Cyclohexane-methylcyclopentane isomerization

Whereas the cyclohexane-methylcyclopentane isomerization involves initial formation of the cyclohexyl (methylcyclopentyl) cation, that is, via protolysis of a C-H bond, it should be mentioned that in the acid-catalyzed isomerization of cyclohexane, up to 10% hexanes are also formed, and this is indicative of C-C bond protolysis (Scheme 6.10). 

The catalytic properties of metal-zirconium phosphate solid has also been investigated (21, 349). The catalysts were prepared by the ion exchange of zirconium phosphate with copper, nickel, and chromium ions. Cataljdic dehydration of 2-propanol was studied at 160°-350°C, with zirconium phosphate itself giving the highest activity, yielding 97% propylene at 230°-240°C. Introduction of Cu +, Ni +, and Cr " decreased the dehydrating properties, and also decreased the catalytic isomerizing properties when tested with the cyclohexane-methylcyclopentane isomerization. The introduction of copper and nickel improved the dehydration properties of zirconium phosphate when tested on ethylbenzene. 

The reaction (equation 76) of the hexenyl radical 47 forming cyclopentyl-methyl radical was discovered independently in several laboratories and has been of pervasive utility in both synthetic and mechanistic studyThe competition between formation of cyclopentylcarbinyl and cyclohexyl radicals favors the former even though the latter is more stable, and this kinetic preference is explained by more favourable transition state interaction. The effects of substituents on the double bond, heteroatoms in the chain, and many other factors on the partitioning between these two paths have been examined. In the gas phase above 300°C, methylcyclopentane has been observed to form cyclohexane via isomerization of cyclopentylmethyl radicals into the more stable cyclohexyl radicals. ... 

Benzene, naphthalene, toluene, and the xylenes are naturally occurring compounds obtained from coal tar. Industrial synthetic methods, called catalytic reforming, utilize alkanes and cycloalkanes isolated from petroleum. Thus, cyclohexane is dehydrogenated (aromatization), and n-hexane(cycli> zation) and methylcyclopentane(isomerization) are converted to benzene. Aromatization is the reverse of catalytic hydrogenation and, in the laboratory, the same catalysts—Pt, Pd, and Ni—can be used. The stability of the aromatic ring favors dehydrogenation. 

The first published report of the isomerization of a saturated hydrocarbon appeared in 1902, when Aschan reported1 that cyclohexane undergoes isomerization to methylcyclopentane with aluminum chloride. Later researchers were, however, unable to repeat this work, and it was not until 1933 that their failures were, as discussed subsequently, explained by Nenitzescu and coworkers.2 3... 

Cyclohexane-methylcyclopentane isomerization21 can be depicted as in Scheme 4.2. The isomerization of substituted cyclopentanes and cyclohexanes to polymethylcyclohexanes similarly occurs by way of a series of consecutive steps... 

Cracking and disproportionation in the reaction of hexane could be suppressed by the addition of cycloalkanes (cyclohexane, methylcyclopentane, cyclopentane).101 Furthermore, 3-methylpentane and methylcyclopentane also reduced the induction period. These data indicate that reactions are initiated by an oxidative formation of alkene intermediates. These maybe transformed into alkenyl cations, which undergo cracking and disproportionation. When there is intensive contact between the phases ensuring effective hydride transfer, protonated alkenes give isomerization products. 

The results obtained in this study indicate that in Al-ffee H-boralite (BOR 1) only weak BrOnsted acid sites (Si—OH—B) are present. They are active only in cyclohexanol dehydration. Their catalytic activity is, however, relatively low. The insertion of A1 into the framework results in the creation of strong Bronsted acid sites. Most probably they are Si—OH—Al, the same as in zeolites. The IR band which could be characteristic of such Si—OH—Al (at about 3610 cm ) was not seen in the spectrum because of the very low concentration of these hydroxyls. The catalytic activity of Si—OH—Al is much higher that of Si—OH - B. Contrary to Si—OH -B, Si—OH— A1 are active in consecutive reactions of cyclohexene (isomerization and disproportionation). Cyclohexene isomerization (to methylcyclopentenes), a typical carbenium ion reaction is catalysed by strong Brdnsted acid sites even at temperatures as low as 450 K. The same strong Bronsted acid sites catalyse also cyclohexene disproportionation (to cyclohexane, methylcyclopentane and coke). Our earlier... 

The combined liquid effluent from the fluidized-bed run was collected, and a complete analysis was made. The data are shown in Table V. The extensive isomerization taking place is of some interest all of the C6 paraffins except diisopropyl are present in the effluent, along with cyclohexane-methylcyclopentane, MCH-dimethylcyclopentane, etc. Nickel is, of course, known to have isomerizing activity, but in addition the catalyst may be functioning as a dual-function isomerization catalyst. 

Isomerization of cyclohexane in the presence of aluminum trichloride catalyst with continuous removal of the lower boiling methylcyclopentane by distillation results in a 96% yield of the latter (54). The activity of AlCl -HCl catalyst has been determined at several temperatures. At 100°C, the molar ratio of methylcyclopentane to cyclohexane is 0.51 (55). 

Some processes use only one reactor (57) or a combination of liquid- and vapor-phase reactors (58). The goal of these schemes is to reduce energy consumption and capital cost. Hydrogenation normally is carried out at 2—3 MPa (20—30 atm). Temperature is maintained at 300—350°C to meet a typical specification of less than 500 ppm benzene in the product at higher temperatures, thermodynamic equiUbrium shifts to favor benzene and the benzene specification is impossible to attain. Also, at higher temperatures, isomerization of cyclohexane to methylcyclopentane occurs typically there is a 200 ppm specification limit on methylcyclopentane content. 

Likewise it is possible to differentiate between substituted and unsubstituted alicycles using inclusion formation with 47 and 48 only the unbranched hydrocarbons are accommodated into the crystal lattices of 47 and 48 (e.g. separation of cyclohexane from methylcyclohexane, or of cyclopentane from methylcyclopentane). This holds also for cycloalkenes (cf. cyclohexene/methylcyclohexene), but not for benzene and its derivatives. Yet, in the latter case no arbitrary number of substituents (methyl groups) and nor any position of the attached substituents at the aromatic nucleus is tolerated on inclusion formation with 46, 47, and 48, dependent on the host molecule (Tables 7 and 8). This opens interesting separation procedures for analytical purposes, for instance the distinction between benzene and toluene or in the field of the isomeric xylenes. 

An example of a parallel-reaction network is the decomposition of cyclohexane, which may undergo dehydrogenation to form benzene and isomerization to form methylcyclopentane, as follows ... 

Fig. 1. Reaction composition profile. Reforming at 794 K, 2620 kPa. Zone A dehydrogenation zone zone B isomerization zone zone C hydrogenation and cracking zone. [Charge stock A, hexane (HEX) , benzene (BENZ) V, cyclohexane (CH) O, methylcyclopentane (MCP).]...

For example, in the ring isomerization reaction, methylcyclopentane forms a methylcyclopentene intermediate in its reaction sequence to cyclohexane. The intermediate can also further dehydrogenate to form methylcyclo-pentadiene, a coke precursor. Bakulen et al. (4) states that methylcyclo-pentadiene can undergo a Diels-Alder reaction to form large polynuclear aromatic coke species. Once any olefinic intermediate is formed, it can either go to desired product or dehydrogenate further and polymerize to coke precursors. This results in a selectivity relationship between the desired products and coke formation as shown on the next page. 

Similar isomerization occurs in the presence of silica-alumina-thoria.104 As it might be expected, this reaction is similar to the isomerization of cyclohexane to methylcyclopentane. Both processes involve the same intermediate cyclohexyl car-bocation, which is formed, however, in different reactions. It may be formed from cyclohexane by hydride ion transfer, or by protonation of cyclohexene. Bicyclic alkenes undergo complex interconversions via carbocations over acidic catalysts.105... 

Nickel as well as platinum and palladium are the most important catalysts used in the liquid or gas phase usually in the temperature range of 170-230°C and pressure of 20-40 atm. Since the reaction is highly exothermic [Eq. (11.91)], efficient heat removal is required to ensure favorable equilibrium conditions and prevent isomerization of cyclohexane to methylcyclopentane ... 

The isomerization of cycloalkanes over SbF5-intercalated graphite can be achieved at room temperature without the usual ring opening and cracking reactions, which occur at higher temperatures and lower acidities.110 In the presence of excess hydrocarbon after several hours, the thermodynamic equilibrium is reached for the isomers. Interconversion between cyclohexane (20) and methylcyclopentane (21) yields the thermodynamic equilibrium mixture [Eq. (5.46)]. 

SbF5—Si02—AI2O3 has been used to isomerize a series of alkanes at or below room temperature. Methylcyclopentane, cyclohexane, propane, butane, 2-methylpropane, and pentane all reacted at room temperature, whereas methane, ethane, and 2,2-dimethylpropane could not be activated.111... 

The formation of C6 and C7 acids along with some ketones was reported in the reaction of isopentane, along with methylcyclopentane and cyclohexane with CO in HF-SbF5 at ambient temperatures and atmospheric pressure.406 Yoneda et al.407 have also found that other alkanes can be carboxylated as well with CO in HF-SbF5. Tertiary carbenium ions, which are produced by protolysis of C—H bonds of branched alkanes in HF-SbF5, undergo skeletal isomerization and disproportionation before reacting with CO. Formation of the tertiary carboxylic acids in the... 

During the reductive isomerization of 7/3-methyl- 14-isoestr-4-ene-3,17-dione 272 in HF SbF5/methylcyclopentane at 0°C, it was found879 that a 1,3-hydride shift occurs followed by kinetically controlled hydride transfer (Scheme 5.91). The mechanism of the reaction was confirmed by employing the deuteriated donor cyclohexane- as well as a specifically deuterium-labeled starting steroid. 

The isomerization of a series of cyclic and bicyclic saturated hydrocarbons over SbF,-intercalated graphite was achieved at or below room temperature without the ring opening and cracking reactions, and the thermodynamic equilibrium was reached for the isomers in all cases (39). Interconversion between cyclohexane and methylcyclopentane also yielded the thermodynamic equilibrium mixture. 

In connection with the research on destructive hydrogenation at the Institute of High Pressures, Maslyanskii (224) passed benzene at 475° under 200 atm. hydrogen over molybdenum oxide (1 mole CeH6 16 moles Ha) to produce 58% methylcyclopentane, 14% cyclohexane, 8% 2-methyl-pentane, 5% n-hexane, and 8% unreacted. Over molybdenum sulfide the product distribution was similar. The preparation of these catalysts was described by him in 1940 (223). Isomerization and other conversions accompanying destructive hydrogenation were also pointed out by Prokopets and by others (257,311,314). 

Pt-promoted Cs2.5 catalyst also is efficient for the skeletal isomerization of n-butane, n-pentane, n-hexane, and n-heptane. Pt-Cs2.5 supported on silica is effective for isomerization of cyclohexane and the hydroisomerization of benzene to methylcyclopentane. ... 

Cyclohexane is known to be isomerized to methylcyclopentane when catalyzed by strong acids. In fact, the SO jT rOi catalyst converts cyclohexane into methylcyclopentane and methylcyclopentane into cyclohexane [119, 142, 143]. The reactions proceed by the monomolecular mechanism via the intermediacy of secondary and tertiary carbenium ions followed by protonated cyclopropanes. 

Benzene and aikyibenzenes are quantitatively converted to cyclohexanes by catalytic hydrogenation. Modem procedures employ liquid-phase hydrogenation over nickel catalysts at 100-200° or over platinum catalysts at room temperature. Nickel catalysts are poisoned by traces of thiophene and water. Small quantities of hydrogen halide increase the effectiveness of platinum catalysts. Isomerization occurs during the reduction of benzene over nickel at 170° the cyclohexane formed is probably contaminated with methylcyclopentane, Partial reduction of benzene to 1,4-dihydrobenzene is accomplished by sodium in liquid ammonia at —45°. ... 

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