Proton Conduction in a MIL-53(Al) Metal-Organic Framework: Confinement versus Host/Guest Interaction

In this contribution, we present and discuss results from a computational study of proton transfers between imidazole molecules confined in a MIL-53(Al) metal-organic framework. We combined molecular-dynamics simulations and a density-functional tight-binding method. The extensive analysis of trajectories resulted in two main competing effects: on the one hand, the one-dimensional channel structure of MIL-53(Al) arranges the imidazole molecules to allow proton exchange by hopping transport; on the other hand, the interactions between the MIL-53(Al) host system and the imidazole molecules influence the free movement retaining the molecules. We find that the retaining leads to an increase in proton transfers, when both vehicle mechanisms and hopping events are considered. Thus, a well-balanced relationship between these two effects is necessary for efficient proton transport in metal-organic frameworks. Furthermore, the lifetime of the transition state could be estimated to be on the order of 100 fs.