This report summarises the work done on monohydroxy aliphatic alcohol upgrading using Au/TiO2 catalysis. The catalysts were initially tested using a plug-flow CO oxidation reactor; complete conversion of a stream of CO flowing over the catalyst bed at a GHSV of approximately 79,500 hr-1 was typically achieved without any required external heating. TEM analysis showed that the freshly prepared catalyst does not contain detectable Aunano clusters, while the spent CO oxidation catalyst had clearly visible nanoparticles with an average size of approximately 1.6 nm. XRD analyses showed that the final pH to which the deposition-precipitation procedure was adjusted had a major role in determining the average nanocluster size. Alcohols were oxidised using the 1% Au/TiO2 catalyst in a plug-flow reactor, with the alcohol vapour being produced by sparging a blended stream of helium and oxygen (typically made up to a total flowrate of 100 ml min-1). The temperature of the alcohol could be adjusted, thereby controlling the vapour mole fraction of alcohol. For methanol oxidation, the primary reaction pathway across the entire range of studied feed compositions was combustion. The onset of combustion occurred dramatically, in the range of 140-160°C. For ethanol oxidation, acetaldehyde selectivity increases and overall conversion decreases as the oxygen content of the feed stream decreases. The kinetics of the catalysed ethanol oxidation showed a compensation effect, described by the equation ln(A) = 0.2032EA + 2.6102 (EA in kJ mol-1). Propanol oxidation demonstrated the highest selectivity towards a value added product (propanaldehyde), with propanaldehyde being formed in significant quantities. However, combustion was still favoured at high temperatures when large excesses oxygen were present. The thermokinetic data calculated for n-propanol oxidation did not exhibit the compensation effect observed in ethanol oxidation; the EA for this reaction was stable at approximately 38 kJ mol-1. In the anaerobic catalysed reactions of ethanol and n-propanol, an oily layer was collected above the water meniscus in a cold trap. This oil could potentially be formed via poly-aldol condensation reactions of the aldehydes produced during oxidation. Though other researchers suggest these condensation reactions typically end in a cyclic dehydration into aromatic compounds, electrospray mass spectrometry found no indication of such products. Control reactions performed using unloaded TiO2 and porous Au (obtained by in-situ reduction of Au2O3) produced different product distributions, all requiring substantially higher reaction temperatures. This suggests that there must be a synergistic effect between the Au and TiO2 substrate which facilitates reactions. Furthermore, the product distributions of the 1% Au/TiO2 catalysed reactions were significantly different from results published by other researchers performing similar oxidations on Au(111) single crystals, where substantially higher selectivity towards value-added products (formaldehyde, acetaldehyde, and propanaldehyde) is typically observed.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:697745 |
| Date | January 2015 |
| Creators | Indar, Devon |
| Contributors | Schroeder, Sven |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/oxidation-of-alcohols-using-heterogeneous-autio2-catalysts(14601b30-96d8-418b-bfb8-107d25490249).html |
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