Nanoparticle supported, magnetically separable manganese porphyrin as an efficient retrievable nanocatalyst in hydrocarbon oxidation reactions

Article abstract of DOI:10.1039/c6ra02123a

A manganese porphyrin, meso-tetrakis(pentafluorophenyl)porphyrinato manganese(iii) acetate, Mn(TPFPP)OAc, was immobilized on silica-coated magnetic Fe₃O₄ nanoparticles functionalized with 3-aminopropyltriethoxysilane (APTS) through the amino propyl linkage using a grafting process in toluene solvent. This enabled the covalent immobilization of Mn(iii) porphyrin via an aromatic nucleophilic substitution reaction, to afford the Fe₃O₄@SiO₂-NH₂@MnPor catalyst. The resulting nanoparticles were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), FT-IR spectroscopy, UV-Vis spectroscopy, elemental analysis (CHN), atomic absorption spectroscopy (AAS), and vibrating sample magnetometry (VSM). The immobilized manganese porphyrin was applied as an efficient and retrievable heterogeneous nanocatalyst in alkane hydroxylation and alkene epoxidation. Leaching and recycling experiments revealed that the prepared nanocatalyst can be recovered, and reused several times, without loss of activity and magnetic properties.

Full text of DOI:10.1039/c6ra02123a

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Nanoparticle supported, magnetically separable manganese porphyrin as an
efficient retrievable nanocatalyst in hydrocarbon oxidation reactions
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Mojtaba Bagherzadeh* and Anahita MortazaviꢀManesh
Chemistry Department, Sharif University of Technology, P.O. Box 11155ꢀ3615, Tehran, Iran.
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* Corresponding author. Tel.: +98 21 66165354; fax: +98 21 66012983;
Eꢀmail address:bagherzadeh@sharif.edu
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Abstract
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A manganese porphyrin, mesoꢀtetrakis(pentafluorophenyl)porphyrinato manganese (III) acetate,
Mn(TPFPP)OAc, was immobilized on silicaꢀcoated magnetic Fe3O4 nanoparticles which
functionalized with 3ꢀaminopropyltriethoxysilane (APTS) through the amino propyl linkage
using a grafting process in toluene solvent. This enabled the covalent immobilization of Mn(III)
porphyrin via an aromatic nucleophilic substitution reaction, to afford the Fe3O4@SiO2ꢀ
NH2@MnPor catalyst. The resulting nanoparticles were characterized by Xꢀray powder
diffraction (XRD), scanning electron microscopy (SEM), FTꢀIR spectroscopy, UVꢀVis
spectroscopy, elemental analysis (CHN), atomic absorption spectroscopy (AAS), and vibrating
sample magnetometry (VSM). The immobilized manganese porphyrin was applied as an
efficient and retrievable heterogeneous nanocatalyst in the alkane hydroxylation and alkene
epoxidation. Leaching and recycling experiments revealed that the prepared nanocatalyst can be
recovered, and reused several times, without loss of activity and magnetic properties.
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Keywords: Manganese porphyrin, Fe3O4 nanoparticle, Magnetically separable nanocatalyst,
Heterogeneous catalysis, Oxidation reactions
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1. Introduction
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Metalloporphyrins are well known to mimic the activity of enzymatic monooxygenases.1,2 In this
regard, metalloporphyrin complexes have been largely employed as valuable biomimetic
catalysts, owing to the critical roles they play in oxygen transfer processes.3 Investigating in this
area is based on different strategies to design selective, stable and high turnover catalytic
systems.4,5 The introduction of secondꢀgeneration metalloporphyrins, which bear bulky and/or
electronꢀwithdrawing substituents at the meso positions of the porphyrin ring, has been among
the main approaches commonly employed with the aim of increasing the stability of the aromatic
heterocycle towards oxidative degradation in reaction condition. Immobilization of expensive
metalloporphyrin catalysts onto supports appears to be a good way to improve their oxidative
stability, selectivity and the catalytic performance because of the support environment and other
advantages with respect to recovery and reuse.6ꢀ11 In other words, supporting metalloporphyrins
provides a physical separation of active sites, thus minimizing catalyst selfꢀdestruction and
metalloporphyrin dimerization.10,12 Furthermore, heterogeneous catalytic oxidations have
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become an important target since their process are used in industry, helping to minimize the
problems of industrial waste treatment. Hence, the immobilization of these biomimetic catalysts
is much desired.
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An attractive approach is the preparation of magnetically separable nanocatalysts.13ꢀ16 In this
manner, silicaꢀcoated magnetic nanoparticles, Fe3O4@SiO2, have been studied extensively due to
their superparamagnetism property, large surface area to volume ratio and easy functionalization.
Homogeneous catalysts immobilized on magnetic nanoparticles (MNPs) surface occupy a unique
position due to combining the advantages of both homogeneous and heterogeneous catalysts.17ꢀ19
Because of large surface area of nanoparticles, high loadings of catalytically active sites are
guaranteed and therefore nanoparticles supported homogeneous catalysts exhibit high catalytic
activity and selectivity.16 In addition, the magnetic properties of the Fe3O4 nanoparticles can
optimize the operations of separation, recycling, and reuse of the heterogeneous catalyst.20
Hence, these nanoparticles make catalyst recovery more practical and faster than conventional
separation methods also reduce solvent consumption. In this way, the important issue of favoring
the green chemistry strategy is also covered.21
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In the present work, we describe the preparation of an efficient heterogeneous catalyst
synthesized by immobilizing manganese porphyrin on functionalized magnetic nanoparticles via
the amino propyl linkage. Also, the catalytic activity of the supported catalyst, Fe3O4@SiO2ꢀ
NH2@MnPor, has been investigated in hydrocarbon oxidation reactions as a recyclable and
sustainable nanocatalyst.
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2. Experimental
2.1. Materials and Methods
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All reagents and solvents were purchased from Merck, Fluka or Aldrich chemical companies.
Iodosylbenzene22 and tetraꢀnꢀbutylammonium hydrogen monopersulfate23 were synthesized
according to the literature procedure.
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Scanning electron microscopy (SEM) was carried out on Philips XL30. FTꢀIR spectra were
recorded as KBr pellets using ABB FTꢀIR spectrophotometer. Measures of pH were carried out
by Mettler Toledo S40 SevenMulti™ pHꢀmeter. Xꢀray diffraction (XRD) pattern was obtained
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by D4 ENDEAVOR diffractometer (Bruker AXS Inc.) with Cu Kα as a radiation source.
Elemental analyses (CHN) were performed using a Heraeus Elemental Analyzer. A Varian
(AA220) flame atomic absorption spectrometer (air/acetylene flame) was used for manganese
ion determinations. UVꢀVis spectra were recorded with a Shimadzu UVꢀ2100 spectrometer.
Magnetic measurement of materials was investigated with a vibrating sample magnetometer
VSM (Meghnatis Daghigh Kavir Company, Iran) at room temperature. Gas chromatographic
(GC) analyses were performed on an Agilent Technologies 6890 N, 19019 Jꢀ413 HPꢀ5, capillary
60 m × 250 µm × 1µm. Brunauer–Emmett–Teller (BET) surface area of the catalyst was
measured by Belsorp mini II instrument at ꢀ196°C.
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2.2. Porphyrin Synthesis and Metallation
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Porphyrins are a group of heterocyclic macrocycle organic compounds. The porphyrin ring
structure is aromatic, with a total of 26 electrons in the conjugated system. Various analyses
illustrate that not all atoms of the ring are involved equally in the conjugation.24 One result of
this large conjugated system is that porphyrin molecules typically have very intense absorption
bands in the visible region. Porphyrins bind metals to form metalloporphyrin complexes. In the
present work, the free base, mesoꢀtetrakis(pentaꢀflourophenyl)porphyrin (H2F20TPP or
H2TPFPP), was prepared25 and metallated with Mn(OAc)2·4H2O to afford the metalloporphyrin
[Mn(TPFPP)OAc].26 The UV–Vis spectrum of homogeneous Mn(TPFPP)OAc was shown in the
Supplementary Information labeled as Fig. S1.
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2.3. Preparation of Fe3O4 Magnetic Nanoparticles
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Fe3O4 nanoparticles were synthesized on the basis of the procedure described previously.19,27,28
In brief, under N2 atmosphere, 5.2 g (19.3 mmol) of FeCl3.6H2O, 2 g (10.0 mmol) of FeCl2.4H2O
and 0.85 mL concentrated HCl were dissolved in 25 mL degassed water. This solution was
added dropwise at room temperature to 250 mL of NaOH solution (1.5 mol/L) under N2. The
reaction mixture was vigorously stirred for 30 min (1300 rpm). The formed black precipitates
were separated using a strong magnetic field (0.5 T magnet) and washed several times with
degassed water. Finally, for storage, Fe3O4 nanoparticles were dispersed in 200 mL degassed
water under N2. Synthesis of Fe3O4 nanoparticles was approved by XRD.
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2.4. Synthesis of Fe3O4@SiO2 Nanoparticles
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In the next step, Fe3O4MNPs were coated with a thin layer of silica.19,29 One gram of freshly
prepared Fe3O4 nanoparticles was added into 30 mL of an aqueous solution of citric acid (0.02
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g/mL), then the pH was adjusted to 5.2 using ammonia, and the mixture was heated to 80–90 C
for 1.5 h. After heating, the pH of the reaction mixture was increased with ammonia to pH=11
and 1.25 mL of tetraethylorthosilicate (TEOS) dissolved in ethanol (12.5 mL) was added
dropwise into the suspension of particles. The mixture was stirred at room temperature for 24 h
to allow the baseꢀcatalyzed hydrolysis and condensation of TEOS monomers on the nanoparticle
surface go to completion. Finally, the dark brown Fe3O4@SiO2 nanoparticles were separated
using a 0.5 T magnet and were washed with distilled water and ethanol. Synthesis of
Fe3O4@SiO2 nanoparticles was confirmed with Xꢀray diffraction analysis.
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2.5. Synthesis of Fe3O4@SiO2-NH2
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In a typical reaction, 5 mL of 3ꢀaminopropyltriethoxysilane (APTS) dissolved in 100 mL ethanol
were added dropewise to the suspension of one gram of the silicaꢀcoated Fe3O4 nanoparticles in
100 mL of distilled water. The pH value of the reaction mixture was increased with KOH to
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pH=11 and the reaction mixture was stirred at 70 C for five hours.15,30 Finally, the brown
precipitates were separated using a 0.5 T magnet and were thoroughly washed with distilled
water to remove any unbound APTS. Functionalization of Fe3O4@SiO2 nanoparticles was
confirmed by FTꢀIR spectroscopy, and elemental analysis (CHN).
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2.6. Synthesis of Fe3O4@SiO2-NH2@MnPor Nanocatalyst
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The process of manganese porphyrin immobilization was conducted by dispersing the
synthesized Fe3O4@SiO2ꢀNH2 (0.10 g) in 10 mL of toluene, containing 4.6×10ꢀ5 mol of
metalloporphyrin. The suspension was refluxed and stirred for 24 h under argon atmosphere. The
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solid was filtered, washed with toluene, CH2Cl2 and ethanol and dried at 60 C overnight.31,32
The quantity of immobilized manganese porphyrin was determined by measuring the manganese
content of the prepared catalyst; Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc], by atomic absorption
spectroscopy (AAS).
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2.7. General Oxidation Procedure
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A typical oxidation reaction using the Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc] nanoparticles as
catalyst is described as follows. In a 10 mL roundꢀbottom flask, the prepared catalyst (0.01 g,
containing 8×10ꢀ4 mmol of MnPor), the nitrogenous base of imidazole as the coꢀcatalyst (0.064
mmol), substrate (0.2 mmol) and oxidant (0.4 mmol) were added in order with a molar ratio of
{catalyst/imidazole/substrate/oxidant}:{1/80/250/500}. The reaction mixture was stirred at room
temperature in a tightly closed flask. The progress of the reaction was monitored in different time
intervals using gas chromatography (GC). The catalyst nanoparticles were collected at the
bottom of the roundꢀbottom flask using a magnet, supernatant carefully decanted and formation
of products was examined by GC. The oxidation products were identified by comparison with
authentic samples. An internal standard method was used to calculate yields. In the GC
experiments, nꢀdecane was used as the internal standard.
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2.8. Reusability of the Catalyst
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The reusability and the stability of the catalyst were studied in repeated oxidation reactions. At
the end of each reaction, the catalyst was magnetically separated from the reaction mixture,
washed with CH2Cl2 and then dried in vacuum at room temperature for 4 h before reusing in the
subsequent oxidation reaction. The catalyst was consecutively reused about six times without
detectable catalyst leaching or significant loss of activity.
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3. Results & Discussion
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As illustrated in Scheme 1, magnetically separable Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc] (1)
nanocatalyst was synthesized by multistep procedure. First, superparamagnetic Fe3O4
nanoparticles were prepared using the coꢀprecipitation method. The synthesis of Fe3O4
nanoparticles was followed by coating the surface with a thin silica layer in order to increase the
functionality and stability of nanoparticles. For this purpose, the silicaꢀcoated MNPs were
obtained by basic hydrolysis and condensation of TEOS on the surface of the Fe3O4
nanoparticles. In the next step, Fe3O4@SiO2 nanoparticles have been surfaceꢀmodified with 3ꢀ
aminopropyltriethoxysilane (APTS) which introduced ꢀNH2 group on to the surface of support.
Eventually, the supported catalyst (1), was prepared by the reaction of Fe3O4@SiO2ꢀNH2 with
Mn(TPFPP)OAc. By the procedure of modification of Fe3O4 nanoparticles surface, covalent
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bonding is a possible approach for this immobilization, because the manganese porphyrin
presents the pentafluorophenyl groups at the mesoꢀpositioned porphyrin ring, which can be
bonded to the functionalized silica by nucleophilic aromatic substitution. This mechanism is
described by the reaction of fluorine atoms from the meso porphyrin groups and pendant amino
groups from the Fe3O4@SiO2 nanoparticles surface.11,32,33
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Scheme 1. Stepꢀbyꢀstep synthesis of the nanocatalyst (1)
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3.1. Characterization of the Catalyst, Fe3O4@SiO2-NH2@[Mn(TPFPP)OAc]
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Fig. 1 shows the XRD patterns of the synthesized compounds as they are shown schematically in
Scheme 1. The Xꢀray diffraction analysis was carried out for the nanoparticles before and after
growing a SiO2 shell and after immobilization of the catalyst. In the XRD pattern of the prepared
Fe3O4, six characteristic peaks (2θ = 30.6, 35.7, 43.8, 54.2, 57.3, and 63.6), corresponding to
(220), (311), (400), (422), (511), and (440) Bragg reflections, respectively, were observed (Fig.
1a). These diffraction peaks are in good agreement with the database in JCPDS file (PCPDFWIN
v.2.02, PDF No. 85ꢀ1436) and reveal that the resultant nanoparticles were pure Fe3O4 without
impurity phases. This XRD pattern is consistent with the pattern previously reported for Fe3O4
samples.19,34 The XRD pattern of Fe3O4@SiO2 core/shell is shown in Fig. 1b. The same
characteristic peaks can also be found in this pattern indicating that the crystalline structure of
Fe3O4 nanoparticles did not change after the surface modification with silica. The weaker peaks
intensity in pattern of Fig. 1b than that of Fig. 1a can be attributed the shielding effect of
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amorphous silica shell. Also, Fig. 1 shows the XRD patterns of the MNPsꢀNH2 and MNPsꢀ
NH2@MnPor (Fig. 1c, 1d) with specific peaks and relative intensity, which completely match
with the standard Fe3O4 sample.
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Fig. 1. XRD pattern of: (a) Fe3O4, (b) Fe3O4@SiO2 (c) Fe3O4@SiO2ꢀNH2 (d) Fe3O4@SiO2ꢀNH2@MnPor
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Scanning electron microscopy (SEM) image of Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc] catalyst
clearly depicts a smooth morphology of the nanocatalyst; particles were well distributed with
diameters around 27 nm and rather high surface area (Fig. 2). Brunauer–Emmett–Teller (BET)
analysis of the nanocatalyst was performed and the obtained N2 adsorption–desorption isotherm
is depicted in Fig. S2. which is located in Supplementary Information. The BET analysis is the
most common method for determining surface areas from nitrogen adsorption isotherms.35 The
measured BET surface area is 81.2 m2 gꢀ1 for the nanocatalyst (1). Therefore, the relatively high
surface area of nanoparticles is beneficial for high loading of catalyst during the complex
anchoring step.
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Fig. 2. The scanning electron microscopy image of Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc]
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Because of the presence of the magnetic core inside, MNPs cannot be analyzed by means of
solid state NMR spectroscopy. Therefore amino functionalized nanoparticles (Fe3O4@SiO2ꢀNH2)
were systematically characterized by FTꢀIR and elemental analysis. Fig. 3 shows the FTꢀIR
spectra of the coreꢀshell magnetic nanoparticles (Fe3O4@SiO2) after being functionalized by
APTS. The FTꢀIR spectrum (Fig. 3a) represents significant absorption bands at about 586 and
632 cmꢀ1 which corresponds to Fe–O vibration modes of Fe3O4. Furthermore, a strong absorption
band at around 1068 cmꢀ1 (between 1000 and 1200 cmꢀ1) and a band at 794 cmꢀ1, which can be
assigned as vibration modes of Si–O–Si,36,37 indicate that the Fe3O4 core is successfully coated
by silica shell. Another bands at around 1622 and 3427 cmꢀ1 are associated with the vibrations of
absorbed water molecules.38
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Additionally, the characteristic peaks at 2854 and 2923 cmꢀ1 ascribed to the CꢀH stretching
vibration of the propyl group in the pendant APTS can be clearly observed in the FTꢀIR
spectrum of Fe3O4@SiO2ꢀNH2, which confirms that APTS molecules have been bonded
successfully to the surface of the silicaꢀcoated MNPs.39 This conclusion is further supported by
the elemental analysis which gave the percentages of C, H, and N to be 5.10%, 1.54%, and
1.53%, respectively. The frequency of NꢀH asymmetric and symmetric stretching vibrations of
the amine group fall in the 3300ꢀ3400 cmꢀ1 range and are obscured by the water band.
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Fig. 3. FTꢀIR spectra of: (a) Fe3O4@SiO2ꢀNH2 (b) Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc], (c) Mn(TPFPP)OAc
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Furthermore, immobilization of metalloporphyrin on APTSꢀcoated magnetic nanoparticles was
exhibited by FTꢀIR spectroscopy. Comparing FTꢀIR spectra of Fe3O4@SiO2ꢀNH2@MnPor (Fig.
3b) and Mnꢀporphyrin (Fig. 3c) revealed that signals at about 759, 941, 987, 1492, 1517 and
1649 cmꢀ1 corresponding to the vibration modes of pure metalloporphyrin are present in the FTꢀ
IR spectra of Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc] nanoparticles. The loading of manganese
porphyrin complex was 0.08 mmol/g of the supported nanocatalyst, determined by AAS
analysis.
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The presence of MnPor on functionalized magnetic nanoparticles was analyzed by means of
UV–Vis spectroscopy (Fig.4). No absorption band was observed in the UV–Vis spectra of
Fe3O4@SiO2ꢀNH2 (Fig.4b, dash line). However, the Soret and Q bands at 475 and 576 nm
respectively were appeared after immobilization process of Mn(TPFPP)OAc on the surface (Fig.
4a). In this way, both of the FTꢀIR and UVꢀVis spectroscopy provide evidences for anchoring of
the manganese porphyrin on APTSꢀcoated Fe3O4 NPs.
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Fig. 4. UV–Vis spectra of: (a) Fe3O4@SiO2ꢀNH2, (b) Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc]
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The magnetic properties of uncoated Fe3O4 nanoparticles, Fe3O4@SiO2, and the Fe3O4@SiO2ꢀ
NH2@[Mn(TPFPP)OAc] catalyst were characterized by a vibrating sample magnetometer
(VSM) under an applied field of ꢀ10 000 to 10 000 Oe at room temperature (Fig. 5). According
to the magnetization curves, the reduced saturation magnetization values observed for the
functionalized nanoparticles (Fig. 5b, 5c) as compared with pure Fe3O4 nanoparticles (Fig. 5a)
were due to the diamagnetic surface coating and surface functionalization of Fe3O4
nanoparticles.40,41 However, both in the case of the coreꢀshell MNPs and of the final
nanocatalyst, such values are still large enough for nanoparticles to be easily separated by an
external magnet.
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Fig. 5. Magnetization curves of: (a) Fe3O4 (b)Fe3O4@SiO2 (c) Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc]
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3.3. Catalytic Experiments
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The present work reports the catalytic activity of Mn(TPFPP)OAc supported on APTSꢀcoated
Fe3O4@SiO2 nanoparticles; Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc] (1), in alkane oxidation in
the presence of two different oxidants; tetraꢀnꢀbutylammonium hydrogen monopersulfate (nꢀ
Bu4NHSO5) and iodosylbenzene (PhIO). Catalytic performance of heterogeneous catalyst (1) in
alkene epoxidation with nꢀBu4NHSO5 was also investigated as shown in Scheme 2.
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Scheme 2. Hydroxylation of alkanes and epoxidation of alkenes catalyzed by Fe3O4@SiO2ꢀNH2@[MnTPFPP] (1)
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It is believed that the employment of imidazoles in metalloporphyrin systems for mimicking the
axial coordination function of cytochromeꢀP450 has led to remarkable improvement in oxidation
reactions.42,43 According to the coꢀcatalytic role of axial base in biomimetic oxidations catalyzed
with manganese porphyrins,44,45 imidazole nitrogenous base was employed as axial ligand in the
mentioned reactions. Imidazole with the ability of σꢀdonating and πꢀdonating, mainly exhibited
higher activity as a coꢀcatalyst, compared to other nitrogenous bases.
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The
oxidation
reactions
were
carried
out
with
molar
ratio
of
{catalyst/imidazole/substrate/oxidant}:{1/80/250/500} at room temperature in CH2Cl2 as the
reaction medium.
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3.3.1. Alkane Hydroxylation catalyzed by Fe3O4@SiO2-NH2@[Mn(TPFPP)OAc]
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One of the typical challenging areas in organic chemistry is the catalytic oxidation of saturated
hydrocarbons under mild conditions.46,47 For this purpose the catalytic performance of
Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc] was investigated in biomimetic oxidation of alkanes (Fig.
6). In order to find the suitable reaction conditions, the ability of two different single oxygen
donors such as tetraꢀnꢀbutylammonium hydrogen monopersulfate (nꢀBu4NHSO5) and
iodosylbenzene (PhIO) was examined in the oxidation of alkanes in dichloromethane medium
after 4 h. The results, which are summarized in Fig. 6, show that nꢀBu4NHSO5 gives the higher
oxidation conversion. This observation could be explained based on the ability of hydrogen
bonding formation between the orthoꢀC–F groups of MnPor with the hydrogen of oxidant in case
of nꢀBu4NHSO5.
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Fig. 6. Oxidation of alkanes catalyzed by Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc] in the presence of two different
oxidants: nꢀBu4NHSO5 and PhIO.
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This heterogenized catalytic system converts different alkanes to their corresponding alcohols
and aldehyde/ketones (Table 1). The oxidation of cyclooctane, cyclohexane, toluene, indane and
diphenylmethane in dichloromethane solution at room temperature catalyzed by immobilized
Mnꢀporphyrin gives respectively cyclooctanone, cyclohexanone, benzaldehyde, 1ꢀindanone and
benzophenone as the main product, as well as smaller amounts of cyclooctanol, cyclohexanol,
benzyl alcohol and 1ꢀindanol. In addition; to study the influence of time in the oxidation process,
the reactions were carried out in two different time intervals. It can be observed that
Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc] is an efficient catalyst for oxidation of alkanes in the
presence of nꢀBu4NHSO5. Therefore, the results of this study show better catalytic performance
of the heterogonous catalyst using nꢀBu4NHSO5 as oxidant in comparison with PhIO in
oxidation reactions both in terms of selectivity and conversion levels.
292
293
294
295
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Table 1. Hydroxylation of alkanes catalyzed by Fe3O4@SiO2ꢀNH2@[MnTPFPP]a
Entry
Alkane
Conversion%a,b
Selectivity to aldehyde/ketone%e
TONf
71 (67)c [64 (60)c]d
62 [60]d
80 [76]d
177.5
1
2
55 (54)c [48 (46)c]d
137.5
52 (49)c [44 (43)c]d
65 (59)c [57 (50)c]d
95 [89]d
73 [70]d
130
3
4
162.5
5
47 (44)c [36 (31)c]d
100 [100]d
117.5
297 aReaction conditions: alkane (0.2 mmol), nꢀBu4NHSO5 (0.4 mmol), catalyst (8×10ꢀ4 mmol), CH2Cl2 (1 mL), Reaction time:
298 20 h. bDetermined by GC. cValues in parentheses were obtained after 4 h. dValues in brackets were obtained in the presence
e
299 of PhIO. Selectivity to (aldehyde/ketone) = ((aldehyde/ketone)/((aldehyde/ketone) + alcohol)) × 100. fTON = (mmol of
300 product)/mmol of catalyst.
301
302
303
304
305
306
307
308
309
A cageꢀcontrolled radical mechanism, the “oxygen rebound” mechanism, has been proposed and
discussed for the manganese(III) porphyrinꢀcatalyzed hydroxylation of alkanes.48ꢀ50 This process
involves two steps: firstly, hydrogen atom abstraction by the active specie MnV(O)P generates a
solventꢀcaged alkyl radical and a hydoxymanganese(IV) species (Scheme 3, pathway A); in the
second step, a subsequent ‘in cage’ reaction leads to the formation of the alcohol and the MnPor
is recovered. However, in competition with the second step, the carbon radicals can escape from
the cage (Scheme 3, pathway B), thus triggering radical processes, with the production of ketone
products.51,52
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311
Scheme 3. Mechanism of the MnPorꢀcatalyzed oxidation of alkanes
312
3.3.2. Alkene epoxidation catalyzed by Fe3O4@SiO2-NH2@[Mn(TPFPP)OAc]
313
314
315
316
317
318
319
320
321
322
323
324
Catalytic oxidation of olefins constitutes an important area in modern chemistry.53ꢀ56 The
transition metalꢀcatalyzed epoxidation reaction provides a convenient route for preparing
epoxides. In this regard, the Fe3O4@SiO2ꢀNH2@[Mn(TPFPP)OAc] was used as an efficient
catalyst for the epoxidation of alkenes with nꢀBu4NHSO5 in CH2Cl2 as a solvent (Table 2). This
catalytic system showed high activity in the oxidation of cyclooctene and afforded the
cyclooctene oxide in 97% after 20 h. In the case of cyclohexene, the major product was
cyclohexene oxide (88%) and 2ꢀcyclohexenꢀ1ꢀone was produced as a minor product (5%).
Epoxidation of styrene yielded 94% of styrene oxide with only a small amount of benzaldehyde
as byꢀproduct after 20 h. The epoxidation of linear alkenes such as 1ꢀheptene and 1ꢀoctene was
also investigated. Linear alkenes which have the lowest electron densities and no conjugated πꢀ
bondings are less reactive than cyclic ones. The best conversion was obtained after 20 h in terms
of calculated turnover numbers (TON) as shown in Table 2.
325
326
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Table 2. Epoxidation of alkenes with nꢀBu4NHSO5 catalyzed by Fe3O4@SiO2ꢀNH2@[MnTPFPP]a
Entry
Alkene
Conversion% a,b
Selectivity to epoxide%
TONf
97 (85)c
100
242.5
1
93d (83)c,d
94e (81)c,e
94
92
232.5
235
2
3
48 (37)c
56 (43)c
100
100
120
140
4
5
328
329
330
aReaction conditions: alkene (0.2 mmol), nꢀBu4NHSO5 (0.4 mmol), catalyst (8×10ꢀ4 mmol), CH2Cl2 (1 mL),
Reaction time: 20 h. Determined by GC. Values in parentheses were obtained after 3 h. The byꢀproduct is allylic
ketone. eThe byꢀproduct is benzaldehyde. fTON = (mmol of product)/mmol of catalyst.
b
c
d
331
332
333
334
335
336
337
338
339
340
341
342
For alkene epoxidation, there are some mechanism reports of parallel reactions occurring besides
the concerted epoxidation mechanism (Scheme 4, pathway A). In these mechanisms, the active
specie MnV(O)P forms a charge transfer complex with the alkene.1,57ꢀ59 As shown in Scheme 4,
two processes can occur: (B) the charge transfer complex generates a radical intermediate, that
will attack the alkene generating the epoxidized product, or (C) the charge transfer complex
generates a solventꢀcaged alkyl radical, which favors the electron transfer from the alkene to the
MnV(O)P, followed by the formation of a carbocation and subsequent oxygen transfer, thus
generating the epoxidized product.48,60 It should be mentioned whenever the reaction goes
through each of the radical pathways (B and/or C), a variety of products may be readily yielded.
According to the negligible difference of conversion and epoxidation yield as shown in Table 2,
epoxide seems to be the sole main product. Therefore a concerted mechanism (pathway A) may
play a role here for the reaction progress.
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344
Scheme 4. Mechanism of the MnPorꢀcatalyzed oxidation of alkenes
345
3.3.3. Catalyst Reuse and Stability
346
347
348
349
350
351
352
353
354
355
The reusability of a heterogeneous catalyst is of prime importance in catalyst design. Under
homogeneous conditions, manganese porphyrin cannot be recovered even once but the
nanomagnetꢀimmobilized Mnꢀporphyrin can be magnetically separated and reused multiple
times without detectable catalyst leaching or significant loss of catalytic activity. In this manner,
for completion of our study, the reusability of the prepared catalyst, Fe3O4@SiO2ꢀ
NH2@[Mn(TPFPP)OAc], was checked in the multiple sequential epoxidation of cyclooctene
with nꢀBu4NHSO5. The catalyst was separated from the reaction mixture after each run using a
magnet, washed with CH2Cl2 and dried before being used again. As shown in Table 3, the
catalyst could be reused for six consecutive times without significant loss of its catalytic activity.
Also, the filtrates were collected and the amounts of Mn leached after each run were determined
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357
by AAS analysis. As illustrated in Table 3, small amounts of manganese were detected in the
filtrates.
358
Table 3. The results of catalyst recovery in the epoxidation of cyclooctene
Run
Conversion% a
Mn leached% a,b
97
92
1.4 (0.06)d
0.8 (0.03)d
1
2
89
88
87
87
0.5 (0.02)d
3
4
5
6
c
c
c
359
360
361
aReaction conditions: cyclooctene (0.2 mmol), nꢀBu4NHSO5 (0.4 mmol), CH2Cl2 (1
mL), Reaction time: 20 h. Determined by AAS. Not detected. Values in parentheses
were reported in ppm.
b c d
362
363
364
365
366
367
368
369
370
Comparison of the catalytic performances presented in this work with those of other similar
studies61ꢀ67 reveals that our catalytic system is superior to some of the previously reported
catalysts in terms of catalytic activity (TON) and reaction conditions. The high turnover number
of our catalyst compared to recently reported protocols61,62 makes the Fe3O4@SiO2ꢀ
NH2@[MnTPFPP] more attractive for oxidation of hydrocarbons. In other words, compared with
these nonporphyrin catalysts, our catalytic system exhibits high catalytic activity in term of high
TON. According to the molar ratio of 1/250/500 for catalyst/substrate/oxidant which is carried
out in this work, the amount of catalyst which is used in each run (0.01 g, containing 8×10ꢀ4
mmol of MnPor) is lower than that of previous works.61ꢀ66
371
372
373
374
375
It is observed that in several articles,62,65 performance of the catalyst in oxidation of various
substrates was inspected under reflux, while the oxidation reactions were carried out at room
temperature in this work. It should be mentioned that the insertion of an oxygen atom within the
C−H of a saturated carbon atom is one of the most difficult reactions to achieve with a catalyst at
room temperature.
376
377
378
Our heterogenized catalytic system, can be recovered and reused in comparison with the
homogeneous complexes.61ꢀ64 Metal complexes are stabilized upon immobilization on solid
supports because of the support environment. Also, the formation of inactive dimer, the major
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380
381
382
383
drawback of homogeneous catalyst, is prevented by site isolation on the surface. Moreover, nonꢀ
magnetic heterogeneous systems66,67 require filtration and centrifugation steps to recover the
catalysts leading to loss of catalytic activity during consecutive cycles. Therefore, the
magnetically supported nanocatalyst, the present work, can be conveniently separated from
reaction solution with a magnet and reused several times without significant loss of activity.
384
4. Conclusions
385
386
387
388
389
390
391
392
393
394
395
396
This work describes the covalent immobilization of Mnꢀporphyrin, 5,10,15,20ꢀ
tetrakis(pentafluorophenyl)porphyrin manganese(III) acetate Mn(TPFPP)OAc, onto magnetic
nanoparticles covered with amino functionalized silica. This magnetically separable
metalloporphyrin was applied as an efficient retrievable heterogeneous nanocatalyst for the
oxidation of a variety of alkanes and alkenes. In other words, this heterogenized catalytic system
could join the catalytic properties of the manganese porphyrin with the magnetic properties of
Fe3O4 nanoparticles. The results show that the dispersion of these superparamagnetic
nanoparticles, which can be reversibly controlled by applying an external magnetic field,
introducing them as the recyclable support matrix in the catalytic oxidations. Because of the
relatively high surface area of these nanoparticles, high loadings of catalytically active sites are
guaranteed and therefore high catalytic activity is provided for the supported metalloporphyrin
catalyst.
397
398
399
400
401
Furthermore, the separation and reuse of the magnetic Fe3O4 nanoparticles were very effective
and economical. Leaching and recycling experiments revealed that nanocatalyst is already
recyclable several times without loss of activity and magnetic properties. The attractive features
of this catalytic system such as easy work up, noticeable reusability and mild reaction conditions
make it particularly suitable for hydrocarbon oxidation reactions.
402
Acknowledgments
403
404
We would like to appreciate the Sharif University of Technology Research Council for research
funding of this project.
405
406
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