Hydrogenation to Cyclohexanones

It has been known that cyclohexanones are formed as intermediates in the hydrogenation of phenols.83,88,123 124 However, the amounts of the ketone intermediates accumulated during the hydrogenation depend greatly on the nature of catalysts and reaction conditions, as well as on the structure of phenols. 

8 MPa H2, based on a quantitative analysis of the reaction mixture by means of gas chromatography.126,127 The results are summarized in Table 11.9. 

It is seen that, over all the three metals, the amounts of methylcyclohexanone intermediates desorbed from the catalyst surface do not differ much between the three 

TABLE 11.9 The Selectivities for the Formation of Cyclohexanone Intermediates in the Hydrogenation of Isomeric Cresols and the Relative Reactivities of the Ketone to Cresol over Ru, Rh, and Pd-C Catalystfti  

Selectivity to Ketone Relative Reactivity of Ketone Intermediate Desorbed to Cresol 

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Another approach to achieve higher conversions is to start from cyclohexene, which is much more reactive than cyclohexane towards autoxidation [6], and can be prepared by hydrogenation of benzene over a ruthenium catalyst [7]. The higher reactivity of cyclohexene also allows for lower reaction temperatures thus further limiting overoxidation. The 2-cyclohexen-l-one product formed by decomposition of cyclohexenyl hydroperoxide can subsequently be hydrogenated to cyclohexanone. The net reaction stoichiometry is the same as the current process. We now report our results on the use of CrAPO-5, CrS-1 and other transition-metal substituted molecular sieves for the decomposition of cyclohexenyl hydroperoxide. 

The process would use N2O to hydrojylate benzene to phenol. The phenol would be hydrogenated to cyclohexanone using available technology. The final step is the currently practiced nitric acid oxidation of cyclohexanol and cyclohexanone, which returns N2O for use in the front end of the process. The stoichiometric balance is close however, either some additional on-purpose N2O or KA would likely be required for a stand alone plant. 

The formation of phenol from benzene using N2O as the oxidant on various metal oxides was demonstrated in the early 1980s. The phenol obtained from benzene oxidation, which is incorporated in the adipic acid production process, can be hydrogenated to cyclohexanone. The nitric acid oxidation of cyclohexanol and cyclohexanone forms N2O which can he recycled, thus closing the N2O cycle. ... 

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