Multivariate Hydrogen-Bonded Organic Frameworks with Tunable Permanent Porosities for Capture of a Mustard Gas Simulant

Precise synthesis of topologically predictable and discrete molecular crystals with permanent porosities remains a long-term challenge. Here, we report the first successful synthesis of a series of 11 isoreticular multivariate hydrogen-bonded organic frameworks (MTV-HOFs) from pyrene-based derivatives bearing −H, −CH₃, −NH₂ and −F groups achieved by a shape-fitted, π–π stacking self-assembly strategy. These MTV-HOFs are single-crystalline materials composed of tecton, as verified by single-crystal diffraction, nuclear magnetic resonance (NMR) spectra, Raman spectra, water sorption isotherms and density functional theory (DFT) calculations. These MTV-HOFs exhibit tunable hydrophobicity with water uptake starting from 50 to 80 % relative humidity, by adjusting the combinations and ratios of functional groups. As a proof of application, the resulting MTV-HOFs were shown to be capable of capturing a mustard gas simulant, 2-chloroethyl ethyl sulfide (CEES) from moisture. The location of different functional groups within the pores of the MTV-HOFs leads to a synergistic effect, which resulted in a superior CEES/H₂O selectivity (up to 94 %) compared to that of the HOFs with only pure component and enhanced breakthrough performance (up to 4000 min/g) when compared to benchmark MOF materials. This work is an important advance in the synthesis of MTV-HOFs, and provides a platform for the development of porous molecular materials for numerous applications.     

In Situ Enzyme Immobilization by Covalent Organic Frameworks

Enzyme immobilization is a widely reported method to favor the applicability of enzymes by enhancing their stability and re-usability. Among the various existing solid supports and immobilization strategies, the in situ encapsulation of enzymes within crystalline porous matrices is a powerful tool to design biohybrids with a stable and protected catalytic activity. However, to date, only a few metal–organic frameworks (MOFs) and hydrogen-bonded organic frameworks (HOFs) have been reported. Excitingly, for the first time, Y. Chen and co-workers expanded the in situ bio-encapsulation to a new class of crystalline porous materials, namely covalent organic frameworks (COFs). The enzyme@COF materials not only exhibited high enzyme loading with minimal leaching, high catalytic activity and selectivity, chemical and long-term stability and recyclability but could also be scaled up to a few grams. Undoubtedly, this work opens new striking opportunities for enzymatic immobilization and will stimulate new research on COF-based matrices.     

Porous Covalent Organic Framework Based Hydrogen-Bond Nanotrap for the Precise Recognition and Separation of Gold

Utilizing weak interactions to effectively recover and separate precious metals in solution is of great importance but the practice remains a challenge. Herein, we report a novel strategy to achieve precise recognition and separation of gold by regulating the hydrogen-bond (H-bond) nanotrap within the pore of covalent organic frameworks (COFs). It is found that both COF-HNU25 and COF-HNU26 can efficiently capture Au^III with fast kinetics, high selectivity, and uptake capacity. In particular, the COF-HNU25 with the high density of H-bond nanotraps exhibits an excellent gold uptake capacity of 1725 mg g⁻¹, which is significantly higher than that (219 mg g⁻¹) of its isostructural COF (COF-42) without H-bond nanostrap in the pores. Importantly, the uptake capacity is strongly correlated to the number of H-bonds between phenolic OH in the COF and [AuCl₄]⁻ in water, and multiple H-bond interactions are the key driving force for the excellent gold recovery and reusability of the adsorbent.     

Fabrication of Pillar-Cage Fluorinated Anion Pillared Metal–Organic Frameworks via a Pillar Embedding Strategy and Efficient Separation of SO2 through Multi-Site Trapping

Flue gas desulfurization is crucial for both human health and ecological environments. However, developing efficient SO₂ adsorbents that can break the trade-off between adsorption capacity and selectivity is still challenging. In this work, a new type of fluorinated anion-pillared metal–organic frameworks (APMOFs) with a pillar-cage structure is fabricated through pillar-embedding into a highly porous and robust framework. This type of APMOFs comprises smaller tetrahedral cages and larger icosahedral cages interconnected by embedded [NbOF₅]²⁻ and [TaOF₅]²⁻ anions acting as pillars. The APMOFs exhibits high porosity and density of fluorinated anions, ensuring exceptional SO₂ adsorption capacity and ultrahigh selectivity for SO₂/CO₂ and SO₂/N₂ gas mixtures. Furthermore, these two structures demonstrate excellent stability towards water, acid/alkali, and SO₂ adsorption. Cycle dynamic breakthrough experiments confirm the excellent separation performance of SO₂/CO₂ gas mixtures and their cyclic stability. SO₂-loaded single-crystal X-ray diffraction, Grand canonical Monte Carlo (GCMC) simulations combined with density functional theory (DFT) calculations reveal the preferred adsorption domains for SO₂ molecules. The multiple-site host–guest and guest-guest interactions facilitate selective recognition and dense packing of SO₂ in this hybrid porous material. This work will be instructive for designing porous materials for flue gas desulfurization and other gas-purification processes.     

Water-Stable Metal Azolate Frameworks Showing Interesting Flexibilities for Highly Effective Bioethanol Dehydration

The ethanol/water separation challenge highlights the adsorption capacity/selectivity trade-off problem. We show that the target guest can serve as a gating component of the host to block the undesired guest, giving molecular sieving effect for the adsorbent possessing large pores. Two hydrophilic/water-stable metal azolate frameworks were designed to compare the effects of gating and pore-opening flexibility. Large amounts (up to 28.7 mmol g⁻¹) of ethanol with fuel-grade (99.5 %+) and even higher purities (99.9999 %+) can be produced in a single adsorption process from not only 95 : 5 but also 10 : 90 ethanol/water mixtures. More interestingly, the pore-opening adsorbent possessing large pore apertures showed not only high water adsorption capacity but also exceptionally high water/ethanol selectivity characteristic of molecular sieving. Computational simulations demonstrated the critical role of guest-anchoring aperture for the guest-dominated gating process.     

A Thiazole-linked Covalent Organic Framework for Lithium-Sulphur Batteries

Lithium-sulphur (Li−S) batteries are a promising alternative power source, as they can provide a higher energy density than current lithium-ion batteries. Porous materials are often used as cathode materials as they can act as a host for sulphur in such batteries. Recently, covalent organic frameworks (COFs) have also been used, however they typically suffer from stability issues, resulting in limited and thus insufficient durability under practical conditions and applications. Herein, we report the synthesis of a crystalline and porous imine-linked triazine-based dimethoxybenzo-dithiophene functionalized COF (TTT-DMTD) incorporating high-density redox sites. The imine linkages were further post-synthetically transformed to yield a robust thiazole-linked COF (THZ-DMTD) by utilizing a sulphur-assisted chemical conversion method, while maintaining the crystallinity. As a synergistic effect of its high crystallinity, porosity and the presence of redox-active moieties, the thiazole-linked THZ-DMTD exhibited a high capacity and long-term stability (642 mAh g⁻¹ at 1.0 C; 78.9 % capacity retention after 200 cycles) when applied as a cathode material in a Li−S battery.     

Metal-Organic Frameworks for Breakthrough Separation of 2-Butene Isomers with High Dynamic Selectivity and Capacity

Developing porous sorbents represents a potential energy-efficient way for industrial gas separation. However, a bottleneck for reducing the energy penalty is the trade-off between dynamic adsorption capacity and selectivity. Herein, we showed this problem can be overcome by modulating the kinetic and thermodynamic separation behaviours in metal–organic frameworks for sieving 2-butene geometric isomers, which are desired for upgrading the raffinates to higher value-added end products. We found that the iron-triazolate framework can realize the selective shape screening of 2-butene isomers assisted by electrostatic interactions at the pore apertures. Further introducing uncoordinated N binding sites by ligand substitution lowered the gas diffusion barrier and greatly boosted the dynamic separation performance. In breakthrough tests under ambient conditions, trans-2-C₄H₈ can be efficiently separated from cis-2-C₄H₈ with a record capacity of 2.10 mmol g⁻¹ with high dynamic selectivity of 2.39.     

Emerging Materials for Interfacial Solar-Driven Water Purification

Solar-driven water purification is considered as an effective and sustainable technology for water treatment using green solar energy. One major goal for practical applications is to improve the solar evaporation performance by the design of novel photothermal materials, with optimized heat localization and water transport pathways to achieve reduced energy consumption for water vaporization. Recently, some emerging materials like polymers, metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and also single molecules were employed to construct novel solar evaporation systems. In this minireview, we present an overview of the recent efforts on materials development for water purification systems. The state-of-the-art applications of these emerging materials for solar-driven water treatment, including desalination, wastewater purification, sterilization and energy production, are also summarized.     

Redox-Bipolar Polyimide Two-Dimensional Covalent Organic Framework Cathodes for Durable Aluminium Batteries

Emerging rechargeable aluminium batteries (RABs) offer a sustainable option for next-generation energy storage technologies with low cost and exemplary safety. However, the development of RABs is restricted by the limited availability of high-performance cathode materials. Herein, we report two polyimide two-dimensional covalent organic frameworks (2D-COFs) cathodes with redox-bipolar capability in RAB. The optimal 2D-COF electrode achieves a high specific capacity of 132 mAh g⁻¹. Notably, the electrode presents long-term cycling stability (with a negligible ≈0.0007 % capacity decay per cycle), outperforming early reported organic RAB cathodes. 2D-COFs integrate n-type imide and p-type triazine active centres into the periodic porous polymer skeleton. With multiple characterizations, we elucidate the unique Faradaic reaction of the 2D-COF electrode, which involves AlCl²⁺ and AlCl₄⁻ dual-ions as charge carriers. This work paves the avenue toward novel organic cathodes in RABs.     

Beyond Solvothermal: Alternative Synthetic Methods for Covalent Organic Frameworks

Covalent organic frameworks (COFs) are crystalline porous organic materials that hold a wealth of potential applications across various fields. The development of COFs, however, is significantly impeded by the dearth of efficient synthetic methods. The traditional solvothermal approach, while prevalent, is fraught with challenges such as complicated processes, excessive energy consumption, long reaction times, and limited scalability, rendering it unsuitable for practical applications. The quest for simpler, quicker, more energy-efficient, and environmentally benign synthetic strategies is thus paramount for bridging the gap between academic COF chemistry and industrial application. This Review provides an overview of the recent advances in alternative COF synthetic methods, with a particular emphasis on energy input. We discuss representative examples of COF synthesis facilitated by microwave, ultrasound, mechanic force, light, plasma, electric field, and electron beam. Perspectives on the advantages and limitations of these methods against the traditional solvothermal approach are highlighted.     

Active Separation of Water Isotopologues by Local Molecular Motion in Microporous Framework Materials

Around 10–15 % of the world's energy consumption is accounted for by the separation and purification of chemicals. Among them is the enrichment and separation of isotopologues which are an essential aspect of modern chemistry. In their recent work, Su et al. demonstrate the separation of water isotopologues by responsive dynamic pore windows in a microporous coordination polymer with unprecedented selectivity based on an elegant mechanism.     

Site-Selective Synthesis and Concurrent Immobilization of Imine-Based Covalent Organic Frameworks on Electrodes Using an Electrogenerated Acid

Imine-based covalent organic frameworks (COFs) are crystalline porous materials with prospective uses in various devices. However, general bulk synthetic methods usually produce COFs as powders that are insoluble in most of the common organic solvents, arising challenges for the subsequent molding and fixing of these materials on substrates. Here, we report a novel synthetic methodology that utilizes an electrogenerated acid (EGA), which is produced at an electrode surface by electrochemical oxidation of a suitable precursor, acting as an effective Brønsted acid catalyst for imine bond formation from the corresponding amine and aldehyde monomers. Simultaneously, it provides the corresponding COF film deposited on the electrode surface. The COF structures obtained with this method exhibited high crystallinities and porosities, and the film thickness could be controlled. Furthermore, such process was applied for the synthesis of various imine-based COFs, including a three-dimensional (3D) COF structure.     

Covalent Organic Frameworks as Emerging Nonlinear Optical Materials

The vastness of organic synthetic strategies and knowledge of reticular chemistry have made covalent organic frameworks (COFs) one of the most chemically and structurally diverse class of materials with potential applications ranging from gas storage, molecular separation, and catalysis to energy storage and magnetism. Recently, this class of porous materials has garnered increasing interest as potential nonlinear optical (NLO) materials. Traditionally, inorganic crystals, small-molecule organic chromophores, and oligomers have been studied for their NLO response. Nevertheless, COFs offer significant advantages over existing NLO materials in terms of higher mechanical strength, thermochemical stability, and extended conjugation. Herein, we discuss crucial aspects, terminology, and measurement techniques related to NLO, followed by a critical analysis of the design principles for COFs with NLO response. Furthermore, we touch on selected potential applications of these NLO materials. Finally, future prospects and challenges of COFs as NLO materials are discussed.     

Covalent Organic Frameworks for Energy Conversion in Photocatalysis

Intensifying energy crises and severe environmental issues have led to the discovery of renewable energy sources, sustainable energy conversion, and storage technologies. Photocatalysis is a green technology that converts eco-friendly solar energy into high-energy chemicals. Covalent organic frameworks (COFs) are porous materials constructed by covalent bonds that show promising potential for converting solar energy into chemicals owing to their pre-designable structures, high crystallinity, and porosity. Herein, we highlight recent progress in the synthesis of COF-based photocatalysts and their applications in water splitting, CO₂ reduction, and H₂O₂ production. The challenges and future opportunities for the rational design of COFs for advanced photocatalysts are discussed. This Review is expected to promote further development of COFs toward photocatalysis.     

Magnetoresistance in Organic Spin Valves Based on Acid-Exfoliated 2D Covalent Organic Frameworks Thin Films

Covalent organic frameworks (COFs), as a burgeoning class of crystalline porous materials, have made significant progress in their application to optoelectronic devices such as field-effect transistors, memristors, and photodetectors. However, the insoluble features of microcrystalline two-dimensional (2D) COF powders limit development of their thin film devices. Additionally, the exploration of spin transport properties in this category of π-conjugated skeleton materials remains vacant thus far. Herein, an imine-linked 2D Py-Np COF nanocrystalline powder was synthesized by Schiff base condensation of 4,4′,4′′,4′′′-(pyrene-1,3,6,8-tetrayl)tetraaniline and naphthalene-2,6-dicarbaldehyde. Then, we prepared a large-scale free-standing Py-Np COF film via a top-down strategy of chemically assisted acid exfoliation. Moreover, high-quality COF films acted as active layers were transferred onto ferromagnetic La_0.67Sr_0.33MnO₃ (LSMO) electrodes for the first attempt to fabricate organic spin valves (OSVs) based on 2D COF materials. This COF-based OSV device with a configuration of LSMO/Py-Np COF/Co/Au demonstrated a remarkable magnetoresistance (MR) value up to −26.5 % at 30 K. Meanwhile, the MR behavior of the COF-based OSVs exhibited a highly temperature dependence and operational stability. This work highlights the enormous application prospects of 2D COFs in organic spintronics and provides a promising approach for developing electronic and spintronic devices based on acid-exfoliated COF thin films.     

Piezochromism in Dynamic Three-Dimensional Covalent Organic Frameworks

Piezochromic materials with pressure-dependent photoluminescence tuning properties are important in many fields, such as mechanical sensors, security papers, and storage devices. Covalent organic frameworks (COFs), as an emerging class of crystalline porous materials (CPMs) with structural dynamics and tunable photophysical properties, are suitable for designing piezochromic materials, but there are few related studies. Herein, we report two dynamic three-dimensional COFs based on aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ) chromophores, termed JUC-635 and JUC-636 (JUC=Jilin University China), and for the first time, study their piezochromic behavior by diamond anvil cell technique. Due to the various luminescent groups, JUC-635 has completely different solvatochromism and molecular aggregation behavior in the solvents. More importantly, JUC-635 with AIE effect exhibits a sustained fluorescence upon pressure increase (≈3 GPa), and reversible sensitivity with high-contrast emission differences (Δλ_em=187 nm) up to 12 GPa, superior to other CPMs reported so far. Therefore, this study will open a new gate to expand the potential applications of COFs as exceptional piezochromic materials in pressure sensing, barcoding, and signal switching.     

2D MOFs-based Materials for the Application of Water Pollutants Removing: Fundamentals and Prospects

Water quality can have serious impacts on human health. One crucial issue of water pollution seriously affects our safety due to the continually emerging of discovered anthropogenic pollutants. The water treatment technologies are persistent improvement to adapt such new contaminants, which accelerates the evolution of materials science to explore solving the problems. Metal-organic Frameworks (MOFs) as the significant porous and multi-dimensional networks has been concerned for toxic pollutant elimination, especially probed the applications of outstanding layered 2D skeletons MOFs-based materials. The emphases of this review highlight the 2D MOFs-based materials used in water remediation and treatment strategies including adsorption and catalysis methods. Further, the prospects and challenges of 2D MOFs-based materials for water treatments applications would be surveyed meticulously for the future research and development.     

Highly Porous Hydroxyapatite/Graphene Oxide/Chitosan Beads as an Efficient Adsorbent for Dyes and Heavy Metal Ions Removal

Dye and heavy metal contaminants are mainly aquatic pollutants. Although many materials and methods have been developed to remove these pollutants from water, effective and cheap materials and methods are still challenging. In this study, highly porous hydroxyapatite/graphene oxide/chitosan beads (HGC) were prepared by a facile one-step method and investigated as efficient adsorbents. The prepared beads showed a high porosity and low bulk density. SEM images indicated that the hydroxyapatite (HA) nanoparticles and graphene oxide (GO) nanosheets were well dispersed on the CTS matrix. FT-IR spectra confirmed good incorporation of the three components. The adsorption behavior of the obtained beads to methylene blue (MB) and copper ions was investigated, including the effect of the contact time, pH medium, dye/metal ion initial concentration, and recycle ability. The HGC beads showed rapid adsorption, high capacity, and easy separation and reused due to the porous characteristics of GO sheets and HA nanoparticles as well as the rich negative charges of the chitosan (CTS) matrix. The maximum sorption capacities of the HGC beads were 99.00 and 256.41 mg g⁻¹ for MB and copper ions removal, respectively.     

多孔氧化物块体材料的制备及其对重金属离子吸附研究进展

目前去除重金属离子的方法较多,其中吸附法因操作简单、经济性好而被广泛使用。多孔氧化物块体材料作为新兴的吸附材料具有比表面积高、机械强度高及可回收性好等特点,但吸附选择性差以及孔结构单一的缺陷限制了其在水体重金属处理中的应用。本文详细叙述了多孔氧化物块体材料的多种制备方法,并分别讨论了各方法的优势及其不足。介绍了近几年研究较多的多孔氧化物块体材料及其制备方法与特性。最后,对多孔氧化物块体材料对Pb、Cd及Cr等重金属离子吸附性能的影响因素及改善措施方面进行了总结分析,并指出多种材料的复合、材料表面的接枝改性及分级多孔结构的构建会增强对重金属离子的吸附性能。期望本文为多孔氧化物块体材料的制备及其在重金属离子吸附方面的应用提供参考。     

疏水多孔硅制备及其对水中有机污染物的吸附

以硅酸钠为硅源,盐酸为催化剂,三甲基氯硅烷(TMCS)为表面改性剂,经溶胶-凝胶和表面改性过程制备出一种疏水性多孔硅材料.采用傅里叶变换红外(FTIR)光谱仪、接触角分析仪、氮气物理吸附仪和扫描电子显微镜(SEM)对其结构和性质进行表征.结果表明:所制备的多孔硅具有分等级孔道结构(中孔-大孔),比表面积为566m2·g-1,孔体积高达2.28cm3·g-1,多孔硅与水的接触角为156°,显示出超疏水特征.对甲苯、汽油、柴油和润滑油的吸附量均可高达自身质量的14倍,丰富的孔道使其在几分钟内即可达到饱和吸附.这种多孔硅在汽油/水混合体系中对汽油具有较高的选择性,同时具有良好的再生能力.经正己烷萃取再生后,多孔硅仍能基本保持初始吸附容量.此方法制备的多孔硅材料在吸附分离污水中的有机物和溢油处理方面具有很好的应用前景.