Benzene over cyclohexane

FIGURE 18 Molecular discrimination of benzene over cyclohexane from selective sorption in a TMA-hectorite film deposited on a QCM sensor. (Based on Ref. 185.)... 

Sakata et al. made porons carbon membrane plates (PCMP) to separate benzene-cyclohexane azeotrope by pervaporation [9]. PCMPs were prepared from phenol resin powder (BELLPEARL S-870, Kanebo) by pressurizing the powder in a hydraulic press. The disk was heated at 300°C for 1 h in air stream and then carbonized at 800°C in nitrogen stream (A) or CO2 stream at 800°C (B) or at 850°C (C). A had only micro-pores, B had much wider pore size distribution than A and C had both micro- and meso-pores. The separation factor (benzene over cyclohexane) of 2.8 was obtained by pervaporation at 60°C. Flux of pure benzene and pure cyclohexane was 0.34 and 0.57 kg/m h, respectively. 

Cyclohexane, produced from the partial hydrogenation of benzene [71-43-2] also can be used as the feedstock for A manufacture. Such a process involves selective hydrogenation of benzene to cyclohexene, separation of the cyclohexene from unreacted benzene and cyclohexane (produced from over-hydrogenation of the benzene), and hydration of the cyclohexane to A. Asahi has obtained numerous patents on such a process and is in the process of commercialization (85,86). Indicated reaction conditions for the partial hydrogenation are 100—200°C and 1—10 kPa (0.1—1.5 psi) with a Ru or zinc-promoted Ru catalyst (87—90). The hydration reaction uses zeotites as catalyst in a two-phase system. Cyclohexene diffuses into an aqueous phase containing the zeotites and there is hydrated to A. The A then is extracted back into the organic phase. Reaction temperature is 90—150°C and reactor residence time is 30 min (91—94). 

The oil that separates is extracted with ether, the extract dried over anhydrous sodium sulfate and then evaporated at reduced pressure. The residue is dissolved in boiling benzene (75 ml) treated with decolorizing charcoal, filtered, treated with boiling cyclohexane (275 milliliters) and cooled to give 22.3 grams of 2,3-dichloro-4-butyrylphenoxyacetic acid. After several recrystallizations from a mixture of benzene and cyclohexane, then from methyl-cyclohexane, next from a mixture of acetic acid and water, and finally from methylcyclo-hexane, the product melts at 110° to 111°C (corr). 

Benzene and cyclohexane are freshly distilled and stored over sodium wire or calcium hydride. 

Benzene hydrogenation was used to probe metal site activity. A 12/1 H2/benzene feed was passed over the catalysts at 700 kPa with a weight hourly space velocity of 25. The temperature was set to 100°C and the conversion of benzene to cyclohexane was measured after 2 hours at temperature. The temperature was then increased at 10°C increments and after two hours, the conversion remeasured. 

FIGURE 1.3 Data for dehydrogenation of cyclohexane to benzene over several Pt single crystals from reference 19. 

Barnett et al. [AIChE J., 7 (211), 1961] have studied the catalytic dehydrogenation of cyclohexane to benzene over a platinum-on-alumina catalyst. A 4 to 1 mole ratio of hydrogen to cyclohexane was used to minimize carbon formation on the catalyst. Studies were made in an isothermal, continuous flow reactor. The results of one run on 0.32 cm diameter catalyst pellets are given below. 

C. The Detection of Cyclohexene Intermediates The postulate that olefins are released from the surface during the hydrogenation of aromatic hydrocarbons has gained considerable support. Madden and Kemball (89) observed cyclohexene during the early stages of the vapor phase hydrogenation (flow system) of benzene over nickel films at 0° to 50°. The ratio of cyclohexene to cyclohexane diminished with time, and little or none of the alkene was detected if the films were annealed at 50° in a stream of hydrogen. 

Hydrogen undergoes catalytic hydrogenation adding to unsaturated hydrocarbons, such as alkenes and alkynes forming alkanes. The reaction is catalyzed by nickel, platinum or palladium catalysts at ambient temperature. Hydrogenation of benzene over platinum catalyst yields cyclohexane, C6H12. 

R16H selectivity and activity kinetics were fit over a wide range of temperature and pressure. Reforming selectivity is shown in Figs. 16 and 17, where benzene and hexane are plotted against C5-, the extent of reaction parameter. The effect of pressure on reforming a 50/50 mixture of benzene and cyclohexane at 756 K is shown in Fig. 16. Selectivity to benzene improves significantly when pressure is decreased from 2620 to 1220 kPa. In fact, at 2620 kPa, hexane is favored over benzene when the C5 yield exceeds 10%. This selectivity behavior can be seen in the selectivity rate constants ... 

For purposes of comparison, it is of interest to consider the results of kinetic studies of the dehydrogenation of cyclohexane to benzene over certain other catalysts. Herington and Rideal (H6) studied the kinetics at 400-450°C. using a chromia-alumina catalyst containing 12% Cr203. These authors concluded that cyclohexene was an intermediate in the reaction, so that desorption and readsorption of cyclohexene were essential... 

Fig. 3. Dehydrogenation of cyclohexane to cyclohexene and benzene over chromia-alumina (H6).

In a more recent study of the dehydrogenation of cyclohexane to benzene over a chromium oxide catalyst at 450°C., Balandin and coworkers (Dl) concluded that benzene was formed by two routes. One of these, the so-called consecutive route, involves cyclohexene as a gas phase intermediate, while the other proceeds by a direct route in which intermediate products are not formed in the gas phase. It was concluded that the latter route played a larger role in the reaction than did the former. These conclusions were derived from experiments on mixtures of cyclohexane and Cl4-labeled cyclohexene, which made it possible to evaluate the individual rates Wi, BY, Wt, and Wz in the reaction scheme... 

The hydrogenation of benzene to cyclohexane, CgH6 +3H2 CgH12, Is carried out at 500 psig and 400 F in the vapor phase over a nickel catalyst. The space velocity is WHSV = 2.0 lb benzene/(h)Ccuft of reactor). A 200% excess of hydrogen is used. Benzene purity is 99% and that of hydrogen is 65 mol%, the balance being methane. Find the total feed rate in the units (a) lbmol/(h)Ccuft of reactor) (b) actual cuft/(h)(cuft of reactor). 

Cyano-10-(3-methanesulfonyloxypropyl)phenthiazine and 4-hydroxypiperidine in toluene were heated under reflux with stirring. The reaction mixture was allowed to cool and water was added. The resulting toluene solution layer was decanted and washed twice with water. The toluene solution was then stirred with 5% hydrochloric acid. The hydrochloride of the desired phenthiazine base precipitated in gummy condition in the aqueous layer. This was decanted and treated with sodium hydroxide (density 1.33). It was then extracted three times with ethyl acetate. The extracts were dried over sodium sulfate, filtered and concentrated in vacuum. A resinous product was obtained. This product was dissolved in a mixture of benzene and cyclohexane and chromatographed on a column containing alumina. The chromatographed product was eluted successively with mixtures of benzene and cyclohexane and then with benzene and finally with a mixture of benzene and ethyl acetate. The eluates were evaporated to yield a crude product. This product was recrystallised from aqueous ethanol (40% water) and yielded 2-cyano-10-[3-(4-hydroxy-l-piperidyl)propyl]phenthiazine as white crystals. 

The large difference in gallery heights for Cr3 53 and Cr1 88-montmorillonites leads to dramatic differences in catalytic reactivity (17). Figure 4 illustrates the conversion of cyclohexane to benzene over both materials at 550°C as a function of reaction time. Both catalysts were pre-reduced under H2 in a continuous flow reactor at 500°C, followed by reaction with cyclohexane (weight hourly space velocity = 3, contact time = 6 sec, He carrier gas.) The clay remains intact at these reaction temperatures as evidenced from the thermal data (17). 

Figure 4. Conversion of cyclohexane to benzene over chromia pillared montmorillonites at 550°C (A)...

Cyclohexane (C6H12) is made by hydrogenation of benzene (over Ni or Pt). Most of it is converted to adipic acid by oxidation, via the intermediaries cyclohexanol and cyclohexanone. 

The benzene yields given by the data of Figures 4 and 5, 87% at 204°C and 88% at 227°C, may be compared with computed equilibrium yields of 13% and 19%, based on inlet conditions. This clearly shows the advantage of the continuous annular chromatographic reactor over, say, a tubular reactor. The comparison is not entirely straightforward, because dilution of the cyclohexane by He carrier as it disperses circumferentially shifts the equilibrium toward products this would have to be taken into account in any quantitative comparison. The data show only partial separation of benzene and cyclohexane. This partial separation must result in partial suppression of the back reaction, and must also contribute to the observed yield enhancement (in addition to the dilution effect). ... 

The sulfur compounds contained as impurities in a substrate or solvent may have a profound effect on hydrogenation, particularly over platinum metals where the amounts of catalyst used are usually much smaller than in the case of base metals. An excellent way to remove such impurities is to treat the sample with Raney Ni at slightly elevated temperatures22 (usually 50-80°C). The impurities in benzene or cyclohexane can thus be removed simply by refluxing with Raney Ni for 0.5 h (see Section 13.3). Granatelli applied this desulfurization with Raney Ni to determine quantitatively as little as 0.1 ppm of sulfur contained in 50 g of nonolefinic hydrocarbons.23... 

Hydrogenation of benzene over acidic catalysts or in the presence of acid results in the formation of the products resulting from alkylation by the intermediate cyclohexene such as cyclohexylbenzene, together with cyclohexane, as shown in Scheme 11.1. Slaugh and Leonard obtained cyclohexylbenzene in high selectivity in the hy-... 

Hall and Cawley hydrogenated biphenyl over pelleted molybdenum sulfide at 350°C and 20-30 MPa H2, and obtained cyclohexylbenzene, benzene and cyclohexane, and dicyclohexyl, along with their isomerides.58... 

Our final example of a complex column is an azeotropic system in which we add a light entrainer to facilitate the separation of two components. The classical example of this type of system is the use of benzene or cyclohexane to break the ethanol-water azeotrope. As shown in Fig. 6.26, the vapor from the top of the column is condensed and fed into a decanter in which the two liquid phases separate. The aqueous phase is removed as product. The organic phase (the light entrainer) is refluxed back to the column. Some of the organic may also be added to the feed stream to alter the composition profiles in the column (if more entrainer is needed lower in the column). Note that the organic level in the decanter is not controlled. A small stream of fresh entrainer ivould be added to make up for any losses of entrainer over a long period... 

The supposed consecutive scheme of the hydrogenation of benzene to cyclohexane over a Ni containing catalyst is... 

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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