Synthesis, Characterization and Catalytic Properties of the Novel Manganese-Containing Amorphous Mesoporous Material MnTUD-1

Abstract

The incorporation of manganese into the amorphous mesoporous material TUD-1 (nSi/nMn = 89−8; Mn content: 1−11 wt %) was achieved by the direct hydrothermal method in the presence of triethanol amine. Nitrogen sorption revealed the mesoporous nature (dP = 4−9 nm) of MnTUD-1 and a decreasing specific surface area with increasing Mn content. FT-IR spectroscopy indicates the incorporation of Mn into the silica framework of TUD-1. As evident from DR-UV−vis and EPR spectroscopy, the Mn atoms in MnTUD-1 exist in the oxidation states +2 or +3. The fraction of Mn2+ is <1 % in the as-prepared samples and drops to zero upon further calcination in oxygen. With increasing Mn content, Mn2O3 as an extraframework species is detected both by XRD and by high-resolution transmission electron microscopy. According to EPR, a fraction of the Mn atoms, decreasing with the nSi/nMn ratio, is antiferromagnetically coupled. The uncoupled, that is, isolated, Mn species in the framework play a predominant role in the catalytic activity of MnTUD-1 in the liquid-phase epoxidation of styrene or trans-stilbene with tert-butyl hydroperoxide as an oxidant. The MnTUD-1 catalysts are highly stable toward leaching of Mn, in contrast to MnOx supported on macroporous silica gel, and they are significantly more active than the previously reported catalysts MnMCM-41 or MnSBA-15.