A novel magnetic hybrid nanomaterial as a highly efficient and selective catalyst for alcohol oxidation based on new Schiff base complexes of transition metal ions

Article abstract of DOI:10.1016/j.molcata.2014.10.024

In this study, novel heterogeneous catalysts of Schiff base complexes of transition metal ions supported on silica coated magnetic metallic cobalt nanoparticles were successfully synthesized. Firstly, silica coated cobalt nanoparticles were synthesized th

Full text of DOI:10.1016/j.molcata.2014.10.024

Accepted Manuscript
Title: A novel magnetic hybrid nanomaterial as a highly
efficient and selective catalyst for alcohol oxidation based on
new Schiff base complexes of transition metal ions
Author: Amir-Reza Judy-Azar Sajjad Mohebbi
PII:
S1381-1169(14)00479-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.10.024
MOLCAA 9326
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Journal of Molecular Catalysis A: Chemical
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Revised date:
Accepted date:
27-5-2014
23-10-2014
26-10-2014
Please cite this article as: A.-R. Judy-Azar, S. Mohebbi, A novel magnetic hybrid
nanomaterial as a highly efficient and selective catalyst for alcohol oxidation based on
new Schiff base complexes of transition metal ions, Journal of Molecular Catalysis A:
Chemical (2014), http://dx.doi.org/10.1016/j.molcata.2014.10.024
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A novel magnetic hybrid nanomaterial as a highly efficient and selective catalyst for alcohol
oxidation based on new Schiff base complexes of transition metal ions
Amir-Reza Judy-Azara, Sajjad Mohebbi b,*
a,b*Department of Chemistry, University of Kurdistan, P.O. Box 66175-416, Sanandaj, Iran
Corresponding author email address: smohebbi@uok.ac.ir, Tel: +98(871) 6624133
Abstract
In this study, novel heterogeneous catalysts of Schiff base complexes of transition metal ions
supported on silica coated magnetic metallic cobalt nanoparticles were successfully synthesized.
Firstly, silica coated cobalt nanoparticles were synthesized through a one-pot reaction. Secondly,
Schiff base complexes of different transition metal ions were immobilized on a magnetic support.
The catalytic activity of catalysts was studied in oxidation of various aromatic alcohols with
environmentally friendly oxidant under different solvents and solvent free conditions. Also, the
effects of parameters such as solvent, amount of oxidant and catalysts, reaction temperature and
utilization efficiency of hydrogen peroxide were investigated. The synthesized catalysts acted
selectively and converted the alcohols to corresponding aldehydes with high yield. The synthesized
catalysts were fully characterized by a verity of techniques such as X-ray diffraction (XRD), EDAX
analysis, scanning electron microscopy and FT-IR analysis. Also, the magnetic properties of these
catalysts were investigated by AGFM. The catalysts could be reused several times without significant
loss in their activities.
Keywords: Schiff base, oxidation, nanoparticles, cobalt, magnetic materials, hybrid materials.
Abbreviations1
1. Introduction
Organic products such as aldehydes and ketones are considerably important in organic syntheses in
laboratories and chemical industries. Also, these products produced from alcohol oxidation are
important intermediates for pharmaceuticals, agricultural chemicals and fine chemicals [1]. In
general, the oxidation process is carried out with stoichiometric amounts of toxic and/or
homogeneous oxidants like MnO4 [2], bromate [3], MnO2, SeO2, and Br2 [4], but these materials
produce a large amount of waste which is unacceptable from the point of green chemistry. So, these
oxidants must be replaced with clean ones. Thus, it necessitates preparing heterogeneous catalysts
which are highly active, selective and recyclable [5].
A big challenge for researchers both in academic and industry area is to expand a greener oxidation
process that is efficient, cheaper and chemoselective [6].
To date, a numerous numbers of heterogeneous catalysts based on metal hydroxides, metal oxides
and metal clusters have been produced for the oxidation of alcohols [7]. But these catalysts have
some drawbacks including tough conditions for preparation, being expensive and low activity for
oxidation of inactive alcohols. Many different transition metals and also noble metals such as Pd [8],
Cu [9], Ru [10], Co [11] and Au [12] have been applied for the oxidation of alcohols. But these kinds
of systems need high loading of reagent and tough reaction conditions. The used noble metals such
as Pd, Ru and Au are expensive and not readily available. Also, the aerobic oxidation reaction of
alcohols has been investigated by transition metal nanoparticles and many of them have shown high
catalytic activity and selectivity [13,14]. In spite of these advances, persistent problems still exist. On
the other hand, some of the oxidation reactions are performed under severe conditions such as high
temperature and high oxygen pressure [15].
1
MCSNPs: magnetic core-shell nanoparticles, MCNPs: magnetic cobalt nanoparticles, SBC: Schiff base complex
1
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The selective oxidation of alcohols into corresponding carbonyl compounds as a functional group
transformation has been occupied an outstanding part in modern synthetic organic chemistry [16].
Due to the importance of aldehydes in the synthesis of fine chemicals such as fragrances or food
additives [17,18] conversion of primary alcohols to aldehydes has been attracting researcher’s
attention. It has been reported that nitroxyl radicals such as 2,2,6,6,-tetramethylpiperiduine-1-oxyl
[19-20] have displayed great efficiency in selective oxidation of alcohols to corresponding aldehydes
and ketones.
Using hydrogen peroxide or molecular dioxygen is a ‘‘green’’ oxidation process and it is the most
attractive oxidation technology for selective organic syntheses. Also, some work has been reported
from Kaneda’s research group for alcohol oxidation using molecular oxygen by ruthenium catalyst
[21], giant palladium cluster complexes [22], Ru/CeO2/CoO(OH) catalyst [23], MnFe1:5Ru0.35Cu0.15O4
catalyst [24], Ruthenium-HAp-Encapsulated Superparamagnetic δ-Fe2O3 Nanocrystallites [25].
Hydrogen peroxide is an environmentally friendly oxidant for a variety of oxidation reactions [26]. It
may overcome the disadvantage of the toxic inorganic oxidants [27]. Besides, in comparison with
other organic peroxides or peracids, hydrogen peroxide is much cheaper and safer.
Among catalysts some of the significant specification of metal complexes of Schiff bases has
developed the coordination chemistry due to the structural variety and preparative accessibility.
Thus, there has been increasing interest in synthesis and characterization of these materials [28-31].
On the other hand, transition metal complexes are highly effective catalysts. Also, they have a high
selectivity in a variety of organic reactions. The main focus of current developments in the field of
catalysis is to immobilize some metal ion complexes either on functionalized solid supports or on
inert materials [32]. Also, due to the activities of metal (II) Schiff-base complexes they are of a great
importance [33,34]. These attractions arise from their important biological, catalytic and magnetic
properties [35-37].
In catalytic technology one of the essential steps is separation and recycling of the catalyst which
mostly affects the overall process. Although the homogeneous catalysts are considerably efficient,
they have drawbacks such as difficulty in separation and reuse of the catalyst. So, fabrication of
novel and efficient heterogeneous catalysts is definitely necessary. Because of this reason
immobilization of homogeneous catalysts on a special support has attracted a great deal of
attention. A possible method is to immobilize active catalyst on the surface of magnetic support that
can be easily separated and recovered from the reaction media using magnet [38,39].
Magnetic nanoparticles have been broadly applied to various applications such as magnetic
resonance imaging, purification/separation of proteins, nucleic acids and magnetic drug targeting
unique magnetic, catalytic and optical properties. These nanoparticles have attracted a great deal of
interest in several areas such as physics, material sciences, chemistry, medicine and biology [41].
In comparison to iron oxide (Fe3O4) particles, magnetic metal nanoparticles such Fe, Co, FePt, or
FeCo are of great significance for some reason. Firstly, there is a high possibility to obtain a narrow
size distribution, whereas in case of iron oxides often rather broad particle size distribution is
obtained. Secondly, magnetic metal nanoparticles possess high saturation magnetization. Thus, due
to the higher magnetization property, cobalt nanoparticles are considered as a potential candidate
for applications [42].
So, the use of magnetically recoverable catalysts has been recently developed in hydrogenation
reactions [43], olefin epoxidation [44], decarboxylative coupling [45] and asymmetric hydrogenation
[46].
In recent years many catalytic systems have been reported for the oxidation of alcohols including
polymer-supported periodic acid under mild aprotic condition [47], ruthenium hydroxides [48], Au,
Pd, and bimetallic AuPd nanoparticle catalysts [49], aerobic oxidation of alcohols by CuCl/TEMPO
[50], cobalt complex with 4’-(2-thienyl)-2,20;60,200-terpyridine ligand [51], aqueous 10% H2O2 and
iron-benzenetricarboxylate metal-organic gel [52], N-hydroxyindole and copper(I) chloride [53],
silver-based catalyst promoted by the presence of ceria [54], copper oxide-supported gold catalysts
2
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[55], dense CO2 [56], Iron(III)-Schiff base-triphenylphosphine complexes [57] and oxidation of benzyl
alcohol catalyzed by carbon nanotubes [58].
However, it is still a big challenge to synthesize highly efficient catalysts for oxidation reactions
under milder conditions using green oxidants.
In this paper, we present preparation and characterization of novel hybrid materials as a
heterogeneous catalysts based on silica coated magnetic nanoparticles with new Schiff base
complexes. One of the advantages of these systems is relatively a strong chemical interaction
between Schiff base transition metal complexes and silica groups immobilized on the surface of
magnetic nanoparticles. On the other hand, due to the superparamagnetic properties of cobalt core,
the prepared hybrid nanomaterials are easily separable from the solution after the completion of
the reaction. So, they can act as magnetically recoverable catalysts. So far, there have been a few
reports of magnetic catalyst such as Nanoparticle-supported and magnetically recoverable palladium
[59] and Ruthenium-Hydroxyapatite-Encapsulated Super-paramagnetic δ-Fe2O3 Nanocrystallites
[25]. There is only one report of immobilizing Schiff base complexes on magnetic base which used as
recyclable catalyst for synthesis of 1,1-diacetates [60]. To the best of our knowledge, this is the first
report of hybrid materials based on magnetic metallic cobalt core as a support that has been
synthesized and applied for alcohols oxidation.
2. Experimental and methods
2.1 Synthesis of magnetic cobalt nanoparticles (MCNPs)
Cobalt nanoparticles were synthesized by a modified method [61]. Cobalt chloride hexahydrate (1 g)
was dissolved in 60 ml doubly distilled water. Then 0.07 g of citric acid was added to the solution
following by argon bubbling into the solution for 2 minutes. In a beaker, 3 g sodium borhydride was
dissolved in 100 ml water and drop wise added to the stock solution under argon bubbling. With the
first drop of NaBH4, the solution color was rapidly turned to dark black indicating the formation of
cobalt nanoparticles.
2.2 Synthesis of magnetic Co@SiO2 core-shell nanoparticles (MCSNPs)
The solution containing MCNPs was left to be stirred for 30 minutes. Then the solution of 200ml
ethanol, 200 µl 3APTES and 800 µl TEOS was added to the solution of cobalt nanoparticles. The
argon atmosphere was cut off and the solution strongly stirred for 24h. After this period, a puffy
black precipitate with strong magnetic property was obtained and separated by magnet and washed
several times with ethanol and water. Then the precipitate was left to be dried at 50 °C in the oven.
Scheme 1. Process for the synthesis of Co@SiO2 nanoparticles
3
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2.3 Synthesis of Schiff base ligand
To synthesize an appropriate ligand with the capability of binding to the surface of MCSNPs,
equimolar amounts of 3-aminopropyl-triethoxysaline (3APTES) and tetraethyl orthosilicate (TEOS)
was mixed as follows: 1 mmol of 3-aminopropyl-triethoxysilane was added to 5ml methanol (A). 1
mmol of 2-hydroxy Benzaldehyde was added to 5mmol methanol (B). Then solution A and B was
mixed and stirred for 30 minutes. A yellow solution obtained by mixing two solutions that are
indicative of the formation of an imine bond in Schiff base ligand.
2.4 Synthesis of Schiff base metal complexes
2.4.1 Synthesis of Schiff base nickel (II) complex, [Ni(II)SBC]
In a typical procedure, A methanolic solution (10 mL) of Ni(acac)2 (1mmol) was added to 10 ml
methanolic solution of Schiff base ligand and the total volume was reached to 40 mL by adding the
same solvent. The mixture was refluxed for an hour to complete the reaction. A bright green
precipitate was obtained and washed with water and ethanol and filtered by Whatman paper and
dried at 60 °C.
2.4.2 Synthesis of Schiff base cobalt (II) complex, [Co(II)SBC]
Schiff base cobalt (II) complex was prepared by a procedure as follows: A methanolic solution (10 ml)
of Co(acac)2 (1mmol) was added to 5 ml methanolic solution of Schiff base ligand and the total
volume was reached to 40 ml by adding the same solvent. The mixture was refluxed for an hour to
complete the reaction. A green precipitate was obtained and washed with water and ethanol and
filtered by Whatman paper and dried at 60 °C.
2.4.3 Synthesis of copper (II) Schiff base complex, [Cu(II)SBC]
As mentioned above, Schiff base copper (II) complex was prepared as same procedure as above by
copper (II) acetylacetonate with this difference that a dark green color precipitate was obtained. The
precipitate was washed with water and ethanol and filtered by Whatman paper and dried at 60 °C.
2.4.4 Synthesis of Zinc (II) Schiff base complex, [Zn(II)SBC]
A same procedure was carried out with zinc (II) acetylacetonate and a bright yellow precipitate was
obtained. The precipitate was washed with ethanol and filtered by Whatman paper and dried at 60
°C.
2.5 Synthesis of magnetic catalysts
2.5.1 Synthesis of Co@SiO2@[Ni(II)SBC] catalyst
The magnetic hybrid nanomaterials were synthesized as following procedures: 0.3 g MCSNPs were
ultrasonically dispersed in 10ml ethanol. Then 0.2 g of Schiff base nickel complex was added to the
solution and total volume was reached to 30 ml with the same solvent. The mixture was refluxed for
10 hours. The obtained light green magnetic precipitate was collected by a magnet and washed with
doubly distilled water and dried in an oven at 60 °C for 12h.
2.5.2 Synthesis of Co@SiO2@[Co(II)SBC] catalyst
0.3 g magnetic Co@SiO2 nanoparticles were added to 10ml ethanolic solution and dispersed
ultrasonically. Then 0.2 g of Schiff base cobalt complex was added to the solution and total volume
was reached to 30 ml with the same solvent. The mixture was refluxed for 10 hours. After this
period, a green magnetic precipitate was obtained. The produced precipitate was collected by a
magnet and washed with doubly distilled water and dried in an oven at 60 °C for 12h.
2.5.3 Synthesis of Co@SiO2@[Cu(II)SBC] catalyst
The same procedure was carried out to synthesize hybrid Co@SiO2@[Cu(II)SBC] nanomaterials. 0.3 g
MCSNPs were added to 10ml ethanolic solution and dispersed ultrasonically. 0.2 g of Schiff base
4
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copper complex was added to the solution and total volume was reached to 30 ml. The mixture was
refluxed for 10 hours. After this period, a light green magnetic precipitate was obtained. The product
was collected by a magnet and washed with water and dried in an oven at 60 °C for 12h.
2.5.4 Synthesis of Co@SiO2@[Zn(II)SBC] catalyst
Same as an above procedure, 0.3 g MCSNPs were added to 10ml ethanolic solution and dispersed
ultrasonically. 0.3 g of Schiff base zinc complex was added to the solution and total volume was
reached to 30 ml of solvent. The mixture was refluxed for 10 hours. After this period, a light yellow
magnetic precipitate was obtained. The product was collected by a magnet and washed with water
and dried in an oven at 60 °C for 12h.
3. Typical procedure for alcohol oxidation
The oxidation process was carried out under identical conditions with special care. The oxidation of
alcohols was carried out with H2O2 in the presence of different synthesized catalysts at atmospheric
pressure and temperature of 50 °C.
The yield of the reactions was determined by extraction method as follows: The heterogeneous
catalyst was separated from the reaction mixture. Then the organic phase was separated by
water/ethyl acetate extraction method and the solvent was dried. Next, in order to isolate the
aldehyde from the remaining alcohol the mixture was stirred in warm n-hexane several times. Thus,
the aldehyde was dissolved in warm n-hexane and isolated from remaining alcohol. The remaining
aldehyde was dried and weighted. The weighted aldehyde was compared with the initial amount of
alcohol used in the oxidation reaction. Then, by a simple calculation the yield of the reaction was
obtained ([amount of obtained aldehyde]/[amount of theoretical weight] ×100).
In order to determine the amount of metal ions Co, Cu, Ni and Zn loaded on magnetic support the
hybrid materials were decomposed by strong acids. For this purpose, 0.1 mg of catalyst, 0.5 ml of
perchloric acid and nitric acid (0.5 ml) solution were mixed. The mixture was heated for 20 minutes.
Then, the metal content of the catalyst was estimated by Atomic Absorption Spectroscopy (AAS).
The Co, Cu, Ni and Zn content of the catalysts estimated by AAS were 9.81, 9.52. 10.12 and 9.26 %wt
respectively (Table 5).
3.1 Sheldon’s test for heterogeneous catalyst
The hot filtration test is a useful method to determine whether the catalyst truly acts as
heterogeneous or not [62]. The hot filtration test was carried out and the result was as follows. The
reaction media were filtered in hot condition and the catalyst remained on the filter paper and the
filtrate came through and the reaction was continued. It was found that the conversion remained
the same of the filtrate. This showed that there was no leaching of the immobilized catalyst. So the
synthesized catalyst is heterogeneous.
4. Results and discussions
Since the presence of the magnetic cobalt core made product analysis by NMR spectroscopy
impossible, the material produced was analyzed by IR spectroscopy and energy dispersive x-ray
analyses. In order to confirm the surface modification of the silica coated cobalt nanoparticles with
Schiff base complexes of metal ions the FT-IR spectra of the prepared magnetic hybrid
nanomaterials were obtained (Fig. S1). The free Schiff base ligand shows υ(C-N) stretch at 1630-1635
cm-1 while this band shifts to lower frequency in the complexes and manifests at 1616-1630 cm-1
which is due to the coordination of the nitrogen with the metals. Also the bands in the range of
5
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2770-2900 cm-1 are related to the C-H stretching vibration of methylene groups. In the FT-IR
spectrum of Co@SiO2 there are some obvious bands. A broad and sharp band at 1089 cm-1 is
attributed to Si-O-Si indicating the formation of silica coated Co nanoparticles. The broad peak at
3469 cm-1 is attributed to the OH groups of silica coated cobalt nanoparticles. The FT-IR spectra of
synthesized hybrid nanomaterials also show some bands indicating the formation of hybrid
materials. Fig. S1a shows the FT-IR spectrum of Schiff base zinc complex anchored from Co@SiO2
nanoparticles. As it is obvious there is a sharp band at 1627cm-1 that is indicative of the coordination
of the nitrogen with the Zn(II) and formation of hybrid material. Also, Figures S1b, S1c and S1d show
the sharp bands at 1629 cm -1, 1627 cm-1 and 1616 cm-1 which are respectively indicative of
coordination of the azomethene nitrogen with Ni(II), Cu(II) and Co(II) metal ions confirming the
formation of hybrid materials. Other bands are nearly the same that were discussed above in the
case of free Schiff base ligand.
In addition to the Sheldon’s test which confirmed that the reaction occurred in a heterogeneous
way, FT-IR spectrum of the catalyst was provided after the reaction done. As it is shown in Fig.S2
there are no any changes in the peaks positions of the catalyst before and after the reaction which is
the confirmation of the heterogeneous catalyst.
Fig. S3 shows the XRD patterns of the synthesized nanomaterials. Fig. S3a shows the XRD pattern of
cobalt nanoparticles. As it is seen the characteristic peak for cobalt appears at 2θ=44.2. In the XRD
pattern of Co@SiO2 and Co@SiO2 coated with verities of Schiff base complexes [Fig. S3b-S3e] the
characteristic peaks of cobalt has not been changed but only the peak intensity.
To further confirm the formation of hybrid nanomaterials energy dispersive x-ray analyses were
provided. Fig. S4 shows the EDAX analyses of synthesized materials. All the analyses confirm the
formation of hybrid nanomaterials. EDAX analysis of Co@SiO2 nanoparticles shows peaks of
elements Co, Si and O indicating the formation of Co@SiO2 and purity of synthesized nanomaterials.
As can be seen the other four analyses in addition of elements such as Co, Si and O show the
elements C, N, Ni, Cu, Co and Zn which is confirming the immobilization of Schiff base complexes on
silica coated Co NPs.
Fig 1. Scanning electron microscopy images of cobalt nanoparticles, silica coated cobalt (Co@SiO2)
NPs and synthesized hybrid nanomaterials
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To study morphology of the synthesized nanomaterials SEM images were taken (Fig. 1). Four images
are representative of four hybrid nanomaterials. They show that the synthesized nanomaterials are
uniformly distributed spherical and are in the range of 30-60 nm.
The magnetic properties of synthesized hybrid nanomaterials were also investigated by alternative
gradient force magnetometer (AGFM) at room temperature. Fig. S5 shows the magnetization curves
of prepared materials. The magnetization curve of magnetic Co@SiO2 nanoparticles shows no
remanence which indicates their superparamagnetic properties. Also, magnetization curves of the
hybrid nanomaterials show superparamagnetic behavior and decreased saturation magnetization
further confirms the immobilization of Schiff base complexes on the silica coated nanoparticles.
Thus, transition metal ion complexes can still be separated from the product by applying an external
magnetic field.
4. Investigation of catalytic activity of synthesized magnetic catalysts
Herein, we are going to report a highly simple method for selective oxidation of alcohols (scheme 2).
Benzyl alcohol was selected as a model substrate for the oxidation process. At first, initial test was
run with catalysts without optimized amounts and the results show that all catalysts are highly active
(table 1). The catalysts acted selectively which is a noticeable advantage. To optimize the reaction
conditions to find the best amounts of catalysts and oxidant different amounts of catalysts and
hydrogen peroxide were used. The catalyst Co@SiO2@[Cu(II)SBC] was used as a model catalyst. The
results have been summarized in table 2-3. Optimized amounts of used catalyst and hydrogen
peroxide were determined 0.04 g and 60 µl respectively. To examine the efficiency of other
synthesized Co@SiO2@Schiff base complex of metal ions the optimized amounts were used in
reaction medium and the product was analyzed after 50 min. The results are shown in table 4. As it
is observed all of the catalysts are highly active and act selectively. Also reaction yield is more than
90 % in the presence of these nano-hybrid catalysts.
To further confirm the activity of catalyst as an important agent in the oxidation process, the
reaction was run in the absence of catalysts. As it is shown (table 3, entry 7) the oxidation reaction is
not completed in the absence of catalysts and the yield of the reaction is only 7%. This shows that
the reaction cannot proceed in the absence of catalysts which indicates that applied catalysts are
crucial for the oxidation reaction.
Photographs of synthesized magnetic hybrid nanomaterials
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Scheme 2. Process of alcohol oxidation with magnetic Schiff base complexes
Table 1. Initial tests: Catalytic results of benzyl alcohol oxidation with H2O2 by various Schiff base ions
complexes supported on Co@SiO2
Entry Oxidant
Catalyst (g)
product
Selectivity (%)
isolated
yield
23
1
2
3
4
H2O2
H2O2
H2O2
H2O2
Co@SiO2@[Zn(II)SBC]
Co@SiO2@[Cu(II)SBC]
Co@SiO2@[Co(II)SBC]
Co@SiO2@[Ni(II)SBC]
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
>99
>99
>99
>99
31
30
34
a Reaction condition: benzyl alcohol (0.5 mmol), hydrogen peroxide 30% (30 µl), catalyst 0.02 g,
reaction time: 50 min, reaction temperature 50 °C.
Table 2. Effect of catalyst amount in the yield of BzOH oxidation reactiona
Entry
H2O2
30% (µl)
30
Co@SiO2@[Cu(II)SBC] (g)
Product
Selectivity
(%)
Isolated
yield
33
1
2
3
4
5
6
7
0.02
0.025
0.03
0.035
0.04
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
>99
>99
>99
>99
>99
>99
>99
30
30
30
30
30
30
38
41
49
58
58
4
0.045
Not used
a Reaction condition: benzyl alcohol (0.5 mmol), reaction time: 50 min, reaction temperature 50 °C.
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Table 3. Effect of hydrogen peroxide concentration in oxidation of BzOHa
Entry
H2O2
30% (µl)
20
Co@SiO2@[Cu(II)SBC] (g)
Product
Selectivity
(%)
isolated
yield
52
1
2
3
4
5
6
7
0.04
0.04
0.04
0.04
0.04
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
>99
>99
>99
>99
>99
>99
>99
30
40
50
60
65
65
58
68
79
91
91
7
0.04
Not used
a Reaction condition: benzyl alcohol (0.5 mmol), reaction time: 50 min, reaction temperature 50 °C.
4.1. Effect of solvents on the oxidation of benzyl alcohol in the presence of Co@SiO2@[Cu(II)SBC]
Effect of different solvents was investigated on the oxidation of alcohols. Reaction proceeding was
monitored by thin layer chromatography (TLC) technique. According to the previous results which
obtained as optimized amounts, 0.04 g catalyst and 60 µl of hydrogen peroxide were used in
different solvents such as water, acetonitrile, EtOH, ethyl acetate and DMF. The results have been
summarized in Table 6. The results show that the best yield is obtained in acetonitryl and ethyl
acetate solvents. It is also seen that benzaldehyde is the only produced product in all solvents and
the selectivity always remains 100% in this system.
To investigate the effect of support on the reaction of alcohol oxidation the reaction was run in the
presence of Co@SiO2 with hydrogen peroxide (60 µl H2O2, 30 %, 0.587 mmol). As it is shown in table
4, the alcohol is not converted to the corresponding aldehyde in the presence of Co@SiO2. This
indicates that the reaction is only proceed in the presence of Schiff base catalysts.
Table 4. Catalytic activity of Schiff base complexes of metal ions in optimized conditions.a
Entry
Catalyst [0.04g]
H2O2 30%
(µl)
Product
Isolated
yield
92
94
91
8
1
2
3
4
5
Co@SiO2@[Co(II)SBC]
Co@SiO2@[Ni(II)SBC]
Co@SiO2@[Zn(II)SBC]
Co@SiO2
60
60
60
60
60
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
benzaldehyde
No catalyst
6
a Reaction condition: benzyl alcohol (0.5 mmol), catalyst 0.04 g, reaction time: 50 min, reaction
temperature 50 °C.
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Table 5. Hydrogen peroxide utilization efficiency of each synthesized catalyst
Entry
Catalysta
H2O2 30%
(µl)
Product
Yield
H2O2
Efficiency %c
78
1
2
3
4
Co@SiO2@[Co(II)SBC]
Co@SiO2@[Ni(II)SBC]
Co@SiO2@[Zn(II)SBC]
Co@SiO2@[Cu(II)SBC]
60
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
92
94
91
91
60
60
60
80
77
77
a Catalyst amount: 0.04 g, hydrogen peroxide 30% (9.79 mmol/ml), c [ Aldehyde]/[H2O2]c × 100 (%)
where [H2O2]c is the concentration of consumed H2O2.
A catalyst may be defined by its Turnover Number (TON) and Turnover Frequency (TOF). Table 5
shows the TON and TOF numbers of the synthesized nanocatalysts. As it is clear the catalysts have
high TON and TOF numbers (Table 5).
Table 6. Turnover number and turnover frequency for the catalysts and the amount of metals loaded
on magnetic support determined by AAS.
entry
catalyst
Metal loaded (wt %) TONa (101) TOFb (101)
1
2
3
4
Co@SiO2@[Co(II)SBC]
Co@SiO2@[Cu(II)SBC]
Co@SiO2@[Ni(II)SBC]
Co@SiO2@[Zn(II)SBC]
Co: 9.81
Cu: 9.52
Ni: 10.12
Zn: 9.26
2.812
3.214
2.647
3.103
3.374
4.285
3.379
3.723
a Moles of substrate converted per moles of metal in the catalyst. b Moles of substrate converted per
mole of metal in the catalyst multiplied by time.
Table 7. Effect of solvent on the oxidation reaction of benzyl alcohol to Benzaldehyde
Entry
catalyst
solvent
product
Time (min) Selectivity (%) isolated
yield
1
2
3
4
Co@SiO2@Cu(II)SBC
Co@SiO2@Cu(II)SBC
Co@SiO2@Cu(II)SBC
acetonitrile benzaldehyde
50
50
50
50
100
100
100
100
97
81
87
91
water
EtOH
benzaldehyde
benzaldehyde
Co@SiO2@Cu(II)SBC ethyl acetate benzaldehyde
5
Co@SiO2@Cu(II)SBC DMF benzaldehyde
50
100
85
Reaction condition: benzyl alcohol (0.5 mmol), hydrogen peroxide 30% (60 µl), catalyst 0.04 g,
reaction temperature 50 °C.
10
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Table 8. Investigation of catalytic activity of Co@SiO2@[Co(II)SBC] catalyst on the oxidation reactiona
entry
1
catalyst
Time
(min)
product
selectivity % Isolated
Yield %
Substrate
HO
O
O
Cl
Cl
Co@SiO2@[Co(II)SBC]
Co@SiO2@[Co(II)SBC]
Co@SiO2@[Co(II)SBC]
Co@SiO2@[Co(II)SBC]
Co@SiO2@[Co(II)SBC]
27-28
24-26
32-33
22-25
25-27
100
100
100
100
100
91
93
94
90
91
HO
HO
2
3
4
5
O
F
F
HO
O
O
O
-O
O
OH
-O
O
O
N+
N+
O
HO
6
7
Co@SiO2@[Co(II)SBC]
Co@SiO2@[Co(II)SBC]
24-26
33-35
100
100
92
83
OH
O
a Reaction conditions: alcohol (0.5 mmol), catalyst (0.04 g), hydrogen peroxide 30 % (60 µl), reaction
temperature 50 °C.
4.2. Effect of temperature on the oxidation reaction
Temperature is another important parameter for the oxidation of alcohols. As it is observed the
above reactions were carried out at 50 °C and the high yields were obtained. To show the effect of
temperature the reaction was run at room temperature to a high temperature in the presence of
optimized amounts of Co@SiO2@[Cu(II)SBC] catalyst and hydrogen peroxide. Fig. 2 shows that the
yield of the reaction is substantially influenced by temperature. By raising the temperature from 22
°C to 60 °C the yield of the reaction is increased from 54 to 96%. Also rising temperature does not
affect the selectivity of the reaction. The selectivity remains 100 % in any condition. This is a great
advantage of this reaction system. It was also shown that no significant increase is observed in the
yield by rising temperature upper than 50 °C. Therefore, it is inferred that the optimized
temperature is 50 °C.
4.3. Effect of time on the oxidation reaction
The effect of time was investigated on the oxidation reaction. The results shown in Fig. 3 indicate
that the initial conversion of benzyl alcohol increases with the processing of reaction and the
maximum conversion of 96% is obtained after 30 min and then remains constant. Also, through the
range of given time the selectivity remains 100%.
11
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Fig. 2 Effect of temperature on the oxidation of benzyl alcohol catalyzed by Co@SiO2@[Cu(II)SBC].
The results of the oxidation of the variety of aromatic and aliphatic alcohols in the presence of the
catalyst Co@SiO2@[Co(II)SBC] have been summed up in table 7. The process was monitored by thin
layer chromatography. As it is seen the selectivity is 100% in any case of alcohol. There is a slight
difference in time of reactions which is probably due to the presence of electron-donating and
electron-withdrawing groups on the benzene ring. Another experiment was carried out with the
same conditions, but the applied catalyst was Co@SiO2@[Zn(II)SBC]. Table S1 shows the catalytic
activity of Co@SiO2@[Zn(II)SBC subjected to different alcohols.
Fig. 3 Effect of time on the conversion of benzyl alcohol to Benzaldehyde in the presence of different
synthesized catalysts
As the results show (table S1), the catalyst is active enough to oxidize the different alcohols to
corresponding aldehyde. The yield of the reaction is high indicating that most of the reactants
(alcohols) have been successfully converted to corresponding aldehydes.
At the end, catalyst Co@SiO2@Ni(II) was chosen in oxidation reaction. The obtained results indicate
that the highly active and the conversion of alcohols to corresponding aldehydes occur about during
half an hour (Table S2). The selectivity is considerably noticeable and remains 100 %. This is the
advantage of these catalysts which can selectively convert different alcohols to corresponding
aldehyde. The results show that catalyst containing Ni(II) metal ion is more active than three others
and converts alcohols to aldehydes in shorter time.
4.4. Reusability of the catalysts
Recycling of the used catalyst is extremely important in industry. To further evaluate the
performance of the magnetic materials, reuse experiments of the catalyst were carried out. The
results are shown in Fig. 4. To run the experiment, 0.04 g of each synthesized catalyst, 60 µl of
12
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hydrogen peroxide and benzyl alcohol were mixed in 100 µl acetonitrile as solvent. The activity of
catalysts decease after using five times, but the selectivity of the catalyst remains 100%.
4.5. Comparison of catalytic activity of synthesized catalysts
It is inferred from table 6-8 that Schiff base complex of Ni(II) metal ion is more active than the others
and convert alcohols to the corresponding aldehydes in a shorter time. The catalytic strength of
synthesized Lewis acids was found to be in the order of Ni(II) > Co(II) > Cu(II) ~ Zn(II). As the order
shows there is not any noticeable difference between catalytic activity of Cu(II) and Zn(II) Lewis
acids. Fig. 3 represents the discussed results. As it is clear the catalyst containing Ni(II) complex
converts alcohols during 21-30 minutes and reaction are completed, but the other three catalysts
are not completing the oxidation reaction during this time and need more time.
Fig 4. Recycling of the four synthesized magnetic catalysts in oxidation reaction
5. Conclusion
Novel hybrid nanomaterials based on silica coated magnetic cobalt nanoparticles were successfully
synthesized. Schiff base complexes of four metal ions were immobilized on the magnetic support by
simple reflux. The synthesized materials were used as catalysts for the oxidation of alcohols. These
hybrid nanomaterials showed outstanding catalytic performance with selective conversion of BzOH
at 50 °C in solvent (free) condition. This high activity and selectivity is most probably related to the
Lewis acidity of the catalysts. In conclusion, using hydrogen peroxide as mild oxidant in the presence
of the synthesized catalysts provides an efficient, easy and safe approach for the oxidation of
alcohols to corresponding aldehydes. It seems these heterogeneous catalysts could run other
organic reactions which need Lewis acid. The study of these reactions is in progress.
Acknowledgment
The financial support rendered by the University of Kurdistan is gratefully acknowledged.
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*Graphical Abstract (for review)
Page 16 of 17

*Highlights (for review)





Novel hybrid nanomaterials were synthesized as an environmentally friendly system.
Variety of Schiff base complexes were simply anchored from magnetic support.
Hybrid materials show superior catalytic activity.
Hybrid nanomaterials act selectively in oxidation reaction.
Oxidation reaction is carried out in shorter time with high yield.
Page 17 of 17