Thermodynamics dehydrocyclization

So far, we have not discussed the overall reactant-product thermodynamics for these dehydrocyclization reactions (based on carbocation intermediates), and these are shown in Table III and Figure 9. 

We have not carried out calculations starting with secondary cations derived from the title alkanes because at a computational level, these will have ground-states and transition-states similar to heptane itself (previously discussed). This will be true even though the most stable carbocations in these branched alkanes will be the corresponding tertiary ions, which in themselves will not be directly involved in dehydrocyclization processes. However, one has to keep in mind that the thermodynamic ground-states in these real catalytic reactions will be the alkanes themselves, and in this regard secondary cations formed from n-octane or 2- (or 3-) methylheptane will not differ much in absolute energy. As shown earlier, a 1,6-closure of 2-methylheptane leads eventually to m-xylene, while 3-methylheptane has eventual routes to both o- and p-xylene. 

Table III. Overall thermodynamics (kcal/mol) for dehydrocyclization via 1-5 and 1-6 fi-H bridged intermediates...

Table 4.1 Thermodynamic parameters of gas-phase reactions of combined dehydrocyclization of light and long alkanes...

Also formed in considerable quantities are C9-C12 polycyclic hydrocarbons, particularly Tetralins and indanes. The formation of these bi-cyclic species is surprising. Under the high hydrogen pressures employed in the hydrocracking reaction, dehydrocyclization is not favored thermodynamically. For example, in Table V, equilibrium calculations indicate that the cyclization of n-butylbenzene to form Tetralin and hydrogen is unfavorable. 

Coke Deposition on the Catalytic Functions and Their Deactivations. The acid and metal functions of the reforming catalysts are balanced to have the highest possible yield in the bifunctional reaction of paraffin dehydrocyclization. Coke is deposited on both acid and metal sites, decreasing their catalytic activities. It is important to know (under commercial conditions) the deactivation on both sites and which controls the reactions and fixes the catalyst s practical cycle. If the rate of reaction is several orders of magnitude higher on one site than on the other, the deactivation of the first site does not modify the rate of the bifunctional reaction and the deactivation of the second will control the whole reaction. If a reaction is so rapid on a catalytic site that it is under thermodynamic equilibrium, the deactivation of the site will not be noticed if the reaction is kinetically controlled, it is possible to follow the site deactivation by means of this reaction change. 

Another major function of a catalyst is to provide reaction selectivity. Under the conditions in which the reaction is to be carried out, there may be many reaction channels, each thermodynamically feasible, that lead to the formation of different products. The selective catalyst will accelerate the rate of only one of these reactions so that only the desired product molecules form with near-theoretical or 100% efficiency. One example is the dehydrocyclization of n-heptane to toluene ... 

The aromatics are more stable than the paraffins at high temperatures (above 400°C). Therefore, the dehydrocyclization of paraffins is thermodynamically very feasible under these conditions. At low temperatures the opening of the aromatic ring and hydrogenation to the paraffin (eg, from benzene to n-hexane) is favored. 

Thermodynamics. The equilibrium constant for dehydrocyclization reaction is very high. For example, in the case of re-hexane transformation to benzene, the reaction is shown in (Scheme 5) and the equilibrinm constant is... 

Dehydrocyclization is highly desirable but, unfortunately, is the least favorable kinetically (relative activity = 1), and thermodynamically favored at high temperatures. Because of their low relative activities, hydrocracking and dehydrocyclization do not operate near equilibrium. The equilibrium of dehydrocyclization is illustrated by the stability diagram in Figure 6.9.3 for the example of n-heptane conversion into toluene (+4H2)- The equilibrium constant is given by ... 

Since dehydrogenation and dehydrocyclization are the most important for the octane number gain, operation conditions favoring these reactions are selected. Increasing the reaction temperature favors the thermodynamics, but accelerates the hydrocracking reactions, leading to a loss in yield. 

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