Nanostructured ruthenium on γ-Al2O3 catalysts for the efficient hydrogenation of aromatic compounds

Free and trioctylamine (TOA)-stabilized ruthenium nanoparticles have been prepared by decomposition of the metal precursor Ru(η6-cycloocta-1,3,5-triene)(η4-cycloocta-1,5-diene) under mild conditions (room temperature, hydrogen atmospheric pressure). The nanoparticles have been deposited on γ-Al2O3 supports having different surface area. The resulting systems are active in the hydrogenation of methyl benzoate to methyl cyclohexanoate with a reaction rate decreasing in the order Ru(TOA)/γ-Al2 O3 (high surface area, catalyst D) > Ru(TOA)/γ-Al2O3 (catalyst C) > Ru/γ-Al2O3 (high surface area, catalyst B) > Ru/γ-Al2O3 (catalyst A). Catalysts A–D are long lived and can be reused without loss of activity; they are considerably more active than a commercial ruthenium on γ-Al2O3 sample. High Resolution Transmission Electron Microscopy analyses of such systems show that the nanoparticles are homogeneously dispersed on the support and that the size distribution decreases in the order catalyst A, 2.9 nm > catalyst B, 2.8 nm > catalyst C, 2.4 nm > catalyst D, 2.3 nm. Based on the easy hydrogenation of the aromatic ring to the cyclohexane derivative, an efficient synthesis of 4-carbomethoxyformylcyclohexane, important starting material in the preparation of pharmaceutical products, from the largely available methyl 4-formylbenzoate, has been set up in the presence of catalyst D.