Cyclohexane to benzene dehydrogenation

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.)...

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. 

TABLE 7.27. Kinetic Parameters for Cyclohexane Dehydrogenation to Benzene Over Metal Catalysts ... 

In 2000, Itoh and Haraya constructed the first CMR and experimentally examined the performance of a dehydrogenation reaction. Asymmetric polyimide hollow fibers were pyrolyzed in a vacuum oven at 1023 K in order to obtain hollow fiber carbon membranes. Their CMR consisted of SS in which 20 carbonized hollow fibers (0.295 mm diameter and 128 mm long) and catalyst pellets (0.5 wt% Pt/Al203) were allocated. The reactor, used for cyclohexane dehydrogenation to benzene at 468 K, showed a fair improvement over equilibrium conversions. In detail, the temperature dependency of the permeation rates showed that the carbon membrane had micropores with an average diameter close to those of the gas molecules and therefore the permeation process was molecular-sieving controlled. The ideal H2/Ar... 

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... 

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). 

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... 

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

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. 

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). 

Some other authors (221) thought that the dehydrogenation of cyclohexane to benzene goes on in such a manner that first cyclohexane dehydrogenates to cyclohexene, and then in the latter a much faster disproportionation of hydrogen occurs, in conformity with the reaction of the so-called irreversible catalysis of Zelinskii with formation of benzene... 

Cyclohexane dehydrogenation represents another classical example for isotopic studies. Balandin s sextet mechanism predicted direct dehydrogenation of cyclohexane over several metals, assuming a planar reactive chemisorption of the reactant. Cyclohexene is also readily dehydrogenated to benzene. The use of hydrocarbons labelled with established the true reaction pathway. T6tenyi and co-workers[ °di] reacted a mixture of [ 0]-cyclohexane and inactive cyclohexene on different metals and measured the specific radioactivity of the fractions (cyclohexane, cH, cyclohexene, cH= and benzene, Bz) in the product at low conversion values (Table The... 

On metallic catalysts, cyclohexene can either be hydrogenated or dehydrogenated to benzene, depending on the temperature and hydrogen pressure the first is favoured at low temperatures, but it can also disproportionate to give cyclohexane and benzene ... 

Thermodynamics. The dehydrogenation of naphthenes to aromatics at typical reforming reactions conditions, such as 500°C, has an equilibrium conversion of 100% as long as the hydrogen concentration is not too high. Figure 3 (9) shows the cyclohexane conversion to benzene as a function of temperature, at different pressures, when the H2/CH initial ratio is 10. At 500°C, only at pressures above 3 MPa, the conversion is not 100%. Reforming pressures in modern reactors is well below this value therefore, the conversion of CH to B will be complete. The equilibrium constant for this reaction is... 

The alkylcyclopentane (AGP) to aromatics process (ACP ACH Ar) is less efficient than ACH dehydrogenation, owing to the slowness of the first step and to ACP ring opening. Under conditions where cyclohexane is converted to benzene with close to 100% efficiency, only 50—75% of methylcyclopentane may be converted to benzene. 

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