Garret Miyake (left) and Anna Wolff in the lab. [Image: Colorado State University College of Natural Sciences]
Researchers at Colorado State University, USA, have reportedly devised a new scheme for light-activated catalysis that offers a more efficient way to transform hydrocarbons into plastics and other everyday chemicals (Science, doi: 10.1126/science.adw1648). Capable of driving high-energy reactions at room temperature, the scientists say the photocatalysis system offers a promising route to reducing the energy needed for chemical manufacture while also generating less pollution.
More energy needed
Light-based catalysis has become a versatile tool for activating chemical reactions in organic compounds, which typically alter the oxidation state of the molecule through the transfer of single electrons. In photoredox reactions light absorbed by the catalyst generates an excited state that can transfer single electrons to the chemical substrate without the need for extreme reaction conditions.
Visible light is preferred to provide greater control over the reaction, but it does not yield single electrons with sufficient energy to drive some of the most industrially important processes. One example is the chemical reduction of a group of aromatic hydrocarbons, called arenes, that is needed for the production of chemicals for plastics and medicine. “Generating these reactions is difficult and energy intensive because the original bonds are so strong,” explains lead author Garret Miyake.
Recent work has suggested that such high-energy reactions could be achieved by coupling the energy of two photons into a single chemical reduction. However, the efficiency of these reactions is often compromised by a process called back electron transfer (BET), in which the free electron recombines with the catalyst before delivering its energy to the substrate.
Replicating photosynthesis
The scientists made the unexpected discovery that a coronene compound made up of seven benzene rings delivered a marked improvement in catalytic activity.
To combat this problem, the researchers took inspiration from photosynthesis, where BET is suppressed by reactions that require a proton to be transferred at the same time as an electron. In their efforts to synthesize a catalyst that could replicate this process, the scientists made the unexpected discovery that a coronene compound made up of seven benzene rings delivered a marked improvement in catalytic activity. This compound was found to reduce 72% of a benzene substrate within four hours, outperforming all the other candidate materials.
Detailed analysis of the coronene compound provides evidence that photoexcitation induces proton-transfer events within the catalyst. These proton-mediated processes generate a highly reactive state within the coronene compound, providing enough energy to activate benzene reduction, while the same mechanism induces a repulsive force between the catalyst and substrate that mitigates against BET.
The researchers also tested the efficacy of the coronene photocatalyst for activating a range of other reactions, in most cases achieving high productivity under mild reaction conditions. “We posit that these advancements will serve as guiding principles for designing efficient photocatalysts and expanding the scope of photoredox catalysis,” they conclude.
