The continuous O-alkylation of alcohols and diols has been studied over solid acid catalysts in supercritical carbon dioxide, scC02, to examine the potential for performing highly selective, clean etherification reactions. An introduction to the application of these reactions with respect to green chemistry is presented in Chapter 1, focusing in particular, on the industrial use of alternative solvents and solid acid catalysts. The apparatus, experimental methods and analytical techniques are described in Chapter 2, and their use is illustrated with reference to the acid catalysed esterification of 1,6-hexanediol in SCC02. Chapter 3 details the etherification of the symmetrical diol substrate, 1,6-hexanediol, which was investigated using primary alcohols (MeOH, EtOH and n-PrOH) as alkylating agents. This system showed remarkable pressure tunability with the desymmetrised mono-ether being favoured at high system pressures, and the bis-ether formed selectively at low pressure. When studying other diols, intermolecular etherification was only observed at primary OH groups, secondary OH groups preferentially underwent elimination or formed cyclic ethers when the substrate structure allowed. The etherification of the commercially important di-hydroxy substrate, isosorbide, was also attempted in scC02, and the results are presented in Chapter 4. The OH groups of isosorbide, an unsymmetrical molecule, exhibited vastly differing reactivity, and achieving good selectivity to the desired product, dimethyl isosorbide (DMI), proved difficult. In addition, the formation of dimethyl ether (DME), an undesirable by-product formed from unreacted MeOH, means that these processes currently have limited commercial potential. Dimethyl carbonate (DMC) has also been studied as a potentially benign reagent for the formation of methyl ethers in scC02, shown in Chapter 5. DMC was observed to decompose over acid catalysts to form DME and C02, but gave excellent yields of the methyl ethers of various substrates. Temperature programmed experiments revealed that the decomposition reaction greatly assisted the methylation of the substrate. This system shows much greater potential for commercial application as highly concentrated substrate solutions can be converted with high yields and selectivities. A brief summary of the conclusions drawn from this Thesis is given in Chapter 6. This Chapter evaluates how well the initial Project aims were met and proposes some future directions for this area of research.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:580384 |
| Date | January 2006 |
| Creators | Gooden, Peter N. |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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