Effect of Ring-Configuration on Pyrolysis and V2O5Catalyzed Oxidation of Polycyclic Aromatic Hydrocarbons

Abstract

Conversion of polycyclic aromatic hydrocarbons (PAHs) through oxidative ring-opening can be used as a potential strategy for upgrading of aromatic-rich fractions of heavy oil and bitumen. At the core of such a process is the oxidation of PAHs to produce quinonoids, further oxidation of quinonoids to form ring-opened carboxylic acids, and finally decarboxylation of the carboxylic acids. The fused ring systems in PAHs include linear and angular ring-configurations, which affect pyrolysis and oxidation selectivity. This study evaluated the effect of the ring-configuration of PAHs on pyrolysis and on oxidation over a metal oxide catalyst (V₂O₅). Pyrolysis takes place in parallel with catalytic oxidation, and so it has an effect on the observed conversion chemistry. Liquid-phase pyrolysis and V₂O₅-catalyzed conversion of anthracene, phenanthrene, anthraquinone, and phenanthrenequinone were studied individually. All experimental work was conducted under a nitrogen atmosphere to avoid autoxidation. Results from high-pressure differential scanning calorimetry (HP-DSC) as well as from batch oxidation reactions indicated differences in reactivities due to the ring-configurations of both PAHs and their corresponding quinonoids. The reactivity sequence over V₂O₅ for PAHs was anthracene > phenanthrene as the increase in π-sextets in anthracene provided additional driving force toward oxidation. On the contrary, the reactivity over V₂O₅ of the corresponding quinonoids was phenanthrenequinone > anthraquinone. The angular configuration of phenanthrenequinone led to the carbonyl groups to be adjacent and to the carbons participating in that C–C bond to have an aliphatic nature, which increased the reactivity of the bond. The angular ring-configuration of phenanthrenequinone favored intramolecular ring-closing reactions, while the linear ring-configuration of anthraquinone exhibited higher stability toward oxidation. Conversion of anthraquinone led to products of oxygen removal via anthrone as well as to some ring-opened products. Compared to pyrolysis, V₂O₅ accelerated the conversion of PAHs and their corresponding quinonoids. Regardless of the ring-configuration, catalytic oxidation of PAHs over V₂O₅ did not favor production of ring-opened products. The study highlighted the importance of a hydrogen source, possibly water, for a successful ring-opening.