Cyclohexanes hydrogenolysis

Cu addition leads to an enhanced rate of benzene production with little or no induction time. That is, the initial rate of cyclohexane hydrogenolysis, relative to the Cu-free surface, is suppressed. Further, Cu reduces the relative carbon buildup on the surface during reaction. Thus, Cu may play a similar role as the carbonaceous layer in suppressing cyclohexane hydrogenolysis while concurrently stabilizing those intermediates leading to the product... 

Goodman and co-workers have also studied the kinetics of methanation and cyclohexane hydrogenolysis on their Cu/Ru(0001) model catalysts. In these cases, as with ethane hydrogenolysis, Cu merely serves as an inactive diluent, suppressing the rates on a simple one-to-one site-blocking basis (161, 162). 

The thermodynamical data for the simplest reaction of cyclohexane hydrogenolysis into n-hexane are presented in Table 1. These data show that the high yields can be theoretically expected at quite low temperatures. 

The 1,3-carbon-carbon bond activation (also called y-H activation pathway) has the advantage in that it explains several observations (1) hexane undergoes hydrogenolysis on an Ir/Si02 catalyst at 200 °C (H2/hexane = 50/1), while no hydrogenolysis of ethane is observed up to 270 °C under similar conditions and (2) the hydrogenolysis of hexane and cyclohexane have common features (vide infra) [168]. 

Table 8 Distribution of Ci or -hexane over Ir/SiOz- to C5 primary products in the hydrogenolysis of cyclohexane -Temp = 200 °C, Hz/Alkane ratio = 50 ...

Secondly, this mechanism (1,3-carbon-carbon bond activation) applies to both acyclic and cychc paraffins such as hexane and cyclohexane (Scheme 40 and Table 8). Kinetic studies on the hydrogenolysis of these alkanes are note-... 

As a case study, the hydrogenolysis of alkanes over Ir/Si02 will be studied in detail, and the product selectivities at zero contact time for the hydrogenolysis of hexane and cyclohexane are shown in Table 8. 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

Fig. 6. Activities of copper-nickel alloy catalysts for the hydrogenolysis of ethane to methane and the dehydrogenation of cyclohexane to benzene. The activities refer to reaction rates at 316° C. Ethane hydrogenolysis activities were obtained at ethane and hydrogen pressures of 0.030 and 0.20 atm., respectively. Cyclohexane dehydrogenation activities were obtained at cyclohexane and hydrogen pressures of 0.17 and 0.83 atm, respectively (74).

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