Carbon-Based Membranes
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Carbon-Based Membranes Tanja Pietraß Abstract Inorganic carbon-based membranes for gas separation comprise materials that are fabricated through pyrolysis of a precursor material (often a synthetic polymer), and the more recently discovered carbon nanotubes. Fabrication, assembly into different architectures, and mechanism of operation are summarized for precursor-based carbon membranes, with a focus on selective surface flow and molecular sieving. Only preliminary work on carbon nanotube-based membranes for gas separation has been published. Their unusual transport properties, however, promise their use in gas separation in the future. In light of this application, structural properties and results relating to flow through these tubular structures are summarized. Keywords: adsorption, carbon, carbonization, hydrogen, membrane, nanofiber, nanotube.
Introduction Rising energy costs will cause membrane technology to play an increasingly important role in reducing the environmental impact and cost of industrial processes, predicts a recent cover story in Chemical and Engineering News.1 Conventional techniques such as fractionated distillation, condensation, and absorption for the purification and separation of gases from mixtures rely on energy-intensive phase changes. In membrane technology, however, all components remain in the gas phase. In addition, industrial plants that incorporate membranes for separation have a smaller footprint and reduced overall costs compared with plants that use traditional distillation separation methods.1 This article focuses on the use of carbon-based materials as the building blocks for membranes for hydrogen separation and purification and merely serves as a summary of the field. More details can be found in References 2 and 3 and references therein. While other types of carbon materials have been studied for many years in this application, carbon nanotubes were only discovered in 1991 by Iijima and co-workers.4 The first report of their potential use as hydrogen storage materials appeared in 1997,5 prompting a dramatic increase in research activity related to the interaction of gases with these unique materials. Their smooth interior surfaces promise higher flow rates through
MRS BULLETIN • VOLUME 31 • OCTOBER 2006
the tubes than through polymeric membranes. Compared with the use of carbon nanotubes as gas storage media and sensors, carbon nanotube membranes have received much less attention.
Inorganic Carbon-Based Membranes Overview The development of porous inorganic membranes—in contrast to polymerbased membranes—began during World War II with the goal of separating uranium isotopes for military applications.2 The interest in inorganic membranes stems from their resistance to high temperature and wear, their chemical inertness, and their well-defined, stable pore structures. Mechanical instability (embrittlement), insufficient permeability or selectivity based on permeability (known as permselectivity), and their high cost, however, still hamper their application in the proce
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