Understanding and Controlling Water Stability of MOF-74

TL;DR

This paper investigates the water-induced instability of MOF-74, revealing that water dissociation at metal centers weakens the framework and reduces CO2 uptake, and proposes modifications to enhance stability under humid conditions.

Contribution

It identifies the water dissociation reaction as the cause of MOF-74 instability and suggests structural modifications to prevent this reaction, improving water stability.

Findings

Water dissociation causes MOF-74 degradation.

OH groups weaken the MOF framework.

Modifying MOF-74 increases water dissociation energy barrier.

Abstract

Metal organic framework (MOF) materials in general, and MOF-74 in particular, have promising properties for many technologically important processes. However, their instability under humid conditions severely restricts practical use. We show that this instability and the accompanying reduction of the CO$_2$ uptake capacity of MOF-74 under humid conditions originate in the water dissociation reaction H$_2$O$\rightarrow$OH+H at the metal centers. After this dissociation, the OH groups coordinate to the metal centers, explaining the reduction in the MOF's CO$_2$ uptake capacity. This reduction thus strongly depends on the catalytic activity of MOF-74 towards the water dissociation reaction. We further show that-while the water molecules themselves only have a negligible effect on the crystal structure of MOF-74-the OH and H products of the dissociation reaction significantly weaken the MOF framework and lead to the observed crystal structure breakdown. With this knowledge, we propose a way to suppress this particular reaction by modifying the MOF-74 structure to increase the water dissociation energy barrier and thus control the stability of the system under humid conditions.