Hydrogenation of Benzene

Benzene hydrogenation is a major petrochemical process. Benzene is readily hydrogenated to cyclohexane using nickel or platinum in fixed beds. Most of the cyclohexane nearly 98% is produced by benzene hydrogenation process. Generally this reaction is carried out at 160 – 220$^{\circ}$C and 25 – 30 atm. Many catalyst such as Ni/alumina and Ni/Pd are used for the reaction.

Benzene hydrogenation has been chosen as a model aromatic substance. This reaction has also been used as model reaction in heterogeneous catalysis by metals where metal-support interactions are involved and the desired product of benzene hydrogenation – cyclohexane is an important chemical intermediate for the synthesis of nylon-66 and nylon-6.

During hydrogenation of benzene some two important observations are made.

1. Partially hydrogenated benzene derivatives were never found – only reaction ends in the formation of cyclohexane.
2. Cyclohexane was dehydrogenated above 200^oC to give the reverse reaction. At higher temperatures benzene cracked to form methane and carbon.

Benzene must be free from sulfur to avoid poisoning the catalyst although the original short uneconomical catalyst were common. Reaction temperature and exotherm can be controlled by evaporation of the product and dilution of the benzene feed with recycled cyclohexane.