Catalytic multiple reactions

Equation 8.3.4 may also be used in the analysis of kinetic data taken in laboratory scale stirred tank reactors. One may directly determine the reaction rate from a knowledge of the reactor volume, flow rate through the reactor, and stream compositions. The fact that one may determine the rate directly and without integration makes stirred tank reactors particularly attractive for use in studies of reactions with complex rate expressions (e.g., enzymatic or heterogeneous catalytic reactions) or of systems in which multiple reactions take place. 

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

Nucleophilic additions of organorhodium species to C=0 and CN multiple bonds constitute important classes of catalytic organic reactions and have experienced significant progress in the last decade. 

The accessible peripheral catalytic groups enable reaction rates that are comparable to those of homogeneous systems, but the periphery-functionalized dendrimers contain multiple reaction sites and may have extremely high local catalyst concentrations, which can lead to cooperative effects in reactions that proceed via a... 

All of the preceding work was for simple, or one step, reactions. The more interesting case of multiple reactions has been studied by de Maria et al. (D15) and by Tichacek (T7). de Maria et al. considered the catalytic oxidation of naphthalene. They found that the consideration of the dispersion effects enabled them to obtain a better design. Tichacek considered the selectivity for several different types of reactions. Naturally, the results were rather complicated, and the statement of general conclusions is rather diflBcult. For small values of the reactor dispersion group, Dl/uL < 0.05, it was found that the fractional decrease in the maximum amount of intermediate formed is closely approximated by the value of Dl/uL itself. For other ranges of the parameters, we refer to the original work (T7). 

These considerations are only valid for isothermal reactors, and we shall see in the next two chapters that the possibility of temperature variations in the reactor can lead to much more interesting behavior. We will also see in Chapter 7 that with catalytic reactors the situation becomes even more complicated. However, these simple ideas are useful guides in the choice of a chemical reactor type to carry out multiple-reaction systems. We will stiU use these principles as the chemical reactors become more complicated and additional factors need to be included. 

In catalytic distillation the temperature also varies with position in the column, and this will change the reaction rates and selectivities as well as the equilibrium compositions. Temperature variations between stages and vapor pressures of reactants and products can be exploited in designing for multiple-reaction processes to achieve a high selectivity to a desired product with essentially no unwanted products. 

We regard the essential aspects of chemical reaction engineering to include multiple reactions, energy management, and catalytic processes so we regard the first seven chapters as the core material in a course. Then the final five chapters consider topics such as environmental, polymer, sohds, biological, and combustion reactions and reactors, subjects that may be considered optional in an introductory course. We recommend that an instmctor attempt to complete the first seven chapters within perhaps 3/4 of a term to allow time to select from these topics and chapters. The final chapter on multiphase reactors is of course very important, but our intent is only to introduce some of the ideas that are important in its design. 

Sundmacher K, Uhde G, Hoffmann U. Multiple reactions in catalytic distillation processes for the production of fuel oxygenates MTBE and TAME analysis by rigorous model and experimental validation. Chem Eng Sci 1999 54 2839-2847. 

Schrock, R.R. (2006) Multiple metal-carbon bonds for catalytic metathesis reactions (Nobel Lecture). Angew. Chem. Int. Ed., 45, 3748. 

Agnew and Potter [6] did the same as Barkelew for heterogeneous catalytic reactors and presented design diagrams to prevent runaway, including also the parameter of the ratio of the tube to the catalyst particle diameters dt/dp. Burghardt and Warmuzinski [7] considered multiple reactions and also took the heat effect of the secondary reaction into account however, they did not study the selectivities achieved in the reactor. 

A familiar example of multiple reaction paths is the catalytic oxidation of ethylene to ethylene oxide, where the by-products water and carbon dioxide are produced both by direct oxidation of ethylene and by further oxidation of ethylene oxide. For this example, the equations may be written in the form... 

Schrock, R. R. Multiple Metal-Carbon Bonds for Catalytic Metathesis Reactions (Nobel Lecture 2005). Adv. Synth. Catal. 135. 2007 349 41-53. 

Catalytic aldol reactions are among the most useful synthetic methods for highly stereo-controlled asymmetric synthesis. In this account we discuss the recent development of a novel synthetic technique which uses tandem enzyme catalysis for the bi-directional chain elongation of simple dialdehydes and related multi-step procedures. The scope and the limitations of multiple one-pot enzymatic C-C bond formations is evaluated for the synthesis of unique and structurally complex carbohydrate-related compounds that may be regarded as metabolically stable mimetics of oligosaccharides and that are thus of interest because of their potential bioactivity. 

Pyrolytic reactions can appear to be much more complicated compared to other reactions. However, this is mainly due to subsequent reactions that occur after the initial elimination step. A common cause of this problem is related to the fact that the reactions do not actually take place in ideal gas phase. Some pyrolytic processes may take place in true condensed phase. Multiple reaction paths and the interaction of the resulting molecules are, therefore, inevitable. Also, additional issues may affect the practical results of a pyrolysis. Some are related to the fact that the true pyrolysis can be associated with reactions caused by the presence (intentional or not) of non-inert gases such as oxygen or hydrogen that may be present during the heating. Also, the pyrolyzed materials may be in contact with non-inert surfaces that can have catalytic effects. In order to diminish these effects in the pyrolysis done for analytical purposes, an inert gas frequently is present during the pyrol ic reaction. 

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