Abstract
The dehydrogenation reaction of hydrocarbons catalyzed by active carbon proceeds via a radical mechanism to produce the corresponding olefins and the atomic hydrogen and molecular hydrogen. The atomic hydrogen often disturbs dehydrogenation reactions, because it decreases the activity of active carbon and induces the reverse reaction of dehydrogenation, namely the hydrogenation reaction of dehydrogenated products. The dehydrogenation catalyzed by active carbon, therefore, seems to include complicated phenomena. However, these phenomena have not yet been clarified completely. In this study, dehydrogenation reactions of cyclohexane (and cyclohexene) catalyzed by active carbon were tried. The dehydrogenation mechanism of cyclohexane and the behavior of an atomic hydrogen on active carbon are discussed.
The reaction was carried out using a fixed-bed flow reactor packed with coconut charcoal. Products were qualitatively and quantitatively analyzed by gas liquid chromatography.
In the dehydrogenation of cyclohexane (Figs. 1-3), cyclohexene and benzene were obtained as main products. Dehydrogenation of cyclohexene, on the other hand, produced not only benzene, but also cyclohexane and formed trace of cyclohexadiene too, especially under mild conditions (Fig. 8, Table 1). These results strongly suggest that the dehydrogenation of cyclohexane procecds through successive reactions of cyclohexane→ cyclohexene→cyclohexadiene→benzene. But cyclohexene seemed to give benzene (and cyclohexane) by a disproportionation reaction, because it undertook the dehydrogenation reaction more easily than cyclohexane and also acted as a hydrogen acceptor.
The atomic hydrogen absorbed on active carbon was easily removed by oxygen (Figs. 1, 4), resulting in the high conversion of cyclohexane. This fact may explained not only by the removal of the hydrogen to give vacant active sites, as well as by the formation of new active sites under the action of the resulting water (Fig. 5). The atomic hydrogen formed as the result of dehydration reactions of cyclohexane and cyclohexene, was defferent from molecular hydrogen in the point of view that the former was so active as to hydrogenate benzene (Table 2).
