Tapping the potential of fragile metals
29 June 2006 (Volume 1 Issue 6)
Chemists around the world have been competing with each other to develop novel metal catalysts. But few are as daring as Zhaomin Hou at RIKEN, who enjoys tackling the extremely difficult elements called rare earth metals.
Working with those metals, Hou has been developing a number of unusual catalysts that could lead to new materials - from films to synthetic rubbers - with much better qualities than existing products.
Rare earth metals form a group of 17 chemical elements in the periodic table, including scandium, yttrium and the lanthanides (see Table). They acquired their name because they were considered scarce when they were first discovered in the late 18th century. Later it was found that rare earth metals exist in abundance, but they are highly brittle, and difficult to mine and extract from other metals. So many chemists have shied away from handling them.
Hou, however, bet on the potential of rare earths as early as the mid-1980s, when he was a graduate student at Kyushu University on the southern Japanese island of Kyushu. It was a time when, in the search for better catalysts than common metals, some chemists were just beginning to learn about rare earths. The research population is still small.
"The study of rare earth metals lags far behind other metals. I found it interesting because few people are researching them, and so many things have yet to be uncovered," says Hou, 44, now Chief Scientist at RIKEN's Discovery Research Institute in Saitama, near Tokyo.
Unexpected move to Japan
Hou's interest in rare earth metals was triggered by his supervisor at Kyushu University, who had just started researching them when Hou arrived from China at his laboratory in 1983. When Hou graduated from the University of Petroleum in eastern China in 1982, he passed a competitive examination to study abroad. At that time, Chinese students lacked the freedom to choose where to study in a foreign country, and the Chinese government decided to send him to Kyushu University, which was famous for applied chemistry.
The hardest part of starting his new life was learning Japanese, recalls Hou, who now speaks the language fluently. In 1989, after Hou earned his PhD, his supervisor introduced him to RIKEN, then the only research institute in Japan to accept postdocs. There were times when Hou thought about returning home, but given the political disorder at that time in China he decided to continue working abroad.
Hou found that RIKEN, with its fine research environment and friendly culture, was where he could maximize his potential. He returned there in 1993 after a two-year fellowship at the University of Windsor in Ontario, Canada, and in 2002 was appointed Chief Scientist at the Organometallic Chemistry Laboratory, which has a longstanding history as one of the most prominent laboratories at RIKEN.
Flash of insight
Over the past few years, Hou's achievements have been impressive. He focuses on catalysts that can promote copolymerization, in which two or more monomers are reacted to become a copolymer that often has better properties than each of original monomers. The major breakthrough came in 2004, when his team developed a catalyst based on scandium1.
In the past, researchers have paid little attention to scandium as a polymerization catalyst because of its difficult characteristics. But based on his experience, Hou thought scandium might have similar or even better properties to titanium, which is widely used as a catalyst for some kinds of polystyrene, a common source of sheet plastic. But titanium makes polystyrene brittle, and therefore difficult to process and this limits its applications. "Polystyrene has good properties, but it's a shame that we couldn't fully enjoy their benefit," Hou says.
Researchers worldwide had been trying in vain to introduce ethylene into stereo-regular polystyrene to make it more durable and elastic. Hou thought a scandium-based catalyst could manage to copolymerize styrene and ethylene in a stereospecific fashion.
By experimenting with different organic compounds, Hou was able to create a special organic support (ancillary ligand) for the scandium, that provided an excellent catalytic system for the preparation of styrene-ethylene copolymers having the desired structures and properties. Hou says the scandium catalyst is quite unique and versatile. It can, for example, co-polymerize cyclic organic compounds (olefins) with ethylene in specific ways to make new polymer materials that can be used for optical lenses, CD discs and films.
By changing the ancillary ligand, Hou has also succeeded in developing new rare earth metal catalysts which can help synthesize polyisoprene, a man-made form of rubber, with wellcontrolled structures and properties superior to those of natural rubber. Although rare earth metals are rather expensive, they have shown very good cost performance because of their high activity and the high value of the resultant products. A joint development with a Japanese chemical company is underway.
The challenge ahead
Hou's challenge has moved to far more complicated catalysts. He has been working hard to develop a new type of catalyst: multinuclear complexes containing both rare earth elements and non-rare-earth elements, based on rare-earth-metal hydride clusters developed recently by his group2. "We are aiming at developing a novel type of catalyst, in which the metal centers with different properties can act cooperatively on reactant molecules", Hou says.
The search for better catalysts is always tough - but Hou never backs off. When he hits a wall, he tries to refresh his mind by sipping a favourite alcoholic beverage and chatting with his fellow researchers. When experiments fail, he discusses the problem thoroughly with colleagues and nails down the issues. Then those efforts come to fruition.
"Rare earth elements have a unique and fantastic potential in many aspects," Hou says. "However, if you handle a rare earth metal in the same way as other metals, you will fail in most cases. But if you treat it meticulously and persistently, you will eventually get a know-how, and it will not be at all a difficult metal. I hope more researches will join in this wonderful world."
