Molecular Transport in Metal Organic Framework Ma- terials

Metal organic frameworks (MOF) materials are a class of porous materials well suited for hydrogen storage and gas separation. While current work on MOFs focuses mostly on the adsorption properties of small molecules, their diffusion is still poorly understood. To elucidate the diffusion process, we study the diffusion of H$_2$, CO$_2$, and H$_2$O in the nano-pores of MOF-74-Mg by combining \emph{ab initio} simulations with infrared (IR) spectroscopy. We present computed adsorption energies and changes in the IR frequencies upon adsorption. We also discuss several diffusion mechanisms and their calculated barriers. We further verify the existence of the debated secondary binding sites for guest molecules and we discuss the role played by H$_2$O. We find that H$_2$O is much more likely to adsorb in the MOF than H$_2$ and CO$_2$, leading to a significant reduction of the adsorption capabilities of the MOF towards these target molecules, and hence resulting in limitations for practical applications. Overall, our computational findings are in very good agreement with experiment and they provide a fundamental understanding of the diffusion processes of small molecules in these nano-porous materials, with implication for the usability of MOFs in gas separation and storage applications.