Trimesic acid–nickel-based metal–organic frameworks (Ni-MOFs) have attracted growing attention since their first synthesis by Yaghi and co-workers in 1996, owing to their potential applications in a wide range of fields, including CO₂ hydrogenation, photocatalysis, batteries, and supercapacitors. Furthermore, Ni-based MOFs are known to undergo structural transformations even under relatively mild conditions, rendering their investigation particularly compelling. In this study, we examine the influence of different co-solvents on the solvothermal synthesis of Ni-BTC. While N,N-dimethylformamide (DMF) is commonly employed as the primary solvent in MOF synthesis, here, it is combined with water and formic acid as co-solvents in order to evaluate their impact on framework formation.
The Ni-BTC samples were synthesized via a straightforward and reliable solvothermal method. Following the mixing of the precursors with the solvent system (DMF, DMF/water, or DMF/formic acid), the resulting solution was heated at 120 °C for 12 h and subsequently washed. The recovered solids were then activated under vacuum at 110 °C overnight. The obtained materials were thoroughly characterized by XRD, N2 adsorption/desorption isotherm, FTIR, UV-Vis, TGA and SEM analysis.
Each solvent formulation produced crystalline structures with distinctive features and well-defined morphologies. When DMF was employed as the sole solvent, the resulting material exhibited a crystalline structure consisting of hexagonal crystals with a stacked 2-D layer, represented by the simplified formula Ni(HBTC)(DMF)₂·xDMF. The incorporation of water as a co-solvent significantly altered the crystallization pathway, leading to the formation of the well-known Ni₃(BTC)₂·12H₂O phase with acicular crystals. In contrast, the presence of formic acid promotes competitive coordination with the metal centers, leading to the formation of alternative architectures with an enhanced surface area. Structural transitions are observed when the material is brought into contact with water, whereas exposure to ambient humidity results in a macroscopic color change.
