SYNTHESIS, STRUCTURAL CHARACTERIZATION, AND HYBRIDIZATION OF METAL–ORGANIC FRAMEWORK-5 (MOF-5) WITH HUMIC ACID: A SUSTAINABLE APPROACH TO FUNCTIONAL COMPOSITE MATERIALS

Abstract

Metal–organic frameworks (MOFs) are highly porous, crystalline materials comprising metal nodes and organic linkers, which can be tuned to produce a variety of pore architectures that are suitable for the storage, catalysis, and removal of pollutants. MOF-5, a member of this family constructed from Zn4O clusters and linked by terephthalate units, is one of the most studied MOFs due to its cubic topology and large surface area, but its practical application is limited by its high moisture sensitivity. Humic acid (HA), a naturally occurring macromolecule with a high concentration of carboxyl, phenolic and hydroxyl groups, was explored for this work as a hybridizing agent that not only provides additional functional chemistry to MOF-5, but also potentially enhances the environmental durability of it. MOF-5 was synthesized via solvothermal method from the reaction between zinc nitrate hexahydrate and terephthalic acid in dimethylformamide, and then the pores were activated by the exchange of solvents with methanol/ethanol. Alkaline extraction combined with acid precipitation was used to isolate the humic acid from natural organic matter, and then the MOF-5@HA composite was successfully fabricated by dispersing HA into activated MOF-5 in an alcoholic medium. Powder X-ray diffraction (PXRD) and Fourier-transform infrared (FTIR) spectroscopy were used to confirm the formation of crystalline, phase-pure MOF-5 and demonstrate the PXRD signature did not change significantly after hybridization with the HA functional groups while spectral evidence was observed for coordination and hydrogen-bonding interactions. Such results show that MOF-5 can be hybridized with a renewable and low-cost organic material without destroying the crystal structure, thus expanding the surface chemistry of the composite which is important for the adsorption of heavy metal and dye, catalysis, and catching of gases. The results are discussed in the context of the ever-expanding literature on MOF–organic hybrids up to 2026.