Topology of silica supported vanadium–titanium oxide catalysts for oxidative dehydrogenation of propane

Abstract

Two-dimensional vanadia and titania surface clusters were hosted on the walls of the hierarchical pore system of mesoporous silica SBA-15. The topology of the catalyst surface was varied by sequential grafting of vanadium and titanium alkoxides generating an extended library of mixed (VO x ) n -(TiO x ) n /SBA-15 catalysts. The surface of the catalysts was analyzed by FTIR, UV-vis, Raman, and NEXAFS spectroscopy. Electron microscopy, X-ray fluorescence, X-ray diffraction, and nitrogen adsorption have been applied to characterize chemical composition, micro-and meso-structure of the materials. Segregation of nano-crystalline vanadia and titania particles was excluded by UV-vis, Raman and NEXAFS spectroscopy. Monolayer coverage of titanium oxide surface species has been achieved in the range between 17 and 19 wt% Ti loading corresponding to 6-8 Ti atoms per nm cat 2 and Si/Ti ratios between 3.3 and 2.8. Up to a critical total metal loading, vanadia is grafted on both the silica surface and surface titania species yielding tetrahedrally coordinated vanadium oxo-species characterized by low nuclearity and moderate catalytic activity. A volcano-type dependency with respect to the propylene space-time yield has been observed in the oxidative dehydrogenation of propane. The maximum in productivity of propylene is attributed to a particular surface topology that is characterized by (VO x ) n islands embedded in a matrix of dispersed titania species forming an almost complete combined vanadia-titania monolayer on the silica surface.

New insight into the structure of dispersed titania by combining normal-mode analysis with experiment

“…The band is attributed to isolated Ti ions in tetrahedral coordination, in agreement with the literature [11,13]. According to previous studies on SBA-15 supported titania the shoulder at 280 nm can be attributed to Ti O Ti bridges and changes in the coordination environment [13]. However, at loading densities close to the one of our sample (0.5 Ti/nm 2 ) predominantly isolated TiO 4 species have been reported based on UV-vis and NEXAFS studies [13].…”

“…The UV-vis analysis reveals a broad absorption band with a peak maximum at 220 nm and a shoulder at around 280 nm. The band is attributed to isolated Ti ions in tetrahedral coordination, in agreement with the literature [11,13]. According to previous studies on SBA-15 supported titania the shoulder at 280 nm can be attributed to Ti O Ti bridges and changes in the coordination environment [13].…”

“…From these studies it can be concluded that, at low content, Ti atoms are predominantly present in tetrahedral coordination, whereas with higher Ti content increasing crosslinking (Ti O Ti) and changes in the coordination gain importance. As reported in the literature for SBA-15 supported titania, at low loadings (0.5 Ti/nm 2 ) predominantly isolated TiO 4 species are present [13]. At higher loadings (e.g.…”

“…Previous studies have addressed the structure of silicasupported titania both experimentally [8][9][10][11][12][13][14] and theoretically [14,8,15]. From these studies it can be concluded that, at low content, Ti atoms are predominantly present in tetrahedral coordination, whereas with higher Ti content increasing crosslinking (Ti O Ti) and changes in the coordination gain importance.…”

Restructuring of silica supported vanadia during propane oxidative dehydrogenation studied by combined synchrotron radiation based in situ soft X-ray absorption and photoemission

“…The vanadium loading of the V x O y /SBA-15 catalysts was varied in the range 2-14 wt-% designated as 2V-14V. Detail description of the catalyst synthesis as well as catalyst characterization is given in Gruene et al [8] and Hamilton et al [9].…”