A Schiff base-modified silver catalyst for efficient fixation of CO2 as carboxylic acid at ambient pressure

Article abstract of DOI:10.1039/c7gc00923b

The fixation of CO₂ as a carboxylic acid is a significant reaction for C-C bond formation in organic synthesis. So far, besides C-H carboxylations using stoichiometric amounts of metals or organometallic reagents, great efforts have been devoted to develop new heterogeneous catalyst, while the catalytic performance of supported metal catalysts is not satisfactory. Herein, a Schiff base-modified silver catalyst was developed for the direct carboxylation of terminal alkynes with CO₂, enabling an efficient and green synthesis of valuable alkynyl carboxylic acids. The reaction can smoothly proceed under atmospheric pressure and low temperature (60 °C) conditions. Moreover, a silver-based catalyst, which was prepared by an in situ reduction route, can be easily prepared, recovered, and reused five times without significant loss of activity due to the stability promoted by the Schiff-base on the support surface. In addition, the markedly negative influence of H₂O and solvent effect on this reaction system has also been discussed.

Full text of DOI:10.1039/c7gc00923b

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A Schiff base modified silver catalyst for efficient and fixation of
CO2 as carboxylic acid at ambient pressure‡
Zhilian Wu,abc† Lei Sun,b† Qinggang Liu,bd Xiaofeng Yang,*b Xue Ye,b Yancheng Hu,b and Yanqiang
Huang*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The fixation of CO2 as carboxylic acid is a significant reaction for building blocks for the construction of coumarins, flavones,
the C-C bond formation in organic synthesis. So far, other than C-H and alkynylarenes.12-15 Thus a variety of methods including the
carboxylations using stoichiometric amounts of metals or hydrolysis of alkynyl bromide and relative derivatives,
organometallic reagents, great efforts have been devoted to oxidative carbonylation of alkynes, and reaction of
develop new heterogeneous catalyst, while the catalytic alkynylmetal species with chloroformate have been developed
performance of supported metal catalysts is not satisfactory. Here, towards their synthesis. Despite these advances, the
a Schiff base modified silver catalyst is developed for the direct exploration of an efficient alternative, that fulfils the principles
carboxylation of terminal alkynes with CO2, enabling an efficient of green chemistry, is still demanded.16 In this context, the
and green synthesis of valuable alkynyl carboxylic acids. The direct carboxylation of terminal alkynes with CO2 is definitely
reaction can proceed smoothly at atmospheric pressure and low the simplest and most straightforward access to alkynyl
temperature (60 °C). Moreover, the silver-based catalyst, which carboxylic acids.17-24
was prepared by an in situ reduction route, can be facile prepared,
Several homogeneous catalytic systems have been reported
easily recovered and reused for five times without a significant in this research area,22, 25-28 however, the further application
loss of activity due to the promoted stability by the Schiff-base on was severely impeded by either the recycling problems or the
the support surface. In addition, the markedly negative influence metal contamination of the products. To date, only a few of
of H2O and solvent effect has also been discussed to this reaction heterogeneous conversion was conducted by the supported
system.
silver catalysts for the direct carboxylation of terminal alkynes
with CO2 to produce alkynyl carboxylic acids.24, 29-31 However,
the catalytic performance of supported metal catalysts is not
satisfactory due to their poor yield of desired product, harsh
reaction conditions, or difficult catalyst preparation. For
example, Finashina’s group recently reported that Ag/F—Al2O3
catalyst only gave a yield of 62.1% phenylpropiolic acid under
The conversion and chemical fixation of CO2 is presently a field
of research with ever-increasing interest in virtue of
environment, energy, and related social issues in the past
decades.1-3 Besides, CO2 is an attractive C1 building block in
organic synthesis due to its intriguing features, such as non-
toxic, renewable, inexpensive, and readily available. Great
efforts have been devoted toward the efficient utilization of
CO2 by chemical methods.4-8 Among various CO2 derivatives,
alkynyl carboxylic acids are not only key structural motifs in
numerous biologically active molecules,9-11 but also versatile
29
50 oC and 60 atm conditions. Bhaumik and his colleague
found that AgNPs/Co-MOF catalyst demonstrated an improved
catalytic efficiency,30 but the complex synthetic route limits its
further application. Therefore, the development of an efficient
and robust heterogeneous catalyst featuring facile preparation
for this type of reactions still remains a great challenge.
In situ reduction route is demonstrated as a green and facile
method for the fabrication of highly dispersed metal clusters
or nanoparticles over the support.32 For example, Bao’s group
recently disclosed that amino groups of APTES
(aminopropyltriethoxylsilane)-modified mesoporous silica can
be used to anchor formaldehyde as the reducing agent, on
which silver precursor could be in situ reduced.33 In particular,
our group have successfully developed a method to synthesize
a new type of Schiff base modified gold catalyst by one-pot
aldimine condensation and the in situ autoreduction of a gold
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precursor,34 and the catalyst can be served as an environment-
friendly and efficient catalyst for H2 production from formic
acid owing to its highly dispersed, small sized gold
nanoparticles. Inspired by the previous reports, the
functionalization of Schiff base over supported silver catalysts
will result in a promoted metal dispersion and would offer
great opportunity for the carboxylation of terminal alkynes
with CO2 to produce alkynyl carboxylic acids.
Herein, we present a general route to synthesize a novel
Schiff base modified silver catalyst by the in situ reduction of
silver precursor under ultrasonic conditions. Such Schiff base
modified silver catalyst can act as a robust and green catalyst
in the conversion of CO2 under low temperature (60 °C) and
atmospheric pressure (1.0 atm) conditions, and show excellent
recyclability in the reactions.
The solid Schiff base was synthesized by one-pot aldimine
condensation of (3-aminopropyl)-triethoxysilane (APTES) with
HCHO. Briefly, a quantitative amount of HCHO solution (37%)
was added into an aqueous solution of APTES with vigorous
stirring at room temperature; after washing and drying, a
Figure 1 Element maps for the Ag/Schiff-SiO2 sample (1c): a) TEM
precipitate was obtained as
a metastable SiO2 support
image; b) Si (green); c) O (cyan); d) Ag (red).
(Scheme 1 A). Next, silver nanoparticles (NPs) were deposited
on the support by direct reduction of the AgNO3 precursor
with metastable aminal on the SiO2 support under ultrasonic
conditions, followed by washing and drying to obtain the final
Schiff base modified silver catalyst (supporting information,
1a
-1d, Ag/Schiff-SiO2, Scheme 1 B). 2a (supporting information,
Ag/SiO2) and 3a (supporting information, Ag/Schiff-SiO2) were
developed as reference catalysts by
strategy.
a NaBH4 reduction
The generated Schiff-SiO2 was confirmed by Fourier
transform infrared spectroscopy (FTIR), in which a strong peak
at 1660 cm-1 ascribed to the stretched vibration of the C=N
group of imine was observed (Figure S1). 29Si CP-MAS NMR and
13C CP-MAS NMR spectra were also used to verify the
evolution of the Schiff base during the whole preparation
procedures. As demonstrated in the 29Si CP-MAS NMR spectra
the Si environments of T2 (-60 ppm), T3 (-67 ppm) and Si-O-
Figure
2 TEM image of the Ag/SiO2-Schiff sample (1c; left) and size
distribution of the Ag NPs in a sample of 1c (Ag NPs size: 6.3±1.9 nm) (right).
C2H5 groups (-50 ppm) were detected according to the NMR
chemical shifts (Figure S2a), showing the APTES hydrolysis and
condensation with HCHO to Schiff-SiO2 solid material with
some alkane-Si moieties retaining. In the 13C CP-MAS NMR
spectra (Figure S2b), the sample of Schiff-SiO2 showed
resonance peaks of carbon chemical shifts at d = 163 and 78
ppm, which were assigned to the -N=C (C1) Schiff base and
aminal N-C-N (C2), respectively, while the metastable aminal
(C2) may be further hydrolysed to -NHCH2OH.34, 35 Powder X-
ray diffraction (PXRD) pattens in Figure S3 show that the as-
prepared catalysts (1a, 1b, 1c and 1d) showed no silver peaks,
indicating finely distributed small Ag nanoparticles. The Ag
nanoparticles in the catalyst of 1c-recycled may slightly
aggregated in the reaction, which displayed by the poor
characterized peaks of Ag (111). However, the catalysts of 2a
Scheme 1 The synthetic scheme employed for preparing Ag/Schiff-SiO2
nano-composite.
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Table 1 Synthesis of 3-phenylpropiolic acid from CO2 and 1-ethynylbenzene[a]
throughout the surface of the Schiff-SiO2 matrix. The TEM
images (Figure 2 and Figures S6–S8) confirmed that the silver
NPs in 1c were homogeneously dispersed and exhibits smaller
sizes (6.3±1.9 nm) than those in 1a
, 1b, and 1d.
Cat.
Base
solvent
T/oC
t/h Yield[b]
/%
TON[c]
The catalytic activities of the Ag/Schiff-SiO2 samples (1a
-
1d,
Entry
2a, and 3a) were then evaluated for the fixation of CO2 with
terminal alkynes into propiolic acids. In the initial research, the
carboxylation of 1-ethynylbenzene was selected as a model
reaction to explore the influence of various parameters on the
reaction. Under the conditions of 60 oC, 1.0 atm of CO2 and 6 h,
reaction in DMSO only gave 37% and 36% yields in the absence
of catalyst and in the presence of Schiff-SiO2, respectively
(Table 1, entry 1-2). If we added the Ag/Schiff-SiO2 catalyst (1c)
into the same reaction, the yield can be promoted to 78%
(Table 1, entry 3). Up to 98% yield of the desired propiolic acid
product was obtained with 0.14 mol% of catalyst 1ꢀc at
ambient pressure in 24 h (Table 1, entry 4), and the catalyst 1c
has the largest TON (705) compared with previous
heterogeneous catalytic systems in table S2. As reference
Cs2CO3
Cs2CO3
DMSO
DMSO
60
60
6
6
37
36
1
2
－
－
None
Schiff-
SiO2
1c
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
NaOH
KOH
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
60
60
60
60
60
60
60
60
60
60
6
24
24
24
24
24
24
24
24
24
78
98
97
82
82
86
89
1
3
561
705
697
590
590
618
640
7
1c
4
2a
5
3a
1a
1b
1d
1c
1c
1c
6
7
8
9
10
11
12
3
22
CsOH·
H2O
Na2CO3 DMSO
1
7
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
60
60
60
60
60
60
60
25
40
50
70
24
24
24
24
24
24
24
24
24
24
24
2
23
39
50
7
13
14
15[d]
17
18
19
20
21
22
23
24
14
165
280
360
50
catalysts, Ag/SiO2
(
2a) was developed with NaBH4 reduction
3a) was developed with NaBH4
K2CO3
DMSO
DMSO
DMF
strategy and Ag/Schiff-SiO2
(
K2CO3
reduction along with stirring strategy. Interestingly, 2a also can
gave good yield (97%) (Table 1, entry 5), but after one cycle,
the Ag loading of recycled 2a catalyst only owned half of the
fresh one, while the recycled 1c catalyst almost possessed the
same Ag loading, therefore the 2a catalyst shown very poor
stability compared to 1c catalyst. While 3a catalyst only can
gave relative poor yield (82%) (Table 1, entry 6), and it can be
explained by PXRD results (PXRD; Figure S3). Besides, the
support of Schiff-SiO2 may be served as enhanced stability and
a significant decrease in the propensity for loss of metals from
the catalysts, during the reaction. Based on the above results,
it can be deduced that the in situ autoreduction route is the
best method to fabricate the nano-silver catalyst among the
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
MeCN
Dioxane
PC
3
22
23
12
50
82
92
165
86
DMSO
DMSO
DMSO
DMSO
360
590
661
[a]Reaction conditions: 1-ethynylbenzene (1.0 mmol), catalyst (1a = 1.17 wt%
Ag/Schiff-SiO2, 1b = 1.36 wt% Ag/Schiff-SiO2, 1c = 1.45 wt% Ag/Schiff-SiO2, 1d =
2.52 wt% Ag/Schiff-SiO2, 2a = 1.20 wt% Ag/ SiO2, 3a = 0.71 wt% Ag/Schiff-SiO2,
Ag: 1-ethynylbenzene = 1:719 mol mol-1), Cs2CO3 (1.5 mmol), CO2 (1.0 atm), 60 °C,
solvent (5 mL), 24 h, unless otherwise noted. [b] Yield based on HPLC. [c] TON =
(moles of product)/(moles of metal in the catalyst), unless otherwise noted. [d]
3.0 mmol CsCl was added to the reaction system.
three strategies. Under the same reaction conditions, 1a, 1b,
and 1d gave 82%, 86% and 89% of the desired product yields,
respectively (Table 1, entries 7-9), and the catalytic activity
decreased in the following order: 1c>1d>1b>1a, which was
accordance with the TEM results. From the catalytic activity
and 3a sintered into bulk Ag particles, which indicated by
sharp characterized peaks of Ag (111), Ag (200), Ag (220) and
Ag (311). Correspondingly, X-ray photoelectron spectroscopy
(XPS) results indicated that the Silver ions were successfully
reduced to Ag (0) nanoparticles by the support of Schiff-SiO2,
and the charge states of silver nanoparticles almost with no
change (Figure S9, Figure S11). Small fraction of Ag+1 species in
the XPS are likely to be the Ag2O (Figure S9, Figure S11),
because the Ag nanoparticles are easily to be oxidized in the
results of 1a-1d, it is demonstrated that the average size of Ag
nanoparticles may be the main reason, which lead to catalytic
performances of the all catalysts with different Ag loadings
being different. Accordingly, a larger surface area of active
sites is available for the substrates, and a better catalytic
performance was achieved with 1c
.
Cs2CO3 is one of the most commonly used bases for many
organic reactions.36 Here, Cs2CO3 also performed as the best
base for this carboxylation reaction of 1-ethynylbenzene with
CO2, which is consistent with the results in previous report.16,
air. Besides, the N species of -N
recycled were quite similar with the 1c (Figure S10, Figure S12).
Catalysts 1ꢀa 1ꢀb 1ꢀc 1d 2a, and 3a have Ag loadings of 1.17,
=C and -NH–C groups in the 1c-
,
,
,
,
19, 24, 37
Among those bases we screened, NaOH, KOH,
1.36, 1.45, 2.52, 1.20 and 0.71 wt%, respectively, as confirmed
by inductively coupled plasma (ICP) analysis (Table S1). To
further study the Ag/Schiff-SiO2 catalysts, the morphology of
the silver NPs deposited on the support of Schiff-SiO2 was
characterized by energy dispersive spectroscopy (EDS) and
transmission electron microscopy (TEM). EDS images (Figure 1)
showed that the Ag NPs were homogeneously distributed
CsOH·H2O, Na2CO3, and K2CO3, only resulted in poor yields
(Table 1, entries 10-14). Surprisingly, if we added 2 mmol CsCl
into the reaction system of K2CO3, the yield can be elevated
from 23% to 39% (Table 1, entries 14-15), and it was proved
that Cs+ could promote the reaction.
In addition, the impact of the polar organic solvents is
thoroughly investigated, and DMSO was confirmed to be the
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Table 2 Substrate scope tests over the 1c catalyst[a]
0.14 mol% 1c,
1.5 equiv. Cs2CO3
Yield
Selective
100
80
60
40
20
100
80
60
40
20
0
HCl
R
R
COOH
+
CO2
DMSO, 60 oC, 24 h
Yield[b]/%
98
Entry
1
Alkyne
Product
2
3
4
5
92
87
93
86
0
0
2
4
6
8
The additive of water / µL
Figure 3 Effect of H2O additive on the 3-phenylpropiolic acid yield and
selectivity. Reaction conditions: catalyst 1c (1.45wt% Ag/Schiff-SiO2, 10
mg), Cs2CO3 (1.5 mmol), 1-ethynylbenzene (1.0 mmol), CO2 (1.0 atm),
DMSO (5 mL), 60 °C, 24 h.
6
7
94
92
best solvent for this reaction. Under the conditions of 60 °C
and 1.0 atm of CO2, 98% yield was obtained in DMSO over
catalyst 1c (Table 1, entry 4). While 50%, 7%, 3%, and 23%
yields was obtained in N,N-dimethylformamide (DMF),
acetonitrile (MeCN), 1,4-dioxane (Dioxane) and propylene
carbonate (PC), respectively (Table 1, entries 17-20). It was
deduced that the greater polar solvent (DMSO) may be more
beneficial for this heterogeneous reaction, similar result also
can be found in the previous report.38 The influence of
temperature on the reaction was also studied, and 60 °C was
proved to be the optimal temperature (Table 1, entry 4, 21-
24) .
[a] Reaction conditions: alkyne (1.0 mmol), catalyst 1c (1.45wt% Ag/Schiff-
SiO2, 10 mg), Cs2CO3 (1.5 mmol), CO2 (1.0 atm), 60 °C, DMSO (5 mL), 24 h.
[b] Isolated yields.
mL DMSO, 60 °C and 1.0 atm CO2, the corresponding
carboxylation products were obtained in promising yields (86-
98ꢀ%) when aromatic alkynes with either electron-donating
group (Table 2, entry 2) or with electron-withdrawing groups
(Table 2, entries 3-4). Even for an alkyne with a hetero
aromatic ring group (Table 2, entry 5), a yield of 86% was
achieved. Both the alkyl- or alkenyl-substituted terminal
alkynes are amenable to the procedure, thus giving the
corresponding alkynyl carboxylic acids in synthetically
satisfactory yields of 94% and 92%, respectively (Table 2, entry
6-7).
Due to unique structure and properties, water have great
impact on the reaction course,39 which might lead to particular
interactions, such as polarity, hydrophobic effect, hydrogen
bonding and trans-phase interactions. Especially, some recent
fundamental discoveries were proved that water could
accelerate reactions under heterogeneous conditions.40-42
However, H2O has markedly negative influence to our reaction
system. When the reaction was conducted in the standard
conditions, it was found that the H2O additive is linearly
related to the propiolic acid yield (Figure 3). It is worth to note
that when 8 μL H2O was added into the reaction system, the 3-
phenylpropiolic acid yield was rapidly dropped from 98% to
56%. A control experiment was conducted by using 3-
phenylpropiolic acid (1.0 mmol) as starting reactant under the
same conditions with CO2 (1.0 atm) in DMSO (5 mL) with H2O
(8 μL) at 60 ◦C for 24 h. A mixture of alkynyl carboxylic acid and
terminal alkyne (ratio is about 5 : 5) was obtained. Therefore,
it is speculated that H2O in the reaction system might facilitate
decarboxylation reaction of alkynyl carboxylic acid.
As a heterogeneous catalyst, Ag/Schiff-SiO2 could not only
be facilely prepared, but also easily separated by
centrifugation or filtration of the reaction mixture. As Shown in
figure S4, the catalyst 1c maintained high activity under the
optimized conditions, only slightly decreased activity was
observed, which may be attributed to the nano Ag particle
slightly aggregated in the reaction, and the decreased amount
of catalyst during the regeneration process, it is deduced that
the N species in Schiff-SiO2 can effectively anchor Ag NPs. The
PXRD pattern (Figure S3), TEM images (Figure S5) and XPS
(Figure S11-S12) of the used Ag/Schiff-SiO2
the same as the fresh catalyst. In short, the Ag/Schiff-SiO2 (1c
demonstrated robust stability and recyclability as
(1c) were almost
)
So as to investigate the generality of the Ag/Schiff-SiO2
catalyst (1c) in the carboxylation reaction, various typical
terminal alkynes were explored (Table 2). Under the standard
conditions: 1c (0.14 mol% of Ag), 1.5 equivalents of Cs2CO3, 5
a
heterogeneous catalyst for the direct carboxylation of terminal
alkynes with CO2 to produce propiolic acids. The total TON for
1c in the beginning 5 runs was 3430, and the number would be
further increased.
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CO2
between silver NPs and Cs2CO3 enormously enhanced the
catalytic activity of this nanocatalyst system.
R
R
R
As the key step in the reaction, the transition state
B in the
Ag
N
O
solvent was investigated by using density functional theory
(DFT) calculations. In this step, the complex of deprotonated is
considered as the reactant. Figure 4 displays the relative free
energy profiles for the Ag8-catalyzed electrophilic attack with
CO2 in DMSO and DMF solvent, respectively. In DMSO, the
deprotonation of phenylacetylene coordinates with CO2over
the alkynyl C≡C triple bond by a moderate energy barrier of
15.1 kcal/mol (EDMSO). Then the product is formed by the
coordination of PhC≡C-COO to Ag8 cluster with an energy
decreasing (-5.1 kcal/mol) against to the reactant. However,
the same reactive path in DMF solvent exhibits an increase of
6.6% in the energy barrier (EDMF=16.1 kcal/mol) comparing to
the reaction in DMSO. The calculations indicate that the silver
NPs could be an effective catalyst for the electrophilic attack
process. Moreover, the greater polar solvent (DMSO) may be
more beneficial for this heterogeneous reaction, which is in
well consistent with our experimental findings.
Ag
Ag
N
Cs2CO3
C
O
A
N
B
R
R
Ag/Schiff-SiO2
O
C
OCs
H2
C
R
D
E
N
N
HN
H3O
Cs2CO3
O
O
O
Ag
C
R
C
Si
Si
Si
OEt
HO
OH
n
OH
O
O
N
OEt
OEt
Scheme
2 Possible reaction mechanism for the Ag/Schiff-SiO2
catalyzed carboxylation of terminal alkyne.
Conclusions
In summary, a Schiff-base modified silver nano-composite
catalyst is successfully synthesized and demonstrated to have
robust activity for the direct carboxylation of terminal alkynes
with CO2 to produce valuable carboxylic acids. H2O has
markedly negative influence on this reaction system. The DFT
calculations indicate that the silver NPs could be an effective
catalyst for the electrophilic attack process. Moreover, the
strongly polar solvent (DMSO) may be more beneficial for this
heterogeneous reaction. The heterogeneous catalyst feature
good catalytic activity, robust recyclability, and excellent
substrate generality in the carboxylation of terminal alkynes
with CO2 under mild conditions. Potential applications of this
catalyst in other areas can be anticipated, for instance, in
organic synthesis, polymer, and particularly for the fixation of
CO2 into chemicals and fuels. Tremendous efforts to elucidate
the catalytic mechanism and to explore new efficient catalysts
are still underway in our lab.
Figure 4 Relative free energies of the energy barrier (ΔE) for the Ag8-
catalyzed electrophilic attack step in DMF (black) and DMSO (red)
solvents.
Acknowledgements
25
Based on our finding and some previous reports,24,
a
The authors acknowledge the financial support from the
National Natural Science Foundation of China (21403218,
21476226, 21503125 & 21506204), China Ministry of Science
and Technology under contact of 2016YFB0600902, Dalian
Science Foundation for Distinguished Young Scholars (2016RJ04),
the Youth Innovation Promotion Association CAS. The authors
also appreciate valuable discussions with Dr Xiaohuan Liu, Dr
Guangfeng Liang, and Miss Xiaoli Yang.
possible mechanism over Ag/Schiff-SiO2 (1c) was proposed and
schematically depicted in Scheme 2. It is speculated that the
silver NPs on Schiff-SiO2 can coordinate to the terminal alkyne
and be deprotonated by Cs2CO3 through acidifying the C–H
bond to form a silveracetylide intermediate
electrophilic attack from carbon dioxide to the coordinated
acetylide to form the transition state , subsequently to form
the silver carboxylate . In the presence of a large amount of
Cs2CO3 and alkyne reactants, the transmetallation occurred
quickly, and the generation of cesium carboxylate and
A, meanwhile,
B
C
D
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Green Chemistry
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DOI: 10.1039/C7GC00923B
A Schiff base modified silver catalyst for efficient and
fixation of CO2 as carboxylic acid at ambient pressure
Zhilian Wu,abc† Lei Sun,b† Qinggang Liu,bd Xiaofeng Yang,*b Xue Ye,b Yancheng Hu,b
and Yanqiang Huang*b
Table of contents entry
A
Schiff base modified silver catalyst is developed for the direct carboxylation of terminal
alkynes with CO2, enabling an efficient and synthesis of valuable alkynyl carboxylic acids.