Transformations of ethanol by homogeneous catalysis for creation of advanced biofuels

• Main Author: Dowson, George Richard Michael
• Published: University of Bristol 2012
• Subjects: 662.6692
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.617929

We propose and elucidate mechanisms of two methods for transformation of ethanol into longer-chain higher alcohols, specifically butanol, for use as advanced biofuels capable of being "drop-in" replacements to petrol. The first method involves dehydration of ethanol to ethylene at low temperatures (below 120°C) using hydroiodic acid in a similar process to methanol carbonylation, allowing heat-sensitive but water-tolerant ethylene dimerizalion catalysts to afford butenes that may be hydrated to butanol. Anionic rhodium halocarbonyl complexes were investigated by NMR-scale ethyl iodide dehydrohalogenation experiments at low temperatures in order to determine the mechanism by which ethanol is dehydrated in the parent screening experiments. These NMR experiments showed that halogen exchange had likely led to some of the yield limitations observed in the screening experiments and that the use of tetraalkylammonium salts was necessary for the catalyst to regenerate and achieve turnover, so far indicating the proposed methanol carbonylation-like route was proceeding. The second method, involving dehydrogenation of ethanol to acetaldehyde with aldol condensation and subsequent hydrogenation affording butanol directly is also discussed, with ruthenium diphosphine catalysts and alkoxide bases found to allow unprecedented yields and selectivity for the homogeneous transformation of ethanol into butanol. Screening experiments for optimising reaction conditions for maximum yield and selectivity were carried out as well as investigations into water tolerance and side product formation. A mechanistic study of this reaction was also carried out, showing that the high selectivity was achieved by control of the aldol condensation of acetaldehyde to form 4- carbon species and the suppress ion of further aldehyde coupling, preventing the formation of longer-chain species. Deuterium labelling, catalyst loading and reaction homogeneity studies were also carried out, indicating a novel on-metal coupling process was occurring, the speculative mechanism of which is proposed and discussed.