N-Heptane, dehydrocyclization

Figure 5 shows the relative catalytic activities for n-heptane dehydrocyclization to toluene and for dehydrogenation of cyclohexane. On this catalyst, dehydrocyclization of paraffins can be produced simultaneously by a monofunctional metallic mechanism and a bifunctional one controlled by the acid function (refs. 16-18). The deactivation produced by a small coke deposition should correspond to the deactivation of the contribution of the metallic mechanism to dehydrocyclization. This linear deactivation for the rest of the coke deposition should correspond to the deactivation of the contribution of the bifunctional acid controlled mechanism. The decrease in dehydrocyclization observed during the lineout period is smaller than the decrease in gas formation (by hydrocracking-hydrogenoly5is)i therefore, the selectivity to aromatic hydrocarbons increases during this period. 

When the same Pt-Re-S/Al202 catalyst was coked in laboratory, the fraction of coke on the metallic function was higher than when the catalyst was coked in the commercial unit, as shown in Figure 2, curves C and A. For this reason, the metal function is deactivated more extensively in the case of laboratory coking the residual activity of the reactions controlled by the metal function is smaller and the initial drop of activity in n-pentane hydrocracking and n-heptane dehydrocyclization is greater. 

W.D. Gillespie, R.K. Herz, E.E. Petersen, and G.A. Somoijai. The Structure Sensitivity of n-Heptane Dehydrocyclization and Hydrogenolysis Catalyzed by Platinum Single Crystals at Atmospheric Pressure. J. Catal. IQ AAl (1981). 

Figure 17C shows the effect of copper and iron on the n-heptane dehydrocyclization. 

As catalytic supports, lanthana has been used in the synthesis of methanol from syngas [32], in the ethane hydrogenolysis and cyclopropane hydrogenation [5], in the n-heptane dehydrocyclization [33] and in the oxidative coupling of methane [34], as well as in diesel soot elimination [35] and diy reforming of methane [36],... 

Stepwise Ce dehydrocyclization was observed over potassia-chromia-alumina as well as potassia-molybdena-alumina catalysts (9, 10). Higher operating temperatures (450°-500°C) of these catalysts facilitate the appearance of unsaturated intermediates in the gas phase. Radiotracer studies indicate a predominant Ce ring closure of C-labeled n-heptane over pure chromia (132,132a). 

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.

It has been reported (115) that n-heptane and n-octane dehydrocyclize upon alloying of palladium with silver. The dehydrocyclization products are to a considerable degree dealkylated. 

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

Beltramini and Trimm (67) utilized Pt-, Sn- and Pt-Sn- supported on y-alumina for the conversion of n-heptane at 500°C and 5 bar. They observed that during six hours less coke per mole of heptane converted was deposited on the Pt-Sn-alumina catalyst than on Pt-alumina however, the total amount of coke formed during six hours was much greater on Pt-Sn-alumina than on Pt-alumina. The addition of tin increased the selectivity of dehydrocyclization. Since hydrocracking and isomerization activity of a Sn-alumina catalyst remained high in spite of coke formation, the authors concluded that there was little support for the suggestion that tin poisons most of the acid sites on the catalyst. These authors (68) also measured activity, selectivity and coking over a number of alumina supported catalysts Pt, Pt-Re, Pt-Ir, Pt-Sn and Pt-... 

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. 

In some cases, a complete estimation of the relative contributions of the various pathways of cyclic and bond shift types requires the simultaneous use of a number of C-labeled molecules. Thus far the most complicated example is the isomerization of 3-methylhexane. This molecule, which dehydrocyclizes in three different ways, to 1,2-dimethylcyclopentane, 1,3-dimethyIcyclo-pentane, and ethylcyclopentane (Scheme 17), may isomerize by 23 different pathways, consisting of both cyclic and bond shift types. In particular, four parallel pathways account for n-heptane, and five for self-isomerized 3-methylhexane (4J). Therefore, even when using all the possible labeled molecules, one cannot distinguish between all the isomerization pathways, since the complete location of C in 3-methylhexane cannot be completely achieved. 

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. 

Data on rates of dehydrocyclization rD and cracking rc of n-heptane at 495°C and 14.6 atm are given in Table 5.2 for platinum-iridium on alumina and platinum-rhenium on alumina catalysts, and also for catalysts containing platinum or iridium alone on alumina (33). The rate rD refers to the rate of production of toluene and C7 cycloalkanes, the latter consisting primarily of methylcyclohexane and dimethylcyclopentanes. The rate of cracking is the rate of conversion of n-heptane to C6 and lower carbon number alkanes. 

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. 

As it is mentioned earlier, there is a significant change in selectivities of toluene and Cg+ aromatics with the addition of zinc. The preferential increase of toluene indicates the possibility of toluene formation from the direet dChydrocyclization/direct aromatization of n-heptane over the Zn/HZSM-5 (Kms), in addition to the cracking-and-oligomerization route (Kai). The direct dehydrocyclization of hydrocarbons (hexane and above) was also reported by Giannetto et al [49] from their studies over Ga-HZSM-5 catalyst. 

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