How to break one of nature’s strongest bonds

2023-06-05 11:17:30

Using pulsed X-rays brings scientists a big step toward developing better catalysts to transform the greenhouse gas methane into a less harmful chemical.

The result, published in the magazine Science ⁽¹⁾reveals for the first time how the carbon-hydrogen bonds of alkanes are broken and how the catalyst acts in this reaction.

Methane, one of the most potent greenhouse gases, is being released into the atmosphere at an increasing rate due to livestock farming and thawing permafrost. Converting methane and longer-chain alkanes into less noxious and, in fact, useful chemicals would eliminate the associated threats and, in turn, make a huge raw material available to the chemical industry.

However, the transformation of methane requires as a first step the breaking of a CH bond, one of the strongest chemical bonds in nature.

Easily break CH links

Forty years ago metallic molecular catalysts were discovered that can easily break CH bonds. All that was found necessary was a brief flash of visible light to “switch on” the catalyst and, as if by magic, the strong CH bonds of nearby passing alkanes are easily broken with almost no energy. Despite the importance of this so-called CH activation reaction, how the catalyst performs this function was unknown for decades.

The research was led by scientists from Uppsala University (Sweden), in collaboration with the Paul Scherrer Institute in Switzerland, Stockholm University, the University of Hamburg and the European XFEL in Germany. For the first time, the scientists were able to directly observe the catalyst at work and reveal how it breaks those CH bonds.

In two experiments conducted at the Paul Scherrer Institute in Switzerland, the researchers were able to follow the delicate exchange of electrons between a rhodium catalyst and an octane CH group as it breaks down.

Using two of the most powerful X-ray sources in the world, the SwissFEL X-ray laser and the Swiss Light Source X-ray synchrotron, the reaction could be followed from start to finish. Measurements revealed initial light-induced catalyst activation within 400 femtoseconds (0.0000000000004 seconds) to final CH bond cleavage after 14 nanoseconds (0.000000014 seconds).

Learn to direct the flow of electrons

“The time-resolved X-ray absorption experiments we have performed are only possible at large-scale facilities such as SwissFEL and the Swiss Light Source, which provide extremely bright and short X-ray pulses. The catalyst is immersed in a dense solution of octane, but taking the perspective of the metal, we were able to specifically choose the CH bond out of hundreds of thousands that are broken,” explains Raphael Jay, a researcher at Uppsala University and lead experimentalist on the study.

To interpret the complex experimental data, theorists from Uppsala University and Stockholm University joined forces and performed advanced quantum chemical calculations.

“Our calculations allow us to clearly identify how the electronic charge flows between the metallic catalyst and the CH group in just the right proportion,” explains Ambar Banerjee, a postdoctoral researcher at Uppsala University and lead theorist of the study. “We can see how the charge The charge flowing from the metal towards the CH bond sticks the two chemical groups together. Instead, the charge flowing in the opposite direction acts like scissors that ends up separating the C atom and the H atom.”

The study solves a forty-year mystery about how an activated catalyst can break strong CH bonds by carefully exchanging fractions of electrons and without the need for enormous temperatures or pressures. With their new tool at hand, the researchers want to learn how to direct the flow of electrons to develop better catalysts for the chemical industry to make something useful out of methane and other alkanes.

References

• ⁽¹⁾ Tracking C–H activation with orbital resolution. Science.