Single gas permeation of thin zeolite (MFI) membranes: theory and analysis of experimental observations

Theoretical expressions for single gas permeation are analysed and evaluated with selected literature and some new experimental data on Silicalite/ZSM-5 membranes. The phenomenological sorption–diffusion (PSD) description (and its equivalent Maxwell–Stefan equation) covers both the microscopic models based on configurational diffusion (CD) with δ=1.0 and on surface diffusion (SD) with δ≥1.24. The ratio δ of the pore diameter over the (kinetic) molecular diameter is important. For 1.24≤δ<3.0 the gas translational diffusion (GT) or activated Knudsen Model is shown to be the preferred microscopic model, which is not covered by the PSD description. The combination of GT and SD mechanisms which is called surface diffusion enhanced micropore permeation (SEMP) model is the only model able to describe the experimentally observed minima in the flux of hydrogen, argon and krypton as a function of temperature as well as the experimentally observed combination of a maximum followed by a minimum in the flux versus temperature curve (methane, ethane). Maxima in the flux versus temperature curves were observed for e.g. C1–C4 alkanes and carbon dioxide and could be described by PSD as well as SEMP models. The observed reversal of the order of the flux magnitudes with increasing temperature and in the activation energies of diffusion in the series Ne>Ar>Kr>Xe at given values of δ close to unity is predicted by the SEMP model.