Impact of pH, base selection, and metal–ligand molar ratio on room-temperature synthesis of Ni and Mg MOFs via direct mixing: challenges and prospects

Abstract

Commonly used methods for MOF synthesis, such as solvothermal, mechanochemical, and electrochemical syntheses each, have their own disadvantages, such as long reaction times and the need for specialized equipment. The direct mixing method is a simple, fast, and cheap alternative that allows for MOF production at room temperature in times as short as 15 min. The room-temperature synthesis via a direct mixing method of Ni and Mg MOFs based on trimesic acid (BTC) and terephthalic acid (BDC) was studied by evaluating the effect on the product of the synthesis parameters: pH, base, stirring time, and metal–ligand molar ratio. It was found that the base used to adjust the pH was a critical factor to ensure the formation and purity of each MOF, where Na⁺ from NaOH could alter the crystal structure of Mg MOFs by incorporating into it due to its size similarity with Mg²⁺, while the ability of K⁺ from KOH to enter the crystal structure was much lesser due to its larger size and allowed for the proper development of the Mg MOFs crystal structure Similarly, a higher molar ratio of the ligand to the metal resulted in the incorporation of the base’s metal into the MOF as an impurity due to an excess of ligand without enough metal to coordinate with. Pure Ni-BTC and Ni-BDC MOFs could be obtained in ~ 15 min, while the Mg-BTC MOF always contained the base-forming metal as an impurity. All the products had irregular morphologies which resulted in lower surface areas and pore volumes when compared to MOFs obtained by other methods. The improvement of these properties is crucial to make the direct mixing method a viable alternative as higher surface areas and pore volumes are beneficial for many of the applications of MOFs, and it was identified that the choice of metal salt precursor plays an important role over these properties. As such, experimentation with different precursors is an important avenue of future research for the improvement of this method.

Graphical Abstract