Defect-free high-silica CHA zeolite membranes with high selectivity for light gas separation

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.