DOI: 10.1002/chem.200802697
Enhanced Pd-Catalyzed Hydrogenation of Olefins within Polymeric
Microreactors under Organic/Aqueous Biphasic Conditions
Yang Lan, Minchao Zhang, Wangqing Zhang,* and Li Yang[a]
The hydrogenation of olefins has fundamental importance
in both academic and industrial fields.[1] Batch hydrogena-
tion carried out in industry or the academic laboratory basi-
cally uses homogeneous or heterogeneous catalysts.[2,3] Ho-
mogeneous catalysts generally have high activity, but with
the disadvantage of hard product separation and catalyst re-
cyclability.[2] On the other hand, besides low activity, hetero-
geneous catalysts need to be removed from the reaction
system by filtration, which may take as long as the chemical
reaction itself.[3] Therefore, research into more efficient cata-
lytic system has been converted to organic/aqueous multi-
phase catalysis, which combines the advantages of both ho-
mogeneous and heterogeneous catalysis.[4,5] Organic/aqueous
biphasic catalysis employing water-soluble transition-metal
complexes or metal nanocatalysts has become popular over
the past few decades,[6–12] because it is easy to realize catalyst
recyclability and product separation by simple means of de-
cantation, since the catalyst is dispersed in aqueous phase
and the product in organic phase. However, while the bipha-
sic conditions facilitate catalyst/product separation, it also
introduces a kinetic barrier due to mass-transport limitations
between the two phases.[4,5] To promote mass transportation,
several approaches including addition of co-solvent,[13] sur-
factant[14] and the use of phase-transfer catalysts[15] and am-
phiphilic catalysts,[16] have been proposed.
the ability to encapsulate and enrich hydrophobic organic
substrates in the aqueous phase. Therefore, there is no need
to add a co-solvent or surfactant, and hydrogenation occurs
efficiently within the microreactor with high local concentra-
tion of reactants. Additionally, the microreactor is stable
and can be possibly used in practical applications for contin-
uous hydrogenation of olefins on a technical scale without
deactivation in activity.
The microreactor schematically shown in Figure 1a is
composed of three parts: 1) the outer corona of polyacryl-
amide (PAM), which is hydrophilic and keeps the micro-
reactor suspending in aqueous phase; 2) the cross-linked
and hydrophobic wall of poly[styrene-co-2-(acetoacetoxy)-
AHCTUNGTREGeNNNU thylmethacrylate] (PS-co-PAEMA), which maintains the
body of the hollow microsphere; and 3) 3.9 nm Pd nanopar-
ticles, which are immobilized in the wall of the hollow mi-
crosphere. The microreactor was synthesized by two general
steps (Figure S1 in the Supporting Information). First,
hollow microspheres were constructed by using several pro-
cedures of seed polymerization and followed by removal of
the seed. The synthesis was tracked by IR (Figure S2 in the
Supporting Information) and solid-state 13C CPMAS NMR
spectroscopy (Figure S3 in the Supporting Information), as
well as TEM (Figure S4 in the Supporting Information).
Second, Pd ions were firstly conjugated with the chelate li-
gands of PAEMA[19] and subsequently reduced by NaBH4
aqueous solution, which consequently resulted in formation
of 3.9 nm Pd nanoparticles immobilized in the walls of the
hollow microspheres (Figure 1). Herein, it is deduced that
the Pd nanoparticles are selectively embedded in the wall of
the hollow microspheres, since the chelate segment of
PAEMA is located predominantly at the wall of the hollow
microspheres. Powder X-ray diffraction (XRD) patterns
(Figure S7 in the Supporting Information) also confirm the
formation of Pd nanoparticles.
Reactions within a suitable micro- or nanoreactor[17] can
be promoted possibly due to high local concentration of re-
actants.[18] Herein, we propose a microreactor of a water-
soluble hollow polymeric microsphere with Pd nanoparticles
immobilized in the wall for the hydrogenation of olefins
under organic/aqueous biphasic conditions. The present mi-
croreactor possesses an important advantage in that it has
[a] Y. Lan, M. Zhang, Prof. W. Zhang, L. Yang
Key Laboratory of Functional Polymer Materials of Ministry of Edu-
cation
The microreactor can be easily dispersed in aqueous
phase due to the hydrophilic PAM corona (Figure S6 in the
Supporting Information). Besides, the unique structure of
the microreactor endows itself with an important advantage;
that is, the microreactor can encapsulate and enrich organic
Institute of Polymer Chemistry, Nankai University
Tianjin 300071 (P.R. China)
Fax : (+86)22-23503510
Supporting information for this article is available on the WWW
3670
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Chem. Eur. J. 2009, 15, 3670 – 3673