The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and Application Transport into zeolite nanosheets: Diffusion equations put to test

Ultrathin porous materials, such as zeolite nanosheets, are prominent candidates for performing cataly-sis, drug supply, and separation processes [1, 2] in a highly efficient manner due to exceptionally short transport paths. Predictive design of such processes requires the application of diffusion equations that were derived for macroscopic, homogeneous surroundings to nanoscale, nano-structured host sys-tems. Because of the obvious dissonance between requirements and assumptions, we tested analytical solutions of Fick’s diffusion equations for their applicability to methane transport into two different zeolite structures (Figure 1b) under instationary conditions. The two central questions were: Do the conventional diffusion equations break down at the nanoscale? And, what is the reason to the (possi-ble) limitation of applicability? Figure 1: (a) Scheme of TrMD. (b) Deviation of TrMD transport diffusivities from EMD predictions. Computationally demanding transient molecular dynamics (TrMD) simulations were performed, par-alleling conditions of diffusion experiments with constant sorbate supply (Figure 1a). Transient con-centration profiles from TrMD were fitted to standard analytical solutions of Fick’s diffusion equations [3] to yield transport diffusivities, DT, and surface permeabilities, α: c(t,z)/c ∞ = 1−∑∞i {2L exp[−γ2i DT t/(δ/2)2]cos[γiz/(δ/2)]/[(γ2i +L2+L)cosγi]}, (1