Periodic mesoporous organosilica grafted palladium organometallic complex: Efficient heterogeneous catalyst for water-medium organic reactions

Article abstract of DOI:10.1002/aoc.1696

Water-medium organic reactions were studied over periodic mesoporous silica (PMO) containing Pd(II) organometallic complex. This heterogeneous catalyst was achieved by Pd(II) compound coordinated with the PPh₂-ligand onto the pore surface of ph

Full text of DOI:10.1002/aoc.1696

Full Paper
Received: 8 April 2010
Revised: 29 May 2010
Accepted: 6 June 2010
Published online in Wiley Online Library: 24 August 2010
(wileyonlinelibrary.com) DOI 10.1002/aoc.1696
Periodic mesoporous organosilica grafted
palladium organometallic complex: efficient
heterogeneous catalyst for water-medium
organic reactions
Jianlin Huang and Fang Zhang∗
Water-medium organic reactions were studied over periodic mesoporous silica (PMO) containing Pd(II) organometallic
complex. This heterogeneous catalyst was achieved by Pd(II) compound coordinated with the PPh2-ligand onto the pore surface
of phenylene-bridged PMO support. This catalyst displayed ordered mesoporous channels, which ensured the high dispersion
of Pd(II) active sites and the convenient diffusion of reactant molecules into the pore channels. Meanwhile, the phenyl group in
the pore wall of PMO could enhance the surface hydrophobicity which promoted the adsorption of organic reactant molecules
on the catalyst in aqueous environment. As a result, this elaborated catalyst exhibited comparable activity and selectivity with
the corresponding PdCl2(PPh3)2 homogeneous catalyst in the water-medium organic reactions, and could be used repeatedly,
c
showing a good potential in industrial applications. Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Keywords: periodic mesoporous silica; heterogeneous catalyst; water-medium; PdCl2(PPh3)2; immobilization
Introduction
alytic activity. Meanwhile, the organic groups anchoring on the
pore surface might easily leach off during liquid phase cataly-
sis, leading to poor catalyst durability.[9] The recent discovery
of the periodic mesoporous organosilicas (PMOs), built from
bridged organosilane precursors, wherein organic group R is
an integral part of the mesoporous wall, opened up a new av-
enue for organo-modified mesoporous materials. Unlike organic
group-functionalized mesoporous silicas, the organic groups are
homogeneously distributed in the framework of PMOs, which
could provide a novel platform for water-medium organic re-
actions in a designated way by altering the pore structure and
chemical composition.[10]
In this work, we report a facile approach to a novel Pd(II)
organometalliccatalystimmobilizedonaphenylene-bridgedPMO
support. As expected, this catalyst exhibited superiority over other
heterogeneous catalysts and even equivalent catalytic efficiency
to homogeneous catalysts during aqueous organic reactions.
Furthermore, this heterogeneous catalyst can be reused at least
five times with only slight loss of catalytic activity.
In nature, numerous biological processes occur in aqueous
environments with unparalleled catalytic activity and selectivity.[1]
In contrast, scientists require the aid of toxic, volatile organic
solvents to complete synthetic procedures, which inevitably
risks environmental pollution and human health problems.[2]
Accordingly, increasing social pressure is requiring scientists to
find green chemical processes that can be conducted in water.
The use of water as a solvent in organic reactions offers significant
environmentaladvantagesbecausewateristhecleanestandsafest
solvent.[3] Manyorganicreactionshavebeenfacilitatedinaqueous
media with satisfactory activity and selectivity. However, most of
them are performed by homogeneous catalysts, which usually
have disadvantages in separation and recycling, thus adding to
the cost and even causing environmental pollution by heavy
metallic ions.[4]
The alternative approach for water-medium chemical trans-
formations is immobilized heterogeneous catalysts.[5] Although
the efficiency of heterogeneous catalysis is generally inferior to
that of homogeneous systems, the advantages of this kind of
catalyst include their easy recovery from reaction mixtures and
their reusability. The major dilemma for the heterogeneous cat-
alytic system is the limited solubility of most organic molecules
in water.[6] We and other groups employed the functionalized
mesoporous silica as a support to address the above limita-
tion. It was found that the functionalization of silica support
with organic groups could strengthen the surface hydropho-
bicity, which facilitated the adsorption of organic reactants on
the catalyst, leading to the enhanced activity and selectivity.[7,8]
However, the incorporation of the organic groups in the pore
channels might limit the diffusion of reactant molecules due to
the blockage of pore channels, which is unfavorable for cat-
Experimental
Catalyst Preparation
The diphenylphosphine-functionalized phenylene (Ph)-bridged
PMO was prepared via co-condensation of bis(triethoxysilyl)
∗
Correspondence to: Fang Zhang, Department of Chemistry, Shanghai Normal
University, 100 Guilin Road, Shanghai 200234, China.
E-mail: zhangfang@shnu.edu.cn
Department of Chemistry, Shanghai Normal University, 100 Guilin Road,
Shanghai 200234, China
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Appl. Organometal. Chem. 2010, 24, 767–773
Copyright ꢀ 2010 John Wiley & Sons, Ltd.

J. Huang and F. Zhang
benzene (BTESB) and 2-(diphenylphosphino) ethyltriethoxysilane
(DPPES), denoted as PPh2 –PMO–Ph. In a typical synthesis, 1.0 g
P123and3.0 gKClweredissolvedin40 ml0.067 MHClaqueousso-
lutionandstirredfor2 h,followedbyadding1.0 gBTESBand0.21 g
DPPES. After stirring for 24 h at 313 K, the mixture was transferred
intoanautoclaveandallowedhydrothermaltreatmentat373 Kfor
another24 h. Themolarratiooforganosilane : P123 : KCl : HCl : H2O
in the mixture was 1.0 : 0.034 : 7.8 : 2.43 : 415. The white solid
product was recovered by filtration and dried in vacuum, fol-
lowed by extraction in 500 ml ethanol solution at 353 K for
24 h to remove surfactant molecules and other adsorbed organic
residues.
The Pd(II) organometallic catalyst immobilized on the
PPh2 –PMO–Ph support was prepared by coordinating Pd(II) ions
with the PPh2-ligand. Briefly, 1.0 g PPh2 –PMO–Ph was added to
30 ml toluene solution containing 0.20 g PdCl2(PPh3)2 and stirred
for 12 h at room temperature under an argon atmosphere. The
yellow solid was washed with toluene until the filtrate became
colorless, followed by Soxlet-extraction with dichloromethane
to remove Pd(II) species that physisorbed on the support. The
final catalyst (Pd–PPh2 –PMO–Ph) with a Pd(II) loading of 1.1
wt% was dried under vacuum at 353 K. For comparison, both the
PPh2 –SBA-15 and the PPh2 –SBA-15–Ph were prepared according
to the method reported previously.[7,8] The Pd(II) organometal-
lic complex was immobilized on these two supports in the
same way as described above, and the as-prepared catalysts
were denoted as Pd–PPh2 –SBA-15 and Pd–PPh2 –SBA-15–Ph,
respectively.
Figure 1. IR spectra of (a) the PMO-Ph and (b) the PPh2-PMO-Ph samples.
at 373 K in N2 flow, from which both the reaction conversion
and the selectivity were calculated using internal standards.
The reproducibility was checked by repeating each result at
least three times and was found to be within acceptable limits
( 5%).
In order to determine the catalyst durability, the
Pd–PPh2 –PMO–Ph catalyst was allowed to settle down after
each run of reactions and the clear supernatant liquid was de-
canted slowly. After being washed with toluene, the catalyst was
re-used with fresh charge of solvent and reactant for subsequent
reaction under the same conditions. The content of Pd species
leached off from the heterogeneous catalyst was determined by
ICP analysis.
Characterization
The Pd(II) loading was determined by inductively coupled plasma
optical emission spectrometer (ICP, Varian VISTA-MPX). The X-ray
powder diffraction (XRD) patterns were recorded on a Rigaku
D/Max-RB diffractometer with CuKα radiation. Morphologies
were observed by transmission electron microscopy (TEM, Jeol
JEM2010). Fourier transform infrared (FTIR) spectra were collected
with a Nicolet Magna 550 spectrometer by using the KBr
method. N2 adsorption isotherms were measured at 77 K using
a Quantachrome Nova 4000e analyzer after being outgassed at
373 K overnight. The specific surface area (SBET), average pore
diameter (DP) and pore volume (VP) were calculated based on
BET method and BJH model, respectively. Solid-state NMR spectra
wereobtainedonaBrukerAV-400instrument.Thermalgravimetric
analysis (TGA) was performed with a Perkin-Elmer Pyris Diamond
TG analyzer under an air atmosphere with a heating ramp of
10 K/min. The surface electronic states were analyzed by X-ray
photoelectron spectroscopy (XPS, PHI5000 versa probe). All the
binding energy values were calibrated using C1S = 284.8 eV as a
reference.
Results and Discussion
Characterizations of Catalysts
The FTIR spectra (Fig. 1) revealed that, in comparison with
the PMO–Ph, the PPh2 –PMO–Ph displayed three additional
absorbance bands around 2981, 698 and 1437 cm−1, indicative
of the asymmetric and symmetric stretching modes of the
C–H bond[11] and the -H out-of-plane deformation of the
monosubstituted benzene ring as well as the vibration from
the P–C bond, showing the successful incorporation of the PPh2-
CH2-CH2- groups into the PMO–Ph support.[12] These groups
replaced the surface OH- groups, leading to considerable decrease
in the absorbance at 3440 cm−1 characteristic of Si-OH groups.
The P–Ph vibration band was normally observed in the range
from 1090 to 1130 cm−1, which could not be resolved due
to the overlap by the intense peak resulted from the Si–O
vibration.
As shown in Fig. 2, the XPS spectra demonstrated that all the
Pd species in the Pd–PPh2 –PMO–Ph sample were present in
bivalence state, corresponding to the binding energy (BE) of
337.3 eV in the Pd3d5/2 level. In comparison with the BE of the
Pd(II) in Pd(PPh3)2Cl2,[13] the BE of the Pd(II) in Pd–PPh2 –PMO–Ph
shifted negatively by 0.70 eV, which was could attributed to the
stronger electron-donation ability of the PPh2 –CH2-CH2 –ligand
to the Pd(II) than that of the PPh3 –ligand. This could be easily
Activity Test
The Barbier reactions were carried out at 323 K in a 25 ml round-
bottomed flask. In a typical run, a catalyst containing 0.051 mmol
Pd, 0.050 ml benzaldehyde, 0.20 ml allyl bromide, 5.0 ml distilled
water and 0.45 g SnCl2 were mixed and allowed to react for 12 h
under reflux. Then the solution was extracted by ether and dried
by MgSO4, followed by filtrating solid catalyst and evaporating
solvent. Reaction products were quantitatively analyzed on a
gas chromatograph (GC, Agilent 1790) equipped with an FID
and a JWDB-5 95% dimethyl 1-(5%)-diphenylpolysiloxane column
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PMO grafted palladium organometallic complex
Figure 2. XPS spectra of the Pd(PPh3)2Cl2 and the Pd-PPh2-PMO-Ph
samples.
understood by considering the conjugated π –π system between
P and Ph, which could dilute the electron density on the P atom.[14]
Thus, it could be concluded that Pd(II) complex was anchored
onto the PMO support by the coordinated bonding approach
(Scheme 1).
Figure 3 shows solid-state NMR spectra of the
Pd–PPh2 –PMO–Ph sample. The 29Si MAS NMR spectrum dis-
played three resonance peaks up-field corresponding to T3
(δ = −81 ppm), T2 (δ = −72 ppm) and T1 (δ = −63 ppm),
where Tm = RSi(OSi)m –(OH)3−m (m = 1–3). No Qn peaks are
observed, where Qn = Si(OSi)n –(OH)4−n (n = 2–4) suggest-
ing that all the Si species were covalently bonded with carbon
atoms.[15] From the 13C CP-MAS NMR spectrum, one could see
two peaks around 13 and 33 ppm, attributed to two C atoms
in the PPh2-CH2-CH2-group. Meanwhile, an intense peak around
138 ppm was assigned to the C atoms in the benzene ring.[16]
The peak at 58 ppm was assigned to the C atoms in the C2H5O
group connected with Si due to the incomplete hydrolysis.[17]
Other peaks denoted by asterisks were attributed to rotational
sidebands.[18] No significant resonance peaks in the range of
67–77 ppm were observed, implying the complete removal of
P123 surfactant in the Pd–PPh2 –PMO–Ph sample. Meanwhile,
the 31P MAS NMR spectrum displayed a strong peak at 21.4 ppm
and a shoulder peak around 24.3 ppm, which could be as-
signed to two kinds of P atoms in the Pd(II)–PPh2 and the
Figure 3. Solid state NMR spectra of the Pd-PPh2-PMO-Ph sample.
Scheme 1. Schematic image of the formation of Pd-PPh2-PMO-Ph catalyst.
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Appl. Organometal. Chem. 2010, 24, 767–773
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J. Huang and F. Zhang
Table 1. Structural properties and catalytic performances of different Pd(II) organometallic catalystsa
O
OH
Pd(II)
H
+
Br
H O
2
Catalyst
Pd loading (wt%)
SBET (m2/g)
DP (nm)
VP (cm3/g)
Conversion (%)
Selectivity (%)
Yield (%)
PPh2 –PMO–Ph
–
777
538
184
389
145
4.2
3.6
1.9
5.1
1.7
0.78
0.48
0.15
0.57
0.10
–
–
–
Pd–PPh2 –PMO–Ph
Pd–PPh2 –SBA-15–Ph
Pd–PPh2 –SBA-15
Pd–PPh2 –PMO–Phb
1.1
92
76
54
64
92
94
89
89
84
71
41
57
0.50
0.60
0.90
a Reaction conditions: a catalyst containing 0.051 mmol Pd(II), 0.050 ml benzaldehyde, 0.20 ml allyl bromide, 0.25 mmol 2-phenylethanol as internal
standard, 5.0 ml H2O, 2.0 mmol SnCl2, reaction temperature = 323 K, reaction time = 12 h. The conversion was calculated by benzaldehyde.
b The catalyst after being used repetitively six times.
Figure 4. TG and DTA curves of the Pd-PPh2-PMO-Ph sample.
Figure 5. N2 adsorption-desorption isotherms of (a) the PPh2-PMO-Ph and
(b) the Pd-PPh2-PMO-Ph samples.
Pd(II)–PPh3 coordination bonds.[19] These results further con-
firmed the successful incorporation of Pd(II) active sites in the
PMO–Ph network. This was further confirmed by the TG/DTA anal-
ysis. As shown in Fig. 4, besides weight loss due to the removal
of physisorbed water, the Pd–PPh2 –PMO–Ph sample displayed
three exothermic peaks at 611, 673 and 811 K with weight loss
around 8.8, 11.2 and 9.2% owing to the subsequent combustions
of the PPh2-CH2-CH2-, PPh3- and Ph-groups anchored on the sil-
ica support,[20] which was consistent with the weights of these
organic groups calculated theoretically.
Figure 5 revealed that both the PPh2 –PMO–Ph and the
Pd–PPh2 –PMO–Ph samples displayed type IV N2 adsorp-
tion–desorption isotherms with the H1 hysteresis loop, which
was characteristic of typical mesoporous structure.[21] The im-
mobilization of the Pd(II) organometallic catalyst had no signif-
icant influence on the shape of the N2 adsorption–desorption
isotherm, but caused little decrease in the SBET, DP and VP
(see Table 1) due to the occupation of the Pd(II) complexes
in the pore channels. Meanwhile, the small-angle XRD pat-
terns (Fig. 6) showed three well-resolved peaks indicative of
the (100), (111) and (200) diffractions, suggesting the rela-
tively high ordered degree of the mesoporous structure.[22]
The TEM images (Fig. 7) further confirmed that both the
PPh2 –PMO–Ph and the Pd–PPh2 –PMO–Ph samples exhib-
ited long-range ordered 2D p6mm hexagonal mesoporous
channels.[23]
Table 1 summarizes the catalytic parameter of different Pd(II)-
based catalysts during water-medium Barbier reaction between
benzaldehydeandallylbromide.ThePd–PPh2 –SBA-15–Phexhib-
ited much higher activity and selectivity than Pd–PPh2 –SBA-15,
showing the promoting effect of phenyl group. The incorporation
of the Ph-groups into the SBA-15 greatly enhanced the surface
hydrophobicity, which could promote the adsorption on the
catalyst and thus improve both the activity and the selectiv-
ity. The Pd–PPh2 –PMO–Ph exhibited similar selectivity to the
Pd–PPh2 –SBA-15–Ph, implying that the nature and the chemi-
cal environment of the Pd(II) active sites in these two catalysts
were essentially equivalent. However, the Pd–PPh2 –PMO–Ph
was much more active than the Pd–PPh2 –SBA-15–Ph, which
could be understood by considering the different ways of Ph-
functionalization. The Ph-groups in the Pd–PPh2 –PMO–Ph were
embedded into the silica walls rather than anchored in the
pore channels. However, the Ph-groups in the Pd–PPh2 –SBA-
15–Ph were occupied the pore channels, leading to an abrupt
decrease in SBET, DP and VP (see Table 1). Accordingly, the
Pd–PPh2 –SBA-15–Ph limited the diffusion and adsorption of
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PMO grafted palladium organometallic complex
Figure 8. Recycling test of the Pd-PPh2-PMO-Ph catalyst in water-medium
Barbier reaction. Reaction conditions are given in Table 1.
by Sheldon et al. When the benzaldehyde conversion reached
30%, the reaction mixture was filtered to remove the solid
catalyst and then the mother liquor was allowed to react
for another 12 h under the same conditions.[24] No significant
change in either the conversion or the yield was observed,
indicating that the active phase was not the dissolved Pd(II)
species leached from the supports. Therefore it is reasonable to
conclude that the present catalysis was really heterogeneous in
nature.
Figure 6. Small angle XRD patterns of (a) PPh2-PMO-Ph and (b) Pd-PPh2-
PMO-Ph samples.
The Pd–PPh2 –PMO–Ph could be easily separated from the
reaction mixture by filtration and used for the subsequent run
of reactions. As shown in Fig. 8, the catalyst could be used
repetitively five times without significant decrease in the catalytic
efficiency. However, further increase in the recycled uses caused
an abrupt decrease in the catalytic efficiency. As shown in Table 1,
the activity decreased by 20% while the selectivity remained
almost unchanged in sixth run of recycled tests. One possible
reason was the leach of Pd(II) active sites since the ICP analysis
revealed that 3.5% Pd(II) species had leached after reaction six
times (see Table 1). Another important reason was damage to
ordered mesoporous structure, due to the amphour pore wall of
ourmesoporouscatalyst, whichcouldnotstandthehotwaterover
the long running time. As shown in Fig. 9, the Pd–PPh2 –PMO–Ph
after being used five times displayed a significant deformation
of the N2 adsorption–desorption isotherm. Meanwhile, the small-
angle XRD patterns in Fig. 10 demonstrated an abrupt decrease
in the ordering degree of mesoporous structure. The damage
of ordered mesoporous structure caused an abrupt decrease
in the SBET, DP and VP, which might limit the diffusion of
reactant molecules in the pore channels and their adsorption
on Pd(II) active sites, leading to a decrease in the catalytic
activity.[25]
Figure 7. TEM images of (a) PPh2-PMO-Ph and (b) Pd-PPh2-PMO-Ph.
reactant molecules due to steric hindrance, corresponding to the
lower activity. The Pd–PPh2 –PMO–Ph catalyst was further tested
in the Barbier reactions using a different substance (Table 2),
which also exhibited matchable activity and selectivity with the
Pd(PPh3)2Cl2 homogeneous catalyst. The aliphatic aldehydes and
ketones (butyraldehyde, cinnamylaldhyde and acetophenone)
were also tested by our catalyst (Pd–PPh2 –PMO–Ph); however,
the final results showed very poor catalytic performance. The
control experiments that were catalyzed by homogeneous cata-
lysts [PdCl2(PPh3)2] displayed similar results, which demonstrated
that the above substrate is different to activate in our system.
To expand our catalyst application area, we further tested the
Pd–PPh2 –PMO–Ph catalyst in the water-medium Sonogashira
and Heck reactions. As shown in Table 3, the results showed high
catalytic efficiency in the two typical carbon–carbon coupling
reactions.
Conclusions
In summary, this work developed a new approach to designing
Pd(II) organometallic catalysts immobilized on the phenyl-
functionalized PMO (Pd–PPh2 –PMO–Ph). The ordered meso-
porous structure ensured the high dispersion of Pd(II) active
sites and also diminished the diffusion limit of organic re-
actant molecules. The phenyl-groups embedded in the silica
walls could promote the adsorption of organic molecules on
Pd(II) active sites, especially in aqueous solution. As a re-
sult, the Pd–PPh2 –PMO–Ph exhibited comparable activity and
To determine whether the heterogeneous or the dissolved
homogeneous Pd(II) species were the real catalysts responsible
for the allylation reaction between benzaldehyde and allyl
bromide, the following procedure was carried out, as proposed
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Appl. Organometal. Chem. 2010, 24, 767–773
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
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J. Huang and F. Zhang
Table 2. Catalytic performances of the Pd–PPh2 –PMO–Ph and Pd(PPh3)2Cl2 in different Barbier reactionsa
Entry
1
Catalyst
R
X
Product
Conversion (%)
92
Selectivity (%)
Yield (%)
84
Pd–PPh2 –PMO–Ph
H
Br
92
OH
Pd(PPh3)2Cl2
96
94
94
78
90
73
2
3
Pd–PPh2 –PMO–Ph
CH3
Br
Br
OH
H3C
Pd(PPh3)2Cl2
91
94
90
95
82
89
Pd–PPh2 –PMO–Ph)
Cl
OH
Cl
Pd(PPh3)2Cl2
99
89
92
88
91
78
4
5
Pd–PPh2 –PMO–Ph
H
H
Cl
OH
Pd(PPh3)2Cl2
93
94
96
85
89
80
Pd–PPh2 –PMO–Ph
I
OH
Pd(PPh3)2Cl2
98
97
95
a Reaction conditions are shown in Table 1.
Table 3. Catalytic performaces of the Pd–PPh2 –PMO–Ph catalyst in
water-medium Sonogashira and Heck reactions
Sonogashira reactiona
Heck reactionb
Conversion Yield
Conversion
(%)
Yield
(%)
Entry
R
(%)
(%)
1
2
3
H
76
74
79
74
73
74
79
72
90
79
72
90
OCH3
NO2
Reaction conditions: a 0.0048 mmol Pd(II), 0.24 mmol phenylacetylene,
0.26 mmol iodobenzene, 0.15 mmol n-decane as internal standard,
0.72 mmol DBU, 5.0 ml distilled water, 0.078 mmol CuI, reaction tem-
perature = 353 K, reaction time = 3 h; b 0.0060 mmol Pd(II), 0.20 mmol
styrene, 0.24 mmoliodobenzene, 0.40 mmolEt3N, 0.15 mmoln-decane
as internal standard, 5.0 ml distilled water, reaction temperature =
353 K, reaction time = 5 h.
Figure 9. N2 adsorption-desorption isotherms of the fresh Pd-PPh2-PMO-
Ph and the Pd-PPh2-PMO-Ph after being used repetitively for 5 times.
selectivity with the corresponding PdCl2(PPh3)2 homogeneous
catalyst, and could be used repetitively, which could reduce
the cost and also diminish the environmental pollution from
heavy metallic ions. Other immobilized organometallic catalysts
could also be designed based on the present method, which
offered more opportunities for clean organic syntheses in water
medium.
Acknowledgment
This work was supported by the National Natural Science Foun-
dation of China (50943048), the Chinese Education Committee
(20070270001), the Special Doctorial Project (20070270001), the
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PMO grafted palladium organometallic complex
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Figure 10. Small-angle XRD patterns of the fresh Pd-PPh2-PMO-Ph and the
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