Ultrafine silver nanoparticles supported on a covalent carbazole framework as high-efficiency nanocatalysts for nitrophenol reduction

Article abstract of DOI:10.1039/c9ta02457c

A novel conjugated microporous polymer (CMP) material CZ-TEB was synthesized with a carbazole analogue and 1,3,5-triethynylbenzene. It possessed a high specific surface area, excellent thermal stability and layered-sheet morphology. Furthermore, ultrafine silver nanoparticles were successfully immobilized on CZ-TEB, thus preparing a nanocatalyst Ag⁰@CZ-TEB. To evaluate its catalytic performance, Ag⁰@CZ-TEB was exploited in the reduction reaction of nitrophenols, a family of priority pollutants. Ag⁰@CZ-TEB exhibited high catalytic ability, convenient recovery and excellent reusability. Strikingly, the normalized rate constant (k_nor) of the reduction reaction of 4-NP to 4-AP is as high as 21.49 mmol^-1 s^-1. This result shows a significant improvement over all previously reported work. We purposed to use a "capture-release" model to explain the high catalytic ability of Ag⁰@CZ-TEB. This explanation is supported by further experimental results that agree well with the "capture-release" model.

Full text of DOI:10.1039/c9ta02457c

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C9TA02457C
N
N
Ag NPs
Ag0@CZ-TEB
N
Knor =21.49 mmol-1 s-1
O2N
OH
H2
N
OH
Toxic waste
Industrial material
A novel covalent micro/macro porous polymer (CMP), CZ-TEB, is synthesized and then Ag
NPs are immobilized on it, the normalized rate constant (Knor) of Ag0@CZ-TEB catalyzed
reduction reaction of 4-NP to 4-AP reaches up to 21.49 mmol-1 s-1
.

JPolueransael odfoMnaotetraiadljsuCshtemmairsgtriynsA
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DOI: 10.1039/C9TA02457C
COMMUNICATION
Ultrafine Silver Nanoparticles Supported on a Covalent Carbazole
Framework as High-Efficiency Nanocatalysts for Nitrophenol
Reduction
Received 00th January 20xx,
Accepted 00th January 20xx
Wei Gong, Qianqian Wu, Guoxing Jiang and Guangji Li*
DOI: 10.1039/x0xx00000x
A novel conjugated microporous polymer (CMP) material CZ-TEB aroused great interest due to their excellent performance as
was synthesized with carbazole analogue and 1,3,5- nanocatalysts in catalytic processes.5,6 This advanced method can
triethynylbenzene. It possessed high specific surface area, excellent minimize the agglomeration of UMNPs and make the catalytic
thermo-stability and layered-sheet morphology. Furthermore, materials easy to clean and reuse. Such composite nanocatalysts
ultrafine silver nanoparticles were successfully immobilized on the provide exciting possibilities to achieve the application of UMNPs
CZ-TEB, so preparing a nanocatalyst Ag0@CZ-TEB. To evaluate its catalyst, and also opportunities for the further investigation of both
catalytic performance, the Ag0@CZ-TEB was exploited in the new and existing reactions.
reduction reaction of nitrophenols, a family of priority pollutants.
Constructed by diverse organic building blocks, porous organic
The Ag0@CZ-TEB exhibited high catalytic ability, convenient polymers (POPs), which are classified as polymers of intrinsic
recovery and excellent reusability. Strikingly, the normalized rate microporosity (PIMs),7-10 covalent organic framework (COFs),11-15
constant (knor) of the reduction reaction of 4-NP to 4-AP is as high conjugated micro- and meso-porous polymers (CMPs),16-19 are an
as 21.49 mmol-1 s-1. This result shows a significant improvement emerging class of porous materials. Compared with traditional
over all previously reported work. We purposed to use a “capture- porous materials such as zeolites and porous carbon, POPs are
release” model to explain the high catalytic ability of the Ag0@CZ- inherently designable owing to the diversity of functional building
TEB. This explanation is supported by the further experimental blocks. It implies that the POPs with various structures can be
results that agree well with the “capture-release” model.
designed and synthesized to meet the multiple purposes. Moreover,
POPs are usually constructed by light element such as nitrogen and
carbon, thus exhibiting relative light-weight and atom-efficiency. The
studies on POPs materials have been expanded to their functional
applications in many fields, including storage, separation, and
purification of gas,20-22 sensing device23, 24 and photoelectricity.25-28
As one of the most promising fields, the POPs-supported catalysts
have also been continuously studied.17,29,30 Some researchers found
that the catalytic reaction proceeding in confined spaces performed
better than traditional catalytic reaction.31,32 This finding implies that
porous materals can be used as efficient supported materials for
metal catalyst. Sharing the same advantages, the porous structure
and diverse building blocks in CMPs, which have non-negligible effect
on the material property, makes it possible to meet different
purposes, especially in supported metal heterogeneous catalyst. 33
In this contribution, a novel conjugated micro-porous
polymer (CMP) with rigid 2D chemical structure, CZ-TEB, was
synthesized using carbazole analogue (CZ) and 1,3,5-
triethynylbenzene (TEB) as building blocks. Further analysis
indicated that CZ-TEB possessed excellent thermo-stability and
high Brunauer-Emmett-Teller (BET) surface area. Moreover, Ag0
nanoparticles were immobilized on the CZ-TEB, thus obtaining
a new silver catalyst Ag0@CZ-TEB. The catalytic activity and
Introduction
Ultrafine metal nanoparticles (UMNPs) materials are a kind of metal
nanoparticles with narrow size distribution. Taking advantages of the
high surface-to-volume ratio and high-density active sites, UMNPs
are considered as a promising atom-efficient catalyst.1 However,
owing to the high surface energy, UMNPs are thermodynamically
unstable and easy to aggregate; the surface of UMNPs is particularly
prone to contamination via the direct attachment of capping
molecules with strong chemical interactions. These defects make
UMNPs lose their catalytic activity, recyclability and reusability
during catalytic reactions.2-4 Despite some pioneering work has been
made to overcome these shortcomings, it is still a major challenge to
design and fabricate highly efficient and easily reusable UMNPs.
Recently, porous framework supported UMNPs materials have
* Department of Polymer Science and Engineering, School of Materials Science and
Engineering, South China University of Technology, Guangzhou 510640,
Guangdong, P. R. China. E-mail: gjli@scut.edu.cn
† Electronic Supplementary Information (ESI) available: detailed experimental
procedures, full characterization data and the other experimental results. See
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C9TA02457C
4-nitrophenol
Toxic waste
4-aminophenol
Ag0@CZ-TEB
important industrial material
Scheme 1 Illustration of synthetic pathway and pore structure of CZ-TEB for silver nanoparticle immobilization. C, grey; N, blue; H, white;
silver nanoparticles, red.
reusability of the as-prepared nanocatalyst Ag0@CZ-TEB were (FT-IR) (Fig. 1a) and 13C cross-polarization magic-angle spinning
studied using the reduction of an environmental pollutant 4- nuclear resonance (13C CP/MAS NMR) (Fig. 1e). According to the FT-
nitrophenol as a model reaction.
IR spectrum of the CZ-TEB, the nearly completed disappearance of R-
C≡C-H resonance at 3275 cm-1 indicates that most of the compound
5 has been used up. Correspondingly, the weak stretching vibration
bands around 2197 cm-1 is observed, implying the existence of R-C≡C-
R in the CZ-TEB. In the 13C CP/MAS NMR spectrum of the CZ-TEB (Fig.
1e), there are three intense peaks between 120 to 150 ppm assigned
to the aromatic carbons and two minor peaks at ca. 115 and 93 ppm,
indicating the existence of alkynyl groups.
However, in spite of all efforts, the powder X-ray diffraction (PXRD)
for the CZ-TEB within the 2θ range of 5-60° shows a dispersion peak.
It follows that the CZ-TEB is amorphous rather than crystalline. The
layered-sheet morphology of the CZ-TEB was observed by field-
emission scanning electron microscopy (FE-SEM) (Fig. 2a). It can be
attributed to the π-π stacking between the aromatic ring structure of
adjacent layers. Besides, the CZ-TEB exhibited robust thermal
stability. Only 13.2 wt% loss was detected up to 700 °C as revealed
by thermal gravimetric analysis (TGA) (Fig. 1d).
Results and discussion
The synthesis of CZ-TEB was achieved via Sonagashira coupling
reaction of carbazole analogue 2 with 1,3,5-triethynylbenzene (5), as
shown in Scheme 1. The results of the reaction condition screening
indicated that among the products, the CZ-TEB, which was
synthesized in the mixed solvent of DMF/diisopropylamine (DIPA) at
80 °C for 3 days using tris(dibenzylideneacetone) dipalladium(0)
(Pd2(dba)3) as a catalyst, possesses the highest specific surface area,
1600 m2 g-1 (Table S1†, Enrty 5; for more details of condition
screening, see ESI). It is noteworthy that the different palladium
catalysts have a huge impact on the BET surface area of the CZ-TEB,
while the solvents do not the case. The as-prepared CZ-TEB was
further characterized by Fourier transform infrared spectroscopy
a)
b)
e)
8,11
5,14,15
6,7,9,10,12,13
c)
d)
*
*
*
* 3,4
* 1,2
Fig. 1 a) FT-IR spectra of compounds 2 (black), 5 (red), and CZ-TEB (blue); b) N2 adsorption-desorption analysis; c) pore-size-distribution of
CZ-TEB; d) TGA curve of CZ-TEB; e) 13C CP/MAS NMR spectrum of CZ-TEB.
2 | J. Name., 2012, 00, 1-3
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Owing to the fact that the CZ-TEB can be easily disVpieewrsAerdticlienOlonlwine-
polar solvent or aprotic solvent, but aggregaDteOiIn: 1p0r.1o0t3ic9/sCo9vTleAn0t24su57cCh
as water. it could be inferred that the ideal model reaction for
evaluating the catalytic performance of the prepared nanocatalyst
Ag0@CZ-TEB should have the following features: (a) the substrate
molecules can enter the pores of the catalyst and interact with the
metal or catalytically active sites in the pores; (b) once the reaction
is completed, the product can easily detach from the catalyst surface
so as to make the catalytically active site available for another
substrate molecule. Based on the above inference, the reduction
reaction of nitrophenols in water should be a model reaction for
evaluating the catalytic performance of Ag0@CZ-TEB, where
nitrophenols are hydrophobic and the reduction products or
aminophenols are prone to form hydrogen bonds with water,
thereby easily detaching from the catalyst surface. Besides, this
reaction is one of the widely used reactions for evaluating silver-
loaded catalysts. On the other hand, nitrophenols are toxic
nitroaromatics and also persistent organic pollutants in industrial
wastes. The chemical and biological stability makes them difficult to
a)
b)
3μm
3μm
c)
d)
Fig. 2 a) SEM image of the as-prepared CZ-TEB; b) SEM image of
Ag0@CZ-TEB after five catalytic runs; c) TEM image of Ag0@CZ-TEB;
d) Size distribution of silver nanoparticles (statistical data for over
100 particles).
a)
As an important structural feature of POPs, the porosity of the CZ-
TEB was also investigated. First, the nitrogen sorption experiment at
77 K was conducted. The specific surface areas and pore dimensions
were evaluated based on both BET and Langmuir methods,
respectively. As shown in Fig. 1b, the nitrogen adsorption-desorption
isotherms of the CZ-TEB show a typical type IV adsorption at low
relative pressure (P/P0 < 0.2) with an steep rised curve, implying that
nitrogen molecules occupy the small pores. The adsorption gradually
rises over the remaining relative pressure (0.2 < P/P0 < 0.85), another
steep rise occurs at relative high pressure (P/P0 > 0.85). This
increasing adsorption of nitrogen can be attributed to the inter-layer
gap.34 The pore size distribution of the CZ-TEB was calculated by
using a nonlocal density functional theory (NL-DFT) method. It can be
seen from the result in Fig. 1c that the CZ-TEB exhibits a wide pore
size distribution, as observed in purely amorphous porous
polymers.35,36 Meanwhile, the specific surface area of the CZ-TEB
determined by BET method and Langmuir method, respectively, is
1600 m2 g-1 and 1537 m2 g-1, respectively (Fig. S15†); and the total
pore volume was determined to be 2.09 cm3 g-1 (P/P0 = 0.99).
b)
Like other POPs materials, the CZ-TEB with high specific surface
area and nanoscale porous structure may be an appropriate
supported material for catalytic metal. Therefore, a new silver
catalyst Ag0@CZ-TEB was prepared using the CZ-TEB as a supported
framework for silver nanoparticles (Scheme 1, for the synthesis
details, see ESI). High-resolution TEM image (HR-TEM) confirmed the
Ag nanoparticles were in crystalline phase with a crystal plane
spacing of 0.236 nm, which corresponds to the [111] lattice plane of
Ag0 NPs (Fig. S9f†). X-ray photoelectron spectroscopy (XPS) analysis
also confirmed the presence of Ag0 nanoparticles in the Ag0@CZ-TEB
in which the characteristics of Ag0 species were observed at binding
energies of 368 eV (3d5/2) and 374 eV (3d3/2), respectively (Fig. S17†).
The loading of Ag0 was determined by Inductively Coupled Plasma-
Atomic Emission Spectrometry (ICP-AES) to be 5.1 wt%.
Fig. 3 a) UV-vis specta of Ag0@CZ-TEB catalyzed reduction reaction
of 4-NP to 4-AP; b) UV-vis specta of the corresponding reduction
reaction as control experiments, where unsupported Ag NPs or CZ-
TEB was used as catalyst, respectively.
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DOI: 10.1039/C9TA02457C
a)
b)
c)
d)
Fig. 4 a) Pseudo-first-order plot of -ln A versus time at ambient temperature and pressure; b) rate constant k for the reduction reactions of
4-NP in five consecutive runs; c) FT-IR spectra of fresh Ag0@CZ-TEB and Ag0@CZ-TEB after 5 consecutive runs. d) comparison of normalized
rate constant with other studies.
degrade in a natural environment. Many methods such as microbial s-1(Fig. 4a), with a normalized rate constant (knor) of 21.49 mmol-1 s-1
degradation, extraction, adsorption, catalytic oxidation or reduction (Table 1, Entry 1).
have been used in the treatment of nitrophenols. It is noteworthy
In order to clarify the catalysis of Ag0@CZ-TEB, a set of control
that the aminophenols produced via the reduction reaction of experiments were carried out under the same conditions. In the
nitrophenols is a class of synthetic intermediates for manufacturing control experiments conducted without the nanocatalyst Ag0@CZ-
pharmaceuticals, dyes, polymer stabilizers and imaging agents.37 TEB, the absorbance at ca. 400 nm hardly changed with time,
Hence, the reduction of nitrophenols to aminophenols is considered showing an extremely slow progress of nitrophenol reduction.37,39-41
to be a very promising way to treat nitrophenols. Currently, the In the case of using the unsupported Ag NPs, the reaction was
reduction reactions of nitrophenols using expensive Au, Pt or Pd as completed after 2 h (Fig. 3b), exhibiting a much slower reaction rate
catalysts have been studied extensively; but there is not much than in the case of using Ag0@CZ-TEB as a catalyst. To our surprise, a
research on the relatively cheap Ag catalysts.38 Therefore, novel very weak absorbance peak at ca. 295 nm representing 4-AP was
silver nanocatalysts with the characteristics of cheapness, high observed after the CZ-TEB catalyzed reduction reaction of 4-NP was
efficiency and reusability are of great significance for the reduction
of nitrophenols into aminophenols; and the potential applications in
the environmental management and sustainable chemistry also
make them attract great interest.
Table 1 Data on the rate constants of the reduction reactions of
nitrophenols with different sterically hindered substituent groups
using Ag0@CZ-TEB as nanocatalyst.
T
(°C)a
RT
RT
RT
RT
RT
RT
K
knor
(mmol-1 s-1)
21.49
20.30
0.36
Entry
Substrate
Scheme 2 The schematic diagram of “capture-release“ reaction
model. brown, porous catalyst; red dot, 4-NP; blue dot, 4-AP,
green dot, H2O; blue arrow, hydrogen bond.
(s-1)
1
2
3
4
5
6
4-NP
2-Me-4-NP
2-Cl-4-NP
2-Br-4-NP
2-NP
1.99×10-2
1.88×10-2
6.67×10-4
4.17×10-4
4.23×10-4
2.46×10-4
conducted for 20 min (Fig. 3b). It suggests that the supported
material CZ-TEB could have weak catalytic ability. However, the
catalytic ability of CZ-TEB (8.9 × 10-4 mg-1 s-1) had negligible impact
on that of Ag0@CZ-TEB. Compared with the reported studies on the
reduction of 4-NP to 4-AP using silver as a metal catalyst,42-44 this
normalized rate constant indicates a significant increase in the
catalytic activity of the prepared nanocatalyst Ag0@CZ-TEB (Fig. 4d).
What’s more, the Ag0 loading in the nanocatalyst Ag0@CZ-TEB was
calculated to be only 5.1 wt%.
0.23
0.23
0.13
3-NP
aT = 251°C
The reduction of 4-nitrophenol (4-NP) was performed in the
presence of large excess NaBH4 (100 equivs to 4-NP) and the
prepared nanocatalyst Ag0@CZ-TEB, with distilled water as solvent.
The reaction was monitored by UV-vis spectra. As shown in Fig. 3a,
the characteristic absorbance peak at ca. 316 nm of 4-NP switched
to ca. 400 nm after the addition of NaBH4, and the corresponding
reaction solution turned from yellow to fluorescent yellow. After
adding Ag0@CZ-TEB, a new characteristic peak appeared at ca. 295
nm, confirming the generaion of 4-aminophenol (4-AP).37 As the
reaction proceeded, the absorbance intensity at ca. 400 nm
decreased gradually, while the peak intensity at ca. 295 nm increased
gradually. It indicates a constantly increase of the generated 4-AP.
After only 2 min reaction time, the absorbance peak at ca. 400 nm of
4-NP disappeared, implying the the reaction was completed (Fig. 3a).
The reactions were considered as pseudo-first order in the presence
of large excess NaBH4 and the apparent rate constant value was
calculated from the ploting slope of -lnA400 against time to be 0.0199
Theoretically, The catalytic reaction in Ag0@CZ-TEB nanocatalyst
follows the Langmuir−Hinshelwood mechanism, in which BH4− ions
will be absorbed on the surface of Ag nanoparticles, and 4-NP will be
“captured“ by the pores on the CZ-TEB surface. The BH4− adsorption
on the surface of the nanocatalyst further formate the active
hydrogen species, which activates the hydrogenation of the
“captured“ 4-NP leading to the formation of 4-AP. Comparing with
reported work37 where Ag nanoparticles of the similar size was used
as a catalyst, the reduction reaction catalyzed by Ag0@CZ-TEB has
better catalytic performence. We attributed the high reaction rate
for the reduction of 4-NP to 4-AP to high porosity and surface area of
the nonacatalyst.31,32 It is assumed that this result can be explained
by the “capture-release“ reaction model as illustrated in Scheme 2.
The substrate can be simplfied as a ball and the porous catalyst as a
pocket. If the pocket dimension matches with the substrate ball, the
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substrate can be “captured“ by the pocket. Once the substrate 4-NP family of priority pollutants, at ambient temperatureVaienwdAprtircelesOsunlrinee.
is “captured“, it will be reducted in a confined space. In this way, the The studies indicate that the nanocatalystDAOgI0:@10C.1Z0-3T9E/BC9iTsAo0f24h5i7gCh
substrate molecules can get higher probability of achieving a efficiency, stability and reusability. Strikingly, the normalized rate
“effective collison“ with catalyst and NaBH4 according to the reaction constant (knor) for the reductoin reaction of 4-NP to 4-AP reaches up
collision theory. Since the reduction product 4-AP is more prone to to 21.49 mmol-1 s-1.
form hydrogen bond with H2O than 4-NP, the 4-AP molecules, once
generated, will immediately move out of the “pocket“, and then the
“vacated pocket“ is available to capture 4-NP molecules again. Such
a “capture-release“ process will continue until the substrate is used
up, thereby keeping the “pocket“ of the nonacatalyst from being
Conflicts of interest
There are no conflicts to declare.
occupied by the product. One of the key points in this model is the
size match of the substrate with the pocket. This speculation is
Acknowledgements
confirmed by the experiments on the catalytic reduction of
nitrophenols with different steric hindrance. As shown in Table 1, it
is obvious that the reduction reaction of 2-Me-4-NP with a relative
small steric hindrance shows slightly lower k than that of 4-NP (Table
1, Entry 1-2); the reduction reactions of 2-Cl-4-NP and 2-Br-4-NP with
a bulky steric hindrance proceeded much more slowly than that of 4-
NP (Table 1, Entry 3-4). Based on an analysis of the molecular size of
the different substrates, the difference in the reaction rate given in
Table 1 results from the match in size between the substrate and the
“pocket“ of the nanocatalyst or the pores on the CZ-TEB. In this
reduction reaction, the substrate 4-NP, calculated as ca. 1.4 nm from
the long axis, can match fairly with the “pocket“, calculated by
theoretically model of single unit in CZ-TEB as 1.7 nm from the long
axis. That is to say, the reaction rates of the reduction of nitrophenols
with different steric hindrance decreases with an increase in the size
of substituent group. Therefore, these results well agree with
We would like to thank the researchers in the Shiyanjia Lab
(www.shiyanjia.com) for their helping with XPS and BET
analysis.
Notes and references
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As the Ag0@CZ-TEB had been proved to be an efficient catalyst for
the reduction of 4-NP, the reduction of the isomers of 4-NP, 2-NP and
3-NP, were also investigated using the Ag0@CZ-TEB as a catalyst,
respectively (Table 1, Entry 5-6). The results show that the reduction
reactions of 3-NP and 2-NP are obviously much less efficient than
that of 4-NP. It is no surprise that the negative charges on nitroxides
in 4-NP are delocalized throughout the benzene rings more easier
than 2-NP and 3-NP due to both inductive effect and conjugative
effect.45
As a high-efficiency nanocatalyst for the nitrophenol reduction,
the stability and reusability of Ag0@CZ-TEB are of importance for the
practical applications. Therefore, the nanocatalyst Ag0@CZ-TEB was
used in five consecutive reactions of 4-NP reduction. The
nanocatalyst was recycled by centrifugation and washing with
ethanol three times and then used for the next run. After five
consecutive runs, there are only 9% decline in the apparent rate
constant for Ag0@CZ-TEB catalyzed reduction reaction of 4-NP (Fig.
4b) and 0.1 wt% loss of Ag NPs loaded onto the CZ-TEB, which was
detected by the ICP-AES; and the structure of Ag0@CZ-TEB possesses
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Conclusions
A novel covalent porous framework CZ-TEB with high BET surface
area and robust thermo-stability was successfully constructed. On
this basis, a new nanocatalyst Ag0@CZ-TEB was prepared by
immobilizing ultrafine Ag0 nanoparticles on the CZ-TEB. Furthermore,
the Ag0@CZ-TEB is exploited in the reduction of nitrophenols, a
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