Magnetically recoverable porphyrin-based nanocatalysts for the effective oxidation of olefins with hydrogen peroxide: A comparative study

Article abstract of DOI:10.1039/d0nj04190d

In this paper, preparation, characterization and catalytic applications of metalloporphyrin-based magnetic nanocatalysts were investigated. meso-Tetrakis(4-carboxyphenyl)porphyrinatoiron(iii) chloride (Fe(TCPP)Cl) and meso-tetrakis(4-carboxyphenyl)porphyrinatomanganese(iii) acetate (Mn(TCPP)OAc) were separately immobilized onto the surface of amine functionalized magnetic nanoparticles (Fe3O4/SiO2/NH2) via covalent attachment. The obtained nanocatalysts were characterized using FT-IR and UV-Vis and atomic absorption spectroscopy, X-ray powder diffraction (XRD), vibrating sample magnetometry (VSM), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). The catalytic efficiency of Fe3O4/SiO2/NH2-Fe(TCPP)Cl and Fe3O4/SiO2/NH2-Mn(TCPP)OAc for the green oxidation of alkenes with H2O2 was investigated in a comparative manner. The Mn-porphyrin based magnetic nanocatalyst shows higher catalytic efficiency compared to the Fe-porphyrin. In addition, the prepared magnetic nanocatalyst exhibited excellent reusability and could be reused at least five times without significant leaching or loss of activity. This journal is

Full text of DOI:10.1039/d0nj04190d

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
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Magnetically recoverable porphyrin-based nanocatalyst for the effective oxidation of
olefins with hydrogen peroxide: a comparative study
Saeed Rayati*, Dana Moradi and Fatemeh Nejabat
Abstract
In this paper, preparation, characterization and catalytic application of metalloporphyrin-based magnetic nanocatalysts were investigated.
meso‐tetrakis(4‐carboxyphenyl) porphyrinatoiron (III) chloride (Fe(TCPP)Cl) and meso-tetrakis(4‐carboxyphenyl) porphyrinatomanganese
(III) acetate (Mn(TCPP)OAc) were separately immobilized of onto the surface of amine functionalized magnetic nanoparticles
(Fe3O4/SiO2/NH2) via covalent attachment. The obtained nanocatalysts were characterized by FT-IR and UV-Vis and atomic absorption
spectroscopy, X-ray powder diffraction (XRD), vibrating sample magnetometry (VSM), thermogravimetric analysis (TGA), transmission
electron microscope (TEM). The catalytic efficiency of Fe3O4/SiO2/NH2-Fe(TCPP)Cl and Fe3O4/SiO2/NH2-Mn(TCPP)OAc for the green oxidation
of alkenes with H2O2 were investigated in a comparative manner. Mn-porphyrin based magnetic nanocatalyst shows higher catalytic
efficiency compared to the Fe--porphyrin. In addition, the preparedmagnetic nanocatalyst exhibited excellent reusability and could be reused
at least for five times without significant leaching or loss of activity.
structural and high magnetic properties could get more attention as
valuable materials for the attachment of metal complexes to prepare
reusable nanocatalyst.28-32
Introduction
Oxidation of olefins is one of the great interests of the
pharmaceutical and industrial chemistry due to the importance of
valuable oxygen containing products.1-3 To achieve unique products
with excellent yield and selectivity, oxidation of olefins were
conducted over various metal complexes as catalysts in
homogeneous and heterogeneous systems and many scientists were
greatly focused on designing effective catalysts for oxidation
reactions.4-7 Cytochrome P450 plays an important role in oxidation
reactions of metabolic processes and participate in various
oxygenations.8 Metalloporphyrins can be an ideal biomimetic model
of CY-P450 to study details of biological systems and to improve the
development of synthetic catalysts.9-14 Although the catalytic
reactions in the homogeneous systems exhibit high activity (because
of its large surface area), but these kind of catalysts have many
disadvantages such as economic drawbacks regard to the difficulties
of the separation and the contamination of the products.15-18
Recently, a great deal of effort was devoted to the application of
nanomaterials i.e. nanosheets,19-21 nanotubes22-24 and nanoparticles
25-27 as support for the preparation of various effective heterogenized
catalyst. Apart from the well-known organic and inorganic support,
functionalized magnetic nanoparticles because of their chemical,
By considering and discussing the environmental issues, many
attempts were focused on the applying of the principles of green
chemistry in the catalytic reactions.33-34 Recently, employing
hydrogen peroxide as a biodegradable, cheap, safe and green
oxidant for the catalytic oxidation reactions is an ideal method for
the development of green chemistry in the chemical synthesis.34-37 In
this study, in continuing our studies about preparation of porphyrin
based nanocatalysts and comparing the effect of metal center on the
catalytic efficiency of nanocatalysts, Mn and Fe porphyrins
immobilized onto the surface of amine functionalized magnetic
nanoparticles as efficient catalysts for the oxidation of olefins with
H2O2.
Experimental
Chemicals and Instruments
.
Chemical reagents and solvents were purchased from Merck, Aldrich,
Scharlau or Fluka and used without further purification.
UV-Vis spectra were recorded with a Lambda 25 Perkin Elmer
spectrophotometer from 400-700 nm. Fourier transform infrared
(FT-IR) spectra were recorded (KBr pallets) on an ABB Bomem: FTLA
2000-100 in the range of 400-4000 cm-1. Manganese and iron
content were determined by using a Varian AA240 atomic absorption
aDepartment of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-
1618, Tehran 15418, Iran. E-mail: rayati@kntu.ac.ir
Fax: +98 21 22853650; Tel: +98 21 22850266
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of substrate, 0.12 mmol of imidazole (ImH) and 0.7 mL H2O2 as an
DOI: 10.1039/D0NJ04190D
oxidant were added in 1 mL acetonitrile. The reaction was stirred at
spectrometer. The morphology features of solid samples were
investigated by transmission electron microscopy (TEM; Zeiss-
View Article Online
EM10C-100 KV). The crystalline structure of obtained samples was room temperature for an appropriate time and at the end of the
identified by X-ray powder diffraction (XRD) using a Panalytical Xpert
PRO X Ray Diffractometer (Xpert Pro MPD) with Cu-kα radiation
(λ=0.15418) at 40kV and 40mA. Thermal stability of samples was
investigated by the thermal gravimetric analysis (Mettler-Toledo TGA
851e). Magnetic properties of materials determined on a BHV-55
vibrating sample magnetometer (VSM) at room temperature. Purity
determinations of the products were accomplished by GC-FID on an
Agilent 7890B instrument using a SAB‐5 capillary column (phenyl
methyl siloxane 30m × 0.32 mm × 0.25 μm).
oxidation reaction, the heterogenized catalyst was separated from
the reaction media by a magnet and gas chromatography was used
to analyze the reaction progress.
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Results and discussion
Preparation of nanocatalyst
The heterogenized nanocatalysts Fe3O4/SiO2/NH2@M(TCPP) (M is
manganese or iron) were prepared as illustrated in Scheme 1.
Synthesis of nanocatalyst [Fe3O4/SiO2/NH2-M(TCPP)]
Porphyrins and metalloporphyrins were synthesized by using Adler’s
method (See Supporting information).38,39 Silica coated magnetic
nanoparticles (Fe3O4/SiO2) was prepared and functionalized with 3-
aminopropyltriethoxysilane (APTS) as reported previously (See
Supporting information).40-44 Metalloporphyrins (Fe(TCPP)Cl or
Mn(TCPP)OAc) were covalently attached to the surface of
Fe3O4/SiO2/NH2 to prepare magnetic nanocatalysts in the presence
of TBTU/DIPEA.37,38 In brief, metalloporphyrin (0.6 g of Fe(TCPP)Cl or
Mn(TCPP)OAc) was dissolved in DMF (40 mL), 2-(1H-benzotriazole-1-
yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU) (0.4 g),
(N,N′-diisopropylamine) (DIPEA) (0.3 g) and Fe3O4/SiO2/NH2 (1 g )
were added to the solution and the mixture was stirred for about 48
h at room temperature. The supported magnetic nanocatalyst was
collected by a magnet, washed several times with DMF and ethanol
and dried at 50 °C for about 20 hours.
Fe3O4 nanoparticles synthesized via precipitation method, coated by
silica and functionalized by amine groups as reported previously.43
The “one pot” amidic reaction of carboxylic groups of
metalloporphyrins and amine groups of magnetic supports was
carried out using TBTU as a coupling reagent and DIPEA as a base in
DMF at room temperature. TBTU is an uronium salt commonly used
in amidation and esterification reactions, especially in basic
solutions.24,44
Characterization of the catalyst Fe3O4/SiO2/NH2-MTCPP
The amount of attached metalloporphyrins onto the surface of
Fe3O4/SiO2/NH2 was determined by UV-Vis and atomic absorption
spectroscopy (AAS). Based on the results, each gram of
Fe3O4/SiO2/NH2-FeTCPP(Cl) and Fe3O4/SiO2/NH2-MnTCPP(OAc)
contain 500 and 520 µmol of metalloporphyrin respectively.
General procedure for alkene oxidation
The FT-IR spectra of the synthesized nanomaterials are presented in
Figure 1. The strong absorption bands at 468 cm-1, 586 cm-1 and 669
cm-1 are related to stretching vibration of Fe-O in Fe3O4, which could
be detectable in all samples (Figure 1).
Oxidation reaction of olefins in the presence of Fe3O4/SiO2/NH2-
M(TCPP) (M: Fe or Mn) as catalyst is elaborated as follows: In a test
tube, 0.0025 mmol of the prepared nanocatalyst (0.005 g), 0.2 mmol
Scheme 1. Schematic preparation of magnetic nanocatalyst.
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Figure 2. XRD pattern of (a) Fe3O4/SiO2/NH2-Mn(TCPP)OAc and (b) Fe3O4/SiO2/NH2-
Fe(TCPP)Cl.
Figure 1. FTIR spectra of (a) Fe3O4/SiO2/NH2, (b) Fe3O4/SiO2/NH2-FeTCPP(Cl) and (c)
Fe3O4/SiO2/NH2-MnTCPP(OAc).
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The strong absorption bands at 1090 cm-1 and 790 cm-1 are assigned
to anti-symmetric stretching of O-Si-O and symmetric stretching of
this band, which reveal that the Fe3O4/SiO2 has been successfully
prepared. In addition, the peaks at 1630 cm-1 and 3420 cm-1 can be
attributed to amino functionalizing of Fe3O4/SiO2. After
metalloporphyrin immobilization, a new absorption band could be
detectable around 1650 cm-1, which evidently indicates the
formation of amide bond between carboxyl groups (-COOH) of
metalloporphyrins and amine groups (-NH2) of the magnetic support
(Figure 1, b and c). Moreover, the absorption band at 1397 cm-1 is
attributed to the vibrations of metalloporphyrin .43,45
-10000
-5000
0
5000
10000
-10
-20
-30
-40
Support
Fe-Catalyst
Mn-Catalyst
Magnetic field (Oe)
Figure 3. Magnetization curve of Fe3O4/SiO2/NH2, Fe3O4/SiO2/NH2-Mn(TCPP)OAc and
Fe3O4/SiO2/NH2-Fe(TCPP)Cl.
To investigate the crystalline structure and phase purity of
nanomaterials, (XRD) patterns of Fe3O4/SiO2/NH2-FeTCPP(Cl) and
Fe3O4/SiO2/NH2-MnTCPP(OAc) were recorded and presented in
Figure 2. Based on the results, the main peaks 2θ values of 31, 36,
43, 53, 57 and 63 deg. corresponding to (220), (311), (400), (422),
(511) and (440) plans of cubic inverse spinel Fe3O4 39,41 are similar in
the pattern of the prepared magnetic nanocatalysts and solid
supports (Figure SI) with similar lattice constant (Table SI). The
observed diffraction peaks are in good agreement with database of
JCPDS file (Ref. Code 96-900-2322) and revealed the stability of
crystalline structure of magnetic nanoparticles during the
subsequent modification.
Moreover, based on Debye–Scherrer equation (D = 0.9λ/B cos θ), the
average crystallite size of Fe3O4/SiO2/NH2, Fe3O4/SiO2/NH2-
Mn(TCPP)OAc and Fe3O4/SiO2/NH2-Fe(TCPP)Cl were calculated to be
18, 21 and 28 nm, respectively. Based on the results after porphyrin
deposition, the average particle sizes of magnetic nanoparticles were
increased and less crystallinity was observed; which can be
attributed to the attachment of metalloporphyrins onto the surface
of Fe3O4/SiO2/NH2.
The magnetic characteristics of Fe3O4/SiO2/NH2 and the
nanocatalysts were evaluated by vibrating sample magnetometer
(VSM), which illustrates ferromagnetic behaviour of the prepared
samples (Figure 3). The prepared nanomaterials are
superparamagnetic and no coercivity (HC) and remanent
magnetization (MR) were observed in the curves (Figure 3). The
saturated magnetization values of Fe3O4/SiO2/NH2, Fe3O4/SiO2/NH2-
Fe(TCPP)Cl and Fe3O4/SiO2/NH2-Mn(TCPP)OAc are 36, 22 and 20
amu/g respectively. Although because of diamagnetic behavior of
the surface, the magnetization of the nanocatalysts is diminished
compared to the Fe3O4, but the prepared nanocatalysts still respond
sufficiently to the external magnetic field.46-48
To study thermal stability of magnetic nanocatalysts, thermo-
gravimetric analysis (TGA) of Fe3O4/SiO2/NH2-Fe(TCPP)Cl and
Fe3O4/SiO2/NH2-Mn(TCPP)OAc were investigated under O2
atmosphere (100 mL/min) at the heating rate of 10 ℃/min in the
range between room temperature and 800 °C (Figure 4). The thermal
behaviour of magnetic nanocatalysts show a weight loss at about 130
°C, which is due to the loss of physically adsorbed water molecules.
The next weight loss between 170 °C and 280 °C could be attributed
to desorption of water molecules; which have chemical bond with
SiO2. In the TGA curves of nanocatalysts, the weight loss between 320
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°C and 800 °C could be related to decomposition of organic parts,
metalloporphyrin and formation of metal oxide.49-50
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Figure 4. TGA Diagram of (a) Fe3O4/SiO2/NH2-Mn(TCPP)OAc and (b) Fe3O4/SiO2/NH2-
Fe(TCPP)Cl.
Figure 5 shows the UV-Vis spectra of dispersed Fe3O4/SiO2/NH2,
Fe3O4/SiO2/NH2-Fe(TCPP)Cl and Fe3O4/SiO2/NH2-Mn(TCPP)OAc in
ethanol. The UV-Vis spectra of magnetic nanocatalyst displayed the
typical Soret band of metalloporphyrins (420 nm for iron porphyrins
and 470 nm for manganese porphyrins respectively) and there is no
detectable peak in this region in the UV-Vis spectrum of
Fe3O4/SiO2/NH2 (Figure 5, c). These data indicated metalloporphyrins
successfully immobilized onto the surface of Fe3O4@SiO2NH2
without any distortion or aggregation.
The morphology of original Fe3O4/SiO2/NH2, Fe3O4/SiO2/NH2-
Fe(TCPP)Cl and Fe3O4/SiO2/NH2-Mn(TCPP)OAc were characterized by
transmission electron microscope (TEM) which are shown in Figure
6. The TEM images show uniformity and homogeneous spherical
morphology of all three samples with little aggregation. The
core/shell nanostructures of magnetic nanomaterials clearly are
detectable in the TEM images and shows that the core of Fe3O4
totally covered by silica layer and organic materials (Figure 6, (b)).
Figure 6. TEM image of (a) Fe3O4/SiO2/NH2, (b) Fe3O4/SiO2/NH2-Mn(TCPP)OAc and (c)
Fe3O4/SiO2/NH2-Fe(TCPP)Cl.
Catalytic activity of magnetic nanocatalysts
The catalytic properties of two magnetic nanocatalysts was
investigated for the green oxidation of olefins with H2O2 and the
effect of metal centre of metalloporphyrins on the catalytic efficiency
of the prepared nanocatalysts were compared. The oxidation of
cyclooctene with hydrogen peroxide was used as a model reaction;
to achieve the best reaction condition, different parameters such as
solvent, reaction time, temperature, amount of imidazole and the
amount of H2O2 were studied and the catalytic efficiency of Mn-
catalyst and Fe-catalyst were compared in all cases.
Figure 5. UV-Vis spectra of (a) Fe3O4/SiO2/NH2, (b) Fe3O4/SiO2/NH2-Fe(TCPP)Cl and (c)
Fe3O4/SiO2/NH2-Mn(TCPP)OAc
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The highest conversion was obtained by addition of 0V.7iewmALrtioclfeHO2nOlin2e,
DOI: 10.1039/D0NJ04190D
but the conversion was decreased by increasing the amount of H2O2
up to 1 mL, which could be due to decomposition of nanocatalyst in
the presence of higher amount of the oxidant (Table 1, entry 3-4)
Green oxidation of cyclooctene with H2O2 in the present of magnetic
nanocatalysts was carried out in different times and the results are
presented in Figure 8. After three hours acceptable conversion was
obtained in the presence of Mn-catalyst (80%), while very little
amount of epoxy cyclooctane was achieved in the presence of Fe-
catalyst (9%) (Figure 8). By continuing the reaction for up to 10 hours,
the oxidation reaction almost completed in the presence of
Fe3O4/SiO2/NH2-Mn(TCPP)OAc, but the kinetics of the catalytic
oxidation reaction with H2O2 in the presence of Fe3O4/SiO2/NH2-
Fe(TCPP)Cl was slow and only 44% of epoxy cyclooctene was
achieved after 24 hours. However, after 48 hours the oxidation
reaction completed in the presence of Fe3O4/SiO2/NH2-Fe(TCPP)Cl.
Mn-Catalyst
Fe-Catalyst
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Solvent
Figure7. The effect of solvent on the oxidation of cyclooctene with H2O2 in the presence
of magnetic nanocatalysts. The molar ratio of Catalyst:Cyclooctene:ImH is (1:80:50),
H2O2(1 mL), r.t, Time: 5 h
In order to investigate the effect of solvent on the catalytic oxidation
of cyclooctene with H2O2 in the present of magnetic nanocatalysts,
different solvents such as ethanol, methanol, acetone,
dichloromethane, dichloroethane and acetonitrile were applied and
the results are presented in Figure 7. Based on the results, the
highest conversion was obtained in acetonitrile (60%), no significant
product was obtained in dichloroethane and dichloromethane (1%),
since aqueous hydrogen peroxide is insoluble in organic solvents and
two-phase system were obtained in these solvents. It should be
noted that epoxy cyclooctane was the sole product of this oxidation
reaction and the reaction did not proceed in the absence of the
catalyst.
According to the literature, presence of a nitrogenous base as an
axial base increases the catalytic efficiency of metalloporphyrins for
the oxidation reaction.51 Oxidation of cyclooctene with H2O2 in the
presence of magnetic nanocatalysts in different amount of imidazole
was studied and the results are presented in Table 2.
The obtained results reveal that optimal ratio of catalyst:ImH is 1:50
and more amount of ImH inhibited the reaction significantly, which
could be due to the formation of inactive six coordinate species, i.e.
M(por)(ImH)2.52
To achieve the best amount of oxidant for the green oxidation of
cycloocetene, different amount of H2O2 were utilized while other
parameters were maintained constant for the oxidation reaction in
CH3CN (Table 1). Hydrogen peroxide is an inactive oxidant for the
epoxidation reaction at room temperature; no significant amount of
the products was detectable for the oxidation reaction in presence
of three equivalent of the oxidant (Table 1, entry 1) and excess
amount of H2O2 was required to achieve the acceptable results
(Table 1, entry 2-4).
Table 1. Optimization of the amount of oxidant on the oxidation of cyclooctene with
H2O2 in the presence of magnetic nanocatalysts.
Conversion %a
Entry
H2O2 (mL)
Figure 8. Optimization of time on the oxidation of cyclooctene with H2O2 in the presence
of magnetic nanocataltsyts. Reaction condition: The molar ratio of
Catalyst:Cyclooctene:ImH is (1:80:50), H2O2: 0.7 mL, CH3CN: 1 mL, r.t.
Mn-catalyst
Fe-catalyst
2
1
0.061
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0.4
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a Reaction condition: The molar ratio of catalyst: cyclooctene: ImH is (1:80:50), CH3CN: 1
mL, Time: 5 h, temperature: 25 °C.
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Table 2. Optimization of amount of ImH on the oxidation of cyclooctene with H2O2 in the
presence of magnetic nanocatalysts at room temperature.
Table 4. Oxidation of various olefins with H2O2 in the presence of magnetic nanocatalysts
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at room temperature.
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Entry
ImH:Catalyst
Conversion%a
Conversion%a (Selectivity%b)
No.
1
Alkene
Time (h)
Mn-Catalyst
85 (100)
Fe-Catalyst(c)) (c)
74 (100)
Mn-catalyst
Fe-catalyst
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9
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3
87 (100)
75 (93)
89 (84)
74 (78)
97 (100)
96 (73)
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100 (84)
69 (87)d
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a
Reaction conditions: The molar ratio of Catalyst:Cyclooctene:ImH is (1:80:X),
H2O2: 0.7 mL, CH3CN: 1 mL, Time: 5 h.
3
5
78 (64)
83 (50)
66 (97)d
71 (94)d
The effect of reaction temperature on the oxidation of cyclooctene
with H2O2 in acetonitrile in the presence of magnetic nanocatalysts
were investigated (Table 3). The results illuminate that low
temperature has a negative effect on the kinetic of the oxidation
reactions and low amount of epoxy cyclooctane was achieved in the
presence of both Fe and Mn nanocatalysts at 5 °C (Table 3). Based on
the results, Mn-catalyst shows the best catalytic efficiency at room
temperature and low amount of the product was obtained in higher
or lower temperature. But on the contrary, the catalytic efficiency of
Fe3O4/SiO2/NH2-Fe(TCPP)Cl increase by increasing the temperature
and the best result was obtained at reflux (Table 3).
5
6
7
8
3
5
3
5
3
5
3
5
3
80 (47)
85 (33)
19 (63)
72 (92)
87 (57)
99 (48)
16 (75)
86 (50)
10 (100)
53 (94)d
55 (94)d
40 (82)d
60 (78)d
31 (100)
67 (82)
83 (31)
90 (64)
32 (100)
To generalize the applicability of the prepared magnetic
nanocatalysts for the oxidation of olefins with hydrogen peroxide,
oxidation of various olefins was carried out under the optimum
reaction condition in the presence of both Fe-catalyst and Mn-
catalyst and the results are presented in Table 4. Excellent results
were obtained for the oxidation reaction with H2O2 in the presence
of both Fe-catalyst and Mn-catalyst for all the olefins.
9
10
3
5
31 (100)
39 (100)
18 (100)
22 (100)
Table 3. Effect of temperature on oxidation of cyclooctene with H2O2 in the presence of
magnetic nanocatalysts.
a
Reaction condition: Catalyst:substrate:ImH(1:80:50), Solvent:CH3CN:1 mL,
H2O2:0.7 mL, r.t. b Selectivity to the corresponding epoxide. c Based on 0.005 mmol
d
Conversion %a
of the catalyst, Reflux, CH3CN: 2 mL, H2O2: 1.4 mL..
corresponding aldehyde.
Selectivity to the
Entry
Temperature (℃)
Time (h)
Mn-Catalyst
Fe-Catalyst
1
2
3
4
5
25
3
5
3
5
3
5
3
5
3
3
1
1
4
15
30
32
74
97
The major product for the oxidation of olefins with H2O2 in the
presence of Mn-catalyst was the corresponding epoxide, but the
same result was not obtained in the presence of Fe-catalyst and in
some cases, the corresponding aldehyde was the major product. The
sole product of the oxidation of cyclooctene, 1-octene and cis-
stilbene with H2O2 was their corresponding epoxide in the presence
of both catalysts and for the oxidation of cyclohexene and indene,
the corresponding epoxide was the major product. In the oxidation
of styrene, α-methylstyrene, 4-methylstyrene, 4-chlorostyrene and
4-metoxystyrene the corresponding aldehyde were detected (by GC)
as the major product in the oxidation reaction in the presence of Fe-
85
87
36
37
34
36
45b
Reflux b
a
b
Reaction condition: Catalyst:cyclooctene:ImH (1:80:50). CH3CN: 1 mL, H2O2: 0.7 mL.
Based on , 0.005 mmol of the catalyst, CH3CN: 2 mL, H2O2: 1.4 mL.
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catalyst, which indicates that ring-opening of epoxide can be
proceeded under the reflux condition. Furthermore, it seems that
the catalytic efficiency of Fe3O4/SiO2/NH2-Mn(TCPP)OAc for
epoxidation with hydrogen peroxide is strongly dependent on the
electronic property of substrates not the steric effects of the
substrates (Table 4, Entry 3-7); but the catalytic efficiency of Fe-
catalyst for the oxidation of various olefins does not follow a specific
order. In addition, the lowest conversion was reached for the
oxidation of 1-octene as a linear olefin due to the lack of electron
density and conjugated π-bonding system.
Table 6. The result of reusability of Fe3O4/SiO2/NH2-MnTCPP(OAc) for oxidation of
View Article Online
DOI: 10.1039/D0NJ04190D
cyclooctene with H2O2.
Run
Conversion %a
Mn-leaching % b
1
87
0
0
2
64
60
60
58
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3
0.8
0.3
3
To prove the catalytic efficiency of the supported nanocatalysts,
oxidation of cyclooctene with H2O2 was carried out in the absence of
catalyst, in the presence of unsupported metalloporphyrins and in
the presence of supported nanocatalysts in the same condition
(Table 5). Based on the results, no significant product was obtained
after 5 hours in the absence of catalyst (1%), which indicate the
critical role of the catalyst in the oxidation reaction (Table 5, entry 1).
Although the reaction proceeded well in the presence of
unsupported Mn(TCPP)OAc (conversion 75%); but only 8% of epoxy
cyclooctane was obtained in the present of Fe(TCPP)Cl. By utilizing
heterogeneous catalysts in the oxidation reaction, not only higher
amount of epoxy cyclooctene were obtained, but also magnetic
nanocatalysts were easily separated from the reaction media by an
external magnet to use for the next run. It is clearly detectable that
the rate of oxidation increases by the attachment of
metalloporphyrin onto the surface of a magnetic nano-support.
4
5
a
Reaction condition: The molar ration of catalyst:cyclooctene:ImH is (1:80:50),
Solvent:CH3CN:1 mL, H2O2:0.7 mL, 7 h, r.t.
b Mn-leaching was determined by AAS.
Based on the results, the prepared magnetic nanocatalyst showed
excellent reusability in five consecutive cycles with a little loss of
activity (Table 6).
To study the stability of magnetic nanocatalyst, after each catalytic
reaction, Mn leaching of Fe3O4/SiO2/NH2-Mn(TCPP)OAc was
evaluated by AAS and the results showed little amount of manganese
in the filtrate; which reveals the stability of the prepared magnetic
nanocatalyst (Table 6). Moreover, the XDR pattern of the used
catalyst indicated that magnetic nanocatalyst is quite stable and no
significant change was identified in the XRD pattern of
Fe3O4/SiO2/NH-Mn(TCPP)OAc (Figure 9).
The stability and reusability of magnetic nanocatalyst was
investigated by recycling experiment for the oxidation of cyclooctene
with H2O2 in the presence of Fe3O4/SiO2/NH2-Mn(TCPP)OAc in
acetonitrile. After each catalytic cycle, the catalyst separated with an
external magnet, washed with acetonitrile and acetone (2 mL, 2×2
time), dried at 60 °C and directly used in the next run.
Table 5. Oxidation of cyclooctene with H2O2 in the present of supported and
unsupported catalysts at room temperature.
Entry
Catalyst
Conversion%a
TON
1
None
1
-
2
3
4
5
6
Fe3O4/SiO2/NH2
Fe(TCPP)Cl
12
8
-
6.4
Mn(TCPP)OAc
72
87
11
57.6
69.6
8.8
Fe3O4/SiO2/NH2-Mn(TCPP)OAc
Fe3O4/SiO2/NH2-Fe(TCPP)Cl
Figure 9. XRD pattern of (a) amine functionalized magnetic support, (b) fresh catalyst and
(c) reused catalyst.
a
Reaction condition: The molar ration of catalyst:cyclooctene:ImH is (1:80:50),
Solvent:CH3CN:1 mL, H2O2:0.7 mL, r.t, reaction time: 5 h.
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According to the literature, a possible mechanism was proposed for nanoparticles under the oxidation reaction even in the presence of
View Article Online
DOI: 10.1039/D0NJ04190D
the oxidation of olefins with hydrogen peroxide (Scheme SI). In the excess amount of the oxidant. The ease of separation by an external
catalytic cycle of metalloporphyrins for epoxidation with H2O2, two magnet and reusability of the catalyst for at least five runs without
common intermediate have been suggested (a six-coordinate active significant leaching or decreasing the catalytic activity were merits of
species and a high valent metal-oxo (M=O)), which are responsible the prepared nanocatalysts.
for oxygen transfer in a parallel process (Scheme SI, (a) and (b)).
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Acknowledgements
To evaluate the catalytic efficiency of the prepared nanocatalysts,
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This work was supported by K.N. Toosi University of Technology
research council and Iran National Science Foundation (INSF) [grant
No. 98026139].
the catalytic results were compared with the previous reported data
for epoxidation of cyclooctene in the presence of porphyrin-based
magnetic nanocatalysts (Table 7). Appropriate reaction time and the
use of green oxidant like H2O2 for epoxidation in the mild condition
are the considerable advantages of this research compared to some
other previously reported data.
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A simple method for the functionalization of magnetic nanoparticles, which prDoOdI:u1c0.e10d3V9bie/Dwy0ANrtiJc0le4O19nl0inDe
the attachment of metalloporphyrin onto the surface of amine functionalized silica-coated
magnetic nanoparticles via amide bond.
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