The UV−visible spectra of the MnOx/SBA-15 catalysts, along with references SBA-15 and MnOx, are shown in Fig. 3. The prepared SBA-15 has no PHA-680632 in the 200−800 nm range. The MnOx prepared shows a broad peak at 500 nm assigned to 6A1g to 4T2g crystal field transitions of Mn2+ as observed in Mn3O4 and MnO . Manganese-loaded catalysts showed an additional absorption in the 210–280 nm regions, which is assigned to charge–transfer transitions from O2− to tetrahedral Mn3+, i.e. in the framework of SBA-15 . Obviously, the UV–vis spectrum of 6 wt% MnOx/SBA-15 is different from that of 17 wt% MnOx/SBA-15. The absorption in the 210–280 nm regions is relatively pronounced, indicating that the proportion of the tetrahedrally oxygen-coordinated Mn3+ is higher for 6 wt% MnOx/SBA-15. It is noted that the O2− to Mn3+ electron transition in Mn3O4 in which Mn is octahedrally coordinated with oxygen usually exhibits a typical band at 320 nm . Unfortunately, the assignments may be somewhat hampered by overlap of signals corresponding to tetrahedral oxygen coordinated Mn3+ (270 nm) and cannot be ruled out. Additionally, a broad band near 400–500 nm originates from the O2− → Mn4+ charge-transfer transition, which indicates that the Mn4+ species is presented in MnOx/SBA-15 .