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

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.

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

Lioubov Kiwi, Fabio Rainone, Albert Renken

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)]

2003

Transient kinetics and in situ spectroscopic techniques for the characterisation of V/Ti-oxide catalysts in toluene partial oxidation

Fabio Rainone

Development of selective oxidation catalysts is a major concern of modern chemical industry. Among several systems, VTi-oxide based catalysts showed the most promising and versatile, able to catalyse key reactions like oxidation of o-xylene to phtalic anhydride and the abatement of NOx for pollution control. Extensive investigation in the past decades concluded that supported vanadia is present as a layer of molecularly-defined surface compounds VxOy, ("monolayer species") on top of the carrier oxide which are the catalytically active sites. Despite all the efforts to correlate the activity of VTi-oxide with its structure, the catalytic role of the different active species in the selective oxidation of hydrocarbons and how to tune their properties are still open questions. The goal of the present thesis is the in situ characterisation of V/Ti-oxide by transient-response chemical (feed step-response, temperature-programmed analyses) and spectroscopic (infrared and Raman) techniques to answer these important questions by using toluene partial oxidation as test reaction. First, the identification and elucidation of the role of the vanadia species was carried out, using Raman spectroscopy and activity tests. The strongly-bound monolayer species which cannot be dissolved by acid treatment were found to have isolated vanadate structure and be catalytically active in toluene partial oxidation. They have redox properties close to more condensed vanadate ("polymeric") species, which are also active. Vanadium present as crystalline V2O5 is not directly active, but plays a role in the surface reduction mechanism. By in situ DRIFTS, it has been shown that V/Ti-oxide active sites work with a coupled mechanism of product desorption and site reoxidation. Re-oxidation can occur either by reaction with gas-phase O2 or by oxygen spillover from bulk V2O5. Acidity associated with V-O-V bridges in monolayer species is likely responsible for the reversible deactivation of the catalysts caused by strongly held surface coke. Second, the modification of the acid/base catalyst properties by K addition and the formation of surface V oxide species were studied. The presence of K modifies the molecular structure of the vanadia surface species and forms new K-containing ones. These species are difficult to reduce in hydrogen and they are not catalytically active. Their presence causes a diminished catalytic activity. At the same time, Lewis and Brønsted acidity is depressed. This eliminates the phenomenon of deactivation. The excessive basicity caused by potassium addition has a detrimental effect on selectivity in benzoic acid, which is strongly held on the surface because of their intrinsic acidity and undergoes overoxidation to CO2. Finally, the VTi-oxide was supported on structured supports based on woven fibreglass. The use of structured beds is interesting in chemical engineering due to their advantages in terms of increased mass and heat trasfer, reduced pressure drop and optimal flow distribution. In order to reach optimum performance, the rather inert silica surface must be modified by adding an alumina layer on top of the fibres. This modification is essential for an optimal support because of insufficient dispersion of the titania layer on silica due to phase repulsion between the two oxides. The structured catalysts possess comparable activity and identical selectivity to the studied non-structured VTi-oxide catalyst with the same content of vanadia layers.

EPFL2004

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

Transient kinetics and in situ spectroscopic techniques for the characterisation of V/Ti-oxide catalysts in toluene partial oxidation

Fabio Rainone

EPFL2004