Coupling, of reactions

A proven approach to enhancing chemical conversion in the reaction zone is to carry out another reaction in the permeate zone to consume the permeate, thus creating a greater driving force for the permeate through the membrane. Examples have been given in 

Chapter 8 to demonstrate the benefits of coupling a second reaction on the permeate side to tl first on the retentate side. 

Conjugation of two reactions in a single reactor comparimentalizcd by a membrane reduces the number of degrees of freedom compared to having two independent reactors. For example the reaction stoichiometry may require the addition or removal of the permeating species which potentially can lower the maximum total conversion. Likewise heat may need to be added to or taken away from one of the reaction zones. 

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Corrosion of lead starts at the equilibrium potential of the negative electrode. It induces self-discharge of the positive electrode on account of the following couple of reactions Discharge of the positive electrode... 

In kinetic analysis of coupled catalytic reactions it is necessary to consider some specific features of their kinetic behavior. These specific features of the kinetics of coupled catalytic reactions will be discussed here from a phenomenological point of view, i.e. we will show which phenomena occur or may occur, and what formal kinetic description they have if the coupling of reactions is taking place. No attention will be paid to details of mechanisms of the processes occurring on the catalyst surface from a molecular point of view. 

It should be noted that many practically important catalytic transformations (such as isomerization of or hydrocracking of paraffins), which are presumed to proceed via consecutive mechanisms, are performed on multifunctional catalysts, with which the coupling of reactions in the sense just discussed may not necessarily occur. The problem of the selectivity of some models of polystep reactions on these catalysts has been discussed in detail by Weisz (56). 

Previously acrylonitrile had proved to be inert towards transition metal catalysed cross- and self-metathesis using ill-defined multicomponent catalysts [lib]. Using the molybdenum catalyst, however, acrylonitrile was successfully cross-metathesised with a range of alkyl-substituted alkenes in yields of40-90% (with the exception of 4-bromobut-l-ene, which gave a yield of 17.5%). A dinitrile product formed from self-metathesis of the acrylonitrile was not observed in any of the reactions and significant formation (>10%) of self-metathesis products of the second alkene was only observed in a couple of reactions. 

Coupling of Reaction and Mass Transfer in Ideal Reactors... 

The activity of elemental carbon as a metal-free catalyst is well established for a couple of reactions, however, most literature still deals with the support properties of this material. The discovery of nanostructured carbons in most cases led to an increased performance for the abovementioned reasons, thus these systems attracted remarkable research interest within the last years. The most prominent reaction is the oxidative dehydrogenation (ODH) of ethylbenzene and other hydrocarbons in the gas phase, which will be introduced in a separate chapter. The conversion of alcohols as well as the catalytic properties of graphene oxide for liquid phase selective oxidations will also be discussed in more detail. The third section reviews individually reported catalytic effects of nanocarbons in organic reactions, as well as selected inorganic reactions. 

Gryaznov, V. M., V. S. Smirnov, L. K.. Ivanova and A. P. Mishchenko. 1970. Coupling of reactions resulting from hydrogen transfer through the catalyst. Dokl. Akad. Nauk SSSR. 190(1) 144-147. 

The hanging component of the reaction network W is such Aj e that for all reactions = 0. This means that all reaction rates do not depend on concentration of A,. The hanging reaction is such element of with number r that r s only if — ly for some number X. An example of hanging components gives the last component A for the triangle network (79). An example of hanging reactions gives a couple of reactions (80) if they do not affect any other reaction. 

The second law requires that the total entropy production in a system of several reactions be positive for a closed system removed from equilibrium. However, in the case of thermodynamic coupling of reactions (26), it is not necessary that individual reaction entropy productions be positive. Apparently such reaction systems have not yet been considered in connection with natural water systems. 

The close-coupling of reaction and separation units for driving equilibrium reactions to complete conversion, though... 

Nucleophilic trapping agents used in the Type II Ac-Pd process are not limited to MeOH and other alcohols. A wide range of heteroatom and carbon nucleophiles may be used as in the cases of the Type II cyclic carbopalladation processes terminated by various nucleophilic reagents (Sect. 2.1.2). A couple of reactions shown in Scheme 58 [ 145] provide additional examples of heterocycles synthesis via Type II Ac-Pd process terminated by cross-coupling. 

The coupling of a permselective membrane with a packed bed of catalyst pellets (Fig. 5b) has been one of the most widely studied membrane reactor setups. Generally, the catalyst fixed bed is enclosed on the tube side of a porous membrane, although several cases can be found in the literature in which permselective tubular membranes have been inserted at regularly spaced intervals into the packed bed of catalyst pellets (e.g.. Ref. 25). The most interesting property of this membrane reactor type is that the amount of catalyst and the membrane surface area can be varied almost independently within wide ranges, so as to optimize the coupling of reaction and separation. 

Bernal MP, Coronas J, Menendez M, and Santamarfa J. Coupling of reaction and separation at the microscopic level Esterification processes in a H-ZSM-5 membrane reactor. Chem Eng Sci 2002 57 1557-1562. 

A catalytic reaction is the result of a cyclic process that consists of many elementary reaction steps. The essence of a catalytic reaction is that the catalytic reactive center reappears after each cycle in which reactant molecules are converted into products. Since zeolites are microporous systems, a special feature is the coupling of reaction at the protonic centers with diffusion of the molecules through the micropores to and from the zeohte exterior. The zeolite catalytic cycle is sketched in Fig. 1. To reach the catalytic reactive center, molecules have to adsorb in the mouth of a micropore and diffuse to the catalytic center, where they can react. Product molecules have to diffuse away and, once they reach the micropore mouth, will desorb. Clearly, then, one has to complement quantum-chemical information on reactivity, concerned with the interaction of zeolite protons with reactants, with information on diffusion and on adsorption of reactants and products. 

This negative value for the AG indicates that the isomerization of DHAP to GAP is exergonic and can occur spontaneously when these species are present at the above concentrations. Note that AG for this reaction is negative, although AG° is positive. It is important to stress that whether the AC for a reaction is larger, smaller, or the same as AG depends on the concentrations of the reactants and products. The criterion of spontaneity for a reaction is AG, not AG°. This point is important because reactions that are not spontaneous based on AG can be made spontaneous by adjusting the concentrations of reactants and products. This principle is the basis of the coupling of reactions to form metabolic pathways (Chapter 15). 

It is noteworthy that the redox couples of reactions 1-3 [Eq. (1)] are perfectly reversible provided the scan rate is sufficient (>100 mV s 1). The gliding motions, either for the divalent or the monovalent complex, are slow on the time scale of the voltammetry measurements (lines 2 and 4). 

The acetal 142 is lithiated with. see-Bui. i and TMEDA and formylated with DMF to give, after workup, the aldehyde 141. The next couple of reactions happen in one pot under the same set of reaction conditions. The thiol is reacted with NaH and the anion added to a solution of the aldehyde 141. After workup the product is the target material.24... 

The coupling of reactions is not always possible, as there is often the problem of incompatibility of conditions for all the reactions. This has, however, not impeded use of the technique for the analysis of a range of biochemicals. 

The free energy change (AG) of a process is a measure of its spontaneity. The free energy of a system decreases in a spontaneous process that is, if ASuniv > 0, AGsys < 0. In a coupling of reactions, a spontaneous step with a larger negative AG drives a nonspontaneous step with a smaller positive AG. 

When studying a multistep reaction, chemists often find that a nonspontaneous step is driven by a spontaneous step in a coupling of reactions. One step supplies enough free energy for the other to occur, as when the combustion of gasoline (spontaneous) supplies enough free energy to move a car (nonspontaneous). 

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