N, N -Dimethylation of nitrobenzenes with CO2 and water by electrocatalysis

Article abstract of DOI:10.1039/c7sc01058c

We have proposed a strategy for the synthesis of N,N-dimethylanilines from nitrobenzene and its derivatives, CO₂, and water via an electrochemical reaction under ambient conditions. H⁺ generated from H₂O was used as the hydrogen source. Pd/Co-N/carbon, in which the Pd nanoparticles were supported on Co-N/carbon, was designed and used as the electrocatalyst. It was found that the electrocatalyst was very efficient for the reaction in MeCN solution with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim]Tf₂N) as the supporting electrolyte and 1-amino-methylphosphonic acid (AMPA) as the thermal co-catalyst. A series of control experiments showed that Pd/Co-N/carbon and AMPA cooperated very well in accelerating the reaction. This synthetic route has some obvious advantages, such as using CO₂ and water as the reactants, ambient reaction conditions, and high yields of the desired products. This opens up a way to synthesize chemicals by the combination of an electrocatalyst and a thermal catalyst with organic compounds, CO₂, and water as the reactants.

Full text of DOI:10.1039/c7sc01058c

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N,N-Dimethylation of Nitrobenzenes with CO2 and Water by
Electrocatalysis
Xiaofu Sun,a,b Qinggong Zhu,a Jiayin Hu,a,b Xinchen Kang,a,b Jun Ma,a Huizhen Liu,a,b and Buxing
Han*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We proposed a strategy to synthesize N,N-dimethylanilines from nitrobenzene and its derivatives, CO2, and water via
electrochemical reaction under ambient conditions. H+ generated from H2O was used as hydrogen source. Pd/Co-
N/carbon, in which Pd nanoparticles were supported on Co-N/carbon, were designed and used as electrocatalysts. It was
found that the electrocatalysts were very efficient for the reaction in MeCN solution with 1-butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide ([Bmim]Tf2N) as the supporting electrolyte and 1-amino-methylphosphonic acid (AMPA)
as thermal co-catalyst. A series of control experiments showed that Pd/Co-N/carbon and AMPA cooperated very well in
accelerating the reaction. This synthetic route has some obvious advantages, such as using CO2 and water as the reactants,
ambient reaction condition, and high yields of the desired product. This opens a way to synthesize chemicals by the
combination of electrocatalyst and thermal catalyst with organics, CO2, and water as reactants.
www.rsc.org/
have also been studied using H2 as reducing agent at high
temperature and high pressure (Fig. S1).17 Obviously, searching
for new reducing agents and performing the methylation
Introduction
Carbon dioxide (CO2) is an abundant, inexpensive, harmless
and renewable C1 resource.1,2 CO2 chemistry has emerged as
one of the most significant branches of chemistry. Owing to its
thermodynamic stability and kinetic inertness, well designed
activation of CO2 and thermodynamic driving force are
required for efficient conversion.3ꢀ5 Transformation of CO2 into
different chemicals have been reported, such as direct
hydrogenation of CO2 to organic acids and alcohols.6,7 In
parallel, direct introduction of CO2 into organic substrates is
also an attractive goal in modern chemistry,8ꢀ11 and exploration
of new routes is very interesting.
NꢀMethylation and N,Nꢀdimethylation reations are very
important in chemical industry, which can obtain a series of
useful intermediates for the synthesis of many valuable
products, such as dyes, pesticides and perfumes.12ꢀ20
Traditionally, they can be synthesized through the methylation
of amines with methanol or formaldehyde.19,20 Using CO2 as C1
reaction of nitrobenzenes under mild conditions are desirable.
Electrochemical method has been used to synthesize
organic molecules from the past century.21ꢀ25 It possess some
obvious advantages, such as mild conditions, high functional
group tolerance, and innate scalability and sustainability.21
Water is an ideal hydrogen source. H+ can be obtained via the
oxygen evolution reaction (OER) at the anode electrode and
tranferred to cathode electrode.26 It is well known that the
efficiency and selectivity of electrochemical reactions depend
strongly on the properties of electrodes and electrolytes, and
their coupling.22 Therefore, designing a suitable electrocatalyst
with abundant active sites and high electrical conductivity is the
key to promoting electrocatalytic reactions.
Use of CO2 and water as reactants simultaneously in
organic reactions is very attractive. Herein, we developed a
strategy for synthesis of N,Nꢀdimethylanilines by N,Nꢀ
dimethylation of nitrobenzene or its derivatives with CO2 using
H2O as the hydrogen source. In the reactions, electrochemical
reaction and thermal reaction were combined with Pd/Coꢀ
N/carbon as electrocatalyst and 1ꢀaminoꢀmethylphosphonic
acid (AMPA) as thermal catalyst. It was demonstrated that
Pd/CoꢀN/carbon and AMPA had exellent synergistic effect and
the reactions proceeded efficiently under ambient condition and
high yields of the desired product could be reached. As far as
we know, this is the first work on synthesis N,Nꢀ
dimethylanilines using nitrobenzene (or its derivatives), CO2,
and water as reactants.
resource is
a very promising route to synthesize N,Nꢀ
dimethylanilines. In general, H2 or PhSiH3 has been used as
reducing agent for the Nꢀmethylation reaction of anilines.12ꢀ18
Anilines are usually synthesized by the hydrogenation of
nitrobenzenes. Recently, direct Nꢀmethylation of nitrobenzenes
a.Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid and
Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, P.R.China, E-mail: hanbx@iccas.ac.cn
b.University of Chinese Academy of Sciences, Beijing 100049, P.R.China
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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temperature and the ratio of sodium citrate (stabilizing agent) to
PdCl2. The size distributions of the Pd nanoparticles in different
catalysts are shown in Fig. S3, which were obtained from
counting more than 200 particles in the transmission electron
microscopy (TEM) images. The support had specific surface
area of 380 m2 gꢀ1 determined by N2 adsorption/desorption
method. The elemental analysis of CoꢀN/carbon support was
conducted by inductively coupled plasma optical emission
spectroscopy (ICPꢀOES). The results showed that the contents
of Co, C and N element in the support are 37.69 wt%, 30.73
wt% and 1.05 wt%, respectively. The surface composition of
the support detected by Xꢀray photoelectron spectroscopy (XPS)
is given in Table S1.
Results and discussion
The Hꢀtype reaction cell used was composed of a cathode,
a platinum anode, and an Ag/Ag+ reference electrode (Fig. S2),
which was similar to that utilized in the previous works for
electroreduction of CO2.27ꢀ31 The cathode and anode
compartments were separated by a Nafion 117 proton exchange
membrane (PEM). H2SO4 aqueous solution was used as anodic
electrolyte. In the reaction, H+ from water could be transferred
from anode compartment to cathode compartment through the
PEM, which acted as the hydrogen source for the reaction. The
detailed description of the apparatus is given in the ESI.
Fig.
1 shows highꢀresolution transmission electron
microscopy (HRꢀTEM) images of Pd2.2/CoꢀN/carbon. It can be
seen that the Pd nanoparticles were uniformly deposited on the
CoꢀN/carbon support (Fig. 1A). Elemental distribution
mappings also illustrate the coꢀexistence of Pd and Co elements
and Pd nanoparticles were dispersed on the support
homogeneously (Fig. 1B). The typical Pd (111) plane with the
characteristic lattice spacing of 0.23 nm could be observed (Fig.
1C). XPS spectra (Fig. 1D) showed the chemical nature of
catalyst, including the peaks belonging to Pd0 (Pd 3d: 340.7 and
335.4 eV), Pd2+ (Pd 3d: 343.0 and 336.9 eV), CoꢀN (Co 2p:
780.1 eV), CoꢀO (Co 2p: 795.4 eV), pyridinic N (N 1s: 399.5
eV) and graphitic N (N 1s: 402.2 eV).36,38 The results provided
direct evidence that most Pd nanoparticles existed in the form
of Pd0, and some electrochemical active sites for CO2 reduction
such as pyridinic N and CoꢀNx moieties39,40 existed in the
support. Furthermore, the actual Pd loadings in all catalysts
were 18.3 ± 0.6 wt%, as measured by ICPꢀOES as listed in
Table S2. TEM and HRꢀTEM images of the other Pdx/Coꢀ
N/carbon catalysts with different Pd sizes are shown in Fig. S4.
To prepare the electrodes, Pdx/CoꢀN/carbon catalysts were
suspended in acetone with Nafion Dꢀ521 dispersion to form a
homogeneous ink with the aid of ultrasound, which was spread
onto carbon paper (CP) to obtain the working electrodes. Ionic
liquids (ILs) are efficient supporting electrolytes for the
reduction of CO2.27,41 The initial screening was performed for
the methylation reaction of nitrobenzene 1a with CO2 and H+
from H2O over Pd2.2/CoꢀN/carbon using MeCN containing 0.5
Fig. 1 Structural and elemental analysis of Pd2.2/Co-N/carbon. A)
TEM image of Pd2.2/Co-N/carbon. B) Corresponding elemental
mappings of Pd2.2/Co-N/carbon (scale bar: 50 nm). C) HR-TEM
image of Pd2.2/Co-N/carbon. The scale bar of inset of C is 1 nm. D)
XPS spectra of Pd 3d, Co 2p and N 1s orbits of Pd2.2/Co-N/carbon.
Pd is an efficient electrocatalyst for the reduction of CO2
to CO or formate.32,33 It is known that nanoporous carbon
materials have high specific surface area, high chemical and
thermal stability, and good conductivity.34,35 Zeolitic
imidazolate frameworks (ZIFs) have been reported to be
outstanding carbon precursors.36,37 They can incorporate N
atoms and metal species into the carbon lattice, which can
enhance electric conductivity and electronꢀdonor tendency, and
act as the hard template during the carbonization process.36
Meanwhile, the residual metals can also act as coꢀcatalyst. We
designed a new electrocatalysts Pd/CoꢀN/carbon using ZIF as
carbon and nitrogen precursors, and detailed preparation
procedures are given in the SI. Very briefly, CoꢀN/carbon
support was first prepared by the carbonization of the Coꢀ
ZIF/graphene oxide (GO) at 800 oC under an Ar
atmosphere.The Pdx/CoꢀN/carbon electrocatalysts were
obtained by immobilizing Pd nanoparticles on the support, in
which x denotes the average size of Pd particles in nm. The size
of Pd nanoparticles could be controlled by the reduction
M
1ꢀbutylꢀ3ꢀmethylimidazolium bis(trifluoromethylsulfonyl)
imide ([Bmim]Tf2N) as the electrolyte in the presence of
AMPA. As expected, the reduction potential was crucial for the
conversion of 1a (Table 1, entries 1ꢀ6). The most effective
potential was ꢀ2.3 V vs. Ag/Ag+, and afforded full conversion
o
of 1a with a 92 % yield of N,Nꢀdimethylaniline 1d in 30 C.
o
Lowering the reaction temperature to 20 C resulted in a yield
of only 65 % after 10 h (Table S3). Increasing the temperature
o
o
o
to 40 C, 50 C and 60 C resulted in 81 %, 72% and 58 %
yields of 1d (Table S3) due mainly to the lower solubility of
CO2 in the electrolyte. Therefore, 30oC is an optimal
temperature. Table 1 (entries 5, 7ꢀ10) and Fig. S5 indicated that
reactant 1a could be converted completely after 10 h.
Subsequently, conversion of 1a was also studied using MeCN
containing other ILs as the supporting electrolytes, but their
performances were not as good as [Bmim]Tf2N (Table 1,
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entries 5, 11ꢀ16). Furthermore, the solutions of [Bmim]Tf2N 10). Diꢀsubstituted nitrobenzene such as 5ꢀnitroꢀmꢀxylene (12a),
with AMPA in DMSO, DMF, MeNO2 and 1,4ꢀdioxane were 2,6ꢀdimethylnitrobenzene (13a) and 3ꢀmethylꢀ4ꢀnitroanisole
also used as the electrolytes, and lower yields of 1d were
(14a) also showed high reactivity, which gave the desired
obtained (Table S4). The results indicated that [Bmim]Tf2N in products in 75ꢀ82 % yields (Table 2, entries 11ꢀ13). To our
MeCN solution was the best electrolyte.
delight, N,Nꢀdimethylation of benzonitrile and its derivatives
could also be carried out with moderate yields (Table S5).
Table 1. Electrocatalytic methylation of nitrobenzene with CO2 and
water over Pd2.2/Co-N/carbon.[a]
Table 2. Electrocatalytic methylation of substituted nitrobenzenes
with CO2 and water over Pd2.2/Co-N/carbon.[a]
Entry
1
Substrates
Products
Yield / %[b]
75
2a
2d
Entry
1
Electrolyte[b]
[Bmim]Tf2N
[Bmim]Tf2N
[Bmim]Tf2N
[Bmim]Tf2N
[Bmim]Tf2N
[Bmim]Tf2N
[Bmim]Tf2N
[Bmim]Tf2N
[Bmim]Tf2N
[Bmim]Tf2N
[Bmim]PF6
E / V[C]
-1.9
-2.0
-2.1
-2.2
-2.3
-2.4
-2.3
-2.3
-2.3
-2.3
-2.3
-2.3
-2.3
-2.3
-2.3
-2.3
t / h
10
10
10
10
10
10
1
Yield / %[d]
13
36
68
89
92
92
7
74
71
2
3
3a
2
3d
3
4
4a
4d
5
6
7
90
87
82
82
86
4
5
6
7
8
5a
6a
7a
5d
8
5
56
85
92
81
82
85
23
15
30
9
8
10
11
12
13
14
15
16
12
10
10
10
10
10
10
6d
7d
[Bmim]BF4
[Bmim]TfO
[Bmim]ClO4
[Bmim]NO3
[Bmim]H2PO4
N
8a
Cl
8d
[a] Reaction conditions: nitrobenzene (1.0 mmol), AMPA (0.06 mmol), CO2 (0.1
MPa), temperature (30 oC); [b] Electrolyte (30 mL) is CO2-saturated MeCN
containing 0.5 M IL; [c] All potentials are reported with respect to Ag/Ag+. [d]
Yield determined by 1H NMR spectroscopy.
9a
9d
Fig. S6 summarizes the yields of 1d from the
electrochemical conversion of 1a over the Pdx/CoꢀN/carbon
catalysts with different Pd particle sizes at different potentials.
The electronic properties of nanoparticles can be tuned by
controlling the sizes.42,43 Generally, smaller nanoparticles have
the lower dꢀband center, which in turn results in a decrease in
its adsorption energy.44 With respect to the reduction of CO2,
lowering of the dꢀband center reduces the binding energy of
hypothetical intermediates,45 thereby enhancing the rate of CO2
reduction and the conversion of nitrobenzene.
78
9
10a
10d
76
78
10
11
11d
11a
12a
NO2
12d
The methylation of various substituted nitrobenzenes over
Pd2.2/CoꢀN/carbon were investigated (Table 2). The electronꢀ
donating methyl, methoxy and methylthio groups in the 4ꢀ
position of aromatic ring of 1a reduced the reactivity with
yields of 75 % (2d), 74 % (3d) and 71 % (4d), respectively
(Table 2, entries 1ꢀ3). For halogenꢀsubstituted nitrobenzenes,
the desired products were obtained in 82ꢀ90 % yields (Table 2,
entries 4ꢀ8). When 4ꢀnitrobiphenyl 10a and 2ꢀnitrobiphenyl 11a
were used, the corresponding products were generated in 78 %
82
75
12
13
13d
13a
14a
14d
(
10d) and 76 % (11d) yields, respectively (Table 2, entries 9ꢀ
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[a] Reaction conditions: substituted nitrobenzene (1.0 mmol), AMPA (0.06 mmol),
can be adsorbed on the electrode surface and the CO2 is
ꢀ
CO2 (0.1 MPa), electrolyte (30 mL, CO2-saturated MeCN containing 0.5
M
reduced to CO2• , which forms COads after receiving the second
[Bmim]Tf2N), -2.3 V vs Ag/Ag+, 30 oC, 10 h; [b] Yield determined by 1H NMR
electron. Pd nanoparticles facilitate these processes.32 After
accepting an electron and proton, COads can be adsorbed on the
spectroscopy.
support, and pyridinic N as well as CoꢀNx moieties can further
Some control experiments were conducted in order to get
some evidences to study the mechanism. Fig. S7 shows that
very small amount of AMPA could promote the reaction for
producing N,Nꢀdimethylaniline efficiently. However, only
aniline was formed in the absence of AMPA (Fig. S8), and
aniline could not be further converted even extending the
reaction time (Table S6). These results indicate that the AMPA
is necessary and acts as a coꢀcatalyst. To further understand the
role of AMPA, NMR analysis was performed on aniline and its
mixture with AMPA, respectively (Fig. S9), it was found that
1H signal of NꢀH in aniline shifted downfield from 4.88 to 5.11
31,39ꢀ40
drive COads to generate CHOads
.
The next step is the
insertion of CHOads into ArNH2Pd* to form adsorbed Nꢀ
phenylformamide in the presence of the Brønsted base, AMPA.
ArNH2Pd* can be obtained through the pathway of
nitrobenzene → nitrosobenzene → Nꢀphenylhydroxylamine →
aniline on the electrode surface. It is usually observed in the
electrochemical reduction of nitrobenzene to aniline.47 The
adsorbed Nꢀphenylformamide can be quickly hydrogenated into
Nꢀmethylaniline. Following this, Nꢀmethylaniline can be
adsorbed onto the electrode surface and reacted with CHOads to
form the adsorbed NꢀmethylꢀNꢀphenylformamide. Finally, the
protonation of the adsorbed NꢀmethylꢀNꢀphenylformamide
leads to the formation of N,Nꢀdimethylaniline.
To further verify the reaction mechanism proposed, we
conducted the experiments using some intermediates in Fig. 2
as starting reactants, including aniline, Nꢀphenylformamide, Nꢀ
methylaniline and NꢀmethylꢀNꢀphenylformamide (Fig. S10).
92 % N,Nꢀdimethylaniline, 5 % Nꢀmethylaniline and a small
amount of aniline, Nꢀphenylformamide and NꢀmethylꢀNꢀ
phenylformamide can be detected in reaction (I). In reaction (II),
the products contained 93 % N,Nꢀdimethylaniline, about 3 %
1
ppm due to mixing with AMPA. The shift of H signal of NꢀH
in aniline was also affected by the amount of AMPA and the
result was consistent with the reaction result that the yield of
the product increased with the increase of the amount of AMPA
from 0 to 0.06 mmol (Fig. S7). From these results, it can be
concluded that AMPA as a Brønsted base could activate the
proton on the aniline to promote the formation of N,Nꢀ
dimethylaniline. In this way, the C atom on the intermediates
obtained from CO2 electroreduction could show an easier
electrophilic attack to the N atom on aniline, i.e. AMPA acted
as the coꢀcatalyst to promote the reaction of aniline with the
intermediate from CO2 electroreduction to form Nꢀ
phenylformamide or NꢀmethylꢀNꢀphenylformamide. As a result,
the next reaction that NꢀmethylꢀNꢀphenylformamide reacted
with H+ from water to synthesize N,Nꢀdimethylaniline could
proceed successfully. More details will be discussed in the
mechanism part below.
Nꢀmethylaniline as well as
a
small amount of Nꢀ
phenylformamide and NꢀmethylꢀNꢀphenylformamide. The
results were similar to those in reaction (I), suggesting that
nitrobenzene was first converted to aniline completely in a
short time. In reactions (III) and (V), when Nꢀphenylformamide
or NꢀmethylꢀNꢀphenylformamide was used as the strating
reactant, the desired products Nꢀmethylaniline or N,Nꢀ
dimethylaniline could be obtained. At the same time, most N
-
methylaniline could be transformed to N,Nꢀdimethylaniline
with the aid of AMPA in reaction (IV). In addtion, a small
amount
of
Nꢀphenylformamide
or
NꢀmethylꢀNꢀ
phenylformamide as reaction intermediates could be also
detected in reactions (III) and (IV). All these evidences support
the proposed reaction mechanism.
Conclusions
In conclusion, N,Nꢀdimethylanilines can be synthesized by
methylation of nitrobenzene (or substituted nitrobenzenes) with
CO2 and water. Pd/CoꢀN/carbon electrode and the electrolyte
composed of MeCN, [Bmim]Tf2N, and AMPA is very efficient
electrolysis system at ambient condition. The N,Nꢀ
dimethylation of a range of substrates, including nitrobenzene,
substituted nitrobenzene and benzonitrile, can proceed
smoothly with satisfactory yields. In the reaction, Pd/Coꢀ
N/carbon catalyzes the reduction of nitrobenzene to aniline
quickly, which is further converted to the N,Nꢀdimethylaniline
catalyzed cooperatively by electrocatalyst Pd/CoꢀN/carbon and
thermal catalyst AMPA. We believe that this route has potential
of application for producing N,Nꢀdimethylanilines due to some
Fig. 2. Possible pathway for electrochemical reaction of
nitrobenzene with CO2 and water to form N,N-dimethylaniline of
over Pd/Co-N/carbon.
On the basis of the experimental results and the related
knowledge in the literature, we propose
a speculative
mechanism for N,Nꢀdimethylation of nitrobenzene, which is
shown schematically in Fig. 2. Two parallel processes, CO2 and
nitrobenzene hydrogenation occur in the cathode compartment
in the first step. For CO2 activation and reduction, a [Bmimꢀ
CO2]+ complex can form quickly via the hydrogen bonding
interaction between CO2 and IL cation,46 which can reduce the
reaction barrier for electron transfer to CO2.27 [BmimꢀCO2]+
obvious
advantages.
In
addition,
combination
of
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electrocatalysts and thermal catalysts can realize other reactions N/carbon with H2 for 12 h in 0.1 M HCl aqueous solution at 25
that cannot be conduct with only electrocatalysts or thermal oC.
catalysts.
Materials characterization
XPS analysis was performed on the Thermo Scientific
ESCALab 250Xi using 200 W monochromatic Al Kα radiation.
The 500 ꢁm Xꢀray spot was used. The base pressure in the
analysis chamber was about 3×10ꢀ10 mbar. Typically, the
hydrocarbon C1s line at 284.8 eV from adventitious carbon was
used for energy referencing. The actual loading of Pd in the
catalysts was determined by ICPꢀOES (VistaꢀMPX). The N2
adsorption/desorption isotherms of the CoꢀN/carbon nanosheets
were determined using a Quadrasorb SIꢀMP system. The
microstructures of the catalysts were studied using JEOLꢀ
2100F HRꢀTEM operated at 200 kV.
Preparation of electrode and electrochemical reaction
To prepare an electrode, the corresponding Pdx/CoꢀN/carbon
catalyst was suspended in acetone with Nafion Dꢀ521
dispersion to form a homogeneous ink assisted by ultrasound,
which was spread onto carbon paper (CP: 1×1 cmꢀ2) to obtain
the working electrodes. The loading of catalyst was 2.0 ± 0.1
mg cmꢀ2. Before experiment, all the auxiliary electrodes were
sonicated in acetone for 3 min and then washed with water and
ethanol, followed by drying in N2 atmosphere.
An electrochemical workstation (CHI 6081E, Shanghai
CH Instruments Co., China) was used in all the experiments.
The electrolysis experiments were conducted in a typical Hꢀ
type cell that was similar to that used in the previous works,28,29
which is schematically shown in Fig. S2. It consisted of a
cathode (working electrode), an anode (platinum gauze
auxiliary electrode), and an Ag/Ag+ reference electrode. The
cathode and anode compartments were separated by a Nafion
117 proton exchange membrane. 0.5 M H2SO4 aqueous solution
was used as anodic electrolyte. H+ can be transferred from
anode compartment to cathode compartment through Nafion
117 proton exchange membrane, which is the hydrogen source.
Under the continuous stirring, CO2 was bubbled through the
catholyte (2 mL/min) for 30 min before electrolysis. Then, the
electrolysis experiments were carried out with CO2 bubbling (2
Experimental section
Preparation of Co-ZIF/GO
In this work, GO was prepared by modified Hummer method
according to the reference.48 1.2 mmol of Co(NO3)2
•6H2O and
1 mmol of 2ꢀmethylimidazole were dissolved in 12 mL and 20
mL of methanol, respectively. Co(NO3)2 solution was added
into the 2ꢀmethylimidazole solution to obtain a clear purple
solution under continuous stirring. Then, 10 mL of GO
dispersion (10 mg GO in 10 mL methanol/water, v/v, 4:1) was
immediately added to the above purple solution. After stirring
for 3 h, the precipitation was collected by centrifugation and
o
washing with methanol, followed by drying at 50 C for 10 h.
CoꢀZIF/GO was obtained.
Preparation of Co-N/carbon support
The resulting CoꢀZIF/GO precursor was heated in the tube
furnace at 10 °C minꢀ1 in argon (Ar) medium to 800 °C and
kept at this temperature for 3 h. Then the composites were
immersed in 2 M HCl aqueous solution for 24 h. The
precipitation was collected by centrifugation and washing with
water, followed by drying at 80 oC for 24 h. CoꢀN/carbon
support was obtained, of which the elemental composition is
given in Table S1.
Preparation of Pdx/Co-N/carbon catalysts
Pd/CoꢀN/carbon catalysts were synthesized with sodium
borohydride as a reductive agent, sodium citrate as a stabilizing
agent and CoꢀN/carbon as support. The sizes of the Pd
nanoparticles in the catalysts were controlled by varying the
ratio of sodium citrate to PdCl2 as well as the reduction
temperature.32 The average particle size was determined by
counting more than 200 particles from TEM images. Herein,
preparation of Pd2.2/CoꢀN/carbon is described in detail. 0.5
mmol of PdCl2 in 0.1 M HCl solution and 4 mmol sodium
citrate were dissolved in 200 mL water under continuous
stirring for 30 min. Then, 200 mg asꢀprepared CoꢀN/carbon
support was added into the mixture with sonication for 1 h.
Then, 50 mL of 0.1 M sodium borohydride aqueous solution
mL/min). In
nitrobenzene and 0.06 mmol AMPA were added into 30 mL
electrolyte (CO2ꢀsaturated MeCN containing 0.5
a
typical reaction procedure, 1.0 mmol
M
o
o
[Bmim]Tf2N). The reaction was performed at 30 C for desired
time under magnetic stirring. After the reaction, MeCN in the
system was removed by rotary evaporation and the IL was
extracted by ether. The isolated product was obtained and
purified by column chromatography using petroleum
ether/ethyl acetate (150:1).
was added into the above suspension dropwise at 0 C under
vigorous stirring to reduce the Pd2+. After stirring for 8 h, the
black precipitation was collected by centrifugation and washing
with water for 3 times, Pd2.2/CoꢀN/carbon was obtained after
freeze drying.
The other Pd/CoꢀN/carbon catalysts were prepared by
controlling the reduction temperature and adjusting the ratio of
sodium citrate (stabilizing agent) to PdCl2. For Pd3.6/Coꢀ
N/carbon and Pd4.5/CoꢀN/carbon, the temperatures to reduce
Pd2+ were 25 oC and 60 oC, respectively, and the ratio of
sodium citrate to PdCl2 was the same as that for preparing
Pd2.2/CoꢀN/carbon. For Pd6.4/CoꢀN/carbon and Pd7.9/Coꢀ
N/carbon, the ratios of sodium citrate to PdCl2 were 2 and 0,
respectively, and the reduction temperature was 25 oC.
Pd10.2/CoꢀN/carbon was obtained by bubbling Pd3.6/Coꢀ
Product analysis
The gaseous product of electrochemical experiments was
collected and analyzed by gas chromatography (GC, HP
4890D), which was equipped with FID and TCD detectors
using helium as the internal standard. For methylation of
nitrobenzene, the liquid product was analyzed by 1H NMR
(Bruker Avance III 400 HD spectrometer) in chloroformꢀd with
TMS as an internal standard.
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DOI: 10.1039/C7SC01058C
ARTICLE
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29 Q. Zhu, J. Ma, X. Kang, X. Sun, H. Liu, J. Hu, Z. Liu and B.
Han, Angew. Chem. Int. Ed., 2016, 55, 9012.
Acknowledgements
30 S. Zhang, P. Kang, S. Ubnoske, M. K. Brennaman, N. Song,
The authors thank the National Natural Science Foundation of
China (21533011, 21403253, 21673248) and Chinese Academy
of Sciences (QYZDYꢀSSWꢀSLH013).
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