Preparation of microcapsule-supported palladium catalyst using SPG (Shirasu Porous Glass) emulsification technique

Article abstract of DOI:10.1016/j.cclet.2010.01.027

A new method for the preparation of microcapsule-supported palladium catalyst was described. The highly monodisperse crosslinked polystyrene microcapsules containing phosphine ligand were synthesized by the self-assembling of phase separated polymer (SaPSeP) method using diphenyl(4-vinylphenyl)phosphine and divinylbenzene as a monomer and crosslinking agent, respectively, and 2,2′-azobisisobutyronitrile (AIBN) as an initiator within the droplets of oil-in-water (O/W) emulsions, which were prepared by using the Shirasu Porous Glass (SPG) membrane emulsification technique. The prepared microcapsule-supported palladium catalyst exhibited high catalytic activity for Heck reaction and can be reused several times without loss of activity.

Full text of DOI:10.1016/j.cclet.2010.01.027

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 979–982
www.elsevier.com/locate/cclet
Preparation of microcapsule-supported palladium catalyst using
SPG (Shirasu Porous Glass) emulsification technique
b
a
a,
Ying Liu a, Xiu Juan Feng a, , De Cai Bao , Kai Xiao Li , Ming Bao
*
*
a State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
b School of Science of Chemistry and Food, Bohai University, Jinzhou 121000, China
Received 3 November 2009
Abstract
A new method for the preparation of microcapsule-supported palladium catalyst was described. The highly monodisperse cross-
linked polystyrene microcapsules containing phosphine ligand were synthesized by the self-assembling of phase separated polymer
(SaPSeP) method using diphenyl(4-vinylphenyl)phosphine and divinylbenzene as a monomer and crosslinking agent, respectively,
and 2,20-azobisisobutyronitrile (AIBN) as an initiator within the droplets of oil-in-water (O/W) emulsions, which were prepared by
using the Shirasu Porous Glass (SPG) membrane emulsification technique. The prepared microcapsule-supported palladium
catalyst exhibited high catalytic activity for Heck reaction and can be reused several times without loss of activity.
# 2010 Xiu Juan Feng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Microcapsule-supported palladium catalyst; SPG emulsification technique; Heck reaction
The methods for the preparation of polymer-supported catalysts can broadly be separated into two groups [1,2]: in
the first, a polymer is prepared by first using a monomer bearing ligand moiety, before the catalyst is anchored to the
polymer through a complexation reaction between the transition metal atom and ligand moiety on the polymer. In the
second, a polymer is modified by a ligand, and then the catalyst is attached to the polymer through the method
mentioned above. The metal complexes of polymer-supported catalysts are usually linked to the polymer chain, and
protrude into the reaction solvent. This leads to the loss of catalyst activity when they are recovered by filtration and
recycled due to the leaching of active catalyst species from polymeric support.
To resolve the above-mentioned problem, we researched on the preparation of microcapsule-supported palladium
catalyst using Shirasu Porous Glass (SPG) emulsification technique [3]. The principle of the preparation is
schematically described in Scheme 1. At first, monodispersed size-controllable microcapsules containing phosphine
ligands were prepared by the using diphenyl(4-vinylphenyl)phosphine and divinylbenzene as a monomer and
crosslinking agent, respectively. Palladium was then attached onto microcapsules inside. These newly formed
microcapsules acted as microreactors, in which the catalysis will occur. Even if palladium species breaks off from one
phosphine ligand, it may attach to another phosphine ligand inside of the microcapsule. Therefore, the microcapsule-
supported palladium catalyst should be very stable, and could be reused several times without loss of activity. As
* Corresponding authors.
E-mail addresses: xiujuan.feng@163.com (X.J. Feng), mingbao@dlut.edu.cn (M. Bao).
1001-8417/$ – see front matter # 2010 Xiu Juan Feng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.01.027

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Y. Liu et al. / Chinese Chemical Letters 21 (2010) 979–982
Scheme 1. Schematic illustration for the preparation of the microcapsule-supported palladium catalyst.
expected, the microcapsule-supported palladium catalyst exhibited high catalytic activity in Heck reaction of various
aryl iodides with methyl acrylate in toluene/H2O (v/v = 4:1). Here we wish to report our primary results.
1. Experimental
The emulsification procedure was carried out by utilizing a hydrophilic SPG membrane emulsification kit with
average pore size of 2.8 mm. Prior to emulsification, the membrane was soaked in deionized water containing a small
amount of sodium dodecylsulfate (SDS) for over 30 min after ultrasonic treatments. Then, the SPG membrane was
installed into the SPG membranes module as a filter media. The dispersed phase consisting of diphenyl(4-
vinylphenyl)phosphine (10 mmol, 2.88 g), divinylbenzene (DVB, 2.5 mmol, 0.72 g), 2,20-azobisisobutyronitrile
(AIBN, 3 wt% of monomers), and toluene (2.4 mL) was poured into a pressure tight vessel, and allowed to permeate
into the continuous phase containing polyvinyl alcohol (PVA, 1.0 wt%), SDS (0.3 wt%), and deionized water (60 mL)
through the hydrophilic membrane under nitrogen gas pressure.
The obtained emulsion was transferred into a three-neck bottle glass that had a condenser, a magnetic stirrer and a
nitrogen inlet nozzle. Emulsion polymerization was carried out for 24 h at 70 8C under N2 atmosphere. After the
polymerization had finished, the reaction mixture was cooled to room temperature and methanol was added for the
precipitation of the polymer particles. The polymer particles were then washed with deionized water, methanol,
acetone, dichloromethane, and methanol successively, and dried in a vacuum. The white powder of the microcapsule
particles was obtained in 52% yield (1.87 g).
A mixture of Pd2(dba)3 (250 mg), microcapsules (1.0 g), and CH2Cl2 (20 mL) was stirred at room temperature
under N2 atmosphere for 3 h. The desired microcapsule-supported palladium catalyst was obtained by ligand-
exchange reaction, and the content of Pd in the microcapsules was determined by ICP (Inductively Coupled Plasma)
analysis after the microcapsules were filtrated, washed with CH2Cl2, and dried in a vacuum.
Aryl iodide (0.5 mmol), methyl acrylate (2.0 equiv.), n-Bu3N (3.0 equiv.), microcapsule-supported Pd(0) catalyst
(1.0 mol% Pd), and solvent (toluene/H2O, v/v = 4:1, 2 mL) were added to a Schlenk reactor. The resulting mixture was
stirred at 90 8C for 24 h under N2 atmosphere. After completion of the reaction, the mixture was allowed to cool to
room temperature, and the catalyst was collected by filtration, washed with acetone, water, and ethanol, then dried
under vacuum and reused. The organic filtrate was evaporated under vacuum to give a crude product. The crude
product was purified by flash chromatography using silica gel column (eluent:EtOAc/Petroleum = 15:1) to give the
desired product, which was characterized through 1H NMR spectroscopy. The 1H NMR data of methyl cinnamate are
shown here (400 MHz, CDCl3): d 3.78 (s, 3H), 6.41 (d, 1H, J = 16.0 Hz), 7.35–7.36 (m, 3H), 7.48–7.51 (m, 2H), 7.66
(d, 1H, J = 16.0 Hz).
2. Results and discussion
The highly monodisperse cross-linked polystyrene microcapsules containing phosphine ligand were synthesized by
the self-assembling of phase separated polymer (SaPSeP) method using diphenyl(4-vinylphenyl)phosphine and DVB

Y. Liu et al. / Chinese Chemical Letters 21 (2010) 979–982
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Fig. 1. The SEM image (left) and the TEM image (right) of the microcapsules prepared using SPG emulsification technique.
as a monomer and crosslinking agent, respectively, and AIBN as an initiator within the droplets of oil-in-water (O/W)
emulsions [4], which were prepared by using the SPG membrane emulsification technique. When toluene was utilized
as a solvent in the organic phase, lowest ratio of phosphorus to carbon on the surface of the microcapsule (0.89%) was
observed by energy dispersive X-ray (EDX) spectrometry combined with scanning electron microscope [5], which
indicated that 90% of phosphine ligands existed inside the microcapsule (ratio of phosphorus to carbon on the whole
microcapsule was 8.90% determined by EDX combined with transmission electron microscope) [6]. The scanning
electron microscope (SEM) image and the transmission electron microscope (TEM) image were shown in Fig. 1. As
shown in Fig. 1 (left), the size of the prepared microcapsules was uniform. The TEM image (Fig. 1, right) clearly
indicated that the microcapsules possess a hollow structure.
The microcapsule-supported palladium catalyst was obtained by treating the microcapsules with a solution of
Pd2(dba)3 in dichloromethane. ICP analysis revealed that the ratio of Pd was 2.17 wt%, and the EDX determination
shows that no Pd existed on the surface of microcapsule, which clearly indicated the Pd was completely immobilized
inside the microcapsule.
The microcapsule-supported palladium catalyst was then employed in the Heck reaction to evaluate its catalytic
activity and stability [7]. Heck reactions of various aryl iodides with methyl acrylate were tested under optimized
conditions, the catalysis results of which are summarized in Scheme 2. The reaction of iodobenzene with methyl
acrylate offered the corresponding product methyl cinnamate in excellent yield (98%). When the 4-methy-
iodobenzene was examined a satisfactory reaction yield was obtained (82%). Reactions of 4-methoxy-iodobenzene, 4-
cyano-iodobenzene, and 4-acetyl-iodobenzene provided corresponding products in moderate yields (71–74%). The
content of the soluble palladium species in the reaction of iodobenzene with methyl acrylate was only 0.036 wt%
determined by ICP, which indicated that the catalysis mainly occurred inside of the microcapsules. The microcapsule-
supported palladium catalyst was also repeatedly used in the reaction of iodobenzene with methyl acrylate, and the
yields were 98% in run 1, 96% in run 2, and 92% in run 3. Finally, it was found that this catalyst could not promote the
reaction of bromobenzene with methyl acrylate.
In conclusion, we present a new method for the preparation of microcapsule-supported palladium catalyst. The Pd
species were anchored inside of the microcapsules, and therefore the microcapsules acted as microreactors. It was
considered that the substrates entered the microcapsule through its pores, and then which were transformed to products
by the catalysis of palladium. The microcapsule-supported palladium catalyst obtained can be reused several times
without loss of catalytic activity.
Scheme 2. Heck reaction of aryl iodides with methyl acrylate catalyzed by microcapsule-supported palladium catalyst.

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Y. Liu et al. / Chinese Chemical Letters 21 (2010) 979–982
Acknowledgments
We are grateful to the National Natural Science Foundation of China (No. 20572010) for financial support. This
work was partly supported by the program for Changjiang Scholars and Innovative Research Teams in Universities
(No. IRT0711).
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