Nature magazine recently published an article by Cornell chemists that demonstrates a new way to convert nitrogen, which makes up 70 percent of the air, to ammonia.
“Right now, you take N2 to ammonia and then ammonia goes to what we’ll call value-added … organic nitrogen molecules,” said Prof. Paul Chirik, chemistry and chemical biology. He explained that those organic molecules are then made into everything from medicines to rocket fuels to fertilizers. Until now, nitrogen has been converted to ammonia using some form of the venerable Haber-Bosch process.
“Fertilizer made from this ammonia is estimated to be responsible for sustaining roughly 40 percent of the world’s population and is the source for 40 to 60 percent of the nitrogen in the human body,” said Prof. Michael D. Fryzuk, chemistry, University of British Columbia, in a commentary letter that appeared in the same issue of Nature magazine. The process is also very energy intensive, he said.
“One percent of all energy produced in this world goes to that production,” said research associate Emil Lobkovsky, chemistry and chemical biology.
“It’s a hard bond to break and … the molecule is non-polar so that means it doesn’t really stick to stuff,” Chirik said, explaining why the Haber-Bosch reaction requires so much energy to break apart the two nitrogen atoms.
However, Chirik, along with Lobkovsky and former graduate student Jaime Pool Ph.D. ’04, has discovered a way to convert nitrogen using much less energy.
“Where [we] make ammonia is at 85 degrees centigrade, which is a lot lower than 400 degrees centigrade,” he said of the new process. Most industrial uses of the Haber-Bosch process require temperatures around 400 degrees centigrade, and up to 100 times the regular atmospheric pressure.
“We didn’t start out trying to make ammonia,” Chirik said.
“What we were trying to do was take transition metal compounds, and the one we focused on was zirconium, [in order to] to cut out the middle-man … so we could take N2 from the atmosphere and turn it into some organic molecules, so we don’t have to go through the ammonia,” he explained.
However, instead of skipping the ammonia step of the process, Chirik’s group ended up making ammonia.
“What happened was we started studying these zirconium compounds and we ended up making ammonia under mild conditions and people got really excited,” Chirik said.
However, both he and Pool made it clear that their process probably would never replace the Haber-Bosch process.
“The reaction that we report makes ammonia stoichiometrically, and what that means is for every metal, we got out one ammonia,” Chirik said. “What the Haber-Bosch reaction does is catalytic, so what that means is that you get many many ammonias per one metal.”
Pool confirmed that statement, but explained that their new N2 reaction has other uses.
“Our system is very unlikely to ever replace the currently used Haber-Bosch process,” she said. “However, it could offer insight into the formation of new N-H bonds under relatively mild conditions.”
She added that Nitrogen-hydrogen bonds are important because they form the basis for the ammonia molecule and are part of many other molecules.
Chirik elaborated on how their process could offer such insight.
“The reason why it’s important is because we can watch the chemistry happen,” he said.
Unlike the Haber-Bosch process, Chirik’s group can grow crystals from their reaction, allowing them to observe it. To make their observations, they used nuclear magnetic resonance spectroscopy, a technique similar to MRI scanning, and x-ray crystallography.
“The difference between the x-ray method and some other ones is that we don’t need any up-front information about the possible structure. So it’s completely unbiased information about molecular structure,” said Lobkovsky. With the information provided by the NMR and crystallography data, researchers can study the reaction to better understand nitrogen bonding.
“The real important thing here is … the clues as to how we would actually make N-H bonds, and that’s what we think the future of our research is,” Chirik said. “We’ve learned how to make N-H bonds and now we can learn to make other kinds of bonds too.”
However, mastering the new conversion reaction in the first place wasn’t easy.
“Honestly, the work was pretty challenging,” Pool said. “I had never worked with air sensitive materials before graduate school and we quickly realized that I had to go to greater extents to ensure that everything was dry enough for this chemistry.”
“This is hard chemistry,” Chirik added, “and you need someone who is very talented. [Jaime] is one of those people, and some lesser people probably couldn’t have done it. … It tells you how great the graduate students are that we get here.”
“After the experience in Paul’s laboratory, I would highly recommend it to other students considering graduate school,” Pool advised. “I found myself hating to go home at night because I always wanted to know the answers to the questions that we were asking.”
Archived article by Sarah Colby