Gas permeation and separation in ZSM-5 micromembranes

Freestanding Sil-1 and ZSM-5 micromembranes of Si/Al ratios ranging from 14 to ∞ were fabricated on silicon. The zeolite synthesis was adjusted to obtain micromembranes of similar microstructures (i.e., thickness, orientation, crystal grain size, morphology and intergrowth) and prepared membranes display comparable helium gas permeance. The low temperature removal of the organic TPA+ molecules by ozone enabled the preparation of low defect zeolite micromembranes that exhibit superb H2/nC4 (i.e., ≥100) and nC4/iC4 (i.e., ≥30) permselectivity ratios. The effects of aluminum content of the ZSM-5 micromembranes were investigated for single, binary and ternary gas permeation. Surface diffusion is important at low temperature and the single gas permeances of hydrogen, argon, nitrogen and methane were larger than Knudsen. The deviation from Knudsen increased with the Si/Al ratio and was largest for Sil-1. Propane–nitrogen separation in the zeolite micromembranes is mainly molecular sieving and display weak dependence on the aluminum content of the zeolite, where the separation of the ternary H2–CH4–CO2 mixture is by preferential adsorption and shows strong dependence on the Si/Al ratio of the zeolite.