Diffusion of Small Molecules in Metal Organic Framework Materials

Abstract: Ab initio simulations are combined with in situ infrared spectroscopy to unveil the molecular transport of H$_2$, CO$_2$, and H$_2$O in the metal organic framework MOF-74-Mg. Our study uncovers---at the atomistic level---the major factors governing the transport mechanism of these small molecules. In particular, we identify four key diffusion mechanisms and calculate the corresponding diffusion barriers, which are nicely confirmed by time-resolved infrared experiments. We also answer a long-standing question about the existence of secondary adsorption sites for the guest molecules, and we show how those sites affect the macroscopic diffusion properties. Our findings are important to gain a fundamental understanding of the diffusion processes in these nano-porous materials, with direct implications for the usability of MOFs in gas sequestration and storage applications.