Palladated halloysite hybridized with photo-polymerized hydrogel in the presence of cyclodextrin: An efficient catalytic system benefiting from nanoreactor concept

Article abstract of DOI:10.1002/aoc.4776

Considering the excellent performance of halloysite as a catalyst support and in an attempt to benefit from the concept of nanoreactors in the catalysis, an innovative catalytic system has been designed, in which acrylamide and bis-acrylamide were photo-p

Full text of DOI:10.1002/aoc.4776

Received: 14 September 2018
DOI: 10.1002/aoc.4776
Revised: 23 November 2018
Accepted: 26 November 2018
F U L L P A P E R
Palladated halloysite hybridized with photo‐polymerized
hydrogel in the presence of cyclodextrin: An efficient
catalytic system benefiting from nanoreactor concept
Samahe Sadjadi1
| Mohammad Atai2
1 Gas Conversion Department, Faculty of
Petrochemicals, Iran Polymer and
Petrochemical Institute, PO Box
14975‐112, Tehran, Iran
2 Polymer Science Department, Iran
Polymer and Petrochemical Institute, PO
Box 14975‐112, Tehran, Iran
Considering the excellent performance of halloysite as a catalyst support and in
an attempt to benefit from the concept of nanoreactors in the catalysis, an
innovative catalytic system has been designed, in which acrylamide and bis‐
acrylamide were photo‐polymerized in the presence of palladated halloysite.
The novel precipitation photo‐polymerization method avoided the formation
of an extended polymeric network, but led to the formation of co‐polymer on
the halloysite periphery. The co‐polymer exhibited good swellability in aque-
ous media and formed hydrogel. This hydrophilic environment around cata-
lytic palladated halloysite can be considered as a nanoreactor that can
concentrate the substrate and bring them into the vicinity of the palladated
halloysite. This catalytic system was used for promoting hydrogenation of
hydrophobic nitro arenes in aqueous media. To avoid immiscibility of hydro-
phobic substrates and hydrophilic nature of the nanoreactor, that emerged
from swelling of hydrogel, β‐cyclodextrin (CD) was utilized as phase transfer
agent. The results confirmed high catalytic activity of this catalytic system.
Even highly hydrophobic substrates could tolerate hydrogenation under this
protocol to furnish the corresponding product in high yield. Finally, the contri-
bution of both CD and hydrogel to the catalysis was confirmed. Moreover,
studying the recyclability of the catalyst as well as Pd leaching proved the high
recyclability of the catalyst and low leaching of Pd nanoparticles.
Correspondence
Samahe Sadjadi, Gas Conversion
Department, Faculty of Petrochemicals,
Iran Polymer and Petrochemical Institute,
PO Box 14975‐112, Tehran, Iran.
Email: samahesadjadi@yahoo.com;
s.sadjadi@ippi.ac.ir
Mohammad Atai, Polymer Science
Department, Iran Polymer and
Petrochemical Institute, PO Box
14975‐112, Tehran, Iran.
Email: m.atai@ippi.ac.ir
KEYWORDS
catalyst, halloysite, hydrogel, hydrogenation, photo‐polymerization
1 | INTRODUCTION
One of the mostly used oligosaccharides for catalytic
purposes is β‐cyclodextrin (β‐CD). The extensive use
Hydrogenation of nitro arenes to the corresponding ani-
lines is one of the most important hydrogenation reac-
tions, mostly catalyzed in the presence of Pd‐based
catalysts.[1–4] As Pd is one of the precious metals, its
recovery and recycling are economically important. To
decrease the required amount of Pd and facilitate its
recovery, heterogenation of Pd by using heterogeneous
supports is suggested.[5–11]
of this compound is due to its unique conical scaffold
that provides a hydrophobic cavity and hydrophilic
exterior, biocompatibility, non‐toxic nature, and cost‐
effectiveness.[12] The remarkable feature of CD that
allows development of environmentally benign proce-
dures in aqueous media is its capability for embedding
hydrophobic substrates and transferring them to
the aqueous media.[13–16] Taking advantage of this
Appl Organometal Chem. 2019;e4776.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4776

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SADJADI AND ATAI
feature, various aqueous biphasic catalysts have been
developed.[17,18]
Taking these results into account and considering the
fact that despite the presence of hydroxyl groups on the
surface of Hal, it still has a relatively weak adhesion
with Pd nanoparticles due to the absence of chemical
interaction,[19,27] design and preparation of a novel
Hal‐hydrogel catalytic system for hydrogenation of nitro
arenes was targeted. It is assumed that the polymeric
functionality on Hal not only can result in efficient
immobilization of Pd nanoparticles due to the electro-
static reactions of Pd nanoparticles with many amide
functionalities on the polymer backbone, but also the
nano‐environment aroused from polymer swelling
in aqueous media could effectively concentrate the
substrates and enhance the reaction rate. To circumvent
the immiscibility of hydrophobic substrates and hydro-
philic nature of the nanoreactor and allow the reactions
to be carried out in aqueous media at low temperature,
CD was used as a phase transfer agent. To prepare
the catalyst, Hal was first functionalized with
3‐(trimethoxysilyl) propyl methacrylate and then used
for immobilization of Pd nanoparticles to furnish
Pd/Hal‐V. Subsequently, acrylamide (AAM) and bis‐
acrylamide (BisAAM) were co‐polymerized in the pres-
ence of Pd/Hal‐V via a novel procedure, precipitation
photo‐polymerization. On the contrary of conventional
co‐polymerization methods that led to the formation of
an extended polymeric network, precipitation photo‐
polymerization could control the co‐polymerization pro-
cess in a way that the co‐polymer was formed on the
periphery of Pd/Hal‐V. The catalytic activity of the
resulting hybrid, Pd/Hal‐H, in the presence of β‐CD as
a co‐catalyst, was examined for hydrogenation of nitro
arenes. To establish the contribution of hydrogel and
CD in the catalysis, several model catalysts were pre-
pared, and their catalytic activities compared with that
of the catalyst. Moreover, the recyclability of the catalyst
as well as Pd leaching were investigated.
Use of clays as supports for immobilization of the
catalytic species and development of heterogeneous cat-
alyst is well established.[19] In this line, the utility of
naturally occurring biocompatible halloysite nanoclay,
Hal, that is a dioctahedral 1:1 layered aluminosilicate
[Al2Si2O5(OH)4·nH2O] has attracted growing attention,
and recently many Hal‐based heterogeneous (photo) cat-
alysts with outstanding catalytic performances have
been reported.[20–22] It is worth noting that apart from
individual Hal, various Hal‐based hybrid systems have
also been designed and applied for catalytic pur-
poses.[23–25] Notably, the physical and chemical features
of Hal such as tubular morphology, high chemical and
mechanical stability, inert and non‐toxic nature, oppo-
sitely charged inner and outer surface, and the capability
of surface functionalization render Hal a promising clay
not only in the field of catalysis, but also for many scien-
tific domains such as drug delivery, composites and
polymer science.[25–40]
Hydrogels are water‐swellable synthetic or natural 3D
crosslinked hydrophilic polymeric networks with high
biocompatibility and mechanical strength. These soft
and porous polymeric materials that mostly contain the
hydrophilic moieties such as hydroxyl, sulfonic, carbox-
ylic and amidic functional groups can swell in aqueous
media. However, they are not water soluble due to their
physical and/or chemical properties. Hydrogels are exten-
sively used in a diverse range of applications, ranging
from chemical separation to biomedical engineering and
drug delivery systems.[41–43]
There are a few reports on the Hal‐hydrogel compos-
ites, mostly used for controlled release,[41] reinforcement
of hydrogel[43] and soft tissue engineering.[42] To the best
of our knowledge, there is only one report on the cata-
lytic application of Hal‐hydrogel, in which the covalent
hybrid of thermo‐responsive poly(N‐isopropylacrylamide)
(PNIPAAM) and Hal was used for the immobilization of
Pd nanoparticles and development of Hal‐PNIPAAM/
PdNPs as a catalyst for the Suzuki reaction.[25] It was
believed that due to the hydrophilic nature of the swelled
form of the hydrogel at lower temperature (temperatures
lower than the lower critical solution temperature) it was
unsatisfactory for hydrophobic substrates, and superior
catalytic activity was achieved at elevated temperature.
Following our study on the Hal‐based heterogeneous
catalyst,[23] recently we have developed hybrid systems
composed of Hal and CD. The study of the catalytic activ-
ity of these hybrid systems proved that they could benefit
from the chemistry of both components, and the presence
of CD as a phase transfer agent could increase the rate of
the reactions in aqueous media.[23,24,44]
2 | EXPERIMENTAL
2.1 | Instruments
To characterize the catalyst, Brunauer–Emmett–Teller
analysis, thermogravimetric analysis (TGA), field emis-
sion scanning electron microscopy (FE‐SEM), transmis-
sion electron microscopy (TEM), X‐ray diffraction
(XRD), Fourier transform‐infrared spectroscopy (FT‐IR)
and inductively coupled plasma‐atomic emission spec-
troscopy (ICP‐AES) were employed. To record TEM
images of the catalyst and estimate the average Pd parti-
cle sizes, Tecnai microscope (200 kV) was used. For this
analysis, the catalyst was dispersed in water and, after

SADJADI AND ATAI
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evaporation of the solvent, the TEM images were
recorded. The apparatus used for recording FE‐SEM was
MIRA3 TESCAN. The structure of the catalyst was stud-
ied by using XRD (Siemens, D5000, Cu Kα radiation from
a sealed tube). Moreover, the XRD pattern of the catalyst
was compared with that of the pristine Hal. To verify for-
mation of the catalyst and studying the effect of the
recycling on the structure of the catalyst, the FT‐IR spec-
tra (Spectrum 65, Perkin‐Elmer) of the pristine Hal, and
fresh and recycled catalysts were recorded and compared.
To study the thermal stability of the catalyst and measur-
ing the content of polymer and organosilane, TGA
(Mettler Toledo) was performed under N2 atmosphere
by using a heating rate of 10°C min−1. The textural prop-
erties of the catalyst were studied by recording N2
adsorption–desorption isotherm using BELSORP Mini II
apparatus. To perform this analysis, the catalyst was
pre‐heated at 200°C for 3 hr. Finally, Pd loading and
leaching of the catalyst were calculated using the ICP‐
AES method (Varian, Vista‐pro).
method. Typically, to a stirring suspension of Hal‐V
(1.2 g) in dry toluene (15 mL), Pd (OAc)2 (0.02 g) was
slowly added and the resulting mixture was vigorously
stirred at ambient temperature for 10 hr. The reduction
of Pd (II) to Pd (0) was carried out by using NaBH4 as
reducing agent. More precisely, a mixture of NaBH4 in
toluene and methanol (10 mL, 0.2 N) was added to the
suspension of Pd (II)‐Hal‐V in a dropwise manner, and
the resulting mixture was stirred for 5 hr. After that, the
black solid was filtered, washed with methanol repeatedly
and dried in the oven at 80°C overnight.
2.2.4 | Optimization of initiator
concentration
To find the optimum initiator concentration for the
photo‐polymerization of AAM, different concentrations
of 0.5, 1.0, 1.5 and 2 wt.% of CQ and DMAEMA were used
as photoinitiator system in the AAM solution. The results
showed an increase in the conversion of the AAM by
increasing the initiator concentration up to 1 wt.% with
no further increase in the conversion upon enhancing
the initiator concentration. Therefore, further experi-
ments were performed using CQ and amine at a concen-
tration of 1 wt.% .
2.2 | Materials and methods
2.2.1 | Materials
The chemicals used for synthesis of the catalyst and
performing hydrogenation reaction included AAM and
BisAAM, obtained from Merck (Germany); Hal, nitro
arenes, camphorquinone (CQ), dimethyl aminoethyl
methacrylate (DMAEMA), 3‐(trimethoxysilyl) propyl
methacrylate, Pd (OAc)2, NaBH4, MeOH and toluene,
purchased from Sigma–Aldrich (Germany).
2.2.5 | Optimization of concentration of
the crosslinker
To optimize the concentration of the crosslinker in the
polymerization system, BisAAM was incorporated in
concentrations of 1, 2, 3, 4 and 5 wt.% in the monomer
solution. Having polymerized the AAM and the
crosslinking agent, the product was washed with acetone
and dried, and its swelling behavior in de‐ionized (DI)
water was then examined. It was found that the speci-
mens containing the crosslinker up to 2 wt.% were easily
swollen in the water forming a very‐low‐strength hydro-
gel. At concentrations of 4 wt.% and higher, the reaction
resulted in a polymer network with high crosslink den-
sity, which hardly swelled in DI water. The water uptake
of the crosslinked PAAm specimens containing 2, 3, 4
and 6% BisAAM as crosslinker are summarized in
Table 1.
2.2.2 | Surface functionalization of Hal
with 3‐(trimethoxysilyl) propyl methacry-
late: Hal‐V
Initially, surface modification of Hal was carried out
through our previously reported procedure.[24] Briefly,
3‐(trimethoxysilyl) propyl methacrylate (4 mL) was added
to the well‐dispersed suspension of Hal (2.5 g) in dry tol-
uene (50 mL). The resulting mixture was then refluxed at
110°C overnight. Upon completion of the reaction, the
precipitate was filtered off, washed several times with
dried toluene and dried at 90°C overnight.
The crosslinker at 3 wt.% resulted in a hydrogel with
high water sorption and a discrete structure that is desir-
able as a substrate for the catalyst. Therefore, the catalyst
(palladium‐containing hydrogel) was prepared using 3
wt.% BisAAM as crosslinker.
2.2.3 | Immobilization of Pd nanoparti-
cles: Pd/Hal‐V
Immobilization of Pd nanoparticles on the surface of Hal‐
V was simply accomplished via the wet impregnation

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SADJADI AND ATAI
TABLE 1 The water uptake of the crosslinked PAAm specimens
containing 2, 3, 4 and 6 wt.% BisAAM as crosslinker was deter-
mined according to the method described in Section 2.2.6
BisAAM
wt.%
2
3
4
6
Water uptake Could not be measured as the
730 460 400
(%)
polymer in water formed a
continuous sticky and mostly
sol structure.
The structure is not suitable as
a substrate for the catalyst.
BisAAM, bis‐acrylamide.
2.2.6 | Precipitation photo‐polymerization
of AAM: Synthesis of Pd/Hal‐H
A solution of 25 wt.% of AAM monomer in acetone was
prepared; 3 wt.% of BisAAM (based on the AAM mono-
mer) as crosslinking agent and 20 wt.% of Pd/Hal‐V
(based on the AAM monomer) were added to the solu-
tion. The suspension was stirred for 1 hr, and then
ultra‐sonicated (20 kHz, 100 W) for 10 min with intervals
of 7 s on and 3 s off. Then, 1 wt.% CQ and 1 wt.%
DMAEMA (both based on the AAM monomer) were
added to the suspension in the dark. The mixture was
then inserted into a glass reactor equipped with con-
denser and mechanical stirrer. The solution in the reactor
was then irradiated using a homemade blue light source
for 30 min. The light source consisted of LED lamps with
emission wavelength of 470 ( 5) nm and power of 50 W
(Figure 1). The reactor was irradiated on both sides hav-
ing applied the light sources while the solution was stir-
ring. Considering the fact that acetone is a good solvent
for AAM and BisAMM but a non‐solvent for their co‐
polymer, the polymer chains are precipitated as the reac-
tion is progressed.
FIGURE 1 The photo‐polymerization reaction set up
2.2.7 | General procedure for the hydro-
genation of nitro arenes
Typically, nitro arene (1 mmol), CD (1 wt. %) and catalyst
(2 wt. %) were placed in DI water (2 mL). The reaction
mixture was then heated up to 50°C, and then H2 gas
(1 bar) was purged into the glass reactor under stirring.
The progress of the reaction was traced by using thin‐
layer chromatography. Upon completion of the reaction,
the catalyst was filtered off and the corresponding amine
product was achieved by evaporating water. Considering
the fact that all the products were previously reported,
their identification was carried out by comparing
boiling/melting points and FT‐IR spectra with those of
the authentic sample. To recycle the catalyst, the recov-
ered catalyst was washed with water and dried at 90°C
overnight.
Upon completion of the reaction, the precipitated
solid was washed with fresh acetone and centrifuged to
separate the Pd/Hal‐H. The washing and centrifugation
were repeated three times to remove any unreacted
monomer, crosslinker and initiator system. The polymer
was then dried and used for further investigation. The
process of synthesis of the catalyst has been schematically
presented in Figure 2.
The content of Pd in the structure of the catalyst was
calculated using ICP‐AES analysis. The procedure for
preparing ICP sample included digestion of organic moi-
ety through treatment of a known amount of the catalyst
with concentrated acid solution (mixture of concentrated
nitric and hydrochloric acids). Using this methodology,
the content of Pd was estimated to be 0.1 wt%.
2.2.8 | Catalyst water uptake
To determine the catalyst swelling ratio in water (water
uptake), 0.01 g of the catalyst was placed in a jar and
10 g DI water was added to it. After 24 hr the excess water

SADJADI AND ATAI
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FIGURE 2 The schematic procedure
for the synthesis of the catalyst
was removed by filter paper and the water uptake was
calculated as follows:
w1 − w0
water uptake ðwt:%Þ ¼
× 100
w0
where w0 is the initial weight of the catalyst and w1 is the
weight of the swollen catalyst after removal of excess
water.
3 | RESULTS AND DISCUSSION
3.1 | Catalyst characterization
FIGURE 3 Fourier transform‐infrared (FT‐IR) spectra of pristine
Hal, Hal‐V and the catalyst
Initially, the FT‐IR spectra of Pd/Hal‐H and Hal‐V were
recorded and compared with that of pristine Hal
(Figure 3). As the comparison of the three FT‐IR spectra
reveals, the Hal characteristic bands, i.e. the bands at
1033 cm−1 (Si–O stretching), 540 cm−1 (Al‐O‐Si vibra-
tion), 3697 cm−1 and 3624 cm−1 (internal ‐OH groups),
1645 and 2923 cm−1[28] are observed in the FT‐IR spectra
of both Pd/Hal‐H and Hal‐V, indicating that the Hal
structure is preserved upon incorporation of the silane
group and polymer. Notably, the FT‐IR spectrum of
Hal‐V is very similar to that of pristine Hal. This issue
can be attributed to the low content of V on the Hal‐V
hybrid (as confirmed by TG analysis, vide infra). More-
over, most of the characteristic bands of V (i.e. the bands
related to the –C=C and –Si‐O) overlapped with those of
Hal. However, a small band at 1734 cm−1 can be detected
that can be assigned to –C=O functionality. It is worth
noting that in the FT‐IR spectrum of the catalyst the
intensities of the Hal characteristic bands decreased.
Moreover, an additional band at 1690 cm−1 is seen that
can be assigned to the amidic carbonyl groups in the
polymeric network.
To further characterize the structure of Pd/Hal‐H, the
XRD pattern of the catalyst was obtained and compared
with that of pristine Hal (Figure 4). The XRD pattern of
Hal is well established, and shows peaks at 2θ = 12°,
19.9°, 24.7°, 26.6°, 35.1°, 38.3°, 55° and 62.7° (JCPDS
No. 29–1487).[45,46] As depicted, the characteristic peaks
of Hal can also be detected in the XRD pattern of the cat-
alyst. However, their intensities are decreased. It is worth

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SADJADI AND ATAI
in the course of synthesis of the catalyst was only 20
wt.%, it was expected that similar to previous reports
Pd/Hal‐V was entrapped within the polymeric network.
However, both FE‐SEM and TEM images (Figures 5 and
6) revealed that using the photo‐polymerization proce-
dure, the polymeric network was formed just on the
periphery of the Pd/Hal‐V tubes. More precisely in both
images, the Hal tubes are observable as the main
FIGURE 4 X‐ray diffraction (XRD) patterns of pristine Hal and
the catalyst
noting that no significant shift or change of the interlayer
distance was observed in the XRD pattern of Pd/Hal‐H
compared with that of pristine Hal, implying that the
polymeric chains were grown on the surface of Hal and
form the crosslinked network. It is worth noting that in
the XRD pattern of the catalyst the characteristic peaks
of Pd nanoparticles were not observed. According to the
literature,[47] the low content of Pd (as confirmed via
ICP analysis) and its high distribution can justify this
observation.
Next, the morphology of Pd/Hal‐H was studied by
recording the TEM and FE‐SEM images. According to
the literature, in which composite of Hal and hydrogel
was synthesized via conventional polymerization
method,[41] Hal was dispersed in a vast polymeric net-
work. In this work, as the used amount of the Pd/Hal‐V
FIGURE 6 Transmission electron microscopy (TEM) images of
the catalyst
FIGURE 5 The field emission scanning electron microscopy (FE‐SEM) images of the catalyst

SADJADI AND ATAI
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morphology and the polymers are detected around the
tubes. In other word, in the SEM and TEM images of
the catalyst an extended polymeric network was not seen.
This observation can be attributed to the use of the photo‐
polymerization procedure in the presence of Pd/Hal‐V. In
detail, Pd/Hal‐V that is a grayish powder can absorb the
light and reduce the yield of the photo‐polymerization
reaction. On the other hand, the photo‐polymerization
reaction time was low. Hence, the reaction time and
condition prevented from formation of an extended
polymeric network. Notably, in the TEM image of the
catalyst the Pd nanoparticles are observable. Using TEM
images, the average Pd particle size was estimated to be
5 nm.
To estimate the content of the polymer on the struc-
ture of the Pd/Hal‐H and study its thermal stability, TG
analysis was exploited. In Figure 7, the thermogram of
the catalyst is illustrated and compared with that of pris-
tine Hal and Hal‐V.
The thermogram of pristine Hal is in good accord
with the literature,[48,49] and exhibited two main weight
losses, one (~150°C) assigned to loss of water and another
one (~550°C) related to Hal dehydroxylation. The ther-
mogram of Hal‐V also shows the above‐mentioned
weight losses. Furthermore, an additional weight loss
(~3 wt.% at about 300°C) is observable that can be
assigned to the degradation of organosilane.
Polymer percentage in in the catalyst
À
Á
¼ 100 − 37:7=82:3*100 ¼ 54:2 wt:%
Finally, the nitrogen adsorption–desorption isotherm
of Pd/Hal‐H was recorded to study how hybridization
with hydrogel can affect the surface properties of Hal.
The results confirmed the similarity of the isotherm of
Pd/Hal‐H (Figure 8) with that of the pristine Hal
(Figure S1). More precisely, both isotherms showed type
II isotherms with H3 hysteresis loops.[46] The comparison
of the specific surface area of pristine Hal (51 m2 g−1
)
with Pd/Hal‐H showed that upon introduction of poly-
mer on the surface of Hal, the specific surface area did
not change significantly (the specific surface area of the
catalyst was 54 m2 g−1). This observation can be due to
the porous nature of the polymer. Moreover, the total
pore volume of the catalyst (12.4 cm3) was slightly higher
than that of pristine Hal (11.7 cm3). On the other hand,
the pore diameter of the catalyst (28.6 nm) was remark-
ably larger than that of pristine Hal (5.4 nm). This obser-
vation can show that on the polymeric network both
large and small pores are present. Totally, it seems precip-
itated polymers on the halloysite substrate provide higher
porosity on the surface of the nanoparticles.
The measurement of the water swelling of the catalyst
showed a water uptake of 750 wt.% based on the catalyst,
which would be about 1380 wt.% based on the polymer
part of the catalyst.
It is worth noting that in this work precipitation photo‐
polymerization was used as a fast procedure for formation
of polymer. As in the method, the polymer is precipitated
Similarly, the thermogram of the catalyst exhibits the
weight losses of pristine Hal. Moreover, additional weight
losses related to the organosilane and polymer are
detectable.
According to the thermogram of Hal, about 82.3 wt.%
of the pristine Hal remains at 800°C, and the remaining
ash in the thermogram of the catalyst is 37.7 wt.%. The
polymer percentage of the catalyst is then calculated as
follows:
FIGURE 7 The thermogravimetric analyses (TGA) of pristine
Hal, Hal‐V and the catalyst
FIGURE 8 N2 adsorption–desorption isotherm of the catalyst

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SADJADI AND ATAI
from solvent and is easily separated from the polymeriza-
tion medium, which significantly facilitates the work‐up
step. Furthermore, the method provides a porous texture
on the catalyst substrate, which is of great importance for
the catalytic activity of the final catalyst. Under conven-
tional polymerization methods, Hal is entrapped within
the hydrogel network,[41] while in the precipitation
photo‐polymerization method the formation of hydrogel
can be controlled in a way that it can only be formed
around the Hal tube (as confirmed by TEM). The reason
for employing the photo‐polymeriation method was the
fact that the preliminary catalytic experiments showed
low catalytic activity of Pd/H (vide infra). Moreover, the
accessibility of the substrates to the catalytic active sites
could be limited in a catalyst in which Hal was dispersed
in an extended and bulky hydrogel network.
hydrophobic substrates. More precise comparison of the
catalytic activities of the catalysts showed that the cata-
lytic activity of the catalyst (Pd/Hal‐H) was the highest
compared with that of Pd/Hal‐V and Pd/H. As the con-
tents of Pd in all catalysts were similar and both H and
Hal were hydrophilic, the superior catalytic activity of
Pd/Hal‐H can indicate the possible synergism between
H and Hal.[24]
The comparison of entries 6 and 7, in which different
amounts of CD were used as co‐catalyst, implies that only
a low amount of CD (1 wt.%) was adequate for promoting
the reaction, and further increasing this value to 2 wt.%
did not affect the yield of the reaction.
Next, the effect of Pd content of the catalyst on the
catalytic activity of the catalyst was investigated by pre-
paring the catalysts with different loading of Pd ranging
from 0.03 to 0.2 wt.% (samples with 0.03, 0.1, 0.2 wt.%),
and the comparison of the catalytic activities of the sam-
ples with that of the catalyst showed that there was not a
linear relationship between the Pd content of the catalyst
and the observed catalytic activity, and the best result was
achieved with the catalyst with 0.1 wt.% Pd. As Pd nano-
particles are the main catalytic species for hydrogenation,
the lower catalytic activities of the catalysts with lower
content of Pd than 0.1 wt.% were attributed to the lack
of catalytic active sites for promoting the reaction. In
the case of catalysts with higher content of Pd that
showed lower catalytic activities, it was assumed that
use of a higher amount of Pd could lead to aggregation
of the Pd and a decrease in the catalytic activity. To verify
this assumption, the TEM images of that sample were
recorded (Figure S4) and compared with that of the cata-
lyst. The comparison of the average Pd particle sizes indi-
cated the aggregation of Pd nanoparticles in the case of
use of higher content of Pd, confirming the importance
of the average Pd particle size on the catalysis.
In the following, to elucidate whether this catalyst is
selective toward the nitro group and whether this
protocol can be generalized to the steric and highly
hydrophobic substrates, hydrogenation of two more
substrates, highly hydrophobic 1‐nitronaphthalene and
4‐nitroacetophenone, that possess two functional
groups was investigated (Table 3). As illustrated, the
reactivity of the substrates follows the order of 4‐
nitroacetophenone > nitrobenzene > 1‐nitronaphthalene.
The lower catalytic activity observed in the case of 1‐
nitronaphthalene can be attributed to its steric nature.
In other words, the encapsulation of this substrate into
the cavity of CD and formation of the inclusion complex
is more tedious than small substrates; hence, phase
transferring to the aqueous media is retarded and lower
yield of the desired product is furnished. As shown, two
other substrates, nitrobenzene and 4‐nitroacetophenone,
3.2 | Catalytic activity
To initiate the studies of the catalytic performance of the
catalytic system and the role of hydrogel and CD in the
catalysis, two control catalysts, including Pd/Hal‐V (see
SI) and Pd/H, with similar Pd contents were synthesized
(see SI, Figures S2 and S3). Moreover, one hydrogenation
reaction, hydrogenation of nitrobenzene, was selected as
a model reaction and the optimization of the reaction
condition was carried out (see SI). Subsequently, the com-
parison of the catalytic activity of Pd/Hal‐V, Pd/H and
Pd/Hal‐H in the presence and absence of CD was accom-
plished (Table 2). As tabulated in Table 2, in all cases,
addition of CD led to a significant increase in product
yield. This observation confirms the probability of forma-
tion of an inclusion complex between CD and
TABLE 2 The efficiency of various catalytic systems for hydro-
genation of nitrobenzenea
Catalyst CD
amount amount Time Yield
Entry Catalyst
(wt.%)
(wt.%)
(hr)
(%)b
1
2
3
4
5
6
7
Pd/Hal‐V
Pd/Hal‐V + CD
Pd/H
2
2
2
2
2
2
2
–
1
–
1
–
1
2
2
2
2
2
2
2
2
30
60
40
63
60
95
95
Pd/H + CD
Pd/Hal‐H
Pd/Hal‐H
Pd/Hal‐H
aReaction condition: substrate (1 mmol), catalyst, H2O (2 mL), hydrogen
flow of 50 mL min−1, agitation (800 rpm) at 50°C in 1 hr.
bIsolated yields.
CD, cyclodextrin.

SADJADI AND ATAI
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TABLE 3 Hydrogenation of nitro compound under Pd/Hal‐H+ CD catalysisa
In the presence of CD
Without CD
Time (hr)
Amount of
cat. (wt.%)
Amount of
CD (wt.%)
Entry
Substrate
Time (hr)
Yield (%)b
Yield (%)b
1
2
3
Nitrobenzene
2
2
2
1
1
1
2
3
3
95
85
98
2
3
3
60
60
70
1‐Nitronaphthalene
4‐Nitroacetophenone
aReaction condition: substrate (1 mmol), catalyst (2 wt.%), H2O (2 mL), hydrogen flow of 50 mL min−1, agitation (800 rpm) at 50°C.
bIsolated yields.
CD, cyclodextrin.
exhibited comparative yields, indicating that the presence
of other comparative functional groups (i.e. carbonyl)
on the aromatic ring did not affect the catalytic activity
and the catalyst was totally selective toward the nitro
group and hydrogenation of the carbonyl group did not
take place.
recycling, however, the slope of the loss of catalytic activ-
ity was higher and, upon the eighth recycling, significant
catalytic activity loss was observed.
To elucidate whether the catalyst preserved its struc-
ture in the course of recycling, the FT‐IR spectrum of
the recycled Pd/Hal‐H was recorded (Figure 10). As
depicted in Figure 10, the FT‐IR spectrum of the recycled
catalyst showed the characteristic bands of the fresh one.
This observation indicated that the structure of the cata-
lyst was not diminished in the course of recovery and
recycling. However, as can be seen in Figure 10, the
intensities of the bands in two peaks are different, and
some broadening is observed in the FT‐IR spectrum of
the recycled catalyst.
Next, the Pd leaching of Pd/Hal‐H was measured by
using ICP. It is worth noting that the leaching of the Pd
nanoparticles for the first five runs of the reaction was
negligible (2.3 wt.% Pd content of the catalyst) and then
slowly increased, and upon the eighth recycling signifi-
cant Pd leaching (5.7 wt.% Pd content of the catalyst)
was observed.
3.3 | Catalyst recyclability
The final features of the catalyst that were studied were
catalyst recyclability and Pd leaching. As described in
the Experimental section, after completion of the first
reaction run of hydrogenation of nitrobenzene, the cata-
lyst was separated, washed and dried. Then, the recov-
ered catalyst was applied for promoting the next run of
the reaction under similar reaction conditions. The
recycling was repeated for eight consecutive reaction
runs, and the yield of aniline (model product) for each
cycle was measured and compared with that of fresh cat-
alyst. In Figure 9, the results of recycling experiments are
presented. As depicted, for the first five recycling experi-
ments, the catalyst showed high recyclability and cata-
lytic activity decreased by only 10%. Upon further
Finally, the TEM analysis of the recycled catalyst was
performed to elucidate whether recycling could provoke
any change in the morphology and Pd particle size
(Figure 11). As shown in Figure 11, the Hal tubes in the
recycled catalyst are observable and the morphology of
the catalyst is similar to that of the catalyst. Hence, it
can be concluded that recycling did not cause significant
FIGURE 10 The comparison of the Fourier transform‐infrared
(FT‐IR) spectra of fresh and recycled catalyst
FIGURE 9 The results of catalyst recyclability

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SADJADI AND ATAI
ORCID
Samahe Sadjadi https://orcid.org/0000-0002-6884-4328
REFERENCES
[1] T. Sugimura, K. Wakabayashi, H. Nakagama, M. Nagao, Can-
cer Sci. 2004, 95, 290.
[2] H. U. Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner, M.
Studer, Adv. Synth. Catal. 2003, 345, 103.
[3] M. Cargnello, V. V. T. Doan‐Nguyen, T. R. Gordon, R. E. Diaz,
E. A. Stach, R. J. Gorte, P. Fornasiero, C. B. Murray, Science
2013, 341, 771.
[4] G. Sevjidsuren, S. Zils, S. Kaserer, A. Wolz, F. Ettingshausen,
D. Dixon, A. Schoekel, C. Roth, P. Altantsog, D. Sangaa, C.
Ganzori, J. Nanomater. 2010, 2010, 1.
FIGURE 11 Transmission electron microscopy (TEM) image of
the recycled catalyst
[5] P. G. N. Bouchenafa‐Saib, P. Verhasselt, F. Addoun, V. Dubois,
Appl. Catal. A Gen. 2005, 286, 167. Appl. Catal. A Gen. 2005,
286, 167
morphology change. Regarding the Pd nanoparticle aver-
age size, it was found that recycling several times could
lead to slight aggregation of Pd nanoparticles and an
increase of Pd particle size to 8 nm.
[6] X. Yu, M. Wang, H. Li, Appl. Catal. A Gen. 2000, 202, 17.
[7] R. Giordano, P. Serp, P. Kalck, Y. Kihn, J. Schreiber, C. Marhic,
J.‐L. Duvail, Eur. J. Inorg. Chem. 2003, 2003, 610.
[8] C.‐H. Li, Z.‐X. Yu, K.‐F. Yao, S.‐F. Ji, J. Liang, J. M. Cat. A
Chem. 2005, 226, 101.
4 | CONCLUSION
[9] P. Sangeetha, K. Shanthi, K. S. Rama Rao, B. Viswanathan, P.
Selvam, Appl. Catal. A Gen. 2009, 353, 160.
Taking advantage of the swellability of hydrogels that
form a hydrophilic nano‐environment and the capability
of CD for formation of inclusion complex with hydropho-
bic substrates and serving as a phase transfer agent, as
well as the high performance of Hal, a novel catalytic
system was prepared through precipitation photo‐
polymerization of AAM and BisAAM in the presence of
palladated halloysite. The characterization of the catalyst
confirmed that using the photo‐polymerization method, a
polymeric sheath was formed on the surface of Hal that
highly swelled in water. Investigation of the catalytic
activity of the catalyst for the hydrogenation of nitro
arenes in the presence of CD as phase transfer agent con-
firmed the high activity and selectivity of the catalyst.
Moreover, the contribution of both hydrogel and CD to
the catalysis was proved. Notably, Pd/Hal‐H exhibited
high recyclability and low Pd leaching.
[10] N. Arora, A. Mehta, A. Mishra, S. S. Basu, App. Clay Sci. 2018,
151, 1.
[11] Y. Chen, L. Li, L. Zhang, J. Han, Colloid Polymer Sci. 2018, 296,
567.
[12] S. Manivannan, R. Ramaraj, Chem. Eng. J. 2012, 210, 195.
[13] S. Noël, B. Légera, A. Ponchel, F. Hapiot, E. Monflier, Chem.
Eng. Trans. 2014, 37, 337.
[14] L. Bai, F. Wyrwalski, J. F. Lamonier, A. Y. Khodakov, E.
Monflier, A. Ponchel, Appl. Catal. B 2013, 381, 138.
[15] A. Gogoi, K. C. Sarma, Mater. Chem. Phys. 2017, 194, 327.
[16] S. Menuel, B. Léger, A. Addad, E. Monflier, F. Hapiot, Green
Chem. 2016, 18, 5500.
[17] F. Hapiot, H. Bricout, S. Menuel, S. Tilloy, E. Monflier, Catal.
Sci. Technol. 2014, 4, 1899.
[18] F. Hapiot, E. Monflier, Catalyst 2017, 7, 173.
[19] Y. Liu, H. Guan, J. Zhang, Y. Zhao, J.‐H. Yang, B. Zhang, Int. J.
Hydrogen Energy 2018, 43, 2754.
ACKNOWLEDGEMENT
[20] Y. Liu, J. Zhang, H. Guan, Y. Zhao, J.‐H. Yang, B. Zhang, Appl.
Surf. Sci. 2018, 427, 106.
The authors appreciate partial financial supports from
Iran Polymer and Petrochemical Institute. S.S. fully
appreciates Prof. Heravi's support.
[21] A. Maleki, Z. Hajizadeh, R. Firouzi‐Haji, Microporous Mesopo-
rous Mater. 2018, 259, 46.
[22] N. Sahiner, S. Butun Sengel, Fuel Sci. Technol. 2017, 158, 1.
[23] S. Sadjad, M. M. Heravi, M. Malmir, Carbohydr. Polym. 2018,
186, 25.
CONFLICT OF INTEREST
There are no conflicts to declare.
[24] S. Sadjad, M. Atai, Appl. Clay Sci. 2018, 153, 78.

SADJADI AND ATAI
11 of 11
[25] M. Massaro, V. Schembri, V. Campisciano, G. Cavallaro, G.
Lazzara, S. Milioto, R. Noto, F. Parisi, S. Riela, RSC Adv.
2016, 6, 55 312.
[41] X. Lin, X. Ju, R. Xie, M. Jiang, J. Wei, L. Chu, Chin. J. Chem.
Eng. 2013, 21, 991.
[42] M. A. Bonifacio, G. P, A. M. Ferreira, S. Cometa, E. De Giglio,
Carbohydr. Polym. 2017, 163, 280.
[26] P. Pasbakhsh, G. J. Churchman, Natural Mineral Nanotubes:
Properties and applications, Apple Academic Press, Oakville
2015.
[43] S. Azmi, S. I. Abd Razak, M. R. Abdul Kadir, N. Iqbal, R.
Hassan, N. H. Mat Nayan, A. H. Abdul Wahab, S.
Shaharuddin, Soft Mat. 2017, 15, 45.
[27] M. Massaro, G. Lazzara, S. Milioto, R. Noto, S. Riela, J. Mater.
Chem. B 2017, 5, 2867.
[44] S. Sadjad, Appl. Organomet. Chem. 32. https://doi.org/10.1002/
aoc.4211
[28] M. Massaro, C. G. Colletti, G. Lazzara, S. Milioto, R. Noto, S.
Riela, J. Mater. Chem. A 2017, 5, 13 276.
[45] H. Zhu, M. L. Du, M. L. Zou, C. S. Xu, Y. Q. Fu, Dalton Trans.
2012, 41, 10 465.
[29] J. Tully, R. Yendluri, Y. Lvov, Biomacromolecules 2016, 17, 615.
[46] P. Yuan, P. D. Southon, Z. Liu, M. E. R. Green, J. M. Hook, S. J.
[30] Y. Zhang, A. Tang, H. Yang, J. Ouyang, Appl. Catal. A 2016,
119, 8.
Antill, C. J. Kepert, J. Phys. Chem. C 2008, 112, 15 742.
[47] S. Mallik, S. S. Dash, K. M. Parida, B. K. Mohapatra, J. Colloid
Interface Sci. 2006, 300, 237.
[31] G. Cavallaro, G. Lazzara, S. Milioto, F. Parisi, Chem. Rec. 2018,
18, 1.
[48] S. Bordeepong, D. Bhongsuwan, T. Pungrassami, T.
[32] P. Yuan, D. Tan, F. Annabi‐Bergaya, Appl. Clay Sci. 2015,
112–113, 75.
Bhongsuwan, Songklanakarin J. Sci. Technol. 2011, 33, 599.
[49] L. Zatta, J. E. F. da Costa Gardolinski, F. Wypych, Appl. Clay
Sci. 2011, 51, 165.
[33] Y. Zhang, A. Tang, H. Yang, J. Ouyang, Appl. Clay Sci. 2016,
119, 8.
[34] J. Zhang, D. Zhang, A. Zhang, Z. Jia, D. Jia, Iran Polym. J. 2013,
22, 501.
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
[35] B. Szczepanik, P. Rogala, P. M. Słomkiewicz, D. Banaś, A.
Kubala‐Kukuś, L. Stabrawa, Appl. Clay Sci. 2017, 149, 118.
[36] B. Szczepanik, P. Słomkiewicz, Appl. Clay Sci. 2016, 124–125,
31.
[37] R. Zhai, B. Zhang, L. Liu, Y. Xie, H. Zhang, J. Liu, Catal.
Commun. 2010, 12, 259.
How to cite this article: Sadjadi S, Atai M.
Palladated halloysite hybridized with photo‐
polymerized hydrogel in the presence of
cyclodextrin: An efficient catalytic system
benefiting from nanoreactor concept. Appl
Organometal Chem. 2019;e4776. https://doi.org/
10.1002/aoc.4776
[38] S. Kumar‐Krishnan, A. Hernandez‐Rangel, U. Pal, O. Ceballos‐
Sanchez, F. J. Flores‐Ruiz, E. Prokhorov, O. Arias de Fuentes,
R. Esparza, M. Meyyappan, J. Mater. Chem. B 2016, 4, 2553.
[39] D. Tan, P. Yuan, F. Annabi‐Bergaya, D. Liu, L. Wang, H. Liu,
H. He, Appl. Clay Sci. 2014, 96, 50.
[40] N. Sabbagh, A. Akbari, N. Arsalani, B. Eftekhari‐Sis, H.
Hamishekar, Appl. Clay. Sci. 2017, 148, 48.