Isolating the Role of the Node-Linker Bond in the Compression of UiO-66 Metal–Organic Frameworks

Abstract

Understanding the mechanical properties of metal–organic frameworks (MOFs) is essential to the fundamental advancement and practical implementations of porous materials. Recent computational and experimental efforts have revealed correlations between mechanical properties and pore size, topology, and defect density. These results demonstrate the important role of the organic linker in the response of these materials to physical stresses. However, the impact of the coordination bond between the inorganic node and organic linker on the mechanical stability of MOFs has not been thoroughly studied. Here, we isolate the role of this node–linker coordination bond to systematically study the effect it plays in the compression of a series of isostructural MOFs, M-UiO-66 (M = Zr, Hf, or Ce). The bulk modulus (i.e., the resistance to compression under hydrostatic pressure) of each MOF is determined by in situ diamond anvil cell (DAC) powder X-ray diffraction measurements and density functional theory (DFT) simulations. These experiments reveal the distinctive behavior of Ce-UiO-66 in response to pressures under 1 GPa. In situ DAC Raman spectroscopy and DFT calculations support the observed differences in compressibility between Zr-UiO-66 and the Ce analogue. Monitoring changes in bond lengths as a function of pressure through DFT simulations provides a clear picture of those which shorten more drastically under pressure and those which resist compression. We hypothesize that the presence of ∼10% Ce3+ in the nodes of Ce-UiO-66 may contribute to the weakening of the node–linker coordination, manifesting in the distinct behavior under pressure. This study demonstrates that changes to the node–linker bond can have significant ramifications on the mechanical properties of MOFs.