Hydrothermally stable silica-alumina composite membranes for hydrogen separation

A thin layer (30-40 nm) of a dual-element silica-alumina composition was deposited on a porous alumina support by chemical vapor deposition (CVD) in an inert atmosphere at high temperature. Prior to CVD, an intermediate layer of gamma-alumina was coated on the macroporous alumina support. The intermediate layer was prepared by the dip-coating and calcination of boehmite sols of different sizes to give a graded structure that was substantially free of defects. The resulting supported composite membrane had high permeance for hydrogen in the order of 2-3 x 10(-7) mol m(-2) s(-1) Pa-1 at 873 K with selectivities of H-2 over CH4, CO and CO2 of 940, 700 and 590, respectively. The membrane operated by a hopping mechanism involving jumps of permeating molecules between solubility sites. The presence of aluminum improved the hydrothermal stability of the membranes for periods in excess of 500 h at 873 K in 16% steam, allowing the permeance to remain above 10(-7) mol m(-2) s(-1) Pa-1, although with decreased selectivities. (C) 2007 Elsevier B.V. All rights reserved.