An Overview Several Possible Green Routes to AA, Some Sustainable, Others Not

Co A1PO-18 and CoAl PO-34,1 -hexanol is the maj or product during the initial stages of the reaction, but this is converted into 1-hexanal and hexanoic acid 1,6-hexandiol, 1,6-hexanedial and AA appear later. However, the formation of the 1,6-oxidized products was not kinetically related to that of the compounds oxidized at one C atom only. Therefore, attack at the two terminal C atoms occurred concurrently, because the amount and dispersion of Co atoms accommodated on the iimer walls of the AlPO framework were such that two Co(III) ions were separated by about 7-8A Clearly, this is affected by the Co/P ratio, which is the main parameter influencing the Co dispersion and hence the distribution of products. With CoAPO-18, a selectivity of 33.6% to AA was obtained at a 9.5% w-hexane conversion, at 100 °C after 24 h, with oxygen as the oxidant, the major by-products being hexanoic acid and 2-hexanone. The overall terminal selectivity achieved was 65%. 

Despite the relevance of these findings and the implications that they may have [43q, r] these excellent figures were not confirmed using catalysts with an identical composition and structure, namely, MnAPO-5 and MnAPO-18 [43s, t]. It was reported that n-hexane oxidation turnover rates (per redox-active Mn center) by oxygen were similar on MnAPO-5 and MnAPO-18, because the reactant may rapidly diffuse and reach the active site, regardless of the pore size in the microporous structure. No regiospedficity was detected for w-hexane oxidation to alkanols, aldehydes and ketones (7-8% terminal selectivity), and the relative reactivity of primary and secondary C—H bonds in w-hexane was identical in both catalysts and similar to that predicted from relative C—H bond energies in n-hexane. The selectivity to terminal adds was very low. 

An Overview Several Possible Green Routes to AA, Some Sustainable, Others Not 

To date, it seems that the most sustainable approach is the one that combines the use of cheap raw materials, for example, cyclohexane, benzene or phenol, with oxygen as the terminal oxidant. Within this context, a process that does not use acetic acid in the aerial oxidation of the KA Oil into AA or, even better, in the direct oxidation of cyclohexane to AA would represent a significant step forward towards a new and sustainable synthesis. On the other hand, recent examples demonstrate that even the traditional process making use of nitric acid for the oxidation of KA Oil may be turned into an intrinsically green one that is economically sustainable due to the use of the co-produced N2O in down-stream applications. 

Research in this field is still very active, and the various alternative options and strategies currently under investigation increase the likelihood that in the near future a new and sustainable synthetic route will finally be implemented at the commercial level. 

---