Directional Isothermal Diffusion of Acetone in UiO-66 Metal-Organic Frameworks

A goal of developing novel metal-organic frameworks (MOFs) is selectively transporting hazardous chemicals, including chemical warfare agents, through a material that can contain functionalized domains that enhance filtration of the target analyte and reject undesired background gases (e.g., H2O, CO2). To achieve this goal, it is critical to understand the diffusion processes of simple analytes through well-characterized MOFs. Here, we use in situ infrared spectroscopy to investigate the transport of acetone, a simple polar molecule, from the external MOF crystallite surface into the internal pore environment of UiO-66, and vice versa, i.e., inward and outward diffusion. Previous work has shown that small, polar molecules can reversibly and nondestructively bind to the μ3–OH groups located within the tetrahedral pore environment of UiO-66 MOFs, quenching the free OH vibrational band, providing an ideal system to probe molecular diffusion pathways. We find that, following an Arrhenius analysis for the outward diffusion process, the barrier for acetone to overcome short-range interactions and diffuse out of UiO-66 is ∼37 kJ mol–1. Methodologies for extracting diffusion coefficients from such experiments via Fick’s second law are discussed. We find that, depending on the choice of isothermal hold temperatures, we can isolate the direction of analyte transport, i.e., diffusion from the external surface into the internal pores or vice versa. This work reveals how system temperature impacts diffusion processes and provides insight into how a simple, polar molecule is transported throughout UiO-66.