Polysaccharide-based superporous hydrogel embedded with copper nanoparticles: A green and versatile catalyst for the synthesis of 1,2,3-triazoles

Article abstract of DOI:10.1039/c8cy01796d

In this contribution, we a report a facile synthesis and stabilization of copper nanoparticles (CuNPs) in a superporous hydrogel based on chitosan (Cs) and poly(vinyl alcohol) (PVA). The functional groups of Cs and PVA stabilized the CuNPs and, as a result, several properties (e.g. thermal, morphological and liquid uptake) of the final materials exhibited remarkable improvements in terms of stability and catalytic activity. The metallic nature of CuNPs was confirmed by XRD and XPS analyses. From the XRD data, the Cu nanoparticle size was estimated to range from 3 to 7 nm. Moreover, EDS mapping analysis revealed that the CuNPs are homogeneously distributed throughout the hydrogel surface. The Cs/PVA-CuNP hydrogel was found to be an excellent catalyst for the synthesis of 1,2,3-triazoles using phenyl azide and phenylacetylene as model substrates under mild optimized conditions (>90% yield). Reuse studies revealed that this original CuNP-containing catalyst can be employed in at least 5 consecutive cycloaddition reactions without demonstrating a clear loss of efficiency. Moreover, the copper leaching from the hydrogel matrix was negligible during these consecutive reactions under the investigated conditions. The Cs/PVA-CuNP hydrogel could also efficiently catalyze cycloaddition reactions involving different substituted azide and alkyne precursors.

Full text of DOI:10.1039/c8cy01796d

Catalysis
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Polysaccharide-based superporous hydrogel
embedded with copper nanoparticles: a green and
versatile catalyst for the synthesis of 1,2,3-
triazoles†
Cite this: DOI: 10.1039/c8cy01796d
a
a
bc
Jaqueline F. Souza, Gabriel P. Costa, Rafael Luque,
a
a
Diego Alves * and André R. Fajardo
*
In this contribution, we a report a facile synthesis and stabilization of copper nanoparticles (CuNPs) in a
superporous hydrogel based on chitosan (Cs) and polyIJvinyl alcohol) (PVA). The functional groups of Cs
and PVA stabilized the CuNPs and, as a result, several properties (e.g. thermal, morphological and liquid up-
take) of the final materials exhibited remarkable improvements in terms of stability and catalytic activity.
The metallic nature of CuNPs was confirmed by XRD and XPS analyses. From the XRD data, the Cu nano-
particle size was estimated to range from 3 to 7 nm. Moreover, EDS mapping analysis revealed that the
CuNPs are homogeneously distributed throughout the hydrogel surface. The Cs/PVA-CuNP hydrogel was
found to be an excellent catalyst for the synthesis of 1,2,3-triazoles using phenyl azide and phenylacetylene
as model substrates under mild optimized conditions (>90% yield). Reuse studies revealed that this original
CuNP-containing catalyst can be employed in at least 5 consecutive cycloaddition reactions without dem-
onstrating a clear loss of efficiency. Moreover, the copper leaching from the hydrogel matrix was negligible
during these consecutive reactions under the investigated conditions. The Cs/PVA-CuNP hydrogel could
also efficiently catalyze cycloaddition reactions involving different substituted azide and alkyne precursors.
Received 27th August 2018,
Accepted 16th November 2018
DOI: 10.1039/c8cy01796d
rsc.li/catalysis
Taking into account the different catalysts described in the
literature, metal nanoparticles (MNPs) have gained great atten-
Introduction
Considerable efforts from the scientific community towards
environmentally friendly processes and technologies in recent
years have been directed to seeking alternative processes and
tion due to their interesting properties (high reactivity, selectiv-
5,6
ity, controllable facets, size, and shape, among others). In this
context, chemical reactions catalyzed by MNPs offer great ad-
vantages in terms of green credentials as compared to other
conventional catalysts such as high yield, fast reactions and re-
1
,2
syntheses with improved green credentials. Overall, green
chemistry seeks ways to produce and process substances and
materials in a more benign way considering human health
7
usability. Among the transition metals used to synthesize
3
and the environment. Catalysis is probably the most promi-
nanoparticles, copper (Cu) is of particular interest due to its
interesting properties, low-cost preparation and many potential
applications in catalysis. Copper nanoparticles (CuNPs) have
been employed in numerous areas such as nanofluids, biologi-
nent field of green chemistry due to its direct relationship
with the design of chemical substances and processes that re-
duce or eliminate the use and generation of hazardous mate-
4
8,9
rials. This aspect is very attractive from the environmental
cal and optical applications, and nanodevices. Considering
protection viewpoint. Moreover, the use of catalysts and cata-
lytic systems allows saving time and energy, desirable from
an economic viewpoint.
organic synthesis, CuNPs have been successfully utilized as
heterogeneous catalysts in oxidative cross-coupling reactions
and azide–alkyne cycloaddition reactions (“click reactions”).
10–12
Triazoles resulting from this type of “click reaction” are of par-
ticular interest since they are used in several applications (e.g.
dyes, agrochemicals, medicinal chemistry, and so on).
a
Universidade Federal de Pelotas, Brazil. E-mail: diego.alves@ufpel.edu.br,
12,13
andre.fajardo@pq.cnpq.br; Fax: +55 5332757533; Tel: +55 5332757533
b
Departamento de Química Orgánica, Grupo FQM-383, Universidad de Córdoba,
Despite the remarkable properties of MNPs in catalysis,
their use still has some limitations due to issues related to ag-
gregation and coalescence. In nano-sized dimensions, the sur-
face energy of such particles increases and strong attractive
Campus de Rabanales, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396,
Córdoba, Spain
c
Peoples Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya
str., 117198, Moscow, Russia
1
4
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cy01796d
forces cause inherent instability in such systems. MNPs
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undergo agglomeration in order to reduce the energy associ-
ated with their large surface areas. This undesirable phenom-
enon impairs drastically the catalytic activity of MNPs. Numer-
ous studies report elegant strategies to overcome this
drawback and, as noticed, the immobilization of MNPs on
was added to the polymeric solution followed by addition of
50 mg of NaHCO as the gas blowing agent. The system was
3
gently stirred for 30 min and immediately deep-frozen (−20
°C) for 24 h. The frozen hydrogel was collected and im-
mersed in ethanol (200 mL) for 24 h. Finally, cubic samples
1
5,16
3
solid supports is one of the most reliable.
Solid supports
(1 cm ) were cut from the as-prepared hydrogel and then
utilized for this aim are generally engineered from inorganic,
dried under vacuum (37 °C) for 48 h. These samples were la-
beled as Cs-gel.
1
7,18
organic
or
hybrid
materials.
More
recently,
polysaccharide-based materials have been used as supports
for MNPs without any undesirable consequences on their
In situ synthesis of CuNPs
1
9,20
practical use and the environment.
Attractive properties
The synthesis of the CuNPs into the Cs/PVA hydrogel matrix
such as renewable origin, biodegradability, low cost, high mo-
lecular weight and high functionality encourage the use of
polysaccharides for this aim. In this sense, materials designed
from chitosan (Cs), a well-known polysaccharide synthesized
from the total or partial deacetylation of chitin, are good can-
was performed by a two-step methodology: (i) adsorption and
2+
(
ii) reduction of Cu ions. Briefly, hydrogel samples (cubic
3
shape – 1 cm ) were immersed in 50 mL of CuCl
tion (0.05 mol L ) under mild stirring at room temperature
for 24 h. Next, Cu -containing samples (labeled as Cs/PVA-
Cu ) were thoroughly washed with distilled water in order to
2 2
·2H O solu-
−1
2+
2
1,22
didates to support MNPs.
Cs has free amine groups and
2+
hydroxyl groups on its backbone, increasing its ability to inter-
remove weakly physisorbed ions and subsequently immersed
n+
act with metallic ions (M ) and coordinate with MNPs,
−1
in a NaBH
4
solution (0.5 mol L ) at room temperature for 4
2
3
preventing their agglomeration and leaching. Despite these
attractive properties, the low mechanical strength and low hy-
drophilicity of chitosan-based materials additionally require
the formation of composites with synthetic polymers. Herein,
Cs was combined with polyIJvinyl alcohol) (PVA), a synthetic
hydrophilic and non-toxic polymer, in order to obtain a super-
porous hydrogel employed as a solid support for CuNPs. The
synthesis of CuNPs was performed in situ and their catalytic
activity was investigated towards the synthesis of 1,4-
disubstituted-1,2,3-triazoles under eco-friendly conditions.
h. After that, the samples embedded with CuNPs (labeled as
Cs/PVA-CuNP) were collected, washed with distilled water
and dried under vacuum (37 °C) for 48 h. The dried samples
were kept in a desiccator prior to characterization and use.
Hydrogel characterization
2
+
The amount of Cu ions adsorbed per hydrogel sample film
was quantified using the flame atomic absorption spectro-
scopy (FAAS) technique. For this, aliquots of the CuCl ·2H O
2
2
solution (after the adsorption step) and washing medium
were collected and analyzed in a Shimadzu spectrometer in
order to determine the residual concentration of copper. The
instrument was equipped with a copper hollow cathode lamp
and a deuterium lamp for background correction. The operat-
ing parameters were adjusted according to the standard
guidelines of the manufacturers and the atomic absorption
signal was measured as a peak height mode as a function of
Experimental
Materials
−
1
Chitosan (Cs) (M
v
87 000 g mol and 85% deacetylated) was
purchased from Golden-Shell Biochemical (China). PolyIJvinyl
−
1
alcohol) (PVA) (Mw 124 000 g mol and 99% hydrolyzed) and
glutaraldehyde (50 wt/v% in H O) were purchased from
Sigma-Aldrich (USA). CopperIJII) chloride dihydrate (CuCl
2H O, P.A.), sodium borohydride (NaBH , P.A.) and sodium
2
2
2+
an analytical curve (R ≈ 0.999). The amount of Cu in each
2
·
sample was calculated as per eqn (1):
2
4
3
bicarbonate (NaHCO , 99%) were purchased from Vetec (Bra-
zil). Acetic acid (99%) and ethanol (P.A.) were purchased
from Synth (Brazil). Terminal alkynes were purchased from
Sigma Aldrich and used without any purification. Organic

C C

V
2

o
R

Cu  
(1)


w
azides
procedures.
as received without further purification.
were
synthesized
All chemicals of analytical grade were used
according
to
reported
Fourier transform infrared spectroscopy (FTIR) analyses
were performed in Shimadzu IR-Affinity-1 (Japan) equipment
2
4–26
−1
operating in the spectral region of 4000–400 cm with a reso-
−
1
lution of 4 cm . Before spectra acquisition, the samples were
ground with spectroscopic grade KBr and pressed into disks.
Thermogravimetric analyses (TGA) were performed in a
Shimadzu DTG60 analyzer (Japan) operating in a temperature
Synthesis of the superporous hydrogel
A Cs-solution was prepared by dissolving 250 mg of the poly-
saccharide in acetic acid solution (1 v/v%) under magnetic
stirring for 12 h. In parallel, a second solution was prepared
by dissolving 100 mg of PVA in distilled water under stirring
at 80 °C for 12 h. The PVA-solution was subsequently cooled
down to room temperature and mixed with the Cs-solution.
The resulting solution was homogenized under magnetic stir-
ring for 1 h. Glutaraldehyde (50 μL), the crosslinking agent,
−
1
range of 25–500 °C with a scanning rate of 10 °C min under
−
1
a N2IJg) atmosphere (flow of 20 mL min ). X-ray diffraction
(XRD) patterns were recorded from powder samples using a
Siemens D500 diffractometer (Germany) equipped with a Cu-
Kα radiation source at 30 kV and 20 mA. The XRD patterns
were obtained in a scanning range of 5–50° with a scanning
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−
1
rate of 1° min . The surface elemental information of Cs/
PVA-CuNP (binding energy and oxidation states) was deter-
mined using a Thermo Scientific Escalab 250Xi (USA) X-ray
photoelectron spectrometer (XPS) at an excitation energy of
Al-Kα (1486.6 eV). The XPS spectrum was obtained in the
preparation chamber of the equipment with a base pressure
dium thermostated at different temperatures (25–75 °C).
Again, the samples were kept immersed under these condi-
tions for 24 h. The maximum swelling at equilibrium for
each experiment was calculated using eqn (3). Each experi-
ment was performed in triplicate.
−
9
below 5 × 10 mbar at 293 K for 60 min. The energy scale
was calibrated by assigning 284.6 eV to the adventitious C 1s
peak and 530.8 and 532 eV to O 1s. Scanning electron micros-
copy (SEM) images were recorded at 15 kV using a Jeol JSM-
General experimental procedure for the synthesis of 1,4-
disubstituted 1,2,3-triazoles
Aryl azides 1 (0.157 mmol), terminal alkynes 2 (0.157 mmol),
the Cs/PVA-CuNP hydrogel (10 mol%, 1 mg of Cu per hydro-
gel sample) and a mixture of EtOH/H O (1 : 1) (1.5 mL) were
2
added to a glass vial. Then, the heterogeneous reaction mix-
ture was sonicated for 6 h at room temperature in an ultra-
sonic bath. After the total disappearance of the starting mate-
rials, ethyl acetate (3 mL) was added and the reaction
mixture was sonicated for additional 5 min. The reaction
mixture was then separated from Cs/PVA-CuNP using a Pas-
teur pipette. This procedure was then repeated four times
6
610LV microscope (USA) equipped with an energy-dispersive
X-ray (EDX) analyzer. Before SEM visualization, the samples
were gold-coated by sputtering. N2IJg) adsorption–desorption
isotherms were obtained using a Micromeritics Tristar II Kr-
3
020 (USA) gas adsorption analyzer using samples previously
heated to 120 °C under vacuum for 20 h. The Brunauer–
Emmett–Teller (BET) specific surface area was examined
using the adsorption data.
The total porosity of each hydrogel sample was examined
4
and the combined extracts were dried over MgSO and con-
2
7
using the liquid displacement method. Dry samples were
immersed in a known initial volume of acetone (V ) at room
temperature for 24 h. Following acetone impregnation, the
volume of the set sample-acetone was determined (V ). Then,
the acetone-impregnated hydrogel sample was withdrawn and
the remaining liquid volume was determined (V ). The total po-
centrated under vacuum. The crude products obtained were
subsequently purified by column chromatography on silica
gel using a mixture of hexane/ethyl acetate as the eluent to af-
ford the desired products 3a–h. All the products were charac-
terized by nuclear magnetic resonance (NMR) spectroscopy in
1
2
1
3
a Bruker Avance DPX 400 spectrometer at 400 MHz ( H) and
rosity (in %) was calculated as per eqn (2). This experiment
was performed in triplicate for each sample.
13
at 100 MHz ( C). All the NMR spectra were acquired using
CDCl3 or DMSO-d6 as the deuterated solvent and tetra-
methylsilane (TMS) was used as the internal standard. The
recovered Cs/PVA-CuNP sample was dried under vacuum and
could be reused directly in subsequent reactions.

V1 V


3
Porosity

%


100
(2)
 2
V V3
Results and discussion
Hydrogel characterization
Swelling experiments
The water uptake capacity of the as-prepared hydrogels was
investigated by swelling experiments conducted in distilled
water at room temperature and mild stirring (100 rpm).
Briefly, dry samples (50 mg) were weighed (w ) and then im-
o
mersed in the swelling medium. At pre-determined time in-
tervals, the swollen samples were withdrawn and weighed again
The chemical structural properties of the as-prepared hydro-
gels (with and without Cu specimens) were examined by FTIR
(
Fig. 1a). As noticed, the Cs/PVA spectrum shows the charac-
teristic bands of the starting materials (Cs and PVA) with
21,28,29
some discrepancies.
The broad band centered at 3423
−1
cm is associated with the O–H stretching of the hydroxyl
groups of Cs and PVA. Moreover, this band overlaps with the
band of the N–H stretching of the amino groups of Cs. The
t
(w ). Before this step, the excess liquid on the sample surface
was carefully drained. The swelling degrees (in %) at differ-
ent immersion times (t) were calculated as per eqn (3). This
experiment was performed in triplicate for each sample.
−1
band at 1655 cm is assigned to the imine bond (CN)
30
formed due to the crosslinking of Cs with glutaraldehyde.
−
1
The band at 1559 cm is assigned to N–H deformation (am-
−
1

w  wo  100
ide II band) while the band at 1412 cm is attributed to C–N
stretching. The bands in the 1150–1000 cm region are as-
cribed to the C–O–C bending and C–OH stretching of Cs and
t
Swelling

%


(3)
3
0
−1
wo
3
1
2+
The effect of the pH and temperature of the swelling me-
dium on the maximum water uptake was also investigated in
separate experiments using similar procedures. Considering
the pH effect, the pH of the swelling medium was adjusted to
the C–O stretching of PVA. In parallel, the Cu -containing
2
+
hydrogel (Cs/PVA-Cu ) spectrum shows an obvious modifica-
tion as compared to the spectrum of the pristine hydrogel. In
general, the different vibrational modes of the amino and hy-
−
1
2+
different values (2–12) using HCl (1.0 mol L ) or NaOH (1.0
droxyl groups in the Cs/PVA-Cu spectrum were shifted to
−
1
mol L ) solutions. Samples were kept in these swelling me-
dia at room temperature for 24 h. The effect of the tempera-
ture was investigated using distilled water as the swelling me-
distinct wavenumber regions or they showed different absor-
bance intensities as a result of the interaction of these groups
2
+
−1
with Cu ions. For instance, the band at 3423 cm (O–H
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and 30.6°. These diffraction peaks suggest that Cs/PVA shows
a semi-crystalline structure likely due to the inter and intra-
molecular interactions that take place between Cs and PVA
3
5
chains. These interactions were confirmed by FTIR analysis.
On the other hand, the XRD patterns recorded for the CuNP-
containing hydrogel exhibit an obvious decrease in the dif-
fraction peaks associated with the Cs–PVA interaction proba-
bly due to the interaction between the CuNPs and hydrogel
matrix. The presence of CuNPs in the hydrogel matrix dis-
rupts the H-bonding between the amino and hydroxyl groups
of Cs and PVA. Moreover, the appearance of three peaks at 2
theta ≈ 43.4°, 50.6°, and 74.8° corresponding to the (111),
(
200), and (220) crystalline planes of Cu was observed. These
planes are in agreement with the face-centered cubic (FCC)
3
6
phase of Cu [JCPDS 04-0836] and confirm that the CuNPs
were successfully synthesized.
In addition, the diffraction pattern of Cs/PVA-CuNP re-
veals the presence of impurity peaks at 2 theta ≈ 36.2°, 54.0°,
3
7
and 61.5° associated with CuO species. Considering these
diffraction peaks assigned to metallic copper, the average
particle size was estimated by using the Debye–Scherrer equa-
3
8
tion. Herein, the in situ synthesized CuNPs exhibited a par-
ticle size ranging from 3 to 7 nm. This particle size is compa-
rable to or even smaller than other values reported in
2
1,37,39
previous studies employing similar protocols.
Fig. 1c shows the high-resolution XPS spectrum of Cu 2p
recorded from the Cs/PVA-CuNP surface. As observed, the
spectrum displays peaks corresponding to Cu 2p and Cu
3
/2
3
7,40
2p
1/2 spin–orbit components.
These peaks were deconvoluted into two peaks each due
to the presence of reduced Cu species and CuO species. The
peaks centered at 932.1 and 951.8 eV are assigned to reduced
Cu species while the peaks centered at 933.8 and 953.6 eV
Fig. 1 (a) FTIR spectra and (b) XRD patterns recorded from Cs/PVA
and Cs/PVA-CuNP hydrogels. (c) XPS spectrum of Cu 2p of the Cs/
PVA-CuNP hydrogel.
3
7
are due to CuO species. Moreover, the broad peaks ob-
served between 965–960 eV and 947–940 eV are associated
with the shake-up satellite lines of CuO 2p1/2 and CuO 2p3/2
,
−
1
40
and N–H stretching modes) was shifted to 3445 cm due to
a reduction of intra and intermolecular H-bonds. The bands
associated with N–H and C–N deformation modes and C–OH
stretching decreased in intensity likely due to the interaction
respectively.
Despite the detection of different valence
states of copper, the main species is reduced Cu (90.1%). The
presence of CuO species can be credited to the incomplete re-
2
+
duction of Cu ions previously adsorbed on the Cs/PVA ma-
trix. In addition, the possibility related to the re-oxidation of
CuNPs due to their exposure to air should be considered as
2
+
21,32
between the amino and hydroxyl groups with Cu ions.
These results were in good agreement with FAAS analysis,
which demonstrated that the Cs/PVA hydrogel sample is able
4
1
stated by Feng et al. No peaks corresponding to Cu O speci-
2
2
+
to adsorb and retain 1.0 ± 0.2 mg of Cu (n ≈ 6). After the re-
mens (529.6 eV) were observed in the XPS spectra of Cu 2p or
O 1s (not shown here). The low CuO content in the Cs/PVA-
CuNP sample as inferred from XPS analyses is in agreement
with the XRD analyses that detected the tiny presence of cop-
per oxidized phases as compared to metallic copper
reflections.
2
+
duction of Cu ions, band changes associated with the
amino and hydroxyl groups of Cs/PVA are more pronounced
as noticed in the Cs/PVA-CuNP spectrum. As demonstrated
by other authors, these groups play a leading role in the sta-
3
3,34
bilization of nanoparticles.
7
Additionally, the new band at
46 cm could be assigned to the loading effect of metallic
Cu in the hydrogel matrix, confirming the successful forma-
−
1
2+
The thermal stability of the Cs/PVA, Cs/PVA-Cu and Cs/
PVA-CuNP hydrogels was investigated by TGA/DTG analyses
(Fig. S1a and b†). The TGA curves recorded for all the hydro-
gels show an initial stage of weight loss in the temperature
range of 50 °C and 130 °C due to the elimination of moisture
and volatile compounds. The TGA curve of Cs/PVA exhibits
additional stages of weight loss assigned to the elimination
2
1
tion of such particles.
XRD analysis was performed in order to identify the differ-
ent crystalline phases present in Cs/PVA and Cs/PVA-CuNP
hydrogels. As depicted in Fig. 1b, the XRD pattern of Cs/PVA
shows some diffraction peaks at 2 theta ≈ 11.7°, 19.5°, 20.3°,
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2
−1
of hydroxyl side groups of PVA (140–340 °C), thermal decompo-
sition of the linear and crosslinked moieties of Cs (350–460 °C)
creased to 12 m g for Cs/PVA-CuNP. Moreover, the estimated
total porosity of this sample decreases to 77%. This reduction
of these morphological parameters can be attributed to the
presence of CuNPs, which increases the crosslinking effect in
the hydrogel matrix due to interactions that take place between
these particles and the polymers. In addition, some granular
materials are deposited on the pore surface of Cs/PVA-CuNP,
which is not observed in the SEM image taken from Cs/PVA.
Obvious visual discrepancies are also noticed between these
two hydrogel samples (Fig. S1a and b†). For example, the pris-
tine hydrogel shows a yellowish color proceeding from Cs while
the CuNP-containing hydrogel shows a dark color typically ob-
2
1,42
and polyene backbone degradation of PVA (above 490 °C).
Although the TGA curves recorded from the Cu-containing
2+
hydrogels (Cs/PVA-Cu and Cs/PVA-CuNP) show some similar-
ity, some clear discrepancies can be noticed. For instance, the
2+
DTG curve for Cs/PVA-Cu exhibits a sharp peak centered at
2
+
2
37 °C suggesting that Cu ions adsorbed on the hydrogel ma-
trix decrease its thermal stability as compared to the pristine
2
+
hydrogel (see Fig. S1b†). The presence of Cu ions disrupts the
inter and intramolecular H-bonds in the hydrogel matrix reduc-
2+
ing the thermal energy required to degrade the Cs/PVA-Cu
4
3
2+
16
sample. Conversely, after the reduction of Cu ions, the func-
tional groups available in the hydrogel (amino and hydroxyl
groups) strongly interact with CuNPs in order to reduce their
great surface energy. Therefore, the thermal stability of Cs/PVA-
CuNP increases as compared to the other hydrogel samples. As
mentioned in other studies, the interactions between nano-
particles and the functional groups of the hydrogel act as addi-
tional crosslinking points hampering the thermal degradation
served for hydrogels embedded with metal nanoparticles.
SEM/EDS-analysis was used to investigate the elemental
composition of Cs/PVA-CuNP. According to the EDS spectrum
recorded from this sample, the presence of signals related to
the elements that compose the polymers (Cs and PVA) and
signals confirming the presence of Cu in the hydrogel com-
position is observed (Fig. S3a†). Additionally, the EDS map-
ping image confirms that the CuNPs are homogeneously dis-
tributed throughout the sample (Fig. S3b†). The red dots
represent the elemental map distribution of the CuNPs. This
is a remarkable result since it confirms that the Cs/PVA ma-
trix is an efficient device to immobilize CuNPs, preventing ag-
glomeration and other undesirable results.
44,45
of the hydrogel matrix.
The morphology of the Cs/PVA and Cs/PVA-CuNP hydrogels
was investigated by means of SEM. The images taken from the
hydrogel samples confirm that Cs/PVA has a rough and super-
porous structure with interconnected pores (Fig. 2a and b).
This type of structure can be associated with the protocol used
4
6
in the hydrogel synthesis. Preliminary experiments revealed
that the hydrogels prepared using just Cs or PVA did not ex-
hibit this morphology. The Cs/PVA sample possessed irregu-
larly shaped pores with an average size of 177.9 ± 38.7 μm,
while the total porosity of this sample was around 87%. The
surface area examined by using the BET adsorption isotherm
was 17 m g for Cs/PVA. On the other hand, although Cs/PVA-
CuNP still preserves the morphological characteristics of Cs/
PVA (high porosity and interconnected pores), some discrepan-
cies are observed (Fig. 2c and d). As evaluated, the average pore
size decreased to 135.8 ± 22.9 μm while the surface area de-
Swelling properties
The ability to absorb and retain large amounts of liquid is
the most iconic property of hydrogels. This ability is particu-
larly interesting in catalytic systems engineered from metal
nanoparticles and hydrogels because the liquid uptake profile
of this set will regulate the contact between the catalysts em-
2
−1
1
6
bedded in the hydrogel matrix and the starting materials.
Herein, the liquid uptake capacity of the Cs/PVA and Cs/PVA-
CuNP hydrogels was examined by a swelling experiment. The
swelling curves presented in Fig. 3 reveal that at short immer-
sion times (<15 min), both hydrogels showed fast swelling
rates likely due to the hydrophilic groups distributed
throughout the hydrogel surface. The large pore sizes
Fig. 2 SEM images recorded from Cs/PVA hydrogel (a and b) and Cs/
Fig.
3 Swelling kinetics curves of Cs/PVA and Cs/PVA-CuNP
PVA-CuNP hydrogel samples (c and d) at different magnifications.
hydrogels in distilled water at room temperature.
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4
7
presented by these hydrogels also favor the liquid uptake.
swelling rate was observed at pH 12, which can be assigned
to the low availability of functional groups on the hydrogel
matrix to interact with water molecules due to interactions
between these groups and the CuNPs. Importantly, the
CuNP-containing hydrogel exhibited a low swelling ability as
compared to the pristine hydrogel. These results strengthen
the discussion that the CuNPs enhance the crosslinking den-
sity of the hydrogel matrix because of the coordination of the
particles with the functional groups of Cs and PVA.
After that, the swelling curves exhibited completely different
patterns. For Cs/PVA, the swelling rate slowed down and the
equilibrium was achieved close to 100 min after immersion.
As time goes on, the liquid uptake process ends and the max-
imum swelling rate calculated for Cs/PVA is 382%. In con-
trast, the swelling curve of Cs/PVA-CuNP achieves the equilib-
rium more quickly (20 min of immersion) and the maximum
swelling rate calculated for this sample is 208%. As demon-
strated by FTIR and thermal analyses, CuNPs interact with
the hydrophilic groups of the Cs/PVA matrix and, as a conse-
quence, the availability of such groups to interact with water
molecules is impaired. Additionally, the interaction between
the particles and the hydrogel matrix acts as an extra
crosslinking point reducing the expansion of the polymeric
network and limiting the liquid diffusion into the hydrogel.
These effects explain why the Cs/PVA-CuNP hydrogel shows
lower swelling ability as compared to the pristine hydrogel.
Other similar studies also report this effect caused by filler
In parallel, the effect of the temperature of the swelling
medium was also investigated for both hydrogel samples
(Fig. S4b†). As noticed, an increase of temperature from 25 to
75 °C increases the maximum swelling rate of Cs/PVA, partic-
ularly from 55 °C. In general, the increase of the temperature
causes an increment in the kinetic energy of the polymer
chains that form the hydrogel matrix and disrupts some inter
5
0
and intramolecular interactions among them. Thus, free hy-
drophilic groups can interact with the water molecules, in-
creasing the swelling rate. In contrast, the effect due to the
increase of the temperature of the swelling medium is more
discrete for Cs/PVA-CuNP. Overall, slight variations were ob-
served in the maximum swelling rates of the Cs/PVA-CuNP
hydrogel with the increase of temperature. The additional
crosslinking points that result from the coordination of
CuNPs and the hydrogel matrix impair the mobility of the
polymer chains even at high temperatures. At high tempera-
ture (75 °C), a slight decrease in the swelling rate was ob-
served for the CuNP-containing hydrogel. It can be suggested
that under such conditions, the catalytic nature of Cu dam-
ages the crosslinking that keeps the hydrogel matrix stable
and also can promote the loss of fragments of polymer
3
9,45
materials on hydrogel swelling properties.
The effects of the pH and temperature of the swelling me-
dium were also investigated. Overall, these two factors had a
clear influence on the maximum swelling of both hydrogels
Cs/PVA and Cs/PVA-CuNP). Under strong acidic conditions
pH < 2), both hydrogels showed a high swelling rate due to
(
(
protonation of the amino groups of Cs, a typical behavior
reported in different studies focused on Cs-based hydro-
4
8,49
gels.
The positively charged groups in the hydrogel bulk
phase cause the expansion of the polymeric network, increas-
ing liquid uptake (Fig. S4a†). The increase in the pH of the
medium results in a decrease of the swelling rate (4 < pH <
4
2
8
) for both hydrogels likely due to the reduction of the pro-
chains. These experiments reveal that both hydrogels show
pH- and temperature-sensitive swelling, which are very attrac-
tive features particularly in catalysis applications.
tonated amino groups. In this stage, the expansion of the
polymeric network is less pronounced and at neutral pH con-
ditions, it no longer occurs. As a result, the amount of liquid
that diffuses into the hydrogel matrix decreases considerably.
Under alkaline pH conditions (pH ≥ 8), no significant varia-
tion in the swelling rate was observed for Cs/PVA. However,
for the Cs/PVA-CuNP hydrogel, a decrease in the maximum
Catalytic activity: synthesis of 1,4-disubstituted-1,2,3-triazoles
Cs/PVA-CuNP was subsequently examined as the catalytic sys-
tem to promote azide–alkyne cycloaddition reactions.
a
Table 1 Optimization of the reaction conditions for the synthesis of 3a
b
Entry
1
Solvent
Time (h)
Temperature (°C)
Yield of 3a (%)
tBuOH/H
tBuOH/H
tBuOH/H
tBuOH/H
2
2
2
2
O (1 : 1)
O (1 : 1)
O (1 : 1)
O (1 : 1)
6
6
6
6
6
50
50
50
50
50
65
83
n.d.f
c
2
d
f
3
4
e
n.d.
92
c
5
2
EtOH/H O (1 : 1)
a
Reactions were performed with phenyl azide 1a (0.157 mmol), phenylacetylene 2a (0.157 mmol), and Cs/PVA-CuNP (10 mol%, 1 mg of Cu per
b
c
hydrogel sample) under an air atmosphere using 1.5 mL of solvent and an ultrasonic bath. Yields are given for isolated products. Different
extraction processes (see in the text). Reaction was performed without Cs/PVA-CuNP. Reaction was performed with the chitosan hydrogel in
the absence of copper. n.d. = not detected.
d
e
f
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combined extracts were dried over MgSO and concentrated
4
under vacuum. Using this extraction procedure, the desired
product 3a was obtained in 83% yield after flash chromatog-
raphy on silica gel (Table 1, entry 2).
In order to prove the importance of the catalyst in this re-
action, blank runs (in the absence of Cs/PVA-CuNP) were
performed, providing no formation of the desired product 3a
(
Table 1, entry 3). We observed the same result when the re-
action was performed using only the chitosan hydrogel in the
absence of copper (Table 1, entry 4). In both cases, the
starting materials were fully recovered (Table 1, entries 3 and
4). An excellent result was achieved using a mixture of EtOH/
H O (1 : 1) as the solvent, which gave 3a in 92% yield after pu-
2
rification (Table 1, entry 5). From Table 1, the optimum reac-
tion conditions to obtain 1,4-diphenyl-1H-1,2,3-triazole (3a)
are presented in entry 5, using 1a (0.157 mmol), 2a (0.157
mmol) and 10 mol% of Cs/PVA-CuNP in a mixture of EtOH
and H O (1 : 1) (1.5 mL) as the solvent, sonicated in an ultra-
2
sonic bath under air for 6 h.
After the reaction optimization, a study regarding the re-
covery and reuse of the catalyst was performed. In order to
investigate the reusability of the catalyst, six consecutive reac-
tions were performed using the same Cs/PVA-CuNP sample.
Upon product 3a formation (1st reaction), we added ethyl ac-
etate (3 mL) to the reaction mixture which was sonicated for
Fig. 4 Photographic images taken from the reaction system after
each step: (A) the as-prepared catalyst placed in the vial; (B) the cata-
lyst in the presence of azide and alkyl substrates (t ≈ 0 h); (C) at the
end of the cycloaddition reaction (t ≈ 6 h); and (D) the recovered and
regenerated catalyst placed in the vial ready for the next reaction run.
5
min, followed by separation from the catalyst using a Pas-
teur pipette. This procedure was repeated five times and the
1
1
Inspired by the pioneering report of Huisgen on the ther-
mal 1,3-dipolar cycloaddition of azides with terminal alkynes,
a number of catalytic strategies employing transition metals,
extracted products were dried over MgSO and concentrated
4
under vacuum. The recovered Cs/PVA-CuNP was dried under
vacuum and reused directly in the next reaction run. The
progress of this reaction was monitored visually as the prod-
uct crystallized out from the reaction medium. Fig. 4
5
1,52
especially copper (CuAAC),
have been employed to selec-
tively synthesize 1,2,3-triazoles. However, the search for effi-
cient methods using appropriate and environmentally cata-
lytic systems for the selective synthesis of 1,2,3-triazoles still
remains a challenge in synthetic chemistry. Thus, we started
our studies using phenyl azide (1a) and phenylacetylene (2a)
as model substrates in reactions carried out in an ultrasonic
bath (Table 1).
In the initial experiment, phenyl azide (1a),
phenylacetylene (2a) and Cs/PVA-CuNP (10 mol%, 1 mg of Cu
per hydrogel sample) were mixed together in a mixture of
2
tBuOH and H O (1 : 1) and sonicated under an air atmo-
sphere for 6 h. This reaction was carried out in a vial and
monitored by TLC until the total disappearance of the
starting materials. After this time, the mixture was diluted
in water (3 mL) and washed 3 times with ethyl acetate (3 × 3
mL). The obtained product was purified by flash chromatog-
raphy on silica gel affording the desired compound 3a in
6
5% yield (Table 1, entry 1).
Subsequently, the reaction was carried out under identical
conditions, although with a change in the extraction proce-
dure. After the total disappearance of the starting materials,
ethyl acetate (3 mL) was added and the reaction mixture was
additionally sonicated for 5 minutes. The reaction mixture
was then separated from Cs/PVA-CuNP using a Pasteur pi-
pette. This procedure was then repeated four times and the
Fig. 5 1,4-Diphenyl-1H-1,2,3-triazole (3a) yield (a) and percentage of
copper leaching (b) after each consecutive reaction run.
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Table 2 Scope and generality of the reactiona
b
Entry
1
Aryl azide 1
Alkyne 2
Product 3
Yield (%)
92
1
1
a
2
2
a
a
3a
2
84
b
3
3
b
c
3
4
65
86
1
1
c
2
2
a
a
d
3
3
d
e
5
85
1
1
1
e
a
a
2
2
a
6
7
82
54
b
3
3
f
g
2
2
c
8
55
d
1
a
3h
a
Reactions were performed with aryl azides 1 (0.157 mmol), terminal alkynes 2 (0.157 mmol), Cs/PVA-CuNP (10 mol%, 1 mg of CuNP per
sample) at room temperature for 6 h under an air atmosphere using a mixture of EtOH/H
using an ultrasonic bath. Yields are given for isolated products.
2
O (1 : 1) (1.5 mL). All the reactions were performed
b
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illustrates each experimental step. The simplicity of this pro-
cess should be highlighted.
According to the experimental data collected after each reac-
tion, it was observed that a good level of efficiency was
Taken together, these findings further confirm that Cs/
PVA-CuNP is a promising reusable catalyst for the synthesis
of 1,2,3-triazoles via cycloaddition reactions.
maintained even after the catalyst was reused for six consecu- Conclusions
tive times. Overall, the 1,4-diphenyl-1H-1,2,3-triazole (3a) yield
Copper nanoparticles (CuNPs) were synthesized and stabi-
varied from 92% to 70% after five successive reaction runs,
which suggests that the catalytic efficiency of Cs/PVA-CuNP is
slightly affected after reuses (Fig. 5a). At the sixth reaction, the
efficiency of the catalyst showed a noticeable reduction and, as
consequence, the yield of compound 3a dropped to 55%.
lized in a chitosan/polyIJvinyl alcohol) (Cs/PVA) hydrogel ma-
trix using a simple two-step adsorption/reduction protocol.
As assessed by different techniques, the CuNPs embedded
into the Cs/PVA matrix influenced the thermal, morphologi-
cal and swelling properties of the final materials as compared
to the Cs/PVA hydrogel. XRD and XPS analyses confirmed the
metallic nature of the CuNPs, with nanoparticle sizes esti-
mated to be in the 3–7 nm range. SEM/EDS analyses demon-
strated that the Cs/PVA-CuNP hydrogel exhibits a super-
porous structure and the CuNPs are homogeneously
distributed throughout the hydrogel surface. Catalytic experi-
ments proved that Cs/PVA-CuNP is an excellent catalyst for
the synthesis of 1,2,3-triazole using phenyl azide and
phenylacetylene as model substrates (92% yield). Reuse stud-
ies revealed that this original CuNP-containing catalyst can
be employed in at least 5 consecutive cycloaddition reactions
without a significant loss of efficiency. FAAS analysis demon-
strated that the Cu leaching from the hydrogel matrix was
minor (ca. 4 wt%) during these consecutive reactions. Finally,
the Cs/PVA-CuNP hydrogel also efficiently catalyzed the cyclo-
addition reactions involving different substituted azide and
alkyne precursors.
In order to investigate the reduction efficiency, copper
leaching from Cs/PVA-CuNP into the reaction medium was
measured. For this, just after each reaction, an aliquot of the
reaction medium was collected and the amount of copper
leached was quantified by FAAS. The percentage of copper
leached from Cs/PVA-CuNP after each reaction run is
depicted in Fig. 5b. It should be noticed that these percent-
ages were calculated considering the initial copper content in
the Cs/PVA-CuNP sample after each reaction. These results
revealed that insignificant amounts (less than 1.2% w/w per
reuse step) of copper were leached from the catalyst to the re-
action medium. Overall, the Cs/PVA matrix showed a remark-
able supporting ability for copper, losing ca. 4 wt% Cu after
six reuses. Leaching of the metal into the reaction medium
can result in some drawbacks related to product purifica-
5
3
tion. Conversely, the loss of small amounts of copper may
explain the decrease observed for the 3a yield after consecu-
tive reaction runs performed. We hypothesize that the
leaching can be attributed to the removal of copper species
immobilized on the catalyst surface and/or due to the experi-
mental procedures carried out for the recovery of the catalyst.
The scope of our methodology was subsequently extended
to different aryl azides (1a–e) and terminal alkynes (2a–d) to
check the versatility of Cs/PVA-CuNP in azide–alkyne cycload-
dition reactions. The results presented in Table 2 unequivo-
cally confirm that our experimental protocol worked well for
different substituted azides and alkynes, providing the de-
sired products in moderate to good yields.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are grateful for the financial support and scholarships
from the Brazilian agencies CNPq (Grants. number 310943/
2016-7 and 305974/2016-5) and FAPERGS (PRONEM – Grant.
number 16/2551-0000240-1). CNPq is also acknowledged for
the fellowship to D. A. and A. R. F. This study was financed in
part by the Coordenação de Aperfeiçoamento de Pessoal de
Nível Superior – Brasil (CAPES) – Finance Code 001. The pub-
lication has been prepared with support from RUDN Univer-
sity Program 5-100.
The reactions of phenylacetylene (2a) with aryl azides
containing electron-withdrawing (EWG) substituents 1b–c
and electron-donating groups (EDG) 1d–e at the aryl moiety
yielded the corresponding products 3b–e in 65–86% yields
(Table 2, entries 2–5). A little decrease in yields was observed
using 4-chloro-phenyl azide as a substrate, which provided
the desired product 3c in 65% yield (Table 2, entry 3). The re-
activity of phenyl azide (1a) with different terminal aryl al-
kynes (2b–c), under otherwise identical reaction conditions,
was subsequently investigated. In general, the reactions were
found to be sensitive to the electronic conditions in the aryl
ring of the terminal alkynes, in which an alkyne containing
an EDG gave a better yield when compared to an alkyne
containing an EWG on the aromatic ring (Table 2, compare
entry 6 with 7). The reaction performed with 1-hexyne (2d)
furnished exclusively the respective 4-butyl-1-phenyl-1H-1,2,3-
triazole (3h) in 55% yield (Table 2, entry 8).
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