Composition and operation of hydrogen-selective amorphous alloy membranes

Catalytic membrane reactors, composed of a suitable hydrogen-selective membrane and water-gas-shift catalyst, can be used to produce a pure hydrogen stream from coal-derived syngas with high yield. The membrane must combine chemical and physical stability in the syngas environment at elevated temperatures, with rapid hydrogen permeation and low cost. Amorphous alloy membranes based on a combination of early transition metals and late transition metals are an alternative to current palladium alloy membranes, offering hydrogen permeance and selectivity that is comparable to palladium alloys, plus the potential advantages of high resistance to crystalline hydride formation and lower cost. Amorphous alloy membranes have been reported that are capable of sustained operation at 400 °C, which makes them suitable for use in a CMR based on a metal-oxide catalyst. The development of membranes capable of sustained operation at 700 °C, and thus suitable for use with metallic catalysts, remains a significant technological hurdle, due to the tendency of amorphous alloys to crystallize after prolonged use at elevated temperatures, even below the measurable crystallization or glass transition temperatures. Here we provide a critical review of amorphous alloys, their manufacture and properties, and compare them with other types of membrane reactors. Economic and operational considerations are discussed in detail, which allows informed selection of amorphous alloy compositions for high-temperature separation of coal-derived syngas.