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Aug. 11, 2006

Rare-earth elements show their mettle

A rare breed of metal clusters help make ethylene from carbon monoxide

A cornerstone of the chemical industry is the ability to convert one chemical compound into another. As well as making existing processes more efficient, researchers also work to develop completely new reactions. Efforts often focus on catalysts that can either speed up a known reaction or promote an otherwise unknown one. Organometallic compounds—substances containing both organic groups and metal atoms—often fill this role, but the potential of some metallic elements is only now being explored.

Zhaomin Hou from RIKEN’s Discovery Research Institute in Wako is an advocate for the little-used rare-earth metals—a group of elements that comprises scandium, yttrium and the lanthanides. “The aim of our research is to search for new catalysts for more selective and efficient chemical transformations,” says Hou, “or for new processes that cannot be achieved by previously known catalysts.”

The reaction of carbon monoxide with hydrogen to give a mixture of liquid hydrocarbons and their oxygen-containing derivatives—the Fischer-Tropsch reaction—has been known for over 80 years. This process is industrially important because it is used to make a synthetic petroleum substitute from either coal or natural gas. However, as Hou points out, “the catalyst system is rather complicated, the reaction gives a mixture of different products and little is known about the reaction mechanism”.

Now, Hou and coworkers have developed rare-earth metal complexes that can quickly and cleanly convert carbon monoxide into ethylene. This process is important because ethylene is a versatile precursor that is used in the manufacture of other chemicals such as polyethylene or polyvinyl chloride (PVC). “The formation of ethylene from this reaction shows, for the first time, that carbon monoxide can be hydrogenated selectively into ethylene under mild conditions,” comments Hou.

The metal clusters contain either four yttrium or lutetium atoms that are held together with bridging hydrogen atoms. Carefully controlled studies using isotopically labeled carbon monoxide allowed Hou to identify intermediates in the reaction pathway and deduce the order in which the chemical bonds were being broken and formed. This insight into the reaction mechanism, which is reported in the Journal of the American Chemical Society1, may shed light on how other similar reactions work.

This study demonstrates the unique reactivity of rare-earth metal clusters and highlights their potential as chemical catalysts. “This work will give us a better understanding of how carbon monoxide is converted into useful higher molecular weight chemicals,” says Hou, “and may eventually lead us to make a better catalyst system.”

References

  • 1. Shima, Y. & Hou, Z. Hydrogenation of carbon monoxide by tetranuclear rare earth metal polyhydrido complexes. Selective formation of ethylene and isolation of well-defined polyoxo rare earth metal clusters. Journal of the American Chemical Society 128, 8124–8125 (2006). doi: 10.1021/ja062348l

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