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 (NH3-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 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.