Adsorption of cumene

The adsorption of cumene and inhibitors on active cracking sites follows a Langmuir type of isotherm. This means that there is little or no interaction among the chemisorbed molecules on the surface. This might be expected to be the case as studies of the chemisorption of the inhibitor quinoline by similar catalyst (14) show that the surface is sparsely covered with active sites (<5% of internal surface area covered with chemisorbed quinoline at 315°C.). In addition, the active sites are homogeneous with respect to adsorption energies. 

A conceptual model depicting the sequences of steps in this platinum-catalyzed reaction is shown in Figure 10-10. Figure lO-lOis only a schematic representation of the adsorption of cumene a more realistic model is the formation of a complex of the -it orbitals of benzene with the catalytic surface, as shown in Figure 10-11. 

The rate expression for the adsorption of cumene as given in Equation (10-22) is... 

To answer this question we shall assume that the adsorption of cumene is indeed rate-limiting, derive the corresponding rate law, and then check to see if it is consistent with experimental observation. By assuming that this (or any other) step is rate-limiting, we are considering that the reaction rate constant of this step (in this ease k/ ) is small with respeet to the specific rates of the other steps (in this case ks and The rate of adsorption is... 

Combining Equations (10-41) and (10-37), we find that the rate law for the catalytic decompositon of cumene, assuming that the adsorption of cumene is the rate-limiting step, is... 

The initial entropy of cumene adsorption on a commercial aluminosilicate was measured to be about —750JmoP K , whereas the gas-phase entropy of cumene at 298 K is 389 J moP K (709). This seemingly inconsistent result appears to be caused by the dissociative adsorption of cumene at low coverages on this catalyst. In this case, the measured heat corresponds to a combination of heats of adsorption and reaction. Higher coverages produced lower, nearly constant heats and entropies of adsorption. These entropies correspond to the loss of between two and three degrees of translational freedom. The adsorption of benzene on these samples did not show abnormally... 

Adsorption of cumene on Hp at room temperature gives rise to two sharp UV absorptions at 220 and 260 nm (Fig. 6), whose intensities change slightly with time.Upon evacuation at increasing temperature a new band appeared at 330 nm (at 473 K), which shifted to 345 nm at 573 K. Prolonged evacuation at this temperature blacked sample. 

In order to investigate changes in the characteristics of the mordenite as aluminum was extracted, studies were made of the adsorption of cumene, the cracking and hydrocracking of cumene, and the isomerization of 1-butene. 

When cumene hydroperoxide is heated to cumene cracking temperatures, at least partial decomposition probably occurs. Thus, it is desirable to measure the inhibitor action of the decomposition products of cumene hydroperoxide. Some low-temperature thermal decomposition products which were identified (by chemical and mass spectroscopic analyses) are acetophenone, phenyl-dimethyl-carbinol, a-methylstyrene, phenol, acetone, and methyl alcohol. Kharasch, Fono, and Nudenberg (7) obtained similar results. According to them, the chief decomposition products at 158° are acetophenone and phenyl-dimethyl-carbinol, with acetophenone becoming relatively more important at higher temperatures. Since the temperature used for the cracking reaction is above 300°, acetophenone is probably the most important decomposition product. The equilibrium constant for the adsorption of cumene hydroperoxide and some of the individual decomposition products on catalytic sites are included in Table I. 

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