METAL COMPOSITE MEMBRANES FOR HYDROGEN SEPARATION

Further advances in the field of gas separation membranes will require materials with higher permeation selectivities than the currently available polymers. Metal membranes offer such an alternative. Because the mechanism for gas permeation through metals differs from that through polymers, metal membranes with very high selectivities can be made. Gas permeation through these membranes is very slow, however, and membranes with adequate fluxes are difficult to make. High membrane cost has also limited development because some of the best membrane materials are precious metals. Membrane Technology and Research, Inc. is pursuing a new approach to preparing metal membranes that offer the high selectivity inherent to permeation through metals with the high fluxes typical of polymeric membranes. We have prepared ultrathin metal composite membranes by sputter-deposition of a 76 atom% palladium/24 atom% silver alloy layer onto a conventional polymeric gas separation membrane. Preparation of membranes with different thicknesses of the metal layer (250-1,000 angstrom) under different deposition conditions (75-260 angstrom/min) showed that the best membranes are obtained at high metal deposition rates. These membranes have a hydrogen flux of 1 x 10(-5) cm3 (STP)/cm2 s cmHg at room temperature (25-degrees-C) and a hydrogen/carbon dioxide selectivity greater than 100. In contrast, the best commercial polymeric membranes have a hydrogen/carbon dioxide selectivity of 6. The hydrogen flux through the membrane increases with temperature, but the fluxes of all other gases are unchanged. The net effect is an increase in membrane hydrogen flux and selectivity at high temperature. We have demonstrated that the membranes are stable for over 6 weeks continuous operation at room temperature.