Advances in reticular materials for flame retardant polymers

Construction of MOFs-based nanocomposites and their application in flame retardant polymers: A review

Mixed-matrix membranes (MMMs) have emerged as a promising approach for developing new, stable, and highly effective gas and liquid separation materials. MMMs combine porous crystalline framework materials, such as Metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and hydrogen-bonded organic frameworks (HOFs), as fillers incorporated in a polymer matrix. This article comprehensively reviews MMM research, discussing the structure and properties of MOFs, COFs, and HOFs and their attractiveness for use in MMMs. The article also reviews the use of mixed matrix filtration membranes with MOFs, COFs, and HOFs for various water treatment and gas separation applications. The potential of MMMs for meeting the needs of different industries is demonstrated through the discussion of specific examples. Overall, this article highlights the significant potential of MMM technology for developing next-generation separation materials and attempts to cover the most recent progress in the design and deployment of MOFs, COFs and HOFs-based MMMs, as are the remaining obstacles and prospects. This work also highlights the enormous potential of these materials for separation applications and raises attention toward the economic aspect and market diffusion of such MMMs.

Metal–Organic Frameworks–Based Flame-Retardant System for Epoxy Resin: A Review and Prospect

Two porous hydrogen-bonded organic frameworks (HOFs) based on arene sulfonates and guanidinium ions are reported. As a result of the presence of ionic backbones appended with protonic source, the compounds exhibit ultra-high proton conduction values (σ) 0.75× 10(-2) S cm(-1) and 1.8×10(-2) S cm(-1) under humidified conditions. Also, they have very low activation energy values and the highest proton conductivity at ambient conditions (low humidity and at moderate temperature) among porous crystalline materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). These values are not only comparable to the conventionally used proton exchange membranes, such as Nafion used in fuel cell technologies, but is also the highest value reported in organic-based porous architectures. Notably, this report inaugurates the usage of crystalline hydrogen-bonded porous organic frameworks as solid-state proton conducting materials.

Metal clusters exhibit diverse structures, emerging functions, and applications; thus, incorporating clusters into metal–organic frameworks (MOFs) brings tremendous merits. Although the constructio...

Strategic design of solid-state proton-conducting electrolytes for application in anhydrous proton-exchange membrane fuel cells (PEMFCs) has gained burgeoning interest due to a spectrum of advantageous features, including higher CO tolerance and ease in the water management systems. Toward this direction, crystalline materials like metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), and polyoxometalates (POMs) are emerging PEM materials, offering strategic structural engineering through crystallography, thus enabling ultrahigh anhydrous proton conductivity up to 10-2-10-1 S/cm. This Perspective highlights significant progress achieved thus far with such crystalline platforms in the domain of anhydrous proton conduction across a wide temperature window (sub-zero to above 100 °C). Based on their structural backgrounds, these platforms are categorized into four classes (viz. MOFs, COFs, HOFs, and POMs) with a detailed evolutionary timeline since their emergence early in 2009. Insightful discussions with a key focus on the strategies undertaken to attain anhydrous proton conductivity along with implementation in fuel cell technology through membrane electrode assembly are presented. A section on "Critical Analysis and Future Prospects" provides decisive key viewpoints on those overlooked issues with future endorsement (e.g., performance assessment with CO tolerance analysis and fuel cell test stand) for further development while comparing them with other anhydrous platforms from both academic and industrial perspectives.

Functional Scaffolds from AIE Building Blocks

Enantiomers typically show different pharmacological, toxicological, and physiological properties. Thus, the preparation of enantiopure compounds is of great significance for human health and sustainable development. Compared with asymmetric catalysis, enantiomeric separation is simpler, faster, and more efficient; as such, it has become the preferred method for obtaining pure enantiomers. At present, enantiomeric separation methods mainly include chromatography, nanochannel membrane separation, selective adsorption, and recrystallization. In particular, gas chromatography (GC) plays an important role in enantioseparation because of its high sensitivity, excellent reproducibility, and outstanding processing capacity for various enantiomers. The stationary phase is key to the separation efficiency of GC, and more efficient, stable, and cost-effective materials that could serve as stationary phases are constantly being explored. Organic frameworks, such as covalent organic frameworks (COFs), metal-organic frameworks (MOFs), porous organic cages (POCs), metal-organic cages (MOCs), and hydrogen-bonded organic frameworks (HOFs), possess large specific surface areas, high porosities, tunable pore sizes, and easy functionalization, rendering them promising candidates for the separation of mixed analytes. Research has shown that the use of organic frameworks as stationary phases for GC results in excellent column efficiency and high resolution for various analytes, including n-alkanes, n-alcohols, polycyclic aromatic hydrocarbons, positional isomers, and organic fluorides. Furthermore, organic frameworks can be prepared as chiral stationary phases for GC by the intelligent introduction of a chiral moiety, thereby enabling the efficient separation of enantiomers. Synthetic strategies for chiral organic frameworks are primarily categorized as post-synthesis or bottom-up approaches. In general, the post-synthesis strategy can introduce various chiral sites to the framework; however, the distribution of chiral sites may not be uniform, and the ordered framework may be destroyed during the post-synthesis process. The bottom-up strategy allows for the uniform and precise distribution of chiral sites in the framework, but the synthesis of chiral monomers and the constraint between asymmetry and crystallinity limit its development. Chiral induction has been proposed as an alternative strategy for synthesizing chiral organic frameworks. The use of this strategy has led to the successful preparation of organic frameworks with abundant chiral sites and excellent crystallinity. Dynamic coating and in situ growth are the main approaches used to transform the as-prepared chiral organic frameworks into stationary phases. Notably, the in situ growth approach can yield chiral COF/MOF-coated capillary columns that provide high resolution for the separation of enantiomers with excellent repeatability and reproducibility. Nevertheless, owing to the slightly complex pretreatment process and the difficulty of synthesizing chiral organic frameworks, the in situ growth approach has not yet been widely applied. Owing to their excellent solvent processing performance, POCs, MOCs, and HOFs can be easily coated on the inner walls of columns to form membranes via dynamic or static coating. A series of enantiomers have been successfully separated and analyzed by immobilizing chiral COFs, MOFs, POCs, MOCs, and HOFs on GC capillary columns, demonstrating the great potential of chiral organic frameworks for enantiomeric separation. In general, the mechanisms by which chiral organic frameworks recognize enantiomers could be mainly categorized as van der Waals interactions, hydrogen bonding, π-π interactions, and size-exclusion effects. While molecular simulations can offer some insights into these recognition mechanisms, clarifying these mechanisms based on effective characterization remains challenging. In summary, organic frameworks show outstanding advantages for enantiomer separation. Given breakthroughs in synthetic strategies for chiral organic frameworks and the in-depth study of chiral recognition mechanisms, chiral organic frameworks may be expected to become an important aspect in the field of chiral materials, further realizing the large-scale analysis and production of chiral analytes. A total of 64 references, most of which are from the American Chemical Society, Springer Nature, Wiley Online Library, and Elsevier databases, are cited in this review.

Recent advances in hydrogen bonded organic frameworks and their derived materials for electrocatalytic water splitting

Although metal organic frameworks (MOFs) and covalent organic frameworks (COFs) have been extensively used as fluorescent-based antibiotic sensors, newly developed hydrogen-bonded organic frameworks (HOFs) are largely unexplored toward this direction. To realize this, the luminescent HOFs must be stable in water as the analytes are mostly found in water-based effluents in environments. In addition, HOFs should be equipped with specific recognition sites in order to direct the discrimination among the antibiotics. Herein, we report a 3D porous HOF, IITKGP-HOF-6, constructed from an aromatic-rich tetratopic carboxylic acid (H4L), which exhibits excellent hydro and prolonged open-air stability (7 and 15 days, respectively). IITKGP-HOF-6 was explored for the highly selective detection of nitrofurans (NFs) family of antibiotics in aqueous medium exhibiting a remarkably low detection limit of 0.75 μM for nitrofurazone (NFZ) through luminescence quenching. Photoinduced electron transfer driven by the presence of low-lying charge-transfer excited state below to the and Forster energy transfer between H4L donor and NFZ acceptor are confirmed to be responsible for observed quenching using detailed quantum-chemical studies. This work demonstrates the usage of HOFs as sensory materials toward antibiotics in aqueous medium along with a clear understanding into the sensing mechanism at the molecular level.

Crystalline porous molecular frameworks formed through intermolecular hydrogen bonding are often called hydrogen-bonded organic frameworks (HOFs) by analogy to metal organic frameworks (MOFs) and covalent organic frameworks (COFs). Although the origin may go back to the 1960s, HOFs have recently been investigated as a new family of functional porous materials. In this review, HOFs composed of tritopic, tetratopic, and hexatopic carboxylic acid derivatives are reviewed by considering structural aspects such as isostructurality. These derivatives typically form H-bonded hcb, dia, sql, hxl, and pcu networks depending on the numbers, positions, and orientations of the carboxy groups in the molecule. We show detailed structures for selected HOFs indicating the low-dimensional networks formed through H-bonding of the molecule and higher-dimensional structures formed by assembly of the network. The networks can be designed and predicted from the molecular structure, while the latter is still difficult to design. We hope that this review will contribute to the well-controlled construction of HOFs.

Ammonium polyphosphate (APP) is often used to construct intumescent flame retardant systems together, but it is faced with the problem of adding too much flame retardant. This paper envisions the construction of a modified APP flame retardant (TR@ZIFAPP) with a sandwich structure by the method of covering, which, in order to endow APP flame retardant with better flame retardant effect. By constructing a metal catalytic layer and a triazine carbon formation layer, the modified APP flame retardant has both catalytic carbon formation and phosphorus–nitrogen synergistic flame retardant functions. TR@ZIFAPP is expected to be used in flame‐retardant polymer composites with low addition requirements, adding it to epoxy resin (EP) reveals that the TR@ZIFAPP flame retardant could achieve the optimal flame retardant effect when the addition amount was only 3 wt%. The UL‐94 rating reached V‐0 grade, and the two off‐fire auto‐ignition time levels were only 0.7 s. The analysis of its flame retardant mechanism showed that the flame retardant in EP was mainly through the rapid formation of carbon to isolate oxygen and dilute the flammable gas to reach a strong flame retardant effect.

ConspectusMembrane technology plays an increasingly important role for sustainable development of our society owing to its huge capability to tackle the energy crisis, water scarcity, environmental pollution, and carbon neutrality. To fully unlock the potential of membranes, it is in high demand to develop advanced membrane materials that significantly outperform conventional polymer membrane materials in separation performance and long-term stability. The emergent covalent organic frameworks (COFs) have been deemed as potent membrane materials because of their unique structure and properties in comparison with polymers, zeolites, and metal organic frameworks (MOFs). (i) First, the highly tunable and ordered crystalline pore structure, high porosity, and excellent stability render COFs an ideal membrane material. COFs are more stable than MOFs and, in some cases, are even more stable than zeolite. Moreover, it is easier to introduce functional groups into the COF nanochannels compared with zeolite and MOFs. Further, COFs are ideally suitable for constructing ordered nanochannels with size in the range of 0.6–3 nm which is difficult to be realized by other materials. (ii) Second, along with the unremitting discovery of diverse platform chemistries such as reticular chemistry, the in-depth understanding of nucleation/growth mechanisms of COFs as well as the rapid progress of manufacturing technologies and various routes to fabricating COF membranes with favorable physical and chemical structures inside the nanochannels are being actively exploited. COFs generally show better membrane-formation ability owing to their abundant 2D structures, which make it easier to fabricate ultrathin membranes compared with zeolite and MOFs. (iii) Last, a great number of COF membranes exhibit exceptionally high separation performance and stability, establishing their position as the next-generation membranes.In this Account, we discuss three types of engineering toward COF membranes based on Schiff base reaction for high-efficiency molecules/ion separations, i.e., reticular engineering, crystal engineering, and nanochannel engineering. First, we discuss the reticular engineering of COF membranes with a focus on the bond types, chemical structure, and architecture design. The membrane-formation ability and methods of COFs are also analyzed. Second, we discuss the crystal engineering of COF membranes with a focus on the key thermodynamical and kinetic factors to drive the disorder-to-order transition where we attempt to dig deeper into the crystallization habit of COF membranes. Third, we discuss nanochannel engineering of COF membranes with a focus on the construction and modulation of the physical and chemical microenvironments of nanochannels for efficient and selective transport of molecules/ions. Last, we conclude with a perspective on the opportunities and major challenges in the R&D of COF membranes, targeting at identifying the future directions.

Polymer composites are widely used in various fields of production and life, and the study of preparing environmentally friendly and flame retardant clay/polymer composites has gradually become a global research hotspot. But how to efficiently surface modify clay and apply it to the field of flame retardant polymers is still a potential challenge. One of the most commonly used surface modification methods is the modification of clay with silane coupling agents. The hydrolysable groups of the silane coupling agent first hydrolyze to generate hydroxyl groups. These hydroxyl groups then undergo a condensation reaction with the hydroxyl groups on the surface of the clay, allowing for organic functional groups to be grafted onto the clay surface. The organic functional groups and polymer matrix react to generate chemical bonds so that the composite material's interface is more closely combined. Thus, the dispersion of clay in the organic polymer material and the compatibility of the two is better, which improves the flame retardant effect of the composite material. This paper introduces the classification of a silane coupling agent and the mechanism and process of silane coupling agent-modified clay, outlines the mechanism of silane coupling agent-modified clay flame retardant polymers, reviews the research results on flame retardant polymers of various clays after surface treatment with silane coupling agents in recent years, and highlights the synergistic flame retardant effect of clay and flame retardant organized by silane coupling agents. Finally, it is found that the current research in the field of silane coupling agent-modified clay in flame retardants is focused on the modification of montmorillonite, sepiolite, attapulgite, and kaolinite by KH-550, KH-560, and KH-570, and the development trends in this field are also prospected.

Flame retardant effect and mechanism of a novel DOPO based tetrazole derivative on epoxy resin