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A New Degree Of Separation For Rare-Earth Metals

Recycling: Chemists design and prepare a nitroxide ligand that selectively binds neodymium over dysprosium to recover the metals

by Stephen K. Ritter
June 8, 2015 | A version of this story appeared in Volume 93, Issue 23

Rare-earth metals are important for making many electronic devices and permanent magnets, yet commodity amounts of the elements can be in short supply. This situation has prompted metal suppliers to ramp up recycling of consumer products as an alternative to mining. To that end, chemists have developed several approaches for separating mixtures of rare-earth metals so they can be reblended in the necessary proportions for different applications. In the latest example, Eric J. Schelter and coworkers at the University of Pennsylvania report the design and synthesis of a nitroxide-based ligand that selectively binds neodymium over dysprosium in mixtures of the two elements derived from permanent magnets (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201501659). The nitroxide ligand forms a size-sensitive aperture when it binds the metals. In solutions of neodymium and dysprosium triflate salts, the ligand prefers the larger neodymium cation, forming a soluble complex as the less soluble complex formed from the smaller dysprosium cation precipitates. The team developed a complete recycling process to recover the two metals and reuse the ligand. According to the researchers, the new approach offers a simpler, less expensive alternative to redox chemistry, acid leaching processes, and ionic liquid extraction currently being used and explored for recycling rare earths.

Chemical structure of a neodymium nitroxide complex
This tripodal nitroxide ligand (left) wraps up neodymium (right) to separate it from dysprosium.


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