M Baltes

Stabilized MCM-48/VOx catalysts: synthesis, characterization and catalytic activity

Abstract The high synthesis cost, the poor hydrothermal stability of the pore structure and the extensive leaching of the active metal sites in liquid phase heterogeneous catalysis are three of the most important drawbacks for the industrial use of the newly developed mesoporous crystalline materials (MCM-41, MCM-48, HMS, FSM, …). In this study, we present an integrated approach that tackles these three problems simultaneously. High quality MCM-48 is prepared by a method of surfactant extraction and recuperation. The surface of the material is stabilized using a bifunctional silane, rendering the surface essentially hydrophobic, but leaving sufficient secondary anchoring sites for a further activation with VO(acac) 2 and final calcination. The final catalyst is thoroughly characterized and its catalytic and mechanical behavior is evaluated. It is shown that these materials can withstand severe hydrothermal conditions and that the leaching of the catalytic active species in liquid media is very strongly reduced. The obtained catalysts are active and selective and can be regenerated without significant loss of activity.

Synthesis and characterization of alumina-supported vanadium oxide catalysts prepared by the molecular designed dispersion of VO(acac)2 complexes

Alumina-supported vanadium oxide catalysts have been prepared by the molecular designed dispersion method, using the vanadyl acetylacetonate complex (VO(acac)2). The complex has been adsorbed on the support from solution, followed by thermal conversion into the corresponding supported vanadium oxide catalyst. The formation of the VO(acac)2–support material has been studied by both spectroscopic (FTIR, FT-Raman, ESR, UV–VIS–DRS; DRS: diffuse reflectance spectroscopy) and chemical methods. The present study reveals that VO(acac)2 complexes react preferentially with the hydroxy groups of the alumina, and only to a limited extent with the coordinatively unsaturated Al3+ sites. In addition, the deposition of the VO(acac)2 complex on the alumina surface yields different supported vanadium configurations as a function of the surface loading, due to both ligand exchange and hydrogen bond interactions. The thermal conversion of the adsorbed VO(acac)2 complexes into the supported vanadium oxide catalyst has been studied by TGA, GC-FTIR and in situ IR, which provides additional information on the VO(acac)2–support interaction. Characterization of these supported vanadium oxide catalysts by FTIR, FT-Raman and UV–VIS–DRS reveals the formation of highly dispersed VOx functionalities.

The Adsorption of VO(acac)2 on a Mesoporous Silica Support by Liquid Phase and Gas Phase Modification to Prepare Supported Vanadium Oxide Catalysts

Supported vanadium oxide catalysts are prepared by adsorption and subsequent calcination of the vanadyl acetylacetonate complex on silica by liquid phase and gas phase modification. The influence of the pretreatment temperature and the effect of the solvent in the liquid phase are discussed. Two types of gas phase deposition processes are used: flow-type reactions and vacuum deposition. The bonding mechanism, the influence of pretreatment temperature of the support and the influence of the reaction temperature are investigated by FTIR, XRD, TGA and chemical analysis. After calcination the obtained vanadium oxide layer is characterized by XRD and UV-Vis diffuse reflectance spectroscopy. The gas phase modification enables the creation of well dispersed supported VOx catalysts. Loadings up to 1.4 mmol g-1 (7 wt% V) without the formation of a crystalline fraction can be achieved. The selective oxidation of methanol to formaldehyde is used as a probe reaction to assess the catalytic activity and selectivity of the catalysts. It is shown that not the concentration of vanadium species, but their surface configuration is the determining factor in catalytic reactions.

The Uses of Polynuclear Metal Complexes to Develop Designed Dispersions of Supported Metal Oxides: Part II. Catalytic Properties

In many catalytic reactions, the configuration, dispersion andensemble size of the supported catalytic species have a significant impacton the activity, selectivity and product distribution of the catalyst. Byusing the method of molecular design, it becomes possible to fundamentallyoptimize these catalytic reactions, eliminating side reactions and improvingthe activity or selectivity of the catalyst, provided that the detailedreaction mechanisms and the structural dependencies are known. In thismanuscript, several examples are presented that illustrate the dramaticeffect of the ensemble size of the supported catalytic species on thecatalytic behavior. It is evidenced how supported metal oxides may beprepared to have quite different catalytic properties.