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

Abstract

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 z X 2 I z pathway with oxo±peroxo species [S z ~quaternary ammonium ion surfactant, in this paper cetyltrimethylammonium (CTMA z ), X 2 ~Cl 2 and/ or anionic peroxo species, I z ~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 21 , 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 re¯ectance 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 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.

Self-Assembled Complexes of Non-cross-linked Amphiphilic Polymeric Ligands with Inorganic Species: Highly Active and Reusable Solid-Phase Polymeric Catalysts

“…H 2 O 2 under the organic solvent-free conditions was examined. [30][31][32][33][34][35][36][37][38][39][40][41][42] We were pleased to find that PWAA showed a very high catalytic activity on epoxidation. In the presence of 2.7ϫ10…”

Low-Temperature Active Pt-catalysts Supported on Mesoporous Silica and Metal-Silicates for Cleaning Diesel-NOx Exhausts

“…MPS and the metal-silicates probably possess the Brønsted acid-sites, while γ-alumina contains Lewis acid-sites. The observation of the Brønsted acid-sites of transition-metal inserted MCM-41 was unsuccessful because of really ineffective pyridine adsorption FT-IR assessment (Zhang et al, 1998;Briot et al, 2000;Parvulescu et al, 2003). Although the measurement of acid-sites of MPS and the metal-silicates by the above standard method is very difficult, the support effect is undoubtedly due to the strong Brønsted acid-sites locally distributed on the support surfaces.…”

Tungsten-Based Mesoporous Silicates W-MMM-E as Heterogeneous Catalysts for Liquid-Phase Oxidations with Aqueous H2O2

“…However, incorporation of tungsten into mesoporous silicates is not a trivial task since tungsten is considerably larger than silicon, with an ionic radius of 0.42 Å, compared to 0.26 Å [19]. Thus, tungsten-containing mesoporous molecular sieves usually have a serious drawback of the active metal leaching under turnover conditions of liquid-phase oxidation [20][21][22][23][24]. Numerous efforts have been focused on synthesizing materials in which the active tungsten species are well isolated and anchored on the support through W-O-Si covalent bonds.…”

“…Numerous efforts have been focused on synthesizing materials in which the active tungsten species are well isolated and anchored on the support through W-O-Si covalent bonds. In recent years, tungsten has been incorporated into high surface area mesoporous molecular sieves, such as MCM-41 [21,[25][26][27][28][29][30][31], MCM-48 [32,33], SBA-15 [34][35][36], SBA-16 [23], HMS [37,38], MCF [39,40], KIT-5 [41], KIT-6 [24,42], and TUD-1 [43].…”

“…The higher W content resulted in the formation of extra-framework crystalline WO 3 , as detected by XRD and Raman spectroscopy. To improve the dispersion of tungsten oxide species in the silicate matrix, an oxoperoxo route was developed by Brégeault et al [21,25]. Indeed, excess H 2 O 2 generated low condensed oxoperoxometalate species in the gel, thus avoiding formation of extra-framework crystalline WO 3 during calcination [46].…”