The selectivity of consecutive reactions

We consider the reaction scheme of eq. (3.54) and define the kinetics of the desired and the undesired reaction respectively  

Here a negative sign indicates a net formation rate. 

Consecutive reactions in batch (plug flow) reactors 

It appears that a simple analytical solution is only possible when / = x = 1. 

For Ae situation of constant density a simple solution is found. In analogy with eq. (3.14) we find for plug flow reactors (if / / ) 

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The obtained catalysts showed similar activity in cyclopentadiene hydrogenation. The selectivity data in cyclopentadiene hydrogenation are given in Table I. The selectivity of consecutive reaction was determined as a rate ratio of cyclopentene/cyclopentane formation. On the contrary, the chitosan modification influenced essentially the selectivity of the catalyst on it basis. 

The catalysts used till now to examine the SCILL concept (e.g., Ni) may not be optimal for the specific test reaction. For example, Pd is more appropriate for the hydrogenation of COD to COE than Ni with COE, with yields of more than 90% [5, 6] compared to Ni with about 30% (see Chapter 11) nevertheless, Ni can be used to show the general influence of an IL-coated porous catalyst on the selectivity of consecutive reactions. 

Pore difTusion also has an impact on the selectivity of consecutive reactions (Section 14.4.2.5). 

Generally speaking, the frequency of the on-off cycled liquid feed must increase as the heat production increases, the reaction rate increases, the selectivity of consecutive reactions is important and when depletion of the liquid-phase reactant is of considerable influence. A high-low cycled liquid feed may bring about the same effect as increasing cycled liquid feed frequencies at on-off cycling. 

From product distribution analysis it could be concluded that larger particles present higher selectivity to glycerate due to the reduction of consecutive reaction, i.e. oxidation of glycerate to tartronate, remaining glycolate amount being almost stable. 

An excellent demonstration of the tunability of ionic liquids for catalysis is provided by an investigation of the dimerization of 1-butene (235). A Ni(cod)(hfacac) catalyst (Scheme 23) was evaluated for the selective dimerization of 1-butene after it was dissolved in various chloroaluminate ionic liquids. Earlier work on this reaction with the same catalyst in toluene led to the observations of low activity and difficult catalyst separation. In ionic liquids of varying acidity, little catalytic activity was found. However, a remarkable activity was achieved by adding a weak buffer base to an acidic ionic liquid. The reaction took place in a biphasic reaction mode with facile catalyst separation and catalyst recycling. A high selectivity to the dimer product was obtained because of a fast extraction of the Cg product from the ionic liquid phase, with the minimization of consecutive reaction to give trimers. Among a number of weak base buffers, a chinoline was chosen. The catalyst performance was compared with that in toluene. The catalyitc TOF at 90°C in toluene was... 

The electrosorption of reactive intermediates and of organic molecules at this interface is generally weak, due to physical adsorption. Nonetheless, in particular if the reactive intermediates are so reactive that they do not survive for much longer than 10-9 sec and therefore cannot escape from the electrode surface, the chemical composition of the adsorbate layer being different from that of the bulk electrolyte composition influences the course of consecutive reactions and their yields and selectivities decisively. 

Fig. 4. Selectivity S as function of the conversion for the case of consecutive reactions, which both are of zero order solid lines, no interaction dotted lines, no segregation.

Catalysts similar to those claimed by Union Carbide were later studied by Bordes and coworkers [4], and by Burch and coworkers [5]. Merzouki et al. [4a, b] proposed that the Mo/V/Nb/O catalyst is made up of (VNbMo)5014-type microdomains in a M0O3 matrix. At 200 °C, a selectivity of 45% to acetic acid and 45% to ethylene was obtained at 25% ethane conversion an increase of temperature caused a loss in selectivity to acetic acid in favor of that to ethylene. Burch and Swarnakar [5a] compared the reactivity of Mo/V/O and Mo/V/Nb/O systems. The former contained M0O3, Mo6V9O40 and Mo4V6025 crystalline compounds, while the latter also contained Mo3Nb2On, the most intense diffraction line of which occurred at 4.01 A The addition of Nb increased both activity and selectivity, and the formation of Mo3Nb201 i was proposed to account for the increase in performance. The product distribution was independent of the conversion, indicating the absence of consecutive reactions. 

The presence of consecutive reactions involving the desired product, and the reactivity of the product towards unselective oxidizing attacks occurring at the catalyst surface. For example, the development of electrophilic adsorbed oxygen species can be detrimental for selectivity. 

Additional aggravating circumstances arise from the fact that chemical steps which are transfer limited will proceed differently in the industrial plant than on laboratory-scale. The selectivity of multiple reactions such as competing consecutive and parallel reactions depend very much on the extent of micro-mixing in the system. These facts are well known from Chemical Reaction Engineering textbooks. Conversely, these reactions are carried out to obtain details about the extent of micro- and macro-mixing in stirring. 

High selectivity produces high yields of a desired product while suppressing undesirable competitive and consecutive reactions. This means that the texture of the catalyst (in particular pore volume and pore distribution) should be improved toward reducing limitations by internal diffusion, which in the case of consecutive reactions rapidly reduces selectivity. 

The occurrence of consecutive reactions, leading to combustion, which lower the selectivity to MA when the alkane conversion, is increased. At n-butane conversions, up to 60-70%, the extent of the consecutive reaction to give combustion products is not substantial, but the decrease in selectivity becomes dramatic when the conversion exceeds 70-80%. This observation has been attributed to the development of local catalyst overheating associated with the highly exothermic oxidation reactions and to the poor heat-transfer properties of the catalytic material. This problem is obviously more important in fixed-bed rather than mixed (fluidized) reactors, in which the heat transfer is faster. 

Table 1 compares the steady-state conversion at 260C and the selectivity in MAA, acetone and propylene at 260C and in correspondence of total IBA conversion, for the compounds calcined at 320C. The Kx salts exhibited comparable catalytic behaviors, with some differences in the IBA conversion and in the maximum selectivity to MAA, the most active and selective catalysts being the ammonium-salified (Ko) and the K2 these were the samples characterized by the lowest values of surface area before reaction. Moreover, they were the only samples which did not exhibit a decrease in selectivity approaching total IBA conversion. This effect can be related to the absence of consecutive reactions of MAA combustion, which instead occurred in the high sur ce area compounds. 

Chatsiriwech et al.[2] have shown that the conversion of equilibrium-limited reactions can be increased in a PSR. In the present contribution we explore the possibilities to enhance the selectivity in consecutive reactions through pressure swing operation. Potentially interesting applications include hydrogenations which are catalyzed at temperatures suitable for adsorbents. 

Imperfect mixing can also affect the selectivity for consecutive reactions of the following type, which are sometime called consecutive-parallel reactions ... 

Especially for equilibrium limited and consecutive reactions, reactive distillation offers advantages. Higher selectivities in the case of consecutive reactions can be achieved by low local product concentrations if the product is removed from the reaction zone directly by distillation. Conversions higher than equilibrium conver-... 

The effects of concentration levels on the selectivity of complex reactions can most readily be seen by considering a few examples. We begin with the two basic cases parallel and consecutive reactions. For the parallel reactions... 

A further benefit of this LLTP technique is the suppression of consecutive reactions. This advantage is described by the general equation (1), in which starting components A and B usually yield the products C and D, but also an tmdesired consecutive product E. If the catalyst and the products C and D form only one phase, a further reaction to E cannot be avoided. If, however, the catalyst is dissolved in the aqueous phase and the products C and D in the second organic phase, no further reaction can proceed. Consequently, by using the LLTP-technique, the products C and D are formed with high selectivity. 

A rather important criterion for the duration of the liquid-off period may be the selectivity for consecutive reactions as the hydrogenation of phenylacetylene to styrene and ethylbenzene in which styrene is the wanted product. During the liquid-off mode, the hydrogenation of phenylacetylene to styrene will take place preferentially. However the formed styrene will not be removed from the location of reaction, and the hydrogenation to the unwanted ethylbenzene will become more and more important. Therefore, to avoid selectivity problems, there will be an upper limit of the dry period. 

The selectivity of rapid reactions may be sensitive to mixing conditions, particularly in the case of competitive and competitive-consecutive reactions. A detailed description of all the relevant phenomena may be quite complicated. However, we can see a priori that good mixing and well controlled feed rates will generally be favourable for the selectivity of semi-batch processes, hi section 3.4 the relation between the selectivity and the degree of conversion was calculated for a few types of reaction pairs, for batch (or plug-flow) reactors, and for CSTR s. In sections 5.2.2 and 5.2.3 the influence of incomplete micro-mixing on selectivity was briefly discussed, for turbulent and laminar flow, respectively. 

Characteristic Reaction Time. As shown in Example 13-1, mixing can affect the selectivity of a reaction, not just the rate. Reactions that show selectivity are usually two-step reactions which are either consecutive or parallel. One reaction is usually so fast that it is mixing controlled. The second reaction has a characteristic time constant of the order of the local mixing time. The reaction time is usually given by... 

Also from the examples shown in Fig. 5 (the transient case where no step is clearly rate determining) it is evident that the selectivity of the consecutive reaction A —> B —> C, as estimated from the curves, will be in... 

The increase of selectivity in consecutive reactions in favor of the intermediate product may be sometimes extraordinarily high. Thus, for example, in the already cited hydrogenation of acetylene on a platinum and a palladium catalyst (45, 46) or in the hydrogenation or deuteration of 2-butynes on a palladium catalyst (57, 58), high selectivities in favor of reaction intermediates (alkenes) are obtained, even though their hydrogenation is in itself faster than the hydrogenation of alkynes. 

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). 

The selectivity of the gas phase amination is highly dependent on the type of zeolite as shown in Figure 11 for five zeolites all containing 3 wt % of Cu. When the zeolite becomes more spacious (L, Beta) the consecutive reaction increases which is understandable. Over Cu-Y reduction predominates. 

Beneficial micro reactor properties mainly refer to improving heat management as a key for obtaining a partial reaction which is part of a consecutive sequence, when large heats are released by reaction steps other than the partial one (see also Sectoin 3.3.1). Tkn even more import selectivity issue refers to the suppression of side reactions, which relate to the other fimctionality present in the reactant. This, again, profits from improved heat transfer. 

Consecutive reactions, E < Ei. The selectivity of the desired product decreases with temperature. However, a low temperature disfavours the reaction rate. A nonuniform temperature-time profile should be applied to maximize reactor productivity (see Fig. 5.4-72). At the start, no desired product is present in the reaction mixture. The temperature should then be as high as possible to keep the rate of P formation high. During the course of reaction, the amount of P in the reaction mixture increases. Therefore, the temperature should be lowered to minimize the rate of formation of the unwanted product from the desired product. 

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