STUDY OF THE STRUCTURAL STABILITY OF MOF HKUST-1 UPON AIR EXPOSURE

Metal-Organic Frameworks (MOFs) constitute a class of microporous crystalline compounds which have attracted the attention of whole the scientific community in the last years. They involve the presence of metallic groups and organic linkers as connectors. The reasons of interest principally lie in their huge surface area, in the large pore volume and in the possibility to functionalize the organic linkers also after the synthesis of the material. The cleverness of research groups all around the world has carried to the synthesis of more than 70,000 varieties of such hybrid compounds interesting in many application fields. One of the most known and studied MOFs for its chemical, structural and magnetic properties is HKUST-1, belonging to the class of carboxylate MOFs. These properties are such to make it a model for the whole class of carboxylate compounds. Its metallic groups, a Cu(II) dimer, is connected by BTC (benzene 1,3,5-tricarboxylate) linker molecules to form a three dimensional porous network. The unit constituted by a Cu(II) dimer coordinated by four carboxylate bridges is called paddle-wheel moiety. The properties of HKUST-1 make it suitable for a lot of fields as gas storage as hydrogen and methane, purification of air and water, capture of fuel streams, catalysis, drug delivery and so on. Previous studies pointed out that activated HKUST-1 is strongly hydrophilic and that the binding sites on Cu(II) ions are the primary adsorption sites for polar molecules such as water. This affinity of HKUST-1 with respect to water is very interesting for many applications, such as in biomedical field, in the packing and transporting of gaseous industrial streams to avoid corrosive and condensation effects, and to increase the efficiency of CO2 capture. In spite of these interesting applications, it was shown that the crystalline structure of the HKUST-1 can be seriously damaged by a prolonged exposure to moist environment. Recent studies concerning a prolonged exposure to air moisture have demonstrated the ability of water molecules to start the hydrolysis process of Cu-O bonds involved in the paddle-wheel units. The consequence is a damage of the crystalline structure. This property limits not only the applications where HKUST-1 is directly exposed to moisture or water but also all those uses where water is present just as impurity. Our study aims to characterize in great detail the decomposition process of HKUST-1 upon a long exposure to air, unveiling all the main stages involving the crystalline matrix. To accomplish this objective, we have taken advantage of various spectroscopic experimental techniques which have permitted us to characterize the system at atomic level. Thanks to this approach, we have revealed three different stages of the interaction of HKUST-1 with air moisture and we have determined the equilibrium structures of the paddle-wheel units in each of these stages.