Chemists Take a Vital Step Towards Greener Food Production

The Haber-Bosch ammonia synthesis process is possibly the most significant (if least familiar) industrial advancement of the 20^th century.

It defeated food scarcity by enabling mass production of fertilizers, which were utilized to strengthen food harvests throughout the world.

However, the production of ammonia — which is considered to be the building block for ammonium nitrate fertilizer — produces a tricky byproduct down the line: carbon dioxide. Over two tons of carbon is produced for every ton of fertilizer.

It accounts for approximately 1.4% of global carbon dioxide emissions. Although the process combated mass starvation, it also started increasing the planet’s load of greenhouse gases.

At present, one main aim for researchers is de-coupling food production from carbon. Partially, this implies determining a method to generate fertilizer via carbon-free ammonia synthesis. Can this be performed without Haber-Bosch?

Paul Chirik, who is the Edwards S. Sanford Professor in Chemistry, has taken a crucial step towards this chance with a special, basic method for the synthesis of chemical bonds. He and the scientists in his laboratory have made use of visible light to push the development of weak element-hydrogen bonds, which form the basis of the challenge since they are so hard to create.

The laboratory’s proof-of-concept study sets out an easy technique that includes shining blue light on an iridium catalyst to allow the development of weak bonds at or near thermodynamic potential — that is, with no huge outlays of energy or without a carbon byproduct. The study was reported recently in the Nature Chemistry journal.

The big breakthrough here is being able to take light and then promote a chemical reaction to make a bond that’s really weak, that you couldn’t do without an external stimulus. In the past, that stimulus has been coupled with making waste or consumption of electricity. Here, we’re doing it with light.

Paul Chirik, Edwards S. Sanford Professor in Chemistry, Princeton University

Chirik continued, “We have this world of metal catalysts that have done amazing things—they’ve made ammonia, they’ve made drugs, they’ve made polymers. Now, we can do even more with them when we start looking at what happens when these catalysts absorb light.”

“So, you’re taking something that did really cool chemistry before and you’re juicing it with another 50 kilocalories. A whole world opens up. Suddenly, there’s a new class of reactions we can think about doing,” added Chirik.

Shine a Light

E-H bonds are nothing but a way of indicating any bonds that could be made between hydrogen and another element. The strengths of E-H bonds are highly dependent on the chemical structure of each element; several of these bonds tend to be weak. This implies that they are unsteady and prone to break easily and form hydrogen (H₂).

A majority of the chemical reactions are pushed by the formation of strong bonds, as energy is discharged when more stable products are developed. It is the assembly of weak bonds that tends to be responsible for the difficulty.

The Chirik laboratory has discovered an approach to make a weak bond by shining light on a catalyst; it is iridium here.

This is how the approach works: The scientists selected a representative organic molecule, called anthracene, which serves as a kind of platform on which the chemistry occurs within the reaction flask.

When blue light is shone on iridium within the flask, it gets “excited,” that is, it exhibits energy to push the reaction. In this condition, it comes across the anthracene molecule and carries a hydrogen atom to create a weak bond. Then, the iridium catalyst triggers hydrogen gas, thereby completing the cycle.

The use of hydrogen gas rather than carbon-based hydrogen sources — extensively employed in organic synthesis earlier — offers a potentially lasting method of making weak chemical bonds without producing a carbon byproduct.

Yoonsu Park, a postdoctoral research associate in Chirik’s laboratory and study lead author, and Sangmin Kim, a 2021 PhD graduate of the laboratory, have collaboratively discovered the concept of using photochemistry by examining weak bonds that emerge in other reactions and extrapolating their lessons.

Greg Scholes, the William S. Tod Professor of Chemistry, and his graduate student Lei Tian — both additional authors on the study — offered an understanding of the role of blue light by making use of a range of laser experiments.

Also, Park found which metal catalyst in the wide periodic table would be highly effective in performing the preferred reaction. Park jumped off from earlier laboratory work performed with rhodium, which is another unique and costly metal catalyst, and quickly chose iridium.

Although researchers are not yet prepared to jettison Haber-Bosch, the Chirik lab’s proof-of-principle is a crucial early step.

We haven’t made ammonia yet catalytically. We have a long way to go on that goal. But it’s this idea of learning how to make these weak bonds that is so important. The thing I like about this research is, it’s different. It’s fundamental chemistry, as basic as you can get. Nobody’s opening a plant on this research tomorrow. But we’re really excited about the concept, and we really hope that other people do this.

Paul Chirik, Edwards S. Sanford Professor in Chemistry, Princeton University

This study was financially supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Catalysis Science Program (DE-SC0006498), and the Andlinger Center for Energy and the Environment at Princeton University.

Journal Reference:

Park, Y., et al. (2021) Visible light enables catalytic formation of weak chemical bonds with molecular hydrogen. Nature Chemistry. doi.org/10.1038/s41557-021-00732-z.

Source: https://www.princeton.edu/