| Summary: | The enantioselective hydrogenation of ethyl pyruvate to ethyl lactate (the Orito reaction) has been studied over cinchonidine-modified Pt/graphite moderated by the adsorption of S, Bi, Rh and Pd onto the Pt surface. The effect of the moderating elements on reaction rate (at 293 K, 30 bar, 100% conversion) has been examined in detail. The types of Pt site ({111}-terraces, {100}-terraces, steps) at which the moderating element is adsorbed has been determined by use of cyclic voltammetry. The effect of heating Pt/graphite in H2/Ar at temperatures between 450 and 900 K was investigated. Sintering occurred, as observed by high resolution transmission electron microscopy, causing a reduction in activity due to a decrease in surface area. A complex variation in the enantiomeric excess (ee) occurred: which reflected the relative abundance of step sites (giving high ee), {111 {-terraces (modest ee) and {100{-terraces (low ee) on Pt particles of various size. S-adsorption favoured terrace occupation and the remaining step sites provided an increased ee. By contrast adsorption of Bi, Rh and Pd onto Pt/graphite resulted in preferential step site occupation at first and to a decrease in ee. In the case of Bi only, very low doses resulted in preferential {100}-terrace occupation and to a transient increase in ee. Rh and Pd, unlike Bi, are potentially catalytically active metals. Rh, like Pt, favoured R-lactate formation (in the presence of cinchonidine) whereas Pd favoured S-lactate formation. The effect of Pd was therefore more extreme than that of Rh. The effect on the activity of Pt/graphite of the adsorption of S, Bi, Rh, and Pd is described in detail. For adsorption of the Bi, reaction rate passed through a maximum with increasing coverage as initial step site blocking led to a reduction in polymer formation and to a 'cleaner' surface this was followed by deactivation as monolayer formation was approached and active surface area was reduced. In the case of Pd adsorption on Pt, the Pd catalysed the formation of a particular polymer that strongly poisoned reaction. Novel experiments were performed in which ethyl pyruvate hydrogenation was catalysed by Pd/graphite moderated by the adsorption of Pt. This appears to be the first study of the catalytic effects of the adsorption of Pt onto Pd . Activity increased with increasing concentration of Pt, becoming similar to that of Pt/graphite as full surface coverage was achieved. This investigation has mapped the relative importance of {111}-terraces, {100}- terraces and elementary steps in the determination of rate and enantiomeric excess in this classic enantioselective reaction, and has provided models that can be tested in future research.
|
|---|
