Selective hydrogenation of benzoic acid over Au supported on CeO₂ and Ce_0.62Zr_0.38O₂: Formation of benzyl alcohol

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 = 1.5–2 nm) Au particles, which is consistent with CO adsorption measurements. Incorporation of Au facilitated partial support reduction during TPR to 573 K 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 Cdouble bond; length as m-dashO 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.