Flower-like AgNPs@m-MgO as an excellent catalyst for CO2 fixation and acylation reactions under ambient conditions

Article abstract of DOI:10.1039/c8nj02286k

Synthesis of fine chemicals from the chemical fixation of CO₂ is one of the attractive research areas of today to utilise greenhouse gas CO₂ in a greener pathway. A flower-like silver nanoparticle grafted mesoporous magnesium oxide (AgNPs@m-MgO) nanocomposite has been prepared by a facile in situ pathway. The materials are characterised by XRD, FTIR, UV-vis, TG-DTA, FESEM, HR-TEM and N₂ adsorption-desorption studies. This AgNPs@m-MgO material showed a mesoporous nature with good surface area. It indicated brilliant catalytic activity for both the carboxylation of terminal alkynes by chemical fixation of CO₂ (1 atm) and the acylation of various amines by utilizing acetic acid as a reagent in solvent-free conditions showing yields up to 98% and 99% of the propiolic acid and acetamide products, respectively. The reusability of this catalyst has also been verified and it showed high recycling efficiency for both the reactions together with no considerable catalyst deactivation.

Full text of DOI:10.1039/c8nj02286k

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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ARTICLE
Flowerlike AgNPs@m-MgO as an excellent catalyst for CO2
fixation and acylation reactions under ambient conditions
Arpita Hazra Chowdhury, Swarbhanu Ghosh, Sk. Manirul Islam*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Synthesis of fine chemicals from the chemical fixation of CO2 is one of the attractive research areas of today to utilise
greenhouse gas CO2 in a greener pathway. Flowerlike silver nanoparticles grafted mesoporous magnesium oxide
(AgNPs@m-MgO) nanocomposite has been prepared by a facile in-situ pathway. The materials are characterised by XRD,
FTIR, UV-vis, TG-DTA, FESEM, HR-TEM and N2 adsorption–desorption studies. This AgNPs@m-MgO material showed
mesoporous nature with good surface area. It indicated brilliant catalytic activity for both the carboxylation of teterminal
alkynes by chemical fixation of CO2 (1 atm) and the acylation of various amines by utilizing acetic acid as reagent in solvent
free condition showing yield upto 98% and 99% of the propiolic acid and acetamide products respectively. The reusability
of this catalyst has also been verified and it showed high recycling efficiency for both the reactions together with no
considerable catalyst deactivation.
www.rsc.org/
of this problem, immobilization of metal nanoparticles should be
done by introducing an appropriate solid support to enhance the
stability of the material [23]. In the present work, we have chosen
mesoporous magnesium oxide as the solid support because
recently mesoporous MgO has shown high potential for carbon
dioxide adsorption due to its high porosity and tunable nature of
pores [24].
1. Introduction
Excessive discharge of greenhouse gases resulting global warming is
one of the major environmental threats world is facing today.
Among these greenhouse gases, CO2 contributes the most to global
warming. This major environmental issue has attracted the
attention of researchers to capture and utilize CO2 as C1 reagent to
synthesize fine chemicals. Now days, several research groups has
been developed terminal alkyne carboxylation using CO2 to
synthesize propiolic acids [1-4] which are important compounds in
medicinal chemistry [5-7] and are used in the formation of
alkynylarenes or aminoalkynes through decarboxylative cross-
coupling reactions [8-10]. Although oxidation of alcohols or
aldehydes and hydrolysis of bromide derivatives are some well
known procedures to prepare propiolic acids [11-13], but catalytic
insertion of CO2 to terminal alkynes is the most clear-cut and
environment friendly method to synthesize propiolic acids [14].
Several homogeneous catalytic systems using silver (I) salt for
terminal alkynes carboxylation are reported in literature [15-18]. Cu
can also activate the triple bond of alkynes. Cu(I)@C heterogeneous
system is reported as an efficient and cheap catalyst for
carboxylation of terminal alkynes [19].There are only few works
done on silver nanoparticle based heterogeneous catalytic systems.
Nanoparticles have high surface area to volume ratio and posses
large number of open active sites on their surfaces. These beneficial
features make the nanoparticles much more reactive [20-22] than
Acylation of amines is significantly important conversion used in
organic synthesis in both industrial and medicinal chemistry.
Generally, acylating agents like acetic anhydride and acetyl chloride
are used in the presence of acidic [25, 26] and basic [27, 28] catalyst
to synthesize acetamides. The drawbacks of these reagents are that
they are toxic, hygroscopic and corrosive in nature. Therefore, the
use of acetic acid is economically as well as environmentally
advantageous over the use of acetic anhydride and acetyl chloride.
Now days, the increasing demand of products with high yield and
low waste in chemical industries attracts the attention of
researchers to develop efficient heterogeneous catalytic systems
for different organic synthesis to achieve environmentally benign
synthesis with good yields, high selectivity and good reusability of
the catalyst.
In the present work, we have synthesized silver nanoparticles
grafted mesoporous magnesium oxide (AgNPs@m-MgO)
nanocomposite by a facile and cost effective in-situ synthetic
method and applied it as a reusable, environmentally benign and
cost effective heterogeneous catalyst in the carboxylation of an
extensive variety of terminal alkynes through chemical fixation of
CO2 under mild reaction conditions. We have also studied its
catalytic activity for acylation of different types of amines utilizing
acetic acid as acylating reagent to gain high atom economy. To the
best of our knowledge, there is no report on AgNPs@m-MgO as a
catalyst used in these two above mentioned reactions.
their metal salts. The major issue with the metal nanoparticles is
that they get destabilized by highly active surface atoms. To get rid
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spectrophotometer (UV-3101PC, Shi-madzu). The N2 adsorption-
desorption analysis of Ag@MgO sample was performed by using a
ASIQ MP BET analyzer, Quantachrome Instruments, USA. The
Barrett–Joyner–Halenda (BJH) method was applied to estimate the
pore size distribution of the material. The FESEM study of the
material was performed by using Zeiss, Supra™ 35VP (Oberkochen,
Germany) instrument and the TEM study was done by using Tecnai
G2 30ST (FEI). Energy dispersive analysis of X-ray spectroscopy (EDS)
coupled to TEM was done to get information about the elements
present in the sample. At the end of the reactions, the obtained
products were studied by a Varian 3400 gas chromatograph.
2. Experimental
2.1. Materials
Magnesium nitrate hexahydrate (Mg(NO3)2,6H2O), ammonium
hydroxide (NH4OH), silver nitrate (AgNO3), sodium sulfate (Na2SO4),
sodium bicarbonate (NaHCO3), sodium chloride (NaCl) and sodium
borohydride (NaBH4) were bought from Merck, India. All alkynes,
amines, acetic acid (99-100%), Polyvinylpyrrolidone (PVP) and
solvents were bought from Sigma-Aldrich. Deionized (DI) water was
utilized all through the investigation.
2.2. Synthesis of mesoporous MgO (m-MgO)
2.5. Procedure of carboxylation of terminal alkynes using
AgNPs@m-MgO Catalyst
Mesoporous magnesium oxide (MgO) was synthesized utilizing
magnesium nitrate hexahydrate as precursor salt via simple
template-free ammonia precipitation route. In a round bottom
flask, a 0.02 mol magnesium nitrate hexahydrate aqueous solution
was taken and ammonium hydroxide solution was added dropwise
to it under stirring condition at room temperature, maintaining
NH4OH to Mg mole ratio at 4:1. Then the mixture in the tightly
closed round bottom flask was stirred at 70 °C for 6 h. After 6 h, the
heating was stopped and the mixture was stirred slowly for the next
20 h at room temperature. The white precipitate was obtained,
which was collected by centrifugation, washed with DI water for
multiple times. The collected product was dried at 60 °C for 4 h. The
desired mesoporous MgO nanomaterial was obtained after
calcining the as prepared sample at 450 °C for 3 h.
Carboxylation reaction of terminal alkynes was done in a 25 mL
round bottom flask, equipped with rubber stopper penetrated by a
balloon which contained CO2 under 1 atmosphere, containing a
mixture of 1.0 mmol of terminal alkynes, 1.2 mmol Cs2CO3, 3 mL
DMF and 50 mg of AgNPs@m-MgO catalyst. After stirring the
mixture at 70 0C for 12 h, the system was cooled to room
temperature and 5 mL 2(N) potassium carbonate solution was
mixed with the reaction mixture through stirring for 30 minutes.
CH2Cl2 was added to the reaction mixture. Then the aqueous layer
was acidified with concentrated HCl to pH=1. Ethyl acetate was
used for extraction. At the end of the process, the solid catalyst was
filtered out from the combined organic layers which were washed
with saturated NaCl solution. Then it was dried over anhydrous
Na2SO4 and filtered. GC analysis of the product was done to get the
yield of the acid product. The progress of the reaction was checked
by the gas chromatographic technique.
2.3. Synthesis of silver nanoparticles grafted mesoporous
magnesium oxide (AgNPs@m-MgO) nanocomposite
The AgNPs@m-MgO nanocomposite was prepared by in-situ
synthesis of silver nanoparticles in MgO matrix. 200 mg of MgO was
sonicated in 10 mL H2O for 1 h followed by addition of 0.1 mmol of
AgNO3 to the MgO suspension. After stirring for 1 h, 5 mL aqueous
solution of 0.04g PVP was added as stabilizing agent. Then 2 mL 1
(M) aqueous solution of NaBH4 was added as a reducing agent
followed by 10 minutes stirring of the reaction mixture. The product
was then isolated through centrifugation. DI water and ethanol
were used as washing solvents. The washed product was dried at 60
°C in vacuum for 2 h to obtain the AgNPs@m-MgO nanocomposite
material.
2.6. Procedure for acylation of amines using AgNPs@m-MgO
catalyst
Acylation of amine was conducted by mixing 3 mmol of aniline with
6 mmol acetic acid with 20 mg of AgNPs@m-MgO catalyst in a 25
mL round bottom flask, fitted with a condenser. The reaction was
0
done at 120 C for 3 h. After cooling the mixture at r.t, the work up
was carried out by adding ethyl-acetate to the reaction mixture.
The organic part was gathered after separating the solid insoluble
catalyst through centrifugation. The organic part was then washed
with H2O and saturated NaHCO3 solution and afterward dried over
anhydrous Na2SO4 followed by evaporation of the solvent to obtain
the needed product. The product was analysed by GC analysis to
find out the yield of the acylated product. The progress of the
conversion of amine to respective N-acylated product was checked
by the gas chromatographic technique.
3. Results and discussion
3.1. Characterization of AgNPs@m-MgO, m-MgO and the as-
prepared materials
Scheme 1. Schematic illustration of the preparation of AgNPs@m-MgO
nanocomposite material
3.1.1. X-ray diffraction (XRD) Analysis
The XRD pattern depicted in Figure 1(a) indicates the presence of
characteristics peaks of Magnesium Hydroxide [Mg(OH)2, JCPDS:84-
2163] in the as-prepared sample which is on calcination at 450 0C
gives MgO material. The wide angle XRD pattern Figure 1(b)
signifies the presence of characteristic peaks of cubic MgO crystal
(JCPDS card no 01-1235). The XRD pattern of the AgNPs@m-MgO
nanocomposite (Figure 1(c)) indicates the presence of metallic silver
and cubic MgO with small amount of Mg(OH)2. The crystalline peaks
at 2θ = 38.4, 44.5, 64.4, 77.4 and 81.5 are due to (111), (200), (220),
(311) and (222) lattice planes, respectively, of FCC Ag (JCPDS Card
No. 04-0783) [29]. The crystallite sizes of the Ag nanoparticles (14-
2.4. Characterization
X-ray diffraction (XRD) analysis of the samples were done with
Philips X’Pert Pro PW 3050/60 powder diffractometer equipped
with Ni-filtered Cu-Ka radiation (k = 0.15418 nm). The DT and TG
analysis (Netzsch STA 449C, Germany) of the material was done
from 300 to 1000 °C in air atmosphere. The FTIR analysis (Perkin-
Elmer FT-IR 783 spectrophotometer) of the materials were done to
get the information about characteristic vibration bands of them.
The UV–Vis spectra was recorded by using
a UV–vis–NIR
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20 nm) are the XRD pattern of the catalyst (Figure 1c) using the 3.1.3. UV-vis Analysis
ARTICLE
Scherrer
equation.
UV-vis absorption spectra of the AgNPs@m-MgO nanocomposite is
depicted in Figure 3. Both the MgO and Ag nanoparticles showed
consistent absorption over the UV wavelength range. The
absorption band at 426 nm appeared due to surface plasmon
resonance of Ag Nps [29]. The absorption peak at 235 nm peak
arises due to the excitation of O2− surface anions on the edge of the
MgO nanocrystal.
(101)
(001)
(a)
(b)
(200)
(102)
(220)
(110)
(111)
(111)
(103)
(201)
(222)
(311)
10
20
30
40
50
60
70
80
10
20
30
40
50
60
70
80
235 nm
2θ in degree
2
θ
in degree
(111)
(c)
426 nm
Ag
Mgo
(200)
Mg(OH)2
(001)
(220)
(110)
(200)
300
400
500
600
700
800
(220)
(311)
(222)
(311)
Wavelength (nm)
Figure 3. UV-vis spectra of the AgNPs@m-MgO sample.
(222)
10
20
30
40
50
60
70
80
90
3.1.4. Thermogravimetric (TG) and Differential Thermal Analysis
(DTA)
2θ in degree
Figure
AgNPs@m-MgO materials (c).
1. XRD patterns of the as-prepared Mg(OH)2 (a), m-MgO (b) and
The TG-DTA plots of the as-prepared Mg(OH)2 sample are depicted
in Figure 4. The TG analysis indicates two step mass loss where the
first step is about 6% upto 320 0C, which is occurred due to the
removal of surface adsorbed water showing an endothermic peak
at around 120 0C in the DTA curve. The second step is about 31% in
3.1.2. FTIR Analysis
The FTIR spectra of the as-prepared, calcined and composite
samples are depicted in Figure 2. In the FTIR spectra of Mg(OH)2,
the sharp peak at 3695 cm-1 is due to the presence of structural
hydroxyl groups and the broad peak at 3450 cm-1 is due to the
hydroxyl group, adsorbed on the surface respectively [30]. It is
interesting to notice that the 3695 cm-1 band is completely vanished
in the case of calcined sample due decomposition of Mg(OH)2 to
MgO. Physisorbed water, present in as-prepared, calcined and
composite samples causes the characteristic band at 1648 cm−1
which is obtained from the bending vibration mode. The
characteristic Mg–O vibration bands between 900-450 cm-1 are
confirmed for the calcined and composite samples [31]. In the FTIR
spectra of AgNPs@m-MgO nancomposite sample, the slight
presence of 3695 cm−1 band (shown in inset of Figure 2) because of
non-bonded OH groups indicates that the surface of nanocrystalline
magnesium oxide becomes partially hydroxylated during its
preparation [32].
0
the temperature range between 3200 to 450 C, which is occurred
due to dehydroxylation of Mg(OH)2 showing the endothermic peak
at 358 0C in the DTA plot [33]. This 31% mass loss acquired from the
TGA plot is almost similar to the anticipated hypothetical mass loss
of 30.9% calculated from the stoichiometric conversion of Mg(OH)2
to MgO phase.
0.0
100
ꢀ0.4
90
ꢀ0.8
80
70
ꢀ1.2
60
ꢀ1.6
200
400
600
800
1000
Ag@MgO
MgO
(0
C)
Temperature
Mg(OH)
Figure 4. DTA-TG plots of the Mg(OH)2sample.
2
3500
3750
4000
3.1.5. Surface area measurement
Figure 5 represents the N2 adsorption-desorption isotherm of the
AgNPs@m-MgO nanocomposite material. The isotherm showed
type III isotherm with H3 type hysteresis loop. It suggested the
presence of slit like pores of nanoscale dimensions in the
AgNPs@m-MgO sample, which is also indicated by the BJH pore size
distribution plot [34], depicted in the inset of Figure 5. The BJH pore
size distribution plot indicated that the material has a pore size
distribution centered at 3.9 nm which falls in the mesoporous
region (2 - 50 nm). The estimated BET surface area, peak pore
500 1000 1500 2000 2500 3000 3500 4000
ꢀ1
Wavenumber (Cm
)
Figure 2. FTIR spectra of the as-prepared Mg(OH)2, m-MgO and AgNPs@m-MgO
samples.
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diameter and total pore volume of the AgNPs@m-MgO material are
83.2 m2 g-1, 3.9 nm, 0.81 cm3 g-1 respectively.
600
0.05
500
0.04
0.03
400
0.02
300
0.01
0.00
200
0
5
10
15
20
25
30
Pore diameter (nm)
100
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure
(
p/p )
0
Figure 5. N2 adsorption–desorption isotherm of the AgNPs@m-MgO sample at
77 K. Pore size distribution is shown in the inset.
3.1.6. Microscopic Analysis
Figure 7. HR-TEM images of AgNPs@m-MgO sample (a,b) at two different
magnifications, (c) lattice fringes, (d) SAED pattern.
In order to analyze the morphology of the AgNPs@m-MgO material
FESEM study was done. The corresponding high magnified images
are shown by arrow marks. From the FESEM images (Figures 6a and
6b) of the nanocomposite sample, flower like morphology of the
AgNPs@m-MgO composite material can be observed. In a possible
growth mechanism, at the time of the synthesis of the as-prepared
Mg
Element weight (%)
O(K)
42.74
52.72
4.53
Mg(K)
Ag(L)
O
-
sample, the adsorption of the free ions (NO3 , OH-) on the main
crystallographic plane of the material surface through hydrogen
bonding or through loose coordination with Mg+2 [30] leads to the
aggregation of the MgO nanoparticles in a 2D fashion forming the
petals of the flower like morphology of the sample.
Ag
0
5000
10000
TEM images (Figures 7a and 7b) suggested the inter-particle
porosity for this AgNPs@m-MgO nanomaterial. Figure 7c and 7d
represent the fringe and SAED pattern of AgNPs@m-MgO sample,
respectively. The SAED pattern indicates that the particles have
polycrystalline nature with different diffraction planes. The
concentric diffraction rings relating to (111), (200), and (222) planes
coordinated well with the Ag planes obtained from XRD. Diffraction
rings corresponding to (200) matched with MgO plane. In the HR-
TEM of the AgNPs@m-MgO sample, the d-spacing is calculated 0.23
nm which corresponds to (111) plane of Ag indicating the presence
of Ag nanoparticles (shown with dotted circle) (Fig. 7c). The
constituent elements Mg, O and Ag of the composite material are
clearly observed from the EDAX pattern, depicted in Figure 8. 4.53
wt% Ag nanoparticles was present in the nanocomposite sample.
Energy (ev)
Figure 8. EDAX pattern of AgNPs@m-MgO sample.
3.2. Catalytic Activity
3.2.1. Carboxylation of terminal alkynes catalyzed by AgNPs@m-
MgO
The AgNPs@m-MgO nanocomposite has been used as catalyst for
the CO2 fixation on terminal alkynes in presence of DMF as solvent
at 1 atm pressure of CO2 (Scheme 2).
Scheme 2. AgNPs@m-MgO catalysed Carboxylation of terminal alkynes
3-phenylpropiolic acid synthesis from the carboxylation reaction
between 1-ethynylbenzene and carbon dioxide is chosen as the
model reaction (Scheme 3). In order to optimize the yield of the
product, we have varied different parameters such as different
types of bases, amount of base (Table 1), different types of solvents
and reaction time (Table 2). The roles of all of these parameters are
very crucial in this carboxylation reaction as shown in Table 1 and
Table 2.
Figure 6. FE-SEM images of AgNPs@m-MgO sample at two different
magnifications.
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may be due to DMF is a good solvent for Cs2CO3 as well as a good
solvent for CO2 also (DMF is a weak base) [35]. It is seen from Table
2 that the yield of the desired 3-phenylpropiolic acid product was
67% (Table 2, entry 8) when the reaction was done without any
solvent even after 24 h of reaction time. The reaction showed
moderately good yield in 6 hours of reaction time (Table 2, entry 1).
With increasing time of the reaction, the yield of the reaction
increases (Table 2, entries 1-4) and the reaction was completed
within 12 h reaction time with a very high yield of the needed
product (Table 2, entry 4).
Scheme 3. AgNPs@mMgO catalysed Carboxylation of 1-ethynylbenzene
Table 1 showed the effect of different kind of bases on the
carboxylation reaction. The reaction was carried out using bases like
K2CO3, Na2CO3 and Cs2CO3 for 12h at 700C temperature. Among the
bases Cs2CO3 showed the highest yield for the needed propiolic acid
product. The amount of Cs2CO3 has been varied and the highest
yield (98%) for the 3-phenylpropiolic acid product was obtained by
using 1.2 mmol Cs2CO3 (Table 1, entry 3). Reactions with less than
1.2 mmol of Cs2CO3 showed incomplete conversion of 1-
ethynylbenzene and gave relatively lower yield of the carboxylated
product (Table 1, entries 1,2). Reaction with more than 1.2 mmol of
Cs2CO3 showed slightly lower yield of the needed product (Table 1,
entry 3). Therefore 1.2 mmol Cs2CO3 is the optimized amount of
base. Table 1 showed that negligible amount of the desired
propiolic acid product was obtained in absence of any base and the
catalyst AgNPs@m-MgO (Table 1, entries 7 and 8).
Table2. Effect of solvent and time on carboxylation of 1-
ethynylbenzenea.
Entry
Solvent
Time (h)
Yield (%)b
1
DMF
DMF
6
52
65
87
98
55
51
2
3
4
5
6
8
DMF
10
12
12
12
Table1. Effect of base and amount of base on carboxylation of 1-
ethynylbenzenea.
Entry
Base
Amount of
Yield (%)b
DMF
base (mmol)
DMSO
CH3CN
Cs2CO3
Cs2CO3
0.8
1
52
78
1
2
7
Dioxane
-
12
24
60
67
3
4
5
6
Cs2CO3
Cs2CO3
Na2CO3
K2CO3
1.2
1.5
1.2
1.2
98
95
32
27
8c
aReactions carried out at atmospheric pressure (1 atm) of CO2 using
1-ethynylbenzene (1.0 mmol), AgNPs@m-MgO (50 mg), Cs2CO3 (1.2
mmol), reaction temp. (70 0C), solvent (3 mL). bGC yield. Without
c
solvent.
After screening all the parameters, the reaction condition where
the reaction was carried out with 1.2 mmol Cs2CO3 at 70 0C
temperature for 12 h using 50 mg of catalyst and DMF (3 mL) as
solvent at 1 atmospheric pressure of CO2 with 98% yield of 3-
phenylpropiolic acid product (Table 2, entry 4) is established as the
optimized reaction condition. We have studied the effect of
reaction time on 1-ethynylbenzene to 3-phenylpropiolic acid
conversion using AgNPs@m-MgO catalyst and the data is shown in
Figure 9. A very fast conversion was observed with increasing
reaction time. A linear increment of the conversion with reaction
time for the primary 12 hours of reaction was observed and then it
remained constant at 98% of conversion for the next 2 h of reaction
time. The selectivity of the 3-phenylpropiolic acid product remained
around 100% throughout the entire reaction period. So, Figure 9
confirmed that 12 h reaction time is sufficient enough to carry out
this chemical fixation reaction of CO2 at 70 0C temperature over
nanocrystalline AgNPs@m-MgO surface.
7c
8d
-
-
Trace
Trace
Cs2CO3
1.2
aReactions carried out at atmospheric pressure (1 atm) of CO2 using
1-ethynylbenzene (1.0 mmol), AgNPs@m-MgO (50 mg), base,
reaction temp. (70 C), DMF (3 mL), 12 h. GC yield. Without base.
dWithout catalyst.
0
b
c
Various solvents were used to understand the effect of solvents on
the carboxylation reaction. The reaction was carried out in solvents
like DMF, DMSO, CH3CN and dioxane (Table 2, entries 4-7). Among
all the solvents DMF showed best performance (Table 2, entry 4) as
the medium for this carboxylation reaction. The probable reason
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aReactions carried out at atmospheric pressure (1 atm) of CO2 using
terminal alkynes (1.0 mmol), AgNPs@m-MgO (50 mg), Cs2CO3 (1.2
mmol), reaction temp. (70 0C), DMF (3 mL), 12 h. balkyne (0.5
mmol).cGC yield.
Based on some previous reports [36, 37] a probable mechanistic
pathway for the CO2 fixation reaction over terminal alkynes forming
the respective propiolic acids in the presence of AgNPs@m-MgO
nanocatalyst is shown in Figure10. The terminal alkyne is first
activated by silver followed by the formation of silver acetylide
which is the key intermediate (A) for the silver catalysed C-H
activation of terminal alkynes, through the reaction between
terminal alkyne and AgNPs@m-MgO nanocomposite in presence of
Cs2CO3 base. Mesoporous MgO does not act only as a simple
support. It activates the CO2 molecule by adsorbing carbon dioxide
on its surface [24]. Subsequently, silver propynoate (B)
intermediate is formed through the insertion of CO2 into the sp-
hybridized Ag–C bond. This silver propynoate reacts with another
terminal alkyne and Cs2CO3 forming cesium propynonate and
regenerates silver acetylide (A). At the end of the reaction,
acidification leads to the generation of the needed propiolic acid
product.
Figure 9. Effect of reaction time on 1-ethynylbenzene to 3-phenylpropiolic acid
conversion over AgNPs@m-MgO catalyst.
The CO2 fixation reaction using AgNPs@m-MgO catalyst was carried
out on a variety of substrates such as 1-ethynylbenzene, 1-ethnyl 4-
methoxybenzene, 1-ethynyl 3-methylbenzene, 1-ethynyl 3-
aminobenzene and 1,4-diethynylbenzene under optimised
conditions. From summarised experimental results in Table 3, it is
observed that most of the terminal alkynes showed excellent yield
of the respective propiolic acid products with high selectivity. -CH3, -
OMe and –NH2 as the electron-donating groups on 1-
ethynylbenzene resulted in slightly less yields than 1-
ethynylbenzene (Table 3, entries 2-4). Similar yields were obtained
with 1-ethynyl-4-methoxybenzene and 1-ethynyl 3-aminobenzene
(Table 3, entries 2 and 4). On the other hand, relatively lower yield
was observed in case of 1,4-diethynyl benzene than the other
substrates.
Table 3. AgNPs@m-MgO catalysed Carboxylation of terminal
alkynesa
Entry
Alkyne
Product
Yieldc
(%)
Figure 10. Probable mechanistic pathway for terminal alkyne carboxylation over
AgNPs@m-MgO catalyst
3.2.2. Acylation of amines catalyzed by AgNPs@m-MgO
1
98
The AgNPs@m-MgO catalysed acylation reaction between amines
and acetic acid in solvent free condition is shown in Scheme 4.
2
3
93
96
Scheme 4. AgNPs@m-MgO catalysed acylation of amines
The reaction between aniline and acetic acid to synthesize
acetanilide is selected as the model reaction (Scheme 5). We have
varied different parameters like catalyst amount and reaction time
to optimize the yield of acetanilide product. Both the parameters
played very important role in this acylation reaction as shown in
Table 4.
4
93
5b
89
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amine),
m-choloro aniline (amine with electron withdrawing group)
and cyclohexylamine (aliphatic amine). Table 5 indicates that
aniline, benzyl amine and cyclohexylamine (aliphatic amine) (Table
5, entries 1, 2 and 5) showed brilliant yields of the respective
Scheme 5. AgNPs@m-MgO catalysed acylation of aniline
acylated products. N-methyl aniline (hindered amine) and
m-
choloro aniline (amine with electron withdrawing group) showed
comparatively slower reaction with comparatively lower yield of the
needed product (Table 5, entries 3, 4).
Table 4. Effect of amount of catalyst and reaction time on acylation
of anilinea
Table 5. AgNPs@m-MgO catalysed Acylation of aminesa
Entry
Amount of
Time (h)
Yield (%)b
Yieldb
(%)
catalyst (mg)
Entry
Amine
Product
Time
(h)
3
20
0.5
48
1
2
3
4
5
6
7
8
9
1
99
20
20
20
15
10
5
1
2
65
81
2
3
4
3
4
4
97
82
87
3
99
3
77
3
54
3
32
25
3
99
5
3
96
0
12
Trace
aReactions carried out using aniline (3.0 mmol), acetic acid (6.0
mmol), AgNPs@m-MgO, reaction temp. (120 0C). bGC yield.
aReactions carried out using aniline (3.0 mmol), acetic acid (6.0
mmol), AgNPs@m-MgO (20 mg), reaction temp. (120 0C). bGC yield.
The amount of catalyst has been varied from 5 to 20 mg and it can
be observed from the Table 4 (entries 4-7) that with increasing the
catalyst amount the yield of acetanilide increases and the best yield
(99%) for the acetanilide was obtained by using 20 mg of
AgNPs@m-MgO catalyst (Table 4, entry 4). There is no significant
effect on the yield of the acylated product can be observed after
additional increase in catalyst amount (Table 4, entry 8). It can also
be observed from the Table 4 that negligible amount of the needed
acetanilide product was obtained in absence of the catalyst after 12
h of reaction (Table 4, entry 9). The reaction showed moderate yield
in only half an hour reaction time (Table 4, entry 1). With increasing
time of the reaction, the yield of the reaction increases (Table 4,
entries 1-4) and the reaction was completed only in 3 h reaction
time with a very high yield of the needed product (Table 4, entry 4).
After checking the parameters, the reaction condition where the
reaction was carried out with 3 mmol aniline, 6 mmol acetic acid at
A probable mechanistic pathway for the acylation reaction of
amines forming the respective acylated products in the presence of
AgNPs@m-MgO nanocatalyst and acetic acid is shown in Figure 11.
The mesoporous MgO helps the silver nanoparticles to be well
distributed over it, which makes the AgNPs more available to
activate the acetic acid molecules. The oxygen atom of the carbonyl
group of acetic acid is first activated by silver nanoparticles of the
AgNPs@m-MgO catalyst followed by the nucleophilic attack of the
amine lone pair to the carbonyl carbon of acetic acid, which leads to
dehydration from the complex forming the respective acylated
product [38].
0
120 C temperature for 3 h with 20 mg of AgNPs@m-MgO catalyst
(Table 4, entry 4) with 99% yield of acetanilide product is
established as the optimized reaction condition.
The acylation reaction using AgNPs@m-MgO catalyst was carried
out on various types of substrates under optimised reaction
conditions, like aniline, benzyl amine, N-methyl aniline (hindered
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the catalyst. ICP-AES analysis of the filtrate, collected from hot
filtration was performed, which indicated the presence of very
much negligible amount of leached active metal in the solution.
These observations strongly suggest that there was approximately
no leaching out of the active silver metal from the MgO support
during the reaction and the AgNPs@m-MgO catalyst is completely
heterogeneous in nature.
Figure 11. Probable mechanistic pathway for acylation of amines over
AgNPs@m-MgO catalyst.
3.3. Recyclability of AgNPs@m-MgO catalyst
One of the most important factors for heterogeneous catalysis is
catalyst life. We have checked the reusability of the AgNPs@m-
MgO catalyst for both the CO2 fixation and acylation reactions. After
finishing the reactions, the AgNPs@m-MgO catalyst was isolated
from the reaction mixture by centrifugation. Then the impurities
were removed from the isolated catalyst by washing it with DI
water and ethyl acetate for multiple times and dried in vacuum
oven at 600C for 2 h. The successfully recovered catalyst was then
reused for 5 times in CO2 fixation and acylation reactions in
optimised conditions showing no considerable change in yields for
both the products, which are shown in Figure 12. The XRD data
(Figure S1) and TEM images (Figure S2) of the recycled catalyst
confirm that there is no significant structural change happened for
the catalyst after recycling. The XRD pattern shows that the MgO
gets little bit more hydroxylated during recycling process as the
characteristic peaks of Mg(OH)2 become little bit more prominent in
the recycled catalyst. This observation suggests that the flower like
AgNPs@m-MgO nanocatalyst has excellent recycling ability for CO2
fixation and acylation reactions.
Figure 13. Heterogeneity test indicating no active metal leaching into the filtrate
solution.
Table 6 shows a comparative study of the present catalytic system
with some other reported catalytic systems. It can be observed
from the table that AgNPs@m-MgO nanocatalyst showed
comparatively better yield of the desired products with short
reaction time at comparatively higher reaction temperature than
the other reported catalysts for both propiolic acid synthesis using
CO2 and acylation reaction using CH3CO2H.
Table 6. Comparative study of AgNPs@m-MgO catalyst with other
reported catalytic systems
Reaction
Catalyst
Reaction
Condition
1-
Time Yield Reference
(%)
Propiolic
acid
bridged
bis(amid
ate)
24 h
94
[39]
ethynylbenz
ene (1.0
mmol),
Synthesis
rare-
earth
Cs2CO3
(2.0 mmol),
Catalyst
(0.04 mmol),
CO2
metal
amides
(1 atm),
DMSO (5
mL).
Figure 12. Recyclability chart of flower like AgNPs@m-MgO catalyst under the
optimized conditions.
3.4. Heterogeneity Test
To understand the heterogeneous nature of the AgNPs@m-MgO
Ag@MIL-
101
1-
15 h
96.5
[40]
catalyst,
a
hot filtration test [19] was carried out on the
hours of
ethynylbenz
ene (1
carboxylation reaction of 1-ethynylbenzene. After
8
reaction 65% yield was obtained. Then the catalyst was filtered out
from the reaction mixture and then the reaction was performed for
further 6 hours in absence of the catalyst. It can be observed from
Figure 13 that the yield of the reaction stopped at approximately
65%. Therefore no further reaction was happened after filtering out
mmol),
catalyst (70
8 | J. Name., 2012, 00, 1-3
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mg), Cs2CO3
(1.5 mmol),
DMF (5 mL),
50 0C 1 atm
of CO2
Acknowledgements
SMI thankfully acknowledges Department of Science and
TechnologyDST-SERB(projectreferenceno. EMR/2016/004956),
New Delhi, Govt. of India, Board of Research in Nuclear
Sciences (BRNS), Project reference No: 37(2)/14/03/2018-
BRNS/37003, Govt. of India and Council of Scientific and
Industrial and Research, CSIR (project reference no
02(0284)2016/EMR-II dated 06/12/2016) New Delhi, Govt. of
India for providing financial support. AHC acknowledges
University of Kalyani, for providing her personal research grant
through URS fellowship. IHC is thankful to UGC, Government of
India for her fellowship. SMI acknowledges University of
Kalyani, India for providing personal research grant. We
sincerely acknowledge the DST and UGC, New Delhi, Govt. of
India, for providing grant under FIST, PURSE and SAP program
to the Department of Chemistry, University of Kalyani.
AgNPs@
1-
12 h
98
This Study
m-MgO
ethynylbenz
ene (1
mmol),
catalyst (50
mg), Cs2CO3
(1.2 mmol),
DMF (5 mL),
70 0C 1 atm
of CO2
Notes and references
1
2
3
4
5
6
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Graphical Abstract
Flowerlike AgNPs@m-MgO as an excellent catalyst for CO2
fixation and acylation reactions under ambient conditions
Arpita Hazra Chowdhury, Swarbhanu Ghosh, Sk. Manirul Islam,*
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India