Introduction As early as 1950, membranes were con- sidered for a variety of gas separations such as the removal of oxygen from air, helium from natural gas, and hydrogen from petroleum refinery gas. 1 Still, it was nearly 25 years later before commercial membranes that could economically per- form these separations were developed, 2 with hydrogen recovery being one of the first widespread applications of gas separation membranes. 3 These hydrogen separations include adjustment of the H 2 /CO ratio in synthesis gas (a mixture of hydrogen and carbon monoxide that is often obtained from the steam reforming of natural gas or the gasification of coal), removal of hydrogen from various hydrocarbon streams, and the removal of hydrogen from purge gases in ammonia production and other petrochemical processes. 2–6 Membranes compete with pressure swing adsorption (PSA) and cryogenic systems in hydrogen recovery separa- tions. PSA uses special adsorbents, such as zeolites, to preferentially adsorb the unde- sired components at high pressures, thereby purifying the hydrogen gas. Cryo- genic systems utilize very low tempera- tures to selectively condense undesired components to purify hydrogen. Since the late 1980s, membranes have been compet- itive with these other technologies over a wide range of operating conditions. 2 This review will take a look at the work that has been done with polymeric membranes for hydrogen separations and then sug- gest some desirable directions for the future development of technology in this area. History of Polymeric Membranes for Hydrogen Separations Despite earlier considerations of the use of membranes for industrial gas separa- tions, it was the late 1970s before DuPont pioneered the use of small-diameter hollow-fiber membranes. 4 However, the productivity of the first-generation hollow fibers was too low to provide economical widespread gas separations. This obstacle was overcome when Monsanto Co. de- veloped multicomponent polysulfone hollow-fiber membranes for hydrogen recovery. 5 By limiting the dense, selective region of the fibers to a very thin section, transport through the fibers was greatly increased. These asymmetric membranes were successfully implemented in industrial-scale hydrogen recovery from ammonia purge gases. This technology was soon followed by the Separex ® spiral- wound cellulose acetate membranes de- veloped by Separex Corp. for similar separations as well as for natural gas pu- rification and dehydration. 6 The cellulose acetate membranes provided better per- formance because of their greater resis- tance to hydrocarbon impurities. By the mid-1980s, membranes were being imple- mented more frequently and in further applications, including hydrogen recov- ery from recycling refinery gas. 7,8 In Japan, Ube introduced a polyimide membrane with the best heat- and solvent-resistance properties of its time. Seibu Oil’s Onoba City refinery was the first commercial ap- plication of these membranes. 2 Table I summarizes the hydrogen transport prop- erties of the first-generation commercial polymer membranes for gas separation. Recently, hydrogen recovery has gained attention, because of the discussions of a hydrogen economy, as the leading candi- date to supply the increasing energy re- quirements of industrialized nations. 9–15 An attractive alternative to fossil fuel com- bustion for power generation is the proton-exchange membrane (PEM) fuel cell, which uses hydrogen as fuel to con- vert chemical energy to electrical energy. 10 In addition to higher fuel efficiency than MRS BULLETIN • VOLUME 31 • OCTOBER 2006 745 Polymer Membranes for Hydrogen Separations John D. Perry, Kazukiyo Nagai, and William J. Koros Abstract The development of a hydrogen-based economy would generate a substantial necessity for efficient means of collecting hydrogen with a relatively high purity. Membrane separations play a major role in the separation of hydrogen gas from various gas mixtures, and this article discusses the use of polymeric materials to produce these membranes. After a review of the historical use of polymeric membranes and some background information regarding mechanisms of gas transport in membranes, this article will review the work that has been done in the two major classes of hydrogen separation membranes: hydrogen-selective membranes and hydrogen-rejective membranes. In hydrogen-selective membranes, the very small size of the hydrogen molecule is exploited to allow rapid diffusion of hydrogen through the membrane while excluding other gases. Hydrogen-rejective membranes use the significantly higher sorption of other gases to overcome the advantages of the small size of the hydrogen molecule. The discussion of these two types of membranes will be followed by a presentation of the current state of the art with regard to polymeric membranes for hydrogen separation and a discussion of the predictions for future applications and advancements in this area. Keywords: hydrogen, membrane, polymer. Table I: Hydrogen Separation Factors of First-Generation Commercial Gas Separation Membranes. Membrane Material(s) Developer H 2 /N 2 H 2 /CO H 2 /CH 4 Ref. Polysulfone silicone rubber Monsanto 39 23 24 5 Polyimide Ube 35.4 30 … 88 Cellulose acetate Separex 33 21 26 2, 6 www.mrs.org/bulletin