Dehydrogenation kinetic rate data

As a practical example of the interpretation of kinetic rate data to distinguish between more than one reaction mechanism, consider the dehydrogenation of ethanol (i.e., CH3CH2OH, [A]) to acetaldehyde (i.e., CH3CHO, [C]) and hydrogen (i.e., H2, [D]). The reversible chemical reaction that occurs on a catalytic... 

Note that the mathematical form of the model implies that the rate-limiting step is a dual-site surface reaction between chemisorbed hydrogen and chemisorbed butyraldehyde, and that the reverse reaction is the monomolecular dehydrogenation of chemisorbed butanol. Models of this sort should not be overinterpreted from a mechanistic standpoint. Kinetics models are at best ambiguous indicators of mechanism in that several models typically fit the data equally well. 

A quantitative kinetic model, denominated TC4, for the catalytic conversion of n-butane is proposed. The model considers 56 elementary reactions, six of them were chosen to occur in heterogeneous phase. The TC4 model can be used to predict the product distribution and the heterogeneous rate constants for a wide range of conditions and on different catalyst types. The model can fit also the experimental data from the isobutane dehydrogenation reaction. A plot, that we have denominated "the graphic s performance of a catalyst", is proposed for the evaluation of the maximum yield of a catalyst with a minimum of experimental data. 

The data required for a kinetic formulation of the deactivation of the main reaction are probably best collected in a differential reactor. Some extrapolation to zero time is required when the reaction rate of the main reaction cannot be observed at zero coke content. The procedure can be hazardous with very fast coking, of course. In their study of butene dehydrogenation, Dumez and Froment [1976] were able to take samples of the exit stream of stabilized operation of the fixed bed reactor after 2 minutes, while the observations extended over more than 30 minutes. 

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