Transport Diffusion Measurements in ZIF‐8 Thin Films by Quartz Crystal Microgravimetry: Pitfalls and Opportunities

ABSTRACT Mass transport in nanoporous materials directly affects their performance in applications, as seen in adsorption‐based separations where diffusion kinetics limit the maximum production rate. Nevertheless, diffusion kinetics have been understudied, largely due to the difficulties in measuring them accurately. Under well‐defined conditions, gravimetric uptake experiments are well‐suited for studying the mass transfer behavior of guests in nanoporous materials. In this work, we illustrate the pitfalls and opportunities in the experimental determination of intrinsic transport diffusivities by microgravimetry. To do so, the behavior of a series of alcohols (n‐propanol, n‐butanol), ketones (acetone, butanone), and alkanes (n‐pentane, n‐hexane) was studied in ZIF‐8 thin films deposited on quartz crystal microbalance sensors. For all studied guest molecules, the sorption rate proved strongly concentration‐dependent, resulting in diffusivity values spanning several orders of magnitude. Using this data set, we illustrate how erroneous conclusions can easily result when insufficient care is taken in acquiring or analyzing the uptake data. To prevent such issues, which are unfortunately common in reported data, a set of guidelines was formulated to extend the IUPAC recommendations for diffusion measurements to the specific case of quartz crystal microgravimetry. These guidelines cover the setup, the adsorbent, the experimental design to elucidate the model requirements, heat and bed effects, and the contribution of surface barriers. At the same time, the novel data reported here substantially broadens the existing knowledge of transport diffusion in ZIF‐8.