Aqueous acidic hydrogen peroxide as an efficient medium for tungsten insertion into MCM-41 mesoporous molecular sieves with high metal dispersion

TLDR: In this article, two sets of WOx/SiO2-MCM-41 precatalysts (A and B) were prepared and compared, and the results suggest that the novel preparation method (route A) has a marked effect on the dispersion of the WOx species on and into the silica matrix.

Abstract: Two sets of WOx/SiO2-MCM-41 precatalysts (A and B) were prepared and compared. Tungsten-containing MCM-41 mesoporous molecular sieves have been synthesized with tetraethyl orthosilicate and tungsten precursors, tungstic acid, 'H2WO4', or sodium tungstate, in the presence of cetyltrimethylammonium chloride (CTMACl) micelles as template in aqueous acid solution. In route A, H2O2 was added to avoid the formation of iso- (or hetero-) polyoxometalates in the counter-ion-mediated S+X−I+ pathway with oxo–peroxo species [S+ = quaternary ammonium ion surfactant, in this paper cetyltrimethylammonium (CTMA+), X− = Cl− and/or anionic peroxo species, I+ = inorganic silicate precursor generating silanols or their protonated forms]. This procedure is compared with methods involving or generating polyoxometalates without peroxo moieties (route B) to determine whether the metal loading and/or dispersion is improved by H2O2. The native materials were calcined in air (1 K min−1, isothermal at 920 K for 4 h) in order to decompose the organic template. The resulting molecular sieves have been characterized by chemical analysis, powder X-ray diffraction, UV-visible diffuse reflectance spectroscopy, nitrogen sorption isotherms, TEM and EDX analysis, and Raman spectrometry. High incorporation levels ((Si∶W)exp molar ratio ≈30∶1) with a nearly homogeneous distribution of the dopants can be obtained only by route A. The spectral differences between the materials are correlated with their catalytic reactivities for cyclooctene epoxidation with an anhydrous H2O2/t-BuOH mixture at room temperature. The results suggest that the novel preparation method (route A) has a marked effect on the dispersion of the WOx species on and into the silica matrix.