Metal-Organic Frameworks for the Separation of O2 from Air

Pure molecular oxygen is important to many industrial processes such as oxy-fuel combustion, steel production, on-board oxygen generation in military aircraft and medical oxygen concentrators. Industrial-scale oxygen separation is conventionally carried out by cryogenic air separation. Cryogenic air separation produces high quality O2 (99% oxygen purity), however, it is energy intensive, expensive and requires large housing facilities. A competing technology, pressure swing adsorption (PSA), can carry out O2 separation on a smaller scale, but its application is limited to processes that require lower purity O2. PSA processes rely on adsorbents such as zeolites, to capture N2 from air, however, zeolites are costly, inefficient and unable to produce high oxygen purity.

Seeking a more effective alternative to zeolites, Sandia researchers looked to Metal-Organic Frameworks (MOFs). MOFs are crystalline, porous materials in which a metal center is bound to organic molecules by mild self-assembly chemical synthesis. During synthesis, MOFs can be modified to be highly selective for a certain molecule. The MOFs designed by Sandia researchers are designed to be highly selective for O2 over N2, resulting in the simultaneous separation of nitrogen and other air components from oxygen. When incorporated into the PSA process, MOFs have the ability to produce a purer form of oxygen, rivaling the purity from cryogenic air separation. MOFs offer better sorbent purity, fewer structural defects, and can store greater amounts of adsorbates compared to zeolites. Additionally, MOFs provide the ability to perform O2 separation under mild conditions (ambient pressure and temperature), potentially revolutionizing the swing adsorption process and increasing the potential application fields.