Cyclohexane dehydrogenation over nickel catalysts

Cusumano et al. (128) studied the reaction over Pt on alumina and on silica supports and concluded that the TOF was about the same for both catalysts, which did show quite different atomic rates AR. The later work of Sinfelt et al. (269) on reactions over copper-nickel alloys led also to the suggestion that cyclohexane dehydrogenation over Ni does not require a large ensemble of surface atoms and thus may be structure insensitive on a geometric basis. For ethane hydrogenolysis studied on the same CuNi alloys, it was found that the activity decreased much more rapidly than did the fraction of Ni atoms on the surface of the alloys. This implies that ethane hydrogenolysis requires an ensemble of surface atoms and should show antipathetic structure sensitivity. This reaction will be discussed in connection with Fig. 15 (below). 

The same catalysts which permit the addition of elementary hydrogen to a double bond are able to accelerate the opposite process—dehydrogenation, or elimination of hydrogen—when the temperature is altered. Thus cyclohexane is decomposed into benzene and hydrogen when passed over nickel or palladium black at about 300° (Sabatier, Zelinsky). The equilibrium... 

Figure 5.2.6 I Effect of alloy composition on the rates of ethane hydrogenolysis and cyclohexane dehydrogenation on Ni-Cu catalysts. (Figure from Catalytic Hydrogenolysis and Dehydrogenation Over Copper-Nickel Alloys by J. H.

Another synthesis of pyrogaHol is hydrolysis of cyclohexane-l,2,3-trione-l,3-dioxime derived from cyclohexanone and sodium nitrite (16). The dehydrogenation of cyclohexane-1,2,3-triol over platinum-group metal catalysts has been reported (17) (see Platinum-GROUP metals). Other catalysts, such as nickel, rhenium, and silver, have also been claimed for this reaction (18). 

Another dehydrogenation process was introduced before 1944 for converting methyl-cyclohexane to toluene over a tungsten-nickel sulfide catalyst. Substantially higher hydrogen partial pressures are employed in this process than in hydroforming. 

In the field of hydrocarbon conversions, N. D. Zelinskii and his numerous co-workers have published much important information since 1911. Zelinskii s method for the selective dehydrogenation of cyclohexanes over platinum and palladium was first applied to analytical work (155,351,438,439), but in recent years attempts have been made to use it industrially for the manufacture of aromatics from the cyclohexanes contained in petroleum. In addition, nickel on alumina was used for this purpose by V. I. Komarewsky in 1924 (444) and subsequently by N. I. Shuikin (454,455,456). Hydrogen disproportionation of cyclohexenes over platinum or palladium discovered by N. D. Zelinskii (331,387) is a related field of research. Studies of hydrogen disproportionation are being continued, and their application is being extended to compounds such as alkenyl cyclohexanes. The dehydrocyclization of paraffins was reported by this institute (Kazanskil and Plate) simultaneously with B. L. Moldavskil and co-workers and with Karzhev (1937). The catalysts employed by this school have also been tested for the desulfurization of petroleum and shale oil fractions by hydrogenation under atmospheric pressure. Substantial sulfur removal was achieved by the use of platinum and nickel on alumina (392). 

The effect on activity for the dehydrogenation reaction is very different from that for the hydrogenolysis reaction. In the case of ethane hydrogenolysis, adding only 5 at.% copper to nickel decreases catalytic activity by three orders of magnitude. Further addition of copper continues to decrease the activity. However, the activity of nickel for dehydrogenation of cyclohexane is affected very little over a wide range of composition, and actually increases on addition of the first increments of copper to nickel. Only as the catalyst composition approaches pure copper is a marked decline in catalytic activity observed. 

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