Sustainable synthesis of Metal-Organic Frameworks (MOF)

Abstract

The research presented in this work investigates the use of green solvents in the synthesisof a new-class of hybrid porous materials, named metal-organic frameworks (MOFs), which are formed by the combination of metallic centres and organic linkers. The fluid technology based on supercritical carbon dioxide (scCO2) has been studied as a promising alternative for safer and cleaner synthesis over the conventional solvothermal conditions. In particular, the specific goal stablished for this research is to successfully synthesize two widely-known compounds with MOF structures denominated HKUST-1 and ZIF-8 that will demonstrate the applicability of the method. The reactions were carried out in a reactor vessel at the conditions of 200 bar, 70 ºC for 20 hours in a medium of scCO2 with selected additives. The reactant quantities were used in stoichiometric ratios according to the formula unit of each MOF. The physicochemical and textural properties of the prepared samples were measured and compared to those obtained from conventional methods. The two products have been successfully synthesized with a high-degree of purity as demonstrated by the elemental analysis. The structure of HKUST-1 corresponds to a face-centred cubic lattice with an observed surface area of 1290 m2·g-1, in accordance with previous reported data. ZIF-8 was also obtained with the same structure than the one described in the literature, but with outstanding values of surface area (1730 m2·g-1), which overpass most of the values reported with other synthetic approaches. In the last part of this report, an explanation based on the ultraviolet-visible (UV-Vis) spectra and the Crystal Field Theory is provided as a tool to understand the observed colour change in HKUST-1