A systematic approach is described for the fabrication of defect-free high-silica zeolite membranes with CHA (SSZ-13) topology. Home-made hydrothermally-synthesized CHA seeds were coated on porous α-alumina substrates with a pore diameter of 80 nm and by means of a further hydrothermal treatment a zeolite membrane layer was formed. In order to obtain a thin and defect-free zeolite layer, the influence on the final microstructure of seed concentration during coating, coating method (rubbing, dip- or spin-coating) and crystal growth time was investigated. The template removal procedure was optimized to avoid the formation of cracks or defects. For an optimal thermal treatment, using a step-wise temperature increase to 500 °C, the membranes exhibit CO₂/CH₄ permselectivities of 25–30 with CO₂ permeances of around 2 x 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at 22 °C and 2 bar of pressure difference. O₂ plasma pre-treatment prior to template removal increased the CO₂/CH₄ permselectivity to 176, while maintaining the same CO₂ permeance values when no pre-treatment was used. The SF₆ permeances, both at low (22 °C) and high (200 °C) temperatures, were below the detection limit (2 x 10⁻¹⁰ mol m⁻² s⁻¹ Pa⁻¹), which in return results in very high N₂/SF₆ permselectivities of more than 700.

The transformation of microporous, amorphous silica membranes into b-oriented silicalite-1 (MFI) zeolite layers via in-situ crystallisation was investigated. The effect of synthesis parameters, such as the type and concentration of the silica precursor, crystallisation time and temperature, on the morphology of silicalite-1 (MFI) zeolite layers was studied. By optimizing these parameters, silicalite-1 zeolite layers were formed from the already-deposited silica layers, which promotes the crystallisation from the surface in the preferred b-orientation. The use of a monomeric silica precursor, which has slower hydrolysis kinetics than a colloidal one, resulted in the formation of zeolite crystals via heterogeneous nucleation on the surface and suppressed the formation of crystal nuclei in the liquid media via homogeneous nucleation, which then would further deposit onto the surface in a random orientation. Lastly, by optimizing the crystallisation time and temperature of the synthesis, thickness, coverage and orientation of silicalite-1 zeolite layers were controlled.

We demonstrate that b-oriented MFI (Mobil Five) zeolite membranes can be manufactured by in situ crystallization using an intermediate amorphous SiO₂ layer. The improved in-plane growth by using a zeolite growth modifier leads to fusion of independent crystals and eliminates boundary gaps, giving good selectivity in the separation of CO₂/Xe mixtures. The fast diffusion of CO₂ dominates the overall membrane selectivity toward the CO₂/Xe mixture. Because of the straight and short [010] channels, the obtained CO₂ permeation fluxes are several orders of magnitude higher than those of carbon molecular sieving membranes and polymeric membranes, opening opportunities for Xe recovery from waste anesthetic gas.