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

Büker, J. and Angel, S. and Salamon, S. and Landers, J. and Falk, T. and Wende, H. and Wiggers, H. and Schulz, C. and Muhler, M. and Peng, B.

Volume: 12 Pages: 3594-3605
DOI: 10.1039/d2cy00505k
Published: 2022

Nanoparticulate CoxFe3−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 Co3c2+ surface sites, which are identified to be the most active sites for 2-cyclohexene-1-hydroperoxide decomposition in cyclohexene oxidation. © 2022 The Royal Society of Chemistry

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