Gold catalysts supported on TiO₂-nanotubes for the selective hydrogenation of p-substituted nitrobenzenes

Gold nanoparticles supported on titania nanotubes (TiO₂NT) were synthesized and employed as an efficient catalysts for the selective hydrogenation of nitrobenzenes. Materials characterization by N₂ adsorption-desorption isotherms, XRD, HRTEM, DRS UV–vis, and XPS revealed that structured materials with high metal dispersion were obtained. The catalytic properties of these materials were tested using nitrobenzene and eight p-substituted analogs as model compounds with the aim of gaining insight into the role of electron-withdrawing and electron-donating substituents on the rate and selectivity of hydrogenation to the corresponding p-substituted anilines. Catalytic data showed pseudo first order kinetics for all compounds, with minimum formation of reaction intermediates and absence of condensation side products. Quantum chemical computational calculations demonstrated that the experimental kinetic constants (k) for the series of nitrobenzenes under study were described by a multilinear regression equation using the substituent Hammet sigma constant (σ), and the calculated solvation energy of the reactants (ΔG_solv) as predictor variables. The catalyst activity-structure correlation revealed that electronic effects are critical for the reaction kinetics, and that electron-withdrawing groups increase the hydrogenation rates over electron-donating substituents. Solvation plays also a relevant role as less solvated species interact better with the catalyst surface and react faster than highly solvated substrates. These results are valuable to design novel efficient strategies for the selective hydrogenation of nitrocompounds.