Interface structural effect of ruthenium-cerium oxide nanocomposite on its catalytic activity for selective oxidation of bioterpenes-derived p-cymene

An efficient catalyzed liquid-phase selective oxidation of p-cymene to tertiary cymene hydroperoxide (TCHP) and its derivative p-methylacetophenone (PMAP) was achieved over a nanostructured ruthenium-cerium oxide (Ru/CeO₂) composite catalyst with remarkable activity using a green molecular oxygen oxidant. Both of the composite catalysts were prepared from CeO₂ oxide with different particles sizes [i.e., bulk CeO₂ (Ru/b-CeO₂) and nanosized CeO₂ (Ru/n-CeO₂)]. The n-Ru/CeO₂ exhibited high catalytic activity for the free-radical driven oxyfunctionalization reaction of the inert C-H bond in p-cymene to afford up to 55% conversion within 5 h at 65% TCHP and 26% PMAP selectivity. The combined ratio of the TCHP/PMAP selectivity was dependent on the reaction time and the amount of p-cymene conversion. An increase in the reaction time resulted in converted large amount of TCHP to PMAP. The better catalytic performance of the Ru/n-CeO₂ catalyst compared to Ru/b-CeO₂ was partially due to the catalyst redox and oxygen vacancies effect. Both the reduced and oxidized forms of Ru/n-CeO₂ were investigated to gain insight into the active form of the catalyst. The reduced Ru/n-CeO₂ catalyst exhibited poor performance compared to the high catalytic performance of the oxidized form. This result demonstrated the significant concomitant effect of the balanced Ru⁴⁺/Ru³⁺ and Ce⁴⁺/Ce³⁺ redox combination to facilitate the free-radical driven mechanism via the excitation of O₂ to activate the substrate inert C-H. The Ru/n-CeO₂ catalyst was highly recyclable with consistent activity after its sequential re-use. The catalytic performance of Ru/n-CeO₂ was correlated to its structural characteristics based on the results from complimentary techniques, such as powder XRD, Raman spectroscopy, HRTEM, BET SA nitrogen physisorption, FESEM, H₂-TPR and EDX. According to the characterization results, the enhanced catalytic performance of the Ru NPs loaded on CeO₂ with nanosized particles was due to intrinsic metal-metal oxide interactions, enhanced redox cycles and high oxygen mobility resulting from structural defects of n-CeO₂ that form oxygen vacancies compared to dispersed on the bulk CeO₂ with large particles. The catalytic activity exhibited by Ru/CeO₂ provides encouraging results to add to the class of active and stable catalysts for the activation of typical alkyl aromatic hydrocarbons, such as the p-cymene inert C-H bond, for the selective introduction of an oxygen atom.