A facile synthesis strategy to couple porous nanocubes of CeO2 with Ag nanoparticles: An excellent catalyst with enhanced reactivity for the 'click reaction' and carboxylation of terminal alkynes

Article abstract of DOI:10.1039/c8nj00665b

Oleic acid and oleylamine capped 3-D porous CeO₂ nanocubes were synthesized by the hot injection method. The surface of the nanocube was modified with 2,4-dimethylphenol (DMP). The reducing and capping abilities of DMP to form Ag nanoparticles were explored to form noble metal nanoparticles over the porous CeO₂ nanocubes support at room temperature. The surface modification and formation of Ag nanoparticles over the CeO₂ support were thoroughly characterized with FTIR, UV-Vis spectroscopy, XRD, XPS, TEM, HRTEM, STEM, EDX and chemical mapping. This CeO₂-Ag nanocomposite was found to have enhanced catalytic activities in the 'click reaction' as compared to unsupported Ag nanoparticles and porous CeO₂ nanocubes; it was also found to have optimized catalytic activity in the carboxylation of terminal alkynes when Cs₂CO₃ was used as the base in DMF solvent at 80 °C. The reaction can give the desired product in yields as high as 98% under several experimental conditions via the green pathway. Recycling data showed that the catalyst could be reused five times without considerable loss of its activity and without any deterioration of physical conditions as observed by various characterization methods like TEM, EDX, XRD and XPS.

Full text of DOI:10.1039/c8nj00665b

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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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A facile synthesis strategy to couple porous nanocube of CeO2
with Ag nanoparticles: An excellent catalyst with enhanced
reactivity for ‘click reaction’ and carboxylation of terminal alkynes
Received 00th January 20xx,
Accepted 00th January 20xx
Subhasis Das,a Paramita Mondal,b Swarbhanu Ghosh,b Biswarup Satpati,c Sasanka Deka,d Sk.
DOI: 10.1039/x0xx00000x
Manirul Islam*b and Tanushree Bala*a
www.rsc.org/
Abstract: Oleic acid and oleylamine capped 3-D porous CeO2 nanocubes were synthesized by hot injection method. The
surface of the nanocube was modified with 2, 4-dimethylphenol (DMP). Reducing and capping abilities of DMP to form Ag
nanoparticles were explored to form these noble metal nanoparticles over porous CeO2 nanocube support at room
temperature. Surface modification and formation of Ag nanoparticles over CeO2 support were thoroughly characterized
with FTIR, UV-Vis spectroscopy, XRD, XPS, TEM, HRTEM, STEM, EDX and chemical mapping. This CeO2-Ag nanocomposite
was found to have enhanced catalytic activities in click reaction as compared to unsupported Ag nanoparticles and porous
CeO2 nanocube. It was also found to have optimized catalytic activity in the carboxylation of terminal alkyne when Cs2CO3
0
was used as the base in DMF solvent at 80 C. The reaction could yield the desired product as high as 98% under several
experimental conditions via a green pathway. Recycling data showed that the catalyst could be reused five times without
considerable loss of its activity and without any deterioration of physical conditions as observed by various
characterizations like TEM, EDX, XRD and XPS.
has evolved as one of the very promising routes owing to its ability
to catalyze different types of reactions, easy accessibility and cost.
CeO2 is the most abundant rare earth metal oxide. Defects in their
1. Introduction:
With the advancement of technology, various types of metal oxide
nanoparticles have dragged the attention of scientific research
community owing to their diverse applications. Among the various
applications, catalysis can be considered to be in focus due to its
importance in industries. In this respect, CeO2 mediated catalysis
crystal structure due to oxygen vacancies determine the properties
which in turn dictate the applications in many cases. Moreover,
easy reversible conversion between Ce4+/Ce3+ redox couple by
releasing (deoxygenation) / accepting (oxygenation) oxygen, which
is formally named as oxygen storage capacity (OSC) of ceria, makes
it precious for various catalytic reactions.1-7 Thus there is a healthy
demand for CeO2 which is reported to be synthesized following
number of experimental routes, e.g. co-precipitation process,8
hydrothermal or solvothermal techniques,9 microemulsion based
synthesis,10 sol-gel11 and solution combustion synthesis (SCS)
procedures12 etc. CeO2 nanoparticles are already reported to have
successful use in the fields of colorimetric bioassays,13 gas
sensors,14 adsorbents,15 low temperature water gas shift reaction,16
oxygen permeation membrane systems,17 fuel cells,18,19 solar cells,20
luminescence,21 remediation of environmental pollutants via
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photocatalysis and several catalytic reactions.22 This wide range of catalysts can be developed for the synthesis of 1, 2, 3-triazole,
applications of nanosized CeO2 is found to depend on the shape and besides the reports available in the literature.
size of the particles. This has driven the contemporary researchers
CeO2-Ag nanocomposite is also found to exhibit good catalytic
towards the synthesis of CeO2 particles with various shapes and activity in the carboxylation of terminal alkyne when Cs2CO3 is used
0
dimensions, such as nanorods,23 nanowires,24 nanotubes,25 as a base in DMF solvent at 80 C. Alkynyl carboxylic acids are one
nanopolyhedrons,26 three-dimensional flower-like structures27 and of the very important classes of compounds due to its wide
hollow structures28 etc. Enhanced activity of CeO2 nanoparticles can applications in academic and pharmaceutical laboratories. It is also
also be achieved by doping different rare earth metals (like Gd, rigorously used as the starting materials for the synthesis of other
Zr)29,30 due to extraordinary synergetic effect between CeO2 and organic compounds. Carboxylation of terminal alkyne with CO2 as
metal. This synergetic phenomenon is known as strong metal- C1 building blocks has gained considerable momentum due to its
support (ceria) interaction effect (SMSI). Apart from rare earth abundance, nontoxicity and cost effectiveness. Chemically inert CO2
metals, noble metal/CeO2-supported catalyst is also well known for fixation reaction requires activation of CO2 before using it in
SMSI.31,32 Though several synthetic routes are available for the carboxylation reactions. The choice of catalysts is quite huge in
synthesis of pure CeO2, only a few reports are available till date for number as per the current literature e.g. transition metals, amines,
formation of noble metal-supported over CeO2.33-35 Further, most of N-heterocyclic carbenes43,44 and frustrated Lewis pair,45 noble
these reports require high temperature, hazardous materials or metals combined with various types of matrices,46-48 alkali metal
specialized technique like laser ablation,36 which makes these halides49 etc. We report such reactions with the very high yield of
syntheses difficult to practice in industries or by others.
the desired product (as high as 98%) under various combinations of
In this report, Ag nanoparticles are synthesized over 3-D experimental parameters with CeO2-Ag nanocomposites which
porous CeO2 nanocubes whose surface is deliberately modified with seems to be very promising and quite different from the reports
2, 4-dimethylphenol (DMP), a known reducing agent of Ag+ ion to available till date.
metallic Ag.37 Successful surface modification with DMP plays the
Recycling data for both types of reactions (Click reaction and
key role of dragging Ag+ ions from aqueous AgNO3 solution to carboxylation reaction) were also encouraging to show that the
organic phase containing CeO2 nanocubes and then helps to reduce catalyst could be reused five times without considerable loss of its
them to form Ag nanoparticles site selectively on the surface of activity and without any deterioration of physical conditions as
CeO2 nanocubes at room temperature.
proved from various characterizations like TEM, EDX, XRD and XPS.
This CeO2-Ag nanocomposite is found to have better catalytic
activity in click reaction as compared to unsupported Ag
nanoparticles and porous CeO2 nanocubes. Actually, 1,4-
disubstituted 1,2,3-triazoles are an important class of heterocyclic
compounds which are found to have the wide range of industrial
applications such as corrosion inhibitors, agrochemicals, optical
brighteners, photographic materials38 as well as in the field of
biology.39 Azide-alkyne [3+2] cycloaddition (AAC) reaction to form
1,2,3-triazole is typically called as “click reaction” which is mainly
catalysed by Cu(I) and is believed to proceed via Cu(I)-acetylide
intermediate produced from Cu(I) and terminal alkyne.40 High yield,
increase in reactivity even at low concentration and regioselectivity
are the main advantages of catalysed AAC reaction over thermally
induced reaction. Recently Cu(I)-modified zeolites41 and Cu-Fe
bimetallic nanoparticles42 were also set up as the catalyst for this
kind of reaction. Thus it is always appreciated if a new class of
2. Experimental section:
Chemicals used:
Selenium powder (Se, 99.99%), sulphur powder (S, 99.99%) and
trioctylphosphine (TOP, 99%) were purchased from Strem
chemicals, USA. Cadmium oxide (CdO, 99.5%), oleyl amine (OLAM
70%), AgNO3, 1-octadecene (ODE, 90%), oleic acid (OLAC, 90%), 2,
4-dimethylphenol (DMP) and trioctylphosphine oxide (TOPO, 99%)
were purchased from Sigma Aldrich, USA. n-hexylphosphonic acid
(HPA, >97%) and n-Octadecylphosphonic acid (ODPA, >97%) were
purchased from Plasma Chem, Germany. Anhydrous methanol,
toluene and other solvents as well as bases were purchased from
Thomas baker, India. All the chemicals were used as received
without any further purification.
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Synthesis of porous ceria nanocube:
water and poured drop-wise to the solution. After mixing it, NaN3
3-Dimensionally self-assembled porous CeO2 nanocubes were (1.2 mmol) was added into the solution and kept under magnetic
prepared initially following a reported method.5 In the synthesis stirring for 10 min. Then alkyne (1.2 mmol) and CeO2-Ag
procedure, 1 mmol of Ce(NO3)3·6H2O (0.434 g), 2 ml of oleylamine nanocomposites (20 mg) were added in the reaction mixture with
(OLAM), 2 ml of oleic acid (OA), 4 ml of octadecene, and 2.5 mmol nonstop stirring for 3 h at room temperature. Then, the CeO2-Ag
of 1,12-dodecanediol (DDOL) (OA/OLAM/DDOL ratio = 1:1:2.5) were nanocomposites was separated by filtration and cleaned with H2O,
added in a three-necked round-bottom flask (50 ml). The flask was EtOH and acetone and extraction of other parts of the reaction
heated to 100 °C under vigorous magnetic stirring. The reaction mixture were performed with ethyl acetate. The aqueous part
mixture was kept at this condition for 30 min under vacuum to present in the mixture was separated by Na2SO4 and the reaction
remove water and oxygen. Then, the solution was heated to 300 °C mixture was added to a small amount of EtOH. After that, the
at a heating rate of approximately 15 °C min−1 under nitrogen separation of pure product was performed by crystallization to
atmosphere and kept at this temperature for another 60 min. As obtain the resultant 1,4-disubstituted 1,2,3-triazoles.
soon as the reaction was complete, the heating mantle was
removed and the reaction mixture was cooled to room temperature General procedure of carboxylation reaction:
naturally. The product was precipitated by adding acetone and A 50 ml high-pressure reactor was charged with DMF (5.0 ml), 1.0
purified by repeated centrifugation.
mmol of terminal alkynes, CeO2-Ag nanocomposites (30 mg), and
Cs2CO3 (1.5 mmol). After that CO2 (1 atm) was introduced into the
reaction mixture under stirring at 80 0C for 14 h. After completion
Synthesis of CeO2-Ag nanocomposite:
For this purpose, the surface of the as-synthesized CeO2 nanocube of the reaction, the reaction mixture was cooled to room
was modified first with 2, 4-dimethyl phenol (DMP). Typically, 10 ml temperature and transferred to the 2(N) potassium carbonate
of 10-1 (M) DMP in toluene was added to 15 ml of porous ceria solution (5 ml) under stirring for 30 min. The mixture was washed
nanocube solution and was stirred overnight. The solution then was with CH2Cl2 and the aqueous layer was acidified with concentrated
centrifuged to get the solid after decanting off the colorless HCl to pH=1 at low temperature, then extracted with ethyl acetate.
supernatant. The precipitate was re-dispersed in toluene The combined organic layers were washed with saturated NaCl
maintaining a concentration of 0.12 mg/ml. 10 ml of this solution solution, dried over Na2SO4 and filtered. The solvent was removed
was stirred with 10 ml aqueous solution of 10-3 (M) AgNO3 in in vacuum to afford the isolated acid product. The products were
presence of 5×10-5 (M) KOH solution, expecting a complete phase analyzed by Varian 3400 gas chromatograph (GC data) by their
transfer for all Ag+ ions. But it was found only 80% of the whole retention times in comparison with the authentic samples.
amount got phase transferred (estimated by measuring the
concentration of the residual AgNO3 after phase transfer, the data
3. Characterisation:
not shown in the paper). The top layer i.e. the organic phase of the
biphasic mixture was separated using separating funnel where the
Fourier Transform Infrared (FTIR) Spectroscopy measurements:
surface bound DMP molecules acted as reducing agent to deposit
FTIR spectra were recorded from the pellet of the samples prepared
Ag nanoparticles on CeO2. Further calculation showed 0.74 gm of Ag
with dry KBr at every step of the reaction on a Perkin Elmer
was deposited over 1 gm of CeO2.
Spectrum-One Spectrometer operated in the diffuse reflectance
mode at a resolution of 4 cm-1. The spectra of pristine porous ceria
General procedure for the synthesis of 1,4-disubstituted 1,2,3-
nanocube were also recorded for comparison.
triazoles:
In a 25 ml round bottom flask aromatic amine (1 mmol) was poured
UV-Vis spectroscopy:
0
and kept in an ice bath of the temperature of 0–5 C. A solution of
The spectra were measured from porous ceria nanocubes and
conc. H2O-HCl in 1:1 ratio (V/V) was poured slowly and stirred for 5
CeO2-Ag nanocomposite in toluene using
a Jasco UV-Vis
min. Then sodium nitrite (1 mmol) was taken in 1 ml of chilled
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spectrophotometer (V570 UV-VIS-NIR) operated at a resolution of 2
nm.
4. Results and discussion:
CeO2 nanoparticles synthesized by hot injection technique were
already well characterized. It was established that each nanocube
could be considered to be loose assembly of several small CeO2
nanocrystals, which gave rise to porous nature to the cubes.5, 50
Successful reproduction of this structure was evidenced by TEM
images, Supporting Information SI-1. The size of the cube was
estimated to be 44-46 nm. The smaller building units of these cubes
were the CeO2 nanocrystals of irregular size and shape which could
be seen clearly at the higher resolution of TEM, Supporting
Information SI-1. The composition of the material was established
from EDX analysis (Supporting Information SI-1) which had the
signatures of Ce, O, C and Cu. C and O signals came from the
organic capping ligands whereas Cu signal was from the TEM grid.
The absence of any other elements confirmed the purity of the
material.
X-ray diffraction (XRD) measurements:
XRD measurements of porous CeO2 nanocubes and CeO2-Ag
nanocomposite were recorded from the dry samples using Philips
PW 3071 diffractometer operating at 40 kV and at a current of 30
mA with Cu Kα radiation.
X-ray photoelectron spectroscopy (XPS):
XPS measurement was made using a custom-built ambient pressure
XPS system from Physical Electronics (Model Name – Phi 5000
VersaProbe II) and equipped with unique monochromator using Al
Kα anode (1486.6 eV). The spectra were recorded at the pass energy
of 50 eV.
Transmission Electron Microscopy (TEM):
Transmission electron microscopy (TEM) performed using an FEI,
Tecnai G2 F30, S-Twin microscope operating at 300 kV. The
microscope was also equipped with a high-angle annular dark field
(HAADF) microscopy in scanning mode. The compositional analysis
was performed using energy dispersive X-ray (EDX, EDAX Inc.) in the
same microscope. Samples of both porous CeO2 nanocube and
CeO2-Ag nanocomposites were prepared by placing one drop of the
dilute solution on carbon-coated copper grids and were dried
thoroughly in the air.
The growth of Ag nanoseeds from Ag+ ions on CeO2 nanocubes
could be initiated site selectively, only if, the reducing agent was
present on the surface and/or inside the porous CeO2 nanocubes.
Here, CeO2 nanocubes were capped by oleic acid and oleylamine
and these capping ligands were unable to serve this purpose. Thus
surface modification was carried out judicially with DMP molecules
since phenolic OH group was reported to initiate reduction of Ag+ to
Ag nanoparticles.37 Surface modification step did not have any
detrimental effect on the shape or size of the porous CeO2
nanocubes as observed in TEM images of CeO2 nanocubes modified
with DMP, Supporting Information SI-2.
Nuclear Magnetic Spectroscopy (NMR):
NMR spectra were monitored on
a Bruker AMX-400 NMR
The success of the surface modification was established from
FTIR analyses, Fig. 1. To comprehend the change due to surface
capping by DMP, FTIR spectra of as- synthesized CeO2 (curve 1),
DMP modified CeO2 (curve 2) and after the addition of AgNO3 to
this DMP modified CeO2 (curve 3) were compared thoroughly. The
as synthesized CeO2 nanocubes were capped with oleic acid and
oleyl amine. Several characteristic bands corresponding to both
these capping agents were identified in its FTIR spectra, curve 1, Fig.
1. Two sharp peaks at 2857 cm-1 and 2924 cm-1 appeared due to
CH2 symmetric and asymmetric stretching vibrations of oleic acid
whereas a small shoulder peak at 3007 cm-1 was the =C-H
(methylene H) stretching frequency of oleic acid. A broad peak in
the range of 3434 cm-1 was due to N-H stretching vibration of the
spectrophotometer (400 MHz for 1H NMR) using tetramethylsilane
as internal standard.
Gas Chromatography (GC):
The gas chromatography (GC) of the samples were recorded by
using a Varian 3400 gas chromatograph equipped with a 30 m CP-
SIL8CB capillary column and a flame ionization detector. The
products for carboxylation reactions were quantified by Varian
3400 gas chromatograph (GC data) by comparing with the authentic
samples.
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amine group of oleylamine. The peaks below 1500 cm-1 mostly merged with the N-H stretching in this case. Thus a broad band was
appeared due to various modes of combinations of C-C stretching, observed centered at 3554 cm-1. These observations concluded the
C-O stretching, CH2 deformation and so on. The peak for carboxylic successful surface modification of CeO2 by DMP. Curve 3 retained
acid group of pure oleic acid was reported at 1709 cm-1 and –NH2 almost same features as that of curve 2 with minute damping for
scisssoring mode at 1593 cm-1 for pure oleylamine. C-O symmetric the phenolic –OH stretching at 3554 cm-1 and a bifurfacted band at
stretching of –COO- group of oleic acid was found at 1710 cm-1 1105 and 1155 cm-1. This pointed out towards the interaction of
whereas the peak at 1527 cm-1 could be the shifted –NH2 scisssoring phenolic-OH group with new species, here presumably Ag+.57-59 It
mode of oleylamine. Literature showed an association between established that DMP molecules being on the surface of CeO2 phase
oleic acid and oleylamine could not be overlooked when both are transferred Ag+ from precursor solution and subsequently reduced
present together and in such cases peaks were reported to appear it to Ag0. Such interaction with Ag+ and phenolic –OH group was
between 1400-1600 cm-1 which could be assigned to C-O symmetric also reported by other groups.37, 60
and asymmetric stretching of –COO- group. Thus, in this study, the
band at 1434 cm-1 could be identified as carboxylate asymmetric
stretching.51, 52
Fig. 2 (A) UV-Vis and (B) XRD plots of CeO2 (curve 1) and CeO2-Ag
nanocomposite (curve 2).
The successful reduction of Ag+ to form Ag nanoparticles was
primarily confirmed from UV-Vis spectra and XRD, Fig. 2. CeO2
nanocube (curve 1, Fig. 2A) showed absorption peak in the UV
Fig. 1 FTIR spectra of oleic acid and oleylamine capped CeO2 (curve
1), DMP-modified CeO2 (curve 2) and Ag nanoparticles supported
over CeO2 (curve 3) obtained from the pellet prepared with dry KBr.
transition from O2- (2p) to Ce4+ (4f).61 The direct band gap of CeO2
region centered at 312 nm which appeared due to the charge
nanocube was measured 3.22 eV from the absorption spectra
After modifying the surface of CeO2 with DMP, a distinct
which was the characteristic band gap of the semiconductors. In
change was observed in curve 2 as compared to curve 1. The new
peaks were identified at 730, 880, 1105 and 1230 cm-1 which could
comparison to the pristine CeO2 nanocube, there appeared a broad
absorption peak from 320 nm to 550 nm with the maximum
be assigned to out of plane C=C of the benzene ring, out of plane C-
absorption at 420 nm when Ag+ was added to the surface modified
H, deformation vibration of O-H group and Ph-O stretching
respectively due to dimethyl phenol.53-56 The O-H stretching was
CeO2 nanocube (curve 2, Fig. 2A). This absorption peak at 420 nm
expected in the region around 3400 cm-1 which was probably
was already well-known as the characteristic peak of Ag
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nanoparticles due to surface plasmon resonance (SPR). The 881.9 eV and 900.5 eV corresponded to d5/2 and d3/2 components
formation of Ag nanoparticles could be explained due to the respectively for Ce4+ oxidation state. Separation between these two
presence of DMP (the reducing agent of Ag+ ions) on the surface of components was 18.4 eV. Other peaks could be assigned as the
CeO2 nanocube.
satellite peaks of CeO2 denoted by ‘S’ in Fig. 3A. O 1s curve (Fig. 3B)
XRD was performed for both the samples i.e. CeO2 nanocube was fitted with a major component at 531.9 eV and a minor
(curve 1, Fig. 2B) as well as CeO2-Ag nanocomposites (curve 2, Fig. component at 529.08 eV. The major component of O 1s came from
2B) to support the formation and to confirm the crystalline nature. CeO2 nanocube while the minor component had probably appeared
XRD profile of pristine CeO2 could be indexed with fcc structure of from DMP and other capping agents. Presence of oleylamine even
bulk CeO2 and the peaks were marked for various (hkl) planes after the formation of CeO2-Ag nanocomposites was confirmed
according to JCPDS card #34-0394. Size of the CeO2 nanocube was from the N 1s curve (Fig. 3C), which was fitted with single Gaussian
measured 39.04 nm from the XRD peak of (200) planes using and had the peak at 401.01 eV. Ag 3d curve (Fig. 3D) had been
Scherrer formulae. The size matched very closely to the average fitted with two components. The peaks at 367.5 eV and 373.5 eV
size of the nanocube as obtained from TEM images. After the with a spin-orbit coupling factor of 6 eV corresponded to d5/2 and
formation of CeO2-Ag nanocomposites, there appeared a sharp d3/2 of metallic Ag. A set of minor components at 368.6 eV and
peak at 2θ = 39.10 which was the fcc (111) plane of Ag (curve 2, Fig. 374.7 eV BE could be allocated to d5/2 and d3/2 components of Ag+
2B). Size of the Ag nanoparticles was measured 6.01 nm using respectively62 which might reflect some unreduced Ag+ ions and/or
Scherrer formulae for the (111) peak at 2θ = 39.10. Other two small Ag bound to DMP where interaction with phenolic oxygen might
peaks of Ag at 44.80 and 64.80 were corresponded to presence (200) shift the peak to higher BE.63, 64
and (220) planes of fcc Ag respectively. The peaks for CeO2
The success of the synthesis should vastly depend on the
remained undisturbed in curve 2. All the observations suggested proper surface modification by DMP. Ag+ was expected to get
the formation of small Ag nanoparticles along with CeO2 nanocube reduced site selectively by surface bound DMP only to generate
without any clue about the real position of Ag nanoparticles.
CeO2-Ag nanocomposites. The site selective growth of Ag
nanoparticles could only be proved by TEM analysis in combination
with EDX, Fig. 4. Fig. 4A-C represented the CeO2-Ag nanocomposites
at different magnifications. All the micrographs clearly showed
formation of Ag nanoparticles on the surface of CeO2 nanocubes as
darker dots. The porous cubical nature was not disturbed due to
the formation of Ag nanoparticles. Size of the Ag nanoparticles was
found to vary from 3.26 nm to 5.95 nm which was found to be in
good agreement with the XRD analysis. HRTEM analysis (Fig. 4D)
showed two types of fringes. The lattice spacings were calculated to
be 2.7 Å and 2.2 Å. The first one was for CeO2 (200) planes whereas
the second one can be corroborated to be (111) planes of fcc Ag
nanoparticles. EDX analysis (Fig. 4E) was done from the same area
of Fig. 4A. Sharp peaks of Ag confirmed its formation. The peaks for
Ce, O and C came from ceria and the organic capping agents. Cu
peak was due to the grid material. For a detailed distribution of
atomic content inside the porous CeO2 nanocubes, elemental
mapping of Ce and Ag was performed using drift corrected EDX
imaging using STEM-HAADF mode, Fig. 5. The STEM-HAADF image
in Fig. 5A and the corresponding chemical maps for Ce and Ag
acquired using Ce-L and Ag-L energy, were presented in Fig. 5B and
Fig. 3 XPS analysis from CeO2-Ag nanocomposite for (A) Ce 3d, (B) O
1s and (C) N 1s and (D) Ag 3d.
XPS analysis confirmed the presence of Ag in its metallic state
along with CeO2, Fig. 3. Ce 3d spectra (Fig. 3A) was best fitted with
six components among which the peaks at binding energy (BE)
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5C, respectively. It was observed that the position of Ce and Ag
overlapped when Fig. 5B and 5C were considered together. This Study of Catalytic Activity:
undoubtedly established the formation of Ag nanoparticles on the Click reaction between phenyl azide and phenyl acetylene
surface of the CeO2 nanocubes.
1. Conc. HCl:H2O (1:1)
NaNO2 (0o - 5o C)
R
NH2
R1
N3
N
N
Ag-Ceria
Catalyst
3 h, RT
N
2. NaN3
R
R
R
Scheme 1. Synthesis of 1, 4-disubstituted 1,2,3-triazole.
aReaction conditions: phenyl acetylene (1.2 mmol); aromatic amine
(1equiv); sodium nitrite (1mmol), NaN3 (1.2 mmol); solvent (10 ml);
20 mg of CeO2-Ag nanocomposites catalyst, room temperature.
bIsolated yields.
Click reaction between phenyl azide generated in situ from aniline
and phenylacetylene in water was chosen as the model reaction to
check the catalytic ability of CeO2-Ag nanocomposites. The reaction
was carried out at room temperature for 3 h in presence of 20 mg
of the catalyst (Scheme 1). A promising result with 98% yield of 1, 4-
disubstituted-1,2,3-triazole as product prompted to execute further
catalytic activity of this catalyst in the click reaction. It was
important to mention that no product was detected in absence of
catalyst. The reaction was also carried out in the presence of native
CeO2 and Ag nanoparticles under similar experimental conditions
but the yields were quite poor to report. Thus, CeO2-Ag
nanocomposites were considered to to form triazole product
efficiently. Results were summarized in Supporting Information SI-3.
To ensure the role of CeO2 support, Ag nanoparticles were
embeded over two different supports, hyper-crosslinked polymer
(HCP) and mesoporous (MP) TiO2. These two catalysts were
screened for the click reaction of phenyl azide and phenylacetylene.
From the result, presented in Supporting Information SI-3, it could
be easily concluded that CeO2-Ag nanocomposites had better
catalytic ability than the rest. The same test reaction was also
performed with some unsupported silver salts to compare the
efficacy of supported catalyst. These silver salts were capable to
catalyse the reaction, but yield was found to be very low and the
reaction consumed more time. Due to large surface-to-volume ratio
of Ag metal nanoparticle than corresponding bulk metal salts, it
might improve the catalytic activity.
Fig.
4 (A-C) Transmission electron microscopic image of Ag
nanoparticles supported over CeO2 at different magnification. (D)
HRTEM image of the same clearly showing the lattice plane of Ag
and CeO2. (E) EDX as obtained from the area of (A).
Fig.
5 (A) Scanning TEM-HAADF image of Ag nanoparticles
supported over CeO2. (B-C) EDX maps obtained from the same area
in Fig. 5A using Ce-L and Ag-L energy respectively.
After obtaining some crucial insight into the effect of different
catalyst, the study was extended to check the effect of solvent,
reaction temperature, reaction time and the amount of catalyst on
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the reaction. Among several solvents chosen as the reaction observed with 20 mg catalyst. Increasing the catalyst amount from
medium, lowest amount of product was detected in toluene 20 mg to 30 mg did not alter the reaction yield. Temperature
medium with some unwanted side product (Supporting Information showed no such effect on the activity of the catalyst. A reaction at
SI-4, Entry 1). Using polar solvents such as DMF, DMSO, THF and higher temperature did not significantly increase the yield. The
ACN, the yield of the product increased (Supporting Information SI- dependence of the conversion to the product on reaction time was
4, Entry 2-5). But it is interesting that highest yield was obtained presented in Fig. 6B. Decreasing the reaction time from 3 h to 2 h
when the reaction was conducted in water medium (Supporting resulted in significantly diminished yields while increasing the
Information SI-4, Entry 6) rather than in organic solvents. Water as reaction time from 3 h to 4 h led no further improvement of yield of
a reaction medium could be considered as clean, non- hazardous the product.
and safe. So we performed all the reactions in the water medium.
A series of reactions with various substituted aniline and
alkynes under optimized reaction conditions were tested to prove
the versatility of the protocols with CeO2-Ag nanocomposites as the
catalyst. The results were presented in Table 1. At first, the reaction
of substituted azides (generate in situ from anilines) and
phenylacetylene was carried out. A number of the functional group
on the aromatic ring of anilines were well tolerated and afforded
the desired product in good to excellent yields. The presence of
electron donating or electron withdrawing group on the azide could
not significantly affect the yield of the reaction (Table 1, Entry 6).
Ortho-substituted azide took slightly longer reaction time to reach
the goal (Table 1, Entry 2 and 3). Substituted phenyl acetylene was
also treated with the phenyl azide in standard reaction condition
resulting in the formation of 1, 2, 3-triazole derivatives (Table 1,
Entry 7-8). It was noteworthy that different combination of
substituted azides and substituted phenyl acetylene were applied
and they reacted with similar efficiencies with each other forming
the desired product in presence of CeO2-Ag nanocomposites as
catalyst.
Fig. 6 Effect of catalyst amounta (A) and timeb (B) on Click reaction.
aReaction conditions: phenyl acetylene (1.2 equiv); aromatic amine
(1equiv); Water (10 ml); CeO2-Ag nanocomposite catalyst, 3 h at
room temperature.
bReaction conditions: phenyl acetylene (1.2 equiv); aromatic amine
(1equiv); Water (10 ml); CeO2-Ag nanocomposite catalyst (20 mg) at
room temperature.
c Both Isolated yields.
Next, the effect of the amount of catalyst was also evaluated
for the model reaction. The quantity of the active metal had a
significant effect on this reaction (Fig. 6A). Different amount of
catalyst, 10 mg to 30 mg, were tested and the best result was
Table 1 Click reaction between different kinds of aromatic amine derivatives and phenyl acetylene catalyzed by CeO2-Ag nanocomposite.a
1. Conc. HCl:H2O (1:1)
NaNO2 (0o - 5o C)
R1
NH2
R1
N3
N
N
Ag-Ceria
Catalyst
3 h, RT
2. NaN3
N
R
R
R
Entry
R
R1
Products
Time (h)
Yieldb (%)
1
H
H
3
98
N
N
N
8 | J. Name., 2012, 00, 1-3
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2
2-NO2
H
4
95
N
N
N
NO2
3
4
5
2-I
H
H
H
4
3
3
89
97
95
N
N
N
I
3-OH
3-Cl
N
N
N
HO
N
N
N
Cl
6
7
4-OMe
2-NO2
H
3
4
98
89
MeO
N
N
N
OMe
4-OMe
N
N
N
NO2
OMe
8
3-OH
4-OMe
3
94
N
N
N
HO
aReaction conditions: phenyl acetylene (1.2 equiv); aromatic amine (1equiv); Water (10 ml); 20 mg CeO2-Ag nanocomposite catalyst at
room temperature. Products were characterized by NMR spectroscopy (Supporting Information SI-5). b Isolated yields.
Comparison with other reported system
Table 2 Comparison of catalytic activity of the present catalyst in the ‘click’ reactions with other reported catalyst systems.
Entry
Catalyst
Reaction conditions
Phenyl acetylene, benzyl azide, caprylic
acid, PhMe, 900C
Yield (%)
Ref.
1
Silver acetate complex
79
65
2
3
Cu(MeCN)4PF4 and Silver acetylide
Silver(I)acetate complex (ligand: 2-
diphenylphosphino-N,N,-diisopropylcarboxamide)
Ag2CO3
Pyridine, azide, 24h, RT.
Phenyl acetylene, benzyl azide), caprylic
acid, PhMe, 48 h, RT.
75
95
66
67
4
5
Phenyl acetylene, NMP, ethyl 2-
isocyanoacetate, 80 0C.
89
98
68
CeO2-Ag nanocomposites
Aniline, phenyl acetylene, water, 20 mg of
CeO2-Ag, 3 h, RT.
This
study
exhibited higher yields compared to those in the literature.65-68
Moreover in the present study, reactions were conducted at room
temperature with shorter reaction time. Most importantly, here the
One-pot click reaction of aniline with phenylacetylene under
heterogeneous conditions over a variety of catalysts had already
been reported. Table 2 provides a comparison of the results
obtained for this present catalytic system with those reported in
the literature.65-68 From Table 2, it was seen that present catalyst
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coupling reactions were carried out in water using CeO2-Ag Entry 8-11). Among the chosen solvents, DMF had the best yield of
nanocomposites as catalyst.
propiolic acid, as high as 98% (Supporting Information SI-6, Entry 3).
As DMF could act as a weak base, it also increased the solubility of
CO2 and hence reaction rates.69 Other solvents gave moderate to
low conversions. The reaction did not take place at all in toluene
medium (Supporting Information SI-6, Entry 11). After optimizing
solvent and base, the effect of temperature was estimated
(Supporting Information SI-6, Entry 12-15). Increasing the
Carboxylation reaction of terminal alkyne:
Ag Ceria catalyst
OH
O
HCl
Base
Solvent
80 0
R
H
+
CO2
R
C
C
Scheme 2. CeO2-Ag nanocomposites catalyzed carboxylation of
terminal alkynes.
aReaction conditions: alkyne (1.0 mmol), 30 mg of CeO2-Ag
nanocomposite, Cs2CO3 (1.5 mmol), CO2 (1.0 atm), 80 0C, DMF (5
ml), 12 h, bIsolated Yield.
0
temperature from room temperature to 60 C, the yield of product
was just low to moderate. The most favorable reaction temperature
0
was thus concluded to be 80 C (Supporting Information SI-6, Entry
Several successful methodologies for the carbonylation reaction
had been reported by Islam and his group in recent years.
Generally, toxic carbon monoxide was used as the C1 source in
carbonylation reaction. In recent years, the attention had been
shifted to CO2 as the C1 source in different organic reactions for its
non-toxic nature and low cost for its resource. Here CeO2-Ag
nanocomposites were used to investigate its catalytic activity for
the carboxylation of terminal alkynes using CO2 as the C1 source via
a green pathway (Scheme 2). To optimize reaction parameters,
carboxylation of 1-phenylethyne with CO2 was considered as a
model reaction. The reaction was chemoselective and highly
dependent on the reaction conditions, i.e. catalyst, solvent, base,
nature of the nucleophile and also the pressure of CO2. At the first
step, 1-ethynylbenzene was taken as the substrate and was
converted to 3-phenylpropiolic acid using K2CO3 as a base in DMF
solvent medium under 1 atm CO2 pressure. The reaction was
conducted in a 50 ml autoclave at 80 0C for 12 h and the yield of the
product (49%) was quite reasonable. To increase the yield of the
3). It was interesting to note that above 80 0C, yield decreased
considerably. This might be due to decomposition of silver
propynoate intermediate through decarboxylation at the higher
temperature. Also, the reaction was conceded under different time
ranging from 8 h to 14 h and complete conversion (98%) was
obtained at 12
h of reaction time and beyond (Supporting
Information SI-6, Entry 3, 15 and 19).
Fig.
7 Effect of amount of catalyst on carboxylation of 1-
phenylethyne.
aReaction conditions: alkyne (1.0 mmol), CeO2-Ag nanocomposites,
Cs2CO3 (1.5 mmol), CO2 (1.0 atm), 80 0C, DMF (5 ml), 12 h. b Isolated
yield.
product
a
number of reaction conditions were varied
systematically. It was demonstrated that base had a marked impact
on the reaction (Supporting Information SI-6, Entry 1-7). A blank
experiment revealed that no reaction occurred in absence of the
base. Several inorganic and organic bases were examined to
optimize the influence of the base on the catalytic activity. E.g.
Na2CO3, K2CO3, Cs2CO3, DBU and DBN were used and the best result
was obtained with Cs2CO3. Et3N and KOtBu were least efficient for
this reaction. It was found that Cs2CO3 as a base furnished the
Amount of catalyst was also varied to investigate the effect of it
on product yield (Fig. 7). A catalyst (CeO2-Ag nanocomposite)
amount of 30 mg afforded the production of propiolic acid with
98% yield. Yield was decreased sharply when catalyst amount was
reduced from 30 mg to 20 mg. The additional increase in catalyst
amount from 30 mg to 40 mg had no appreciable effect on the
yield. The optimized catalytic system was next successfully applied
to a range of terminal alkynes. The results were summarized in
Table 3. Different terminal alkynes were explored in CO2 medium
and converted into the corresponding analogous caboxylative
0
product with maximum yields (98%) in DMF at 80 C. The reaction
was carried out in several polar solvents like, DMF, DMSO, THF and
dioxane and also in the non-polar solvent like toluene to examine
the effect of solvent on the reaction (Supporting Information SI-6,
10 | J. Name., 2012, 00, 1-3
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products with high yields. Different electronic and steric silver propiolate intermediate was formed by the insertion of CO2
substituents of the terminal alkynes were well tolerated under into the polar Ag–C bond of silver acetylide. In the last step, there
present reaction conditions. The phenyl ring of 1-phenylethyne was a nucleophilic attack by another terminal alkyne molecule at
substituted with electron withdrawing and electron donating silver propiolate intermediate, silver acetylide was regenerated and
groups were smoothly converted to the analogous product. the desired product was obtained (after acidification) (Scheme 3).
However, when strong electron donating or strong electron The 3rd step was slowed down when strong electron withdrawing
withdrawing group were present in phenyl ring, the yield of the group was present at aryl ring. From the reaction mechanism, it was
corresponding product was somehow lower than 1-phenylethyne. found that there was opposite electronic effect in the first and third
Methoxy (-OMe) as the strong electron-donating groups or nitro (- step and from overall assessment, it might be concluded that
NO2) as the strong electron-withdrawing group on 1- moderately donating group performed better compared to strong
ethynylbenzene resulted in slightly low yields than 1- donating or withdrawing group.
ethynylbenzene (Table 3, Entry 3 and 4).
CO2
Table
3 CeO2-Ag nanocomposites catalyzed carboxylation of
a
terminal alkynes with CO2
.
O
R
H
R
Ag
Ag-Ceria
R
C
Entry
Alkynes
1-phenyl
ethyne
Product
3-phenyl propiolic
acid
Yieldb (%)
OAg
Cs2CO3
CsHCO3
H3O+
1
98
O
R
H
R
C
OH
2
3
4
1-ethynyl-4-
methyl
3-
p
-tolyl propiolic
acid
96
92
90
Scheme
3 Proposed mechanism of CeO2-Ag nanocomposite
benzene
catalyzed carboxylation of alkyne.
1-ethynyl-4-
methoxy
benzene
3-(4-methoxy
Comparison with other reported system
phenyl) propiolic
acid
Table 4 Comparative catalytic activity of the present catalyst system
with other reported system for carboxylation reactions.
1-ethynyl-4-
3-(4-nitro phenyl)
propiolic acid
nitro benzene
Entry
Catalyst
Reaction conditions
Yield Ref.
(%)
aReaction conditions: alkyne (1.0 mmol), 30 mg of CeO2-Ag
nanocomposite, Cs2CO3 (1.5 mmol), CO2 (1.0 atm), 80 0C, DMF (5
ml), 12 h, bGC Yield. Products were characterized by GC by
comparing with authentic samples.
1
Ag@MIL-
101
1-ethynylbenzene (1.0
mmol), catalyst (70 mg),
Cs2CO3 (1.5 mmol), CO2 (1.0
atm), 50 0C, DMF (5 ml), 15
h.
96
70
It was seen that, in present catalytic system moderately
donating group (methyl, -CH3) gave the higher product yield (Table
3, Entry 2) compared to strong donating or withdrawing group. This
fact could be explained by the reaction mechanism. The present
carboxylative coupling reaction occurred in three major steps. In
the first step, silver acetylide intermediate was generated in
presence of the base. It was the key step of this reaction. Terminal
alkyne was deprotonated by the strong base and silver acetylide
was produced by the attachment of silver metal from leading
catalyst. This step was facilitated by the strong electron
withdrawing group present in the phenyl ring. In the next step,
2
Bridged
bis
(amidate)
rare-earth
metal
1-ethynylbenzene (1.0
mmol), Cs2CO3 (2.0 mmol),
Catalyst (0.04 mmol), CO2 (1
atm), DMSO (5 ml), 24h.
94
71
amides
Copper-
NHC
3
4
1-ethynylbenzene (1.0
mmol), Cs2CO3 (1.2 mmol),
CO2 (2.5 atm), DMF (5 ml),
1200C, 14 h.
95
82
72
73
Ag-NHC I
1-phenylethyne (2.0 mmol),
Ag-NHC I (0.02 mmol, 1
mol% Ag), Cs2CO3 (3.0
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Heterogeneity Test
mmol), DMF (10 ml), rt, 16 h
alkyne (1.0 mmol), 30 mg
A typical hot filtration test was performed in the carboxylation
5
CeO2-Ag
nanocom-
posites
98
This
work reaction of 1-phenylethyne to investigate whether the reaction
proceeded in a heterogeneous or a homogeneous fashion. The
reaction was carried out for 5 hr when 28% yield was obtained.
After that the catalyst was separated from the reaction mixture by
filtration and then without catalyst, the reaction was conducted for
CeO2-Ag nanocomposite
,
Cs2CO3 (1.5 mmol), CO2 (1.0
atm), 80 0C, DMF (5 ml), 12
h.
Carboxylation of terminal alkynes with CO2 under heterogeneous
conditions over a variety of catalysts had been studied earlier. Table
4 provided a comparison of the results obtained for our present
catalytic system with those reported in the literature.70-73
another 5 hr. It was observed that no reaction occurred without
catalyst and the yield remained unaltered indicating no leaching of
active metal from the support. This proved the heterogeneous
nature of the catalyst. For further confirmation, ICP-AES analysis
was carried out on filtrate of the reaction mixture to detect leached
metal in the solution. No silver metal was detected in the filtrate
which supported the result of the hot-filtration and confirmed the
same conclusion about the heterogeneity of the catalyst.
Catalyst reusability
It was also important to check the physical conditions of the
catalyst after the reaction. Thus TEM, EDX, XRD and XPS were
performed with the recovered catalyst (Supporting Information SI-
7-9). It was found that the structure of the catalyst remained almost
unaltered, the presence of CeO2 nanocubes was distinct along with
the surface-bound Ag nanoparticles (Supporting Information SI-7).
EDX in Supporting Information SI-4 had the signals from Ce, Ag, O, C
as expected. Cu came from the TEM grid. XRD demonstrated very
sharp and well-defined peaks for fcc CeO2 and fcc Ag (Supporting
Information SI-8). Thus the crystalline nature of the matrix i.e. CeO2
nanocubes, as well as Ag, remained unchanged under the reaction
conditions. A detailed XPS (Supporting Information SI-9) analysis
showed signal for Ce4+ (Ce 3d) from CeO2 same as Fig. 3A, indicating
no alteration in the matrix after catalysis. O 1s showed two peaks,
one from CeO2 and the other from the capping agents. N 1s signal
was also found which confirmed the presence of the capping agent
oleylamine even after 5 cycles of catalysis. Most importantly Ag 3d
signals could be fitted with the peaks at 367.5 eV and 373.5 eV with
a spin-orbit coupling factor of 6 eV corresponded to d5/2 and d3/2 of
metallic Ag. There was no sign of Ag+ in the recycled catalyst.
Fig. 8 Recycling of CeO2-Ag nanocomposite catalyst.
Recycling of heterogeneous catalysts could be considered as a
key issue to determine the economic and environmental benefit in
chemical and pharmaceutical industries. The reusability of CeO2-Ag
nanocomposites as the catalyst in the click reaction and
carboxylation reaction of 1-phenylethyne were checked (Fig. 8). As
the reaction was completed, the catalyst was removed by plain
filtration and washed with methanol followed by acetone then
dried under low pressure at 50 0C. Under optimum reaction
conditions, the next run was tested using the recovered catalyst.
The performance of the recycled catalyst over five successive runs
was shown in Fig. 8. The catalytic efficacy of the recycled catalyst
remained almost same even after five cycles.
5. Conclusion:
In this report, synthesis of Ag nanoparticles supported over 3-D
porous CeO2 nanocubes was shown at room temperature by
modifying the surface of CeO2 with DMP. Reducing and capping
12 | J. Name., 2012, 00, 1-3
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ability of DMP towards Ag nanoparticles played the key role in the
successful synthesis of the CeO2-Ag nanocomposites. This
nanocomposite was found to show extremely promising catalytic
activity in click reaction as well as in carboxylation of terminal
alkyne via a green pathway. All probable reactions parameters were
varied to establish the optimized condition of the reaction to have
3169.
6 E. C. Su and W. G. Rothschild, J. Catal., 1986, 99, 506.
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combinations of reaction parameters had the yield as high as 98%.
The catalyst was very effectively reused for several cycles plausibly
Primus, C. M. Teodorescu, C. Solans and M. S. Dominguez, Phys.
Chem. Chem. Phys., 2011, 13, 17135.
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Conflicts of interest
E. Comini and G. Sberveglieri, Nanotechnology, 2007, 18
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There are no conflicts to declare.
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Acknowledgements
207.
SD thanks University Grant Commission (UGC) for a research
fellowship. SD and TB thank Department of Science and Technology
(DST, India) for funding through Fast Track Scheme (SR/FT/CS-
106/2011) and Centre for Research in Nanoscience and
Nanotechnology (CRNN), University of Calcutta for instrumental
support. PM is thankful to the University Grant Commission (UGC)
for funding under Dr. D.S. Kothari post-doctoral research fellowship.
SMI gratefully acknowledges the Department of Science and
Technology, DST-SERB, (Project sanction No. EMR/2016/004956),
New Delhi, India, and the Council of Scientific and Industrial
Research, CSIR, [Project sanction No. 02(0284)/16/EMR-II), Date:
06-12-2016] for financial support.
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New Journal of Chemistry
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DOI: 10.1039/C8NJ00665B
A facile synthesis strategy to couple porous nanocube of CeO2 with Ag
nanoparticles: An excellent catalyst with enhanced reactivity for ‘click
reaction’ and carboxylation of terminal alkynes
Subhasis Das, Paramita Mondal, Swarbhanu Ghosh, Biswarup Satpati, Sasanka Deka, Sk. Manirul Islam and
Tanushree Bala
Surface of the CeO2 nanocube was modified with DMP which helped to drag Ag+ ions to the
organic medium and also to reduce to Ag nanoparticles over the nanocubes. This CeO2-Ag
nanocomposites was found to have excellent catalytic properties towards the synthesis of 1,4-
disubstituted 1,2,3 triazole from phenyl azide and phenyl acetylene. It also efficiently catalyzed
the carboxylation of terminal alkyne.