50th Anniversary Perspective: Polymers and Mixed Matrix Membranes for Gas and Vapor Separation: A Review and Prospective Opportunities

Postcombustion Carbon Capture Using Thin-Film Composite Membranes

Climate change due to anthropogenic carbon dioxide emissions (e.g., combustion of fossil fuels) represents one of the most profound environmental disasters of this century. Equipping power plants with carbon capture and storage (CCS) technology has the potential to reduce current worldwide CO₂ emissions. However, existing CCS schemes (i.e., amine scrubbing) are highly energy-intensive. The urgent abatement of CO₂ emissions relies on the development of new, efficient technologies to capture CO₂ from existing power plants. Membrane-based CO₂ separation is an attractive technology that meets many of the requirements for energy-efficient industrial carbon capture. Within this domain, thin-film composite (TFC) membranes are particularly attractive, providing high gas permeance in comparison with conventional thicker (∼50 μm) dense membranes. TFC membranes are usually composed of three layers: (1) a bottom porous support layer; (2) a highly permeable intermediate gutter layer; and (3) a thin (<1 μm) species-selective top layer. A key challenge in the development of TFC membranes has been to simultaneously maximize the transmembrane gas permeance of the assembled membrane (by minimizing the gas resistance of each layer) while maintaining high gas-specific selectivity. In this Account, we provide an overview of our recent development of high-performance TFC membrane materials as well as insights into the unique fabrication strategies employed for the selective layer and gutter layer. Optimization of each layer of the membrane assembly individually results in significant improvements in overall membrane performance. First, incorporating nanosized fillers into the selective layer (poly(ethylene glycol)-based polymers) and reducing its thickness (to ca. 50 nm) through continuous assembly of polymers technology yields major improvements in CO₂ permeance without loss of selectivity. Second, we focus on optimization of the middle gutter layer of TFC membranes. The development of enhanced gutter layers employing two- and three-dimensional metal-organic framework materials leads to considerable improvements in both CO₂ permeance and selectivity compared with traditional poly(dimethylsiloxane) materials. Third, incorporation of a porous, flexible support layer culminates in a mechanically robust high-performance TFC membrane design that exhibits unprecedented CO₂ separation performance and holds significant potential for industrial CO₂ capture. Alternative strategies are also emerging, whereby the selective layer and gutter layer may be combined for enhanced membrane efficiency. This Account highlights the CO₂ capture performance, current challenges, and future research directions in designing high-performance TFC membranes.

Mixed matrix membranes for hydrocarbons separation and recovery: a critical review

The separation and purification of light hydrocarbons are significant challenges in the petrochemical and chemical industries. Because of the growing demand for light hydrocarbons and the environmental and economic issues of traditional separation technologies, much effort has been devoted to developing highly efficient separation techniques. Accordingly, polymeric membranes have gained increasing attention because of their low costs and energy requirements compared with other technologies; however, their industrial exploitation is often hampered because of the trade-off between selectivity and permeability. In this regard, high-performance mixed matrix membranes (MMMs) are prepared by embedding various organic and/or inorganic fillers into polymeric materials. MMMs exhibit the advantageous and disadvantageous properties of both polymer and filler materials. In this review, the influence of filler on polymer chain packing and membrane sieving properties are discussed. Furthermore, the influential parameters affecting MMMs affinity toward hydrocarbons separation are addressed. Selection criteria for a suitable combination of polymer and filler are discussed. Moreover, the challenges arising from polymer/filler interactions are analyzed to allow for the successful implementation of this promising class of membranes.

Graphene-Based Membranes for CO2/CH4 Separation: Key Challenges and Perspectives

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.

Mixed-Matrix Membranes Formed from Multi-Dimensional Metal-Organic Frameworks for Enhanced Gas Transport and Plasticization Resistance

Mixed-matrix membranes (MMMs) formed by incorporating metal-organic frameworks (MOFs) into polymers have a general limitation in that the MOFs are typically formed into rather simple dimensionalities (such as 1D, 2D, or 3D). Each design approach has intrinsic-albeit independent-benefits, such as network percolation (1D), access to high-aspect ratios (2D), and ease of processability (3D). However, a design strategy is needed to combine multiple dimensionalities and, thereby, access the full range of transport and compositing benefits of these high-performance materials. Herein, a facile method to form multi-dimensional HKUST-1 nanoparticles is introduced by using a modulator to tune the MOF nucleation and growth mechanism. At 30 wt % multidimensional MOF loading, the MMM shows CO₂ permeabilities of approximately 2500 Barrer, which represents a 2.5-fold increase compared to that of a pure polymer without a large loss of selectivity for CO₂ /CH₄ and CO₂ /N₂ . Additionally, almost no plasticization pressure response is observed for CO₂ up to 750 psi, suggesting an unusual stability to high activity feeds.

Alternatives to Cryogenic Distillation: Advanced Porous Materials in Adsorptive Light Olefin/Paraffin Separations

As primary feedstocks in the petrochemical industry, light olefins such as ethylene and propylene are mainly obtained from steam cracking of naphtha and short chain alkanes (ethane and propane). Due to their similar physical properties, the separations of olefins and paraffins-pivotal processes to meet the olefin purity requirement of downstream processing-are typically performed by highly energy-intensive cryogenic distillation at low temperatures and high pressures. To reduce the energy input and save costs, adsorptive olefin/paraffin separations have been proposed as promising techniques to complement or even replace cryogenic distillation, and growing efforts have been devoted to developing advanced adsorbents to fulfill this challenging task. In this Review, a holistic view of olefin/paraffin separations is first provided by summarizing how different processes have been established to leverage the differences between olefins and paraffins for effective separations. Subsequently, recent advances in the development of porous materials for adsorptive olefin/paraffin separations are highlighted with an emphasis on different separation mechanisms. Last, a perspective on possible directions to push the limit of the research in this field is presented.

Synthesis of Soluble Metal Organic Framework Composites for Mixed Matrix Membranes

A general, green, efficient, and easily scalable methodology has been developed to more effectively incorporate (disperse) metal-organic frameworks (MOFs) into polymer technologies via solid state synthesis of any MOF nanocrystals within soluble mesoporous polymers. The resulting solid hybrid materials (pellets) can be directly transformed into colloidal MOF polymeric suspensions (inks) by simple dissolution in organic solvents. The straightforward use of novel colloidal MOF polymeric inks as ultimate additive for mixed matrix membranes resulted in unprecedented snakeskin microstructure exhibiting outstanding selectivity for CO₂ over N₂ (>100) from post-combustion flue gas at very low and well-dispersed MOF nanocrystal concentrations ranging from 1 to 7 wt %. This novel methodology brings one of the most versatile routes yet reported to transform any MOF into more functional forms that can be directly integrated into any conventional polymer technology at the commercial scale.

Polymers with Side Chain Porosity for Ultrapermeable and Plasticization Resistant Materials for Gas Separations

Polymer membranes with ultrahigh CO₂ permeabilities and high selectivities are needed to address some of the critical separation challenges related to energy and the environment, especially in natural gas purification and postcombustion carbon capture. However, very few solution-processable, linear polymers are known today that access these types of characteristics, and all of the known structures achieve their separation performance through the design of rigid backbone chemistries that concomitantly increase chain stiffness and interchain spacing, thereby resulting in ultramicroporosity in solid-state chain-entangled films. Herein, the separation performance of a porous polymer obtained via ring-opening metathesis polymerization is reported, which possesses a flexible backbone with rigid, fluorinated side chains. This polymer exhibits ultrahigh CO₂ permeability (>21 000 Barrer) and exceptional plasticization resistance (CO₂ plasticization pressure > 51 bar). Compared to traditional polymers of intrinsic microporosity, the rate of physical aging is slower, especially for gases with small effective diameters (i.e., He, H₂ , and O₂ ). This structural design strategy, coupled with studies on fluorination, demonstrates a generalizable approach to create new polymers with flexible backbones and pore-forming side chains that have unexplored promise for small-molecule separations.

Transition from a 1D Coordination Polymer to a Mixed-Linker Layered MOF

A novel copper(II) metal-organic framework (MOF) has been synthesized by modifying the reaction conditions of a 1D coordination polymer. The 1D polymer is built by the coordination between copper and 2,2'-(1 H-imidazole-4,5-diyl)di-1,4,5,6-tetrahydropyrimidine (H-L1). The geometry of H-L1 precludes its ability to form extended 3D framework structures. By adding 1,4-benzenedicarboxylic acid (H₂BDC), a well-studied linker in MOF synthesis, we achieved the transition from a 1D polymer chain into porous 2D layered structures. Hydrogen bonding between L1 and BDC directs the parallel stacking of these layers, resulting in a 3D structure with one-dimensional channels accessible by two different pore windows. The preferred growth orientation of the crystal produces prolonged channels and a disparity in pore size distribution. This in turn results in slow diffusion processes in the material. Furthermore, an isoreticular MOF was prepared by substituting the BDC linker by 2,6-naphthalenedicarboxylic acid (H₂NDC).

Thermally Stable Cross-linked P84 with Superior Membrane H2/CO2 Separation Properties at 100 °C

Polymers with a strong size-sieving ability and superior H₂/CO₂ selectivity are of great interests for pre-combustion CO capture at 100 °C or above. Polyimides (such as Matrimid^® and 6FDA-durene) have been cross-linked using diamines and show superior H₂/CO₂ selectivity. However, these cross-linked polymers cannot be used for the pre-combustion CO₂ capture because of the lack of thermal stability at 100 C. Herein we demonstrate that commercial P84™ can be chemically cross-linked using 1,4-butanediamine (BuDA) to achieve robust H₂/CO₂ separation properties at 100 °C to 150 °C. The cross-linked P84 were thoroughly evaluated using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The effects of the cross-linking time on the physical properties and H₂/CO₂ separation properties at various temperatures were determined and interpreted using a free volume model. An exemplary sample based on P84 crosslinked by BuDA for 6 h exhibits a H₂ permeability of 47 Barrers (1 Barrer = 3.35 × 10^-16 mol m/m²·s·Pa) and H₂/CO₂ selectivity of 14 at 100 °C, which is on the Robeson's upper bound, indicating their potential for practical applications.

Maximizing Ether Oxygen Content in Polymers for Membrane CO2 Removal from Natural Gas

Membrane materials for CO₂ removal from natural gas are based on glassy polymers with a high CO₂/CH₄ diffusivity selectivity. However, these polymers suffer from competitive sorption by heavy hydrocarbons that decreases CO₂ permeability and physical aging that reduces gas permeability with time. We circumvent these issues by designing rubbery, solubility-selective polymers with a ratio of ether/ester oxygen to carbon as high as 0.8 through the use of 1,3-dioxolane and 1,3,5-trioxane. The ether/ester oxygen groups interact favorably with CO₂ but do not interact with CH₄, leading to a high CO₂/gas solubility selectivity that is unaffected by heavy hydrocarbons in the raw natural gas. These polar groups are incorporated in short branches to yield an amorphous and rubbery nature, leading to high gas permeability that is stable over time. A polymer with an O/C ratio of 0.71 (P71) shows a mixed-gas CO₂ permeability of 320 Barrers and a CO₂/CH₄ selectivity of 21 in the simulated natural gas at 50 °C, which is independent of the hexane content and above the upper bound for CO₂/CH₄ separation at 50 °C.

High-Frequency Mechanical Behavior of Pure Polymer-Grafted Nanoparticle Constructs

Polymer-grafted nanoparticle (GNP) membranes show increased gas permeability relative to pure polymer analogs, with this effect evidently tunable through systematic variations in the grafted polymer chain length and grafting density. Additionally, these materials show less deleterious aging effects relative to the pure polymer. To better understand these issues, we explore the solid-state mechanical properties of GNP layers using quartz crystal microbalance (QCM) spectroscopy, which operates under conditions (≈5 MHz) that we believe are relevant to gas transport. The GNP's high-frequency storage moduli exhibit a characteristic increase with increasing nanoparticle (NP) core loading, consistent with past work on the reinforcement of polymers physically well mixed with bare NPs. However, these GNPs show a substantial, nonmonotonic decrease in loss as a function of chain length (at fixed grafting density), with the loss minimum corresponding to the chain length with the maximum gas permeability. We speculate that this feature corresponds to a dynamical transition, where the GNP membranes go from a jammed solid (colloid-like) to liquid-like (polymer-controlled) behavior with increasing chain length.

Solvation Effects on the Permeation and Aging Performance of PIM-1-Based MMMs for Gas Separation

Membranes are particularly attractive for lowering the energy intensity of separations as they eliminate phase changes. While many tantalizing polymers are known, limitations in selectivity and stability slightly preclude further development. Mixed-matrix membranes may address these shortcomings. Key to their realization is the intimate mixing between the polymer and the additive to eliminate nonselective transport, improve selectivity, and resist physical aging. Polymers of intrinsic microporosity (PIMs) have inherently promising gas transport properties. Here, we show that porous additives can improve transport and resist aging in PIM-1. We develop a simple, low-cost, and scalable hyper-cross-linked polymer (poly-dichloroxylene, pDCX), which was hydroxylated to form an intimate mixture with the polar PIM-1. Solvent variation allowed control of physical aging rates and improved selectivity for smaller gases. This detailed study has allowed many interactions within mixed matrix membranes to be directly elucidated and presents a practical means to stabilize porous polymers for separation applications.

Sorption Thermodynamics of CO₂, H₂O, and CH₃OH in a Glassy Polyetherimide: A Molecular Perspective

In this paper, the sorption thermodynamics of low-molecular-weight penetrants in a glassy polyetherimide, endowed with specific interactions, is addressed by combining an experimental approach based on vibrational spectroscopy with thermodynamics modeling. This modeling approach is based on the extension of equilibrium theories to the out-of-equilibrium glassy state. Specific interactions are accounted for in the framework of a compressible lattice fluid theory. In particular, the sorption of carbon dioxide, water, and methanol is illustrated, exploiting the wealth of information gathered at a molecular level from Fourier-transform infrared (FTIR) spectroscopy to tailor thermodynamics modeling. The investigated penetrants display a different interacting characteristic with respect to the polymer substrate, which reflects itself in the sorption thermodynamics. For the specific case of water, the outcomes from molecular dynamics simulations are compared with the results of the present analysis.

Modelling Mixed-Gas Sorption in Glassy Polymers for CO₂ Removal: A Sensitivity Analysis of the Dual Mode Sorption Model

In an effort to reduce the experimental tests required to characterize the mixed-gas solubility and solubility-selectivity of materials for membrane separation processes, there is a need for reliable models which involve a minimum number of adjustable parameters. In this work, the ability of the Dual Mode Sorption (DMS) model to represent the sorption of CO₂/CH₄ mixtures in three high free volume glassy polymers, poly(trimethylsilyl propyne) (PTMSP), the first reported polymer of intrinsic microporosity (PIM-1) and tetrazole-modified PIM-1 (TZ-PIM), was tested. The sorption of gas mixtures in these materials suitable for CO₂ separation has been characterized experimentally in previous works, which showed that these systems exhibit rather marked deviations from the ideal pure-gas behavior, especially due to competitive effects. The accuracy of the DMS model in representing the non-idealities that arise during mixed-gas sorption was assessed in a wide range of temperatures, pressures and compositions, by comparing with the experimental results available. Using the parameters obtained from the best fit of pure-gas sorption isotherms, the agreement between the mixed-gas calculations and the experimental data varied greatly in the different cases inspected, especially in the case of CH₄ absorbed in mixed-gas conditions. A sensitivity analysis revealed that pure-gas data can be represented with the same accuracy by several different parameter sets, which, however, yield markedly different mixed-gas predictions, that, in some cases, agree with the experimental data only qualitatively. However, the multicomponent calculations with the DMS model yield more reliable results than the use of pure-gas data in the estimation of the solubility-selectivity of the material.

Membranes with Intrinsic Micro-Porosity: Structure, Solubility, and Applications

Microporous polymer membranes have been widely studied because of their excellent separation performance. Among them, polymers of intrinsic micro-porosity (PIMs) have been regarded as a potential next-generation membrane material for their ultra-permeable characteristics and their solution-processing ability. Therefore, many reviews have been reported on gas separation and monomers for the preparation of PIMs. This review aims to provide an overview of the structure-solubility property. Different structures such as non-network and network macromolecular structure made of different monomers have been reviewed. Then their solubility with different structures and different separation applications such as nanofiltration, pervaporation, and gas/vapor separation are summarized. Lastly, we also provide our perspectives on the challenges and future directions of the microporous polymer membrane for the structure-property relationship, anti-physical aging, and more.

Preparation of Mixed Matrix Membranes Containing ZIF-8 and UiO-66 for Multicomponent Light Gas Separation

Mixed matrix membranes (MMMs) containing zeolitic imidazolite framework-8 (ZIF-8) and UiO-66 as microporous fillers were prepared and evaluated their potential for the separation of a gas mixture produced by a methane reforming process. Hydrothermal synthesis was performed to prepare both the ZIF-8 and UiO-66 crystals, with crystal sizes ranging from 50 to 70 nm for ZIF-8 and from 200 to 300 nm for UiO-66. MMMs were prepared with 15% filler loading for both MMM (ZIF-8) and MMM (UiO-66). MMM (UiO-66) exhibited H2 permeability of 64.4 barrer and H2/CH4 selectivity of 153.3 for single gas permeation, which are more than twice the values that were exhibited by a neat polymer membrane. MMM (ZIF-8) also showed better separation properties than that of a neat polymer membrane with H2 permeability of 27.1 barrer and H2/CH4 selectivity of 123.2. When a gas mixture consisting of 78% Ar/18% H2/4% CH4 flowed into the membranes at 5 bar, the H2 purity increased to as high as 93%. However, no improvement in the mixture gas separation performance was achieved by the MMMs as compared to that of a neat polymer membrane.

Zeolitic Imidazolate Framework Membranes for Light Olefin/Paraffin Separation

Propylene/propane and ethylene/ethane separations are performed by energy-intensive distillation processes, and membrane separation may provide substantial energy and capital cost savings. Zeolitic imidazolate frameworks (ZIFs) have emerged as promising membrane materials for olefin/paraffin separation due to their tunable pore size and chemistry property, and excellent chemical and thermal stability. In this review, we summarize the recent advances on ZIF membranes for propylene/propane and ethylene/ethane separations. Membrane fabrication methods such as in situ crystallization, seeded growth, counter-diffusion synthesis, interfacial microfluidic processing, vapor-phase and current-driven synthesis are presented. The gas permeation and separation characteristics and membrane stability are also discussed.

Defining the optimal criterion for separating gases using polymeric membranes

Polymeric membranes are efficient in separating gas mixtures, typically by exploiting the sieving mechanism. What controls the sieve size of a given polymer matrix is unclear, although one line of thought implies that the local cage size, defined by the dynamic motions of the glassy polymer matrix, is the relevant metric. Here, we use coarse-grained molecular dynamics simulations and show that the sieve size is defined by a static cavity size controlled by polymer chain stiffness (a packing-driven metric) combined with the local cage-like motions of the polymer host. The best separation performance for a pair of gases is when this combined metric is roughly half way between the diameters of the gases in question, with the static and dynamic quantities contributing roughly equally. For the various models simulated we find the existence of an upper bound correlation which passes through this optimal point and has a slope expected from the Freeman model, namely , where the d's correspond to the kinetic diameters of the gases in question. Our results thus demonstrate that the relevant free volume size that affects gas transport in these condensed phases is defined by both static and dynamic measures.

Membrane Filtration with Liquids: A Global Approach with Prior Successes, New Developments and Unresolved Challenges

After 70 years, modern pressure-driven polymer membrane processes with liquids are mature and accepted in many industries due to their good performance, ease of scale-up, low energy consumption, modular compact construction, and low operating costs compared with thermal systems. Successful isothermal operation of synthetic membranes with liquids requires consideration of three critical aspects or "legs" in order of relevance: selectivity, capacity (i.e. permeation flow rate per unit area) and transport of mass and momentum comprising concentration polarization (CP) and fouling (F). Major challenges remain with respect to increasing selectivity and controlling mass transport in, to and away from membranes. Thus, prediction and control of membrane morphology and a deep understanding of the mechanism of dissolved and suspended solute transport near and in the membrane (i.e. diffusional and convective mass transport) is essential. Here, we focus on materials development to address the relatively poor selectivity of liquid membrane filtration with polymers and discuss the critical aspects of transport limitations. Machine learning could help optimize membrane structure design and transport conditions for improved membrane filtration performance.

Interfacial Engineering in Metal-Organic Framework-Based Mixed Matrix Membranes Using Covalently Grafted Polyimide Brushes

Good interfacial compatibility is the key to realize the full potential of metal-organic framework-based mix matrix membranes for gas separation. Here we report a new approach that uses polyimide brushes covalently grafted on the MOF surface to engineer the MOF-polymer interface. Benefiting from the strong brush-brush interaction, polyimide grafted MOF particles can form a stand-alone membrane at 88 wt % MOF loading without the addition of polymeric matrix. Compared to traditional mixed-matrix membranes, the modified membranes exhibit improved ductility up to 472%, reduced interfacial tearing phenomenon under shear force, decreased matrix chain mobility, and improved plasticization resistance against CO₂. Most importantly, with increasing MOF loading, only the modified membranes exhibit simultaneous increase of selectivity and permeability for CO₂/N₂ and CO₂/CH₄ separation, following the trend predicted by the modified Maxwell model.