Highly dispersible and magnetically recyclable poly(1-vinyl imidazole) brush coated magnetic nanoparticles: An effective support for the immobilization of palladium nanoparticles

Article abstract of DOI:10.1039/c5nj02576a

A heterogeneous recoverable catalyst was prepared via the complexation of palladium onto the surface of magnetic nanoparticles coated by a poly(1-vinyl imidazole) brush. The stable, active and reusable catalyst was proven to be highly active in aerobic oxidation of primary and secondary alcohols with excellent yields. Only 0.1 mol% of the catalyst was used to oxidize 1 mmol of primary and secondary alcohols. The catalyst was readily recovered and reused up to 10 times under the described reaction conditions without significant loss of activity.

Full text of DOI:10.1039/c5nj02576a

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Highly Dispersible and Magnetically Recyclable Poly(1-Vinyl
Imidazole) Brush coated Magnetic Nanoparticle: an Effective
Support for Immobilization of Palladium Nanoparticles
Ali Pourjavadia*, Niloofar Safaiea, Seyed Hassan Hosseinia, Craig Bennettb
a Polymer Research Laboratory, Department of Chemistry, Sharif University of Technology, Tehran, Iran
b Department of Physics, Acadia University, Wolfville, Nova Scotia, Canada
E-mail address: purjavad@sharif.edu; Phone/fax: (982)166165311
Abstract
A heterogeneous recoverable catalyst was prepared via complexation of palladium onto the
surface of magnetic nanoparticles coated by poly(1-vinyl imidazole) brush. The stable, active
and reusable catalyst was proven to be highly active in aerobic oxidation of primary and
secondary alcohols with excellent yields. Only 0.1 mol% of catalyst was used to oxidize 1 mmol
of primary and secondary alcohols. The catalyst was readily recovered and reused up to 10 times
under the described reaction condition without significant loss of activity.
Keywords: Magnetic catalyst, Palladium catalyst, Oxidation
1

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Introduction
Selective oxidation of alcohols to corresponding aldehydes or ketones is one of the necessary and
common transformations in organic synthesis from the academic and industrial point of view due
to its applications as a valuable intermediate for pharmaceuticals, dyes, fragrances and flame
retardants.1-4 As a result, there are a large number of available methods however many of the
most commonly used methods generate an intolerably high amount of toxic waste which violates
green chemistry regulations.5-7
With the economic and environmental concerns, the use of molecular oxygen as an inexpensive,
readily available and clean oxidant has received considerable attention as a desirable green
process for selective oxidation of alcohols.8,10 In this regard, several transition metals such as,
cobalt,11 copper,12 gold,13 iron,14 osmium,15 ruthenium,16 vanadium17 and palladium18 have been
used as homogeneous catalysts in oxidation reactions based on molecular oxygen. Among them,
Pd based catalysts are the most promising choice due to high efficiency and selectivity in the
alcohol oxidation reaction.19 However, these methods have encountered many problems and
challenges including recyclability and reusability issues, the high pressure of O2 especially in
flammable organic solvents, usage of costly or toxic additives, difficult catalyst separation and
the use of a high amount of catalyst.
To overcome the separation problem of homogenous catalysts, the concept of heterogenizing of
homogeneous catalyst has emerged to combine advantages of both a homogeneous and
heterogeneous catalyst.20-24 In order to achieve this, active species (transition metals) are
immobilized onto the solid supports to form a heterogeneous catalyst. Various support materials
25
have been used for immobilization of Pd such as Al2O3, mesoporous silica,26cellulose,27
30
nanocrystalline starch,28 graphene,29 hydroxylapatite
,
and carbon.31 However, these supported
2

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catalysts suffer from major drawbacks, including high metal leaching, low metal loading, low
activity, tedious work-up procedure, difficulties in synthetic process and recovery.
A novel approach to solve the mentioned problems of traditional Pd supported catalysts is
immobilization of Pd on polymeric supports.32 Several types of polymers have been used as
support for immobilization of Pd.33-37 Although, these Pd supported polymeric systems are
efficient but relatively low stability at high temperature, high leaching and low catalytic activity
limit their applications. Accordingly, with the aim of increased ease of recovery, increased
loading and stability of the catalyst, non-toxic magnetic nanoparticles as an environmentally-
friendly material could be a desirable choice to immobilize polymeric support.38
In this study, we report the synthesis and application of poly(vinyl imidazole)/Pd coated
magnetic nanoparticles as an efficient catalyst. In this catalytic system, magnetic nanoparticles
are coated by a polymeric support which bears palladium nanoparticles. Strong complexation
could dramatically enhance catalytic activity for selective oxidation of alcohols in water
medium. In addition, easy separation and recovery of the catalyst due to magnetic properties
present an interesting alternative to other synthetic oxidation methods.
Experimental
Reagents and analysis
Ferric chloride hexahydrate (FeCl3.6H2O), ferrous chloride tetrahydrate (FeCl2.4H2O), ammonia
(30%) and methyl acrylate were obtained from Merck, 1-Vinylimidazole, tetraethoxysilane
(TEOS) and 3-aminopropyltriethoxysilane (APTES) were obtained from Aldrich. 1-
Vinylimidazole was distilled before use and 2′-Azobisisobutyronitrile (AIBN, Kanto, 97%) was
recrystallized in ethanol.
3

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Thin layer chromatography (TLC) was employed with silica gel 60 F254 plates and visualized by
UV light. FT-IR spectra of the samples were taken using an ABB Bomen MB-100 FT-IR
spectrometer. The samples were powdered and mixed with KBr to make pellets.
Thermogravimetric analysis (TGA) was acquired under a nitrogen atmosphere with a TGA Q 50
Thermogravimetric analyzer. Transmission electron microscopy (TEM) images were taken with
a Philips CM30 electron microscope. The morphology of the catalyst was observed using a
MIRA3 TESCAN scanning electron microscope (SEM).
Synthesis of amine functionalized magnetic nanoparticles (MNPs)
6.80 g of FeCl3.6H2O and 2.50 g of FeCl2.4H2O were added to 300 mL deionized water in a 250
mL round bottom flask under nitrogen atmosphere at room temperature; 70 mL ammonia
solution was then added dropwise into the mixture while it was vigorously stirred. When the
color of solution changed to black, the magnetite participates were separated with external
magnetic force and then washed several times with deionized water. 3 g of Fe3O4 was suspended
in 500 mL of 4/1 ethanol/water mixture by ultrasonication and pH was adjusted to 10 by adding
ammonia solution. Afterwards, 20 mL of tetraethoxysilane (TEOS) was added dropwise to the
magnetic solution (during 3 h) and the mixture was stirred using a mechanical stirrer under
nitrogen atmosphere at 50 °C. The stirring was continued for another 3 h. Then, the silica coated
magnetic nanoparticles (Fe3O4@SiO2) were magnetically separated and washed three times with
deionized water and ethanol. The final dark brown product was dried under vacuum at 50 °C for
24 h.
3 g of dry silica coated magnetic nanoparticles were suspended in 100 mL of 4/1 ethanol/water
mixture and the mixture was sonicated for 30 min and a solution of 6 mL of 3-
4

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aminopropyltriethoxysilane (APTES) added and heated at desired temperature (80 °C) for 24 h.
After completion of the reaction, the solution was allowed to cool to room temperature. Finally,
Fe3O4@SiO2@APTES (noted as MNPs) were separated by an external magnet and washed three
times with dry methanol and dried under vacuum for 24 h.
Synthesis of poly(1-vinylimidazole-co-methyl acrylate)
2 g of 1-vinylimidazole and 0.5 g of methyl acrylate were dissolved in 5 mL methanol in a 50
mL round bottom flask. The content of the flask was deoxygenated under argon for 20 min and
AIBN was added to initiate polymerization. The flask was equipped with a condenser and placed
in an oil bath at 75 °C for 18 h. After completion of the reaction, solvent was evaporated and the
solid product was washed five times with water (5×30 mL) and the final copolymer was dried in
vacuum at 50 °C.
Synthesis of the catalyst (MNP@PVim@Pd)
MNP (0.3 g) and poly(1-vinylimidazole-co-methyl acrylate) (1.0 g) were loaded into a 25 mL
round bottom flask and 10 mL ethanol was added to the mixture. The mixture was sonicated for
20 min and then deoxygenated under argon for another 20 min. The flask was equipped with a
condenser and placed in an oil bath at 80 °C for 24 h. The desired solid product (MNP@PVim)
was magnetically separated and washed four times with methanol (4× 20 mL) and dried under
vacuum at 50 °C.
To increase the solubility of PdCl2 in water, PdCl2 (0.044 g) and an excess amount of KCl (0.075
g) were dissolved in 10 mL deionized water in a 25 mL round bottom flask. The mixture was
stirred at room temperature for 1 h until an orange solution was formed. MNP@PVim (0.5 g)
5

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was then added into the solution and the mixture was stirred at room temperature for 24 h.
Subsequently, 0.094 g of NaBH4 was added to the mixture to reduce Pd (II) to Pd (0). The
reaction product (MNP@PVim@Pd) was magnetically separated, washed five times with
methanol (5×30 mL) and dried under vacuum at 50 °C.
General procedure for selective oxidation of alcohols catalyzed by MNP@PVim@Pd
The activity of the supported Pd catalyst was investigated for selective oxidation of alcohols
using atmosphere pressure as a source of oxygen (aerobic oxidation). 1 mmol of alcohol, 1 mmol
of potassium carbonate and 2 mL water were loaded into a 25 mL round bottom flask, and the
flask was equipped with a condenser. 5 mg of MNP@PVim@Pd was then added to the mixture
which was vigorously stirred for an appropriate time under reflux condition at 100 °C in an oil
bath. After completion of the reaction (monitored by TLC), the reaction was terminated and the
catalyst was magnetically separated. Following this, the catalyst was washed three times with
methanol (5×30 mL) and dried for another run. The organic product was extracted by hot ethyl
acetate and analyzed by gas chromatography (GC).
Result and Discussion
Co-precipitation of iron (II) and iron (III) in alkali solution was used to synthesis Fe3O4
nanoparticles with excellent magnetism which permits the catalyst to be recovered easily.
Surface modification of Fe3O4 with SiO2 was performed with TEOS in order to enhance its
chemical stability in the next synthesis steps. Coating of the MNP surface with APTES is
essential for covalent attachment of copolymer onto MNP surface. Magnetic nanoparticles
coated poly (1-vinyl imidazole-co-methyl acrylate) was prepared via co-polymer grafting to the
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surface of functionalized MNPs. This approach would be useful in high temperature applications
due to the presence of polymer chains which could prevent aggregation of MNPs. Palladium
ions were loaded by complexation of palladium chloride and co-polymer chains on the surface of
MNPs, which is depicted in Scheme 1. Afterward, NaBH4 was applied to reduce Pd (II) and
resulted in the desired magnetic poly (1-vinyl imidazole-co-methyl acrylate)-immobilized Pd
nanoparticles.
Such an approach for synthesis of heterogeneous catalyst increases the loading level of
palladium ions, due to the polymer coating, as relative to a conventional heterogeneous catalyst.
As a result, a low percentage amount of catalyst is required in comparison with the substrate
which makes it an effective protocol for industrial applications. Moreover, polymer coating
prevents agglomeration of Pd nanoparticles on the surface of MNPs.
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Scheme 1 Synthesis and structure of MNP@PVim@Pd.
At this step, FT-IR was utilized to characterize the catalyst structure. Fig. 1, shows the FT-IR
spectra of Fe3O4 (a), Fe3O4@SiO2 (b), MNP (c), poly(1-vinyl imidazole-co-methyl acrylate) (d)
and MNP@PVim (e). The FT-IR spectrum of Fe3O4 reveals a stretching mode vibration of Fe-O
at 640 cm-1 and Fe3O4@SiO2 shows an additional peak at 1100 cm-1 which verifies the presence
of Si-O in the structure (Fig. 1a, 1b). Coating of APTES onto the surface of Fe3O4 is confirmed
by the stretching mode vibration of C-O at 1011 cm-1 and C-N at 1364 cm-1 in Fig. 1b.
8

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Furthermore, N-H bending modes at 1622 cm-1 and aliphatic C-H bands at 2859 cm-1 and 2922
cm-1 clearly confirm coating of APTES onto the surface of Fe3O4. The FT-IR spectrum of
copolymer (Fig. 1c) displays characteristic peaks of imidazole rings at 1497 cm-1 and 1650 cm-1
which are attributed to C=C and C=N, respectively. Moreover, a weak peak at 2930 cm-1 is
representative of aliphatic C-H band which clearly indicates the presence of the imidazole ring.
In addition, a new band at 1729 cm-1 is attributed to C=O vibration mode which affirmed the
presence of methyl acrylate. After polymer coating onto the surface of MNP, all specific peaks
of MNP and polymer are easily observed (Fig. 1d). Moreover, a peak at 1690 cm-1 verifies the
formation of amides in the catalyst structure. These results confirmed that MNPs were properly
coated by co-polymer.
Fig. 1 FT-IR spectrum of Fe3O4 (a), Fe3O4@SiO2 (b), MNP (c), poly(vinyl imidazole-co-methyl
acrylate) (d), MNP@PVim (e).
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The thermal stability and organic content of MNP@PVim was evaluated by thermal gravimetric
analysis (Fig. 2). From the weight loss in the TGA curve of MNP (Fig. 2Ia), the loading amount
of APTES is 0.51 mmol g-1. Based on the TGA curve of MNP@PVim (Fig. 2Ib), the content of
co-polymer on the surface of MNP is about 20 wt%. In the DTG curve of the MNP@PVim three
main peaks were seen. Loss of adsorbed water molecules can be seen below 100 °C. Two other
peaks at 280 °C and 390 °C are attributed to methyl acrylate and imidazole parts in the polymer
structure, respectively. These results proved that the catalyst is thermally stable and could be
used under harsh reaction condition at high temperature (under 500 °C).
Fig. 2 (I) TGA curve of MNP (a) and MNP@PVim (b); (II) DTG curve for MNP@PVim.
The X-ray diffraction pattern of MNP@PVim@Pd is shown in Fig. 3, with characteristic peaks
and relative intensity, which completely matches with the standard Fe3O4 (red lines) (JCPDC file
No. 19-0629). These results confirmed that the modifications did not produce a phase change in
the Fe3O4 particles. The XRD pattern of Pd nanoparticle cannot be seen in Fig. 3 due to the
amount of Pd nanoparticles is less than MNPs.
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Fig. 3 The XRD pattern of MNP@PVim@Pd
Vibrating sample (VSM) has been employed to measure the magnetic properties of the catalyst.
The VSM curves of MNP@PVim@Pd and Fe3O4 are depicted in Fig. 4 which show small
coercivities and indicate the superparamagnetic nature of both materials. The saturation
magnetization of MNP@PVim@Pd is smaller than bare MNP due to polymer coating of MNPs.
However, the magnetization of the catalyst is high enough to ease the recovery process.
Fig. 4 Magnetization curves of MNP (a) and MNP@Pvim@Pd (b).
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The surface morphology of MNP@PVim@Pd was investigated using SEM which is illustrated
in Fig. 5a. The SEM image of the catalyst shows the presence of well dispersed polymer coated
magnetic nanoparticles. Energy-dispersive X-ray spectroscopy (EDS) analysis of the catalyst
directly confirmed the presence of Pd in the catalyst structure (Fig. 5b).
Fig. 5 SEM image of MNP@PVim@Pd (a); EDS analysis of MNP@PVim@Pd (b).
The TEM image of magnetic nanoparticles is represented in supporting information (Fig. 1s).
The TEM image of MNP@PVim@Pd is shown in Fig. 6. The TEM image of the catalyst shows
that palladium nanoparticles with the average size of 3 nm are dispersed in the polymeric matrix.
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Fig. 6 TEM image of MNP@PVim@Pd
Gel permeation chromatography (GPC) curve of the copolymer is shown in the Fig. 7. As seen in
the GPC curve of the copolymer, a single peak is observed, which suggests the monodispersion
of Mw and the absence of neither homopolymer nor monomers. Based on the GPC curve, Mw and
PDI of the copolymer are 7500 g/mol and 2.4, respectively.
Fig.7 GPC curve of copolymer.
Atomic absorption spectroscopy (AAS) is a suitable method to determine the loading amount of
transition metals. It was found that the loading amount of Pd in the catalyst is 0.31 mmol g-1.
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These results established the high capability of imidazole groups to coordinate Pd ions in the
structure of the catalyst.
The catalytic performance of MNP@PVim@Pd was investigated in the oxidation of alcohols. In
order to obtain the best catalytic activity, several optimizations, such as catalyst loading, solvent,
temperature, time and different bases were examined to obtain the best reaction condition for
oxidation of alcohols. Oxidation reaction of alcohols can be conducted in a broad range of
organic solvents. However, production of high amount of toxic waste restricts its utilization in
practical scale. Hence, water as a universal solvent with low cost and intrinsically green could
stand out in the role of chosen solvent. Therefore, we focused our attention to optimize catalyst
loading, base, temperature and time in water medium (Table 1). As seen in Table 1, no
significant amount of product was observed when no catalyst has been used even at higher
temperature (Entries 1, 2). The result of entry 3 shows that MNP@PVim has no catalytic activity
in the absence of palladium, however the use of 0.6 mol% of MNP@PVim@Pd gave 84% yield
at 70 °C. It was found that reduction in the amount of catalyst and temperature could reduce the
yield of products (Entries 4 to 8). As seen in the Table 1, increasing the temperature to 100 °C
improved the oxidation yield up to 99% (Entry 9). With the goal of using a lower amount of
catalyst, applying 0.1 mol% of MNP@PVim@Pd at 100 °C was designated for further
optimization of the model reaction (Entry 10). Several other bases were also tested which gave
lower yields (Entries 11 to 16). In solvent free condition, no substantial product was obtained
(Entry17). This could be related to difficulties in mixing of substrates under the solvent free
condition. In order to accentuate the expediency of water as a solvent, some other organic
solvents were also tested and the results are shown in entries 18 to 22. To investigate the
capability of the catalyst in large scale application, very small amount of the catalyst was used to
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oxidize benzyl alcohol. 10 mmol of reactants were utilized while only 0.1 mol% of catalyst was
consumed to give 92% yield in longer reaction time (30 h) (Entry 23).
Table 1 Control experiment for oxidation of benzyl alcohol.a
Cat. Loading
(mol%)
-
T
Time
(h)
24
24
24
24
24
24
24
24
24
18
18
18
18
18
18
18
18
18
18
18
18
18
30
Yieldb
(%)
<1c
<3c
20
84
81
76
54
43
99
99
99
75
80
65
27
73
40
62
65
53
24
52
92
Entry
Catalyst
Solvent
Base
(oC)
70
1
2
3
4
5
6
7
8
9
-
-
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
neat
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
CH3ONa
NaOH
Et3N
-
100
100
70
70
70
MNP@PVim
10 mg
0.6
0.3
MNP@PVim@Pd
MNP@PVim@Pd
MNP@PVim@Pd
MNP@PVim@Pd
MNP@PVim@Pd
MNP@PVim@Pd
0.1
0.09
0.03
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
70
70
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
10 MNP@PVim@Pd
11 MNP@PVim@Pd
12 MNP@PVim@Pd
13 MNP@PVim@Pd
14 MNP@PVim@Pd
15 MNP@PVim@Pd
16 MNP@PVim@Pd
17 MNP@PVim@Pd
18 MNP@PVim@Pd
19 MNP@PVim@Pd
20 MNP@PVim@Pd
21 MNP@PVim@Pd
22 MNP@PVim@Pd
23 MNP@PVim@Pdd
tBuOK
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
CH3CN
DMF
Toluene
THF
tBuOH
H2O
a Reaction condition: Benzyl alcohol (1 mmol), base (1 mmol), solvent (2 mL).
b GC yield.
c Mainly recovery of the starting materials.
d 10 mmol scale of reactant.
The scope of the selective oxidation reaction of alcohols catalyzed by MNP@PVim@Pd with
respect to the various primary and secondary alcohols was also explored (under the optimized
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reaction condition). It was revealed that the catalyst is highly selective since no benzoic acid was
produced. Wide diversity of benzyl alcohol substrates including electron withdrawing and
electron donating groups were readily oxidized under optimized reaction condition to
corresponding benzaldehydes with excellent to good yields. Electron withdrawing groups like Cl
and NO2 substitutes required longer reaction time with lower yields in comparison with electron
donating groups (Entries 4 to 11). Furthermore, oxidations of three kinds of secondary alcohols
under the same reaction condition were also examined and satisfactory conversion was achieved
(Entries 2, 3 and 12). Some primary aliphatic alcohols were also tested (Entries 13-18). These
kinds of alcohols showed lower yields in compare with aromatic ones. These results reveal the
efficiency of MNP@PVim@Pd for the selective oxidation of alcohols.
Table 2 Oxidation of alcohols catalyzed by MNP@PVim@Pd.a
Entry
1
Substrates
Time (h)
18
Yield (%)b
99
2
3
17
17
89
87
4
5
6
7
19
19
20
20
90
92
85
84
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8
9
17
21
83
80
10
11
22.5
22.5
77
75
12
21
90
13
14
15
16
17
18
22
22
22
22
22
22
78
73
74
74
74
74
a Reaction condition: Alcohol (1 mmol), K2CO3 (1mmol), MNP@PVim@Pd (0.1
mol%), H2O (2 mL), 100 oC.
b Yields were determined by GC.
An important aspect of heterogeneous catalyst in large scale application is the recyclability and
reusability of the exerted catalyst. To explore this potential, oxidation of benzyl alcohol in the
optimized reaction condition was carried out. After completion of the reaction, the catalyst was
easily separated and was used for the next run. As seen in Fig. 8, the catalyst was reused for 10
runs without significant loss of activity.
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Fig. 8 Recycling experiment.
To verify whether the observed catalytic activity is derived from leached metal species or solid
support, the typical metal leaching test was performed. To demonstrate the heterogeneity of the
catalyst, the oxidation of benzyl alcohol under optimized reaction condition was conducted.
After 6 h, the reaction was stopped and the catalyst was completely separated with external
magnetic force. The reaction mixture allowed stirring for 18 h without catalyst and no more
benzaldehyde was obtained (Fig. 9). After separation of catalyst, the reaction mixture was
analyzed by induction coupled plasma (ICP-AES) and no palladium specie was detected. These
results corroborate the exact heterogeneity of MNP@PVim@Pd. Moreover, no remarkable
change in the catalyst structure was seen in FT-IR spectrum and TEM image of the recycled
catalyst (See supporting information Fig S3 and Fig S4) which truly establishes the stability of
the catalyst even after 10 subsequent runs.
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Fig. 9 Leaching experiment. Blue line: reaction in the presence of catalyst and Red line: reaction
after removal of catalyst.
Conclusion
In summary, we have developed a novel and green method to oxidize primary and secondary
alcohols to corresponding aldehydes or ketones via magnetically reusable palladium catalyst.
The resulting catalyst demonstrated high activity and selectivity in the oxidation of alcohols with
an ideal green process under atmospheric pressure in water medium. The catalyst was recovered
easily by an external magnet and reused in subsequent runs (10 runs) without significant loss of
activity. The operational simplicity, purity and high yields of products, absence of organic
solvents, use of atmospheric pressure of molecular oxygen as an oxidant and the high
recyclability of the catalyst (up to 10 runs) validate the potential of this catalyst for large scale
applications.
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Supporting information
Supporting information may be found in the online version of this article.
Acknowledgments
We are grateful for financial support by the Iran's National Elites Foundation.
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Graphical Abstract
Highly Dispersible and Magnetically Recyclable Poly(1-Vinyl
Imidazole) Brush coated Magnetic Nanoparticle: an Effective
Support for Immobilization of Palladium Nanoparticles
A heterogeneous recoverable catalyst was prepared via complexation of palladium onto the surface of magnetic
nanaoparticles coated by poly(1-vinyl imidazole) brush. The stable, active and reusable catalyst was proven to be
highly active in aerobic oxidation of primary and secondary alcohols with excellent yields. Only 0.1 mol% of
catalyst was used to oxidize 1 mmol of primary and secondary alcohols.