Deactivation kinetics of V/Ti-oxide in toluene partial oxidation

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Abstract

Deactivation kinetics of a V/Ti-oxide catalyst was studied in partial oxidation of toluene to benzaldehyde (BA) and benzoic acid (BAc) at 523–573 K. The catalyst consisted of 0.37 monolayer of VOx species and after oxidative pre-treatment contained isolated monomeric and polymeric metavanadate-like vanadia species under dehydrated conditions as was shown by FT Raman spectroscopy. Under the reaction conditions via in situ DRIFTS fast formation of adsorbed carboxylate and benzoate species was observed accompanied by disappearance of the band of the monomeric species (2038 cm−1) (polymeric species were not controlled). Slow accumulation of maleic anhydride, coupling products and/or BAc on the surface caused deactivation of the catalyst during the reaction. Temperature-programmed oxidation (TPO) after the reaction showed formation of high amounts of CO, CO2 and water. Rate constants for the steps of the toluene oxidation were derived via mathematical modelling of reaction kinetics at low conversion and constant oxygen/toluene ratio of 20:1. The model allows predicting deactivation dynamics, steady-state rates and selectivity. The highest rate constant was found for the transformation of BA into BAc explaining a low BA yield in the reaction.

Introduction

Direct selective catalytic oxidation of toluene to benzaldehyde (BA) and benzoic acid (BAc) is a reaction of industrial interest. The V/Ti-oxides are known as catalysts for this reaction [1]. However, high yields of BA and BAc have not been reached up today due to the toluene oxidation to COx and H2O.

Steady-state kinetics of toluene oxidation over vanadia catalysts has been a subject of multiple studies [2], [3], [4], [5], [6], [7], but kinetic modelling of the reaction was performed only for bulk crystalline V2O5 as a catalyst [7]. Supporting of vanadia on titania does not only result in advantage in higher surface area but provides the formation of new two-dimensional vanadia species which could be considered as active phase in selective oxidation of toluene [8], [9], [10].

In our earlier work [9], [10], [11], [12], [13], the toluene oxidation over V/Ti-oxides was studied by transient response methods and catalysts were characterised via different techniques including FT Raman spectroscopy, HRTEM, XPS, 51V MAS NMR, TPR and XRD. Deactivation of the catalyst due to carbonaceous residue formation was observed during the reaction [9], but has not been investigated in detail. The basics of deactivation kinetics in heterogeneous catalysis are considered by Froment and Bischoff [14].

The main objective of the present work is to study regularities of deactivation during the toluene oxidation over a well-characterised V/Ti-oxide catalyst. Spectroscopic in situ DRIFT and FT Raman techniques are applied as well as temperature-programmed oxidation (TPO) and transient response method. Conversion and selectivity to BA, BAc and side products (maleic anhydride (MA), COx) were described by a formal kinetic model.

FT-IR spectroscopy has been already applied to study toluene interaction with V/Ti-oxide and oxidation of adsorbed products [15], [16], [17], [18], [19], [20], but up to our knowledge the investigation of the reaction in the presence of gaseous oxygen under dynamic conditions by spectroscopic in situ methods has not been reported yet.

Section snippets

Catalyst preparation

Laboratory TiO2 support was prepared by hydrolysis of the tetrapropyl orthotitanate (>98%, Fluka). XRD and Raman spectroscopy studies showed that the support possesses an anatase structure. Atomic emission spectroscopy did not indicate any potassium and sodium (<0.01 wt.%). Monolayer catalyst with 1.8 wt.% V was prepared via well-known grafting technique [1], [21] by three steps of VOCl3 vapour deposition on the surface of the TiO2, followed by hydroxylation and drying. After the calcination for

Structure of vanadia species and catalytic activity

Raman spectrum of the pre-oxidised catalyst under dehydrated conditions is shown in Fig. 1. The catalyst contains two types of vanadia species: isolated monomeric species (1033 cm−1) and polymeric metavanadate-like species (920 cm−1). In both of these species pentavalent vanadium atoms are tetracoordinated [22]. No bulk crystalline V2O5 (994 cm−1 region) is observed.

Conversion of toluene on time-on-stream is shown in Fig. 2. It is seen that the conversion decreases reaching a steady-state within 80

Conclusions

Deactivation of V/Ti-oxide catalyst has been studied in toluene oxidation by transient response method, DRIFTS in situ and kinetic modelling. Formation of benzoate/carboxylate species is a fast process. Accumulation of MA, coupling products and/or BAc on the surface is a slow process responsible for deactivation. Deactivation is found stronger at lower temperatures. A reaction network has been proposed. Formal rate constants for the routes of the toluene transformation were determined by

Acknowledgements

The authors gratefully acknowledge the Swiss National Science Foundation for the financial support. The work of S.I. Reshetnikov has been supported by the Russian Foundation for Basic Research (Grant no. 01-03-32790).

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