A Gentler Route to Ammonia?

Ammonia, the eye-stinging, nostril-searing household cleaner, is also used in metal working and drug manufacture, and it is the essential starting point for most fertilizers. To manufacture it, tightly bound pairs of nitrogen atoms must be split and then recombined with hydrogen. That's an energy-intensive process that's been done the same way for 90 years. Now, a new technique shows that it's possible to turn the notoriously unreactive nitrogen molecules into ammonia with less effort.

Breaking up is hard to do for nitrogen molecules. The industrial Haber-Bosch process used to manufacture ammonia takes 1000 atmospheres of pressure, temperatures of 500°C, and an iron catalyst to strong-arm the nitrogen molecules into splitting and recombining with hydrogen.

Several times during the past century, chemists dreamed up new reactions, only to see them fail in the lab. Now, in the 5 February issue of Nature, a group reports the synthesis of ammonia in a pentane solution at just one atmosphere and an almost comfortable 45°C. Paul Chirik and colleagues at Cornell University in Ithaca, New York, inserted nitrogen molecules between two zirconium metal complexes. This had been done before by other researchers, but in this case, the zirconium complexes were slightly smaller than usual, allowing them to snuggle a bit closer to each other and to the nitrogen.

This subtle change made all the difference. The zirconium sapped the loyalty of the electrons bonding the nitrogen pairs, weakening the link between the two atoms and letting hydrogen weasel in and latch onto the nitrogen. By raising the temperature to 85°C and adding more hydrogen, the researchers got some of the paired nitrogen atoms to add two more hydrogens; then, each nitrogen atom left its partner completely to form ammonia (NH3).

The new reaction reveals intimate details of the reaction between hydrogen and nitrogen that could eventually lead to low-temperature ways to make ammonia, says Christopher Cummins of the Massachusetts Institute of Technology in Cambridge. Does this bode a kinder, gentler future for ammonia production? Probably not, says Michael Fryzuk of the University of British Columbia in Vancouver. He cautions that the reaction will probably never compete commercially with the Haber-Bosch approach because companies are already so heavily invested in the older process.

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