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A Cheap Way to Chop up Nitrogen
14 December 2009 (All day)
Nitrogen atoms are needed to make many important chemicals from drugs to fertilizers. But getting those atoms into chemicals is challenging, because nitrogen molecules are tough nuts to crack. They consist of two atoms sharing a stubborn triple bond, which chemists can break up only by scorching them with temperatures of up to 500°C. And that results in the simple chemical ammonia, which needs further processing to produce more complicated compounds. Now chemists have bypassed the energy-intensive reaction and devised a new one that splits molecular nitrogen at room temperature and synthesizes a common fertilizer.
Nitrogen gas (N2) makes up 78% of the air you breathe. Since the early 20th century, chemists have combined nitrogen and hydrogen with an iron catalyst at 500°C to produce ammonia, a process called the Haber-Bosch process. In the mid-1990s, inorganic chemists discovered a gentler way to force nitrogen's molecular break-up using compounds that contained molybdenum. Pairs of these metal compounds donate six electrons to nitrogen--two for each of nitrogen's three bonds. Others have since created similar compounds that follow the same strategy. In 2004, chemist Paul Chirik of Cornell University made his own version based on zirconium.
Chirik's molecules split nitrogen and added hydrogen to form ammonia, mimicking the Haber-Bosch process at a much more reasonable temperature of 85°C. But the procedure is not completely clean: To make hydrogen gas, chemists start with methane, a fossil fuel, and release carbon dioxide as a byproduct. So Chirik and colleagues wanted to find a reaction that skipped ammonia synthesis altogether and went right from nitrogen gas to a useful chemical.
The solution involves clamshell-like organic molecules that hold an atom of the metal hafnium in their opening. Two of these molecules can lock a nitrogen molecule between their metal atoms and break the first two of its bonds. That's as far as hafnium atoms, which have only two electrons each to donate, can go. To break the final bond, the chemists turned to another molecule with an even stronger triple bond, carbon monoxide. When they bubbled this gas into a flask with their hafnium molecules, two carbon monoxide molecules split the nitrogen and formed new bonds to create oxamide, a common fertilizer, the scientists reported online 13 December in Nature Chemistry. Oxamide is a simple chemical with six atoms: two carbon, two oxygen atoms that are both from the incoming carbon monoxide molecules, and two individual nitrogen atoms. The hafnium molecules "pinned [nitrogen] down, pulled its bond apart, and got it ready to do chemistry," Chirik says. "The carbon monoxide is the straw that breaks the camel's back."
Exactly how the reaction works is not yet clear, Chirik says. He and his colleagues want to understand this mechanism and improve on it, because the reaction is currently not catalytic: The hafnium compounds can do the reaction only once, because freeing oxamide from the molecules leaves them in a state that can't bind nitrogen again. To create molecules that can react over and over may require the chemists to move down the periodic table to find a new metal that doesn’t clutch onto oxamide as tightly, Chirik says.
Splitting molecular nitrogen with the help of carbon monoxide is "unexpected and completely unprecedented," says inorganic chemist Michael Fryzuk of the University of British Columbia, Vancouver, in Canada. This new combination of reactions is "super elegant," says chemist Christopher Cummins of the Massachusetts Institute of Technology in Cambridge, and the process could allow molecular nitrogen to be a reagent in other synthetic reactions, he adds.