Cyclohexane dehydrogenation—

is the reaction structure sensitive or insensitive According to Boudart, supported catalysts have dispersed metallic particles of different sizes d. With increasing or decreasing particle sizes, the concentration of accessible atoms or sites varies significantly, indicating surface structural changes. Structure sensitive reactions (SSRs) are those reactions where the intrinsic reaction rate relative to the accessibility of surface sites or the turnover frequency (TOF) varies with particle sizes or diameters, but it does not vary for structure insensitive reactions (SIRs). It means, for SSRs, the TOF depends on particle sizes, or dispersion of particles or on the accessible surface sites of the catalyst. On the other hand, for SIRs, the activity or TOF is independent of the particle sizes. 

Dehydrogenation reactions are known as SIRs, while hydrogenolysis reactions are SSRs. Some reactions include several steps simultaneously with different reactions, which are structure sensitive or insensitive. 

Benzene is the final product for all tests. The activity or TOP values remain constant around 1 s suggesting that the rate is proportional to accessible active sites. From the Arrhenius plot, one determines the activation energy which is also constant. 

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Fig. 6. Activities of copper-nickel alloy catalysts for the hydrogenolysis of ethane to methane and the dehydrogenation of cyclohexane to benzene. The activities refer to reaction rates at 316° C. Ethane hydrogenolysis activities were obtained at ethane and hydrogen pressures of 0.030 and 0.20 atm., respectively. Cyclohexane dehydrogenation activities were obtained at cyclohexane and hydrogen pressures of 0.17 and 0.83 atm, respectively (74).

Catalytic reformers. Catalytic reforming is an important step to improve the quality of gasoline. During the reforming process, naphthens are dehydrogenated to aromatics. As a representative example, hydrogen is produced by cyclohexane dehydrogenation to benzene as follows ... 

The rate enhancement for cyclohexane dehydrogenation observed for submonolayer copper deposits may result from changes in the geometric (6) and the electronic (8) properties of the copper overlayer relative to bulk copper. Alternatively, the two metals may catalyze different steps of the reaction cooperatively. For example, dissociative adsorption on bulk copper is unfavorable because of an activation barrier of approximately 5 kcal/mol (33). 

Figure 2. Relative rate of reaction vs. surface Cu coverage on Ru(0001) for cyclohexane dehydrogenation to benzene. PT = 101 Torr. Hj/cyclohexane = 100. T = 650 K. (Data from ref. 10.) (Reprinted with permission from ret 42. Copyright 1986 Annual Reviews, Inc.)...

The addition of Cu to Ru(0001) results in a dramatic enhancement of the rate of cyclohexane dehydrogenation, despite the fact that Cu is much less active for this reaction than is Ru. 

Itoh, N., Y. Shindo, K. Haraya and T. Hakuta. 1988. A membrane reactor using microporous glass for shifting equilibrium of cyclohexane dehydrogenation. J. Chem. Eng. Japan 21(4) 399-404. 

Examples 22-25, all for cyclohexane dehydrogenation over Pt, are similar to the examples just discussed in that the data are all for surface reactions. Also with each of them the log L value calculated for Step 5 is between 8 and 10. We reported similar results and log L calculations (109) and suggested, using independent evidence of Boudart (110), that the site density is not as low as our TST calculations indicate. Thus, the log L calculation can be used to demonstrate the existence of a complexity that might not otherwise be detected, in this case, the possibility that cyclohexene is an intermediate. 

The reaction might be more complicated than indicated. The existence of an intermediate, as postulated in cyclohexane dehydrogenation, is sometimes a possibility. 

Stepwise cyclohexane dehydrogenation revealed the possible importance of unsaturated intermediates in benzene formation 48). 

Cyclohexane dehydrogenates rather rapidly to benzene. Its rearrangement has not been reported over pure metals until now. Cg Ring opening is negligible over platinum and palladium 48, 5i) slight hexane formation was reported over carbon supported rhodium, iridium, and, especially, osmium and ruthenium (702), as well as over nickel on alumina (99). 

A variety of alkanes can be dehydrogenated with catalysts such as 1. Whilst COA dehydrogenation usually stops at the COE stage, cyclohexane dehydrogenation affords benzene in good yields [7]. Substituted cyclohexanes are dehydrogenated at both exocycHc and endocyclic positions for example, ethylcyclohexane dehydrogenation produces styrene as the major product (Equation 12.1). 

In the case of reversible reaction, Hougen and Watson proposed the similar semi-empirical equation (HW equation). For instance, for the reaction of cyclohexane dehydrogenation this equation has the form... 

Figure 8.20 Activity of Ni-Cu alloys towards cyclohexane dehydrogenation and ethane hydrogenolysis. (Following Sinfelt, 1977.)...

Cyclohexane forms a (9 X 9) surface structure on the Ag(l 11) crystal face and a (j j) surface structure on the Pt(l 11) crystal face at around 200 K. This latter surface structure corresponds to the (001) surface orientation of the monoclinic bulk crystal structure of the molecule. On heating the platinum crystal face to 450 K a ( ) surface structure forms that is identical to the surface structure formed by cyclohexene monolayers at the same temperature. It appears that cyclohexane dehydrogenates at elevated temperatures on platinum to form the same species or that of cyclohexene. 

With respect to the atomic arrangements of the surfaces, the adsorption of cyclohexane occurs very similarly on (111) and (110) planes, in the former case as a nondissociative complex of symmetry C3v as is often the case, the results on the (100) face [of Pt(100)] are qualitatively different. On Ni the (111) face is less reactive for cyclohexane dehydrogenation than the stepped and kinked [5(111) X (110)] plane. 

We have used our Single Turnover (STO) reaction sequence to characterize dispersed metal catalysts with respect to the numbers of alkene saturation sites, double bond isomerization sites, and hydrogenation inactive sites they have present on their surfaces (ref. 13). Comparison of the product composition observed when a series of STO characterized Pt catalysts were used for cyclohexane dehydrogenation with those observed using a number of instrumentally characterized Pt single crystal catalysts has shown that the STO saturation sites are comer atoms of one type or another on the metal surface (ref. 10). 

Fig. 21. (a) Cyclohexane dehydrogenation to benzene (O) and hydrogenolysis to n-hexane (A) as a function of step density, (b) Cyclohexane dehydrogenation to benzene and hydrogenolysis to n-hexane as a function of kink density at a constant step density of 2.0 x 1014/cm2. 

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