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Clean and rapid synthesis of microporous mixed- metal MOF-74 materials via mechanochemistry

Porous metal-organic framework materials (MOFs) are nowadays one of the most investigated areas of materials science, with potential application in catalysis, storage and separation, drug delivery, sensing, light harvesting, and other. (Li et al., 1999) Among others, MOF-74 materials attracted special attention due to their robustness, stability, open metal sites, and microporous honeycomb structure. MOF-74 materials are highly modular, with different M2+ metal cations known to successfully form monometallic MOF-74 structure, and also showing different physicochemical properties depending on the metal cation used. In some cases small portion of metal nodes in MOF-74 materials were exchanged by other cation, after soaking the prepared M1-MOF-74 in solution containing the other metal(II) cation. Such materials were reported to have 1-5 % of the other metal in the structure and much better catalytic properties, (Sun et al, 2015) but the controlled synthesis of bimetallic MOF-74 materials still remains a real challenge. Here we present the application of mechanochemistry, ( James S. L. et al., 2012) i.e. reactions between solids induced by mechanical force, for a rapid and controllable synthesis of monometallic and bimetallic MOF-74 materials in gram quantities, using only small volume of green solvent additives such as water or alcohols. In situ synchrotron X-ray powder diffraction monitoring (Užarević et al, 2015 ; Friščić et al, 2013) revealed that the mechanochemical formation of MOF-74 proceeds through different intermediate polymeric phases, most of which are inaccessible from solution procedures.(Julien et al, 2016) It was possible to isolate and characterize these intermediates, and to use them for controllable synthesis of various bimetallic MOF-74 materials based on Zn, Cu, and Co cations. The obtained M1M2-MOF-74 materials show homogeneous distribution of metal cations, excellent porosity and have significantly different magnetic properties than their monometallic analogues. Herein presented results demonstrate how mechanochemistry, combined with in situ monitoring methods, can open a pathways towards microporous metal-based materials which are completely inaccessible by standard solution procedures, and also provided a new entry in this highly investigated and exciting field of material chemistry.

mechanochemical synthesis ; microporous metal-organic materials ; MOF-74

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