Enhanced Pd-catalyzed hydrogenation of olefins within polymeric microreactors under organic/aqueous biphasic conditions

Article abstract of DOI:10.1002/chem.200802697

A microreactor of a water-soluble hollow polymeric microsphere with Pd nanoparticles immobilized in the wall was proposed for the hydrogenation of olefins under organic and aqueous biphasic conditions. It was found that the microreactor was stable and was used in practical application for continuous hydrogenation of olefins on a technical scale without deactivation in activity. It was composed of three parts, such as the outer corona of polyacrylamide (PAM), the cross-linked and hydrophobic wall of poly[styrene-co-2-(acetoacetoxy) -ethylmethacrylate] (PS-co-PAEMA), and 3.9 mm Pd nanoparticles. Hollow microspheres were constructed using several procedures of seed polymerization along with removal of the seed for the synthesis of the microreactor. It was observed that the microreactor dispersed easily in aqueous phase due to the presence of the hydrophilic PAM corona. The hydrogenation of CA with H ₂ was also evaluated to investigate hydrogenation of olefins within the microreactor.

Full text of DOI:10.1002/chem.200802697

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
under http://dx.doi.org/10.1002/chem.200802697.
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Chem. Eur. J. 2009, 15, 3670 – 3673

COMMUNICATION
reactants since the hollow microsphere provides hydropho-
bic microenvironment for organic reactants in water. Herein
the encapsulation of a poorly water-soluble cinnamic alcohol
(CA) and a hydrophobic styrene is typically introduced.
After the hollow microspheres are introduced into the satu-
rated CA aqueous solution and stirred for 1 h, the CA con-
centration located in the hollow microspheres is almost
270 times higher than that in water, and the encapsulated
CA takes up about 52% of vacancy volume of the hollow
microsphere (Figure S5 in the Supporting Information). For
the more hydrophobic styrene, the corresponding values are
5070 times and 100%, respectively. The difference in encap-
sulation and enrichment of CA and styrene may be attribut-
ed to their polarity disparity.
The merits mentioned above make the microreactor a po-
tential candidate for organic/aqueous biphasic catalysis. The
entire concept of hydrogenation within the microreactor
under organic/aqueous biphasic condition is pictorially
shown in Figure 2. The olefin forms one phase, meanwhile
Figure 2. Schematic hydrogenation within the microreactor under organ-
ic/aqueous biphasic conditions.
water that contains the microreactor forms the other. The
molecules of olefin firstly diffuse into water and then are en-
capsulated and concentrated into the microreactor, in which
the hydrogenation takes place. Then the product diffuses
into water and is further extracted into the organic phase.
Clearly, the hollow microspheres accelerate the diffusing of
the reactants from the organic phase to the aqueous phase
and further concentrate them within the microreactor
through encapsulation, which is the reason for efficient hy-
drogenation.
To explore hydrogenation within the microreactor under
organic/aqueous biphasic condition, the hydrogenation of
CA with H2 at 358C was evaluated as a typical example. It is
found that the conversion increases almost linearly along
with time, showing a quasi-zero-order kinetics to CA con-
centration (Figure 3). This phenomenon can be explained by
the fact that the local concentration of CA encapsulated
within the microreactor is high enough during the process of
Figure 1. Schematic structure (a) and TEM images (b and c) of the
microreactor.
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W. Zhang et al.
Table 1. Hydrogenation of olefins within the microreactor under organic/
aqueous biphasic conditions.[a]
Entry Substrate
Product
Conditions Yield TOF
[d]
S/C[b]/t [h]/ [%][c]
T [8C]
N
]
1
2000/3/35
100
930
2
2000/1/35
2000/1/35
2000/2/35
100 2150
100 2150
100 1449
3[e]
4
5[f]
6[g]
7[h]
8[i]
2000/6/35
122/5/80
449/4/50
4000/1/90
100
99
100
204
–
448
Figure 3. Time-dependence conversion of CA within the microreactor
under organic/aqueous biphasic conditions. Reaction conditions: aqueous
dispersion of the microreactor (20.0 mL) containing the Pd catalyst
(1.67ꢁ10À5 mol); CA (4.47 g, ꢀ4.3 mL, 3.33ꢁ10À2 mol); 358C; 1 atm H2.
100 2000
9
2000/14/35
2000/2/35
99.5 219
10
100 1099
100 1136
hydrogenation under the present organic/aqueous biphasic
conditions.
11
2000/2/35
5000/1/75
Hydrogenation of several typical aliphatic, aromatic, and
alicyclic olefins within the microreactor under organic/aque-
ous biphasic conditions at 358C was further explored. All
hydrogenation reactions were performed at 358C, since all
olefins used in the present study are liquid at this tempera-
ture. Hydrogenation of all the olefins proceeded efficiently
and almost quantitive yields were achieved (Table 1). For
aliphatic olefins, butyl methacrylate is totally converted to
butyl butyrate in 3 h, and the butyl acrylate, which has less
hindered double bonds, is converted to butyl propionate in
1 h (entries 1 and 2). Of all the aromatic olefins, styrene, the
polarity of which is the smallest, possesses the fastest con-
version rate and the highest turnover frequency (TOF; en-
tries 4–10). Compared the activity of the present microreac-
tor with several reference metal nanocatalysts of Pd[20,21] and
Pt[3e] and even the Rh[22] catalyst under other similar condi-
tions, the present microreactor is very competitive (en-
tries 5–8 and 12). It is deemed that the efficient hydrogena-
tion is ascribed to the high local concentration of olefins en-
capsulated within the microreactor. To confirm this hypothe-
sis, hydrogenation of styrene employing the reference cata-
lyst of 4.1 nm Pd nanoparticles immobilized on the solid
microsphere (Figure S8 in the Supporting Information)
under other similar conditions was tested, resulting in a
much lower TOF than those of the microreactor (entry 5).
Furthermore, the present microreactor used in hydrogena-
tion under organic/aqueous biphasic conditions has an ad-
vantage that it avoids the bothersome separation of catalyst
or the product purification, which is usually necessary in the
batch hydrogenation employing a typical heterogeneous Pd
catalysts, such as Pd/C[23] and Pd nanoparticles deposited on
mesoporous silicas,[24–26] or in the hydrogenation under mul-
tiphasic condition employing a phase-transfer catalyst[27,28]
(Figure S9 in the Supporting Information). In the present
experiment, after the hydrogenation is completed, the or-
ganic phase just contains the product, which can be separat-
12[j]
91
–
[a] Reaction conditions: aqueous dispersion of the microreactor
(20.0 mL) containing the Pd catalyst (1.67ꢁ10À5 mol); substrate (3.33ꢁ
10À2 mol); 358C; 1 atm H2. [b] S/C=the molar ratio of substrate to cata-
lyst. [c] Yield measured by 1H NMR spectroscopy or HPLC. [d] TOF is
measured as moles hydrogenated olefin per mole Pd per hour from the
initial reaction time. [e] Yield at the eighth catalyst cycle. [f] The refer-
ence catalyst of 4.1 nm Pd nanoparticles immobilized on the solid micro-
sphere (Figure S8 in the Supporting Information). [g] The reference Pd
catalyst in reference [20]. [h] The reference Pd catalyst in reference [21];
[i] The reference Rh catalyst in reference [22]; [j] The reference Pt cata-
lyst in reference [3e].
ed simply by decantation and no further purification is
needed; therefore it has possible practical application for
continuous hydrogenation of olefins on technical scale (Fig-
ure S9 in the Supporting Information).
The reusability of the microreactor was also tested by
using the hydrogenation of butyl acrylate as a typical exam-
ple. After 1 h the hydrogenation was completed (Figure S10
in the Supporting Information), the organic phase was re-
moved by simple decantation, and butyl acrylate was reload-
ed. It is found that the microreactor can be used for at least
eight times without any decrease in activity (entry 3 in
Table 1, and Table S1 in the Supporting Information). The
organic phase was analyzed by atomic absorption spectros-
copy (AAS) and no Pd catalyst, which might have leached
into the organic phase, was detected. Moreover, the long-
time endurance of the microreactor is also judged by the hy-
drogenation of CA when the CA/Pd ratio is increased to
40000, which affords a turnover number (TON, mole prod-
uct per mol Pd) of 14400 in 240 h (Figure S11 in the Sup-
porting Information).
In conclusion, a microreactor of hollow polymeric micro-
sphere with Pd nanoparticles immobilized in the wall has
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Biphasic Catalysis
COMMUNICATION
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Keywords: biphasic
microreactors · nanoparticles · palladium
catalysis
·
hydrogenation
·
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Received: December 20, 2008
Published online: February 26, 2009
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