Understanding and Controlling Water Stability of MOF-74

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$\to$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.

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• Cluster Assisted Water Dissociation Mechanism in MOF-74 and Controlling it Using Helium
• Chemistry in confined spaces: Reactivity of the Zn-MOF-74 channels