This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Making complex molecules often means joining smaller chemical building blocks together, a process facilitated by catalysts, compounds that make such reactions possible or speed them up without being consumed themselves. Many widely used reactions also require chemists to modify their starting molecules before joining them.
These added chemical "handles" make the molecules reactive, but preparing them means extra steps and, often, additional waste. An alternative is C–H functionalization, which modifies ubiquitous carbon–hydrogen bonds already present in a molecule. Such modifications provide a more direct route to the desired structures, but are currently based on the use of large amounts of precious metals as catalysts, such as palladium, which can neither be recovered nor recycled.
Until now, this promising approach was not sustainable in the context of chemical production. At NCCR Catalysis, a team led by Jérôme Waser at EPFL and Javier Pérez-Ramírez at ETH Zurich has now developed a reusable single-atom palladium catalyst for C–H functionalization, leading to the construction of carbon–carbon bonds, which are central to many pharmaceuticals and other fine chemicals. Their research is published in the Journal of the American Chemical Society.
In this type of catalyst, palladium atoms are dispersed across a solid support and isolated from each other. That way, they keep high activity, but the solid catalyst can easily be recovered after the reaction by simple filtration. To develop this new process, the team had to overcome significant challenges in catalyst stability, activity and recyclability: They achieved a cumulative turnover number above 200, meaning that each molecule of catalyst could be used to generate more than 200 molecules of the product, one order of magnitude higher than the current best catalyst (20 turnovers).
The method still has limitations. The catalyst cannot simply be reused indefinitely, and repeated regeneration changes the support. But the work acts as a proof of concept that single-atom catalysts can carry out demanding C–H functionalization reactions while retaining the precious metal (in this case palladium), and that understanding how they deactivate can provide practical routes to regeneration.
"The work was very long and complex, as we first had to find conditions for the catalyst to be active but still stable, then understand why it was losing its activity, and find a way to reactivate it to recycle it," says Waser. "It was possible only by combining the expertise in organic reactions of the EPFL group with the expertise in heterogeneous catalyst design of the ETHZ group." "This is a fantastic example of integration within NCCR Catalysis. Without this community and students ready to embrace an interdisciplinary view of science, this collaboration would probably never have happened.
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