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novel approaches for synthesizing materials with large surface areas and high porosity. The initial MOF
structure, designated as MOF-5, was synthesized in 1999 by researchers at the University of Michigan. This
2+
material comprised Zn ions coordinated with 1,4-benzenedicarboxylate (H BDC) ligands and exhibited a
2
2
remarkable surface area exceeding 2,900 m /g . In 2013, MOFs were introduced as fillers into the polymer
[1]
matrix for the first time, thereby enhancing low-humidity proton conductivity . This breakthrough sparked
[2]
a surge of interest in MOFs, prompting researchers to explore their potential for diverse applications such as
gas storage, catalysis, and drug delivery. Since then, thousands of MOFs have been synthesized using
various metal ions and ligands to create materials with distinct properties and functions. MOFs have
garnered attention as potential flame retardants due to their high surface area, tunable pore size, thermal
stability, abundance of transition metals, and flexible structure.

The application of MOFs as flame retardants can be traced back to research conducted in the late 2010s. In
2017, Hou et al. successfully synthesized iron-based and cobalt-based MOFs, which were subsequently
incorporated into PS as flame retardants. The results demonstrated a significant improvement in both the
thermostability and flame retardancy of the PS composites . Subsequently, researchers have investigated
[3]
diverse categories of MOFs for their flame-retardant properties [3-10] . Table 1 summarizes some MOF flame
retardants for different polymer matrices [11-23] . Recently, (zeolite imidazolate framework) ZIF series of MOFs
have been intensively explored as potential flame retardants in various polymer matrices due to their ease of
synthesis. MOFs contain flammable organic ligands in their structures, leading to limited efficiency, and
therefore, modification or co-blending with other flame retardants is often required to boost their
fire-retardant efficiency, e.g., performing better in UL-94 testing, especially at lower loading levels. It is
accepted that MOFs usually reduce heat release rates and slow the combustion process of polymers via
catalytic carbonization of transition metals within MOFs during combustion. However, there remain some
unknowns on the impacts of structural compositions of MOFs on their catalyzing carbonization and modes
of action. Additionally, they have explored the potential of incorporating MOFs as additives into existing
flame-retardant materials to enhance their efficacy [Figure 1]. The use of MOFs as flame retardants is a
relatively new and emerging area of research, and there are still ongoing debates regarding their cost-
effectiveness compared to traditional flame retardants.

However, MOFs have several advantages over traditional flame retardants, such as their high porosity, high
surface area, and tunable properties. These properties allow MOFs to be tailored to specific applications and
offer superior flame-retardant performance. While MOFs may be relatively expensive compared to
traditional flame retardants, their unique properties and superior flame-retardant performance may
outweigh these costs in certain applications. The flame retardancy of MOFs is attributed to two primary
mechanisms: gas-phase and condensed-phase. The gas-phase mechanism involves the liberation of
non-flammable gases, such as water, carbon dioxide, and nitrogen. These gases can dilute the flammable
gases and reduce the concentration of fuel and oxidizer in the flame zone [24,25] . The condensed-phase
mechanism involves the formation of a protective layer on the surface of the polymer, which acts as a
physical barrier and reduces the combustion rate. This protective layer is formed by thermal decomposition
of MOFs, releasing metal oxides or metal phosphates that react with the polymer to form a char layer. The
char layer functions as an insulating layer, reducing heat transfer between flame and polymer and
preventing fire spread [26-29] .

One may consider the potential benefits of utilizing MOFs as flame retardants, such as their capacity to
enhance fire safety without adversely affecting the properties of the applied material. However, there are still
some challenges associated with large-scale production and application of MOFs as flame retardants, as well
as potential risks that come with using these materials. In this review, we aim to provide valuable guidance