the gas permeability and selectivity of 1000 hMOF/polymer MMMs. The ML models that we developed accurately predict the uptake and diffusion properties of He, H2, N2, and CH4 gases in MOFs and will significantly accelerate the assessment of separation performances of MOF membranes and MOF/polymer MMMs. These models will also be useful to direct the extensive experimental efforts and computationally demanding molecular simulations to the fabrication and analysis of membrane materials offering high performance for a target gas separation.