Poisoning, homogeneous catalyst

Apart from the activation of a biphasic reaction by extraction of catalyst poisons as described above, an ionic liquid solvent can activate homogeneously dissolved transition metal complexes by chemical interaction. 

On the other hand, uniform or homogeneous catalyst poisoning presumes that the poison precursor species has full access to the catalyst interior before deactivation begins. There is no dif-fusional resistance for this species. This will be more likely to occur when the pores are large, the catalyst pellets small, and the intrinsic deactivation rate is low. In addition smaller poison precursor molecules will be able to diffuse more rapidly into the catalyst interior. Here the Thiele modulus for poison laydown h will be small, and in the limit, zero. 

Homogeneous catalysts often have the advantages of better catalyst reproduci bility and better selectivity. They are also less susceptible to catalyst poisoning (heterogeneous catalysts are usually poisoned by small amounts of sulfur, often found in rubber stoppers, or by sulfur-containing compounds, such as thiols and sulfides). On the other hand, heterogeneous catalysts are usually easier to separate from the reaction mixture. 

Phases gas-liquid, gas-liquid catalytic solid, gas-liquid plus catalytic solid minimizes catalyst poisoning, lower pressure than fixed bed. Used for hydrogenation reactions and MTBE and acrylamide production. For example, 90% conversion via reactive distillation contrasted with 70% conversion in fixed-bed option. Liquid with homogeneous catalyst etherification, esterification. Liquid-liquid HIGEE for fast, very fast, and highly exothermic liquid-liquid reactions such as nitrations, sulfonations, and polymerizations. Equilibrium conversion <90%. Use a separate prereactor when the reaction rate at 80% conversion is >0.5 initial rate. The products should boil in a convenient temperature range. The pressure and temperature for distillation and reaction should be compatible. 

One reason that is often cited for studying supported homogeneous catalysts is the relative ease of separation of catalyst from reactants. However for this factor to have practical significance it is essential that metal elution is minimal and very few studies on elution have been carried out in the past. ° Elemental analyses on the catalyst before and after reaction are insufficiently accurate to detect small losses. Moreover, arguments for low Rh loss on the basis of no change in activity on re-use of catalyst are not valid as the reaction is normally under mass transfer control. As many catalyst-support systems contain residual halide which is a known catalyst poison, if simultaneous leaching of halide and metal occurs then little change in activity may result. 

The knowledge of the colloidal chemistry of gold was used by Kim and Turkevich(106) to prepare monodisperse palladium particles in diameter greater than 75 A. The number of surface atoms determined by electron microscopy was found to be equal to the number of catalytic centers determined by poison titration for the athylene hydrogenation reaction. The surface of the palladium catalyst was homogenous. The velocity of the catalytic reaction was found to be proportional to the number of surface atoms. All surface atoms were active. 

Polymer-bound catalysts share some of the limitations intrinsic in both classical homogeneous and heterogeneous catalysis. Like soluble catalysts, the range of reactions they facilitate is rather limited. As with heterogeneous catalysts it is often difficult to learn the intimate details of ligand coordination, mechanism, and catalyst poisoning. Normally, such interpretations rely heavily upon the characteristics of soluble complexes, though it is clear the correspondence is imperfect. 

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