Unique characteristics of MnOx-incorporated mesoporous silicate, Mn-FDU-5, prepared via evaporation induced self assembly

摘要

Mn containing 3D mesostructured FDU-5 (Ia3d) silicates with varying Si/Mn ratios were synthesized for the first time via evaporation induced self assembly (EISA) technique under acidic conditions at room temperature, employing Pl23 triblock copolymer as a structure directing agent. This method facilitates complete incorporation of higher amounts of Mn in the framework and extraframework locations of FDU-5 even under highly acidic synthesis conditions. The Mn-FDU-5 samples possessed surface areas of 300-500 m(2)/g, pore volumes of 0.42-0.55 cm(3)/g and narrow pore size distributions of 4.1-4.9 nm. Homogeneous dispersion of Mn species and aggregated Mn oxide clusters were evidenced from FIB-SEM micrographs. Complementary analytical techniques such as diffuse reflectance UV-Vis, FTIR, TPR and EPR analyses provide insights into the nature of the different types of Mn species (Mn2+, Mn3+ and Mn3O4 nanoparticles) that co-exist in FDU-5. The Mn-FDU-5 material is shown to be active for the epoxidation of trans-stilbene (TS, similar to 60 % conversion) to trans-stilbene epoxide (similar to 64 % selectivity) with TBHP as oxidant. Although the activity of Mn-FDU-5 (similar to 45 % TS conversion) is similar to those observed with MCM-41 and MCM-48 supports containing similar amounts of Mn (1 wt%), higher epoxide selectivity (similar to 64 %) was observed with Mn-FDU-5. The mixed oxidation states of Mn (Mn2+, Mn3+) along with the extraframework Mn3O4 are found to be beneficial in catalyzing TS epoxidation.