Hydrogenation of Benzene to Cyclohexene

As noted in Section 10.12, the partial hydrogenation of benzene to cyclohexene is an economically attractive first step in the synthesis of polyamides, because it is more easily separated from unreacted benzene than is cyclohexane. The reaction is however a difficult one to accomplish, because cyclohexene is normally much the more easily hydrogenated, so its appearance in any significant amount depends upon constructing a catalyst which is in effect selectively poisoned for the hydrogenation of the alkene but not for the more strongly adsorbed benzene. There have been many attempts to modify metals (mainly ruthenium ) to secure this result, in ways that are now briefly recounted. 

A practical large-scale process would be better conducted in a continuous mode, for which vapour-phase reaction is preferable, so this has attracted some attention. The presence of water is still essential, but other modifiers such as ethylene glycol (1,2-dihydroxy-ethane) and e-caprolactam have proved beneficial. A good understanding of the role of these various parameters and modifiers is not yet available. 

---

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). 

Progress is being made in the search for catalysts to hydrogenate aromatic systems (see Section VII). This area is likely to become increasingly important if coal, which contains polyaromatic compounds, is utilized more for production of petrochemicals. Stereospecific production of fully m-C6D6H6 from perdeuterobenzene has been reported catalysts for selective hydrogenation of benzene to cyclohexene would be valuable. 

The partial hydrogenation of benzene to cyclohexene is difficult to carry out with heterogeneous metal catalysts, but it has been achieved by using pentammineos-mium(II).171 The reaction requires the presence of Pd(0), which itself is ineffective as a catalyst for the hydrogenation of benzene ... 

The partial hydrogenation of benzene to cyclohexene is of great industrial importance. A possible application of the product cyclohexene is its transformation to cyclohexanone. It was shown that ruthenium may give rise to increased selectivity toward cyclohexene.426 In addition, studies with various Ru-B catalysts demonstrated that boron effectively promotes the activity and selectivity of ruthenium for cyclohexene.427 428... 

Selective Hydrogenation of Benzene to Cyclohexene Obtaining trans substituted cyclohexanes suggested that desorbed cyclohexenes were intermediates in the hydrogenation of benzenes. The isolation of cyclohexene and substituted cyclohexenes from the hydrogenation of benzene and substituted benzenes was first reported for hydrogenations run over a Ru/C catalyst, but the maximum olefin concentrations observed in this early work were only 0.2-3.4%.7... 

Caprolactam (world production of which is about 5 million tons) is mostly produced from benzene through three intermediates cyclohexane, cyclohexanone and cyclohexanone oxime. Cyclohexanone is mainly produced by oxidation of cyclohexane with air, but a small part of it is obtained by hydrogenation of phenol. It can be also produced through selective hydrogenation of benzene to cyclohexene, subsequent hydration of cyclohexene and dehydrogenation of cyclohexanol. The route via cyclohexene has been commercialized by the Asahi Chemical Company in Japan for adipic acid manufacturing, but the process has not yet been applied for caprolactam production. 

The hydrogenation of benzene to cyclohexene, follovv ed by the hydration of cycloolefin, vas developed by Asahi, and is currently employed by this company and some Chinese producers as the first step in the manufacture of AA in 1990 Asahi built a plant vdth a capacity of 60 000 tons yr. The partial hydrogenation reaction product is a mixture of unreacted benzene, cyclohexene and by-product cyclohexane. Figure 7.2 sho vs a simplified fio v sheet of the Asahi process. 

Partial hydrogenation of benzene to cyclohexene is possible on supported Ru catalyst, or on an unsupported Ru catalyst poisonned by FeCl3, FeS04, TiC, or O-i-Ruthenium catalysts allow selective reduction of anthracene derivatives bridged by a... 

Properties of Sol-Gel Derived Ru/Cu/Si02 Catalysts and Role of Water in the Selective Hydrogenation of Benzene to Cyclohexene with the Catalysts... 

Therefore, a study has been made of the batch-wise partial hydrogenation of benzene to cyclohexene over ruthenium as a catalyst in the presence of an aqueous zinc sulphate solution in this case the reaction is performed in a stirred autoclave at 423 K and a total pressure of... 

Scheme 6.7 Partial hydrogenation of benzene to cyclohexene by Ru(o) NPs in BMI.PF6 at 75°C and under 4atm of hydrogen (constant pressure) [80],...

Similarly, using Ru MNPs dispersed in BMI.PF, selective hydrogenation of benzene to cyclohexene could be detected only at very low conversions (entry 12, Table 13.1) [13]. 

The Asahi process involves initial partial hydrogenation of benzene to cyclohexene using heterogeneous ruthenium catalyst. The cyclohexene hydration proceeds with >99% selectivity at 10-15% conversion (Figure 11.11) [44]. 

Silveira ET, Umpierre AP, Rossi LM, Machado G, Morais J, Soares GV, Baumvol DR, Texeira SR, Finchtner PFP, Dupont J (2004) The partial hydrogenation of benzene to cyclohexene by nanoscale ruthenium catalysts in imidazolium ionic liquids. Chemistry 10 3734-3740... 

The partial hydrogenation of multiple bonds includes chemical reactions of industrial interest for example, the partial hydrogenation of benzene to cyclohexene used to synthesize several compounds, from which the main is nylon production, the partial hydrogenation of 1,5-cyclooctadiene (COD) to cyclooctene (COE), which is an important industrial intermediate product for manufacturing special polymers such as poly-octenamers via ring-opening metathesis polymerization, which is used as a modifier in mbbers and thermoplastics (Sato, Aoki, Noyori, 1998 Rylander, 1985 Schmidt Schomacker, 2007). 

In 1995, Chauvin et al. [48] reported the role of ILs in asymmetric synthesis. Malhotra et al. [49] synthesized chiral ILs for enantioselective transformations. Recently, they reported that Ru nanoparticles dispersed in imidazolimn ILs are efficient catalysts for selective partial hydrogenation of benzene to cyclohexene. 

Niwa S., Mizukami F., Isoya S., Tsuchiya T., Shimizu K., Imai S., Imamura J. Partial hydrogenation of benzene with ruthenium catalysts prepared by a chemical mixing procedure— preparation and properties of the catalysts. J. Chem. Tech. Biotechnol. 1986a 36 236-246 Niwa S., Mizukami F., Kuno M., Takeshita K., Nakamura K., Tsuchiya T, Shimizu K., Imamura J. Selective hydrogenation of benzene to cyclohexene with new ruthenium catalysts prepared by a chemical mixing procedure, J. Mol. Cat. 1986b 34 247-249... 

A water-soluble ruthenium complex, HRu(C0)Cl(TPPMs)3-2H20, was used for hydrogenation of olefins in the biphasic system decalin-water [155], Ruthenium catalyst immobilized on a hydrophilic microporous resin turned out to be more selective for partial hydrogenation of benzene to cyclohexene than the same catalyst immobilized on charcoal. 

Hronec, M. Cvengrosova, Z. Kralik, M. Palma, G. Corain, B. (1996) Hydrogenation of benzene to cyclohexene over polymer-supported ruthenium catalysts, J. Mol Catal A -Chem., 105,25-30. 

---