N,N-Dimethylformamide solvothermal strategy: From fabrication of palladium nanoparticles supported on reduced graphene oxide nanosheets to their application in catalytic aminocarbonylation reactions

Article abstract of DOI:10.1016/j.apcata.2015.02.026

A facile N,N-dimethylformamide (DMF) solvothermal strategy has been developed to fabricate palladium nanoparticles supported on reduced graphene oxide nanosheets (SRGO-Pd), which efficiently catalyze the aminocarbonylation of aryl halides. The palladium n

Full text of DOI:10.1016/j.apcata.2015.02.026

Accepted Manuscript
Title: N, N-dimethylformamide solvothermal strategy: From
fabrication of palladium nanoparticles supported on reduced
graphene oxide nanosheets to their application in catalytic
aminocarbonylation reactions
Author: Ying Zhang Huaming Sun Weiqiang Zhang Ziwei
Gao Peng Yang Jian Gu
PII:
S0926-860X(15)00118-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.02.026
APCATA 15263
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
26-11-2014
29-1-2015
15-2-2015
Please cite this article as: Y. Zhang, H. Sun, W. Zhang, Z.G.P. Yang, J. Gu,
N, N-dimethylformamide solvothermal strategy: from fabrication of palladium
nanoparticles supported on reduced graphene oxide nanosheets to their application
in catalytic aminocarbonylation reactions, Applied Catalysis A, General (2015),
http://dx.doi.org/10.1016/j.apcata.2015.02.026
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Highlights
1. We propose a strategy for applications ranging from the fabrication to the use.
2. DMF could play the multiple role during the solvothermal process.
3. Palladium particles could be tuned by the solvothermal temperture.
4. A new protocol for a ligand- and CO-free aminocarbonylation was demonstrated.
5. SRGO-Pd catalysts demonstrated excellent catalytic activity and recyclability.
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N, N-dimethylformamide solvothermal strategy: from fabrication of palladium
nanoparticles supported on reduced graphene oxide nanosheets to their application in
catalytic aminocarbonylation reactions
Ying Zhang, Huaming Sun*, Weiqiang Zhang, Ziwei Gao*, Peng Yang, Jian Gu
Graphical Abstract
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N, N-dimethylformamide solvothermal strategy: from fabrication
of palladium nanoparticles supported on reduced graphene oxide
nanosheets to their application in catalytic aminocarbonylation
reactions
Ying Zhang, Huaming Sun*, Weiqiang Zhang, Ziwei Gao,* Peng Yang, Jian Gu
Key Laboratory of Applied Surface and Colloid Chemistry, MOE; School of Chemistry and
Chemical Engineering, Shaanxi Normal University, Xi'an 710119, China
*Corresponding author.
E-mail address: hmsun@snnu.edu.cn (H. M. Sun), zwgao@snnu.edu.cn (Z. W. Gao).
ABSTRACT: A facile N, N-dimethylformamide (DMF) solvothermal strategy has been
developed to fabricate palladium nanoparticles supported on reduced graphene oxide
nanosheets (SRGO-Pd), which efficiently catalyze the aminocarbonylation of aryl halides.
The palladium nanoparticles were uniformly anchored on the reduced graphene oxide
nanosheets by a one-step DMF solvothermal approach. During the process, the palladium
particles on the reduced graphene oxide nanosheets were tuned by the solvothermal
temperture. DMF did not only play the role of the reductant for graphite oxide and Pd2+, but
also restrained the aggregation of the graphene sheets and stablized the palladium
nanoparticles. Under DMF solvothermal conditions, SRGO-Pd catalysts demonstrated an
excellent catalytic activity and good recyclability for the aminocarbonylation reaction of aryl
halides without using carbon monoxide and any other additive. The SRGO-Pd catalyst could
be recycled at least 8 times with sustained activity. Such a strategy represents a promising
route for applications ranging from the fabrication of graphene-supported materials to the use
as catalysts.
Keywords: Pd nanoparticles; Graphene; Aminocarbonylation reactons; Catalysis;
Solvothermal method.
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1. Introduction
Various fabrication strategies have been developed for preparing palladium nanoparticles
as heterogeneous or semi-heterogeneous catalysts for numerous catalytic reactions. [1-6] The
high dispersion and precise size controlling are required to improve the catalytic activity and
reduce the consumption of the catalysts. A high dispersion and precise size control are
required to improve the catalytic activity and reduce the consumption of the catalysts. The
aggregation of bare palladium nanoparticles, unfortunately, prohibits a fine tailoring of the
morphology and catalytic activity of the corresponding particles. For this reason, many
attempts have been made to immobilize or stabilize Pd nanoparticles on a wide variety of
supports. [7-11]
Graphene is an attractive supporting material for catalyst loading owing to its fast 2D
electron-transfer kinetics, large surface area, and high mechanical, thermal and chemical
stability. Several graphene-based palladium nanoparticles have been used as catalysts with
high activity and selectivity for carbon-carbon bond forming reactions. [12-14] However,
there are still a number of limitations to the practical application of most graphene-supported
metal nanocatalysts. First, the strong π–π stacking interactions between graphene nanosheets
can lead to serious aggregation and restacking, which inevitably blocks active catalytic sites,
and thus significantly hampers the catalytic activity. [15, 16] Second, owing to their high
surface energy, metal nanoparticles are typically unstable and tend to sinter into larger species,
especially under harsh reaction conditions such as at the high temperatures required for many
catalytic reactions and subsequent detoxification processes. This sintering results in a
dramatic decrease in catalytic activity. To overcome these limitations, the dispersibility and
stability of graphene-supported metal nanocatalysts must be improved by developing novel
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methods in order to securely anchor palladium nanoparticles onto graphene supports. More
recently, efforts have been made to retain the structure of the graphene network, such as
keeping the graphene solvated by the vacuum filtration method [17], thus developing a range
of 3D graphene-based composites by spray methods [18] and preparing
graphene-nanosheet-supported metal nanoparticles covered by mesoporous silica layers. [19]
Thanks to the unique features such as a very high self-generated pressure inside the sealed
reaction vessel and containment of volatile products, the solvothermal approach [20] has in
recent decades become a well-known and powerful technique for fabricating inorganic solid
materials in which the solvent plays a crucial role. DMF is commonly used as a polar aprotic
solvent with a high boiling point in the preparation of metastable catalysts via the
solvothermal approach, either separately or cooperatively, taking advantages of the easy
operation and the fact that it is environmentally benign and low-cost. [21]
The important role played by DMF as a solvent and reaction medium has motivated studies
related to its redox activity and stability. [21-24] Recently, some researchers found that GO
dispersion could be directly reduced to graphene sheet dispersion in DMF. [22, 25, 26] There
are, however, only few reports on the direct preparation of graphene-based metal
nanoparticles by the simple solvothermal reduction method. As a consequence, the DMF
solvothermal approach is highly desirable for catalyst loading to fabricate graphene-supported
Pd nanoparticle catalysts.
This work presents how the DMF solvothermal approach can be successfully used for
fabricating Pd nanoparticles supported on reduced graphene oxide nanosheets (SRGO-Pd).
Thanks to it, one can effectively avoid the close restacking of graphene nanosheets. The
obtained catalyst maintains an excellent dispersing stability in DMF. The solvothermal
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method offers significant advantages for the preparation since no reducing agents or
stabilizing agents are required. More importantly, based on this DMF solvothermal situation，
SRGO-Pd efficiently catalyzes the aminocarbonylation of aryl halides without the need for
any ligand or base and can be reused whilst maintaining the inherent catalytic activity. The
critical advance is the use of DMF as a resource of carbon monoxide and as an amide source.
It acts both as a solvent and as a catalyst stabilizer, thereby facilitating an otherwise
challenging transformation. To the best of our knowledge, this is the first report on
applications of Pd nanoparticles supported on graphene to catalyze aminocarbonylation
reactions.
The preferred position for Scheme 1.
2. Experimental
2.1. Fabrication of SRGO-Pd catalyst under DMF solvothermal conditions
In the experiments, GO was prepared by the oxidation of high-purity graphite powder
(99.9999%, 200 mesh) with H2SO4/KMnO4 according to the method of Hummers and
Offeman [27]. After repeated washing of the resulting yellowish-brown cake with hot water,
the powder was dried at room temperature under vacuum overnight. For the preparation of
SRGO-Pd: 40 mg of the dried GO was sonicated in DMF until a homogeneous yellow
dispersion was obtained. The solution was transferred to an autoclave after adding an
appropriate amount of palladium acetate and heated at a certain temperature for 6 h. The
SRGO-Pd were separated by using an Eppendorf 5804 centrifuge operated at 8000 rpm for 15
min and dried overnight under vacuum. The same method was also used for the preparation of
SRGO.
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2.2. General procedure for the aminocarbonylation of aryl halides under DMF
solvothermal conditions
SRGO-Pd200 was sonicated in DMF until a homogeneous yellow dispersion was obtained.
Aryl halides (2 mmol) was added to the reaction mixture with stirring over a period of 15 min.
The reaction mixture was transferred to an autoclave and heated at 200°C for 6 h. After
completion, the reaction mixture was cooled to room temperature and filtered. The filtrate was
poured into a saturated solution of NaHCO3. The aqueous layer was then extracted with ethyl
acetate. The combined organic layers were dried over MgSO4 and the solvents were removed
in vacuo to afford the crude product which was purified by column chromatography using
petroleum ether/ethyl acetate as eluent. The same procedure was applied to the aryl halides.
2.3. Procedure for recycling the catalysts
The catalyst obtained after filtration was washed with DMF to remove any organic
impurities. The resulting catalyst was dried in oven at 50°C for 6 h and used for the next cycle.
The catalyst was dissolved in DMF and sonicated until a homogeneous yellow dispersion was
obtained. Iodobenzene was then added; The reaction was transferred to an autoclave and
heated at 200°C for 6 h. After the completion of the reaction, the reaction mixture was cooled
to room temperature and filtered. The Pd nanoparticle catalyst was then directly transferred to
another Teflon-lined stainless-steel autoclave along with fresh reagents for the next run. This
procedure was repeated for every run, and the percent conversion of product was determined
by the means of GC-MS spectroscopy.
3. Results and discussion
3.1. Fabrication of homogeneous SRGO-Pd dispersion via the DMF solvothermal approach
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The fabrication strategy for preparing SRGO-Pd is shown in Scheme 1 (for the detailed
experimental steps, please see the Experimental Section). A simple solvothermal process was
employed for preparing SRGO-Pd through a heated DMF reduction of GO and palladium
acetate in a sealed Teflon-lined stainless-steel autoclave at a certain temperature. A
homogenous SRGO-Pd dispersion in DMF was obtained after several hours. Different
temperatures, i.e., 160, 180, and 200°C, were used to prepare SRGO-Pd dispersions, and these
homogeneous dispersions were denoted SRGO-Pd160, SRGO-Pd180, and SRGO-Pd200,
respectively.
As shown in Fig. S1, the color of the dispersion changed from a yellowish brown (Fig.
S1(A)) into deep black after the solvothermal reduction (Fig. S1(B), (C), (D)). Furthermore,
an intense odor resembling that of an amine could was given off from the SRGO-Pd
dispersions. To our surprise，the as-prepared SRGO-Pd dispersion in DMF was very stable
and did not aggregate, even after more than 1 month.
Meanwhile, solvothermal reduction of GO and palladium acetate was also carried out in
other solvents, such as methanol, ethanol, THF, DMSO, and ethylene glycol. However, in
these cases, no stable homogeneous dispersions could be obtained. Therefore, the DMF
solvent played an important role in the preparation of homogeneous dispersions of SRGO-Pd
in the solvothermal reduction process.
The preferred position for Fig. 1.
Fig. 1 (A-C) represents the TEM micrographs of the SRGO-Pd catalyst prepared at three
solvothermal temperatures. Wrinkles of SRGO-Pd, a characteristic feature of the graphene
sheets, can be observed. As shown in Fig. 1 (A-C), the individual Pd nanoparticles were well
separated from each other and nicely spread out on the surface of the graphene sheets. In
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contrast, Pd particles prepared in the control experiments without addition of GO at 180 °C
tended to aggregate (Fig. 1(D)). The results thus prove that graphene has a potential for
regulating growth processes of Pd nanoparticles. The good distribution of Pd nanoparticles on
the graphene sheets guaranteed efficient catalytic properties of SRGO-Pd. The size of the Pd
nanoparticles increased with the temperature, indicating that the solvothermal temperature
could be very useful for tuning the size and dispersion of the Pd nanoparticles on graphene.
Morphology of the catalysts was also examined by SEM. From Fig. S2(a), an SEM image of
the graphene oxide shows GO nanosheets are soft and can easily be imbedded small
nanoparticles. In contrast to Fig. S2a in which any smaller particulate is not detected, many
round-shaped Pd nanoparticles, which are decorated on some graphene sheets, can be clearly
observed in the SEM image of SRGO-Pd (Fig. S2(b-d)). The palladium content of the
obtained SRGO-Pd amounted to 354 ug mg -1 as verified by ICP-MS. The obtained catalysts
are listed in Table S3.
The preferred position for Fig. 2.
Fig. 2 displays the XRD patterns of the initial graphite powder, the prepared GO, the
SRGO and SRGO-Pd samples obtained by the DMF solvothermal method. The initial
graphite powder showed the typical sharp diffraction peak at 2θ=26.6° with the corresponding
d-spacing of 3.34 Å. The GO sample presented no diffraction peaks from the parental graphite
material and only a new broad peak at 2θ=11.7° with a d-spacing of 7.56 Å could be observed.
This suggested that the distance between the carbon sheets had increased due to the insertion
of interplanar oxygen functional groups. [28-30] After performing the solvothermal process
on the GO, the XRD patterns of the resultant graphene displayed the disappearance of the
11.7° peak confirming the reduction of the GO sheets. A similar XRD spectrum was observed
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for the SRGO-Pd sample prepared by the simultaneous reduction of GO and palladium
acetate using the solvothermal method. In the X-ray diffraction pattern of SRGO-Pd, there
were three main peaks at scattering angles of 39.9, 46.5 and 68.0, corresponding to the
crystalline planes of the face-centered cubic (fcc) structured palladium (JCPDS No. 46-1043),
thus indicating that Pd2+ had been reduced to Pd0.
The representative ATR-FTIR peaks (Fig. S4) of the oxygen-containing functional groups
of GO, which include the bands at 1055 cm-1 (C-O stretching vibrations), 1393 cm-1 (tertiary
C-OH groups stretching), and 1726 cm-1 (C=O stretching of COOH groups situated at the
edges of the GO sheets), were absent in the ATR-FTIR spectrum of SRGO-Pd synthesized
from GO, which indicates the reduction of these functional groups. A new absorption band at
1570 cm-1 attributed to the skeletal vibration of the graphene sheets appeared in the
ATR-FTIR spectrum of SRGO-Pd.[20]
The preferred position for Fig. 3.
In the survey spectrum (Fig. 3A), we can observe that the intensity of O1s decreased
significantly from GO to SRGO-Pd200, indicating a considerable deoxygenation by the
solvothermal process. Table S5 lists the atomic composition of GO and SRGO-Pd200
obtained by XPS. From the data in Table S5, it can be found that the C/O atomic ratio
increased from 2.36 for GO to 6.20 for SRGO-Pd200, which is comparable to the C/O ratio of
chemically reduced GO. [29, 31, 32] The XPS spectrum of GO in the C1s region (Fig. 3B)
clearly indicates a considerable degree of oxidation of the carbon atoms of the different
functional groups: the carbon atoms in C=C (284.8 eV), the carbon atoms in C–O (285.7 eV),
the epoxy carbon atoms (286.9 eV), the carbonyl carbon atoms (287.7 eV), and the carboxyl
carbon atoms (288.8 eV). [30, 33] After the solvothermal treatment, SRGO-Pd200 showed
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only one predominant peak at 284.8 eV (Fig. 3C), which could be attributed to graphitic
carbon systems. Moreover, the tail of this signal at a high binding energy indicated that a
small amount of carbon atoms was still covalently bound with the oxygen remaining in the
SRGO-Pd200 after the solvothermal treatment.
Fig. 3D displays the XPS spectra of the Pd-3d electron in the SRGO-Pd catalysts. The data
show that Pd nanoparticles were present as Pd (0), which is consistent with the observed
binding energies of 335.1 eV (Pd (0) 3d5/2) and 340.5 eV (Pd(0) 3d3/2). [33, 34]
Above all, SRGO-Pd was successfully and directly synthesized by a facile one-step DMF
solvothermal reaction, where the nanomaterial fabrication and reduction of GO sheets to
graphene sheets could occur simultaneously. This DMF solvothermal process opens up new
avenues to one-pot fabrication of graphene-supported metal composites.
3.2. The key role of the N, N-dimethylformamide solvothermal in fabricating SRGO-Pd
catalyst
In this work, the DMF solvothermal played an important role in the fabrication of
SRGO-Pd catalysts. A solvothermal mechanism was proposed according to Fig. 4. Firstly，
the reduction mechanism of GO and Pd2+ in the above process may be the result of: 1) thermal
reduction, which has been reported for the reduction of GO in other solvents at high
temperature, and 2) the production of the reductant CO from DMF during the solvothermal
process. It is well known that carbon monoxide is a strong reducing agent, which can
efficiently remove oxygen from many compounds. [25, 35] Therefore, the reduction process
consists of simultaneous thermal and chemical reduction.
Secondly, the stabilization mechanism might be the resultant derivative from DMF during
the solvothermal process. In the solvothermal process, DMF can be hydrolyzed into
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NH(CH3)2 and CO (reaction formula in Fig. 4). Carbon monoxide can be confirmed by the in
situ mass spectrometer. (Fig. S6) On the other hand, NH(CH3)2 can be confirmed by the
intense odor of amine given off by the dispersion after the reaction. As a result of protonation,
NH(CH3)2 becomes positively charged in the form of [NH2(CH3)2]+ (reaction formula in Fig.
4). The XPS results show that there remained a small amount of carboxyl groups on the
SRGO-Pd (Fig. 3C), and that [NH2(CH3)2]+ could interact with the negatively charged –COO-
on the edge of GO sheet driven by electrostatic interaction. With the alkyl groups modified on
the edges of the SRGO-Pd sheets, the latter were stabilized, and their dispersibility enhanced.
The existence of a nitrogen species was verified by XPS analysis for the N1s peak at 400 eV
(Fig. 3a), and the increase at 285.7 eV was partially due to the C–N in the [NH2(CH3)2]+ (Fig.
3c). The in-situ produced [NH2(CH3)2]+ acted as a stabilizer for the SRGO-Pd, wherefore,
without any formed stabilizer in the dispersion of methanol, ethanol, THF, DMSO, and
ethylene glycol, the carbon sheets aggregated together due to the π-π interaction between the
sheets. On the other hand, DMF molecules were efficient stabilizers of the metal
nanoparticles. [21, 36]
Consequently, DMF could serve as a solvent, an endogenous reducing agent and a
self-generated stabilizer during the solvothermal process.
The preferred position for Fig. 4.
3.3. Catalytic activity of SRGO-Pd toward aminocarbonylation during the N,
N-dimethylformamide solvothermal approach
As mentioned, DMF is a much better solvent that others used and can be employed as a
source of carbon monoxide and dimethylamine originating from its decomposition at boiling
point. This observation has led to the use of DMF as a carbonylating agent.
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In our experiment we investigated the catalytic activity of SRGO–Pd for the
aminocarbonylation in DMF as the solvent under solvothermal conditions without any
additive. In a model reaction, we employed iodobenzenes as the reactants with DMF as the
solvent but were surprised to find that dimethylamide, rather than biphenyl, was the
predominant product, formed in good yield after solvothermal treatment at 200 °C for 6 h
(Scheme 1).
Control experiments under the open condition (the aminocarbonylation reaction of
iodobenzene using DMF as the solvent) were performed with conventional thermal heating
using 0.5 mol % of the catalyst. Here, only a small yield of 2% was observed after 6 h of
refluxing in flasks at 160 °C. This result clearly demonstrated the effect of the solvothermal
condition on increasing reaction rates by providing a direct and rapid heating source for
aminocarbonylation. This observation is consistent with recent reports on the selective heating
of the surfaces of heterogeneous catalysts and the formation of hot-spots by microwave
irradiation which can lead to nonequilibrium local heating at the surface of the metal
nanoparticle catalysts.[33, 37] Compared with the example reported by Kazushi Hosoi et
al.[38] and Nasser Iranpoor et al.[39], a palladium-catalyzed coupling reaction of DMF with
aryl halides could be successfully carried out in the absence of phosphoryl chloride to afford
the corresponding tertiary amides in good yields.
The preferred position for Fig. 5.
To better understand the solvothermal approach, we performed experiments at four
different temperatures, from 140 °C to 200 °C, bearing in mind that the boiling point of DMF
is 153 °C. Moreover, we prolonged the reaction time to 6 h. As shown in Fig. 5, the yield
increased with an increasing temperature. At 140 °C, very little product was formed, which
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was due to the slow reaction rate at this low temperature. However, when the temperature was
increased to 160 °C, the yield was 62%. According to the GC-MS analysis result, a small
amount of biphenyl appeared, indicating that the side reaction could occur at 160 °C. Upon
increasing the temperature to 180 °C, a yield of 82% was observed, and when further raising
the temperature to 200 °C, the yield was nearly 100%. This demonstrates that the
solvothermal approach can restrain side reactions at suitably high temperatures. We thus infer
that the C–I bond of the substrate was activated, thus decreasing the activation energy of the
self-coupling reaction, which has the advantage of increasing the reaction rate of the desired
reaction over that of the side reaction.
The preferred position for Table 1.
This finding encouraged us to study in more detail the scope and limitations of DMF as a
potential carbon monoxide precursor for carbonylation reactions. A series of aryl halides were
screened, and the results are presented in Table 1. The in-situ carbon monoxide generation
methodology worked efficiently with iodobenzene and more electron-rich aryl iodides,
whereas aminocarbonylation of electron-deficient aromatic systems failed. Although only a
limited number of examples are presented here, we believe that the method will attract
attention due to its experimental simplicity and the fact that no external carbon monoxide gas
and ligand need to be added. It should serve as a convenient alternative to other existing
methodologies for aminocarbonylation of aromatics,ꢀin particular for large-scale reactions.
The preferred position for Table 2.
Table 2 illustrates the catalytic activity of the catalysts at different catalyst loadings for the
aminocarbonylation reaction of iodobenzene under solvothermal conditions in the absence of
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additives. To our great delight，SRGO–Pd showed a superior activity at lower concentrations.
With 0.5 mol% loading, the catalysts displayed a nearly complete conversion (98%) of
iodobenzene into the product. Similarly, when 0.05 mol% loading was used, the catalysts still
worked effectively, giving rise to a conversion of 89% for SRGO–Pd. However, further
reducing the concentrations provided some insight into the relative reactivity. With the lowest
loading of 0.005 mol%, the reaction was completed after 6 h with SRGO–Pd, affording a
yield of 50% of the product. These results demonstrate the remarkable catalytic activity of
SRGO–Pd.
Although the real mechanism remains unclear at this stage, in agreement with previous
studies on the catalytic aminocarbonylation reactions and our current observations, [8, 9,
38-45] the catalytic mechanism shown in Scheme 2 can be proposed. It is well established
that DMF decarbonylates under solvothermal heating conditions to yield carbon monoxide
and dimethylamine without base. However, the present methodology is, to the best of our
knowledge, the first example where the carbon monoxide is both liberated from the solvent
and thereafter utilized in an efficient organic carbonylation procedure in the absence of base.
The preferred position for Scheme 2.
The DMF solvent has been shown to be an excellent source of carbon monoxide and
dimethylamine, with both gases serving as building blocks in the SRGO-Pd-catalyzed aryl
amide synthesis, in the absence of a strong base under solvothermal conditions. Based on this,
it can be concluded that the current SRGO-Pd nanoparticles under solvothermal DMF provide
an efficient strategy for the aminocarbonylation of aryl halides.
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3.4. Recycling the SRGO-Pd catalysts
A significant practical application of heterogeneous catalysis lies in the ability to easily
remove the catalyst from the reaction mixture and reuse it for subsequent reactions until the
catalyst is sufficiently deactivated. Thus, the ability to recycle the SRGO–Pd catalysts was
studied for the aminocarbonylation reaction of iodobenzene utilizing 0.5 mol% of these
catalysts at 200 °C for 6 h under solvothermal conditons. After each reaction, the catalyst was
recovered by simple washing with DMF and re-used in a subsequent run.
The preferred position for Fig. 6.
As shown in Fig. 6, the results indicated that SRGO–Pd can be reused 8 times without
significant loss in the catalytic activity. In order to address priority of the SRGO-Pd over
other catalysts, control experiments (the aminocarbonylation reaction of iodobenzene using
commercialized 10 wt% Pd/C, activated carbon/Pd and SRGO-Pd as the catalysts) were
performed under solvothermal conditons. (Fig. S7) Analysis of the reaction mixture by
ICP-MS after the 8th run also displayed a very low palladium leaching of 107 ppm. The
results demonstrated that the DMF solvothermal approach could efficiently prevent leaching
of Pd. TEM images of SRGO-Pd200 after 0 (a), 4 (b) and 8 (c) runs are shown in Fig. S8. As
can be seen from Fig. S8, the size of Pd nanoparticles on the surface of the graphene sheets
slightly increased with an increasing recycling time. However, well distributed Pd
nanoparticles on the graphene sheets can still observed after eight runs. (Fig. S8 (c)) The
results indicate that DMF plays an important role in improving the dispersibility of Pd. This is
the reason why the SRGO-Pd catalyst could be recycled at least 8 times with sustained
activity.
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4. Conclusions
In conclusion, we here propose a simple yet general DMF solvothermal strategy for
applications ranging from the fabrication of palladium nanoparticle catalysts supported on
reduced graphene nanosheets to the use as aminocarbonylation catalysts. Our experiments
demonstrated that the prepared SRGO-Pd dispersions were stable for at least one month
without any surfactant/polymer stabilization. In the process, the DMF played versatile roles:
as a green reactive medium, a reductant and a stabilizer. DMF solvothermal hydrolysis
reactions can take place under these conditions leading to the generation of
aminocarbonylation products. A simple and highly efficient protocol for a ligand- and
CO-free aminocarbonylation of aryl iodides or bromides catalyzed by SRGO-Pd catalyst in
DMF under solvothermal conditions was thus demonstrated. The catalyst could be readily
recovered and reused. We expect it to be widely used in applications of graphene-based
catalyst preparation and coupling transformations.
Acknowledgements
Ying Zhang and Huaming Sun contributed equally to this work. We acknowledge the 111
Project (B14041), Innovative Research Team in University of China (IRT1070), the grant
from National Natural Science Foundation of China (21271124, 21446014), the Fundamental
Research Funds for the Central Universities (No. GK201302015) and National Training
Programs of Innovation and Entrepreneurship for Undergraduates (201410718005) for
financial support. We also thank Prof. Weiqiang. Zhang, Prof. Guofang. Zhang for useful
discussions.
Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/.
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Legends
Scheme 1. Representation of SRGO-Pd catalyzed aminocarbonylation.
Scheme 2. Proposed mechanism for SRGO-Pd catalyzed aminocarbonylation.
Fig. 1. TEM images of SRGO-Pd160 (a), SRGO-Pd180 (b), SRGO-Pd200 (c), Pd200 without
GO.
Fig. 2. XRD patterns of graphite (A), GO (B), SRGO200 (C), and SRGO-Pd200 (D) samples.
Fig. 3. (A) XPS survey spectrum of GO and SRGO-Pd, and XPS spectra of (B) C1s region of
GO, (C) C1s region of SRGO-Pd and (D) Pd-3d electron of SRGO-Pd.
Fig. 4. Mechanism of the formation of SRGO-Pd.
Fig. 5. Effect of temperature on the reaction.
Fig. 6. Recycling test for reactions.
Table 1. SRGO-Pd-catalyzed aminocarbonylation of aryl halidesa.
Table 2. Effect of Pd loading on reaction
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Scheme 1. Representation of SRGO-Pd catalyzed aminocarbonylation.
21
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Scheme 2. Proposed mechanism for SRGO-Pd catalyzed aminocarbonylation.
22
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Fig. 1. TEM images of SRGO-Pd160 (a), SRGO-Pd180 (b), SRGO-Pd200 (c), Pd200 without
GO.
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Fig. 2. XRD patterns of graphite (A), GO (B), SRGO200 (C), and SRGO-Pd200 (D) samples.
24
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Fig. 3. (A) XPS survey spectrum of GO and SRGO-Pd, and XPS spectra of (B) C1s region of
GO, (C) C1s region of SRGO-Pd and (D) Pd-3d electron of SRGO-Pd.
25
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O
C
CH3
CH3
CH3
CH3
H
N
CO
H
N
CH3
NH2
H3C
HOOC
COO
NH2
C
HOOC
OOC
CH3
H3
Pd2+
DMF solvothermal process
H3C
CH3
NH2
OOC
HO
O
OH
OH
COOH
HOOC
COONH2
H3C
OH
COO NH2
COOH
CH3
H3C
CH3
Pd nanoparticle
Fig. 4. Mechanism of the formation of SRGO-Pd.
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Fig. 5. Effect of temperature on the reaction.
27
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Fig. 6. Recycling test for reactions
28
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Table 1. SRGO-Pd-catalyzed aminocarbonylation of aryl halidesa.
O
0.5 mmol% SRGO-Pd
DMF, 200°C
R-X
R
N
Yieldb
(%)
Entry
1
Aryl halides
Product
O
CH3
I
90
72
67
58
62
N
N
CH3
O
CH3
CH3
Br
2
3
4
5
O
I
CH3
N
CH3
H3CO
H3CO
O
I
CH3
N
CH3
H3C
H3C
O
CH3
CH3
I
N
aReaction conditions: aryl halides (2 mmol), SRGO–Pd (0.5 mol%, 3 mg), DMF (10 mL),
solvothermal heating at 200 °C for 6 h. bIsolated yield (%)
29
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