N/O-doped carbon as a "solid ligand" for nano-Pd catalyzed biphenyl- and triphenylamine syntheses

Article abstract of DOI:10.1039/c7cy00231a

A series of N/O-doped porous carbon supported nanopalladium catalysts have been successfully prepared, in which the N/O doped carbons were controllably produced via polypyrrole/furan synthesis followed by carbonization. These catalysts exhibit good performance in biphenylamine and triphenylamine syntheses with nitrobenzene and cyclohexanone as starting materials. Their catalytic activity can be tuned efficiently by the N/O functional groups on the carbon surface. TEM, XRD, XPS and laser Raman methods were applied to probe the structure of these catalysts. These results indicate that the Pd nanoparticles were supported on N/O-doped porous carbon via the "coordination" between Pd nanoparticles and N/O functional groups including O-CO, CN and tertiary nitrogen, and better catalytic performance was obtained if carbon with the highest N-species loading was used as the support. In addition, a mechanistic study proved that the reaction starts with the catalytic reduction of nitrobenzene with cyclohexanone as the hydrogen source. During this reaction, aniline was formed and the cyclohexanone was transformed into phenol. Then biphenylamine and triphenylamine were generated through the reaction of aniline and cyclohexanone. This work should facilitate the controllable preparation of carbon supported nanocatalysts with specific activity, and open up a promising pathway for the development of new methodologies for N-containing fine chemical synthesis.

Full text of DOI:10.1039/c7cy00231a

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PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
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N/O-Doped Carbon as “Solid Ligand” for Nano-Pd Catalyzed
Biphenyl- and TriphenylAmine Synthesis
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a,b
a
a
*a
Shaofeng Pang, Yujing Zhang, Yongji Huang, Hangkong Yuan and Feng Shi
DOI: 10.1039/x0xx00000x
A series of N/O-doped porous carbon supported nanopalladium catalysts have been successfully prepared, in which the
N/O doped carbons were controllably produced via polypyrrole/furan synthesis and followed by carbonization. These
catalysts exhibit nice performance in biphenylamine and triphenylamine synthesis with nitrobenzene and cyclohexanone
as starting materials. Their catalytic activity can be tuned efficiently by the N/O functional groups on the carbon surface.
TEM, XRD, XPS and Laser Raman methods were applied to probe the structure of these catalysts. These results indicate
that the Pd nanoparticles were supported on N/O-doped porous carbon via the "coordination" between Pd nanoparticles
and N/O functional groups including O-C=O, C=N and tertiary nitrogen, and better catalytic performance was obtained if
carbon with the highest N-species loading was used as support. In addition, mechanistic study proves that the reaction
starts with the catalytic reduction of nitrobenzene with cyclohexanone as the hydrogen source. During this reaction,
aniline was formed and the cyclohexanone was transformed into phenol. Then biphenylamine and triphenylamine were
generated through the reaction of aniline and cyclohexanone. This work should facilitate the controllable preparation of
carbon supported nanocatalysts with specific activity, and open up a promising pathway for the development of new
methodologies for N-containing fine chemical synthesis.
www.rsc.org/
focused on the exploitation and development of active
heterogeneous catalysis systems to integrate the high activity
of homogeneous catalysis and easy separability of
1
. INTRODUCTION
The controllable preparation of heterogeneous catalyst from
1
molecular level has been intensively pursued by chemists,
4
1-48
-11 heterogeneous catalysis.
Unfortunately, it remains a
challenge in catalysis study. Inspired by the discussions above,
the development of controllable preparation of carbon
while it is difficult to be achieved due to the complicated
structure of traditional inorganic supports. Nano-carbon
materials with well-defined molecular structures, including
4
9-56
supports with different N/O functional groups
might
present a potential to build heterogeneous catalysts with
7-60
amorphous
carbon,
ordered
mesoporous
carbon,
2-16
5
1
comparable activity to homogeneous catalysts,
6
because N-
graphite/graphene (oxide) and carbon nanotubes, etc,
1
widely applied in catalytic transformations.
are
It might
1-68
69, 70
7-25
or O-
containing ligands are widely used in
homogeneous reactions such as palladium-catalyzed
dehydrogenation or hydrogenation reactions.
Polypyrrole is an extensively studied polymer that
possesses high nitrogen content and leaves a high amount of
71-74
carbonaceous residue after pyrolysis (yield ≈ 60 wt%),
which then leads to an effective way for N-doped carbon
materials preparation. Nevertheless, to the best of our
knowledge, polyfuran has been less extensively studied.
present a promising way to design and prepare nano-carbon
materials with molecularly defined structure containing
specific elements or functional groups, which can adjust the
properties of the carbon materials as supports. Among them,
N/O-doped nano-carbon materials can be regarded as N/O-
substituted graphite and becomes a promising source for the
production of free-standing nitrogen or oxygen-rich carbon
2
6-30
materials.
As it is well known, the N- and O-functional Oxypolymerization is an efficient way in mixing-polymerization
7
5-79
groups play key roles in homogeneous catalysis and they can of pyrrole and furan,
strongly influence the catalytic efficiency in specific N/O ratios can be obtained followed by carbonization, which
and N,O-rich carbons with tunable
3
1-40
can be used as “solid ligand” to instead of N/O-containing
ligands in catalysts. In this way, a heterogeneous catalysis
system with high activity and selectivity for specific reaction
might be established. In addition, the proportion of N and O
functional groups of the final solid carbons can be adjusted by
changing the ratios of pyrrole and furan in the initial stage. If
nano-palladium particles were further supported on the “solid
ligand”, heterogeneous catalyst with specific activities might
be achieved. In this work, a series of carbon materials with
different N/O ratios were prepared as support or "solid ligand"
reactions.
Over a long time, more works have been
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Scheme 1. Preparation of Pd/C-py, Pd/C-py/fu and Pd/C-fu through the palladium nanoparticles supported on functionalized porous carbons
for nanopalladium, which were denoted as Pd/C-py, Pd/C- washed for several times with HCl (10 wt%) to remove the
py/fu and Pd/C-fu, respectively (Scheme 1)
.
inorganic salts and then washed with deionized water. Finally,
the activated carbon was dried in air at 120 °C and 2.1 g
carbon material was produced with a carbonization ratio of
Arylamines, especially biphenyl- and triphenylamine, are
considerable importance in a variety of industrial processes, so
the development of simple and economic methods for their
preparation is of significant interest. In addition, the
introduction of functional groups with specific patterns around
the aromatic rings of arylamines is at the forefront of fine
chemical synthesis, and it entails a strong potential for
industrial developments. Among the methodologies for
3
5%.
Typical procedure for C-fu preparation: 6 g furan was added
to a chloroform solution of FeCl (0.5 M, 400 mL) and the
3
mixture was magnetically stirred for 24 h to synthesize
polyfuran. The polyfuran was then separated by filtration and
washed with chloroform/methanol solution (V chloroform : V
methanol = 1 : 1) and HCl (10 wt%). The polyfuran was dried in air
at 80 °C, and the polyfuran yield was about 100%. Following,
the polyfuran was activated by heating a polyfuran-KOH
8
0, 81
arylamines synthesis,
the reaction of nitrobenzene and
cyclohexanone, which involves the aniline generation via
hydrogen transferring from cyclohexanone to nitrobenzene
and followed by the coupling reaction of aniline and
cyclohexanone, can be a good choice for the controllable
synthesis of diphenylamine and triphenylamine. So it was
chosen as the model reaction to test the catalytic performance
of the “solid ligand” supported nanopalladium catalysts.
mixture (KOH/Polypyrrole = 2, w/w) under N
2
(99.999%) at 600
−
C for 1 h (heating rate: 3 °C·min ). Finally, the reactor was
1
°
cooled to room temperature under the N2 flow, and C-py was
obtained. The sample was then thoroughly washed for several
times with HCl (10 wt%) to remove the inorganic salts and then
washed with deionized water. Finally, the activated carbon
was dried in air at 120 °C and 1.8 g carbon material was
2
. EXPERIMENTAL SECTION
7
1-74
Typical procedure for C-py preparation :
6 g pyrrole was produced with a carbonization ratio of 30%.
added to a solution of FeCl3·6H2O (0.5 M, 400 mL) and the Typical procedure for C-py/fu-3 preparation: A mixture of 4.5
mixture was magnetically stirred for 2 h to synthesize g pyrrole and 4.5 g furan was added to a chloroform solution
polypyrrole. The polypyrrole was then separated by filtration of FeCl
and washed with deionized water/methanol (V water : V methanol
stirred for 24 h to synthesize polypyrrole/furan. The polymer
: 1) and HCl (10 wt%). The polypyrrole was then dried in air was then separated by filtration and washed with
3
(0.5 M, 400 mL) and the mixture was magnetically
=
1
at 80 °C. The final polypyrrole yield was about 100%. The chloroform/methanol solution (V chloroform : V methanol = 1 : 1) and
polypyrrole was chemically activated by heating a polypyrrole- HCl (10 wt%). The polymer was dried in air at 80 °C and the
KOH mixture (polypyrrole/KOH = 2, w/w) under N
2
flow polymer yield was about 100%. The polymer was activated by
99.999%) at 600 °C for 1 h (heating rate: 3 °C·min ). Finally, heating a polymer-KOH mixture (KOH/Polymer = 2, w/w) under
−1
(
−
1
the reactor was cooled to room temperature under the N
flow, and C-py was obtained. The sample was then thoroughly Finally, the reactor was cooled to room temperature under the
2
2
N (99.999%) at 600 °C for 1 h (heating rate: 3 °C·min ).
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N2 flow, and C-py was obtained. The sample was then Pd/C-py/fu-2, Pd/C-py/fu-3, Pd/C-py/fu-4, Pd/C-py, Pd/C-py-x,
2
thoroughly washed for several times with HCl (10 wt%) to Pt/C-py and Ru/C-py were 745 m /g, 767 m /g, 839 m /g, 906
2
2
2
remove the inorganic salts and then washed with deionized m /g, 1128 m /g, 1571 m /g, 2602 m /g, 1446 m /g and 1870
2
2
2
2
2
water. Finally, the activated carbon was dried in air at 120 °C m /g. The loadings of all catalyst samples were determined by
and 2.1 g carbon material was produced with a carbonization ICP-AES. The metal loadings of Pd/C-fu, Pd/C-py/fu-1, Pd/C-
ratio of 35%.
py/fu-2, Pd/C-py/fu-3, Pd/C-py/fu-4, Pd/C-py, Pd/C-py-x, Pt/C-
Typical procedure for carbon supported nanocatalysts py and Ru/C-py were 7.4 wt%, 7.6 wt%, 7.3 wt%, 7.5 wt%, 7.4
preparation: 0.7 g carbon and 240 mL deionized water were wt%, 7.4 wt%, 7.3 wt%, 6.7 wt% and 7.1 wt%, respectively.
mixed in
temperature for 1 h. Then, 2.1 mL H
added dropwise and the mixture was further stirred for 4 h. by using a Bruker ARX 400 or ARX 100 spectrometer at 400
a
round-bottom flask and stirred at room Elemental analysis (N, C, O, and H) of the samples was carried
2
PdCl (0.282 M) was out on a Vario EL microanalyzer. NMR spectra were measured
4
1
13
Then 3 mL NaOH (5 M) was added dropwise and the mixture MHz ( H) and 101 MHz ( C). All spectra are reported in ppm
was stirred for 2 h. Following, 14 mL formaldehyde solution relative to tetramethylsilane referenced to the residual solvent
(38 wt%) was added as reducing agent at 80 °C. The reaction peaks.
mixture was stirred overnight, and the catalyst was then Synthesis of triphenylamine: 0.5 mmol nitrobenzene, 4 mmol
separated by filtration, and washed with deionized water and cyclohexanone, 30 mg catalyst and 5 mL toluene were added
dried in air at 80 °C for 5 h. The catalyst sample was denoted to a 38 mL pressure tube equipped with a magnetic stirrer and
o
as Pd/C-py/fu-3. Different catalysts were prepared with the reacted at 160 C for 36 h under Ar. Then the reaction mixture
same procedure, and denoted as Pd/C-fu, Pd/C-py/fu-1, Pd/C- was cooled to room temperature. The crude reaction mixture
py/fu-2, Pd/C-py/fu-3, Pd/C-py/fu-4, Pd/C-py, Pd/C-py-x, Pt/C- was concentrated in vacuo and purified by column
py and Ru/C-py. Pd/C-py catalyst being used for three runs was chromatography to give triphenylamine in 96% isolated yield.
denoted as Pd/C-py-R.
Catalyst characterization: Transmission electron microscopy cyclohexanone, 100 mg catalyst and 2 mL toluene were added
TEM) characterization was carried out by using a Tecnai G2 to a 15 mL pressure tube equipped with a magnetic stirrer and
Synthesis of diphenylamine: 0.5 mmol nitrobenzene, 6 mmol
(
o
F30 S-Twin transmission electron microscope operating at 300 reacted at 80 C for 36 h under Ar. Then the reaction mixture
kV. Single-particle EDX analysis was performed by using a was cooled to room temperature. The crude reaction mixture
Tecnai G2 F30 S-Twin Field Emission TEM in STEM mode. For was concentrated in vacuo and purified by column
TEM investigations, the catalysts were dispersed in ethanol by chromatography to give biphenylamine in 82% isolated yield.
ultrasonication and deposited on carbon-coated copper grids. Purification procedure: The crude products were subjected to
XRD measurements were conducted by using a STADIP silica gel column chromatography (60, 230-400 mesh supplied
automated transmission diffractometer (STOE) equipped with by Qingdao Haiyang Chemical and Special Silica Gel Co, Ltd).
an incident beam curved germanium monochromator with Eluting with ~75 mL of Petroleum Ether, followed by 200 : 1
CuKa1 radiation. The catalyst samples were dried in air and (Petroleum Ether : EtOAc).
pressed on a glass slide for analysis. The XRD patterns were
scanned in the 2
ϴ
range of 10–80°. For data interpretation, 3. RESULTS AND DISCUSSION
the software WinXpow (STOE) and the database of powder
diffraction file (PDF) of the International Centre of Diffraction
Data (ICDD) were used. The X-ray photoelectron spectroscopy
Performance of the catalysts. As
a proof-of-concept
application of the “solid ligand” catalyst, such a model reaction
could directly reflect the catalytic activity of the activated
metal nanoparticles with different ratios of oxygen/nitrogen
doped carbon materials. The performance of the prepared
catalysts for synthesis of triphenylamine via the reaction of
nitrobenzene and cyclohexanone was tested first, as shown in
Table 1. Obviously, the Pd/C-py exhibits better catalytic
performance, and no triphenylamine was observed if Pd/C-fu
was used as catalyst (Table 1, entries 1 and 6). With the
increasing of the mass ratios of pyrrole during polymer
synthesis, the catalytic performance can be improved
significantly and 99% nitrobenzene conversion with 96%
triphenylamine yield were obtained when Pd/C-py was used as
the catalyst (entries 1-6). Therefore, the presence of nitrogen
but not oxygen group is essential to gain high activity. To our
delight, the catalyst can be recovered for 3 runs without
obvious catalyst activity decreasing (entry 6). The Pd loading
maintained 7.3 wt% after being used for three runs. It is
almost the same as the fresh catalyst, which is 7.4 wt%,
suggesting no palladium leaching occurred during the reaction.
(XPS) measurements were carried out by using a VG ES-CALAB
2
10 instrument equipped with a dual Mg/Al anode X-ray
+
source, a hemispherical capacitor analyzer, and a 5 keV Ar ion
gun. All spectra were recorded by using nonmonochromatic
MgKa (1253.6 eV) radiation. The samples were fixed to a
stainless steel sample holder by using double-sided adhesive
carbon tape. The electron binding energy was referenced to
the C1s peak at 284.8 eV. The peaks were fitted by Gaussian–
Lorentzian curves after a Shirley background subtraction. For
quantitative analysis, the peak area was divided by the
element-specific Scofield factor and the transmission function
of the analyzer. The background pressure in the chamber was
-7
less than 10 Pa. Raman spectra were measured with 532nm-
Edge by using LabRAM HR Evolution (HORIBA Jobin Yvon
S.A.S.). Nitrogen adsorption–desorption isotherms were
measured at 77 K by using a Quanta chrome autosorb iQ
2
instrument. The pore-size distribution was calculated from the
desorption isotherm by using the Barrett, Joyner, and Halenda
(BJH) method. The BET surface areas of Pd/C-fu, Pd/C-py/fu-1,
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a
a
Table 1. Results of catalyst screening and reaction conditions optimization.
Table 2. EA analysis results of different catalysts.
Entry
Catalyst
Pd/C-fu
N
C
O
H
NO
2
O
1
2
3
4
5
6
7
8
9
0.16
2.15
3.78
3.96
4.85
5.03
8.23
8.01
8.72
70.34
64.51
62.42
59.02
52.77
66.41
62.25
70.92
69.72
19.42
23.31
24.11
25.87
30.89
18.21
18.14
9.37
1.52
1.55
1.71
1.87
2.25
1.48
2.21
2.53
2.61
Cat, Ar, Solvent
+
N
Pd/C-py/fu-1
Pd/C-py/fu-2
Pd/C-py/fu-3
Pd/C-py/fu-4
Pd/C-py-x
Pd/C-py
b
Entry
Catalyst
Pd/C-fu
Py/Fu
Sel.
Con.
Y.
c
d
e
%
%
%
--
1
2
3
4
5
6
0:1
1:4
2:3
1:1
4:1
1:0
--
4
26
49
75
99
99
99/
Pt/C-py
Pd/C-py/fu-1
Pd/C-py/fu-2
Pd/C-py/fu-3
Pd/C-py/fu-4
Pd/C-py
2
Ru/C-py
9.71
22
55
67
17
55
67
96/
a
Elemental analysis (N, C, O, and H, wt%) of the samples was carried out on a
Vario EL microanalyzer.
Catalyst characterizations. In order to reveal the relationship
between the catalyst structure and catalytic performance,
these catalysts were further characterized by TEM, XRD, XPS
and Laser Raman. First, the morphologies and microstructures
of the catalysts were examined by TEM, and the images were
97/
g
g
g
93
99
92
f
7
8
9
Pd/C-py-x
1:0
1:0
1:0
--
79
99
78
Pt/C-py
Ru/C-py
Pd
--
--
--
--
--
--
shown in Figure 1
.
1
1
1
0
1
2
--
--
--
PdO
--
--
--
--
Pd/XC-72R
--
--
17
--
a
0
.5 mmol nitrobenzene, 4 mmol cyclohexanone, 30 mg catalyst, 5 mL toluene,
o
b
1
60 C (reaction temperature), 36 h, Ar. the mass ratios of pyrrole and furan for
c
2
d
carbon materials preparation. BET surface area (m /g). Nitrobenzene
e
f
o
g
conversion determined by GC-MS. Isolated yield. Pyrolyzed at 700 C. The
catalyst was used at the 3rd run.
Thus the N-rich carbon from polypyrrole exhibits the best
performance as “solid ligand” to support nano-palladium in
triphenylamine
synthesis
from
nitrobenzene
and
cyclohexanone. In addition, Pt/C-py and Ru/C-py were
prepared with the same procedures and were used as the
catalysts under the same reaction conditions but no reaction
was observed (entries 8-9). So palladium is the suitable active
species in this reaction. As control reactions, the activity of
PdO and Pd powder (APS 0.25-0.55 micron) were tested under
the same reaction conditions (entries 10-11), and no reaction
was observed, too. Thus, PdO and Pd powder can not be used Figure 1. TEM images of Pd/C-fu (a), Pd/C-py/fu-1 (b), Pd/C-py/fu-2 (c), Pd/C-
py/fu-3 (d), Pd/C-py/fu-4 (e) and Pd/C-py (f). The insets are the corresponding
as catalyst directly. Noteworthy, the better catalytic PSD histograms.
performance of Pd/C-py should not be attributed to its higher
2
It can be seen clearly that the Pd nanoparticles are well
dispersed on the surface of these porous carbon supports, and
no obvious Pd nanoparticles aggregation can be observed.
Statistical analysis of particle size distribution (PSD) was
carried out by measuring the number of Pd nanoparticles on
the picture (insets of Figure 1). The average Pd diameters of
Pd/C-fu, Pd/C-py/fu-1, Pd/C-py/fu-2, Pd/C-py/fu-3, Pd/C-py/fu-
BET surface area, which was 1571 m
/g, because only 78%
triphenylamine yield was obtained if Pd/C-py-x was used as the
2
catalyst, which BET surface area was 2602 m
In consideration of the EA analysis results of different catalysts
Table 2), it can be imagined that the presence of suitable
/g (entries 6-7).
(
amount of N/O groups should be the key factor for the high
activity. Finally, Pd/XC-72R was prepared with a commercial
active carbon (XC-72R) as support. However, only small
amount of biphenylamine was generated and almost no
triphenylamine was observed when Pd/XC-72R was used as
the catalyst. It can be imagined that the presence of N-groups
on porous carbons is essential to gain the high activity. The N-
groups might behave as the ligand to modulate the activity of
nano-palladium.
4
, Pd/C-py and Pd/C-py-R were 3.93 nm, 3.51 nm, 2.89 nm,
.83 nm, 2.8 nm, 2.3 nm and 2.3 nm, respectively. A more
2
detailed analysis on PSD reveals that with the increase of
nitrogen content of supports, the proportion of Pd
nanoparticles sizes within 1-4 nm increased (from 72% to
9
2%). It may be indicated that a stronger metal-support
interaction (SMSI) between Pd nanoparticles and the nitrogen
functional groups compared to oxygen functional groups.
4
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Energy dispersive X-ray spectroscopy (EDX) elemental line indicates the stability of the catalyst during reaction (Figure
scans over different catalysts (Figure S2) illustrate that the S6).
well-distribution of O, N, and Pd in different samples. On the
Typical XPS survey scans of the catalysts were shown in
other hand, high-resolution TEM (HR-TEM) images of these Figure S5, revealing that catalyst Pd/C-fu contains C, O and Pd
catalysts were shown in Figure S3, and the crystal lattice plane as the main elements and N/O-doped porous carbons catalysts
distance of Pd(111) (0.245 nm) and Pd(200) (0.194 nm) were also contains N element. O1s spectra of these series of O-
observed, which are consistent with the XRD diffraction doped, N/O-doped carbon materials and catalysts
patterns of the catalyst (Figure 2)
.
demonstrate the binding energy of oxygen functional groups,
i.e., 532 eV, 532.6 eV and 533.8 eV in C-fu, C-py/fu-3 and C-py
(Figure 3a-c). The binding energy of 532 eV is assigned to the
oxygen in carboxylate (COO−). The binding energy of 532.6 eV
is attributed to hydroxyl group (C-OH) of alcohol, phenol,
aliphatic ether and carbonyl group(C=O). The binding energy of
5
33.8 eV demonstrates oxygen of aryl ethers or carboxylic
acids (O=C-O). For O1s spectra of the catalysts, the binding
energy of 535 eV is related to chemisorbed oxygen in
8
2-84
carboxylic groups or water (H-O-H).
Interestingly, no
obvious O1s binding energy shift can be seen in O-doped, N/O-
doped carbon materials along with the increasing of nitrogen
content, indicating the increase of nitrogen functional groups
had no effect on oxygen functional groups by thermal
treatment. Whereas, the peak at 532 eV shifts to 531.6 eV,
when O-doped and N/O-doped carbon materials are
introduced with the palladium nanoparticles (Figure 3d-i). This
shift indicates an increase of electron density of the O atoms
by the charge transfer from Pd species to O-C=O group.
Meanwhile, it is noted that no shift of binding energy were
observed for the peaks of 532.6 eV and 533.8 eV before and
Figure 2. XRD diffraction patterns of Pd/C-fu (a), Pd/C-py/fu-1 (b), Pd/C-py/fu-2
c), Pd/C-py/fu-3 (d), Pd/C-py/fu-4 (e) and Pd/C-py (f).
(
More information can be gained from the XRD after Pd loading, respectively. The results of O1s spectra
characterization results. Sharp diffraction peaks of Pd/C-fu suggest that the local electronic structure of O-C=O in
o
o
o
o
centered at 36.025 , 40.118 , 46.658 and 68.119 associated carboxylate can be significantly modified by the interaction
with the C (401), Pd (111), Pd(200) and Pd(220) can be with a large amount of Pd active ingredient, and other oxygen
observed (Figure 2a). Interestingly, these diffraction peaks functional groups may not have obvious interaction with the
turned weaker with the increasing of nitrogen content in the Pd species.
supports, and no obvious diffraction peaks assigned to Pd
It is worth mentioning that O1s spectra of Pd/C-py-N
nanoparticles can be observed from Pd/C-py. These results (Figure 3j) (without reduction by formaldehyde) is similar with
suggest that the increasing of N-groups on the carbon supports Pd/C-py. This result showed that the reduction process had no
can influence the morphology of palladium nanoparticles effect on the oxygen functional groups that already exist in
significantly. These results were further confirmed by the N/O-doped carbons. N/O-doped porous carbons are always a
selected area electron diffraction (SAED) patterns. In local complicated system because of the coexistence of various N
regions, the Pd/C-fu showed diffraction rings of porous species, and it is still under debate as to which type of nitrogen
carbons with diffraction spots assigned to the diffraction functionality is responsible for the electronic interaction. Thus,
patterns of Pd (111), Pd(200) and Pd(220), and the Pd/C-py/fu- a series of N/O-doped carbon materials and catalysts with
1
with the diffraction patterns of Pd(111) and Pd(220) (Figure different nitrogen content were prepared to investigate the
S4). However, there are no diffraction spots regarding crystal interaction between the N species and Pd nanoparticles
Pd nanoparticles from Pd/C-py/fu-2, Pd/C-py/fu-3, Pd/C-py/fu- (Figure 4). The N species in C-py/fu-3 and C-py are composed
4
and Pd/C-py can be observed, illustrating ultra-small Pd by three types of N, which are attributed to the C=N bond
nanoparticles with low crystallinity on the porous carbons with appeared at 398.6 eV, the tertiary nitrogen at 400.1 eV among
the increasing of nitrogen content of the supports. Based on s-triazine rings and graphitic N-oxide at 400.9 eV,
8
5-87
aforementioned TEM, EDX, HR-TEM, XRD, and SAED analyses, respectively
(Figure 4a-b).
it further suggested that Pd nanoparticles have a strong
interaction with nitrogen functional groups compared with
oxygen functional groups. Finally, based on the TEM, HR-TEM
and XRD characterization results of Pd/C-py-R, it can be seen
that the catalyst structure is not changed remarkably, which
A noticeable shift of the peaks of C=N bond and tertiary
nitrogen from 398.6 eV and 400.1 eV to 398.3 eV and 399.8 eV
after loading of palladium nanoparticles (Figure 4c-g). Thus,
the introduction of Pd specie into the carbon material
generates an increased electron density of N specie, indicating
an electron-deficient chemical state of Pd nanoparticles. Yet
for all that, there is no obvious shift of binding energy in the
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Figure 3. O1s spectra of C-fu (a), C-py/fu-3 (b), C-py (c), Pd/C-fu (d), Pd/C-py/fu-1 (e), Pd/C-py/fu-2 (f), Pd/C-py/fu-3 (g),Pd/C-py/fu-4 (h),Pd/C-py (i) Pd/C-fu and Pd/C-
py without reduction by formaldehyde aqueous solution is labeled as Pd/C-py-N (j)
Figure 4. N1s spectra of C-py/fu-3 (a), C-py (b), Pd/C-py/fu-1 (c), Pd/C-py/fu-2 (d), Pd/C-py/fu-3 (e), Pd/C-py/fu-4 (f), Pd/C-py (g) and Pd/C-py-N (h)
structure of graphitic N-oxide before and after Pd loading. The Pd3d XPS spectra of the catalysts present two peaks at
These results may be beyond all expectations, because it is about 335.5 and 340.8 eV, corresponding to the spin-orbit split
reported that the graphitic N-oxide plays a crucial role in the doublet of Pd3d5/2 and 3d3/2, respectively (Figure 5a-f). The
8
8-94
dehydrogenation and oxygen reduction
graphitic are expected to show stronger π bonds of aromatic peaks (Figure 5a), including the predominant metallic Pd and
due to N-doped Pd3d spectra of Pd/C-fu are de-convoluted into two groups of
9
5-99
rings. It implies that the electronic interaction is not Pd oxide.
occurred between graphitic N-oxide and Pd nanoparticles.
Accordingly, it can be observed obviously that N1s spectra of be attributed to metallic Pd (Peak-1),
Pd/C-py is similar with Pd/C-py-N, indicating the reduction
process has absolutely no influence on the existing nitrogen
The peaks at 335.5 eV (Pd3d5/2) and 340.8 eV (Pd3d3/2) can
1
00, 101
and peaks at 336.2
eV (Pd3d5/2) and 341.7 eV (Pd3d3/2) can be assigned to the Pd
1
02
species interacted with the O-C=O groups (Peak-2).
functional groups, too (Figure 4h)
.
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Interestingly, when the N species are introduced into the
Cat Pa ll ey as si es dS oc ni eo nt ca ed j &u s Tt me ca hr gn i on sl ogy
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Raman spectra of Pd/C-py and Pd/C-fu were given in Figure 6
.
As for the Pd/C-fu (Figure 6b), two remarkable peaks at 1369
-1
and 1600 cm assigned to the well-defined D and G bands can
be observed. In the case of Pd/C-py (Figure 6a), broader D
-1
band centered at 1363 cm is caused by multiple overlapping
-1
peaks (the mode of C–N vibration at 1285 cm , the mode of
107-109
-1
C=N at 1400 cm and the D mode of graphene),
whereas
the G band centered at 1607 cm arises from the overlap of
-1
1
10, 111
the G bands of grapheme.
The high intensity of the D
band of Pd/C-py and Pd/C-fu clearly indicates the presence of
defects in the carbon material, which should be caused by
1
12
N/O-doping or O-doping in the porous carbons supports.
Compared with Pd/C-fu, the ID/IG ratio in Pd/C-py becomes
greater, which is not only caused by more disordered structure
involving C-N bonds, but also ascribed to the high baseline of
Pd/C-py in the region of the D band.
Figure 6. FT-Raman spectra of Pd/C-py (left) and Pd/C-fu (right).
All the above discussions imply both nitrogen functional
Figure 5. Pd3d spectra of Pd/C-fu (a), Pd/C-py/fu-1 (b), Pd/C-py/fu-2 (c), Pd/C- groups and oxygen functional groups coexisted in porous
py/fu-3 (d), Pd/C-py/fu-4 (e), Pd/C-py (f) and Pd/C-py-N (g)
carbons, so an illustration of the catalyst structures were given
in Scheme 2.
Carbon support, new peaks at 337.8 eV (Pd3d5/2) and 343.3 eV
(Pd3d3/2) (Peak-3) can be observed obviously in Pd/C-py/fu-1
and Pd/C-py (Figure 5b-f). In the case of Pd/C-py (Figure 5f)
,
the local electronic structure of Pd nanoparticles can be
significantly affected by the interaction with a large amount of
nitrogen functional groups, and resulting in Peak-4 with
binding energies at 338.2 eV (Pd3d5/2) and 343.7 eV (Pd3d3/2).
Whereas, it is also worth pointing out that no shifting with the
already existing peaks (peak-1 and peak-2) of Pd 3d binding
energy toward the low-energy side can be observed, although
nitrogen content is gradually increased from Pd/C-fu to Pd/C-
py. Meanwhile, there is no shift of the binding energy of O1s
Scheme 2. Pictorial description of Pd/C-fu, Pd/C-py/fu and Pd/C-py.
Scope and limitations. It has been reported that palladium-
catalyzed synthesis of arylamines from nitrobenzene and
cyclohexanone could also be successfully converted to the
(Figure 3d-i) and N1s (Figure 4c-g), too. In one word, these
results suggested that the Pd species might coordinate with
03-106
o
1
corresponding substituted diphenylamine at 150 C with good
1
the O-C=O groups, C=N and tertiary nitrogen groups,
and
13, 114
isolated yields.
However, it is still difficult to synthesize
these functional carbons behave as solid ligands to support
palladium nanoparticles. In consideration of the results of the
model reaction together (Table 1 entries 1-6), the interaction
between the nitrogen functional groups and Pd species might
be responsible for the higher activity of Pd/C-py. Also, Pd/C-
py-R was characterized by XPS and almost the same O1s, N1s
and Pd3d spectra as the fresh Pd/C-py were observed. So the
surface structure of the catalyst is stable enough during the
triphenylamine. Therefore, with the optimized reaction
conditions in hand, we examined the scope of the reaction
with a range of nitrobenzene and cyclohexanone derivatives as
starting materials (Scheme 3). The desired triphenylamine
derivatives were formed in good yield when nitrobenzene or 4-
nitrotoluene was used in combination with cyclohexanone or
4
-methylcyclohexanone, and the isolated yields of c1, c6 and
c7 were 96%, 90% and 90%, respectively. The substituting
positions on the aromatic ring of nitrobenzene did significantly
affect the reaction. The yield of c4 was only 31% when 2-
nitrotoluene and cyclohexanone were used as the starting
materials. It needs to be mentioned that the reaction can not
progress well if nitrobenzene or cyclohexanone derivatives
reaction (Figure S6)
.
Raman spectroscopy is
a
powerful technique to
characterize the structures and electronic properties of
catalytic materials, particularly to determine the defects, and
1
07-109
the ordered and disordered structures of carbon.
The FT-
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with electron-withdrawing or big substituting groups were not influence the reactivity of nitrobenzene, and 75% yield of
used as the starting materials. For example, only 61%, 47% and the desired product (d10) was obtained.
6
4% yields of c9-11 can be obtained. Even though, this
a
Scheme 4. Synthesis of diphenylamine derivatives
unprecedented sequence offers a synthetically powerful
method for the synthesis of triphenylamine derivatives.
a
Scheme 3. Synthesis of triphenylamine derivatives
d1, 82%
d3, 70%
d2, 83%
d4, 62%
d2, 80%
d5, 60%
c1, 96%
c2, 77%
c5, 89%
c3, 74%
c6, 90%
d6, 90%
d7, 54%
d8, 80%
c4, 31%
d9, 84%
d10, 75%
a
Isolated yield.
Reaction Mechanism Exploration. In the previous study with
aniline and cyclohexanone as starting materials, it has been
considered that 2-cyclohexen-1-one may be the possible
c7, 90%
c8, 82%
c9, 61%
1
15-118
reaction intermediate.
Therefore, in order to get insights
into the mechanism of the Pd/C-py catalyzed reaction of
nitrobenzene with cyclohexanone to synthesize diphenylamine
and triphenylamine, the reaction of nitrobenzene with
cyclohexanone or 2-cyclohexen-1-one was carried out with
different time intervals (Table 3). Clearly, 90% nitrobenzene
conversion, 70% diphenylamine yield and 19% triphenylamine
obtained at 12 h with cyclohexanone as starting material
(Table 3, entry 1). If the reaction was extended to 24 h, 14%
diphenylamine and 84% triphenylamine yields were obtained
c10, 47%
c11, 64%
a
Isolated yield.
In order to show the generality of this catalyst system, (entry 3). Finally the yield of triphenylamine reached 96% at 36
next, we explore the Pd/C-py-catalyzed synthesis of
(entry 5). If 2-cyclohexen-1-one was used as starting
substituted diphenylamine with nitrobenzene
and material, 32% biphenylamine and 45% triphenylamine was
cyclohexanone derivatives as the starting materials (Scheme obtained at 24 h (entries2 and 4), but their yields changed to
. To our delight, an array of substituted diphenylamine 53% and 31%, respectively, at 36 h (entry 6). Interestingly, 2-
h
4)
o
derivatives were obtained with good yields at 80 C, including cyclohexen-1-one has been consumed, and remarkable
products with methyl, tert-butyl, phenyl, and ester substituting amount of cyclohexanone and phenol were generated at 24 h.
groups on the aromatic rings (d1-d10). The steric-hindered 2- These results suggested that 2-cyclohexen-1-one should not be
nitrotoluene proceed effectively to 2-methyl-N-phenylaniline the reaction intermediate in this reaction, and it can be
(
d4) in 62% yield, and even the 3-nitrotoluene with 4- transformed into cyclohexanone and phenol in the presence of
Pd/C-py catalyst.
methylcyclohexanone affords the corresponding product (d7
)
with 54% yield. Further, the presence of acetate group does
8
| J. Name., 2012, 00, 1-3
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Table 3. Control experiments of nitrobenzene and cyclohexanone or 2-cyclohexen-1- determine the quantities of starting materials and products at
a
one
different reaction times (Table 1, entry 6, and Figure 7). 1.1
O
µmol 2-cyclohexen-1-one, 1.4 µmol aniline and 0.5 µmol N-
cyclohexylideneaniline were formed in 1 h. Following, the
content of 2-cyclohexen-1-one and aniline decreased
remarkably at 2 h, i.e., 0.35 µmol 2-cyclohexen-1-one, 1 µmol
aniline and 0.175 µmol N-cyclohexylideneaniline can be
observed inside the reaction mixture. Meanwhile, the yields of
diphenylamine (43%) and triphenylamine (3%) has an obvious
increase. Notably, N-cyclohexylideneaniline can not be
observed obviously at 5 h, whereas, 0.23 µmol 2-cyclohexen-
1-one and 0.6 µmol aniline were still maintained. These result
imply that N-cyclohexylideneaniline may be the reaction
intermediate. As the reaction continued, the content of 2-
cyclohexen-1-one, aniline and N-cyclohexylideneaniline
decreased, but the amount of phenol increased slightly, and
7.5 µmol phenol formed at 23 h. The content of phenol has
maintained 7.5 µmol until the reaction was completed. These
(
A)
NO
2
H
N
Pd/C-pyrrole
and
N
or
+
O
(
C)
(D)
(
B)
b
c
c
Entry
Reactants
t/h
12
12
24
24
36
36
Con.%
Y(C).%
70
Y(D).%
1
2
3
4
5
6
A
B
A
B
A
B
90
99
99
99
99
99
19
12
84
45
96
31
60
14
32
3
53
a
0
.5 mmol nitrobenzene, 4 mmol cyclohexanone (A) or 2-cyclohexen-1-one (B),
NO
2
o
b
3
0 mg Pd/C-py, 5 mL toluene, 160 C (reaction temperature), Ar. conversion of
O
O
NH
2
c
nitrobenzene determined by GC-FID with external standard method. GC yields.
Pd/C-py
+
+
Catalytic hydrogen transfer
NO
In addition, the above results also reveal the reversible
transformation between triphenylamine and diphenylamine
2
(trace)
NH
2
(1)
O
OH
Pd/C-py
(a).
(
Table 3, entries 2, 4 and 6, Scheme 5). Triphenylamine can be
formed by reaction of diphenylamine and cyclohexanone
route A), and diphenylamine also can may be obtained via the
Catalytic hydrogen transfer
(trace)
NH
2
O
(
N
+
(2)
decomposition of triphenylamine with the co-generation of
phenol (route B). These results also suggested that the
concentration of cyclohexanone is a crucial factor for the Pd/C-
py-catalyzed hydrogen transfer reaction. Route A would be the
main pathway if the concentration of cyclohexanone is high
enough, otherwise triphenylamine can be converted into
biphenylamine in this catalytic system.
(
trace)
NO
2
NH
2
H
N
N
Pd/C-py
+
Catalytic hydrogen transfer
(
trace)
O
2
N
(b).
nsfer
ra
H
H
NH
2
O
y
N
+
O
-
OH
N
d/C-p
gen t
route:A
route:B
N
P
N
+
hydro
tic
ly
ata
C
Scheme 5. Reversible transformation between triphenylamine and
diphenylamine
Scheme 6. process-A (a) and process-B (b) of the catalytic hydrogen transfer
processes.
In order to clarify the reaction process, the reaction of
nitrobenzene and cyclohexanone was traced by GC-FID to
NO
2
O
H
N
30mg Pd/C-py
+
o
+
5
mL toluene, 160 C
N
Figure 7. The model reactions with different time intervals (from 1 h to 36 h). The content of starting materials and intermediates determined by GC-FID with external
standard method.
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results implicate that the reaction process may be
composed of two reaction processes (Scheme 6). In the
reaction-A (Scheme 6a), 2-cyclohexen-1-one and aniline
were generated via transfer hydrogenation. Next, 2-
cyclohexen-1-one reacted with nitrobenzene to form
phenol and aniline. Meanwhile, aniline reacted with
cyclohexanone, and N-cyclohexylideneaniline generated for
reaction-B (Scheme 6b). In addition, the content of phenol
maintained unchanged at 7.5 µmol from 23 h to the
completion of the reaction. So it was deduced that reaction-
A almost stopped when reaction-B started.
Following, we verified the role of N-cyclohexylideneaniline
in the reaction, so it was used as the starting material
directly with Pd/C-py as catalyst (Table 4). Obviously, N-
cyclohexylideneaniline can be converted into diphenylamine
and N-cyclohexylaniline in 1 h. Noteworthy, the reaction
finished in 1 h and then the yields of diphenylamine and N-
cyclohexylaniline maintained stable (Table 4, entries 2-4)
.
That signifies N-cyclohexylideneaniline can be converted
into the desired product very fast and N-
cyclohexylideneaniline might be the key intermediate.
Based on these results, a possible reaction mechanism is
a
Table 4. Catalytic transformation of N-cyclohexylideneaniline
proposed (Scheme 7)
. For the dehydrogenation of
cyclohexanone to 2-cyclohexen-1-one in the reaction
process-A, deprotonative coordination of cyclohexanone to
Pd with the assistance of N/O-doped porous carbons affords
Pd-enolate-H species, followed by β-H elimination to give 2-
cyclohexen-1-one. Then the catalytic dehydrogenation of 2-
cyclohexen-1-one to phenol occurred through a catalytic
cycle (cycle-1), which was similar to the dehydrogenation of
cyclohexanone to 2-cyclohexen-1-one. Meanwhile, the
catalytic hydrogenation of a small amount of nitrobenzene
b
c
c
Entry
t/h
0.5
1
Con.%
20
Y (B).%
6
Y (C).%
13
1
2
3
4
99
31
64
to aniline by Pd
aniline generated during reaction-A will react with
cyclohexanone quickly and generate N-
-2H can then regenerate the catalyst. The
2
99
31
63
cyclohexylideneaniline. Then, reaction-B were happened
between N-cyclohexylideneaniline and nitrobenzene to give
diphenylamine as product with the concomitant formation
of the Pd–2H species. The Pd–2H species and nitrobenzene
would continue the reaction, and the active catalyst would
be regenerated for the next cycle (cycle-2).
4
99
32
66
a
o
0
.5 mmol N-cyclohexylideneaniline, 30 mg Pd/C-py, 5 mL toluene, 160 C
b
(
reaction temperature), Ar. Conversion of N-cyclohexylideneaniline
c
determined by GC-FID with external standard method. GC yields.
Scheme 7. Proposed reaction mechanism.
immobilize nanopalladium, which can be an active catalyst
for biphenylamine and triphenylamine synthesis through the
reaction of nitrobenzene and cyclohexanone. The catalytic
performance can be tuned efficiently via tuning the N/O
group contents and ratios on the carbon material surface,
4
. CONCLUSIONS
In summary, we have developed a general method for
the precise construction and functionalization of porous
carbon materials with tunable N/O functional groups via
pyrrole/furan polymerization and pyrolization. These N/O
doped carbons can behave as support and "solid ligand" to
1
0 | J. Name., 2012, 00, 1-3
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which can coordinate with the Pd nanoparticles to modulate
the size and electronic properties of Pd nanoparticles via the
electron-donating from Pd nanoparticles to the N/O species.
In addition, mechanistic studies prove that this reaction starts
15. X. Duan, Z. Ao, H. Sun, S. Indrawirawan, Y. Wang, J. Kang,
F. Liang, Z. H. Zhu and S. Wang, ACS Appl. Mat. Interfaces,
2
015, 7, 4169-4178.
6. H. Yang, X. Cui, X. Dai, Y. Deng and F. Shi, Nat. Commun.,
015, 6.
7. D. R. Dreyer, H. P. Jia and C. W. Bielawski, Angew. Chem.
Int. Ed., 2010, 49, 6813-6816.
8. B. Frank, M. Morassutto, R. Schomäcker, R. Schlögl and D.
S. Su, ChemCatChem, 2010, 2, 644-648.
9. J. McGregor, Z. Huang, E. P. Parrott, J. A. Zeitler, K. L.
Nguyen, J. M. Rawson, A. Carley, T. W. Hansen, J.-P.
Tessonnier and D. S. Su, J. Catal., 2010, 269, 329-339.
1
1
1
1
2
with
the
transferhydrogenation
reaction
between
nitrobenzene and cyclohexanone to generate aniline. Then
aniline would react with cyclohexanone to form N-
cyclohexylideneaniline, and biphenylamine could be
synthesized through its further reaction with nitrobenzene.
This work should open up a promising pathway for the
development of new method for arylamine synthesis, and
structure and activity defined heterogeneous catalysts for fine
chemical synthesis.
20. A. Rinaldi, J. Zhang, B. Frank, D. S. Su, S. B. Abd Hamid and
R. Schlögl, ChemSusChem, 2010, 3, 254-260.
2
2
1. H. Chen, M. He, Y. Wang, L. Zhai, Y. Cui, Y. Li, Y. Li, H.
Zhou, X. Hong and Z. Deng, Green Chem., 2011, 13, 2723-
2
726.
Acknowledgements
2. D. R. Dreyer, H.-P. Jia, A. D. Todd, J. Geng and C. W.
This work was financially supported by the National Natural
Science Foundation of China (21633013) and Youth Innovation
Promotion Association CAS.
Bielawski, Org. Biomol. Chem., 2011, 9, 7292-7295.
23. H. Huang, J. Huang, Y.-M. Liu, H.-Y. He, Y. Cao and K.-N.
Fan, Green Chem., 2012, 14, 930-934.
2
4. Y. Chen, J. Zhang, M. Zhang and X. Wang, Chem. Sci.,
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