Page 228 - Read Online

Page 10 of 17 Hou et al. Microstructures 2023;3:2023039 https://dx.doi.org/10.20517/microstructures.2023.37

Figure 4. Ligand design of MOF-based flame retardants.

Harmlessness to the human body and environment-friendly recycling have become the prerequisites for the
future development of flame retardants. Traditional flame retardants, such as halogenated compounds, have
been found to be detrimental to both human health and the environment. Phosphorous flame retardants are
commonly utilized in a diverse range of consumer goods, such as electronics, furniture, and construction
materials, to mitigate the risk of fire. Nevertheless, these compounds have been associated with various
potential hazards: (1) Phosphorus-based flame retardants have been found to accumulate in the
environment, including soil, water, and air samples. They can also bioaccumulate in aquatic organisms and
pose a toxic threat to fish and other wildlife; (2) Evidence suggests that exposure to certain phosphorus-
based flame retardants may negatively impact human health. Therefore, developing effective ligands
without the use of phosphorus or halogen presents a significant challenge. Overall, the utilization of
biofunctional MOFs as flame retardants presents a promising avenue for research that could result in the
development of safer and more environmentally sustainable materials. However, further investigation is
necessary to comprehensively comprehend the properties and potential applications of these materials.

MOF DERIVATIVES TREATMENT AS FLAME RETARDANTS
As previously mentioned, the flammability of MOF ligands is a limiting factor in their inherent flame-
retardant properties. Therefore, post-treatment techniques are often required to enhance their fire-resistant
capabilities [Figure 5].

One common post-processing technique for MOFs involves the modification or removal of organic ligands.
In MOFs, ligand removal can be achieved through an acid-base reaction between the MOF and solvent,
which entails protonation of the organic ligand by a strong acid followed by detachment from the metal
center. Generally, metal ions or clusters are linked by organic ligands in MOFs. These ligands usually
contain carboxylic or amino functional groups that can serve as proton acceptors or donors, respectively.
When a MOF is exposed to an acidic solution, the protons in the acid can interact with the functional
groups on the ligands, resulting in protonation of the ligand. Subsequently, weakened affinity for metal
centers allows easy detachment from metal ions or clusters. The liberated metal sites are then available for