Homogeneous catalyst kinetic model

In the majority of catalytic reactions discussed in this chapter it has been possible to rationalize the reaction mechanism on the basis of the spectroscopic or structural identification of reaction intermediates, kinetic studies, and model reactions. Most of the reactions involve steps already discussed in Chapter 21, such as oxidative addition, reductive elimination, and insertion reactions. One may note, however, that it is sometimes difficult to be sure that a reaction is indeed homogeneous and not catalyzed heterogeneously by a decomposition product, such as a metal colloid, or by the surface of the reaction vessel. Some tests have been devised, for example the addition of mercury would poison any catalysis by metallic platinum particles but would not affect platinum complexes in solution, and unsaturated polymers are hydrogenated only by homogeneous catalysts. 

The kinetic modeling study of l-(4-isobutylphenyl)ethanol (IBPE) carbonylation nsing a homogeneous palladium complex has been reported by Chaudhari. The three key steps (formation of the active substrate formation of the active catalyst catalytic carbonylation of active substrate) were studied in detail. The average carbonylation rate depends on several factors, so a dynamic analysis, where the concentrations of both the catalyst species and the intermediates were varied, was carried out. 

The units of space velocity are the reciprocal of time. Usually, the hourly volumetric feed-gas flow rate is calculated at 60 °F (15.6 C) and 1.0 atm (1.01 bar). The volumetric liquid-feed flow rate is calculated at 60 F (15.6 °C). Space velocity depends on the design of the reactor, reactor inlet conditions, catalyst type and diameter, and fractional conversion. Walas [7] has tabulated space velocities for 102 reactions. For exanple, for the homogeneous conversion of benzene to toluene in the gas phase, the hoiuly-volumetric space velocity is 815 h . This means that 815 reactor volumes of benzene at standard conditions will be converted in one hoiu. Although space velocity has limited usefulness, it allows estimating the reaction volume rapidly at specified conditions. Other conditions require additional space velocities. A kinetic model is more useful than space velocities, allowing the calculation of the reaction volume at different operating conditions, but a model requires more time to develop, and frequently time is not available. 

Many models use only simple kinetics to describe the gas-phase reactions without taking radical transfer between the two phases into account. This may be an oversimplification, since radical transfer may strongly influence the results of the calculations. Moreover, homogeneous combustion kinetics, even without the presence of a catalyst, are extremely complex, with already over 300 known primary reactions for methane. 

The first kinetic model for propagation in homogeneous systems was proposed by Ewen [47], assuming that the propagation took place as shown in Fig. 9.18. This scheme, shown for Cp2Ti(IV) polymerization of propylene, is representative of the kinetics for dl of the polymerizations with Group IVB metallocenes. In the scheme, species 1 and 4 represent coordinatively unsaturated Ti(IV) complexes that are-formally 16-electron pseudo-tetrahedral species, species 2 represents the interacting catalyst/cocatalyst combination, while intermediate 3 is shown with the monomer coordinated... 

Further development of kinetic models for the OCM process followed the path of addition of a limited number of heterogeneous steps (first of all— initiation or generation of primary methyl radicals) to homogeneous schemes of methane oxidation (Aparicio et al, 1991 Hatano et al, 1990 McCarty et al, 1990 Shi et al, 1992 Vedeneev et al, 1995 Zanthoff and Baerns, 1990). There was certain logic in such an approach since the most efficient OCM catalysts are almost exclusively oxides with no transition metal ions (some Mn-contain-ing oxide systems are the only exception), any reactions in adsorbed layers at such temperatures can be neglected. In the framework of such models some substantial features of the process could be described. For instance, they predicted the limit in the C2-hydrocarbon yield close to that reliably observed experimentally over the most efficient catalysts (20-25%). 

By combining the homogenous catalytic mechanism with interfacial reaction and assuming the addition of olefin to the active catalyst species as the rate-controlling step, a semi-empirical kinetic model could also be proposed, which was also in good agreement with the experimental data. 

D+ = Ru(bpy)3+, Mn04, IO4, Pb02, etc.) catalysed by homogeneous catalysts (Co2+, Mn++ generated in situ). The kinetics and mechanism of manganese system Mn2+/ MnO " has been investigated in detail as a suitable model for the role of Mn in photosynthesis. 

In the section 2.3. we presented catalysis by complexes of transition metals and argued that this type of homogeneous catalysis is gaining importance in industrial processes. Kinetic models of several reactions will be considered in the next paragraph. Here we would like to address reactions where the metal ions act as catalysts. As an example of such a process, hydrogen peroxide decomposition will be considered. Transition metals, mainly manganese, iron and copper, are effcient catalysts for this reaction. 

When studying the kinetics of heterogeneous reactions or when designing a large catalytic reactor, there are more factors to consider than when dealing with homogeneous reactions. For a solid-catalyzed reaction, the rate depends on the reactant concentrations at the catalyst surface, but these are not the same as the bulk concentrations, because some driving force is needed for mass transfer to the surface. If the catalyst is porous, as is usually the case, there are further differences in the concentration between the fluid at the external surface and the fluid in the catalyst pores. Models must be developed to predict the surface concentrations as functions of the partial pressures or concentration in the gas or liquid, and the rate expression can then be written in terms of the fluid concentrations. 

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