Asymmetric Hydrogenation Over Supported Nickel Catalysts

Bennett, Alan (1992) Asymmetric Hydrogenation Over Supported Nickel Catalysts. PhD thesis, University of Glasgow.

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Abstract

The use of Ni/SiO2 catalysts modified in aqueous solutions of optically pure (R,R) tartaric acid for the asymmetric hydrogenation of methyl acetoacetate to methyl 3-hydroxybutanoate has been studied in detail. This study into the effect of the modification variables, namely tartaric acid concentration, temperature and pH on the enantiodifferentiating ability of Ni/SiO2 catalysts, has shown that each of these variables exert an influence on the extent of nickel corrosion during modification as well as the enantiodifferentiating ability of the resultant catalyst. In contrast to previous reports in the literature, aimed at a specification of standard conditions for nickel catalyst modification, this study has shown that, due to the corrosive nature of modification, highest values for enantioselectivity are achieved when specific conditions are derived for the individual catalyst. Ni/SiO2 catalysts of higher nickel loading are more resistant to the inherent corrosiveness of the tartaric acid modification procedure and, in consequence, are capable of exhibiting greater enantiodifferentiating ability than catalysts which have low nickel loading. The adsorption of tartaric acid results in the formation of a nickel/tartaric acid surface complex, a quantity of which can be degraded to a nickel (II) tartrate complex which is washed into solution. From the results of this study a mechanism for the enantioselective and racemic hydrogenation of methyl acetoacetate to methyl 3-hydroxybutanoate is suggested. It is proposed that a nickel/tartaric acid complex is formed on the surface during modification and that this complex controls the stereochemistry of the adsorbed substrate which is hydrogenated. Bare nickel sites are still present on the catalyst surface after modification and these are considered to be responsible for hydrogen activation and for racemic hydrogenation. It is also proposed that spill-over of hydrogen from the bare nickel sites to the nickel/tartaric acid/adsorbed methyl acetoacetate complex is a prerequisite for enantio- selective hydrogenation. The enhancement in enantioselectivity when sodium bromide is used as a co-modifier has been demonstrated to be the result of a partial poisoning of non-selective sites remaining on the catalyst surface after modification.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: G Webb
Keywords: Organic chemistry, Inorganic chemistry
Date of Award: 1992
Depositing User: Enlighten Team
Unique ID: glathesis:1992-75439
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 20:07
Last Modified: 19 Nov 2019 20:07
URI: http://theses.gla.ac.uk/id/eprint/75439

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