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

Abstract

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 H 2 . 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.

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

“…However, the polymeric species probably contain acid hydroxyl groups associated with the V-O-V bridges [1], which are responsible for coking and deactivation of the catalyst. Similar bridges exist in V 2 O 5 also introducing Brønsted acidity [26]. This correlates with the increase of deactivation with vanadia coverage (Fig.…”

“…K-doping decreases mainly the selectivity of BAc formation in favour of maleic anhydride and CO x formation. Similar effect of potassium on vanadia species, catalytic activity, reducibility and acidity was observed at much higher vanadia loading (2 ML) [6,26].…”

“…This catalyst, as was mentioned, contains the K-doped monomeric species and KVO 3 , but does not contain the polymeric species and V 2 O 5 with Brønsted acidity. Additionally, K-doping makes vanadia species more difficult to reduce than the undoped ones as was shown by TPR [6,26].…”

Transient kinetics of toluene interaction with V/Ti-oxides in anaerobic conditions

“…The presence of K was found on the surface of these catalysts by XPS [12]. It is important that K affects the structure of vanadia species, acid-base properties of the catalysts as well as reactivity of surface oxygen [9,12,14,15].…”

“…For the sub-monolayer catalysts the following bands are present: 1020, 980-1000 and 940-960 cm −1 . They are assigned to the K-perturbed, K-doped monomeric vanadia species and bulk "amorphous" KVO 3 , respectively, basing on the FTRaman and TPR study of the catalysts intentionally doped by potassium [12,15]. Vanadium in all of these species is tetra-coordinated.…”