Synthesis, Characterization, and Epoxidation Activity of Tungsten-Incorporated SBA-16 (W-SBA-16)

Abstract

Tungsten, in varying amounts, was incorporated into a SBA-16 structure via a one-pot direct synthesis method under an acidic medium using Pluronic F127 triblock co-polymer as a template and n-butanol as a co-surfactant. Tetraethyl orthosilicate (TEOS) and sodium tungstate were used as the Si and W sources, respectively. The resulting materials (denoted as W-SBA-16) are characterized for structural ordering, textural properties, and types of tungsten incorporation by techniques such as small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), N2 sorption, high-resolution transmission electron microscopy (HR-TEM), diffuse-reflectance ultraviolet–visible light (DR-UV-vis) microscopy, temperature-programmed reduction in a hydrogen atmosphere (H2-TPR), and temperature-programmed desorption of ammonia (NH3-TPD). The surface area (823–354 m2/g) and pore volume (0.71–0.44 cm3/g) of the W-SBA-16 materials are found to decrease with an increase in tungsten loading (from 2.7 wt % to 30.4 wt % of the total sample). Isolated framework WO4 species and octahedrally coordinated polytungstate species are observed at all tungsten loadings, while bulk WO3 species are observed only at higher tungsten loadings. The W-SBA-16 materials display significant acidity that is tunable with tungsten loading, and they selectively catalyze the epoxidation of cyclohexene to cyclohexene oxide with H2O2 as an oxidant. The fact that bulk WO3 alone does not catalyze the reaction implies that the framework-incorporated W species and/or the polytungstate species are responsible for the observed catalysis. For this reaction, three-dimensional cubic mesostructured catalysts (W-SBA-16, W-KIT-6, and W-KIT-5) perform better than two-dimensional mesostructured (W-SBA-15) material. The problem of gradual tungsten leaching must be overcome for these catalysts to have practical utility.

Synthesis, characterization, and visible-light-driven photocatalytic performance of W-SBA15

“…On the other hand, the excessive addition of tungsten source easily promotes the form transformation from incorporated tungsten species to isolated tungsten species or low oligomeric tungsten oxide species, and finally to crystalline WO 3 . It has been reported that incorporated tungsten species, isolated tungsten species, and low oligomeric tungsten oxide species, rather than crystalline WO 3 were powerful active sites for a variety of reactions [31,32,46,49]. In our study, three above tungsten species forms were indeed beneficial to the enhancement of photocatalytic efficiency, possibly owing to the quite intimate connection between framework of SBA15 and tungsten species, thus facilitating to transfer photogenerated electrons to SBA15 matrix and further transfer them by means of electron delocalization capacity from SBA15 framework [30], effectively avoiding the recombination of charge carriers.…”

Sn-based catalysts for Baeyer-Villiger oxidations by using hydrogen peroxide as oxidant

“…Among the reported cubic mesoporous silica materials, SBA-16 appears to be one of the best candidates as catalytic support or absorbent because of the enhanced thermal stability due to the thick wall, economical synthesis with inexpensive silica sources, and large pores [67][68][69][70]. Sn incorporated SBA-16 (Sn-SBA-16), as an ordered cubic-like mesoporous structure, gives Lewis acid sites through the framework substitution for silica matrix.…”

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].…”

“…Tungsten-based W-SBA-16 [23], W-KIT-5 [41] and W-KIT-6 [42] materials, obtained by hydrothermal synthesis under acidic conditions, contained framework [WO 4 ] 2− tetrahedral species and octahedrally coordinated polytungstate at low W loadings. However, these materials suffered from tungsten leaching under turnover conditions of catalytic oxidation with aqueous H 2 O 2 [23,41,42].…”