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Despite of its importance and attractivity to industrial application, the heterogeneously catalyzed selective oxidation of ethane to ethylene is far from being fully exploited [1]. One reason is the complexity of the network of parallel and consecutive reactions via olefins and oxygenation products to CO2 and water. The main objective of the presented project is a detailed description of the performance of an alumina-supported vanadium oxide catalyst and its kinetic behaviour in this reaction network.
Vanadium oxide catalysts are active in partial ethane oxidation in the temperature range from 300-600 °C. While VV oxidation state is responsible for complete oxidation of ethane to CO2, the presence of VIV favours selective oxidation, for which a reaction mechanism
ethane + V(IV) -> ethylene +V(III) – [H]2
V(III) – [H]2 + [O]lattice -> V(IV) + H2O
can be suggested. The experiments clearly identify lattice oxygen as an important component for the oxidative dehydrogenation reaction, and shows that the selectivity towards ethylene increases with decreasing oxygen concentration, as it is to be expected. Yielding in the absence of O2 to C2H4 as the single product. The experiments suggest a sophisticated reaction network, and yield to a reliable number of rate constants, activation energies and prefactors.
For a detailed analysis of the heterogeneously catalyzed reaction supported by the experiments, five different steps were taken into account. Three different types of oxygen species are assumed to be involved:
2 C2H6 + O2 -> 2 C2H4 + 2 H2O (Lattice oxygen) (1)
C2H6 + 3.5 O2 -> 2 CO2 + 3 H2O (Dissociated surface oxygen) (2)
C2H4 + 2 O2 -> 2 CO + 2 H2O (Dissociated surface oxygen) (3)
C2H4 + 3 O2 -> 2 CO2 + 2 H2O (Dissociated surface oxygen) (4)
2 CO + O2 -> 2 CO2 (Non-dissociated surface oxygen) (5)
Based on these reactions and their experimentally determined rate constants, a realistic model for the system could be formed. Reaction 1 can be described by redox mechanism as first proposed by Mars and van Krevelen [2]. The reactions 2 to 5 on the catalyst surface are expressed by Langmuir-Hinshelwood type equations. Details concerning on the reaction models will be given on the poster.
Acknowledgements
The financial support by the German Research Foundation (DFG) is gratefully acknowledged.
References
[1] M.A Banares, Catalysis Today 51 (1999) 319-348.
[2] P. Mars and D.W. van Krevelen, Chem. Eng. Sci. 3 (1954) 41-59.
