Abstract
Nanostructured vanadia model catalysts supported by silica were synthesized in a multi-step procedure as well as through incipient wetness impregnation. Afterwards, the samples were characterized and tested in the oxidative dehydrogenation (ODH)of propane. Silica supports used were mesoporous SBA-15 and Aerosil 300. The multi-step synthesis includes a surface functionalization and an ion exchange with decavanadate ions. One aim of this study was the investigation of the in uence between two support materials and between different synthesis methods concerning the vanadia structure and to find a correlation to their catalytic behavior.
Therefore, highly dispersed vanadia species with a similar vanadium density of 0.7 V atoms/nm2 were prepared. The samples were thoroughly characterized by nitrogen adsorption-desorption, small-angle XRD, TEM, XPS, Raman- and UV-vis spectroscopy. Furthermore, reactivity was tested with TPR besides catalytic test.
It could be shown that the multi-step procedure has a stabilizing effect on the mesoporous material. After mechanical, thermal and hydrothermal treatment, the sample treated with surface functionalization shows a higher stability than blank SBA-15. The blank SBA-15 shows a signiffcant decrease of the BET surface area already at a mechanical treatment already at 75 MPa, whereas a signi cant change of the surface area in the multi-step samples appears not until at 376 MPa. After pressure treatment at 752 MPa no mesoporous structure can be observed anymore for blank SBA-15, but for the multi-step sample it is in parts still observable. The impregnated samples show the same behavior as blank SBA-15. The enhanced stability has a positive influence on reaction behavior. The multi-step sample pressed at 752 MPa shows in the ODH of propane a higher selectivity towards propene than the impregnated sample treated in the same way. This can be explained by accessibility of active sites within the samples.
In the impregnated sample are more active sites blocked than in the multi-step
sample, due to the complete loss of mesoporous structure.
When comparing both support materials and the different synthesis methods, an increase of the degree of polymerization can be observed in the following order: VxOy/SBA-15 multi-step - VxOy/SBA-15 impregnated - VxOy/A300 multi-step - VxOy/A300 impregnated. It can be said that the spectroscopic data show a more distinct influence on the degree of polymerization of the support materials, whereas TPR data show clear differences among the synthesis methods. In the oxidative dehydrogenation of propane, no differences could be observed concerning the selectivities at similar conversions.
This leads to the assumption that the reactivity is highly influenced by the
composition of the gas phase.
Another issue in this study were first investigations on an SBA-15/titanium system, which can be applied as a support material for vanadia. For this purpose, several loadings of titania were impregnated on SBA-15 and characterized. Additionally, a Ti/SBA-15 sample with vanadium was synthesized, spectroscopically characterized and tested in the oxidative dehydrogenation of propane. The results show for conversion and selectivity a similar behavior at a reaction temperature of 450 °C as vanadium on blank SBA-15, but clear differences at a reaction temperature of 500 °C. For V-Ti/SBA-15 at a reaction temperature of 500 °C a signiffcant increase of conversion of propane can be observed.
