Half etching of ZIF-67 towards open hollow nanostructure with boosted absorption ability for toxic smoke and fume in epoxy composites

Metal-organic frameworks (MOFs) are favored in the field of flame retardancy due to the catalytic effect of metal nodes on char layer formation and the synergistic flame-retardant effect of organic ligands containing elements such as nitrogen and phosphorus. However, the inherent microporosity of MOFs limits their adsorption efficiency for toxic smoke and flammable gases. In this work, an organic phosphorus-modified MOF with a distinctive nanostructure of hierarchically porous (P-Co-MOF/ZIF) was successfully synthesized. In brief, an amino-functionalized zeolitic imidazolate framework (NH₂-ZIF) was initially synthesized through a ligand substitution reaction with ZIF-67. Subsequently, organic phosphorus flame retardants were grafted on NH₂-ZIF, and the acidic substances generated during this process were used to synchronously half etch ZIF, resulting in a ZIF with a high specific surface area and unique nanostructure. Through this simple synthetic method, the catalytic ability of transition metals in ZIF is preserved, and organic phosphorus flame retardants are incorporated into ZIF, resulting in the synergistic flame-retardant effect of phosphorus and nitrogen. Additionally, its unique hierarchically porous nanostructure can effectively enhance the adsorption of volatile products during the combustion process, thereby offering outstanding flame retardancy and smoke suppression effects for epoxy resin (EP). The results indicate that adding 2 wt% P-Co-MOF/ZIF to EP can increase the limiting oxygen index value to 29.5%. Furthermore, the peak of heat release rate, total heat release, and total smoke production of the composite material can decrease by 43.3%, 37.9%, and 38.1%, respectively, compared to EP. Therefore, this work will provide new inspiration for designing functional nanostructures and synthesizing efficient flame retardants.