Benzene from methylcyclopentane

Fig. 8. Yields of ring enlargement products (percentage of effluent) as a function of the hydrogen content of the carrier gas. Pulse system catalyst, 0.4 g platinum black T = 360°C. (a) Cscyclics (methylcyclopentane plus methylcyclopentene) from ethylcyclobutane (97a). (b) Ce cyclics (mainly benzene) from methylcyclopentane (91, 91a).

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. 

Fig. 1. Yields of benzene and methylcyclopentane from n-hexane (mole % in the effluent) as a function of the hydrogen percentage in the carrier gas (the other component being He). Pulse system, catalyst 0.4 g Pt black, T = 360°C (27a).

In order to produce additional evidence for the above mecheuiism for aromatization over Ga203 HZSM-5 catalysts the reactions of n-hexene, 1,5 hexadiene, methylcyclopentane, methylcyclopentene, cyclohexene, cyclohexadiene at 773 K over H-2SM-5 and Ga-HZSM-5 were comparatively studied. In these exj riments low pressure and low contact were employed to observe the primary kinetic products uncomplicated by secondary reactions. The relative rates of the formation of benzene from the various hydrocarbons cited above are listed in Table 4. 

Burch and coworkers (50-53) have reported on the use of Pt-Sn catalysts for hydrocarbon conversions. It was concluded that no proper alloys of Pt and Sn were formed so that this could not account for the changes in the catalytic properties imparted by the presence of Sn (50). Burch and Garla concluded for their catalysts that (i) n-hexane is isomerized by a bifunctional mechanism, (ii) benzene and methylcyclopentane are formed directly from n-hexane at metal sites, and (iii) the conversion of methylcyclopentane requires acidic sites (51). It was also concluded that the Sn (II) ions modified the Pt electronically with the result that self-poisoning by hydrocarbon residues is reduced. However, these later observations were based upon conversions at one bar. 

Sexton et al. (66) also examined the activity for the dehydrogenation of cyclohexane and conversion of methylcyclopentane of a series of PtSn alumina catalysts where the Sn/Pt ratio was varied. They found that the activity decreased as the Sn/Pt ratio increased. Selectivity for benzene formation from methylcyclopentane increased to a maximum at ca. 1.5 to 2.5 wt.% Sn (Sn/Pt = 4.9 to 8.2) and then declined. These conversions were conducted at normal pressures. 

Hindin et al. (18) published data showing benzene formation to proceed readily from methylcyclopentane over mechanical mixtures of platinum bearing particles and silica-alumina, at atmospheric pressure and near 500°C. temperature. Under these conditions the equilibrium constant for conversion of a cyclopentane to a cyclopentene is of the order of unity. Consequently, the first step, if it is catalyzed by X, can itself proceed with... 

Pt black, pulse system, carrier gas H2, T = 673 K. Averages of 4 or 5 runs, scattering 1 to 8%. After Ref. 46. Methylcyclopentane (MCP) and benzene were incompletely separated. MCP prevailed from 3-methylpentane and benzene from hexane. 

Hexane refers to the straight-chain hydrocarbon, C H branched hydrocarbons of the same formula are isohexanes. Hexanes include the branched compounds, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, and the straight-chain compound, / -hexane. Commercial hexane is a narrow-boiling mixture of these compounds with methylcyclopentane, cyclohexane, and benzene (qv) minor amounts of and hydrocarbons also may be present. Hydrocarbons in commercial hexane are found chiefly in straight-mn gasoline which is produced from cmde oil and natural gas Hquids (see Gasoline AND OTHER MOTOR fuels Gas,natural). Smaller volumes occur in certain petroleum refinery streams. 

The catalytic performances obtained during transalkylation of toluene and 1,2,4-trimethylbenzene at 50 50 wt/wt composition over a single catalyst Pt/Z12 and a dualbed catalyst Pt/Z 121 HB are shown in Table 1. As expected, the presence of Pt tends to catalyze hydrogenation of coke precursors and aromatic species to yield undesirable naphthenes (N6 and N7) side products, such as cyclohexane (CH), methylcyclopentane (MCP), methylcyclohexane (MCH), and dimethylcyclopentane (DMCP), which deteriorates the benzene product purity. The product purity of benzene separated in typical benzene distillation towers, commonly termed as simulated benzene purity , can be estimated from the compositions of reactor effluent, such that [3] ... 

The formation of methylcyclopentane becomes favorable at above 323°C (600 K). At this temperature, that of benzene is even more so. Cyclohexane is thermodynamically unstable from 223°C (500 K) upward. 

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. 

Another application of the Platforming process has been to produce aromatic hydrocarbons. Of particular interest is the production of benzene. It has been shown (10) that yields up to 92% of theoretical based on methylcyclopentane and cyclohexane content can be obtained from light straight-run fractions. Similarly, toluene and xylenes are produced in high yields from the corresponding C7 and Cg fractions. 

Component 1 in Singapore buildings was correlated with compounds associated with humans and their activities. Human effluents have been reported to contain isoprene (Ellin et al, 1974) while tetrachloroethylene is a VOC found in dry-cleaned clothes worn by building occupants (Wallace, Pellizzari and Wendel, 1991) or from the use of consumer products (Sack et al., 1992). Tetradecane, benzaldehyde, o-xylene, naphthalene are emissions from dry process photocopiers (Leovic et al., 1996). Component 2 with high loadings ofn-decane, n- undecane, toluene, styrene, n-nonane, 1,2,4-trimethyl benzene probably reflects the emissions of carpets and vinyl floorings (Yu and Crump, 1998). Component 3 was primarily correlated with heptane and methylcyclopentane, which could be due to the emissions of water-based paints. Finally, component 4 was associated with 2-methylpentane, hexane, cyclohexane, methylcyclohexane and limonene, which is reflective of the emissions of air fresheners and cleaning products (Sack et al., 1992). 

For relief and reassurance, Table 5.13 shows the relative energies of some isomers calculated at modest levels, namely HF/3-21G1 1, HF/6-31G, and MP2/6-31G. For a reality check, we also see values from G3(MP2) and experiment (experiment fulvene/benzene, [229/230] cyclopropane/propene, [231/231] dimethyl ether/ethanol, [232/233] methylcyclopentane/cyclohexane, [230/234]). The energy differences chosen for this illustration are enthalpy differences, because differences in heats of formation yield these, and heats of formation represent the most extensive compilations of experimental energy quantities relevant to our... 

Turning from the polemics surrounding the Kekule case, it is refreshing to read of real experiments and genuine problems in the laboratory. The complexities associated with early attempts to reduce benzene to cyclohexane by HI included the production of methylcyclopentane, whose boiling temperature was nearly that of hexane, and which originated in an unsuspected rearrangement.141... 

In a further study, Muller and Gault (94) reported that isomerization of 2,3-dimethylbutane on thick platinum films yielded, as well as the expected bond-shift initial products (2-methylpentane and 2,2-dimethylbutane), substantial amounts of 3-methylpentane, n-hexane, and methylcyclopentane even at 273°C. Clearly, this is another example of a multistep mechanism. On the same basis, isomerization of 2,2-dimethylbutane should give only 3-methylpentane, 2,3-dimethylbutane, and 2-methylpentane as initial products in fact, Muller et al. report that n-hexane, methylcyclopentane, and benzene represented 15% of their initial products at 275°C. Somewhat in contrast to the situation for Pt/Al203, the number of surface reactions before desorption appeared to be no greater than two or three. It turns out that in the formation of 3-methylpentane the distribution was best explained by the succession of a bond shift and cyclic mechanism. This is quite distinct from the formation of n-hexane where two consecutive bond shifts occur. Perhaps in consequence of this difference, they conclude, a marked variation with temperature of the product distributions is observed. 

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