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

Two sets of WO(x)/SiO(2)-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, "H(2)WO(4)", or sodium tungstate, in the presence of cetyltrimethylammonium chloride (CTMACl) micelles as template in aqueous acid solution. In route A, H(2)O(2) 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 H(2)O(2). 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 approximate to 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 H(2)O(2)/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 WO(x) species on and into the silica matrix.