Kinetics in Heterogeneous Systems

In this Chapter, a heterogeneous system is one in which the reactants are present in at least two phases. The discussion will concentrate on two such conditions, two-phase gas/liquid systems and three-phase gas/liquid/solid systems. Chemists tend to favor homogeneous conditions, with the reactants all in one phase, because they provide more controlled and reproducible conditions. However, heterogeneous conditions are often preferred in industrial processes because of the ease of separating the catalyst from the products. In many mechanistic studies, heterogeneity adds a complicating feature to be avoided, but there are times when this cannot be done, or when it happens unexpectedly. 

In gas/liquid systems, the gas often has limited solubility in the liquid which contains the other reagents. As a consequence, there can be problems of mass transport of the gaseous reactant from the gas to the liquid phase. Mass transport can limit the concentration of the gas in the liquid and/or become a rate-limiting feature of the system. These features can confuse interpretations of product distributions and rate laws. 

The gas/liquid/solid systems generally involve reactants in the gas and liquid phases and a catalyst as the solid phase. In some cases, the solid may be produced from initially homogeneous conditions, and a question arises as to whether the real catalyst is the original species added or the solid product formed under the reaction conditions. There are further questions about the factors that may control the rate of the catalytic process. 

In the chemistry laboratory, these systems ate most often encountered with the gases Hj or CO reacting with substrate and possibly a catalyst in the liquid phase. For the mechanistic interpretation of kinetic observations, an important factor is the rate of mass transfer of the gas to the liquid phase. The rate of gas absorption into the liquid is typically represented as a first-order process, driven by the difference between the saturated gas concentration [G(o]f and the concentration at any time [G(ol, as given by 

Application of mass balance conditions to the total moles of G in the gas and liquid phases at any time and at the end of the process, along with the ideal gas law gives 

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Giovanoli, R. Briitsch, R. (1974) Dehydration of y-FeOOH Direct observation of the mechanism. Chimia 28 188-191 Giovanoli, R. Briitsch, R. (1975) Kinetics and mechanisms of the dehydration of y-FeOOH. Thermochim. Acta 13 15-36 Giovanoli, R. Cornell, R.M. (1992) Crystallization of metal substituted ferrihydrites. Z. Pflanzenemahr. Bodenk. 155 455-460 Giovanoli, R. Briitsch, R. Stadelmann, W. (1975) Thermal decomposition of y- and a-FeOOH. In Barrett, P. (ed.) Reaction kinetics in heterogeneous systems. Elsevier Amsterdam, 302-313... 

Description of polymerization kinetics in heterogeneous systems is complicated, even more so given that the structure of complex formed is not very well defined. In template polymerization we can expect that local concentration of the monomer (and/or initiator) can be different when compared with polymerization in the blank system. Specific sorption of the monomer by macromolecular coil leads to the increase in the concentration of the monomer inside the coil, changing the rate of polymerization. It is a problem of definition as to whether we can call such a polymerization a template reaction, if monomer is randomly distributed in the coil on the molecular level but not ordered by the template. 

Perhaps one of the most unknown areas in kinetics of soil chemical processes is redox dynamics (Chapter 8). Some work on reductive dissolution of manganese oxides [Mn(III/IV)] and oxidation of As(V), Cr(III), and Pu(III/IV) by oxides has appeared. However, a comprehensive understanding of redox kinetics in heterogeneous systems is lacking. 

To look at the kinetics in heterogeneous systems, we consider the step of adsorbing a gaseous molecule A onto an active site s to form an adsorbed species As. The adsorption rate constant is ka. The process is reversible, with a desorption rate constant kd ... 

Effect of mass transfer on kinetics in heterogeneous systems... 

When chemical reactors have more than one phase, the problem increases in complexity because the reaction and mass transfer processes interact and another time constant is introduced. The interaction is governed by the relative rates of the reaction and mass transfer. In some cases, chemical reactions are mass transfer rate controlled (very fast chemical reactions) and in others, they are reaction kinetics controlled (very slow chemical reactions) however, in reality very few reactions strictly fit this classification. Thorough discussions of this problem are given in Refs. . Classifications of the relative contributions of mass transfer and reaction kinetics in heterogeneous systems... 

Latex particle size is a key parameter for emulsion polymerization, with strong impact on the final properties of the polymers synthesized and, in general, on the polymerization kinetics in heterogeneous systems. An experimental methodology which combines a continuous-flow system and an ESR time sweep experiment was extended by Parker et al. [144] to study the effects of varying latex particle size on polymerization kinetics andradical distribution in the case of MMA emulsion... 

Astarita (1967), Levenspiel (1972), and Doraiswamy and Sharma (1984) describe an effective framework in which to evaluate the relative contributions of mass transfer and reaction kinetics in heterogeneous systems. This classification is as follows. 

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