H₂ separation using defect-free, inorganic composite membranes

Defect-free, microporous Al₂O₃/SAPO-34 zeolite composite membranes were prepared by coating hydrothermally grown zeolite membranes with microporous alumina using molecular layer deposition. These inorganic composite membranes are highly efficient for H₂ separation: their highest H₂/N₂ mixture selectivity was 1040, in contrast with selectivities of 8 for SAPO-34 membranes. The composite membranes were selective for H₂ for temperatures up to at least 473 K and feed pressures up to at least 1.5 MPa; at 473 K and 1.5 MPa, the H₂/N ₂ separation selectivity was 750. The H₂/CO₂ separation selectivity was lower than the H₂/N₂ selectivity and decreased slightly with increasing pressure; the selectivity was 20 at 473 K and 1.5 MPa. The high H₂ selectivity resulted either because most of the pores in the Al₂O₃ layer were slightly smaller than 0.36 nm (the kinetic diameter of N₂) or because the Al₂O₃ layer slightly narrowed the SAPO-34 pore entrance. These composite membranes may represent a new class of inorganic membranes for gas separation.