Structure and reaction studies on vanadium molybdenum mixed oxides
The correlation of the structure and the catalytic activity of vanadium molybdenum oxide catalysts were examined by X-ray diffraction and temperature programmed reduction measurements. The experiments revealed the presence of two metastable vanadium molybdenum mixed oxides, h-(V,Mo)O3 and (V,Mo)2O5, and an optimal vanadium molybdenum ratio of 3:7 for the yield of acrylic acid for acrolein.
Introduction
The V–Mo–O system was chosen to determine the relation of structure and catalytic selectivity. As an example the production of acrylic acid by partial oxidation of acrolein is selected. This process is well understood on a macroscopic scale [1], and kinetic measurements indicate that the reaction can be described by the Mars-van-Krevelen mechanism proposed almost 50 years ago [2].
In contrast, on a microscopic scale, the single steps of the mechanism are not known in detail, and not even the catalytic active phase(s) or site(s) have been identified unambiguously.
In previous works different oxide species are reported to be part of the active phase system in these catalysts. Andrushkevich [3] revealed MoO3 and V2O4 as major components of the V–Mo–O-catalysts and reported that the catalytic activity is related to the content of V4+. Tichy et al. [4] identified VMo3O11, whereas Schlögl and coworkers [5] have proposed VMo4O14 as active phase. Both mixed oxides build layer structures, belonging to the shear structures. Schlögl and coworkers [5] explain that these structures are able to integrate and remove oxygen by a transition from corner linked octahedra into edge sharing regions. Also pure vanadiumpentoxide is used as catalyst in industrial processes, a common example is the production of sulphuric acid.
To determine the catalytically active phases and the conditions of their formation and transformation samples are prepared differently. One mixed oxide series was fused from the metal oxides, vanadium pentoxide and molybdenum trioxide, in order to explore the thermodynamic stable phases. Two other sample series were prepared by crystallisation or spray-drying from the ammonium salt solution followed by a calcination step, a common way to prepare catalysts for industrial use. In the mixed oxide system under investigation the vanadium to molybdenum ratio was varied over the complete composition range. Phase analysis by X-ray diffraction was carried out for every sample as well as a characterisation by temperature programmed reduction (TPR).
Section snippets
Sample preparation
Three groups of samples prepared by different routes were examined. In this paper the sample composition is always given by VxMoyOz, with x+y=1; z=2.5x+3y. Oxygen defects are assumed but not taken into account.
The first group of samples was prepared by a combination of liquid and solid phase reaction (group 1). Mixtures of the pure oxides V2O5 (Alfa Aesar, >99.995%) and MoO3 (Alfa Aesar, >99.998%) were fused in the desired composition at 800 °C in a sealed silica ampoule to obtain a homogeneous
Phase composition
The X-ray analysis of the group 1 samples are in a good agreement with the phase diagram by Volkov et al. [8] (cf. Fig. 1). Two vanadium molybdenum mixed oxides are described: An α-phase as a molybdenum-doped vanadium pentoxide and a β-phase given as V7/3−yMo5/6−1/2yO8 with 0.33≤y≤0.40.
The results of the XRD experiments revealed the presence of the α-phase up to a molybdenum level of y=0.08 with the structure of vanadium pentoxide [9]. Vanadium pentoxide crystallises in the orthorhombic system,
Conclusions
Our experiments showed that the preparation route of fusing the pure oxides lead to thermodynamically stable mixed oxides, which do not produce a significant amount of acrylic acid for acrolein.
The catalysts prepared by the precursor route consist primarily of two different vanadium molybdenum mixed oxides, a hexagonal h-(V,Mo)O3 and (V,Mo)2O5, which is structurally related with vanadium pentoxide. The TPR-measurements with 5% acrolein in inert gas as reducing argent reveal an optimal
Acknowledgements
The authors would like to thank Dr. H. Ehrenberg and Dr. A. Drochner (TU Darmstadt) for helpful discussions. Financial support by the Deutsche Forschungsgemeinschaft DFG, SPP 1091, and the Fonds der Chemischen Industrie is gratefully acknowledged.
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