One-step synthesis of three-dimensional graphene/multiwalled carbon nanotubes/Pd composite hydrogels: An efficient recyclable catalyst for Suzuki coupling reactions

Article abstract of DOI:10.1039/c5ta00280j

In this paper, a simple one-pot hydrothermal procedure to create three-dimensional (3D) graphene/multiwalled carbon nanotubes/Pd (G/MWCNTs/Pd) composite hydrogels with a unique porous nanostructure was reported. During the formation of the G/MWCNTs/Pd composites, 2D graphene sheets and 1D MWCNTs were self-assembled to form interconnected porous microstructures and ultrafine Pd NPs were in situ grown on the 3D carbon-based skeleton simultaneously. In the as-obtained nanocomposites, MWCNTs not only prevented the close restacking of graphene sheets to increase the specific surface area but also provided an additional transport path to the catalyst surfaces which facilitated reactant transport. The G/MWCNTs/Pd composites proved to be an efficient, recyclable, and robust catalyst for the Suzuki cross-coupling reactions under mild aerobic conditions, which was attributed to the 3D macroporous framework with high specific surface area, numerous activation sites, and efficient transport pathways for improving the catalytic performance. Moreover, catalyst separation could be easily achieved by simple filtration, and the catalyst could be reused for at least six runs without significant loss in catalytic activity. Additionally, nearly no Pd species was released from the G/MWCNTs/Pd composites during the catalytic reactions, showing the heterogeneity in the present catalysis system.

Full text of DOI:10.1039/c5ta00280j

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One-step synthesis of three-dimensional graphene/multiwalled carbon
nanotubes/Pd composite hydrogel: an efficient recyclable catalyst for
Suzuki coupling reactions
Hui-qing Song a, Qian Zhu a, Xue-jing Zheng a, Xing-guo Chen a, b, *
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
In this paper,
a simple oneꢀpot hydrothermal procedure to create threeꢀdimensional (3D)
graphene/multiwalled carbon nanotubes/Pd (G/MWCNTs/Pd) composite hydrogel with unique porous
nanostructure was reported. During the formation of the G/MWCNTs/Pd composites, 2D graphene sheets
10 and 1D MWCNTs were selfꢀassembled to form interconnected porous microstructures and ultrafine Pd
NPs were in situ grown on the 3D carbonꢀbased skeleton simultaneously. In the asꢀobtained
nanocomposites, MWCNTs not only prevented the close restacking of graphene sheets to increase the
specific surface area but also provided an additional transport path to the catalyst surfaces which
facilitated reactant transport. The G/MWCNTs/Pd composites proved to be an efficient, recyclable, and
15 robust catalyst for the Suzuki crossꢀcoupling reactions under mild aerobic conditions, which was
attributed to the 3D macroporous framework with high specific surface area, numerous activation sites,
and efficient transport pathways for improving the catalytic performance. Moreover, catalyst separation
could be easily achieved by simple filtration, and the catalyst could be reused at least six runs without
significant loss in catalytic activity. Additionally, nearly no Pd species was released from the
20 G/MWCNTs/Pd composites during the catalytic reactions, showing the heterogeneity in the present
catalysis system.
45 the catalytic cycles. Among the various supports studied, the
flexible graphene that provides much larger surface area for the
dispersion of Pd NPs makes it quite promising for catalysis.
Graphene, a twoꢀdimensional (2D) monolayer of sp2ꢀ
hybridized carbon atoms patterned in a hexagonal lattice, with the
50 advantages of large specific surface area, outstanding electrical
conductivity, superior mechanical flexibility, and easy
1. Introduction
It is well known that palladium (Pd)ꢀcatalyzed Suzuki crossꢀ
25 coupling transformations constitute a widely used method for the
C–C bond formation in organic synthesis and industrial
processes.1, 2 Currently the Suzuki crossꢀcoupling reactions are
performed mostly in homogeneous Pd catalytic systems in spite
of the fact that it requires toxic and expensive phosphine ligands,
30 which brings about unnecessary waste and hazard to the
environment. Furthermore, further removal of Pd from the
reaction systems is highly desirable due to the restriction of the
catalysts in the industrial reuse. Therefore, from the viewpoint of
green chemistry, heterogeneous catalysts centered around the use
35 of Pd NPs are wellꢀrecognized because they have more
advantages, such as the ease of workꢀup, the easy separation of
products, and the recovery and regeneration of catalysts.1ꢀ4
Unfortunately, Pd NPs without stabilization are prone to
agglomerate due to their high surface areaꢀtoꢀvolume ratio, which
40 may lead to deactivation of catalysts.5, 6 A promising strategy to
solve the issue could be the development of heterogeneous
systems where active Pd species are immobilized on solid
supports, which could maximize the active surface area of
catalysts and improve their catalytic activity and durability during
8
functionalization, have attracted widespread attention.7, It is
generally believed that the grapheneꢀbased catalysts not only
raise the nanosized catalyst surface area for electron transport but
55 also provide better mass transport of reactants to catalysts.9
However, in grapheneꢀbased composites, the graphene sheets
easily form irreversible agglomerates driven by the strong Van
der Waals interaction, which greatly compromises the superiority
11
of the intrinsic high specific surface area of graphene.10,
60 Recently, threeꢀdimensional (3D) carbon nanostructures made by
connecting 2D planar graphene and 1D carbon nanotubes (CNTs)
opened a new avenue for enhancing the surface area of
materials.12ꢀ15 In the resulting 3D porous carbon nanostructures,
the MWCNTs can efficiently inhibit the aggregations of the
65 graphene sheets and maximize the surface area of 3D
nanostructures for increasing the accessibility of the reactants to
16, 17
the catalyst surfaces.10,
Besides, the 3D graphene/CNT
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hybrids consist of abundance oxygenꢀcontaining functional
groups, which can grip nanoparticles tight on the carbon
framework and form a robust nanoarchitecture. Considerable
yellow. The asꢀobtained suspension was washed with 5% HCl
and then deionized water by repeated centrifugation. Then the
product was dialyzed through a dialysis membrane (retained
effort has been devoted to fabricating the 3D graphene/CNT 60 molecular weight: 8ꢀ14 kDa) for a week to completely remove
5
hybrids via various methods, including chemical vapor
deposition,18, 19 microwave method,13, 14 atomic layer deposition,9
etc. However, some methods suffer from multistage synthesis
processes, processing difficulties or expensive apparatus, etc.
Recently, Wang et al.20 reported the 3D PdꢀCNTꢀGH composites
the residual salts and acids, and the suspensions were vacuumꢀ
dried at 65 °C to get GO sheets.
2.3 Preparation of 3D G/MWCNTs/Pd composite hydrogel
Before use, the raw MWCNTs (0.5 g) were chemically
65 shortened and carboxylated by treat with concentrated HNO3 (50
mL) at 60 °C under vigorous stirring for 12 h, producing
carboxylic acid groups at the defect sites.22 Then the mixture was
filtered and washed with deionized water, the obtained solid was
employed as purified MWCNTs in the next steps.
The selfꢀassembled 3D G/MWCNTs/Pd composites were
synthesized via a facile oneꢀstep hydrothermal process. In a
typical experiment, a 30 mL GO (1.5 mg mLꢀ1) aqueous
dispersion was mixed with 15 mg MWCNTs (the mass ratio of
GO to MWCNTs was 3:1) with the aid of ultrasonication for 2 h
75 to form a homogeneous GO/MWCNTs suspension. Subsequently,
7.0 mg PdCl2 was added to the above dispersion and sonicated for
0.5 h to achieve the electrostatic adsorption of Pd2+ ions on
GO/MWCNTs, and the pH of the mixture was adjusted to about
9.0 with 1 mol/L NaOH aqueous solution. After that, 1.8 g
10 via a selfꢀassembled process, while the Pd NPs were about 30 nm
in size. Generally, the intrinsic catalytic properties of metal NPs
closely depend on their dimension, size and morphology.21
Hence, it is worthwhile to exploit a simple, lowꢀcost, green, and
scalable selfꢀassembly method to form 3D carbon nanostructures
15 with controlled NPs in size.
70
In this paper, we reported a facile oneꢀstep hydrothermal
approach to prepare macroscopic graphene/multiwalled carbon
nanotubes/Pd (G/MWCNTs/Pd) composite hydrogel with 3D
interconnected networks under the selfꢀassembly of GO sheets
20 and MWCNTs, in which Pd NPs were in situ simultaneously
grown on the 3D carbon framework. In this architecture,
MWCNTs could bridge adjacent graphene sheets and restrain the
aggregation of graphene sheets, resulting in a high contact area
between the reactants and catalysts which could greatly improve
25 the catalytic activity. The resulting 3D G/MWCNTs/Pd
composites, when served as the catalyst for the Suzuki crossꢀ
coupling reactions, exhibited remarkable catalytic performances
due to high porosity, high surface area, and robust chemical and
mechanical stability. Additionally, the 3D G/MWCNTs/Pd
30 composites showed wonderful cycling stability.
80 glucose as reducing agent was added to form
a stable
homogeneous solution by sonication for 30 min, then the
resulting mixture was sealed tightly in a Teflonꢀlined stainless
steel autoclave and subjected to an elevated temperature of 120
°C in air oven for 24 h. After cooling to room temperature, the asꢀ
85 obtained black gelꢀlike 3D cylinder was immersed in deionized
water overnight to remove the residual ion and then freezeꢀdried
into an aerogel for further use. For comparison, the MWCNTs/Pd
without any additional GO and the 3D rGO/Pd hydrogel without
any additional MWCNTs were fabricated using the similar
90 procedures to that of G/MWCNTs/Pd. The Pd content was found
to be 3.55 wt% for G/MWCNTs/Pd, 3.76 wt% for MWCNTs/Pd
and 4.42 wt% for rGO/Pd by inductively coupled plasma atomic
emission spectrometry (ICPꢀAES).
2. Experimental section
2.1. Materials
Graphite powder was purchased from SigmaꢀAldrich. Raw
multiwalled carbon nanotubes (MWCNTs) were supplied from
35 Chengdu Organic Chemicals Co., Ltd. (Chengdu, China).
Palladium chloride (PdCl2, >99%) was obtained from Sun
Chemical Technology Co., Ltd. (Shanghai, China). Glucose was
commercially available from Sinopharm Chemical Reagent Co.,
Ltd. (Shanghai, China). All the organic substrates were acquired
40 from J&K Scientific Ltd. and Aladdin Reagent Co., Ltd. All other
chemical reagents were used as received without further
purification. Deionized water was used throughout the
experiments.
2.4. Characterization
95
Powder Xꢀray diffraction (XRD) patterns were performed on
a D/MAXꢀ2000 diffractometer (Rigaku, Japan) with Cu Kα
radiation (λ= 1.514178 Å). Raman spectra were obtained using a
Dilor LABRAMꢀ1B multichannel confocal microspectrometer
with a 532 nm laser excitation. FTꢀIR spectra were recorded on a
100 Nicolet Nexus 670 fourier transform infrared spectrometer (FTꢀ
IR, America) using KBr pellets. The microstructures of the
samples were characterized by the fieldꢀemission scanning
electron microscopy (FESEM, Sꢀ4800, Japan) and the
transmission electron microscopy (TEM, Hitachiꢀ600, Japan)
105 with an energy dispersive Xꢀray analyzer (EDX). N2 adsorption–
desorption isotherms were acquired on a TriStar II 3020 analyzer
(Micromeritics, USA). The specific surface areas (SBET) were
calculated using the Brunauer–Emmett–Teller (BET) model and
the poreꢀsizeꢀdistribution curves were obtained from the
110 adsorption branches using the nonꢀlocal density functional theory
(NLDFT) method. Xꢀray photoelectron spectra (XPS)
measurements were carried out on a PHIꢀ5702 multifunctional
spectrometer equipped with an Al Kα exciting source. The Pd
2.2. Preparation of graphene oxide (GO)
45
GO was prepared by oxidation of natural graphite powder
according to a modified Hummers method.7 Briefly, graphite
powder (1 g) and NaNO3 (0.5 g) were added to concentrated
sulfuric acid (23 mL) under stirring in an ice bath. Under
vigorous agitation, KMnO4 (3.0 g) was slowly added into the
50 cooled acid solution mixture maintaining temperature below 20
°C. Then, the ice bath was removed and the reaction system was
stirred at 35 °C for about 30 min. Next, 46 mL of deionized water
was added slowly, and the mixture was further stirred for 1 h with
increasing the temperature to 98 °C. Finally, 140 mL of deionized
55 water was added followed by a slow addition of 10 mL of H2O2
(30%), turning the color of the solution from darkꢀbrown to
2
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content of the catalysts was measured by inductively coupled 55 mechanical strength of the selfꢀassembled structure under the
plasma atomic emission spectrometry (ICPꢀAES, Thermo Jarrel
Ash, Franklin, MA, USA).
hydrothermal condition.25 Meanwhile, an efficient selfꢀassembly
driven by the hydrophobic, electrostatic repulsion and the πꢀπ
interactions may occur and generated a 3D structure of composite
hydrogel.
2.5. General procedure for the Suzuki coupling reactions
5
The Suzuki coupling reactions were carried out in a 25 mL
threeꢀneck flask under air atmosphere. In a typical experiment,
aryl halide (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2
mmol), catalyst (0.5 mol% of Pd) and EtOH/H2O (v/v=1:1, 8 mL)
were mixed and stirred at 60 °C for a specified time. The reaction
60
10 progress was monitored by thin layer chromatography (TLC) at
regular intervals. After the completion of the reaction, the catalyst
was separated out by simple filtration and the filtrate was
extracted with ethyl acetate, followed by analyzed by gas
chromatography (GC, Varian 450) equipped with a capillary
15 column (30 m, 0.25 mm i.d., CPꢀ5) and an FID detector. N2 was
used as the carrier gas and the column temperature was
maintained at 100 °C. The reaction yield was calculated through
using nꢀdodecane as internal standard, which was reported as
average value from at least three independent experiments.
20
For recycling, the catalyst was easily recovered by filtration
and then washed thoroughly with diethylether, followed by
deionized water and finally by dichloromethane, and then dried
under vacuum at 40 °C for the next run.
2.6. Hot filtration test
25
The separated hot filtration test was conducted by carrying
out the reaction of bromobenzene with phenylboronic acid under
the optimized conditions. After 30 min, the reaction mixture was
filtered at the reaction temperature in order to avoid possible
precipitation of leached Pd species on the catalyst surface upon
Scheme 1 Schematic illustration of the fabrication process of the 3D
G/MWCNTs/Pd composite hydrogel.
30 cooling. Then the reaction solution was further stirred for another
2 h at 60 °C, and the reaction progress was examined by GC
analysis.
In order to further investigate the crystal phase and structure
65 information of the materials, the XRD patterns of the asꢀobtained
GO, MWCNTs and the G/MWCNTs/Pd composites were shown
in Fig. 1. The XRD pattern of GO showed a typical 2θ diffraction
peak at 9.9° indexed to the (002) crystal face with the dꢀspacing
of 0.90 nm, which is much larger than the dꢀspacing (0.34 nm) of
70 the pristine graphite (002) reflection. The expansion of the
interlayer distance is attributed to the oxidizationꢀinduced
expansion, due to the presence of oxygenꢀcontaining functional
groups on the graphene sheets and some atomic scale structural
defects.26 And, the characteristic (002) diffraction peak of the
75 MWCNTs was observed at 25.9°, which corresponds to an
interlayer distance of 0.35 nm. The pattern of the G/MWCNTs/Pd
2.7. Solid-phase poisoning test
The solidꢀphase poisoning test was performed using
35 iodobenzene and phenylboronic acid as coupling partners and
adequate amount of poly(4ꢀvinylpyridine) as a poisoning agent
was introduced to the reaction under the optimized Suzuki
conditions.
3. Results and discussion
40
A schematic illustration of the preparation process of the
selfꢀassembled 3D G/MWCNTs/Pd composite hydrogel was
shown in Scheme 1, and the detailed experimental steps for the
resulting composites were listed in the Experimental Section.
Because of the abundant oxygenꢀcontaining functional groups,
composites exhibited
a broad reflection at about 25.8°
corresponding to the graphiteꢀlike structure (002) with the dꢀ
spacing (0.35 nm), which indicated that GO can be reduced
80 during the hydrothermal conditions and was also a sign of a more
disordered stacking and uniform dispersion of the graphene
sheets in the asꢀprepared 3D framework.27, 28 Furthermore, the
characteristic (002) peaks from MWCNTs and graphene sheets
were overlapped, also suggesting the good graphitization of GO
85 through the hydrothermal treatment. Besides this, the wellꢀ
resolved peaks at 40.1°, 46.6°, 68.1° for the G/MWCNTs/Pd
composites could be assigned to the (111), (200) and (220) lattice
planes of a faceꢀcentered cubic (fcc) Pd crystalline structure
(JCPDS no. 05ꢀ0681), and no new phase was detected in the
45 GO Fig. S1a) and MWCNTs (Fig. S1b) could disperse well in
(
water by sonication to produce a homogeneous suspension and
MWCNTs are adsorbed onto the GO surface or inserted into the
interspacing of GO sheets, which is important for forming the
uniform 3D structure.17 During the formation of composite
50 hydrogels, several reactions could happen: firstly, the surface
oxygenꢀcontaining functional groups and defects on GO and
MWCNTs can act as nucleation sites and bind with Pd2+ ions by
24
electrostatic force.23,
Then, the Pd2+ ions could be in situ
reduced to Pd(0) NPs by glucose, which also increased the
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G/MWCNTs/Pd composites. And the average crystallite size of
process. The result was in accordance with that of Raman
Pd calculated using the DebyeꢀScherrer equation is about 7.9 nm, 45 spectroscopy analysis. In addition, it is also found that the bands
slightly larger than the TEMꢀdetermined size. In addition, the
XRD patterns of rGO/Pd and MWCNTs/Pd (Fig. S2) were
investigated and they revealed the indicative diffraction peaks of
Pd crystals, indicating the Pd NPs were well anchored on the rGO
and MWCNTs, respectively.
of GO at 1621 cmꢀ1 ascribed to the skeletal vibrations of
unoxidized graphitic domains redꢀshifted to 1574 cmꢀ1 after the
hydrothermal reduction, which may be due to the addition of
multiwalled carbon nanotubes and improvement of the
50 conjugation degree of graphene and MWCNTs.16
5
Then the elemental compositions and the chemical state of
Pd particles in the G/MWCNTs/Pd composites were investigated
by Xꢀray photoelectron spectroscopy (XPS). From the survey
scan spectra in Fig. 3a, it can be seen that the G/MWCNTs/Pd
55 composites displayed a distinct Pd signal compared with the XPS
spectrum of GO/MWCNTs, as expected. In the highꢀresolution
Pd 3d XPS spectrum (see Fig. 3b), the binding energy of the
doublet peak at 335.7 eV (assigned to Pd 3d5/2) and 341.0 eV
(assigned to Pd 3d3/2) can be attributed to the Pd (0) state.32
60 Furthermore, no peaks corresponding to Pd2+ in the XPS
spectrum were observed, which indicated that Pd2+ irons have
been reduced to Pd NPs by glucose during the hydrothermal
process.25 In the C1s spectrum of GO/MWCNTs (Fig. 3c), four
pronounced deconvoluted binding energy peaks were identified
65 as sp2 carbon at 284.6 eV, C−O at 286.6 eV, C=O at 288.2 eV and
O−C=O at 289.7 eV.9, Compared with GO/MWCNTs, most
33
Fig.
1 XRD patterns of asꢀprepared GO, MWCNTs and the
oxygenꢀcontaining functional groups were reduced by glucose in
the formation process of the G/MWCNTs/Pd composite hydrogel,
which was verified by the increasing intensity of sp2 CꢀC bonds
70 at 284.6 eV and the decreasing intensity of the oxygenꢀcontaining
carbon (C−O at 286.6 eV, carbonyl C=O at 288.2 eV and
carboxyl O−C=O at 289.7 eV),34 as shown from the deconvoluted
C1s spectra of the asꢀprepared G/MWCNTs/Pd composites in
Fig. 3d. In agree with XRD and Raman spectra measurements,
75 they all indicated that the G/MWCNTs/Pd composites presented
effective reducing not only for oxygenꢀcontaining functional
groups but also for the decorated Pd NPs.
10 G/MWCNTs/Pd composites.
Raman spectroscopy, a powerful tool in studying structural
changes of carbonaceous materials, was employed to gain
information about the vibrational properties of GO, MWCNTs
and the G/MWCNTs/Pd composites. As shown in Fig. 2a, two
15 prominent characteristic peaks at around 1355 and 1590 cmꢀ1
were presented in the Raman spectra, which can be ascribed to
the D and G peaks of the carbon materials, respectively. The G
band is assigned to the E2g vibrational mode of sp2 carbon atoms
patterned in hexagonal lattice corresponding to ordered sp2
20 bonded carbon, whereas the D band is attributed to structural
defects in the crooked graphene sheet and the disorder carbon.29,
30
As known, the intensity ratio of the D band to the G band
(ID/IG) in the Raman spectrum is used as an indicator of the
disorder degree and the average size of the sp2 domains in
25 graphitic materials.27 It is found that the value of ID/IG of in the
3D G/MWCNTs/Pd composites was 1.24, which increased
markedly when compared with those of GO (0.92) and MWCNTs
(0.99), indicating the improvement of the disordered levels. The
more disordered carbon structure in the G/MWCNTs/Pd
30 composites could be explained that the removal of oxygenꢀ
containing functional groups led to a decrease in the average size
of the inꢀplane sp2 domains after the hydrothermal treatment.28
The FTꢀIR spectra of GO and the prepared G/MWCNTs/Pd
composites in the region of 4000–500 cmꢀ1 were shown in Fig.
35 2b. In the IR spectrum of GO, the absorption peaks at 3430, 1728,
1621, 1384, 1224 and 1067 cmꢀ1 were observed, which can be
indexed to the stretching vibrations of O−H, C=O in carboxyl
group, C=C in aromatic ring, C−OH, carboxy C−O and epoxy
30, 31
C−O stretching vibrations, respectively.12,
However, the
40 oxygen functional group peaks at 1728, 1384 and 1067 cmꢀ1 were
almost disappeared in the spectrum of the G/MWCNTs/Pd
composites, indicating the effective reduction of GO to the
reduced graphene oxide by glucose during the hydrothermal
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macropores ranging from submicrometer to several micrometers.
From Fig. 4a and b, it also showed that MWCNTs were well
distributed on the surface of graphene sheets as well as inserted
20 into the graphene sheets, which acted as bridges to effectively
prevent graphene sheets from restacking and enlarged the spaces
among sheets, leading to a loose architecture.35 Compared with
G/MWCNTs/Pd, rGO/Pd (Fig. S3a) diaplayed a similar porous
structure, whereas MWCNTs/Pd (Fig. S3b) consisted of
25 randomly orientated and entangled MWCNTs with Pd NPs
anchored on them. The detailed structure of the G/MWCNTs/Pd
composites was further examined by TEM measurements. As
presented in Fig. 4c, Pd NPs were uniformly attached on the
graphene surface and multiwalled carbon nanotubes, and the
30 average diameter was about 4.5ꢀ6.0 nm (Fig. S4a). The image
shown in Fig. 4c also presented that the MWCNTs were
dispersed on the graphene surface and/or inserted into the
interspacing the graphene sheets to form a 3D structure
framework, validating the introduction of MWCNTs onto the
35 graphene sheets.9, 35 According to the previous reports,10, 36, 37 the
interaction between metallic NPs with graphene layers can
enhance their interface contact and suppress the dissolution and
agglomeration of NPs, thereby promoting the chemical activity
and stability of the composites. The lattice spacing of the Pd NPs
40 detected in the highꢀresolution TEM (HRTEM) image (Fig. 4d)
was calculated to be 0.22 and 0.19 nm, which is in accordance
with the (111) and (200) planes of Pd crystals, respectively.38 The
EDX analysis in Fig. S5 showed that the G/MWCNTs/Pd
composites consisted of C, O, and Pd elements, indicating the
45 successful fabrication Pd NPs on the carbon skeleton. For
comparison, we also presented the TEM images of the rGO/Pd
and MWCNTs/Pd composites (Fig. S3cꢀf). TEM and HRTEM
images of rGO/Pd and MWCNTs/Pd revealed the crystalline Pd
NPs with the size of 4.5ꢀ6.5 nm (Fig. S4b and c) were well
50 anchored on the graphene layers and MWCNTs, respectively.
Fig. 2 (a) Raman spectra of GO, MWCNTs and the G/MWCNTs/Pd
composites and (b) FTꢀIR spectra of GO and the G/MWCNTs/Pd
composites.
5
Fig. 3 XPS spectra of GO/MWCNTs and the G/MWCNTs/Pd composites:
(a) survey scan, (b) Pd 3d spectrum of the G/MWCNTs/Pd composites,
(c) C 1s spectrum of GO/MWCNTs and (d) C 1s spectrum of the
G/MWCNTs/Pd composites.
Fig. 4 SEM images of the G/MWCNTs/Pd composites (a and b).
Representative TEM (c) and HRTEM (d) images of the G/MWCNTs/Pd
composites. Inset shows the corresponding SAED pattern.
10
The morphology and structure of the selfꢀassembled 3D
G/MWCNTs/Pd nanostructures were characterized by means of
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). Fig. 4a and b showed the corresponding
SEM images of the asꢀprepared G/MWCNTs/Pd composite
55
The Brunauer–Emmmett–Teller (BET) specific surface area
and porous structure of the G/MWCNTs/Pd composites were
further investigated by nitrogen adsorptionꢀdesorption analysis,
with typical isotherms shown in Fig. 5. It can be seen that the
G/MWCNTs/Pd composites featured the type IV isotherms with
15 hydrogel, revealing
a wellꢀdefined and interconnected 3D
60 an H3ꢀtype hysteresis loop (P/P0 =0.45−1.0), confirming the
graphemeꢀbased network microstructure with continuous
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presence of mesoporous and macroporous structures in the
adopted as the solvent, the reaction was sluggish (Table 1, entries
composites.19 Based on the above results, we think that the 45 4); however, the addition of H2O to DMF or EtOH led to a
G/MWCNTs/Pd composites is porous, which is consistent with
the microstructure observed by the SEM images. As
profound increase in activity. Notably high activity of the catalyst
G/MWCNTs/Pd can be achieved by using the mixed solvent
DMF/H2O (v/v=1:1) and EtOH/H2O (v/v=1:1) (Table 1, entries 8
and 9), due to the good solubility of organic substrates and
a
5
comparison, those of the rGO/Pd and MWCNTs/Pd composites
have also been provided, and they exhibited the typical type IV
isotherms, indicating the porosity of the samples.31 The BET 50 inorganic base in coꢀsolvents. The mixed solvent endowed the
surface areas and pore volume of the G/MWCNTs/Pd composites
were measured to be 142.3 m2 gꢀ1 and 0.52 cm3 gꢀ1, which are
base with good solubility in water for activating arylboronic acid,
resulting in a higher reactivity in aqueous medium. And the result
was consistent with previous work on solvent effects in the
Suzuki reaction.39 Among the solvents screened, EtOH/H2O
10 evidently superior to that of rGO/Pd (87.8 m2 gꢀ1and 0.35 cm3 gꢀ1)
and MWCNTs/Pd (77.3 m2 gꢀ1 and 0.31 cm3 gꢀ1), respectively. It
turned out that MWCNTs can effectively prevent the aggregation 55 (v/v=1:1) was the preferred solvent for the crossꢀcoupling
of graphene sheets to increase the specific surface area of the
reaction with the highest yield of >99% (Table 1, entry 9). Next,
various bases, such as Cs2CO3, Na2CO3, KOH, Na3PO4ꢁ12H2O,
NaAc and Et3N, were investigated for the coupling reaction under
the same conditions. And, the results indicated that inorganic
G/MWCNTs/Pd composites after the introduction of
35
15 MWCNTs.17,
The pore size distributions of the composites
obtained on basis of the nonꢀlocal density functional theory
(NLDFT) method also deliver a consistent picture in the pore size 60 bases were more effective than organic bases like NaAc and Et3N
viewpoint. As shown in Fig. 5bꢀd, the G/MWCNTs/Pd and
rGO/Pd composites displayed the pores in the range of 3ꢀ100 nm
20 and most of the pores in MWCNTs/Pd lay in 10ꢀ100 nm range,
which is associated with different porous architectures resulting
(Table 1, entries 14 and 15), probably due to two factors: (1)
partial inhomogenity of organic bases in aqueous phase and (2)
inorganic bases were more effective in activate arylboronic acid
towards the formation of a boronate complex which can facilitate
from the different building blocks.31 Combining the results with 65 the subsequent transmetallation process to enhance the reactive
SEM measurements, the larger specific surface area and the
unique hierarchical porous feature of the G/MWCNTs/Pd
25 composites would provide efficient transport pathways to the
interior cavities, thus leading to better catalytic performance.
activity as compared to organic bases in the reaction system.
Among the tested bases, it seemed that K2CO3 and Cs2CO3 were
preferred for the reaction system (Table 1, entries 9 and 10),
because they accomplished activation of the palladium better than
70 other bases.40 Finally, K2CO3 was chosen as the base for the
coupling reaction because of its economic material benefit. As
demonstrated above, the Suzuki coupling reactions were carried
out at 60 °C in air using K2CO3 as base and EtOH/H2O (v/v=1:1)
as reaction medium.
75 Table 1 Optimization of the reaction conditions for Suzuki crossꢀcoupling
reaction of iodobenzene with phenylboronic acid catalyzed by
G/MWCNTs/Pda
Entry Solventc
Base
CH3CN* (35.94) K2CO3
THF* (7.58) K2CO3
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
20
20
20
20
20
20
20
20
15
15
15
15
25
20
23
44
34
72
86
Acetone* (20.70) K2CO3
H2O☆ (78.36)
K2CO3
DMF* (36.31)
MeOH☆ (32.66)
EtOH☆ (24.55)
DMF/ H2O
EtOH/H2O
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
Na2CO3
KOH
Fig. 5 N2 adsorption/desorption isotherms (a) and pore size distribution of
G/MWCNTs/Pd (b), rGO/Pd (c) and MWCNTs/Pd (d).
8
9
98
30 Catalytic performance
>99
>99
91
83
68
10
11
12
13
14
15
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
To evaluate the catalytic activity of the asꢀprepared 3D
G/MWCNTs/Pd composites, Suzuki coupling reaction one of the
representative Pdꢀcatalyzed reactions was carried out as a model
reaction. It is worth pointing out that base and solvent can
35 remarkably influence the reactivity of the Suzuki coupling
reaction. Here, we chose iodobenzene and phenylboronic acid as
the model substrates to optimize the reaction conditions under
aerobic condition. As showed in Table 1, poor yields were
obtained when the model reaction was performed in polar aprotic
40 solvents, such as CH3CN, THF, acetone and DMF (Table 1,
entries 1ꢀ3 and 5). In contrast, the relatively high yields were
achieved with the use of polar protic solvents, such as MeOH and
EtOH (Table 1, entries 6 and 7). Interestingly, when H2O was
Na3PO4ꢁ12H2O 15
NaAc
Et3N
15
15
35
56
a Reaction conditions: iodobenzene (1 mmol), phenylboronic acid (1.2
mmol ), base (2 mmol), catalyst (0.5 mol%), solvent (8 mL, for mixed
solvent, the volume ratio is 1:1) at 60 °C under air.
b GC yields. c the dielectric contant under 25 °C.
* represents the polar aprotic solvent
☆ represents the polar protic solvent
80
With the optimal reaction conditions in hand, the scope of
the G/MWCNTs/Pdꢀcatalyzed Suzuki reactions was examined by
6
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DOI: 10.1039/C5TA00280J
Table 2 Scope of the Suzuki crossꢀcoupling reaction catalyzed by
coupling various substituted aryl halides with phenylboronic acid,
and the results were summarized in Table 2. The data presented
in Table 2 showed that aryl iodides and aryl bromides were
converted effectively to the corresponding coupling products with
good yields. Aryl iodides bearing an electronꢀdonating or an
electronꢀwithdrawing group exhibited high reactivity and
afforded satisfactory yields within 15ꢀ30 min (Table 2, entries 1ꢀ
7). Fortunately, the catalyst G/MWCNTs/Pd also exhibited
excellent catalytic activity for aryl bromides with substituents
10 such as −CH3, −NO2, −CN, −OCH3, −OH and −NH2
corresponding to respective biaryl products with high yields
(Table 2, entries 8ꢀ15). Further studies indicated that the less
active chlorobenzene and its derivatives coupled with
phenylboronic acid needed a longer time to obtain a moderate
15 yield (Table 2, entries 16ꢀ18). It is wellꢀknown that substituents
in the aryl halides have an effect on the Suzuki crossꢀcoupling
reactions. In agreement with previous studies,41, 42 aryl halides
with electronꢀwithdrawing substituents gave higher yields than
that of electronꢀdonating ones. However, aryl iodides containing
20 the −COOH group proceeded less efficiently than other electronꢀ
withdrawing substituents, which may be attributed to the decrease
in basicity of the reaction medium.41 It is noteworthy that 2ꢀ
iodophenol with −OH group and 2ꢀbromonitrobenzene with –
NO2 group, which are sterically hindered, offered good yields but
25 not as excellent as theirs paraꢀsubstituted aryl halides (Table 2,
entries 4, 5, 10, 11). The result revealed that the paraꢀsubstituted
aryl halides showed better reactivity than orthoꢀsubstituted aryl
halides. As a contrast, the catalytic performances of an equal
amount of rGO/Pd and MWCNTs/Pd for the Suzuki coupling
30 reaction were also studied (Table 2). From these results, it is
40 different supported Pdꢀbased catalysts under optimized conditionsa
5
Yieldb (%)
Entry
R
X
Time (h)
G/MWCNTs rGO/Pd MWCNTs/Pd
/Pd
1
2
3
4
5
6
7
8
H
I
I
I
I
I
I
I
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
1/4
1/4
1/4
1/4
1/4
1/5
1/2
1
1
1
1.5
1.5
1.5
2
>99
96
91
86
92
97
91
95
96
98
89
90
92
88
94
46
33
62
76
81
68
64
71
80
73
78
83
84
69
71
75
67
73
14
8
64
68
55
46
54
71
60
69
73
76
62
65
65
59
63
9
4ꢀCOCH3
4ꢀNH2
2ꢀOH
4ꢀOH
4ꢀNO2
4ꢀCOOH
H
4ꢀCN
4ꢀNO2
2ꢀ NO2
4ꢀCH3
4ꢀOCH3
4ꢀOH
4ꢀNH2
H
9
10
11
12
13
14
15
16
17
18
2
6
6
6
4ꢀOCH3
4ꢀNO2
6
13
19
a Reaction conditions: iodobenzene (1 mmol), phenylboronic acid (1.2 mmol ),
K2CO3 (2 mmol), catalyst (0.5 mol%), EtOH/H2O (v/v=1:1, 8 mL) at 60 °C
under air.
b GC yields.
Recycling of the catalyst
Recycling is one of the most attractive advantages of the
heterogeneous catalyst for practical applications, here we chose
phenylboronic acid and iodobenzene as model substrates to test
45 the reusability and lifetime of the G/MWCNTs/Pd catalyst under
the optimized conditions mentioned above. As illustrated in Table
3, the catalyst was reused directly up to six consecutive cycles
and remained the excellent catalytic efficiency without loss of
catalytic selectivity and activity. The excellent recyclability of the
50 catalyst could be attributed to the strong interaction between Pd
NPs and oxygenꢀcontaining functional groups arising from
carbonꢀbased material.43 The TEM measurement of the recovered
G/MWCNTs/Pd catalyst (after 6 runs) indicated that Pd NPs were
still well dispersed without obvious aggregation (Fig. S6), which
55 further confirmed the high catalytic activity and good stability of
the catalyst. Another key factor to be investigated is the stability
of the catalyst. To examine of the stability of the G/MWCNTs/Pd
catalyst, the Suzuki coupling reaction of iodobenzene and
phenylboronic acid was repeated using the asꢀprepared catalyst
60 for three months. As shown in Fig. S7, the catalyst exhibited
similarly high catalytic activity in three months, suggesting that
the G/MWCNTs/Pd catalyst was very stable which was not airꢀ
sensitive and can be used within three months without obvious
loss of activity.
found that G/MWCNTs/Pd exhibited
a more pronounced
catalytic activity for the Suzuki coupling reaction than the
rGO/Pd and MWCNTs/Pd catalyst under the optimal reaction
conditions. In view of the three composites, the enhanced
35 catalytic performance of G/MWCNTs/Pd can be attributed to the
effect of porous structure on the effective access of the substrates
to the exposed active sites with fast diffusion, which was closely
related to its higher BET surface area.
65 Hot filtration and leaching test
Metal leaching of the catalyst is of critical importance and in
order to test if Pd species was leaching out from the catalyst
during reaction, a hotꢀfiltration test was performed by coupling
bromobenzene with phenylboronic acid. When the reaction was
70 stirred for 30 min (71% yield, GC), the solid catalyst was
removed and the residual activity of the mother liquar was
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DOI: 10.1039/C5TA00280J
studied. Moreover, no significant change in either the
bromobezene conversion or the crossꢀcoupled biphenyl was
sites for the leached Pd to form a complex and deactivated the
active Pd species, thereby reduced the yield of the product.
observed in the filtrate. And, ICPꢀAES analysis of the filtrate 30 However, it is noted that the yield did not apparently change,
confirmed the absence of Pd species in the liquid phase.
Subsequently, the catalyst after six cycles was further analyzed
by ICPꢀAES. The results showed that the Pd content in the
G/MWCNTs/Pd composites after six runs was 3.46 wt% and the
Pd content in the fresh catalyst was 3.55 wt%. And the amount of
the leached Pd collected after six cycles in the reaction mixture
indicating the Suzuki coupling reactions in the experiment were
performed via a heterogeneous catalytic way. We could deduce
that the catalyst was heterogeneous in nature and no significant
leaching of Pd occurred during the coupling reaction.
In order to evaluate the efficiency of the G/MWCNTs/Pd
catalyst, we compared the results obtained in this work with those
5
35
10 was 20 ppb, the smaller amount of Pd leaching suggested the
strong affinity between Pd NPs with carbon skeleton which
effectively prevented the Pd active sites from leaching. The
reported elsewhere of recoverable Pdꢀbased efficient
3, 38, 45ꢀ53
heterogeneous catalysts.
Taking iodobenzene reacting
with phenylbronic acid as an example, the results were listed in
results demonstrated that the effect of trace amounts of Pd 40 Table 4. It can be seen that the present catalyst exhibited a better
leaching on the present catalyst system can be neglected and the
15 carbon support bound Pd NPs were responsible for the catalytic
activity.
catalytic effect with the highest apparent turnover frequencies
(TOF) of 799 hꢀ1 in comparison to other reported ones (Table 4,
entry 12). Although some of them also obtained high yields, some
toxic solvents (such as CH2Cl2, DMF, or DME) were used or
45 longer reaction time or relatively high temperature was needed.
These chemicals were less favorable than the ethanol used in this
study. The G/MWCNTs/Pd catalyst seemed to be the best
candidate, showing an excellent efficiency for the Suzuki crossꢀ
coupling reactions in terms of yield, cost, reaction time,
50 selectivity, and reusability. Furthermore, we also compared the
catalytic activities of G/MWCNTs/Pd with different supported Pd
catalysts for the Suzuki crossꢀcoupling reaction of aryl halide
with phenylboronic acid,54ꢀ57 and the reactions were carried out at
60 °C in air using K2CO3 as base and EtOH/H2O (v/v=1:1) as
55 reaction medium. As demonstrated in Table S1, the catalyst
G/MWCNTs/Pd exhibited the better catalytic activity.
Table 3 Reusability of the G/MWCNTs/Pd catalyst for the Suzuki crossꢀ
coupling reaction of iodobenzene with phenylboronic acid
Runs
Yield
1
>99
2
99
3
97
4
96
5
97
6
94
Solid-phase poisoning test
20
To ascertain the heterogeneous nature of the catalyst used in
our system, the solidꢀphase poisoning test was performed. Up to
now, various poisons have been used to hinder the catalytic
activity of homogeneous versus heterogeneous catalysts. In the
study, we employed poly(4ꢀvinylpyridine) as an effective Pd
25 scavenger that could selectively coordinate and trap the
homogeneous Pd species.44 If Pd leached out during the arylꢀ
coupling reaction, then poly(4ꢀvinylpyridine) provided binding
Table 4 Catalytic performances of different Pdꢀbased catalysts in the coupling of iodobenzene and phenylboronic acid
Entry
1
2
3
4
5
6
7
8
Catalyst
Pd/MFC
Pd NPs
Pd–Fe3O4
Pd@MagꢀMSN
Pd/CNTꢀSiC
Pd@CNPCs
MUAꢀPd
PdꢀCNTꢀEDꢀOH
Pd(0)/MCoSꢀ1
Pd/NiFe2O4
sugarꢀderived PdNPs
G/MWCNTs/Pd
Solvent
EtOH
H2O
DME/H2O=3:1
CH2Cl2
EtOH/H2O=4:1
DMF/H2O=1:1
DMF
DMF
H2O
Base
Temp (°C)
reflux
80
reflux
80
60
50
90
110
70
Time (h)
1
24
24
6
1
1.5
8
24
5
1/12
20
1/4
Yield (%)
100
97.1
99
85
98
96.7
>99
94
98
97
TOFa (hꢀ1)
324.6
40.5
41.3
14.2
350
644.6
6.2
13.1
98
Ref.
3
38
45
46
47
48
49
50
51
52
53
this work
K2CO3
Na2CO3
Na2CO3
K2CO3
K3PO4
K2CO3
NaOH
Na2CO3
K2CO3
Na2CO3
K2CO3
K2CO3
9
10
11
12
DMF/H2O=1:1
iPrOH
EtOH/H2O=1:1
90
100
60
196
4.4
799
88
>99
a The apparent TOF value was measured as moles of product with per mole of Pd catalyst per hour.
70 easy separation, recycling, airꢀstability, low cost and without the
use of phosphine ligands, the 3D G/MWCNTs/Pd catalyst was
highly desirable to address the environmental concerns. On the
basis of the results, the 3D G/MWCNTs/Pd composite materials
may promise huge potential for applications in industrial
75 catalysis.
4. Conclusions
60
In summary, the 3D G/MWCNTs/Pd composite hydrogel
was fabricated successfully via a facile oneꢀpot selfꢀassembled
approach, accompanied by an in situ growth process, where
ultrafine Pd NPs were wellꢀdispersed on the surface of the 3D
carbon skeleton. The 3D G/MWCNTs/Pd composites endowed
65 macroporous structure, large surface area and good durability,
resulting in excellent catalytic activity and selectivity toward the
Suzuki crossꢀcoupling reaction with high yields. More
significantly, the catalyst could be recovered easily and reused at
Acknowledgements
The authors are grateful for financial support from the National
Natural Science Foundation of China (No. 21375053) and Special
Doctorial Program Fund from the Ministry of Education of China
least six runs without significant loss of the activity. Due to its 80 (No. 20130211110039).
8
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Journal of Materials Chemistry A
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DOI: 10.1039/C5TA00280J
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