Parallel reaction mechanism

The following problem is formulated as an optimization problem. A batch reactor operating over a 1-h period produces two products according to the parallel reaction mechanism A — B, A — C. Both reactions are irreversible and first order in A and have rate constants given by... 

Overall refined series-parallel reaction mechanism for PC and Fe-assisted PC reaction... 

Perhaps the most important metal metal bonded reaction from the point of view of catalysis is addition of hydrogen. In spite of years of study, the exact mechanism of hydrogenation of even dicobalt octacarbonyl is not ftdly understood. Oxidative addition see Oxidative Addition) of hydrogen to metal-metal bonds is generally slower than oxidative addition to mononuclear complexes that have vacant sites. Two principal mechanisms have been proposed in this chemistry. The first relies on cleavage of the metal-metal bond to generate radicals (equation 75). The second mechanism involves dissociation of CO to generate a coordinatively unsaturated complex that retains the metal metal bond (equation 76). In spite of the apparent simphcity of these two possibilities, the reaction mechanism is unknown and it is likely that parallel reaction mechanisms occur. 

Figure 11.5. f-l Reactant mole fraction versus time group for parallel reaction mechanism with exponential activity function (from Froment and Bischoff i5 ). 

In a normal system there are always parallel reaction mechanisms and succeeding reaction steps, and so it may be impossible to interpret measured data. 

The most frequent use of DBMS is for studies of possible fuels in fuel cells. Figure 5 shows the faradaic and ion currents for CO2 and methylformate during methanol oxidation at carbon-supported Pt nanoparticles. Note that the formation of methylformate starts at a slightly lower potential than that of CO2. The ratio of the CO2 formation rate to the faradaic current yields a current efficiency of 90 % in this case. Under flow and at smooth Pt electrodes, the current efficiency for CO2 remains at 30 % for all flow rates [4]. This proves the parallel reaction mechanism suggested by Bagotsky [30]. One path leads to formaldehyde and formic acid. Under flow, these molecules diffuse away fi om the electrode, while under stagnant conditions as in the pores of a porous electrode, they are further oxidized to CO2. The other path leads to CO2 via adsorbed CO and is independent of flow rate. 

Rate surface for parallel reaction mechanism with exponential deactivation function. From Froment and Bischoff [1961],... 

Let us consider now dependence of selectivity on the particle size within the framework of the geometrical concept for a cube discussed above. This concept, if applicable, allows us to separate reactions on edges, terraces, and comers each having distinct values of rates towards a particular reactant. Let us consider as an example a parallel reaction mechanism A —> B, A C where the rates towards the reactants are given by... 

The reaction mechanism depends on the chemistry of the active oxidant and chemical contaminants. Multiple sequential and parallel reaction steps occur frequently. Partial oxidation produces noxious byproducts. 

Every chemist is aware of the fact that this case is not unique in chemical nomenclature it is the result of continuous and parallel developments in chemistry Accordingly, rules of systematic nomenclature — compounds, reactions, mechanisms - will also develop continuously. 

To this point we have focused on reactions with rates that depend upon one concentration only. They may or may not be elementary reactions indeed, we have seen reactions that have a simple rate law but a complex mechanism. The form of the rate law, not the complexity of the mechanism, is the key issue for the analysis of the concentration-time curves. We turn now to the consideration of rate laws with additional complications. Most of them describe more complicated reactions and we can anticipate the finding that most real chemical reactions are composites, composed of two or more elementary reactions. Three classifications of composite reactions can be recognized (1) reversible or opposing reactions that attain an equilibrium (2) parallel reactions that produce either the same or different products from one or several reactants and (3) consecutive, multistep processes that involve intermediates. In this chapter we shall consider the first two. Chapter 4 treats the third. 

This reaction cannot be elementary. We can hardly expect three nitric acid molecules to react at all three toluene sites (these are the ortho and para sites meta substitution is not favored) in a glorious, four-body collision. Thus, the fourth-order rate expression 01 = kab is implausible. Instead, the mechanism of the TNT reaction involves at least seven steps (two reactions leading to ortho- or /mra-nitrotoluene, three reactions leading to 2,4- or 2,6-dinitrotoluene, and two reactions leading to 2,4,6-trinitrotoluene). Each step would require only a two-body collision, could be elementary, and could be governed by a second-order rate equation. Chapter 2 shows how the component balance equations can be solved for multiple reactions so that an assumed mechanism can be tested experimentally. For the toluene nitration, even the set of seven series and parallel reactions may not constitute an adequate mechanism since an experimental study found the reaction to be 1.3 order in toluene and 1.2 order in nitric acid for an overall order of 2.5 rather than the expected value of 2. 

The catalytic steam-reforming process of methanol on Cu/ZnO/Ab03 catalyst primarily produces hydrogen and carbon dioxide. In addition, the minor quantities of carbon monoxide are also produced. This mechanism is explained in terms of parallel reactions [11]. 

The catalyst performance depends on the H2 to CCI2F2 feed ratio. The selectivities to CH2F2 and CHCIF2 are influenced by the H2 to CCI2F2 feed ratio, while the selectivity to methane is independent of this ratio. We have previously proposed a reaction mechanism with serial reactions on the catalyst surface and minor readsorption of the intermediate products, which is depicted in figure 8 [4,5]. Thus the kinetics of the reaction follows mainly parallel reaction pathways, in which the selectivities are not influenced by the conversion, and a... 

Figure 8. Reaction mechanism with apparent parallel reaction kinetics.

Ensemble or Third-Body Ejfects. These effects concern the selective blockage of a particular adsorption site by adatom deposition. This can be advantageous when the reaction mechanism contains parallel paths that can be affected differently by blocking particular sites. In some cases, the undesired reaction needs more than one free adjacent site (ensemble), and can be inhibited by blocking particular sites without decreasing the reactivity of the surface for the catalyzed reaction. 

For either of the ternary complex mechanisms described above, titration of one substrate at several fixed concentrations of the second substrate yields a pattern of intersecting lines when presented as a double reciprocal plot. Hence, without knowing the mechanism from prior studies, one can not distinguish between the two ternary complex mechanisms presented here on the basis of substrate titrations alone. In contrast, the data for a double-displacement reaction yields a series of parallel lines in the double reciprocal plot (Figure 2.15). Hence it is often easy to distinguish a double-displacement mechanism from a ternary complex mechanism in this way. Also it is often possible to run the first half of the reaction in the absence of the second substrate. Formation of the first product is then evidence in favor of a doubledisplacement mechanism (however, some caution must be exercised here, because other mechanistic explanations for such data can be invoked see Segel, 1975, for more information). For some double-displacement mechanisms the intermediate E-X complex is sufficiently stable to be isolated and identified by chemical and/or mass spectroscopic methods. In these favorable cases the identification of such a covalent E-X intermediate is verification of the reaction mechanism. 

Montanari el al., for example, studied a Co—H-MFI sample through FT-IR spectroscopy of in situ adsorption and coadsorption of probe molecules [o-toluonitrile (oTN), CO and NO] and CH4-SCR process tests under IR operando conditions. The oTN adsorption and the oTN and NO coadsorption showed that both Co2+ and Co3+ species are present on the catalyst surface. Co3+ species are located inside the zeolitic channels while Co2+ ions are distributed both at the external and at the internal surfaces. The operando study showed the activity of Co3+ sites in the reaction. The existence of three parallel reactions, CH4-SCR, CH4 total oxidation and NO to NOz oxidation, was also confirmed. Isocyanate species and nitrate-like species appear to be intermediates of CH4-SCR and NO oxidation, respectively. A mechanism for CH4-SCR has been proposed. On the contrary, Co2+ substitutional sites, very evident and predominant in the catalyst, which are very hardly reducible, seemed not to play a key role in the SCR process [173],... 

Thus mechanism B, which consists solely of bimolecular and unimolecular steps, is also consistent with the information that we have been given. This mechanism is somewhat simpler than the first in that it does not requite a ter-molecular step. This illustration points out that the fact that a mechanism gives rise to the experimentally observed rate expression is by no means an indication that the mechanism is a unique solution to the problem being studied. We may disqualify a mechanism from further consideration on the grounds that it is inconsistent with the observed kinetics, but consistency merely implies that we continue our search for other mechanisms that are consistent and attempt to use some of the techniques discussed in Section 4.1.5 to discriminate between the consistent mechanisms. It is also entirely possible that more than one mechanism may be applicable to a single overall reaction and that parallel paths for the reaction exist. Indeed, many catalysts are believed to function by opening up alternative routes for a reaction. In the case of parallel reaction paths each mechanism proceeds independently, but the vast majority of the reaction will occur via the fastest path. 

MATHEY Phosphorus-Carbon Heterocyclic Chemistry The Rise of a New Domain McKILLOP Advanced Problems in Organic Reaction Mechanisms OBRECHT Solid Supported Combinatorial and Parallel Synthesis of Small-Molecular-Weight Compound Libraries... 

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