Japanese scientists have unleashed the power of water to improve the selectivity and green credentials of the synthetic route to industrially-important phenols.

Hisao Yoshida and colleagues from Nagoya University have developed a photocatalyst that activates water to oxidise functionalised aromatic rings in a selective manner.

Phenols are currently made using a multistep process which consumes a significant amount of energy. Selectivity is another well-known disadvantage of this route, as the side chains often oxidise in preference to the aromatic ring. Much research has been done into developing one-step, environmentally-friendly and more efficient routes which involve the direct oxidation of aromatic rings. However to date selectivity has remained an issue. 

Now, Yoshida has overcome this using a platinum-loaded titanium oxide catalyst that is activated by illumination with the appropriate wavelength of light. The activated catalyst then converts water into an oxygen species that selectively reacts with an aromatic ring to make the required phenol, with hydrogen as the waste product.

Added bonuses of this route include mild reaction conditions - room temperature and atmospheric pressure - and removal of the need for expensive or hazardous oxidants. Additionally Yoshida found that the intensity and wavelength of the light can be varied to finely control the reaction products.

Yoshida recognises that the rate of the reaction needs to be improved before this can be a useful industrial process. However, he says that 'our findings provide an important principle which may be widely valuable for many kinds of undiscovered chemical syntheses, especially selective oxidation.'

Stephen Poulston, an expert in photocatalysis at Johnson Matthey, Reading, UK, agrees: 'It is encouraging to see potential applications for photocatalysis beyond the more well-established research.' 'I think the challenge with this reaction, which is common to most photocatalytic processes, is how to scale the reaction up and how to increase the reaction rate,' he adds.

May Copsey