An integrated materials approach to ultrapermeable and ultraselective CO2 polymer membranes

Nanofilm Composite Membranes of Bottlebrush Poly(1,3-Dioxolane) Plasticized by Poly(Ethylene Glycol) for CO2/N2 Separation

Poly(1,3-dioxolane) has emerged as a leading membrane material for post-combustion CO2 capture due to its high ether oxygen content and strong affinity toward CO2. However, they are often cross-linked to inhibit crystallization, which makes them impossible to fabricate into industrial thin-film composite membranes. Herein, soluble and high molecular weight bottlebrush polymers (bPDXLA) are synthesized using reversible addition-fragmentation chain transfer polymerization and demonstrate the feasibility of fabricating nanofilm (≈100 nm) composite membranes (NCMs). Furthermore, bPDXLA can be plasticized using a miscible additive of poly(ethylene glycol) dimethyl ether (PEGDME) to improve CO2 permeability while retaining good CO2/N2 selectivity. For example, adding 20 mass% PEGDME improves CO2 permeance from 930 to 1300 GPU and decreases CO2/N2 selectivity from 74 to 53 at 25 °C; the membrane exhibits stable separation performance competitive with state-of-the-art commercial membranes. This work unveils a practical approach to designing uncross-linked, highly polar polymers for practical membrane gas separation and highlights a facile way to enhance performance by incorporating miscible plasticizers using industrial manufacturing processes.

Synthesis innovations for crystallizing covalent organic framework thin films on biological and non-biological substrates

Thin film technology has emerged as a pivotal field with numerous industrial applications. Depending on their properties-such as magnetic characteristics, conductivity, architectural structure, stability, and functional backbones-thin films are widely utilized in optoelectronics, thin-film coatings, solar cells, energy storage devices, semiconductors, and separation applications. However, for all these applications, thin films must be securely attached to specific substrates, and substrate compatibility with both the thin film and the film-growth process is crucial for optimal performance. In this review, we emphasize the significance of growing thin films, particularly covalent organic framework (COF) thin films, on suitable substrates tailored for various applications. For separation technologies, polymer thin films are commonly fabricated on porous polymeric or metal-based membranes. In contrast, thin films of metals and metal oxides are typically deposited on conducting substrates, serving as current collectors for energy storage devices. Semiconductor thin films, on the other hand, are often grown on silicon or glass substrates for transistor applications. Emerging COF thin films, with their tunable properties, well-defined pore channels, and versatile functional backbones, have demonstrated exceptional potential in separation, energy storage, and electronic and optoelectronic applications. However, the interplay between COF thin films and the substrates, as well as the compatibility of growth conditions, remains underexplored. Studies investigating COF thin film growth on substrates such as metals, metal oxides, glass, silicon, polymers, ITO, and FTO have provided insights into substrate properties that promote superior film growth. The quality of the film formed on these substrates significantly influences performance in applications. Additionally, we discuss the stabilization of biological substrates, like peptide-based biomimetic catalysts and enzymes, which often suffer from instability in non-aqueous environments, limiting their industrial use. Growing COF membranes on these biological substrates can enhance their stability under harsh conditions. We also highlight techniques for growing COF membranes on biological substrates, ensuring the preservation of their structural integrity and functional properties.

Behavior, mechanisms, and applications of low-concentration CO2 in energy media

This review explores the behavior of low-concentration CO2 (LCC) in various energy media, such as solid adsorbents, liquid absorbents, and catalytic surfaces. It delves into the mechanisms of diffusion, adsorption, and catalytic reactions, while analyzing the potential applications and challenges of these properties in technologies like air separation, compressed gas energy storage, and CO2 catalytic conversion. Given the current lack of comprehensive analyses, especially those encompassing multiscale studies of LCC behavior, this review aims to provide a theoretical foundation and data support for optimizing CO2 capture, storage, and conversion technologies, as well as guidance for the development and application of new materials. By summarizing recent advancements in LCC separation techniques (e.g., cryogenic air separation and direct air carbon capture) and catalytic conversion technologies (including thermal catalysis, electrochemical catalysis, photocatalysis, plasma catalysis, and biocatalysis), this review highlights their importance in achieving carbon neutrality. It also discusses the challenges and future directions of these technologies. The findings emphasize that advancing the efficient utilization of LCC not only enhances CO2 reduction and resource utilization efficiency, promoting the development of clean energy technologies, but also provides an economically and environmentally viable solution for addressing global climate change.

Polymer-MOF Network Enabling Ultrathin Coating for Post-Combustion Carbon Capture

Permeance-selectivity trade-off and high temperature resilience are key challenges in development of membranes for post-combustion carbon capture. While mixed matrix membranes (MMMs) consisting of polymers and metal-organic frameworks (MOFs) offer the potential to address the challenges, they are limited by the low loading of MOFs in the thin film layer. Herein, we propose an inverse synthesis strategy to form polymer-MOF networks by copolymerizing monomers with functionalized UiO-66 nanoparticles. This process yields a finely dispersed, easily processable solution, enabling defect-free, thin polymer-MOF coatings with up to 40 wt % MOF loading within the polyethylene oxide-based polymers on polyacrylonitrile supports. The membrane with 40 wt % MOF demonstrated a 212 % increment in CO2 permeance at 25 °C and maintained a selectivity of 20 at 60 °C, which is attributed to the stable diffusivity selectivity of MOFs at high temperature. Furthermore, the membrane was evaluated with mixed gas and 83 % relative humidity (RH) at 60 °C, achieving a CO2 permeance up to 2793 GPU and a CO2/N2 selectivity of 21.6. This work offers insights into the design of practical mixed matrix membranes, which not only paves the way towards energy efficient carbon capture from flue gas, but also provides more possibilities for other applications.

A review of metal-organic frameworks and polymers in mixed matrix membranes for CO2 capture

Polymeric membranes offer an appealing solution for sustainable CO2 capture, with potential for large-scale deployment. However, balancing high permeability and selectivity is an inherent challenge for pristine membranes. To address this challenge, the development of mixed matrix membranes (MMMs) is a promising strategy. MMMs are obtained by carefully integrating porous nano-fillers into polymeric matrices, enabling the simultaneous enhancement of selectivity and permeability. In particular, metal-organic frameworks (MOFs) have gained recognition as MMM fillers for CO2 capture. Here, a review of the current state, recent advancements, and challenges in the fabrication and engineering of MMMs with MOFs for selective CO2 capture is proposed. Key considerations and promising research directions to fully exploit the gas separation potential of MOF-based MMMs in CO2 capture applications are highlighted.

Packing Engineering of Zirconium Metal-Organic Cages in Mixed Matrix Membranes for CO2/CH4 Separation

Metal-organic cages (MOCs) have been considered as emerging zero-dimensional (0D) porous fillers to generate molecularly homogeneous MOC-based membrane materials. However, the discontinuous pore connectivity and low filler concentrations limit the improvement of membrane separation performance. Herein, we propose the dimension augmentation of MOCs in membranes using three-dimensional (3D) supramolecular MOC networks as filler materials in mixed matrix membranes (MMMs). We further explore the packing engineering of MOC networks to produce distinct polymorphs (α and β phases) for tailoring membrane performance. Synchrotron X-ray absorption and positron annihilation lifetime spectroscopy were employed to differentiate distinct MOC polymorphous networks within membranes. Gas permeation tests revealed that the corresponding MMMs showed superior CO2/CH4 separation performance, exceeding the Robeson upper bound. Our proposed approach is expected to enrich the repertoire of reticular chemistry pertaining to molecular-based networks.

Direct Preparation of Ultrathin Polymer Membranes on Porous Substrates for the Separation of Helium From Methane

Ultrathin polymer membranes on porous substrates exhibit excellent gas and ion permeability and have important applications in many fields, such as membrane separation and batteries. However, there is still a lack of facile and general methods for the direct preparation of ultrathin polymer membranes on porous substrates, especially from polymer solutions. Within this work, a new strategy to fabricate centimeter-size ultrathin polymer membranes (thickness down to 16 nm) is presented directly on porous supports by using the liquid-liquid interfacial spin-coating technique. The method allows the preparation of ultrathin polymer membranes with a wide range of polymers, and the membranes are smooth and intact without cracks. The helium/methane separation performance is evaluated. As the membrane thickness increased from 40 to 180 nm, the He/CH4 selectivity increased and then decreased. The ultrathin membrane provides a good He/CH4 selectivity of 8.8 and He permeance of 1 × 105 GPU, which is the highest value reported so far and more than 150 times higher than that of membranes with He/CH4 selectivity above eight in the literature. This new approach provides the possibility to explore the great potential of ultrathin membranes for separation.

Rubbery organic frameworks (ROFs) toward ultrapermeable CO2-selective membranes

The capture of CO2 is of high interest in our society representing an essential tool to mitigate man-made climate warming. Membrane technology applied for CO2 capture offers several advantages in terms of energy savings, simple operation, and easy scale-up. Glassy membranes are associated with low gas permeability that negatively affect on their industrial implementation. Oppositely, rubbery membranes offer high permeability, but their selectivity is low. Here we report rubbery organic frameworks (ROFs) combining the high permeability of soft matrices with the high sieving selectivity of molecular frameworks. The best performing membranes provide a CO2/N2 selectivity up to 104 with a CO2 permeability up to 1000 Barrer, representing relevant performances for industrial implementation. Water vapors have a positive effect on CO2 permeability, and the CO2/N2 selectivity is higher than in dry conditions, as most of CO2 gas emissions are present in fully humidified gas streams. The synergetic high permeability/selectivity performances are superior to that observed with current state-of-the-art polymeric membranes.

CO2 capture technology based on gas hydrate method: a review

At present, the problem of global warming is becoming increasingly serious, and one of the main culprits is the increasing amount of carbon dioxide emissions. Although the traditional carbon capture technologies can reduce the concentration of CO2 in the atmosphere, it has a series of problems such as high energy consumption, high cost, low efficiency or unfriendly environment. Hydrate-based carbon dioxide separation are considered to be a technology with great application and development prospects. Compared with the traditional method of carbon dioxide separation, the hydrate method has the advantages of simple process, low energy consumption and environmental friendliness. This review introduces the advantages of hydrate method compared with traditional carbon capture technologies, expounds the theory of carbon dioxide capture by hydrate, and the strengthening and improvement techniques of hydrate method, including thermodynamic promoter, kinetic promoter and mechanical reinforcement, and introduces the practical application of hydrate method in various fields.

Molecular Dynamics Simulations of Gas Transport Properties in Cross-Linked Polyamide Membranes: Tracing the Morphology and Addition of Silicate Nanotubes

This study employs molecular dynamics (MD) simulations to fundamentally provide insight into the role of cross-link density in the CO2 separation properties of interfacially polymerized polyamide (PA) membranes. For this purpose, two atomistic models of pure polyamide membranes with different cross-link densities are constructed by MD simulations to conceptually determine how the fractional free volume of polyamide affects the gas separation performance of the membrane. The PA membrane with a lower cross-link density (LCPA) shows a higher gas diffusion coefficient, a lower gas solubility coefficient, and a higher gas permeability than the PA membrane with a higher cross-link density (HCPA). Moreover, the pristine and modified silicate nanotubes (SNTs) as the fast gas transport channels are incorporated into the polyamide membranes to assess the effect of the SNT/PA interface chemistry on the CO2 separation properties of the membranes. SNTs are systematically modified by three modifying agents with different CO2-philic groups and different interfacial interaction energies with the polyamide matrix. The results of MD simulations demonstrate that the incorporation of silicate nanotubes into the PA matrix increases the gas diffusivity and permeability and decreases the CO2/gas selectivity. Moreover, the membranes containing modified SNTs possessing high CO2-philicity and high SNTs/PA interfacial interactions show a high CO2 separation performance.

Organosiloxane membranes for heavy aromatic oil fractionation

The industrial separation of hydrocarbons relies on distillation. Organic solvent nanofiltration can provide an energy-efficient alternative. We prepared high performance organosiloxane membranes for fractionation of heavy aromatics. They achieved a high permeance up to 0.13 L m-2 h-1 bar-1, with a rejection rate of 88.7% for hydrocarbons with five aromatic rings.

Controlling Anti-Penetration Performance by Post-Grafting of Fluorinated Alkyl Chains onto Polystyrene-block-poly(vinyl methyl siloxane)

Polydimethylsiloxane (PDMS) has been widely used as a surface coating material, which has been reported to possess dynamic omniphobicity to a wide range of both polar and nonpolar solvents due to its high segmental flexibility and mobility. However, such high flexibility and mobility also enable penetration of small molecules into PDMS coatings, which alter the chemical and physical properties of the coating layers. To improve the anti-penetration properties of PDMS, a series of fluorinated alkyl segments are grafted to a diblock copolymer of polystyrene-block-poly(vinyl methyl siloxane) (PS-b-PVMS) using thiol-ene click reactions. This article reports the chemical characterization of these model fluorosilicone block copolymers and uses fluorescence measurements to investigate the dye penetration characteristics of polymer thin films. The introduction of longer fluorinated alkyl chains can gradually increase the anti-penetration properties as the time to reach the maximum fluorescence intensity (tpeak) gradually increases from 11 s of PS-b-PVMS to more than 1000 s of PS-b-P(n-C6F13-VMS). The improvement of anti-penetration properties is attributed to stronger inter-/intrachain interactions, phase segregation of ordered fluorinated side chains, and enhanced hydrophobicity caused by the grafting of fluorinated alkyl chains.

Mechanically stable polymer molecular sieve membranes with switchable functionality designed for high CO2 separation performance

The development of high-performance membranes selective for carbon dioxide is critically important for advancing energy-efficient carbon dioxide capture technologies. Although molecular sieves have long been attractive membrane materials, turning them into practical membrane applications has been challenging. Here, we introduce an innovative approach for crafting a polymeric molecular sieve membrane to achieve outstanding carbon dioxide separation performance while upholding the mechanical stability. First, a polymer molecular sieve membrane having high gas permeability and mechanical stability was fabricated from a judiciously designed polymer that is solution-processable, hyper-cross-linkable, and functionalizable. Then, the carbon dioxide selectivity was fine-tuned by the subsequent introduction of various amine-based carriers. Among the diverse amines, polyethyleneimine stands out by functionalizing the larger pore region while preserving ultramicropores, leading to improved carbon dioxide/dinitrogen separation performance. The optimized membrane demonstrates exceptional carbon dioxide/dinitrogen separation performance, outperforming other reported polymer molecular sieve membranes and even competing favorably with most carbon molecular sieve membranes reported to date.

Low-Loading Mixed Matrix Materials: Fractal-Like Structure and Peculiarly Enhanced Gas Permeability

Mixed matrix materials (MMMs) containing metal-organic framework (MOF) nanoparticles are attractive for membrane carbon capture. Particularly, adding <5 mass % MOFs in polymers dramatically increased gas permeability, far surpassing the Maxwell model's prediction. However, no sound mechanisms have been offered to explain this unusual low-loading phenomenon. Herein, we design an ideal series of MMMs containing polyethers (one of the leading polymers for CO2/N2 separation) and discrete metal-organic polyhedra (MOPs) with cage sizes of 2-5 nm. Adding 3 mass % MOP-3 in a polyether increases the CO2 permeability by 100% from 510 to 1000 Barrer at 35 °C because of the increased gas diffusivity. No discernible changes in typical physical properties governing gas transport properties are detected, such as glass transition temperature, fractional free volume, d-spacing, etc. We hypothesize that this behavior is attributed to fractal-like networks formed by highly porous MOPs, and for the first time, we validate this hypothesis using small-angle X-ray scattering analysis.

Ultrapermeable Gel Membranes Enabling Superior Carbon Capture

Membrane technology is rapidly gaining broad attraction as a viable alternative for carbon capture to mitigate increasingly severe global warming. Emerging CO2 -philic membranes have become crucial players in efficiently separating CO2 from light gases, leveraging their exceptional solubility-selectivity characteristics. However, economic and widespread deployment is greatly dependent on the boosted performance of advanced membrane materials for carbon capture. Here, we design a unique gel membrane composed of CO2 -philic molecules for accelerating CO2 transportation over other gases for ultrapermeable carbon capture. The molecular design of such soft membranes amalgamates the advantageous traits of augmented permeation akin to liquid membranes and operational stability akin to solid membranes, effectively altering the membrane's free volume characteristics validated by both experiments and molecular dynamics simulation. Surprisingly, gas diffusion through the free-volume-tuned gel membrane undergoes a 9-fold improvement without compromising the separation factor for the superior solubility selectivity of CO2 -philic materials, and CO2 permeability achieves a groundbreaking record of 5608 Barrer surpassing the capabilities of nonfacilitated CO2 separation materials and exceeding the upper bound line established in 2019 even by leading-edge porous polymer materials. Our designed gel membrane can maintain exceptional separation performance during prolonged operation, enabling the unparalleled potential of solubility-selective next-generation materials towards sustainable carbon capture.

Graphene-Oxide-Modified Metal-Organic Frameworks Embedded in Mixed-Matrix Membranes for Highly Efficient CO2/N2 Separation

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.

Enhanced CO2 Reactive Capture and Conversion Using Aminothiolate Ligand-Metal Interface

Metallic catalyst modification by organic ligands is an emerging catalyst design in enhancing the activity and selectivity of electrocatalytic carbon dioxide (CO2) reactive capture and reduction to value-added fuels. However, a lack of fundamental science on how these ligand-metal interfaces interact with CO2 and key intermediates under working conditions has resulted in a trial-and-error approach for experimental designs. With the aid of density functional theory calculations, we provided a comprehensive mechanism study of CO2 reduction to multicarbon products over aminothiolate-coated copper (Cu) catalysts. Our results indicate that the CO2 reduction performance was closely related to the alkyl chain length, ligand coverage, ligand configuration, and Cu facet. The aminothiolate ligand-Cu interface significantly promoted initial CO2 activation and lowered the activation barrier of carbon-carbon coupling through the organic (nitrogen (N)) and inorganic (Cu) interfacial active sites. Experimentally, the selectivity and partial current density of the multicarbon products over aminothiolate-coated Cu increased by 1.5-fold and 2-fold, respectively, as compared to the pristine Cu at -1.16 VRHE, consistent with our theoretical findings. This work highlights the promising strategy of designing the ligand-metal interface for CO2 reactive capture and conversion to multicarbon products.

High-Performance FAU Zeolite Membranes Derived from Nano-Seeds for Gas Separation

In this study, high-performance FAU (NaY type) zeolite membranes were successfully synthesized using small-sized seeds of 50 nm, and their gas separation performance was systematically evaluated. Employing nano-sized NaY seeds and an ultra-dilute reaction solution with a molar composition of 80 Na2O: 1Al2O3: 19 SiO2: 5000H2O, the effects of synthesis temperature, crystallization time, and porous support (α-Al2O3 or mullite) on the formation of FAU membranes were investigated. The results illustrated that further extending the crystallization time or increasing the synthesis temperature led to the formation of a NaP impurity phase on the FAU membrane layer. The most promising FAU membrane with a thickness of 2.7 µm was synthesized on an α-Al2O3 support at 368 K for 8 h and had good reproducibility. The H2 permeance of the membrane was as high as 5.34 × 10-7 mol/(m2 s Pa), and the H2/C3H8 and H2/i-C4H10 selectivities were 183 and 315, respectively. The C3H6/C3H8 selectivity of the membrane was as high as 46, with a remarkably high C3H6 permeance of 1.35 × 10-7 mol/(m2 s Pa). The excellent separation performance of the membrane is mainly attributed to the thin, defect-free membrane layer and the relatively wide pore size (0.74 nm).

Solid-solvent processing of ultrathin, highly loaded mixed-matrix membrane for gas separation

Mixed-matrix membranes (MMMs) that combine processable polymer with more permeable and selective filler have potential for molecular separation, but it remains difficult to control their interfacial compatibility and achieve ultrathin selective layers during processing, particularly at high filler loading. We present a solid-solvent processing strategy to fabricate an ultrathin MMM (thickness less than 100 nanometers) with filler loading up to 80 volume %. We used polymer as a solid solvent to dissolve metal salts to form an ultrathin precursor layer, which immobilizes the metal salt and regulates its conversion to a metal-organic framework (MOF) and provides adhesion to the MOF in the matrix. The resultant membrane exhibits fast gas-sieving properties, with hydrogen permeance and/or hydrogen-carbon dioxide selectivity one to two orders of magnitude higher than that of state-of-the-art membranes.

Carbon Capture Membranes Based on Amorphous Polyether Nanofilms Enabled by Thickness Confinement and Interfacial Engineering

Thin-film composite membranes are a leading technology for post-combustion carbon capture, and the key challenge is to fabricate defect-free selective nanofilms as thin as possible (100 nm or below) with superior CO₂/N₂ separation performance. Herein, we developed high-performance membranes based on an unusual choice of semi-crystalline blends of amorphous poly(ethylene oxide) (aPEO) and 18-crown-6 (C6) using two nanoengineering strategies. First, the crystallinity of the nanofilms decreases with decreasing thickness and completely disappears at 500 nm or below because of the thickness confinement. Second, polydimethylsiloxane is chosen as the gutter layer between the porous support and selective layer, and its surface is modified with bio-adhesive polydopamine (<10 nm) with an affinity toward aPEO, enabling the formation of the thin, defect-free, amorphous aPEO/C6 layer. For example, a 110 nm film containing 40 mass % C6 in aPEO exhibits CO₂ permeability of 900 Barrer (much higher than a thick film with 420 Barrer), rendering a membrane with a CO₂ permeance of 2200 GPU and CO₂/N₂ selectivity of 27 at 35 °C, surpassing Robeson's upper bound. This work shows that engineering at the nanoscale plays an important role in designing high-performance membranes for practical separations.

ZIF-62 glass foam self-supported membranes to address CH4/N2 separations

Membranes with ultrahigh permeance and practical selectivity could greatly decrease the cost of difficult industrial gas separations, such as CH₄/N₂ separation. Advanced membranes made from porous materials, such as metal-organic frameworks, can achieve a good gas separation performance, although they are typically formed on support layers or mixed with polymeric matrices, placing limitations on gas permeance. Here an amorphous glass foam, a_gfZIF-62, wherein a, g and f denote amorphous, glass and foam, respectively, was synthesized by a polymer-thermal-decomposition-assisted melting strategy, starting from a crystalline zeolitic imidazolate framework, ZIF-62. The thermal decomposition of incorporated low-molecular-weight polyethyleneimine evolves CO₂, NH₃ and H₂O gases, creating a large number and variety of pores. This greatly increases pore interconnectivity but maintains the crystalline ZIF-62 ultramicropores, allowing ultrahigh gas permeance and good selectivity. A self-supported circular a_gfZIF-62 with a thickness of 200-330 µm and area of 8.55 cm² was used for membrane separation. The membranes perform well, showing a CH₄ permeance of 30,000-50,000 gas permeance units, approximately two orders of magnitude higher than that of other reported membranes, with good CH₄/N₂ selectivity (4-6).

Rapid Fabrication of High-Permeability Mixed Matrix Membranes at Mild Condition for CO2 Capture

Mixed matrix membranes (MMMs), conjugating the advantages of flexible processing-ability of polymers and high-speed mass transfer of porous fillers, are recognized as the next-generation high-performance CO₂ capture membranes for solving the current global climate challenge. However, controlling the crystallization of porous metal-organic frameworks (MOFs) and thus the close stacking of MOF nanocrystals in the confined polymer matrix is still undoable, which thus cannot fully utilize the superior transport attribute of MOF channels. In this study, the "confined swelling coupled solvent-controlled crystallization" strategy is employed for well-tailoring the in-situ crystallization of MOF nanocrystals, realizing rapid (<5 min) construction of defect-free freeway channels for CO₂ transportation in MMMs due to the close stacking of MOF nanocrystals. Consequently, the fabricated MMMs exhibit approximately fourfold enhancement in CO₂ permeability, i.e., 2490 Barrer with a CO₂ /N₂ selectivity of 37, distinctive antiplasticization merit, as well as long-term running stability, which is at top-tier level, enabling the large-scale manufacture of high-performance MMMs for gas separation.

Strategies to improve the mass transfer in the CO2 capture process using immobilized carbonic anhydrase

High carbon dioxide (CO₂) concentration in the atmosphere urgently requires eco-friendly mitigation strategies. Carbonic anhydrase (CA) is a high-quality enzyme protein, available from a wide range of sources, which has an extremely high catalytic efficiency for the hydration of CO₂ compared with other catalytic CO₂ conversion systems. While free CA is costly and weakly stable, CA immobilization can significantly improve its stability and allow enzyme recycling. However, gaseous CO₂ is significantly different from traditional liquid substrates. Additionally, due to the presence of enzyme carriers, there is limited mass transfer between CO₂ and the active center of immobilized CA. Most of the available reviews provide an overview of the improvement in catalytic activity and stability of CA by different immobilization methods and substrates. However, they do not address the limited mass transfer between CO₂ and the active center of immobilized CA. Therefore, by focusing on the mass transfer process, this review presents CA immobilization strategies that are more efficient and of greater environmental tolerance by categorizing the methods of enhancing the mass transfer process at each stage of the enzymatic CO₂ capture reaction. Such improvements in this green and environmentally friendly biological carbon capture process can increase its efficiency for industrial applications.

Ratiometric Luminescent Thermometer Based on the Lanthanide Metal-Organic Frameworks by Thermal Curing

The high-performance optical thermometer probes are of great significance in diverse areas; lanthanide metal-organic frameworks (Ln-MOFs) are a promising candidate for luminescence temperature sensing owing to their unique luminescence properties. However, Ln-MOFs have poor maneuverability and stability in complex environments due to the crystallization properties, which then hinder their application scope. In this work, the Tb-MOFs@TGIC composite was successfully prepared using simple covalent crosslinking through uncoordinated -NH₂ or COOH on Tb-MOFs reacting with the epoxy groups on TGIC {Tb-MOFs = [Tb₂(atpt)₃(phen)₂(H₂O)]_n; H₂atpt = 2-aminoterephthalic acid; phen = 1,10-phenanthroline monohydrate}. After curing, the fluorescence properties, quantum yield, lifetime, and thermal stability of Tb-MOFs@TGIC were remarkably enhanced. Meanwhile, the obtained Tb-MOFs@TGIC composites exhibit excellent temperature sensing properties in the low-temperature (S_r = 6.17% K^-1 at 237 K), physiological temperature (S_r = 4.86% K^-1 at 323 K), or high-temperature range (S_r = 3.88% K^-1 at 393 K) with high sensitivity. In the temperature sensing process, the sensing mode of single emission changed into double emission for ratiometric thermometry owing to the back energy transfer (BenT) from Tb-MOFs to TGIC linkers, and the BenT process enhanced with the increase of temperature, which further improved the accuracy and sensitivity of temperature sensing. Most notably, the temperature-sensing Tb-MOFs@TGIC can be easily coated on the surface of polyimide (PI), glass plate, silicon pellet (SI), and poly(tetrafluoroethylene) plate (PTFE) substrates by a simple spraying method, which also exhibited an excellent sensing property, making it applicable for a wider T range measurement. This is the first example of a postsynthetic Ln-MOF hybrid thermometer operative over a wide temperature range including the physiological and high temperature based on back energy transfer.

Covalent Organic Frameworks: The Rising-Star Platforms for the Design of CO2 Separation Membranes

Membrane-based carbon dioxide (CO₂ ) capture and separation technologies have aroused great interest in industry and academia due to their great potential to combat current global warming, reduce energy consumption in chemical separation of raw materials, and achieve carbon neutrality. The emerging covalent organic frameworks (COFs) composed of organic linkers via reversible covalent bonds are a class of porous crystalline polymers with regular and extended structures. The inherent structure and customizable organic linkers give COFs high and permanent porosity, short transport channel, tunable functionality, and excellent stability, thereby enabling them rising-star alternatives for developing advanced CO₂ separation membranes. Therefore, the promising research areas ranging from development of COF membranes to their separation applications have emerged. Herein, this review first introduces the main advantages of COFs as the state-of-the-art membranes in CO₂ separation, including tunable pore size, modifiable surfaces property, adjustable surface charge, excellent stability. Then, the preparation approaches of COF-based membranes are systematically summarized, including in situ growth, layer-by-layer stacking, blending, and interface engineering. Subsequently, the key advances of COF-based membranes in separating various CO₂ mixed gases, such as CO₂ /CH₄ , CO₂ /H₂ , CO₂ /N₂ , and CO₂ /He, are comprehensively discussed. Finally, the current issues and further research expectations in this field are proposed.

Biocatalytic Membranes for Carbon Capture and Utilization

Innovative carbon capture technologies that capture CO₂ from large point sources and directly from air are urgently needed to combat the climate crisis. Likewise, corresponding technologies are needed to convert this captured CO₂ into valuable chemical feedstocks and products that replace current fossil-based materials to close the loop in creating viable pathways for a renewable economy. Biocatalytic membranes that combine high reaction rates and enzyme selectivity with modularity, scalability, and membrane compactness show promise for both CO₂ capture and utilization. This review presents a systematic examination of technologies under development for CO₂ capture and utilization that employ both enzymes and membranes. CO₂ capture membranes are categorized by their mode of action as CO₂ separation membranes, including mixed matrix membranes (MMM) and liquid membranes (LM), or as CO₂ gas-liquid membrane contactors (GLMC). Because they selectively catalyze molecular reactions involving CO₂, the two main classes of enzymes used for enhancing membrane function are carbonic anhydrase (CA) and formate dehydrogenase (FDH). Small organic molecules designed to mimic CA enzyme active sites are also being developed. CO₂ conversion membranes are described according to membrane functionality, the location of enzymes relative to the membrane, which includes different immobilization strategies, and regeneration methods for cofactors. Parameters crucial for the performance of these hybrid systems are discussed with tabulated examples. Progress and challenges are discussed, and perspectives on future research directions are provided.

Truly combining the advantages of polymeric and zeolite membranes for gas separations

Mixed-matrix membranes (MMMs) have been investigated to render energy-intensive separations more efficiently by combining the selectivity and permeability performance, robustness, and nonaging properties of the filler with the easy processing, handling, and scaling up of the polymer. However, truly combining all in one single material has proven very challenging. In this work, we filled a commercial polyimide with ultrahigh loadings of a high-aspect ratio, CO₂-philic Na-SSZ-39 zeolite with a three-dimensional channel system that precisely separates gas molecules. By carefully designing both zeolite and MMM synthesis, we created a gas-percolation highway across a flexible and aging-resistant (more than 1 year) membrane. The combination of a CO₂-CH₄ mixed-gas selectivity of ~423 and a CO₂ permeability of ~8300 Barrer outperformed all existing polymer-based membranes and even most zeolite-only membranes.

Constructing Dual-Transport Pathways by Incorporating Beaded Nanofillers in Mixed Matrix Membranes for Efficient CO2 Separation

Mixed matrix membranes (MMMs) have attracted significant attention in the field of CO₂ separation because MMMs have potential to overcome an undesirable "trade-off" effect. In this study, the beaded nanofillers of ZIF-8@aminoclay (ZIF-8@AC) were synthesized using an in situ growth method, and they were doped into a Pebax MH 1657 (Pebax) matrix to fabricate MMMs for efficient CO₂ separation. The beaded structure was formed by ZIF-8 particles joined together during the process of AC coating on the ZIF-8 surface. ZIF-8@AC played a vital role in the improvement of gas separation performance. It was mainly attributed to the following reasons: First, the inherent micropores of ZIF-8 constructed the internal pathways for gas transport in the beaded nanofillers, benefiting the improvement of gas permeability. Second, the staggered AC layers constructed the external pathways for gas transport in the beaded nanofillers, increasing the tortuosity of gas transport for larger molecules and favoring the improvement of gas selectivity. Therefore, the internal and external pathways of ZIF-8@AC co-constructed the dual-transport pathways for CO₂ transport in MMMs. In addition, the abundant amino groups of the beaded nanofillers provided abounding carriers for CO₂, facilitating CO₂ transport in the dual-transport pathways. Therefore, the CO₂ separation performance of Pebax/ZIF-8@AC-1 MMMs was significantly improved. The CO₂ permeability and CO₂/CH₄ separation factor of Pebax/ZIF-8@AC-1-7 MMM were 620 ± 10 Barrer and 40 ± 0.4, which were 2.3 and 1.6 times those of a pure Pebax membrane, respectively. Furthermore, the CO₂/CH₄ separation performance of Pebax/ZIF-8@AC-1-7 MMM overcame successfully the "trade-off" effect and approached the 2019 upper bound. It is a novel strategy to design a beaded nanofiller doped into MMMs for carbon capture.

Multidimensional Building Blocks for Molecular Sieve Membranes

Chemical separations aiming for high-purity commodities are critical to modern society. Compared to distillation, chemical absorption, and adsorption, membrane separation is attractive for its energy efficiency, ease of operation, and compact footprint. Molecular sieve membranes (MSMs) are broadly defined as membranes that are constructed from intrinsically and artificially porous materials. On the basis of our recent studies, this Account will first summarize the evolution of MSMs from the viewpoint of dimensionality of building blocks, which fundamentally determines the stacking architectures, intercrystalline gaps, and mass transfer channels of MSMs. Intergrowth of three-dimensional (3D) crystals as primary building blocks gives rise to classical MSMs. However, the poor connection between crystals inherent to those membranes results in intercrystalline gaps that are catastrophic for separation selectivity. We adopted a variety of strategies to close the crystal boundary gaps, including microwave synthesis, electrochemical-ionothermal synthesis, and modular integration. These efforts make us better understand the structure-performance relationship in membranes and create solutions for industrial processes. Excitingly, we first scaled-up the microwave synthesis of a Linde type A (LTA) zeolite membrane and built the world's largest ethanol dehydration membrane unit with an annual capacity of 100,000 tons. MSMs can also be made of two-dimensional (2D) nanosheets as primary building blocks. Those strike a balance between permeation rate and selectivity because the nanometer thickness ensures the minimization of the mass-transfer resistance of the membrane and the layer-by-layer stacking mode can significantly reduce the intercrystalline gaps. By publishing our first report on metal-organic framework (MOF) nanosheet membranes in Science, we committed to establishing top-down and bottom-up methods for assembly of laminae. Once the stacking, orientation, and connection between the layers are meticulously controlled, nanosheet building blocks with diversity open the door for ultrapermeable and selective MSMs. We recently proposed a supramolecule array membrane (SAM) with zero-dimensional (0D) molecules as primary building blocks, which has great potential to absolutely eliminate intercrystalline gaps in membranes. In contrast to the classical transport through nanopores of membranes, selective transport through the intermolecular spacing of supramolecules is creatively realized within the SAM, which marks a new breakthrough in ultraprecise sieving of molecules with tiny differences in size and revolutionizes MSMs in regard to stacking modes, intercrystalline gaps, and transport channels. MSMs have proven to be successful in diverse applications and have triggered wide interest. A unique perspective on the dimensionality evolution of building blocks will accelerate the progress of MSMs. The synergy of multidimensional MSMs will be a positive response to fundamental bottlenecks and industrial questions of membranes and will unlock the potential of membranes to displace the existing separation technologies in the future.

Critical Assessment of Membrane Technology Integration in a Coal-Fired Power Plant

Despite the many technologies for CO2 capture (e.g., chemical or physical absorption or adsorption), researchers are looking to develop other technologies that can reduce CAPEX and OPEX costs as well as the energy requirements associated with their integration into thermal power plants. The aim of this paper was to analyze the technical and economic integration of spiral wound membranes in a coal-fired power plant with an installed capacity of 330 MW (the case of the Rovinari power plant-in Romania). The study modeled energy processes using CHEMCAD version 8.1 software and polymer membranes developed in the CO2 Hybrid research project. Thus, different configurations such as a single membrane step with and without the use of a vacuum pump and two membrane steps placed in series were analyzed. In all cases, a compressor placed before the membrane system was considered. The use of two serialized stages allows for both high efficiency (minimum 90%) and CO2 purity of a minimum of 95%. However, the overall plant efficiency decreased from 45.78 to 23.96% and the LCOE increased from 75.6 to 170 €/kWh. The energy consumption required to capture 1 kg of CO2 is 2.46 MJel and 4.52 MJth.

Facilitated transport membrane with functionalized ionic liquid carriers for CO2/N2, CO2/O2, and CO2/air separations

CO₂ separations from cabin air and the atmospheric air are challenged by the very low partial pressures of CO₂. In this study, a facilitated transport membrane (FTM) is developed to separate CO₂ from air using functionalized ionic liquid (IL) and poly(ionic liquid) (PIL) carriers. A highly permeable bicontinuous structured poly(ethersulfone)/poly(ethylene terephthalate) (bPES/PET) substrate is used to support the PIL-IL impregnated graphene oxide thin film. The CO₂ separation performance was tested under a mixture feed of CO₂/N₂/O₂/H₂O. Under 410 ppm of CO₂ at 1 atm feed gas, CO₂ permanence of 3923 GPU, and CO₂/N₂ and CO₂/O₂ selectivities of 1200 and 300, respectively, are achieved with helium sweeping on the permeate side. For increased transmembrane pressure (>0 atm), a thicker PIL-IL/GO layer was shown to provide mechanical strength and prevent leaching of the mobile carrier. CO₂ binding to the carriers, ion diffusivities, and the glass transition temperature of the PIL-IL gels were examined to determine the membrane composition and rationalize the superior separation performance obtained. This report represents the first FTM study with PIL-IL carriers for CO₂ separation from air.

Membrane Separation Processes and Post-Combustion Carbon Capture: State of the Art and Prospects

Membrane processes have been investigated for carbon capture for more than four decades. Important efforts have been more recently achieved for the development of advanced materials and, to a lesser extent, on process engineering studies. A state-of-the-art analysis is proposed with a critical comparison to gas absorption technology, which is still considered as the best available technology for this application. The possibilities offered by high-performance membrane materials (zeolites, Carbon Molecular Sieves, Metal Oxide Frameworks, graphenes, facilitated transport membranes, etc.) are discussed in combination to process strategies (multistage design, hybrid processes, energy integration). The future challenges and open questions of membranes for carbon capture are finally proposed.