Theoretical Insights into the Initial Hydrolytic Breakdown 'of HKUST-1

HKUST-1, i.e., Cu-3 (BTC)(2) (BTC = benzene-1,3,5-tricarboxylate), has been considered as one of the very promising copper-paddlewheel-based metal-organic frameworks (CP-MOFs) for a variety of practical applications such as gas separation and heterogeneous catalysis. The poor hydrolytic stability of HKUST-1 in humid conditions, especially in an aqueous environment, however, severely hinders its practical applications. Understanding the hydrolytic mechanism of HKUST-1 is crucial for the improvement of hydrolytic stability. In the present work, the hydrolytic breakdown mechanism of a Cu-Cu metal node at HKUST-1 in the presence of water molecules was investigated using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. AIMD simulation trajectories show that the presence of water molecules induces pronounced oscillations of all Cu-O bonds at the Cu-Cu node, leading to the precursor state for the initial Cu-O bond breaking. The hydrolytic breakdown mechanism that involves water adsorption, ligand displacement, and water dissociation steps under various water concentrations (14 water molecules per Cu-Cu node) was studied using DFT. It had been found that Cu-O bond breaking via ligand displacement with water substitution is quite facile. The complete ligand detachment with breaking of two Cu-O bonds must involve the dissociation of adsorbed water molecules. The complete detachment of two ligands at the Cu-Cu node needs at least three water molecules. The detailed hydrolytic breakdown mechanism provides fundamental insights into the structural collapse of the HKUS-1 framework.