Directly using air to synthesize valuable C–N–O chemicals

N₂ and O₂ are the main components of air. They are abundant, low-cost, and attractive raw materials for making nitroaromatic C-N-O chemicals, which are important precursors for pharmaceuticals and agrochemicals. However, the traditional industrial route is long and energy-intensive. It first converts N₂ to ammonia through the Haber-Bosch process at 400 to 500 °C and 150 to 200 bar. The ammonia is then oxidized to nitric acid by O₂ through the Ostwald process at 600 to 900 °C. Finally, nitroaromatic chemicals are produced by nitration within a mixture of concentrated nitric acid and sulfuric acid. This route consumes large amounts of energy and generates hazardous waste.

A one-step process that directly uses air as the nitrogen and oxygen source under mild conditions is highly desirable. The main difficulty is that N₂ is too inert. N₂ activating normally requires harsh conditions. However, under such conditions, O₂ and the aromatic carbon source are far more reactive than N₂ and would lead to overoxidation and poor selectivity toward the desired products. The key challenge is therefore to control the activation of reactants and achieve selective C-N and N-O coupling without unwanted overoxidation.

To address this challenge, the team headed by Prof. Dehui Deng in Dalian Institute of Chemical Physics, Chinese Academy of Sciences developed a plasma-ultrasound catalytic (PUC) process, and enables direct conversion of air and phenol into valuable C-N-O products under mild conditions. This process couples non-thermal plasma with ultrasound and a CeO₂ catalyst in a phenol solution, and achieves an 8.1% overall yield and 92.5% selectivity for nitroaromatic chemicals, mainly nitrophenol and nitrosophenol at 5 °C and 1 atm.

Mechanistic studies based on in-situ characterizations developed by their team and theoretical calculations explain how the process works. Air plasma generates reactive oxygen species and NO_x species. These species then enter the phenol solution and trigger the subsequent reactions. The plasma-derived oxygen-containing radicals oxidize phenol to phenoxy radicals. The NO_x species then nitrate these radicals to form nitroaromatic products. In the liquid phase, ultrasound and the CeO₂ catalyst work together and open a heterogeneous catalytic pathway on the catalyst surface. This pathway enables additional formation of NO_x from N₂ and N₂O and promotes the following nitration reactions.

A key design feature is that the process separates the activation of air from the activation of phenol. Plasma and ultrasound act in different regions in one reactor. This separation prevents phenol from being overactivated and suppresses overoxidation of the products. As a result, the process can achieve high selectivity when directly using air as the raw material.

“Our study establishes a new PUC process for efficient, selective synthesis of valuable C–N–O chemicals directly from air and carbon substrates under mild conditions.” said Prof. Deng. “This work demonstrates a promising paradigm for the selective synthesis of C–N–O chemicals.”