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Free keywords:
selective oxidation; oxidative dehydrogenation; ethane
Abstract:
The oxidation of ethane was studied over a VOx/γ-Al2O3 catalyst (1.4 % V). Ethylene, CO and CO2 were observed as the main products. To clarify the relations in the reaction network additionally the oxidation of ethylene and CO was investigated for the same range of operation conditions.
Based on the results obtained a reaction network of ethane oxidative dehydrogenation can be proposed consisting of five partial reactions. Ethane reacts in two parallel reactions to ethylene (1) and directly to CO2 (2). From the formed ethylene two further parallel pathways lead to CO (3) and CO2 (4). Thus, ethylene oxidation (reaction 3) is the only source for CO observed during ethane oxidation. Finally, the consecutive oxidation of CO to CO2 (5) is a part in the network.
To understand the role of the catalyst, experiments were carried out comparing the performance of the VOx/γ-Al2O3 catalyst with the pure γ-alumina support and a FeOx/γ-Al2O3 catalyst. From these experiments it can be concluded, that ethane conversion correlates with the basic properties of the catalyst. Ethylene selectivity shows an opposite trend. Ethylene formation from ethane consumes lattice oxygen, which can only provided by redox sites of the catalyst. In contrast, deep oxidation reactions of both hydrocarbons (reactions 3 and 4) do not dependent on the presence of redox sites, but only on the presence of gas phase oxygen. Reactions 1 - 4 can be considered as surface reactions. CO oxidation (reaction 5) occurs primarily again via a heterogeneous redox mechanism under consumption of lattice oxygen, but there is additionally non-catalytic gas-phase oxidation. Two sources of carbon formation were identified, ethylene pyrolysis under oxygen absence and Boudouard reactions from CO.
