Nanoparticle-supported and magnetically recoverable organic-inorganic hybrid copper(ii) nanocatalyst: A selective and sustainable oxidation protocol with a high turnover number

Article abstract of DOI:10.1039/c4ra06599a

A magnetically recoverable copper-based nanocatalyst was prepared from inexpensive starting materials. With a particle size between 20 to 30 nm, it was shown to catalyze the oxidation of benzylic alcohols. The catalyst exhibited a high turnover number (TON) and excellent selectivity. The catalyst was characterized by several techniques, such as XRD, HR-TEM, SAED, EDS, FT-IR, VSM, and BET surface area. Factors affecting the reaction parameters, such as the substrate to oxidant molar ratio, weight of the catalyst, reaction time, etc., were investigated in detail. The reusability of the catalyst was examined by conducting repeat experiments with the same catalyst; it was observed that the catalyst displayed no significant changes in its activity even after seven cycles for the aerobic, as well as for the peroxide, oxidation of benzyl alcohol. Furthermore, the heterogeneous nature, easy recovery, and reusability, makes the present protocol highly beneficial for addressing environmental concerns and industrial requirements. This journal is

Full text of DOI:10.1039/c4ra06599a

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Nanoparticle-supported and magnetically
recoverable organic–inorganic hybrid copper(II)
nanocatalyst: a selective and sustainable oxidation
protocol with a high turnover number†
Cite this: RSC Adv., 2014, 4, 41111
a
b
a
*
Puran Singh Rathore, Rajesh Patidar and Sonal Thakore
A magnetically recoverable copper-based nanocatalyst was prepared from inexpensive starting materials.
With a particle size between 20 to 30 nm, it was shown to catalyze the oxidation of benzylic alcohols.
The catalyst exhibited a high turnover number (TON) and excellent selectivity. The catalyst was
characterized by several techniques, such as XRD, HR-TEM, SAED, EDS, FT-IR, VSM, and BET surface
area. Factors affecting the reaction parameters, such as the substrate to oxidant molar ratio, weight of
the catalyst, reaction time, etc., were investigated in detail. The reusability of the catalyst was examined
by conducting repeat experiments with the same catalyst; it was observed that the catalyst displayed no
significant changes in its activity even after seven cycles for the aerobic, as well as for the peroxide,
oxidation of benzyl alcohol. Furthermore, the heterogeneous nature, easy recovery, and reusability,
makes the present protocol highly beneficial for addressing environmental concerns and industrial
requirements.
Received 3rd July 2014
Accepted 13th August 2014
DOI: 10.1039/c4ra06599a
www.rsc.org/advances
this study, the objective was to develop an efficient catalytic
system for the aerobic or H2O2 oxidation of various alcohols
Introduction
under solvent-free conditions.
The selective oxidation of alcohols to their corresponding
carbonyl compounds is one of the most useful reactions in
industrial processes.1 Oxidation reactions are usually difficult
and typically require stoichiometric amounts of toxic heavy-
metal salts or expensive catalysts involving noble metals, such
as gold, ruthenium, rhodium, or palladium.2–5 Traditionally,
non-catalytic methods with stoichiometric, toxic, corrosive, and
expensive oxidants, such as permanganate, dichromate, and
peroxy acids, under stringent conditions of high pressure and/
or temperature have been widely used for alcohol oxidation.6–9
These reactions are also oen carried out with high concen-
trations of bases and environmentally unfriendly organic
solvents. Therefore, much attention has been paid to the
development of heterogeneous catalytic systems that use clean
and atom-efficient oxidants like molecular oxygen or H2O2
without organic solvents.10–13 Among these systems, the solvent-
free aerobic oxidation of alcohols using molecular oxygen or air
as the oxidant has become more attractive,14–16 due to positive
effects in terms of cost, safety, and environmental impact. In
The copper-catalyzed oxidation of organic compounds has
attracted signicant attention in recent years,17 owing to the
high demand for mild and efficient oxidation catalysts.18
Copper is an abundant metal in the Earth's crust and its redox
properties make it ideally suited for catalytic oxidation
processes, provided that the electron-transfer processes can be
controlled by an appropriate ligand set. Therefore, a number of
copper-catalyzed aerobic oxidation systems have been well-
established.19 They employ copper salts in combination with
2,2,6,6-tetramethyl-piperidyl-1-oxy (TEMPO),20 and various N
ligands such as 2,20-bipyridine (Bpy),20c,h 1,4-diazabicyclo[2.2.2]
octane (DABCO),20g and 4,40-trimethylene-dipyridine (TMDP).20f
However, they are homogeneous20 in nature and an additional
base is oen needed,20c,d,h which limits their application in the
oxidation of the base sensitive alcohols.
Over the last few years, magnetic nanoparticles (e.g. Fe3O4)
have been extensively investigated as inorganic supports for the
synthesis of organic–inorganic hybrid materials. They are
potential alternatives to conventional materials, being robust,
readily available, high-surface-area heterogeneous catalyst
supports.21 They offer an added advantage of being magnetically
separable, thereby, eliminating the requirement for catalyst
ltration aer completion of the reaction. Most importantly,
when magnetic nanoparticles are used as supports, the size of
the support materials decreases to the nanometer scale, and all
aDepartment of Chemistry, Faculty of Science, The M. S. University of Baroda,
Vadodara, 390002, India. E-mail: chemistry2797@yahoo.com; Fax: +91-0265-
2429814; Tel: +91-0265-2795552
bAnalytical Discipline and CIF, Central Salt and Marine Chemicals Research Institute
(CSIR-CSMCRI), G. B. Marg, Bhavnagar, 364002, Gujarat, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra06599a
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of the catalytic sites on the external surface of the particles are dihydroxyphenylalanine (LD) in water to yield the Fe3O4–LD,
accessible to the substrates.22
and these obtained nanoparticles were further metallated with
Hence, modied magnetic nanoparticles have received a lot copper nitrate in ethanol to achieve the nal copper complex
of attention as a support for the incorporation of different graed magnetically recoverable nanoparticles (Fe3O4–LD–Cu)
transition metal ions.23 However, most of these techniques (Scheme 1).
require several reaction steps to introduce functional groups to
The High-Resolution Transmission Electron Microscopy
the magnetic surface and oen involve the use of organosilica (HR-TEM) images of the Fe3O4–LD–Cu nanocatalyst shows a
precursors as an organic shell to prepare a suitable support for somewhat spherical morphology, with some cubic partials, and
trapping the metal ions.24 The latter are not only very expensive an average size range of 20–30 nm (Fig. 1A). Fig. 1B and C are
and toxic, but also involve complicated synthesis methods. the HR-TEM images of typical Fe3O4–LD–Cu at different
Therefore, from both an environmental and economic point of magnications. The nanoparticles, depicted in Fig. 1C and D,
view, the preparation of modied magnetic nanoparticles via a have a discrete core–shell structure, and a uniform magnetic
simple method and without using organoalkoxysilane core with a diameter of 10–15 nm surrounded by a 2–3 nm thick
compounds is highly desirable. In addition, although Fe3O4- LD organic shell. The high resolution images in Fig. 1E show
containing high performance catalysts for the oxidation of well developed lattice fringes, with the fringes extending
alcohols are reported,25 the application of copper metal catalysts throughout the particle, conrming the monocrystalline nature
based on modied magnetic nanoparticles as heterogeneous of the individual particles. The distance between adjacent
catalysts in the oxidation of alcohols is not reported.
lattice fringes was measured as 0.233 nm in Fig. 1E and corre-
In continuation of our efforts toward the development of sponds to the 311 reection. The selected area electron
efficient nanoparticle-assisted catalysis,26 herein, an Fe3O4–L- diffraction (SAED) pattern shown in Fig. 1F corresponds to the
3,4-dihydroxyphenylalanine (LD) nanocomposite containing higher order reections of the Fe3O4–LD–Cu nanocatalyst. The
Cu2+ ions (Fe3O4–LD–Cu) was prepared via a simple method as a white spots, as well as the bright diffraction rings, indicate that
novel heterogeneous magnetic catalyst. The main goal of this the nanoparticles produced by the above-stated method are
catalytic synthesis was to introduce a novel and efficient readily highly crystalline.
available copper as an effective catalyst, based on organic
The crystalline structures of Fe3O4, Fe3O4–LD, and Fe3O4–
molecule-graed magnetic nanoparticles to expand the use of LD–Cu were analysed by powder X-ray diffraction (XRD). As
these types of nanocomposites for catalytic oxidation reactions. displayed in Fig. 2A to C, all the samples show diffraction peaks
A literature survey reveals that this type of catalytic system is less at around 30.1ꢀ, 35.2ꢀ, 43.1ꢀ, 53.5ꢀ, 57.4ꢀ, and 62.7ꢀ 2q, corre-
reported27 for the aerobic and H2O2 oxidation of alcohols. sponding to the spinel structure of Fe3O4,29 and which can be
Initially, we used benzyl alcohol as the model system, to assigned to the diffractions of the (220), (311), (400), (422), (511),
simplify the analysis and to accelerate the screening speed. and (440) faces of the crystals, respectively. The relative inten-
Consequently, the optimized conditions were used for the sities of the diffraction peaks matched well with the standard
synthesis of various aromatic aldehydes.
XRD data of the Joint Committee on Powder Diffraction Stan-
dards (JCPDS) card number (19-0629) for Fe3O4 crystal with a
spinel structure, which is consistent with the TEM results. In
addition, the XRD patterns depict similar diffraction peaks,
which indicate that the nanocomposite was synthesized without
damaging the crystal structure of the Fe3O4 core. In addition,
the broad diffraction peak in the range of 2q between 10ꢀ and
Results and discussions
Catalyst characterization
We used magnetite nanoparticles (Fe3O4), which were prepared
by the co-precipitation method.28 These were reacted with L-3,4-
Scheme 1 Schematic representation of the synthesis of Fe3O4–LD–Cu nanocatalyst.
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Fig. 1 HR-TEM images of magnetic Fe3O4–LD–Cu nanocatalyst at different magnifications (A) 50 nm, (B) 20 nm, (C) 10 nm, and (D) 2 nm,
showing the particle size distribution. The resolved lattice fringes and SAED pattern of Fe3O4–LD–Cu are (E) and (F), respectively.
also shows that different reaction conditions during the
synthesis did not affect the crystallinity and morphology of the
Fe3O4 nanoparticles throughout the process.
Fourier transform infrared spectroscopy (FT-IR) seems to be
the best technique to characterize the functionalization and
modication of magnetic nanoparticles. The FT-IR spectra of
the LD, Fe3O4–LD, and Fe3O4–LD–Cu nanocomposites were
recorded to conrm the modication of the magnetite surface
with the LD and metal ions (Fig. 3A–C). The IR spectra of LD
(Fig. 3A) show a strong peak around 3404 cmꢁ1, due to the O–H
stretching vibrations of catechol, and at 1652 cmꢁ1, due to the
C]O stretching of carboxylic acids. The presence of magnetite
nanoparticles in Fe3O4–LD is observable by the strong adsorp-
tion band at about 602 cmꢁ1, corresponding to the Fe–O
vibrations (Fig. 3B). It is also clear that the strong O–H
stretching vibrations of catechol, which are generally present at
3400 cmꢁ1, are absent in the spectrum of Fe3O4–LD (Fig. 3B).
Instead, the broad band around 3106 cmꢁ1 seems to be due to
O–H stretching of carboxylic acids. Moreover, on moving from
Fig. 2 XRD pattern of (A) Fe3O4, (B) Fe3O4–LD, and (C) Fe3O4–LD–Cu
nanocatalyst.
LD to Fe3O4–LD, a signicant reduction in the intensity of the
O–H stretching and bending vibrations bands is observed.
According to Fig. 3B, the successful Fe3O4 surface modication
30ꢀ can be attributed to the amorphous material coated on the
magnetic nanoparticles.30 According to the XRD results, it can
with LD moieties is veried. In terms of Fe3O4–LD–Cu (Fig. 3C),
a red shi of the band at 1652 cmꢁ1 is observed (1652 cmꢁ1
/
be concluded that the Fe3O4 nanoparticles were successfully
1623 cmꢁ1), which is probably characteristic of the asymmet-
rical uctuations of the carbonyl group aer interaction with
the metal ions. The intensity of the band at 3106 cmꢁ1, corre-
sponding to the O–H bond stretching of carboxylic acids of LD
(Fig. 3B), decreases in the spectrum of the Fe3O4–LD–Cu
(Fig. 3C), indicating a strong interaction between the oxygen
donors and the metal ions. The appearance of new bands in the
region of 575 cmꢁ1 can be attributed to n(M–N), while the
stretching frequency bands in the 447 cmꢁ1 region correlate to
c(M–O), which conrmed the coordination through nitrogen
and oxygen.31 The FT-IR data conrmed that the nitrogen and
coated with LD and LD–copper. The XRD pattern clearly depicts
that there is no change in the topological structure and in the
inherent properties of Fe3O4 before and aer the coating with
LD. On assessment of the diffractograms of LD-encapsulated
and LD–copper complex graed nanoparticles, the very distin-
guishable FCC peaks of the magnetite crystal were not changed,
which means that these particles have phase stability, but there
is a slight decrease in the intensity with broadening of the
corresponding peak of LD (Fig. 2C). This can be attributed to the
lowering of the scattering contrast between the walls of the
Fe3O4 framework and the organic moiety attached over Fe3O4. It
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Fig. 3 FT-IR spectra of (A) LD, (B) Fe3O4–LD, and (C) Fe3O4–LD–Cu nanocatalyst.
carboxyl oxygen atoms are found to be involved in coordination
with the metal ion in complexes. Overall, the IR results suggest
that Fe3O4–LD–Cu was successfully immobilized onto the
surface of the magnetic nanoparticles.
The weight percentage of the copper content in the prepared
nanocatalyst was obtained using AAS, and sample digestions
were carried out in a microwave oven at 500 watt for 10 min with
a constant pressure programme with 5 mL aqua regia. The
volume of the ltrate was then adjusted to 100 mL using double
deionized water. Reference solutions for copper measurements
were made with a high degree of analytical purity to obtain the
calibration curves. 0.927% copper content in the catalyst was
quantied using a calibration curve in duplicate for each
sample. Energy Dispersive X-ray Analysis (EDX) also conrms
the ratio of 1.20 : 98.80 (Cu/iron oxide) (Fig. 4). The EDX anal-
ysis of Fe3O4 nanoparticles indicate that the well-cleaned nal
product is mostly composed of O and Fe (Fig. 4A), and in the
case of Fe3O4–LD–Cu nanocatalyst, O, Fe, and Cu with no other
signal appearing in Fig. 4B.
The magnetic properties of the synthesized Fe3O4 nano-
particles, Fe3O4–LD, and Fe3O4–LD–Cu were analyzed by
vibrating sample magnetometry (VSM). The eld-dependent
magnetization curves shown in Fig. 5 indicate the magnetiza-
Fig.
nanocatalyst.
4
EDX patterns of (A) Fe3O4–LD, and (B) Fe3O4–LD–Cu
tion as a function of applied magnetic eld, measured at room
temperature. The coercivity values of the Fe3O4 nanoparticles,
Fe3O4–LD, and Fe3O4–LD–Cu were 74, 73.65, and 71 G, respec-
tively. In spite of these low magnetization values with respect to
the magnetization of pure Fe3O4 nanoparticles,23 which are due
to the decrease in the surface moments of the magnetite
nanoparticles by the diamagnetic LD coating over the Fe3O4
nanoparticles and by the graing of the metal–ligand complex
over Fe3O4–LD, it they are still sufficient for magnetic separa-
tion by a conventional magnet. The above-mentioned TEM
images also conrmed the encapsulation and graing of the
organic layer over the Fe3O4 nanoparticles. Another important
parameter for the practical applications of nanoparticles is
revealed from the enlarged VSM curve shown in Fig. 5D. The
hysteresis loops of the powdered materials showed almost
negligible magnetic hysteresis, with both the magnetization
and demagnetization curves passing through the origin, which
clearly indicates the superparamagnetic nature of the materials.
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Cu2+ ions. Nevertheless, for a better understanding of the
oxidation state of copper, the binding energy obtained from the
catalyst was compared with the Cu2+ 2p3/2 and 2p1/2 peak
positions. The binding energies of the main peaks are
about 934.59 eV and 942.27 eV due to Cu 2p3/2 and Cu 2p1/2
,
respectively. Since these lie within the range given in litera-
ture,32 it can be concluded that the oxidation state of copper in
the catalyst is +2.
Catalytic activity
Having synthesized and characterized the Fe3O4–LD–Cu nano-
composite, its role as a heterogeneous catalyst was then evalu-
ated for the oxidation of alcohols. The oxidation was carried
out with air, as well as with H2O2, as an oxidant. In order to
optimize the reaction conditions and to obtain the best catalytic
activity, the oxidation of benzyl alcohol was chosen as a model
reaction. In this regard, different reaction parameters, such as
solvent, temperature, and amount of catalyst, were investigated
(Table 1).
Fig. 5 Magnetization curves obtained by VSM at room temperature for
(a) Fe3O4, (b) Fe3O4–LD, and (C) Fe3O4–LD–Cu nanocatalyst, and (d)
inset: enlarged image near the coercive field.
It is seen from the Table 1 (entry-1) that the presence of a
catalyst is required for the oxidation of benzyl alcohol. In order
to investigate the role of Cu2+ in the catalyst, the model reaction
was carried out under the optimized reaction conditions in the
presence of Fe3O4 and Fe3O4–LD as catalysts (Table 1, entry-2).
No progress in the reaction was observed even aer ve
hours. Thus, it can be concluded that Cu2+ incorporated onto
the magnetic nanocomposite plays a pivotal role in the solvent-
free aerobic oxidation, as well as in the H2O2 promoted oxida-
tion, of benzyl alcohol.
As far as the effect of solvent is concerned, the model reac-
tion was performed in several solvents, as well as in solvent free
conditions (Table 1, entries 3 to 7). It was observed that the best
yield and selectivity were obtained when the reaction was con-
ducted under solvent-free conditions (entries 8 and 9). More-
over, the effect of the catalyst amount on the oxidation of benzyl
alcohol was also investigated, by varying the amounts of the
catalyst (Table 1, entries 8 to 10). As can be seen, when the
amount of catalyst was increased from 15 to 25 mg (entries 8
and 9), the product yield increased from 31% to 60% in air
oxidation and 52% to 58% in the H2O2 oxidation, which is
probably due to the availability of more acid sites. Furthermore,
the percentage yield remained stable with between 25 mg to 50
mg catalyst, with a reduction of about 12% in benzaldehyde
selectivity, which may be due to over-oxidation of the substrate
at high amounts of the catalyst (entries 9 and 10). According to
the results, 25 mg was chosen as the optimum amount of
catalyst, due to the best yield and selectivity, for the further
steps (Table 1, entry 9, Scheme 2).
This also means that the magnetic material can only be aligned
under an applied magnetic eld, but it will not retain any
residual magnetism upon removal of the eld. Thus, the above
discussed Fe3O4 nanoparticles appear to be suitable as the
support for the catalyst.
The Brunauer–Emmett–Teller (BET) surface area of
a
magnetic Fe3O4–LD–Cu sample was determined to be as high as
70.41 m2 gꢁ1 (ESI†). Similarly, the BJH adsorption and desorp-
tion cumulative surface area of the pores are 73.36 m2 gꢁ1 and
73.06 m2 gꢁ1, respectively (ESI†). The Barrett–Joyner–Halenda
(BJH) pore size of the NPs was determined to be 14.37 nm, and
the total pore volumes 0.253 cm3 gꢁ1 (ESI†).
In order to provide further evidence of the formation of Cu2+
complex over the iron oxide core and to determine the oxidation
state of copper, the XPS resolution copper 2p spectroscopy
results of Fe3O4 and Fe3O4–LD–Cu were compared (ESI†). The
signals of Cu2+ were very faint in the XPS spectrum of the Fe3O4–
LD–Cu nanocomposite (Fig. 6), because of the low loading of
The inuence of reaction temperature on the catalytic
activity was investigated by several separate reactions under the
same reaction conditions. As represented in Table 1, in air
oxidation, the reaction yield does not increase with temperature
(entries 11, 12, and 14), but the selectivity to benzaldehyde
decreases signicantly because of the over-oxidation at high
temperatures. In the case of H2O2 oxidation of benzyl alcohol,
ꢀ
ꢀ
Fig. 6 X-ray photoelectron spectra of Fe3O4–LD–Cu nanocatalyst.
when the temperature increases from 25 C to 70 C (Table 1,
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Table 1 Effects of different solvent, temperature, and amount of catalyst on the aerobic and H2O2 oxidation of benzyl alcohol in the presence of
Fe3O4–LD–Cu catalyst
Air oxidation
H2O2 (30% v/v)
Conversion
(% ꢃ2 by GC)
Selectivity
(% ꢃ1 by GC)
Conversion
(% ꢃ2 by GC)
Selectivity
(% ꢃ1 by GC)
Entrya
Temperature (ꢀC)
Catalyst (mg)
Solvent
1
2
3
4
5
6
7
8
25–30
25–30
25–30
25–30
25–30
25–30
25–30
25–30
25–30
25–30
50
—
—
—
Water
Ethanol
Trace
20
12
16
49
25
20
31
60
59
58
58
59
60
99
99
88
88
99
90
92
99
99
88
85
81
81
80
Trace
35
30
20
48
22
35
52
62
60
72
95
95
95
99
92
70
71
94
75
72
95
96
96
96
96
96
91
Fe3O4/Fe3O4–LD
25
25
25
25
25
15
25
50
25
25
50
25
Acetonitrile
Dichloromethane
Methanol
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
9
10
11
12
13
14
70
70
90
a
Reaction conditions: benzyl alcohol (10 mmol), benzyl alcohol : H2O2 (30% v/v) mole ratio ¼ 1 : 1.1, solvent ¼ 2 mL, reaction time ¼ 5 h, the main
by-product was benzoic acid.
Scheme 2 Oxidation of benzyl alcohol to benzaldehyde in optimum reaction conditions; (A) air and (B) H2O2.
entries 9 and 12), the reaction yield also increases from 58% to gives useful information about the immobilization process and
95%, with a 96% selectivity of benzaldehyde. According to the catalytic stability along the catalytic cycles. The reusability of
results, the reaction temperature of 25 ꢀC to 70 ꢀC appears to be the catalyst was tested by carrying out repeated runs of the
the optimum reaction temperature range for aerobic oxidation reaction on the same batch of the catalyst in the model reaction
and H2O2 oxidation, respectively. According to the above, the (Scheme 2A and B). In order to regenerate the catalyst, aer each
results in Table 1 entries 9 and 12 are the optimum conditions cycle, it was separated by an external magnet (Fig. 7A) and
for the air and H2O2 oxidation of benzyl alcohol, respectively washed several times with acetone. Then, it was dried in an oven
ꢀ
(Scheme 2A and B).
at 60 C and reused in the subsequent run. The results show
In order to assess the efficiency of the NPs further, the that the Fe3O4–LD–Cu magnetic catalyst could be reused up to
quantity of H2O2 and the reaction time were also optimized seven times with no signicant loss of activity/selectivity (Table
(ESI†). Aer optimization of the reaction conditions (Table 1 3). It should be mentioned that there was very low Cu2+ leaching
and ESI†), the catalytic activity of magnetic Fe3O4–LD–Cu during the reaction and the catalyst exhibited high stability
nanocatalyst was further explored with other benzylic alcohols. even aer seven recycles (Table 3). Aer that, however, the
The Fe3O4–LD–Cu catalyst exhibited good activity and selectivity reaction time increased with each successive recycling experi-
in the solvent-free aerobic oxidation, as well as in the H2O2 ment, rising from 5 h to 6 h nally in air oxidation. A similar
oxidation of different benzylic alcohols (Table 2). All of the result was also obtained in the H2O2 oxidation of benzyl alcohol
investigated benzylic alcohols with either electron-donating or (Table 3). This may be due to a gradual loss of the catalytic
electron-withdrawing substituents can be oxidized into their activity of the nanocatalyst with the number of runs, which may
corresponding aldehydes in high yields with excellent selectivity be due to various reasons. One of the reasons may be surface
(entries 2–13). In all cases, the turnover number (TON) was as modication due to the deposition of matter during the reac-
high as ꢂ1850 to 2500 for H2O2 oxidation and ꢂ830 to 1700 for tion. The HR-TEM images of the nanocatalyst were recorded
the air oxidation of various benzylic alcohols.
aer the 7th run of air (Fig. 7B) and H2O2 oxidation (Fig. 7C) of
The recycling and recovery of used catalyst is one of the most benzyl alcohol. The HR-TEM images display an agglomeration
important criteria for industrial based catalyst systems, and of NPs due to deposited matter (Fig. 7B and C).
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Table 2 Aerobic and H2O2 oxidation of various benzylic alcohols over Fe3O4–LD–Cu nanocatalyst
Air oxidationb
a
H2O2 (30% v/v)
Entryc
Reactant
Product
Conversion/TON
90/2500
Selectivity
96
Conversion/TON
Selectivity
99
1
52/1444
36/1000
2
75/2083
79
98
3
4
69/1916
68/1888
89
92
30/833
98
99
40/1111
5
6
7
90/2500
92/2555
92/2550
96
98
95
59/1638
62/1722
61/1694
99
99
99
8
9
86/2388
85/2361
96
96
59/1555
60/1666
99
99
10
11
12
78/2166
70/1944
72/2000
98
92
95
52/1444
49/1361
52/1444
99
99
99
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Table 2 (Contd.)
Air oxidationb
a
H2O2 (30% v/v)
Entryc
Reactant
Product
Conversion/TON
Selectivity
96
Conversion/TON
Selectivity
99
13
72/2000
47/1305
a
Reaction conditions: alcohol (10 mmol), atmospheric air, 25 mg catalyst (0.02 wt% or 0.003 mmol copper, obtained by AAS for Fe3O4–LD–Cu),
reaction time and temperature, 12 h and 25–30 ꢀC, respectively. Acetonitrile was used as a solvent (when alcohol was solid and air was used as
b
the oxidant) ¼ 2 mL. Reaction conditions: alcohol (10 mmol), alcohol : H2O2 (30% v/v) mole ratio ¼ 1 : 1.1, 25 mg catalyst, reaction time, and
c
temperature, 12 h and 70 ꢀC, respectively. The main by-product was benzoic acid, TON was calculated on the basis of copper ions estimated
by AAS, isolated yield aer column chromatography, and selectivity by GC.
Fig. 7 (A) Reaction mixture of benzaldehyde (a) before, and (b) after magnetic separation by a simple magnet. HR-TEM of reused magnetic
Fe3O4–LD–Cu nanocatalyst after (B) air oxidation, and (C) H2O2 oxidation.
Table 3 Air and H2O2 oxidation of benzyl alcohol to benzaldehyde over Fe3O4–LD–Cu nanocatalyst under optimum conditions (recycling
experiments)
b
Air oxidationa
Time (h)
H2O2 (30% v/v)
Conversion
(% ꢃ2 by GC)
Selectivity
(% ꢃ1 by GC)
Conversion
Selectivity
Cycle
Time (h)
(% ꢃ2 by GC)
(% ꢃ1 by GC)
1
2
3
4
5
6
7
5
5
5
5
5
6
6
60
59
60
59
58
59
58
99
99
99
98
99
99
99
4
4
4
4
5
5
6
95
95
94
95
94
94
94
96
96
95
95
94
95
96
a
Reaction conditions: alcohol (10 mmol), atmospheric air, 25 mg catalyst and reaction temperature 25–30 ꢀC. b Reaction conditions: alcohol (10
mmol), alcohol : H2O2 (30% v/v) mole ratio ¼ 1 : 1.1, 25 mg catalyst, reaction temperature 70 ꢀC.
It is evident from Table 4 that the Fe3O4–LD–Cu nano-
catalyst is highly efficient in catalyzing the air, as well as H2O2,
oxidation of benzylic alcohols and gave products in good yields
with a high turnover number (TON) in comparison to previous
literature reports.5,20e,f,23,25d,e Hence, the catalytic efficiency of
the present catalytic system is considered remarkable in terms
of the mild reaction conditions, low catalyst costs, short
reaction time, high reaction yield, and easy recovery of the
catalyst.
Hot ltration test
A hot-ltration based leaching test was conducted to exclude any
homogeneous catalytic contribution or lixiviation of the catalytic
species in the catalyzed reaction. First, an AAS analysis of the post
reaction mixture aer catalyst separation was conducted. The
results revealed that the concentration of Cu(II) ions in the
supernatant corresponded to a negligible catalyst leaching (<0.01
ppm), in so far as there was hardly any change in the amount of
Cu compared with the fresh catalyst.
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Table 4 Comparison of the results of the present system with the recently published catalytic systems for the oxidation of alcohols to aldehyde
Entry
Catalytic system
Reaction conditions
% Yield by GC
Ref.
1
2
3
4
5
6
7
Fe3O4/MPA–PHEA–Cr composite
Cu–NHC–TEMPO
TEMPO/CuCl2/bapbpy
Nano-g-Fe2O3
Polymer supported palladium catalyst
Magnetically recoverable Au NP
Our catalyst
O2-bubbling, 45 ꢀC, 4 h
60
70
18
18
99
85
(a) 60
(b) 96
23
20e
20f
25d
5
25e
This work
C6H5Cl, 80 ꢀC, air, 15 h
t-BuOK, acetonitrile–methanol (2 : 1), rt, air, 7 h
H2O2, 75 ꢀC, 12 h
K2CO3, water, 100 ꢀC, 6 h
Toluene, K2CO3, 100 ꢀC, bubbling O2, 6 h
(a) Air, 25–30 ꢀC, 5 h
(b) H2O2, 70 ꢀC, 4 h
Synthesis of nano-Fe3O4–L-DOPA–copper complex (Fe3O4–LD–Cu)
Conclusion
Amino acid functionalized nano-Fe3O4 (1 g) was dispersed in a
water–ethanol mixture (1 : 1). To this, an aqueous solution of
Cu(NO3)2$3H2O (120 mg) was added. Triethyl amine (TEA)
solution in water was added dropwise to bring the pH of this
mixture to 6. The reaction mixture was then stirred for 24 h at
room temperature. The product was allowed to settle, washed
several times with water and acetone, and then dried under
In summary, a facile route to the synthesis of a copper containing
Fe3O4–L-3,4-dihydroxyphenylalanine nanocomposite was repor-
ted without using any organosilane precursors. This catalyst then
catalyzed the oxidation of alcohols with high TON and excellent
selectivity. Also, being magnetically separable, the requirement
for catalyst ltration aer completion of the reaction was elimi-
nated, which is an additional sustainable attribute of this
oxidation protocol. The magnetic catalyst exhibited high catalytic
activity/selectivity in the solvent-free aerobic oxidation, as well as
in the H2O2 oxidation, of alcohols. Further investigation and
modication are under progress in our laboratory.
ꢀ
vacuum at 60 C for 2 h.
Details of experimental procedure for the Fe3O4–LD–Cu
catalyzed oxidation of alcohol
H2O2 as oxidant. Benzyl alcohol (10 mmol) was heated with
30% v/v H2O2 (11 mmol) at 70 ꢀC in the presence of 25 mg
(0.02 wt% or 0.003 mmol copper by AAS) of Fe3O4–LD–Cu
catalyst and stirred at that temperature for 4 h. The progress of
Experimental
Materials
Benzyl alcohol, other alcohols, FeCl3$6H2O, FeCl2$4H2O, the reaction was monitored by thin layer chromatography
ammonium hydroxide, Cu(NO3)2$3H2O, TEA, H2O2, ethanol, (TLC). On completion of the reaction, the mixture was cooled to
and L-3,4-dihydroxyphenylalanine (LD) were purchased from room temperature and the catalyst was removed by an external
Merck Mumbai, India. All the solutions were prepared using magnet. The liquid organic product was analysed by a gas
double-distilled and demineralized water.
chromatograph (GC) to calculate the level of benzyl alcohol
conversion and the benzaldehyde selectivity. For isolation, the
product was extracted with ethyl acetate. The combined organic
layer was washed with water and brine solution and then nally
dried over anhydrous Na2SO4. Evaporation of the solvent le the
crude benzaldehyde with a 96% selectivity, which was then
puried by column chromatography over silica gel (ethyl ace-
tate : hexane 5 : 95 v/v) to provide pure benzaldehyde in a 87%
yield (1H-NMR spectroscopic data provided ESI†). The oxidation
of other alcohols was carried out in a similar manner.
Air oxygen as oxidant. In a typical reaction, benzyl alcohol
(1 mL, 10 mmol) and the magnetic catalyst (25 mg) were loaded
into a two-neck round bottom ask. The mixture was then
exposed to air to allow oxygen ow into the mixture to initiate
the reaction. Aerwards, the reaction mixture was stirred under
solvent-free conditions with air oxygen for 4.5 h. The conversion
and selectivity were determined by GC. The product was iso-
Synthesis of magnetic nanoferrites
Fe3O4 nanoparticles were synthesized by a co-precipitation
method as reported previously.28 FeCl3$6H2O (6.95 g) and
FeCl2$4H2O (10 g) were dissolved in 50 mL of deionized water
ꢀ
and stirred at 50 C for 30 min under a nitrogen atmosphere.
Then, ammonium hydroxide (25%) was added slowly to adjust
the pH of the solution to 10. The reaction mixture was then
continually stirred for 1 h at 60 ꢀC. The precipitated nano-
particles were separated magnetically, washed with water until
the pH reached 7, and then dried under vacuum at 60 ꢀC for 2 h.
The obtained magnetic nanoferrite (Fe3O4) was then used for
further chemical modication.
Surface modication of nanoferrites
Nano-Fe3O4 (1 g) was dispersed in 10 mL water by sonication for lated by following the similar workup as above. The oxidation of
30 min. L-DOPA (1 g) dissolved in 5 mL of water was added to other alcohols was carried out in a similar manner. In the case
this solution and again sonicated for 2 h. The amino acid of solid reactants, acetonitrile was used as a solvent.
functionalized nanomaterial was then isolated by an external
All experiments were repeated three times and the repro-
magnet, washed with water, and then dried under vacuum at ducibility conrmed. The products puried by short-path silica
ꢀ
60 C for 2 h.
gel chromatography (0–25% ethyl acetate in hexane, v/v) were
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1
analyzed by gas chromatography (GC) and 1H-NMR spectros- GC), and H NMR (BRUKER 400 MHz) with those of authentic
copy (ESI†). The recyclability of the NPs was also tested. The NPs samples or reported data.
were recovered by an external magnet and washed with water,
followed by methanol and again water. Finally, they were dried
at 60 C under vacuum and used directly for the next round of
Acknowledgements
ꢀ
reaction without further purication.
The authors are grateful to the Solvay group, new Research,
Development and Technology Centre at Savli, Gujarat, India, for
nancial assistance.
Characterizations methods
The amount of copper in the catalyst and in the supernatant was
estimated by atomic absorption spectroscopy (AAS) on an AA
6300: Shimadzu (Japan) atomic absorption spectrometer using
an acetylene ame. The optimum parameters for the Cu
measurements are: wavelength ¼ 324.7 nm; lamp current ¼ 2
mA; slit width ¼ 0.2 nm; and fuel ow rate ¼ 0.2 L minꢁ1. The
powered X-ray diffraction (XRD) patterns were recorded with a
PanAlytical (model; Empyrean) ‘X’PERT-PRO XRPD of Cu Ka
radiation (l ¼ 0.15406 nm) on an advanced X-ray power
diffractometer. Samples were prepared by pressing dried
powder, and the patterns were collected at a scanning rate of 2ꢀ
per min and 2q ranging from 0 to 80ꢀ. The surface area and
porosity of the nanocatalyst were measured by a volumetric
adsorption system (Micromeritics Instrument corporation, USA,
model ASAP 2010) using N2 adsorption/desorption isotherms at
77 K up to 1 bar. Prior to the measurements, the samples were
activated (degassed) by heating at the rate of 1 K minꢁ1 up to
383 K under vacuum. The temperature, as well as vacuum, was
maintained for seven hours prior to the measurements. The
surface area was calculated by the Brunauer–Emmett–Teller
(BET) method, while the porosity was calculated by the Barrett–
Joyner–Halenda (BJH) method. High-Resolution Transmission
Electron Microscopy (HR-TEM) was carried out using a Jeol
(Jem-2100) electron microscope operated at an acceleration
voltage of 200 kV. For this purpose, a dry powered sample was
dispersed in methanol and ultrasonication treatment was
applied to it for 30 min. Aerward, the sample was deposited
onto a carbon-coated grid at room temperature and it was
allowed to air-dry (about 6 hours). Selected area electron
diffraction patterns (SAED) and Energy-dispersive X-ray spec-
troscopy (EDX/EDS) were also investigated from the electron
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