Benzene hydrogenation - cyclohexane

Cyclohexane is an important intermediate in the production of Nylon 6 via caprolactam, or Nylon 66 via adipic acid. Demand for cyclohexane rose considerably with the production of nylon fibers, which began in 1937 with Du Pont, followed by IG Farbenindustrie, It is obtained either by refining appropriate petroleum fractions, or by benzene hydrogenation. 

Cyclohexane is produced predominantly by hydrogenation of benzene. For thermodynamic reasons, the temperature should not exceed 220 °C, since the benzene content then becomes too large, because of the position of the equilibrium. 

Whereas older hydrogenation processes were operated with sulfur-resistant catalysts because of the use of sulfur-containing benzene, the new methods, which use nickel and noble metal catalysts, require a very pure benzene with a sulfur 

1 Main reactor 2 Finishing reactor 3 Separator 4 Stabilizer 5 Separator 

The reaction temperature varies between 200 and 350 °C, with a pressure in the reactor of 30 bar the hydrogen/benzene ratio is 3 1. 

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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 dipole moment varies according to the solvent it is ca 5.14 x 10 ° Cm (ca 1.55 D) when pure and ca 6.0 x 10 ° Cm (ca 1.8 D) in a nonpolar solvent, such as benzene or cyclohexane (14,15). In solvents to which it can hydrogen bond, the dipole moment may be much higher. The dipole is directed toward the ring from a positive nitrogen atom, whereas the saturated nonaromatic analogue pyrroHdine [123-75-1] has a dipole moment of 5.24 X 10 ° C-m (1.57 D) and is oppositely directed. Pyrrole and its alkyl derivatives are TT-electron rich and form colored charge-transfer complexes with acceptor molecules, eg, iodine and tetracyanoethylene (16). 

A route to phenol has been developed starting from cyclohexane, which is first oxidised to a mixture of cyclohexanol and cyclohexanone. In one process the oxidation is carried out in the liquid phase using cobalt naphthenate as catalyst. The cyclohexanone present may be converted to cyclohexanol, in this case the desired intermediate, by catalytic hydrogenation. The cyclohexanol is converted to phenol by a catalytic process using selenium or with palladium on charcoal. The hydrogen produced in this process may be used in the conversion of cyclohexanone to cyclohexanol. It also may be used in the conversion of benzene to cyclohexane in processes where benzene is used as the precursor of the cyclohexane. 

High purity cyclohexane is manufactured by hydrogenating benzene at 400-500°F and 500 psig. Some cyclohexane was earlier produced by fractionating naphtha but its purity of 85-90% was too low to compete with 99-t- percent purity from benzene hydrogenation. A number of cyclohexane processes based on benzene hydrogenation are available. 

In the petrochemical field, hydrogen is used to hydrogenate benzene to cyclohexane and benzoic acid to cyclohexane carboxylic acid. These compounds are precursors for nylon production (Chapter 10). It is also used to selectively hydrogenate acetylene from C4 olefin mixture. 

The hydrogenation of benzene produces cyclohexane. Many catalyst systems, such as Ni/alumina and Ni/Pd, are used for the reaction. General reaction conditions are 160-220°C and 25-30 atmospheres. Higher temperatures and pressures may also be used with sulfided catalysts ... 

Older methods use a liquid phase process (Figure 10-11). ° New gas-phase processes operate at higher temperatures with noble metal catalysts. Using high temperatures accelerates the reaction (faster rate). The hydrogenation of benzene to cyclohexane is characterized by a highly exothermic reaction and a significant decrease in the product volume... 

Figure 10-11. The Institut Francais du Petiole process for the hydrogenation of benzene to cyclohexane " (1) liquid-phase reactor, (2) heat exchanger, (3) catalytic pot (acts as a finishing reactor when conversion of the main reactor drops below the required level), (4) high-pressure separator, (5) stabilizer.

Consider the hydrogenation of benzene to cyclohexane, which takes place by the step-by-step addition of two H atoms per step ... 

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. 

The total hydrogenation of benzene derivatives represents an important industrial catalytic transformation, in particular with the conversion of benzene into cyclohexane, a key intermediate in adipic acid synthesis, which is used in the production of Nylon-6,6 (Scheme 1). This reaction is still the most important industrial hydrogenation reaction of monocyclic arenes [1]. 

Hydrogenation of benzene to cyclohexane was effected in a fixed bed reactor at 210-230°C, but a fall in conversion was apparent. Increasing the bed temperature by 10°C and the hydrogen flow led to a large increase in reaction rate which the interbed cooling coils could not handle, and an exotherm to 280°C developed, with a hot spot of around 600° C which bulged the reactor wall. 

Hydrar A catalytic process for hydrogenating benzene to cyclohexane. It is conducted in the vapor phase with a fixed-bed reactor. The catalysts are based on platinum and modified by lithium an alternative nickel-based catalyst is also used. Developed by UOP, subsequently renamed HB Unibon. 

Hytoray [Hydrogenation Toray] A process for hydrogenating benzene to cyclohexane. Developed by Toray. 

Penex-Plus A petroleum refining process that combines the Penex process with a process for hydrogenating benzene to cyclohexane. Developed by UOP for reducing the benzene content of gasoline first offered for license in 1991. 

Several ruthenium systems catalyze the hydrogenation of aromatic rings, and this topic is detailed in Chapter 16. An early example reported by Bennett and coworkers was that of RuHCl( 76-C6Me6)(PPh3), which catalyzed the hydrogenation of benzene to cyclohexane at 25 °C, 1 bar H2 [69]. Since ruthenium colloids are very active for this reaction under certain conditions, there is evidence that at least some of the reported catalysts are heterogeneous [70]. 

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