Rita Herbert

Stabilization of Mesoporous Silica SBA‐15 by Surface Functionalization

Since its discovery in 1998, mesoporous silica SBA-15 has attracted a great deal of attention as new potential supporting material. 2] It has the structure of a molecular sieve with uniform hexagonal channels (narrow pore size distribution, 5– 30 nm) and high surface area (about 800 m g ). Due to the numerous silanols on the pore walls, a large number of catalytically active centers can be incorporated. It has been reported that mesoporous silica SBA-15 incorporated with transition metals show outstanding catalytic activity. However, mesoporous silica materials have not been widely used as catalysts or catalyst supports in industry yet. Apart from the high costs, one reason is the poor mechanical and hydrothermal stability of mesoporous silicas under the conditions of industrial applications. Conditions for producing pellets are in the pressure range of 10–400 MPa, which is already critical for the pore structure of mesoporous materials. Also, for applications in selective oxidation catalysis, very harsh working conditions are typically encountered (400 8C, steam, etc.). Functionalization of mesoporous silica materials has been widely studied in the literature. 16] An increase in mechanical and hydrothermal stability has been reported previously for MCM-48 by functionalization using dichlorodimethylsilane. Herein we report on a new procedure to stabilize mesoporous silica SBA-15 by surface functionalization leading to an increase in mechanical, thermal and hydrothermal stability. It is demonstrated that such stabilization significantly improves the catalytic performance of a supported vanadium oxide catalyst in propane oxidative dehydrogenation (ODH). The surface functionalization step is part of a multi-step procedure, 19] which includes a grafting step with 3-aminopropyltrimethoxysilane (APTMS), followed by a reaction of the amino groups and HCl in aqueous solution (step 2) and an ion exchange with transition metal ions, which herein is butylammonium decavanadate (step 3). Finally, the material is calcined, thereby removing all organic residues in contrast to the approach taken previously. Scheme 1 summarizes the first two steps (functionalization) of this procedure.

Synthesis, Characterization and Catalysis of Nanostructured Vanadia Model Catalysts for Partial Oxidation of Propane

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 influence between two support materials and between different synthesis method 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, Ramanand 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 trated with surface functionalization shows a higher stability than blank SBA-15. The blank SBA-15 shows a significant decrease of the BET surface area already at a mechanical treatment already at 75 MPa, whereas a significant 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 in-