Magnetically separable CuFe2O4/reduced graphene oxide nanocomposites: As a highly active catalyst for solvent free oxidative coupling of amines to imines

Article abstract of DOI:10.1039/c6ra08868f

In this report, a very simple and straightforward one step coprecipitation method was used for decorating CuFe₂O₄ nanoparticles (NPs) over graphene sheets. The as synthesized CuFe₂O₄/RGO nanocomposites (CFRNCs)

Full text of DOI:10.1039/c6ra08868f

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Magnetically separable CuFe2O4/reduced graphene
oxide nanocomposites: as a highly active catalyst
for solvent free oxidative coupling of amines to
imines†
Cite this: RSC Adv., 2016, 6, 53430
*
Ritu Dhanda and Mazaahir Kidwai
In this report, a very simple and straightforward one step coprecipitation method was used for decorating
CuFe2O4 nanoparticles (NPs) over graphene sheets. The as synthesized CuFe2O4/RGO nanocomposites
(CFRNCs) were fully characterized by XRD, TEM, HRTEM, XPS, FTIR, Raman, ICP-AES and SEM-EDS
analysis which shows well controlled small sized (ꢀ7 nm from Scherrer formula) CuFe2O4 NPs with
dense and compact loading over RGO sheets. The magnetic study reveals the superparamagnetic
behavior of ferrite NPs which provides an extra benefit in catalytic recyclability. The synthesized CFRNCs
were found to be highly active in aerobic oxidative coupling of amines to desired imines achieving
excellent yields (88–95%) under mild reaction conditions. Moreover the role of Cu, Fe species and their
interaction with RGO sheets was well studied separately by synthesizing CuO/RGO, Fe2O3/RGO, bare
CuFe2O4 and RGO samples, then catalytic mechanism was proposed based upon various controlled
reaction results. The CFRNCs were recycled up to 4th cycle under similar catalytic conditions and found
to be highly efficient without any accountable loss in activity and stability. This outstanding activity of
CFRNCs can be attributed to synergistic coupling between Cu/Fe species, high surface area due to the
presence of RGO sheets and small sized surfactant free CuFe2O4 NPs which led to the use of the
present nanocatalyst in many more industrially important catalytic applications.
Received 6th April 2016
Accepted 19th May 2016
DOI: 10.1039/c6ra08868f
www.rsc.org/advances
materials have been designed for catalyzing this coupling
reaction.5–10 However, various key challenges like harsh reaction
1 Introduction
conditions, low selectivity and involvement of organic solvents
still exist. Since the separation and purication of unstable
imines and catalyst is very difficult and brings environmental
issues hence it is very desirable to develop highly selective and
cost-effective heterogeneous catalysts which can work under
solvent-free conditions and easily separated aer reaction.
Keeping environmental issues in mind, various magnetic
nanomaterials were designed for organic synthesis reactions
which are easy to recover and reusable for various cyclic runs.
Among magnetic NPs, ferrites with general formula MFe2O4 (M
is divalent transition metal, such as Mn, Fe, Co, Ni, Cu, Zn etc.)
are one of the best nanomaterials for catalytic reactions because
their properties can be varied by changing the identity of the
divalent M2+ ion. Among the ferrites, CuFe2O4 has received
great attention due to large abundance, low cost, environmental
friendly nature and highly active Cu(II) cation for catalysis
reaction. In recent years, CuFe2O4 NPs were used in various
catalysis reactions like steam reforming, nitroaromatic reduc-
tion, dye degradation, acyloxylation, phenol hydroxylation
etc.11–20 In general, it has been found that loading of these
ferrites NPs on reduced graphene oxide (RGO) sheets leads to
highly active and selective catalysts.21–24
The environmental problems concerning organic waste have
increased due to industrial development and waste accumula-
tion which pollutes the environment and is harmful to human
health. Due to these concerns, solvent free organic synthesis are
of great importance in industrial processes with minimum
organic waste.1,2 Among various solvent free couplings, amine to
imine conversion is one of the important processes due to
application of its products in the synthesis of diverse interme-
diates for a variety of ne chemicals, pharmaceuticals and
molecular motors.3,4 Among the various reported methods,
oxidative coupling of amines to imines provides an attractive
alternative for imine synthesis. Recently, various heterogeneous
Green Chemistry Research Laboratory, Department of Chemistry, University of Delhi,
North campus, New Delhi-110007, India. E-mail: kidwai.chemistry@gmail.com; Fax:
+91-11-27666235
† Electronic supplementary information (ESI) available: XRD spectra for
CuO/RGO, bare CuFe2O4 and GO samples. SEM-EDS analysis for CuFe2O4/RGO
nanocomposites. Survey spectrum of XPS and high resolution C 1s spectrum.
The TGA analysis of CuO/RGO NCs. The low resolution TEM analysis for
CuFe2O4/RGO NCs with varying graphene content and bare CuFe2O4 NPs. 1H
and 13C NMR of synthesized imine compounds and table showing comparison
of present catalyst with earlier reports. See DOI: 10.1039/c6ra08868f
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Since, graphene is two-dimensional sp2 carbon network CFRNCs with 30 wt% graphene is as follows: 100 mg of GO was
which has remarkable electronic conductivity (105 to 106 S mꢁ1), dispersed in 80 mL of water via ultra-sonication. Subsequently,
large surface area (2000–3000 m2 gꢁ1), excellent absorptivity, 1 mmol of Cu(NO3)2$3H2O and 2 mmol Fe(NO3)3$9H2O (molar
ultrathin thickness, superior structural exibility, low ratio of Cu : Fe ¼ 1 : 2) was dissolved in 20 mL of water and then
manufacturing cost, exceptional mechanical, optical, thermal dropwise added to the GO dispersion. The resulting reaction
and magnetic properties which makes it a most promising mixture was magnetically stirred for 10 minutes. Then freshly
support material for use in diverse horizons like sensors, elec- prepared aqueous NaOH (8 M) was added drop wise to the
tronics, electrochemical energy storage, efficient catalysis etc.25–31 reaction mixture to maintain pH 10. The whole setup was
Graphene sheets can be prepared in large scale by oxidative maintained at room temperature and stirred for 15 minutes.
exfoliation of common graphite to graphene oxide (GO) via Then resulted mixture was reuxed at 80 ꢂC for next 2.5 h. The
Hummer's method and followed by chemical or thermal resulted dark brown product was isolated by centrifugation,
reduction. Various metals and metal oxides NPs were anchored washed with water for three times and then dried in oven at
on graphene sheets via co-reduction, co-precipitation method 50 ꢂC. For comparison, bare CuFe2O4 NPs were also synthesized
which showed enhanced properties and improved functional- using similar procedure without addition of GO.
ities due to interaction between NPs and graphene sheets.21–24,32
Recently, CuFe2O4/RGO nanocomposites were synthesized
under hydrothermal condition using high temperature and long
reaction time.21–24 However, in this report we have synthesized
reduced graphene oxide supported CuFe2O4 NPs via facile and
inexpensive co-precipitation approach at low reaction tempera-
ture. The synthesized CFRNCs were well characterized by XRD,
TEM, HRTEM, SEM-EDAX, VSM, XPS, ICP-AES, FTIR and Raman
spectroscopy. The CuFe2O4 NPs were found to be in pure phase,
small sized and homogeneously distributed all over graphene
sheets. The NCs exhibits excellent magnetic properties that help in
easier separation of present nanocatalyst. The CFRNCs were
applied in solvent free oxidative coupling of amines to imines and
found to be highly active and selective for desired product.
Moreover, this catalyst was found applicable for oxidative coupling
of both electron donating and electron withdrawing groups con-
taining amines. The NCs can be magnetically recycled and showed
very good recyclability for amine to imine conversion for contin-
uous cyclic runs which may lead to the use of present nanocatalyst
in many more industrially important catalytic applications.
2.3. Synthesis of CuO/RGO NCs
For synthesis of CuO/RGO with similar percentage loading
(ꢀ30 wt% graphene), 3 mmol of Cu(NO3)2$3H2O was taken and
followed by similar reaction procedure as used in synthesis of
CFRNCs.
2.4. Synthesis of hematite-reduced graphene oxide
nanocomposites (NCs)
The reduced graphene oxide (RGO) supported alpha-Fe2O3
nanoparticles (30 wt% graphene) were synthesized via the
earlier reported method.34
2.5. Synthesis of RGO nanosheets
For synthesis of reduced graphene oxide sheets, 100 mg of GO
was dispersed in 100 mL of water via ultra-sonication. Then
resulted mixture was reuxed at 80 ꢂC for next 2.5 h. The
resulted dark brown product was isolated by centrifugation,
washed with water for three times and then dried in oven at
ꢂ
50 C.
2 Experimental
2.1. Chemicals and materials
2.6. Catalytic oxidative coupling of benzylamine to imine
The oxidation of benzylamine was performed using O2 as
oxidant under solvent free conditions. Typically 15 mg of cata-
lyst and 1 mmol of benzylamine were taken in a 10 mL round
bottom ask. The reaction mixture was then heated at 60 ꢂC for
the required time under O2 balloon and the progress of reaction
was monitored by thin layer chromatography (TLC). Aer
reaction, catalyst was magnetically separated and the ltrate
was passed through the basic alumina-packed column using
a mixture of ethyl acetate and hexane (1 : 9) as eluent. The imine
derivatives obtained by this procedure were characterized by 1H
and 13C NMR.
Copper(II) nitrate trihydrate (99%), iron(III) nitrate nonahydrate
(98%), 4-chlorobenzylamine (98%) and natural ake graphite
were purchased from Sigma Aldrich USA. 2-Methylbenzylamine
(98%), 4-(triuoromethyl)benzylamine (98%), 4-uorobenzyl-
amine (97%), 3-(triuoromethyl)benzylamine (97%), 2-chlor-
obenzylamine (96%), 2-methoxybenzylamine (98%), 3-
methoxybenzylamine (98%), 4-methylbenzylamine (98%),
4-methoxybenzylamine (98%), 3,4-dichlorobenzylamine (96%)
and benzylamine (99%) were from Alfa Aesar. Sodium hydroxide
pellets (97.5%) was from Thomas baker. All chemicals were used
as received without any further purication. Deionized water
was used in all experiments. GO was synthesized from natural
graphite powder following improved Hummers method.33
2.7. Characterization
The phase purity and crystal structure of synthesized samples
were examined using powder X-ray diffraction (XRD, Bruker D8
Advance diffractometer) with Cu Ka as a radiation source (l ¼
2.2. Synthesis of CuFe2O4 nanoparticles on RGO nanosheets
The CuFe2O4/RGO NCs with different RGO (15, 30, 45, 60 wt%) 1.5406 A) at a scanning rate of 3 minꢁ1. The TEM and HRTEM
content has been synthesized by one pot coprecipitation images were obtained from Phillips Technai G2 30 transmission
method. The typical experimental procedure for synthesis of electron microscope operating at 200 kV accelerating voltage.
ꢂ
˚
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TEM samples were prepared by putting a drop of sample planes (see Fig. S1a†) while aer reduction GO exhibits a broad
dispersion in ethanol on carbon coated copper grid and peak at 20ꢂ corresponding to (002) planes of graphene
allowing the solvent to evaporate at room temperature. sheet.21,22,33 However, no RGO peaks was found in XRD spectrum
Elemental composition was determined by SEM-EDS and ICP- of CFRNCs indicating that GO was exfoliated due to crystal
AES. EDS was recorded on energy dispersive spectrometer growth of CuFe2O4 NPs between interlayer of graphene oxide
attached with JEOL JSM-6610LV scanning electron microscope sheets.21–23 The diffraction peaks at 2q ¼ 30.2, 35.5, 37.5, 43.2,
(SEM). ICP-AES was done using ARCOS system from M/s. 47.7, 57.3, 62.4 & 71.2ꢂ can be indexed to 220, 311, 222, 400, 331,
Spectro, Germany. The X-ray photoelectron spectroscopy 511, 440 and 620 planes of cubic CuFe2O4 spinel structure
measurement was performed using SPECS XPS system. (JCPDS 25-0283). The broad diffraction peaks of CFRNCs indi-
Magnetic properties of CuFe2O4/RGO nanocomposite sample cates smaller size of NPs which was found 7 nm from Scherrer
were measured by vibrating sample magnetometer (Micro- formula. For controlled catalytic studies, bare CuFe2O4 and
Sense EV9) at room temperature (20 ꢃ 1 ꢂC). Raman study was CuO/RGO were synthesized separately under similar reaction
done using Renishaw inVia Raman spectrometer equipped conditions and characterized by powder XRD analysis (see
with a laser having a wavelength of 514 nm. Fourier transform Fig. S1b and c† for XRD spectra). The bare copper ferrite NPs
infrared (FTIR) spectra (KBr disk, 4000–400 cmꢁ1) were exhibits diffraction peaks at 29.9, 35.4, 36.9, 57.8 and 62.2ꢂ
recorded on a Perkin Elmer FT-IR 2000 spectrophotometer.
which corresponds to cubic CuFe2O4 spinel structure while
peaks 38.7 and 48.5ꢂcan be indexed as impurity peak corre-
sponding to heterogeneous growth of CuO particles (JCPDS 80-
1268). These ndings indicates that GO sheets not only stabilize
the NPs but also helps in controlled synthesis of pure CuFe2O4
NPs. The CuO/RGO sample exhibit diffraction peaks at 32.4,
35.4, 38.4, 48.3, 53.4, 69.1, 61.5, 66.1, 67.7, 72.3 and 74.7ꢂ can be
assigned to monoclinic CuO (JCPDS card no. 48-1548) without
any impurity peaks.
3 Result and discussion
The phase purity and crystal structure of CuFe2O4 NPs and
successive reduction of GO to RGO was examined by powder
XRD studies. Fig. 1a display the XRD spectrum of CFRNCs
where all the peaks can be assigned to cubic CuFe2O4 spinel
structure without any impurity peaks of copper oxides (Cu2O or
CuO).21–24 From XRD studies, it was found that GO exhibits two
peaks at 2q ¼ 10.03 and 42.5ꢂ corresponding to (001) and (002)
The effective reduction of GO to RGO and successful
synthesis of RGO supported CuFe2O4 NPs can be further
Fig. 1 (a) Powder XRD spectrum of CFRNCs in the range of 10 to 75ꢂ, (b) FTIR and (c) RAMAN spectra of GO, RGO and CFRNCs samples. (d)
Hysteresis loop for CFRNCs.
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explained by the FTIR and Raman studies. Fig. 1b represents the RGO (ID/IG ¼ 1.14) and GO (ID/IG ¼ 0.81) which suggests that
FTIR spectra of GO, RGO and CFRNCs samples. The FTIR conjugated graphene network (sp2) is re-established aer
spectrum of GO clearly shows the abundance of oxygen con- loading of CuFe2O4 NPs on GO sheets resulting increased ID/IG
taining groups. For GO, the broad absorption at 3425 cmꢁ1 can value for nanocomposites.33
be assigned to O–H stretching vibration. The other peaks at
Magnetic properties of as synthesized CFRNCs were inves-
1725, 1395 and 1125 cmꢁ1 appears due to C]O stretching tigated at room temperature (20 ꢃ 1 ꢂC) using a vibrating
mode, O–H bending mode and C–O (alkoxy) stretching mode. sample magnetometer with an applied magnetic eld of
The peaks centered at 2856 and 2925 cmꢁ1 in the GO spectrum ꢁ22 000 Oe to 22 000 Oe and corresponding hysteresis loop is
also remain prominent in RGO and CFRNCs can be assigned to shown in Fig. 1d which clearly reveals superparamagnetic
–CH2 stretching vibrations of graphene sheets. The peak at 1630 nature for nanocomposites representing behavior of ferrite
cmꢁ1 in GO is due to the stretching vibration of absorbed water structure.
molecule on graphene oxide sheets while the absence of this
The size, morphology and crystal structure of CFRNCs were
peak in CFRNCs indicates complete removal of adsorbed water further studied by TEM and HRTEM. From low resolution TEM,
molecules.35 The FTIR spectrum of both RGO and CFRNCs it was observed that CuFe2O4 NPs are distributed all over RGO
sample show a decrease in absorption intensity for O–H group sheets, however some agglomerations were also found which
(3425 cmꢁ1) and absence of absorption peak corresponding to may be due to magnetic interaction between surfactant free
C]O (1725 cmꢁ1), C–O (1125 cmꢁ1) stretching vibrations and CuFe2O4 NPs and high percentage loading (Fig. 2a and b). The
O–H bending (1395 cmꢁ1) vibrations. These results clearly NPs were not observed outside the RGO sheets indicating very
indicate that GO has been reduced up to a great extent. A new good interactions between NPs and RGO sheets. The average
absorption peak at 602 cmꢁ1 in case of CFRNCs can be assigned particle size for CuFe2O4 NPs supported on RGO sheets was
to metal oxygen bonds.
found ꢀ12 ꢃ 2 nm with addition of very small CuFe2O4 crys-
Raman spectroscopy is a powerful technique in character- tallites. For further study, HRTEM analysis was done on
ization of structural properties of carbon based materials.
Fig. 1c represents the Raman spectra of GO, RGO and CFRNCs.
As shown in Raman spectrum, GO display two prominent peaks
at 1354 cmꢁ1 and 1600 cmꢁ1 corresponding to D and G band of
carbon respectively. The small shiing was observed in D and G
bands of RGO and CFRNCs indicating increase in disorderness
and removal of oxygen containing functional groups from gra-
phene oxide sheets. It is well known that D band can be
assigned to disordered structure or defects in graphene sheets
while G band is associated with E2g mode observed for sp2
carbon domains.36 The calculated intensity ratio of D and G
band (ID/IG) for CFRNCs (ID/IG ¼ 1.2) shows higher value over
Fig. 2 (a and b) Low resolution TEM images, (c) high resolution TEM
showing interplanar distance (inset: 2D-FFT showing exposure of (311) Fig. 3 High resolution XPS spectrum of (a) Cu 2p and (b) Fe 2p levels in
plane) and (d) SAED pattern for as prepared CFRNCS.
CFRNCs.
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The elemental composition and surface oxidation state of
CFRNCs were further examined by XPS. Fig. S4a† display the
survey spectrum of CFRNCs which reveals the presence of C, O,
Cu and Fe as the main element. For further investigation, high
resolution XPS was done for C 1s, Cu 2p and Fe 2p core levels
and spectrum is shown in Fig. S4b† and 3(a and b) respectively.
High resolution deconvoluted C 1s spectrum display four peaks
centered at 284.8, 286.5, 288.2, 289.2 eV corresponding to C–C/
C]C/C–H, C–OH, C]O and O]C–O bonds of sp2 carbon of
RGO sheets respectively.33 For C 1s spectrum, high intensity of
C–C/C]C/C–H peak was found over C–OH, C]O and O]C–O
peaks indicating exclusive reduction of GO to RGO. The Cu 2p
core level spectrum show Cu 2p3/2 and Cu 2p1/2 peaks centered
at 934.7 and 944.6 eV binding energies which is in conformity
with the earlier reported results.21–23 The Fe 2p core level spec-
trum display two peaks at 711.2 and 724.8 eV which can be
assigned as 2p3/2 and 2p1/2 peaks. Additionally, Fe 2p spectrum
show a weak satellite at 718.3 eV which indicates presence of
Fe3+ cations as earlier reports of copper ferrite.21–23 Hence ob-
tained results indicate controlled synthesis of pure spinel
CuFe2O4 NPs supported on reduced graphene oxide sheets.
Scheme 1 Oxidative coupling of benzylamine to N-benzylidene-1-
phenylmethanamine.
CFRNCs which reveals crystalline structure of as synthesized
RGO supported CuFe2O4 NPs. The interplanar spacing (dhkl) was
calculated from adjacent lattice fringes was found to be 0.25 nm
which can be indexed to (311) plane of cubic spinel CuFe2O4
(Fig. 2c).21,22 The two dimensional Fast-Fourier transform (2D-
FFT) pattern calculated for CuFe2O4/RGO sample is shown in
Fig. 2c inset also conrmed the exposure of (311) plane for
CuFe2O4 nanoparticles. The SAED pattern of CFRNCs is shown
in Fig. 2d which represents the crystalline nature of the particles
with the cubic structure, where the (220), (311), (400), (511) and
(440) lattice planes were clearly indexed in agreement with XRD
results. The elemental analysis was carried out on CuFe2O4/
RGO nanocomposites using SEM EDS and obtained results are
shown in Fig. S2.† The EDS results conrmed the presence of
Cu, Fe, C and oxygen as main elements. The Cu/Fe ratio ob-
tained from EDS was found to be consistent with stoichiometric
elemental ratios for CuFe2O4. The carbon element detected in
EDS would come from RGO sheets while oxygen was detected
from CuFe2O4 NPs and residual oxygen containing functional
groups on RGO sheets. The actual elemental composition and
percentage loading for CFRNCs were further analyzed by ICP-
AES study. ICP analysis of 10 mg sample showed 1.96, 3.47
mg of Cu and Fe elements (i.e. 19.6% and 34.7%) respectively
which means 7.4 mg of CuFe2O4 was loaded on graphene sheets
(i.e. 26% graphene content). ICP ndings further conrmed
that Cu and Fe are present in 1 : 2 molar ratio which is
consistent with stoichiometry of CuFe2O4 structure. The
percentage loading of CuO/RGO was studied by thermogravi-
metric technique which display 23 wt% graphene content in
NCs (see Fig. S3†).
3.1. Catalytic study
To explore the catalytic activity of as synthesized CFRNCs,
solvent free oxidative coupling of benzylamine in presence of O2
balloon was chosen as a model reaction for developing the
optimal reaction conditions (Scheme 1). First the reaction was
performed with CuFe2O4/RGO NCs at 60 ꢂC and progress of the
reaction was monitored by thin layer chromatography. The
obtained imine product was well characterized by 1H, 13C NMR
and mass spectrometry (see Fig. S7–S9†). The CFRNCs showed
excellent activity (>95%) and remarkable selectivity for benzyl-
amine coupling to desired imine product aer 8 h reaction at 60
ꢂC. It has been found that reaction does not proceed in presence
of reduced graphene oxide sheets and under catalyst free
conditions indicating urgent need of the CFRNCs for diben-
zylamine product formation. Furthermore, the amount of
Fig. 4 Comparing the catalytic reactivity with (a) different catalyst amount, (b) effect of intrinsic CuFe2O4 loading and (c) with different catalysts.
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centers. In addition, the p–p stacking and electrostatic inter-
actions between amine and RGO sheets increases the adsorp-
tion of reactant on the catalyst and causes increased collision
frequency between reactants leading to a high conversion rate.
However, decrease in conversion rate with more graphene
content (>30 wt%) might be due to lower active content of
CuFe2O4 to provide sufficient active copper/iron centers for
oxidative coupling reaction. A signicant decrease in imine
formation (up to 16%) was found under N2 environment indi-
cating crucial role of O2 for happening of catalytic reaction (see
1
Fig. S10† for H-NMR data).
Fig. 5 Proposed catalysis mechanism for aerobic oxidative coupling
of amines to imines in presence of CFRNCs.
3.2. Comparison with separately synthesized catalysts
To understand the catalytic mechanism and high activity of
CFRNCs, different nanocatalysts CuO/RGO, Fe2O3/RGO, bare
CuFe2O4 and RGO sheets were synthesized under similar reac-
tion condition and catalytic experiments were performed with
aerobic oxidative coupling of benzylamine under solvent free
conditions (see Fig. 4c). Here CuO/RGO, Fe2O3/RGO both cata-
lyzed the reaction and showed imine formation up to 56 and
18% respectively indicating both Cu(II) and Fe(III) as an active
centers for coupling reactions. However it can be noticed that
CuFe2O4/RGO NCs with similar loading exhibit higher activity
over CuO/RGO and Fe2O3/RGO NCs. Based upon these results,
the higher activity of CFRNCs may be due to best synergistic
effect between CuFe2O4 NPs/graphene sheets and combined
activity of both Cu(II) and Fe(III) metal centers. Similar type of
synergistic role of Cu and Fe centers was further supported by
earlier reports which explain the better activity of CFRNCs over
CuO/RGO and Fe2O3/RGO NCs.15,37 The bare CuFe2O4 NPs
showed only 23% conversion which may be due to absence of
graphene sheets causing agglomerated NPs, hence lower
surface area (see low resolution TEM in Fig. S6†) while a trace
amount of imine product was found in presence of RGO and
catalyst free conditions. Based upon above controlled experi-
ments, the high catalytic activity of present CFRNCs nano-
catalyst can be ascribed to following factors (1) metal–metal
synergy between Cu(II) and Fe(III) metal centers, (2) high surface
catalyst was optimized by studying yield obtained versus
amount of catalyst used (see Fig. 4a).
Since no signicant conversion was observed with high
catalyst loading of 20 mg while reducing the amount to 15 mg is
sufficient to obtain 95% yield of desired imine product with
some aldehyde and unidentied impurities just in 8 h. To check
the effect of active metal loading, we have synthesized CuFe2O4/
RGO NCs with 15, 30, 45 and 60 wt% graphene by varying the
metal precursor amount and characterized by low resolution
TEM imaging (see Fig. S5†). As synthesized nanocatalysts were
applied in benzylamine oxidation reaction and obtained results
are included in Fig. 4b. Since no benzylamine conversion was
detected in absence of catalyst, which indicated that CuFe2O4
NPs were responsible for the oxidation reaction. However, all
the CuFe2O4/RGO samples showed obvious catalytic activity for
amine oxidative coupling. When the RGO content in the cata-
lysts increased from 0 to 60%, the benzylamine conversion rate
rst increased from 23 to 95% and then decreased to 56%. This
showed that the introduction of RGO enhanced the catalytic
activity of the nanocomposites which is due to uniform dis-
persity of CuFe2O4 NPs on RGO sheets and prevented them from
agglomerating, thus increasing the number of Cu/Fe active
Fig. 6 (a) Recyclability plot showing isolated yield of imine product and (b) powder XRD spectrum of magnetically recovered CFRNCs after 4th
catalytic run.
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Table 1 Oxidative coupling of various amines to imines over CFRNCsa
area due to small sized surfactant free CuFe2O4 NPs and pres-
ence of two dimensional graphene sheets and (3) graphene
sheets holds organic substrates by p–p interaction with
aromatic rings and hence increase the catalytic activity.
3.3. Plausible mechanism
Entry
R
T (h)
Yieldb
Based upon above controlled experiments and earlier reports,
catalytic mechanism of CFRNCs for conversion of amine to
imine was proposed which was shown in Fig. 5. In the initial
phase both oxygen and amine molecules were activated by both
Fe(III) and Cu(II) metal centers and transformed into oxygen and
amine radical species which later react to form H2O2 and ben-
zylimine intermediates.3,6 The in situ formed H2O2 may oxidize
another molecule of benzylamine to imine intermediate and get
converted into water which later hydrolyze imine to aldehyde.3
Since the benzylimine intermediate is unstable and its decom-
position was further accelerated by Lewis acidic nature of ferrite
nanocatalyst. It has been well studied that imine intermediate
can decomposed by two pathways, (1) react with another
nucleophilic benzylamine molecule to form nal product or (2)
rst hydrolyzed to aldehyde and followed by reaction with
another benzylamine molecule to form nal desired dibenzyl-
amine product.8,9 This hypothesis was again proved from NMR
results (from reaction course) which showed presence of benz-
aldehyde.6 The formation of free radical was also investigated by
using 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) as free
radical trap which reduces benzylamine conversion up to 2–3%
aer its addition. This dramatic decrease in conversion
conrms the free radical mechanism for this reaction.
1
2
3
4
5
6
7
8
H
4-Cl
4-F
8
8
8
10
8
8
8
9
10
11
10
10
95
93
92
90
91
90
94
88
89
89
90
87
2-Me
4-CF3
4-OMe
4-Me
3-OMe
2-Cl
3,4-Cl
3-CF3
2-OMe
9
10
11
12
a
Reaction conditions: amine (1 mmol), catalyst (15 mg), O2 balloon,
60 ꢂC. b Isolated yield.
and EWGs (–Cl, –CF3, –F) on the phenyl ring did not affect the
rate of reaction which indicate that there is no clear electronic
role of substituents.3,4 However, for ortho and meta substituted
groups, CFRNCs showed slower reactant conversion which
explain stearic effect of substituents. Hence all these
outstanding results indicate wide applicability of CFRNCs for
such oxidation reactions with excellent activity and high recy-
clability. Since earlier various heterogeneous materials have
been reported for amine to imine conversion, so we have
compared our catalyst with some recent metal based heteroge-
neous catalysts and obtained results are entered in Table S1.†
These results indicate that as synthesized CFRNCs are amongst
the best heterogeneous catalysts in terms of mild solvent free
reaction conditions with high recyclability. Since the TOF values
was found higher for Au based catalysts, however inexpensive-
ness, solvent free catalysis and magnetic recyclability of
CuFe2O4/RGO NCs makes it better for industrial purpose over
these catalysts.
3.4. Recyclability and heterogeneity of CFRNCs
To further check the stability and recyclability of CFRNCs,
similar catalyst was used up to four consecutive runs under
similar reaction condition for aerobic oxidative coupling of
benzylamine. Aer each catalytic cycle, CFRNCs were recovered
by magnetic separation, washed, dried and employed for next
cycle. The CFRNCs showed very high activity (yield 89%) for
amine coupling even aer 4th cycle without any accountable loss
of activity. Fig. 6a represents the obtained yield aer each
successive cycle. The recovered catalyst was characterized by
powder XRD which showed similar pattern with fresh sample
indicating stability of spinel structure (see Fig. 6b). Further, ICP
analysis also showed very minute change in percentage loading
(0.09%) of copper ferrite NPs. This outstanding stability and
recyclability of CuFe2O4/RGO nanocomposites can be ascribed
to very good interaction between ferrite NPs and graphene
sheets which prevents agglomeration.
4 Conclusions
CuFe2O4 NPs were in situ synthesized on graphene sheets via
surfactant free one pot co-precipitation method which exhibited
excellent catalytic activity towards self-coupling of amine to
imines. The synthesis of NCs was found well controlled by
various characterization technique like XRD, TEM, ICP, EDS,
FTIR, RAMAN and XPS studies. The CFRNCs catalyzed the
3.5. Substrate scope
With the optimized reaction conditions, we have explored the aerobic amine coupling under neat conditions just in 8–11 h to
substrate scope of present nanocatalyst for oxidative coupling get 88–94% isolated yield with excellent selectivity. The CFRNCs
reaction and results are shown in Table 1 (see 1H and 13C NMR were separated magnetically aer catalysis reaction and recy-
in Fig. S11–S32†). The present CFRNCs tolerate wide range cled up to four consecutive runs without any accountable loss in
functional groups and results into desired product with excel- activity and stability. In addition, role of Cu, Fe species and
lent yield for ortho, meta, para substituted electron donating graphene sheets in oxidative amine coupling was studied
and electron withdrawing groups. The EDG (–CH3 and –OCH3) separately by synthesizing CuO/RGO, Fe2O3/RGO, bare CuFe2O4
53436 | RSC Adv., 2016, 6, 53430–53437
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´
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Acknowledgements
Ritu Dhanda thanks UGC, India for providing senior research
fellowship. Authors also thank USIC, DU and SAIF-AIIMS for
instrumentation facility and DU R&D for nancial support.
Authors also acknowledge SAIF-IIT Mumbai for ICP-AES and IIT
Delhi for XPS study.
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