Oxidative cracking of three to five-member ring polycyclic aromatic hydrocarbons in subcritical and supercritical water

Polycyclic aromatic hydrocarbons (PAH) are refractory structures common in heavy hydrocarbons. Thermal cracking in supercritical water (SCW) is limited but PAH can be completely oxidised if an oxidant is added. By restricting oxidant supply to substoichiometric amounts, this study aims to achieve partial oxidation as a route to useful chemicals, such as mono- and bi-aromatics. Oxidative cracking reactions of anthracene, pyrene and benzo[a]pyrene in subcritical and SCW were studied. PAH conversions well above 90 % were achieved along a fast heating ramp in a batch reactor. This quick initial oxidation took place predominantly in inner rings, weakening the aromatic structure and increasing cracking reactivity. This oxidation-cracking pathway became dominant in the SCW region, producing mostly oxygenated compounds with fewer aromatic rings. On the other hand, competing reactions leading to polymerization were favoured in the subcritical water region. PAH reactivity was found to follow the order anthracene > benzo[a]pyrene > pyrene.