Partial oxidation of toluene to benzaldehyde and benzoic acid over model vanadia/titania catalysts

Lioubov Kiwi, Fabio Rainone, Albert Renken
2003
Article de conférence

Pure and K-doped vanadia/titania catalysts have been studied by transient response method and in situ DRIFTS during toluene partial oxidn. They were characterized by FT-Raman spectroscopy, TPR, TPD of pyridine and XPS. In order to elucidate the role of different vanadia species for catalysis, the study was carried out in aerobic and anaerobic conditions. Monolayer species (monomeric and polymeric) are responsible for the catalytic activity and selectivity. Bulk V2O5 present in the catalysts with high concn. of vanadia supplies oxygen for the monolayer species easily reducible by toluene. This oxygen is necessary for the products formation and desorption in the absence of gaseous oxygen. Potassium at low concn. diminishes the total rate of toluene oxidn. and suppresses deactivation provided by cocking. This is due to transformation of the polymeric species to the not active K-doped monomeric ones. [on SciFinder (R)]

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Lioubov Kiwi, Fabio Rainone, Albert Renken
2003
Article de conférence

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https://infoscience.epfl.ch/record/84432?ln=fr

À propos de ce résultat

Concepts associés (11)

Concepts associés

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Publications associées (10)

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Partial oxidation of toluene to benzaldehyde and benzoic acid over model vanadia/titania catalysts: Role of vanadia species

Lioubov Kiwi, Fabio Rainone

Pure and K-doped vanadia/titania prepared by different methods have been studied in order to elucidate the role of vanadia species (monomeric, polymeric, bulk) in catalytic toluene partial oxidation. The ratio of different vanadia species was controlled by treating the catalysts in diluted HNO3, which removes bulk vanadia and polymeric vanadia species, but not the monomeric ones, as was shown by FT-Raman spectroscopy and TPR in H2. Monolayer vanadia species (monomeric and polymeric) are responsible for the catalytic activity and selectivity to benzaldehyde and benzoic acid independently on the catalyst preparation method. Bulk V2O5 and TiO2 are considerably less active. Therefore, an increase of the vanadium concentration in the samples above the monolayer coverage results in a decrease of the specific rate in toluene oxidation due to the partial blockage of active monolayer species by bulk crystalline V2O5. Potassium diminishes the catalyst acidity resulting in a decrease of the total rate of toluene oxidation and suppression of deactivation. Deactivation due to coking is probably related to the Brønsted acid sites associated with the bridging oxygen in the polymeric species and bulk V2O5. Doping by K diminishes the amount of active monolayer vanadia leading to the formation of non-active K-doped monomeric vanadia species and KVO3.

2004

Chargement

DRIFTS and transient-response study of vanadia/titania catalysts during toluene partial oxidation

Lioubov Kiwi, Fabio Rainone, Albert Renken

The role was studied, of bulk V2O5 in vanadia/titania catalysts in partial oxidn. of toluene. Two catalysts with 1.8 and 11.1% of V were studied, by in situ DRIFTS and transient-response methods. Bulk V2O5 was found by FT-Raman spectroscopy only in the 11.1% V/TiO2 catalyst while monolayer vanadia species (monomeric and polymeric) were obsd. in both samples. Toluene interactions in the presence of gaseous oxygen showed that the selectivity to benzaldehyde and benzoic acid was similar for the two catalysts. The nature of adsorbed species was also similar. Toluene interaction with oxidized samples in the absence of gaseous oxygen led to rapid formation of coordinatively adsorbed benzaldehyde (.apprx.1625 cm-1) and benzoate/carboxylate species (1600-1400 cm-1), accompanied by the disappearance of monomeric vanadia species (.apprx.2037 cm-1). The catalyst contg. V2O5 showed higher level of toluene conversion to benzoic acid than the catalyst without V2O5. Large quantities of gaseous byproducts (COx, benzaldehyde, H2O) were formed when using the V2O5 contg. catalysts, while almost none were obsd. in the absence of V2O5. Thus, bulk V2O5 supplies oxygen to the vanadia active species; also, oxygen is involved in formation and desorption of intermediates and products. [on SciFinder (R)]

2003

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Influence of Potassium Doping on the Formation of Vanadia Species in V/Ti Oxide Catalysts

Lioubov Kiwi, Fabio Rainone, Albert Renken

The influence of potassium on the formation of surface vanadia species on V/Ti oxide catalysts contg. from 0.2 to 5 monolayers of vanadia (K/V at. surface ratio ?1) has been investigated by temp. programmed redn. in hydrogen and by FT-Raman spectroscopy under dehydrated conditions. In the pure catalysts, monomeric and polymeric (metavanadate-like) species, "amorphous" and bulk cryst. V2O5 were detected depending on the surface vanadia loading. In the K-doped catalysts, vanadia species formed on the surface depend also on the K/V at. ratio. Even at small K/V ratios, K inhibits the formation of the polymeric species in favor of the "K-doped" and/or "K-perturbed" monomeric species. These species possess lengthened V:O bonds with respect to the monomeric species in the undoped V/Ti oxides. At K/V = 1, the "K-doped" monomeric species and "amorphous" KVO3 are mainly present on the surface. Redn. of vanadia forms in the K-doped catalysts takes place at higher temps. than in the catalysts where potassium was absent. The monomeric and polymeric species, which are the active sites in partial catalytic oxidn., have the lowest redn. temp. Vanadia species formed on the com. titania, contg. K, were also elucidated. The catalysts were characterized via XPS, high-resoln. transmission electron microscopy, and Brunauer-Emmett-Teller surface area measurements. [on SciFinder (R)]

2001

Chargement

Catalyse

vignette|Intérieur du musée de la catalyse à Widnes en Angleterre. En chimie, la catalyse (du grec ancien : , « détacher ») se réfère à l'accélération ou la réorientation de la cinétique de réaction

Spectroscopie Raman

La spectroscopie Raman (ou spectrométrie Raman) et la microspectroscopie Raman sont des méthodes non destructives d'observation et de caractérisation de la composition moléculaire et de la structure

Oxygène

L'oxygène est l'élément chimique de numéro atomique 8, de symbole O. C'est la tête de file du groupe des chalcogènes, souvent appelé groupe de l'oxygène. Découvert indépendamment en 1772 par le Suéd