Fabio Rainone

Catalytic oxidation of toluene with molecular oxygen over Cr-substituted mesoporous materials

AbstractThesynthesesofthermallystablechromium-incorporatinghexagonalmesoporousaluminophosphateandcubicCr-MCM-48have been reported. Characterization of the catalysts was made using low angle XRD, N 2 adsorption, UV-VISDRS, thermalanalysis, ICP-AES and ESR. Mesoporous Cr-AlPO 4 and Cr-MCM-48 catalysts have been found to be active for the vapourphase oxidation of toluene with molecular oxygen. Mesoporous Cr-AlPO 4 is found to exhibit both acidic and redox propertiesand hence oxidation as well as dealkylation reactions are taking place in a concerted manner, whereas, the cubic silicateanalogue acts as a pure redox catalyst.© 2002 Elsevier Science B.V. All rights reserved. Keywords: Chromium; Aluminophosphates; Mesoporous materials; Partial oxidation 1. IntroductionAluminophosphate materials, recently developed ascrystalline microporous materials are of potential useincatalysis[1].Flanigenetal.[2]reportedtheincorpo-ration of various elements into the framework sites ofaluminophospahtes and the resulting systems are ac-tiveforvariouscatalytictransformations.However,thepore dimensions of these materials are not sufficient toaccommodate a broad spectrum of organic moleculesin their cavities. With the first discovery of M41S bythe Mobil researchers, a series of mesoporous ma-terials with pore dimensions greater than 20A havebeen synthesised using structure-directing templates

Implication of the acid-base properties of V/Ti-oxide catalyst in toluene partial oxidation

The work presents the effect of K-doping on V/Ti-oxides taking into account: the surface acid–base properties and the structure of surface vanadia species in respect to the catalyst performance and deactivation. The structure of active surface species determines redox properties, which are related to the catalytic performance by the Mars–van Krevelen mechanism. The reducibility of surface vanadia is studied by temperature-programmed reduction (TPR) in H2. The molecular structure of surface vanadia is determined by FT-Raman spectroscopy in a controlled atmosphere. Surface acid–base properties are characterised via temperature-programmed desorption (TPD) of pyridine with mass spectrometric analysis of the products. Transient response techniques with continuous monitoring of the composition of gaseous phase are applied to follow the catalyst surface transformations. Evolution of benzaldehyde (BA) formed during interaction of toluene with the pre-oxidised catalyst (without gaseous oxygen) gives information about the nucleophilicity of surface oxygen. Addition of potassium to surface vanadia leads to an increased oxygen nucleophilicity, resulting in a higher selectivity towards BA formation. In general, increase in surface basicity decreases catalytic activity, but at the same time the catalyst deactivation due to coking is suppressed. This allows catalyst optimisation in view of a better control of the partial oxidation process.

Structural and catalytic properties of vanadia-based fibrous catalysts in toluene partial oxidation

Structured vanadia-based catalysts in the form of woven fabrics were successfully synthesised by incipient-wetness impregnation and sol–gel methods. The catalysts were investigated by Raman, X-ray photo-electron spectroscopies as well as high-resolution transmission electron microscopy (HRTEM). The spectroscopic results revealed that the elemental fibres consisted of a silica core covered by vanadia/titania. The modification of fibre silica surface by alumina resulted in a support with an increased dispersion of active vanadia/titania layer. Crystalline anatase was not found to be necessary for the formation of active surface vanadia species. Drying of titania layer without calcination led to a higher vanadia dispersion on titania. Structured fibrous vanadia-based catalysts demonstrated comparable activity and identical selectivity towards the products of toluene partial oxidation (benzaldehyde and benzoic acid) as granulated vanadia/titania with the same vanadium coverage. The transient response technique was applied to elucidate the role of the support on the active species. Thus, the V 2O5/TiO2/SiO2 woven fibre fabrics show promise for hydrocarbon partial oxidation, providing advantages due to its open macro-structure suitable for design of structured catalytic beds.

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

The work was aimed on the determination of the role of bulk V2O5 present in vanadia/titania catalysts for toluene partial oxidation. Two catalysts with 1.8 and 11.1 wt% 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 wt% V/TiO2 catalyst while monolayer vanadia species (monomeric and polymeric) in both samples. Toluene interaction 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 the oxidised samples in the absence of gaseous oxygen showed rapid formation of coordinatively adsorbed benzaldehyde (∼1625 cm−1) and benzoate/carboxylate species (1600–1400 cm−1), accompanied by the disappearance of the monomeric vanadia species (∼2037 cm−1). The catalyst containing V2O5 could convert toluene more deeply than the catalyst without V2O5, namely to adsorbed benzoic acid (∼1670 cm−1). Additionally, large quantities of gaseous products (COx, benzaldehyde, H2O) were obtained with the former catalyst, while almost none with the latter. Thus, bulk V2O5 could supply oxygen for the monolayer vanadia species easily reducible by toluene. This oxygen is required for the product formation and desorption.