Parallel reaction paths

In continuation of a previous work (1), catalytic hydrogenation of cinnamaldehyde has been studied in slurry phase using a high-pressure autoclave. A series of carbon powder (CP)-supported Pt catalysts with widely varying Pt dispersion and Pt location on the support has been used in the study. The purpose has been to find out how the location of the metal on the support and its dispersion affect the two parallel reaction paths, namely the hydrogenation of the C=0 and C=C bonds. 

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. 

Several reaction mechanisms have been presented for the electroxidation of methanol. Breiter considered the reaction on platinum in add to follow parallel reaction paths, namely. 

Over the years, TAP systems have been used in catalyst industry research ranging from determining selectivity and activity of well-defined surfaces to distinguishing between sequential or parallel reaction paths. The capabilities of the TAP system are varied and can perform applications such as TPD, TPSR, TPO and TPR. In pulse experiments, the adsorption of reactants provides detailed information on the thermal stability of adsorbed intermediates. Fig. 4 shows the general parameters of the TAP system. 

Parallel reaction paths must be postulated (i.e., more than one ionized form of the enzyme must react). 

Kurimura et al. (22) studied the oxidation of Fe +(EDTA) and Fe +(NTA) by dissolved oxygen in aqueous solutions and suggested that the oxidation proceeds by two parallel reaction paths, with both pro-tonated and unprotonated chelates reacting. The reaction mechanisms suggested (22,23) are as follows ... 

Experimental measurements of absorption fluxes and colour development for the gas-liquid reaction between sulphur trioxide and dodecylbenzene have been carried out in a stirred cell absorber. A model with two parallel reaction paths representing sulphonation and discolouration has been applied to analyse the exothermic absorption accompanying conversions up to 70%. The results show that the two reactions have similar activation energies and that temperature increases greater than 100°C occur at the interface during absorption. The absorption enhancement factor exhibits a maximum value as liquid phase conversion proceeds. 

At this point, it should be emphasized that steps 2-6 are not linked in a sequential manner. Instead, diffusion and reaction inside the pore system, steps 3-5, occur simultaneously, as observed in the macroporous pellet as well. Moreover, the opportunities for a reactant molecule either to be converted at the outer surface of the crystallite or to enter the porous structure also represent parallel reaction paths. 

This effect is in agreement with the findings of Ashmore et To explain the anomalously fast rate of the decomposition of NO2, these workers postulated that parallel reaction paths were occurring, one the usual molecular path and the other by a free radical mechanism. If this is correct then the principle of microscopic reversibility requires that the oxidation of NO also proceeds by two paths ... 

Based on their isomerization experiments in polar solvents, Whitten et al. stated that thermal Z—> E isomerization of donor/acceptor-substituted azobenzenes proceeds by rotation. Later, Shin and Whitten modified this point of view and saw a dual mechanism active dependent on solvent and donor strength, Asano and coworkers,from their experiments under pressure, inferred a dependence of the mechanism on solvent and even parallel reaction paths along these mechanistic coordinates. 

The stereochemistry of chlorination of tran.v-stilbene with phosphorus pentachloride is dependent on the solvent system135. Anti addition is predominant in hexane, whereas syn addition is observed in dichloromethane. This observation is explained by two parallel reaction paths. 

The possibility of anharmonic effects has also been investigated. The initial nonequilibrium distributions are listed in Table III. Transition probabilities are tabulated in Table IV. The reaction model chosen is one where the reactant state consists of only six levels. However, two final or product states are assumed via parallel reaction paths. Each product state consists of only two levels. The two reactions are distinguished by two distinct sets of rate constants. For example, = /cj g = 0.0010 and 2.7 = = 0.0050. . . , whereas = k i = 0.0CI06 and k = 2,10 = 0.0030. Results for cases hv = 2.0000, a = 0.0080 and fihv = 2.0000, oc = 0.0200 are shown in Figs. 4 and 5, respectively. These results indicate that no significant difference was found between the two cases of (ihv = 2.0000 with different values of a, the anharmonicity parameter. 

PER/SEC] Perlmutter-Hayman, B., Secco, F., Kinetics and equilibria in the system pyrophosphate-nickel(ll). Evidence for two parallel reaction paths influence of ionic strength on the ion pair constant, Isr. J. Chem., 11, (1973), 623-633. Cited on pages xxiv, 207, 208, 209, 210, 285, 355, 361, 362, 363, 426. 

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