Oxidative ring-opening has been suggested as an alternative strategy to increase the hydrogen-to-carbon ratio of materials rich in multinuclear aromatics. The reaction sequence involves the addition of oxygen, with subsequent elimination as carbon dioxide and not as water. In this way, oxygen selectively removes carbon and not hydrogen. The first two steps in the selective oxidative ring-opening of multinuclear aromatics, are both oxidation steps and the focus of this investigation. Although free radical oxidation can be used to produce quinonoids and to perform further oxidation leading to ring-opening and formation of carboxylic acids, free radical oxidation is not selective. It was shown the use of a catalyst is necessary to promote selective oxidation accompanied by cleavage of the aromatic rings, as well as to limit free radical side-reactions. Of relevance to this study is that the reactivity towards oxidation for linear and angular multinuclear aromatic ring-configurations is different. This study investigated the effect of ring-configuration on oxidation. Catalytic oxidation of anthracene, anthraquinone, phenanthrene, and phenanthrenequinone over a V2O5 metal oxide catalyst was investigated. Results from high-pressure differential scanning calorimetry (HP-DSC) analysis indicated differences in reactivity due to the initial interaction with oxygen. Oxidation of anthracene took place at a lower temperature when compared with phenanthrene; whereas, the opposite behavior was observed for the corresponding quinonoid species (anthraquinone and phenanthrenequinone). A detailed analysis of the reaction products from the oxidation of these quinonoids showed that production of ring-opened compounds was obscured by the contribution of ring-closing and free-radical addition reactions. Ring-closed products dominated the product distribution. The study indicated that the selection of the catalyst for oxidation cannot be made independently, but must also consider the catalysis of the subsequent steps leading to the selective elimination of carbon dioxide. The study also indicated that multinuclear aromatic ring-configuration affected oxidation kinetics to quinonoids and further oxidation of quinonoids to carboxylic acids differently.

Oxidative ring-opening of multinuclear aromatics: Effect of ring-configuration on oxidation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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