(173ac) Condensed Phase Catalytic Conversion of Benzyl Alcohol to Phenol Using High-Frequency Ultrasound: A Combined Experimental and DFT Investigation

Phenol is an essential compound to produce polymers, such as nylons (polyamides), polycarbonates and phenolic resins. The production of phenol is typically through the Cumene process, and the excessive acetone produced as the byproduct hampers the economics of the process. Therefore, it is necessary to develop an alternative method for phenol synthesis. Hence, in this work, we attempted the oxidation of benzyl alcohol on a CuO nanocatalyst in aqueous medium using high-frequency ultrasound, to produce phenol. Experimental results showed phenol selectivity of 73% during the initial conversion and a drop in selectivity as conversion increased. Without ultrasound, the same selectivity towards phenol and reaction kinetics could only be achieved if hydrogen peroxide was added to the reaction medium, which suggested the possible role of ultrasound in water dissociation and the participation of the water in the reaction. DFT calculations were performed to explore the reaction energetics, the role of the catalyst and water and the key intermediates involved in the reaction. The results revealed the benzyl alcohol was first oxidized to form benzaldehyde and the C-C cleavage between the aldehyde group and benzene ring was favored kinetically and thermodynamically on the CuO surface. It confirmed the low selectivity toward benzaldehyde in the experiments. The key participant in the reaction was the hydroxyl group dissociated from water by ultrasound. It promoted the oxidation of benzyl alcohol, also incorporated itself into the benzene ring. This reaction mechanism was further confirmed by the isotope labelling experiments that showed the OH group in phenol was not coming from benzyl alcohol but from water dissociation. As the surface lattice oxygen was not consumed during the C-H and O-H activation, the catalyst remained stable throughout the process. This combined experimental and computational investigation showed a novel and promising route to produce phenol from benzyl alcohol.