Researchers conduct comprehensive review on preparation of MOF-based flame retardants via coordination bond cleavage

Since the first report of metal-organic framework (MOF)-based flame retardants in 2017, this research area has exploded. However, improving the flame retardant performance of MOFs and expanding their application areas are still significant challenges. The physicochemical properties of MOFs are closely dependent on their topology, pore properties, and chemical composition, which can be modulated by targeted design.

Directly related to synthesis, post-synthesis strategies for MOFs, incl Ion exchange, ligand modification, and acid/base etching have greatly expanded their scope and potential of application. Methods based on coordination bond cleavage of MOFs have proven to be very useful in modulating the structure and have attracted extensive research in the field of flame retardants.

Ye-Tang Pan’s team at the Beijing Institute of Technology (BIT) focuses on MOF-based design and fabrication. Flame retardants and multifunctional flame retardant polymer nanocomposites. This work presents the main progress of MOF-based flame retardants so far from three aspects: the development and challenges of MOF-based flame retardants, the design of efficient MOF-based flame retardants through coordination bond breaking, and the application of flame-active MOFs. . retardant field.

Is published I Industrial Chemistry and Materials.

MOFs applied alone do not give polymer composites high limiting oxygen index (LOI) values ​​and UL-94 vertical combustion ratings, due to the single flame retardant element, low percentage of flame retardant elements, flammability of ligands, and microporous. – Dominant pore structure that is difficult to fully exploit.

“In this review, we review for the first time synthetic post-hoc methods involving coordination bond cleavage in the flame retardant field to prepare composites and structures with or without disrupting the reticulation chemistry of the parent matrix of MOFs. can be done.” Ye said. -Tang Pen, Professor, Beijing Institute of Technology, China, “We conclude with a critical look at the applications, challenges, and future prospects of this emerging and emerging field.”

Template derivatization of MOFs is considered an effective strategy for the preparation of structurally active materials. However, commonly produced metal-carbon alloys are dramatically dependent on uncontrolled heat treatment and consume energy. Expensive organic ligands break down into gases and expand at high temperatures, with internal structural contraction, leaving the carbon framework with rare functional components.

Therefore, the pseudomorphic cleavage of MOFs based on the ion/ligand exchange strategy is more convenient, gentle and controllable, which is also increasingly investigated in many fields. MOFs have inherent flame retardant capabilities, i.e., large specific surface area, well-defined pore structure, and tunable physicochemical properties, and the above strategies also provide viable insights into the flame retardant functionalization of MOFs. Is.

The acid-base strength mismatch causes the pH value for the corresponding salt to drop far from 7 during hydrolysis. Alkaline imidazolate ligands in ZIFs are prone to protonation by H.⁺ The salt-containing aqueous solution is released, after which the framework breaks down.

The group began correlative research in early 2017 using aluminum nitrate (aq.) to attack ZIF-8 aggregates to form honeycomb-like aluminum hydroxide flakes. Removal of the rhombic dodecahedra with the remainder of the outer coating layer results in mesoporous Al(OH).₃ which can be further loaded with phosphorus-based flame retardant to improve the fire safety of epoxy resin (EP) superior to commercial counterparts.

One of the main reasons why the flame emission of MOFs alone is exceptional is that they contain a large number of flammable ligands in their structure. The flame retardant functional substitution of primitive ligands by post-synthesis ligand exchange strategy provides a good idea to improve the flame retardant performance of MOFs.

For ZIFs containing basic ligands, acidic compounds are more likely to break their coordination bonds, caused by ionized H.⁺ protonated ligands. The process of dissociation of coordination bonds of MOFs under alkaline conditions can be simplified as a ligand exchange process in which coordination groups in solution are replaced by anions/molecules such as OH.^– And H₂O. Thermodynamically, MOFs are more inclined to remain in the crystalline state if the coordination bond between the metal ion and the ligand (ML) is stronger than that between OH.^– Or someone else The ligand anions

Furthermore, inspired by the encapsulation strategy and the acid etching strategy, the group reported for the first time that the condensation of certain compounds with an acid-forming tendency achieved simultaneous etching of ZIF-67 during the encapsulation process. used to do, which is suitable for the encapsulation process. Improvement of flame retardancy and synthesis efficiency for formulated fillers.

As porous materials, MOFs can absorb smoke particles and toxic gases produced during polymer combustion. However, MOFs as flame retardants often face the problem of insufficient char formation capacity. Loading of functional fillers is an effective strategy to solve this problem. Efficient loading of guests can be realized by manufacturing carriers with graded porous or hollow structures.

This research group fabricated bird’s nest-like hierarchical porous nanocages with an effective loading of triphenyl phosphate up to 35.8 wt%, and the prepared polyurea composites showed good durability in terms of flame retardant properties. In addition, designing MOFs with open nanostructures can improve their ability to trap smoke particles. Toxic gases and smoke particles are more easily captured by MOFs through physical and chemical adsorption.

The easy functionalization of MOFs also creates convenient conditions for grafting target molecules. Important approaches include substitution reactions between amino-functionalized MOFs and flame retardants containing phosphorus-chlorine bonds. Addition reaction between double-bond functionalized MOFs and flame retardants containing phosphorus-hydrogen bonds; and salt formation reactions between amino-functionalized MOFs and flame retardants containing phosphate ester bonds.

Flame retardants derived from MOFs have excellent flame retardant advantages, and coordination bond cleavage is an effective means to improve the flame retardancy of MOFs as well as expand their functional applications.

“In this work, we systematically summarize and outline the direct or indirect breaking of coordination bonds based on conformational relationships and functionalization of highly effective MOFs as flame retardants, as well as future prospects and challenges. This It is also hoped that this work will rapidly guide researchers in the field and influence their next research, said Professor Penn.