Alternatives for the Synthesis of OlOne

Two variants are currently employed for the synthesis of Ol/One: (i) the hydrogenation of phenol to cyclohexanol and cyclohexanone and (ii) the hydration of cyclohex-ene to cyclohexanol; cyclohexene is synthesized by the selective hydrogenation of benzene. Cyclohexanol is then oxidized to AA using the same process as that employed for the nitric acid oxidation of the KA Oil.

The hydrogenation of phenol has been adopted by Solutia and Radici. This process has some advantages, particularly for smaller scale manufacturers and for companies that are large-scale manufacturers of phenol. The equipment needed for KA Oil manufacture from phenol is less complex and the process is safer than that based on cyclohexane oxidation, resulting in reduced investment costs. Moreover, current hydrogenation technology allows one to directly obtain a mixture ofcyclohexanol and cyclohexanone with the desired ratio. By increasing the percentage of ketone it is possible to save hydrogen in this step and nitric acid during oxidation. The hydrogenation process is very selective and the final product is extremely pure, if compared with KA Oil stemming from cyclohexane oxidation. This could render AA purification simpler.

The hydrogenation of benzene to cyclohexene, followed by the hydration of cycloolefin, was 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 with a capacity of 60 000tonsyr—1. The partial hydrogenation reaction product is a mixture of unreacted benzene, cyclohexene and by-product cyclohexane. Figure 7.2 shows a simplified flow sheet of the Asahi process.

Original work done at the University of Delft [4a, b] disclosed the possibility of performing the selective hydrogenation of benzene to cyclohexene. Thermodynam-ically, total hydrogenation to cyclohexane is much more favorable than partial hydrogenation; therefore, under normal conditions, it is very difficult to stop the reaction at the monoolefin. This method is based on a catalyst consisting of Pt or Ru powder, coated with a layer of an aqueous solution of zinc sulfate. The reaction is performed in bulk benzene; since the catalyst is surrounded by the aqueous phase, those organic molecules that are better soluble in the aqueous phase are preferentially hydrogenated. Cyclohexene is less soluble in the aqueous phase than benzene, and hence as it forms it migrates preferentially to the organic phase, preventing further hydrogenation [4c]. Cyclohexene is obtained with 80% selectivity (the remainder being cyclohexane), at 70-75% benzene conversion.

Asahi has been investigating solvent mixtures suitable for extractive distillation to make the separation of cyclohexene practical [5]. Cyclohexene is hydrated to cyclo-hexanol on a ZSM-5 based catalyst. With respect to the conventional process from

Figure 7.2 Simplified flow sheet of the Asahi process for benzene hydrogenation to cyclohexene.
Cyclohexane Production Process Flowsheet
Scheme 7.1 Summary of the current processes for AA production, starting from benzene.

cyclohexane, the theoretical consumption of hydrogen is reduced by one-third, and fewer by-products are formed.

Scheme 7.1 summarizes the various industrial processes currently employed for AA production from benzene.

Alternative Homogeneous Catalysts for Cyclohexane Oxidation to Ol/One

In the literature, homogeneous catalysts other than Co complexes have been reported for the oxidation of cyclohexane to Ol/One with either oxygen (air) or hydroperoxides, sometimes offering selectivity comparable to that achieved with the Co-based catalyst. The aim is to find catalysts that permit high cyclohexane conversion while maintaining high selectivity to cyclohexanol and cyclohexanone. Indeed, with oxygen, a coreductant (typically an aldehyde) that is more readily oxidizable by oxygen into peroxide than the substrate is often added to the reaction medium. In this way, the substrate is not oxidized by oxygen, but will react with the resulting peroxide through a homolytic or a heterolytic mechanism. Often, the reactivity is greatly affected by the presence of additives [6a].

Examples of alternative systems investigated in the literature include:

1. Keggin-type polyoxometalates, POMs, can act either as redox-type oxidants towards the substrate, furnishing nucleophilic O2_ species that are then reversibly reox-idized by molecular oxygen under mild conditions (P/Mo/V POMs), or may catalyze radical autoxidation, or may even activate hydroperoxides, depending on their composition. For instance, Fe2Ni-substituted P/W POMs facilitate the oxidation of cyclohexane to Ol/One with air at atmospheric pressure [6b-d], whereas Co(Fe)-substituted P/W POMs catalyze the oxidation with t-BuOOH [6e]. Fe-substituted polyoxotungstates are active catalysts for the oxidation of cyclohexane with oxygen at atmospheric pressure and 120 ° C with high selectivity to Ol/One, although with low conversion [6f]. A Pt/C-P/Wpolyoxometalate mixed heterogeneous-homogeneous system catalyzes the oxidation in the presence of a H2-O2 mixture, at 35 °C, in acetonitrile solvent, via intermediate formation of hydrogen peroxide (HP). In the absence of the polyoxometalate, the Pt/C catalyst rapidly converts the H2-O2 mixture into water, without yielding organic products [6g].

2. Cu-based systems have been widely investigated as catalysts for the oxidation of cyclohexane under relatively mild conditions, using various oxidants like HP, t-BuOOH, peracetic acid and oxygen. Catalysts are based on Cu(I) and Cu(II) complexes, with various types ofligands [6h-r]. However, most of them still provide very low yields and low selectivity, require rather expensive and environmentally unfriendly components, are active only in the presence of various additives or coreductants or involve complicated syntheses, thus being inaccessible on a large scale. Nevertheless, a remarkable 39% yield (overall to Ol/One) was obtained with polynuclear Cu triethanolamine complexes [6p-q] in the oxidation of cyclohexane with aqueous HP in acidic medium (liquid biphasic catalysis) at room temperature in acetonitrile at atmospheric pressure. The highest yield to Ol/One reported (69%), however, was obtained with a bis-(2-pyridylmethyl)amine Cu(II) complex and with HP as the oxidant [6t], in acetonitrile solvent under mild conditions. Yields achieved with O2, on the other hand, are usually lower than 5% [6l, s].

3. Various iron salts and mononuclear Fe or binuclear Fe complexes with a N,O environment, biomimetic to methane monooxygenase complexes, have been applied to the oxidation of cyclohexane with various oxidants [6u,v,7a-g], but their catalytic activity is usually modest, with the exception of a hexanuclear Fe(III) compound derived from p-nitrobenzoic acid, which gives the highest total yield to Ol/One of about 30% [7a]. Moreover, most of these complexes are often unstable and very expensive. A hexanuclear heterotrimetallic Fe/Cu/Co complex bearing two Cu(m-O)2Co(m-O)2Fe cores, prepared by self-assembly, oxidizes cyclohexane with aqueous HP, with a maximum yield to Ol/One of 45%, virtually total selectivity to the two compounds, and preferred formation of cyclohexanol [7h]. The remarkable activity of the Fe/Cu/Co cluster was associated with the synergic effect of the three metals.

4. Vanadium-based complexes and salts have been used for oxidation either with oxygen in the presence of coreductants [7i], with peroxyacetic acid in acetonitrile [7j], or with HP/O2 combined oxidants [7k-m].

5. The addition of an alkyl nitrite (e.g., isoamyl nitrite) as a promoter in the aerial oxidation of cyclohexane in the presence of a Co/Mn catalyst led to an improvement in the conversion and in the selectivity to Ol/One and AA, compared to the unpromoted reaction [7n]. For instance, at 120 °C and 9 atm oxygen pressure, 9.7% cyclohexane conversion was obtained in 3 h reaction time, with selectivity to Ol/One of 65% and to AA of 35%. In the absence of the nitrite, the conversion was less than 4%.

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