The Reactivities of Polyaromatic Hydrocarbons in Catalytic Hydrogenation over Supported Noble Metals

Abstract The reactivities of several polyaromatic hydrocarbons (pyrene: P, fluoranthene: FL, anthracene: An, fluorene: F, acridine: Ac, and carbazole: C) in the catalytic hydrogenation were studied and compared over supported noble metals (Pt, Pd, Rh, and Ru on a carbon or alumina support). The specific reactivity and selectivity were found to be strongly dependent upon the combination of the catalyst species and the substrate. The catalysts exhibited the activity order of Rh > Pd >> Pt > Ru for tricyclic aromatics of An, F, Ac, and C, although the catalysts provided very variable selectivities of the partially hydrogenated products which were produced via both consecutive and competitive routes, apparently according to the type of ring-skeleton and the presence of a nitrogen atom. The reactivity of the starting substrates and intermediates are compared to the quantum chemical reactivity indices, calculated based on MNDO (modified neglect of diatomic overlap). The values of the LUMO electron density, the π-bond order, and the stability of the product appear to govern the reactivity and selectivity of the hydrogenation, depending upon the combination of the catalyst species and the substrate. No hydrogenation of a substrate proceeds when its reactivity indices are less than the threshold value, which is subject to the activity of the catalyst and the reaction conditions defining product selectivity via both competitive and consecutive schemes.