Selective hydrogenation of benzoic acid over Au supported on CeO2 and Ce0.62Zr0.38O2: Formation of benzyl alcohol

Abstract

The gas-phase hydrogenation of benzoic acid was studied over Au supported on CeO2 and Ce0.62Zr0.38O2 (CZ). HAADF-STEM has established formation of nanoscale (mean=1.5–2nm) Au particles, which is consistent with CO adsorption measurements. Incorporation of Au facilitated partial support reduction during TPR to 573K where the presence of Zr increased oxygen mobility, resulting in a greater degree of Ce4+ reduction (to Ce3+) in Au/CZ, as demonstrated by oxygen storage capacity and XPS measurements. Hydrogenation of an aqueous benzoic acid feed generated benzaldehyde and benzyl alcohol with a higher rate over Au/CZ that is attributed to the action of oxygen vacancies, which activate the carboxyl function for hydrogen attack. A parallel/consecutive kinetic model has been applied to quantify catalytic selectivity. A concerted (single step) conversion is proposed for Au/CeO2 that involves bridging interaction of the benzoate with Ce cations and Au nanoparticles with hydrogen addition. A stepwise conversion on Au/CZ is achieved via a Mars and van Krevelen mechanism with benzoic acid activation at an oxygen vacancy and reaction with surface hydrogen to generate benzaldehyde as a reactive intermediate that is converted to benzyl alcohol via nucleophilic CO attack. Switching from an aqueous to ethanolic feed increased rate due to greater oxygen vacancy availability with higher selectivity to benzaldehyde and appreciable toluene formation over Au/CZ.