Kinetics of Catalysis

FIGURE 18.14 The decomposition of hydrogen peroxide, H2O2, to water and oxygen is catalyzed by adding a very small amount of transition metal oxide (a), (b) The water evolves as steam because of the heat given off in the reaction. 

Catalysis are classified into two types homogeneous and heterogeneous. In homogeneous catalysis the catalyst is present in the same phase as the reactants, as when a gas-phase catalyst speeds up a gas-phase reaction, or a species dissolved in solution speeds up a reaction in solution. Chlorofluorocarbons and oxides of nitrogen are homogeneous catalysts responsible for the destruction of ozone in the stratosphere. These reactions are examined in more detail in Section 20.5. A second example is the catalysis of the oxidation-reduction reaction 

The Ag ions are not permanently transformed by this reaction because those used up in the first step are regenerated in the second they play the role of catalyst in significantly speeding the rate of the overall reaction. 

In heterogeneous catalysis the catalyst is present as a phase distinct from the reaction mixture. The most important case is the catalytic action of certain solid surfaces on gas-phase and solution-phase reactions. A critical step in the production of sulfuric acid relies on a solid oxide of vanadium (V2O5) as catalyst. Many other solid catalysts are used in industrial processes. One of the best studied is the addition of hydrogen to ethylene to form ethane  

The process occurs extremely slowly in the gas phase but is catalyzed by a platinum surface (Fig. 18.15). 

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The kinetics of catalysis of cyclotrimerization was studied on the model system phenyl isocyanate/ace-tonitrile (solvent). Acetonitrile (AN, 99.64%, from Vinstron Corp.) was purified by refluxing with phosphorus pentoxide (5 g/1), then with calcium hydride (2 g/1) followed by distillation under nitrogen. Phenyl isocyanate was obtained from the Upjohn Company with a purity of 99.5%, and was purified by distillation. Tolylene diisocyanates (2,4 and 80/20 2,4/2,6 isomers) were obtained from the Mobay Chemical Co., and were purified by distillation. Cyclic sulfonium zwitterions (SZ) were obtained from the Dow Chemical Co. 

It is important to study chemical kinetics of catalysis in dispersed liquid-liquid systems. Therefore it is necessary to consider the following (14-16) ... 

The insolubility of enzymes in monophasic organic systems has a controlling influence on the kinetics of enzymatic catalysis in organic media. Insolubilized enzymes are subject to intraparticle and external diffusional limitations which can mask the true, intrinsic kinetics of catalysis. These limitations are particularly severe for highly active and purified enzymes such as horseradish peroxidase. One way to overcome this problem is to increase the surface area of the enzyme in contact with the organic solvent. 

Peroxidases (EC 1.11.1.7) catalyze the reduction of hydrogen peroxide or alkyl hydroperoxides while a wide range of substrates act as electron donors. The mechanism of peroxidase catalyzed reactions has been intensively studied (see reviews [201,207-210]). The kinetics of catalysis reveals a ping-pong mechanism. In the first step, the peroxide binds to a free coordination site of iron (Fe ) and is reduced to water (or an alcohol ROH) in a rapid two-electron process, whereby compound I is formed as the stable primary intermediate ... 

The acid-protease from Rhizopus chinensis was first isolated by Fukumoto, Tsuru, and Yamamoto (1). Its optimum pH for catalytic activity was shown to be between 2.9 and 3.3 (1). Although its complete sequence has not yet been determined, some limited sequence data are available - particularly that of the 39 amino-terminal residues (2,3) and of the residues in the immediate vicinity of the catalytically active aspartic acid residues (4,5). These data show that this enzyme has substantial sequence homology with porcine pepsin. Investigations of the kinetics of catalysis (6,7) have led to proposals of an extended subsite specificity. 

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

Influence of the Adsorption Isotherm on the Kinetics of Heterogeneous Catalysis... 

Left side of Fig. 4 shows a ribbon model of the catalytic (C-) subunit of the mammalian cAMP-dependent protein kinase. This was the first protein kinase whose structure was determined [35]. Figure 4 includes also a ribbon model of the peptide substrate, and ATP (stick representation) with two manganese ions (CPK representation). All kinetic evidence is consistent with a preferred ordered mechanism of catalysis with ATP binding proceeding substrate binding. 

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

A micelle-bound substrate will experience a reaction environment different from bulk water, leading to a kinetic medium effect. Hence, micelles are able to catalyse or inhibit organic reactions. Research on micellar catalysis has focused on the kinetics of the organic reactions involved. An overview of the multitude of transformations that have been studied in micellar media is beyond the scope of this chapter. Instead, the reader is referred to an extensive set of review articles and monographs" ... 

Studies of micellar catalysis of himolecular reactions of uncharged substrates have not been frequent" ". Dougherty and Berg performed a detailed analysis of the kinetics of the reaction of 1-fluoro-2,4-dinitrobenzene with aniline in the presence of anionic and nonionic surfactants. Micelles induce increases in the apparent rate constant of this reaction. In contrast, the second-order rate constant for reaction in the micellar pseudophase was observed to be roughly equal to, or even lower than the rate constant in water. 

Acid—Base Catalysis. Inexpensive mineral acids, eg, H2SO4, and bases, eg, KOH, in aqueous solution are widely appHed as catalysts in industrial organic synthesis. Catalytic reactions include esterifications, hydrations, dehydrations, and condensations. Much of the technology is old and well estabhshed, and the chemistry is well understood. Reactions that are cataly2ed by acids are also typically cataly2ed by bases. In some instances, the kinetics of the reaction has a form such as the following (9) ... 

Ca.ta.lysts, A catalyst has been defined as a substance that increases the rate at which a chemical reaction approaches equiHbrium without becoming permanently involved in the reaction (16). Thus a catalyst accelerates the kinetics of the reaction by lowering the reaction s activation energy (5), ie, by introducing a less difficult path for the reactants to foUow. Eor VOC oxidation, a catalyst decreases the temperature, or time required for oxidation, and hence also decreases the capital, maintenance, and operating costs of the system (see Catalysis). 

Effects of transport processes cannot be ignored in investigations aimed at more fundamental aspects of kinetics and catalysis. The interaction of chemical and physical processes was noticed a long time ago. M. V. Lomonosov mentioned in 1745 ... 

The role that acid and base catalysts play can be quantitatively studied by kinetic techniques. It is possible to recognize several distinct types of catalysis by acids and bases. The term specie acid catalysis is used when the reaction rate is dependent on the equilibrium for protonation of the reactant. This type of catalysis is independent of the concentration and specific structure of the various proton donors present in solution. Specific acid catalysis is governed by the hydrogen-ion concentration (pH) of the solution. For example, for a series of reactions in an aqueous buffer system, flie rate of flie reaction would be a fimetion of the pH, but not of the concentration or identity of the acidic and basic components of the buffer. The kinetic expression for any such reaction will include a term for hydrogen-ion concentration, [H+]. The term general acid catalysis is used when the nature and concentration of proton donors present in solution affect the reaction rate. The kinetic expression for such a reaction will include a term for each of the potential proton donors that acts as a catalyst. The terms specific base catalysis and general base catalysis apply in the same way to base-catalyzed reactions. 

Analysis of the kinetics of this catalysis points to the protonated imine as the key intermediate. 

In analyzing the behavior of these types of tetrahedral intermediates, it should be kept in mind that proton-transfer reactions are usually fast relative to other steps. This circumstance permits the possibility that a minor species in equilibrium with the major species may be the major intermediate. Detailed studies of kinetics, solvent isotope effects, and the nature of catalysis are the best tools for investigating the various possibilities. 

Much of the study of kinetics constitutes a study of catalysis. The first goal is the determination of the rate equation, and examples have been given in Chapters 2 and 3, particularly Section 3.3, Model Building. The subsection following this one describes the dependence of rates on pH, and most of this dependence can be ascribed to acid—base catalysis. Here we treat a very simple but widely applicable method for the detection and measurement of general acid-base or nucleophilic catalysis. We consider aqueous solutions where the pH and p/f concepts are well understood, but similar methods can be applied in nonaqueous media. 

A recent definition of catalysis that is based on dier-modynamics was advanced by the Subcommittee on Chemical Kinetics, Physical Chemistry Division, lUPA ... 

An interesting method, which also makes use of the concentration data of reaction components measured in the course of a complex reaction and which yields the values of relative rate constants, was worked out by Wei and Prater (28). It is an elegant procedure for solving the kinetics of systems with an arbitrary number of reversible first-order reactions the cases with some irreversible steps can be solved as well (28-30). Despite its sophisticated mathematical procedure, it does not require excessive experimental measurements. The use of this method in heterogeneous catalysis is restricted to the cases which can be transformed to a system of first-order reactions, e.g. when from the rate equations it is possible to factor out a function which is common to all the equations, so that first-order kinetics results. 

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