How Can a Hydrophobic MOF be Water‐Unstable? Insight into the Hydration Mechanism of IRMOFs

Abstract

We report an ab initio molecular dynamics study of the hydration process in a model IRMOF material. At low water content (one molecule per unit cell), water physisorption is observed on the zinc cation but the free⇄bound equilibrium strongly favors the free state. This is consistent with the hydrophobic nature of the host matrix and its type-V isotherm observed in a classical Monte Carlo simulation. At higher loading, a water cluster can be formed at the Zn(4)O site and this is shown to stabilize the water-bound state. This structure rapidly transforms into a linker-displaced state, where water has fully displaced one arm of a linker and which corresponds to the loss of the material's fully ordered structure. Thus an overall hydrophobic MOF material can also become water unstable, a feature that has not been fully understood until now.

Emerging adsorptive removal of azo dye by metal–organic frameworks

“…It is Scheme 2. Schematic of ligand displacement mechanism in the presence of water for IRMOF materials (Toni et al, 2012). noteworthy that this kind of MOF readily adsorbs both cationic and anionic dyes in liquid phase even though it hardly adsorbs nitrogen at low temperatures. A comparative adsorption study indicated that the adsorption capacity of MOF-235 is also much higher than that of AC, such as MIL-101 and MIL-53 for MO removal.…”

“…Their results indicated that ligand displacement is the dominant mechanism in the degradation of IRMOFs in the presence of water molecules. Scheme 2 shows their proposed mechanism of ligand displacement in two successive steps, first water binding to the metallic cluster and then linker displacement (Toni et al, 2012).…”

“…Toni et al (Toni et al, 2012) investigated the water stability of the IRMOF family and asked "How can a hydrophobic MOF be water-unstable? ", via a simulation work.…”

Origin of highly active metal–organic framework catalysts: defects? Defects!

“…76 From direct first-principles simulations at various water loadings, the ligand displacement was also defined as the main route for the framework decomposition of hydrophobic IRMOFs. 77 It has been found that in addition to the water molecule involved in the ligand displacement, additional water stabilized both the hydrated metal species and the displaced ligand. Coudert and coworkers furthermore reported that ZnO clusters act as hydrophilic defects in hydrophobic MOFs.…”

Synergistic Effect of Mixed Oxide on the Adsorption of Ammonia with Metal–Organic Frameworks

“…[54][55][56][57] Similar to the parent HT material, MMO retains a large amount of water upon desorption, 15.2 mmol/g, illustrating the strong interaction of MMO with water. As shown in previous studies, 38 UiO-66 and UiO-66-NH 2 adsorb large amounts of water, considerably more than HT, 18.5 and 19.5 mmol/g, respectively, yet retain less than 2.6 mmol/g after desorption indicating weak and reversible interactions with water.…”