Halloysite nanoclay decorated with 2-amino pyrimidine functionalized poly glycidyl methacrylate: An efficient support for the immobilization of Pd nanoparticles

Article abstract of DOI:10.1016/j.jssc.2018.12.051

Taking advantage of the synergistic effects between polymer (P) and halloysite clay (Hal) as well as the capability of heteroatom-containing polymers for anchoring nanoparticles and suppressing their leaching, a novel support composed of Hal and 2-amino p

Full text of DOI:10.1016/j.jssc.2018.12.051

Journal of Solid State Chemistry 271 (2019) 59–66
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Halloysite nanoclay decorated with 2-amino pyrimidine functionalized poly
glycidyl methacrylate: An efficient support for the immobilization of Pd
nanoparticles
Samahe Sadjadia,⁎, Fatemeh Koohestanib, Naeimeh Bahri-Lalehc,⁎, Khadijeh Didehbanb
a
Gas Conversion Department, Faculty of Petrochemicals, Iran Polymer and Petrochemical Institute, PO Box 14975-112, Tehran, Iran
Department of Chemistry, Payame Noor University, Tehran, Iran
Polymerization Engineering Department, Iran Polymer and Petrochemical Institute (IPPI), P.O. Box 14965/115, Tehran, Iran
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Halloysite
Polymer
Hybrid
Pd nanoparticles
Sonogashira reaction
Taking advantage of the synergistic effects between polymer (P) and halloysite clay (Hal) as well as the
capability of heteroatom-containing polymers for anchoring nanoparticles and suppressing their leaching, a
novel support composed of Hal and 2-amino pyrimidine (Py) functionalized poly glycidyl methacrylate (P) was
prepared through initial functionalization of Hal followed by polymerization with glycidyl methacrylate and
functionalization with 2-amino pyrimidine. The support (Hal-P-Py) was then used for the immobilization of Pd
nanoparticles to afford a heterogeneous catalyst (Pd@Hal-P-Py) with high catalytic activity for Sonogashira
coupling reaction under mild and eco-friendly reaction condition. Preparing various control catalysts, i.e. Pd@
Hal-P, Pd@Hal, Pd@P and Pd@P-Py, and comparing their catalytic activities with that of the catalyst, the
contribution of polymer and 2-amino pyrimidine to the catalysis was confirmed. Noteworthy, the catalyst was
recyclable and could be recovered and used up to six times with insignificant loss of the catalytic activity and Pd
leaching. Moreover, hot filtration experiment confirmed the heterogeneous nature of the catalysis.
1. Introduction
with the catalytic species through ionic, hydrophobic interactions as
well as covalent, non-covalent or hydrogen bridges and suppress their
Recently, development of heterogeneous catalysts through immo-
bilization of catalytic species on the halloysite nanotubes, Hal, has
received increasing interest and several research groups focused on
disclosing different Hal-based catalysts for catalyzing diverse range of
chemical transformations [1–5]. It is worth noting that the unique
tubular morphology of Hal [4,6,7], as well as its bio-compatibility,
availability, high thermal and mechanical resistance, chemical compo-
sition (general formula of Al2Si2O5(OH)4·nH2O with Al-OH octahedral
sheet groups on the interior surface and Si-O-Si groups on the exterior
surface) and charged nature [8–12] render Hal a distinguished support
for catalytic purposes. To provide better dispersion of catalytic species
on the surface of Hal and/or furnish bifunctional catalysts, Hal can be
hybridized with other materials or surface functionalized on one or
both inner and outer surfaces [1,13,14].
leaching and consequently enhance the recovery of the catalyst [18–
21]. Recently, some hybrid systems of Hal-polymer have been disclosed
and employed for the catalytic purposes. One of the first examples was
reported by Riela and Lazzara, in which Hal was decorated with
thermo-responsive poly(N-isopropylacrylamide) and then the hybrid
system was applied for immobilization of Pd nanoparticles [1]. This
research group also reported Pd immobilized on hybrid system based
on thiol functionalized Hal and highly cross-linked imidazolium salts
[22]. Moreover, Pd nanoparticles immobilized on ternary hybrid
system composed of Hal, polyacrylamide and cyclodextrin has also
been reported [23]. The results confirmed that incorporation of
polymer not only could suppress leaching of nanoparticles, but also
could improve the catalytic activity of the final catalyst through
synergistic effects between Hal and polymer.
Functionalized polymers, including glycidyl methacrylate polymers
[15] have been extensively used as catalyst support for development of
heterogeneous catalysts [16,17]. It is known that polymers as support
can effectively improve the dispersion of the catalytic species.
Moreover, the functionalities on the polymer backbone can interact
One of the most important class of Pd-catalyzed chemical transfor-
mations is C–C coupling reaction. These reactions, such as Heck,
Sonogashira and Suzuki reactions have been widely applied for the
synthesis of natural products as well as diverse range of synthetic
compounds and organic building blocks [24]. Classically, these reac-
⁎
Corresponding authors.
E-mail addresses: s.sadjadi@ippi.ac.ir (S. Sadjadi), N.Bahri@ippi.ac.ir (N. Bahri-Laleh).
http://dx.doi.org/10.1016/j.jssc.2018.12.051
Received 7 November 2018; Received in revised form 12 December 2018; Accepted 26 December 2018
Available online 27 December 2018
0022-4596/ © 2018 Elsevier Inc. All rights reserved.

S. Sadjadi et al.
Journal of Solid State Chemistry 271 (2019) 59–66
tions have been catalyzed using homogeneous Pd-based catalysts along
with copper co-catalysts and phosphorous ligands [25]. Taking the
environmental considerations and economic issues into account,
development of co-catalyst and ligand free protocols in the presence
of heterogeneous catalysts gained tremendous attentions in the last
decade. In this line, various heterogeneous Pd catalysts have been
prepared through immobilization of Pd on various supports such as
clays, silica, metal-organic frameworks etc. [26–30].
(3 g) and KPS (1.8 mg) as polymerization initiator were added to the
aforementioned suspension. Subsequently, the reaction mixture was
stirred under N2 atmosphere at 60 °C for 12 h. Upon completion of
polymerization, the solid was filtered, washed several times with
distilled water to remove oligomers and un-reacted monomers and
dried at 80 °C overnight. For further purification, the product was
purified by Soxhlet with chloroform for 72 h. Finally, Hal-P was
achieved after dying at 70 °C for 12 h.
In the following of our study on Hal-based heterogeneous catalysts
[31–34] and encouraged by the catalytic results of Hal-polymer hybrid
systems [23], herein, we wish to report a novel Hal-polymer hybrid
system, Hal- poly glycidyl methacrylate, that was further functionalized
with 2-amino pyrimidine and applied for the immobilization of Pd
nanoparticles to afford a novel catalyst, Pd@Hal-P-Py, with the utility
for Sonogashira coupling reaction. In the following, the effects of
incorporation of polymer and 2-amino pyrimidine on the catalytic
activity are investigated. Moreover, the recyclability of the catalyst as
well as Pd leaching are studied.
2.3.3. Synthesis of Hal-P-Py
2-amino pyridine (4.01 g) and NaOH (2 M, 2 mL) were added to the
suspension of Hal-P (6 g) in acetonitrile (2 mL). Then, the resulting
mixture was refluxed at 24 h. At the end of the reaction, Hal-P-Py was
filtered, washed with acetonitrile and dried at 80 °C overnight.
2.3.4. Immobilization of Pd nanoparticles on Hal-P-Py: Pd@Hal-P-Py
To incorporate Pd nanoparticles on Hal-P-Py, Hal-P-Py (6 g) was
added in toluene (20 mL) containing Pd (0.18 g). The resulting mixture
was then stirred for 24 h. To reduce Pd (II) to Pd (0), a solution of NaBH4
in a mixture of toluene and methanol (10 mL, 0.2 N) was applied. In
detail, the reducing agent was added to the Pd(II)@Hal-P-Py in a
dropwise manner and then the mixture was stirred for 24 h. Finally,
the solid was filtered, washed with toluene and dried at 70 °C. Schematic
representation of the procedure for the synthesis of the catalyst is
illustrated in Fig. 1. For the synthesis of the control catalysts, Pd@Hal-
P, Pd@Hal, Pd@P and Pd@P-Py, the same procedure was applied, except
the used support was Hal-P, Hal, P and P-Py respectively.
2. Experimental section
2.1. Materials
The chemicals used for the preparation of Pd@Hal-P-Py and
studying its catalytic activity included Hal, 3- (tri-methoxysilyl) propyl
methacrylate, glycidyl methacrylate, 2-amino pyrimidine, potassium
peroxide sulfate (KPS), Pd(OAc)2, toluene, sodium hydroxide, NaBH4,
distilled water, acetonitrile, chloroform and MeOH, all was purchased
from Sigma-Aldrich and used as received.
2.4. Catalytic test: typical procedure for Sonogashira reaction
2.2. Instruments and characterization techniques
A mixture of acetylene (1.2 mmol), halobenzene (1. mmol), catalyst
(1.5 mol%) and K2CO3 (2.0 mmol in 5.0 mL water) in aqueous media
(2:1 mixture of water: EtOH) was heated at 75 °C for appropriate
reaction time. The reaction was monitored by TLC and upon comple-
tion of the reaction, the catalyst was easily filtered, washed with EtOH
several times and dried in oven at 90 °C overnight. On the other hand,
the organic layer in filtrate was extracted with diethyl ether (15 mL).
The desired product was purified by column chromatography over
silica gel (Scheme 1).
Verification of formation of the catalyst was achieved using various
techniques, including TGA, TEM, BET, XRD, FTIR and ICP-AES.
Tecnai microscope (200 kV) was applied for recording TEM images.
To record the images, the catalyst was dispersed in water and the
analysis was performed after evaporation of the solvent. The BET
analysis of the pristine Hal and the catalyst was accomplished by using
BELSORP Mini II instrument. To perform this analysis, the samples
were pre-heated at 100 °C for 3 h. XRD patterns of pristine Hal and
that of the catalyst were recorded by applying a Siemens, D5000. Cu Kα
radiation from a sealed tube. FTIR spectra of fresh and recycled
catalyst, pristine Hal and polymer decorated Hal were obtained by
using PERKIN-ELMER- Spectrum 65 instrument. Thermogravimetric
analysis (TGA) was carried out under N2 atmosphere over the range of
50–800 °C by using METTLER TOLEDO thermogravimetric analysis
apparatus with heating rate of 10 °C min−1. The content of Pd in the
catalyst and amount of Pd leaching upon catalyst recycling were
estimated by using ICP analyzer (Varian, Vista-pro).
3. Result and discussion
3.1. Catalyst characterization
In Fig. 2 the morphology of Pd@Hal-P-Py and Hal are illustrated.
According to the literature [35], pristine Hal exhibited tubular mor-
phology. The comparison of two TEM images showed that the
morphology of the catalyst is distinguished from that of pristine Hal.
In the TEM images of Pd@Hal-P-Py, apart from dark short tubes,
polymeric sheet can be observed. Moreover, the dark small spots are
indicative of Pd nanoparticles. Noteworthy, the average Pd particle size
was estimated to be 5 nm.
2.3. Synthesis of the catalyst
2.3.1. Functionalization of Hal with 3-(trimethoxysilyl)propyl
methacrylate: synthesis of Hal-A
The formation of Pd@Hal-P and Pd@Hal-P-Py was investigated by
recording FTIR spectra of both materials and comparing them with
that of Hal, Fig. 3. According to the literature [33], the pristine Hal
characteristic bands included the bands at 3697 cm−1 and 3623 cm−1
(inner surface -OH and inner -OH groups), 540 cm−1 (Al-O-Si vibra-
tion), 1035 cm−1 (Si–O stretching) as well as the bands at 2928, 1698,
1635 cm−1. The FTIR spectra of both Pd@Hal-P and Pd@Hal-P-Py
showed the characteristic bands of Hal, implying that the Hal structure
did not collapse upon polymerization process and Pd immobilization.
The precise comparison of these two spectra with that of pristine Hal
revealed that in Pd@Hal-P and Pd@Hal-P-Py spectra, an additional
band at 1463 cm−1 is emerged that can be attributed to the –C˭C
functional groups of the polymeric backbone. Moreover, the observed
band at 1726 cm−1 can be assigned to the esteric (-C˭O) functionalities
Hal was first surface functionalized with 3- (tri-methoxysilyl)
propyl methacrylate. To this purpose, Hal (5 g) and 3- (tri-methox-
ysilyl) propyl methacrylate (4.5 mL) were added into toluene (60 mL)
and the mixture was ultrasounded for 20 min. Then, the resulting well-
dispersed suspension was refluxed at 125 °C for 24 h. After that, the
obtained precipitate was washed with toluene for three times and dried
at 80 °C overnight.
2.3.2. Decoration of Hal surface with polyglycidyl methacrylate:
synthesis of Hal-P
Hal-A (3 g) was suspended in distilled water under ultrasonic
irradiation for half an hour. Then, glycidyl methacrylate monomer
60

S. Sadjadi et al.
Journal of Solid State Chemistry 271 (2019) 59–66
Fig. 1. Schematic representation of the procedure for the synthesis of the catalyst.
in the polymer. The quantitative studies also showed that the content of
–C˭C in the FTIR spectrum of Pd@Hal-P-Py is higher than that of Pd@
Hal-P, confirming the conjugation of 2-amino pyrimidine moiety.
In the following, nitrogen adsorption–desorption isotherm of Pd@
Hal-P-Py was recorded, Fig. 4a, and compared with that of pristine Hal
(Fig. 4b). The results established that the type of two isotherms were
Scheme 1. Sonogashira reaction.
Fig. 2. a: TEM images of the catalyst and b: the TEM image of the pristine Hal.
61

S. Sadjadi et al.
Journal of Solid State Chemistry 271 (2019) 59–66
Fig. 5. Thermograms of the catalyst and pristine Hal.
Fig. 3. FTIR spectra of the catalyst, pristine Hal and Hal-P.
similar, type II isotherms with H3 hysteresis loops. The comparison of
the specific surface area of Pd@Hal-P-Py and that of pristine
Hal, however, demonstrated that the specific surface area of the
catalyst (5.2 m2 g−1) was significantly lower than that of pristine Hal
(51 m2 g−1), indicating that large portion of exterior surface of Hal is
covered by the polymer.
To further characterize Pd@Hal-P-Py, ICP analysis was exploited.
In this line, the sample for ICP analysis was prepared by digesting the
polymeric moiety in the concentrated acidic solution. The result
indicated low Pd loading of the catalyst (0.5 wt%).
Next, the thermogram of the catalyst was obtained and compared
with that of pristine Hal, Fig. 5. The comparison of these two
thermograms revealed that Pd@Hal-P-Py exhibited more mass losses
steps and its thermal stability was lower than that of pristine Hal. In
detail, thermogram of the catalyst not only showed the mass losses at
150 °C and 550 °C that are due to the loss of the structural water and
dehydroxylation of the Hal matrix respectively [36,37], but also
exhibited an additional mass loss step at 310 °C that can be attributed
to the degradation of polymer and organosilane. Comparing two
thermograms, the content of the organic moiety was calculated to be
~30 wt%.
To elucidate whether Hal structure would be preserved upon
introduction of polymer and Pd immobilization, the XRD pattern of
pristine Hal was compared with that of Pd@Hal-P and Pd@Hal-P-Py,
Fig. 6. As depicted in both XRD patterns of Pd@Hal-P and Pd@Hal-P-
Py, the characteristic bands of pristine Hal (2θ = 14.3°, 22.0°, 26.6°,
28.6°, 37.1°, 40.4°, 57.5° and 64.7° (JCPDS No. 29–1487)) [38,39] can
be observed, implying that the structure of Hal in both samples was
preserved. It can also be found out that in both XRD patterns, the
position of the characteristic bands are similar and no displacement
and shift is observable, indicating that the interlayer distance remained
intact. This observation can confirm that the polymer and Pd nano-
particles were located on the exterior surface of Hal nanotubes.
Notably, in the comparison of the XRD patterns of the catalyst and
that of pristine Hal showed that the intensities of the characteristic
bands of Hal in the XRD pattern of the catalyst decreased significantly.
Fig. 4. a: N2 adsorption-desorption isotherm of the catalyst and b: the N2 adsorption-
desorption isotherm of Hal.
Fig. 6. XRD patterns of pristine Hal, Hal-P and the catalyst.
62

S. Sadjadi et al.
Journal of Solid State Chemistry 271 (2019) 59–66
Table 1
Optimization of reaction conditions in the Sonogashira coupling reaction of iodobenzene with phenylacetylenea.
Entry
Pd loading wt%
Pd average particle size (nm)
Catalyst loading (mol%)
Condition (solvent/temperature °C)
Time (h:min)
Base
Yieldb (%)
1
2
3
4
5
6
7
8
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.2
0.3
0.7
0.2
0.3
0.7
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
1
H2O:EtOH (2:1)/75
H2O:EtOH (4:1)/75
H2O:EtOH (1:2)/75
H2O:EtOH (1:4)/75
H2O
EtOH
Toluene
CH3CN
H2O:EtOH (2:1)/100
H2O:EtOH (2:1)/25
H2O:EtOH (2:1)/50
H2O:EtOH (1:4)/75
H2O:EtOH (1:4)/75
H2O:EtOH (1:4)/75
H2O:EtOH (2:1)/75
H2O:EtOH (2:1)/75
H2O:EtOH (2:1)/75
H2O:EtOH (2:1)/75
H2O:EtOH (2:1)/75
H2O:EtOH (2:1)/75
H2O:EtOH (2:1)/75
H2O:EtOH (2:1)/75
H2O:EtOH (2:1)/75
1:15
1:25
1:10
1:10
1:30
1:10
1:40
1:30
1:10
1:45
1:30
2
1:35
1:15
1:34
1:30
1:15
2
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KOH
NaOH
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
90
80
85
84
80
86
75
80
83
71
78
75
80
90
82
86
90
79
82
88
81
88
86
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
2
1.5
1.5
1.5
1.5
1.5
1.5
2
5
4.5
4.8
6.5
4.5
4.8
5.8
1:40
1:15
1:40
1:30
1:15
2
1
a
Reaction condition: aryl halide (1.0 mmol), terminal alkyne (1.2 mmol), base (2 mmol), solvent (5 mL).
Isolated yield.
b
Considering the fact that the content of Pd in the catalyst is relatively
catalytic activity, the Pd particle sizes of the samples were estimated
and compared, Table 1. The results, Table 1, indicated that in the case
of the catalyst with 0.7 wt% Pd loading, the Pd particle size was bigger
than other samples. This observation showed that both Pd content and
Pd average particle size can affect the catalytic activity. In detail, low
amount of Pd resulted in lower catalytic activity, while high amount of
Pd loading resulted in bigger Pd particles and consequently lower
catalytic activity. According to the result, the optimum loading of Pd
was found to be 0.5 wt%.
low and the characteristic bands of Pd are of low intensity [40], in the
XRD pattern of the catalyst that the intensities of the bands are
significantly decreased, the characteristic bands of Pd nanoparticles
were not observable.
3.2. Catalytic activity
In the following, the catalytic activity of the catalyst was studied. To
Next, it was studied whether hybridization of Hal with polymer can
improve the catalytic activity of the final catalyst. In this line, two
control catalysts, Pd@Hal and Pd@P-Py were synthesized and their
catalytic activities for the model Sonogashira reaction were compared
with that of the catalyst, Table 2. The results indicated that both control
catalysts exhibited moderate catalytic activities that were inferior to
that of the catalyst, confirming that hybridization of Hal and P-Py could
improve the catalytic activity. This observation can indicate the
synergism between polymer and clay. Next, it was investigated whether
incorporation of 2-amino pyrimidine (Py) can affect the catalytic
activity. In this line, two other control catalysts, Pd@Hal-P and Pd@
P, were synthesized. The comparison of the catalytic activity of Pd@
Hal-P for the model reaction was compared with that of the catalyst,
Table 2. The result proved the superior catalytic activity of the latter.
Moreover, the comparison of the catalytic activities of Pd@P and Pd@
P-Py showed that Pd@P-Py exhibited higher catalytic activity. These
results indicated that the presence of Py can slightly improve the
catalytic activity of the final catalyst. According to the literature [41], as
introduction of Py in the backbone of the catalyst can increase the
this purpose,
a Pd-catalyzed reaction, Sonogashira C-C coupling
reaction, was selected and the catalytic activity of Pd@Hal-P-Py for
promoting a typical Sonogashira reaction (reaction of iodobenzene and
phenyl acetylene) in the presence of K2CO3 as base, catalytic amount of
the catalyst (1.5 mol%) at 75 °C in aqueous media (mixture of water
and EtOH) was studied. The result showed that in this condition, the
desired product was furnished in high yield (90%). Notably, increase of
the amount of the catalyst did not affect the yield of the reaction,
Table 1. Moreover, examining various solvents including water,
acetonitrile and toluene, confirmed that the mixture of water and
EtOH was the solvent of the choice, Table 1. Having optimum reaction
condition in hand, the effect of Pd loading was also studied. In this line,
several control samples with Pd loadings of 0.2, 0.3 and 0.5 wt% were
synthesized (the loading of Pd was verified by using ICP) and their
catalytic activities for promoting the model reaction under optimum
reaction condition was studied, Table 1. The results indicated that very
low loading of Pd led to the longer reaction times and lower yields of
the desired product, while use of higher Pd loading did not have a
positive effect on the catalytic activity of the catalyst and the catalyst
with 0.7 wt% Pd showed slightly lower catalytic activity compared to
that of the catalyst with 0.5 wt% Pd loading. On the other hand, the
results of use of higher quantities of the catalysts with lower Pd loading
(0.2 and 0.3 wt%) confirmed that although higher amount of those
catalysts improved the catalytic activities, the observed catalytic
activities were lower than that of the catalyst with 0.5 wt% Pd loading.
Moreover, the result showed that use of lower content of the catalyst
with high Pd loading (0.7 wt%) resulted in lower yield of the desired
product. These results proved that loading of Pd is not the only
determining factor on the catalytic activity. As Pd particle size can be
considered as a crucial parameter on the catalytic activity and affect the
Table 2
Comparison of the catalytic activities of the control catalysts with that of the catalyst.
Entry
Catalyst
Reaction time (h:min)
Yield (%)b
1
2
3
4
5
Pd@Hal
Pd@P
Pd@P-Py
Pd@Hal-P
Pd@Hal-P-Py
2
2
70
60
64
82
90
1:30
1.:15
1:15
b
Isolated Yield.
63

S. Sadjadi et al.
Journal of Solid State Chemistry 271 (2019) 59–66
Table 3
Pd@Hal-P-Py catalyzed Sonogashira reaction of various halides with terminal alkynesa [41].
1
2
Entry
R
R
X
Time (h:min)
Yield (%)
1
2
3
4
5
6
7
8
H
p-Me
Ph
Ph
Ph
Ph
Ph
Ph
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
Ph
I
I
I
I
I
I
I
I
I
I
I
I
Br
Cl
1:15
2:25
2:25
2:10
2:15
2:20
1:50
1:50
2:30
2:00
2:15
2:20
3:15
3:25
90
85
85
82
81
80
85
82
95
75
85
80
70
60
p-OMe
p-COMe
p-NO2
1-naphtalene
H
p-OMe
p-Me
p-COMe
p-NO2
1-naphtalene
H
9
10
11
12
13
14
H
Ph
a
Reaction condition: aryl halide (1.0 mmol), terminal alkyne (1.2 mmol), Pd@Hal-P-Py (1.5 mol%), K2CO3 (2.0 mmol) in water:EtOH (5.0 mL) at 75 °C.
number of heteroatoms, especially nitrogen atoms, it can increase of
the electrostatic interaction of Pd nanoparticles and the support and
consequently anchor Pd nanoparticles more effectively [15]. To verify
this assumption, the Pd loadings of Pd@Hal-P and Pd@Hal-P-Py were
calculated via ICP analysis. The results showed that the Pd loading in
the catalyst (0.5 wt%) was slightly higher than that of the Pd@Hal-P
(0.46 wt%).
Confirming high catalytic activity of the catalyst for the model
Sonogashira reaction, the generality of this protocol was examined for
other aryl halides and terminal alkynes, Table 3. As tabulated, various
substrates with diverse range of electronic and steric features could
undergo the reaction to furnish the desired products in relatively short
reaction time and high yields. As shown, the reaction of aromatic
alkynes was more efficient than aliphatic ones. Moreover, use of aryl
iodide led to the higher yields compared to aryl chloride and bromides.
This is due to the lower activities of aryl chloride and bromides.
Noteworthy, among various aryl iodide, the ones with electron with-
drawing groups and lower steric hindrance resulted in the higher
yields.
cost-effective base at relatively low reaction temperature to afford the
product in short reaction time. Notably, compared to the homogeneous
catalysts, Pd@Hal-P-Py is recyclable.
3.3. Catalyst recyclability
The final part of this study was devoted to the investigation of the
recyclability of the catalyst and studying the efficiency of Hal-P-Py
support for suppressing Pd leaching. As described in the Experimental
section, the recovered catalyst was washed, dried and used for the next
consecutive reaction run. The results of the recycling tests, Fig. 7,
showed that upon the second reaction run, the catalyst preserved its
catalytic activity, while further recycling up to sixth run led to slight
decrease of the catalytic activity. Upon the seventh recycling, a
significant loss of the catalytic activity was observed.
To study the reason behind the loss of catalytic activity at seventh
reaction run and elucidate whether recycling could triggered Pd
leaching, the catalyst recycled for seven runs was subjected to the
ICP analysis. The result indicated that the Pd leaching for the catalyst
recycled for seven runs was considerable.
In the following the merit of this protocol was studied by compar-
ison of the catalytic activity of Pd@Hal-P-Py with some of the
previously reported catalytic protocols, Table 4. As illustrated, the
yield of the desired product using Pd@Hal-P-Py is higher or compara-
tive to the tabulated catalysts. Moreover, using this catalyst,
the reaction could proceed in aqueous media in the presence of
To further characterize the recycled catalyst, the TEM image of the
recycled catalyst after seven reaction run was recorded, Fig. 8.
Comparing the TEM images of the fresh and recycled catalysts, it can
be concluded that recycling did not cause significant aggregation of Pd
nanoparticles.
Table 4
Comparison of catalytic activity of the present catalyst with the other reported catalysts in the Sonogashira coupling reaction between iodobenzene and phenylacetylene.
Entry
Catalyst
Reaction conditions
Yield (%)
Time (h:min)
[Ref]
1
2
3
4
5
6
7
8
9
Pd@Hal-P-Py
Pd(OAc)2
NiCl2·6H2O
Pd-LHMS-3
Pd/MgLa
NHC-palladium complex
Fe3O4@SiO2/Schiff base/Pd(II)
I-Pd
H2O: EtOH (2:1)/75 °C
MeCN/DABCO/air
n-Bu4-NBr/NaOH/EG/120 °C
Hexamine/H2O/Reflux
DMF/Et3N/80 °C
DMSO/K3PO4/100 °C
DMF/K2CO3/90 °C
CH3CN/Et3N/Reflux
DMF/piperidine/110 °C
DMF/K2CO3/120 °C
90
100
86
85
90
81
93
80
83
93
1.15
18
2
10
10
1
This work
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
1
24
24
1:25
Nano Pd@Fe3O4
polymeric N-heterocyclic carbene–Pd complex-grafted silica
10
[50]
64

S. Sadjadi et al.
Journal of Solid State Chemistry 271 (2019) 59–66
References
[1] M. Massaro, V. Schembri, V. Campisciano, G. Cavallaro, G. Lazzara, S. Milioto,
R. Noto, F. Parisi, S. Riela, Design of PNIPAAM covalently grafted on halloysite
nanotubes as support for metal-based catalysts, RSC Adv. 6 (2016) 55312–55318.
[2] M. Massaro, C.G. Colletti, G. Lazzara, S. Milioto, R. Noto, S. Riela, Halloysite
nanotubes as support for metal-based catalysts, J. Mater. Chem. A 5 (2017)
13276–13293.
[3] L. Jiang, Y. Huang, T. Liu, Enhanced visible-light photocatalytic performance of
electrospun carbon-doped TiO2/halloysite nanotube hybrid nanofibers, J. Colloid
Interface Sci. 439 (2015) 62–68.
[4] A. Maleki, Z. Hajizadeh, R. Firouzi-Haji, Eco-friendly functionalization of magnetic
halloysite nanotube with SO3H for synthesis of dihydropyrimidinones,
Microporous Mesoporous Mater. 259 (2018) 46–53.
[5] Y. Liu, J. Zhang, H. Guan, Y. Zhao, J.-H. Yang, B. Zhang, Preparation of bimetallic
Cu-Co nanocatalysts on poly (diallyldimethylammonium chloride) functionalized
halloysite nanotubes for hydrolytic dehydrogenation of ammonia borane, Appl.
Surf. Sci. 427 (2018) 106–113.
Fig. 7. The results of recyclability of the catalyst.
[6] M. Massaro, G. Lazzara, S. Milioto, R. Noto, S. Riela, Covalently modified halloysite
clay nanotubes: synthesis, properties, biological and medical applications, J. Mater.
Chem. B 5 (2017) 2867–2882.
[7] L. Gómez, J.L. Hueso, M.C. Ortega-Liébana, J. Santamaría, S.B. Cronin, Evaluation
of gold-decorated halloysite nanotubes as plasmonic photocatalysts, Catal.
Commun. 56 (2014) 115–118.
[8] P. Pasbakhsh, G.J. Churchman, Natural Mineral Nanotubes Properties and
Applications, Taylor & Francis Group, 2015.
[9] K.C. Christoforidis, M. Melchionna, T. Montini, D. Papoulis, E. Stathatos,
S. Zafeiratos, E. Kordoulie, P. Fornasiero, Solar and visible light photocatalytic
enhancement of halloysite nanotubes/g-C3N4 heteroarchitectures, RSC Adv. 6
(2016) 86617–86626.
[10] P. Liu, M. Zhao, Silver nanoparticle supported on halloysite nanotubes catalyzed
reduction of 4-nitrophenol (4-NP), Appl. Surf. Sci. 255 (2009) 3989–3993.
[11] W. Ma, H. Wu, Y. Higaki, A. Takahara, Halloysite nanotubes: green nanomaterial
for functional organic-inorganic nanohybrids, Chem. Rec. 18 (2018) 1–15.
[12] D. Zhao, Z. Cheng, Z. Nan, Synthesis of Co/N-HNTs composites and investigation
on its catalytic activity for H2 generation, Mater. Chem. Phys. 183 (2016) 615–621.
[13] M. Massaro, S. Riela, P. Lo Meo, R. Noto, G. Cavallaro, S. Milioto, G. Lazzara,
Functionalized halloysite multivalent glycocluster as a new drug delivery system, J.
Mater. Chem. B 2 (2014) 7732–7738.
[14] M.M. Massaro, S. Riela, G. Cavallaro, C.G. Colletti, S. Milioto, R. Noto, F. Parisi,
G. Lazzara, Palladium supported on Halloysite-triazolium salts as catalyst for
ligand free Suzuki cross-coupling in water under microwave irradiation, J. Mol.
Catal. A 408 (2015) 12–19.
[15] A.M. Trzeciak, E. Mieczynska, J.J. Ziolkowski, W. Bukowski, A. Bukowsa,
J. Noworo I, J. Okal, Palladium(0) nanoparticles encapsulated in diamine-modified
glycidyl methacrylate polymer (GMA-CHDA) applied as catalyst of Suzuki–Miyaura
cross-coupling reaction, New J. Chem. 32 (2008) 1124–1130.
Fig. 8. TEM image of the recycled catalyst.
Finally, to investigate the nature of the catalysis, the hot filtration
test was performed. The results confirmed that the catalysis was
heterogeneous and Pd leaching and re-disposition on the Hal-P-Py
support did not occur in the course of the reaction.
[16] B. Tamami, S. Ghasemi, Modified crosslinked polyacrylamide anchored Schiff
base–cobalt complex: a novel nano-sized heterogeneous catalyst for selective
oxidation of olefins and alkyl halides with hydrogen peroxide in aqueous media,
Appl. Catal. A 393 (2011) 242–250.
[17] H. Mahdavi, A. Zirakzadeh, J. Amani, Modified cross-linked polyacrylamide
supported palladium salts as a new heterogeneous catalyst for Heck reaction, React.
Funct. Polym. 67 (2007) 716–722.
4. Conclusion
A novel heterogeneous catalyst (Pd@Hal-P-Py) was designed and
prepared through functionalization of Hal with 3- (tri-methoxysilyl)
propyl methacrylate followed by polymerization with glycidyl metha-
crylate, functionalization with 2-amino pyrimidine and Pd immobiliza-
tion. The study of the catalytic activity of Pd@Hal-P-Py for promoting
Sonogashira reaction confirmed high catalytic activity of the catalyst
under mild reaction condition. Moreover, the catalyst showed high
recyclability up to six recycling runs with slight loss of the catalytic
activity and Pd leaching. The study of the role of polymer and 2-amino
pyrimidine in the catalysis was carried out by preparing control
catalysts, i.e. Pd@Hal-P, Pd@Hal, Pd@P and Pd@P-Py, and comparing
their catalytic activities with that of the catalyst. The results indicated
the synergistic effect between polymer and Hal. Moreover, it was found
that incorporation of 2-amino pyrimidine led to better catalytic activity
and lower Pd leaching that can be due to the electrostatic interaction of
multi-nitrogen atoms in the backbone of P-Py with Pd nanoparticles.
Notably, the hot filtration test indicated the heterogeneous nature of
the catalysis.
[18] B. Tamami, A. Fadavi, Amino group immobilized on polyacrylamide: an efficient
heterogeneous catalyst for the Knoevenagel reaction in solvent-free and aqueous
media, Catal. Commun. 6 (2005) 747–751.
[19] Y. Zhang, X.-Y. Quek, L. Wu, Y. Guan, E.J. Hensen, Palladium nanoparticles
entrapped in polymeric ionic liquid microgels as recyclable hydrogenation catalysts,
J. Mol. Catal. A 379 (2013) 53–58.
[20] T. Ishida, Y. Onuma, K. Kinjo, A. Hamasaki, H. Ohashi, T. Honma, T. Akita,
T. Yokoyama, M. Tokunaga, M. Haruta, Preparation of microporous polymer-
encapsulated Pd nanoparticles and their catalytic performance for hydrogenation
and oxidation, Tetrahedron 70 (2014) 6150–6155.
[21] H. Hagio, M. Sugiura, S. Kobayashi, Immobilization of a platinum catalyst using the
polymer incarcerated (PI) method and application to catalytic reactions, Synlett
(2005) 0813–0816.
[22] M. Massaro, C.G. Colletti, G. Buscemi, S. Cataldo, S. Guernelli, G. Lazzara,
L.F. Liotta, F. Parisi, A. Pettignano, S. Riela, Palladium nanoparticles immobilized
on halloysite nanotubes covered by a multilayer network for catalytic applications,
New J. Chem. 42 (2018) 13938–13947.
[23] S. Sadjad, M. Atai, Ternary hybrid system of halloysite nanotubes, polyacrylamides
and cyclodextrin: an efficient support for immobilization of Pd nanoparticles for
catalyzing coupling reaction, Appl. Clay Sci. 153 (2018) 78–89.
[24] B.-N. Lin, S.-H. Huang, W.-Y. Wu, C.-Y. Mou, F.-Y. Tsai, Sonogashira reaction of
aryl and heteroaryl halides with terminal alkynes catalyzed by a highly efficient and
recyclable nanosized MCM-41 anchored palladium bipyridyl complex, Molecules
15 (2010) 9157–9173.
[25] R. Chinchilla, C. Nájera, Recent advances in Sonogashira reactions, Chem. Soc. Rev.
40 (2011) 5084–5121.
[26] R.L. Oliveira, W. He, R.J.M.K. Klein Gebbink, K.P. de Jong, Palladium nanopar-
ticles confined in thiolfunctionalized ordered mesoporous silica for more stable
Heck and Suzuki catalysts, Catal. Sci. Technol. 5 (2015) 1919–1928.
[27] Y. Wang, A.V. Biradar, C.T. Duncan, T. Asefa, Silica nanosphere-supported shaped
Pd nanoparticles encapsulated with nanoporous silica shell: efficient and recyclable
Acknowledgment
The authors appreciate partial financial supports from Iran Polymer
and Petrochemical Institute and Payame Noor University.
65

S. Sadjadi et al.
Journal of Solid State Chemistry 271 (2019) 59–66
nanocatalysts, J. Mater. Chem. 20 (2010) 7834–7841.
[28] Y. Uozumi, F. Sakurai, The Suzuki–Miyaura coupling using a rasta resin supported
Pd catalyst, Synfacts 8 (2012) 0574.
[29] S. Gao, N. Zhao, M. Shu, S. Che, Palladium nanoparticles supported on MOF-5: a
highly active catalyst for a ligand- and copper-free Sonogashira coupling reaction,
Appl. Catal. A 388 (2010) 196–201.
[30] B. Basu, S. Paul, An improved preparation of mesoporous silica-supported Pd as
sustainable catalysts for phosphine-free Suzuki–Miyaura and Heck coupling
reactions, Appl. Organomet. Chem. 27 (2013) 588–594.
[31] N. Bahri-Laleh, S. Sadjad, A. Poater, Pd immobilized on dendrimer decorated
halloysite clay: computational and experimental study on the effect of dendrimer
generation, Pd valance and incorporation of terminal functionality on the catalytic
activity, J. Colloid Interface Sci. 531 (2018) 421–432.
[32] S. Sadjad, M. Akbari, E. Monflier, M.M. Heravi, B. Leger, Pd nanoparticles
immobilized on halloysite decorated with cyclodextrin modified melamine-based
polymer: a promising heterogeneous catalyst for hydrogenation of nitroarenes, New
J. Chem. 42 (2018) 15733–15742.
[40] S. Sadjadi, G. Lazzara, M. Malmir, M.M. Heravi, Pd nanoparticles immobilized on
the poly-dopamine decorated halloysite nanotubes hybridized with N-doped porous
carbon monolayer: a versatile catalyst for promoting Pd catalyzed reactions, J.
Catal. 366 (2018) 245–257.
[41] S. Sadjadi, N. Bahri-Laleh, CuI@amine-functionalized halloysite as an efficient
heterogeneous catalyst for promoting A3 coupling reaction under ultrasonic
irradiation: a combination of experimental and DFT simulation, J. Porous Mater.
25 (2018) 821–833.
[42] J.-H. Li, Y. Liang, Y.-X. Xie, Efficient palladium-catalyzed homocoupling reaction
and sonogashira cross-coupling reaction of terminal alkynes under aerobic condi-
tions, J. Org. Chem. 70 (2005) 4393–4396.
[43] N. Nowrouzi, M. Zarei, NiCl2·6H2O: an efficient catalyst precursor for phosphine-
free Heck and Sonogashira cross-coupling reactions, Tetrahedron 71 (41) (2015)
7847–7852.
[44] A. Modak, J. Mondal, A. Bhaumik, Pd-grafted periodic mesoporous organosilica: an
efficient heterogeneous catalyst for Hiyama and Sonogashira couplings, and
cyanation reactions, Green Chem. 14 (10) (2012) 2840–2855.
[33] S. Sadjad, G. Lazzara, M. Malmir, M.M. Heravi, Pd nanoparticles immobilized on
the poly-dopamine decorated halloysite nanotubes hybridized with N-doped porous
carbon monolayer: a versatile catalyst for promoting Pd catalyzed reactions, J.
Catal. 366 (2018) 245–257.
[34] S. Sadjad, M.M. Heravi, M. Malmir, Pd@HNTs-CDNS-g-C3 N4: a novel hetero-
geneous catalyst for promoting ligand and copper-free Sonogashira and Heck
coupling reactions, benefits from halloysite and cyclodextrin chemistry and g-C3N4
contribution to suppress Pd leaching, Carbohydr. Polym. 186 (2018) 25–34.
[35] Z. Zhou, J. Ouyang, H. Yang, A. Tang, Three-way catalytic performances of Pd loaded
halloysite-Ce0.5Zr0.5O2 hybrid materials, Appl. Clay Sci. 121–122 (2016) 63–70.
[36] S. Bordeepong, D. Bhongsuwan, T. Pungrassami, T. Bhongsuwan, Characterization
of halloysite from Thung Yai District, Nakhon Si Thammarat Province, in Southern
Thailand, Songklanakarin, J. Sci. Technol 33 (2011) 599–607.
[37] L. Zatta, J.E.F. da Costa Gardolinski, F. Wypych, Raw halloysite as reusable hetero-
geneous catalyst for esterification of lauric acid, Appl. Clay Sci. 51 (2011) 165–169.
[38] H. Zhu, M.L. Du, M.L. Zou, C.S. Xu, Y.Q. Fu, Green synthesis of Au nanoparticles
immobilized on halloysite nanotubes for surface-enhanced Raman scattering
substrates, Dalton Trans. 41 (2012) 10465–10471.
[39] P. Yuan, P.D. Southon, Z. Liu, M.E.R. Green, J.M. Hook, S.J. Antill, C.J. Kepert,
Functionalization of halloysite clay nanotubes by grafting with γ-aminopropyl-
triethoxysilane, J. Phys. Chem. C 112 (2008) 15742–15751.
[45] A. Cwik, Z. Hell, F. Figueras, A copper-free Sonogashira reaction using a Pd/MgLa
mixed oxide, Tetrahedron Lett. 47 (18) (2006) 3023–3026.
[46] L. Yang, Y. Li, Q. Chen, Y. Du, C. Cao, Y. Shi, G. Pang, Sonogashira/hydroarylation
sequential reactions: catalyzed by NHC–Pd complexes, Tetrahedron 69 (25) (2013)
5178–5184.
[47] M. Esmaeilpour, J. Javidi, F.N. Dodeji, M.M. Abarghoui, M(II) Schiff base
complexes (M = zinc, manganese, cadmium, cobalt, copper, nickel, iron, and
palladium) supported on superparamagnetic Fe3O4@SiO2 nanoparticles: synthesis,
characterization and catalytic activity for Sonogashira–Hagihara coupling reac-
tions, Transit. Met. Chem. 39 (7) (2014) 797–809.
[48] V. Polshettiwar, P. Hesemann, J.J.E. Moreau, Palladium containing nanostructured
silica functionalized with pyridine sites: a versatile heterogeneous catalyst for
Heck, Sonogashira, and cyanation reactions, Tetrahedron 63 (29) (2007)
6784–6790.
[49] J. Chung, J. Kim, Y. Jang, S. Byun, T. Hyeon, B.M. Kim, Heck and Sonogashira
cross-coupling reactions using recyclable Pd–Fe3O4 heterodimeric nanocrystal
catalysts, Tetrahedron Lett. 54 (38) (2013) 5192–5196.
[50] B. Tamami, F. Farjadian, S. Ghasemi, H. Allahyari, Synthesis and applications of
polymeric N-heterocyclic carbene palladium complex-grafted silica as a novel
recyclable nano-catalyst for Heck and Sonogashira coupling reactions, New J.
Chem. 37 (7) (2013) 2011–2018.
66