Abstract

Two sets of WO_x/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 WO_x species on and into the silica matrix.