Alkanes, laser flashes and BESSY's X-ray vision

An international research team has succeeded in observing an intermediate step in the catalysis of alkanes. By understanding these reactions, existing catalysts can be optimized in the future and new ones found, for example to convert the greenhouse gas methane into valuable raw materials for industry.

Our relationship with methane is ambivalent. On the one hand, the lightest alkane is a highly potent greenhouse gas. It fuels the greenhouse effect a good 28 times more than carbon dioxide. On the other hand, it is also an interesting raw material. Not only for burning in a gas heating system. Once the C-H bond, i.e. the bond between carbon and hydrogen atoms, has been broken, the greenhouse gas can be used to produce a wide range of basic materials for industry.

Sounds like a good solution. However, it is precisely this bond in methane that has presented chemists with a major challenge for decades: it is one of the strongest bonds in nature. It has long been possible to break it using catalysts. However, it is still unclear how this works in detail. An international research team led by scientists from Uppsala University, together with colleagues from the Universities of Hamburg and Stockholm, the Max Born Institute and the Helmholtz-Zentrum Berlin, has now been able to observe the intermediate stages of this catalysis live.

Snapshots of the Sigma complex

"Over fifty years ago, researchers discovered that the C-H bonds break when special metal catalysts are added and then irradiated with visible light," explains Raphael Jay from Uppsala University, the lead experimentalist of the study. "What exactly happens during this process, i.e. how the alkane molecules approach the metal catalysts and remain attached to them, was a mystery until today."

The research teams wanted to solve this problem. As a catalyst, they chose chromium hexacarbonyl, a highly symmetrical, saturated and unreactive system with a chromium atom in the center. Although this does not lead to the complete splitting of the alkanes, it does lead to an important intermediate step in the reaction - the formation of the sigma complex.

"We split off a carbonyl group with a UV pulse, creating a kind of opening," explains Raphael Jay. "The metal becomes highly reactive, desperately wants to bind something and therefore captures a C-H group. This docks as a whole to the metal atom. We call this the sigma complex."

He and his team observed the individual steps from switching on the light to the finished complex in the laser laboratory of the Huse Group at the University of Hamburg. Using optical light pulses, they recorded the reaction in ultra-short snapshots. These showed that the catalyst was activated after less than 100 femtoseconds. This is so short that even light can only travel three hundredths of a millimeter in this time. It becomes very hot and its components vibrate around the chromium atom. Only when the vibrations stop can the alkane approach the catalyst and form the sigma complex. This happens in eight picoseconds - a time span in which the light travels around 2.4 millimeters.

X-ray beam and liquid leaf

"We used optical spectroscopy to trace the steps to the sigma complex," explains Raphael Jay. "We then wanted to characterize the resulting bonds with X-rays." To do this, they came to HZB in Berlin for another experiment. Because the beamline UE52-SGM with the end station AXSYS-NEXAFS at BESSY II is simply predestined for this, says the physicist, who already conducted research on it for his dissertation. "I don't know of any other synchrotron in the world with a setup like the one we have chosen," he says.

And indeed, the experiment is quite a challenge. Firstly, the researchers chose longer-chain alkanes because, unlike methane, they are liquid under normal ambient conditions. This makes them easier to analyze with the X-ray beam. However, soft X-rays are absorbed by the carbon in the alkanes. The solution consisted of two nozzles positioned at a fixed angle close to each other, which sprayed the alkanes with the catalyst into the experimental chamber. "When the two liquid jets collide in the vacuum chamber, they form a wafer-thin liquid leaf," explains Mattis Fondell from the HZB Institute Methods and Instrumentation for Research with Synchrotron Radiation, who developed the experimental setup. "This leaf is thin enough that it does not completely absorb the soft X-ray light. Based on the strength of the absorption for different wavelengths of X-ray light, we can look specifically at certain chemical bonds in the molecules." In this way, the researchers scanned the sigma complex in the liquid with high sensitivity and recorded how the bond between the metal and alkane forms.

"Next, we want to understand how the structure of the catalyst and the metal in its center influence the way it is switched on and how it interacts with alkanes," Raphael Jay gives an outlook. "This will make it possible to better control and adapt its behavior in C-H bond activation reactions."

Kai Dürfeld / Science and technology journalist