Microporous Organic Materials for Membrane-Based Gas Separation

Microporous Materials in Polymer Electrolytes: The Merit of Order

Solid-state batteries (SSBs) have garnered significant attention in the critical field of sustainable energy storage due to their potential benefits in safety, energy density, and cycle life. The large-scale, cost-effective production of SSBs necessitates the development of high-performance solid-state electrolytes. However, the manufacturing of SSBs relies heavily on the advancement of suitable solid-state electrolytes. Composite polymer electrolytes (CPEs), which combine the advantages of ordered microporous materials (OMMs) and polymer electrolytes, meet the requirements for high ionic conductivity/transference number, stability with respect to electrodes, compatibility with established manufacturing processes, and cost-effectiveness, making them particularly well-suited for mass production of SSBs. This review delineates how structural ordering dictates the fundamental physicochemical properties of OMMs, including ion transport, thermal transfer, and mechanical stability. The applications of prominent OMMs are critically examined, such as metal-organic frameworks, covalent organic frameworks, and zeolites, in CPEs, highlighting how structural ordering facilitates the fulfillment of property requirements. Finally, an outlook on the field is provided, exploring how the properties of CPEs can be enhanced through the dimensional design of OMMs, and the importance of uncovering the underlying "feature-function" mechanisms of various CPE types is underscored.

Effective C4 Separation by Zeolite Metal-Organic Framework Composite Membranes

Inorganic zeolites have excellent molecular sieving properties, but they are difficult to process into macroscopic structures. In this work, we use metal-organic framework (MOF) glass as substrates to engineer the interface with inorganic zeolites, and then assemble the discrete crystalline zeolite powders into monolithic structures. The zeolites are well dispersed and stabilized within the MOF glass matrix, and the monolith has satisfactory mechanical stabilities for membrane applications. We demonstrate the effective separation performance of the membrane for 1,3-butadiene (C4H6) from other C4 hydrocarbons, which is a crucial and challenging separation in the chemical industry. The membrane achieves a high permeance of C4H6 (693.00±21.83 GPU) and a high selectivity over n-butene, n-butane, isobutene, and isobutane (9.72, 9.94, 10.31, and 11.94, respectively). This strategy opens up new possibilities for developing advanced membrane materials for difficult hydrocarbon separations.

Superlative Porous Organic Polymers for Photochemical and Electrochemical CO2 Reduction Applications: From Synthesis to Functionality

To mimic the carbon cycle at a kinetically rapid pace, the sustainable conversion of omnipresent CO2 to value-added chemical feedstock and hydrocarbon fuels implies a remarkable prototype for utilizing released CO2. Porous organic polymers (POPs) have been recognized as remarkable catalytic systems for achieving large-scale applicability in energy-driven processes. POPs offer mesoporous characteristics, higher surface area, and superior optoelectronic properties that lead to their relatively advanced activity and selectivity for CO2 conversion. In comparison to the metal organic frameworks, POPs exhibit an enhanced tendency toward membrane formation, which governs their excellent stability with regard to remarkable ultrathinness and tailored pore channels. The structural ascendancy of POPs can be effectively utilized to develop cost-effective catalytic supports for energy conversion processes to leapfrog over conventional noble metal catalysts that have nonlinear techno-economic equilibrium. Herein, we precisely surveyed the functionality of POPs from scratch, classified it, and provided a critical commentary of its current methodological advancements and photo/electrochemical achievements in the CO2 reduction reaction.

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.

Micro and Nanoporous Membrane Platforms for Carbon Neutrality: Membrane Gas Separation Prospects

Recently, carbon neutrality has been promoted as a potentially practical solution to global CO2 emissions and increasing energy-consumption challenges. Many attempts have been made to remove CO2 from the environment to address climate change and rising sea levels owing to anthropogenic CO2 emissions. Herein, membrane technology is proposed as a suitable solution for carbon neutrality. This review aims to comprehensively evaluate the currently available scientific research on membranes for carbon capture, focusing on innovative microporous material membranes used for CO2 separation and considering their material, chemical, and physical characteristics and permeability factors. Membranes from such materials comprise metal-organic frameworks, zeolites, silica, porous organic frameworks, and microporous polymers. The critical obstacles related to membrane design, growth, and CO2 capture and usage processes are summarized to establish novel membranes and strategies and accelerate their scaleup.

Breakthrough applications of porous organic materials for membrane-based CO2 separation: a review

Over the last decades, porous organic materials (POMs) have been extensively employed in various industrial approaches including gas separation, catalysis and energy production due to possessing indisputable advantages like great surface area, high permeability, controllable pore size, appropriate functionalization and excellent processability compared to traditional substances like zeolites, Alumina and polymers. This review presents the recent breakthroughs in the multifunctional POMs for potential use in the membrane-based CO2 separation. Some examples of highly-selective membranes using multifunctional POMs are described. Moreover, various classifications of POMs following with their advantages and disadvantages in CO2 separation processes are explained. Apart from reviewing the state-of-the-art POMs in CO2 separation, the challenges/limitations of POMs with tailored structures for reasonable application are discussed.

A Microporous Poly(Arylene Ether) Platform for Membrane-Based Gas Separation

Membrane-based gas separations are crucial for an energy-efficient future. However, it is difficult to develop membrane materials that are high-performing, scalable, and processable. Microporous organic polymers (MOPs) combine benefits for gas sieving and solution processability. Herein, we report membrane performance for a new family of microporous poly(arylene ether)s (PAEs) synthesized via Pd-catalyzed C-O coupling reactions. The scaffold of these microporous polymers consists of rigid three-dimensional triptycene and stereocontorted spirobifluorene, endowing these polymers with micropore dimensions attractive for gas separations. This robust PAE synthesis method allows for the facile incorporation of functionalities and branched linkers for control of permeation and mechanical properties. A solution-processable branched polymer was formed into a submicron film and characterized for permeance and selectivity, revealing lab data that rivals property sets of commercially available membranes already optimized for much thinner configurations. Moreover, the branching motif endows these materials with outstanding plasticization resistance, and their microporous structure and stability enables benefits from competitive sorption, increasing CO2 /CH4 and (H2 S+CO2 )/CH4 selectivity in mixture tests as predicted by the dual-mode sorption model. The structural tunability, stability, and ease-of-processing suggest that this new platform of microporous polymers provides generalizable design strategies to form MOPs at scale for demanding gas separations in industry.

Water-Mediated Hydrogen Bond Network Drives Highly Crystalline Structure Formation of Crown Ether-Based Covalent Organic Framework for Sr Adsorption

Covalent organic frameworks (COFs) with crown ether units have drawn great attention due to their potential applications in adsorption, catalysis, and sensing. However, employing crown ethers to construct COFs is still challenging in light of the flexible nature of macrocycles. Here, a highly crystalline one-dimensional covalent organic framework (1D-18C6-COF) with crown ether units on the ribbon edge was synthesized. The water-mediated hydrogen bond network and π-π stacking hold the 1D COF ribbons together. The combination of experimental and DFT studies demonstrated that the hydrogen bond network plays a crucial role in the structure crystallinity. The 1D-18C6-COF was applied as an adsorbent for strontium, and it exhibited rapid kinetics with good selectivity. In the competitive adsorption experiment, a separation factor of 1900 was achieved, representing one of the largest values for cesium/strontium separation. This work provides new insights into the design and functional exploration of crystalline COFs with flexible units.

Performance Improvement of an STS304-Based Dispensing Needle via Electrochemical Etching

In this study, we explored the formation of micro-/nanosized porous structures on the surface of a needle composed of STS304 and examined the effect of conventional needles and needles capable of liquid ejection. Aqua regia, composed of HCl and HNO3, was electrochemically etched to form appropriately sized micro-/nanoporous structures. We observed that when dispensing liquids with low surface tension, they do not immediately fall downward but instead spread over the exterior surface of the needle before falling. We found that the extent of spreading on the surface is influenced by an etched porous structure. Furthermore, to analyze the effect of surface tension differences, we dispensed liquids with varying surface tensions using etched needles. Through the analysis, it was confirmed that, despite the low surface tension, the ejected droplet volume and speed could be stably maintained on the etched needle. This indicates that the spreading phenomenon of the liquid on the needle surface just before ejection can be controlled by the micro/nanoporous structure. We anticipate that these characteristics of etched needles could be utilized in industries where precision dispensing of low-surface-tension liquids is essential.

Function-Led Design of Covalent-Organic-Framework Membranes for Precise Ion Separation

Insufficient access to clean water and resources has emerged as one of the most pressing issues affecting people globally. Membrane-based ion separation has become a focal point of research for the generation of fresh water and the extraction of energy elements. This Review encapsulates recent advancements in the selective ion transport of covalent organic framework (COF) membranes, accomplished by strategically pairing diverse monomers to create membranes with various pore sizes and environments for specific purposes. We first discuss the merits of using COF materials as a basis for fabricating membranes for ion separation. We then explore the development of COF membranes in areas such as desalination, acid recovery, and energy element extraction, with a particular emphasis on the fundamental principles of membrane design. Lastly, we address both theoretical and practical challenges, as well as potential opportunities in the targeted design of ion-selective membranes. The goal of this Review is to stimulate future investigative efforts in this field, which is of significant scientific and strategic importance.

Kilogram-Scale Fabrication of a Robust Olefin-Linked Covalent Organic Framework for Separating Ethylene from a Ternary C2 Hydrocarbon Mixture

One-step adsorptive purification of ethylene (C2H4) from a ternary mixture of acetylene (C2H2), C2H4, and ethane (C2H6) by a single material is of great importance but challenging in the petrochemical industry. Herein, a chemically robust olefin-linked covalent organic framework (COF), NKCOF-62, is designed and synthesized by a melt polymerization method employing tetramethylpyrazine and terephthalaldehyde as cheap monomers. This method avoids most of the disadvantages of classical solvothermal methods, which enable the cost-effective kilogram fabrication of olefin-linked COFs in one pot. Furthermore, NKCOF-62 shows remarkably selective adsorption of C2H2 and C2H6 over C2H4 thanks to its unique pore environments and suitable pore size. Breakthrough experiments demonstrate that polymer-grade C2H4 can be directly obtained from C2H2/C2H6/C2H4 (1/1/1) ternary mixtures through a single separation process. Notably, NKCOF-62 is the first demonstration of the potential to use COFs for C2H2/C2H6/C2H4 separation, which provides a blueprint for the design and construction of robust COFs for industrial gas separations.

Tetramine-Based Hyperbranched Polyimide Membranes with Rigid Crosslinker for Improved Gas Permeability and Stability

Triamine-based HBPI membranes are known for high gas separation selectivity and physical stability, but their permeabilities are still very low. In this study, we utilized a tetramine monomer called TPDA (N,N,N',N'-tetrakis(4-aminophenyl)-1,4-benzenediamine) as a crosslinking center and incorporated an additional diamine comonomer called DAM (2,4,6-trimethyl-1,3-diaminobenzene) to enhance gas separation performance, especially gas permeability. The findings demonstrated that the resultant 6FDA-DAM/TPDA membranes based on tetramine TPDA exhibited a greater amount of free volume compared to the triamine-based HBPI membranes, resulting in significantly higher gas permeabilities. Furthermore, the higher concentration of DAM component led to the generation of more fractional free volumes (FFV). Consequently, the gas permeabilities of the 6FDA-DAM/TPDA membranes increased with an increase in DAM content, with a minimal compromise on selectivity. The enhanced gas permeabilities of the 6FDA-DAM/TPDA membranes enabled them to minimize the footprint required for membrane installations in real-world applications. Moreover, the 6FDA-DAM/TPDA membranes exhibited remarkable durability against physical aging and plasticization, thanks to the incorporation of a hyperbranched network structure.

Facile and Simple Post Treatment Ball Milling Strategy for the Production of Low-Cost TiO2 Composites with Enhanced Photocatalytic Performance and Applicability to Construction Materials

A facile and cost-effective approach assisted by ball milling (BM) of commercial titanium dioxide (TiO₂), has been utilized to develop cheaper and efficient construction materials. At least three of the commercial and cheaper TiO₂ samples (BA01-01, BA01-01+ and R996, designated as A1, A4 and R1, respectively) were selected and subjected to BM treatment to enhance their photocatalytic efficiencies, if possible. It was noted, that the samples A1, A4 and R1 were typical composites of TiO₂ and calcium carbonate (CaCO₃) and contained varying proportions of anatase, and rutile phases of TiO₂ and CaCO₃. Two of the highly efficient commercial TiO₂ samples, Degussa P25 (simply designated as P25) and ST01 (Ishihara Ind.) were selected for making benchmark comparisons of photocatalytic efficiencies. The BM treated TiO₂ samples (designated as TiO₂-BM with respect to A1, A4 and R1) were evaluated for photocatalytic efficiencies both in both aqueous (methylene blue (MB)) and gaseous (NO_x) photodegradation reactions. Based on detailed comparative investigations, it was observed that A1-BM photocatalyst exhibited superior photocatalytic performances over A4-BM and R1-BM, towards both MB and NO_x photodegradation reactions. The difference of NO_x photodegradation efficiency between the mortar mixed with A1-BM and that mixed with ST01, and P-25 at 15% were 16.6%, and 32.4%, respectively. Even though the mortar mixed with A1-BM at 15% composition exhibited a slightly lower NO_x photodegradation efficiency as compared to mortar mixed with the expensive ST01 and P-25 photocatalysts, the present work promises an economic application in the eco-friendly construction materials for air purification considering the far lower cost of A1. The reasons for the superior performance of A1-BM were deduced through characterization of optical properties, surface characteristics, phase composition, morphology, microstructure and particle size distribution between pristine and BM treated A1 using characterization techniques such as diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction analysis, field emission scanning electron microscopy and particle size analysis.

Pyro-layered heterostructured nanosheet membrane for hydrogen separation

Engineering different two-dimensional materials into heterostructured membranes with unique physiochemical properties and molecular sieving channels offers an effective way to design membranes for fast and selective gas molecule transport. Here we develop a simple and versatile pyro-layering approach to fabricate heterostructured membranes from boron nitride nanosheets as the main scaffold and graphene nanosheets derived from a chitosan precursor as the filler. The rearrangement of the graphene nanosheets adjoining the boron nitride nanosheets during the pyro-layering treatment forms precise in-plane slit-like nanochannels and a plane-to-plane spacing of ~3.0 Å, thereby endowing specific gas transport pathways for selective hydrogen transport. The heterostructured membrane shows a high H₂ permeability of 849 Barrer, with a H₂/CO₂ selectivity of 290. This facile and scalable technique holds great promise for the fabrication of heterostructures as next-generation membranes for enhancing the efficiency of gas separation and purification processes.

Two-dimensional nanomaterial MXenes for efficient gas separation: a review

Transition metal carbides/nitrides (MXenes) are emerging two-dimensional (2D) materials that have been widely investigated in recent years. In general, these materials can be obtained from MAX phase ceramics after intercalation, etching, and exfoliation to obtain multilayer MXene nanosheet structures; moreover, they have abundant end-group functional groups on their surface. In recent years, the excellent high permeability, fine sieving ability and diverse processability of MXene series materials make the membranes prepared using them particularly suitable for membrane-based separation processes in the field of gas separation. 2D membranes enhance the diversity of the pristine membrane transport channels by regulating the gas transport channels through in-plane pores (intrinsic defects), in-plane slit-like pores, and planar to planar interlayer channels, endowing the membrane with the ability to effectively sieve gas energy efficiently. Herein, we review MXenes, a class of 2D nanomaterials, in terms of their unique structure, synthesis method, functionalization method, and the structure-property relationship of MXene-based gas separation membranes and list examples of MXene-based membranes used in the field of gas separation. By summarizing and analyzing the basic properties of MXenes and demonstrating their unique advantages compared to other 2D nanomaterials, we lay a foundation for the discussion of MXene-based membranes with outstanding carbon dioxide (CO₂) capture performance and outline and exemplify the excellent separation performances of MXene-based gas separation membranes. Finally, the challenges associated with MXenes are briefly discussed and an outlook on the promising future of MXene-based membranes is presented. It is expected that this review will provide new insights and important guidance for future research on MXene materials in the field of gas separation.

Fine-Tuned Ultra-Microporous Metal-Organic Framework in Mixed-Matrix Membrane: Pore-Tailoring Optimization for C2 H2 /C2 H4 Separation

Effective separation of ethyne from ethyne/ethylene (C₂ H₂ /C₂ H₄ ) mixtures is a challenging and crucial industrial process. Herein, an ultra-microporous metal-organic framework (MOF) platform, Cd(dicarboxylate)₂ (ditriazole), with triangular channels is proposed for high-efficiency separation of C₂ H₂ from C₂ H₄ . The targeted structures are constructed via a mixed-ligand strategy by selecting different-sized ligands, allowing for tunable pore sizes and volumes. The pore properties can be further optimized by additional modification via pore environment tailoring. This concept leads to the successful synthesis of three ultra-microporous Cd-MOFs (JLU-MOF87-89). As intended, C₂ H₂ uptake and C₂ H₂ /C₂ H₄ selectivity gradually increase with progressively optimizing the pore structure by adjusting ligand length and substituents. JLU-MOF89, functionalized with methyl groups, features the most optimal pore chemistry and shows selective recognition of C₂ H₂ over C₂ H₄ , owing to the framework-C₂ H₂ host-guest interactions. Furthermore, JLU-MOFs are fabricated into mixed-matrix membranes for C₂ H₂ /C₂ H₄ separation. C₂ H₂ permeability and C₂ H₂ /C₂ H₄ permselectivity are substantially enhanced by ≥400% and ≥200%, respectively, after hybridization of JLU-MOF88 and JLU-MOF89 with a polyimide polymer (6FDA-ODA). These membranes can work efficiently and are stable under different conditions, demonstrating their potential in actual ethyne separation.

Thermally Crosslinked Hydrogen-Bonded Organic Framework Membranes for Highly Selective Ion Separation

The weak bonding energy and flexibility of hydrogen bonds can hinder the long-term use of hydrogen-bonded organic framework (HOF) materials under harsh conditions. Here we invented a thermal-crosslinking method to form polymer materials based on a diamino triazine (DAT) HOF (FDU-HOF-1), containing high-density hydrogen bonding of N-H⋯N. With the increase of temperature to 648 K, the formation of -NH- bonds between neighboring HOF tectons by releasing NH₃ was observed based on the disappearance of the characteristic peaks of amino groups on FDU-HOF-1 in the Fourier transform infrared (FTIR) and solid-state nuclear magnetic resonance (ss-NMR). The variable temperature PXRD indicated the formation of a new peak at 13.2° in addition to the preservation of the original diffraction peaks of FDU-HOF-1. The water adsorption, acid-base stability (12 M HCl to 20 M NaOH) and solubility experiments concluded that the thermally crosslinked HOFs (TC-HOFs) are highly stable. The membranes fabricated by TC-HOF demonstrate the permeation rate of K⁺ ions as high as 270 mmol m^-2 h^-1 as well as high selectivity of K⁺/Mg²⁺ (50) and Na⁺/Mg²⁺ (40), which was comparable to Nafion membranes. This study provides guidance for the future design of highly stable crystalline polymer materials based on HOFs.

Collagen Fiber-Based Advanced Separation Materials: Recent Developments and Future Perspectives

Separation plays a critical role in a broad range of industrial applications. Developing advanced separation materials is of great significance for the future development of separation technology. Collagen fibers (CFs), the typical structural proteins, exhibit unique structural hierarchy, amphiphilic wettability, and versatile chemical reactivity. These distinctive properties provide infinite possibilities for the rational design of advanced separation materials. During the past 2 decades, many progressive achievements in the development of CFs-derived advanced separation materials have been witnessed already. Herein, the CFs-based separation materials are focused on and the recent progresses in this topic are reviewed. CFs widely existing in animal skins display unique hierarchically fibrous structure, amphiphilicity-enabled surface wetting behaviors, multi-functionality guaranteed covalent/non-covalent reaction versatility. These outstanding merits of CFs bring great opportunities for realizing rational design of a variety of advanced separation materials that were capable of achieving high-performance separations to diverse specific targets, including oily pollutants, natural products, metal ions, anionic contaminants and proteins, etc. Besides, the important issues for the further development of CFs-based advanced separation materials are also discussed.

Research progress on extraction technology and biomedical function of natural sugar substitutes

Improved human material living standards have resulted in a continuous increase in the rate of obesity caused by excessive sugar intake. Consequently, the number of diabetic patients has skyrocketed, not only resulting in a global health problem but also causing huge medical pressure on the government. Limiting sugar intake is a serious problem in many countries worldwide. To this end, the market for sugar substitute products, such as artificial sweeteners and natural sugar substitutes (NSS), has begun to rapidly grow. In contrast to controversial artificial sweeteners, NSS, which are linked to health concepts, have received particular attention. This review focuses on the extraction technology and biomedical function of NSS, with a view of generating insights to improve extraction for its large-scale application. Further, we highlight research progress in the use of NSS as food for special medical purpose (FSMP) for patients.

Core-Shell Covalently Linked Graphitic Carbon Nitride-Melamine-Resorcinol-Formaldehyde Microsphere Polymers for Efficient Photocatalytic CO2 Reduction to Methanol

Photocatalytic reduction of CO₂ with light and H₂O to form CH₃OH is a promising route to mitigate carbon emissions and climate changes. Although semiconducting metal oxides are potential photocatalysts for this reaction, low photon efficiency and leaching of environmentally unfriendly toxic metals limit their applicability. Here, we report metal-free, core-shell photocatalysts consisting of graphitic carbon nitride (g-C₃N₄, CN) covalently linked to melamine-resorcinol-formaldehyde (MRF) microsphere polymers for this reaction. Covalent linkage enabled efficient separation of photo-generated carriers and photocatalysis. Using 100 mg of a photocatalyst containing 15 wt % CN, a CH₃OH yield of 0.99 μmol·h^-1 was achieved at a reaction temperature of 80 °C and 0.5 MPa with external quantum efficiencies ranging from 5.5% at 380 nm to 1.7% at 550 nm. The yield was about 20 and 10 times higher than that of its components CN and MRF, respectively. Characterization with X-ray photoelectron spectroscopy, transmission electron microscopy, and bulk and surface elemental analyses supported the formation of a core-shell structure and the charge transfer in the C-N bond at the CN-MRF interface between the methoxy group in the 2,4-dihydroxylmethyl-1,3-diphenol part of MRF and the terminal amino groups in CN. This enhanced ligand-to-ligand charge transfer resulted in 67% of the photo-excited internal charge transferred from CN to the hydroxymethylamino group in MRF, whose amino group was the catalytic site for the CO₂ photocatalytic reduction to CH₃OH. This study provides a series of new metal-free photocatalyst designs and insights into the molecular-level structure-mediated photocatalytic response.

Zwitterionic iodonium species afford halogen bond-based porous organic frameworks

Porous architectures characterized by parallel channels arranged in honeycomb or rectangular patterns are identified in two polymorphic crystals of a zwitterionic 4-(aryliodonio)-benzenesulfonate. The channels are filled with disordered water molecules which can be reversibly removed on heating. Consistent with the remarkable strength and directionality of the halogen bonds (XBs) driving the crystal packing formation, the porous structure is stable and fully preserved on almost quantitative removal and readsorption of water. The porous systems described here are the first reported cases of one-component 3D organic frameworks whose assembly is driven by XB only (XOFs). These systems are a proof of concept for the ability of zwitterionic aryliodonium tectons in affording robust one-component 3D XOFs. The high directionality and strength of the XBs formed by these zwitterions and the geometrical constraints resulting from the tendency of their hypervalent iodine atoms to act as bidentate XB donors might be key factors in determining this ability.

Hydrogen Production and Its Applications to Mobility

Hydrogen has been identified as one of the key elements to bolster longer-term climate neutrality and strategic autonomy for several major countries. Multiple road maps emphasize the need to accelerate deployment across its supply chain and utilization. Being one of the major contributors to global CO₂ emissions, the transportation sector finds in hydrogen an appealing alternative to reach sustainable development through either its direct use in fuel cells or further transformation to sustainable fuels. This review summarizes the latest developments in hydrogen use across the major energy-consuming transportation sectors. Rooted in a systems engineering perspective, we present an analysis of the entire hydrogen supply chain across its economic, environmental, and social dimensions. Providing an outlook on the sector, we discuss the challenges hydrogen faces in penetrating the different transportation markets. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Ultralarge Free-Standing Imine-Based Covalent Organic Framework Membranes Fabricated via Compression

Demand continues for processing methods to shape covalent organic frameworks (COFs) into macroscopic objects that are needed for their practical applications. Herein, a simple compression method to prepare large-scale, free-standing homogeneous and porous imine-based COF-membranes with dimensions in the centimeter range and excellent mechanical properties is reported. This method entails the compression of imine-based COF-aerogels, which undergo a morphological change from an elastic to plastic material. The COF-membranes fabricated upon compression show good performances for the separation of gas mixtures of industrial interest, N2 /CO2 and CH4 /CO2 . It is believed that the new procedure paves the way to a broader range of COF-membranes.

Homogeneously Mixing Different Metal-Organic Framework Structures in Single Nanocrystals through Forming Solid Solutions

Pore engineering plays a significant role in the applications of porous materials, especially in the area of separation and catalysis. Here, we demonstrated a metal-organic framework (MOF) solid solution (MOSS) strategy to homogeneously and controllably mix NU-1000 and NU-901 structures inside single MOF nanocrystals. The key for the homogeneous mixing and forming of MOSS was the bidentate modulator, which was designed to have a slightly longer distance between two carboxylate groups than the original tetratopic ligand. All of the MOSS nanocrystals showed a uniform pore size distribution with a well-tuned ratio of mesopores to micropores. Because of the appropriate pore ratio, MOSS nanocrystals can balance the thermodynamic interactions and kinetic diffusion of the substrates, thus showing exceedingly higher separation abilities and a unique elution sequence. Our work proposes a rational strategy to design mixed-porous MOFs with controlled pore ratios and provides a new direction to design homogeneously mixed MOFs with a high separation ability and unique separation selectivity.

Preparation of Butadiene-Bridged Polymethylsiloxane/Ethylcellulose/1-Carboxymethyl-3-methylimidazolium Chloride Ternary Composite Membranes for Gas Separation

Excessive CO2 emissions have resulted in global warming and are a serious threat to the life of people, various strategies have been implemented to cut carbon emissions, and one of them is the use of a gas separation membrane to capture CO2 effectively. In this experiment, the butadiene-bridged polymethylsiloxane (BBPMS)/ethyl cellulose (EC)/ionic liquid (IL) ternary composite membranes were prepared by EC as a substrate, BBPMS, and IL as additives in tetrahydrofuran under high-speed stirring and coated on the membrane. The membrane structure was characterized by a Fourier transform infrared spectrometer and scanning electron microscope, and the membrane properties were tested by a membrane tensile strength tester, thermal weight loss analyzer, and gas permeability meter. The results show that the surface of the ternary composite membrane is dense and flat with a uniform distribution, and the membrane formation, heat resistance, and mechanical properties are good. The permeability coefficient of the ternary composite membrane for CO2 reached 1806.03 Barrer, which is 20.00 times higher than that of the EC/IL hybrid matrix membrane. The permeability coefficient of O2 reached 321.01 Barrer, which is 19.21 times higher than that of the EC/IL membrane. When the doping amount of BBPMS is 70-80%, the O2/N2 gas permeation separation of the BBPMS/EC/IL ternary composite membrane is close to the Robertson 2008 curve. It is always known that in the gas separation process the membrane material is the most crucial factor. The success of this experiment points to a new direction for the preparation of new membrane materials.

Recent Progress in Mixed-Matrix Membranes for Hydrogen Separation

Membrane separation is a compelling technology for hydrogen separation. Among the different types of membranes used to date, the mixed-matrix membranes (MMMs) are one of the most widely used approaches for enhancing separation performances and surpassing the Robeson upper bound limits for polymeric membranes. In this review, we focus on the recent progress in MMMs for hydrogen separation. The discussion first starts with a background introduction of the current hydrogen generation technologies, followed by a comparison between the membrane technology and other hydrogen purification technologies. Thereafter, state-of-the-art MMMs, comprising emerging filler materials that include zeolites, metal-organic frameworks, covalent organic frameworks, and graphene-based materials, are highlighted. The binary filler strategy, which uses two filler materials to create synergistic enhancements in MMMs, is also described. A critical evaluation on the performances of the MMMs is then considered in context, before we conclude with our perspectives on how MMMs for hydrogen separation can advance moving forward.

Hotpots and trends of covalent organic frameworks (COFs) in the environmental and energy field: Bibliometric analysis

Covalent organic frameworks (COFs) have attracted extensive attention due to their low density, adjustable structure, functionalization, and good stability. This paper systematically and comprehensively describes to qualitatively and quantitatively the progress, trends, and hotspots of COFs in the environmental and energy fields from the perspective of bibliometrics. Herein, based on the Web of Science database, a total of 2589 articles from 2005 to October 6, 2020, were collected. Thereafter, co-occurrence, co-citation analysis, and cluster analysis were conducted using CiteSpace and VOSviewer software. The results indicated that COFs research shows the characteristics of rapid growth. The active countries were mainly USA, Germany, Japan, China, and India. More than half of the top 20 active institutions were from China. The research hotspots in this field were systematically elaborated, including synthesis, adsorption, catalysis, membrane, sensor, and energy storage. Research has shown that various COFs are reasonably designed, synthesized, and used in different applications. For example, when COFs are used for photocatalysis, groups containing photocatalytic active sites are integrated into COFs to improve photocatalytic activity. Finally, some challenges were proposed, that are beneficial to the rapid and balanced development of the COFs field. For instance, the preparation methods still need to be further improved for mass production and there is an imbalance in environmental applications such as fewer sensor and membrane applications. We believe that this study provides a comprehensive and systematic overview of the environmental and energy applications of COFs for future investigations.

Construction of Stable Helical Metal-Organic Frameworks with a Conformationally Rigid "Concave Ligand"

A helical metal-organic framework was prepared by using a conformationally rigid tetratopic benzoic acid ligand with binding units pointing toward each other (concave ligand). To avoid the obvious intramolecular interactions between binding units, matching spacing groups were applied to introduce atropic repulsion, thereby allowing the formation of extended frameworks for the first time. With this new ligand design, a helical-shaped MOF with significantly improved air and moisture stability was successfully prepared, thus providing a new strategy for ligand design toward porous material constructions.

A 2D Graphitic-Polytriaminopyrimidine (g-PTAP)/Poly(ether-block-amide) Mixed Matrix Membrane for CO2 Separation

Poly(ether-block-amide)/g-PTAP mixed matrix membranes (MMMs) were developed by incorporating different wt.% (1-10%) of a novel 2D g-PTAP nanofiller and its effects on membrane structure and gas permeability were studied. The novel 2D material g-PTAP was synthesized and characterized by various analytical techniques including field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Raman spectroscopy. The fabricated MMMs were investigated to study the interaction and compatibility between Pebax and g-PTAP. The MMMs showed an effective integration of g-PTAP nanofiller into the Pebax matrix without affecting its thermal stability. Gas permeation experiments with MMMs showed improved CO₂ permeability and selectivity (CO₂ /N₂ ) upon incorporation of g-PTAP in the Pebax polymer matrix. The maximum CO₂ permeability enhancement from 82.3 to 154.6 Barrer with highest CO₂ /N₂ selectivity from 49.5 to 83.5 were found with 2.5 wt.% of nanofiller compared to neat Pebax membranes.

Organic molecular sieve membranes for chemical separations

Molecular separations that enable selective transport of target molecules from gas and liquid molecular mixtures, such as CO2 capture, olefin/paraffin separations, and organic solvent nanofiltration, represent the most energy sensitive and significant demands. Membranes are favored for molecular separations owing to the advantages of energy efficiency, simplicity, scalability, and small environmental footprint. A number of emerging microporous organic materials have displayed great potential as building blocks of molecular separation membranes, which not only integrate the rigid, engineered pore structures and desirable stability of inorganic molecular sieve membranes, but also exhibit a high degree of freedom to create chemically rich combinations/sequences. To gain a deep insight into the intrinsic connections and characteristics of these microporous organic material-based membranes, in this review, for the first time, we propose the concept of organic molecular sieve membranes (OMSMs) with a focus on the precise construction of membrane structures and efficient intensification of membrane processes. The platform chemistries, designing principles, and assembly methods for the precise construction of OMSMs are elaborated. Conventional mass transport mechanisms are analyzed based on the interactions between OMSMs and penetrate(s). Particularly, the 'STEM' guidelines of OMSMs are highlighted to guide the precise construction of OMSM structures and efficient intensification of OMSM processes. Emerging mass transport mechanisms are elucidated inspired by the phenomena and principles of the mass transport processes in the biological realm. The representative applications of OMSMs in gas and liquid molecular mixture separations are highlighted. The major challenges and brief perspectives for the fundamental science and practical applications of OMSMs are tentatively identified.

MOF-in-COF molecular sieving membrane for selective hydrogen separation

Covalent organic frameworks (COFs) are promising materials for advanced molecular-separation membranes, but their wide nanometer-sized pores prevent selective gas separation through molecular sieving. Herein, we propose a MOF-in-COF concept for the confined growth of metal-organic framework (MOFs) inside a supported COF layer to prepare MOF-in-COF membranes. These membranes feature a unique MOF-in-COF micro/nanopore network, presumably due to the formation of MOFs as a pearl string-like chain of unit cells in the 1D channel of 2D COFs. The MOF-in-COF membranes exhibit an excellent hydrogen permeance (>3000 GPU) together with a significant enhancement of separation selectivity of hydrogen over other gases. The superior separation performance for H2/CO2 and H2/CH4 surpasses the Robeson upper bounds, benefiting from the synergy combining precise size sieving and fast molecular transport through the MOF-in-COF channels. The synthesis of different combinations of MOFs and COFs in robust MOF-in-COF membranes demonstrates the versatility of our design strategy.