Facile Synthesis of Small Gold Nanoparticles Stabilized by Carbon Nanospheres for Selective Hydrogenation of 4-Nitrobenzaldehyde

Article abstract of DOI:10.1002/bkcs.11859

We report the facile synthesis of small gold nanoparticles stabilized by carbon nanospheres (Au@C) for highly active and selective hydrogenation of 4-nitrobenzaldehyde. The formation of such a Au@C unique structure can be attributed to the high distribution of Au(III) species in the melamine-formaldehyde resin nanospheres, which were formed via the coordination of Au(III) with the abundant imino groups of melamine-formaldehyde pre-polymer. After the subsequent condensation and confined in situ reduction, small gold nanoparticles, and carbon nanospheres were formed simultaneously. Under the reaction conditions of 80 °C and 1.6 MPa, 99.3% conversion and >99% selectivity were achieved for the hydrogenation of 4-nitrobenzaldehyde to 4-aminobenzaldehyde.

Full text of DOI:10.1002/bkcs.11859

Article
DOI: 10.1002/bkcs.11859
BULLETIN OF THE
M. Liu et al.
KOREAN CHEMICAL SOCIETY
Facile Synthesis of Small Gold Nanoparticles Stabilized by Carbon
Nanospheres for Selective Hydrogenation of 4-Nitrobenzaldehyde
*
Minghui Liu, Jiasheng Wang, Yingcen Liu, Rongwen Lu, and Shufen Zhang
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, PR China.
*E-mail: lurw@dlut.edu.cn
Received April 17, 2019, Accepted August 1, 2019, Published online August 30, 2019
We report the facile synthesis of small gold nanoparticles stabilized by carbon nanospheres (Au@C) for
highly active and selective hydrogenation of 4-nitrobenzaldehyde. The formation of such a Au@C unique
structure can be attributed to the high distribution of Au(III) species in the melamine-formaldehyde resin
nanospheres, which were formed via the coordination of Au(III) with the abundant imino groups of
melamine-formaldehyde pre-polymer. After the subsequent condensation and confined in situ reduction,
small gold nanoparticles, and carbon nanospheres were formed simultaneously. Under the reaction condi-
ꢀ
tions of 80 C and 1.6 MPa, 99.3% conversion and >99% selectivity were achieved for the hydrogenation
of 4-nitrobenzaldehyde to 4-aminobenzaldehyde.
Keywords: Au nanoparticles, Stabilization, 4-Nitrobenzaldehyde, Polymers, Selective hydrogenation
Introduction
the abundant imino groups in the melamine-formaldehyde
pre-polymer and the acidity of chloroauric acid (HAuCl4):
usually the formation of melamine-formaldehyde resin (den-
oted as MF) nanospheres from its pre-polymer is catalyzed
by nitric acid or acetic acid. Specifically, Au(III) complexed
MF nanospheres(Au(III)@MF) were synthesized in aqueous
Small-sized gold nanoparticles exhibit high catalytic activ-
ity in various catalytic reactions.1–4 However, they are
extremely unstable as a result of their decreased size.5,6
Many strategies have been used for the stabilization of gold
nanoparticles, such as introducing suitable capping agents
or dispersing gold nanoparticles on support materials.7–10
Capping agents may cause deactivation of gold
nanoparticles or lose the protective capability at elevated
temperature due to decomposition.11,12 So, the ability to
manipulate the synthesis and stabilization of gold
nanoparticles on supports would enable us to better control
of the activity and stability, allow new insights into chemi-
cal reactions on gold nanoparticles and thus broaden the
range of their potential applications.13,14 Among different
support materials used for gold nanoparticles stabilization,
carbon has been recognized as an excellent support because
of their high stability and high porosity.15,16 Carbon
supported Au nanoparticles are typically prepared via hard
templating approaches, i.e., by adsorbing gold ions on a
hard template, followed by coating with carbon layer and
removing the template with chemical etching or calcina-
tion.17 Other routes include the incorporation of gold
nanoparticles in carbon nanospheres that are previously pre-
pared.18 Nevertheless, these methods have common draw-
backs that the process is complicated and the reaction
condition is very harsh.
ꢀ
solution at 50 C using a one-pot method, and then carbon-
ized and reduced with hydrogen to obtain Au nanoparticles
that are well-distributed in porous carbon nanospheres
(Au@C). The Au@C nanospheres were applied to catalyz-
ing the hydrogenation of 4-nitrobenzaldehyde. The catalyst
showed a high conversion and excellent selectivity of nearly
100% for 4-aminobenzaldehyde.
Experimental
Materials. Formaldehyde aqueous solution (37–41%),
melamine, chloroauric acid, ethanol, ethyl acetate,
4-nitrobenzaldehyde, and pyridine were all purchased from
Sinopharm Chemical Reagent Co., Ltd, China. All
chemicals were of analytical grade and used as received
without further purification.
Preparation of Au@C Catalyst. Melamine (0.252 g,
2 mmol), formaldehyde aqueous solution (1.5 mL), and
deionized water (40 mL) were added to a 100 mL round-
bottomed flask and stirred at 50 ꢀC for 0.5 h. After the
solution became clear which indicated the formation of pre-
polymer, 0.02 mL of HAuCl4 (0.5 mol/L) was sup-
plemented, and the solution was further stirred for 4 h for
the formation of polymer nanospheres (MF nanospheres).
The nanospheres were harvested by centrifugation at
6000 rpm for 10 min, washed with 20% ethanol for three
times, and dried at 100 ꢀC for 10 h. After heating at 400 ꢀC
In this work, we designed a pre-polymer complexing
strategy to synthesize carbon supported gold catalyst with
small size of gold nanoparticles and mesoporous structure
of carbon support. The synthetic protocol is inspired by a
revisit on the coordination ability of Au(III) species with
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for 4 h at ambient pressure with 95% N2 and 5% H2, car-
bon nanospheres with well-distributed gold nanoparticles
were obtained.
melamine-formaldehyde pre-polymer was obtained. After the
addition of HAuCl4 aqueous solution, the Au(III) complexed
with imino groups in the melamine-formaldehyde pre-
polymer,19 forming a complex matrix (according to the UV–
vis spectrum in Figure S1, File S1 in Supporting Information).
Simultaneously, H+ that HAuCl4 brought in catalyzed the
polymerization of pre-polymer,20 leading to the formation of
nanospheres with evenly distributed Au(III) due to the previ-
ous coordination of Au(III) species with the imino groups in
the melamine-formaldehyde pre-polymer (Au(III)@MF). After
the carbonization and hydrogen reduction, carbon nanospheres
with well-distributed uniform gold nanoparticles (Au@C) were
then obtained.
Figure S1 showed the morphology and size distribution
of Au@MF. The SEM image (Figure S2(a)) and TEM
image (Figure S2(b)) of Au@MF showed the morphology
of nanospheres. From Figure S1(c), we can see the
Au@MF nanospheres with an average diameter of
133 Æ 46 nm. The 13C NMR and FT-IR spectrum of
Au@MF nanospheres (Figure S3) proved that a condensa-
tion reaction occurs between the hydroxyl groups of the
melamine formaldehyde pre-polymer to form a network
structure, and further form colloidal nanospheres.
Catalytic Reaction.
The selective hydrogenation for
4-nitrobenzaldehyde was carried out as follows: 33 mg of
4-nitrobenzaldehyde, 48 μL of pyridine, 5 mg of Au@C, and
3 mL of water were added to a high pressure reactor. Tꢀhe reac-
tor was sealed, stirred at 500 rpm, and incubated at 80 C with
oil bath and 1.6 MPa H2. After the reaction was finished (3 h),
the reactor was cooled to room temperature. The suspension
was extracted with 10 mL of ethyl acetate for three times. The
catalysts were separated with filtration. The solution was dried
with anhydrous sodium sulfate and the solvent was removed
through rotary evaporation. The product was analyzed by high
performance liquid chromatography (HPLC, 6130 MSD,
Agilent, Santa Clara, CA, USA), mobile phase A is the aqueous
phase (containing 0.6% acetic acid and 0.3% triethylamine);
mobile phase B is methanol. The flow ratio was VA: VB = 2:1;
the flow rate was 0.8 mL/min; the detection wavelength was
250 Æ 4 nm; and the injection volume was 3 μL.
For the recycle test, the catalysts was washed with etha-
ꢀ
nol three times and dried under 50 C for 10 h. The test
process is the same as above.
Characterization. Analysis and characterization of the
Au@C and selective hydrogenation reaction mainly use the
following instrumentation methods: Transmission electron
microscopy (TEM) images were obtained using a JEM-
2000 (JEOL, Tokyo, Japan) transmission electron micro-
scope at 120 kV. X-ray diffraction (XRD) analysis was
carried out on a Shimadzu XD-3A X-ray diffractometer
(Nakamura-ku, Nagoya, Japan) with Cu Kα radiation, scan-
ning speed of 6ꢀ/min, span of 0.02ꢀ, and scanning angle of
10ꢀ–90ꢀ. Thermo VG ESCALAB 250 was employed to
obtain X-ray photoelectron spectroscopy (XPS) using a Al-
Kα X-ray source running with a power of 150 W
(at 15 kV). The Au content was determined by inductively
coupled plasma-optical emission spectrometer (ICP-OES,
Optima 2000 DV, PerkinElmer Inc., Waltham, MA, USA).
Figure 1(a) and (b) showed the morphology of Au@C
and size distribution under TEM, respectively. It was
observed that gold nanoparticles distributed evenly on the
carbon nanospheres. The average size of gold nanoparticles
was 2.60 Æ 0.87 nm, implicating the high activity of the
catalyst for the small size of gold nanoparticles. From
Figure 1(c), we can see the carbon nanospheres with an
average diameter of 91 Æ 26 nm, endowing the catalyst
with the ability to provide a short reaction path for the cata-
lytic reaction. Figure 1(d) is the EDS spectrum of Au@C
which indicates the existence of C, N, Cu, and Au. The sig-
nal of Cu comes from lacey support film.
Electronic states of the surface elements of Au@C were
investigated using X-ray photoelectron spectroscopy (XPS)
analysis (as shown in Figure 2). There are four typical
peaks of C 1s, N 1s, O 1s, and Au 4f in the full spectrum
of Au@C. The C 1s peak of the Au@C can be attributed to
C C (ca. 284.6 eV) and C–O/C–N (ca. 286.2 eV).21,22
The N 1s spectrum of the Au@C could be formed by
two overlapping peaks that are assigned to pyrrole N
(398.4 eV) and pyridinic N (400.2 eV), respectively.22,23
Besides, the narrow scan spectrum of Au 4f shows two
peaks with binding energies of 87.5 and 83.9 eV, which
Results and Discussion
Under the aforementioned process, small gold nanoparticles
stabilized by carbon nanospheres (Au@C) were success-
fully prepared with uniform size. Scheme 1 shows the pro-
cess for the synthesis of Au@C. Melamine was supplemented
with formaldehyde with a mixing ratio of 1:12 (mol/mol) and
Scheme 1. Procedure for the synthesis of Au@C catalyst.
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Figure 1. (a) TEM image of Au@C, (b) size distribution histogram of Au nanoparticles, (c) size distribution histogram of carbon
nanospheres (d) EDS spectrum.
Figure 2. (a) Survey XPS spectrum of Au@C, and high resolution spectra for the elements of (b) Au, (c) C, and (d) N.
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Figure 3. (a) XRD pattern of Au@C and (b) adsorption and desorption isothermal curve (inset is the pore size distribution calculated by
Barrett–Joyner–Halenda (BJH) model).
can be contributed by Au 4f5/2 and Au 4f7/2 in Au(0),
respectively.24
Figure 3(a) illustrates the XRD pattern of gold
nanoparticles which showed diffraction peaks at 38ꢀ, 45ꢀ,
65ꢀ, 78ꢀ, and 82ꢀ, corresponding to the lattice planes (111),
(200), (220), (311), and (222), respectively (according to
JCPDS: 04-0784). Figure 3(b) shows the adsorption and
desorption isothermal curves of Au@C with a simulation
based on BJH model.
Scheme 2. Hydrogenation of 4-nitrobenzaldehyde.
A
type IV adsorption–desorption isothermal was
observed, indicating a mesoporous structure of Au@C,
which is benefit for making the catalyst accessible for reac-
tant and the product releasable after the hydrogenation.
Au@C possessed a specific surface area of 169.4 m2/g.
There are two peaks in the illustration (inset of Figure 3
(b)). The pore size of the small peak is 2.8 nm, which can
be attributed to the main pore size of the mesoporous mate-
rial. The pore size of the large peak is 22.6 nm, which can
be attributed to the texture void space between the
nanospheres, i.e., that is, the stacked pores, and this is con-
sistent with the fact that a large amount of adsorption
occurs in the high pressure region in the adsorption curve
and the obvious hysteresis loop. No pores larger than
20 nm were observed in the TEM image, confirming that
the large peak indicated stacked pores.
Performance of the Au@C in selective hydrogenation of –
NO2 and –CHO groups was further evaluated using
4-nitrobenzaldehyde as a substrate (as shown in Scheme 2).
For comparison, control samples including the carbonized
resorcinol-formaldehyde nanospheres25 (denoted as C) and the
carbonized melamine-formaldehyde colloidal nanospheres
(denoted as C-MF) were also tested.20 Figure 4 showed that
the conversion of 4-nitrobenzaldehyde was only 1% with a
selectivity of 11.8% for 4-aminobenzaldehyde when C was
used. The conversion and selectivity increased slightly to
1.5% and 14.9%, respectively, when C-MF was used. In
contrast, both conversion and selectivity were dramatically
improved when Au@C was used as the catalyst for this
reaction. The high conversion (99%) and selectivity
Figure 4. Catalytic activity of different materials for selective
hydrogenation of 4-nitrobenzaldehyde.
(99.3%) indicates an excellent catalytic performance of Au@C
in generating 4-aminobenzaldehyde from 4-nitrobenzaldehyde
at 353 K. A comparison between our study and the
recently reported literature is given in Table S1. The
results indicated that the Au@C was an active and effec-
tive catalyst.
A major advantage of Au@C is that it can be recovered
easily from liquid phase reaction system. As shown in
Figure 5(a), conversion efficiency and selectivity of the cat-
alyst were still high (99.3% and 99% respectively) after five
times of reuse. Besides, no breakage and aggregation of
carbon nanospheres or gold nanoparticles were observed
under STEM (Figure 5(b)). Therefore, the Au@C catalyst
has an excellent stability under high pressure, which is cru-
cial for a catalyst application.
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Figure 5. (a) Recycle test for catalytic selective hydrogenation of 4-nitrobenzaldehyde and (b) STEM of Au@C after reaction.
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In this study, melamine-formaldehyde pre-polymer was used as
a starting material, which complexed with Au(III) and polymer-
ized to form nanospheres with evenly distributed Au(III). After
carbonization and reduction under standard gas (95% N2 and
5% H2), small gold nanoparticles stabilized by carbon
nanospheres (Au@C) were obtained. The composition of the
product was characterized with various analytical methods. The
Au@C showed a high catalytic activity and high selectivity for
hydrogenation of 4-nitrobenzaldehyde to 4-aminobenzaldehyde.
This study provides a concise approach for preparing porous
carbon supported well-distributed and stable metal nanoparticle
catalysts.
Acknowledgments. The present study was supported by
National Natural Science Foundation of China (U1608223,
21576044, 21536002, and 21576039), the Science Fund for
Creative Research Groups of the National Natural Science
Foundation of China (21421005).
Supporting Information. Additional supporting informa-
tion may be found online in the Supporting Information
section at the end of the article.
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