Consecutive reaction mechanisms

Figure 2.S. Concentrations of reactant, intermediate, and product for a consecutive reaction mechanism for different rate constants.

High selectivity of 4,4 -DIBP was observed in the catalysis of HM. The selectivity of 4,4 -DIBP was constant during the reaction with the accumulation of 2- and 3-IPBP and decrease of the selectivity of 4-IPBP. These results show that the alkylation proceeds by a consecutive reaction mechanism. The alkylation of 4-IPBP occurred regloselectlvely to give 4,4 -DIBP. Other Isomers, 2- and 3-IPBP, do not participate in the reaction because these Isomers are too sterically bulky to enter the pore of HM. On the other hand, catalyses of HY and HL were nonselective for the formation of 4,4 -DIBP. Three isomers of IPBP take part in the alkylation, which is controlled by the electronic factor of reactant molecules at low temperatures and by the stability of product molecules at higher temperatures. 

As discussed at the beginning of this section, the difficulty of activating short alkanes causes selectivity problems in these oxidation reactions. Intermediates formed in the, usually, consecutive reaction mechanisms are less stable than propane against further oxidation, and total oxidation and cracking occur as side reactions limiting the selectivity compared with the first vanadia-based catalysts... 

A consecutive reaction mechanism was also proposed by Cleaves and Centi (11). This mechanism was based on experimental work to back up the theoretical calculations of Schitt and Jorgensen. Although the proposed intermediates were not detected under reaction conditions, they have been observed with fuel-rich gas feeds and under conditions of transient reactor operation. Using a TAP (temporal analysis of products) reactor, the researchers detected products in the following order of formation butane —> butene butadiene furan. However, the... 

A consecutive reaction mechanism was proposed by Zhang-Lin et al. (25,26). The mechanism was based on kinetics data calculated for the oxidation of butane, 1-butene, 1,3-butadiene, and furan catalyzed by (VO)2P207 and VOPO4 phases. In conbast to the results of the TAP investigations, the kinetics data suggested that furan is not an intermediate in butane oxidation, but is an intermediate in butadiene oxidation. The differences observed in the oxidation of butane and the unsaturated hydrocarbons lead to questions about the validity of extrapolating butene and butadiene oxidation results to the butane oxidation. 

Wang (4) et al studied regeneration kinetics on a zeolite catalyst using pulse and continuous flow techniques Theyproposed a parallel-consecutive reaction mechanism... 

In these systems, the donor and acceptor diffuse together to give a precursor complex, D A, whose formation is described by the equilibrium constant Kp. Electron transfer, characterized by rate constant eTj occurs within the associated donor-acceptor pair, converting the precursor complex to successor complex D A. Subsequent separation of the oxidized donor (D+) and reduced acceptor (A ) from the successor complex is described by. s- The rate of m/ermolecular electron transfer depends not only on the factors that influence kpj but also on factors affecting the formation of the precursor complex [19]. More quantitatively, as described by Eq. 2, the expression for intermolecular electron transfer has the form of a consecutive reaction mechanism described by an observed rate constant (A obs) consisting of rate constants for diffusion (A ) and the activated electron transfer. 

Figure I1.5./-3 Reactant mole fraction versus time group for consecutive reaction mechanism with exponential deactivation function from Froment and Bischoff [35]). 

It further follows from Froment and Bischoff s study that, for a given bed length, both the point and the average carbon content increase with increasing space time (or decrease with space velocity) for the consecutive reaction mechanism, but decrease for the parallel mechanism (increase in terms of space velocity). Eberly s data [142] also indicate that the power in the Voorhies relation depends on the space time or on the liquid hourly space velocity. Another consequence of this analysis is shown in Fig. 11.5.f-5 for a parallel reaction. In the absence of fouling, and for isothermal conditions, the maximum rate of reaction A - / is always at the reactor entrance. This is not necessarily true when the catalyst is foul ... 

This greatly simplifies analysis of the system since the two rates differ only in the fundamental rate constant and the concentration of the reactants. Computer simulation of the consecutive reaction mechanism allows one to determine the 2/ 1 needed to achieve the desired yield of C2H4. Figure 6 shows how the selectivity to B varies as a function of conversion of A for various 2/ 1 ratios. (Since oxidative activation of ethane to give an ethyl radical results in an ethylene product, it only serves to consume oxidant and can be ignored. If direct ethane conversion to CO2 occurs this will reduce C2 yield.) The results in Figure 4 are fit well by a 2/ 1 about 6. 

Experimental test of this mechanism was conducted by performing a competition study with ethylene/methane mixtures in the tubular reactor. The results, summarized in Table 5, demonstrate that ethylene oxidation competes readily with methane oxidation under the experimental conditions of the electrocatalytic cell. The ratios of 2/ 1 calculated for these experiments are 4.0 and 4.6. This is in reasonable agreement with the ratio derived from the methane coupling experiments. Thus, the consecutive reaction mechanism can be applied successfully to systems of this type. The inescapable conclusion is that methane dimerization is limited by the relative rates of methane and ethylene activation. 

Consecutive reaction mechanisms are well-known basic mechanisms in chemical kinetics. The simplest example of such a reaction scheme is A A B C. Its kinetic model is represented as follows... 

The consecutive reaction mechanism is characterized by a maximum in the concentration of intermediate B (in contrast with the parallel mechanism A B, A C), which can be found by equating the time derivative of B to zero. For ki 7 k2, the maximum normalized concentration of B is given by... 

Consequently, if k =fe, the concentrations of A and B at tB,max are equal if k > fe, the concentration of B exceeds that of A and if k [Pg.376]

Let us consider dimethyl ether formation from methanol, which proceeds through a consecutive reaction mechanism I 1. Figure 4.13a illustrates the reaction intermediates for the first reaction step in which the C-0 bond in methanol is cleaved. The calculated reaction energy diagram for this reaction is shown in Fig. 4.13b. The reaction products that form are water and adsorbed methoxy. 

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