Reduced Graphene Oxide Supported Ag Nanoparticles: An Efficient Catalyst for CO2 Conversion at Ambient Conditions

Article abstract of DOI:10.1002/cctc.202000738

A highly efficient carboxylative cyclization of propargylic alcohols with CO₂ under atmospheric pressure catalyzed by silver (0) nanoparticles decorated reduced graphene oxide (Ag-rGO) is reported. Ag-rGO was fully characterized by scanning electron microscope spectra (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectra, Raman spectra and X-ray photoelectron spectroscopy (XPS). Notably, Ag-rGO can be also applied to the construction of other value-added chemicals (β-oxopropylcarbamates and 2-oxazolidinones) from CO₂ at ambient conditions. In addition, Ag-rGO is stable and reusable, which shows the potential for the practical application for CO₂ capture and utilization (CCU).

Full text of DOI:10.1002/cctc.202000738

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Reduced Graphene Oxide Supported Ag Nanoparticles: An
Efficient Catalyst for CO2 Conversion at Ambient Conditions
Authors: Xiao Zhang, Kai-Hong Chen, Zhi-Hua Zhou, and Liang-Nian
He
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To be cited as: ChemCatChem 10.1002/cctc.202000738
Link to VoR: https://doi.org/10.1002/cctc.202000738
01/2020

10.1002/cctc.202000738
ChemCatChem
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Reduced Graphene Oxide Supported Ag Nanoparticles: An
Efficient Catalyst for CO2 Conversion at Ambient Conditions
Xiao Zhang§[a], Kai-Hong Chen §[a], Zhi-Hua Zhou[a], Liang-Nian He*[a]
[a] X. Zhang, Dr. K.-H. Chen, Z.-H. Zhou, Prof. Dr. L.-N. He
State Key Laboratory and Institute of Elemento-Organic Chemistry
College of Chemistry
Nankai University
Tianjin 300071 (P.R. China)
E-mail: heln@nankai.edu.cn
§
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: A highly efficient carboxylative cyclization of propargylic
alcohols with CO2 under atmospheric pressure catalyzed by silver (0)
nanoparticles decorated reduced graphene oxide (Ag-rGO) is
reported. Ag-rGO was fully characterized by scanning electron
microscope spectra (SEM), transmission electron microscopy (TEM),
Fourier transform infrared (FT-IR) spectra, Raman spectra and X-ray
photoelectron spectroscopy (XPS). Notably, Ag-rGO can be also
applied to the construction of other value-added chemicals (β-
oxopropylcarbamates and 2-oxazolidinones) from CO2 at ambient
conditions. In addition, Ag-rGO is stable and reusable that shows the
potential for the practical application for CO2 capture and utilization
(CCU).
applications in supercapacitors, energy, and photoelectric
technology.[9] Particularly, in catalytic research, graphene has
been widely used as an excellent supporter for metal or metal
oxide nanoparticles because of its large specific surface area.[10]
In consideration of the excellent dispersion of graphene to metal
particles and the outstanding stability,[11] we report here fully
characterized reduced graphene oxide (rGO) supported Ag NPs
catalysts (Ag-rGO) for the reaction of propargylic alcohols with
CO2 under atmosphere pressure. Ag-rGO-3 exhibits considerably
high activity in this reaction, and the activity can maintain after
reusing five times. Remarkably, this catalyst also works well for
the construction of β-oxopropylcarbamates and 2-oxazolidinones
from CO2 (1 bar).
Introduction
Results and Discussion
With increasing global warming effect caused by CO2, great
efforts have been made to reduce CO2 emissions.[1] Indeed, CO2
is a nontoxic, abundant, and sustainable C1 feedstock for organic
synthesis through the substitution of fossil-based processes.[2]
Thus, CO2 capture and utilization (CCU) has attracted enormous
interest around the world.[3] In this context, chemical fixation of
CO2 into value-added products through the formation of C-C, C-
O and C-N bonds is of great significance. Unfortunately,
transformation of CO2 at mild conditions still remains a great
challenge because of the thermodynamic stability and kinetic
inertness of CO2.
Scheme 1. Preparation of Ag-rGO nanocomposite.
α-Alkylidene cyclic carbonates are important moieties of
natural products and applied widely as intermediates in organic
synthesis.[4] The synthesis of α-alkylidene cyclic carbonates from
carboxylative cyclization of propargylic alcohols with CO2 is an
attractive approach to CO2 upgrading.[5] Although many molecular
catalysts have been developed for this transformation, most of
them suffer from tedious and cost-effective separation process.
With excellent activity and reusability, heterogeneous catalysts
have been broadly applied in industry. In the field of CCU, a few
of supported metal catalysts, such as porous polymer or covalent
organic frameworks (COF) supported Ag nanoparticles (NPs),[6]
Ag-porous polymer, and Cu-metal-organic framework (Cu-
MOF)[8] have been applied in the reaction between propargylic
alcohols and CO2. Nevertheless, it is still desirable to develop a
robust (especially during recycle progress) heterogeneous
catalyst with high efficiency and selectivity.
Firstly, Ag-rGO was prepared from the reduction of graphene
oxide and AgNO3,[9a] as illustrated in Scheme 1. Then, the
morphology and structure of this material was investigated with
transmission electron microscopic (TEM). As shown in Figure 1a-
c, the Ag NPs are homogeneously dispersed in an approximately
size of 30~50 nm onto the slightly wrinkled graphene sheets. The
measured interplanar spacing for the lattice fringes is 0.235 nm in
Figure 1d, which correspond to the (111) and (200) lattice plane
of Ag, respectively. In addition, homogeneously dispersed Ag
NPs can also be identified through energy-dispersive X-ray (EDX)
spectroscopy (Figure 2). The crystalline phases of GO and Ag-
rGO-3 were characterized through X-ray diffraction (XRD)
measurement, and results are given in Figure 3a. A board peak at
2θ ≈ 20o origins from the amorphous graphitic carbon. Meanwhile,
four strong and sharp diffraction peaks that are assigned to the
(111), (200), (220), and (311) crystalline planes of the face-
centered cube of Ag NPs can be observed at 38.1°, 44.2°, 64.5°,
Owing to its excellent thermal, mechanical and electrical
properties, graphene has attracted widespread interest for
1
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77.5°. All of these results confirm the
Figure 3. (a) XRD patterns of GO, Ag-rGO-3 and standard PDF
of Ag nanoparticles. (b) Raman spectra of GO and Ag-rGO-3. (c)
FT-IR spectra of GO and Ag-rGO-3. (d) TGA spectra of GO and
Ag-rGO-3.
Figure 1. (a) and (b) TEM images of prepared Ag-rGO-3. (c) Ag
nanoparticles size distribution of Ag-rGO-3. (d)TEM image of
silver nanoparticle on graphene oxide sheets.
Then, X-ray photoelectron spectroscopy (XPS) was also
performed to further illustrate the composition and chemical state
of GO and Ag-rGO-3 (Figure 4a). Only the peaks corresponding
to C and O are found for GO, but additional peaks originating from
Ag can be found for Ag-rGO-3. In the core level spectrum in Figure
4b, the peaks at 367.9 eV (Ag 3d5/2) and 373.9 eV (Ag 3d3/2
)
indicate the presence of Ag (0). However, no Ag (I) can be found
in the Ag-rGO-3. The high-resolution of C 1s peak displays three
main peaks at binding energy of 284.6, 285.5, and 286.3 eV,
which can be assigned to C–C, C–O–C, and C–OH, respectively
(Figure 4c and d). Thus, the partial reduction of oxygen-containing
groups is also identified from the increasing peak at 284.6 eV from
GO to Ag-rGO-3.
Figure 2. (a) SEM images of Ag-rGO-3. (b), (c) and (d) EDS
mapping images of Ag-rGO-3.
presence of Ag NPs in the rGO surface.[11a]
Moreover, structural changes of graphene matrix after
decorating process was further identified. Raman spectra of GO
and Ag-rGO-3 have two characteristic peaks (Figure 3b), which
are D band (1370 cm-1) and G band (1600 cm-1), respectively.[9a]
The intensity of these two bands (ID/IG) increases from 0.74 to
1.08, indicating an increase in the disorder level of the graphene
matrix after anchoring Ag NPs. Additional, FT-IR spectroscopy
was carried out to compare the functional groups of the GO and
Ag-rGO-3 (Figure 3c). The vibration of C–OH (3368 cm-1, 1215
cm-1 and 1365 cm-1), COO-H (2950 cm-1), C=O (1731 cm-1), and
C–O (1060 cm-1) can be observed in the spectra of GO, indicating
a large number of C–OH, -COOH, and C-O-C functional groups
on the surface of GO. However, these peaks vanish (1731 cm-1,
1365 cm-1, and 1215 cm-1) or diminish (3357 cm-1 and 1048 cm-1)
after reducing to Ag-rGO-3. Clearly, most oxygen-containing
groups are reduced while remaining some C–OH and C–O-C
groups during reduction.[11c] Furthermore, inductively coupled
plasma (ICP) and thermal gravimetric analysis (TGA) suggest that
the Ag content on Ag-rGO-3 is 8.0 mmol·g-1 (Figure 3d).
Figure 4. (a) XPS survey spectra of GO and Ag-rGO-3. (b) High-
resolution Ag 3d spectra of Ag-rGO-3 and standard spectra of Ag
nanoparticles. C1s XPS spectra of (c) GO (d) Ag-rGO-3.
Encouraged by the homogeneously dispersed Ag NPs on Ag-
rGO-3, it was applied in the construction of 4,4-dimethyl-5-
methylene-1,3-dioxolan-2-one (2a) from 2-methylbut-3-yn-2-ol
(1a) and CO2 (Table 1). At first, using Ag-rGO-3 as catalyst,
reaction conditions were optimized and showed that the best
reaction output could be obtained with 10 wt% catalyst and 2 mol%
o
[N4444][Triz] under 30 C for 12h (Table S1 and S2 in supporting
2
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Table 2 Substrate scopea
information). To identify the role of different components (rGO
matrix and Ag NPs) of Ag-rGO-3 on catalytic activity, controlled
experiments were conducted under standard conditions. As
expected, not desired product 2a was observed without catalyst
or using GO and rGO as catalyst, indicating that Ag species is the
real active catalyst species (Table 1, entry 1-3). On the other hand,
AgNO3 and Ag NPs afforded 2a in only 58% and 20% yield,
respectively, whereas the yield could be improved when using Ag-
rGO as catalyst (Table 1, entry 4-7). Clearly, the activity of Ag
species is enhanced after dispersed on the surface of rGO. The
effect of molar ratio of Ag NPs/rGO on the reaction was further
investigated (Table 1, entry 5-7). When the Ag NPs loading was
5.9 mmol·g-1, only 54% of 2a was observed (Table 1, entry 5). An
increase in the yield of 2a could be found when increasing the
loading of Ag NPs on rGO (Table 1, entry 6 and 7). However, a
further increasing of Ag NPs loading leads to the seveal
aggregation of Ag particles. Thus, the best reaction outcome is
obtained in the presece of Ag-rGO-3, in which the loading of Ag
NPs is 8.0 mmol/g (Table 1, entry 7).
Entry
1
Substrate
Product
Yield (%)b
87
1a
2a
92
90
2
3
1b
2b
1c
1d
1e
2c
Table 1 The catalytic performance of different catalysts
82
4
for the catalyzed synthesis of alkylidene carbonates
a
from CO2
2d
2e
95(93c)
5
aReactions was carried out with 1 (1 mmol), Ag-rGO-3 (6.7 mol %
Ag based on 1), [N4444][Triz] (0.02 mmol, 6.2 mg) at 30 oC for 12 h
under atmospheric CO2 pressure. bDetermined by 1H NMR with
1,3,5-trimethoxybenzene as an internal standard. cIsolated yield.
Ag NPs loading
Entry
Catalyst
Yield/%c
(mmol/g)b
-
1
2
3
4
5
6
7
8
-
0
GOd
rGOd
-
0
Recyclability is an indispensable aspect for heterogeneous
catalytic performance. We explored the reusability and stability of
Ag-rGO-3 in Figure 5. After five runs, the activity of Ag-rGO-3 can
maintain and merely 0.02 ppm Ag, detected by ICP, presents in
the reaction solution. Furthermore, no significant change is
observed for the structure of reused Ag-rGO-3 from TEM and
XRD (Table S1, Supporting Information). More importantly,
according to the XPS result in Figure 6, Ag species is still in the
form of Ag (0) after five successive reactions. Then, the
cyclization reaction stopped after hot filtration for the purpose of
removing Ag-rGO-3 catalyst was conducted (Figure 7). This
reaction is stopped after hot filtration of the Ag-rGO-3 at about 70%
yield. Collectively, these results demonstrate the structure and
activity of Ag-rGO-3 can be maintained during the reaction.
-
0
AgNO3
Ag NPs
Ag-rGO-1
Ag-GO-2
Ag-GO-3
-
58
20
65
72
87
8.0
5.9
7.2
8.0
aReaction conditions: 1a (84.1 mg, 1 mmol), catalyst (6.7 mol%
Ag based on 1a), [N4444][Triz] (6.2 mg, 0.02 mmol), CO2 (1 bar),
12 h, 30 oC. bICP results. cDetermined by GC with biphenyl as
internal standard. d10 wt% catalyst relative to 1a was added.
After establishing the optimized reaction conditions, we
examined the activity of Ag-rGO-3 using various propargylic
alcohols to further assess the substrate scope (Table 2). In
general, this protocol works well for various propargylic alcohols,
resulting in good to excellent yield of corresponding α-alkylidene
cyclic carbonates. Substrates with different alkyl chain such as
methyl, ethyl afforded desired products in good yields (2a and 2b).
The bulky-group-substituted (isopropyl and cyclohexyl) substrate
furnished the corresponding product 2c and 2d in 90% and 82%
yield, respectively. The reduced yield of 2d might be ascribed to
the increased steric hindrance. Furthermore, substrate bearing
phenyl ring was studied under standard reaction conditions,
affording 2e in 95% yield.
Figure 5. Recovery of Ag-rGO-3 catalyst for the carboxylative
cyclization reaction.
3
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and atmospheric pressure of CO2 can proceed smoothly in the
presence
of
Ag-rGO-3,
furnishing
the
desired
β-
oxopropylcarbamates in 86% yield (Scheme 3). Product 3 might
come from a tautomerization of intermediate Ⅲ, which origins
from the nucleophilic addition of the amine to the carbonyl group
of the carbonate 2 (Scheme 2).
Ag 3d5/2
Ag 3d3/2
362 364 366 368 370 372 374 376 378 380
Binding Energy (eV)
Scheme 3. Three-component reaction of propargylic alcohols,
secondary amines, and CO2. Reaction conditions: 1a (84.1 mg, 1
mmol), diethylamine (73.1 mg, 1 mmol) Ag-rGO-3 (6.7 mol% Ag
based on 1a), [N4444][Triz] (6.2 mg, 0.02 mmol), CO2 (1 bar), 12 h,
30 oC.
Figure 6. XPS patterns of high-resolution Ag 3d spectra of
recovered Ag-rGO-3.
90
80
70
60
2-Oxazolidinones are a class of heterocyclic compounds that
have a wide range of applications as chemical intermediates,
chiral auxiliaries, and antibacterial agents.[13] Carboxylative
cyclization of propargylic amines with CO2 is a promising
approach to prepare 2-oxazolidinones.[14] Subsequently, we
continued to explore the reaction of various propargyl amines with
CO2 (Table 3). All propargylic amines 4a−d conducted well and
produced the corresponding 2-oxazolidinones in good to excellent
yield. Benzyl derived propargylic amines (4a) or the substrate with
a methyl group on benzyl position (4b) reacted well with CO2,
delivering 5a and 5b in similarly high yields. The propargylic
amine with phenylethyl substituted (4c) almost provided a
quantitative of desired product 5c. Replacing the benzyl with a
cyclohexyl group (4d) can improve the yield of 5d to 97%.
50
Leaching test
10 wt% catalyst
40
30
2
4
6
8
10
12
Time (h)
Figure 7. Leaching test.
On the basis of our results and related literature reports,[5]
a
plausible mechanism is illustrated in Scheme 2. Initially,
propargylic alcohol (1) couples with CO2 to generate the
carbonate intermediate I in the presence of Ag NPs and a base.
Then, an intramolecular ring-closing reaction occurs on the alkyne,
which is activated by the Ag NPs on Ag-rGO to offer the
corresponding cyclic carbonate 2 with releasing of the Ag-rGO
catalyst. The large surface of rGO makes it easier for Ag NPs to
coordinate with 1a, which is the key factor ensuring high activity.
a
Table 3 Carboxylative cyclization of propargyl amines with CO2
Yield
Entry
1
Substrate
Product
(%)b
90(87[c])
4a
5a
2
92
4b
5b
3
4
99
97
Scheme 2. Plausible mechanism for Ag-rGO-catalyzed
carboxylative cyclization of propargylic alcohols and CO2.
4c
4d
5c
5d
In order to further increase the utility of this methodology in
CO2 utilization, three-component reaction (1a, diethylamine, and
CO2) and the formation of 2-oxazolidinones from CO2 were
performed using Ag-rGO-3 as catalyst (Scheme 2 and Table 2).
Although it can undoubtedly broaden the application of CO2,
typical three-component reaction involving CO2, secondary amine,
and propargylic alcohols requires high temperature and CO2
pressure.[5i-j, 12] To our delight, the reaction of 1a, diethylamine,
aReactions were carried out with 4 (0.5 mmol), Ag-rGO-3 (6.7 mol % Ag
based on 4), [N4444][Triz] (0.02 mmol, 6.2 mg) at 30 oC for 12 h under 1 bar
CO2. bDetermined by 1H NMR with 1,3,5-trimethoxybenzene as an internal
standard. cIsolated yield.
4
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Conclusion
This work was financially supported by National Natural Science
Foundation of China (21975135, 21672119), and China
Postdoctoral Science Foundation (2018M641624).
In conclusions, rGO supported Ag NPs (Ag-rGO) was disclosed
as an efficient catalyst for the synthesis of alkylidene carbonates
from CO2 under atmospheric pressure. The structure and
component of Ag-rGO has been fully characterized, suggesting
the homogeneously dispersed Ag (0) species on rGO surface.
Notably, Ag-rGO-3 also exhibits excellent performance for the
three-component reaction (propargylic alcohols, diethylamine,
and CO2) and the reaction of propargyl amines with CO2 at
ambient conditions. More importantly, Ag-rGO can be easily
reused at least five times without obvious activity losing or
structure damage. The present work reveals a promising
approach for the utilization of CO2 in industry.
Keywords: carboxylative cyclization • carbon dioxide • graphene
oxide• heterogeneous catalyst • sliver nanoparticles
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Materials
Carbon dioxide (99.99%) was purchased from Liquefied Air (Tianjin) Co.,
Ltd. All reagents were purchased from Aladdin or Energy Ltd. and used as
received without further purification.
Synthesis of GO and Ag-rGO.[15]
Graphite oxide (GO) was synthesized by a modified Hummers method.[9a]
A total of 50 mg of GO was added to 30 mL of ethylene glycol and
sonicated for 2 h to disperse the GO well. A total of 200 mg of AgNO3 was
dissolved in a mixture of 5 mL H2O and 15 mL ethylene glycol. This mixture
was then added to the GO/ethylene glycol mixture and stirred for 2 h at
50 °C. Next, 40 mL of 0.1 mol/L sodium borohydride solution was added
slowly and the reaction mixture was heated for 2 h at 110 °C. After the
reaction, a black solid was observed at the bottom of flask. Then, the
reaction mixture was filtered and the product was washed with distilled
water and dried under vacuum.
Catalyst characterization techniques.
Transmission electron microscopy (TEM) was performed with a Tecnai G2
F20 transmission electron microscope operating at 200 KV. Field emission
scanning electron microscopy (SEM) was carried out with a MERLIN
Compact that was assembled with an energy-dispersive X-ray
spectrometer (EDS). X-ray photoelectron spectroscopy (XPS) was
performed with an Axis Ultra DLD instrument using Al Kα radiation (1486.7
eV, 150 W). X-ray diffraction (XRD) patterns were obtained with a
MimFLex600 diffractometer at a scanning rate of 0.5°/min using Cu Kα
radiation in the 2θ range of 5°–80°. FT-IR was recorded with a Bruker
Tensor27 FT-IR spectrophotometer using KBr pellets with the range of
4000–400 cm-1. Thermogravimetric analysis (TGA) was performed with an
Axis Ultra DLD instrument that was fitted with nitrogen purge gas with a
heating rate of 5 °C/min from 0–800°C. Raman spectra were recorded on
a Renishaw inVia Raman Microprobe with 514.5 nm laster excitation from
200 to 2500 cm-1.
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Cyclization of Propargylic Alcohols.
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Propargylic alcohol (1a, 84.1 mg, 1 mmol), Ag-rGO-3 (8.4 mg, 10 wt%
relative to 1a) and base (0.05 mmol) were combined in a 10-mL Schlenk
tube. Then, the Schlenk tube was attached to a CO2 balloon and the
solution was stirred for 12 h at 30 °C. After the reaction completed, the
balloon of CO2 was taken off. The liquid phase was then diluted with 10 ml
CH2Cl2, and biphenyl (20 mg) was added to be used as an internal
standard. The yield of products was determined by gas chromatography
(GC). The Ag-rGO catalyst was separated by centrifugation. Then it was
washed with ethanol three times and dried at 60 °C for 4 h for subsequent
use. GC analyses were carried out on a Shimadzu GC-2014 that was
equipped with a capillary column (RTX-50, 30 m × 0.25 μm) with a flame
ionization detector. 1H NMR spectra were recorded using 400 MHz
spectrometers with CDCl3 as a solvent referenced to CDCl3 (7.26 ppm).
13C NMR was recorded in CDCl3 (77.00 ppm) at 100.6 MHz.
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Entry for the Table of Contents
Heterogeneous silver catalysis: well-
defined reduced graphene oxide
supported Ag nanoparticles are
developed for CO2 conversion. This
catalyst is efficient and reusable for
various CO2 utilization processes.
Xiao Zhang, Kai-Hong Chen, Zhi-Hua
Zhou, Liang-Nian He*
Page No. xxx– Page No.xxx
Reduced
Supported Ag Nanoparticles: An
Efficient Catalyst for CO2
Conversion at Ambient Conditions
Graphene
Oxide
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