Dehydrogenation to benzene

Cyclohexane can be dehydrogenated to benzene very cleanly under the same conditions with the same copper-silver catalyst, as can 2-propanol to acetone. These catalysts almost certainly act by virtue of an oxide layer on the metal. 

The second aromatization reaction is the dehydrocyclization of paraffins to aromatics. For example, if n-hexane represents this reaction, the first step would be to dehydrogenate the hexane molecule over the platinum surface, giving 1-hexene (2- or 3-hexenes are also possible isomers, but cyclization to a cyclohexane ring may occur through a different mechanism). Cyclohexane then dehydrogenates to benzene. 

Cyclohexane is a colorless liquid, insoluble in water but soluble in hydrocarbon solvents, alcohol, and acetone. As a cyclic paraffin, it can be easily dehydrogenated to benzene. The dehydrogenation of cyclohexane... 

Figure 9.2 Variation of the rate of cyclohexene dehydrogenation to benzene with gold coverage at Pt(100) at 373 K. (Reproduced from Ref. 8).

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 ... 

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.)...

Moro-Oka et al. (1976) have reported that the oxidation of 9,10-dihydroanthracene by K02 solubilized in DMSO by 18-crown-6 gives mainly the dehydrogenated product, anthracene. Under the same conditions, 1,4-hexadiene is dehydrogenated to benzene. The authors proposed a mechanism in which the superoxide ion acts as a hydrogen-abstracting agent only. The oxidations of anthrone (to anthraquinone), fluorene (to fluorenone), xanthene (to xanthone) and diphenylmethane (to benzophenone) are also initiated by hydrogen abstraction. 

The naphthene isomerization process has been applied also to the conversion of meth-ylcyclopentane to cyclohexane for subsequent dehydrogenation to benzene. Shell s Wilmington, Calif., refinery has been operating commercial equipment on this basis since March 1950 (18). 

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. 

Even though n-hexane is a minority hydrogenolysis product, it is a reliable measure of the degree of hydrogenolysis because of its ease of mass spectro-metric detection and it is not formed in a background reaction with the walls of the reaction chamber. Besides the saturated hydrogenolysis products and benzene, we found the olefinic products cyclohexene, ethylene, and propylene. Cyclohexene is an intermediate in the dehydrogenation to benzene and its various reactions will be discussed separately in the next section. The olefinic product distribution of ethylene propylene cyclohexene benzene is 10 1 0.5 1. 

Fig. 26. Cyclohexene dehydrogenation to benzene as a function of (a) step density and (b) kink density. Standard reaction conditions.

Dehydrogenation activity has been demonstrated for Rh, Co, and Ni forms of zeolites X and Y (123-125), Both cyclohexane and tetralin dehydrogenation to benzene and naphthalene, respectively, have been used as test reactions. For NiX zeolites, unreduced Ni2 + ions were considered (124) to be the active centers. The incorporation of Ca2+ ions into the zeolite... 

The results presented in Fig. 17 for diffusion flames and those from shock tubes clearly indicate that fuel structure does indeed play a role in a fuel s tendency to form particulates—in significant contrast to the results observed in premixed flames. One may conclude, then, that a fundamental knowledge of a fuel s pyrolysis chemistry [51, 76] will allow one to predict its relative tendency to soot with respect to the results presented in Fig. 17. For example, cyclohexadienes are known to dehydrogenate to benzene during pyrolysis and, indeed, the data in... 

Naphthene isomerization has been applied also to the conversion of methylcyclopentane to cyclohexane for subsequent dehydrogenation to benzene (24). 

The adsorption of 1,3-cyclohexadiene on Pt(lll) leads to irreversible dehydrogenation to benzene below 400 K, the majority of which is further dehydrogenated at higher temperatnres to form a carbonaceons residue [48], No 1,3-cyclohexadiene desorbed dnring TPD from the Sn/Pt(l 11) alloys, but the monolayer was converted with 100% selectivity to prodnce gas-phase benzene. No carbon was left on the alloy snrfaces after TPD as determined by AES. The ensemble requirement for cyclohexadiene dehydrogenation on these alloys is at most a few (4-5) Pt atoms (see Pig. 2.1). 

Cyclohexane can be dehydrogenated to benzene and also benzene can be hydrogenated to cyclohexane, but these reactions cannot be controlled to give the intermediate olefin and diene. Cyclohexene and cyclohexadiene exhibit the characteristic addition reactions of unsaturated hydrocarbons. Cyclohexatriene, or benzene, shows unexpected properties in that it exhibits saturated properties imder ordinary conditions and gives some of the addition reactions of uhsaturated hydrocarbons with extreme difficulty. 

Over TaS2 and intercalates MTa3Se (M = Fe, Co, Ni) cyclohexene (373-673 K) in hydrogen isomerized selectively to 1-methylcyclopentene and in nitrogen dehydrogenated to benzene. Intercalation of Fe and Ni increased activity without changing selectivity, but Co caused activity to decrease. The intercalated metals also decreased the rate of deactivation of the catalyst. 

In addition to hydrogen chemisorption and ethane hydrogenolysis, the reactions of cyclohexane provide a useful chemical probe for investigating ruthenium-copper aggregates. On pure ruthenium, two reactions of cyclohexane are readily observed dehydrogenation to benzene and hydrogenolysis to lower carbon number alkanes. The product of the latter reaction is predominantly methane, even at very low conversions. 

Catalysts such as these are referred to as dual functionar because the platinum and the acid sites are discrete components. They cooperate in promoting the desired over-all reactions, but each appears to be responsible for certain steps. Details of the reaction mechanisms will be discussed later, but the concept of dual functionality can be illustrated by the simple reaction shown in Figure 3. Cyclohexane is dehydrogenated to benzene via cyclohexene. While the second dehydrogenation step is quite rapid, some of the cyclohexene is also trapped on acid sites and isomerized, appearing in the product largely as methyl-cyclopentane. The dual functional nature of the catalyst can be demon-... 

Fewer metals fall into the grouping of catalysts of sextet dehydrogenation than into that of metal catalysts whose activity is accounted for by the principle of preservation of the valence angle. In aecordance with the theory, cyclohexane could not be dehydrogenated to benzene on manganese which possesses a more complicated structure, A12 (204) the olefin bond is hydrogenated over Mn (204). 

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