Connecting Defects and Amorphization in UiO-66 and MIL-140 Metal-organic Frameworks: A Combined Experimental and Computational Study

TL;DR

This study combines experimental and computational methods to understand how different MOFs, UiO-66 and MIL-140, undergo amorphization through bond breaking, revealing the structural changes and defect formation during collapse.

Contribution

It provides new insights into the amorphization mechanisms of MOFs by linking experimental NMR and PDF studies with DFT calculations, highlighting the role of defects and structural distortions.

Findings

UiO-66 retains Zr6O4(OH)4 clusters after amorphization

MIL-140 frameworks experience significant backbone distortion

Amorphous products preserve some inorganic-organic bonding motifs

Abstract

The mechanism and products of the structural collapse of the metal-organic frameworks (MOFs) UiO-66, MIL-140B and MIL-140C upon ball-milling are investigated through solid state 13C NMR and pair distribution function (PDF) studies, finding amorphization to proceed by the breaking of a fraction of metal-ligand bonding in each case. The amorphous products contain inorganic-organic bonding motifs reminiscent of the crystalline phases. Whilst the inorganic Zr6O4(OH)4 clusters of UiO-66 remain intact upon structural collapse, the ZrO backbone of the MIL-140 frameworks undergoes substantial distortion. Density functional theory calculations have been performed to investigate defective models of MIL-140B and show, through comparison of calculated and experimental 13C NMR spectra, that amorphization and defects in the materials are linked.