Stabilized Fe3O4 magnetic nanoparticles into nanopores of modified montmorillonite clay: A highly efficient catalyst for the Baeyer-Villiger oxidation under solvent free conditions

Article abstract of DOI:10.1039/c5gc02668g

In situ generation of Fe₃O₄ magnetic nanoparticles (Fe₃O₄@AT-mont.) into the nanopores of modified montmorillonite (AT-mont.) clay has been carried out. Modification of the montmorillonite was done by acid (4 M HCl) activation under controlled conditions for generating nanopores, which act as a "host" for the Fe₃O₄ nanoparticles. The synthesized Fe₃O₄@AT-mont. was characterized by PXRD, TEM, SEM-EDX, XPS, VSM and surface area analysis. The average particle size of Fe₃O₄ nanoparticles was found to be around 10 nm and exhibit a face centered cubic (fcc) lattice geometry. Fe₃O₄@AT-mont. showed efficient catalytic activity for the Baeyer-Villiger oxidation of various cyclic and aromatic ketones in the presence of hydrogen peroxide as an oxidant at room temperature under solvent free conditions and exhibited conversion of up to 98%. The catalyst was magnetically recovered and recycled up to the third run without any significant loss of efficiency.

Full text of DOI:10.1039/c5gc02668g

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Stabilized Fe3O4 magnetic nanoparticles into
nanopores of modified montmorillonite clay:
a highly efficient catalyst for the Baeyer–Villiger
oxidation under solvent free conditions†
Cite this: DOI: 10.1039/c5gc02668g
Pallab Kumar Saikia, Podma Pollov Sarmah, Bibek Jyoti Borah, Lakshi Saikia,
Kokil Saikia and Dipak Kumar Dutta*
In situ generation of Fe3O4 magnetic nanoparticles (Fe3O4@AT-mont.) into the nanopores of modified
montmorillonite (AT-mont.) clay has been carried out. Modification of the montmorillonite was done by
acid (4 M HCl) activation under controlled conditions for generating nanopores, which act as a “host” for
the Fe3O4 nanoparticles. The synthesized Fe3O4@AT-mont. was characterized by PXRD, TEM, SEM-EDX,
XPS, VSM and surface area analysis. The average particle size of Fe3O4 nanoparticles was found to be
around 10 nm and exhibit a face centered cubic (fcc) lattice geometry. Fe3O4@AT-mont. showed efficient
catalytic activity for the Baeyer–Villiger oxidation of various cyclic and aromatic ketones in the presence
of hydrogen peroxide as an oxidant at room temperature under solvent free conditions and exhibited
conversion of up to 98%. The catalyst was magnetically recovered and recycled up to the third run
without any significant loss of efficiency.
Received 6th November 2015,
Accepted 22nd January 2016
DOI: 10.1039/c5gc02668g
www.rsc.org/greenchem
altering the morphology of the support. Recently, porous
substances with a high surface area like montmorillonite
1. Introduction
The development of nano-sized metal and metal oxide par- clay minerals, zeolites, charcoals containing nanosize
ticles has been intensively pursued because of their unique channels etc. have been used for the stabilization of metal
catalytic properties and a wide range of applicability. The nanoparticles.12–19 Due to stringent and growing environ-
emergence of efficient catalytic systems of nanoparticles in mental regulations, the chemical industry needs the develop-
organic synthesis has been a challenge for researchers.1–8 The ment of eco-friendly and sustainable synthetic methods.
nano-catalysts contain highly active surfaces and provide the Montmorillonite is environmentally benign, cheap, and an
advantages of high atom efficiency, simplified isolation of pro- easily available alumino-silicate clay mineral of the smectite
ducts and easy recovery and recyclability of the catalysts.2–5 group. It exhibits some unique properties such as high cation
However, metal nanoparticles are prone to aggregate during exchange capacity, intercalation, swelling etc. which allow
catalytic reactions owing to their high surface energy, which modifications of the structural properties by acid activation.
decrease the catalytic efficiency. This problem of aggregation The acid activated montmorillonite clay is partially delami-
has led to the importance of using various stabilizers or nated and exhibits a higher surface area containing both
supports.9–11 The stabilizer/support plays an important role micro and mesopores, and can be utilized as a host for the
in controlling the particle size, controlling the morphology, synthesis of metal nanoparticles in the desired size and
dispersion as well as the activity of the synthesized shape.16,18
nanomaterials.8–10 Controlled and precise growth of nano-
The heterogeneous metal nano-catalysts provide consider-
particles with the desired shape and size can be tuned by able advantages like easy isolation of products, efficient recov-
ery and recyclability of catalysts. Moreover, in recent years the
strategy of magnetic separation of catalysts has attracted much
Materials Science Division, CSIR-North East Institute of Science and Technology,
attention over filtration or centrifugation as it prevents the loss
Jorhat 785006, Assam, India. E-mail: dutta_dk@rrljorhat.res.in;
of the catalyst.20,21 Hence, Fe3O4 is extensively studied as a
Fax: (+)91 376 2370011
magnetically recoverable catalyst (MRC) for various organic
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5gc02668g
transformations.22–26
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The Baeyer–Villiger oxidation is of considerable interest in tophenone, and 4-nitroacetophenone were purchased from
organic chemistry because the products, lactones or esters, are Alfa Aesar, U.K. and were used as supplied.
important synthetic intermediates in the agrochemical, chemi-
Powder XRD spectra were recorded on a Rigaku, Ultima IV
cal and pharmaceutical industries.27–29 A variety of carbonyl X-ray diffractometer of 2θ range 2–80° using a Cu-Kα source
compounds, such as ketones, cyclic ketones, benzaldehydes (λ = 1.54 Å). The surface area, pore volume, and average pore
and carboxylic acids can be transformed into various oxidised diameter were measured with the Autosorb-1 (Quantachrome,
products, such as esters, lactones, phenols and anhydrides USA). Surface areas of the samples were measured by adsorp-
respectively through this reaction.30,31 The oxidants used in tion of nitrogen gas at 77 K and applying the Brunauer–
the traditional Baeyer–Villiger oxidation are organic peroxy Emmett–Teller (BET) calculation. Prior to adsorption, the
acids which potentially produce large amounts of harmful samples were degassed at 250 °C for 3 h. Pore size distri-
waste. Different types of homogeneous, heterogeneous, and butions were derived from desorption isotherms using the
enzyme catalysts are reported for the Baeyer–Villiger oxidation, Barrett–Joyner–Halenda (BJH) method. Scanning electron
and are often carried out with expensive, hazardous peracids, microscopy (SEM) images and energy dispersive X-ray spectro-
including in situ generated organic peroxides formed from scopy (EDX) results were obtained with Leo 1430 VP
aldehydes and molecular oxygen, leading to the formation operated at 3 and 10 kV. Prior to examination, the samples
of one equivalent of the corresponding carboxylic acid.32–35 were coated with gold. Transmission electron microscopy
Recent efforts have led to the development of a greener hetero- (TEM) and high resolution transmission electron microscopy
geneous catalytic system with hydrogen peroxide as the sole (HRTEM) images were recorded on a JEOL JEM-2011 electron
oxidant.36–42
microscope and the specimens were prepared by dispersing
We report here, the synthesis of Fe3O4 magnetic nano- the powdered samples in isopropyl alcohol, placing them on
particles supported montmorillonite clay (Fe3O4@AT-mont.) a carbon coated copper grid and allowing them to dry. The
and their catalytic Baeyer–Villiger oxidation of cyclic and Fe contents were determined by using an Atomic Absorption
aromatic ketones using H2O2 as the oxidant. The Fe3O4-nano- Spectrometer (AAS), Perkin Elmer, AAnalyst-700 instru-
particles serve as an efficient green and heterogeneous cata- ment. The reaction products were analyzed by GC (Chemito
lyst for the oxidation of cyclic and aromatic ketones with 8510, FID).
excellent yields and selectivity under mild solvent free con-
ditions. The advantages of this catalytic system are the,
2.2. Preparation of support
solvent free conditions, low cost of the metal and magnetic Purified powdered mont. (8 g) was taken in a round bottom
separation of the catalyst. Moreover the catalyst can be flask and 400 ml 4 M hydrochloric acid was added to it. The
reused several times without any significant loss in its cata- resulting dispersion was refluxed for 1 h. After cooling, the
lytic activity.
supernatant liquid was discarded and the activated mont. was
repeatedly centrifuged with deionised water until the pH of
the water becomes nearly 7. The resulting mass was dried in
an air oven at 50 5 °C and designated as AT-mont.
2. Experimental
2.1. Materials and methods
2.3. Preparation of Fe3O4 magnetic nanoparticles supported
on AT-mont.
Bentonite (procured from Gujarat, India) containing quartz, 1 g powdered AT-mont. was dispersed in 60 ml of double dis-
iron oxide etc. as impurities was purified by a sedimentation tilled water in a beaker and to it 9 mmol of FeCl3 was added,
technique to collect the <2 µm fraction. The basal spacing which was then heated to about 60 °C. Few drops of 2 M
(d001) of the air dried sample was about 12.5 Å. The surface NaOH solution was added to the mixture until precipitation
area of the purified montmorillonite (mont.) was determined starts and the mixture was allowed to stir at 60 °C for 15 min.
by N2 adsorption analysis and found to be 101 m2 g−1. The 10 ml aqueous solution of NaBH4 (2 mmol) was added and the
analytical oxide composition of the bentonite determined was mixture was stirred for 30 min. A bar magnet is held at the
SiO2: 49.42%; Al2O3: 20.02%; Fe2O3: 7.49%; MgO: 2.82%; CaO: bottom of the beaker and all the nanoparticles get stuck to the
0.69%; loss on ignition (LOI): 17.51%; and others (Na2O, K2O bottom and the aqueous part was poured out. Using this tech-
and TiO2): 2.05%. The purified mont. is then treated with 4 M nique, the nanoparticles are repeatedly washed with distilled
hydrochloric acid so as to leach out the excess cations and water and finally dried in a vacuum desiccator. The material
increase the surface area. All operations were carried out in a was designated as Fe3O4@AT-mont.
N2 environment. H2O2 (30% w/v) was purchased from M/S
Merck, Germany and used without further purification. FeCl3,
2.4. Catalytic oxidation of ketones
NaBH4, cyclopentanone, cyclohexanone, cyclohex-2-ene-1-one, Ketone (3 mmol), H2O2 [2 ml, 18 mmol (approx.)] and
2-methylcyclohexanone, 3-methylcyclohexanone, 4-methyl- 20 mg Fe3O4@AT-mont. were added and the reaction mixture
cyclohexanone, cycloheptanone and cyclooctanone were pur- was stirred at room temperature for 6 h. To study the recycl-
chased from Sigma-Aldrich, USA and all reagents were used as ability, the used catalyst was separated from the reaction
supplied. Acetophenone, 4-methylacetophenone, 4-hydroxyace- mixture simply with
a magnetic needle retriever. The
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separated catalyst was washed with acetone and dried in
desiccators before using it in the subsequent run. The
product which contains water as a byproduct was removed
by using sodium sulphate. The conversion of the reaction
was determined using GC.
3. Results and discussion
3.1. Characterization of support
The parent mont. exhibited an intense basal reflection at a
2θ value of 7.06° corresponding to a basal spacing of 12.5 Å
in PXRD analyses [Fig. 1]. The intensity of basal reflection
decreases considerably after
1 h acid activation, which
implies that the layered structure of the clay is disrupted and
also a low intense broad reflection of 2θ in the range 20–30°
confirmed the formation of amorphous silica.12 The N2-
sorption type-IV isotherm with a H3 hysteresis loop at
P/P0 ∼ 0.4–0.9 [Fig. 2] for AT-mont. confirms the presence of
both micro- and mesopores with an average pore diameter of
∼3.4 nm. AT-mont. possesses a high surface area up to
Fig. 2 BET isotherm and BJH pore distribution (inset) of AT-mont.
face centered cubic (fcc) lattice of the Fe3O4 nanoparticles,
which are indexed to the spinel structure of pure stoichio-
metric Fe3O4 (JCPDS Card No. 19-0629).43 The BET surface
area and total pore volume of Fe3O4@AT-mont. were found
to be less compared to AT-mont [Table 1]. The appreciable
decrease of the surface area and the pore volume after sup-
porting Fe3O4 nanoparticles might be due to blocking of
some of the pores by Fe3O4 nanoparticles. In addition, the
presence of Fe3O4 nanoparticles may cause complexities in
porosity measurement with nitrogen sorption, as the electro-
static forces between an adsorbate (i.e. nitrogen) and the
metallic surface may affect the measured values to some
extent. However, the increase of the pore diameter may be
due to rupture of some of the pore walls to generate bigger
pores during the formation of the Fe3O4 nanoparticles and
during catalytic reactions. The SEM-EDX analysis also con-
firms the presence of Fe3O4 nanoparticles on the surface of
the AT-mont. Fig. 5(a) and (b) along with the other elements of
the clay matrix. The elemental dot mapping [Fig. 6] clearly
shows the homogeneous distribution of Fe throughout the
support. The TEM images of Fe3O4@AT-mont. Fig. 7 reveal
that most of the Fe3O4 nanoparticles are dispersed on AT-
mont. with average particle size around 10 nm. The XPS
spectra [Fig. 8] of Fe3O4 give two peaks at 710.8 eV and 724.2
eV, which are characteristic of Fe 2P3/2 and Fe 2P1/2 respecti-
vely. On deconvolution of the XPS peaks it is observed that
the peak at 710.8 consists of two peaks at 709. 5 and 710.6
which are respectively for FeO and Fe2O3 components of
Fe3O4.47 The mean relative area of each constituent peak
assigned to FeO and Fe2O3 is 32.28% and 67.72% respectively
which is consistent with the theoretical ratio for Fe3O4 (FeO
33% and Fe2O3 67%). A very low intensity peak centered at
719 eV44,45 is observed in the deconvoluted XPS spectrum
[Fig. 8] indicating the presence of a negligible amount
of γ-Fe2O3. The room temperature magnetization hysteresis
curve of Fe3O4@AT-mont. Fig. 9 shows a saturation magneti-
346 m2 g−1 (surface area for parent mont. is 101 m2 g−1
)
with a pore volume of ∼0.3 cm3 g−1 [Table 1]. The increase
of the surface area as well as the pore volume is due to the
leaching of Al3+ from the clay matrix during acid activation
forming a porous clay matrix. The differential volume versus
pore diameter plot [Fig. 2 (inset)] indicates a narrow pore
size distribution. The SEM image of AT-mont. Fig. 3(a)
showed the formation of pores on the clay surface. The EDX
pattern of the surface [Fig. 3(b)] revealed the presence of pre-
dominant amounts of Si compared to Al on the surface. It
therefore indicates the absence of Fe on the surface of the
AT-mont.
3.2. Characterization of Fe3O4@AT-mont.
Powder XRD analysis of Fe3O4@AT-mont. Fig. 4 shows six
peaks at 2θ values 20.76, 30.16, 35.66, 43.08, 57.06 and 62.6
due to the (111), (220), (311), (400), (511), (440) indices of a
Fig. 1 Powder XRD of parent mont. and AT-mont.
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Table 1 Surface properties of the AT-mont. and Fe3O4@AT-mont. catalysts
Surface properties of support/catalysts
BET surface
Average pore
diameter (nm)
Pore volume
Samples
area (m2 g−1
)
(cm3 g−1
)
AT-mont.
346
186
3.4
4.8
0.3821
0.3001
Fe3O4@AT-mont.
1
2
3
166
124
124
6.5
9.5
12.2
0.2957
0.2730
0.2430
Fig. 3 (a) SEM image of AT-mont. and (b) EDX spectra of AT-mont.
zation (Ms) of 55.01 emu g−1 with near zero remanence (Mr)
and coercivity (Hc) confirming its superparamagnetic
nature.43,46 The Fe3O4@AT-mont. can be manipulated by an
external magnet which is necessary for magnetic separation.
The Fe content in 100 mg of Fe3O4@AT-mont. is found to be
33.3 mg.
3.3. Catalytic activity
The catalytic activity of the synthesized Fe3O4@AT-mont. was
investigated towards the Baeyer–Villiger oxidation of different
cyclic and aromatic ketones for different time periods, using
hydrogen peroxide as the oxidizing agent at room temperature
and good to excellent results were obtained. The highest con-
version of 98% was obtained after 6 h of stirring at room temp-
erature [Table 2]. The catalytic activity was investigated towards
the same reaction using water (2 ml) as the only solvent,
Fig. 4 Powder XRD pattern of Fe3O4@AT-mont.
Fig. 5 (a) SEM image of Fe3O4@AT-mont. and (b) EDX pattern of Fe3O4@AT-mont.
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Fig. 6 Elemental dot mapping of Al, Si, O and Fe on the Fe3O4@AT-mont. surface.
Fig. 8 XPS of Fe3O4@AT-mont. and the results of deconvolution. (Red:
experimental XPS spectrum; black: best curve fit; blue: 2P3/2 spectrum
for Fe2O3; green: spectrum for FeO, brown: spectrum for γ-Fe2O3;
purple: 2P1/2 spectrum for Fe3O4).
Fig. 7 TEM image of Fe3O4 nanoparticles on AT-mont.
however no conversion was found. Therefore, H2O2 clearly
stands out as the oxidizing agent of our choice by considering
its high conversion rate, greener nature and environmental
acceptability. To check the effect of the support, the reaction
was also carried out using neat mont. and AT-mont. as cata-
lysts without loading of Fe3O4 magnetic nanoparticles, but no
conversion was observed.
dation of cyclic and aromatic ketones were studied. All the
selected cyclic and aromatic ketones yield the desired ester
with a considerably good conversion. In general, the cyclic
ketones showed a higher conversion than the aromatic ones.
This may be due to the ring strain present in the cyclic
ketones making them less stable than the aromatics. More-
over, the resonance effect present in the aromatic ketones
may also be a factor for stabilizing them which is absent in
the cyclic ones. The cyclic ketone i.e. cyclohexanone (entry 2)
The versatility and limitations of various substrates as well
as the efficiency of the catalyst for the Baeyer–Villiger oxi-
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Table 2 Baeyer–Villiger oxidation of cyclic and aromatic ketones using
MRC, Fe3O4@AT-mont. under solvent free conditions at room
temperature
Conversiona
(%)
Selectivity
(%)
Entry
1
Substrate
Product
86
100
2
98 (1st run)
96 (2nd run)
96 (3rd run)
94 (4th run)
100
100
100
100
3b
4c
5
—
—
83
—
—
90
Fig. 9 Room temperature magnetization hysteresis curve of
Fe3O4@AT-mont.
6
80
92
showed the maximum conversion of 98% to its corres-
ponding ester i.e. caprolactone while the aromatic aceto-
phenone (entry 11) exhibited a lower conversion of 88% than
the corresponding ester. A possible reaction pathway for the
Baeyer–Villiger oxidation of ketone with H2O2 as an oxidant
is shown in Scheme 1. The possible active site for the reac-
tion is considered to be Fe(III), as in the presence of H2O2,
Fe(II) easily gets oxidized to Fe(III).47 A comparison of different
reported catalysts with the present work is shown in Table 3.
It is clear that the present catalyst shows a better catalytic
activity.
The recyclability of the catalyst, Fe3O4@AT-mont., was
investigated in the oxidation of cyclohexanone and aceto-
phenone. The catalyst was separated from the reaction mixture
simply with the help of a magnetic needle retriever [Fig. 10].
The recyclability results, clearly indicate that the catalyst
remained active without any significant loss in efficiency
[Table 2]. The recovered catalyst was further investigated
through N2-sorption isotherm studies. The surface areas of the
recovered catalysts decrease to a certain extent compared to
186 m2 g−1 of the freshly prepared catalyst [Table 1]. The BJH
pore size of the recovered catalysts increases to a certain extent
as compared to the fresh catalyst [ESI Fig. 1†]. The decrease of
the surface area and the pore volume after catalytic runs is due
to the rupture of some of the pore walls resulting in bigger
pores which reflects in the higher pore diameter value. The Fe
content in the recovered catalyst after the third run was esti-
mated using AAS and found to be 30.1 mg per 100 mg of the
catalyst, which is almost the same as that of the fresh catalyst.
To check the heterogeneity, a filtration test was performed for
entry 2. The reaction was allowed to run for three hours and
then the catalyst was separated from the reaction mixture and
again it was allowed to run without the catalyst for next four
hours. It was found that the conversion after the first three
hours remained unchanged until the end of the reaction after
separation of the catalyst. This confirms that, no leaching of
7
8
9
84
77
77
94
96
97
10
11
76
98
88 (1st run)
85 (2nd run)
84 (3rd run)
84 (4th run)
90
90
90
90
12
13
14
84
75
77
88
87
85
Substrate : H2O2 = 1 : 6. a From GC analysis. b Parent mont. as a catalyst.
c AT-mont. as a catalyst; time 6 h.
Scheme 1 A possible reaction pathway for the Baeyer–Villiger oxi-
dation with H2O2 as an oxidant.
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Table 3 Comparison of the present catalyst with some reported catalysts for the Baeyer–Villiger oxidation of ketones
Sl no.
Catalyst
Solvent
Oxidant
Temp. (°C)
Time (h)
Conversion/yield (max.)
1
2
3
4
5
Sn-supported on clay30
Sn exchanged hydrotalcites38
Mg-Al mixed oxide41
1,2-Dichloroethane
Acetonitrile
1,4-Dioxane
Acetonitrile
Solvent free
H2O2
H2O2
H2O2
H2O2
H2O2
Refluxing temperature
70
70
Room temperature
Room temperature
2
4
12
24
6
100
58
88.7
92
Oragnoselinium42
Fe3O4@AT-mont. (present work)
98
Fig. 10 Different steps showing the recovery of the Fe3O4@AT-mont. catalyst after reaction using a magnetic needle retriever.
any active species of the catalyst takes place during the cata-
lytic reaction.
Acknowledgements
The authors are grateful to Dr D. Ramaiah, Director, CSIR-
North East Institute of Science and Technology, Jorhat-785006,
Assam, India for his kind permission to publish the work. The
authors thank Dr P. Sengupta, Head, Materials Science Divi-
sion, CSIR-NEIST, Jorhat, for his constant encouragement.
Thanks are also due to CSIR, New Delhi [Project No. CSC-0135
and 0125] for the financial support. The author P. K. Saikia
thanks CSIR, New Delhi for providing the fellowship as a
project fellow under Project No. CSC-0135.
4. Conclusion
The Fe3O4@AT-mont. composite has been synthesized by the
in situ generation of the Fe3O4 magnetic nanoparticles into
the nanopores of modified montmorillonite clay and was
characterized by PXRD, TEM, SEM-EDX, XPS, VSM and
surface area analysis. The average particle size of Fe3O4
nanoparticles was found to be around 10 nm and showed
efficient catalytic activity for the Baeyer–Villiger oxidation of
various ketones to the corresponding products in the pres-
ence of hydrogen peroxide as an oxidant at room tempera-
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