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

摘要

A series of highly ordered mesoporous cobalt-alumina catalysts (denoted as OMCA-x) with a variety of n(Al)(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 nAlCo 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 superox oradical-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.