Heterogeneous catalytic kinetics selectivity

One of the most important requirements for catalytic reactions in fine chemicals applications is proper selectivity, which in a broad sense should be understood as chemo-, regio- and enantioselectivity. Kinetic analysis of complex reaction schemes, where the proper selectivity dependence is the key point of analysis, is still more an exception, than a rule. The main objective is to bring the knowledge of chemical reaction engineering of catalytic reactions to organic chemistry, in particular stereochemical and enantioselective reactions. In what follows heterogeneous catalytic reactions are considered. 

In the case of three phase heterogeneous catalytic reactions, the rate of the process and its selectivity can be determined either by intrinsic reaction kinetics or by external diffusion (on the gas-liquid and gas-solid interface) as well as by internal diffusion through the catalyst pores. Careful analysis of mass transfer is important for the elucidation of intrinsic catalytic properties, for the design of catalysts, and for the scale up of processes. 

The kinetics of heterogeneous-catalytic epoxidation of cycloh cene (CH) by tert-butyl hydroperoxide (TBHP) was investigated [202]. Catalysts were prepared by impregnating the ion-exchange resin, Amberlite IRS-84 in H-form, with the acidic solution of ammonium molybdate. The concentration of Mo was 0.35 mmole/g resin. The reaction kinetics were studied in the absence of solvent and at high molecular ratios of CH and TBHP. The reaction proceeded by pseudo-zero-order in respect of CH. Reaction selectivity was 90- 95 7o U. the initial rate could be determined from the rate of TBHP consumption. It was found that the reaction order was 1.8 for TBHP and 1 for the catalyst. Analysis of EPR spectra of catalysts before and after the reaction showed partial oxidation of Mo to Mo. The authors supposed a stepwise reaction mechanism in which the interaction between CH and a complex of Mo and TBHP is considered to be the slow and irreversible step. In this complex. Mo was present in the oxidized state. 

The problem whether the periodical operation of a technical reactor with input concentrations which change periodically provides higher selectivities and yields than stationary operating was examined with the example of benzene oxidation into malein anhydride [103], A rather complex example was the oxide-hydrogenation of isobutyric aldehyde to methacrolein [100], Based on dynamic experiments a reaction scheme is proposed and estimation of kinetic parameters of the main reaction using an Eley-Rideal type rate equation was carried out. The examples revealed that the wave-front analysis provides valuable qualitative and quantitative kinetic information of heterogeneous catalytic reactions. 

In this chapter, we revisit the subject of reaction/transport interactions in heterogeneous catalysts, this time from a quantitative standpoint. The topic must be examined from two perspectives. First, a researcher that is studying the kinetics of a heterogeneous catalytic reaction (or reactions) must ensure that his or her experiments are free of transport effects. In other words, the experiments must be conducted under conditions where intrinsic chemical kinetics determines the reaction rate(s). The researcher may have to make calculations to estimate the magnitude of heat and mass transport influence. He or she may also have to carry out diagnostic experiments in order to define a region of operation where transport does not affect the reaction rate and selectivity. 

Through such chemisorption studies, the values of >, have been determined not only by geometric accessibility, but also by the chemical heterogeneity of the surface. This can result in abnormal values of D, and demonstrates the scale effect on the kinetics and selectivity of catalytic reactions. For such studies, Farin and Anvir [213] derived the equations that can be applied for characterization of supported catalysts ... 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

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 ... 

Selectivity in catalysis is one of the most important factors to be controlled by researchers. Selectivity can be controlled in several ways such as by structural, chemical, electronic, compositional, kinetic and energy considerations. Certain factors may be more important in homogeneous catalytic reactions rather than heterogeneous reactions and vice versa. In most cases, however, little distinction will be made regarding the control of product selectivity for these two major types of catalysts. 

In heterogeneous catalysis reactants have to be transported to the catalyst and (if the catalyst is a porous, solid particle) also through the pores of the particle to the active material. In this case all kinds of transport resistance s may play a role, which prevent the catalyst from being fully effective in its industrial application. Furthermore, because appreciable heat effects accompany most reactions, heat has to be removed from the particle or supplied to it in order to keep it in the appropriate temperature range (where the catalyst is really fully effective). Furthermore, heterogeneous catalysis is one of the most complex branches of chemical kinetics. Rarely do we know the compositions, properties or concentrations of the reaction intermediates that exist on the surfaces covered with the catalytically effective material. TTie chemical factors that govern reaction rates under these conditions are less well known than in homogeneous catalysis. Yet solid catalysts display specificities for particular reactions, and selectivity s for desired products, that in most practical cases cannot be equaled in other ways. Thus use of solid catalysts and the proper (mathematical) tools to describe their performance are essential. 

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