Mn-containing porous silicates as catalysts for the solvent-free selective oxidation of alkyl aromatics to aromatic ketones with molecular oxygen

摘要

Mn-containing redox molecular sieves, i.e., highly crystalline microporous MnS-1 (0.6 wt% Mn, n(Si)/ n(Mn) = 160) and ordered mesoporous silicates MnMCM-41 and MnMCM-48 (1.8 wt% Mn, n(Si)/ n(Mn) = 51), were synthesized. The porosity of the products was determined by X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The evidence for the presence of isolated Mn sites incorporated into the silicate framework was obtained by Mn K-edge XANES and EXAFS analysis, which provided direct information on valence state and local environment of Mn in these products. The analysis revealed the presence of predominantly Mn~(3+) cations located at a 3-fold coordination in the silicate framework indicating Lewis acid sites in microporous (MnS-1) and mesoporous (MnMCM-41, MnMCM-48) silicate structures. The micro- and mesoporous silicates containing framework Mn~(3+)-sites selectively catalyze the oxidation of alkyl aromatics in benzylic position to aromatic ketones in the absence of any initiator using molecular oxygen as the terminal oxidant under mild, solvent-free, liquid-phase conditions.