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The synthesis of adipic acid from cyclohexene utilising green procedures

Rogers, Owen 2021. The synthesis of adipic acid from cyclohexene utilising green procedures. PhD Thesis, Cardiff University.
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The possibility of replacing cyclohexane in the adipic acid process with cyclohexene is assessed in this investigation. This is accomplished using green procedures such as low temperatures, solventless conditions and green oxidants. This cyclohexene-based process would proceed in 2 steps, in which the first step would be an epoxidation of cyclohexene to form cyclohexanediol (CHD), using O2 or H2O2. The second step would be the aerobic oxidative cleavage of CHD to adipic acid. The aerobic epoxidation of cyclohexene was achieved using Au supported on graphite or graphene. A comparison of the effect on reactivity between impregnation (WI) and sol immobilisation (SI) preparation methods was assessed and testing was conducted using solventless and radical initiator-free conditions. The selectivity to the epoxide could be enhanced using a WO3 co-catalyst and subsequently hydrolysed to CHD with the addition of water, where a maximum yield of 17 % to the diol was achieved. The comparison of preparation methods showed that SI catalysts were more reproducible and far more active, however, selectivity towards CHD was lower than for the WI catalyst by 8.0 %. In addition, this study also reveals that the formation of CHD was limited by the formation of the allylic hydroperoxyl species where a maximum selectivity of 50 % was attainable. A further study showed the aerobic oxidation of cyclohexene could also be achieved using graphitic oxide catalysts in a non-metal approach with selectivity observed towards allylic products. To improve selectivity to CHD, H2O2 was utilised alongside Au / Cs3PW12O40 and Au / Cs4SiW12O40 catalysts, where the phosphotungstic acid supports displayed significantly more activity than their Si counterparts. Using solventless conditions and a sequential addition of H2O2, a conversion of 14.4 % and 100 % selectivity to CHD was reached after 8 h. However, the effect of Au was found to be minimal, with an increase of 5.4 % and 3.4 % conversion observed for Au / Cs2PW12O40 and Au / Cs3PW12O40 respectively. For the oxidative cleavage of CHD to adipic acid (AA) Na2V6O15, CuxV2O6 (x = 0.25, 0.3 0.5) and AgVO3 were prepared. The addition of sodium, copper or silver to the vanadium oxide structure was to reduce the leaching observed when compared to V2O5. Leaching could be decreased to 5% for AgVO3 and was also significantly reduced with Cu0.33V2O5, compared to 88.4% with V2O5. Na was less successful in stabilising the material with 36.8 % of the total V found to leach into solution. However, despite significant stabilisation of vanadium in the bronze structures, we show that as little as 7.6 ppm of a homogeneous vanadium species in the reaction solution can cause the selective oxidation of 2-hydroxycyclohexanone to AA, with a yield of 1.5 %. The homogeneous ii vanadium species was discovered to be in the +5 oxidation state and 51V NMR studies determined that the predominant species was a decavanadate molecule which was unbound to other reaction products or metal

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 14 October 2021
Last Modified: 03 Aug 2022 01:39

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