For almost a century, industrial chemists have had to rely on hellishly high temperatures and gas pressures to cleave the tenacious chemical bond that holds together each two-atom nitrogen molecule. That done, chemists can use the nitrogen from the atmosphere to make fertilizers, explosives, and other modern products. Now, researchers have devised a way to split nitrogen molecules under milder conditions within liquids, a step that may inaugurate a variety of energy-efficient techniques for creating nitrogen-bearing substances.
The triple bond between a nitrogen molecule’s atoms is one of the strongest chemical attractions around. In nature, only the megavoltage of a lightning bolt and the potent enzymes in some soil bacteria and fungi can split nitrogen molecules, says chemist Paul J. Chirik of Cornell University.
It was early in the last century that the German chemists Fritz Haber and Carl Bosch invented and developed an economic method for splitting nitrogen molecules, a feat for which they each received a Nobel prize. Today, manufacturers use the Haber-Bosch process to generate more than 100 million tons of ammonia annually for the chemical and fertilizer industries.
Chemists have long sought substitutes for the Haber-Bosch process, which works most efficiently at temperatures between 400°C and 500°C and at pressures around 400 times that of the atmosphere at sea level. Chirik and his colleagues now propose one less-extreme alternative.
The key to the new reaction is a subtle modification to a zirconium-bearing substance that includes two copies of a bulky chemical group—called a methylated cyclopentadienyl—that’s based on a five-atom ring of carbon. With a methyl group, or CH3, attached to every carbon atom in these rings, the molecules are so large that they can attach to only one end of a nitrogen molecule.
Chirik and his colleagues found that when one methyl group was removed from each ring, the complex was just the right size to snuggle up to both atoms in a nitrogen molecule and give an electron to each atom. This double donation, which can take place in an organic solvent at 100°C and atmospheric pressure, disrupts the triple bond. Further reactions in the same solution then finish the job of splitting the nitrogen molecules and adding hydrogen atoms to make ammonia. Chirik’s team reports its feat in the Feb. 5 Nature.
The relative ease and possible energy efficiency of the new nitrogen-splitting technique may still not be enough to displace the Haber-Bosch process, says Michael D. Fryzuk, a chemist at the University of British Columbia in Vancouver. Many companies have large investments tied up in equipment to carry out that operation.
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Chirik agrees but notes that the new, liquid-based technique may enable researchers to develop energy-efficient methods for making nitrogen-containing ingredients that go into pharmaceuticals, dyes, and other industrial chemicals.