Exploring Window Effects: Revealing Unusual Diffusion Patterns in SSZ-39 (AEI) Zeolite via Moment Analysis of <i>n</i>-Alkane Adsorption Using Inverse Gas Chromatography

The molecular-sized cages of the Na-SSZ-39 small-pore zeolite (AEI-type) result in a high level of confinement of adsorbing hydrocarbon molecules, accompanied by large energetic interactions and important entropic effects. Our previous work has reported the manifestation of cage effects, leading to unusual variations in Henry adsorption constants with increasing alkane chain length. According to simulation studies, adsorptive cage effects are directly linked to unusual transport effects first described by Gorring as window effects, yet these remain challenged due to a lack of experimental proof. In this study, inverse gas chromatography is used to investigate the correlations between adsorption and transport properties of C5–C10 n-alkanes on Na-SSZ-39. Moment analysis of chromatographic peak broadening revealed an unusual trend in transport properties with alkane chain length, showing a maximum at n-octane, caused by the specific cage geometry of this zeolite. Variation of the pellet size and carrier gas suggests that the measured transport properties are related to the apparent diffusion coefficients. The unusual trend in these transport properties reveals the manifestation of window effects in the AEI framework. Additionally, an inverse relationship between the Henry adsorption constants and diffusion coefficients is observed experimentally and confirms longstanding theoretical predictions.