Selective Oxidation of Benzyl Alcohol in Water With Cobalt-Exchanged Zeolite-NaY Catalyst.

A low-cost and highly selective benzyl alcohol (BA) oxidation process in water was established through the activation of peroxymonosulfate (PMS) by cobalt-exchanged zeolite-NaY catalyst, Co-NaY. Different physicochemical and spectroscopic analyses confirmed the existence of cobalt(II) species as Co(OH)2 in zeolite-NaY framework. Zeolite-NaY played a pivotal role in trapping the free radicals, activating the PMS, and stabilizing the active Co(peroxo) species, Co(O-O). The involvement of Co(O-O) species in the reaction mechanism was evidenced by characteristic spectroscopic analyses, including the FTIR band at 926 cm-1 and the Raman band at 983 cm-1, attributed to the ν(O-O) vibration through Fermi resonance. The formation of cobalt(oxo)(hydroxy) complex as the intermediate redox active centre was believed to help the BA oxidation reaction to proceed through a non-free radical pathway. Noninvolvement of the free radical in the course of the reaction was evident from the electron paramagnetic resonance spectroscopic analysis. Benzaldehyde was selectively obtained up to 85% through the benzylic CH bond activation process in water making it a green reaction. Density functional theory calculations further supported the proposed mechanism, indicating that the reaction is thermodynamically favourable at 50°C in water. The catalyst exhibited good recyclability, maintaining product selectivity over three cycles.