Fabrication of palladium nanoparticles as effective catalysts by using supramolecular gels

Article abstract of DOI:10.1016/j.cclet.2015.09.009

Two-component supramolecular gels were made through self-assembly of tetrazolyl derivatives and Pd(OAc)₂. The robust gels indicated high storage modulus (>10,000 Pa) and loss modulus, which were studied by rheological measurements. The formed P

Full text of DOI:10.1016/j.cclet.2015.09.009

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CCLET 3433 1–5
Chinese Chemical Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
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Original article
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Fabrication of palladium nanoparticles as effective catalysts by using
supramolecular gels
Q1 Wei Zhang a,b, Zhi-Gang Xie a,
*
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a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
b University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Two-component supramolecular gels were made through self-assembly of tetrazolyl derivatives and
Pd(OAc)2. The robust gels indicated high storage modulus (>10,000 Pa) and loss modulus, which were
studied by rheological measurements. The formed Pd nanoparticles (ꢀ9 nm) obtained during the
formation of the gel showed effective catalytic hydrogenation of nitrobenzene and could be recovered
and reused without loss of activity.
Received 15 July 2015
Received in revised form 22 August 2015
Accepted 8 September 2015
Available online xxx
ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Pd nanoparticles
Self-assembly
Supramolecular gels
Catalysts
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1. Introduction
frameworks to avoid the aggregation [23,24]. Synthesis of metal 31
nanoparticles supported by robust frameworks usually was 32
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Supramolecular chemistry and molecular self-assembly have
been widely used to fabricate various functional new materials [1–
8]. For example, supramolecular gels (SGs) obtained from low-
molecular-weight gelators via non-covalent interactions have
received considerable attention because of their potential applica-
tions in drug delivery and chemical sensor [9–21]. The application
of gels is dependent on their mechanical properties in a great
extent. For example, the gel for the culture of isolated stem cells
needs to have the elastic modulus of about 104 Pa to provide the
enough mechanical resistance, which promotes cell adhesion and
spreading [22]. Compared with the covalently bonded gels, SGs
generally lack enough mechanical strength and viscoelasticity due
to the relatively weak and reversible non-covalent interactions. So
it still remains a challenge to control the mechanical strength for
developing functional gels.
finished by the direct nanoparticle encapsulation or post- 33
modification of frameworks [25–27]. However, these methods 34
possess multiple steps and harsh reaction conditions, like high 35
temperature and pressure, or even a large amount of catalysts. It is 36
necessary to develop a straightforward method to prepare the 37
framework-supported metal nanoparticles [28]. The unique 38
stable gels are suitable matrices for the synthesis of metal 39
nanoparticles. We envisioned the ultrafine metal nanoparticles 40
could be obtained during the preparation of gels. In this work, we 41
reported the in situ formation of Pd nanoparticles during the 42
process of gel formation.
43
Tetrazoles and their derivatives are interesting materials and 44
have been used in various areas, such as materials science, 45
pharmaceutics and biological fields due to their easy functiona- 46
lization. Recently, two-component SGs were prepared based on the 47
tetrazolyl derivatives by hydrogen bonds and coordination 48
chemistry and used for self-healing materials and oil spill recovery 49
[29,30]. In our recent work, we have reported synthesis of cross- 50
linked polymers by the chemistry of 2,5-disubstuted tetrazoles 51
The synthesis of metal nanoparticles for heterogeneous
catalysis has received great attention because of their unique
catalytic properties and wide-ranging applicability. However,
metal nanoparticles with ultra-small size are prone to aggregate
because of their high surface energy. Much effort has been devoted
to immobilization of metal nanoparticles inside the porous
[31,32].
52
Herein, SGs were synthesized by simple two-component 53
supramolecular chemistry, and showed quite high mechanical 54
strength by changing the non-covalent interaction. More impor- 55
tantly, the ultrafine palladium nanoparticles were obtained during 56
the self-assembly of gel, and could be used as effective heteroge- 57
*
Corresponding author.
neous catalysts for hydrogenation of nitrobenzene.
58
E-mail address: xiez@ciac.ac.cn (Z.-G. Xie).
http://dx.doi.org/10.1016/j.cclet.2015.09.009
1001-8417/ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: W. Zhang, Z.-G. Xie, Fabrication of palladium nanoparticles as effective catalysts by using
supramolecular gels, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.09.009

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2. Experimental
30 min and filtered. The filtrate was adjusted to pH 5 with addition
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of HCl (1 mol/L) solution and filtered to give pale grayish powder,
which was washed with distilled water until no Clꢁ remained. 1H
NMR spectrum was deposited in Fig. S1 in supporting information.
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All starting materials were purchased from Aldrich and Fisher
and used without further purification, unless otherwise noted.
Anhydrous solvents were dried by standard procedures. The
thermogravimetric analysis (TGA) was performed by a Netzch Sta
449c thermal analyzer system at the heating rate of 10 8C/min in N2
atmosphere. The FT-IR spectra were measured by a Nicolet Impact
410 Fourier transform infrared spectrometer. TEM micrographs
were recorded using JEM 1011 with an acceleration voltage of
100 kV. The specimens were prepared by gently placing the sample
on a surface of a carbon-coated copper grid, dried for 2 h at room
temperature, and then subjected to observation. High-resolution
transmission electron microscopy (HR-TEM) images were measured
on a Tecnai G2 F20 S-TWIN electron microscope operated at 200 kV.
Particle size data were obtained by analyzing TEM images using
software Nano Measurer 1. 2. 5. SEM micrographs were performed
on JEOL JXA-840 under an accelerating voltage of 15 kVX and the
elemental mapping was also characterized on it under an
accelerating voltage of 20 kVX. For SEM imaging, a drop of freshly
prepared sample solution was cast onto a silicon slice, and then Au
(1–2 nm) was sputtered onto the grids to prevent charging effects
and to improve the image clarity. 1H NMR spectra were recorded at
400 MHz in DMSO-d6 and CDCl3 as internal standard with TMS, and
the solid-state 13C NMR spectra were recorded at 5 kHz. The Pd
content of the sample was determined by inductively coupled
plasma atomic emission spectroscopy (ICP-AES) analysis with an
ICPX series II, Thermoscien-tific. Rheology test was also performed
to investigate the sol–gel transition of the gelation process by a
dynamicstress-controlledUS302Rheometer (AntonPaar) equipped
with a parallel plate (diameter = 25 mm, gap = 0.5 mm) in oscillato-
ry mode. After the samples were loaded, the edge of the plate was
overlaid with a layer of low viscosity silicone oil to minimize the
evaporation of solvent. The storage moduli G0 and loss moduli G00
werethenmeasured asa functionoftimeata frequencyof1 Hzanda
strainof1%, and thesamples wereincubatedat37 8C for 30 minwith
60 data points were collected over the entire process.
2.2. Synthesis of SG-1
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PBTZ (13.9 mg, 0.065 mmol) and Pd(OAc)2 (14.7 mg, 0.065 mmol)
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were dissolved in DMF (100
mL) in a Scintillation vial by heating
up to 100 8C. Then the two solutions were mixed together quickly.
The brown opaque gel could be achieved within about 30 s when
the mixture was cooled down to the room temperature. The
sample was simply confirmed by the ‘‘stable to inversion of a test
tube’’ method.
2.3. Catalytic test
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In a typical run of catalytic activity test of SG-1, nitrobenzene
(0.5 mmol) and catalysts were added to ethanol (2 mL) under the
atmosphere of H2 (1 atm), then the solution was stirred at 25 8C for
1 h. SG-1 (4 mg), Pd/C (10%, 10 mg), Pd(OAc)2 or PBTZ was used as
catalysts, respectively. After the reaction was completed (moni-
tored by TLC), the mixture was centrifuged at 5000 rpm and the
solid was washed with dichloromethane (3 ꢂ 5 mL). Then the
product was collected and the conversions were determined by 1H
NMR spectroscopy. For comparatively and recycle test, the catalyst
was isolated by centrifugation after the same cycle and washed
with dichloromethane (3 ꢂ 5 mL). Then the isolated catalyst was
used for the next cycle reaction with further treatment and the
process was repeated for 3 times. For the Suzuki-Miyaura coupling
reactions, SG-1, 1,3,5-tribromobenzene or 2-bromopyridine
(1.0 mmol), phenylboronic acid (1.5 mmol), potassium carbonate
(276 mg, 2.0 mmol), and SG-1 (5 mg, 1.2 mol%) were added to a
pressure flask with dry toluene (4 mL). Then the reaction mixture
was stirred at 120 8C at ambient atmosphere. After 2 h, the mixture
was centrifuged, and the solid was washed with dichloromethane
(3 ꢂ 5 mL). The combined organic phase was washed with water
(3 ꢂ 15 mL) to remove potassium carbonate. The organic phase
was then evaporated under reduce pressure to leave the crude
products which were further purified by column chromatography
over silica gel to obtain the desired products.
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2.1. Synthesis of 5,5-(1,4-phenylene)bis(1H-tetrazole) (PBTZ)
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A mixture of terephthalonitrile (10 mmol), sodium azide
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(60 mmol) and triethylamine hydrochloride (60 mmol) in dry
toluene (70 mL) was heated under reflux for three days. The
reaction mixture was mixed with NaOH solution (1 mol/L,
100 mL), stirred for 30 min and filtered, then the aqueous part
was acidified (pH 1) with concentrated HCl and filtered to give the
crude powder of 1. The crude powder was suspended in NaOH
solution (50 mL, 1 mol/L), stirred at ambient temperature for
3. Results and discussion
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The ligand 5,50-(1,4-phenylene)bis(1H-tetrazole) (PBTZ) was
prepared according to literature method [33]. SG-1 was formed
immediately after mixing PBTZ and Pd(OAc)2 in DMF at 100 8C. As
shown in Fig. 1, inversion tests were used to confirm formation of
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Fig. 1. Picture of SG-1 and a schematic representation of the DMF-mediated possible coordination mode between the PBTZ and the Pd2+. Color code: C(gray), N(blue), O(red),
H(white) of the ligand PBTZ, Pd(yellow), and the OAcꢁ1 anion was omitted for clarity (drawn in ChemBio3D Ultra 12.0 with MM2 stimulation with the lowest energy).
Please cite this article in press as: W. Zhang, Z.-G. Xie, Fabrication of palladium nanoparticles as effective catalysts by using
supramolecular gels, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.09.009

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W. Zhang, Z.-G. Xie / Chinese Chemical Letters xxx (2015) xxx–xxx
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Table 1
Hydrogenation of nitrobenzene.a
Entry
Catalyst/dosage (mg)
Conversion (%)b
1
2
3
4
5
6
7
SG-1,
4
10
4
98
>99
98
98
98
0
Pd/C(10% Pd)
SG-1, reuse 1,
SG-1, reuse 2
SG-1, reuse 3
Pd(OAc)2
4
4
2
PBTZ
4
0
Fig. 2. HR-TEM (scale bar: 10 nm) (a) and SEM (scale bar: 40
mm) (b) of SG-1. The
a
inside in (b) shows the corresponding EDS mapping images of the selected region of
Reaction condition: nitrobenzene (0.5 mmol) and 1 mol% of catalyst, ethanol
the SG-1.
(2 mL), 1 atm of H2, 25 8C, 1 h.
b
Conversions were determined by 1H NMR spectroscopy.
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SG-1 with color of brown. The gel formation was ascribed to the
cooperative hydrogen bonding interaction between tetrazoles and
Pd. In order to obtain an aerogel of the SG, the wet gel was
immersed sequentially in methanol and water to exchange the
DMF, and then was freeze-dried. The lyophilized powder of SG-1
was insoluble in all of organic solvents tested and water, even in
concentrated hydrochloric acid. SG-1 was characterized by
transmission and scanning electron microscopy (TEM, HR-TEM
and SEM), infrared spectroscopy (IR), thermogravimetric analysis
(TGA), solid-state 13C NMR spectrum, (ICP-AES) analysis and
rheological measurements.
The TEM images of SG-1 (Fig. S2 in Supporting information)
indicated the SG-1 constituted with nanoparticles with average
diameter of several nanometers. Small Pd nanoparticles with low
dispersity could be seen in whole horizon. In addition, HR-TEM was
used to see the details of Pd nanoparticles as shown in Fig. 2a.
Uniform spherical Pd particles can be seen obviously with average
diameter of 9 nm. SEM shows rough surface of SG-1 (Fig. S3 in
Supporting information), and the EDS mapping gives clear
homogeneous distribution of Pd element in SG-1 (Fig. 2b). The Pd
content in SG-1 determined by ICP-AES is 26 wt%, which is similar to
the calculated value of 24 wt%. The IR spectra of SG-1 exhibited
obvious different patterns with PBTZ, indicating the interaction
between PBTZ and Pd as shown in Fig. 3a. Solid-state 13C cross
polarization/magic angel spinning nuclear magnetic resonance (CP/
MAS/NMR) spectroscopy was used to confirm the composition of
SGs. As shown in Fig. 3b, two characteristic signals for benzene and
tetrazole carbon in SG-1 appeared at 128 and 163 ppm, respectively.
TGA analysis showed that SG-1 were stable up to 250 8C at the
heating rate of 10 8C/min in N2 atmosphere, as shown in Fig. 4a.
The mechanical intensity of the gel is an important parameter
for their potential application. Rheological measurements are used
to study the behavior of the gels when they are exposed to 178
mechanical stress. The ‘‘storage’’ modulus G0 represents the ability 179
of the deformed materials to ‘‘snap back’’ to its original geometry, 180
while the ‘‘loss’’ modulus G00 means the tendency of a materials to 181
flow under stress [34]. The storage modulus G0 and loss modulus G00 182
was measured as a function of time. As shown in Fig. 4b, the G0 183
values exceeded the G00 value by about 1 order of magnitude for SG- 184
1, which was indicative of an elastic gel rather than viscous 185
material [35–37]. It is worthy to mention that G0 for SG-1 is more 186
than 10,000 Pa, which is seldom seen for supramolecular gels 187
[38,39]. This high mechanical intensity of SG-1 benefits to the 188
potential application in cell adhesion, but the biocompatibility 189
needs to be investigated in details.
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As revealed by HR-TEM, the ultrafine Pd nanoparticles were 191
obtained after freeze-drying the wet gel of SG-1. The catalytic 192
performances of SG-1 were investigated by hydrogenation of 193
nitrobenzene at room temperature under 1 atm of H2 with 194
0.1 mol% of catalysts. The conversion of the nitrobenzene was 195
determined by integrating the signals of the reactant (d 8.19) and 196
the product ( 7.12) in the crude reaction mixtures (Fig. S4 in 197
d
supporting information). As shown in Table 1, SG-1 afforded 198
almost complete conversion of nitrobenzene to aniline within 1 h, 199
which is comparable to the commercial Pd/C catalysts with same 200
content of Pd. We studied the recyclability and reusability of the 201
SG-1 catalyst. SG-1 could be readily recovered from the reaction via 202
filtration or centrifugation. The same amount of recovered SG-1 203
showed no decrease of conversion for hydrogenation of nitroben- 204
zene after recycling three times. Morphologies of SG-1 revealed by 205
TEM do not show obvious changes in Fig. S5 in supporting 206
information. The control experiments using only Pd(OAc)2 or PBTZ 207
as catalyst did not form any product. These results revealed the SG- 208
1 possessed high catalytic activity and could be reused easily. The 209
Fig. 3. FT-IR (a) and solid-state (b) 13C NMR spectra of SG-1.
Please cite this article in press as: W. Zhang, Z.-G. Xie, Fabrication of palladium nanoparticles as effective catalysts by using
supramolecular gels, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.09.009

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W. Zhang, Z.-G. Xie / Chinese Chemical Letters xxx (2015) xxx–xxx
Fig. 4. TGA (a) and storage modulus and loss modulus as a function of time (b) of SG-1.
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applicability of the SG-1 catalysts in Suzuki coupling reaction was
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4. Conclusion
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In summary, two-component supramolecular gels were pre-
pared by using tetrazolyl derivatives and Pd(OAc)2 by the metal
coordination and hydrogen bonding interactions. The Pd nano-
particles obtained during the formation of SG-1 were shown to be
highly active and recyclable heterogeneous catalysts toward
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Acknowledgment
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This work was supported by the National Natural Science
Foundation of China (No. 91227118).
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Appendix A. Supplementary data
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230
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2015.09.009.
231
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Please cite this article in press as: W. Zhang, Z.-G. Xie, Fabrication of palladium nanoparticles as effective catalysts by using
supramolecular gels, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.09.009