Solution processing of crystalline porous material based membranes for CO2 separation.

The carbon emission problem is a significant challenge in today's society, which has led to severe global climate issues. Membrane-based separation technology has gained considerable interest in CO2 separation due to its simplicity, environmental friendliness, and energy efficiency. Crystalline porous materials (CPMs), such as zeolites, metal-organic frameworks, covalent organic frameworks, hydrogen-bonded organic frameworks, and porous organic cages, hold great promise for advanced CO2 separation membranes because of their ordered and customizable pore structures. However, the preparation of defect-free and large-area crystalline porous material (CPM)-based membranes remains challenging, limiting their practical use in CO2 separation. To address this challenge, the solution-processing method, commonly employed in commercial polymer preparation, has been adapted for CPM membranes in recent years. Nanosheets, spheres, molecular cages, and even organic monomers, depending on the CPM type, are dissolved in suitable solvents and processed into continuous membranes for CO2 separation. This feature article provides an overview of the recent advancements in the solution processing of CPM membranes. It summarizes the differences among the solution-processing methods used for forming various CPM membranes, highlighting the key factors for achieving continuous membranes. The article also summarizes and discusses the CO2 separation performance of these membranes. Furthermore, it addresses the current issues and proposes future research directions in this field. Overall, this feature article aims to shed light on the development of solution-processing techniques for CPM membranes, facilitating their practical application in CO2 separation.

An Ultrastable Bifunctional Electrocatalyst Derived from a Co2+-Anchored Covalent-Organic Framework for High-Efficiency ORR/OER and Rechargeable Zinc-Air Battery.

It remains a great challenge to develop alternative electrocatalysts with high stability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, a bifunctional electrocatalyst composed of hollow CoOx (Co3O4/CoO) nanoparticles embedded in lamellar carbon nanofibers is derived from a Co2+-anchored covalent-organic framework. The as-fabricated electrocatalyst (CoOx@NC-800) exhibits a half-wave potential (E1/2) of 0.89 V with ultrahigh long-term stability (100% current retention after 3000 CV cycles). Together with promising OER performance, the CoOx@NC-800 based reversible Zn-air battery displays a small potential gap (0.70 V), superior to that of the commercial 20% Pt/C + RuO2. The density functional theory (DFT) calculations reveal that the remarkable electrocatalytic performance and stability of CoOx@NC-800 are attributed to the optimized adsorption of the *OOH intermediate and reduced free energy of the potential-limiting step. This study establishes the functionalization of COF structure for fabrication of high-performance carbon-based electrocatalysts.

PANa/Covalent organic framework composites with improved water uptake and proton conductivity.

It is a challenge to effectively load proton carriers in COFs to improve their proton conductivity. Herein, we report a series of COF-based composites, PANa@UCOF-x (PANa: sodium polyacrylate, x: weight percentage of PANa), which were prepared by coating different proportions of superabsorbent PANa on the COF surface based on an in situ reaction strategy. Since PANa can greatly enhance the enrichment of water molecules in one-dimensional (1D) channels of COFs, these COF-based composites exhibit superprotonic conduction. At 80 °C and 95% relative humidity (RH), the proton conductivity of PANa@UCOF-10, PANa@UCOF-28 and PANa@UCOF-40 reaches 1.6 × 10-2, 5.1 × 10-2, and 1.1 × 10-1 S cm-1, respectively, which is 4-5 orders of magnitude higher than 7.4 × 10-7 S cm-1 of the original UCOF. This work not only develops a new method to improve the water content of the COF channels, but also proves the important role of ordered channels in constructing effective proton conduction pathways.

Single-Atom-like B-N3 Sites in Ordered Macroporous Carbon for Efficient Oxygen Reduction Reaction.

On the premise of cleanliness and stability, improving the catalytic efficiency for the oxygen reduction reaction in the electrode reaction of fuel cells and metal-air batteries is of vital importance. Studies have shown that heteroatom doping and structural optimization are efficient strategies. Herein, a single-atom-like B-N3 configuration in carbon is designed for efficient oxygen reduction reaction catalysis inspired by the extensively studied transition metal M-Nx sites, which is supported on the ordered macroporous carbon prepared by utilizing a hydrogen-bonded organic framework as carbon and nitrogen sources and SiO2 spheres as a template. The co-doping of B/N and ordered macroporous structures promote the metal-free material high oxygen reduction catalytic performance in alkaline media. DFT calculations reveal that the B-N3 structure played a key role in enhancing the oxygen reduction activity by providing rich favorable *OOH and *OH adsorption sites on the B center. The promoted formation of *OH/*OOH intermediates accelerated the electrocatalyst reaction. This study provides new insights into the design of single-atom-like nonmetallic ORR electrocatalysts and synthesis of ordered macroporous carbons based on hydrogen-bonded organic frameworks.

Constructing Porous Carbon Electrocatalysts from Cobalt Complex-Decorated Micelles of Mesoporous Silica for Oxygen Reduction/Evolution Reaction.

The construction of a porous carbon structure with a high specific surface area is conducive to enhanced electrocatalytic activity due to the accessibility of active sites and improvement of the mass transfer. Herein, we explored the possibility of using micelles of mesoporous silica (MCM-48) as the carbon source to generate porous carbon under the confinement of MCM-48 channels. The complexes formed by Co2+ and 4,4'-bipyridine were in situ incorporated into the micelles to derive Co-related active sites (Co-Nx, Co, and Co3O4) for catalyzing the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). After pyrolysis in the N2 atmosphere and subsequent removal of the MCM-48 skeleton, the target porous carbon electrocatalyst was obtained, which exhibited promising performance for both ORR and OER and has great potential as the cathode material for Zn-air battery application. This work not only confirms the effectiveness of using the micelles of MCM-48 as the carbon source for preparing the porous carbon materials, but also provides a new platform for design and synthesis of structurally controllable materials for energy-related electrocatalytic applications.

Recent progress in pristine MOF-based catalysts for electrochemical hydrogen evolution, oxygen evolution and oxygen reduction.

Among various kinds of materials that have been investigated as electrocatalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), metal-organic frameworks (MOFs) has emerged as a promising material for electrocatalyzing these vital processes owing to their structural merits that integrate advantages of both homogeneous and heterogeneous catalysts; however there is still big room for their improvement in terms of inferior activity and poor conductivity, as well as the ambiguity of real active sites. In this review, advanced strategies with the aim of solving the activity and conductivity problems are summarized as microstructure engineering and conductivity improvement, respectively. The structural evolution of some MOFs and their real active species has also been discussed. Finally, perspectives on the development of MOF materials for HER, OER and ORR electrocatalysis are provided.

One-step Ethylene Purification from an Acetylene/Ethylene/Ethane Ternary Mixture by Cyclopentadiene Cobalt-Functionalized Metal-Organic Frameworks.

The separation of ethylene (C2 H4 ) from a mixture of ethane (C2 H6 ), ethylene (C2 H4 ), and acetylene (C2 H2 ) at normal temperature and pressure is a significant challenge. The sieving effect of pores is powerless due to the similar molecular size and kinetic diameter of these molecules. We report a new modification method based on a stable ftw topological Zr-MOF platform (MOF-525). Introduction of a cyclopentadiene cobalt functional group led to new ftw-type MOFs materials (UPC-612 and UPC-613), which increase the host-guest interaction and achieve efficient ethylene purification from the mixture of hydrocarbon gases. The high performance of UPC-612 and UPC-613 for C2 H2 /C2 H4 /C2 H6 separation has been verified by gas sorption isotherms, density functional theory (DFT), and experimentally determined breakthrough curves. This work provides a one-step separation of the ternary gas mixture and can further serve as a blueprint for the design and construction of function-oriented porous structures for such applications.

Scalable crystalline porous membranes: current state and perspectives.

Crystalline porous materials (CPMs) with uniform and regular pore systems show great potential for separation applications using membrane technology. Along with the research on the synthesis of precisely engineered porous structures, significant attention has been paid to the practical application of these materials for preparation of crystalline porous membranes (CPMBs). In this review, the progress made in the preparation of thin, large area and defect-free CPMBs using classical and novel porous materials and processing is presented. The current state-of-the-art of scalable CPMBs with different nodes (inorganic, organic and hybrid) and various linking bonds (covalent, coordination, and hydrogen bonds) is revealed. The advances made in the scalable production of high-performance crystalline porous membranes are categorized according to the strategies adapted from polymer membranes (interfacial assembly, solution-casting, melt extrusion and polymerization of CPMs) and tailored based on CPM properties (seeding-secondary growth, conversion of precursors, electrodeposition and chemical vapor deposition). The strategies are compared and ranked based on their scalability and cost. The potential applications of CPMBs have been concisely summarized. Finally, the performance and challenges in the preparation of scalable CPMBs with emphasis on their sustainability are presented.

Micelles of Mesoporous Silica with Inserted Iron Complexes as a Platform for Constructing Efficient Electrocatalysts for Oxygen Reduction.

Iron, N-codoped carbon materials (Fe-N-C) are promising electrocatalysts toward oxygen reduction reactions due to their high atom utilization efficiency and intrinsic activity. Nanostructuring of the Fe-N-C materials, such as introducing porosity into the carbon structure, would be conducive to further increasing the exposure of active sites as well as improving the mass transfer. Herein, we explore the potential of iron complex-functionalized micelles of mesoporous SiO2 as a platform for constructing porous Fe-N-C materials. The classical three-dimensional MCM-48 was selected as a proof-of-concept example, which was utilized as the hard template, and cetyltrimethylammonium bromide micelles inside it played the role of the main carbon source. Fe-Nx sites were derived from Fe-1,10-phenanthroline complexes in the micelles introduced by in situ incorporation of 1,10-phenanthroline and post Fe2+ insertion in an aqueous solution. After thermal annealing in a nitrogen atmosphere and subsequent removal of the MCM-48 framework, a carbon material that possesses porous structural features with uniformly dispersed Fe-Nx sites (MPC@PhFe) was obtained, which shows superior ORR activity in a 0.1 M KOH solution and great potential for Zn-air battery applications as well. This work demonstrates the feasibility as well as the effectiveness of turning micelles of mesoporous SiO2 into porous carbon structures and might offer a universal strategy for manufacturing carbon materials for future application in energy storage and conversion.

Single-crystal-to-single-crystal transformation and proton conductivity of three hydrogen-bonded organic frameworks.

We report the cyclic single-crystal-to-single-crystal transformation of three hydrogen-bonded organic frameworks (HOFs), induced by the change of temperature and humidity, which clearly reveals that the -SO3-and -NH2 groups in UPC-H7 and UPC-H8 facilitate the diffusion of water molecules into their anhydrous structures to form hydrous UPC-H9. Their proton conductivity was studied under different humidity at varying temperature, showing that the proton conductivity is closely related to water molecules entering the crystal structures arising from the hydrogen bonded reorganization in combination with the triaxial single-crystal proton conductivity tests.

Optimizing Multivariate Metal-Organic Frameworks for Efficient C2H2/CO2 Separation.

Adsorptive separation of acetylene (C2H2) from carbon dioxide (CO2) promises a practical way to produce high-purity C2H2 required for industrial applications. However, challenges exist in the pore environment engineering of porous materials to recognize two molecules due to their similar molecular sizes and physical properties. Herein, we report a strategy to optimize pore environments of multivariate metal-organic frameworks (MOFs) for efficient C2H2/CO2 separation by tuning metal components, functionalized linkers, and terminal ligands. The optimized material UPC-200(Al)-F-BIM, constructed from Al3+ clusters, fluorine-functionalized organic linkers, and benzimidazole terminal ligands, demonstrated the highest separation efficiency (C2H2/CO2 uptake ratio of 2.6) and highest C2H2 productivity among UPC-200 systems. Experimental and computational studies revealed the contribution of small pore size and polar functional groups on the C2H2/CO2 selectivity and indicated the practical C2H2/CO2 separation of UPC-200(Al)-F-BIM.

Cross-Linking between Sodalite Nanoparticles and Graphene Oxide in Composite Membranes to Trigger High Gas Permeance, Selectivity, and Stability in Hydrogen Separation.

Thin membranes (900 nm) were prepared by direct transformation of infiltrated amorphous precursor nanoparticles, impregnated in a graphene oxide (GO) matrix, into hydroxy sodalite (SOD) nanocrystals. The amorphous precursor particles rich in silanols (Si-OH) enhanced the interactions with the GO, thus leading to the formation of highly adhesive and stable SOD/GO membranes via strong bonding. The cross-linking of SOD nanoparticles with the GO in the membranes promoted both the high gas permeance and enhanced selectivity towards H2 from a mixture containing CO2 and H2 O. The SOD/GO membranes are moisture resistance and exhibit steady separation performance (H2 permeance of about 4900 GPU and H2 /CO2 selectivity of 56, with no degradation in performance during the test of 50 h) at high temperature (200 °C) under water vapor (4 mol %).

An ultrafast responsive NO2 gas sensor based on a hydrogen-bonded organic framework material.

We report the development of a new type of organic semiconductor gas sensor based on a porphyrin-based hydrogen-bonded organic framework (HOF). Owing to the orderly porous structures, the decoration with rich amino sites and the n-type semiconductor nature, this HOF-based sensor exhibits selective NO2 sensing performance with ultra-fast response/recovery rates (17.6 s/15.4 s over 100 ppb) and a limit of detection lower than 40 ppb, together with high sensitivity, good reproducibility, and long-term stability at room temperature. This study demonstrates that HOF-based materials have potential application prospects in gas sensing, thereby offering a new way of thinking for the design and development of sensors.

Fabrication of a Hydrogen-Bonded Organic Framework Membrane through Solution Processing for Pressure-Regulated Gas Separation.

Ordered and flexible porous frameworks with solution processability are highly desirable to fabricate continuous and large-scale membranes for the efficient gas separation. Herein, the first microporous hydrogen-bonded organic framework (HOF) membrane has been fabricated by an optimized solution-processing technique. The framework exhibits the superior stability because of the abundant hydrogen bonds and strong π-π interactions. Thanks to the flexible HOF structure, the membrane possesses the unprecedented pressure-responsive H2 /N2 separation performance. Furthermore, the scratched membrane can be healed by the treatment of solvent vapor, achieving the recovery of separation performance.

Fine-Tuning the Pore Environment of the Microporous Cu-MOF for High Propylene Storage and Efficient Separation of Light Hydrocarbons.

Ethylene (C2H4) and propylene (C3H6) are important energy sources and raw materials in the chemical industry. Storage and separation of C2H4 and C3H6 are vital to their practical application. Metal-organic frameworks (MOFs) having adjustable structures and pore environments are promising candidates for C3H6/C2H4 separation. Herein, we obtained a Cu-based MOF synthesized by H3TTCA and pyrazine ligands. By adding different functional groups on the ligands within the MOFs, their pore environments are adjusted, and thus, the C3H6 storage capacity and C3H6/C2H4 separation efficiency are improved. Eventually, the fluoro- and methyl-functionalized iso-MOF-4 exhibits a better gas storage and C3H6/C2H4 separation performance compared with iso-MOF-1 (nonfunctionalized), iso-MOF-2 (fluoro-functionalized), and iso-MOF-3 (methyl-functionalized). A record-high C3H6 uptake of 293.6 ± 2.3 cm3 g-1 (273 K, 1 atm) is achieved using iso-MOF-4. Moreover, iso-MOF-4 shows excellent repeatability, and only 3.5% of C3H6 storage capacities decrease after nine cycles. Employing Grand Canonical Monte Carlo (GCMC) simulations, it is indicated that iso-MOF-4 preferentially adsorbs C3H6 rather than C2H4 at low pressure. Single-crystal X-ray diffraction on C3H6-adsorbed iso-MOF-4 crystals precisely demonstrates the adsorption positions and arrangement of C3H6 molecules in the framework, which is consistent with the theoretical simulations. Remarkably, gas sorption isotherms, molecular simulations, and breakthrough experiments comprehensively demonstrate that this unique MOF material exhibits highly efficient C3H6/C2H4 separation. Additionally, iso-MOF-4 also possesses efficient separation of C3H8/CH4 and C2H6/CH4, indicating its promising potential in storage/separation of light hydrocarbons in industry.

Temperature controlled diffusion of hydroxide ions in 1D channels of Ni-MOF-74 for its complete conformal hydrolysis to hierarchical Ni(OH)2 supercapacitor electrodes.

Conformal hydrolysis of MOF precursors is a promising strategy to prepare hierarchical metal hydroxide electrode materials on a large scale with low cost and high efficiency. However, a complete transformation is challenging due to the normal "outside-in" conversion process. After studying the hydrolysis of Ni-MOF-74, which has regular 1D channels, we suggest that the transformation to Ni(OH)2 can occur simultaneously outside and within the precursor depending on the treatment temperature. Molecular dynamics simulations reveal that a higher temperature weakens the steric effects of OH- ions and facilitates the diffusion in the regular channels, and therefore, a complete transformation from Ni-MOF-74 to Ni(OH)2 is achieved. It is for the first time demonstrated that the 1D channels of MOFs are utilized for the complete conformal hydrolysis of Ni-MOF-74 to Ni(OH)2 electrode materials. Meanwhile, we also perform pioneering work illustrating that the complete conformal hydrolysis is the key to the improved supercapacitor performances of the MOF-derived Ni(OH)2 electrodes. The prepared Ni(OH)2 electrode under the optimized conditions has a specific capacity of 713.2 C g-1 at a current density of 1 A g-1, which is at least 28% larger than those of the Ni(OH)2 prepared at other temperatures. The detailed analyses based on CV and EIS of the obtained Ni(OH)2 electrodes indicate that the residual MOFs within electrodes due to incomplete hydrolysis significantly influence the diffusion length and diffusion efficiency of OH-, drastically lowering the supercapacitor performances.

TiO2-Coated Interlayer-Expanded MoSe2/Phosphorus-Doped Carbon Nanospheres for Ultrafast and Ultralong Cycling Sodium Storage.

Based on multielectron conversion reactions, layered transition metal dichalcogenides are considered promising electrode materials for sodium-ion batteries, but suffer from poor cycling performance and rate capability due to their low intrinsic conductivity and severe volume variations. Here, interlayer-expanded MoSe2/phosphorus-doped carbon hybrid nanospheres coated by anatase TiO2 (denoted as MoSe2/P-C@TiO2) are prepared by a facile hydrolysis reaction, in which TiO2 coating polypyrrole-phosphomolybdic acid is utilized as a novel precursor followed by a selenization process. Benefiting from synergistic effects of MoSe2, phosphorus-doped carbon, and TiO2, the hybrid nanospheres manifest unprecedented cycling stability and ultrafast pseudocapacitive sodium storage capability. The MoSe2/P-C@TiO2 delivers decent reversible capacities of 214 mAh g-1 at 5.0 A g-1 for 8000 cycles, 154 mAh g-1 at 10.0 A g-1 for 10000 cycles, and an exceptional rate capability up to 20.0 A g-1 with a capacity of ≈175 mAh g-1 in a voltage range of 0.5-3.0 V. Coupled with a Na3V2(PO4)3@C cathode, a full cell successfully confirms a reversible capacity of 242.2 mAh g-1 at 0.5 A g-1 for 100 cycles with a coulombic efficiency over 99%.

N,P-Doped carbon with encapsulated Co nanoparticles as efficient electrocatalysts for oxygen reduction reactions.

Exploring efficient non-noble ORR catalysts as alternatives to Pt-based catalysts are highly demanded for their possible application in fuel cells and rechargeable metal-air batteries. Herein, we demonstrate a rational design and synthesis of a N, P-doped carbon with encapsulated Co nanoparticles as efficient electrocatalysts for ORR. The catalyst is derived from a mixture of Co-MOF and triphenylphosphine with a mass ratio of 3 : 1 by pyrolysis in N2 atmosphere at 700 °C. The catalyst exhibited a superior ORR catalytic performance among its counterparts in 0.1 M KOH with onset and half-wave potentials of 0.88 V and 0.80 V, a much larger limiting current density of -5.93 mA cm-2 that surpassed commercial 20% Pt/C, in addition to its durability and resistance to methanol. This enhanced ORR activity of the catalyst can be attributed to the synergistic effect between Co NPs and N, P atoms, the relatively large contact surface, more exposed active sites and good electrical conductivity. This study would provide some new ideas for the design and construction of promising carbon-based non-precious metal electrocatalysts for future practical fuel cell applications.

Green Hydrogen Separation from Nitrogen by Mixed-Matrix Membranes Consisting of Nanosized Sodalite Crystals.

Nanosized sodalite (Nano-SOD) crystals were used as active filler to prepare mixed-matrix membranes (MMMs) for promoting the H2 /N2 gas-separation performance. The Nano-SOD crystals with extremely small crystallites (40-50 nm) were synthesized from a colloidal suspension free of organic structural directing agent and uniformly dispersed in the polyetherimide (PEI) matrix. The Nano-SOD filler with a suitable aperture size (2.8 Å) allowed only H2 molecules to pass through and rejected the N2 , thus improving the selectivity of the membranes. The high dispersion of Nano-SOD crystals in the polymer matrix and the interactions between the inorganic and organic phases greatly improved the membrane separation performance and minimized interfacial holes. The MMMs showed a high H2 permeability (≈7155.1 Barrer at 25 °C under atmospheric pressure) and an ideal H2 /N2 selectivity factor of approximately 16.9 in a single gas test. Moreover, in a gas mixture (H2 /N2 , 25-100 °C), the selectivity factor increased significantly to approximately 30.9. The high stability of the MMMs, which consist of highly dispersed Nano-SOD crystals in a PEI matrix for H2 /N2 separation (6 weeks continuous test), makes them an important material for ammonia synthesis applications that require and also release a large amount of H2 .

Green Fabrication of Ultrathin Co3O4 Nanosheets from Metal-Organic Framework for Robust High-Rate Supercapacitors.

Two-dimensional cobalt oxide (Co3O4) is a promising candidate for robust electrochemical capacitors with high performance. Herein, we use 2,3,5,6-tetramethyl-1,4-diisophthalate as a recyclable ligand to construct a Co-based metal-organic framework of UPC-9, and subsequently, we obtain ultrathin hierarchical Co3O4 hexagonal nanosheets with a thickness of 3.5 nm through a hydrolysis and calcination process. A remarkable and excellent specific capacitance of 1121 F·g-1 at a current density of 1 A·g-1 and 873 F·g-1 at a current density of 25 A·g-1 were achieved for the as-prepared asymmetric supercapacitor, which can be attributed to the ultrathin 2D morphology and the rich macroporous and mesoporous structures of the ultrathin Co3O4 nanosheets. This synthesis strategy is environmentally benign and economically viable due to the fact that the costly organic ligand molecules are recycled, reducing the materials cost as well as the environmental cost for the synthesis process.