Permeation of single gases in thin zeolite MFI membranes

The permeation of a series of gases with widely different Lennard-Jones kinetic diameters and sorption properties is investigated as a function of feed pressure (up to 100 kPa) and temperature (298–473 K) with two different methods. The membrane system studied consists of an MFI (silicalite) top-layer (thickness 3 μm) directly grown on an α-alumina support. Flux values of molecules with a kinetic diameter, d_k, <0.45 nm (H₂, CO₂ and n-alkanes (CH₄ to n-butane)) are in the range of 1.5^–15 mmol m⁻² s⁻¹. Size exclusion is found for o-xylene (d_k=0.68 nm) and 2,2-dimethylbutane (dk=0.60 nm). Flux values are obtained for iso-butane, SF₆ and benzene, intermediate between those for small molecules and those for molecules with size exclusion. Maxima in the curve of flux versus temperature are found for gases with non-linear Langmuir-type adsorption curves as a function of pressure. The temperature of maxima at 100 kPa pressure increases in the order of CO₂<ethane<propane<butane and shifts to higher values with increasing pressure.