Preparation and application of SBA-15-supported palladium catalyst for Heck reaction in supercritical carbon dioxide

Article abstract of DOI:10.1016/j.cclet.2010.12.044

A new method for the preparation of SBA-15-supported palladium catalyst for Heck reaction in supercritical carbon dioxide was presented. The newly formed SBA-15-supported palladium catalyst (Ph-SBA-15-PPh₃-Pd) exhibited high catalytic activity for the Heck reaction of 4-nitrobromobenzene with methyl acrylate. The catalyst can be reused several times without a loss of activity.

Full text of DOI:10.1016/j.cclet.2010.12.044

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 643–646
www.elsevier.com/locate/cclet
Preparation and application of SBA-15-supported palladium
catalyst for Heck reaction in supercritical carbon dioxide
a
b
a,
Xiu Juan Feng a, , Mei Yan , Xin Zhang , Ming Bao
*
*
a State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
b School Environmental Science & Technology, Dalian University of Technology, Dalian 116023, China
Received 9 September 2010
Abstract
A new method for the preparation of SBA-15-supported palladium catalyst for Heck reaction in supercritical carbon dioxide was
presented. The newly formed SBA-15-supported palladium catalyst (Ph-SBA-15-PPh3-Pd) exhibited high catalytic activity for the
Heck reaction of 4-nitrobromobenzene with methyl acrylate. The catalyst can be reused several times without a loss of activity.
# 2011 Xiu Juan Feng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: SBA-15-supported palladium catalyst; Heck reaction; Supercritical carbon dioxide
The Heck reaction is a powerful method for the construction of new carbon–carbon bonds in the synthesis of fine
chemicals, agrochemicals, and active pharmaceutical intermediates as well as functional materials [1]. And the Heck
reaction is normally catalyzed by palladium complexes in organic solvents under homogeneous conditions.
Homogeneous systems are known to often suffer from some drawbacks: it is difficult to separate the coupling product
from the catalyst and organic solvent, the expensive palladium catalyst cannot be recycled, and the organic solvent can
cause potential environmental contamination when it is volatilized. Therefore, it is desirable to develop an efficient,
recyclable catalytic system that uses a green solvent.
Recently, great progress has been achieved with the use of heterogeneous palladium catalysts for Heck reactions
using supercritical carbon dioxide (scCO2). These include silica-supported Pd nanoparticles [2], polyurea-
encapsulated Pd [3], and polymer-supported Pd [4]. However, heterogeneous palladium catalysts act as reservoirs for
highly active soluble forms of Pd in many cases, which may lead to the leaching of active catalyst species [5].
In this paper, we report our preliminary results on the preparation and application of a SBA-15-supported palladium
catalystfor Heck reactioninscCO2 forthe firsttime (Scheme1). Themesoporousmaterials, Ph-SBA-15 and Ph-SBA-15-
PPh3, as well as the desired catalyst, Ph-SBA-15-PPh3-Pd, were synthesized. This newly formed SBA-15-supported
palladium catalyst (Ph-SBA-15-PPh3-Pd) is expected to act as a nanoreactor, in which catalysis can occur. Even if the
palladium species breaks off from one phosphine ligand, it may attach to another phosphine ligand inside the SBA-15.
1. Experimental
Mesoporous material Ph-SBA-15-PPh3 was synthesized according to the modified method (Scheme 1) [6,7]. A
mixture of Pluronic P123 (EO20PO70EO20, 4.0 g), diluted HCl solution (6 wt%, 120 g), and deionized water (30 mL)
* Corresponding authors.
E-mail addresses: xiujuan.feng@163.com (X.J. Feng), mingbao@dlut.edu.cn (M. Bao).
1001-8417/$ – see front matter # 2011 Xiu Juan Feng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.12.044

[()TD$FIG]
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X.J. Feng et al. / Chinese Chemical Letters 22 (2011) 643–646
Scheme 1. Schematic illustration for the preparation of SBA-supported palladium catalyst.
was stirred at room temperature untill the Pluronic P123 template was dissolved completely. After the temperature of
the resulting solution was enhanced to 40 8C, 8.4 g of Si(OEt)4 was added dropwise to the solution. After stirring for
24 h, the resultant suspension was transferred into an autoclave. The autoclave was placed under static conditions at
100 8C for 24 h. After the temperature was reduced to room temperature, the precipitated solid was isolated by
filtration, successively washed with deionized water (100 mL) and ethanol (25 mL), and dried at 80 8C under vacuum
for 5 h. The solid (2.0 g) obtained was treated with (MeO)3SiPh (2.0 mL) in toluene (50 mL) in the presence of
pyridine (2.0 mL) at 115 8C for 2 h. After the mixture was cooled to room temperature, white precipitates were
isolated by filtration. Then, it was successively washed with ethanol, ethyl ether, and acetone, and dried at 80 8C under
vacuum for 5 h. The Pluronic P123 template was removed by an extraction with scCO2 to obtain Ph-SBA-15 (1.6 g).
The mesoporous materials Ph-SBA-15 (1.5 g) such obtained was treated with ligand (MeO)3SiCH2CH2CH2SCH2
C6H4PPh2 (1.5 g) in toluene (80 mL) in the presence of pyridine (1.5 mL) at reflux under N2 atmosphere for 24 h. The
mixture was slowly cooled to room temperature, and then the precipitated white solid was isolated by a filtration,
washed with methanol, ethyl ether, acetone, and hexane successively, and dried at 80 8C under vacuum for 5 h. The
mesoporous materials Ph-SBA-15-PPh3 was eventually obtained as white powders (1.4 g).
Mesoporous materials Ph-SBA-15-PPh3 (1.0 g) was treated with a solution of Pd2(dba)3 (0.1 g) in CH2Cl2 (50 mL)
at room temperature under N2 atmosphere for 24 h. Such palladium species were tethered within the cavity of the
mesoporous materials Ph-SBA-15-PPh3 via ligand-exchange reaction. The content of Pd in the mesoporous materials
was determined by inductively coupled plasma (ICP) analysis to be 1.04 wt% after the resultant khaki solid was
filtrated and washed with CH2Cl2 (10 mL Â 2) as well as dried at 80 8C under vacuum for 2 h.
The supported palladium catalyst Ph-SBA-15-PPh3-Pd (153.6 mg, 3 mol% Pd), 4-nitrobromobenzene (101.0 mg,
0.5 mmol), methyl acrylate (172.2 mg, 2.0 mmol), and (n-Bu)4NOAc (180.9 mg, 0.6 mmol) were placed in a 25 mL
stainless steel pressure vessel with a magnetic stir bar under nitrogen atmosphere. The vessel was purged with carbon
dioxide three times, and then 13 g of carbon dioxide was filled into this vessel. It was then heated to 120 8C for 24 h. The
pressure increased to 23 MPa at this temperature. The vessel was allowed to cool to room temperature and was then
ventedintoethylacetate(30 mL). Thecrudeproductwaspurifiedbyflashchromatographyusingsilicagelcolumntogive
the desired product (101.5 mg, 98%). (E)-Methyl 3-(4-nitrophenyl) acrylate: 1H NMR (400 MHz, CDC13/DMSO-d6): d
8.25 (d, 2H, J = 8.8 Hz), 7.72 (d, 1H, J = 16.0 Hz), 6.67 (d, 2H, J = 8.8 Hz), 6.56 (d, 1H, J = 16.0 Hz), 3.84 (s, 3H). The
catalyst was collected, washed with water, ethanol, and dichloromethane, then dried in a vacuum and reused.
2. Results and discussion
Fig. 1 shows the UV–vis spectra of the SBA-15 and Ph-SBA-15. Compared with SBA-15, Ph-SBA-15 has an
obvious absorption peak attributed to the benzene ring at 263 nm. This reveals that the external surface of SBA-15 has
been successfully modified with (MeO)3SiPh.
The N2 sorption isotherms of Ph-SBA-15 and Ph-SBA-15-PPh3-Pd are displayed in Fig. 2. Both materials exhibit a
type-IV isotherm pattern with an H2 hysteresis loop, which is characteristic of the mesoporous structure. The ordered
mesoporous structure of Ph-SBA-15, Ph-SBA-15-PPh3, and Ph-SBA-15-PPh3-Pd were further confirmed by X-ray
diffraction (XRD, Fig. 3). All three curves obtained display an intense peak and two weak peaks, which match well
with the pattern of SBA-15 silica reported in literature [8].

[()TD$FIG]
X.J. Feng et al. / Chinese Chemical Letters 22 (2011) 643–646
645
[()TD$FIG]
Fig. 1. UV–vis spectra of SBA-15 and Ph-SBA-15.
600
Ph-SBA-15-PPh3-Pd
Ph-SBA-15
500
400
300
200
100
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0 )
Fig. 2. N2 adsorption–desorption isotherms for Ph-SBA-15 and Ph-SBA-15-PPh3-Pd.
The transmission electron microscopy (TEM) shows well-ordered hexagonal pore channels in the Ph-SBA-15-
PPh3-Pd catalyst (Fig. 4, left). The TEM image also reveals arrays in the Ph-SBA-15-PPh3-Pd catalyst when viewed
perpendicular to the channel axis (Fig. 4, right). Finally, inductively coupled plasma (ICP) analysis indicates that the
ratio of Pd on the Ph-SBA-15-PPh3-Pd catalyst is 1.04 wt%.
[()TD$FIG]
Fig. 3. XRD patterns of Ph-SBA-15, Ph- SBA-15-PPh3, and Ph-SBA-15-PPh3-Pd.

[()TD$FIG]
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X.J. Feng et al. / Chinese Chemical Letters 22 (2011) 643–646
Fig. 4. TEM image of Ph-SBA-15-PPh3-Pd: (left) taken along the parallel channels; (right) vertical to the parallel channels.
[()TD$FIG]
Scheme 2. Heck reaction of 4-nitrobromobenzene with methyl acrylate.
The SBA-15-supported palladium catalyst was then employed in a Heck reaction involving 4-nitrobromobenzene and
methyl acrylate to evaluate its catalytic activity and stability. The results are shown in Scheme 2 [9]. The reaction of 4-
nitrobromobenzene with methyl acrylate proceeded smoothly under optimized conditions (3.0 mol% Pd, 0.5 mmol of 4-
nitrobromobenzene, 2.0 mmol of methyl acrylate, 0.6 mmol of nBu4NOAc, scCO2, 120 8C, 24 h) to give the desired
product in 98% yield (run 1). The SBA-15-supported palladium catalyst was also repeatedly used in the reaction of 4-
nitrobromobenzene with methyl acrylate. The same 96% yield of product was obtained in runs 2 and 3.
In conclusion, we have presented a new method for preparing SBA-15-supported palladium catalyst for Heck
reaction in scCO2. Our study on the catalyst reuse suggests that the Ph-SBA-15-PPh3-Pd catalyst is very stable and can
be reused several times without a loss of catalytic activity. It was considered that this newly formed catalyst acts as
expected as a nanoreactor, in which catalysis occurred, and even if the palladium species breaks off from one
phosphine ligand, it may attach to another phosphine ligand inside the SBA-15. Therefore, our catalyst is very stable
and reusable in comparison with previously reported heterogeneous palladium catalyst.
Acknowledgments
We are grateful to the National Natural Science Foundation of China (No. 20603005) and the Specialized Research
Fund for the Doctoral Program of Higher Education (No. 20090041110012) for their financial support.
References
[1] (a) N.T.S. Phan, M. Van Der Sluys, C.W. Jones, Adv. Synth. Catal. 348 (2006) 609;
(b) F. Alonso, I.P. Beletskaya, M. Yus, Tetrahedron 61 (2005) 11771;
(c) I.P. Beletskaya, A.V. Cheprakov, Chem. Rev. 100 (2000) 3009.
[2] S. Saffarzadeh-Matin, F.M. Kerton, J.M. Lynam, et al. Green Chem. 8 (2006) 965.
[3] S.V. Ley, C. Ramarao, R.S. Gordon, et al. Chem. Commun. (2002) 1134.
[4] R.S. Gordon, A.B. Holmes, Chem. Commun. (2002) 640.
´
[5] A. Corma, D. Das, H. Garcıa, et al. J. Catal. 229 (2005) 322.
[6] S. Wang, D.G. Choi, S.M. Yang, Adv. Mater. 14 (2002) 1311.
[7] R. van Grieken, G. Calleja, G.D. Stucky, et al. Langmuir 19 (2003) 3966.
[8] D. Zhao, J. Feng, Q. Huo, et al. Science 279 (1998) 548.
[9] Because we have recently researched on the palladium-catalyzed Stille type C–C bond forming reaction, the Stille reaction was first examined to
evaluate the catalytic activity and stability of the SBA-15-supported palladium catalyst. However, we found the desired Stille reaction did not
occurred at all, and the starting materials were recovered. It may be due to the bulky structures of allyltributylstannane and phenyltributyl-
stannane. S. Lu, Z. Xu, M. Bao, et al. Angew. Chem. Int. Ed. 47 (2008) 4366.