M.Sc. / The main objective of this study was to identify and optimise the homogeneous catalytic systems for Pd(II) catalysed alkene acetoxylation in acetic acid to directly yield allyl or vinyl acetates as opposed to the usual aldehydes or ketones. Part of the intention of this study was to investigate potentially suitable re-oxidants and reaction conditions suitable for industrial application. The synthesis of butenyl acetate (via 1-butene acetoxylation) in particular is regarded as a potential value-adding opportunity for solvents producers. For safety considerations the study was performed with liquid alkenes (cyclohexene, cyclopentene, cycloheptene, cyclooctene and 1-hexene). Cyclohexene was used for the bulk of these studies. One of the most important and pioneering reactions in this field is the similar system for the oxidation of alkenes to ketones (the Wacker process). The related reaction, oxidative acetoxylation, is the result of the discovery of the Wacker process. The problems associated with both these reactions is the difficulty in re-oxidising the catalyst Pd once it has been reduced in the catalytic process from Pd(II) to Pd(0). Various reaction systems have been developed to improve these processes. Some of the systems that have been developed in the acetoxylation of alkenes were investigated. From the studies it has become obvious that for ease of Pd(0) re-oxidation a co-catalyst, benzoquinone, is essential for the catalytic process. This system employing a co-catalyst required another oxygen efficient re-oxidant to oxidize hydroquinone once reduced from benzoquinone in the oxidation of Pd(0). The re-oxidant would in turn be oxidized by oxygen. Various types of re-oxidants such as Cu(II) salts, heteropolyacids and metal macrocycles (e.g. Schiff base complexes and phthalocyanine metal complexes) were investigated in the multi-step electron transfer process. The most promising of the systems was the Pd(OAc)2/ benzoquinone/ heteropolyacid (H5PMo10V2O40 .34H2O)/ O2 system. From the studies it was apparent that the type of the re-oxidant can influence the yield of the product. Various other parameters were found to influence the reaction outcome. The type of Pd(II) salt was found to be influential in the reaction, for instance Pd(OAc)2 was found to be a better catalyst than Pd(CF3CO2)2. The catalyst loading was found to improve the yield iv when increased whilst this was not trivial since Pd is expensive the system needed to have as low catalyst loading as possible. The type of alkene used dictated the rate of the reaction and the product distribution. It was found that the conditions used for cyclohexene were not transferable to other alkenes without changing certain parameters to suit the alkene in question. Cycloalkenes acetoxylation was found to proceed without the addition of the strong nucleophile additive NaOAc, whilst for 1-hexene acetoxylation the reaction did not proceed without the additive. The product distribution was found to also differ between cycloalkenes and 1-hexene. For cyclohexene the by-products observed were the disproportionation products cyclohexane and benzene. In the case of 1-hexene the by-products were 2-hexanone (the Wacker reaction) and 2-hexenes (isomerisation). The operating temperature also played a role in the reaction outcome. In some instances the increase in reaction temperature negatively affected the reaction whilst in other cases it improved the reaction. Oxygen pressure also influenced the reaction to a lesser extent, with an increase in pressure favouring the reaction.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:6762 |
| Date | 08 April 2010 |
| Creators | Mdakane, Bongani Patrick |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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