Unveiled the Source of the Structural Instability of HKUST Powders upon Mechanical Compaction: Definition of a Fully Preserving Tableting Method

Abstract

Metal–organic frameworks (MOFs) are getting closer to finally being used in commercial applications. In order to maximize their packing density, mechanical strength, stability in reactive environments, and many other properties, the compaction of MOF powders is a fundamental step for the application field of research of these extraordinary materials. In particular, HKUST-1 is among the most promising and studied MOF. Contrary to what reported so far in the literature, here we prove that the tableting of HKUST-1 powders without any damage of the lattice is possible and easy to get. For the first time, this kind of investigation has been performed exploiting its peculiar magnetic properties with the aid of electron paramagnetic resonance spectroscopy. Indeed, they have allowed us to explore in detail all the smallest changes induced in the paramagnetic paddle-wheel units by the application of the mechanical pressure on the material. This original approach has permitted us to unveil the main source of structural instability of HKUST-1 during compaction, that is, the water molecules adsorbed by the powdered sample before tableting and finally to establish a proper compaction protocol. Our conclusions are also fully supported by the results obtained with powder X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, water sorption isotherms, and surface area estimation with the Brunauer–Emmett–Teller method, which prove that the tablet of HKUST-1 obtained by this new protocol actually preserves the crystal structure and porosity of the pristine powders. A morphological characterization has also been conducted with a combined use of optical and atomic force microscopies.