Insights in adsorptive cage and window effects in zeolites from a 1D energy barrier model

Abstract

Non-monotonous trends in the Henry adsorption coefficient with respect to carbon number during the adsorption of n-alkanes are often categorized under the term “cage effects”. Recent experimental studies have shown that such effects emerge for a variety of cage-type CHA and AEI zeolite structures. Furthermore, observations of unusual kinetic effects (“window effects”) have been linked to this cage effect but remain controversial. Still, cage and window effects have been proposed to be generic for structures containing larger features (cages), separated by narrow openings. In this work, we present a simple 1D model based on rectangular free energy profiles for the individual CH_x beads that compose the n-alkane chain. The descriptive model offers a prediction of both Henry and diffusion coefficient trends. Using the model, recent observations can be rationalized from the perspective of adsorption in larger cages and narrow openings. It is shown that the local minimum in Henry coefficient coincides with a local maximum in diffusion coefficient. Although the model does not account for even more complex adsorption mechanisms such as molecular coiling within cages, it demonstrates how such unusual effects can be predicted using only straightforward assumptions.

Graphical Abstract