Hydrogen peroxide is a familiar disinfectant found in most people’s medicine cabinet. But H2O2’s bigger role is in industry, where it is used to bleach paper and as an oxidant to make commodity chemicals such as propylene oxide. However, there isn’t an ideal way to make bulk quantities of this key reagent.
Researchers at Osaka University, in Japan, have now designed an approach for producing H2O2 that could become the ideal process chemists have been seeking (ACS Catal. 2014, DOI: 10.1021/cs401208c).
Currently, the industry-leading anthraquinone process is an indirect, energy-intensive approach that requires sequential oxidation, distillation, and hydrogenation steps and has a high operating cost. Alternative metal-nanoparticle-catalyzed processes produce H2O2 directly and more selectively. But they require the combination of H2 and O2 gases, which can be hazardous in a large-scale process. Methods that use metal oxide photocatalysts to convert alcohols to H2O2 avoid H2, but they have low selectivity for H2O2.
The Osaka University team, led by Yasuhiro Shiraishi, found that a graphitic carbon nitride photocatalyst produces H2O2 with 90% selectivity. The catalyst, made from inexpensive cyanamide, is a polymeric material consisting of layered sheets of triple-triazine units. When activated by visible light, it strips hydrogen from ethanol and then traps and protonates O2 to form H2O2 at room temperature.
Shiraishi says that with further catalyst optimization the reaction won’t need ethanol but can rely on water as the hydrogen source, with the option of operating in sunlight without artificial lighting. “If achieved, that would truly be a green process for economical and safe H2O2 synthesis,” Shiraishi says. “And it could lead to a new strategy of using in situ generated H2O2 for sunlight-driven oxidations in organic synthesis.”
The selectivity of the process is clearly a big improvement for using photocatalysts to generate H2O2, says Graham J. Hutchings of Cardiff University, in Wales, whose group has developed highly selective metal nanoparticle catalysts for direct H2O2 production. The amount of H2O2 produced so far is still low, Hutchings notes, and the process is slower than catalytic approaches using H2. “It is a step forward and an elegant approach,” Hutchings adds. “But there is still much to do in terms of efficiencies.”
“Cheap hydrogen peroxide is one of the most demanded chemicals on earth, say for water disinfection or for simple cleaning and bleaching processes,” says Markus Antonietti of the Max Planck Institute of Colloids & Interfaces. “If you can do it with 90% selectivity from sunlight, water, and oxygen, this could be a real game-changer for decentralized, device-based chemicals generation.”