Kinetic heterogeneously catalyzed hydrogenation

A higher form of interpretation of the effect of solvents on the rate of heterogeneously catalyzed reactions was represented by the Langmuir-Hinshelwood kinetics (7), in the form published by Hougen and Watson (2), where the effect of the solvent on the reaction course was characterized by the adsorption term in the kinetic equation. In catalytic hydrogenations in the liquid state kinetic equations of the Hougen-Watson type very frequently degrade to equations of pseudo-zero order with respect to the concentration of the substrate (the catalyst surface is saturated with the substrate), so that such an interpretation is not possible. At the same time, of course, also in these cases the solvent may considerably affect the reaction. As is shown below, this influence is very adequately described by relations of the LFER type. 

Bond (1987) covers the basic principles of catalysis, adsorption on solid surfaces, chemisorption at metal and oxide surfaces, the kinetics of catalyzed reactions, the quantitative aspects of catalysis by metals and the structure, preparation and use of heterogeneous catalysts. The book also discusses the application of catalysts in different fields including energy conservation, production of hydrocarbon feedstocks, bifunctional catalysts in petroleum industry, oxidation catalysts in the petrochemical industry, heavy inorganic industry, hydrogenation of multiple bonds and catalysts used in atmospheric pollution control. 

The kinetics of the heterogeneously catalyzed gas phase hydrogenation of 1-hexene on a Ni catalyst was studied in an almost isothermal ( 1 K) tubular fixed bed reactor (Pachow, 2005). Here we only look at the determination of the reaction order of hexene by the integral method and determine the influence of mass and heat transfer phenomena. 

Table 4.11.1 Experimental conditions used to determine the intrinsic kinetics of the heterogeneously catalyzed gas-phase hydrogenation of 1-hexene.

To summarize, transient kinetic experiments are an estabHshed and valuable tool in the investigation of heterogeneously catalyzed gas-phase reactions. For liquid-phase systems, transient studies are much rarer than for gas-phase systems. It is probably related to slower dynamics and the fact that the intrinsic kinetic phenomena can be obscured by mass transfer effects and catalyst deactivation. As an illustration, we will consider three-phase continuous enantioselective hydrogenation of ethylbenzoylformate (Fig. 7.13) leading to two products over a platinum catalyst on an alumina support [1]. 

Liquid phase hydrogenation catalyzed by Pd/C is a heterogeneous reaction occurring at the interface between the solid catalyst and the liquid. In our one-pot process, the hydrogenation was initiated after aldehyde A and the Schiff s base reached equilibrium conditions (A B). There are three catalytic reactions A => D, B => C, and C => E, that occur simultaneously on the catalyst surface. Selectivity and catalytic activity are influenced by the ability to transfer reactants to the active sites and the optimum hydrogen-to-reactant surface coverage. The Langmuir-Hinshelwood kinetic approach is coupled with the quasi-equilibrium and the two-step cycle concepts to model the reaction scheme (1,2,3). Both A and B are adsorbed initially on the surface of the catalyst. Expressions for the elementary surface reactions may be written as follows ... 

Similar equipment for applications on the laboratory scale has been reported (and has recently been commercialized) (69-72). Most of the reported applications had the aim of investigating kinetics of chemical reactions as indicated by changes in liquid-phase concentrations. The equipment can typically be used at elevated temperatures and pressures. Applications to heterogeneous catalytic reactions include investigations of the enantioselective hydrogenation of exocyclic a,p-unsaturated ketones catalyzed by Pd/C in the presence of (A)-proline (73) and the esterification of hexanoic acid with octanol catalyzed by a solid acid (the resin Nafion on silica) (74). 

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. 

Carbon monoxide (C=0) and hydrogen (H2) produce methanol (CH3OH) on a ruthenium-exchanged zeolite-Y catalyst within a mbular reactor. Propose a mechanism and develop a kinetic rate law which accounts for the fact that a heterogeneous surface-catalyzed reaction occurs within the internal pores of the zeolite catalyst. Hint There are no hydrogen-hydrogen bonds in methanol. 

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