Water coordination and dehydration processes in defective UiO-66 type metal organic frameworks

Abstract

The UiO-66 metal organic framework is one of the most thermally and chemically stable hybrid materials reported to date. However, it is also accepted that the material contains structurally embedded defects, which may be engineered to enhance properties towards specific applications such as catalysis, sensing, etc. The synthesis conditions determine to a large extent the level of perfection of the material and additionally the catalytic activity may be enhanced by post-synthesis activation at high temperature under vacuum, in which defect coordinating species (H₂O, HCl, monocarboxylic modulators, etc.) evaporate. The molecular level characterization of defects is extremely challenging from both theoretical and experimental points of view. Such experimental endeavor was recently proposed via experimental SXRD measurements, also unraveling the coordination of water on the Zr–O–Zr defect sites [Angew. Chem., Int. Ed., 2015, 54, 11162–11167]. The present work provides a theoretical understanding of defect structures in UiO-66(Zr) by means of periodic density functional theory calculations and ab initio molecular dynamics simulations. A range of defect structures are generated with different numbers of missing linkers. For each of the defects, the free energetic and mechanical stability is discussed and the coordination of water and charge balancing hydroxide ions is studied. For catalysis applications, the material is mostly pretreated to remove water by dehydration reactions. For each of the proposed defect structures, mechanistic pathways for dehydration reactions of the Zr-bricks are determined employing nudged elastic band (NEB) calculations. During the dehydroxylation trajectory, loose hydroxyl groups and terephthalate decoordinations are observed. Furthermore, dehydration reactions are lower activated if terephthalate linkers are missing in the immediate environment of the inorganic brick. The creation of defects and the dehydration processes have a large impact on the mechanical properties of the material, which is evidenced by lower bulk moduli and elastic constants for structures with more defects.