An efficient nanoscale heterogeneous catalyst for the capture and conversion of carbon dioxide at ambient pressure

Article abstract of DOI:10.1002/anie.201409103

Silver nanoparticles were successfully supported on the zeolite-type metal-organic framework MIL-101 to yield Ag@MIL-101 by a simple liquid impregnation method. For the first time, the conversion of terminal alkynes into propiolic acids with CO₂ was achieved by the use of the Ag@MIL-101 catalysts. Owing to the excellent catalytic activity, the reaction proceeded at atmospheric pressure and low temperature (50 8C). The Ag@MIL-101 porous material is of outstanding bifunctional character as it is capable of simultaneously capturing and converting CO₂ with low energy consumption and can be recovered easily by centrifugation.

Full text of DOI:10.1002/anie.201409103

Angewandte
Chemie
DOI: 10.1002/anie.201409103
Carbon Dioxide Fixation Very Important Paper
An Efficient Nanoscale Heterogeneous Catalyst for the Capture and
Conversion of Carbon Dioxide at Ambient Pressure**
Xiao-Huan Liu, Jian-Gong Ma,* Zheng Niu, Guang-Ming Yang, and Peng Cheng*
Abstract: Silver nanoparticles were successfully supported on
the zeolite-type metal–organic framework MIL-101 to yield
Ag@MIL-101 by a simple liquid impregnation method. For the
first time, the conversion of terminal alkynes into propiolic
acids with CO2 was achieved by the use of the Ag@MIL-101
catalysts. Owing to the excellent catalytic activity, the reaction
proceeded at atmospheric pressure and low temperature
(508C). The Ag@MIL-101 porous material is of outstanding
bifunctional character as it is capable of simultaneously
capturing and converting CO2 with low energy consumption
and can be recovered easily by centrifugation.
crystalline materials[10–14] and have well-defined pores and
mechanically robust walls, which can thus act as a template or
scaffold to direct the formation of NPs with tunable size and
uniform shape as well as to efficiently stabilize MNPs without
blocking their surface by strongly binding ligands.[15] Herein,
we describe the design and synthesis of an efficient, easily
regenerated MNP-MOF nanoscale heterogeneous material,
namely Ag@MIL-101, which can act as a bifunctional mate-
rial and achieve both the capture and the high-efficient
conversion of CO2 at low temperature (508C) and 1 atm in
a single process. To the best of our knowledge, this is the first
example of an active and reusable catalyst for the carbox-
ylation of terminal alkynes with CO2 by using MOFs as
a matrix and template.
T
he emission of CO2 has become an urgent issue of our
decade as it causes global warming and a consequent series of
environmental problems.[1] Many efforts have been directed
towards the efficient utilization of CO2 by chemical methods,
which may also provide access to high-value products from
a non-toxic, renewable, and low-cost resource.[2] One of the
best strategies for CO2 conversion is the synthesis of propiolic
Ag@MIL-101 catalysts with different Ag loadings were
prepared via
a simple impregnation–reduction method
(Figure 1) by immobilizing Ag NPs in MIL-101(Cr),[16]
À
acids through the C H bond activation of terminal alkynes
with CO2 as a C1 building block[3] because the alkynyl
carboxylic acid products can serve as important synthetic
intermediates[4] for further applications in medical chemistry
as well as organic synthesis[5] to give coumarins, flavones,
aminoalkynes, alkynylarenes, and arylidene oxindoles.[6] Sev-
eral procedures and catalysts, including both homo-[3c–e,7] and
heterogeneous catalytic[3a] systems, have been developed in
this area, but either reusability problems or synthetic
complications limit the further application of these catalytic
systems. The design and synthesis of efficient, inexpensive,
and easily prepared catalysts for this type of reactions are only
at the beginning and urgently require further developments.
Recently, the application of metal–organic frameworks
(MOFs) for utilizing CO2 as a C1 building block[8] or for
encapsulating metal nanoparticles (MNPs)[9] has received
tremendous attention. MOFs are an attractive class of porous
Figure 1. Synthesis of Ag@MIL-101.
which was selected as the support not only because of the
very large pore size (2.9–3.4 nm) and high specific surface
area, but also because of its long-term chemical stability in
various organic solvents, water, and air.[16,17] The obtained
Ag@MIL-101 catalysts were characterized by X-ray photo-
electron spectroscopy (XPS; Supporting Information, Fig-
ure S1) and powder X-ray diffraction (PXRD; Figure S2).
Catalysts 1a, 1b, 1c, and 1d have Ag loadings of 1.66, 2.58,
4.16, and 6.97 wt%, respectively, as confirmed by inductively
coupled plasma (ICP) analysis.
[*] X.-H. Liu, Dr. J.-G. Ma, Dr. Z. Niu, Prof. Dr. G.-M. Yang,
Prof. Dr. P. Cheng
N2 and CO2 sorption were measured for samples of 1a–
1d. Significant decreases in the amount of both N2 and CO2
sorption were observed for all of the Ag-loaded samples in
comparison with MIL-101. The decrease in the surface areas
should be attributed to the incorporation of the Ag NPs into
the pores of MIL-101 and/or a block of Ag NPs located on the
framework surface of MIL-101 (Figure S3). The CO2 sorption
properties of samples 1a–1d were investigated at room
temperature and 1 atm (Figure 2). The uptake of CO2 reached
64.25, 61.50, 60.37, and 63.95 mggÀ1 for 1a, 1b, 1c, and 1d,
respectively (Table S1); these values are comparable to those
Department of Chemistry
Key Laboratory of Advanced Energy Material Chemistry
Nankai University and Collaborative Innovation Center of Chemical
Science and Engineering (Tianjin)
Tianjin 300071 (P. R. China)
E-mail: mvbasten@nankai.edu.cn
pcheng@nankai.edu.cn
[**] This project was financially supported by the “973 program”
(2012CB821702), the NSFC (21301099, 21331003 and 21421001),
the MOE (IRT13022 and 13R30), and the 111 Project (B12015).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409103.
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product yields depended on the amount of 1c used in the
reaction (entries 5–7). When the amount of 1c was increased
to 100 mg, the yield decreased to 94.1%, which was slightly
lower than the yield obtained with a dosage of 70 mg.
Then, we intensively surveyed the influence of other
organic solvents on the catalytic system, which confirmed
DMF as a superior solvent for this system in comparison with
CH3CN, NMP, and DMSO (entries 9–11) because DMF was
a better solvent for both Cs2CO3 and CO2 (DMF is a weak
base) than the other organic solvents.[3d] It is noteworthy that
the yield dropped to 25.2% when the base was changed to
K2CO3 under otherwise identical conditions (entry 12), indi-
cating that Cs2CO3 was a superior base for this direct
carboxylation reaction of terminal alkynes with CO2.[3d,e]
When MIL-101 or Cs2CO3 were used without Ag, the catalytic
systems gave phenylpropiolic acid in poor yields of 21.9% or
3.8%, respectively (entries 14 and 15).
With the optimized conditions (1.5 equiv Cs2CO3, 1 atm
CO2 DMF, 508C) in hand, several typical alkyne substrates
were subjected to this carboxylation reaction (Table 2).
Under the standard conditions, the corresponding products
were obtained in excellent yields (96.5–98.7%) when aro-
matic alkynes with either electron-donating (CH3, OCH3) or
electron-withdrawing (Cl) substituents were employed. Even
for an alkyne with a heterocyclic group (thiophene), 98.5%
yield was achieved.
Figure 2. CO2 adsorption/desorption (298 K) of MIL-101(Cr) and 1a–
1d.
reported for MOF materials with good CO2 uptake properties
under identical conditions.[18]
The catalytic activities of the prepared samples (1a–1d)
were determined for the fixation of CO2 with terminal alkynes
into propiolic acids. In the initial investigation, the carbox-
ylation of 1-ethynylbenzene was selected as a model reaction
to study the influence of various parameters on the reaction.
Table 1 shows the generation of 3-phenylpropiolic acid from
CO2 (1 atm) and 1-ethynylbenzene at 508C in the presence of
Ag@MIL-101 with different Ag loadings. For 70 mg of 1a–d
(entries 1–4), 3-phenylpropiolic acid was obtained in good
yields (67.5–96.5%). Catalyst 1c with 0.027 mmol of Ag
afforded the highest product yield, and the catalytic activity
decreased in the following order: 1c > 1d > 1b > 1a. The
Table 2: Synthesis of propiolic acid derivatives from CO2 and terminal
alkynes with catalyst 1c.[a]
Entry
Alkyne
Product
Yield[b] [%]
96.5
1
2
3
4
5
98.4
Table 1: Synthesis of 3-phenylpropiolic acid from CO2 and 1-ethynyl-
benzene with catalysts 1.[a]
98.2
98.7
Entry
Catalyst (mg)
Ag [mmol]
Solvent
Yield[b] [%]
98.5
1
2
3
4
5
6
7
1a (70)
1b (70)
1c (70)
1d (70)
1c (25)
1c (50)
1c (100)
1c (70)
1c (70)
1c (70)
1c (70)
1c (70)
1c (70)
1c (0)
0.011
0.017
0.027
0.045
0.010
0.019
0.039
0.027
0.027
0.027
0.027
0.027
0.027
0
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
CH3CN
NMP
DMSO
DMF
DMF
DMF
DMF
DMF
67.5
71.9
96.5
90.9
34.1
80.8
94.1
86.2
71.8
86.5
92.1
25.2
53.3
3.8
[a] Reaction conditions: alkyne (1.0 mmol), 1c (70 mg, 2.7 mol% of Ag),
Cs2CO3 (1.5 mmol), CO2 (1.0 atm), 508C, DMF (5 mL), 15 h. [b] Yield of
isolated product.
8[c]
9
As a heterogeneous catalyst, Ag@MIL-101 could not only
be simply prepared, but also easily separated from the
reaction solution by centrifugation. As shown in Figure 3,
catalyst 1c could be regenerated and reused at least five times
without any obvious decreases in catalytic activity in the
subsequent reaction under the optimized conditions. The
PXRD pattern (Figure S2) showed that the framework of
catalyst 1c was well maintained after the catalytic reaction in
DMF, which confirms the stability of the Ag@MIL-101
catalysts during the reaction.
10
11
12[d]
13[e]
14[f]
15
16[g]
MIL-101(Cr) (70)
1c (70)
0
21.9
0
0.027
[a] Reaction conditions: 1-ethynylbenzene (1.0 mmol), catalyst, Cs2CO3
(1.5 mmol), CO2 (1.0 atm), 508C, solvent (5 mL), 15 h. [b] Yield of
isolated product. [c] 12 h. [d] K2CO3. [e] 258C. [f] In the absence of Ag.
[g] In the absence of CO2. NMP=N-methyl-2-pyrrolidone.
To further investigate and explain the excellent catalytic
activity of the Ag@MIL-101 catalysts, the morphology of the
2
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Ag loading, as was observed for ZIF-8 with MNPs.[9e]
The average size of the Ag NPs in 1c is 1.4 Æ 0.4 nm
(Figure 5) and in good agreement with the size of the
mesoporous cavities in MIL-101, indicating that the large
pore sizes of MIL-101 are desirable for the metal precursor
AgNO3 to diffuse and for the small Ag NPs to deposit.
Furthermore, the MOF matrix also plays several other
important roles in achieving the excellent catalytic activity
of Ag@MIL-101: 1) As a good CO2 adsorption material,
Ag@MIL-101 can efficiently capture CO2 as a bifunctional
catalyst and improve the local concentration of CO2 around
the catalytic centers inside the pores of the framework to
promote the activity of the catalyst. 2) The framework and
pores act as a “microreactor” to provide a favorable environ-
ment for the reaction of CO2 with terminal alkynes. The
outstanding Lewis acidity of the MIL-101 framework, which
is due to the unsaturated chromium sites, could lead to
a preference for the adsorption of aromatic substrates[20] and
therefore enhance the reactivity of catalyst 1c. 3) The MOF
structure stabilizes the whole material and leads to good
regeneration properties. After entering the channels of MIL-
101, the terminal alkynes can coordinate to the Ag NPs and be
Figure 3. Recycling tests with catalyst 1c for the carboxylation of
terminal alkynes with CO2.
À
deprotonated by Cs2CO3 through acidifying the C(sp) H
bond to generate a silver acetylide intermediate. Afterwards,
À
CO2 inserts into the C Ag bond to form the carboxylic acid
products, as previously proposed.[3c,e]
In summary, we have synthesized the novel nanoscale
heterogeneous catalyst system Ag@MIL-101 by a simple
liquid impregnation method. The porous material is capable
of simultaneously capturing and converting CO2. This is the
first example of stable MOF-supported catalysts for the
À
transformation of CO2 into carboxylic acids through C H
bond activation of terminal alkynes. These heterogeneous
catalysts feature good catalytic activity, high stability, and
excellent reusability in the carboxylation of terminal alkynes
with CO2 at mild conditions (508C and 1 atm of CO2).
Potential applications of this catalytic system in various areas
can be anticipated, for example, in synthetic and industrial
Figure 4. Element maps for a Ag@MIL-101 sample (1c): a) SEM
image; b) Cr (green); c) O (indigo); d) Ag (purple).
Ag NPs immobilized in MIL-101 in
1a–1d was characterized by energy-
dispersive spectroscopy (EDS) and
transmission electron microscopy
(TEM). EDS images (Figure 4 and
Figures S4–S6) confirm the homoge-
neous three-dimensional distribu-
tion of the Ag NPs throughout the
interior of the MIL-101 matrix. The
TEM images (Figure 5 and Figur-
es S7–S9) show that the Ag NPs in
1c were sufficiently dispersed and
restricted to smaller sizes than those
in 1a, 1b, and 1d, so that a larger
surface area is available for the
reactants, and a higher catalytic
performance is achieved with 1c.[19]
The aggregation of NPs in 1d (Fig-
ure S9) leads to its lower catalytic
Figure 5. TEM image of a Ag@MIL-101 sample (1c; left) and size distribution of the Ag NPs in
activity, even though 1d has a higher a sample of 1c (right).
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Received: September 14, 2014
Published online: && &&, &&&&
Keywords: carbon dioxide fixation · heterogeneous catalysis ·
.
metal–organic frameworks · propiolic acids · silver nanoparticles
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Communications
Carbon Dioxide Fixation
X.-H. Liu, J.-G. Ma,* Z. Niu, G.-M. Yang,
P. Cheng*
&&&& — &&&&
An Efficient Nanoscale Heterogeneous
Catalyst for the Capture and Conversion
of Carbon Dioxide at Ambient Pressure
Silver nanoparticles were supported on
the zeolite-type metal–organic framework
MIL-101 by a simple liquid impregnation
method to yield Ag@MIL-101. CO2 and
terminal alkynes could be converted into
propiolic acids in the presence of the very
active Ag@MIL-101 catalyst at atmos-
pheric pressure and low temperature
(508C).
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These are not the final page numbers!