Free energy of ligand removal in the metal-organic framework UiO-66

TL;DR

This study investigates the energetics and mechanisms of ligand removal in UiO-66, revealing low defect formation energy, potential structural instability, and the role of molecular aggregates in defect processes.

Contribution

It provides atomistic insights into defect formation and ligand removal mechanisms in UiO-66, combining forcefield and quantum chemical methods.

Findings

Low defect formation free energy consistent with high defect concentrations

Identification of structural instability at higher defect levels

Role of molecular aggregates in driving ligand loss

Abstract

We report an investigation of the "missing-linker phenomenon" in the Zr-based metal-organic framework UiO-66 using atomistic forcefield and quantum chemical methods. For a vacant benzene dicarboxylate ligand, the lowest energy charge capping mechanism involves acetic acid or Cl-/H2O. The calculated defect free energy of formation is remarkably low, consistent with the high defect concentrations reported experimentally. A dynamic structural instability is identified for certain higher defect concentrations. In addition to the changes in material properties upon defect formation, we assess the formation of molecular aggregates, which provide an additional driving force for ligand loss. These results are expected to be of relevance to a wide range of metal-organic frameworks.