Heterogeneous-homogeneous catalytic complications

All of the above discussion is strictly applicable only to homogeneous gas phase reactions. Usually the above considerations do apply reasonably well to non-polar liquids and nonpolar solutions, although normal Z values may be an order of magnitude less than for gas reactions. Reactions in solids are often much more complex, since they are usually heterogeneous, involve catalytic effects, reactions at preferential sites (dislocations, etc), and nucleation phenomena. These complicated processes are quite beyond the scope of the present article. For some description of these phenomena, and further references, the reader should consult Refs 9, 10 11... 

In contrast to heterogeneous catalysis, classical homogeneous catalysis takes place in the bulk of a solvent. However, due to the lack of a phase boundary it is much more complicated in this case to isolate the fully dissolved transition metal complex from the desired reaction product. Often, distillation fails due to the thermal instability of the dissolved catalyst or the fact that unselective reactions occur in the bottom of the distillation column. Catalyst recovery and recycling strategies for homogeneous catalytic processes can therefore sometimes be rather complicated... 

Kinetics of heterogeneous catalysis has received much attention. Its mathematical theory is well advanced, largely thanks to extensive work of Boudart, Temkin, and others. On the other hand, heterogeneous catalysis has to deal not only with the same kind of difficulties homogeneous catalysis faces, but with the added complications of surface properties, adsorption/desorption equilibria and rates, and mass transfer to and from catalytic sites, phenomena whose effects often are more important than those of actual kinetics of the reaction on the surface. 

Studies on the mechanisms of catalytic and non catalytic reac tions undertaken over the past 15-20 years have led to significant progress in the theory of reaction mechanisms. Most of the reactions involving homogeneous, metal-complex, and enzymatic catalyses were shown to be no less complex in terms of their mechanism compared with the mechanisms of radical chain processes. Infact, they appear to be much more complicated. Numerous examples of complicated mechanisms can be found in the literature. At present, multiroute mechanisms (with 2 to 4 reaction routes), involving as many as 8 intermediates and up to 12 elementary steps, are widely known to exist even in heterogeneous catalysis by metals and nonmetals where the simplest two-step schemes have hitherto been very popular. The existence of many routes and elementary steps is the most important general feature of the mechanisms of catalytic and also many noncatalytic reactions. 

Heterogeneous catalysis has to deal not only with the catalyzed reaction itself but, in addition, with the complexities of surface properties (different crystal surfaces, different catalytic sites), possible segregation of adsorbates (so-called island formation), contamination or deterioration of catalytic sites, and adsorption and desorption equilibria and rates. Moreover, mass transfer to and from the reaction site is a factor more often than in homogeneous catalysis. In practice, these complications may affect behavior more profoundly than does the kinetics of the surface reaction itself. A practical and balanced kinetic treatment therefore uses simplifications and approximations much more generously than was done in the preceding chapters. Excellent textbooks on the subject are available [G1-G7], so coverage here can remain restricted to a critical overview and indications showing when and how concepts and methods developed in the earlier chapters can be useful. 

Kinetics. If the kinetics and mechanism of the reaction in homogeneous solution are known, they can be translated at least approximately into the heterogenized system [30,37]. The principal difference between the homogeneous and heterogenized reactions is that the catalyst is distributed uniformly over the entire system in the former, but is concentrated in a part of it in the latter, a distinction of physics rather than chemistry. Provided the reaction is first order in the catalyst, as is usually true, the same amount of catalytically active species—say, hydrogen ions or complexed cobalt metal atoms—could be expected to produce approximately the same rate in both systems. This is a very crude, but useful hypothesis to start with. Complicating facets of the heterogenized reactions are ... 

The papers in the present volume include contributions dealing with catalytic effects in homogeneous media which, in some respects, are less complicated than heterogeneous catalytic reactions. 

A mechanism of a catalytic reactions is a sequence of elementary steps, the rate of which can be described by for instance, transition state theory. Catalytic specie react with a catalyst forming complexes as described in Chapters 5-7 respectively for homogeneous, enzymatic and heterogeneous catalysis. From these rather complicated reaction sequences rate laws should be derived which could be then compard with experimental data. 

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