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reaction period of 3 h and its eep value also increases from 83.4%
To further demonstrate the advantages of this core–shell
to 93.9%. A lower fresh concentration means less substrate and catalyst, other acyl donors, such as IPA and vinyl octanoate (VO)
an excessive catalyst, which will lead to faster racemization and are applied to the DKR of 1-PE instead of VA. As shown in Fig. 2,
side reaction rates, not only for the substrates, but also for the when VA is replaced by IPA and VO, similar to that using
products. Therefore, changing the concentration of substrates VA, this core–shell (Hb-PDDA@CALB)MSs achieve a higher
and reaction temperatures could adjust the reaction rates of selectivity of product and ees compared to those catalyzed by
racemization and transesterification and make them match well the mixed catalysts, indicating a decrease of the racemization
with each other.
rate in the core–shell catalyst system. However, the results
It is worthy of note that all the results in Fig. 1 and Tables 1 of eep are very different using the two acyl donors. In the
and 2 imply that a high ees, i.e. a slow racemization process IPA system, the eep observed upon catalysis by the core–shell
during DKR results in a high selectivity and ee of products. The (Hb-PDDA@CALB)MSs is increased by 52.1% compared to that
slow racemization can decrease not only the transformation observed for the mixed catalysts (Fig. 2a). However, when VO is
between R- and S-alcohol isomers but also the formation of used as the acyl donor (Fig. 2b), no notable difference in the eep
by-products as well as the further racemization of the product is observed for the two kinds of catalysts and both of them stay
R-ester. Obviously, its influence on the latter is more significant. at a high level (nearly 100%). Obviously, such a high eep in the
Therefore, this core–shell (Hb-PDDA@CALB)MS catalyst exerts a clear IPA system should be owed to the protection effect of the CALB
structural advantage. On the one hand, the CALB shell can timely shell on products. But for the system using VO as the acyl
remove the R-formed substrate and avoid its meaningless racemiza- donor, the size of its corresponding R-ester is so large that it is
tion inside the nanozeolite core, which will reduce the production of difficult to access the acidic sites of nanozeolites even if they
by-products. On the other hand, the CALB shell can work as a are not coated. Therefore, no difference in the eep is observed
protective layer and limit the diffusion of the substrate and product in the two catalytic systems. Furthermore, the advantages of
R-ester toward the nanozeolite core, and decrease their racemization this core–shell catalyst can be also verified by the DKR results of
and side reaction rates occurring in the external surface and micro- other substrates, i.e. 1-(p-tolyl) ethanol and 1-(4-bromophenyl)-
pores of nanozeolites. Thus, a much better result of DKR is achieved ethanol using VA as the acyl donor (Table S3†).
using this core–shell catalyst. And a slightly lower DKR rate can
In summary, a core–shell nanozeolite@enzyme bifunctional
be simply compensated by extending the reaction time. In addition, catalyst is constructed, and applied to the DKR of aromatic
the DKR result of 1-PE catalyzed by only PDDA coated Hb-ZMSs secondary alcohols. All the results indicate that a slow racemization
[(Hb-PDDA)MSs] and Novozyms435 catalysts further indicates that rate is the key to an ideal DKR process (a high selectivity and
this core–shell structure is indispensable for a high selectivity and ee stereoselectivity of products). Because the enzyme shell immobilized
of product during the DKR process (Fig. S3†).
on the acidic nanozeolite can timely remove R-formed substrates
and limit the accessibility and diffusion of substrates/products
towards the acidic sites and so decreases the racemization rate
of the whole DKR process, this core–shell bifunctional catalyst
displays clear advantages of high selectivity and ee of products,
especially when acyl donors with a low molecular weight are
used. Although the advantages of this core–shell bifunctional
catalyst are only explored for the DKR of aromatic secondary
alcohols, it provides an ideal example for the performance
improvement of catalysts just by the reasonable localization
of different catalytically active sites in one catalyst.
This work was supported by the 973 Program (2013CB934101),
STCMS (11JC1400400, 08DZ2270500 and 09DZ2271500) and NSFC
(21171041).
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Fig. 2 The DKR results of 1-PE catalyzed by the core–shell catalyst using
(a) IPA and (b) VO as acyl donors at 50 1C. The concentrations of 1-PE and
acyl donor are 50 and 100 mmol LÀ1, respectively. The reaction time is 3 h
in the (Hb-PDDA@CALB)MS system and 0.25 h (a) and 0.5 h (b) in the
mixed catalyst system.
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Chem. Commun., 2014, 50, 9501--9504 | 9503