An efficient and selective benzylic oxidation of tetralin to 1-tetralone on Cu(II) immobilized Γ-Fe2O3@SBA-15 magnetic nanocatalyst in green water medium without base or additives

Article abstract of DOI:10.1016/j.mcat.2018.04.030

A novel Cu(II)/γ-Fe₂O₃@SBA-15 magnetic nanocatalyst has been synthesized aiming at possessing high surface area, magnetic property, highly dispersed and stabilized Cu(II) complex using SBA-15 by sequential magnetization and Cu(II) immobilization with Fe(acac)₃ and Cu(OAc)₂ precursors. The nanocatalyst was well authenticated by different techniques like BET surface area, X-ray diffraction, X-ray photoelectron spectroscopy, FT-IR, vibrating sample magnetometer and thermogravimetric analysis. Cu(II)/γ-Fe₂O₃@SBA-15 was found to be effective for the benzylic oxidation of tetralin to 1-tetralone using TBHP at mild reaction conditions in green water medium even in absence of any bases or additives. The heterogeneity of catalyst was confirmed by hot filtering test and further reused for at least 4 times without significant loss in catalytic activity. Ease of catalyst separation, its reusable capacity, wider applicability of alkyl aromatics and importantly reaction operation in water as green medium by elimination of hazardous organic solvents are the commendable merits of this catalyst.

Full text of DOI:10.1016/j.mcat.2018.04.030

MolecularCatalysis453(2018)74–84
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
An efficient and selective benzylic oxidation of tetralin to 1-tetralone on Cu
(II) immobilized γ-Fe2O3@SBA-15 magnetic nanocatalyst in green water
medium without base or additives
Chinna Krishna Prasad Neelia, Hari Prasad Reddy Kannapua, Venkat Narayana Kalevarub,
Seetha Rama Rao Kamarajua, David Raju Burria,⁎
a
Catalysis Laboratory, I & PC Division, Indian Institute of Chemical Technology, Hyderabad, 500007, India
Leibniz Institute for Catalysis at University Rostock, Albert-Einstein-Str. 29a, 18059, Rostock, Germany
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
A novel Cu(II)/γ-Fe2O3@SBA-15 magnetic nanocatalyst has been synthesized aiming at possessing high surface
area, magnetic property, highly dispersed and stabilized Cu(II) complex using SBA-15 by sequential magneti-
zation and Cu(II) immobilization with Fe(acac)3 and Cu(OAc)2 precursors. The nanocatalyst was well authen-
ticated by different techniques like BET surface area, X-ray diffraction, X-ray photoelectron spectroscopy, FT-IR,
vibrating sample magnetometer and thermogravimetric analysis. Cu(II)/γ-Fe2O3@SBA-15 was found to be ef-
fective for the benzylic oxidation of tetralin to 1-tetralone using TBHP at mild reaction conditions in green water
medium even in absence of any bases or additives. The heterogeneity of catalyst was confirmed by hot filtering
test and further reused for at least 4 times without significant loss in catalytic activity. Ease of catalyst se-
paration, its reusable capacity, wider applicability of alkyl aromatics and importantly reaction operation in
water as green medium by elimination of hazardous organic solvents are the commendable merits of this cat-
alyst.
γ-Fe2O3@SBA-15
Cu nanocatalyst
alkylaromatics
benzylic oxidation
Water
1. Introduction
not only for its structural and textural characteristics like high surface
area, controllable porosity, excellent hydrothermal stability, tuneable
Magnetic nanoparticles attained significant interest owing to their
unique physicochemical characteristics [1] and broad applications in
various fields like hyperthermia [2], magnetic resonance imaging [3],
drug delivery [4], protein separations [5], fluid transport [6], catalysis
[7], magnetic biosensors [8] and magnetic refrigeration [9]. Magnetic
nanocrystals like magnetite (Fe3O4) and maghemite (γ-Fe2O3) are being
engaged in high-density magnetic storage [10] and biomedical sciences
[11].
surface chemistry [17] and good accessibility [18], but also for its po-
tential applications like absorption, separation, catalysis etc. [19]. In-
troduction of super-paramagnetic iron oxide nanoparticles into the
pores of silica may assist to combine the unique characteristics of me-
soporous silica with the potentiality of magnetic properties. Usually, γ-
Fe2O3 nanoparticles are synthesized by laser pyrolysis [20], vaporiza-
tion-condensation [21], Co-precipitation [22,23], thermal decomposi-
tion [24] and post grafting techniques [25].
Catalyst recovery and reuse are the two most essential features for
several catalytic processes. Catalyst separation by filtration or cen-
trifugation involves tedious workup procedure wherein a loss in sub-
stantial amount of nanocatalyst is inevitable. At this juncture, isolation
and separation of nanocatalyst can be easily achieved by the develop-
ment of magnetically separable nanocatalysts which has been emerging
recently as an effective technique [12]. Recently, catalytic magnetic
nanomaterials have been synthesized using a variety of methodologies
and have been examined for various organic transformations [13–16].
Mesoporous silica SBA-15 has been exploring as a catalyst support
1-tetralone is an essential intermediate for the production of dyes,
pharmaceuticals and agrochemicals [26]. Traditionally, 1-tetralone is
synthesized with a good selectivity by an intramolecular Friedel-Crafts
reaction [27]. The homogeneous catalytic oxidation of tetralin is car-
ried out by various soluble copper [28] and cobalt [29] systems. Bi-
phasic autoxidation of tetralin has been catalyzed by executing surface-
active tetramethyl ethylenediamine complexes of chromium, manga-
nese and nickel catalysts [30] with low yields to 1-tetralone. Compared
to above soluble systems, a heterogeneous catalyst with high advantage
for recovery and reusability is greatly desirable.
⁎
Corresponding author.
E-mail addresses: narayana.kalevaru@catalysis.de (V.N. Kalevaru), david.iict@gov.in (D.R. Burri).
https://doi.org/10.1016/j.mcat.2018.04.030
Received 11 January 2018; Received in revised form 14 April 2018; Accepted 22 April 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

C.K.P. Neeli et al.
MolecularCatalysis453(2018)74–84
Of late, hexagonal mesoporous aluminophosphate, MCM-41 and
2.4. Preparation of diamine γ-Fe2O3@SBA-15
AlPO4-5 matrix engaged transition metal (Fe, Co, and Cr) catalysts were
used in this oxidation [31]. More recently, the selective oxidation of
tetralin was reported with CrMCM-41 [32] and CrSBA-15 [33] cata-
lysts. However, these catalytic systems include the usage of hazardous
and carcinogenic organic solvents as reaction medium. In view of green
chemistry principles, the use of water as reaction medium has several
advantages such as non-toxicity, non-flammability, cheap, abundant
and exhibits many unique physico-chemical properties with consider-
able rate acceleration compared to conventional solvents.
The diamine functionalization on γ-Fe2O3@SBA-15 was carried out
by post-synthetic approach as follows. Typically, 1 g of γ-Fe2O3@SBA-
15 was degassed for 3 h at 150 °C in vacuum and dispersed in 50 mL of
dry toluene under N2 atmosphere, then 3 mL of N-[3-(Trimethoxysilyl)
propyl] ethylenediamine was added and the mixture was refluxed for
24 h. The solid product was recovered by filtration under vacuum,
washed with 150 mL of toluene and dried at 100 °C in an air oven for
12 h, designated as Diamine γ-Fe2O3@SBA-15.
In continuation of our ongoing research work on the selective
benzylic oxidation over copper catalyst [34], we intend to develop an
efficient and magnetically retrievable copper nanocatalyst for selective
benzylic oxidation of alkyl aromatics in water as green solvent condi-
tions. Herein, we report the synthesis of Cu(II) immobilized γ-Fe2O3@
SBA-15 nanocatalyst and its evaluation in the selective benzylic oxi-
dation of tetralin to 1-tetralone using TBHP under mild reaction con-
ditions with green water as solvent medium. In addition, different
catalytic reaction parameters such as solvent effect, temperature effect,
oxidant to substrate mole ratio, effect of catalyst amount and reaction
time are systematically studied. Moreover, applicability of alkyl aro-
matics, heterogeneity and recyclability of the catalyst has been deli-
neated.
2.5. Cu(II) immobilization on diamine γ-Fe2O3@SBA-15
In typical synthesis, 1 g of vacuum dried diamine γ-Fe2O3@SBA-15
was suspended in 50 mL of DCM under stirring conditions. Then 0.143 g
of Cu(OAc)2 was added into the DCM solution and further stirred for
about 12 h at room temperature. The color of the material is changed
from reddish brown to dark brown which indicates the immobilization
of Cu(II) complex. The reaction mixture was filtered off through cen-
trifuge and successively washed with ethanol until the filtrate was
colorless. The resulting catalyst was dried overnight in air at 100 °C
under vacuum and the final obtained material was designated as Cu(II)/
γ-Fe2O3@SBA-15. For only comparison of the tetralin oxidation ac-
tivity, Cu(II) was deposited on γ-Fe2O3@SBA-15 in conventional im-
pregnation method with Cu(OAc)2 and methanol as metal precursor
and solvent respectively.
2. Experimental
2.1. Materials
2.6. Catalyst characterization
All the reagents and chemicals are commercially available and were
employed as received, unless otherwise indicated. P123 (Sigma Aldrich
99%), TEOS (Sigma Aldrich 98%), HCl (Merck 35%), CH3COOH (Merck
99%), HNO3 (SD Fine Chem. Ltd. 69%), Fe(acac)3 (Sigma Aldrich 97%),
N-[3-(Trimethoxysilyl)propyl]ethylenediamine (Sigma Aldrich 97%),
Toluene (SD Fine Chem. Ltd. 99.5%), Cu(OAC)2 (Sigma Aldrich 98%),
DCM (Sigma Aldrich 99%) and Ethanol (Jiangsu Huaxi International
Trade Co Ltd. 99.99%).
The X-ray diffraction (XRD) patterns were recorded at room tem-
perature using an X-ray diffractometer (Ultima-IV, M/s. Rigaku
Corporation, Japan) at 40 kV and 40 mA with a nickel filtered CuKα
radiation. N2 adsorption–desorption isotherms were recorded on a N2
adsorption unit at −196 °C (Quadrusorb-SI V 5.06, M/s. Quantachrome
Instruments Corporation, USA).The samples were out-gassed at 200 °C
for 4 h prior the measurement. Infrared spectra were recorded on a
Spectrum GX (M/s. Perkin Elemer, Germany) FT-IR system, in the scan
range of 4000–400 cm−1. Fe and Cu contents of the sample were ana-
lyzed with a simultaneous ICP-AES allied analytical system (M/s. Perkin
Elemer 3100XL). The XPS analysis was made on a photoelectron
spectrometer (KRATOS Axis 165, Shimadzu, Japan) with Mg Kα ra-
diation (1253.6 eV) and the C 1 s line (284.6 eV) was used as the re-
ference to correct the binding energies (BE). The TGA measurements
were carried out using a TGA Q500 analyzer (M/s. TA instruments,
USA). The samples were heated in air flowof 10 mL min−1 from 27 to
800 °C at a heating rate of 10 °C min−1 with samples mounted on a
platinum sample holder. During the heating period, the weight loss was
recorded as a function of temperature. Magnetization of the powder
samples was measured at RT using a vibrating sample magnetometer
(M/s. MicroSense, USA) using a glass rod wrapped with aluminium foil
as the sample holder. Variation in magnetization and coercive field with
temperature was recorded using VSM software version 9.13G.
2.2. Preparation of SBA-15
The synthesis of parent SBA-15 has been described by Zhao et al.
[35] and in our own published procedures [36–39]. In a typical
synthesis,
a
solution
of
EO20PO70EO20:2 M
HCl:
TEOS:
H2O = 2:60:4.25:15 (mass ratio) was prepared by using P123 triblock
co-polymer (EO20PO70EO20, MWavg≅ 5800) as a structure directing
agent and tetraethylorthosilicate (TEOS) as a silica source, stirred at
40 °C for 24 h. Then, the resulting mixture was subjected to hydro-
thermal treatment at 100 °C for 12 h. The obtained mesoporous silica
with a surfactant was filtered and washed with deionizer water and
dried at 100 °C. The surfactant was then removed from the SBA-15
channels by calcination at 550 °C for 8 h and this template-free SBA-15
was used as a catalyst support.
2.3. Preparation of magnetic silica γ-Fe2O3@SBA-15
2.7. Catalytic activity test for selective benzylic oxidation of tetralin in
water medium
The magnetic silica had been prepared according to previous report
[40] with slight modification. In a typical process, 1.2612 g of Iron(III)
acetylacetonate [Fe(acac)3] was dispersed in mixture of 12 mL of
CH3COOH (A.R.) and 2 mL of HNO3 (A.R.) and then stirred for 4 h until
Fe(acac)3 was completely dissolved to form a sol. Then, 1 g of SBA-15
powder was added to the above sol-gel solution. After stirring for an-
other 4 h at room temperature, the mixture was then left stand over-
night to let the acid evaporate. Then the sample was collected and
In a typical experiment, 50 mg of Cu(II)/γ-Fe2O3@SBA-15 catalyst
was dispersed in 2 mL of water in 10 mL capacity round bottom flask
fitted with a water condenser, which is open to air. Subsequently,
1 mmol tetralin and 3 mmol of TBHP (70% in aqueous solution) were
added and conducted the reaction at 90 °C for 6 h at constant stirring.
After completion of the reaction, catalyst was removed from the pro-
duct mixture by simple magnetic separation. Then, the product mixture
is extracted with ethylacetate followed by drying over anhydrous
MgSO4 to remove traces of water and the samples were analyzed by a
gas chromatograph (GC-17A, M/s. Shimadzu Instruments, Japan) using
calcined at 350 °C for 30 min in air with a heating rate of 2 °Cmin−1
,
yielding to the formation of reddish brown magnetic silica designated
as γ-Fe2O3@SBA-15.
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C.K.P. Neeli et al.
MolecularCatalysis453(2018)74–84
Scheme 1. Schematic representation of Cu(II) immobilized γ-Fe2O3@SBA-15 synthesis.
an Equity-5 capillary column (0.53 mm x 30 m). The products were
confirmed by GC–MS (QP-5050 model, M/s. Shimadzu Instruments,
Japan) equipped with DB-5 capillary column (0.32 mm dia. and 25 m
long, M/s. J & W Scientific, USA).
Table 1
Structural and textural characteristics of (A) SBA-15, (B) γ-Fe2O3@SBA-15, (C)
Diamine γ-Fe2O3@SBA-15 and (D) Cu(II)/γ-Fe2O3@SBA-15 catalysts from N2
adsorption and low angle XRD.
Catalyst SBETa (m2/g) Vtb (cc/g) DBJH (nm) d100 (nm) a0 (nm) tf (nm)
c
d
e
3. Results and discussion
A
B
C
D
794
487
175
142
1.43
0.86
0.29
0.24
7.38
6.20
4.02
3.91
9.98
9.81
9.72
9.59
11.5
11.3
11.2
11.0
4.2
5.1
7.2
7.1
The overall synthesis of Cu(II)/γ-Fe2O3@SBA-15 magnetic nanoca-
talyst has been schematically represented by the following sequential
steps as shown in Scheme 1. The loading of N-propyl-ethylenediamine
in diamine γ-Fe2O3@SBA-15 was determined from carbon, hydrogen
and nitrogen (CHN) elemental analysis and the loading of copper and
Fe in the Cu(II)/γ-Fe2O3@SBA-15 catalyst was estimated by inductively
coupled plasma-optical emission spectrometry (ICP-OES). The N-
propyl-ethylenediamine loading is around 1.58 mmol g−1. The loadings
of Fe in γ-Fe2O3@SBA-15 and Cu(II)/γ-Fe2O3@SBA-15 are 15 and 11 wt
% whereas Cu(II) metal ions in fresh and recycled Cu(II)/γ-Fe2O3@SBA-
15 catalysts were found to be 5.92 and 5.89 wt% respectively, wherein
Cu(II) impregnated γ-Fe2O3@SBA-15 catalyst exhibited Cu content as
5.94 wt%.
a
b
c
d
e
f
BET surface area.
Total pore volume.
BJH pore diameter.
Periodicity of (100) plane of SBA-15 derived from low angle XRD.
Unit cell parameter (a0 = 2 d100/√3).
Pore wall thickness (t = a0−DP).
samples is lower compared to parent SBA-15, which indicates the de-
crease in surface areas. The BET surface area of parent SBA-15 and Cu
(II)/γ-Fe2O3@SBA-15 are 794 and 142 m2/g, respectively. The decre-
ment in surface area can be ascertained to the incorporation of Fe2O3 by
magnetization and its surface functionalization with diamine and Cu(II)
complex. Thus, sequential attachment of organic spacers/linkers on the
pore wall leads to an increase in pore wall thickness from 4.2 to 7.1 nm.
Instead, the decreasing trend in the surface area, pore volume and pore
diameter are also due to partial blockage of certain mesopores in each
step of the catalyst preparation, which additionally confirms the in-
troduction of N-propyl-ethylenediamine and subsequent coordination
of Cu2+ metal ions. Indeed, the presence of some amount of copper
species in the pore wall may not be ruled out.
3.1. N2 adsorption-desorption isotherms
N2 adsorption–desorption isotherms of SBA-15, γ-Fe2O3@SBA-15,
diamine γ-Fe2O3@SBA-15 and Cu(II)/γ-Fe2O3@SBA-15 catalysts are
displayed in Fig. 1, which reveals that all of the catalysts exhibit a type
IV adsorption isotherm with H1 hysteresis loop, characteristic of or-
dered mesoporous materials [41]. The BET surface area, total pore
volume, BJH average pore diameter and pore wall thickness are listed
in Table 1. Noteworthy, the amount of N2 adsorbed on the modified
3.2. X-Ray diffraction analysis
The low-angle XRD patterns of SBA-15, γ-Fe2O3@SBA-15, diamine
γ-Fe2O3@SBA-15 and Cu(II)/γ-Fe2O3@SBA-15 are shown in Fig. 2. The
pristine SBA-15 displays three strong diffraction peaks at 0.91, 1.54 and
1.76° on the 2 theta scale, which are assigned to (100), (110) and (200)
planes of 2D-hexagonal mesophase indicates the formation of ordered
mesoporous SBA-15. The (100) plane of γ-Fe2O3@SBA-15, diamine γ-
Fe2O3@SBA-15 and Cu(II)/γ-Fe2O3@SBA-15 samples are similar, in-
dicating the retention of the 2D-hexagonal mesoporous framework after
functionalization with N-propyl-ethylenediamine and the coordination
of Cu(II) metal ions. The grafting steps did not significantly diminish
the ordered mesoporous structure of SBA-15 but there is a small shift in
the 2 theta and a decrease in signal intensities, which could be attrib-
uted to the lowering of the local order. The decrease in local order is
owing to reduction of scattering contrast between the channels of the
SBA-15 frame work and the functionalized moieties partially filling the
pores [42].
Fig 1. N2 adsorption-desorption isotherms of different samples.
Fig. 3 shows the wide-angle XRD patterns of the corresponding
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C.K.P. Neeli et al.
MolecularCatalysis453(2018)74–84
Fig. 2. Low-angle XRD patterns of different samples.
Fig. 4. XPS spectrum of Cu(II)/γ-Fe2O3@SBA-15.
appear at 716 eV, which is the characteristic peak of Fe+2 species is
absent, suggesting the absence of Fe3O4 in the catalyst [48,49].
Therefore, on the basis of XRD and XPS analysis, the as-prepared FexOy
nanoparticles are mainly composed of γ-Fe2O3. The broad and intense
peaks of Cu(II)/γ-Fe2O3@SBA-15 catalyst at 934.58 and 954.45 eV
binding energies correspond to spin orbit splitting components of Cu
2p3/2 and Cu 2p1/2 and the presence of 2p → 3d satellite peaks of Cu
2p3/2 and Cu 2p1/2 at 943.81 and 962.98 eV indicates existence of Cu
species in +2 oxidation state.
Fig. 3. Wide-angle XRD patterns of different samples.
samples. The broad band centered at 2θ = 22° can be assigned to the
characteristic reflection of amorphous SiO2 (JCPDS 29-0085). All these
samples exhibit sharpened spinel (311) and (440) peaks at 35.5° and
62.9° that are indexed to γ-Fe2O3 phase in accordance with JCPDS
39–1346. The low intensities and broad nature of the these peaks can be
ascribed to the ultrasmall size of iron oxide nanoparticles [43]. The iron
oxide loaded samples have red-brown color which is the characteristic
γ-Fe2O3 phase and highlighted in dotted lines (Fig. 3). The absence of
diffraction peak corresponding to Cu(II) in Cu(II)/γ-Fe2O3@SBA-15
sample indicates the formation of homogeneously dispersed Cu(II)
complex and exhibit the amorphous nature in the matrices of SBA-15.
Further, the existence of γ-Fe2O3 and Cu(II) metal ions on the surface of
SBA-15 were confirmed when the sample was subjected to XPS analysis.
3.4. FT-IR spectra
The FT-IR spectra of pristine SBA-15, γ-Fe2O3@SBA-15, diamine γ-
Fe2O3@SBA-15 and Cu(II)/γ-Fe2O3@SBA-15 are shown in Fig. 5. A
broad band is observed at 3450 cm−1 in all the samples, which in-
dicates typical eOH stretching vibration, the band at 1637 cm−1 is due
to eOH deformation of physisorbed water and the sharp band around
967 cm−1 is associated with SieOeH stretching [50]. The absorption
band at 1090 and 800 cm−1 shows the presence of eSieOeSie asym-
metric and symmetric stretching bands. The band at 590 cm−1 appears
only in the iron oxide incorporated samples is be attributed to the FeeO
stretching which is absent in pristine SBA-15. Upon diamine functio-
nalization, a decrease in the intensity of the bands at 3450, 800 and at
967 cm−1 is observed, indicating the occurrence of reaction between
the isolated silanol groups of γ-Fe2O3@SBA-15 surface with the
methoxy groups of N-[3-(Trimethoxysilyl)propyl] ethylenediamine
functionalizing ligand. The 2938 and 2879 cm−1 bands in γ-Fe2O3@
SBA-15 are assigned to νCH of the −CH2 groups, and the band at
1376 cm−1 is attributed to δCH of the −CH group. The additional bands
in both diamine γ-Fe2O3@SBA-15 and Cu(II)/γ-Fe2O3@SBA-15 samples
at 1472 and 692 cm−1 are assigned to δNH of the −NH groups [51]
which confirms the successful diamine functionalization. However, the
shift in the bands at 1472 and 1376 cm−1 towards lower wavenumber
of 1460 and 1357 cm−1 clearly indicates the covalent immobilization of
3.3. XPS analysis
The X-Ray photoelectron spectrum of Cu(II)/γ-Fe2O3@SBA-15 is
depicted in Fig. 4. Generally, the binding energy of metallic iron ap-
pears around 707 eV and peaks for the oxide form of iron lie between
710 and 723 eV [44,45]. The binding energies observed in the present
study at about 711.17 and 723.97 eV can be assigned to the Fe 2p3/2
and Fe 2p1/2 of Fe3+ ions in γ-Fe2O3 [46,47]. Another peak that ap-
peared at binding energy of 718.40 eV is the characteristic satellite peak
of Fe 2p3/2 of Fe3+ species. The Fe 2p3/2 satellite peak supposed to
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C.K.P. Neeli et al.
MolecularCatalysis453(2018)74–84
Fig. 7. TGA analysis of different samples.
Fig. 5. FT-IR spectra of (a) SBA-15, (b) γ-Fe2O3@SBA-15, (c) Diamine γ-
Fe2O3@SBA-15 and (d) Cu(II)/γ-Fe2O3@SBA-15.
Fe2O3@SBA-15, diamine γ-Fe2O3@SBA-15 and Cu(II)/γ-Fe2O3@SBA-
15 samples. For uncalcined γ-Fe2O3@SBA-15, a marginal 2.52% weight
loss is observed due to the loss of adsorbed water from the surface of
SBA-15. The major 19.98% weight loss was observed around 250 °C due
to the decomposition of Fe(acac)3 moieties anchored on SBA-15. The
minor weight loss is observed for diamine γ-Fe2O3@SBA-15 between
100 and 150 °C, which is due to physically adsorbed water molecules on
the surface of the SBA-15. The major weight loss was observed between
200 and 450 °C of 16.89 and 18.87% for diamine-γ-Fe2O3@SBA-15 and
Cu(II)/γ-Fe2O3@SBA-15 samples which is due to the thermal decom-
position of grafted N-propyl-ethylenediamine and its combination with
Cu(II) complex. Beyond this temperature, the weight loss is more or less
constant. The TGA curves demonstrate the covalent anchoring of or-
ganic moieties on to the surface of γ-Fe2O3@SBA-15. To validate the
covalent immobilization of Cu(II) metal ions to amine containing Ni-
trogen atom, TGA analysis was carried on the catalyst prepared by
physical mixture of individual Cu(OAc)2 and diamine γ-Fe2O3@SBA-15
components, which showed a drastic weight loss of 24.12% in the
temperature range of 100–450 °C. These results reveal that physical
mixing lead to initiate weight loss at low temperature 100 °C due to
surface deposition rather than slow weight loss of 18.87% at higher
initiate temperature 200 °C owing to chemical interaction through
covalent bonding, clearly evidenced the covalent immobilization of Cu
(II) metal ions to amine containing Nitrogen atoms of functionalization
ligands. These observations are in good agreement with FT-IR results
(see Fig. 5).
Cu(II) metal ions with Nitrogen present in amine groups of the func-
tionalization ligand [52]. Further, the confirmation of Cu(II) covalent
immobilization is elucidated by the results obtained from TGA experi-
ment of the sample prepared by physical mixture of individual Cu
(OAc)2 and diamine γ-Fe2O3@SBA-15 components.
3.5. VSM analysis
The magnetic properties of γ-Fe2O3@SBA-15, diamine γ-Fe2O3@
SBA-15 and Cu(II)/γ-Fe2O3@SBA-15 were authenticated using a vi-
brating sample magnetometer with fields of up to 5000 Oe. The hys-
teresis loops of these samples were recorded at temperature of 300 K.
The field-dependent magnetization plots illustrate these samples are
super paramagnetic at 300 K. The magnetization curves at 300 K show
no hysteresis (Fig. 6), which means that these composites exhibit super
paramagnetic behavior at room temperature. Such behavior is believed
to be derived from the γ-Fe2O3 nanoparticles formed in the mesoporous
channels of SBA-15 host silica. The saturation magnetization of these
samples is 2.89 emu g−1 (γ-Fe2O3@SBA-15), 1.46 emu g−1 (Diamine γ-
Fe2O3@SBA-15), and 1.07 emu g−1 (Cu(II)/γ-Fe2O3@SBA-15) and
shows sufficient response to the external magnetic field.
3.6. TGA analysis
Fig. 7 shows the thermogravimetric analysis curves of uncalcined γ-
3.7. Catalytic activity studies
Aiming to utilize water as a green solvent medium, we develop an
efficient and magnetically retrievable catalytic system which is active
for the benzylic oxidation of tetralin to 1-tetralone in the absence of
bases and additives. Despite the formation of several products in the
tetralin oxidation (Scheme 2), selective formation of 1-tetralone is the
main objective of present study. Initially, controlled experiments were
carried without adding the catalyst at reaction conditions. No conver-
sion was observed in the absence of catalyst which proves that parti-
cular benzylic oxidation of tetralin is a catalytic process. Only trace
amount of tetralin conversion (∼ 3%) was observed in case of γ-
Fe2O3@SBA-15 as a catalyst. Cu impregnated on γ-Fe2O3@SBA-15 was
found to be capable of oxidizing tetralin with low conversion (33%).
Then, the catalytic performance of Cu(II)/γ-Fe2O3@SBA-15 was tested
for the synthesis of 1-tetralone by reacting 1 mmol of tetralin (TLN)
with 3 mmol of TBHP (70% in aqueous solution) as an oxidant using
50 mg of Cu(II)/γ-Fe2O3@SBA-15 in the presence of 2 mL water as a
green solvent at 90 °C for 6 h, in which the selectivity of 1-tetralone
Fig. 6. Magnetization as function of applied field for different samples.
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C.K.P. Neeli et al.
MolecularCatalysis453(2018)74–84
Scheme 2. Possible formation of different products in tetralin oxidation.
product was found to be 85% with tetralin conversion 64%. The high
tetralin conversion on Cu(II) immobilized on diamine functionalized γ-
Fe2O3@SBA-15 compared to Cu(II) impregnated γ-Fe2O3@SBA-15 is
due to the homogeneous dispersion of Cu(II) nanoparticles obtained by
the introduction of Cu(II) species with hydrophobic transport medium.
The homogeneous distribution of Cu(II) species is ascertained by low
crystallinity with below the detection limits (< 5 nm) of wide angle
XRD and exhibits amorphous nature is due to existence of Cu(II) metal
ions in complex form.
In order to obtain the optimum reaction conditions, various para-
meters such as effect of solvent, reaction time, mole ratio of TBHP to
tetralin, temperature and catalyst amount that influencing the catalytic
activity were studied and well displayed.
To identify the best solvent, benzylic oxidation of tetralin was car-
ried out in different solvents of 2 mL each using 50 mg of Cu(II)/γ-
Fe2O3@SBA-15 and 2 mmol of TBHP at 90 °C for 6 h. The results are
displayed in Fig. 8, which reveals that toluene and chlorobenzene sol-
vents gave low selectivity 59 and 61% to 1-tetralone (T = O) yielding
other byproducts such as 4-hydroxy-1-tetralone and 1,4-naphthaqui-
none, while acetonitrile and neat systems gave moderate (67%) and low
(58%) selectivity to 1-tetralone with 1-tetralol (T−OH) and naphtha-
lene (NPH) as major byproducts. The selectivity of 1-teralone (74%)
was higher when water was used as a solvent with minor byproduct as
naphthalene. The higher 1-tetralone selectivity was driven through the
hydrophobically incorporated Cu(II) species favoured by non-polar di-
chloromethane solvent onto the surface of γ-Fe2O3@SBA-15. A similar
type of observation was found in the selective hydrogenation of double
bond in α,β-unsaturated carbonyl compounds [39]. These results con-
firm water was the best solvent in terms of selectivity towards 1-tet-
ralone.
Once it is found that the water is the best solvent, the effect of the
TBHP/TLN molar ratio on the conversion of tetralin to 1-tetralone was
studied by varying the ratio from 1 to 4 and the results are depicted in
Fig. 9. The conversion of tetralin was found to be increased from 45 to
52% with increasing the concentration of the oxidant from 1 to 2. The
maximum 1-tetralone selectivity 85% and good tetralin conversion 64%
was obtained when the TBHP/TLN molar ratio is 3. This might be due
to required amount of oxidant molecules. After that, increasing this
ratio to 4, there is a significant decrease in the selectivity to 1-tetralone
with slight increase in the conversion. The decrease in selectivity was
due to the formation of byproducts with more TBHP concentration.
From these results the TBHP/TLN molar ratio of 3 was found to be
optimum.
To determine the optimum catalyst amount, a series of oxidation
reactions were carried by varying the amounts of catalyst viz., 25, 50,
75 and 100 mg with 1 mmol of tetralin, 3 mmol of TBHP and 2 mL of
water at 90 °C for 6 h and results with their TOF values were sum-
marized in Table 2. It was found that the amount of catalyst could re-
markably promote the conversion of the tetralin to the corresponding 1-
tetralone. With the increase in the amount of catalyst from 25 to 50 mg,
augmentation in the conversion to 64% and selectivity to 85% was
observed. A further increase in the catalyst amount influence to de-
crease the selectivity of 1-tetralone with marginal change in the con-
version. Further, turn over frequencies (TOF) were calculated based on
Fig. 8. Effect of solvent in the benzylic oxidation of tetralin.
79

C.K.P. Neeli et al.
MolecularCatalysis453(2018)74–84
Fig. 10. Effect of reaction time in the benzylic oxidation of tetralin.
Fig. 9. Effect of TBHP to tetralin mole ratio in the benzylic oxidation of tetralin.
the reaction time, seven experiments were separately conducted at
different time intervals (1, 2, 3, 4, 5, 6 and 7 h) over the Cu(II)/γ-
Fe2O3@SBA-15catalyst and the results are displayed in Fig. 10. It was
observed that there is a gradual increase in the conversion from 32 to
64% with increase in reaction time from 1 to 6 h. Also, the conversion
of tetralin gradually increases from 1 to 6 h. The selectivity of 1-tetra-
lone also increased from 61 to 85% with increase in reaction time. Si-
multaneously there was a decrease in the selectivity to 1-tetralol from
28 to 10%, which indicates the transformation of 1-tetralol to 1-tetra-
lone with increase in time. Only small amounts of other byproducts of
selectivity 3% were formed which mainly constitutes of 4-hydroxy-1-
tetralone and 1,4-naphthaquinone. No significant change in either
conversion or selectivity with increase in reaction time from 6 to 7 h,
implying 6 h reaction time is optimum.
Table 2
Effect of catalyst amount on the benzylic oxidation of tetralin (TLN) to 1-tet-
ralone (T = O), 1-tetralol (T-OH) and naphthalene (NPH).
Amount (mg) TLN
Conv.
T = O
Sel. (%)
T−OH
Sel. (%)
NPH
Sel.
(%)
Others
Sel. (%)
TOF
(h−1
)
(%)
25
50
75
100
56
64
67
69
77
85
78
76
19
10
08
05
02
02
05
07
02
03
09
12
3.04
1.98
1.24
0.96
the moles of Cu present in the catalyst amount and found that 50 mg of
catalyst amount resulted higher TOF 1.98 h−1 than other except 25 mg
of catalyst amount of TOF 3.04 h−1 since lower moles of Cu was used,
but still afforded low conversion due to insufficient moles of Cu. Hence
the reactions were carried out by employing 50 mg of the catalyst as the
optimum one.
3.8. Reaction mechanism
To understand the pathway of the benzylic oxidation of tetralin to 1-
tetralone with TBHP and Cu(II)/γ-Fe2O3@SBA-15 catalyst, a plausible
reaction mechanism was proposed by conducting the oxidation reaction
at optimized conditions using 4-t-butyl catechol as a free radical sca-
venger. In presence of 4-t-butyl catechol, the TLN conversion was sig-
nificantly dropped from 64 to 12%, which clearly indicate that the
benzylic oxidation of TLN followed the free radical mechanism with Cu
(II)/γ-Fe2O3@SBA-15 catalyst. Based on the published literature [53]
and our present experiment results a mechanism was detailed in
Scheme 3. Initially, C(II) metal ions decompose the TBHP into tertbu-
toxyl free radical that activate the tetralin molecule by the abstraction
of benzylic hydrogen radical. Then, another tertbutoxyl free radical
attacks the activated tetralin radical to form tert-butylperoxy tetralin,
which was converted into tetralin hydroperoxide as intermediate in
presence of polar solvent like water. The direct decomposition of this
intermediate to 1-tetralone is the rate determining step of the reaction.
Particularily, the high selectivity to 1-tetralone was mainly induced by
the high polar solvent which is water in our present study. However, 1-
tetralone formation through 1-tetralol cannot be excluded which pro-
ceeds slowly on long time duration. Naphthalene was produced by the
dehydration followed by dehydrogenation on Cu(II) lewis acidic site
whereas 4-hydroxy-1-tetralone was formed due to over oxidation of 1-
tetralone with TBHP molecule.
To investigate the optimum reaction temperature for the benzylic
oxidation of tetralin, a series of experiments were performed by em-
ploying the described procedure and varying the reaction temperature
and the results were displayed in Table 3. The temperature has a no-
teworthy effect on the selectivity of 1-tetralone. Amongst the four
tested temperatures of 30, 60 and 90 °C, a maximum conversion of
tetralin 64% and high selectivity to 1-tetralone 85% was obtained at
90 °C. A further increase in the reaction temperature (to 100 °C) re-
duced the 1-tetralone selectivity significantly with increase in the
conversion. The decrease in selectivity was due to the fast decomposi-
tion of TBHP resulting over oxidation products. Consequently, the TOF
of 1.98 h−1 obtained at 90 °C was higher when compared to other
temperatures as listed in Table 3. Therefore, we considered 90 °C as the
optimum temperature for the present study.
The reaction time was found to show a significant effect on the
conversion of the tetralin to the corresponding 1-tetralone. To optimize
Table 3
Effect of reaction temperature on the benzylic oxidation of tetralin (TLN) to 1-
tetralone (T = O), 1-tetralol (T-OH) and naphthalene (NPH).
Temperature (°C) TLN
Conv.
T = O
Sel. (%)
T−OH
Sel. (%))
NPH
Sel.
Others
Sel. (%)
TOF
(h−1
)
(%)
(%)
30
60
90
100
36
55
64
71
58
66
85
74
39
28
10
12
01
02
02
06
02
04
03
08
0.74
1.29
1.98
1.85
3.9. Heterogeneity test
To ascertain that the nature of catalytic process is heterogeneous, a
hot filtering test was conducted and the results are displayed in Fig. 11.
80

C.K.P. Neeli et al.
MolecularCatalysis453(2018)74–84
Scheme 3. Plausible reaction mechanism of benzylic oxidation of tetralin.
3.10. Recyclability
The recyclability of Cu(II)/γ-Fe2O3@SBA-15 catalyst was examined
in the benzylic oxidation of tetralin by following the described proce-
dure. After the reaction, the catalyst was recovered from the reaction
mixture by magnetic separation, washed with ethanol and then dried
for 24 h. The recovered catalyst was used for four consecutive runs by
following the same experimental conditions. The reused catalyst was
found to be equally active and afforded almost a similar conversion and
selectivity of the 1-tetralone, as shown in Fig. 12. These results con-
firmed that the developed catalyst can be recycled without significant
loss in catalytic activity. Indeed, the decrease in the activity after 4th
cycle might be due to blockage of pores by the accumulation of reactant
or remained TBHP molecules after the repeated cycles.
3.11. Scope of different alkylaromatics for the benzylic oxidation in water
medium
Fig. 11. Heterogeneity test of Cu(II)/γ-Fe2O3@SBA-15.
Based on the above results, the optimized reaction conditions for the
benzylic oxidation of tetralin were found as 50 mg of Cu(II)/γ-Fe2O3@
SBA-15catalyst, TBHP/tetralin molar ratio as 3, temperature 90 °C,
2 mL of water and time 6 h. The applicability of this protocol was
generalized to a variety of alkyl aromatics and the results with TOF
comparison are summarized in Table 4. All the substrates were effi-
ciently converted to the corresponding ketones with excellent se-
lectivity under the described experimental conditions. The higher ring
systems had afforded moderate conversion and selectivity which re-
veals the applicability of the present catalyst system is good and effi-
cient to the single ring substituted alkylaromatics to ketones in aqueous
medium which acts as eco-friendly solvent system.
Under the optimized reaction conditions, the Cu(II)/γ-Fe2O3@SBA-15
catalyst was removed after a 3 h reaction at tetralin conversion 49%,
and the reaction was continued for further 3 h. No auxiliary conversion
of tetralin (≈ 49%) was detected, which confirms the heterogeneity of
the catalyst. Furthermore, Cu leaching tests were carried out after the
catalytic reaction under optimized conditions by analyzing the filtrate
solution using ICP-AES and did not detect Cu species (< 0.1 ppm). The
Cu content in the recycled catalyst after four runs was 5.89 wt%, which
is very close to that of the fresh catalyst (5.92 wt%), indicating that
there was negligible leaching of Cu during the reaction.
81

C.K.P. Neeli et al.
MolecularCatalysis453(2018)74–84
Fig. 12. Recyclability of Cu(II)/γ-Fe2O3@SBA-15.
3.12. Comparison of Cu(II)/γ-Fe2O3@SBA-15 activity with reported
MIL-101[55], LDHNi-Salen complex [56], CrAlPO4-5 [31] and CrHMA
[31]. Among these reported catalysts CrHMA was found to be the best
catalyst in tetralin oxidation of TOF 3.16 h−1, but unfortunately the
catalytic activity declined immediately in the successive recycle run
and reaction was operated at higher temperature 120 °C in presence of
hazardous organic solvent like chlorobenzene. Recently, mesoporous
supported Cr catalyst viz., CrMCM-41 [32] and particularly CrSBA-15
[33] as reported by Selvaraj etal. was found to be highly active in the
tetralin oxidation with higher TOF 4.75 h−1. However, all the above
catalysts
The catalytic performance of the Cu(II)/γ-Fe2O3@SBA-15 catalyst
for tetralin oxidation to 1-tetralone was compared in terms of activity
and TOF values with previous reported system and depicted in Table 5.
Chen etal. [54] reported the oxidation of hydrocarbons with CrAPO-5
catalyst but the reaction proceeded in long time and obtained very low
TOF 0.04 h−1. Ahn etal. reported the various catalytic systems such as
Table 4
Benzylic oxidation of substituted alkyl aromatics over Cu(II)/γ-Fe2O3@SBA-15 catalyst under aqueous medium.
Entry
Substrate
Product
Conv. (%)
Sel. (%)
90
TOFa
2.52
1
79
2
3
67
62
86
81
2.04
1.78
4
5
64
74
85
71
1.98
1.86
6
7
8
9
65
60
56
52
99
99
68
67
2.27
2.11
1.35
1.24
Reaction conditions: Catalyst = 50 mg, substrate = 1 mmol, t-BuOOH = 3 mmol, H2O = 2 mL, temp = 90 °C, time = 6 h
a
TOF = mol of product formed/moles of Cu utilized per hour.
82

C.K.P. Neeli et al.
MolecularCatalysis453(2018)74–84
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tion reactions, Org. Lett. 9 (2007) 1419–1421.
Table 5
Comparison of Cu(II)/γ-Fe2O3@SBA-15 activity with previous reported sys-
tems.
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Catalyst
Solvent Temp. (°C) TLN
Conv.
T = O
Sel. (%)
TOF
Ref.
(h−1
)
(%)
CrAPO-5
MIL-101
LDH-[Ni-
PhCl
PhCl
PhCl
100
80
100
44
67
45
64
84
64
0.04
0.79
1.31
[54]
[55]
[56]
(sulfonato-
salen)]
CrAlPO4-5
CrHMA
CrSBA-15
CrMCM-41
PhCl
PhCl
PhCl
scCO2
120
120
120
80
61
61
96
65
64
78
82
98
96
85
1.94
3.16
4.75
0.12
1.98
[31]
[31]
[33]
[32]
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Cu(II)/γ-Fe2O3@ H2O
90
SBA-15
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4. Conclusions
Cu(II)/γ-Fe2O3@SBA-15 catalyst is highly effective for the selective
oxidation of tetralin to 1-tetralone even in water medium under mild
conditions without using additives or bases, which is ascertained from
high surface area, mesopore confinement and isolated distribution of
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Acknolwedgements
Chinna Krishna Prasad Neeli acknowledges the Council of Scientific
and Industrial Research (CSIR-UGC), New Delhi, for awarding research
fellowship. The authors thank Dr R Srinivas, Head Analytical Division,
IICT, India, for their kind help with the ICP-OES analysis.
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