N< hexane hydrogenolysis

Pt-Ir Sulfided, A1203 and Si02 supports n-Hexane hydrogenolysis/reforming. Temp, programmed reduction and X-ray diffraction. 

TABLE 7.12. Kinetic Parameters for n-Hexane Hydrogenolysis Over Metal Catalysts... 

We turn now to the metals that are more active than palladium and platinum for the reactions of alkanes with hydrogen. Rhodium catalysts have been the subject of a number of investigations they were active for n-hexane hydrogenolysis between about 420 and 500 K, and were characterised by giving... 

Nevertheless, in another branched reaction, the hydrogenolysis of methylcyclopentane on Pt-AhOs (10% Pt) at 230°C, leading to 2- and 3-methylpentane (n-hexane is not practically formed under the conditions used)... 

Product Distributions from Hydrogenolysis of Propane and n-Hexane over Nickel Film Catalysts ... 

Dehydrocyclization, 30 35-43, 31 23 see also Cyclization acyclic alkanes, 30 3 7C-adsorbed olefins, 30 35-36, 38-39 of alkylaromatics, see specific compounds alkyl-substituted benzenes, 30 65 carbene-alkyl insertion mechanism, 30 37 carbon complexes, 32 179-182 catalytic, 26 384 C—C bond formation, 30 210 Q mechanism, 29 279-283 comparison of rates, 28 300-306 dehydrogenation, 30 35-36 of hexanes over platintim films, 23 43-46 hydrogenolysis and, 23 103 -hydrogenolysis mechanism, 25 150-158 iridium supported catalyst, 30 42 mechanisms, 30 38-39, 42-43 metal-catalyzed, 28 293-319 n-hexane, 29 284, 286 palladium, 30 36 pathways, 30 40 platinum, 30 40 rate, 30 36-37, 39... 

Higher hydrocarbon molecules allow study of the unique cracking pattern of metals. These studies are usually carried out at low conversion to observe only primary hydrogenolysis. Nickel exhibits high selectivity to cleave terminal C—C bonds leading to demethylation that is, it cleaves only bonds that involve at least one primary carbon atom. For example, in the transformation of n-hexane, only methane and n-pentane are formed (180°C, Ni-on-silica catalyst, 0.3% conversion), whereas 2-methylpentane and 3-methylpentane yield methane, n-pentane, and isopentane.260 In the transformation of 2-methylpentane, the n-pentane isopentane ratio is close to 2, which corresponds to the statistical value. Under more forcing conditions, successive demethylations lead eventually to methane as the only product. 

Since the hydrogenolysis of cyclohexane and cyclohexane derivatives is less probable than the thermodynamically favored dehydrogenation to form aromatic compounds, most studies address hydrogenolysis only in connection with aromati-zation as an unwanted side reaction. An interesting observation by Somorjai showed, however, that hydrogenolysis of cyclohexane to form n-hexane becomes competitive with aromatization on Pt single crystals with increasing kink density.302 On a Pt surface where approximately 30% of the atoms in the steps are in kink positions, benzene and n-hexane are formed in 1 1 ratio. 

Fig. 21. (a) Cyclohexane dehydrogenation to benzene (O) and hydrogenolysis to n-hexane (A) as a function of step density, (b) Cyclohexane dehydrogenation to benzene and hydrogenolysis to n-hexane as a function of kink density at a constant step density of 2.0 x 1014/cm2. 

Even though n-hexane is a minority hydrogenolysis product, it is a reliable measure of the degree of hydrogenolysis because of its ease of mass spectro-metric detection and it is not formed in a background reaction with the walls of the reaction chamber. Besides the saturated hydrogenolysis products and benzene, we found the olefinic products cyclohexene, ethylene, and propylene. Cyclohexene is an intermediate in the dehydrogenation to benzene and its various reactions will be discussed separately in the next section. The olefinic product distribution of ethylene propylene cyclohexene benzene is 10 1 0.5 1. 

In alloys with 0-23% Cu the activation energy of the total conversion of n-hexane is only marginally influenced and the observed effects are consequently connected with the preexponential factors. Since the selectivity of nickel diluted with copper is near the value found by Anderson et al. (113) for highly dispersed films, considering a common cause is suggested (60). Anderson assumes that with a large fraction of surface atoms in very small crystals the isolated corner atoms favor the formation of carbocyclic intermediates of isomerization, whereas hydrogenolysis requires two or more adjacent platinum atoms in a crystal plane. 

The chain length of n-alkanes has a marked influence on reactivities for hydroisomerization, and especially for hydrocracking. To a very small extent a methane and ethane abstracting mechanism, probably hydrogenolysis as predicted in a basic work on bifunctional catalysis (14), is found to be superimposed when lower carbon number feeds (C, Cg, Cg) are used. n-Hexane is excluded from ideal hydrocracking. On the Pt/Ca-Y-zeolite catalyst it is cracked via a mechanism that is mainly hydrogeno-lytic. 

Skeletal rearrangement reactions over Pt single crystals have been studied for methyl cyclopentane, 2- and 3-methylpentane350 and for n-hexane.3sl One conclusion351 is that whereas aromatization reactions are very sensitive to surface structure [Pt(l 11)> Pt(100)], isomerization, Cs-cyclization, and hydrogenolysis reactions display little dependence on structure. Temperature and H2 pressure are more important in affecting the selectivity. 

Pt-Re sulfided, A1203 support Hydrogenolysis and reforming of n-hexane and methylcyclopentane. Coke deposition measured. 

FIG. 13. Common intermediate for dehydrocyclization and isomerization of n-hexane and hydrogenolysis of methylcyclopentane (61). 

With linear alkanes having five or more carbon atoms, cyclization becomes possible as well as isomerization and hydrogenolysis. With n-pentane, cyclization is minimal and with n-hexane it does not exceed 25% in the range 470-570 K [6] with the latter molecule, isomerization predominates above 520 K. Product selectivities are particle-size sensitive, and Pt/SiC>2 catalysts having lower dispersion give more hydrogenolysis and cyclization. 

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