Preparation and characterization of thin zeolite MFI membranes on porous supports

Thin (2–7 μm) polycrystalline randomly-oriented zeolite MFI membranes were prepared on an α-Al₂O₃ support by one or more subsequent hydrothermal treatments. Different particle sizes (275–700 nm) in the layer were achieved by changing the synthesis temperature (371–459 K). Zeolite membranes, prepared by two subsequent hydrothermal treatments have an optimum quality. Membranes prepared by a single hydrothermal treatment have defects and zeolite membranes obtained by three or more hydrothermal treatments become too thick and crack during the removal of the template. The zeolite MFI membranes were characterised by permeation measurements using single gases and mixtures of n-butane (n-C₄H₁₀) and iso-butane (i-C₄H₁₀). Permselectivities, Fα, and separation factors, α, for n-C₄H₁₀/i-C₄H₁₀ decrease strongly with an increasing number of defects and hence provide a good indication of membrane quality. The separation, on the other hand, of a CH₄/n-C₄H₁₀ mixture at high occupation of n-C₄H₁₀ is not much affected by small defects. This is ascribed to blocking of those defects by capillary condensation of the butane. Flux values decrease linearly with increasing zeolite crystallite size in the top layer indicating that the effective membrane thickness is only one particle thick. During hydrothermal treatment zeolite particles also grow at the other side of the support but do not decrease significantly the flux through the supported zeolite membrane.