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Selective hydrogenation reactions in the synthesis of fragrances

Conway, Matthew 2021. Selective hydrogenation reactions in the synthesis of fragrances. PhD Thesis, Cardiff University.
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Abstract

The partial reduction of alkyl-substituted indanes to tetrahydroindane products is an intermediate step in the synthesis of musk fragrances. Industrially, this process is performed solventless in a semi-batch reactor at 180 °C under 40 bar hydrogen pressure over a Pd/C catalyst to yield approximately 62–65 wt.% cycloolefins. Demand for downstream products has recently soared, and therefore yield enhancements are desired, which this thesis investigated. Herein, a study of the effect of temperature and hydrogen pressure on the yield of the desired products was carried out and reaction conditions where a significant improvement in yield can be achieved were identified. This yield enhancement could not be attributed to equilibrium or mass transfer limitations. Rather, it was hypothesised that the greater adsorption strength of aromatics compared to cycloalkenes led to greater changes to surface coverages as temperature increased. Whereas higher hydrogen pressure disfavoured isomerisation reactions that afforded more reactive cycloalkenes, which gave a greater trans/cis ratio of saturated products. The effect of metal (where M = Ru, Rh, Pd, Ir, Pt) on the partial reduction reaction was also studied using carbon-supported catalysts prepared in-house. The ordering of selectivity to cycloalkenes of these catalysts was sensitive to the thermal treatments used in their preparation. Catalysts that were only reduced gave cycloalkene selectivities which decreased in the order of Pd > Pt > Rh > Ir > Ru and calcination of these materials led to the selectivity to cyclic olefins of Rh rising above that of Pt. The metal ordering was ascribed to the differences in the reaction mechanism, whilst the beneficial effect of calcination was not conclusively identified. Lower yields of intermediate were obtained using the in-house Pd/C catalyst compared to the industrial catalyst under all conditions examined. This was attributed to the larger particles present in the in-house catalyst promoting the undesirable isomerisation reaction.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 9 May 2022
Last Modified: 09 May 2022 16:19
URI: https://orca.cardiff.ac.uk/id/eprint/149617

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