Ruthenium hydroxide supported on activated alumina for catalytic permanganate oxidation of aniline

Abstract: Ruthenium loaded on CeO₂ or TiO₂ has been proved to be effective catalysts in permanganate oxidation, but the small size of supports led to a difficult separation of catalysts from the aqueous solution. Therefore, ruthenium hydroxide supported on activated alumina (diameter 3–4 mm), i.e. Ru-AA, was synthesized and employed as catalyst in permanganate oxidation of aniline at circumneutral pH for the first time. The characterization for Ru-AA by SEM-EDAX proved the existence of Ru on the surface of AA, but XRD patterns for AA were not affected by the impregnation of ruthenium hydroxide, due to its low concentration, low crystallinity, and good dispersion. Acting as an electron shuttle, Ru-AA significantly improved the apparent second-order rate constant (k_app) of aniline oxidation from 10.3 to 18.9 M^{− 1} s^{− 1} with its concentration increasing from 0.55 to 2.2 g L^{− 1}. Aniline degradation in catalytic oxidation was markedly influenced by pH, and the k_app decreased from 314.3 to 3.3 M^{− 1} s^{− 1} with increasing pH from 4.0 to 9.0. The intermediates of aniline in the catalytic permanganate oxidation were determined by LC–MS/MS analysis. Ru-AA displayed an excellent stability in first 10 consecutive runs, and the regenerated Ru-AA showed a better performance than the spent one after 15 runs, but less catalytic capability than the virgin one due to the leaching of Ru. The schematic mechanism for Ru-AA-catalyzed permanganate oxidation of aniline was proposed finally.