Till Wolfram

Synthesis of MoVTeNb Oxide Catalysts with Tunable Particle Dimensions

Reliable procedures for the controlled synthesis of phase‐pure MoVTeNb mixed oxides with M1 structure (ICSD 55097) and tunable crystal dimensions were developed to study the structure sensitivity of the selective oxidation of propane to acrylic acid. A series of powdered M1 catalysts was successfully prepared on a gram scale by using a hydrothermal‐based route, purification of biphasic M1‐M2 (M2 phase – ICSD 55098) oxide systems, and an innovative approach utilizing a superheated water vapor treatment of calcined precursors. The influence of the preparation technique on the particle morphology and the size is discussed. Detailed experimental studies highlight that the as‐derived catalytic materials were indeed phase‐pure and compositionally uniform MoVTeNb oxide M1 powders, composed of single‐crystalline and structural defect‐free crystals grown along the c axis. The morphologically different catalysts were studied in the selective oxidation of propane to acrylic acid, revealing that active sites appear on the entire M1 surface and illustrating the high sensitivity of catalyst performance on the catalyst synthesis method.

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

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 (VOx)n–(TiOx)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 nmcat2 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 (VOx)n islands embedded in a matrix of dispersed titania species forming an almost complete combined vanadia–titania monolayer on the silica surface.

High performance (VOx)n–(TiOx)m/SBA-15 catalysts for the oxidative dehydrogenation of propane

Grafted VxOy catalysts for oxidative dehydrogenation of propane (ODP) have been studied due to their potential high performance and as model catalysts in the past. We report on a positive synergetic effect capable of considerably enhancing the propene productivities above reported performances. The most productive catalysts were found at metal loadings (V + Ti) close to the monolayer coverage. The 4V/13Ti/SBA-15 catalyst presented a considerably high productivity (6–9 kgpropene kgcat−1 h−1). Moreover, with this catalyst, propene productivity only slightly decreased as a function of propane conversion, indicating that propene combustion toward COx occurs more slowly in comparison to other catalysts exhibiting high propene productivities. A detailed kinetic analysis of the 4V/13Ti/SBA-15 catalyst revealed that high vanadia and titania dispersions are required for high propene productivity.