Chromia on silica and zirconia oxides as recyclable oxidizing system: structural and surface characterization of the active chromium species for oxidation reaction

Chromium oxide samples supported over silica and zirconia were prepared by wet impregnation of the supports with aqueous solution of CrO3. In order to investigate the influence of the preparation method on the chromium dispersion and oxidation state, one sample was prepared by sol–gel technique. The materials were structurally characterized by XRD analysis. The oxidation states of chromium species were investigated by XPS, DRS and TPR/TPO techniques. EPR analyses were also carried out for the sample Cr/SiO2, that due to the low crystalline state exhibits broad XPS signals. In this case, particular attention was paid to the presence of Cr(V) and Cr (III) species and their concentration. The oxidizing performances of the chromia supported reagents have been evaluated for the liquid phase oxidation of benzyl alcohol to benzaldeyde, in absence of molecular oxygen. With the aim to investigate the redox behaviour of anchored Cr and to evaluate the amount of regenerable Cr species (upon TPR/TPO process), oxidation tests were carried out on fresh and aged samples (i.e. after 7 redox cycles up to 600 °C). A correlation was found between the benzyl alcohol conversion and the amount of chromium reduced by TPR of used samples (i.e. after 22 h of reaction).