Current Technologies for AA Production

The industrial process accounting for the total AA production is the oxidation of either a mixture of cyclohexanol and cyclohexanone, the so-called KA Oil (ketone/ alcohol oil or Ol/One), or of cyclohexanol [2]. These compounds can be produced by different methods, the principal one being the oxidation of cyclohexane with air. 

Two-Step Transformation of Cyclohexane to AA Oxidation of Cyclohexane to Ol/One with Air 

Cyclohexane is obtained either by the hydrogenation of benzene, or from the naphtha fraction in small amounts. Its oxidation to the KA Oil dates back to 1893 and was first industrialized by DuPont in the early 1940s. Oxidation is catalyzed by Co or Mn organic salts (e.g., naphthenate), at between 150 and 180 °C and 10-20 atm. Indeed, this reaction is a two-step process (an oxidation and a deperoxidation step), and two variants are currently in use [2,3]. The oxidation step can be performed with or without a catalyst. The deperoxidation step always uses a catalyst (Co(II) or NaOH). The overall performance of both variants is almost identical, although the selectivity in the individual steps may be different. For example, in a first reactor, cyclohexane is oxidized to cyclohexylhydroperoxide the concentration of the latter is optimised by carrying out the oxidation in passivated reactors and in the absence of transition metal complexes, in order to avoid the decomposition of the hydroperoxide. In fact, the synthesis of the hydroperoxide is the rate-limiting step of the process, and, on the other hand, alcohol and ketone are more reactive than cyclohexane. The decomposition of the hydroperoxide is then carried out in a second reactor, in which the catalyst amount and reaction conditions are optimised, thus allowing the Ol/One ratio to be controlled. 

The per-pass conversion is 5-7%, to limit the consecutive reactions in fact, alcohol and ketone are more reactive towards oxygen than cyclohexane. Selectivity to the KA Oil is around 75-80% (product weight ratio Ol/One 60/40) by-products are carboxylic acids (6-hydroxyhexanoic, n-butyric, n-valeric, succinic, glutaric and adipic among others), and cyclohexyl hydroperoxide. Unconverted cyclohexane is recycled [2a, cj  

The chemistry of the reaction is an homolytic autoxidation. Cyclohexanol is formed from the RO radical (R = C Hn). At high concentrations ( 3 mmol dm ), Co may act as an initiator [2g], with, a direct attack on cyclohexane (this reaction does not occur with Mn, in agreement with the lower electrochemical potential of the Mn(II)/ Mn(III) redox couple) [3bj  

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In 1996 Harnly [50] was still optimistic when he concluded that multi-element AAS will ultimately be successful because it will offer figures of merit which are comparable or superior to those available today for single element AAS , and a CS AAS instrument. .. would provide analytical capabilities comparable to ICP-MS with considerably less complexity . In 1999, however, he became more pessimistic [54] when he provokingly concluded The cost of development and construction of the ideal detector would be expensive. In the current economic climate, where development of technology for future products is sacrificed for quarterly profits, the cost of the detector may be sufficient to block the development of multi-element CS AAS, regardless of the enhanced analytical capabilities . 

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