Solvent-Free Oxidation of Benzyl Alcohol Derivatives by In Situ Generated Redox Couple Pd(0)/PdOx Supported on Ceria Nanorods

Benzyl alcohol (BnOH) oxidation to benzaldehyde (PhCHO) is a pivotal industrial reaction. The aerobic oxidation of BnOH in solvent-free conditions is highly compatible with the necessity of low environmental impact. In this research work, palladium oxide (PdOx) supported on ceria nanorods (CeO2-NR), was synthesized, and utilized for aerobic solvent-free oxidation of BnOH derivatives to the corresponding aldehydes. The catalyst, PdOx/CeO2-NR, was characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS), N-2 adsorption-desorption analysis, temperature-programmed reduction with hydrogen (H-2-TPR), and X-ray Photoelectron Spectroscopy (XPS), proving that the PdOx (x > 1) particles were highly dispersed on CeO2-NR and have a strong interaction with the support. The PdOx/CeO2-NR catalyst permitted the aerobic oxidation of various benzyl alcohol derivatives with good conversion, and high selectivity towards the corresponding aldehydes. The presence of electron donating groups (EDG) on the benzylic ring enhanced the reactivity as opposed to the electron withdrawing groups (EWG) which were detrimental for the catalytic activity. During the reaction a partial reduction of the metal produced a Pd(0)/PdOx/CeO2-NR redox couple stable in the reaction condition, more reactive and recyclable. Some mechanistic hypotheses are presented.

Automated summary

Benzyl alcohol oxidation to benzaldehyde is successfully achieved using PdOx/CeO2-NR catalyst under solvent-free conditions. The catalyst exhibited high conversion and selectivity, with electron donating groups on the benzylic ring promoting the reactivity, while electron withdrawing groups were detrimental. The partial reduction of the metal during the reaction resulted in a Pd(0)/PdOx/CeO2-NR redox couple, which enhanced the catalytic activity and recyclability. Some mechanistic hypotheses are proposed.