Determination of Geometry Arrangement of Copper Ions in HKUST-1 by XAFS During a Prolonged Exposure to Air

We present an experimental investigation focused on the local structural changes taking place around Cu 2+ ions in metal–organic framework (MOF) HKUST-1 for different times of exposure to air by XAFS (X-ray absorption fine structure). The analysis involves both XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) regions around the Cu K-edge. Starting from the paddle-wheel structures proposed in literature, a more detailed description of the geometrical environment of Cu 2+ ions has been found. In particular, the paddle-wheel structure of a fresh sample, which means a pristine HKUST-1 material with a single water molecule weakly adsorbed on each Cu 2+ ion, has been disclosed for the first time. Furthermore, after 20 days of exposure to air, relevant structural changes with respect to the pristine sample have been evidenced. An activation process has demonstrated that these local changes are totally reversible, in agreement with a recent model of the decomposition process of HKUST-1 proposed in literature.