Metal-Organic Frameworks (MOFs) are an interesting type of crystalline solids with outstanding microporosity conformed by inorganic units (e.g. atoms, clusters, chains) connected via strong bonds to polydentate organic ligands developing a 3D network. They have been traditionally synthesised by hydro-and solvothermal methods (involving conductive heating of MOF precursors solution in a closed vessel from the outside). Nevertheless, on the roadmap for MOF commercialization, microwaves (MW) assisted synthetic procedures outstand as they use scalable technology and provide pure crystals with short production times (few minutes) compatible with industrial production. The faster reaction time under MW irradiation is due to the inverted heating gradient, as the reagents absorb the irradiation and the reaction mixture is heated inside-out. Nevertheless, as the MW heating is directly related with the interaction of the different molecules with the alternating electric field of the microwaves, minor changes in the reaction mixture could lead to vast different synthetic results, such as different crystallographic phases.
In order to progress in the knowledge of MOF MW-assisted synthesis, a systematic study has been carried out in the Advanced Porous Materials Unit at IMDEA Energy. In this work, among the great diversity organic building units, the linear amino terephthalic acid (H2BDC-NH2) was selected as ligand, as it leads to highly porous structures. The earth abundant non-toxic iron (III) was chosen as inorganic building unit, as in conjunction can promote the polymorphic series of highly porous redox active terephthalates MIL-53, MIL-68,6 MIL-88B and MIL-101. These benchmarked solids have proven exceptional sorption, catalytic and photocatalytic properties, so are ideal candidates for MW-assisted synthesis. The influence of different reaction parameters (e.g., time, temperature, concentration, reaction media), in the formation of the Fe- BDC-NH2 MOFs was considered. We identified the different crystalline structures, determining their crystal size together with the reaction yield and space-time yield, being key parameters for the future large scale production of Fe-BDC-NH2 MOFs. This work has been published in the Materials journal*. Interestingly, it reports for the first time the hydrothermal synthesis of MIL-53(Fe)-NH2 opening the door for its industrial production.
(*) Phase-Selective Microwave Assisted Synthesis of Iron (III) Aminoterephthalate MOFs. Ana Arenas-Vivo, David Avila, Patricia Horcajada. Materials 2020, 13, 1469; doi:10.3390/ma13061469
More information: Patricia Horcajada, Advanced Porous Materials Unit, email@example.com