Abstract
Preparation of dispersed transition metal oxides catalyst with low oxidation state still remains a challenging task in heterogeneous catalysis. In this study, vanadium oxides supported on zeolite SBA-15 have been prepared under hydrothermal condition using V2O5 and oxalic acid as sources of vanadium and reductant, respectively. The structures of samples, especially the oxidation state of vanadium, and the surface distribution of vanadium oxide species, have been thoroughly characterized using various techniques, including N2-physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-visible spectra (UV-Vis) and UV-visible-near infrared spectra (UV-Vis-NIR). It is found that the majority of supported vanadium was in the form of vanadium(IV) oxide species with the low valence of vanadium. By adjusting hydrothermal treatment time, the surface distribution of vanadium(IV) oxide species can be tuned from vanadium(IV) oxide cluster to crystallites. These materials have been tested in the hydroxylation of benzene to phenol in liquid-phase with molecular oxygen in the absence of reductant. The catalyst exhibits high selectivity for phenol (61%) at benzene conversion of 4.6%, which is a relatively good result in comparison with other studies employing molecular oxygen as the oxidant.
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