Highly active PdCu/graphene catalyst for an efficient Suzuki cross-coupling reaction

Article abstract of DOI:10.1039/d0nj04207b

A graphene supported Pd-Cu bimetallic catalyst (Pd-Cu/G) has been successfully fabricated by a simple one-pot solvothermal reaction. X-ray diffraction (XRD) analysis as well as morphology characterization demonstrated that Pd-Cu nanoparticles (NPs) are uniformly distributed on the graphene nanosheets. When adopted as the catalyst for the Suzuki cross-coupling reaction, this hybrid exhibited excellent catalytic activity (99.9% conversion) as well as desirable selectivity (99.9%). After recycling five times, the high yield and selectivity were well maintained, illustrating the high durability of this catalyst. This journal is

Full text of DOI:10.1039/d0nj04207b

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ISSN 1144-2546
PAPER
Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
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DOI: 10.1039/D0NJ04207B
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Highly Active PdCu/Graphene Catalyst for Efficient Suzuki Cross-
coupling Reaction
Xiaojing Ma,a Lingfeng Gao,*a Zhentao Wenga, Hua Yangb and Xu Sun*a
Received 00th January 20xx,
Accepted 00th January 20xx
Abstract: The graphene supported Pd-Cu bimetallic catalyst (Pd-Cu/G) has been successfully fabricated by the simple one-
pot solvothermal reaction. The X-ray diffraction (XRD) analysis as well as the morphology characterization demonstrated
that Pd-Cu nanoparticles (NPs) are uniformly distributed on the graphene nanosheets. When adopted as the catalyst for
Suzuki cross-coupling reaction, this hybrid exhibited excellent catalytic activity (99.9% conversion) as well as desired
selectivity (99.9%). After circulation for five times, the high yield and selectivity were well maintained, illustrating the high
durability of this catalyst.
DOI: 10.1039/x0xx00000x
Considering this, agglomeration of NPs will be effectively
avoided by using graphene nanosheets with large surface areas
as carrier. 23,24
1. Introduction
In recent years, graphene and graphene-based materials have
attracted great attention due to their unique electronic and
structural properties, such as high intrinsic carrier mobility,1,2
high mechanical strength3,4 and thermal stability.5-7 Meanwhile,
benefited from the two dimensional structure character as well
as the large surface area, the graphene can also serve as a
potential support for the fabrication of hybrid nano-materials.8-
2. Experimental Section
Synthesis of the catalyst materials
Synthesis of (Pd-Cu/G). The Pd-Cu/G catalyst with the Pd/Cu ratio
of 1:1 was synthesized via the hydrothermal reaction. The
ethylene glycol solution of PdCl2 (2 mL, 6.7 mg/mL), GO (3 mL,
10 mg/L), CuSO4·5H2O (20 mg) and sodium glutamate (50 mg)
were mixed together in 30 mL of ethylene glycol to form a
homogeneous mixing suspension. Then, under vigorous stirring,
the pH value of solution system was adjusted to 12 by the
dropwise addition of 10 wt% KOH glycol solution. Thereafter,
the solution was put into Teflon-lined autoclave and maintained
at 160 oC for 6 h. After cooled down to room temperature, Pd-
Cu bimetallic nanocubics on 3D graphene nanocomposites were
obtained. The resulted solid was washed with 0.1 mol/L KOH
and deionized water several times, and was freeze-dried for the
following measurements. The Pd-Cu/G catalyst with the Pd/Cu
ratio of 1:3 and 3:1 were fabricated via the similar method
except the ratio of PdCl2 and CuSO4·5H2O were changed. The
experimental details were provided in Supporting Information.
The products were named as Pd-Cu/G-3 and Pd-Cu/G-4,
respectively.
11
Moreover, Graphene-based composite nanomaterials hold
great potential being a promising candidate for applications in
the fields of chemical catalysts, biological polymerization and
energy storage due to their structural characteristics.10-14
The fabrication of biphenyl compounds by Pd catalyzed
Suzuki cross-coupling reaction is of great importance for the
synthesis of agrochemicals, pharmaceutical drug, polymer
materials and functional organic molecules.17-18 However, the
homogenous Pd-catalysts often suffers from high cost and
metal pollution. To date, a variety of heterogeneous Pd-based
alloys catalysts have been reported, which exhibited advanced
activity for the cross-coupling reaction.18-20 For example,
Heugebaert demonstrated that Pd/Au bimetallic NPs would be
a novel catalyst for the Suzuki cross-coupling reaction.21 Xiang
reported that bimetallic Pd-Ni core-shell NPs exhibited desired
catalytic activities in the cross-coupling reaction.22 While,
preventing the aggregation of these heterogeneous NPs is still
a main challenge to further increase the catalytic efficiency.
3. Results and Discussion
a.
School of Chemistry and Chemical Engineering, University of Jinan, No. 336 West
The morphology character of as synthesized hybrid was analysed via
the scanning electron microscope (SEM) as shown in Fig. 1. It can be
clearly observed that a porous three dimensional (3D) framework
without severe agglomeration was successfully constructed as
shown in Fig. 1a. As presented in Fig. 1b, it can be seen that Pd and
Cu nanocubics were uniformly anchored on the GO sheets. It is
Road of Nan Xinzhuang, Jinan 250022, People’s Republic of China.
b. Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell
Technology, School of Chemistry and Chemical Engineering, Liaocheng University,
252059 Liaocheng, People’s Republic of China.
c. Address here.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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While, as for Cu/graphene, it can be seen from Fig. 3c that the Cu NPs
DOI: 10.1039/D0NJ04207B
are of cubic structure with the average diameter of about 50 nm.
proposed that the functional groups on the surface of GO nanosheet,
such as hydroxy group, epoxy group, carboxyl group and so on, would
be ideal sites for the uniform binding of Pd2+ and Cu2+. Then, across
the solvothermal reaction process, the GO would be reduced, thus
leading to the formation of this porous 3D structure. Meanwhile,
along with the reduction of GO, the Pd2+ and Cu2+ were reduced as
well, with the Pd and Cu nanocubics being uniformly anchored on the
graphene sheets Fig. 1b, which finally lead to the formation of Pd-
Cu/G hybrid.
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Fig. 1 SEM images of PdCu/ 3D graphene at different magnifications.
Fig. 2 (a) XRD patterns of Cu/G, Pd-Cu/G and Pd/G. (b) The XPS spectra of survey
spectrum, (c) Pd3d and (d) Cu2p of Pd-Cu/G.
Then, for better analysis the crystal structure, the X-ray diffraction
patterns of Pd-Cu/G nanocomposite was provided in Fig. 2a. For
better comparison, the XRD patterns of Pd/G and Cu/G were
provided in Fig. 2a as well. As presented in all these samples, a broad
peak located at 2θ ≈ 25o was clearly observed, which was
corresponding to the (002) plane of stacked graphene sheets,
illustrating the successful reduction of GO. From the diffraction peak
of Cu/G in Figure. 2a (line a), three diffraction peaks appeared at
42.4o, 50.5o, 73.5o, which was in well consistence with the lattice
structure of Cu (No. 04-0836).27 While, as presented in the XRD
patterns of Pd/G (line (c) in Fig.2a), no significant diffraction peaks
were observed. it meant that Pd/G compound has low crystallinity.
Furthermore, as for the Pd-Cu/G (line (b) in Fig. 2a), all peaks
appeared between the pure Pd(0) and Cu(0) diffraction planes,
indicating the formation of Pd and Cu NPs on graphene nanosheet.
To further verify the formation of Pd and Cu particles, the surface
of Pd-Cu/G nanocomposites was characterized by XPS (Fig. 2b, c and
d). As can be seen from the survey XPS spectral as shown in Fig.2b,
the chemical composition of Cu, Pd and C were well demonstrated.
Meanwhile, the high resolution XPS result for Pd 3d region is
provided in Figure 2c, from which two peaks at 335.2 and 340.2 eV
corresponding to Pd(0) 3d5/2 and Pd 3d3/2 bands are clearly
observed. These peaks demonstrated the existence of Pd(0) on the
Fig. 3 TEM images of (a) Pd-Cu/G, (b) Pd/G, (c) Cu/G. (d–e) corresponding element
mapping images Pd and Cu. (f) the overlap of Pd and Cu EDS mappings of Pd-Cu/G.
As for as Pd-Cu/graphene composite was seen from Fig. 3a,
graphene nanosheet were observed with no obvious
aggregation being observed. It is noteworthy that the
introduction of graphene nanosheet not only prohibited the
surface of the 3D graphene. Fig. 2d provided the Cu 2p spectral, from aggregation of Pd and Cu NPs, but also shortened the distance
which two peaks corresponded to Cu 2p1/2 and Cu 2p3/2 were
clearly observed at 951.4 eV and 931.3 eV, respectively. Meanwhile,
another two tiny peaks at around 953.7 eV and 933.7 eV can be
attributed to 2p1/2 and 2p3/2 of Cu+, respectively. The appearance
of tiny Cu+ on the surface might be owing to the unavoidable surface
oxidation. Above all, based on the XRD and XPS results, it is in
conclusion that the Pd-Cu NPs decorated graphene was successfully
fabricated.
between Pd and Cu particles, thus enhancing the confinement
effect between Pd and Cu. The above results demonstrated that
our hydrothermal method could guarantee the formation of
highly dispersed Pd-Cu NPs, which might be an advanced
heterogeneous catalyst. To further survey the element
composition, the element mapping images of Pd-Cu/G were
provided as shown in Fig. 3d to 3f. It can be seen that Pd and Cu
are almost uniformly dispersed across the whole nanosheet,
further suggesting the successful fabrication of Pd-Cu/G
composite.
To better demonstrate the successful synthesis of Pd-Cu NPs on
graphene nanosheet, the morphology feature of the composite was
observed by transmission electron microscopy (TEM) images. As for
Pd/graphene as exhibited in Fig. 3b, the Pd NPs with the diameter of
about 20 nm are uniformly distributed on the graphene nanosheet.
These synthesized catalysts were investigated in Suzuki cross-
coupling reaction. To optimize the reaction conditions, aryl
bromides and benzene boronic acid was chosen as model
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substrates under the catalysis of various nanocomposites. The from 3ae, the yield of about 99.5% was achieved, illustrating the high
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DOI: 10.1039/D0NJ04207B
corresponding results was provided in Table S1. Firstly, the activity of the catalyst. Finally, the influence of some other active
K2CO3 was adopted as the bases with the mixture of ethanol and groups, such as carbonyl group, carboxylic acid, ester group and
water (1:1) being adopted as the solvent. It can be seen that phenyl group, was also studied. As can be seen from 3af, 3bg and
when Cu/G was used as the catalyst, the reaction was difficult 3ch, all of these derivatives afforded the formation of the desired
to carry out even at 80 oC, with almost no product being products, with the yield reaching about 95%~98%.
obtained (Table S1, entry 2). In comparison, the Pd/G catalyst
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Table 1. Pd-Cu/G catalyzed Suzuki cross-coupling reactiona
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exhibited obviously enhanced catalytic activity, with the
conversion reaching about 70.3% under the same reaction
condition (Table S1, entry 3). The above result confirmed the
important role of Pd for the reaction. Furthermore, when the
Pd-Cu/G compound was used as the catalyst, the yield of about
o
68.3% was achieved even at 50 C (Table S1, entry 8). With
further increment of temperature to about 80 oC, the yield
would reach about 99.9% (Table S1, entry 4), much higher than
other counter parts, suggesting that the synergetic effect
between Pd and Cu is indeed beneficial for the processing of the
reaction. Not only that, Pd-Cu/G exhibited satisfactory yield
with the Pd/Cu ratio of 1:1, compared with Pd-Cu/G-2(74.5%),
Pd-Cu/G-3(30.5%) and Pd-Cu/G-4(56.3%) under the same
reaction condition (Table S1, entry 9, 10, 11). For better
comparison, the mixture of Pd and C (Pd/C(10%)) was adopted
as the catalyst as well, which gives the yield of about 78.6% at
80 oC (Table S1, entry 6), obviously lower than that of Pd-Cu/G
catalyst, further confirming the important role the synergistic
effect between Cu and Pd.
Furthermore, the influence of the bases and the solvent were
also analysed. Firstly, the Na2CO3 was used as the bases to
analysis the reaction process. As shown in entry 5 (Table S1,
entry 5), it can be seen the yield of 88.5% was obtained, which
is somewhat lower than the reaction with K2CO3 acting as bases.
It can be concluded that K2CO3 would be the more favourable
bases for Suzuki cross-coupling reaction. Besides, the ethanol
was also adopted as the reaction solvent for better comparison,
from which only 60.8% yield was obtained at 80 oC, suggesting
that the mixture of ethanol and water would be more favorable
as the reaction solvent (Table S1, entry 7). Based on the above
results, it is in conclusion that the K2CO3 and the mixture of
ethanol and water would be the most suitable solvent.
Meanwhile, to further confirm the high activity of the catalyst
and to expand the scope of the procedure, the reactions
between benzene boronic acid and a series aryl bromides
compounds were investigated under the above mentioned
optimized reaction conditions. As can be seen from Table 1, the
aryl bromides with either electron-donating group (-OCH3, 3ab)
or electron-withdrawing group (-F, 3ac) both offered the
corresponding products with high yields of 99.9% and 99.7%,
respectively. Besides, as shown in 3ad, when p-bromophenol
was adopted as the reactant, the high yield of 96.4% was
achieved as well, with the active hydroxy group in the
substrates p-bromophenol was unchanged, confirming the high
selectivity of the catalyst. Furthermore, the P-tert-
butylbromobenzene was applied as the reactant as well to
analysis the influence of the steric hindrance effect. As can be seen
Fig. 4 (a) Reusability of Pd-Cu/G in catalysed Suzuki reaction. (b) A representative TEM
image of catalysts after the five runs reusability.
The stability is also an important character of an advanced catalyst.
As for the heterogeneous catalyst, it can be easily recycled, which
provides an ideal material platform for the sustainable industrial
application. The stability of Pd-Cu/G catalyst was analysed as shown
in Fig. 4. It can be seen that after five reaction cycle, the yield of this
coupling reaction remains at more than 95%. The TEM image of the
catalyst that had been recycled five times was also provided as Fig.
4b, from which no obvious morphology changes was observed,
confirming the high stability of the catalyst.
4. Conclusions
In summary, Pd-Cu nanocubics on 3D graphene (Pd-Cu/G)
were synthesized via the one-pot solvothermal reaction. The
graphene with 3D porous structural would be a satisfactory
template, thus bringing about the stabilization of Pd and Cu
particles. This novel Pd-Cu/G exhibited high catalytic activity
when adopted as the catalyst for Suzuki cross-coupling
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13 R Fareghi-Alamdari, M Golestanzadeh, O Bagheri. "meso-
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reaction under mild conditions, with the yield and selectivity
being about 99.9%. Furthermore, as the heterogeneous
catalyst, it could be easily separated for cyclic utilization,
keeping more than 95% of yield. The high activity should be
owing to the synergetic effect before Pd and Cu. This work
might provide some clues of the utility of strongly effective
bimetallic catalyst.
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Tetrakis
[4-(methoxycarbonyl)
phenyl]
DOI: 10.1039/D0NJ04207B
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This study was supported by the Natural Science Foundation of
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