Broader Use Of Atmospheric Nitrogen Looms

Attempts to combine atmospheric nitrogen and benzene to create ammonia usually fail because benzene degrades before a chemical reaction can take place. Now, researchers at Yale University, New Haven, Conn., say they have found a way to combine the materials to make aniline, which is a precursor to materials used to make an assortment of synthetic products.

Researchers have long focused on “nitrogen fixation,” a process to create ammonia from atmospheric nitrogen. Yale chemistry professor Patrick Holland and his colleagues believe this nitrogen could play a wider role — if researchers can find ways to make other compounds with atmospheric nitrogen.

“In the long run, we hope to learn how to use the abundant nitrogen in the air as a resource for synthesizing the products needed by society,” notes Holland.

Holland and his colleagues used an iron compound to break down one of the chemical bonds in benzene, and treated the nitrogen with a silicon compound that allowed it to combine with benzene. A recent article in Nature contains more details.

The researchers believe the ability of these iron complexes to generate a hydrocarbyl group on the iron through C–H activation, and then transfer it to an activated N₂, provides a new strategy for coupling hydrocarbons to N atoms from atmospheric N_{2_.}

Overall Reaction

Figure 1. Researchers devised a new method for potential future catalytic systems that couple nitrogen atoms with abundant hydrocarbons. Source: Patrick Holland, Yale University.

“There are many known reactions that break C-H bonds for cross-coupling, and many known reactions for reducing N₂, even though both are difficult and give a limited number of products. By combining these two powerful reactions, we have a way to put nitrogen atoms from the atmosphere into organic compounds, which is new. And, this was only possible because of the new C-N bond forming step,” Holland elaborates.

“Fundamentally, we’re showing a new way of thinking about how to encourage nitrogen to form new bonds that may be adaptable to making other products,” he adds.

The team next would like to improve the energy efficiency of the process and find an electrochemical reduction route rather than use sodium metal. “We would also like to avoid the ‘temperature cycling’ method described in the paper to enable continuous formation of product. Accomplishing these goals will require more detailed understanding of the key steps of C-H bond cleavage, N₂ binding, and H atom loss within the mechanism. We have a three-year grant from the Department of Energy that aims to resolve these issues,” says Holland.

Scaling-up the process for industrial use does pose challenges. “First, the turnover number is very small. Second, it uses sodium metal as a reductant. Third, it requires cycling the temperature between room temperature and -100°C for the current implementation. Obviously, we hope to improve in the future, as noted above,” Holland explains.

“This is a fundamentally new reaction, which will require more research before development into an efficient industrial process is possible. However, it is an exciting idea for the future, to convert arenes into anilines without dependence on nitric acid or high temperatures,” he concludes.