New method for converting nitrogen to ammonia

A research team at Cornell University has succeeded in converting nitrogen into ammonia using a long-predicted process that has challenged scientists for decades.

The achievement involves using a zirconium metal complex to add hydrogen atoms to the nitrogen molecule and convert it to ammonia, without the need for high temperatures or high pressure.

“The value of our work is that we have answered the very basic chemical question of how to take this very inert and unreactive [nitrogen] molecule and get it to a useful form,” says Paul Chirik, Cornell assistant professor of chemistry and chemical biology.

Chirik and his two colleagues reported on the advance in a recent issue of the journal Nature (Vol. 427, Feb. 5, 2004). The research team included Chirik’s former graduate student Jaime Pool and research assistant Emil Lobkovsky.

In an accompanying “News and Views” in Nature , Michael Fryzuk of the University of British Columbia notes that “a remarkable chemical transformation has been discovered that is likely to have important implications for the production of ammonia.” However, Chirik emphasizes that his research group has succeeded only in producing ammonia in a laboratory setting, molecule by molecule, and is not making claims for an industrial process.

Nitrogen makes up 78 percent of the Earth’s atmosphere and, thanks to a 90-year-old industrial process, it can be converted to ammonia-based fertilizer that sustains about 40 percent of the world’s population, according to Fryzuk.

The problem with converting nitrogen into a usable, industrial form is that, although the element is a simple molecule, it is held together by an incredibly strong bond between two atoms. Indeed only carbon monoxide has a stronger bond. But while carbon monoxide easily adheres to other molecules, nitrogen is non-polar and does not attach easily to metals. It also is hard to put electrons into nitrogen molecules, and hard to take them out. The industrial method for converting nitrogen to ammonia, the Haber-Bosch process (after Fritz Haber and Carl Bosch, both Nobel laureates), produces more than 100 million tons

of ammonia annually for the chemical industry and agriculture. The process requires high temperatures and pressure in order for nitrogen and hydrogen to interact over an iron surface, which serves as a catalyst.

The Chirik team, however, was able to break the nitrogen molecule’s atomic bond, using zirconium in a soluble form, at just 45 degrees Celsius (113 degrees Fahrenheit) and add hydrogen atoms to this so-called “dinitrogen bridge.” Complete fixation to ammonia was achieved at 85 degrees Celsius (185 degrees Fahrenheit).

However, Chirik emphasizes that “the chance that anyone will ever replace the Haber-Bosch process is very small.” His group’s discovery could, he believes, be useful in making “value-added nitrogen chemicals, such as hydrazines for rocket fuels or fine chemicals for drug synthesis or dyes.

Fryzuk notes that it has taken so long to achieve the Chirik group’s transformation of nitrogen because, he says, molecular nitrogen “is so chemically inert that even binding it to metal complexes in solution … was a decades-long challenge for inorganic chemists.”

Unlike the Haber-Bosch process, the Chirik group’s transformation of nitrogen does not use a catalyst. Instead the zirconium makes only one ammonia molecule at a time, not vast numbers as in an industrial process, and, as Fryzuk notes, “there is no known homogenous catalyst that can effect this simple process” at low temperatures and pressure. (Instead of acting as a catalyst, the zirconium forms a new complex in which hydrogen atoms are added to the dinitrogen bridge, ultimately forming ammonia.)

Chirik says his group is currently searching for such a catalyst, which would be patentable. “Maybe we can come up with catalytic cycles that don’t make ammonia but make other nitrogen compounds. Arguably that would be more important than making ammonia,” he says.

The title of the Nature article is “Hydrogenation and cleavage of dinitrogen to ammonia with a zirconium complex.” The research was funded by the National Science Foundation.