School of Engineering and Science - Jacobs University

PVP stabilizer. In the work of Bönnemann [114, 115] , no such dependence was found
(ee = 76% over 1.5 nm Pt cluster and 78 % over 3.9 nm, under 1 bar of H 2 pressure).
In our opinion, one of the primary effects causing enhanced ee is a higher concentration
of active chiral sites (“π-bonded” cinchonidine) on the catalyst surface. In the work of
Yang and co-authors cinchonidine adsorbs on the Pt surface from solution (optimum
concentration of cinchonidine [80, 102, 180]) whereas high hydrogen pressure (40 bar)
promotes a surface cleaning (from PVP, AcOH and etc.) for obtaining more space for
flat adsorbed cinchonidine ( cinchonidine in any tilted mode needs, definitely, less free
space for the adsorption). The catalyst prepared according to Bönnemann’s procedure
(sample B) possesses adsorbed cinchonidine in flat and in two tilted modes as
demonstrated in this manuscript and a high pressure of hydrogen was not applied.
Moreover ee=94-95 % was obtained under high (100 bar) H 2 pressure with
conventional 5R94 [180] and Engelhard 4759 [181]- Pt/Al 2 O 3 catalysts (average
particles size 3.6 nm [182]) where the above mentioned size effect seems to be less
probable.
Based on these facts, we would like to conclude here, that it was found that the size of
Pt clusters (in range 1.4- ∞ nm) has no observed influence on the adsorption mode of
the cinchonidine (at least in case of sample B (1.4 nm), sample D (2.3 nm),
conventional Pt/Al 2 O 3 (4 nm) and macroscopic samples from work of Zaera [77] and
Ferri [76]). This is important because it has been reported in the literature that cluster
size does influence both catalytic activity and the observed ee [106].
Thus, it was found that both ‘flat’ and ‘tilted’ modes of adsorbed cinchonidine are
present on Pt nanoclusters and could be an explanation of the fact that this type of
catalyst does not provide the maximum possible enantiomeric excess (99 %).
4.3.3 Investigation of catalytic behaviour of cinchonidine
modified Pt nanoclusters
Investigation of catalytic behavior of cinchonidine modified Pt nanoclusters was
studied with B, D and conventional modified Pt/Al 2 O 3 in the hydrogenation of ethyl
pyruvate.
Initial transient period
Under all reaction conditions, summarized in Table 4-4, enantiomeric excess was found
to depend on the reaction time in a manner similar to that shown in the Fig. 4-13.
During the first moments of hydrogenation (the period of time termed the initial
transient period) enantiomeric excess increases to a maximum value (ee max ) and then
decreases until the entire amount of ethyl pyruvate is converted in the reaction mixture.
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