Synthesis and structural evolution of metal-organic frameworks by reaction-diffusion process at room temperature -

Abstract

Metal organic Frameworks (MOFs) are new class of hybrid materials that are composed of organic linkers and inorganic clusters joined via strong bonds. MOFs have been mainly synthesized via solvothermal, sonochemical, mechanochemical, fast precipitation or slow diffusion techniques. Herein, we report for the first time the synthesis of metal-organic frameworks (MOFs) via a reaction-diffusion process at room temperature. Our novel synthesis system is based on the separation of the reactants by immobilizing the organic linker in a gel matrix and the metal salt solution is added on the top of the gel and allowed to diffuse through the inner electrolyte forming thereby the corresponding MOF crystals. As a proof of concept, MOF-199 structure is considered in this study and it was successfully synthesized via this method. MOF-199 (also known as HKUST-1) was chosen because it is one of the most prominent MOFs due to its considerable high surface area, large pore volume, high thermal and chemical stability especially in water. Beside its novelty, this method is rapid, efficient, scalable, and environmentally friendly. Additionally, it allows us to adjust and control the size distribution and the morphology of the particles. The obtained crystals were subjected to characterization using different instruments and techniques such as XRD, SEM, TGA, BET, FT-IR and 1H-NMR. Furthermore, different experimental conditions were varied in order to examine their effect on the size and the morphology of the MOF crystals. Such conditions include temperature, concentration of the inner, source of the metal in the outer, concentration of the gel and its type. Another discovered advantage of this method was the successful incorporation of two different linkers within the same framework in a controlled manner. Finally, the adsorption performance over methylene blue of MOF-199 was compared to that of mixed-linkers MOF and showed a great enhancement due to the heterogeneity within order introduced in the crystals.