Oxidative cracking of phenanthrene as polycyclic aromatic hydrocarbon model in supercritical water: Reaction pathways at low oxidant concentration

Polyaromatic hydrocarbons (PAH) are present in several industrially relevant streams, including light cycle oil, coal- and bio-derived oils, high-temperature gasification tars, and asphaltenic oils, posing processing challenges due to coke formation and low conversion to valuable products with conventional technologies. This study focuses on oxidative cracking of model compound phenanthrene in supercritical water (SCW) at low oxidant concentration as a route to produce chemicals of industrial interest from PAHs. While some studies have dealt with PAH SCW oxidation, these were carried out in large oxygen excess, aiming to eliminate PAHs through complete oxidation. This work shows that phenanthrene underwent fast oxidation promoted by reactive oxygen species (ROS) from H₂O₂ decomposition, but its conversion leveled off once ROS were consumed. However, the oxygenated species formed continued reacting in SCW over longer timescales. A reaction pathway is proposed based on the evolution of the main intermediate compounds with time and temperature. Anthraquinone was the main product at early reaction stages, with 0 min selectivity above 65% at all temperatures. It further reacted to form xanthone and fluorenone as main intermediates, reaching selectivity of up to 35% and 32% respectively. At later reaction stages, higher selectivity of up to 31% and 34% towards dibenzofuran or fluorene, respectively, indicates in-situ deoxygenation of intermediate products. This pathway showed differences with those measured under large oxygen excess, as oxidation starts in central positions and further reactions lead to a range of products with progressively less oxygen as well as a hydrogen-rich gas, while coke yields remain low.