Types of Catalysis

Catalysis is of crucial importance for the chemical industry and thus chemical engineering. Catalysts applied in industry come in many different forms, from heterogeneous catalysts in the form of porous solids to homogeneous catalysts dissolved in the liquid reaction mixture to biological catalysts in the form of enzymes. 

We may classify catalysis primarily on the basis of nature of species responsible for the catalytic activity  

Molecular catalysis. The term molecular catalysis is used for catalytic systems in which the catalyst entity is a molecular species similar to that of the reacting chemical compound. Chemical compounds such as molybdenum complexes and large molecules such as enzymes are used as catalyst substances in molecular catalysis. Molecular catalysts are mostly seen in homogeneous catalytic systems in which the catalyst and the reacting compound are both in the same phase (liquid phase). However, molecular catalysts are also found in multiphase (heterogeneous) systems, such as those involving attachment of molecular entities to polymers. 

Surface catalysis. As the name implies, surface catalysis occurs on the surface atoms of an extended solid. This often involves surface atoms of dissimilar nature and property and hence different types of catalytic sites (unlike molecular catalysis, in which all the sites are equivalent). Because the catalyst is a solid, surface catalysis is by nature heterogeneous. 

Enzyme catalysis. Enzymes are proteins, polymers of amino acids, which catalyse reactions in living organisms—biochemical and biological reactions. The systems involved may be colloidal, that is, between homogeneous and heterogeneous. Some enzymes are very specific in catalysing a particular reaction (e.g. the enzyme sucrose catalyses the inversion of sucrose). 

A further classification is based on the number of phases in the system homogeneous (1 phase) and heterogeneous (more than 1 phase) catalysis. 

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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. 

Types of catalysis Brief title of rearction Equation of reaction... 

Several types of catalysis are described by terms denoting the catalyst structure or function. 

The catalytic effect of aromatic nitro groups in the substrate and product or in an added inert nitro compoimd (e.g., w-dinitrobenzene in 18) has been observed in the reaction of 2,4-dinitrochlorobenzene with an amine in chloroform. Hydrogen bonding to benzil or to dimethyl sulfone and sulfoxide also provided catalysis. It is clear that the type of catalysis of proton transfer shown in structure 18 will be more effective when hydrogen bonding is to an azine-nitrogen. 

In general acid catalysis, the rate is increased not only by an increase in [SH ] but also by an increase in the concentration of other acids (e.g., in water by phenols or carboxylic acids). These other acids increase the rate even when [SH ] is held constant. In this type of catalysis the strongest acids catalyze best, so that, in the example given, an increase in the phenol concentration catalyzes the reaction much less than a similar increase in [H30 ]. This relationship between acid strength of the catalyst and its catalytic ability can be expressed by the Breasted catalysis equation ... 

Phase-transfer catalysis is a special type of catalysis. It is based on the addition of an ionic (sometimes non-ionic like PEG400) catalyst to a two-phase system consisting of a combination of aqueous and organic phases. The ionic species bind with the reactant in one phase, forcing transfer of this reactant to the second (reactive) phase in which the reactant is only sparingly soluble without the phase-transfer catalyst (PTC). Its concentration increases because of the transfer, which results in an increased reaction rate. Quaternary amines are effective PTCs. Specialists involved in process development should pay special attention to the problem of removal of phase-transfer catalysts from effluents and the recovery of the catalysts. Solid PTCs could diminish environmental problems. The problem of using solid supported PTCs seems not to have been successfully solved so far, due to relatively small activity and/or due to poor stability. 

The problem of. separation of the catalyst from the product in homogeneous catalysis is the main disadvantage of this type of catalysis. In this section, we will summarize how this problem has been tackled in various homogeneously catalysed processes. 

This finding a new type of catalysis will provide a useful hint for the design of molecular structures of interfacially adsorbable and strongly reactive ligands for a speeifie metal ion. 

A different type of catalysis is observed using proline as a catalyst.166 Proline promotes addition of acetone to aromatic aldehydes with 65-77% enantioselectivity. It has been suggested that the carboxylic acid functions as an intramolecular proton donor and promotes reaction through an enamine intermediate. 

By plotting the measured rate constant versus the undissociated acid concentration, one obtains for this type of catalysis a straight line with intercept kx and slope ax = (/cHA + kA-/q). If the procedure is repeated for other ratios, enough information is obtained to permit evaluation of /cHA and kA-. The hydrogen and hydroxide ion concentrations corresponding to a given ratio q may be determined from equation 7.3.12 and the dissociation constant for water. 

Borabenzene complexes of cobalt such as Co(C5H5BPh)(COD) (51) and its 5-ethyl analog show the same type of catalysis but improved activity and chemoselectivity (77). Thus, 51 as the catalyst precursor gave the hitherto best results in the catalytic synthesis of the valuable 2-vinylpyridine from C2H2 and CH2=CHCN (120°C, 51 bar, 2 hours, turnover number 2164) (77,101). Furthermore, this catalyst for the first time allowed the synthesis of pyridine from C2H2 and HCN under mild conditions (110°C, 23 bar, 60 minutes, turnover number 103) (77). 

The chapters cover the following areas (i) use of coordination complexes in all types of catalysis (Chapters 1-11) (ii) applications related to the optical properties of coordination complexes, which covers fields as diverse as solar cells, nonlinear optics, display devices, pigments and dyes, and optical data storage (Chapters 12-16) (iii) hydrometallurgical extraction (Chapter 17) (iv) medicinal and biomedical applications of coordination complexes, including both imaging and therapy (Chapters 18-22) and (v) use of coordination complexes as precursors to semiconductor films and nanoparticles (Chapter 23). As such, the material in this volume ranges from solid-state physics to biochemistry. 

The catalysis of hydrogen peroxide decomposition by iron ions occupies a special place in redox catalysis. This was precisely the reaction for which the concept of redox cyclic reactions as the basis for this type of catalysis was formulated [10-13]. The detailed study of the steps of this process provided a series of valuable data on the mechanism of redox catalysis [14-17]. The catalytic decomposition of H202 is an important reaction in the system of processes that occur in the organism [18-22]. 

Thus, if information is being sought about intermediates for this type of catalysis, it does not make sense to analyze systems that lead to first-order reactions Rather, systems in which the hydrogenation rate is independent of the substrate concentration would be more appropriate. Indeed, for both catalytic systems shown in Figure 10.21, in each case one of the catalyst-substrate complexes could be isolated and characterized by crystal structure analysis (Fig. 10.23). 

Attaching the catalyst molecules to the electrode surface presents an obvious advantage for synthetic and sensor applications. Catalysis can then be viewed as a supported molecular catalysis. It is the object of the next section. A distinction is made between monolayer and multilayer coatings. In the former, only chemical catalysis may take place, whereas both types of catalysis are possible with multilayer coatings, thanks to their three-dimensional structure. Besides substrate transport in the bathing solution, the catalytic responses are then under the control of three main phenomena electron hopping conduction, substrate diffusion, and catalytic reaction. While several systems have been described in which electron transport and catalysis are carried out by the same redox centers, particularly interesting systems are those in which these two functions are completed by two different molecular systems. 

Chapters 4 and 5 are devoted to molecular and biomolecular catalysis of electrochemical reactions. As discussed earlier, molecular electrochemistry deals with transforming molecules by electrochemical means. With molecular catalysis of electrochemical reactions, we address the converse aspect of molecular electrochemistry how to use molecules to produce better electrochemistry. It is first important to distinguish redox catalysis from chemical catalysis. In the first case, the catalytic effect stems from the three-dimensional dispersion of the mediator (catalyst), which merely shuttles the electrons between the electrode and the reactant. In chemical catalysis, there is a more intimate interaction between the active form of the catalyst and the reactant. The differences between the two types of catalysis are illustrated by examples of homogeneous systems in which not only the rapidity of the catalytic process, but also the selectivity problems, are discussed. 

As pointed out above, values of KTS are obtainable from rate data without making any assumptions about the reaction mechanism. Therefore, one may use KTs and its variation with structure as a criterion of mechanism, in the same way that physical organic chemists use variations in other kinetic parameters (Brpnsted plots, Hammett plots, etc.). For present purposes, the value of Kts can be useful for differentiating between the modes of binding in the S CD complex and the TS-CD transition state, between different modes of transition state binding, and hence between different types of catalysis (Tee, 1989). 

Table 13.1 Kinetics of three types of catalysis that are in concert in the one-pot glucose-to-mannitol bio-chemo cascade conversion [19].

The most detailed investigations have been performed by Chen and Chen [59, 62-65], They considered catalyzed and uncatalyzed reactions between different hydroxyl groups at esterification temperatures (180-195 °C) and at polycondensation temperatures (270-290 °C). Their results are illustrated in Figure 2.13 in the form of Arrhenius plots. The type of catalysis and the reaction equation... 

While it is tempting to explain regulatory and cosolvent effects on the basis of conformational changes favorable or unfavorable to enzyme activity, it is much more difficult to demonstrate the actual involvement, amount, and structural details of such changes. Experimental evidence consists in most cases of bits and pieces provided by techniques such as absorption and fluorescence spectroscopy, circular dichroism, and magnetic circular dichroism. These tools work in solution (and, when desired, at subzero temperatures) to investigate not simply empty enzymes but enzyme—substrate intermediates. However, even with this information, the conformational basis of enzyme activity remains more postulated than demonstrated at the ball and stick level, and in spite of data about the number and sequence of intermediates, definition of their approximate nature, rate constants, and identification of the types of catalysis involved, full explanation of any particular reaction cannot be given and rests on speculative hypothesis. 

In this equation, the value of (Red) is a function of the nature of the reductant, its solubility, the crystallinity of solid phases containing it, effects of solubilizing agents, transport limitations, and other factors. Likewise the value of (Ox) is a function of various factors. As discussed in the previous chapter, most redox reactions are very slow and the prevailing conditions are therefore sensitive to catalysis. Three types of catalysis are involved ... 

Sometimes a lower density polyethylene is made with both this type of catalysis or Ziegler-Natta. Branching is controlled by the addition of small amounts of 1-alkenes added to the ethylene. 1-Hexene would give a C4 branch, 1-octene a Ce branch, etc. If enough 1-alkene is used the polymer is called linear low-density polyethylene (LLDPE). It is made by a high-density polyethylene process but branching gives a lower density. 

There are indications that another type of catalysis is present in the reaction between hydroquinone and silver ions in alkaline solution. The increase of rate with increasing hydroquinone concentration is greater than direct proportionality. This situation is similar to that observed in the oxygen oxidation of durohydroquinone (tetramethylhydroquinone) (James and Weissberger, 16) where the quinone formed in the reaction catalyzes subsequent oxidation. A direct check on quinone catalysis of the hydroquinone-silver ion reaction was not made, since quinone is unstable in alkaline solution, particularly in the presence of sulfite which reacts with it. Experiments were made, however, on the reaction between durohydroquinone and silver ion. This reaction shows the same dependence of rate upon the square root of the silver ion concentration as the hydroquinone reaction does. Addition of duroquinone to the reaction mixture produces a definite acceleration, as shown in Table II. 

The catalyst constitutes an electron shuttle between the electrode and the solution, where it is engaged in a direct redox electron transfer with the substrate. This type of catalysis is termed redox catalysis. 

In this case, the reduced form of the catalyst builds up with the substrate a relatively unstable adduct AQ, which then decomposes, eventually after further reduction at the electrode surface or in solution. Finally, either the oxidized form P of the catalyst or its reduced form Q is regenerated. As regards the substrate, its reduction follows the aforementioned E.C. (Electrochemical Chemical) mechanism. This type of catalysis is termed chemical catalysis. Finally, it must be pointed out that the clear-cut distinction between redox catalysis and chemical catalysis might be difficult, but amounts ultimately to a detailed study of the nature of the redox reaction. 

In heterogeneous catalysis we distinguish usually two mechanisms—the acid-base catalysis, which may be of the same type as the amino acid catalysis, and the catalysis by semiconductors and metals. The theory of this last type of catalysis was developed by T. T. Volkenstein in the U.S.S.R., by Germain in France, and by other scientists in Germany and in the United States. This theory is related to what you have indicated for MgO. It is assumed that an electron deficiency or electron excess is introduced as an impurity that creates, ultimately on the surface, a defect that can bind quasi-chemically electron donors or electron acceptors, respectively. 

Comparison of the Rate Equation and the Characteristics of the Two Types of Catalysis, Chemical (or Thermal) Catalysis and Eiectrocataiysis... 

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