Zirconia-supported all-silica zeolite CHA membrane with unprecedentedly high selectivity in humidified CO₂/CH₄ mixture

Continuous and thin all-silica zeolite CHA membranes were prepared successfully on the tubular zirconia/alumina supports by a novel fluoride-free synthesis approach using lithium hydroxide as mineralizer. Membrane synthesis using the uniform and low-viscosity gel displayed good reproductivity. The influences of pressure and temperature on separation data of the membrane in the dry and wet mixtures were comprehensively investigated. The typical membrane had CO₂/CH₄ and CO₂/N₂ selectivities of 170 and 33 and carbon dioxide permeances of 950 × 10⁻⁹ and 1200 × 10⁻⁹ mol/(m² s Pa) [∼3600 GPU, Gas Permeation Unit: 1GPU = 3.348 × 10⁻¹⁰ mol/(m² s Pa)] at 298 K in dry CO₂/CH₄ and CO₂/N₂ (50/50 in mol) mixtures, respectively. The unprecedentedly high CO₂/CH₄ selectivity of 430 was obtained for the zeolite membrane at 298 K in wet CO₂/CH₄ mixture with saturated water vapor. The long-term stability of this membrane was also investigated in humidified mixture with relative humidity of close to 100%. The possible water-gated molecular sieve mechanism in the presence of moisture was proposed.