Window effects in zeolite diffusion and brownian motion over potential barriers

Gorring (1973) reported a phenomenon, the “window effect,” whereby the diffusivities of normal paraffins within zeolite T do not decrease monotonically with increasing carbon number N, as would be expected intuitively. Rather, following an initial decrease with N, the diffusivities exhibit a local minimum at C8 followed by a pronounced local maximum at C12. This article presents a theoretical analysis of the window effect and related phenomena, based on an analogy of the configurational diffusion process with an “equivalent” one‐dimensional Brownian motion of a rod through a periodic sequence of potential barriers. Numerical calculations are found to be in reasonable agreement with Gorring's experimental data and quantify his qualitatively‐stated mechanism, namely that n‐alkanes longer than C8 are too large to fit entirely within the potential wells formed by erionite cages and therefore experience smaller energetic barriers to diffusion.