Selective oxidation of ethane over a VOx/gamma-Al2O3 catalyst: investigation of the reaction network

The oxidation of ethane was studied over a VOx/gamma-Al2O3 catalyst (1.4% V) in a laboratory fixed bed reactor. Ethylene, CO and CO2 were observed as the main products. To analyze the reaction network the oxidation of ethylene and CO was investigated in the same range of operation conditions. Based on the results obtained, a reaction network for 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 over-oxidation is the source for CO observed during ethane oxidation. Finally, the consecutive oxidation Of CO to CO2 (5) is a part of the network. To understand the role of the catalyst, experiments were carried out comparing the performance of the VOx/gamma-Al2O3 catalyst with that of the pure gamma-alumina support and a FeOx/gamma-Al2O3 catalyst. From these experiments it can be concluded that ethane conversion is correlated with the Broenstedt basic properties of the catalyst. Ethylene selectivity reveals an opposite trend. Ethylene formation from ethane consumes lattice oxygen, which can only be provided by redox sites of the catalyst. In contrast, deep oxidation reactions of both hydrocarbons (reactions 2-4) do not depend on the presence of redox sites, but only on the presence of gas phase oxygen. These deep oxidation reactions can be considered as surface reactions. Finally, CO oxidation (reaction 5) occurs primarily again under consumption of lattice oxygen, but under oxygen excess conditions also through surface reactions. In addition, two sources of carbon formation were identified, ethylene pyrolysis under oxygen absence and Boudouard reactions. For the network of the five main reactions, kinetic parameters were estimated. Reaction 1 was described by a Mars van Krevelen mechanism, the reactions 2-5 by Langmuir Hinshelwood equations. Applying this model a satisfactory fit of all experimental data was obtained