Microkinetic analyses, catalytic reaction

In 2001, Mirodatos et al. [89] stressed the importance of transient studies as an alternative to steady continuous reactor operations. A combination of microkinetic analysis together with transient experiments should allow the determination of the global catalytic conversion from elementary reaction steps. Prerequisite for such analysis is the correlation of experimental data with the data of a model. Compliance between the data helps to derive the reaction mechanism. 

More recently a hybrid approach to computer-assisted catalyst synthesis, based on a microkinetic analysis of the catalytic reaction, has been put forward which comprises essentially deterministic but also some non-deterministic features. For the synthesis of a catalyst with high activity and selectivity for a given reaction, the application of a microkinetic analysis has been suggested by Dumesic and co-workers [43-45]. The derivation of the microkinetics is not necessarily based on detailed kinetic experimentation but, by analogy, to similarities with other known catalytic processes. In an ideal situation, the microkinetics of a catalytic reaction are completely defined according to Dumesic and his collaborators when ... 

Dumesic and co-workers have presented various case studies in which they have applied the above concept of microkinetic analysis in order to propose new catalysts or to explain the different catalytic performance of different catalyst formulations for given reactions ... 

The microkinetic analysis is certainly a scientifically interesting approach which will contribute to the identification and selection of catalytic compounds even in more complex situations as described above. One problem still to be solved is the experimental procurement and/or estimation of the parameters used in microkinetic simulations, which limits the wide applicability of the method. Providing kinetic parameters for a complex reaction network from kinetic experiments for an analogous catalyst is a time-consuming process. Despite the availability of modem experimental equipment and efficient computers, a complex reaction demands at least one man year of work [51]. The estimation of parameters by ab initio or semiempirical methods has to be considered with caution because ideal surfaces are usually assumed. 

As applied to catalysis, the microkinetic analysis of catalytic reactions is used most often. This is an instrument of an idealized description of com plex catalytic processes without consideration of the mass transfer that can affect considerably the observed kinetics of the catalytic transformations. The microkinetic analysis with the necessary consideration of the active sites balance for all types of active centers of the catalyst, even though it has several drawbacks, can provide important information about the potential influence of the very different thermodynamic factors. 

Some specific features of catalytic reactions, which are identified via the microkinetic analysis and are what make catalytic and noncatalytic reac tions qualitatively different, are discussed following. One is possible nonco incidence of the rate limiting steps (the process bottleneck ) and rate determining steps, the parameters of the latter being directly present in the expressions that describe the stationary rate of the stepwise process. 

In catalytic stepwise reactions, which involve more complex elementary transformations than scheme (4.4), the rate-determining parameters can be identified through similar considerations. Several examples of simple model schemes of catalytic transformations are given following. These schemes often are used for the microkinetic analysis of particular catalytic transformations and help to reveal the influence of various factors. 

Hydrogenation is an important industrial reaction that often requires the presence of a heterogeneous catalyst to achieve commercial yields. Ethylene, C2H4, is the smallest olefin that can be used to investigate the addition of hydrogen atoms to a carbon-carbon double bond. Even though many experiments and theoretical studies have been carried out on this simple system, the reaction is still not completely understood. Microkinetic analysis provides insights into the relevant elementary steps in the catalytic cycle. 

To conclude, I offer first an encouraging note and finally a word of caution. If during an on-going microkinetic analysis, one of the prospective elementary steps is severely limited by thermodynamics, kinetic coupling may well overcome this apparent obstacle. Yet, it still appears that the discovery of new catalytic reactions and of new catalysts will continue to be the creation of a lone investigator, with unfettered vision and the luck that Pasteur promised to the prepared mind... 

Microkinetics as defined by Dumesic, is the examination of catalytic reactions in terms of elementary chemical reactions that occur on the catalytic surface and their relation with each other and with the surface during a catalytic cycle. This definition can easily be expanded into covering non-catalytic systems as well. Microkinetics, for the most part, has focused on analysis or understanding of the reaction mechanism. The approach, however, also holds the promise of being used to aid in the synthesis of new materials. Microkinetic modeling is now an important tool for many of the practicing reaction engineers. This approach enables one to formulate and follow the detailed concentration profile for most if not all of the reaction intermediates. 

For the heterogeneous catalytic process to be effective, the reactants present in the surrounding fluid phase must be transported to the surfece of the solid catalyst, and after the reaction, the products formed must be carried back from the surface to the bulk fluid. The path of the physical rate processes at the particle scale is divided into two parts, as depicted in the 7-step sequence of the continuous reaction model used in microkinetic analysis ... 

Part II Building on Fundamentals is devoted to skill building, particularly in the area of catalysis and catalytic reactions. It covers chemical thermodynamics, emphasizing the thermodynamics of adsorption and complex reactions the fundamentals of chemical kinetics, with special emphasis on microkinetic analysis and heat and mass transfer effects in catalysis, including transport between phases, transfer across interfaces, and effects of external heat and mass transfer. It also contains a chapter that provides readers with tooisfor making accurate kinetic measurements and analyzing the data obtained. 

The models developed here account for unmeasurable intermediates such as adsorbed ions or free radicals. Microkinetic analysis, pioneered by Dumesic and cowokers"", is an example of this approach. It quantifies catalytic reactions in terms of the kinetics of elementary surface reactions. This is done by estimating the gas-phase rate constants from transition state theory and adjusting these constants for surface reactions. For instance, isobutane cracking over zeolite Y-based FCC catalysts has 21 reversible steps defined by 60 kinetic parameters." The rate constants are estimated from transition state theory... 

Sundmacher and Qi (Chapter 5) discuss the role of chemical reaction kinetics on steady-state process behavior. First, they illustrate the importance of reaction kinetics for RD design considering ideal binary reactive mixtures. Then the feasible products of kinetically controlled catalytic distillation processes are analyzed based on residue curve maps. Ideal ternary as well as non-ideal systems are investigated including recent results on reaction systems that exhibit liquid-phase splitting. Recent results on the role of interfadal mass-transfer resistances on the attainable top and bottom products of RD processes are discussed. The third section of this contribution is dedicated to the determination and analysis of chemical reaction rates obtained with heterogeneous catalysts used in RD processes. The use of activity-based rate expressions is recommended for adequate and consistent description of reaction microkinetics. Since particles on the millimeter scale are used as catalysts, internal mass-transport resistances can play an important role in catalytic distillation processes. This is illustrated using the syntheses of the fuel ethers MTBE, TAME, and ETBE as important industrial examples. 

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