Facile synthesis of highly ordered mesoporous cobalt–alumina catalysts and their application in liquid phase selective oxidation of styrene

Abstract

A series of highly ordered mesoporous cobalt–alumina catalysts (denoted as OMCA-x) with a variety of n_Al/n_Co ratios have been successfully synthesized via an evaporation-induced triconstituent cooperative co-assembly method and were applied to the liquid phase selective oxidation of styrene using difficult-to-activate molecular oxygen as the oxidant at atmospheric pressure. For comparison, a cobalt catalyst with a n_Al/n_Co ratio of 10 supported on ordered mesoporous alumina (denoted as Co/OMA-10) was also prepared by a traditional incipient wetness impregnation (IWI) method. Although both kinds of catalysts retained a unidimensionally ordered mesoporous structure, the textural and catalytic properties of the catalysts were significantly affected by the preparation methods. The characterisation and catalytic results confirmed that the OMCA-x catalysts exhibited a much more highly ordered hexagonal mesostructure, a narrower pore-size distribution, a higher Brunauer–Emmett–Teller surface area and pore volume, and higher catalytic activity and selectivity towards styrene oxide than those of Co/OMA-10. The improved catalytic activity of the OMCA-x catalysts can be attributed to the highly homogeneous dispersion of the Co species within the mesoporous alumina framework in the form of tetrahedrally coordinated divalent Co-oxide moieties, which can efficiently activate molecular oxygen into peroxo and superoxo radical-type active oxygen species. In addition, the incorporation of Co into the mesoporous framework of alumina and the formation of Co–O–Al bonds can effectively increase the content of tetra- and penta-coordinated framework aluminum, and further facilitate the formation of tetrahedrally coordinated Ia type Al–OH species as catalytic centers for styrene epoxidation, which is advantageous in improving the selectivity towards styrene oxide.