Highly CO₂-selective and moisture-resistant bilayer silicalite-1/SSZ-13 membranes with gradient pores for wet CO₂/CH₄ and CO₂/N₂ separations

Surface hydrophobicity of zeolite membrane which is related to moisture resistance increases with the increase of Si/Al ratio in the framework. An all-silica MFI (silicalite-1) layer was grown on aluminium-containing SSZ-13 membrane as a water-proof shield in this study. The formation process of silicalite-1 layer upon SSZ-13 membrane was investigated. And the moisture-resistant mechanism of bilayer silicalite-1/SSZ-13 membrane was discussed. The gradient pores (5.6 Å @ 3.8 Å) of bilayer silicalite-1/SSZ-13 membranes guarantee the separation efficiency. The bilayer membranes displayed similar CO₂ permeances with the original SSZ-13 membranes in the dry mixture because that the modified silicalite-1 layer had larger pores and was CO₂-selective. Moreover, the bilayer membrane displayed higher moisture resistance than the original SSZ-13 membrane did at low temperatures. The best bilayer membrane had CO₂ permeances of 1.5 × 10^-6 and 1.4 × 10^-6 mol m^-2 s^-1 Pa^-1 and CO₂/CH₄ and CO₂/N₂ selectivities of 153 and 22 in dry and equimolar CO₂/CH₄ and CO₂/N₂ mixtures at 298 K, respectively. The CO₂/CH₄ selectivity of the bilayer membranes in the wet mixture were only 19.4% lower than these in the dry mixtures at 323 K, respectively. The decline rate of the bilayer membrane were much lower than that (39.3%) for the monolayer SSZ-13 membrane. The long-term stability of separation performance of bilayer membranes in the wet mixture was also investigated.