Nitrogen (N₂) and oxygen (O₂), the main constituents of air, are attractive feedstocks for high-value nitroaromatic compounds used in pharmaceuticals and agrochemicals. Yet their industrial synthesis currently relies on multi-step, energy-intensive routes under harsh conditions.

A direct, one-step conversion of air into nitroaromatic C–N–O compounds under mild conditions would thus be a more sustainable alternative. The difficulty, however, lies in controlling the activation of the inert reactants: forcing N₂ to react tends to over-activate O₂ and oxidize the aromatic substrate, compromising product selectivity. The central challenge is therefore to achieve controllable activation of the reactants and selective C–N and N–O coupling without over-oxidation.

To address this challenge, a research team led by Profs. DENG Dehui and YU Liang from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), has developed a plasma–ultrasound catalytic (PUC) process. The findings were recently published in National Science Review.

Using CeO₂ as a catalyst in a home-built plasma–ultrasound synergistic reactor, the process co-converts air and phenol into valuable nitroaromatic compounds under mild conditions. At 5 °C and atmospheric pressure, it attained an overall yield of 8.1% and 92.5% selectivity for nitroaromatic products (nitrophenol and nitrosophenol) within one hour.

The researchers further clarified the mechanism. Air plasma produces reactive oxygen species and NO_x, which oxidize phenol to phenoxy radicals (C₆H₅O); these are then converted into nitroaromatic products. At the same time, ultrasound and the CeO₂ catalyst jointly drive a heterogeneous pathway that promotes the formation of NO and NO₂ from N₂/N₂O and their subsequent nitration—effects enabled by the cavitation-enhanced mass transfer and reaction kinetics. By separating the activation of air from that of phenol, the process avoids both excessive activation and over-oxidation.

"Our study establishes a new PUC process for the efficient and selective synthesis of valuable C–N–O chemicals directly from air and carbon substrates under mild conditions," said Prof. DENG.