Unprecedented catalytic performance in amine syntheses: Via Pd/g-C3N4 catalyst-assisted transfer hydrogenation

Article abstract of DOI:10.1039/c8gc00144h

The preparation of amine compounds is very important for both the chemical industry and renewable feedstock processing. Nevertheless, difficulties remain in finding a catalytic system that is sufficiently active and environmentally benign for producing amine compounds. In this work, we report that g-C₃N₄ nanosheets as support materials can significantly boost the efficiency of Pd nanoparticles for the reduction of nitro compounds to primary amines. Using formic acid as a hydrogen donor and water as a solvent, the optimized 5 wt% Pd/g-C₃N₄ catalyst exhibited an unprecedented performance in the conversion of nitrobenzene into aniline (achieving almost full conversion with an extremely high turnover frequency of 4770 h^-1 at room temperature), yielding the best activity ever reported for heterogeneously catalyzing nitro compound reduction. Pd/g-C₃N₄ catalyst was also active for the one-pot reductive amination of carbonyl compounds with nitro compounds to obtain the corresponding secondary amines with excellent selectivity (>90%). We proposed that the protic N-H⁺ and hydridic Pd-H^- on Pd/g-C₃N₄ are the active species for the transfer hydrogenation reaction of nitro compounds. Furthermore, Pd/g-C₃N₄ catalyst was highly stable with a wide scope in the syntheses of various amine compounds. This work will open up a new approach for the transfer hydrogenations of nitro compounds to produce primary or secondary amines in green chemistry.

Full text of DOI:10.1039/c8gc00144h

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Unprecedented catalytic performance in amines syntheses via
Pd/g-C3N4 catalyst–assisted transfer hydrogenation
Xingliang Xu, Jiajun Luo, Liping Li, Dan Zhang, Yan Wang and Guangshe Li*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The preparation of amine compounds is very important for both the chemical industry and renewable feedstock
processing. Nevertheless, it remains difficulties in finding a catalytic system that is sufficiently active and environmentally
benign for producing amine compounds. In this work, we report that g-C3N4 nanosheets as support materials can
significantly boost the efficiency of Pd nanoparticles for the reduction of nitro compounds to primary amines. Using formic
acid as hydrogen donor and water as solvent, the optimized 5 wt% Pd/g-C3N4 catalyst exhibited an unprecedented
performance in the conversion of nitrobenzene to aniline (achieving almost full conversion with an extremely high
turnover frequency of 4770 h-1 at room temperature), showing the best activity ever reported for heterogeneously
catalyzing nitro compounds reduction. Pd/g-C3N4 catalyst was also active for the one-pot reductive amination of carbonyl
compounds with nitro compounds to obtain the corresponding secondary amines with excellent selectivity (>90%). We
proposed that the protic N-H+ and hydridic Pd-H− on Pd/g-C3N4 are the active species for the transfer hydrogenation of
nitro compounds reaction. Furthermore, Pd/g-C3N4 catalyst was highly stable with a wide scope in the synthesis of various
amine compounds. This work will open up a new approach for transfer hydrogenations of nitro compounds to produce
primary or secondary amines in green chemistry
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intermediates and can solve the problem of low yield caused
by the multi-step reaction. Unfortunately, traditional
hydrogenation of nitro compounds to amines with molecular
INTRODUCTION
Amine compounds, especially those of primary and secondary
amines, are the significant raw materials and intermediates,
and are playing remarkable roles in the syntheses of high
value-added chemicals.1,2 Hence, wide extensive interests from
both academic and industrial researchers have been devoted
to explore more efficient and environmentally benign
strategies to synthesize amines. To date, numerous
preparation systems of primary and secondary amines have
been developed, including hydrogenation of nitro
compounds,3,4 reduction amination of amides,5 direct
amination of halides,6 Buchwald-Hartwigand,7 Ullman-
typecarbon-nitrogen cross-coupling reactions,8 as well as the
direct N-alkylation of amines with alcohol.9 Among all these
methods, the synthesis of amines with nitro compounds could
be the convenient and valuable reactions, because both of
reactions do not require the separation and purification of
hydrogen as reductant is widely regarded as an
environmentally unfriendly and harsh synthetic process, such
as the use of toxic organic solvents and high pressure H2,
which greatly limited their industrial applications.10-12
Therefore, it is important to find environmentally friendly
strategies to synthesize amines more efficiently and easily.
Comparing to hydrogenation reactions involving H2,
catalytic transfer hydrogenation can provide a mild and safe
reaction condition13 using the hydrogen atoms supplied from
14
15
16
isopropanol,
hydrazine,
ammonium borane,
carbon
monoxide, 17 and formic acid18 to replace hydrogen molecules.
Among these donors, formic acid (FA), derived from the by-
products of renewable biomass, has a high energy density (4.4
wt %). Additionally, the non-toxic and stable features suggest
that formic acid can be used as a safe and economical carrier
of liquid hydrogen source.19,20 Therefore, if choosing FA as a
proton donor, the preparation of amine compounds might be
realized in the mild conditions without using harsh conditions
like autoclaves or high-pressure hydrogen. More importantly,
in the catalytic transfer hydrogenation reaction, FA undergoes
a process similar to dehydrogenation (HCOOH→H2 + CO2), the
two hydrogen atoms in the formic acid can participate in the
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of
Chemistry, Jilin University, Changchun, P.R. China.
E-mail address: guangshe@jlu.edu.cn Fax: +86-431-85168358
Tel: +86-431-85168358
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See DOI:
10.1039/x0xx00000x
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hydrogenation reaction, and the decomposition only produces
by-product of carbon dioxide .21
Developing new catalyst is the key to achieve a high
catalytic performance in transfer hydrogenation of nitro
compounds with FA. Accordingly, many heterogeneous metal
catalysts have been conducted.21-23 Pd metal is a common
active component in FA dehydrogenation.24 In addition, Pd-
based catalysts also show good activity in hydrogenation
reaction,25 which could be used in the reduction of
unsaturated chemical bonds. Even so, the preparation
methods for such reactions are lack of control, the catalytic
activities need further improvement, and the weak CO
tolerance of Pd may lead to a reduced stability of the catalyst.
An alternative to these shortcomings is to modify the surface
electronic structure of Pd catalyst with suitable supports,
which can stabilize Pd nanoparticles (NPs), but also improve
the activity of Pd NPs under transfer hydrogenation conditions
due to the metal-support electronic interaction. Therefore,
finding suitable and efficient supports is necessary for the
transfer hydrogenations of nitro compounds. We took
graphitic carbon nitride (g-C3N4) as a model support to study
based on its unusual features: (1) g-C3N4 is a rich-N carbon
material, which could modify the surface structure of noble
metal catalysts through providing an enhanced binding ability
and improved basicity;26,27 (2) g-C3N4 has
a typical 2D
nanosheet structure, which is propitious to disperse and
stabilize Pd NPs;28,29 (3) the high stability of g-C3N4 against
chemical attacks from acid, base, and organic solvents enables
its direct use for sustainable chemical reactions,27 and what's
more, and (4) different kinds of chemical environments for N
atoms in g-C3N4 (among of which pyridine nitrogen belongs to
the Lewis base) could promote the cleavage of oxygen-
hydrogen bonds.30 Therefore, g-C3N4-engineered Pd NPs could
achieve high activities in preparation of amine compounds
using FA as the hydrogen donor.
In this contribution
，we designed a series of Pd/g-C3N4
catalysts (1-10 wt% Pd/g-C3N4) by a wet chemical reduction
method. In comparison to the existent catalysts, the
engineered Pd/g-C3N4 catalysts, especially 5 wt% Pd/g-C3N4
showed great improvements in the transfer hydrogenation of
nitro compounds to synthesize primary amine with FA as
hydrogen donor at room temperature. Moreover, the catalyst
was active for the one-pot reductive amination of carbonyl
compounds with nitro compounds to prepare secondary
amines (as summarized in Scheme 1). Moreover, the catalyst
exhibited a superior recyclability and wide scope in the
syntheses of various primary amine or secondary amines.
RESULTS AND DISCUSSION
Characterization of the prepared sample
We began our research by synthesizing g-C3N4 nanosheets
with the thermal decomposition of melamine. The obtained g-
C3N4 nanosheets were then loaded with a certain content of
Pd (1-10 wt% Pd/g-C3N4) by a wet chemical reduction method.
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Fig. 2 FT-IR spectra of the samples: (a) g-C3N4, and Pd/g-C3N4
catalysts with different of Pd loading: (b) 1 wt% Pd, (c) 3 wt%
Pd, (d) 5 wt% Pd, (e) 8 wt% Pd, and (f) 10 wt% Pd.
heterocycle stretching vibration,33,34 while the four at 1241,
1320, 1409, and 1567 cm-1 to aromatic C-N stretching
vibrations.35,36 The peak at 808 cm-1 is associated with the
breathing mode of triazine units.37 The broad band in the
region of 2900-3500 cm−1 can be assigned to the characteristic
absorption peaks of O-H and amine groups.38 FT-IR spectra of
all samples did not show any observable changes before and
after Pd loading, suggesting
a non-covalently coupled
interaction between g-C3N4 and Pd NPs in the composite, as
concluded elsewhere.39 Moreover, BET surface areas for the
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of g-C3N4 and Pd NPs. The XPS spectra of Pd 3d (Fig. 4d) Table 1. Comparisons of catalytic performance for transfer
presents two doublet peaks, corresponding to spin-orbital hydrogenation of nitrobenzene to aniline over various
splitting of Pd 3d5/2 and Pd 3d3/2 for two typies of Pd species. catalystsa.
The stronger peaks at 335.2 and 340.5 eV for Pd 3d5/2 and Pd
3d3/2 are from metallic Pd0,44,45 and that with relative low
intensity at 337.2 and 342.5 eV can be assigned to Pd2+, which
might be the formation of Pd-O or Pd-N bonds on the surface
of catalyst.46 The content of Pd0 estimated by the area ratio of
two doublet peaks is as high as 75% in the total Pd atoms
(Table. S1†). These results demonstrate that the amine
groups in the g-C3N4 carrier can stabilize highly dispersed Pd0
species and prevent their oxidation partly.47
Entry
Catalyst
Time [min]
Con.b
[%]
Sel.b
[%]
1
2
g-C3N4
20
2
trace
99
trace
99
Catalytic synthesis of primary amines
10 wt % Pd/g-C3N4
3
8 wt % Pd/g-C3N4
3
99
99
1) The selection of catalysts. Firstly, in a preliminary catalyst
screening, the catalytic transfer hydrogenation for
nitrobenzene is selected as a model reaction to study. The
experiments were carried out in deionized water at room
temperature over various catalysts by using 3 mmol FA
(Table. 1). Only adding pure g-C3N4, nitrobenzene did not
undergo any reaction (Table. 1 entry 1). Comparatively, all the
Pd/g-C3N4 catalysts (Table 1 entries 2-6) exhibited excellent
catalytic activity, with a conversion of nitrobenzene as high as
99% and the selectivity of anilines about 99%. Moreover, the
reaction rate gradually increased with the increase of the Pd
loading (Fig. S12†). However, the turnover frequency (TOF)
increased significantly to a maximum of 3172 h-1 at the Pd
loading of 5%, and then slightly decreased when continuously
increasing Pd loading to 8% and 10% (Fig. S13†). The 5 wt%
Pd/g-C3N4 gave the highest TOF is ascribed to its containing
high Pd0 (75%) comparing to the samples with light Pd loading
(1 wt% and 3 wt% ) and possessing small Pd particle size to
the samples with high loading (8 wt% and 10 wt% ), as listed
in Table. S1†. We also test the catalytic activity of other noble
metals (Au, Pt, Ru and Rh) supported on g-C3N4 nanosheets
under the same reaction conditions (entries 7-10 in Table. 1),
and found that these catalysts did not show any activity for the
reduction of nitrobenzene. These results suggest that Pd NPs is
the most active metal in the catalytic transfer hydrogenation
for nitrobenzene in the presence of FA.
4
5
5 wt% Pd/g-C3N4
3 wt% Pd/g-C3N4
1 wt% Pd/g-C3N4
5 wt% Au/ g-C3N4
5 wt% Pt/ g-C3N4
5wt% Ru/ g-C3N4
5 wt% Rh/ g-C3N4
5 wt% Pd/CeO2
5 wt% Pd/Al2O3
5 wt% Pd/C
3
9
99
99
99
99
6
20
3
99
99
7
trace
trace
trace
trace
29
trace
trace
trace
trace
99
8
3
9
3
10
11
12
14
15
16c
17e
3
3
3
45
99
3
<5
99
5 wt% Pd/r-GO
5 wt %Pd/g-C3N4
5 wt% Pd/g-C3N4
3
<5
99
20
3
98
99
99
99
[a] Reaction conditions: (1-6) catalyst (20 mg), water (5 mL),
nitrobenzene (1 mmol), FA (3 mmol), 298 K. (7-15) catalyst
(metal 1 mol%), water (5 mL), nitrobenzene (1 mmol), FA (3
mmol), 298 K. [b] Conversions [Con.] and selectivity [Sel.] were
determined by GC-MS. [c] catalyst (metal 0.1 mol%), water (10
mL), nitrobenzene (20 mmol), FA (60 mmol), 298 K. [d] Reuses
of the 5% wt Pd/g-C3N4 catalyst in transfer hydrogenation of
nitrobenzene after 6 runs.
To investigate the effect of the supports on the catalytic
transfer hydrogenation for nitrobenzene, a series of the Pd/
supporters composites with different supporters were
examined in comparison to Pd/g-C3N4 (entries 11-15). The
Pd/C catalyst showed a very low activity for the reaction. This
is mainly because Pd/C catalyst tends to catalyze the
dehydration of formic acid, which cannot effectively break the
oxygen-hydrogen bonds in FA.30 Pd/g-C3N4 exhibited excellent
performance with higher activity and selectivity in catalytic
transfer hydrogenation of nitrobenzene than other catalyst.
This is mainly because g-C3N4 nanosheets with electron-rich
nitrogen present surface Lewis base properties and modify the
electron density of the supported metals, which can promote
performance in the cleavage of oxygen-hydrogen bonds in
FA.30,48 In addition, the formate as intermediate can be
decomposed into carbon dioxygen and the active hydrogen is
absorbed in the surface of Pd NPs.49,50
In order to study whether the reaction can be used in the
industrial production, a gram-scale reaction was studied (Table.
1, entry 16), and the results show that nitrobenzene was
completely converted into aniline within 20 min, which
suggests that the Pd/g-C3N4 has a good prospect in industrial
production of aniline. In addition, the reusability performance
of catalyst is an important property for representing its
stability. The catalyst was recycled at least six times without
considerable change in conversion of nitrobenzene, and the
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Table. 2 Catalytic transfer hydrogenation of nitrobenzene to Table 3. Catalytic transfer hydrogenation of various nitro
anilines over 5 wt% Pd/g-C3N4 in various solventsa
compounds to the corresponding primary amines over 5 wt%
a
Pd/g-C3N4
Entry
Solvent
Volume
[ml]
Time
[min]
Con.[b]
[%]
TOF
h-1
1
2
3
Water
Ethanol
Toluene
5
5
5
3
5
5
99
99
15
3172
1050
154
Entry
R
Time
[min ]
Con.[b]
[%]
Sel.[b]
[%]
1
5
99
99
99
99
4
5
Cyclohexane
DMF
5
5
5
5
21
51
245
592
2
20
6
7
THF
5
1
5
56
99
612
Water
1.5
4770
8
Water
3
2
99
3774
3
4
20
5
98
96
99
99
9
Water
7
5
99
2180
Reaction condition: Nitrobenzene (1 mmol), FA (3 mmol), 5
wt% Pd/g-C3N4(Pd: 1 mol%), 298K
selectivity toward aniline was always above 99% (Table. 1,
entry 17 and Fig.S14†a).
5
6
5
5
99
99
99
99
2) Influence of the solvent. Solvent effects have been an
important issue for liquid phase organic synthetic reactions, so
we further studied the effect of different solvents on catalytic
transfer hydrogenation of nitrobenzene. From Table 2, the
conversion and TOF are significantly different among the
solvents examined. It is noted that the catalytic activity of
reaction in the protic solvents (water, ethanol) is much higher
than that of the aprotic solvents (toluene, cyclohexane, THF). A
possible explanation is that the hydrogenation rate is related
to H-bond donor capability of the protic solvents because the
strong interaction between protic solvents and substrate by
hydrogen bonding lowers the activation energy barrier and
leads to high hydrogenation rate. However, aprotic solvents
could strongly adsorb onto the catalyst surface and block the
active sites, thereby inhibiting the hydrogenation rate.51 More
interestingly, water has the highest TOF (3172 min-1) and gave
a full conversion toward aniline, demonstrating its superiority
as a green functional solvent. Compared to a number of
organic solvents, water may serve as a reaction promoter for
organic synthetic reactions, which is ascribable to its high
hydrogen-bond-donation (HBD) capability and low hydrogen-
bond-acceptance (HBA) capability.52 When the concentration
of nitrobenzene increasing (the volume of water is 1 ml), the
TOF value can reach 4770 min-1, reflecting some of the highest
activity for heterogeneously catalysed nitrobenzene reduction
under similar condiꢀons (Table S2†). These results show that
Pd/g-C3N4 has excellent catalytic performance for the
reduction of nitrobenzene using FA as the hydrogen donor in
water under room temperature.
7
8
30
20
96
92
99
99
9
20
10
93
94
99
99
10
11
12
120
120
95
92
99
99
[a] Reaction condition: Nitro compound (1 mmol), FA (3 mmol),
H2O (5 mL), 5 wt% Pd/g-C3N4(Pd: 1 mol%), 298K.
[b] Conversion and selectivity were determined by GC-MS.
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3) Substrate scope. To illustrate the general applicability of The main reason is that the transfer hydrogenation of C=N
Pd/g-C3N4, the method was extended to a variety of other bond to synthetize secondary amines needs higher higher
nitro compounds, and the results are presented in Table 3.
temperatures. However, when the temperature is much higher
All nitro compounds have high selectivity towards the (above 100 oC), the by-product of benzamide (4) can be
corresponding primary amines at water under room generated, resulting in
temperature over Pd/g-C3N4. However, the nitro compounds benzylaniline (2).
a decrease in selectivity of N-
with electron-withdrawing groups showed lower activity
The scope of the developed method also was explored in
, the
(Table 3 entries 4-7 vs entries 8-10), possibly because the the preparation of secondary amines. From the Table S3
†
electron-withdrawing group restrain the hydrogenation.53 In nitro compounds reacted smoothly with aromatic aldehyde
addition, the steric hindrance also greatly affect the activity of compounds to give secondary amines with excellent selectivity.
the substrates.54 For example, the reaction rate of the ortho- The influence of electronic effects and spatial effects was the
isomer was slower than the para-isomers (Table 3, entries 1-3). same as the preparation of primary amines. The nitro
When the ortho has two methyl groups (Table 3, entry 11 and compounds and aldehyde compounds with electron-
12), it needed a longer time to complete reaction.
withdrawing groups had a slower reaction rates than those
with electron-donating groups. Their reaction activity
markedly decreased with the increase of the steric hindrance
from nitro compounds and aldehyde compounds, respectively.
These obtained secondary amines are important intermediates
in chemical industry. Therefore, the present catalytic system is
highly promising for uses in industrial production of secondary
amines.
Catalytic synthesis of secondary amines
Next, we studied the reductive amination of benzaldehyde
with nitrobenzene was used as a model reaction for the
preparation of secondary amines. In our study, aldehydes were
excessive to nitro compounds (molar ratio=1.5) in order to
promote the condensation step of the aldehyde with primary
amines. Due to the transfer hydrogenation of C=N bond was
much more difficult than that of C=O bond in the nitro group,
the reactions were performed at higher temperatures. As
shown in Fig. 5, the experimental temperatures range is set up
Recyclability of catalyst
A main advantage of heterogeneous catalyst is its good
stability, recyclability and ease in separation. The reusability of
the 5 wt% Pd/g-C3N4 catalyst was investigated using the
reductive amination of benzaldehyde with nitrobenzene as a
model reaction. For each run, the reaction was carried out at
100 oC for 12 h. The catalyst after reaction was filtrated and
washed with water, followed by drying and treatment under
air at 80 oC for 12 h to remove residues. In this way, the 5 wt%
Pd/g-C3N4 catalyst was recycled six times. No considerable
change in conversion of nitrobenzene (the conversions were
ranging from 99% to 97% after 12 h) was observed. Moreover,
the selectivity toward N-benzylaniline was always above 92%
o
from 60 to 110 C. The selectivity of N-benzylaniline (2) and N-
benzylideneaniline (3) were respectively 13% and 36% at 60 °C
for 12 h. At the same time, there can also be a part of aniline
(4) in reaction system. As the temperature increased to 100 oC,
the selectivity of N-benzylaniline (2) reached as high as 95%,
but the selectivity of N-benzylideneaniline (3) decreased to 1%.
(Fig. S14†b), suggesting this catalyst has excellent reusability.
TEM images taken for the 5 wt% Pd/g-C3N4 catalyst after the
fifth catalytic cycle did not show obvious change in
morphology (Fig. S15† a). The XRD patterns and XPS spectras
(Fig. S15 b and c) showed that the structure and the valence
†
state of the 5 wt% Pd/g-C3N4 catalyst after six runs are almost
the same, confirming the high stability and reusability of the 5
wt% Pd/g-C3N4 catalyst under reaction conditions.
A
hot-filtration test was carried out to verify the
heterogeneous nature of the reaction (Fig. S16 †). After
removal of the solid catalyst at 9 h, no reaction proceeded in
the filtrate. The conversion of nitrobenzene increased with
prolonging the reaction time in the presence of the catalyst.
The conversion would be stable at 99% after 9 h. These results
indicate that the catalytic effect in this system results from the
current sample, rather than the leached metal species.
Fig. 5 N-benzylaniline formation from benzaldehyde and
nitrobenzene at different temperatures. Reaction conditions:
benzaldehyde (1.5 mmol), nitrobenzene (1 mmol), 5 wt%Pd/g-
C3N4 catalyst (2 % mol), water (5 mL), FA (4.5 mmol) and time
(12 h). The conversion and the selectivity of the intermediate
and the product were calculated based on nitrobenzene.
Mechanism analysis
In order to explore the mechanism of reaction, we studied the
decomposition rate of FA with and without nitrobenzene (Fig.
S17). In Fig. S17a, the decomposition rate of
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the similar reaction of the reduction of nitrobenzene, reducing
the nitrogen-oxygen double bond to the nitrogen-oxygen bond
and generating N-benzylaniline.
Conclusions
In this work, we have discovered that g-C3N4 supported Pd
catalyst are highly active in the transfer hydrogenation of a
variety of nitro compounds to generate their corresponding
primary amines or secondary amines by using formic acid as
hydrogen donor. Further, the catalysts showed good
recyclability without significant loss of catalytic activity even
up to six runs. Moreover, the contrast experiments reveal that
electronegative nitrogen atoms in g-C3N4 served as the base
sites to generate the Pd-formate species, followed by the
generation of a proton (H+) and a hydride (H−) to reduce the C=
N bonds by Pd NPs as a metal active center. The outstanding
catalytic activity, combining with the high selectivity, broad
substrate scope and recyclability as well as mild reaction
conditions, makes this system to be attractive alternative
pathway for transfer hydrogenations of nitro compounds to
produce their corresponding primary amines or secondary
amines, showing potential applications in chemical industries.
Scheme 2. Proposed mechanism for the preparation of
primary and secondary amine by transfer hydrogenation.
FA is very low in the absence of nitrobenzene. After 3 hours,
only 20% of FA was decomposed. However, after the addition
of nitrobenzene, the utilization of FA is significantly improved.
Experimental section
Synthesis of g-C3N4 nanosheets. g-C3N4 nanosheets were
synthesized according to the procedure reported in
literature.29 Briefly, 10 g of melamine were first heated in a
In 3 minutes, FA can achieve 100% utilization (Fig. S17†b). It is
clearly see that the reaction rate of FA catalytic transfer
hydrogenation is much higher than the rate of formic acid
decomposition. What's more, hydrogen was not observed in
the presence of nitrobenzene, which indicates that the
catalytic transfer hydrogenation reaction is preferred when
nitrobenzene is involved (Fig. S18†).
o
muffle furnace from room temperature to 520 C at a heating
rate of 4 oC/min. After calcination at 520 oC for 2 h, the as-
prepared g-C3N4 was naturally cooled to room temperature,
and ground to powders.
According to the above discussion, the possible
mechanism is proposed as follows (Scheme 2). In the first step,
g-C3N4 nanosheets have the electronegative nitrogen atoms,
which can capture the H+ from the oxygen-hydrogen bond of
FA to generate NH+ and form the Pd-formate intermediate,
which is considerable in the FA decomposition. The formate
anions tended to adsorb on the surface of the Pd nanoparticles,
as the formate anions can be coordinated with the empty d
orbitals of Pd. In the second step, the Pd-formate intermediate
can produce CO2 molecule and PdH-. The third step, when the
hydrogen atoms of NH + and PdH- combined, the hydrogen will
be generated.55 However, the combinations of hydrogen need
to be sufficient energy to drive, so the hydrogen production
rate is very low. When the system exists nitrobenzene, these
Synthesis of Pd/g-C3N4. Pd/C3N4 catalysts with a Pd loading of 1–
10 wt % (based on ICP bulk chemical analysis) were prepared
via a facile wet chemical reduction. Briefly, 0.50 g C3N4
nanosheets was introduced into 300 mL distilled H2O at 25°C
under constant stirring, and an appropriate amount of PdCl2
solution (6 g L-1) was added into the solution. After stirring at
25°C for 1 h, 0.1 M NaOH was slowly added into the mixed
solution to adjust the pH at the constant value of 9.5. During
the subsequent stirring for another 2 h, given amounts of
NaBH4 solution (freshly prepared, nNaBH4: nPd =15:1) was
rapidly injected into the g-C3N4-PdCl2 solution, while
continuously stirring the resultant mixture for 3 h again at 25°C.
Then, the suspension was filtered and extensively washed with
distilled water till chloride species remained in the reaction
solution is free. Eventually, the powers were dried in oven at
50°C for overnight to get Pd/g-C3N4 samples.
+
two parts of the hydrogen in the NH and PdH- will be
preferred react with nitrobenzene nitro group and generate
amino group, resulting in hydrogen does not produce and
nitrobenzene reduction rate is higher in the system. The fourth
step, when the system exists in the benzaldehyde, the aniline
produced in the previous step will undergo a condensation
reaction with benzaldehyde to form N-benzylideneaniline. In
the last step, the generated N-benzylideneaniline will occurs in
General procedure for the preparation of primary amines. Nitro
compounds (1 mmol), 5 wt % Pd/g-C3N4 (20 mg), water (5 mL)
were stirred for 2 min in a 10 mL glass tube at 25 °C. Next, FA
(3 mmol) was added to the reaction mixture. The solution was
stirred at 600 rpm at 25 °C for different periods of time. After
the reaction, the conversion of nitro compounds and the
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Journal Name
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Table of Content
Unprecedented catalytic performance in amines syntheses via
Pd/g-C3N4 catalyst–assisted transfer hydrogenation
Xingliang Xu, Jiajun Luo, Liping Li, Dan Zhang, Yan Wang and Guangshe Li*
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin
University, Changchun, P.R. China.
E-mail address: guangshe@jlu.edu.cn Fax: +86-431-85168358
Pd/g-C3N4 catalyst exhibited a superb performance in the transfer hydrogenation of
nitro compounds to generate their corresponding primary and secondary amines with
formic acid as the hydrogen donor in aqueous solution.