Dehydrocyclization of n-heptane

Stepped surfaces withstand cyclic oxidation-reduction treatments (146) like [111] and some other low-index planes. Steps have either [311] or [110] structures. They are claimed to be the only places where orbital hybridization does not take place (136). No wonder that such platinum (138) and iridium (147) surfaces have enhanced activity in Cg dehydrocyclization of n-heptane. 

Eischens and Selwood 176) have made a study of the activity of reduced chromia-on-alumina catalysts for the dehydrocyclization of n-heptane. The activity per unit weight of chromium was found to increase sharply at concentrations below about 5 wt. % Cr activities were measured down to 1.9 wt. % Cr concentration at which point the highest activity was observed. Selwood and Eischens concluded that this effect is due to the fact that the chromia is most dispersed at these low concentrations, in agreement with the present EPR data. However, if a two-site mechanism 177) is necessary for dehydrocyclization, the activity may drop at even lower chromium concentrations due to isolation of individual chiomium spins. 

Fig. 9. Effect of total pressure on dehydrocyclization of n-heptane over platinum-alumina catalyst (H7). Conditions 496°C., H2/HC = 5/1.

A1203 support Dehydrogenation of cyclohexane, dehydrocyclization of n-heptane, chemisorption of H2. 

Fig. 5 (right). Idem Fig. 4. O, dehydrocyclization of n-heptane. O, dehydrogenation of cyclohexane...

Carbon Overlayers and Active Species.—One of the most interesting features of the paper by Nieuwenhuys and Somorjai on dehydrogenation of cyclohexane and dehydrocyclization of n-heptane over (111) and stepped [6(111) x (100)] single crystals of Ir is their observation of carbon overlayers. AES and LEED were employed in the surface study. The stepped Ir surface was three to five times more active in dehydrogenation than the Ir(lll) surface. The dehydrocyclization reaction rate was not sensitive to the face of Ir on which the reaction was conducted. 

As in the case of hydrogenolysis of cyclopentane, the change in selectivity observed in pulse and flow systems for the 1-5 dehydrocyclization of n-heptane to ethylcyclopentane and 1,2-dimethylcyclopentane, for instance, was interpreted by two modes of adsorption involving five or seven carbon atoms in contact with the surface (775) (Fig. 3). 

However, there is also evidence that dehydrocyclization may proceed by another route involving only the metal component of the catalyst. It has been observed that unsupported platinum powders catalyze the dehydrocyclization of n-heptane (21). Also, Dautzenberg and Platteeuw (25) report that dehydrocyclization of n-hexane to benzene occurs over a catalyst in which platinum is supported on a nonacidic alumina. Since bifunctional catalysis with participation of acidic sites is then presumably eliminated, the activity is attributed to the platinum itself. 

The rates of dehydrocyclization of n-heptane for the iridium and platinum-iridium catalysts are more than twice as high as the rate for the platinum catalyst, and almost twice as high as the rate for the platinum-rhenium catalyst. The rates of cracking are also higher for the iridium and the platinum-iridium catalysts. 

It is of interest to compare the structural data, obtained as described above, with actual activity data on the same catalysts. The reaction chosen was the dehydrocyclization of n-heptane to toluene. All catalyst samples were those prepared by impregnation as already described. 

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 ... 

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