Carbon nitride supported palladium nanoparticles: An active system for the reduction of aromatic nitro-compounds

Article abstract of DOI:10.1016/j.apcata.2016.04.004

Synthesis of carbon nitride supported highly dispersed ultrafine palladium nanoparticles has been reported for the reduction of aromatic nitro-compounds using hydrazine hydrate as a reducing agent. As a demonstration, the as-synthesized carbon nitride-palladium composite was shown to be a highly active and chemo-selective for the title reaction. Utilizing the optimized reaction conditions a set of aromatic nitro compounds have been converted to their corresponding amine derivatives with good to excellent yield ranging from 80% to 99%. The catalyst can be used for multiple times without affecting the catalytic performance and can also be stored for a long time at ambient condition maintaining the high catalytic efficiency.

Full text of DOI:10.1016/j.apcata.2016.04.004

Accepted Manuscript
Title: Carbon Nitride supported Palladium nanoparticles: An
active system for the reduction of aromatic nitro-compounds
Author: Debkumar Nandi Samarjeet Siwal Meenakshi
Choudhary Kaushik Mallick
PII:
S0926-860X(16)30174-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.04.004
APCATA 15832
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
7-11-2015
6-4-2016
7-4-2016
Please cite this article as: Debkumar Nandi, Samarjeet Siwal, Meenakshi Choudhary,
Kaushik Mallick, Carbon Nitride supported Palladium nanoparticles: An active
system for the reduction of aromatic nitro-compounds, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2016.04.004
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Carbon Nitride supported Palladium nanoparticles: An active system for the
reduction of aromatic nitro-compounds
Debkumar Nandi, Samarjeet Siwal, Meenakshi Choudhary, Kaushik Mallick [*]
Department of Chemistry, University of Johannesburg, P.O. Box: 524, Auckland Park 2006,
South Africa.
[*] Corresponding author: kaushikm@uj.ac.za (K. Mallick)
Graphical abstract

Research Highlights
1.
Synthesis of the Pd-nanoparticles using sodium borohydride on graphitic carbon nitride
(gCN) support.
2.
Graphitic carbon nitride supported palladium nanoparticles (Pd-g-CN) has been used
as an efficient catalyst for the chemo-selective reduction of aromatic nitro-compounds in
presence of a mild reducing agent.
3.
The catalyst showed high recyclability and can also be stored for a long time at ambient
condition maintaining the high catalytic efficiency.
4.
The optimized reaction condition is useful to prepare two new compounds, such as, 2,
6diaminophenol and 4-(1H-indole-1-yl) aniline, not previously reported in the literature.
Abstract: Synthesis of carbon nitride supported highly dispersed ultrafine palladium
nanoparticles has been reported for the reduction of aromatic nitro-compounds using hydrazine
hydrate as a reducing agent. As a demonstration, the as-synthesized carbon nitridepalladium
composite was shown to be a highly active and chemo-selective for the title reaction. Utilizing
the optimized reaction conditions a set of aromatic nitro compounds have been converted to
their corresponding amine derivatives with good to excellent yield ranging from 80% to 99%.
The catalyst can be used for multiple times without affecting the catalytic performance and can
also be stored for a long time at ambient condition maintaining the high catalytic efficiency.
Keywords: Supported Pd-nanoparticles; reduction of nitro-compounds; graphitic-carbon
nitride
1. Introduction:
Development of a high performance catalyst has attracted extensive attention in the field of
synthetic organic chemistry. Metal nanoparticles are used as an efficient catalyst and it is well
documented that a small particle size with a high surface area with accessible surface can
greatly improve the catalytic performance [1]. However, nanoparticles with high surface area
have an inherent tendency to minimize their energy through agglomeration [2]. To prevent the
agglomeration, the nanoparticles are generally capped with various stabilizers. Palladium
2

nanoparticles have served, in various industries and academic synthetic chemistry laboratories,
as a powerful catalyst for various organic transformation reactions. Commonly used stabilizers
for palladium nanoparticle are ligands [3], polymers [4-6], dendrimers [7] and surfactants [8]
and these stabilizers severely limit the accessibility of the reactant molecule toward the catalyst
surface and thus lowering the catalytic performance. To overcome this issue, metal
nanoparticles are dispersed on various solid supports that can stabilize nanoparticles as well as
increase the accessibility of the metallic surface. The stabilizing of the nanoparticles to a solid
support offers additional advantage towards the catalyst recovery and recyclability process.
Solid supports such as carbon nanotubes [9], metal oxides [10], zeolites [11], clays [12] and
naturally occurring porous diatomite [13] have been used to immobilize palladium
nanoparticles.
It has been reported that palladium nanoparticle in metal-organic framework has been used for
the Sonogashira cross-coupling reaction of aryl and heteroaryl bromides with various alkynes
under copper-free conditions [14]. A magnetically separable palladium nanoclusters, supported
on iron based metal-organic framework, exhibited excellent catalytic activity towards the
reduction of 4-nitrophenol with high recyclability performance [15]. Polymer supported
nanosized palladium catalyzed Sonogashira reaction has been reported in an environmental-
friendly reaction system with ethanol as the solvent and potassium carbonate as a base [16]. In-
situ synthesized conjugated polymer stabilized palladium nanoparticles has also been reported
for Heck and Suzuki coupling reactions where the metal particles within the size range of 2-3
nm are stabilized within the polymer matrix [4-6]. Polypyrrole stabilised palladium
nanoparticles showed good catalytic efficiency for the chemoselective transfer hydrogenation
of , -unsaturated carbonyl compounds with high specificity and selectively for the olefins
or acetylenes only, without affecting the carbonyl moiety, for a wide range of substrates [17].
Carbon nanotube-palladium nanoparticle system was evaluated for the Suzuki and Stille
coupling reactions and the hybrid material showed a significant catalytic activity for both the
reactions [18]. The carbon nanotube supported palladium catalyst was applied for the
hydrogenation of commercial polystyrene under mild reaction conditions. The catalyst
characterization showed that the active metal deposited on the external surface of the nanotube
with a good dispersion [19].
The selective reduction of nitroarenes to anilines is an important reaction in pigment,
agrochemical and pharmaceutical industry. The traditional process for the reduction of
nitroarenes using metallic iron generates large amounts of waste. Thus, hydrogenation of
3

nitroarenes catalyzed by transition metals is now the system of choice for the successful
transformation of anilines [20]. The source of reducing agents, such as, hydrogen, sodium
borohydride, formic acid, hydrazine hydrate along with the metal catalysts can be considered
as the suitable combination for the reduction of nitro groups.
Hydrazine hydrate can be considered as an efficient transfer hydrogenation reagent as it is easy
to handle and generate only nitrogen as a product. In the past few years, several reports have
been published for the reduction of nitroarenes employing the combination of hydrazine and
the metal catalyst. Titanium oxide supported gold nanoparticle catalyzed transfer
hydrogenation reaction has been reported for the reduction of the aryl and alkyl nitro
compounds to the corresponding amines in the presence hydrazine hydrate [21]. Graphene
oxide supported iron oxide nanoparticles showed the performance as a catalyst for alkene
hydrogenation to the corresponding alkanes using hydrazine hydrate as the source of hydrogen
[22]. Hydrazine has also been used as a reductant of aromatic and heteroaromatic nitro
compounds in the presence of several metal catalysts [23-27]. Traditionally for catalytic
hydrogenation reaction, the metal catalysts based on Ru [28], Rd [29], Ni [30], Fe [31], Au
[32], Pt [33], and varieties of reducing agents have been employed. The advantages of using
nanoparticulate palladium as catalyst for a variety of organic reactions have been widely
recognized and attention has also been paid for palladium based catalyst as it performs as an
excellent role for many organic hydrogenation reactions [34-39].
In this current study, we report a simple and effective method for the synthesis of graphitic
carbon nitride (gCN) stabilized palladium nanoparticle catalyst, Pd-gCN, for the reduction of
nitroarene compounds. The advantages of using gCN, as a support system, are (a) inexpensive
raw material (urea) has been used for synthesis, (b) highly recyclable for heterogeneous
catalysis reaction and (c) moderately accurate metal loading ability. For the current aromatic
nitro reduction reaction, the combination of gCN supported palladium catalyst and hydrazine
hydrate (a mild reducing agent) have several beneficial features including the chemo selective
and regioselective product formation and the reaction is very effective for the reduction of a
wide range of nitroarene compounds to aromatic amines. Other than that, an environmentally
benign process as only nitrogen is the bi-product from the reaction. Motivation for this
synthetic protocol is also attributed to an increased purity in the final product and also no
catalyst degradation process has been noticed even after several times of recycling process. The
catalyst was characterized by means of various optical and microscopic techniques, which
4

offered the information about the structural properties and also the size and distribution of the
metal nanoparticles.
2. Experimental:
2.1. Materials: All the chemicals and the solvents used for this experiment were of
analytical purity and used without further purification. Ultra-pure water (specific
resistivity >17 MΩ cm) was used in this experiment wherever required.
2.2. Material characterization: Fourier transform infrared spectroscopy (FTIR)
spectra were collected utilizing a Shimadzu IRAffinity-1 with a spectral resolution
1
of 0.5 cm . Transmission electron microscopy (TEM) studies of the
nanocomposite were carried out at 200kV on a JEOL-200. An energy-dispersive
X-ray analyser (EDX) attached to the TEM was used to determine the chemical
composition of the samples. The X-ray diffraction (XRD) patterns were recorded
on a Shimadzu XD-3A X-ray diffractometer operating at 20 kV using Cu-Kα
radiation (k = 0.1542 nm). The measurements were performed over a diffraction
angle range of 2θ = 20o to 70o. X-ray photoelectron spectra (XPS) were collected
in a UHV chamber attached to a Physical Electronics 560 ESCA/SAM instrument.
Nitrogen adsorptiondesorption isotherms were measured with a Quadra-Sorb S1
apparatus at 77 K. The surface areas were calculated by the Brunauer–Emmett–
Teller (BET) method, and the pore size distribution was calculated from the
desorption branch using the Barett-Joyner-Halenda (BJH) theory. NMR spectra
1
(1H and 13C) were recorded on a Bruker AVANCEIII-400 spectrometer. H NMR
13
(400 MHz) and C NMR (100 MHz) were registered in CDCl3 or DMSO-d6 as
solvent and tetramethylsilane (TMS) as an internal standard. Chemical shifts are
reported in δ units (ppm). All coupling constants (J) are reported in Hertz (Hz).
2.3. General Procedures for the preparation of Pd-gC3N4: In a typical experiment,
15 g of urea was kept in a
for 12h. After that the dried urea was
C. The lemon yellow coloured product was washed several time with distilled
water to remove the residual alkaline species (e.g. ammonia) on the
covered crucible at 80 C under ambient pressure
put in a Muffle Furnace for 4h at 550
sample surface and again dried at 80C for 24 hours under vacuum. In the second
stage of the synthesis, 0.5 g of the resultant product was dispersed in 10 mL of
5

water and 0.1 M of K2PdCl4 (2.34 mL) was added drop wise (5 wt% loading of
Pd). After that, 5 mL of 1 10-3 M NaBH4 solution was added dropwise for the
reduction of palladium salt. At the end, the material was filtered, washed with water
and dried. The resultant material, Pd(0)gCN, was characterized using different
techniques and applied as catalyst for the reduction of nitroarene compounds.
2.4. General Procedure for the reduction of nitroarenes:
In a typical reaction, 5.0 mg of Pd-gCN (5.0 wt% of Pd) catalyst was added to the solution of
1.0 mM of nitroarene in ethanol (2 mL) and 2 mM (1.2 equiv. 0.07 mL) of 60% of hydrazine
hydrate. The mixture was placed into a 10 mL round-bottom flask at the reflux temperature (70
oC) for the 4h and then allowed to cool at room temperature. The resultant material was filtered
and the filtrate was subjected to column chromatography over silica gel to obtain the
corresponding products. For di-nitroarenes substrates 4.0 mM (2.4 equiv. 0.14 mL) of 60% of
hydrazine hydrate solution was used.
2.5. General Procedure for the Ullmann reaction: In a 50 mL round-bottom flask,
pyrrole or pyrazole or indole (2.0 mM), 4-iodonitrobenzene (4.4 mM, 1.2 equiv.),
Pd(PPh3)4 (2.5 mol%) and K2CO3 (2.2 equiv.) were added under continuous
stirring condition. To this, 5.0 mL of dry DMA was then added under reflux
condition at 110 °C for 20 h. After completion of the reaction (monitored by thin-
layer chromatography), the reaction mixture was allowed to cool at room
temperature and diluted with distilled water. After that the solution was extracted
with ethyl acetate and the organic layer was dried over anhydrous MgSO4, which
was subjected to column chromatography to obtain the corresponding products.
3. Result and discussion:
3.1. Characterization of gCN and Pd-gCN: The TEM images (figure 1) show the
film-like morphology with a layered structure of the gCN (A) and palladium
particle decorated gCN, Pd-gCN, (B). The higher magnified TEM image (C)
indicates the palladium nanoparticles, with wide range of size distribution, are
highly dispersed on the gCN surface. The structures of gCN and palladium loaded
gCN films were analyzed by XRD, shown in figure 2A. The strong XRD peak
(002) at 27.25o, for both the samples, is revealing of the graphitic-like layered
crystalline structure in the gCN [40]. The low angle peaks (marked within the
6

circle) are the indication of the construction of the in-planar ordering of the gCN
samples. In the spectrum (b), the peaks located at 39.56o, 45.93o, 67.23o and 81.12o,
correspond to the (111), (200), (220) and (311) planes of an fcc (face-centered
cubic) lattice, respectively. This result confirms that the metallic palladium
nanoparticles formed the fcc crystalline structure. To evaluate the extent of
formation of the metallic state of palladium, X-ray photoelectron spectroscopy
(XPS) measurement was carried out. In the figure 2B, the XPS spectrum of the
palladium shows double peaks with binding energies at 335.0 eV and 340.80 eV,
resulting from the spin-orbital splitting of Pd 3d region (3d5/2 and 3d3/2,
respectively) and both these peaks being distinctive for metallic palladium [41].
The Fourier transform infrared spectroscopy measurement of Pd-gCN revealed a
typical molecular structure of gCN. The vibration bands at 1635, 1555, 1451 and
1
1407 cm were assigned to typical stretching vibration modes of heptazine-
derived repeating units which agree with the FTIR analysis for the g-CN [42]. The
1
intense band at 810 cm represented the out-of-plane bending vibration and the
1
typical characteristic of heptazine rings. The bands at 1317 and 1235 cm
correspond to stretching vibration of connected units of C-N(-C)(-C) (full
condensation) or C-NH-C
1
(partial condensation). The broad band at 3157 cm corresponds to the stretching modes of –
NH2 or =NH groups, which are uncondensed amine groups.
The nitrogen sorption measurements at 77K are routinely used for characterizing porous
materials and the procedure universally known as the Barrett-Joyner-Halenda (BJH) method
which is the most popular technique for the determination of the pore-size distribution of the
support material of the catalyst. The interpretation of physisorption hysteresis is important in
the context of pore structure characterization. According to figure 4, the nitrogen adsorption
and desorption isotherms for both gCN (curve ‘a’) and Pd-gCN (curve ‘b’) samples possess a
typical H3-type hysteresis loop according to the IUPAC (International Union of Pure and
Applied Chemistry) classification. The corresponding pore size distribution curve by the BJH
method demonstrates that the average pore diameter was 20 nm for both gCN and Pd-gCN
samples. The BET surface area of the gCN was found to be 63.70 m²/g, whereas, the
incorporation of palladium nanoparticles on the surface of the gCN enhance the surface area
for Pd-gCN sample to 74.26 m²/g.
7

3.2. Catalytic performance of Pd-gCN for the reduction of nitro group: The
chemoselective reduction of nitro groups has enormous importance in organic
synthesis and for that purpose the development of a nontoxic and clean system is
highly required for a successful transformation for such kind of reaction. The
catalytic property of the Pd-gCN for the reduction of aromatic nitro compounds in
the presence of hydrazine as a reducing agent has been investigated in this present
study. For the success of reduction of nitro groups the solvent plays an important
role in the reaction media. In this study, a series of solvent systems (CH3OH,
C2H5OH, iPrOH, dioxane, TFH, DCM and also the combination of polar and
nonpolar solvent) have been utilized in order to determine the optimum condition
of the reaction (supporting information, Table S1). Among the various solvents,
ethanol / methanol and PdgCN (5.0 mol%) along with hydrazine was found to be
highly efficient for the reduction of nitro benzene to aniline. In the absence of
hydrazine there was no indication for the formation of the product (aniline) (Table
S1, entry 1). Using hydrazine as a reducing agent in the absence of the catalyst, the
desired product has not been achieved (entry 2) in the presence of ethanol. In the
presence of catalyst and hydrazine, both methanol and ethanol showed identical
amount of yield for four hours reaction period (entries 4 and 5). It is also important
to mention that the concentration of the catalyst has a tremendous role regarding
the performance of the reaction. Catalyst with 3 mol% palladium forms the product
with 55% yield after 12h whereas 5 mol% palladium forms the product with 99%
of the total yield after 4h under the same solvent environment. Further increase of
the palladium concentration of the catalyst (entry 6) no significant improvement,
in terms of the duration of the reaction, has been observed. The combination of
ethanol-water (1:1) system was not also effective as ethanol alone (entry 10). The
formic acid-triethylamine system when used for the reduction of nitro benzene a
yield of 40% aniline has been obtained (entry 11).
Based on the above optimized reaction conditions, we have chosen ethanol as a solvent for the
rest of our studies as it is less toxic than methanol. Now we have explored the versatility of the
catalyst, Pd-gCN, and the reducing agent, hydrazine, for the reduction of diverse nitroarene
compounds, and the results are summarized in Tables 1 and 2. All the substrates produced the
expected nitro group reduced product with very good to excellent yields and selectivity.
Initially, we have tried the reduction of nitrobenzene with different substituents (such as, -NH2,
8

-OH, -CH3, -Cl, table: 1, 1-8) and in all the compounds the nitro group has been converted to
amine group with the yields of more than 90%. When nitro group is attached with pyridine
produced pyridine-2-amine with a yield of 90% (9) but for 2-chloro-3nitro pyridine system the
product pyridine-3-amine has been obtained with a yield of 95% with the elimination of the
chloride group (10). For the reduction of 2, 6-dinitrophenol (2.4 equivalent amount of reducing
agent was used) the product 2, 6-diaminophenol (11) has been obtained with a yield of 80%. It
is also important to mention that the reduction reaction using the graphitic carbon nitride
supported palladium catalyst is very specific for nitro group reduction. We have chosen two
compounds, such as, 1-nitro-3-vinylbenzene and metanitrobenzoic acid and observed that 3-
vinylaniline and 3-aminobenzoicacid has been obtained without affecting the vinyl and
carboxylic acid group, respectively (12 and 13).
We also have performed a comparative study for the reduction of nitrobenzene in the presence
of halogen group in the para position using Pd-gCN catalyst. The reduction of
4bromonitrobenzene has produced 4-bromoaniline (14, 82% yield) along with benzidine (15,
7% of yield) and a similar trend has been observed for iodonitrobenzene with the formation of
4-iodoaniline (16, 55% yield) and benzidine (27% yield). For the reduction of
4chloronitrobenzene only 4-chloroaniline was the product with a yield of 95% (17) and no
evidence has been found regarding the formation of benzidine. The suggested mechanism for
the benzidine formation is the oxidative addition of carbon-halogen bond to the metal centre
and followed by reductive elimination of the same which results the formation of 4,
4'dinitrobiphenyl as a side product. Amount of formation of this product is generally has the
inverse relation to the bond strength of C-X bond (Cl>Br>I) and finally this side product is
reduced to benzidine under the present reaction condition.
We also have extended our study for nitro substituted bi-arene system under the reaction
condition as mentioned in table 2. The three compounds, 1-(4-nitrophenyl)-1H-pyrrole, 1-
(4nitrophenyl)-1H-pyrazole and 1-(4-nitrophenyl)-1H-indole, has been synthesized using
Ullmann chemistry from the precursors of pyrrole, pyrazole and indole, respectively, with
4iodonitrobenzene (detail method of synthesis is available in the experimental section). The
reduction of 1-(4-nitrophenyl)-1H-pyrrole and 1-(4-nitrophenyl)-1H-pyrazole produced 4(1H-
pyrrole-1-yl) aniline (18) and 4-(1H-pyrazole-1-yl) aniline (19) with the yield of 91 and 95%
respectively. Again, when 1-(4-nitrophenyl)-1H-indole under went for the similar reduction
condition the product 4-(1H-indole-1-yl) aniline (20) has been obtained and to the best of our
knowledge this is the first kind of report for the synthesis of such compound. The product
9

benzidine (15) with a yield of 93% has also been obtained from the reduction of 4,
4'dinitrobiphenyl. The products N1-phenylbenzene-1, 2-diamine (21), quinolin-5-amine (22),
naphthalene-1, 8-diamine (23), with the yield of 99, 96 and 92% has been achieved due to the
reduction of 2-nitrophenylaniline, 5-nitroquinoline, 1, 8-dinitronapthalene respectively, using
Pd-gCN catalyst and hydrazine as a reducing agent. The ether linkage in 1-nitro-
3phenoxybenzene was unaffected during the reduction reaction and formed the compound
3phenoxyaniline (24). Attempts have been made for the reduction of nitro group in
4nitrobenzaldehyde at different conditions using Pd-gCN catalyst. When hydrazine was used
as a reducing agent, (4-nitrobenzylidene) hydrazine, (scheme: 1, 25), with a yield of 94% has
been obtained, whereas, an acetal product, 1-(diethoxymethyl)-4-nitrobenzene (scheme: 1, 26)
with a yield of 96%, was formed in the presence of formic acid. It has been reported that a Pd-
based catalyst can be used for a dehalogenation reaction [43] whereas in our catalytic system,
the dehalogenation of the haloarene substrates was not affected under the optimised conditions
(Table 1, 5, 14, 16 and 17 with an exception of 10). In the present catalytic hydrogenation
reaction, an interesting activation trend has been observed for 4halonitrobenzene system and
already discussed earlier (14-17).
3.3. Scaling up, durability and the kinetic study of the catalyst: Another important
point concerning in heterogeneous catalysis is the lifetime or durability of the
catalyst and the scaling up of the reaction. Therefore, the recyclability of Pd-gCN
and the scaling up of the hydrogenation reaction of nitrobenzene was investigated
at 70oC under reflux condition and we also have increased the amount of the
catalyst, reducing agent and the nitrobenzene by a factor of ten. A yield of 98% of
the hydrogenated product (aniline) has been achieved. After the first run, the
product was extracted with ethyl acetate, and the residual catalyst was reused
another five times under the same conditions. After each cycle, the catalyst was
recovered by filtrating and washing with ethanol, and then reused under the same
reaction conditions similar to the initial cycle. The results indicated that the used
catalyst was also active without a significant loss of performance (Fig. 5A). At the
end of the sixth cycle, a yield of 87% of the hydrogenated product has been
achieved. The catalyst was again characterized by TEM at the end of the sixth cycle
and a fair amount of particles agglomerations has been noticed in the TEM image
(Fig. 5B). The decrease of 11% of yield of aniline at the end of sixth cycle is due
to the loss of catalyst during the filtration and washing process. The catalyst system
10

involved with metallic palladium, the effect of metal leaching can also be
considered as one of the reasons for deactivation of the catalyst but the ICP-MS
(inductively coupled plasma mass spectrometry) analysis did not deliver any
indication of the leaching of palladium species, from the filtrate, at any stage of the
recyclability study of the catalyst. The decrease of the yield with recycling was
mainly due to the loss of catalyst species during the filtration process, where the
recovery of all the catalyst from the filter paper is a difficult task and also partly
due to the agglomeration of the palladium nanoparticles. The reproducibility of the
catalyst for the recycling experiment was also performed under different condition
and produced almost identical result (supporting information: Reproducibility
study of the catalyst for the recycling performance).
The kinetic study of the hydrogenation reaction, in the presence of the Pd-gCN catalyst (5.0
mol% Pd) at 70 °C for a period of four hours, has been performed for nitrobenzene in the
presence of hydrazine. At the beginning of the reaction, the conversion of nitrobenzene was
slow. In the first 30 min, no evidence of the product (aniline) was detected, which can be
considered as the induction period of the reaction, but after that a slow reduction of
nitrobenzene has been noticed. After 90 min of the reaction 20% nitrobenzene has been
reduced. A uniform and steady conversion has been noticed after 90 min and 90% conversion
was obtained after 210 min. After four hours, an almost full conversion of nitrobenzene to
aniline has been achieved, which is represented graphically (Figure 5C).
3.4. Mechanism for catalytic hydrogenation: The most convincing mechanism for
the catalytic reduction of nitroarene in the presence hydrazine is the electron relay
through the palladium nanoparticles. The decomposition of hydrazine process
involves the release of two protons and two electrons. The energy gap between
HOMO and LUMO is much higher for the nitroarene derivatives, hence hydrazine
cannot reduce the substrates because of its inefficient ability of electron donation.
The success of the reduction process is due to the presence of palladium
nanoparticles which form a π-complex with nitroarene to shorten the energy gap
between HOMO−LUMO so that the reducing agent can transform nitroarene to the
corresponding aminoarene in a facile route [44]. The formation of the π-complex
could be one of the important reasons of induction period for the every cycle of
reaction.
11

In general, the decrease in catalytic activity of the supported nanoparticles has been observed
during the storage due to the surface oxidation of palladium nanoparticles [45]. However, the
Pd-gCN catalyst exhibited excellent storage stability under ambient atmosphere for one month
as we did not find any adverse effect on performance for the current hydrogenation reaction
system. The main reason may be ascribed to the fact that the strong bonding interaction between
nitrogen and palladium surface and the insertion of the particles into the layered structure of
the carbon nitride films that prevent both form the leaching and surface oxidation of the
palladium particles.
4. Conclusion:
In summary, we have synthesized a palladium nanoparticle and carbon nitride based
nanocomposite which is clean, free from stabilizer and acts as a high performance catalyst for
the reduction of the aromatic nitro compounds in the presence of mild reducing agent. In
addition, the catalyst is stable enough due to the strong interactions between the support and
nanoparticles which prevents the leaching of the palladium nanoparticle from the support
system and also proved as a highly recyclable catalyst with minor deactivation after several
cycle of the reaction.
Acknowledgement: The authors (DN, SS, MC and KM) acknowledge financial support from
the Research Committee and the Faculty of Science of the University of Johannesburg. MC
and KM also acknowledge financial support from the National Research Foundation, South
Africa. DN and SS further acknowledge financial support from the Global Excellence and
Stature fellowship from the University of Johannesburg.
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Figure caption:
14

Figure: 1
(A) The TEM image show the film-like morphology with a layered structure of the gCN, (B)
palladium particle decorated gCN and (C) the higher magnified TEM image of the Pd(0)gCN
composite material.
Figure: 2
(A) The strong XRD peak at 27.25o, for both the samples, gCN (spectrum ‘a’) and Pd(0)gGN
(spectrum ‘b’), is revealing of the graphitic-like layered crystalline structure. The low angle
peaks (marked within the circle) are the indication of the construction of the in-planar ordering
of the gCN samples. In the spectrum ‘b’, the reflection peaks located at 39.56o, 45.93o, 67.23o
and 81.12o, correspond to the (111), (200), (220), and (311) planes of an fcc lattice,
respectively. (B) The XPS spectrum of the palladium shows double peaks with binding energies
at 335.0 eV and 340.80 eV, resulting from the spin-orbital splitting of Pd 3d region (3d5/2 and
3d3/2, respectively) and both these peaks being distinctive for metallic palladium.
Figure: 3
FTIR spectrum of the Pd(0)-gGN
Figure: 4
Nitrogen adsorption and desorption isotherms and pore size distribution (in-set) of the gCN (a)
and Pd(0)-gGN (b), respectively.
Figure: 5
(A) The histogram from the recyclability study of the Pd(0)-gCN catalytic system shows that
the used material was active as a catalyst without a significant loss of catalytic performance.
At the end of the sixth cycle, a yield of 87% of the reduced product (aniline) was achieved. (B)
The TEM image of the used catalyst at the end of the sixth cycle reveals the agglomeration of
the palladium nanoparticles. (C) Kinetic study of the reduction of nitrobenzene in the presence
of Pd(0)-gCN as a catalyst and hydrazine as a reducing agent at 70oC for four hours.
15

16

17

18

19

Table: 1. Reduction of various substituted nitrobenzene a, b
a Reaction conditions: nitroarenes (1 mmol), Pd-gCN catalyst (5 mol%), N2H4.H2O (1.2 mmol,
0.07mL), EtOH (2 mL), 70⁰C, 4h reflux. b Isolated yields. c Product obtained from 2-chloro-3-nitro
pyridine. d 2.4 equiv., 0.14 mL of N2H4.H2O has been used.
20

Table: 2. Reduction of various substituted nitro bi-arenes a, b
a Reaction conditions: Nitrobiarene (1 mmol), Pd-gCN catalyst (5 mol%) N2H4.H2O (1.2 mmol,
0.07mL), EtOH (2 mL), 70⁰C, 4h reflux.b Isolated yields. c 2.4 equiv., 0.14 mL of N2H4.H2O has been
used.
Scheme: 1. Reactions involved with 4-nitrobenzaldehyde under different conditions.
21