The heterogeneous gas/solid enantioselective hydrogenation of methyl pyruvate over cinchonidine modified supported platinum catalysts (the Orito reaction) in a differential flow system was studied using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and capillary gas chromatography. The reaction was studied over a temperature range of room temperature to 160°C. The adsorption of methyl pyruvate on a Pt(111) single crystal was also studied using Reflection Adsorption Infrared Spectroscopy (RAIRS). DRIFTS spectra are presented showing that methyl pyruvate absorbs on Pt/alumina catalysts principally via the keto-carbonyl (T1 adsorption mode). At room temperature, methyl pyruvate adsorbs on the unmodified catalyst in a T1 mode with the keto-carbonyl forming a carbon-oxygen-metal bond (COM), with a slight preference for the <i>trans </i>conformer. Over the modified catalyst at room temperature, methyl pyruvate adsorbs with a strong preference for the T1-<i>cis</i><sub>COM </sub>mode. Other modes of adsorption are observed: a <i>cis </i>bidentate mode utilising both carbonyls; a <i>trans </i>mode using the keto-carbonyl and the ester oxygen; and T1 adsorption via the keto-carbonyl lone-pairs. As the temperature is increased, these additional modes increase in population. The unmodified reaction was found to be temperature independent over the range studied while the modified reaction showed changes in conversion and enantioselectivity. The deactivation of the modified reaction is discussed in terms of an enantiodirecting function of the modifier and a ligand accelerating function. The enantiodirection decreases above room temperature, while the rate-enhancing ligand acceleration is stable up to 100°C. Above 100°C, it is proposed that the modifier destructively hydrogenates producing non-volatile side-products that block the active catalyst sites. Possible side-reactions and products are also discussed. The implications of a successful gas/solid Orito reaction are discussed with reference to current literature.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:643400 |
| Date | January 2005 |
| Creators | Cook, Alexander C. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13457 |
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