Poly(2-aminothiazole) as a unique precursor for nitrogen and sulfur co-doped porous carbon: Immobilization of very small gold nanoparticles and its catalytic application

Article abstract of DOI:10.1039/c5ra09272h

In this contribution, we report two important achievements; first the synthesis of poly(2-aminothiaozle) (P2AT) with a plate structure containing nanoparticles by a rapidly initiated strategy, and second, the one-step synthesis of nitrogen and sulfur co-doped porous carbon materials with unique morphology using P2AT as the source of both N and S, through a simple hydrothermal carbonization reaction. The obtained functional carbon may serve to develop new carbon materials for various applications, especially catalysis, because of the excellent properties, such as straightforward synthesis, defined morphology, high porosity, and heteroatom doping. As one of the most important applications, we used the porous carbon materials as a support for immobilizing gold nanoparticles (AuNPs). Benefiting from the synergistic effects of N and S co-doping, very small AuNPs, and unique structural features, the as-obtained material was demonstrated to be a superior catalyst for reduction of nitrobenzenes.

Full text of DOI:10.1039/c5ra09272h

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Poly(2-aminothiazole) as a unique precursor for nitrogen and
sulfur co-doped porous carbon: Immobilization of very small gold
nanoparticles and its catalytic application
Received 00th January 20xx,
Accepted 00th January 20xx
Yasamin Bide, Mohammad Reza Nabid*and Fateme Dastara
DOI: 10.1039/x0xx00000x
In this contribution, we report two important achievements; first the synthesis of poly(2-aminothiaozle) (P2AT) with a
plate structure containing nanoparticles by a rapid initiated strategy, and second, the one-step synthesis of nitrogen and
sulfur co-doped porous carbon materials with unique morphology using P2AT as the source of both N and S, through a
simple hydrothermal carbonization reaction. The obtained functional carbon may serve to develop new carbon materials
for various applications especially catalysis, because of the excellent properties, such as straightforward synthesis, defined
morphology, high porosity, and heteroatom doping. As one of the most important applications, we used the porous
carbon materials as a support for immobilizing gold nanoparticles (AuNPs). Benefiting from the synergistic effects of N and
S co-doping, very small AuNPs, and unique structural features, the as-obtained material was demonstrated to be a
superior catalyst for reduction of nitrobenezenes.
www.rsc.org/
investigated. Among them, sulfur doping in carbon materials is
still quite rare and further research is expected due to its
unique properties.15 According to the density functional theory
1. Introduction
The extensive availability of carbon-based materials and also
their various physicochemical properties such as electrical
conductivity, chemical and thermal stability as well as ability to
approve different morphologies, make them suitable for wide
(DFT) calculations conducted by Qiao et al, a synergistic
interaction of the N- and S-dopants was existed probably
because of an easier polarizability.16
2
Several methods such as in situ doping and post-treatment
have been suggested to introduce heteroatoms into the
carbon matrix.17, 18 Besides the more complicated process, the
main disadvantage of post treatment strategy is the
inhomogeneous distribution of heteroatoms and consequently
ineffective improvement of the bulk properties. In contrast,
the in situ doping method includes the direct carbonization of
heteroatom-containing compounds such as ionic liquids or
polymers which allows to obtain a homogeneous distribution
of heteroatoms into the carbon matrix.19-22
applications such as energy storage,1, catalysis.3-5, sorption,
and chromatography. The high surface area and porosity are
desirable properties for most of these applications and
especially for catalysis.6, 7
So, much efforts have been paid by the researchers to design
new types of functional carbons through special precursors,
surface decoration and meso-/nanopore engineering due to
the their properties.8 The modification of carbon-based
compounds through heteroatom doping may become the
‘‘Next Big Thing’’ in materials science.9 Because of the large
electronegativity of nitrogen (3.04) in comparison with C
(2.55),10 it is the most extensively adopted element among all
doping elements. Improving the electric conductivity and
2-Aminothiazole (2-AT) as
a
nitrogen and sulfur-
containing aromatic heterocyclic compound is a biologically
active material with several useful properties such as anti-
corrosion,23 antimicrobial,24 and antitumor.25 So far, only a
increasing chemical stability are just two advantages of
nitrogen doping.11,
27
12
few reports exist on the polymerization of 2-AT,26, which
Multiple doping as a useful synthetic
represents an emerging field in polymers research due to
special structure of this polymer for modern applications. To
the best of our knowledge, there is no report on employing
rapid mixing reaction route for synthesis of P2AT that
sufficient mixing can be easily attained by stirring or shaking
before the polymerization begins.28-30 In this work, P2AT with a
special plate structure containing nanoparticles was
synthesized using rapid mixing polymerization, and then it was
chosen as a single source precursor for synthesis of N and S co-
doped porous carbon materials ((N, S)-PCM) through a
hydrothermal carbonization route. Structurally, a mesoporous
approach for novel carbon materials modifies the carbon
properties one step further compared to one-type-only
heteroatom. The co-doping of nitrogen and the other
elements such as P, B (with lower electronegativity than C),
14
has been
and S (similar electronegativity with C)13,
a.Faculty of Chemistry, Department of Polymer, Shahid Beheshti University, G.C.,
P.O. Box 1983969411 Tehran, Iran. E-mail: m-nabid@sbu.ac.ir; Fax: +98 21
22431661; Tel: +98 21 29903102.
Electronic Supplementary
DOI: 10.1039/x0xx00000x
Information
(ESI)
available.
See
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and macroporous structure in the (N, S)-PCM is achieved. calculated using the Brunauer–Emmett–Teller (BET) equation
Chemically, the (N, S)-PCM displays a high number of using a PHS-1020(PHSCHINA) device. Pore size distribution was
heteroatom-doping, with N:C and S:C ratios of up to 1:4 and determined by Barrett–Joyner–Halenda (BJH) method. The
1:13 (sulfur and nitrogen contents up to 4.45 and 14.2 at.%), samples were degassed at 150 oC for 20
h before
respectively. We expected that, due to the high affinity of gold measurements. AA-680 Shimadzu (Kyoto, Japan) flame atomic
to S and N, the dual doped carbon support can enhance Au absorption spectrometer (AAS) with a deuterium background
loading with uniform dispersion. Thanks to gold-based corrector was used for determination of the metal. The
catalysts, numerous organic reactions have been reachable conversions were obtained using a Varian 3900 GC.
under simple conditions with both high yields and 2.3. Synthesis of poly(2-aminothiazole)
chemoselectivity.31 It should be notable that the catalytic
The special morphology of P2AT has been synthesized under
activity of gold nanoparticles reduces considerably as the
rapid initiated conditions. In a typical synthesis, 25 ml aqueous
particle size grows beyond 10 nm.32
solution of 2-aminothiazole (0.13 M) was mixed with 25 ml
As our continuing interest in the development of various nano-
aqueous solution of acetic acid (0.13 M) under ultrasonic
catalysts for organic transformations,21,22,33-37 herein, we
stirring for 2 min. Then 25 mL aqueous solution of sodium
employed gold nanoparticles immobilized on N and S co-doped
hypochlorite (0.065 M) as an oxidant was added into the
porous carbon structure (AuNPs@(N, S)-PCM) for catalytic
above mixture quickly and immediately stirring or shaking to
reduction of nitrobenzenes. The presence of hydrogen gas as a
highly flammable substance can pose safety concerns.38 So,
ensure sufficient mixing before polymerization begins. The
o
solutions were cooled in a refrigerator at 10 C for 10 min
the as prepared catalyst was employed for reduction of
before the reactants were mixed. The polymerization reaction
nitrobenzenes with NaBH4 as the hydrogen source because of
was carried out for 12 h at the specified temperature without
its non-flammability, easily hydrolyzable nature and controlled
H2 production rate.39
any disturbance. Reaction mixture was neutralized by 0.01 M
KOH solution. The resulting solid product was isolated by
This (N, S)-PCM not only presents a very promising material for
centrifugation and washed with MeCN (2
× 30 mL) and hot
catalytic applications, but also it can be employed in other
fields such as lithium–air batteries, extraction, sensors, and
drug delivery.
water (2× 30 mL) to separate unreacted monomer and
mineral salts, respectively. Finally, the product was dried
under vacuum at room temperature for 24 h (Scheme 1).
2.4. Synthesis of N and S co-doped porous carbon structure
derived from P2AT
2. Experimental
The
N and S co-doped porous carbon structure was
2.1. Materials
synthesized with a combined hydrothermal and freeze-drying
process. Typically, P2AT was firstly dispersed in water with
ultrasonication reaching a concentration up to 3 mg mL-1. A 5
mL aqueous dispersion of P2AT (3 mg mL−1) was sonicated for
5 min, and then, the mixture was transferred to a teflon-lined
autoclave and hydrothermally treated at 180 °C for 10 h.
Finally, the resulting sample was freeze-dried to obtain the
dual-doped porous carbon structure (Scheme 1).
Tetrachloroauric(III)
acid
trihydrate
99.5%,
sodium
hypochlorite solution, sodium borohydride, and 2-
aminothiazole were purchased from Merck Chem. Co. All other
chemicals were purchased from Aldrich or Merck companies
and used as received without any further purification.
2.2. Instruments and characterization
Fourier transform infrared (FT-IR) spectra were recorded on a
Bomem MB-Series FT-IR spectrophotometer. Transmission
electron microscopy (TEM) was performed by LEO 912AB
electron microscope. Ultrasonic bath (EUROSONIC® 4D
ultrasound cleaner with a frequency of 50 kHz and an output
power of 350 W) was used to disperse materials in solvents.
Thermogravimetric analysis (TGA) was carried out using STA
1500 instrument at a heating rate of 10 ºC min-1 in air. The
elemental analysis (CHN) was done by a conventional
combustion method based on burning off the sample, and
detecting the gases by a thermo conductivity detector (TCD).
X-ray powder diffraction (XRD) data were collected on an XD-
3A diffractometer using Cu Kα radiation. X-ray photoelectron
spectroscopy (XPS) was performed using a VG multilab 2000
spectrometer (ThermoVG scientific) in an ultrahigh vacuum.
Ultraviolet–Visible (UV-Vis) spectra were obtained using a
Shimadzu UV-2100 spectrophotometer. Scanning electron
NaOCl, CH3COOH
180 o
C
S
N
Nitrogen and sulfur co-doped
porous carbon material
NH2
HN
N
S
Hydrothermal
carbonization
Rapid initiated polymerization
n
2AT
P2AT
Scheme 1 Synthesis of N and S co-doped porous carbon structure
derived from P2AT.
2.5. Synthesis of gold nanoparticles immobilized on N and S
co-doped porous carbon structure derived from P2AT
Aqueous solution of HAuCl4.3H2O (1 mg in 2.0 mL) and (N, S)-
PCM (56 mg in 5.0 mL) were mixed and placed in an ultrasonic
bath for 30 min to well disperse metal ions through the
nitrogen and sulfur doped carbon structure. Then 4.8 mL of a
0.1 M NaBH4 solution was added to mixture to reduce and
form gold nanoparticles. After stirring for 4 h, it was filtered
under vacuum, washed well with ethanol and water and dried
under vacuum at 50 °C for 12 h.
2.6. General procedure for the catalytic reduction of
nitrobenzenes
microscope (SEM) was performed on
a Zeiss Supra
55 VP SEM instrument. The specific surface area was
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The reduction of 2-nitroaniline (2-NA) with NaBH4 was carried Fig. S1C. The peaks at 1627 and 614 cm-1 attributed to the C=N
out as a model reaction to examine the catalytic activity and and C-S stretching vibrations, respectively, are observed.
reusability of the AuNPs@(N, S)-PCM catalyst. Amounts of 1.5
mL of deionized water, 500 μL of 2-NA aqueous solution (2
mM), and 100 μL of fresh NaBH4 (0.2 M) were added into a
flask in sequence, followed by addition of 40 μL of catalyst (1
mg mL−1) to the mixture. The progress of the reaction was
monitored by tracking the changes in the UV-vis absorption
spectra of the mixture. After the reaction completion, the
catalyst was removed and then reused in the next cycle. The
catalytic reduction reactions of other nitrobenzenes were
conducted under the same condition of 2-NA.
The catalytic reduction of 2-nitroaniline with H2 was also
investigated. For a typical hydrogenation procedure, 0.02 g
AuNPs@(N, S)-PCM, 1 mmol of 2-NA and 15 ml of deionized
water were charged in the flask. The hydrogenation was taking
place under magnetic stirring and an atmospheric hydrogen
pressure at 30oC.
3. Results and discussion
3. 1. Synthesis and recognition of the catalyst
At first, the polymerization of 2-aminothiazole was
accomplished by the rapid mixing method using acetic acid as
a dopant and sodium hypochlorite as an oxidant with the
special ratios of 1:1 for monomer: dopant and 2:1 for
monomer: oxidant. To confirm the successive synthesis of
P2AT, FT-IR was employed, which is in good accordance with
the literature26 (See Fig. S1A). The morphology of the as-
obtained poly(2-aminothiazole) was investigated by SEM as
shown in Figure 1A-C.
As can be seen in the SEM image, a special plate structure with
the height of approximately 8-10 µm consisting of
nanoparticles was obtained. According to Brunauer–Emmett–
Fig. 1 SEM images of P2AT (A-C), (N, S)-PCM (D-F), and AuNPs@(N, S)-
Teller (BET) method, the surface area of P2AT was calculated
to be 38.54 m2g-1.
PCM (G, H).
Moreover, according to the obtained results from TGA (Fig.
2A), P2AT thermally degrades in four steps. 1.2% weight loss
below 100 oC is attributed to loss of moisture, adsorbed
solvent or monomer. The major weight loss occurred on 200-
600 oC is related to the conversion of P2AT to various subunits
including NH3, HCN, CS2 and carbine residue. DTA curve of
P2AT is given in Fig. S2.
After the synthesis and characterization of P2AT, the
hydrothermal carbonization following by freeze drying process
were employed for synthesis of nitrogen and sulfur co-doped
porous carbon structure. The hydrothermal carbonization
o
reaction was carried out at mild temperature (180 C) and in
pure water inside sealed autoclave and under self-generated
pressure. It has been shown that hydrothermal carbonization
Fig. 2 TGA curves of P2AT (A) and (N, S)-PCM (B).
leads to the more technical and structurally well-defined
charring by a controlled chemical method which justifies its
extensive application to produce various carbonaceous
materials with attractive structures.
Morphological study of (N, S)-PCM was accomplished by SEM
which is shown in Fig. 1 D-F. The pumice like morphology
including pores inside the pores ranges from a several
nanometers to micrometers can be observed in the SEM
images of the synthesized material. One of the most important
FT-IR spectrum of the as-obtained nitrogen and sulfur co-
doped porous carbon material derived from P2AT was given in
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factors for choosing a material as a support for catalytic single peaks, corresponding to sp2-hybridized carbon atoms
application is its surface area. The surface area of (N, S)-PCM bound to neighboring carbon atoms or hydrogen (284.8 eV)
was determined by BET method to be 334.3 m2 g-1. BET testing and the peaks assigned to the electron-poor carbon bound to
also showed a mesoporous and macroporous structure with nitrogen or sulfur (286.0 eV),43 and -N-C(S)=N- (288.5eV),44
the average pore sizes of 2.1 nm from analysis of the further proving that N and S heteroatoms have been doped
adsorption branch by the Barrett–Joyner–Halenda (BJH) into the porous carbon structure. The bonding configurations
method. To investigate the thermal stability of (N, S)-PCM, of N and S atoms in the (N, S)-PCM were also investigated
TGA analysis was performed which showed that the weight based on high-resolution N1s and S2p XPS spectra. The N1s
loss starts at 280 oC (Fig. 2B).
peak in the XPS spectrum can be fitted into three peaks:
In comparison with the XRD pattern of P2AT (Fig. 3A), the pyridinic-like N (398.6), pyrrolic-like N (399.9) and graphitic-
hydrothermal carbonization product represents a strong and like N (401.2), which are typically observed in N-doped carbon
wide peak at 2θ of 24.7o assigned to the hexagonal graphite materials.45 Moreover, the high resolution S2p peak can be
(002) reflection (2θ = 26°).40 The shift to the lower value is due fitted with three different peaks at the binding energies of
to the presence of sulfur atoms, and can be directly associated 164.0 (S2p3/2), 164.9 eV (S2p1/2), attributed to the sulfur
to the much larger size of the sulfur atom than carbon or binding in C–S bonds and conjugated –C=S– bonds, and 168.8
nitrogen.41, 42 Since nitrogen was also doped which compresses eV related to the oxidized sulfur (–SOn–).46
the layers due to higher polarization interactions and so The proposed precursor for N, S dual-doped carbon material
compensates the effect, the change in position of the (002) was compared with recently reported precursors in terms of
peak is lower compared to other sulfur doped reports.
the procedure, heteroatom contents, and surface area, which
the results have been summarized in Table 1. Elemental
analysis (CHN) of (N, S)-PCM shows 55.77% C, 17.36% N, 2.54%
H and 12.81% S. In addition, according to the XPS results,
carbon, nitrogen, sulfur and oxygen contents are 57.93, 14.2,
4.45, and 23.42 at.%, which shows that the (N, S)-PCM displays
a high number of heteroatom-doping, with N:C and S:C ratios
of up to 1:4 and 1:13. Among several important reports, our
precursor presents much higher amounts of heteroatom
doping which makes it unique for various applications.
Moreover, in the most cases, high temperatures and
complicated processes are required to obtain the N, S co-
doped carbon materials, while we use a simple hydrothermal
treatment. Actually, employing P2AT as the precursor provides
the advantages of a single step reaction with only one
compound as a source of N and S under relatively mild
conditions, high contents of the heteroatoms, high surface
area, and gram-scale product. Therefore, due to the one-pot
and straightforward synthesis, high surface area and also the
presence of nitrogen and sulfur-dual doped in the as-prepared
material, it seems that it can be an ideal choice to embed
metal nanoparticles specially for catalytic applications.
Furthermore,
after
the
successful synthesis
and
characterization of (N, S)-PCM, gold nanoparticles were
introduces to the support by the reduction of HAuCl4.3H2O
using NaBH4.
To study the morphology of the (N, S)-PCM after the
immobilization of AuNPs, SEM analysis was accomplished as
shown in Fig. 1G and H. As can be seen, the porous structure
of the carbon material was retained but its morphology was
changed due to the loading of AuNPs on their surfaces.
Transmission electron microscope (TEM) analysis on
AuNPs@(N, S)-PCM reveals the synthesis of ultra-small,
monodisperse AuNPs with diameters between 1–4 nm on the
(N, S)-PCM (Fig. 5). It was expected that due to the presence of
both nitrogen and sulfur in the synthesized material, the
support could stabilize Au nanoparticles with excellent
dispersity.
Fig. 3 XRD patterns of P2AT (A), (N, S)-PCM (B), and AuNPs@(N, S)-
PCM (C).
XPS, as one of the most versatile techniques, was employed to
investigate the composition of the as-synthesized material and
the chemical states of the elements. The XPS spectrum of (N,
S)-PCM has been presented in Fig. 4 which the peaks at 285,
532, 400, and 164 eV related to the C 1s, O 1s, N 1s, and S 2p
are observed indicating the successful doping of S and N into
the porous carbon structure by hydrothermal carbonization of
P2AT. The chemical states of C, N and S in the (N, S)-PCM were
further studied by high resolution XPS. The high-resolution
spectrum of C1s in (N, S)-PCM can be divided into several
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Fig. 4 XPS spectrum of AuNPs@(N, S)-PCM, and high resolution XPS spectra of S 2p, C 1s, and N 1s.
Table 1. Several reported precursors for N, S co-doped carbon materials.
Entry Precursor for nitrogen and sulfur doping
Amount
of
N- Surface area
Ref.
Procedure
doping/Amount of S- m2/g
doping
47
1
Cyanamide (N-doping), FeSO4/7H2O (S- Carbonization
0.9/0.6 (at.%,)
-
doping)
at 1050 oC
48
43
49
50
2
3
4
5
Thiophene/acetonitrile
Cysteine
Thiourea and formaldehyde
Pyrrolysis at 900 oC
Pyrrolysis at 900 oC
Heating at 800 oC
7.6–8.3/0.7–1.6 (at.%)
2.7/2.74 (wt.%)
8.5–17/3–8 (wt.%)
150–240
180.5
81–850
1174 and
1195
Thiazolium salts: 3-methyl-thiazol-3-ium- Carbonization at 1000 oC 5.19–6.17/2.17–3.84
dicyanamide and 3-(cyanomethyl)-thiazol-3-
iumbromide
(wt.%)
51
52
53
6
7
8
S-(2-thienyl)-L-cysteine
carboxaldehyde
or
2-thienyl Pyrrolysis at 900 oC
4.3–5/0.74–1.0 (wt.%)
4.5/2.0 (at.%)
224.5 and 321
157–220
Melamine (N-doping), benzyl disulfide (S- Heating at 900 oC
doping)
Human hair (keratin containing cysteine)
Hydrothermal treatment 2.6–3.1/0.2–1.3 (at.% )
849–897
at 180°C, Heating at
600°C
Annealing at 1000°C
45
54
9
SBA-15, sucrose
and thiourea
Citric acid as
6.53/2.88 (at.%)
-
394
-
10
Hydrothermal
the carbon source, L-cysteine as nitrogen treatment, 200°C
and sulfur source
55
56
11
12
Pyrrole and sulfuric acid
Carbonization at 650-
950 °C
KIT-6 as the template and pyrrole as the Carbonization at
4.8–10/1.7–2.6 (at.%)
978–1021
10.0−4.6/0.94−0.75 (at.%) 1064, 693, and 880
precursor
650−950 °C
depends
temperature
-
on
57
13
14
Graphene oxide, thiourea
Poly(2-aminothiazole)
Annealing at 700°C
Hydrothermal
treatment, 180°C
~6.5/0.63 (at.%)
14.2/4.45 (at.%)
334.3
This
work
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To show the role and performance of the support, and the
possible usability of the support as a metal free catalyst, the
catalytic reduction reaction of 2-nitroaniline was carried out
using (N, S)-PCM under the same reaction conditions.
According to the result, the reaction was completed after 56
min which indicated that the only support can be used as an
efficient metal free catalyst for the reduction reactions. In
2013, Kong et al. reported the metal-free catalytic reduction of
4-nitrophenol to 4-aminophenol mediated by N-doped
graphene. They suggested that the adsorption and activation
of 4-nitrophenol ions only can be carried out by the carbon
atoms next to the doped N atoms due to its weakened
conjugation and higher positive charge density.61 In addition,
the presence of S atoms in the (N, S)-PCM), may probably
enhance the catalytic effect, due to its small band gap or a
metallic form.62 Also, the π–π interactions of the aromatic
group with the support prefers the close proximity of the
reagents to the catalytic sites and establishing an efficient
catalytic reduction.
Fig. 5 TEM images of AuNPs@(N, S)-PCM.
We further characterized the AuNPs@(N, S)-PCM using XRD
(Fig. 3C). (N, S)-PCM shows a broad diffraction peak for C(002)
at 2θ=24.7° and is interpreted in terms of short-range order in
stacked carbon sheets, but in AuNPs@(N, S)-PCM, the
diffraction peak for C(002) is negligible indicating that
significant face-to-face stacking is absent and suggesting that
the carbon sheets were exfoliated due to the loading of AuNPs
on their surfaces.58 In addition, two strong bands are observed
at 2θ = 37.90o and 44.05o, which correspond to the (111) and
(200) crystallographic planes of AuNPs, and two additional
peaks appear at 2θ = 64.47o and 77.50o related to the (220)
and (311) crystallographic planes, respectively,59 which are in
good agreement with reference to fcc structure of metallic
gold (JCPDS File No. 01-1174). The broadening of Bragg’s peaks
shows the formation of nanoparticles. According to the
Scherrer equation based on the (111) peak line-width at half-
maximum intensity, the average size of AuNPs was estimated
to be 2.2 nm which is in good agreement with TEM results.
The Nitrogen adsorption isotherms of P2AT, (N, S)-PCM and
AuNPs@(N, S)-PCM and pore-size distributions obtained from
adsorption branch using the BJH method have been presented
in Fig. S3. According to the BET measurements of AuNPs@(N,
S)-PCM, the surface area of the catalyst with 2.0 nm pore sizes
are estimated to be around 174.9 m2/g, thus decreasing
greatly with Au nanoparticle loading compared to 334.3 m2 g-1
for (N, S)-PCM. This may be caused by a partial blockage of the
(N, S)-PCM pores by AuNPs and/or a partial collapse of the
mesoporous structure.
To reveal the performance of the catalyst, several
nitrobenzenes were subjected to the reduction reaction using
NaBH4 in the presence of AuNPs@(N, S)-PCM as the catalyst
(Table 2, Fig. S5-8) which their complete conversion were
reflected by the color change of the solution, from originally
bright yellow to colorless. Since in the nitroarene reactions,
the formation of diazo side products can significantly decrease
the yields, the yields obtained by GC were also presented
(Table 2). The presence of substituents such as OH, and NH2 in
ortho and para situations leads to the decreased activity
towards reduction because of the electron-donating
properties. The catalytic reduction of 2-nitroaniline with H2
was also investigated which 1,2-benzenediamine was obtained
with yield of 95% after 87 min.
The comparison of various AuNP catalysts of 2NA reduction
with NaBH4 as reducing agent is gathered in Table S1. The as
prepared catalyst indicated good results compared to most of
the reported catalysts in terms of mol% Au and time of
reaction completion. Moreover, the synthesized support could
be successfully employed as a metal free catalyst for catalytic
reduction of 2-nitroaniline.
The catalyst stability and recyclability are important issues in
the supported catalyst, so, we investigated the stability and
recyclability of the as-synthesized catalyst by measuring the
conversions up to six cycles after 4 min for reduction reaction
of 2-nitroaniline as a model reaction (The reaction proceed
was monitored by GC). The results are summarized in Fig. S9,
proving the high stability and recyclability of the catalyst.
Additionally, the leaching of the catalyst were investigated
which interestingly, no leaching of active metal was occurred
due to the high stability of Au nanoparticles.
3.2. Catalytic activity
The catalytic activity of the as-synthesized gold nanoparticles
immobilized on sulfur and nitrogen co-doped porous carbon
was investigated by choosing the reduction reaction of 2-
nitroaniline with NaBH4 as a model reaction. This reaction,
despite being thermodynamically favorable, does not occur
spontaneously due to the presence of a kinetic barrier, but in
the presence of metal nanoparticles as catalyst, the kinetic
barrier was overcome through facilitating electron transfer
from the donor BH4 to the acceptor 2-nitroaniline.60 Since 2-
-
Nitroaniline displays a typical absorbance peak at 410 nm
which decreases as it is converted to 1,2-benzenediamine, the
progress of the reaction could be easily monitored by UV-vis
(Fig. S4). According to the result, 2-nitroeniline was completely
reduced after 12 min.
6 | J. Name., 2012, 00, 1-3
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