In this work, the oxidative dehydrogenation (ODH) of propane to propene was studied on a model catalyst system. The catalysts consisted of highly dispersed vanadium oxides on a mesoporous silica material (SBA-15). To investigate the influence of the support material on catalytic properties of the vanadium oxide species, the SBA-15 was modified with highly dispersed titanium oxide. A large sample matrix was obtained by stepwise variation of the vanadium and titanium oxide loading, in which all possible Si, V, Ti, and mixed element combinations were represented. These catalysts were prepared by chemically attaching ("grafting") metal alkoxides to the silanol groups of SBA-15. A variety of analytical methods (N2 physisorption ("BET"), XRD, FTIR, UV-Vis, Raman and NEXAFS spectroscopy, TGA, TEM and SEM, XRF) was applied to investigate the topology of the vanadium-titanium oxide surface structures and the molecular structure of the vanadium clusters. Furthermore, the influence of the aforementioned characteristics on the ligand effects of the oxidative dehydrogenation of propane to propene was studied. Pure highly dispersed two-dimensional vanadium oxide samples behave very differently in the ODH of propane to propene as compared to samples containing three-dimensional nanocrystals. The mixed V/Ti/SBA-15 system is more complex: the presence of titanium, at constant vanadium loading, leads to a higher activity in the reaction. At higher titanium loading (14 wt.% Ti) in the 4V/14Ti/SBA-15 sample a synergy between the metal species is observed, and correlated with an extremely high space time yield of propene (5 g C3H6 gcat-1 h-1). This synergy is characterized by the formation of combined mixed V-Ti-oxide monolayer. In this joint monolayer the abundance of V O-V- and V-O-Ti bonds is maximized and oligomers with mixed metal central atoms (V and Ti) are formed. The formation of these entities, which occurs via self-organization of constituent elements, is also reflected in the unique electronic and catalytic properties of the material.
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
|Subjects:||Inorganic chemistry, Organic chemistry|
|Keywords:||Oxidative dehydrogenation, Propane|
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