Reactions, classes heterogeneous

As described above, most solid-state reactions are heterogeneous, in the sense that reactant and product are in different solid phases. In many of these, product crystals first appear as nuclei that grow at the expense of the parent crystal. On the other hand, there are some solid-state reactions that are not accompanied by a phase change and for which, therefore, analogy with a solid-state transformation is not plausible. Such reactions are of particular interest in several respects They make possible conversion of a single crystal of reactant to a single crystal of product they enable study, for example by X-ray diffraction, of the structures of the parent and product molecules as functions of the degree of conversion in more or less constant environments and one can elucidate from them the constraints that the parent crystal imposes both on the reaction pathway and on the conformation of the product. It is in connection with the latter that this subject is of particular interest in the present context. This class of processes has been discussed by Thomas (183). 

The electron formed as a product of equation (2.5) will usually be received (or collected ) by an electrode. It is quite common to see the electrode described as a sink of electrons. We need to note, though, that there are two classes of electron-transfer reaction we could have considered. We say that a reaction is heterogeneous when the electroactive material is in solution and is electro-modified at an electrode which exists as a separate phase (it is usually a solid). Conversely, if the electron-transfer reaction occurs between two species, both of which are in solution, as occurs during a potentiometric titration (see Chapter 4), then we say that the electron-transfer reaction is homogeneous. It is not possible to measure the current during a homogeneous reaction since no electrode is involved. The vast majority of examples studied here will, by necessity, involve a heterogeneous electron transfer, usually at a solid electrode. 

Catalysts are usually subdivided into homogeneous and heterogeneous classes. Homogeneous catalysts are soluble in the reaction media. Heterogeneous catalysts make up the bulk of the catalyst market they are the solid catalysts that can be a support material, such as alumina, silica, or silica alumina, but more often some metal salt is added to a formed catalyst support. 

It is probably necessary to make a primary operational distinction of reaction classes based on the phase (or phases) of matter involved thus (1) homogeneous, liquid phase (2) gas phase (3) solid phase (4) heterogeneous. A basic subclassification distinguishes between reactions in which the reactants are chemically different from the prodncts, as in equations (1) and (2), and reactions in which the reactants and prodncts involve the same chemical species, as in equations (3) and (4) when (N4) = ( N4). Eqnations (1) and (2) are examples of cross electron-transfer reactions (or cross-reactions), while eqnations (3) and (4) are examples of self-exchange electron-transfer reactions when (N4) = ( N4). More generally, subclassifications of the primary classes are commonly based on energy or free energy considerations such as ... 

In the discussion of the deactivation of active surfaces in Chapter 3, we made the point that the existence of deactivation changed an entire class of steady-state phenomena into unsteady-state phenomena. This is well illustrated by extending the analysis of diffusion and chemical reaction in heterogeneous catalysis to include deactivation. On a quantitative level a precise analysis can become intricate and difficult, so we will confine the presentation here to a more qualitative level (and it s tough enough at that). 

Catalysts for Selected Classes of Reactions, 156 Heterogenized Homogeneous Catalysts, 163 Role of Solvent in Catalysis by Solids, 168... 

Hydrogenations and oxidations form two important classes of catalytic reactions. In heterogeneous catalysis, the metals from the groups Vin and IB of the periodic system, as well as oxides or sulfides, catalyze such reactions. In view of their unique reaction mechanisms, acid-catalyzed reactions are considered as a separate class, while a fourth category is formed by reactions that are catalyzed by coordination complexes or organometallic complexes in solution, as in homogeneous catalysis. Heterogeneous catalytic reactions will be the focus, however. 

Example DVD10-) Analysis of a Heterogeneous Reaction [Class Problem University of Michigan]... 

Catalysis involves an alternative mechanism in which the catalyst is involved. Catalysis is divided into three classes heterogeneous catalysis, homogeneous catalysis, and enzyme catalysis. Heterogeneous catalysis at the surfaces of solids involves adsorption of the reactants. We discussed the Langmuir theory of adsorption and applied it to heterogeneous catalysis. Homogeneous catalysis involves mechanisms with steps that occur in a single phase, and example reactions were analyzed. 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

Oxo Synthesis. Ad of the synthesis gas reactions discussed to this point are heterogeneous catalytic reactions. The oxo process (qv) is an example of an industriady important class of reactions cataly2ed by homogeneous metal complexes. In the oxo reaction, carbon monoxide and hydrogen add to an olefin to produce an aldehyde with one more carbon atom than the original olefin, eg, for propjiene ... 

Hydroxypyridine (86, R = H) and its derivatives also belong to the class of heterocyclic enols. In benzene and dioxane, 3-hydroxy-pyridine occurs as the neutral molecule (and not as a betaine).Its reaction with diazomethane, in heterogeneous media, gives a mixture of 3-methoxypyridine (86, R = Me) (10%) and l-methyl-3-hydroxy-pyridinium betaine (87) (30%If tert-butanol is used as a... 

It is a misconception that most chemicals are manufactured in organic solvents. Most high-volume bulk chemicals are actually produced in solvent-free processes, or at least ones in which one of the reactants also acts as a solvent. Typical examples of such large-scale processes include the manufacture of benzene, methanol, MTBE, phenol and polypropylene. In addition, some heterogeneous gas-phase catalytic reactions, a class of solvent-free processes, are discussed in Chapter 4. 

From the point of view of chemical metallurgy, the class of heterogeneous reactions in which a fluid (gas or liquid) contacts a solid, reacts with it and transforms it into the product is very important and involves many of the principles pertaining to heterogeneous reactions. Fluid-solid reactions may essentially be represented in the following three ways ... 

Heterogeneous reactions involving a solid supported catalyst form an important class of reactors and require special consideration. As discussed in the previous section, such reactors can be configured in different ways ... 

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