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The kinetics of the hydrogenation of ethyl pyruvate to ethyl lactate on a 5% Pt/Al 2 O 3 catalyst in toluene was investigated both in absence and in presence of the chiral modifier 10,11-dihydrocinchonidine. It was shown that all important prerequisites for obtaining reliable kinetic data for the reaction were fulfilled. The effects on rate and enantiomeric excess of catalyst loading, modifier and substrate concentrations, hydrogen pressure, and temperature were determined for the unmodified and the modified system. The modified reaction was approximately 20–30 times faster than the unmodified reaction. A significant increase of the enantiomeric excess from 1 to 40 bar was noticed. Apparent activation energies were estimated to be 4–6 kcal/mol. Rate equations were developed for various kinetic schemes on the basis of the Langmuir–Hinshelwood–Hougen–Watson formalism and fitted to the kinetic data. Several such schemes described the measured data reasonably well and in most cases explanations other than the one considered to be the most plausible were also in agreement with our data. For the unmodified catalyst, we propose a competitive adsorption of the α-ketoester and hydrogen and the addition of the first hydrogen atom to be rate determining. On a chiral site, the rate determining step (RDS) to the major enantiomer is proposed to be the addition of the second hydrogen, whereas the RDS for the minor enantiomer remains the first H addition. On the basis of this interpretation, different proposals advanced in the literature for the mode of action of the cinchona modified Pt catalyst were compared. |
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