Heterogeneous catalysis catalyst types

Diffusivity and tortuosity affect resistance to diffusion caused by collision with other molecules (bulk diffusion) or by collision with the walls of the pore (Knudsen diffusion). Actual diffusivity in common porous catalysts is intermediate between the two types. Measurements and correlations of diffusivities of both types are Known. Diffusion is expressed per unit cross section and unit thickness of the pellet. Diffusion rate through the pellet then depends on the porosity d and a tortuosity faclor 1 that accounts for increased resistance of crooked and varied-diameter pores. Effective diffusion coefficient is D ff = Empirical porosities range from 0.3 to 0.7, tortuosities from 2 to 7. In the absence of other information, Satterfield Heterogeneous Catalysis in Practice, McGraw-HiU, 1991) recommends taking d = 0.5 and T = 4. In this area, clearly, precision is not a feature. 

This type of co-catalytic influence is well loiown in heterogeneous catalysis, in which for some reactions an acidic support will activate a metal catalyst more efficiently than a neutral support. In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transition metal catalysts dissolved in it. 

Normally in heterogeneous catalysis compensation effect behaviour is obtained either for the same reaction upon using differently prepared catalysts of the same type, or with the same catalyst upon using a homologous set of reactants. In the case of electrochemical promotion (Figs. 4.38 and 4.39) one has the same catalyst and the same reaction but various potentials, i.e. various amounts of promoter on the catalyst surface. 

Catalytic transformations can be divided on the basis of the catalyst-type - homogeneous, heterogeneous or enzymatic - or the type of conversion. We have opted for a compromise a division based partly on type of conversion (reduction, oxidation and C-C bond formation, and partly on catalyst type (solid acids and bases, and biocatalysts). Finally, enantioselective catalysis is a recurring theme in fine chemicals manufacture, e.g. in the production of pharmaceutical intermediates, and a separate section is devoted to this topic. 

In any catalyst selection procedure the first step will be the search for an active phase, be it a. solid or complexes in a. solution. For heterogeneous catalysis the. second step is also deeisive for the success of process development the choice of the optimal particle morphology. The choice of catalyst morphology (size, shape, porous texture, activity distribution, etc.) depends on intrinsic reaction kinetics as well as on diffusion rates of reactants and products. The catalyst cannot be cho.sen independently of the reactor type, because different reactor types place different demands on the catalyst. For instance, fixed-bed reactors require relatively large particles to minimize the pressure drop, while in fluidized-bed reactors relatively small particles must be used. However, an optimal choice is possible within the limits set by the reactor type. 

Transition metal NPs dispersed in ILs are recognized as suitable catalyst systems for many reaction types with both typical homogeneous and heterogeneous catalysis (see Sections 1.3.1-1.3.5). The most investigated reactions are the hydrogenation of multiple bonds and arenes [1, 5, 12, 13, 43, 44, 54, 80, 89, 92], carbon-carbon cross-... 

It is true, however, that many catalytic reactions cannot be studied conveniently, under given conditions, with usual adsorption calorimeters of the isoperibol type, either because the catalyst is a poor heat-conducting material or because the reaction rate is too low. The use of heat-flow calorimeters, as has been shown in the previous sections of this article, does not present such limitations, and for this reason, these calorimeters are particularly suitable not only for the study of adsorption processes but also for more complete investigations of reaction mechanisms at the surface of oxides or oxide-supported metals. The aim of this section is therefore to present a comprehensive picture of the possibilities and limitations of heat-flow calorimetry in heterogeneous catalysis. The use of Calvet microcalorimeters in the study of a particular system (the oxidation of carbon monoxide at the surface of divided nickel oxides) has moreover been reviewed in a recent article of this series (19). 

In typical stereochemical experiments, the reactivity of two or more compounds of the same structure but of different configuration is compared either in separate or competitive experiments. The method has been reviewed several times for heterogeneous catalytic reactions, mostly with respect to reactions of hydrocarbons on metals (16-18). The results concerning eliminations on acidic catalysts have been summarized in an article dealing with the mechanism of this type of reactions (12). Clarke and Rooney (17) have broadened the notion of the stereochemical approach to heterogeneous catalysis when they included into it all work in which mechanistic conclu-... 

Hagen CM, Widegren JA, Maitlis PM, Einke RG. Is it homogeneous or heterogeneous catalysis Compelling evidence for both types of catalysts derived from [Rh(Ti(5)-C5Me5) Cl-2](2)as a function of temperature and hydrogen pressure. J Am Chem Soc 2005 127 4423-4432. 

This chapter is not intended to furnish a comprehensive review of the latest theoretical developments across all types of reaction and catalyst in heterogeneous catalysis. It will instead briefly survey two cross-cutting themes that have in recent years benefited substantially from theoretical insights, especially what first-principles DFT calculations have been able to explain and predict, and that are commanding substantial current interest the design of new catalysts aided by theoretical insights and the elucidation of the molecular level effects of reactive physiochemical environments on catalytic reactivity. The relatively simple and predictable structure of metals... 

In heterogeneous catalysis equations (20b) express the preservation of number of active cites of particular type, for instance we have L(z) = zi +... + — 1 for catalyst with single type of active cites when each of... 

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