Type II Selectivity

Type II selectivity arises with the simultaneous production of two products from a single reactant... 

It is clear that all three types of selectivity are relevant to catalytic hydrogenation reactions and from a consideration of the reaction scheme for alkyne hydrogenation (Fig. 4), it can be deduced that all three factors may be operative simultaneously. Clearly, the selectivity for the formation of the alkene relative to alkane will depend upon a number of factors. If both the alkene and the alkane are formed during one residence of the parent molecule on the surface, the selectivity will depend upon the relative values of k, and k2 (Type II selectivity) and upon the ratio kjk4 (Type II selectivity). Since both of these depend upon the specific properties of the catalyst, they have been termed the mechanistic selectivity factor [38], Once the alkene is produced, the system contains another potential adsorbate and Type I selectivity must be taken into account. It... 

Type II selectivity involves the differentiation between two parallel reactions in which different products are formed by separate paths from the same starting material.33 This type of selectivity is encountered in the hydrogenation of crotonaldehyde to either butyraldehyde or 2 buten-l-ol (Eqn. 5.8). When both reactions are of the same kinetic order changes in mass transport will influence them both to the same extent and there will be no effect on reaction selectivity. When the reactions are of different kinetic orders, that one with the higher order will be more affected by mass transport limitation. 

Wheeler characterized the independent parallel reactions A B and R S 2ls type I selectivity. Type II selectivity refers to parallel reactions of the form... 

Type I selectivity occurs between two different products formed in parallel reactions that differ by at least one reactant. Type II selectivity involves selectivity of one product over another in parallel reactions involving the same reactants. And, Type III selectivity involves the selectivity between products in a series reaction. Both Type I and III selectivities are affected by mass transport rates (22). 

These could refer to the main and poisoning reactions, or to reforming or cracking reactions with different compounds in a multicomponent feed stream. Type II selectivity refers to parallel reactions with a single reactant ... 

Now, if the comparison is not made at equal conversion levels far different results will be obtained. We have seen in Figure 4.21 that for first-order kinetics, the conversion in the CSTR lags behind that of a PFR of comparable total residence time. Since both PFR and CSTR Type II selectivities are equal and independent of conversion, a comparison of yields based on equivalent total residence times would show the CSTR Type II yield always to be inferior up to the limit of = 00, where, of course, the conversions in the two reactor systems-are equal. 

How does the range obtained in problem 14 compare with a similar magnitude of effect in a Type II selectivity (A ) for Si = 10 ... 

Type II selectivity is characterized by parallel (or simultaneous) reactions of a single reactant species. Here the reactant molecule A can be converted into either the desired product B or an undesired product C, symbolized by the reaction scheme ... 

If the two competing reactions are of the same kinetic order (e.g., first order), then the catalyst pore structure will not affect Type II selectivity. This is because at each point in the pore structure the two reactions will proceed at the same relative rate, ki/kt, independent of the partial pressure of A. Hence the yield of the desired product B will be ki/(ki + fcs), independent of the catalyst pore size. (We define yield here as the number of moles of B formed per mole of A reacted.) If, hbwever, the two reactions are of different kinetic order, then selectivity may depend on pore size, since the decrease in partial pressure of A within the pore structure will affect the two competing reaction rates differently. As shown in Fig. 5, the concentration of reactant A will decrease markedly within the catalyst pore structure for a rapid reaction on a catalyst with small pores. If the reaction A—>B were first order... 

Unbound mix Type 1 (dense graded-all crushed) Type II (selective granular material) ... 

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