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

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), N 2 sorption, high-resolution transmission electron microscopy (HR-TEM), diffuse-reflectance ultraviolet–visible light (DR-UV-vis) microscopy, temperature-programmed reduction in a hydrogen atmosphere (H 2 -TPR), and temperature-programmed desorption of ammonia (NH 3 -TPD). The surface area (823–354 m 2 /g) and pore volume (0.71–0.44 cm 3 /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 WO 4 species and octahedrally coordinated polytungstate species are observed at all tungsten loadings, while bulk WO 3 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 H 2 O 2 as an oxidant. The fact that bulk WO 3 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.