Dehydrogenation of Normal Paraffins over Metal Supported Carbon Catalysts

Abstract

Dehydrogenation of n-hexane and other normal paraffins (C₃-C₈) was studied on active carbons impregnated with iron and copper. The active carbon which contained 5wt% of copper (C-Cu) showed higher catalytic activities compared to metal free active carbon (C) for all hydrocarbons tested (Table 1). The maximum activity difference between the two catalysts was observed in the case of n-pentane or n-butane dehydrogenation. The reactivity of n-paraffin increases with the number of carbon atoms in the molecule (Fig. 1). The major dehydrogenated product was the thermodynamically-equilibrated mixture of mono-olefins (in the case of C₃-C₅ paraffins). From normal paraffins with 6 or more carbon atoms, aromatic hydrocarbons were formed as well as mono-olefins and di-olefins (Table 2). Ethylbenzene and o-xylene were the only aromatic hydrocarbons formed from n-octane, suggesting that the six membered ring is directly formed from a straight chain. The activity of iron supported (5wt%) active carbon (C-Fe) was higher than that of C-Cu (Table 4). On C-Fe, hydrogenolysis of n-hexane (forming methane) proceeded besides dehydrogenation. The hydrogenolysis on C-Fe was suppressed by pretreating it with H₂S or alkali carbonate or by alloying it with copper (Table 5).
On C-Cu and C-Fe, the adsorption rates of hydrogen at 400°C were far higher than the rate on C (Fig. 3). The temperature-programmed desorption of the adsorbed hydrogen showed that the hydrogen on C-Fe began to desorb at a temperature level lower by about 200°C than that on C (Fig. 4). The amount of desorbed hydrogen from C-Fe in the temperature range up to 550°C was about ten times of that from C. These results lead us to the conclusion that, on the metal supported carbons, the movement of hydrogen between carbon surface and gas phase is far more rapid than on C (because of the spillover phenomenon); thus, they exhibit higher catalytic activities.