A periodic mesoporous hybrid material with a built-in palladium complex: An efficient catalyst for the Suzuki coupling and alcohol oxidation

Article abstract of DOI:10.1016/j.apcata.2010.05.004

A new periodic mesoporous hybrid material with a built-in Pd-guanidine complex was synthesized in the presence of a structure-directing agent. The N₂ sorption, XRD and TEM characterizations revealed that this hybrid material had a large surface area (>700 m² g^-1), a narrow pore size distribution (centered at 2.7 nm), and a periodic 2D-hexagonal structure. FT-IR, solid state NMR and XPS investigations confirmed that palladium-guanidine complex was covalently integrated with the solid material. The solid material exhibited high activity for the Suzuki coupling reaction of various aryl bromides in an aqueous ethanol solution under air. Furthermore, it was found that the solid material worked well for the aerobic oxidation of benzylic alcohols under ambient pressure of O₂, affording over 99% selectivity. This catalyst could be reused 9 times without a decrease in selectivity. This study supplies a new periodic mesoporous hybrid material with a built-in active palladium sites, which may be applied to other catalytic reactions. Crown Copyright

Full text of DOI:10.1016/j.apcata.2010.05.004

Applied Catalysis A: General 382 (2010) 312–321
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A periodic mesoporous hybrid material with a built-in palladium complex:
An efficient catalyst for the Suzuki coupling and alcohol oxidation
Hengquan Yang , Zhancheng Ma , Yong Qing , Guangyong Xiec,∗, Jie Gaoa,
a,∗
a
b,∗
a
a
a
Li zhang , Jinhua Gao , Lin du
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
School of Chemistry and Chemical Engineering, Jiangsu Polytechnic University, Changzhou 213164, PR China
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education,
a
b
c
South-Central University for Nationalities, Wuhan 430074, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new periodic mesoporous hybrid material with a built-in Pd–guanidine complex was synthesized in the
presence of a structure-directing agent. The N2 sorption, XRD and TEM characterizations revealed that
Received 12 November 2009
Received in revised form 19 April 2010
Accepted 10 May 2010
2
−1
this hybrid material had a large surface area (>700 m g ), a narrow pore size distribution (centered at
.7 nm), and a periodic 2D-hexagonal structure. FT-IR, solid state NMR and XPS investigations confirmed
2
Available online 13 May 2010
that palladium–guanidine complex was covalently integrated with the solid material. The solid material
exhibited high activity for the Suzuki coupling reaction of various aryl bromides in an aqueous ethanol
solution under air. Furthermore, it was found that the solid material worked well for the aerobic oxidation
of benzylic alcohols under ambient pressure of O2, affording over 99% selectivity. This catalyst could be
reused 9 times without a decrease in selectivity. This study supplies a new periodic mesoporous hybrid
material with a built-in active palladium sites, which may be applied to other catalytic reactions.
Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
Keywords:
Mesoporous material
Metal complex
Guanidine
Suzuki coupling
Alcohol oxidation
1
. Introduction
tetraalkoxysilane in the presence of a structure-directed agent,
opens a more promising avenue for the preparation of solid cat-
The Pd-catalyzed reactions such as the Suzuki coupling and alco-
hol oxidation are widely used in modern chemical transformations
1–4]. Various palladium complex catalysts for these reactions have
alysts because of the introduction of an ordered mesoporous
structure into solid catalysts, which facilitates accessibility of the
active sites. Compared to the post-modification method, it endows
the solid catalyst with larger surface area and higher pore volume
owing to the incorporation of organic moieties in the framework
instead of grafting on the surface. Moreover, the co-condensation
method leads to a more homogeneous distribution of active sites
and a larger degree of immobilization (because the hydrolysis and
condensation of silicon precursors occur under basic or acidic con-
ditions) [21]. Along this line, Dufaud et al. synthesized a periodic
mesoporous hybrid material containing a rhodium organophos-
[
been successfully established [5–9]. These metal complexes are
often used as homogeneous catalysts. However, they suffer from
the practical problems such as catalyst separation and catalyst recy-
cling. To address these problems, researchers have immobilized
palladium complexes and nanoparticles on various supports such
as silica, organic polymers and dendrimers to create heterogeneous
catalysts because the heterogeneous systems are easy to handle and
recover [10–18].
Grafting a metal complex catalyst on an inorganic support
is a general method for transforming a soluble metal complex
catalyst to the corresponding solid catalyst. This so-called post-
modification method, however, often leads to a significant decrease
in surface area and even pore blockage of the resulting catalyst
phine complex [31]. This hybrid material showed high activity and
good recyclability in the hydrogenation of alkenes. Corma s group
ꢀ
synthesized a periodic mesoporous hybrid material with a vanadyl
shiff base and carbapalladacycle complexes in the frameworks
[32,33]. These materials also exhibited promising activity towards
the cyanosilylation of benzaldehyde and the Suzuki coupling. How-
ever, in contrast to the ever-expanding number of metal complex
catalysts, the catalytically active mesoporous hybrid materials pre-
pared with this elegant method are rather limited.
[
19–21]. More recently, a new concept for preparing the solid cata-
lyst has been developed with the discovery of periodic mesoporous
materials [22–30]. This method, involving a co-condensation
of two terminally trialkoxysilyl-functionalized complex and
Recently, palladium–tetramethylguanidine complex (PdCl L ,
2
2
L = guanidine) was synthesized and well characterized by its
crystal structure. It was found to be highly active in palladium-
catalyzed reactions [34,35]. Compared with the well-established
∗ Corresponding authors. Tel.: +86 351 7018371; fax: +86 351 7011688.
E-mail address: hqyang@sxu.edu.cn (H. Yang).
0
926-860X/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.05.004

H. Yang et al. / Applied Catalysis A: General 382 (2010) 312–321
313
phosphine, N-heterocyclic carbene and palladacyclic complexes,
palladium–guanidine complex is air-stable, inexpensive, low toxic
and easily accessible via chemical synthesis. Such advantages
would make palladium–tetramethylguanidine complex a potential
catalyst for the practical applications if the complex was immobi-
lized in a proper fashion.
7 ppm compared with that of the unsaturated C (C N) of guanidine,
further indicating a coordination between Pd and guanidine.
2.4. Synthesis of the periodic mesoporous hybrid materials with a
built-in Pd–guanidine complex
Herein, by co-condensation of trialkoxysilyl-functionalized
palladium–guanidine complex and tetraalkoxysilane in the pres-
ence of a structure-directed agent, we attempted to synthesize
a new periodic mesoporous hybrid material with a built-in
palladium–guanidine complex in the framework. The material
can afford fast conversions for the Suzuki couplings of various
aryl bromides in an aqueous ethanol solution under mild condi-
tions. Furthermore, it was found that this material also worked
well for the aerobic oxidation of benzylic alcohols with over 99%
selectivity.
Mesoporous material was synthesized according to the mod-
ified method [33]. A mixture of tetraethylorthosilicate (TEOS),
two terminally trimethoxysilyl-functionalized Pd–guanidine com-
plex (Pd–G) and trimethoxysilyl-functionalized guanidine (G) were
used as siliceous precursor. Cetyltrimethylammonium bromide
(CTAB) was used as a structure-directing agent. The molar propor-
tions of the components in gel were nG: 0.25n Pd–G: (1.0–n–0.25n)
i
TEOS: 0.033 Al(O Pr) : 0.12 CTAB: 8.0 NH :114 H O:10 EtOH,
3
3
2
where n was tuned from 2.5% to 5%, 7.5% and 10%. The molar
ratio of trimethoxysilyl-functionalized Pd complex (Pd–G) to
trimethoxysilyl-functionalized guanidine (G) was kept at 1/4 for
all the cases. After the mixture of CTAB, NH ,H O and EtOH was
3
2
2
. Experimental
stirred at room temperature for 2 h, the mixture of silicon precur-
sors and aluminum isopropoxide was added. After the mixture was
further stirred at room temperature for 30 min, the resulting gel
2.1. Reagents and materials
◦
was transferred to a polyethylene container and heated at 80 C for
PdCl2 were purchased from Sigma Company. Various aryl-
2
4 h. The resulting solid was washed with water and dried in air. The
boronic acids were obtained from Beijing Pure Chemical Co. Ltd.
3-chloropropyl)trimethoxysilane was purchased from Jianghan
structure-directing agent was removed by extraction of the solid
material with a diluted ethanolic HCl solution at a room tempera-
ture for 20 h. According to the molar fraction of G in the total silicon
precursors (n), four materials MS-PdG-G(2.5%), MS-PdG-G(5%), MS-
PdG-G(7.5%) and MS-PdG-G(10%) were synthesized (the number
in parentheses means the G molar percents in the total silicon
precursor).
(
Fine Chemical Company (China, distilled). Cetyltriethylammonium
bromide (CTAB), 1,1,3,3-tetramethylguanidine, aryl bromides and
most of other reagents were obtained from Shanghai Chemical
Reagent Company of Chinese Medicine Group.
2
1
.2. Preparation of
,1,3,3-tetramethyl-2-(3-trimethoxysilylpropyl)-guanidine (G)
2.5. Typical procedures for the Suzuki reaction
[36]
A mixture of bromobenzene (2 mmol), phenylboronic acid
In a dry flask, 1,1,3,3-tetramethylguanidine (17.39 g, 0.151 mol)
(
2.2 mmol), K PO ·7H O (2.2 mmol), ethanol (2 mL), H O (2 mL),
3
4
2
2
and xylene (1.8 mL) were mixed together. After being evacu-
ated and purged with N2 five times, the system was heated
◦
and MS-PdG-G material were stirred at 50 C in air. The reaction
process was monitored by TLC or GC. After the reaction, the mixture
was cooled down to room temperature and repeatedly extracted
with diethyl ether. The combined organic layers were concentrated
and the resulting product was purified by column chromatography
◦
up to 120 C and maintained at this temperature for 2 h, and
then (3-chloropropyl)trimethoxysilane (14.93 g, 0.076 mol)
was added dropwise over 2 h. After being further stirred at
◦
1
20 C for 7 h under N2 atmosphere, the resulting mixture was
1
on silica gel. The product was confirmed by H NMR.
allowed to cool down to room temperature. The formed salt
precipitates (1,1,3,3-tetramethylguanidine·HCl was filtrated
out. The filtrate liquid was distilled under reduced pressure
The 1H NMR data for the coupling products are as follows.
Biphenyl (CDCl , 300 MHz, ppm): ı 7.57 (d, 4H, J = 9 Hz); 7.41–7.46
3
(
m, 4H); 7.34 (t, 2H, J = 7.5 Hz). 4-Methylbiphenyl (CDCl , 300 MHz,
3
and
was thus obtained (yield: ca.18%). 1,1,3,3-Tetramethyl-2-(3-
trimethoxysilylpropyl)-guanidine was denoted as G hereafter. 1
NMR (CDCl , 300 MHz, ppm): ı 0.63 (m, 2H, SiCH ); 1.57 (m, 2H,
1,1,3,3-tetramethyl-2-(3-trimethoxysilylpropyl)-guanidine
ppm): ı 7.52 (d, 2H, 6 Hz); 7.43 (d, 2H, 6 Hz); 7.37 (t, 2H,
Hz); 7.27 (d, 1H, 6 Hz); 7.21(d, 2H, 9 Hz); 2.34 (s, 3H). 4-
Methoxybiphenyl (CDCl , 300 MHz, ppm): ı 7.55–7.60 (m, 4H);
6
H
3
3
2
7
3
1
5
.45 (t, 2H, J = 7.5 Hz); 7.33 (m, 1H); 7.00 (d, 2H, J = 9 Hz); 3.88 (s,
SiCH CH ); 2.58–2,73(m, 12H, NCH ); 3.05 (m, 2H, CH N C); 3.55
2
2
3
2
H). 4-Biphenylcarbaldehyde (CDCl , 300 MHz, ppm): ı 10.07 (s,
3
(
s, 9H, CH O).
3
H); 7.95 (d, 2H, J = 7.8 Hz); 7.75 (d, 2H, J = 7.8 Hz), 7.43-7.66 (m,
H). 2-Methylbiphenyl (CDCl , 300 MHz, ppm): ı 7.09–7.24 (m,
3
2.3. Preparation of two terminally trialkoxysilyl-functionalized
9H); 2.14 (s, 3H). 2-Methoxybiphenyl (CDCl , 300 MHz, ppm): ı
3
Pd–G complex [34]
7.48–7.51 (m, 2H); 7.19–7.36 (m, 5H); 6.86-6.98 (m, 2H); 3.66
(
s, 3H). 4-Acetylbiphenyl (CDCl , 300 MHz, ppm): ı 8.06 (d, 2H,
3
In dried flask, PdCl2 (0.064 g) was added to 1,1,3,3-
tetramethyl-2-(3-trimethoxysilylpropyl)-guanidine (G, 0.1930 g)
a
J = 6 Hz); 7.39–7.68 (m, 7H); 2.67(s, 3H). 4-Cyanobiphenyl (CDCl3,
300 MHz, ppm): ı 7.53–7.64 (m, 4H); 7.50 (d, 2H, J = 7.8 Hz);
in 20 mL of dichloromethane. The system was evacuated and
7.35–7.42 (m, 3H). 4-Trifluoromethylbiphenyl (CDCl , 300 MHz,
ppm): ı 7.67 (s, 4H), 7.57–7.59 (d, J = 6.9 Hz, 2H), 7.38–7.48 (m, 3H).
3
◦
purged four times with N . The mixture was stirred at 55 C for
2
1
2 h and was then cooled to room temperature, leading to a
4-Chlorobiphenyl (CDCl , 300 MHz, ppm): ı 7.53–7.56 (m, 4H),
3
homogeneous solution. The solvent was distilled, affording a Pd–G
complex. The complex was confirmed by UV–vis and 13C NMR spec-
troscopy. Compared with G, the UV–vis spectrum of Pd–G complex
in dichloromethane exhibits two new absorption bands around 370
and 445 nm, which are assigned to the “ligand to metal” charge
transfer transitions (LMCT). In the 13C NMR spectrum, the chemi-
cal shift for the unsaturated C (C N) of the complex has a shift of
7.35–7.50 (m, 5H).
Recycling test for the Suzuki reaction was conducted as follows:
for the first run, the mixture of 10 mmol of bromobenzene, 11 mmol
of phenylboronic acid, 11 mmol of K PO ·7H O, 0.1 mol% Pd with
3
4
2
respect to bromobenzene, 10 mL of water and 10 mL of ethanol was
◦
stirred at 50 C. The reaction was monitored by TLC. At the end of
the reaction the system was cooled down to room temperature

3
14
H. Yang et al. / Applied Catalysis A: General 382 (2010) 312–321
Scheme 1. Synthesis of triethoxysilyl-functionalized palladium–guanidine complex.
and repeatedly extracted with diethyl ether. The obtained cata-
lyst was washed with diethyl ether, water, methanol and acetone
in sequence and dried under vacuum. The recovered catalyst was
weighed again. The fresh solvent and substrates were added, but
the molar ratios of substrates and solvent to Pd remained the same
as in the first run.
40 kV, 30 mA). N2 physical adsorption was carried out on a
micrometritics ASAP2020 volumetric adsorption analyzer (before
the measurements, samples were out gassed at 393 K for 6 h). The
Brunauer–Emmett–Teller (BET) surface area was evaluated from
data in the relative pressure range of 0.05–0.15. The total pore vol-
ume of each sample was estimated from the amount adsorbed at
the highest P/P0 (above 0.99). Pore diameters were determined
from the adsorption branch using the Barrett–Joyner–Halenda
2.6. Typical procedures for the oxidation of benzylic alcohols by
(BJH) method. FT-IR spectra were collected on a Thermo-Nicolet-
O2
Nexus 470 infrared spectrometer. Pd content was analyzed by
inductively coupled plasma-atomic emission spectrometry (ICP-
AES, AtomScan 16, TJA Co.). C and N content analysis was conducted
on vario EL (Elementar). X-ray photoelectron spectra (XPS) were
recorded on a PerkinElmer 5400 ESCA, and the C1S line at 284.9 eV
was used as a reference. TEM micrographs were taken with a JEM-
Benzyl alcohol (2 mmol), K CO (2 mmol), the catalyst MS-PdG-
2
3
G(10%) (1 mol% equiv to benzyl alcohol) and toluene (2 mL) were
combined in a dry flask, and the mixture was stirred at given tem-
perature in air or O2 for a given time. Oxygen gas was introduced
into the flask from an O -balloon under atmospheric pressure. At
2
2
000EX transmission electron microscope at 120 kV. Solid state
the end of the reaction, the liquid obtained by a filtration was
diluted with toluene and then analyzed with GC to determine the
conversion and selectivity.
The recycling test for the oxidation of benzyl alcohol was con-
ducted as follows: for the first cycle, the procedure was the same
as described above. After the first cycle reaction, the solid catalyst
was recovered by a centrifugation, and was washed repeatedly with
dry toluene and acetone. The recovered catalyst was dried under
vacuum and was directly used for the next cycle reaction. For each
NMR spectra were recorded on an Infinityplus 300 MHz spec-
trometer: for 13C CP-MAS NMR experiments, 75.4 MHz resonant
frequency, 4 kHz spin rate, 4 s pulse delay, 1.0 ms contact time,
hexamethyl benzene as a reference compound; for 1 Si CP-MAS
NMR experiments, 79.6 MHz resonant frequency, 4 kHz spin rate,
3
4
.0 s pulse delay, TMS as a reference compound. GC analysis was
conducted on a SP-6800A.
cycle reaction, 0.2 mmol of K CO3 (10% equiv. to the first reaction
cycle) was supplied into the reaction system.
2
3. Results and discussion
3.1. Synthesis of the periodic mesoporous hybrid materials with a
2.7. Characterization and analysis
built-in Pd–guanidine complex
Small-angle powder X-ray powder diffraction was performed
on a Rigaku D/Max3400 powder diffraction system (Cu K␣,
In order to obtain a terminally trialkoxysilyl-functionalized
Pd complex, we devised a simple route for introduction of
Scheme 2. Procedure for the preparation of mesoporous material MS-PdG-G (the molar ratio of Pd–G to G was 1/4).

H. Yang et al. / Applied Catalysis A: General 382 (2010) 312–321
315
two trialkoxysilyl groups onto Pd–tetramethylguanidine complex.
Scheme 1 depicts the route in detail. Tetramethylguanidine was
first subjected to a derivation with chloropropyltrimethoxysi-
lane, leading to 1,1,3,3-tetramethyl-2-(3-trimethoxysilylpropyl)-
guanidine (designated G). Then, G was coordinated with PdCl2,
affording terminally trialkoxysilyl-functionalized Pd–G complex.
For both the Suzuki coupling and alcohol oxidation, it was found
that the supported molecular catalysts were gradually converted
to palladium nanoparticles as the reaction proceeded [37,38]. So,
the presence of an excess of guanidine and uniform nanochannels
on the solid catalyst would be favorable for stabilizing the in situ
formed Pd nanopartilces against growth into larger particles. With
these results in mind, we used trialkoxysilyl-functionalized Pd–G
complex admixed with an excess of trialkoxysilyl-functionalized
guanidine as organosilane precursors (with the molar ratio of
1
/4 for all the cases). The process for synthesis of the periodic
mesoporous hybrid material with a built-in palladium–guanidine
complex is schematically depicted in Scheme 2. The mixtures of G,
i
Pd–G complex, TEOS and Al(O Pr) were used as precursors (a little
3
i
amount of Al(O Pr)3 was added in order to improve the hydrother-
mal stability of materials) [39]. Ethanol–water system was chosen
as a medium to synthesize the mesoporous material because the
presence of a certain amount of ethanol favors the homogeneous
dispersion of organosilane and complex.
The material was synthesized following the modified procedure
for the synthesis of MCM-41. The templates inside the chan-
nels were removed by extraction with an ethanol solution of
HCl. Eventually, periodic mesoporous hybrid materials contain-
ing palladium–guanidine complexes were obtained. By varying the
molar fractions of Pd–G complex and G in the total silicon pre-
cursors, we could synthesize four materials MS-PdG-G(2.5%), MS-
PdG-G(5%), MS-PdG-G(7.5%) and MS-PdG-G(10%) (the number in
parentheses means the G molar percent in the total silicon precur-
sors; the molar ratio of Pd–G to G was kept at 1/4 for all the cases).
Fig. 1. XRD patterns of MS-PdG-G materials with the different content of G.
N sorption isotherms of these four materials and their pore size
2
distribution plots are displayed in Figs. 2 and 3, respectively. The
textural parameters determined by N sorption are summarized in
2
Table 1. Every sample showed a typical type IV isotherm with a
steep capillary condensation step in the range of P/P = 0.25–0.35,
0
which was characteristic of a mesoporous material with an ordered
arrangement of cylindrical pores. As shown in Fig. 3, the samples
investigated had a narrow pore size distribution centered about
3.2. Structural characterization of the periodic mesoporous
2
6–27 Å. From Table 1, one can find that the surface area underwent
hybrid materials with a built-in Pd–guanidine complex
a gradual reduction as the molar fractions of G and Pd–G complex in
the initial silicon precursors increased. Even so, the specific surface
In order to obtain the structural information of the solid mate-
rials on a mesoscopic scale, powder X-ray diffraction (XRD), N2
sorption and TEM were employed to characterize the materials. The
XRD patternsfor MS-PdG-G(2.5%), MS-PdG-G(5%), MS-PdG-G(7.5%)
and MS-PdG-G(10%), are shown in Fig. 1. For MS-PdG-G(2.5%),
its XRD pattern clearly exhibited three diffraction peaks, corre-
sponding to (1 0 0), (1 1 0) and (2 0 0) diffractions of the typical
2
D-hexagonal structure (p6mm mesophase). This is an indication of
a highly ordered mesoporous structure. When the molar fraction of
G in the total silicon precursor increased up to 5% (the molar frac-
tion of Pd–G complex correspondingly increased up to 1.25%), the
intensity of the (1 0 0) diffraction peak slightly decreased together
with the broadenings of the (1 1 0) and (2 0 0) diffraction peaks.
When the molar fraction of G further increased up to 7.5% and 10%
(
the molar fraction of Pd–G complex correspondingly increased up
to 1.875% and 2.5%, respectively), the intensity of the (1 0 0) diffrac-
tion peak further decreased, and the intensities of the (1 1 0) and
(
2 0 0) diffraction peaks became less well resolved. These results
suggest that structural regularity of the synthesized materials dete-
riorate upon increasing the content of organic groups. This may be
explained by the fact that the presence of the bulky organosilane
affects the interactions between silicon oligomers and template
molecules and the assembly ability is thus discounted. Addition-
ally, the (1 0 0) diffraction peaks slightly shift towards low angles
with the increasing of G in the total silicon precursor, indicating that
the dimensions of the unit cell (d spacings) slightly increase. These
observations may also reflect the influence of organic moieties on
the assembly of the silicon species around the template.
Fig. 2. N2 sorption isotherms of MS-PdG-G materials with the different content of G.
MS-PdG-G(5.0%), offset vertically by 200; MS-PdG-G(7.5%), by 400; MS-PdG-G(10%),
by 600.

3
16
H. Yang et al. / Applied Catalysis A: General 382 (2010) 312–321
Table 1
Physical parameters of MS-PdG-G materials with different G contents.
Sa (m2
g
−1
)
Vb (cm3
g
−1
)
Dc (A)
C, Nd (wt%)
Pde (wt%)
Samples
MS-PdG-G(2.5%)
MS-PdG-G(5%)
MS-PdG-G(7.5%)
MS-PdG-G(10%)
876
806
728
718
0.71
0.73
0.84
0.85
26
26
26
27
6.9, 1.2 (5.2, 2.3)
9.4, 2.0 (9.5, 4.2)
12.7,3.2 (13.1, 5.7)
15.1,4.1 (16.3, 7.1)
0.45 (0.96)
0.82 (1.75)
1.18 (2.41)
1.42 (2.96)
a
ET surface area.
b
Single point pore volume calculated at relative pressure P/P0 of 0.99.
BJH method from adsorption branch.
Determined by combustion chemical analysis; the data in parentheses are the theoretical values.
Determined by ICP-AES; the data in parentheses are the theoretical values.
c
d
e
The TEM micrograph of MS-PdG-G(10%) is displayed in Fig. 4.
The incidence directions parallel and perpendicular to the pore axis
are clearly observed in Fig. 4A and B although honeycomb struc-
tures in the partial domain appear. In agreement with the XRD
results, the TEM observation further confirms the periodic ordering
of the synthesized materials.
3.3. Compositional characterization of the periodic mesoporous
hybrid materials with a built-in Pd–guanidine complex
To clarify the compositions of the synthesized materials, we
employed FT-IR, solid state NMR and XPS to further characterize
the synthesized hybrid materials. The FT-IR spectra of MS-PdG-
G(2.5%), MS-PdG-G(5%), MS-PdG-G(7.5%) and MS-PdG-G(10%) are
shown in Fig. 5. The FT-IR spectra of four samples clearly exhibit
−
1
peaks at 2924, 2855 and 1410 cm . The two former ones corre-
spond to the stretching vibrations of C–H bonds and the last is
assigned to the bending vibrations of the C–H bonds (Fig. 5A). A peak
−
1
around 1590 cm was also observed (Fig. 5B), which is assigned to
the stretching vibration of C N. It is worthwhile to note that the
absorption intensity for C N vibration gradually increases as the
fractions of G and Pd–G complex in the initial gel increase (Fig. 5B).
The FT-IR results may indicate that guanidine and Pd–guanidine
complex were successfully incorporated on the materials and sur-
vived the hydrothermal process. The C, N and palladium contents
on the solid materials determined by elemental analysis are also
included in Table 1. It was found that the N and palladium contents
gradually increased as their fractions in the initial gel increased,
which were in agreement with FT-IR results. However, the deter-
mined N and Pd contents were lower than their theoretical values
and ca.50% Pd was introduced onto the mesoporous materials.
The decreased contents may be due to the C N hydrolysis under
base/acid conditions during the course of hydrothermal treatment
Fig. 3. Pore size distribution of MS-PdG-G materials with the different content of G.
2
−1
area of MS-PdG-G(10%) is up to 718 m g . Such surface area value
and pore size may be much higher than those derived from a post-
modification method [21]. The co-condensation method enables
organic moieties to be incorporated in the framework, while a post-
modification method has organic moieties tethered on the surface
of a solid matrix. These differences in the locations of organic
moieties may account for the physical property superiority of the
material obtained with a co-condensation method.
Fig. 4. TEM image of MS-PdG-G(10%). (A) The incidence direction parallel to the pore axis; (B) the incidence direction perpendicular to the pore axis.

H. Yang et al. / Applied Catalysis A: General 382 (2010) 312–321
317
Fig. 6. 29Si CP-MAS NMR spectrum of MS-PdG-G(10%).
XPS elemental survey scans of surface elementals of MS-PdG-
G(10%) revealed that silicon, oxygen, carbon, nitrogen, palladium
and chlorine elementals were present on the materials as expected.
Fig. 8 displays Pd binding energies of PdCl2 and MS-PdG-G(10%).
PdCl2 exhibits two peaks centered at 342.6 and 337.3 eV, which
are assigned to Pd 3d3/2 and Pd 3d5/2, respectively. Compared with
PdCl2, Pd 3d3/2 and Pd 3d5/2 peaks for MS-PdG-G(10%) significantly
shift to 341.5 and 336.5 eV, respectively. The remarkable decrease
in Pd binding energy may imply that there are strong coordination
interactions between Pd and guanidine.
The above characterizations confirm that the synthesized mate-
rials had an ordered mesoporous structure. Pd–guanidine complex
was covalently integrated with the synthesized mesoporous mate-
rials.
3.4. Catalytic activity of the periodic mesoporous hybrid
materials with a built-in Pd–guanidine complex
Fig. 5. (A) FT-IR spectra of the samples; (B) the regional FT-IR spectra of the samples.
a) MS-PdG-G(2.5%); (b) MS-PdG-G(5%); (c) MS-PdG-G(7.5%); (d) MS-PdG-G(10%).
(
3.4.1. The Suzuki coupling
The catalytic activity of the synthesized material was first tested
in the Suzuki coupling. Previous studies showed that solvents and
bases had remarkable influences on the reactivity of the Suzuki
reaction [40–42]. We are interested in using water as a medium
of the Suzuki coupling because using water as solvent repre-
sents a green, safe and cost-effective process [43]. Our preliminary
and template extraction, further leading to the leaching of the frag-
ments containing N and Pd from the gel. The determined C contents
for these four materials were higher with respect to the deter-
mined N and Pd contents. These results probably suggest that a little
amount of templates still remained in the solid materials after sol-
vent extraction. For the material MS-PdG-G(10%), the Pd loading is
up to 1.42 wt% which is sufficiently high for performing a catalytic
reaction.
Figs. 6 and 7 show the 29Si CP-MAS NMR and 13C CP-MAS
29
NMR spectra of MS-PdG-G(10%), respectively. In Fig. 6, the Si
CP-MAS NMR spectrum of MS-PdG-G(10%) shows a “Q” and “T”
silicon series. The signals at −110, −101 and −92 ppm correspond
4
3
2
to Q [Si(OSi) ] and Q [Si(OH)(OSi) ] and Q [Si(OH) (OSi) ] sili-
4
3
2
2
con sites, respectively. The broad peak at −66 ppm is ascribed to
the mixture of T3 [SiC(OSi) ] and T [SiC(OH)(OSi) ] organosilicon
3
2
2
0
sites. The absence of T [SiC(OH) ] sites confirms that the Pd–G and
3
G moieties were covalently integrated with the pore walls. In the
13
C CP-MAS NMR spectrum of MS-PdG-G(10%), three bands were
clearly observed (Fig. 7). The signals centered at 161 and 147 ppm
are ascribed to the unsaturated carbons from Pd–G complex and G
ligand, respectively. The signal at 40 ppm is assigned to the satu-
rated carbon of Pd–G complex and G ligand. The solid state NMR
results are in agreement with the FT-IR findings; they further con-
firm that Pd–G complex and G were covalently incorporated on the
solid materials.
Fig. 7. 13C CP-MAS NMR spectrum of MS-PdG-G(10%).

3
18
H. Yang et al. / Applied Catalysis A: General 382 (2010) 312–321
Table 2
Suzuki coupling reaction catalyzed by MS-PdG-G with different contents of Pd.
.
Samples
MS-PdG-G(2.5%)
MS-PdG-G(5%)
MS-PdG-G(7.5%)
MS-PdG-G(10%)
a
Yield (%)
86
1911
84
1844
77
1755
73
1622
b
−1
)
TOF (h
a
The isolated yield.
b
TOF is calculated according to the following equation: TOF = Nproduct/(Npalladium × t), where N denotes molar number, and t denotes reaction time (h).
screening of solvents revealed that the presence of ethanol could
efficiently promote the Suzuki reaction in water and the 50% aque-
ous ethanol was the best choice for the reaction medium. The
MS-PdG-G(10%) also showed high activity for the Suzuki cou-
plings of other arylboronic acids with different substituents. The
results are listed in Table 4. For p-methylphenylboronic acid and p-
methoxyphenylboronic acid, 96% and 92% yields could be obtained
within 45–60 min (Table 3, entries 8 and 9). When these sub-
stituents changed to o-position, within the same reaction time as
that for p-substituted arylboronic acid, the yields for o-substituted
products were lower than those of the corresponding p-substituted
products (Table 3, entries 10 and 11). The decreased reaction rate
may reflect the steric hindrance effects causing by neighboring
groups. For arylboronic acid with electron-withdrawing groups
◦
catalytic activity tests were carried out at 50 C in air using a 50%
aqueous ethanol solution as a reaction medium and K PO4 as a
3
base. The ratio of Pd to bromobenzene was kept at 0.01 mol% and
the weight of the tested material was varied according to the Pd
content. The coupling reaction results with the four materials are
listed in Table 2. For MS-PdG-G(2.5%), the biphenyl in 86% yield
−
1
was obtained within 4.5 h and the TOF corresponded to 1911 h
.
As the Pd contents of MS-PdG-G(5%), MS-PdG-G(7.5%) and MS-
PdG-G(10%) increased correspondingly to 0.82, 1.18 and 1.42 wt%,
the yield of the product decreased down to 84%, 77% and 73%
within the same reaction time. The catalytic activity of the materi-
als slowly decreases with the increasing of Pd contents. The activity
of the solid catalyst was still lower than that of its homogeneous
counterpart (0.01 mol% of Pd–tetramethylguanidine complex, 97%
yield within 20 min). Even so, MS-PdG-G(10%) gave a TOF value of
such as –Cl and –CF , good yields were still afforded although a
3
longer time was required to complete the reaction (Table 3, entries
12 and 13).
The recyclability of MS-PdG-G(10%) was investigated with the
consecutive Suzuki coupling reactions. Due to the unavoidable loss
of solid catalyst during the course of recovery and washing, the
reaction scale was amplified five times to ensure that enough cat-
alyst of good quality was available for us to perform the several
consecutive recycling reactions, as described in Experimental. The
recycling results of the catalyst MS-PdG-G(10%) are summarized in
Table 4. The catalyst MS-PdG-G(10%) gave a complete conversion
of bromobenzene, and a 95% yield was still achieved within 30 min
under the scale-up conditions. After the first reaction cycle, the cat-
alyst could be recovered by a simple centrifugation and filtration.
After being washed and dried, the recovered catalyst was directly
used for the next reaction cycle. For the second reaction cycle,
biphenyl in 79% yield was still obtained within 2 h. From the third to
the fourth cycle, 78% and 87% yields were achieved although a much
longer reaction time was needed to complete the reaction. In order
to clarify the reason of the decrease in activity of the reused catalyst,
we employed elemental analysis and BET surface area determi-
nation to investigate the catalysts MS-PdG-G(10%) reused three
times. It was found that Pd content of the reused MS-PdG-G(10%)
was 1.31 wt% (namely, 92% of initial Pd on the solid catalyst sur-
vived the repeated reaction cycles). However, the BET surface area
−
1
1
622 h which still represented a fast conversion of bromoben-
zene.
In order to further verify the catalytic performance of MS-PdG-
G(10%), we examined its activity towards other substrates with
diverse substituents. The results are also included in Table 3. At the
Pd loading of 0.1 mol%, MS-PdG-G(10%) afforded a satisfactory yield
for bromides containing –CH and –OCH groups within 35 min–2 h
3
3
(
Table 3, entries 1 and 2). For other bromides with substituents such
as –CHO, –CN, and –COCH3, complete conversions of them were
observed and the corresponding biaryl products in 89–97% yields
were also achieved within 45 min–1.5 h (Table 3, entries 3–5). With
regard to 2-substituted bromides, it took a slightly longer time to
complete the reaction, probably due to steric hindrances, but yields
still exceeded 88% within 2.5–3 h (Table 3, entries 6 and 7).
2
−1
of MS-PdG-G(10%) progressively decreased down to 78.1 m g
after being reused three times. The combined results of the ele-
mental analysis and surface area measurements could indicate that
the decrease in catalytic activity was mainly due to the collapse
of the periodic mesoporous structure and the active Pd species
were thus buried [33]. For the fifth reaction cycle, 81% yield was
still attained within 24 h. Although the activity of the synthesized
material decreased during the recycling, the material represents an
efficient heterogeneous catalyst in view of the high activity under
mild conditions and wide substrate scope [44–47,12].
3.4.2. The aerobic oxidation of alcohols
Encouraged by the good structure of the synthesized hybrid
material and the results obtained in the Suzuki reaction, we
attempted to extend its applications by examining the catalytic per-
formances of MS-PdG-G(10%) in the aerobic oxidation of alcohols.
Selective oxidation of alcohols to carbonyl compounds is one of the
most important transformations in the synthesis of fine chemicals.
Fig. 8. XPS spectra of MS-PdG-G(10%) and PdCl2.

H. Yang et al. / Applied Catalysis A: General 382 (2010) 312–321
319
Table 3
a
The Suzuki coupling of various bromides with arylboronic acids in the presence of the catalyst MS-PdG-G(10%) .
Entry
Substrate 1
Substrate 2
Product
Time
Yieldb (%)
1
30 min
93
2
3
2 h
99
92
45 min
4
5
75 min
1.5 h
97
89
6
2.5 h
88
7
8
9
3 h
93
96
92
45 min
60 min
1
0
45 min
84
1
1
1
2
3
60 min
2.5 h
86
80
76
1
6.5 h
a
The reaction was carried out with 2 mmol of bromobenzene, 2.2 mmol of phenylboronic acid, 2.2 mmol of K3PO4·7H2O, 0.1 mol% Pd with respect to aryl bromides, 2 mL
◦
of water and 2 mL of ethanol at 50 C in air.
b
The isolated yield.
Table 4
Recyclability test of the catalyst MS-PdG-G(10%) in the Suzuki reaction.
.
Cycle
Time
1
2
3
4
5
35 min
95
2 h
79
11.5 h
78
16 h
87
24 h
81
a
Yield (%)
a
The isolated yield.
Currently, of particular interest in this field is the use of molecular
oxygen as oxidant because it represents a green chemical process,
in contrast to the use of toxic and expensive stiochiometric metal
oxidants [48–55].
The results of the consecutive oxidation of benzyl alcohol are
displayed in Table 5. The oxidation reactions were conducted
in toluene under O2 atmosphere using K CO3 as a base. At Pd
2
loading of 1 mol%, the fresh catalyst MS-PdG-G(10%) gave 99%

3
20
H. Yang et al. / Applied Catalysis A: General 382 (2010) 312–321
Table 5
The aerobic oxidation of alcohols in the presence of the catalyst MS-PdG-G(10%).
.
Cycle
1
2
3
4
5
6
7
8
9
Time (h)
Conv. (%)
Selectivity (%)
10
99
>99
16
86
>99
20
98
>99
24
94
>99
27
93
>99
29
98
>99
30
99
>99
30
98
>99
31
98
>99
a
a
The conversion determined with GC.
Scheme 3. The reaction results for the substituted benzylic alcohols.
conversion of benzyl alcohol within 10 h. The selectivity for ben-
zaldehyde was over 99% and the by-product benzoic acid was not
observed.
After the first reaction cycle, the catalyst MS-PdG-G(10%) was
recovered by a simple filtration, washed sequentially with toluene
and acetone, and eventually dried for the next cycle. For the second
reaction cycle, the catalytic activity of MS-PdG-G(10%) decreased.
Acknowledgments
We acknowledge New Teacher Foundation from Education Min-
istry of China (200801081035), Shanxi Natural Science Foundation
for Youths (2009021009), the Natural Science Foundation of China
(20903064), Shanxi University Innovative Experimental Project for
Undergraduates and Jiangsu Key Laboratory of Fine Petrochemistry
for financial supports (KF0802).
8
6% conversion was achieved within 16 h, but the selectivity for
benzaldehyde was still up to 99%. The decrease in catalytic activity
may be explained by the fact that the supported molecular catalyst
Pd–guanidine complex was converted to palladium nanoparticles
as the reaction proceeded [50]. From the third to the eighth reaction
cycle, 93–99% conversion was achieved with a slightly prolonged
time and 99% selectivity was maintained. For the ninth reaction
cycle, 98% conversion was obtained within 31 h. The total TON
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