Asahi cyclohexene

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). 

Because of projected nylon-6,6 growth of 4—10% (167) per year in the Far East, several companies have announced plans for that area. A Rhc ne-Poulenc/Oriental Chemical Industry joint venture (Kofran) announced a 1991 startup for a 50,000-t/yr plant in Onsan, South Korea (168,169). Asahi announced plans for a 15,000-t/yr expansion of adipic acid capacity at their Nobeoka complex in late 1989, accompanied by a 60,000-t/yr cyclohexanol plant at Mizushima based on their new cyclohexene hydration technology (170). In early 1990 the Du Pont Company announced plans for a major nylon-6,6 complex for Singapore, including a 90,000-t/yr adipic acid plant due to start up in 1993 (167). Plans or negotiations for other adipic acid capacity in the area include Formosa Plastics (Taiwan) (171) and BASF-Hyundai Petrochemical (South Korea) (167). Adipic acid is a truly worldwide... 

The partial arene derivative hydrogenation into cyclohexene or cyclohexa-diene as intermediates is also investigated. The process developed by Asahi Chemical Industry in Japan is an example of the selective formation of cyclohexene [6]. In the future, this reaction could be an active area of research due to the potential of the intermediate in organic synthesis. 

Conversion of benzene to cyclohexene by partial catalytic hydrogenation is a very important industrial process, since it provides a new route to cyclohexanol, a precursor of nylon, when combined with hydration of cyclohexene. For example, Asahi Chemical Company of Japan developed a selective bilayer catalytic system including a Ru catalyst, Zr02 and ZnS04 under 50 atm of H2 pressure, a process affording the olefin with up to 60% selectivity after 90% conversion of benzene.72... 

The process developed by Asahi Chemical Industry in Japan [110], and performed in a tetraphasic system combining gas, oil, water and ruthenium particles with an average diameter of 20 nm, is a significant milestone in this area. The selectivity is very high and a yield of 60% in cyclohexene is obtained with this bulk ruthenium catalyst in the presence of zinc as co-catalyst at 150 °C and under 50.4 bar of H2. The cyclohexene produced by this process is used as a feedstock for caprolactam. 

Another pertinent example is provided by the manufacture of caprolactam [135]. Current processes are based on toluene or benzene as feedstock, which can be converted to cyclohexanone via cyclohexane or phenol. More recently, Asahi Chemical [136] developed a new process via ruthenium-catalysed selective hydrogenation to cyclohexene, followed by zeolite-catalysed hydration to cyclo-hexanol and dehydrogenation (Fig. 1.49). The cyclohexanone is then converted to caprolactam via ammoximation with NH3/H202 and zeolite-catalysed Beckmann rearrangement as developed by Sumitomo (see earlier). 

Zeolites have been used as (acid) catalysts in hydration/dehydration reactions. A pertinent example is the Asahi process for the hydration of cyclohexene to cyclo-hexanol over a high silica (Si/Al>20), H-ZSM-5 type catalyst [57]. This process has been operated successfully on a 60000 tpa scale since 1990, although many problems still remain [57] mainly due to catalyst deactivation. The hydration of cyclohexanene is a key step in an alternative route to cyclohexanone (and phenol) from benzene (see Fig. 2.19). The conventional route involves hydrogenation to cyclohexane followed by autoxidation to a mixture of cyclohexanol and... 

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 environmental impact of the cyclohexane oxidation could also be reduced. An alternative is to start from benzene and make a selective hydrogenation to form cyclohexene. Ru-based supported catalysts working in the liquid phase and in the presence of a co-catalysts such as Zn (Asahi Chemical Industry process) are selective in the reaction, with yields up to about 60% [247], but with cyclohexane as the main by-product. Cyclohexene is hydrated in the liquid phase with an MFI zeolite as catalyst at moderate temperature (100-130 °C). This reaction is very selective (>99%). This route was primarily developed for the synthesis of adipic acid, but could be used also to reduce the number of products and separation costs in the production of cyclohexanone. 

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. 

Cyclohexene can be synthesized by partial hydrogenation of benzene, by partial dehydrogenation of cyclohexane, or by dehydrohalogenation of cyclohexyl halides. The hydrogenation of benzene is the most viable route in the Asahi process, cyclohexene is obtained with a 60% yield and 80% selectivity, with the remainder being converted into cyclohexane. Asahi has developed a process for the addition of water to cyclohexene to produce cyclohexanol that can then be oxidized to AA using the conventional nitric acid oxidation. 

Asahi has proposed the oxidation of cyclohexene with O2 as a commercially feasible method for AA production [40j]. The reaction, when catalyzed by isopolyoxomolyb-dates, primarily yields cyclohexene oxide, 1,2-cyclohexandiol and 2-cyclohexene-l-ol. The two former compounds are intermediates in the synthesis of AA. The cyclohexyl hydroperoxide acts as the epoxidizing agent for cyclohexene, finally leading to the generation of the products (Scheme 7.15). 

In recent years the Asahi Corporation has developed a benzene-to-cyclohexene process involving a liquid-liquid two-phase system (benzene-water) with a solid ruthenium catalyst dispersed in the aqueous phase. The low solubility of cyclohexene in water promotes rapid transfer towards the organic phase. An 80000 t annum plant using this process is in operation. Another way to scavenge the intermediate cyclohexene is to support the metal hydrogenation catalyst on an acidic carrier (e. g. silica-alumina). On such a bifunctional catalyst the cyclohexene enters catalytic alkylation of the benzene (present in excess) to yield cyclohexylbenzene [19], which can be converted, by oxidation and rearrangement reactions, into phenol and cyclohexanone. 

For the reasons outlined above, some typical acid-catalysed reactions, such as hydration and etherification, may be better performed over non-microporous acid catalysts, but microporous acids have found uses in this area. Asahi, for example, have established the zeolite-catalysed hydration of cyclohexene as a commercial process, " where in a two-phase reaction mixture (aqueous and non-aqueous layers) the H-ZSM-5 catalyst stays in the aqueous phase but adsorbs enough cyclohexene, because of its relative hydrophobicity, that the reaction proceeds in the zeolite pores. This has the advantage over the previously used cyclohexene/sulfuric acid system that the aqueous layer is not acidic and corrosive. Furthermore, the medium-pore structure impedes etherification to dicyclohexyl ether and the highly siliceous zeolite has long-term stability in boiling water. 

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]. 

Mitsutani, A. and Kumano, S. (1997). Evaluation of New One-step Adipic Acid MFG Process, Special Evaluation Report N° 29, Nippon Cemtech Consulting Inc. K. Yamashita, M. Iwasaki (1995). Production of Cyclohexene, Japanese Patent JP7196538 (Applicant Asahi Chemical Industries). 

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