Facile synthesis of highly ordered mesoporous chromium-alumina catalysts with improved catalytic activity and stability

The highly ordered mesoporous chromium-alumina catalysts (denoted as OMCA(x)) with different n(Al)/n(Cr) ratios of 10 and 20 have been successfully synthesized via an evaporation-induced self-assembly (EISA) pathway associated with thermal treatment and were applied to the liquid phase oxidation of cyclohexane with hydrogen peroxide (H2O2) as an oxidant. For comparison studies, a chromium catalyst with the n(Al)/n(Cr) ratio of 10 supported on an ordered mesoporous alumina support [denoted as Cr/OMA(10)] was also prepared by an incipient wetness impregnation (IWI) method. Although both kinds of catalysts retained a unidimensionally ordered mesoporous structure, textural properties of the catalysts were significantly affected by the preparation method. The characteristic results from XRD, N-2 adsorption, TEM, UV-Vis, Al-27 MNS NMR, and H-2-TPR showed that the OMCA(x) catalysts exhibited much more highly ordered hexagonal mesostructure, narrower pore-size distribution, higher Brunauer-Emmett-Teller surface area and pore volume, and more homogeneous distribution of Cr species in mesoporous alumina framework in the form of tetrahedrally coordinated hexavalent Cr-oxide moieties than those of Cr/OMA(10) synthesized by an IWI method. In addition, in comparison with Cr/OMA(10), OMCA(x) catalysts showed higher catalytic activity and stability for the liquid phase oxidation of cyclohexane.