Two component permeation through thin zeolite MFI membranes

Two component permeation measurements have been performed by the Wicke–Kallenbach method on a thin (3 μm) zeolite MFI (Silicalite-1) membrane with molecules of different kinetic diameters, dk. The membrane was supported by a flat porous -Al2O3 substrate. The results obtained could be classified in six regimes based on differences in: • Occupation degrees, θ, on the external surface. • Occupation degrees, θ, in the zeolite pores. • Mobilities in the zeolite pores. High separation factors are obtained for mixtures of “weakly/strongly adsorbing” gases and for mixtures of “small/large” gases. Separation by size-exclusion is demonstrated for 8 kPa n-C6H14/16 kPa 2,2-dimethylbutane (>600, T=298–473 K) and 0.31 kPa p-xylene/0.26 kPa o-xylene (>200, T=400 K). At low θ (high temperature, low pressures) the permeation of both components in a mixture equals the single-gas permeation of these components. At high θ (low temperature and high pressure) the permeation of both components in mixtures is different from single-gas permeation values. The single-gas permeance of “small” molecules with kinetic diameter dk <0.45 nm ranges from 1×10−8 to 15×10−8 mol m−2 s−1 Pa−1 in the temperature range 298–473 K. The permeation decreases with increasing dk of the molecules.