Structure–activity correlation in aerobic cyclohexene oxidation and peroxide decomposition over CoxFe3−xO4 spinel oxides

Abstract

Nanoparticulate Co_xFe_3−xO₄ (0 ≤ x ≤ 3) catalysts were prepared by spray-flame synthesis and applied in liquid-phase cyclohexene oxidation with O₂ as oxidant. The catalysts were characterized in detail using N₂ physisorption, XRD, TEM, XPS, FTIR, Raman, and Mössbauer spectroscopy. A volcano plot was obtained for the catalytic activity in cyclohexene oxidation as a function of the Co content with a maximum at x = 1. Thus, CoFe₂O₄ achieved the highest degree of cyclohexene conversion and the fastest decomposition rate of the key intermediate 2-cyclohexene-1-hydroperoxide. Kinetic studies and a stability test were performed over CoFe₂O₄, showing that cyclohexene oxidation follows first-order kinetics with an apparent activation energy of 58 kJ mol⁻¹. The catalytic hydroperoxide decomposition during cyclohexene oxidation was further investigated using H₂O₂ and tert-butyl hydroperoxide as simpler surrogates resulting in similar volcano-type correlations. The increase in catalytic activity with increasing Fe content with a maximum at x = 1 is ascribed to the increasing concentration of octahedrally coordinated Co²⁺ cations in the spinel structure leading to the presence of coordinatively unsaturated Co_3c²⁺ surface sites, which are identified to be the most active sites for 2-cyclohexene-1-hydroperoxide decomposition in cyclohexene oxidation.