Ordered Mesoporous CeO2-supported Ag as an Effective Catalyst for Carboxylative Coupling Reaction Using CO2

Article abstract of DOI:10.1002/cctc.201900039

Ag/Mesoporous CeO₂ (Ag/M?CeO₂) catalysts were synthesized through the gas bubbling-assisted membrane reduction (GBMR) method and characterized by XRD, ICP-OES, N₂ adsorption-desorption, XPS, Raman spectroscopy, TEM, HRTEM, HAADF-STEM and CO₂-TPD. Ag/M?CeO₂ catalysts contain uniform mesoporous structure, large surface area and oxygen vacancies, which promote the dispersion of Ag nanoparticles. Density functional theory (DFT) calculations indicate that Ag nanoparticles supported on M?CeO₂ could facilitate the formation of oxygen vacancies, which result in higher CO₂ adsorption capacity. With the most oxygen vacancies and the well-dispersed Ag active species, Ag (3.12 %)/M?CeO₂ exhibits high efficiency for the carboxylative coupling of terminal alkynes, chloride compounds and CO₂ under mild reaction conditions (60 °C, 0.5 MPa), affording a wide range of functionalized 2-alkynoates in good yields.

Full text of DOI:10.1002/cctc.201900039

Accepted Article
Title: Ordered mesoporous CeO2-supported Ag as an effective catalyst
for carboxylative coupling reaction using CO2
Authors: xiao zhang, Dingkun Wang, Meizan Jing, Jian Liu, Zhen Zhao,
Guanhua Xu, Weiyu Song, Yuechang Wei, and Yuanqing Sun
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Ordered mesoporous CeO2-supported Ag as an effective catalyst
for carboxylative coupling reaction using CO2
Xiao Zhang*, Dingkun Wang, Meizan Jing, Jian Liu*, Zhen Zhao*, Guanhua Xu, Weiyu Song,
[a]
Yuechang Wei, and Yuanqing Sun
encapsulated in the porphyrin MOF[11] displayed excellent
catalytic activities for the carboxylative coupling reaction.
However, due to the thermodynamic and kinetic stability of CO2,
the transformation of CO2 to functionalized 2-alkynoates using
heterogeneous catalysts under mild reaction condition remains a
great challenge.
Recently, M-CeO2 served as supporting material has
attracted great interest in the catalysis.[12-15] The well-regularized
mesoporous structure of M-CeO2 not only increases the specific
surface that improves the dispersion of metal nanoparticles,[16-18]
but also facilitates the transport of reactants through the
material.[19,20] For example, Zhou et al. reported that Ni/M-CeO2
showed excellent catalytic activity for CO2 methanation. Owing
to the high specific surface area and ordered mesoporous
structure, Ni active species could be highly dispersed on M-
CeO2. Moreover, Ni/CeO2 possesses oxygen vacancies which
can effectively promote CO2 activation.[21]
Abstract: Ag/Mesoporous CeO2 (Ag/M-CeO2) catalysts were
synthesized through the gas bubbling-assisted membrane reduction
(GBMR) method and characterized by XRD, ICP-OES, N2
adsorption-desorption, XPS, Raman spectroscopy, TEM, HRTEM,
HAADF-STEM and CO2-TPD. Ag/M-CeO2 catalysts contain uniform
mesoporous structure, large surface area and oxygen vacancies,
which promote the dispersion of Ag nanoparticles. Density functional
theory (DFT) calculations indicate that Ag nanoparticles supported
on M-CeO2 could facilitate the formation of oxygen vacancies, which
result in higher CO2 adsorption capacity. With the most oxygen
vacancies and the well-dispersed Ag active species, Ag (3.12%)/M-
CeO2 exhibits high efficiency for the carboxylative coupling of
terminal alkynes, chloride compounds and CO2 under mild reaction
conditions (60 °C, 0.5 MPa), affording a wide range of functionalized
2-alkynoates in good yields.
Herein, inspired by the previous works, we synthesized M-
CeO2 by the hard-template method and loaded different
amounts of Ag to yield Ag/M-CeO2 through the GBMR method.
A variety of terminal alkynes, chloride compounds and CO2 are
catalyzed by Ag (3.12%)/M-CeO2, affording the corresponding
products in good yields (74-91%) under mild reaction condition
(60 °C, 0.5 MPa). In addition, Ag (3.12%)/M-CeO2 shows good
stability and recyclability.
Introduction
During the last few decades, the excessive CO2 emission from
fossil energy conversion has caused global warming and climate
change, which should be dealt with greatest attention.[1] On the
other hand, CO2 is an important C1 source in organic synthesis
owing to its nontoxicity, renewability and accessibility.[2] Thus,
the exploitation of CO2 chemical transformations to high-value
products is identified as an attractive approach to control and
reduce CO2 emissions. To date, many efforts have been
devoted towards CO2-involved synthesis of useful chemicals.[3]
Among them, the synthesis of functionalized 2-alkynoates
through the carboxylative coupling of terminal alkynes,
organohalides and CO2 has attracted much attention,[4] because
the products are key intermediates in the synthesis of a variety
of pharmaceutically important compounds and fine chemicals.[5]
Generally, several homogeneous catalytic systems including
transition-metal salts and complexes (e.g. Cu[6] and Ag[7]) have
been developed for this reaction. Nevertheless, most of these
methods have the problems of the separation and recovery of
the metal. In the effort to develop sustainable processes, a few
heterogeneous catalysts have been introduced in this reaction.
For instance, CuBr loaded on activated carbon,[8] CuCl2
incorporated in polyvinyl imidazolium tri-cationic ionic liquid,[9] Cu
nanoparticles supported on Al2O3,[10] and Ag nanoparticles
Results and Discussion
[22]
M-CeO2 was synthesized using KIT-6 as hard template.
Ag/M-CeO2 catalyst was synthesized by the GBMR method. The
schematic diagram of the GBMR method is presented in
Scheme 1.
[a]
Prof. X. Zhang, D. K. Wang, M. Z. Jin, Prof. J. Liu, Prof. Z. Zhao, G.
H. Xu, Prof. W. Y Song, Prof. Y.C. Wei, Dr. Y.Q. Sun
College of Science, State Key Laboratory of Heavy Oil Processing,
China University of Petroleum-Beijing
18 Fuxue Road, Changping, Beijing(China)
E-mail: zhangxiao@cup.edu.cn, liujian@cup.edu.cn,
zhenzhao@cup.edu.cn
Scheme 1. Preparation of Ag/M-CeO2 by the GBMR method.
Ag/M-CeO2 which have Ag loading of 1.33%, 3.12%, and
5.24% were confirmed by ICP-OES analysis. The wide-angle
XRD patterns of bulk CeO2 (B-CeO2) synthesized without the
addition of hard template, mesoporous CeO2 (M-CeO2) and
Ag/M-CeO2 are illustrated in Figure 1a. The distinct diffraction
Supporting information for this article is given via a link at the end
of the document.
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peaks at 2θ = 28.6°, 33.0°, 47.4°, 56.3°, 69.5° and 76.8° are all
medium and high pressure region (P/P0 = 0.7-1.0), which
indexed to a cubic fluorite structure of CeO2 (space group Fm3m, attributes to type H2. For M-CeO2, the hysteresis loop with the
JCPDS 34-0394) and could be corresponding to the (111), (200), relative pressure ranges (P/P0 = 0.75-1.0) is verified to type H4,
(220), (311), (400) and (331) crystal planes. Compared with the
pure M-CeO2, the wide-angle XRD patterns of Ag/M-CeO2 show
an additional peak at 2θ = 38.2°, associated with (111) plane of
Ag face centered cubic (fcc) phase (JCPDS 65-2871). There is
only one weak diffraction peak attributable to Ag, implying that
Ag nanoparticles are homogeneously distributed on M-CeO2.[12]
The mean sizes of Ag nanoparticles are estimated using
Scherrer equation from line broadening of the Ag (111) plane in
the wide angle XRD patterns (Fig. 1a).[23,24] The mean sizes of
Ag nanoparticles in Ag (1.33%)/M-CeO2, Ag (3.12%)/M-CeO2
and Ag (5.24%)/M-CeO2 are 6.6, 6.5 and 7.1 nm, respectively.
The small angle XRD patterns of all mesoporous samples are
shown in Figure 1b. The characteristic peak at 2θ of around 0.6°
can be clearly observed, ascribed to the (211) diffraction peak of
the cubic Ia3d symmetry of mesoporous silica template. These
results indicate that both M-CeO2 and Ag/M-CeO2 could
maintain the same regular mesostructures as the KIT-6 plates.
indicating the formation of mesopore structures. Ag/M-CeO2 with
different amounts of Ag nanoparticles exhibit similar rising
isotherms within a relative pressure range from 0.4-0.6, which
corresponds to the capillary condensation of the mesopores.
The rising curves at a relative pressure of 0.8-1.0 reflect that
Ag/M-CeO2 possesses the mesopore structure. The pore size
distribution (PSD) profiles of M-CeO2 and Ag/M-CeO2 based on
BJH method (Fig. 2b) also show a typical mesoporous structure
of the material, and the pore sizes are ca.4 nm. The surface
area, pore volume and average pore size of B-CeO2, M-CeO2
and Ag/M-CeO2 are summarized in Table S1. The surface areas
of B-CeO2, M-CeO2, Ag (1.33%)/M-CeO2, Ag (3.12%)/M-CeO2,
Ag (5.24%)/M-CeO2 are 78, 154, 145, 144, and 142 m2/g,
respectively. Due to the well-ordered mesoporous and
interconnected channels of M-CeO2, the BET surface area of M-
CeO2 is much higher than that of B-CeO2. What’s more, the pore
volume of M-CeO2 (0.42 cm3/g) is larger than that of B-CeO2
(0.15 cm3/g). M-CeO2 could promote the dispersion of Ag
nanoparticles[25] and make the reactants diffusion facile[26] with
high surface area and large pore volume. With the increasing of
Ag loading amounts, the BET surface area, pore volume and
average pore size slightly decrease (Table S1).
Figure 1. (a) Wide angle XRD; (b) Small angle XRD.
To detect pore properties and the surface area of B-CeO2,
M-CeO2
and
Ag/M-CeO2,
N2
adsorption–desorption
measurements were carried out at -196 oC. As shown in Fig. 2(a,
b), in accordance with IUPAC classification, N2 adsorption-
desorption isotherms of all prepared samples can be classified
as type IV isotherm. B-CeO2 shows a hysteresis loop in the
Figure. 2 (a) N2 adsorption-desorption isotherms of B-CeO2, M-CeO2 and Ag/
M-CeO2; (b) Pore Diameter distribution.
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The surface composition and chemical state of different
elements were examined using XPS analysis in these samples,
Figure 3a presents the XPS spectra of Ce 3d, O 1s and Ag 3d
for Ag/M-CeO2 samples, and Ce 3d, O 1s for M-CeO2 sample.
For M-CeO2 and Ag/M-CeO2, it can be found that the Ce 3d
level has a rather complex structure and the spectrum can be
separated into ten peaks (u1, u2, u3, u4, u5, v1, v2, v3, v4, v5) with u
and v referring to Ce 3d3/2 and Ce 3d5/2 components (Figure 3b).
The peaks labelled u1, u2, u4, v1, v2, and v4 are characteristic of
Ce4+, while u3, v3, u5, and v5 are assigned to Ce3+. As shown in
Table S2, the calculated surface concentrations of Ce3+ for M-
CeO2, Ag (1.33%)/M-CeO2, Ag (3.12%)/M-CeO2, Ag (5.24%)/M-
CeO2 are 18.0, 28.3, 38.1, and 32.2%, respectively. The
amounts of Ce3+ increase with the addition of Ag nanoparticles
on M-CeO2, suggesting that the interaction between Ag
nanoparticles and CeO2 contributes to a partial Ce4+ → Ce3+
reduction. It has been reported that the formation of oxygen
vacancies is ascribed to the partial reduction of Ce4+ to
Ce3+.[27,28] Ag (3.12%)/M-CeO2 has the highest content of Ce3+,
producing the most oxygen vacancies, which can facilitate the
dispersion of Ag nanoparticles.[12] According to Fig 3c, three
kinds of peaks can be identified by the deconvolution of the O1s
spectra. The peak of O at the lowest binding energy is attributed
to O2- in the CeO2 lattice, the second one with the middle binding
2-
energy is due to the chemisorbed oxygen O2 and the third one
at the highest binding energy is assigned to the adsorbed
molecular water or surface hydroxyl species. It is established
2-
that O2 species are produced by oxygen adsorption on the
surface defects of ceria[29] and the ratio of the chemisorbed
oxygen can be used to evaluate the concentration of the oxygen
vacancies.[30,31]
As shown in Table S2, the ratio of the
2-
chemisorbed oxygen O2 decreases in the order of Ag
(3.12%)/M-CeO2 (36.5%) > Ag (5.24%)/M-CeO2 (27.0%) > Ag
(1.33%)/M-CeO2 (25.9%)> M-CeO2 (22.8%). These results show
that the incorporation of Ag on M-CeO2 enhances the amounts
of oxygen vacancies and Ag (3.12%)/M-CeO2 has the highest
content of oxygen vacancies. The Ag 3d spectra (Fig 3d) give
peaks at binding energies of 368.5 eV (3d5/2) and 374.5 eV
(3d3/2) with the spin energy separation of 6.0 eV which are
assigned to metallic Ag nanoparticles. [12,32]
Figure 3 XPS spectra of M-CeO2 and Ag/M-CeO2: (a) survey XPS spectrum;
(b) Ce 3d; (c) O 1s; (d) Ag 3d.
Raman spectras of M-CeO2 and Ag/M-CeO2 catalysts are
shown in Fig. 4. All these samples exhibit two main Raman
adsorption peaks, an intense peak located at 460 cm-1 and a
weak peak at 596 cm-1. The peak around 460 cm-1 is assigned to
the strong F2g vibration mode.[33,34] Another weak peak is
ascribed to defect-induced (D) mode.[35] According to the
peaking fitting of the D and F2g modes in the Raman spectras,
we can calculate the ratio of ID/IF2g, representing the relative
content of oxygen defect sites.[25,34] The ID/IF2g values of M-CeO2,
Ag (1.33%)/M-CeO2, Ag (3.12%)/M-CeO2, and Ag (5.24%)/M-
CeO2 are 2.57%, 3.53%, 7.87%, and 5.60%, respectively.
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Therefore, supported Ag nanoparticles on M-CeO2 could
increase the oxygen vacancies, and Ag (3.12%)/M-CeO2 has the
most oxygen vacancies.
HRTEM image of Ag (3.12%)/M-CeO2 in Figure. 6a shows
the lattice fringes (d = 0.31 nm) corresponding to the (111)
crystallographic planes of CeO2, and the lattice spacing value of
0.24 nm is assigned to the (111) crystallographic planes of the
metal Ag.[36,37] These results are consistent with the ones
observed from XRD characterization. In the HAADF-STEM
image of Ag (3.12%)/M-CeO2 (Figure 6b), the regular
mesoporous structure and well-arranged morphology are not
changed after loading Ag nanoparticles. The element distribution
of Ag (3.12%)/M-CeO2 is obtained using EDS elemental
mapping analysis. As shown in Figure 7, it can be found that the
Ce and Ag are well distributed throughout Ag/M-CeO2.[25]
Figure 4. Raman spectras of M-CeO2 and Ag/M-CeO2.
The TEM images of M-CeO2 are shown in Figure 5. M-CeO2
displays highly ordered mesoporous structures, which indicate
that M-CeO2 contains a prefect replica structure of KIT-6
template.
Figure 6. (a) HRTEM image of Ag (3.12%)/ M-CeO2; (b) HAADF-STEM image
of Ag (3.12%)/ M-CeO2.
Figure 5.(a,b) TEM images of M-CeO2.
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Figure 7. EDS elemental mapping of Ag (3.12%)/ M-CeO2.
The CO2 adsorption on the stoichiometric CeO2(111), the
reductive CeO2(111), the stoichiometric Agn/CeO2(111) and the
reductive Agn/CeO2(111) were theoretically investigated by DFT
calculations.
We chose the CeO2(111) surface, the most stable surface
termination,[40] which consists of 9 atomic layers (three O-Ce-O
Figure 8 shows the CO2-TPD curves of M-CeO2, Ag
(1.33%)/M-CeO2, Ag (3.12%)/M-CeO2 and Ag (5.24%)/M-CeO2.
For all these samples, the desorption of CO2 occurs in two
temperature regions of 50-200 and 320-460 °C. Calculated
based on the curves in Figure 8, the amounts of desorbed CO2
are shown in Table S3. The total amounts of desorbed CO2
follow the order: Ag (3.12%)/M-CeO2 (0.70 mmol g-1) > Ag
(5.24%)/M-CeO2 (0.59 mmol g-1) > Ag (1.33%)/M-CeO2 (0.50
mmol g-1) > M-CeO2 (0.44 mmol g-1), and the total amounts of
adsorbed CO2 follow the same order.[38,39] It is noted that the
order of CO2 adsorption ability of the samples is in accordance
with the concentration of oxygen vacancies. The result shows
that oxygen vacancies could promote CO2 adsorption.
trilayers)
with
a
3×3
unit
cell
(Figure
9a).
The vacuum space that perpendicular to the surface is set to 15
Å in order to minimize the interaction between distinct slab
surfaces. During geometry optimization, the top six atom layers
of ceria are relaxed, while bottom three layers are fixed in their
bulk positions. The CeO2-supported Ag nanorod system is
modeled by adding a two-layer Ag nanorod on the top of the
stoichiometric CeO2(111) surface (shown in Figure 9b)[41,42]
,
which is denoted as Agn/CeO2(111) (where n represents the
nanorod).
Figure 8. CO2-TPD of (a) M-CeO2, (b) Ag(1.33%)/M-CeO2, (c) Ag(5.24%)/M-
CeO2, (d) Ag(3.12%)/M-CeO2.
Figure 9. Top and side views of (a) CeO2(111) and (b) Agn/CeO2(111). (White
and red spheres represent Ce and O atom, respectively)
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On the stoichiometric CeO2 (111) surface (Figure 10a), CO2
can be adsorbed with low adsorption energy of -0.04 eV (C
terminal adsorption) or -0.07 eV (O terminal adsorption).
Moreover, on the stoichiometric Agn/CeO2(111) surface (Figure
10b), the interface between Ag and CeO2 has no effect on CO2
adsorption, which can be proved by the low adsorption energy (-
0.05 eV or -0.13 eV).
Figure 11. Oxygen vacancy formations on different O sites of Agn/CeO2(111).
(a) Stoichiometric Agn/CeO2(111), (b) Reductive Agn/CeO2(111)–Ov1, (c)
Reductive Agn/CeO2(111)–Ov2, (d) Reductive Agn/CeO2(111)–Ov3.
On the reductive CeO2 (111) surface, CO2 can be adsorbed
on this reductive surface with the energy of -0.26 eV (Figure
12a). Compared to the low CO2 adsorption energy of -0.04 eV
on the stoichiometric CeO2 (111) surface, the presence of
oxygen vacancies in reducible support provides the site for CO2
adsorption.[43,44] On the reductive Agn/CeO2(111) surface, CO2
can be strongly adsorbed with the energy of -1.08 eV. It can be
found that oxygen vacancies are beneficial for CO2 adsorption.
The adsorption structure is shown in Figure 12b. O atom makes
up the oxygen vacancy and is connected with Ce atom, while
the C atom is connected with Ag atom.
Figure 10. The optimized structures of CO2 adsorptions on (a) stoichiometric
CeO2(111) and (b) Agn/CeO2(111). (Black spheres represent C atom.)
Due to the outstanding redox ability of CeO2, it’s very
common that there are large numbers of oxygen vacancies on
CeO2(111) and Agn/CeO2(111) surface.[15-18] Three types of
oxygen vacancies (O-1 type, O-2 type, and O-3 type) can be
formed on Agn/CeO2(111) surface (Figure 11). O-1 type loses
oxygen atom on the ceria surface, while O-2 type and O-3 type
lose oxygen atom lie in the interface of Ag nanorod and CeO2
support. Among them, O-1 type owns the low oxygen vacancy
formation energy (Eov=2.27 eV) (Figure 11b). However, O-2 type
and O-3 type oxygen vacancies in interface are difficult to
generate (Eov=2.41 eV) because the distortion of structure
(Figure 11c and 11d). Therefore, we choose O-1 type oxygen
vacancies on the reductive Agn/CeO2(111) surface. Compared
with the oxygen vacancy formation energy on CeO2(111)
(Eov=2.39 eV) and Agn/CeO2(111) (Eov=2.27 eV) surface, the Ag
nanorod contributes to the formation of oxygen vacancy on ceria
surface.
Figure 12. The optimized structures of CO2 adsorptions on (a) reductive
CeO2(111) and (b) reductive Agn/CeO2(111).
These results can be further supported by the differences of
the calculated charge densities of CO2 adsorption. As shown in
Figure 13(a-c), whether on the stoichiometric CeO2(111) surface,
reductive CeO2(111) surface or stoichiometric Agn/CeO2(111)
surface, there is no charge transfer between CO2 and
corresponding catalysts. However, it can be seen in Figure 13d
that there is significant charge transfer between CO2 and
reductive Agn/CeO2(111) support. So, both the oxygen
vacancies and Ag-Ce interface can influence the CO2 adsorption.
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(entries 2,12 and 13). The influence of CO2 pressure on the
reaction is also studied (entries 14 and 15), and the highest yield
is obtained under the CO2 pressure (0.5 MPa). It is worthwhile
noting that no carboxylative coupling product is observed when
the reaction proceeds in the absence of CO2 (entry 16), showing
that the CO2 unit in carboxylative coupling product comes from
free CO2, rather than the carbonate salts.
Table 1. Synthesis of cinnamyl phenylpropiolate from phenylacetylene(1a),
CO2, and cinnamyl chloride(2a) with catalysts[a]
Entry
Catalyst (mg)
Base
Solvent
Yield (%)[b]
1
2
Ag(1.33%)/M-CeO2 (120)
Ag(3.12%)/M-CeO2 (120)
Ag(5.24%)/M-CeO2 (120)
Ag(3.12%)/M-CeO2 (100)
Ag(3.12%)/M-CeO2 (140)
-
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
THF
81
91
90
83
91
4.2
18
33
58
80
45
80
91
62
91
0
3
Figure 13. Calculated charge density difference of CO2 adsorptions on (a)
CeO2(111), (b) reductive CeO2(111), (c) Agn/CeO2(111) and (d) reductive
Agn/CeO2(111) (yellow and blue isosurfaces represent increasing and
decreasing electron densities, respectively).
4
5
6
7
M-CeO2(120)
The catalytic activities of Ag/M-CeO2 are evaluated by the
carboxylative coupling reaction of phenylacetylene (1a), CO2,
and cinnamyl chloride (2a) (Table 1). It is found that 120 mg
three kinds of catalysts and 2.0 equiv. Cs2CO3, give good yields
of cinnamyl phenylpropiolate (3a) (entries 1-3). Ag (3.12%)/M-
CeO2 represents the best catalytic performance (entries 2),
which may be attributed to the well dispersion of Ag
nanoparticles and the most oxygen vacancies. CO2-TPD and
DFT studies have demonstrated that oxygen vacancies are
beneficial for CO2 adsorption. Moreover it has been reported that
oxygen vacancies can promote CO2 activation.[27,28,45] When the
amount of Ag (3.12%)/M-CeO2 is decreased to 100 mg, the yield
decreases to 83% (entries 4). 140 mg Ag (3.12%)/M-CeO2 gives
the same yield of cinnamyl phenylpropiolate (91%) (entries 5).
The yield could not be further improved by increasing the
amount of catalyst. It is noteworthy that almost no carboxylative
coupling product is observed in the absence of any catalyst
(entry 6). M-CeO2 shows certain catalytic activity for the reaction
(entry 7). The yield (18%) is higher than that (4.2 %) without any
catalyst (entries 7 and 6). The catalytic activity of M-CeO2 can
be attributed to the presence of oxygen vacancies on M-
CeO2.[27,28,45] But the yield is much lower than that (91%) when
Ag (3.12%)/M-CeO2 is used as the catalyst (entries 7 and 2),
suggesting that Ag nanoparticles play an important role in
carboxylative coupling reaction.
8
Ag(3.12%)/M-CeO2 (120)
Ag(3.12%)/M-CeO2 (120)
Ag(3.12%)/M-CeO2 (120)
Ag(3.12%)/M-CeO2 (120)
Ag(3.12%)/M-CeO2 (120)
Ag(3.12%)/M-CeO2 (120)
Ag(3.12%)/M-CeO2 (120)
Ag(3.12%)/M-CeO2 (120)
Ag(3.12%)/M-CeO2 (120)
9
CsOAc
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
10
11
12[c]
13[d]
14[e]
15[f]
16[g]
DMF
DMF
DMF
DMF
DMF
[a] Reaction conditions: phenylacetylene (1a) (1.0 mmol), cinnamyl chloride
(1.5 mmol) (2a),catalyst, base (2.0 mmol), CO2 (0.5 MPa), 60 °C, solvent (5
mL), 24 h. [b] Isolated yield. [c] 20 h. [d] 28 h. [e] CO2 (balloon) [f] 1.0 MPa. [g]
In the absence of CO2.
With the optimized conditions (2.0 equiv Cs2CO3, 0.5 MPa
CO2, DMF, 60 °C , 24 h) in hand, various aryl-substituted
terminal alkynes smoothly undergo the carboxylative coupling
reaction (Table 2, entries 1-4). Both electron-donating group
(CH3 and tBu) and electron-withdrawing (Br and F) group
substituted phenylacetylenes successfully converted into the
corresponding products in good yields. Furthermore, allylic
chlorides, propargylic chlorides and aryl halides are tested under
the optimized conditions (entries 5-9), showing high yields of the
corresponding products. Thus, this novel catalytic system shows
wide generality for the synthesis of various functionalized 2-
alkynoates.
We examined the reusability of Ag(3.12%)/M-CeO2 catalyst
in the reaction. Ag(3.12%)/M-CeO2 catalyst was recovered from
the reaction solution by centrifugation. Then the solid residue
was directly used as recycled catalyst in the subsequent runs
without the addition of more catalyst. In Figure 14, Ag(3.12%)/M-
CeO2 catalyst can be recovered and reused five times without
visible loss in catalytic activities. The loading of Ag in the used
catalysts after five catalytic cycles is 3.08% as monitored by
ICP-OES, and the loading of Ag in the fresh catalyst is 3.12%,
which recommends that there is no obvious leakage of Ag. The
XRD patterns and N2 adsorption–desorption (Figures S1 and
In order to recognize the efficient reaction conditions for
promoting the carboxylative coupling, the reaction is conceded
under various reaction times with different bases and solvents
(Table 1, entries 8-11). In the base screening, Cs2CO3 is the
best base for this reaction (entries 8 and 9). It is confirmed that
DMF is a superior solvent in comparison with THF and DMSO,
because DMF is a weak base which is a better solvent for CO2
(entries 10 and 11).[46] Besides, we carried out the catalytic
reactions for different durations ranging from 20 h to 28 h. It is
found that after 24 h the optimum conversion has been achieved
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10.1002/cctc.201900039
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S2) of the reused catalyst do not exhibit any obvious changes,
which indicates that the spatial structure and regular ordered
mesopores have not been destroyed during the process of the
reaction.
carboxylative coupling reaction is proposed in Scheme 2. Firstly,
Ag nanoparticles of Ag/M-CeO2 can coordinate terminal alkynes
with Cs2CO3 to form silver acetylide intermediate A. Some
reports have demonstrated that Ag nanoparticles supported on
the organic network polymers,[47] schiff base,[48] MOF [46,49] and
metal oxides,[50] which can coordinate and activate terminal
alkynes in the carboxylation of terminal alkynes with CO2.
Table 2. Ag(3.12%)/M-CeO2 catalyzed the carboxylative coupling reaction of
terminal alkynes(1), CO2, and chloride compounds (2) [a]
Meanwhile, CO2 is adsorbed and activated on the oxygen
[27,28,45]
vacancies.
Then, the insertion of CO2 into the nearby
silver acetylide generates the silver carboxylate intermediate B.
Finally, chloride compounds react with the intermediate B
forming the product, and simultaneously release the catalyst to
fulfill the cycle.
Entry
Terminal Alkynes
Chloride Compounds
Yield (%)[b]
1
2
3
4
5
6
7
8
9
(1b)
(2a)
(2a)
(2a)
(2a)
(2b)
(2c)
(2d)
(2e)
(2f)
90
91
84
81
74
75
83
81
85
(1c)
(1d)
(1e)
(1a)
(1a)
(1a)
(1a)
(1a)
Scheme 2 Mechanism proposed for carboxylative coupling reaction by Ag/M-
CeO2
Conclusions
[a] Reaction conditions: Alkyne (1.0 mmol), chloride compounds (1.5 mmol),
Ag(3.12%)/M-CeO2 (120 mg), Cs2CO3 (2.0 mmol), CO2 (0.5 MPa), DMF (5
In this work, highly ordered mesoporous Ag/M-CeO2 catalysts
were prepared by the GBMR method. Ag/M-CeO2 contains more
oxygen vacancies than M-CeO2. CO2-TPD and the DFT
calculations indicate that the Ag nanoparticles induce the
increase of oxygen vacancies, which contribute to CO2
adsorption. Ag (3.12%)/M-CeO2 can catalyze the carboxylative
coupling reaction of terminal alkynes, chloride compounds and
CO2, which provides a convenient approach to straightforwardly
synthesize various functionalized 2-alkynoates in good yields
(74-91%) under mild conditions (60 °C, 0.5 MPa). In addition, Ag
(3.12%)/M-CeO2 can be readily recovered and reused for five
times without significant decrease in catalytic activity.
mL), 60 ℃，24 h. [b] Isolated yield
Experimental Section
Materials
Figure 14. Recovery and reuse of Ag(3.12%)/M-CeO2
Unless otherwise noted, carbon dioxide (99.999%), commercially
On the basis of the experimental results, DFT calculations
and previous reports,[6-11] a plausible reaction mechanism for the
available terminal alkynes, chloride compounds ， Pluronic P123
(EO20PO70EO20,
MW=5800),
Cerium
nitrate
hexahydrate
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(Ce(NO3)3 · 6H2O), silver nitrate (AgNO3) and other all reagents were
used without further purification.
was used as the exciting source. The temperature programmed
desorption of CO2 (CO2-TPD) was carried out in
a Micromeritics
AutoChem II 2920. About 100 mg of the samples were pretreated with
He (30 mL min-1) at 400 °C for 30 min before being cooled to 50 °C.
These samples were saturated with 10 % CO2/He blend for 120 min at
50 °C, and then purged with He for 60 min to remove physical adsorbed
CO2. TPD test was carried out from 50 °C to 700 °C with He at a heating
rate of 10 °C min-1. The desorbed CO2 was continuously monitored by a
thermal conductivity detector.
Synthesis of Catalysts
The mesoporous silica with cubic Ia3d symmetry (KIT-6) was prepared
according to the published procedure[51] using tri-block copolymer
Pluronic P123 (EO20PO70EO20) as template by adding of n-butanol in an
acidic aqueous solution.
M-CeO2 was synthesized using KIT-6 as hard template and Ce(NO3)3
·
Computational Details
6H2O as cerium source, followed by removal of KIT-6 using 2 M NaOH
solution.[22] In a typical synthesis, 4 g of Ce(NO3)3 · 6H2O was dissolved
in 20 ml of ethanol. This solution was incorporated into 1 g of KIT-6 by
the incipient wetness impregnation technique. After the ethanol was
evaporated at 60 °C, the composite was calcined at 400 °C to transfer
the precursor into cerium oxide. The bulk CeO2 (B-CeO2) was prepared
as the above method without the addition of hard template.
All the density functional theory (DFT) calculations were performed with
the Vienna ab initio simulation package (VASP) code.[52,53] The exchange
and correlation energy functional was expressed in the GGA-PBE.[54]
Projector-augmented wave method[55,56] was used to describe the
interactions between ions and electrons. The valence electrons were
solved in the plane-wave basis with a cutoff energy of 400 eV. The
convergence criteria[41] for the energy calculation and structure
optimization were set to 1.0×10-5 eV and a force tolerance of 0.05 eV/Å,
respectively. The Brillouin-zone integration[57] was performed using a 1 ×
1 × 1 Γ-centered k-point mesh with Gaussian smearing set to 0.05 eV. To
accurately describe the localization of Ce 4f electrons, we conducted the
DFT+U calculations with a value of Ueff = 4.5 eV. The setting of Hubbard-
like term (Ueff) follows the approach explored by Fabris et al.,[58]
Cococcioni and De Gironcoli.[59]
Ag/M-CeO2 catalyst was synthesized by the GBMR method. The specific
synthesis process was as follows: an aqueous solution of AgNO3 (10
mg/mL) was added dropwise to the mesoporous CeO2 support under
vigorous agitation for 10 min. The suspension was subjected to
sonication (60 W, 40 kHz) at 25 °C for 3 h to mix the Ag precursor and M-
CeO2 evenly. The mixture solution was driven by a peristaltic pump
(Baoding Lange Co., Ltd.) at a rotation speed of 200 rpm (~360 mL/min)
to form a tubal cycling flow. A mixed solution of NaBH4 and TRIS (1
mg/mL, 1 mL/min) was injected into the membrane reactor with two
ceramic membrane tubes (φ = 3mm × 160 mm, Hyflux Group of
Companies, Singapore) by a constant flow pump (HLB-2020, Satellite
Manufactory of Beijing). At the same time, H2 was also flowed through
the membrane rector with the other two membrane tubes. The metal
precursor solution flowed inside the glass tube reactor and outside the
ceramic tubes. The mixed solution was infiltrated through the abundant
holes (d = 40 nm) on the wall of the ceramic tubes into the glass tube
reactor, where the reduction of metal ions occurred immediately when
the two solutions met. The hydrogen bubbling-assisted stirring operation
(40 mL/min) was developed to vigorously stir the solution and to make
the reaction homogenous. The synthesis process was stopped until
NaBH4 solution was consumed completely. The reaction system was
further vigorously bubbled with H2 for 1 h and kept static for 1 h. Then,
the product was filtered and washed with distilled water. The final
products were dried in an oven at 60 °C overnight.
The adsorption energy is defined by:
Eads = E(adsorbate/surface) − E(adsorbate) − E(surface)
Where, E(adsorbate/surface) is the total energy of a surface interacting
with adsorbate. E(adsorbate) and E(surface) are the energies of the
isolated adsorbate and clean surface, respectively. Therefore, a negative
value means exothermic adsorption. The more negative the adsorption
energy, the stronger the adsorption.
The oxygen vacancy formation energy is calculated by:
EO =Esurface-O +1 EO -Esurface
v
v
2
2
Where Esurface is the energy of perfect surface; Esurface-O is the energy of
defective surface with an oxygen vacancy and EO is the energy of a gas-
phase O2.
v
2
The charge density difference of CO2 adsorption on catalyst surface is
defined as Δρ=ρCO /substrate–ρCO −ρsubstrate, where ρCO /substrate is the charge
2
2
2
density of the catalysts system with CO2 adsorption; ρCO and ρsubstrate are
the charge densities of isolated CO2 and catalysts, respectively.
2
Characterization
General procedure of carboxylative coupling reaction
The crystal structures of the samples were determined by a powder X-ray
diffraction on a Shimadzu XRD 6000 using Ni-filtered Cu Kα (λ = 0.15406
nm). The actual Ag content in the prepared catalysts was determined by
ICP-OES using a Perkin–Elmer OPTIMA 7300V. Specific surface area
was measured by adsorption-desorption of N2 gas at -196 oC with
Micromeretics ASAP 2000 instrument. The Brunauer-Emmett Teller
(BET) method was utilized to calculate the specific surface areas from
the adsorption data. Before the measurements, the samples were
outgassed at 120 ºC for 10 h. The transmission electron microscopy
(TEM) images were obtained with a JEM2100 electron microscope
operated at 200 kV. The high resolution transmission electron
microscopy (HRTEM), the high-angle annular dark-field scaning
transmission electron microscope (HAADF-STEM), and energy
dispersive spectroscopy (EDS) for elemental mapping were carried out
on a Tecnai F20 electron microscope. X-ray photoelectron spectra (XPS)
were recorded on a Thermo Escalab-250XI spectrometer using Al Kα (hv
= 1486.6 eV, 1 eV = 1.603 × 10-19 J) X-ray source. The binging energies
were calibrated internally by the carbon deposit C 1s binding energy at
284.8 eV. Raman spectras were measured by Renishaw inVia Reflex
Raman spectrometer under ambient conditions. The laser line at 532 nm
A 40 mL oven dried autoclave containing a stir bar was charged Ag/M-
CeO2 (120 mg), Cs2CO3 (2.0 mmol), alkyne (1.0 mmol), chloride
compounds (1.5 mmol) and 5 mL dry DMF were added with syringe
respectively after purging the autoclave with CO2 three times. The sealed
autoclave was pressurized to appropriate pressure with CO2. The
reaction mixture was stirred at 60 oC for 24 h, then the autoclave was
cooled to room temperature and the remaining CO2 was vented slowly.
Then the reaction mixture was diluted with water (15 mL) and the solid
residue was separated via centrifugation. Then the mixture was extracted
with ethyl acetate (4 × 20 mL). The combined organic layers were
washed with saturated NaCl solution, dried over Na2SO4 and filtered. The
solvent was removed in vacuum, and the product was isolated by column
chromatography on silica gel.
Acknowledgements
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This work was supported by National Natural Science
Foundation of China (No. 21303264)
Keywords: CO2 • Carboxylative coupling • Ag nanoparticles •
Mesoporous CeO2 • Oxygen vacancies
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Entry for the Table of Contents
FULL PAPER
Xiao Zhang*, Dingkun Wang, Meizan
Jing, Jian Liu*, Zhen Zhao*, Guanhua
Xu, Weiyu Song, Yuechang Wei, and
Yuanqing Sun
Page No. – Page No.
Ordered mesoporous CeO2-
supported Ag as an effective catalyst
for carboxylative coupling reaction
using CO2
Ag/M-CeO2 were synthesized through the gas bubbling-assisted membrane
reduction (GBMR) method. Density functional theory (DFT) calculations indicate
that Ag nanoparticles on M-CeO2 facilitate the formation of oxygen vacancies,
which contribute to CO2 adsorption. Ag/M-CeO2 show excellent performance for
carboxylative coupling reaction using CO2, affording a wide range of functionalized
2-alkynoates in good yields.
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