The tandem catalysis of porous CoW composite oxide: Intensified allylic oxidation of cyclohexene to 2-cyclohexene-1-one

Abstract

The controllable oxidation of the allylic CH bond has great academic and industrial value. Different Co/W ratios of CoW composite oxides were prepared using hydrothermal method and used in the allylic oxidation reaction of cyclohexene containing unstable allylic hydrogen atoms and carbon-carbon double bonds. The relationship between the structure of active sites and catalytic performance was revealed using characterization techniques such as XRD, N2 adsorption/desorption, SEM, TEM, FT-IR, UV–vis, and XPS. The results indicated that the Co(II)/Co(III) sites effectively mediate the first step of the entire oxidation reaction system, which generates hydroxyl radicals and peroxyl radicals through the activation of H2O2 to achieve chain initiation. Subsequently, the cyclohexene underwent a series of chain migration reactions under the action of hydroxyl radicals and peroxyl radicals, resulting in the formation of 2-cyclohexen-1-one and 2-cyclohexen-1-ol. Finally, 2-cyclohexen-1-ol interacted with W(VI) sites to form 2-cyclohexen-1-one via an alcohol-coordinated hydroxyl intermediate. The tandem oxidation reaction of cyclohexene allylic oxidation and 2-cyclohexen-1-ol dehydrogenation was realized by the CoW synergistic catalytic. The synergy between Co and W sites, composed of Co-O-W units on the surface of the CoW oxide with porous channel features, was elucidated. The experimental results showed that under the optimal conditions, using CoW composite oxide (Co/W = 1:1) as the catalyst, a 79.8% conversion rate of cyclohexene was achieved with 93.9% selectivity for the allylic products, among which the selectivity for 2-cyclohexen-1-one was 73.8%.