Magnetic crosslinked copoly(ionic liquid) nanohydrogel supported palladium nanoparticles as efficient catalysts for the selective aerobic oxidation of alcohols

Article abstract of DOI:10.1016/j.apcata.2018.07.009

Nowadays it is still a great sustainable processes challenge to produce efficient, selective and easy magnetic recovery and recycling catalysts for oxidation of alcohols using air as the oxidant. In this work, a new magnetic nanohydrogel comprising [DABCO-allyl][Br] ionic liquid, allyl alcohol and N,N’-methylenebis(acrylamide) is used for stabilization of small and highly uniform palladium nanoparticles of 3–4 nm size MXCPILNHG@Pd. This material has been characterized by Fourier-transform infrared spectroscopy (FTIR), atomic adsorption spectroscopy (AAS), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), SEM-Map, energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectra (XPS), vibrating-sample magnetometer (VSM) and dynamic light scattering (DLS). According to optimization of cross-linking degree and ratio of DABCO-IL, MXCPILNHG-2@Pd is found as a highly selective catalyst in oxidations of primary alcohols to the corresponding aldehydes in toluene and to acids in water. Furthermore, secondary alcohols were reacted efficiently to the corresponding ketones in both toluene and water. Catalyst is magnetically recovered and recycled for several times in both toluene and water and the reused catalysts are characterized by TEM and XPS.

Full text of DOI:10.1016/j.apcata.2018.07.009

Accepted Manuscript
Title: Magnetic Crosslinked Copoly(ionic liquid)
Nanohydrogel Supported Palladium Nanoparticles as Efficient
Catalysts for the Selective Aerobic Oxidation of Alcohols
Authors: Mohammad Gholinejad, Mahmoud Afrasi, Nasser
Nikfarjam, Carmen N a´ jera
PII:
S0926-860X(18)30332-6
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.07.009
APCATA 16735
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
23-3-2018
3-7-2018
5-7-2018
Please cite this article as: Gholinejad M, Afrasi M, Nikfarjam N, N a´ jera C, Magnetic
Crosslinked Copoly(ionic liquid) Nanohydrogel Supported Palladium Nanoparticles as
Efficient Catalysts for the Selective Aerobic Oxidation of Alcohols, Applied Catalysis
A, General (2018), https://doi.org/10.1016/j.apcata.2018.07.009
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Magnetic Crosslinked Copoly(ionic liquid) Nanohydrogel Supported Palladium
Nanoparticles as Efficient Catalysts for the Selective Aerobic Oxidation of Alcohols
ab
a
a
c
Mohammad Gholinejad* , Mahmoud Afrasi , Nasser Nikfarjam *, Carmen Nájera
a
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), P. O. Box
5195-1159, Gavazang, Zanjan 45137-66731, Iran, E-mail: gholinejad@iasbs.ac.ir;
4
nikfarjam@iasbs.ac.ir
bResearch Center for Basic Sciences & Modern Technologies (RBST), Institute for Advanced
Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran
cDepartamento de Química Orgánica and Centro de Innovación en Química Avanzada (ORFEO-
CINQA), Universidad de Alicante, Apdo. 99, E-03080-Alicante, Spain.
th
Dedicated to Prof. Habib Firouzabadi on the occasion of his 75 birthday.
Graphical Abstract
1

Research Highlights



Novel magnetic nanohydrogel supported Pd NPs are synthesized and characterized.
This material showed high efficiency for selective aerobic oxidation of alcohols.
Catalyst was magnetically recovered and recycled for several times
Abstract:
Nowadays it is still a great sustainable processes challenge to produce efficient, selective and
easy magnetic recovery and recycling catalysts for oxidation of alcohols using air as the oxidant.
In this work, a new magnetic nanohydrogel comprising [DABCO-allyl][Br] ionic liquid, allyl
alcohol and N,N'-methylenebis(acrylamide) is used for stabilization of small and highly uniform
palladium nanoparticles of 3-4 nm size MXCPILNHG@Pd. This material has been characterized
by Fourier-transform infrared spectroscopy (FTIR), atomic adsorption spectroscopy (AAS),
thermogravimetric analysis (TGA), transmission electron microscopy (TEM), scanning electron
microscopy (SEM), energy dispersive spectroscopy (EDS), SEM-Map, energy-dispersive X-ray
spectroscopy (EDX), X-ray photoelectron spectra (XPS), vibrating-sample magnetometer (VSM)
and dynamic light scattering (DLS). According to optimization of cross-linking degree and ratio
of DABCO-IL, MXCPILNHG-2@Pd is found as a highly selective catalyst in oxidations of
primary alcohols to the corresponding aldehydes in toluene and to acids in water. Furthermore,
secondary alcohols were reacted efficiently to the corresponding ketones in both toluene and
water. Catalyst is magnetically recovered and recycled for several times in both toluene and
water and the reused catalysts are characterized by TEM and XPS.
Keywords: Magnetic, Nanohydrogel, Palladium, Oxidation, Alcohols
2

1
- Introduction
The oxidation of alcohols to carbonyl compounds is an essential transformation in synthetic
organic chemistry because of obtained aldehydes and ketones are basic precursors for synthesis
of many drugs, vitamins and fragrances[1–3]. In particular, the selective oxidation of primary
alcohols to the corresponding aldehydes or acids has been widely approved as one of the most
important transformations from current chemical industry [4–13]. In the past two decades,
extensive attention has been paid to the use of transition metals as catalysts as a better alternative
than the conventional waste-producing oxidation procedures which require stoichiometric
amounts of toxic inorganic salts [4–28]. Among the different oxidizing agents, oxygen produces
water as the only byproduct and therefore is highly desirable from economic and green chemistry
standpoints. Different complexes and salts of transition metals, such as Fe [29], Ru [30], Co [31],
Cu [32], and Au [33] have been employed for this useful reaction. However, in comparison with
the other transition metals, palladium [34–40] is one of the best efficient catalyst for the aerobic
oxidation of alcohols under homogeneous or heterogeneous reaction conditions [41–61]. Due to
contaminants of toxic palladium with products and also high price of palladium catalysts,
recently many heterogeneous palladium catalysts have been developed for oxidation reaction of
alcohols [62–72]. However, in spite of the great achievements in this field, most of the
heterogeneous catalysts are difficult to separate from the reaction mixture by standard laboratory
methods such as filtration and centrifugation. One approach to solve this problem is the use of
magnetic catalysts which can be easily separated from the reaction mixture by an external
magnetic field and match with green and sustainable chemistry points of view. Along this line,
iron oxide nanoparticles with large ratio of surface area to volume, superparamagnetic behavior
and low toxicity are excellent supports for the stabilization of palladium nanoparticles in
3

different organic transformations [73–89]. However, to date, few magnetite nanoparticles
supported palladium catalysts have been prepared and successfully used in the aerobic oxidation
of alcohols.
Nowadays chemists are interested in using ionic liquids as green solvents, catalysts and reagents.
Despite outstanding physicochemical properties of ionic liquids (ILs) as alternative reaction
media, their widespread utilization as solvent or catalyst are limited by the following
drawbacks[90]: a) consume of large amounts of ILs is relatively costly and may cause
toxicological problems, b) ILs play a minor role in the catalyzed reactions due to their higher
viscosity and c) difficulties in product separation and catalyst recovery because of ILs
homogeneity. To cope with these drawbacks, the concept of supported ionic liquids has been
developed to associate both advantages of ILs and heterogeneous support materials. Several
efforts have been made to immobilize ILs onto the support materials such as mesoporous silica
[
91–93], silica gel [94–96], polymers [97–99] and magnetic nanoparticles [100–102].
Heterogenization of ILs onto the material used as support has some problems such as tedious and
difficult procedures of ILs grafting onto the surface of the support and low thermal stability of
grafted ILs. Also, the normal grafting of ILs onto the solid materials is offering only one
available layer on solid surface for immobilization which in turn leads to the low loading amount
of grafted ILs. Hence, the low loading of ILs causes to use a large amount of solid support
materials which in turn results in consuming a large amount of organic solvents in catalysis
reactions and difficulty in separation/recovery of the used catalysts. Consequently, the available
multi-layer onto the solid material surface for IL immobilization was proposed as a good
technique to enhance IL loading amount [103–107]. But to avoid several boring steps [108]
4

looking for a simple and facile way for applying multi-layer on the solid support materials seems
required.
In the present work, we report the synthesis of a novel magnetically copoly(ionic liquid) network
catalyst in the nanoscale sizes, so-called magnetic x-linked PIL nanohydrogel (MXCPILNHG).
These particles are prepared through the crosslinking copolymerization of a monomer bearing IL
via a miniemulsion polymerization method followed by the loading of palladium nanoparticles
(Pd NPs) to the IL groups in the network. The efficiency of this catalyst has been applied to the
selective aerobic oxidation of alcohols.
2
. Experimental
2
.1. General remarks.
All materials were purchased from Sigma-Aldrich, Acros and Merck Millipore. Reactions were
monitored by gas chromatography Varian CP-3800. H NMR and 13C NMR spectra were
1
recorded at 400 MHz and 100 MHz, respectively on a Bruker Avance HD apparatus in CDCl .
3
Chemical shifts are given on the δ-scale in ppm, and residual solvent peaks were used as internal
standards. X-ray diffraction (XRD) patterns were recorded using Philips X’Pert Pro instrument.
The TEM and SEM mapping images were captured with EOL JEM-2010 and Hitachi S3000N.
(
XPS) analyses were performed using a K-Alpha spectrometer. The weight loss of samples was
measured using a thermogravimetry (NETZSCH STA 409) under an N2 flow rate of 20 ml.min-
with a heating rate of 10 °C·min-1 from 30 to 800 °C. The diluted aqueous suspension of
1
MXCPILNHG-2 particles was set on a clean glass slide and then vacuum-coated with gold.
Digital images of the samples were acquired with Hitachi S4160 FE-SEM operating at 20 kV.
FT-IR study of samples were performed using FT-IR spectrophotometer (Bruker vector 22
-1
spectrophotometer, Germany) by preparing their KBr pellets from 400 to 4000 cm . Magnetic
5

measurements were performed using vibration sample magnetometry (VSM), (MDK Co.
Kashan, Iran) analysis.
2
.2. Preparation of Fe
3
4
O NPs.
FeCl
3
·6 H
2
O (11.0 g) and FeCl
2
·4H O (4.0 g) were dissolved in deionized water (250 mL) and
2
the mixture was stirred using a mechanical stirrer under an argon atmosphere. To the resulting
mixture, aqueous ammonia (25 %, 40 mL) was added slowly over 20 min, and the mixture was
°
stirred at 80 C for 4 h. A black precipitate of Fe
3
4
O was collected by using an external magnet
and washed with deionized water (3 × 20 mL) and ethanol (3 × 20 mL) and finally dried under
vacuum.
2
.3. Preparation of Fe
3
O
4
@SiO .
2
To the 30 min sonicated Fe
aqueous ammonia (25 %, 6 mL) were added and the resulting mixture was stirred for 24 h at
room temperature. Fe @SiO NPs were separated by an external magnet and washed with
EtOH (3×10 mL) dried in oven at 60 °C.
.4. Synthesis of [DABCO-Allyl] [Br].
3
O
4
NPs (1 g) in ethanol (200 mL), tetraethyl orthosilicate (2 mL) and
3
O
4
2
2
To a flask containing a solution of DABCO (24 mmol, 2.69 g) in ethyl acetate (50 mL), allyl
bromide (20 mmol, 1.73 mL) was added and the resulting mixture was stirred for 24 h at room
temperature. Then, the obtained solid was crystalized in diethyl ether and dried in oven at 50 °C.
NMRs characterization verified the successful synthesis of [DABCO-Allyl][Br] (Figures S1 and
1
S2). H NMR: δ 5.97-5.87 (m, 1H), 5.67-5.58 (m, 2H), 3.84 (d, 2H, J=8), 3.35 (t, 6H), 3.12 (t,
1
3
6
H). C NMR: δ 129.07, 123.60, 66.41, 52.00, 44.42.
6

2
.5. Preparation of Fe
3
O
4
@SiO @vinyl.
2
Fe
3
O
4
@SiO
2
(1 g) was sonicated in dry toluene (30 mL) for 30 min, and then
dichloromethylvinylsilane (4 mmol, 0.52 mL) was added under inert conditions. The resulting
mixture was stirred for 24 h at 80 °C under argon protection. Then, the reaction mixture was
subjected to magnetic separation and the solid was separated giving Fe
was washed with EtOH (3×10 mL) and dried in an oven at 60 °C.
3
O
4
@SiO @vinyl which
2
2
.6. Preparation of MXCPILNHG-2.
These nanohydrogels were prepared via an inverse miniemulsion polymerization method.
Fe @SiO @vinyl (0.43 g), [DABCO-Allyl][Br] (2.15 mmol, 0.5 g), allyl alcohol (50 mmol,
.4 mL) and N,N’-methylenebis(acrylamide) as crosslinker (4.34 mmol, 0.67 g) were dispersed
3
O
4
2
3
in water (4.8 mL) with the aid of mechanical stirrer to form the water phase. Benzoyl peroxide as
radical initiator (0.11 g) and Span80 as surfactant (1.19 g) were dissolved in cyclohexane (38.5
mL) to form the oil phase. Then, the water phase was added to the oil phase under vigorous
mechanical stirring at room temperature for 1 h. The resulted inverse emulsion was
miniemulsified using sonication (at amplitude of 50% in pulse mode) for 10 min in an ice-water
bath to avoid unwanted polymerization. The obtained stable miniemulsion was quickly
transferred to a three-necked round-bottomed flask equipped with a condenser, argon inlet and
mechanical stirrer in the oil bath and was purged with argon. Finally, polymerization occurred at
7
0 °C under constant stirring of 500 rpm. After 24 h, the mixture was cooled by leaving it at
room temperature for 1 h. The resulting nanoparticles were separated using an external magnet
and washed several times by cyclohexane and water to remove impurities and unreacted
monomers. The particles were dried in the oven at 50 °C for 12 h and then in a vacuum oven at
5
0 °C for another 12 h.
7

2
.7. Synthesis of MXCPILNHG-2 supported palladium NPs catalyst.
MXCPILNHG-2 (500 mg) was sonicated in water (10 mL) for 15 min. Then, a solution of
Na PdCl (0.047 mmol, 14 mg) in H O (5 mL) was added slowly and the mixture was stirred for
h at room temperature. Then, a solution of NaBH (0.8 mmol, 30 mg in 5 mL H O) was added
2
4
2
1
4
2
during the 15 min and mixture was stirred for 24 h at room temperature under argon atmosphere.
The resulting solid was separated magnetically, washed with water (3 × 20 mL) and ethanol (3 ×
o
2
0 mL), and dried in an oven at 60 C. The loading of Pd on the obtained material was
-1
determined by atomic adsorption spectroscopy analysis to be 0.1 mmol·g .
2
.8. General procedure for the oxidation of alcohol to aldehyde and acid (or ketone):
In a 5 mL glass flask, catalyst (10 mg, containing 0.2 mol% Pd), alcohol (0.5 mmol), K
2
CO
O or toluene (2 mL) were added and reaction mixture was stirred
atmosphere (from a balloon). Then, in the
3
(104 mg, 0.75 mmol) and H
2
o
continuously at 90 C for the desired time under O
2
case of water as solvent, products were extracted with ethyl acetate and the catalyst was
recovered by an external magnet. Yields of desired products were determined by gas
chromatography.
2
.9. Contact angle measurement for series of MXCPILNHG@Pd:
Compact pellets of all the prepared nanoparticles (both crosslink and DABCO-IL series) were
prepared using stainless steel die-set and hydraulic press. Fine powder of nanoparticles was
pressed for 5 minutes to form smooth pellets, 7 mm in diameter. The contact angle was
determined with deionized water, using camera (Canon 70D) and ImageJ2x software. Three
readings were taken in order to obtain an average over the heterogeneity of the surfaces. The
pellets were placed in a vacuum oven at 50 °C for 12 h before measurements.
8

3
. Results and discussion
3
.1. Materials and characterization
The preparation steps for the magnetic hydrogel supported palladium NPs are presented in
Scheme 1. Fe NPs were prepared from the reaction of FeCl ·6H O and FeCl ·4H O using a
simple co-perception method. Core/shell Fe @SiO NPs were obtained from the reaction of
Fe NPs and tetraethyl orthosilicate via a sol–gel process. TEM images showed thickness of
SiO shell to be around 4 nm (Figure S3), and also the results of porosimetry revealed the BET
surface area of 37 m ·g for Fe
core/shell surface via the reaction of dichloro(methyl)vinylsilane with Fe
Fe @SiO @vinyl was allowed to undergo polymerization with the already prepared
DABCO-allyl] [Br] salt, allyl alcohol and N,N'-methylenebis(acrylamide) as crosslinking agent
3
O
4
3
2
2
2
3
O
4
2
3
O
4
2
2
-1
3
O
4
@SiO
2
. Carbon-carbon double bound was introduced on
@SiO NPs. Then,
3
O
4
2
3
O
4
2
[
in a mixture of cyclohexane/water and Span 80 (sorbitan oleate) as nonionic surfactant in the
presence of benzoyl peroxide as an initiator. Final nanomagnetic hydrogel supported Pd NPs
were obtained via the reaction of polymer modified Fe
3
O
4
2
NPs with Na PdCl
4
and NaBH under
4
argon. The loading of Pd on the obtained solid was determined by atomic absorption
spectroscopy analysis to be 0.1 mmol/g. The obtained magnetic composite is referred as
MXCPILNHG@Pd NPs through the text of this article.
The surfaces of prepared magnetic compounds were investigated by FT-IR spectroscopy (Figure
-1
1
). In the case of the FT-IR spectrum of Fe
3
O
4
@SiO peaks at 1097 and 1637 cm related to the
2
Si–O–Fe and bending vibration of the adsorbed water on the surface, respectively, were observed
-1
(
Figure 1A). Besides, a new peak was observed at 1642 cm related to C=C stretching for
Fe @SiO @vinyl (Figure 1B). The characteristic peaks for the MXCPILNHG-2 occurs at
3
O
4
2
-1
3
300-3600 cm for the O-H stretching of allyl alcohol, 2928 and 2855 for the C-H stretching of
9

methylene and methine groups in the backbone of polymer. In addition, peaks at 3064, 1453,
-1
1
655 and 1532 cm were attributed the N-H, C-N, free C=O and H-bonded C=O stretching of
amide, respectively (Figure 1A).
Thermogravimetric analysis (TGA) was performed to study of the thermal properties of
Fe
3
3
O
4
4
@SiO
@SiO
2
2
@vinyl and MXCPILNHG-2 (Figure 2). The TGA thermogram for
@vinyl showed two main weight losses between 25-800 °C (Figure 1). The first
Fe
O
weight loss was attributed to water and or physically adsorbed solvents and the second one is
related to the successful grafting of vinyl groups on the surface of the core-shell magnetic
nanoparticles (Figure 2A, S13). While, the TGA thermogram of the final nanohydrogel displayed
three-step weight-loss profiles which of that the first one was attributed to the water and or
physically adsorbed solvents (Figure 2B, S14). Dequaternization of the ammonium generally
happens in the thermal degradation of ammonium containing polymers and the mechanism of
this phenomenon occurs through two main suggested pathways; nucleophilic substitution and
Hofmann elimination [109–111]. Therefore, one can conclude that the second weight loss is
related to the dequaternization of DABCO mainly followed a reverse nucleophilic substitution
mechanism (Scheme S1). These results are consistent with the finding of Zhang et al. where they
studied the thermal degradation of copolymers based on N-4-vinylbenzyl-N'-alkyl DABCO BrCl
(
VBDCxBrCl) and n-butyl acrylate. They reported the 5 wt.% loss temperature values (T
DABCO salt-containing copolymers ranged from 190 to 290 °C when ionic monomer varied
from 42 to 1 mol % [111]. Whereas, the measured T value for MXCPILNHG-2 with 3.95 mol %
d
) of
d
of DABCO-IL monomer was around 245 °C, indicating the degradation mechanism for our case
follows through a nucleophilic substitution mechanism, but it needs to be studied in more details.
And finally, the third weight-loss step corresponded to the polymer backbone degradation,
1
0

mainly poly(allylalcohol) with initiation and maximum degradation temperatures of around
45.7 and 474 .8 °C, respectively [112]. The residue of around 31 wt. % is also attributed to the
content of core-shell magnetic nanoparticles.
3
Magnetization curves for Fe
palladium were also studied (Figure 3). Results indicate a major decrease in the magnetization
value of Fe NPs@SiO @vinyl with respect to Fe NPs confirming the successful
3
O
4
NPs, Fe
3
O
4
NPs@SiO
2
@vinyl, and magnetic hydrogel supported
3
O
4
2
3
O
4
introduction of silyl and double bond shell. Also, a decrease in the magnetization value of
magnetic hydrogel supported palladium is due to the effective polymerization around
Fe
3
O
4
NPs@SiO @vinyl nanoparticles. However, in all samples Zero coercivity and remanence
2
on the magnetization loop was observed without the presence of hysteresis loop, suggesting the
superparamagnetic nature of the samples (Figure S15).
The field emission scanning electron microscopy (FE-SEM) micrographs revealed a paste-like
morphology for MXCPILNHG-2 particles in the dried form (Figure 4). The size distribution of
particle was obtained by manually measuring of at least 400 particles using JMicrovision 1.2.7
software. Then, the data was fitted by Gaussian function and the average particles size was
̅
calculated by the following equation: 퐷 = ∑ 푑 푛 ⁄∑ 푛 , where 푛 is the number of particles with
푖
푖
푖
푖
diameter 푑 . The average size of around 182 nm was found for these nanoparticles. While,
푖
dynamic light scattering (DLS) analysis revealed a hydrodynamic diameter of around 203nm for
these particles (Figure S4). This difference can be attributed to the swelling capability of the
particles in aqueous media, i.e. hydrogelic properties.
1
1

Transmission electron microscopy (TEM) images of final nanohydrogels showed the presence of
polymer sheets around magnetic nanoparticles. TEM images also indicate incorporation of
uniformly dispersed Pd NPs with the calculated average size of 3.5 nm (Figure 5). Also,
scanning tunneling microscopy (STM) images of MXCPILNHG-2@Pd showed the presence of
the Pd NPs (bright dots) on the surface of the final hydrogel structure (Figure S5). As well, BET
isotherm of MXCPILNHG-2 before and after loading of Pd NPs showed no obvious difference
in curves, indicating no obvious changes in structure (Figure S6-A). The degree of hysteresis
loop, the gap between adsorption curve and desorption curve, in the isotherm of final hydrogel
particle increased with increasing of Pd content (from 0.05 to 0.3 mmol of Pd, see Table S5 and
Figure S6-B) and this is mainly due to capillary condensation effect. In other word the observed
pressure difference between gas adsorption and desorption is probably because of locating of Pd
nanoparticles inside pores in final hydrogel structure (bottle-neck structure) and this effect is
intensified with increasing of Pd nanoparticle (Figure S6-B). Moreover, BET measurements
2
-1
showed that the surface area of MXCPILNHG-2 (25 m .g ) was increased by addition of Pd NPs
2
-1
(
28 m .g ), maybe due to the added surface area by Pd NPs (Table S5). Consequently, one
concluded that Pd NPs were located both inside and surface of the final hydrogel structure.
X-ray diffraction (XRD) analysis of prepared magnetic hydrogel showed related Bragg’s
reflections to the Fe
3
4
O NPs in 2ϴ = 30.2, 35.5, 43.4, 53.5, 57.2, and 62.8° correspond to the
(210), (311), (400), (422), (511), and (440) planes, respectively. In addition, these results showed
Bragg’s reflections related to palladium in 2ϴ = 40.1, 46.8, and 68.2° and also related to silica
shells in 2ϴ = 22° (Figure 6) [113].
The X-ray photoelectron spectrum (XPS) of MXCPILNHG-2@Pd was also studied. XPS study
of Fe 2p region shows two main binding energy peaks area (deconvoluted by six peaks) related
1
2

to 2p3/2 and Fe 2p1/2. Peaks centered at 712.7, 726.2 eV are related to Fe(III) oxidation state
Furthermore the satellites at 719.1 eV and 732.9 eV are related to Fe3+ in the Fe
O phase,
2
3
suggesting that the surface of Fe
3
O
4
was partially oxidized to γ-Fe
2
O
3
. Also peaks at 710.5 and
7
24.3 eV are good agreement with Fe(II) and in the Fe
3
O
4
structure [114,115] (Figure 7A). The
XPS spectra in C 1s region showed three peaks centered at 284.7 and 286.9 and 288.5 eV, which
are related to C-C or C=C, C-N or C-O and C=O forms of carbon, respectively [116,117] (Figure
7
3
B). XPS study in Pd 3d region showed the presence of two intensive doublets at 335.5 and
40.7 eV related to Pd(0) and peaks at 337 and 342.5 eV related to Pd(II) species (Figure 7C).
Results indicated that 74.9 % of palladium is in metallic Pd(0) form [118]. Also, the N1s core
level spectrum shows a main peak at 399.6 eV, which is attributable to the neutral amine and a
minor peak at 402 eV, which is related to positively charged quaternary nitrogen species [119]
(Figure 7D).
Furthermore, energy dispersive spectroscopy (EDS) confirmed presence of different elements
such as Pd, N, Si, Fe, and C were in the structure of magnetic hydrogel supported Pd NPs (Figure
8
). On the other hand, SEM-Map images showed the presence of C, N and Pd in highly
uniformly in magnetic hydrogel structure (Figures S7 and S8).
.2.Catalytic performance
3
Catalytic activity of final Pd loaded magnetic nanohydrogels MXCPILNHG-2@Pd was
examined in the aerobic oxidation of alcohols. Initially, selective aerobic oxidation of benzyl
alcohol in toluene or H
2
O solvents and K
2
CO as the base at 90 °C was selected as benchmark
3
reaction. In order to find nanoparticles with high catalyst efficiency as well as high selectivity
ability, two series of nanoparticles were synthesized. The first series, MXCPILNHG-crosslink,
which of that the mole percent of the methylene bisacrylamide (MBA, as crosslinker) to the total
1
3

content of two monomers was increased from 4.5 to 8.3 and 15.0 %, named MXCPILNHG-1,
MXCPILNHG-2 and MXCPILNHG-3, respectively (Table S1). In this series, it was curious to
find the effect of hydrogel pore size, induced by crosslinking degree, on the catalyst activity and
selectivity of nanoparticles and in the second series (i.e., MXCPILNHG-DABCO-IL), the mole
percent of DABCO-IL to the total content of monomers was decreased from 12.4 to 4.1 and 2.2
%
(named MXCPILNHG-4, MXCPILNHG-2, MXCPILNHG-5) in order to answering the
following question; how the hydrophobicity of nanoparticles affects to catalyst efficiency and
selectivity (Table S2).
In the MXCPILNHG-crosslink series, increasing of the crosslinking degree leads to a decrease in
pore size of the obtained nanohydrogel. Therefore, it is expected that the confined pores in the
higher crosslinking degree can limit the reagent transportation inside the nanohydrogel.
Nevertheless, the yields of the obtained products (benzaldehyde and benzoic acid) in the related
solvents were decreased for MXCPILNHG-3 due to the mentioned compact structure of
nanoparticle (Table S3). Also, the results revealed that the acid product yield was not actually
varied with increasing of crosslinking degree for MXCPILNHG-1 and MXCPILNHG-2 (Table
S3). But for these two catalysts, the aldehyde product yield was increased from 5 to 98 % in
toluene as solvent. It seems that the benzyl alcohol has more tendency toward inside of particles
in MXCPILNHG-2 than in MXCPILNHG-1 being oxidized to aldehyde in toluene as solvent.
This tendency may be related to the hydrophobization effect of the crosslinking process, because
for every linkage, one methylene group and two amide groups (with low capability of H-bond
forming due to the strong resonance) are replaced by two hydroxyl groups. To confirm this
claim, we checked their wettability by measuring contact angle of the particles. The results
1
4

showed that contact angle was increased with crosslinking degree; compare 53, 63 and 74° for
MXCPILNHG-1, MXCPILNHG-2 and MXCPILNHG-3, respectively (Figure S9).
In the next step, we were looking for the optimized content of DABCO-IL in MXCPILNHG-
DABCO-IL series. In the lowest content of DABCO-IL, i.e. MXCPILNHG-5, the yield of both
products was decreased (Table S4).
This reduction in yielding can be related to the low loading of Pd NPs induced by the low
content of DABCO-IL in the nanoparticle. And, the comparison between MXCPILNHG-4 and
MXCPILNHG-2 revealed that the acid product yield was not actually changed, but the aldehyde
product yield was decreased for the sample with the higher content of DABCO-IL, i.e.
MXCPILNHG-4 (Table S4). The reason may lie in the fact of hydrophilization effect of
DABCO-IL monomer containing two hydration sites, namely quarter ammonium. Accordingly,
the tendency of nanoparticles for dispersion in toluene for MXCPILNHG-4 is lower than
MXCPILNHG-2 and the catalyst efficiency for aldehyde product was declined from 98 to 10%
(
Table S4). For further proof, contact angle of particles was measured and its value was
increased with lowering of the DABCO-IL content; compare 54, 63 and 80° for MXCPILNHG-
, MXCPILNHG-2 and MXCPILNHG-5 (Figure S9).
4
Based on the observed data from Tables S3 and S4, it seems that the MXCPILNHG-2 in both
series has the best catalyst efficiency and selectivity and therefore, MXCPILNHG-2 was chosen
as optimized formulation for further studies. To insure that toluene and water are best solvents
for selective oxidation of benzyl alcohol to benzaldehyde and benzoic acid, respectively, the
effect of other solvents such as 1,4-dioxane, CH CN, and DMA in the presence of 0.2 mol% of
3
MXCPILNHG-2@Pd were studied (Table 1, entries 1-3). Results indicated formation of low
yields for the desired oxidation products. Using other bases such as NaOAc, t-BuOK and base-
1
5

free reaction conditions gave low conversion in both toluene and H
addition, lowering the Pd loading, reaction times and temperature afforded low reaction
conversion in both toluene and H O as solvents (Table 1, entries 10-15). It should be noted that
using Fe @SiO @Pd as catalyst reactions gave very low yields and starting material was
2
O (Table 1, entries 4-9). In
2
3
O
4
2
intact (Table 1, entries 16-17).
Having the optimized reaction conditions in hand, aerobic oxidation of different primary
alcohols was studied. Reactions of benzylic alcohols having electron-donating and electron-
withdrawing groups as well as 1-naphthylmethanol in H O and toluene proceed efficiently and
2
selectively acid and aldehydes were obtained (Table 2, entries 1-10). Reaction of 2-
furanylmethanol as the heterocyclic primary alcohol was also studied. It is worth noting that 2-
furanylmethanol is unstable alcohol in water and produce undesirable polymeric products.
However, result of reaction in toluene showed highly selective formation of furfural in 96 %
yield (Table 2, entry 11). Also, oxidation of 1-octanol as an aliphatic alcohol proceeded very
well and products were obtained selectively in excellent yields (Table 2, entry 12). Reaction of
cinnamyl alcohol as allylic alcohol in water gave moderate conversion with excellent selectivity
to cinnamaldehyde while its reaction in toluene gave excellent conversion and selectivity to
cinnamic acid (Table 2, entry 13).
We have also studied the oxidation of secondary alcohol under the optimized reaction conditions.
Results of our study indicated that 1-phenylethanol, 1-phenylpropan-1-ol, 1,2,3,4-
tetrahydronaphthalen-1-ol, benzhydrol, and 4-tert-butylcyclohexanol were performed very well
in both toluene and water and the corresponding ketones were obtained in high to excellent
yields (Table 3, entries 1-5). Reaction of cyclooctanol in water proceed well and cyclooctanone
1
6

was obtained in 88 % yield. However, the reaction in toluene was sluggish and low yield for
corresponding ketone was obtained (Table 3, entry 6).
Catalytic activity and TOF of MXCPILNHG-2@Pd is compared with some of the reported
catalysts in oxidation of benzyl alcohol as a common substrate (Table 4). In spite of high activity
of some reported catalysts, results indicated overall activity of MXCPILNHG-2.
3
.3. Recycling of the catalyst
Finally, recycling of the catalyst was studied for the oxidation reaction of benzyl alcohol under
the optimized reaction conditions. For this purpose, after completion of reaction, catalyst was
easily separated by external magnet and after washing with ethyl acetate was reused in another
batch (Figure S15). Results indicated that the catalyst was recyclable up to 5 consecutive runs
with small decrease in activity (Figure 9). However, yield was dropped to 68 and 74 % in ran 6
and to 35 and 43 % in run 7 in toluene and water respectively. Measurement of Pd content in
each cycle indicated that dropping of yields are match with ratio of Pd leaching during the
recycling (Table S6).
3
.4.1. Characterization of reused catalyst
TEM images of reused catalysts in both toluene (Figure 10A-C) and water (Figure 10D-F) in
different magnification indicated preservation of the catalyst structure and the presence of
̅
̅
partially aggregate Pd NPs in bigger size (퐷 = 7.4 nm for toluene and 퐷 = 9.3 nm for H O) with
2
̅
respect to the Pd NPs in the fresh catalyst (퐷 = 3.5 nm).
1
7

XPS study of the reused catalyst after 5 runs in Pd 3d region showed different content of Pd (II)
and Pd(0) species in water and toluene (Figure 11). While ratio of Pd(0)/Pd(II) was very similar
to fresh catalyst, results for reused catalyst in water showed noticeable increasing of Pd(II) form
with respect to the fresh catalyst. This may due to the good diffusion of oxygen in H
respect to toluene.
2
O with
Conclusion
The new catalyst based on the palladium loaded magnetic nanohydrogel bearing ionic liquid sites
MXCPILNHG@Pd) can be prepared and characterized by different techniques. By optimization
(
of crosslinking degree and DABCO-IL content, MXCPILNHG-2@Pd with 8.3 % mol
crosslinker and 4.1 % mol DABCO-IL showed good activity and selectivity toward oxidation of
primary alcohols to aldehydes and acids in toluene and water respectively. In the case of
secondary alcohols, the catalyst showed also high efficiency in both water and toluene. This
catalyst can be easily recovered by separation with a magnet and reused for at least five cycles
without detriment to its catalytic activity. Structure of reused catalyst in both toluene and water
was studied by various techniques such as TEM and XPS.
Acknowledgements
The authors are grateful to Institute for Advanced Studies in Basic Sciences (IASBS) Research
Council, Zanjan University and Iran National Science Foundation (INSF-Grant number of
9
5844587) for support of this work. The authors are also thankful to the Spanish Ministerio de
Economía, Industria y Competitividad, Agencia Estatal de Investigación (AEI) and Fondo
Europeo de Desarrollo Regional (FEDER, EU) (projects CTQ2016-76782-P and CTQ2016-
1
8

8
1797-REDC), the Generalitat Valenciana (PROMETEOII/ 2014/017) and the University of
Alicante.
1
9

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