Manganese containing MCM-41: Synthesis, characterization and catalytic activity in the oxidation of ethylbenzene

Abstract

A series of Mn–MCM-41 containing manganese from 2 to 10wt.% were synthesized by impregnation method using aqueous manganese(II) acetate solution as the precursor. The samples were characterized by X-ray powder diffraction, nitrogen adsorption–desorption, FT-IR, TG–DTA, temperature programmed reduction, ESR, UV–vis diffuse reflectance spectra and solid state 29Si NMR. X-ray diffraction patterns showed that the hexagonal mesopore structure is retained by Mn–MCM-41 after Mn impregnation. FT-IR results indicated that Mn ions are incorporated into the Si–O framework. TPR patterns revealed the presence of highly dispersed manganese species on the surface of the support. ESR spectra exhibited a well-resolved sextet centered at g=2.0 indicating distorted octahedral symmetry of Mn(II) species in Mn/MCM-41. UV–vis diffuse reflectance spectra showed co-existence of Mn2+ and Mn3+ in Mn–MCM-41 and co-ordinated to Si(IV) by disordered octahedral and tetrahedral environments, respectively. 29Si NMR showed higher degree of condensation of materials in framework. Mn/MCM-41 materials possessed significant catalytic activity for oxidation of ethylbenzene using TBHP as oxidant under mild reaction conditions. 6/Mn–MCM-41 showed 57.7% ethylbenzene conversion, 82.2% and 18% selectivity for acetophenone and benzaldehyde, respectively.