Controlled Synthesis and Characterization of Highly Dispersed Molybdenum Oxide Supported on Silica SBA‐15

Abstract

Silica-supported molybdenum oxide catalysts with high densities of disperse molybdenum oxide species of up to 3.5 atoms nm−2 were prepared by using a novel synthesis approach on the basis of functionalized mesoporous SBA-15. As revealed by using Raman spectroscopy and thermal analysis, the synthesis mechanism consists of an ion exchange of heptamolybdate ions (Mo7O246−) into the porous functionalized framework and decomposition of the intact precursor into smaller fragments during final calcination. The amount of dispersed molybdenum oxide as well as the ratio of dispersed to crystalline molybdenum oxide was determined in detail for Mo densities of 0–13.0 atoms nm−2 by using a combination of X-ray fluorescence (XRF), energy-dispersive X-ray analysis (EDX), and Raman spectroscopy. Up to 3.5 atoms nm−2 dehydrated molybdenum oxide is present exclusively as dispersed species. At higher densities, additional Mo is introduced as crystalline MoO3, and the amount of dispersed species remains unchanged. The structure of the supported molybdenum oxide under ambient conditions largely resembles that of heptamolybdate ions. Dehydration leads to the formation of grafted monomeric and connected molybdenum oxide surface species. Detailed X-ray photoelectron spectroscopy (XPS) analysis revealed an increase in the Mo/Si ratio as well as a positive binding energy shift upon dehydration, indicative of an increase in the dispersion of the supported molybdenum oxide. The combination of Raman and XPS analyses establishes a correlation of the changes in structure and dispersion of the supported molybdenum oxide species

Monitoring Silica Supported Molybdenum Oxide Catalysts at Work: A Raman Spectroscopic Study

“…Deposition of molybdenum oxide results in a smaller specific surface area and pore volume as compared to bare SBA‐15. This behavior indicates that molybdenum oxide is located inside the pores coating the surface of the silica matrix as discussed previously 36. 37…”

“…For low Mo loadings (0.8 Mo nm −2 ) the two molybdenum‐related bands exhibit very similar peak intensities (see also Figure 3) strongly resembling the behavior observed at room‐temperature for dehydrated silica‐supported (dispersed) molybdenum oxide, see for example, ref. 36. In contrast, at higher molybdenum loadings (2.0 Mo nm −2 ), the peak intensity ratio I 991 / I 967 at 500 °C is approximately doubled (see also Figure 4) as compared to room‐temperature spectra, indicating an increase in the monoxo concentration when the catalyst temperature is raised to 500 °C.…”

“…As support material we chose mesoporous silica (SBA‐15) exhibiting a well‐defined structure consisting of hexagonally arranged pores with a pore diameter of 7 nm. For the catalyst preparation an ion‐exchange procedure was employed, which has previously been shown to produce dispersed molybdenum oxide species up to high molybdenum oxide densities (3.5 Mo nm −2 ) 36…”

Active Sites in Olefin Metathesis over Supported Molybdena Catalysts

“…The MoO x /SBA‐15 catalysts (molybdena supported on mesoporous silica SBA‐15; Mo loading 2.1–13.3 wt %) were prepared by an ion exchange protocol . The details of the preparation were described elsewhere .…”

Esterification of cetyl alcohol with palmitic acid over WO 3 /Zr-SBA-15 and Zr-SBA-15 catalysts

“…Among different mesoporous silica materials SBA-15 was chosen for this study because of its high surface area and much thicker walls and consequently higher hydrothermal stability than MCM-41 [22,23]. To improve the surface acidity and broaden the applications, different metal ions have been incorporated into SBA-15 [24][25][26][27].…”