Dehydrogenation of cyclohexene to benzene

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

Primary structure sensitivity resulting from the effect of changing particle size on step and kink density appears therefore to be present here at short reaction times. Secondary structure sensitivity (including the effect of carbonaceous poisoning on the reaction rate) appears not to be present here. Thus Somorjai has reported that the dehydrogenation reaction of cyclohexane to cyclohexane is insensitive to both structural featureSt whereas the dehydrogenation of cyclohexene to benzene la very sensitive to the densities of atomic steps and kinks and the order of the carbonaceous overlayer on the platinum crystal surface. 

Ethylene is more rapidly oxidized than propylene. Furthermore, the substituted ethylenes do not display the dependency on reactivity of allyl C—H bonds shown over bismuth molybdate (Tables XI and XII). It is clear that the C02-producing reaction is favored by unsaturation, but not by allyl hydrogens. In fact, over Pt ter<-butylethylene, without any allyl hydrogen, was oxidized about as fast as the methylethylenes. Dienes and acids were found to inhibit the oxidation of olefins over the metals. Acetone, like acetic acid from ethylene over Pd, is considered a side reaction product rather than an intermediate. The only selective oxidation observed was an oxidative dehydrogenation of cyclohexene to benzene over Pd at —20 to +30° here no CO2 was produced. 

Oxidative dehydrogenation of cyclohexanones with [Pd(CF3C02)2] is believed to occur by the mechanism shown (Scheme 5) in which the palladium remains in the +II oxidation state throughout. A related mechanism has been proposed for the [Pds(PPh)2]-catalyzed oxidative dehydrogenation of cyclohexene to benzene. ... 

For purposes of comparison, it is of interest to consider the results of kinetic studies of the dehydrogenation of cyclohexane to benzene over certain other catalysts. Herington and Rideal (H6) studied the kinetics at 400-450°C. using a chromia-alumina catalyst containing 12% Cr203. These authors concluded that cyclohexene was an intermediate in the reaction, so that desorption and readsorption of cyclohexene were essential... 

In a more recent study of the dehydrogenation of cyclohexane to benzene over a chromium oxide catalyst at 450°C., Balandin and coworkers (Dl) concluded that benzene was formed by two routes. One of these, the so-called consecutive route, involves cyclohexene as a gas phase intermediate, while the other proceeds by a direct route in which intermediate products are not formed in the gas phase. It was concluded that the latter route played a larger role in the reaction than did the former. These conclusions were derived from experiments on mixtures of cyclohexane and Cl4-labeled cyclohexene, which made it possible to evaluate the individual rates Wi, BY, Wt, and Wz in the reaction scheme... 

These results have profound effects for the selective catalytic dehydrogenation of cyclohexane to benzene, a prototypical hydrocarbon conversion reaction. On Pt(lll), the intermediates, cyclohexene and a species, have been identified and the rate constants for some of the sequential reaction steps measured [56]. Adsorption and reaction studies of cyclohexane [39], cyclohexene [44], 1,3-cyclo-hexadiene [48], and benzene [39] on the two Sn/Pt(lll) alloys provide a rational basis for understanding the role of Sn in promoting higher selectivity for this reaction. One example of structure sensitivity is shown in Fig. 2.7, in which a monolayer of cyclohexyl (C H ) was prepared by electron-induced dissociation (EID) of physi-orbed cyclohexane to overcome the completely reversible adsorption of cyclohexane... 

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... 

Fig. 3. Dehydrogenation of cyclohexane to cyclohexene and benzene over chromia-alumina (H6).

The usual step-by-step dehydrogenation of cyclohexane to cyclohexene, cyclohexadi-ene and benzene is a thermodynamically hindered reaction ... 

The mechanism of reaction included the rapid oxy-dehydrogenation of cyclohexane to cyclohexene and benzene. The latter was then transformed to successive products (see the section on benzene ammoxidation). The opening of the ring may occur either for cyclohexene or for benzene itself. 

Much higher yields are found for the oxidation of alkylarenes to aldehydes, ketones and carbon acids by oxygen in the presence of transition metal compounds (MnBr2, CoBr2, CuBr) as catalysts [57]. For example, a yield of 30 % benzaldehyde and only 10 % benzoic acid was found for the oxidation of toluene. Also the dehydrogenation of cyclohexene and cyclohexane to benzene with Pt02 as catalyst was investigated at 375 °C [34]. 

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