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Kinetic studies and impact of vanadia support

The oxidative dehydrogenation of propane (OPD) catalyzed by transition metal oxides was chosen as model reaction for partial oxidation reactions of small hydrocarbons due to its complex reaction network of parallel and consecutive reactions, in which the rate of each single reaction depends very sensitively on the chemical composition, oxidation state and structure of the catalyst. In order to discuss structure-activity relationships, simple activity-selectivity measurements are useless as long as there is no clarity on the molecular reaction mechanism with each catalyst. This project aims in the investigation of the reaction network of ODP over a selected group of vanadia catalysts and the experimental determination of kinetic and thermodynamic parameters. Our criterion for the choice of the catalysts is a very different activity-selectivity relation observed in previous experiments. Industrial vanadia or VPO catalysts may be included in experimental program as references. Since the partial oxidation of hydrocarbons is an extremely exothermic reaction, experiments in tubular reactors are in general disadvantageous for a detailed kinetic analysis. In order to avoid strong temperature profiles in the reactor, reactants have to be diluted or low conversions have to be adjusted. Better experimental conditions are ensured by a gradient-free differential reactor, in which the reaction rate can be measured directly under isothermic conditions. On the basis of results from the partial project B3, an experimental set-up was developed including a differential reactor, which allows measurements in a wide range of experimental conditions (partial pressures, temperature, amount of catalyst). The temperature inside the reactor is controlled by a combination of wall cooling and pre-heating/cooling of the reactant mixture. Model development and determination of kinetic and thermodynamic parameters give independant data for a discussion of the results of structure analysis and theoretical calculation.
In a second part of the project, niobia and niobic acid catalysts will be prepared on porous support and also tested for its activity in ODP. The experimental program begins with the variation of the support in order to test the influence of the support material on the catalytic activity.

The potential of microreaction technology in industrial applications

Microreaction technology is one of the fastest growing fields in modern chemical reaction engineering. Due to exceptionally high surface-to-volume ratios mass and heat transfer is much better than in conventional reactors. Even extremely endo- and exothermic reactions can be carried out under isothermal conditions thereby dramatically improving selectivity and yield. For these advantages microreactors have become a valuable tool for kinetic studies and process development on a laboratory scale.
So far only few attempts have been made to apply microreaction technology in industrial processes. Low capacities, lack of standardization and missing financially feasible concepts have prevented a major breakthrough of this new technology in chemical industry. Therefore a thorough analysis towards the potential of microreactors is needed.
As a model reaction for heterogeneous catalyzed gas-phase reactions the oxidative dehydrogenation of propane is investigated in a microreactor. Alumina supported vanadium serves as the catalytic active component. In a first step experimental results yielding from the microreactor will be compared with results from a conventional tubular reactor to prove the advantageous characteristics of microreaction technology in general. In particular it will be shown that isothermal conditions can be obtained even for high conversion rates. Secondly optimal reaction parameters for maximum propene yield will be determined using both experimental data and computer modeling. Factors controlling selectivity and yield include temperature, residence time, composition of gas mixture and coating method of the reactor’s interior surface.
Based on a thorough understanding of the underlying reaction technical and economic aspects of today’s chemical production environment have to be analyzed to identify crucial factors for a successful introduction of microreaction technology in industrial applications. These factors will be summarized in a list of criteria to simplify and standardize the assessment of microreactors being a suitable technology for industrial applications.

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