Highly efficient microwave assisted synthesis of magnetically separable GO-CoFe2O4 nanocomposite for visible light induced oxidative coupling of benzyl amines

Article abstract of DOI:10.1016/j.jphotochem.2020.112697

The present paper describes an efficient microwave-assisted synthesis of magnetic CoFe₂O₄ and its nanocomposite with graphene oxide (GO-CoFe₂O₄) for photocatalytic oxidative coupling of benzylamines to the corresponding imines using molecular oxygen. The surface morphologies and structures of both CoFe₂O₄ and GO-CoFe₂O₄ nanocomposites were investigated by various spectroscopic techniques including powder X-ray diffraction (XRD), Transmission electron microscopy (HR-TEM), Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption–desorption isotherms, X-ray photoelectron spectroscopy (XPS) and Raman Spectroscopy. Photocatalytic oxidative coupling experiments demonstrated that GO-CoFe₂O₄ exhibited significantly higher efficiency than CoFe₂O₄ nanoparticles, suggesting that GO played an important role in the reaction. Moreover, the magnetic nature of the synthesized nanocomposite provided facile recovery of the material by the influence of an external magnet.

Full text of DOI:10.1016/j.jphotochem.2020.112697

Journal Pre-proof
Highly efficient microwave assisted synthesis of magnetically separable
GO-CoFe2O4 nanocomposite for visible light induced oxidative coupling
of benzyl amines
Aathira M. Sadanandan, Praveen K. Khatri, Suman L. Jain
PII:
S1010-6030(20)30496-2
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112697
JPC 112697
Reference:
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Journal of Photochemistry & Photobiology, A: Chemistry
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Revised Date:
Accepted Date:
11 November 2019
8 June 2020
8 June 2020
Please cite this article as: Sadanandan AM, Khatri PK, Jain SL, Highly efficient microwave
assisted synthesis of magnetically separable GO-CoFe2O4 nanocomposite for visible light
induced oxidative coupling of benzyl amines, Journal of Photochemistry and amp;
Photobiology, A: Chemistry (2020), doi: https://doi.org/10.1016/j.jphotochem.2020.112697
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Highly efficient microwave assisted synthesis of magnetically
separable GO-CoFe2O4 nanocomposite for visible light induced
oxidative coupling of benzyl amines
Aathira M. Sadanandana, Praveen K. Khatria and Suman L. Jain*a
aChemical and Material Sciences Division, CSIR-Indian Institute of Petroleum, Dehradun India 248005
*Email: suman@iip.res.in; Tel: 91-135-2525788; Fax: 91-135-2660202
Graphical Abstract
1

Highlights
 An efficient, rapid and energy saving microwave assisted synthesis magnetic GO-
CoFe2O4 nanocomposite
 Use of enviro-economic molecular oxygen as primary oxidant
 Photocatalytic oxidative coupling of amines to imines under ambient conditions
 Heterogeneous photocatalyst with facile recovery by using an external magnet
Abstract
The present paper describes an efficient microwave-assisted synthesis of magnetic CoFe2O4 and
its nanocomposite with graphene oxide (GO-CoFe2O4) for photocatalytic oxidative coupling of
benzylamines to the corresponding imines using molecular oxygen. The surface morphologies and
structures of both CoFe2O4 and GO-CoFe2O4 nanocomposites were investigated by various
spectroscopic techniques including powder X-ray diffraction (XRD), Transmission electron
microscopy (HR-TEM), Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption–
desorption isotherms, X-ray photoelectron spectroscopy (XPS) and Raman Spectroscopy.
Photocatalytic oxidative coupling experiments demonstrated that GO-CoFe2O4 exhibited
significantly higher efficiency than CoFe2O4 nanoparticles, suggesting that GO played an
important role in the reaction. Moreover, the magnetic nature of the synthesized nanocomposite
provided facile recovery of the material by the influence of an external magnet.
Keywords: Photocatalysis, hybrid material, oxidative coupling, molecular oxygen, magnetic
separation
2

1. Introduction
Exploitation of solar light to induce organic transformations in the presence of a photo-active
catalyst is the subject of immense scientific interest and importance owing to the transparency of
most of the organic molecules in the visible spectrum [1-6]. Use of this renewable energy (sunlight)
not only makes the synthesis cost efficient but also provides an ideal solution for environment
related issues originated by energy intensive chemical processes [7-9]. Imines are the important
building blocks that have extensively used as intermediates in the synthesis of biologically
important compounds and as liquid crystals in the field of analytical and polymer chemistry [10-
11]. Conventionally imines are synthesized by the condensation of an aldehyde with an amine in
an organic solvent under reflux conditions [12-13]. Oxidative coupling of amines over photo-
active catalyst using molecular oxygen is a green and energy efficient approach for the synthesis
of imines under mild reaction conditions [14-21]. In this context, Chen et al demonstrated a
carbonyl modified carbon nitride photocatalyst for the oxidative coupling of amines using
molecular oxygen [22]. Su et al reported oxidative coupling of amines to imines using mesoporous
graphitic carbon nitride as a photocatalyst under visible light irradiation [23]. Lang and co-workers
reported the visible light conducted selective oxidation of amines to imines using dye sensitized
titania and TEMPO with atmospheric air pressure [16]. In a most recent approach, a porphyrinic
based metal organic framework (PCN 222) was utilized efficiently as a photocatalyst for oxidative
coupling of diverse amines upon visible light irradiation at ambient conditions [20]. Kumar et al
demonstrated the use of iron bipyridine- graphitic carbon nitride as a hybrid catalyst for the
photoactivity of aerobic oxidative coupling for benzylamines [14].
Cobalt ferrite, one kind of spinel ferrite (MFe2O4, M = metal cation) is a chemically and thermally
stable magnetic material having high magnetic hardness and magneto crystalline anisotropy [24-
3

28]. Owing to its activity in the visible light, it has been extensively applied in photo-catalytic
reactions [29-30]. Recently Li and co-workers developed a magnetically separable photo-catalyst
TiO2/CoFe2O3 by the sol gel and electro-spinning methods for photocatalytic degradation of
methylene blue under UV-Vis light [31]. In an another approach by Wang et al degradation of
methylene blue, methyl orange, rhodamine etc was achieved by a highly visible light active cobalt
ferrite/graphene nanocomposite with different degree of graphene content [32]. The graphene
oxide plays a significant role in improvement of photo-catalytic activity of cobalt ferrite under
visible light irradiation. In an another development, Haw and co-workers reported a novel
magnetically recoverable TiO2-cobalt ferrite nanocomposite for the photo-degradation of
methylene blue [30]. Inspired by these reports, we synthesized a nanocomposite containing
graphene oxide and cobalt ferrite (GO-CoFe2O3) for oxidative coupling of amines to
corresponding imines under visible light. Owing to the presence of oxygen functionalities in GO,
the synthesized nanocomposite offered better dispersion in organic solvents. So far, a number of
methods such as solvothermal, ultrasonic have been explored for the synthesis of nanocomposites
of CoFe2O4 with GO for various applications [29, 33-35].
During the present study, a novel microwave assisted methodology adopted for the synthesis of
GO-CoFe2O4 nanocomposite that make its synthesis simple, rapid, cost effective and energy
efficient. The synthesized nanocompoiste was used as a visible light active photocatalyst for the
oxidative coupling of benzylamines to corresponding imines using molecular oxygen as an oxidant
in high to excellent yields (Scheme 1).
4

Scheme1. Oxidative coupling of benzyl amine using GO-CoFe2O4 as a photocatalyst
2. Experimental
2.1 Materials used
Cobalt nitrate hexahydrate (≥98%), iron (III) nitrate nonahydrate (≥98%), ethylene glycol (99.8%),
4-fluoro benzylamine (97%), 4-bromobenzylamine (96%), aniline (≥99.5%) were purchased from
Sigma Aldrich and used without further treatment. Benzylamine (98+%), 4-methoxy benzylamine
(98+%), 4-chloro benzylamine (97+%), 4-methyl benzylamine (98%), 3-chloro benzylamine
(97+%), 4-chloro3-fluoro benzylamine (98%), 4-bromo aniline (98+%), 4-chloro aniline (98%)
were purchased from Alfa Aesar and urea (99.5%) was procured from Rankem, Ranbaxy India
and used as received.
2.2 Techniques used
X-ray diffraction of the sample was recorded over a Bruker D8 advanced diffractometer, with a
voltage of 40 kV with Cu Kα radiation of a wavelength of 1.5418 Ao. The FT-IR spectra of the
samples were recorded by Perkin Elmer Fourier Transform Infrared spectrophotometer in which
potassium bromide was used to make the transparent pellet for analysis. UV-Visible spectroscopic
analyses of the samples were done by Perkin Elmer Lambda-19 UV-VIS-NIR spectrophotometer
where barium sulfate was used as a baseline of standard reflectance. The surface area and pore
analysis were done by a Micrometrics ASAP2010 instrument in which nitrogen adsorption and
desorption isotherm using liquid nitrogen at 77 K. XPS measurements were done by a system
(ESCA+ omicron nanotechnology, Oxford instruments Germany). HR-TEM and FE-SEM were
used to analyze the morphology of the sample. HR TEM images of the samples were recorded by
high-resolution transmission electron microscopy (JEM 2100), an operating voltage of 200 kV and
LaB6 was used as an electron source in it. In field emission scanning electron microscopy (Quanta
5

200F, FEI) a range of detectors including Secondary Electron (SE) and Energy Dispersive X-ray
analysis (EDS), etc were used with accelerating voltage of 15kv. Raman spectra of the samples
were recorded by HR 800 Jobin-Yvon Raman spectrometer with a spectral range of 4000 cm-1 to
100 cm-1. Thermal gravimetric analysis of the photocatalyst was done by a Perkin Elmer TG-DTA
o
o
o
instrument over a temperature range of 30 C to 800 C with a rise of 10 C per minute under a
nitrogen atmosphere.
2.3 Synthesis of CoFe2O4
The desired nanocomposite was synthesized by a microwave-assisted approach. In a typical
synthesis, ferric nitrate (3.85g, 16mmol) and cobalt nitrate (5.82g, 32 mmol) were dissolved in a
minimum amount of deionized water followed by the addition of 3 g of urea and the resulting
mixture was allowed to stir for 20-30 min. Further, 50 ml of ethylene glycol and 30 ml water were
o
added and the mixture was refluxed in a Milestone microwave reactor at 110 C with an energy
input of 1000 W for 4 h. After completion of the reaction, the solid was separated by centrifugation
and washed with distilled water, followed by ethanol. Finally, the obtained material was dried and
calcined at 300 oC for 4 h.
2.4 Synthesis of the GO-CoFe2O4photocatalyst
The graphene oxide was prepared by modified Hummers method [36]. The nanocomposite was
synthesized by placing 2g of graphene oxide and 0.2 g of CoFe2O4 in a round bottom flask with a
mixture containing 80 ml water and ethylene glycol (in a 1:1 volume ratio). The reaction mixture
was refluxed by microwave irradiation in a microwave reactor at 160 oC by an energy input of 900
watts for 5 h. After completion of the reaction, the mixture was cooled and the nanocomposite was
filtered off, collected and washed with deionized water. Finally, the material was dried at 80 oC in
6

a vacuum oven for 12 hrs. Similarly, other GO-CoFe2O4 samples were prepared by varying the
loading of CoFe2O4 with GO
2.5 Photocatalytic oxidative coupling of benzylamines
Benzylamine (90.5 mmol, 0.054g), 3 ml of acetonitrile and 50 mg of photocatalyst were charged
into a 50 ml round bottom flask. The reaction mixture was allowed to stir at room temperature in
the presence a source of visible light under 1 atm pressure of molecular oxygen for 12 h. A 20-
watt LED lamp (Model No. HP-FL-20W-F-Hope LED Opto-Electric Co., Ltd (Shenzhen, China,
λ ≥ 420 nm) was used for the photocatalytic experiments. Intensity of the light at vessel was found
to be 75 W/m2 as measured by intensity meter. The formation of product and completion of the
reaction was confirmed by thin layer chromatography (TLC). At the end of the reaction, the
photocatalyst was separated out by an external magnet. The photocatalyst did not show any
leaching as confirmed by ICP-AES analysis of the recovered material. The product was isolated
and purified by column chromatography over silica gel. The pure products were confirmed by GC-
MS and 1H NMR analysis.
3. Results & Discussion
3.1 Synthesis and characterization of the photocatalyst
GO-CoFe2O4 nanocomposite was synthesized by the microwave heating of a mixture of GO and
CoFe2O4 as illustrated in Scheme 2.
7

Scheme 2: Preparation of GO-CoFe2O4
The changes in the chemical functionalities during the synthesis of nanocomposite were
determined by FT-IR spectroscopy. The FT-IR spectra of GO, CoFe2O4 and GO-CoFe2O4 are
displayed in figure 1. The FT-IR spectrum of GO reveals characteristic vibrational bands at 1072
cm-1, 1614 cm-1, 1733 cm-1, 2911 cm-1 and 3397 cm-1 corresponding to the C-O stretching, OH
bending, C=O stretching (carboxyl or carbonyl), C-H stretching and OH stretching modes of
vibration respectively. The emergence of an intense peak between 550 cm-1 to 600 cm-1 in the FT-
IR spectrum of CoFe2O4 attributed to the Co-O and Fe-O stretching vibration modes of metal-
oxygen bonds. Absorption bands appeared at 1618 cm-1 and 3411 cm-1 are ascribed to the –OH of
the absorbed water moieties on the surface of the nanocomposite. In the spectrum of GO-CoFe2O4,
the appearance of characteristic vibrational signatures of both components, i.e. GO and CoFe2O4
at 2927 cm-1(C-H stretching), 1711 cm-1 (stretching vibration of C=O) and 555 cm-1 (Co-O, Fe-O)
suggested the interaction of GO with cobalt ferrite. A slight shifting in the frequency values of the
final nanocomposite compared to characteristic bands of the pure samples such as GO and
CoFe2O4 [29] indicated the changes in the chemical behavior during the microwave irradiation.
8

Figure 1. FT-IR spectra of samples (a) GO, (b) CoFe2O4 and (c) GO- CoFe2O4
The crystalline phases of GO, CoFe2O4 and GO-CoFe2O4 were examined by powder X-ray
diffraction (XRD) pattern as depicted in Figure 2. A characteristic peak at 2θ value of 11.16
corresponding to the 001 crystal plane of GO. Peak position in the XRD pattern of CoFe2O4 at 2θ
values of 18.10, 30.5, 35.22, 43.08, 50.02, 57.4 and 62.9 are assigned to (111), (220), (311), (400),
(422), (511) and (440) crystal planes respectively with reference to JCPDs card no. 22-1086 [37].
The absence of characteristic peak of GO in the XRD pattern of GO-CoFe2O4 suggested the
crystalline growth of cobalt ferrite between the stacked layers of GO resulted in the destruction of
systematic stacking of GO sheets [37]. Moreover, the emergence of a new broad peak at 2θ value
9

of 25.4o related to the 002 plane suggested the reduction of GO to rGO during the synthesis under
microwave irradiation [38-39].
Figure 2. XRD patterns of samples a) GO, b) CoFe2O4 and c) GO-CoFe2O4
UV-Vis spectra of the CoFe2O4 and GO-CoFe2O4 are illustrated in Figure 3. The UV-Vis
absorption spectra of both CoFe2O4 and GO-CoFe2O4 exhibited absorption band from UV to
visible region. It can be seen clearly that neat CoFe2O4 exhibits the absorption in a broad range
10

from 200 nm to 400 nm. However, after the incorporation of GO into the CoFe2O4, the
nancomposite shows high absorbance in both near UV and visible light region credited to the π-π*
and n-π* transitions. The increased intensity of absorption band at 220-280 nm region in GO-
CoFe2O4 might be due to the additional transition of aromatic C-C bonds of GO. This is a clear
indication of the presence of extensive conjugated sp2 carbon network in the final material. The
less intense absorption band around 300-350nm was assigned to the n-π* transitions of C=O bonds
present in the GO-CoFe2O4 nanocomposite. Moreover, GO-CoFe2O4 showed little humps at 400-
550 nm regions and long absorption band in comparison to pure CoFe2O4, which indicated the
higher visible light activity of the nanocomposite.
Figure 3.UV- Vis spectra of the samples.
The band gap energy of the CoFe2O4 and GO-CoFe2O4 were determined by tauc plots as depicted
in figure 4. The value of band gap energy of ~2.4 eV was observed for CoFe2O4; whereas 2.11 eV
11

was estimated for the GO-CoFe2O4. The lower band gap along with the higher absorption in visible
region for GO-CoFe2O4 confirmed the higher photocatalytic efficiency of the nanocomposite.
Figure 4.Tauc plot of the a) CoFe2O4 and b) GO-CoFe2O4.
Elemental composition of the synthesized nanocomposite was studied by using X-ray
photoelectron spectroscopy (Fig. 5) The complete survey scan of XPS analysis confirms the
presence of desired elements C1s, O1s, Fe2p and Co2p as shown in Figure 5a. Four characteristic
XPS peaks of carbon C1s at binding energy values of 284.26 eV, 285.17 eV, 286.32 eV and 288.66
eV are attributed to a sp2carbon atom, C-C, C-O, and C=O respectively (Fig. 5b). The O1s exhibits
three key peaks at binding energy values 527.92 eV (Co-O, Fe-O/C-O-C), 529.11 eV (C-OH) and
531.05 eV (C=O) as displayed in Figure 5c. The Fe2p (Fig.5d) and Co2p (Fig.5e) peaks are observed
near to 711 eV and 782 eV respectively. The Fe2p spin-orbit split peak observed at 709.12 eV (Fe
2p3/2 ) and 723.0 eV(Fe 2p1/2) with a difference of 13.22 eV while the spin-orbit split peaks of Co2p
are found at 781.30 eV( Co2p3/2) and 796.52 eV(Co2p1/2) with separation energy of 15.2 eV. These
values are in well agreement with the reported literature [40-41].
12

Figure 5. XPS of GO- CoFe2O4. Where a) survey scan, b) C1s, c) O1s, d) Fe 2p and e)Co 2p.
HR-TEM images of GO-CoFe2O4 as shown in Figure 6 reveal spherical or oval shaped
morphology and fine structure of the sample. Growth of nanomaterial with a uniform distribution
13

over the surface of graphene oxide layer can be seen clearly in the image at scale of 200 nm (Fig
6a) [32]. SAED pattern as shown in figure 6b further confirmed the formation of nanocomposite.
Figure 6d provides the detailed view of crystalline fringes (0.259 nm) of interlayer spacing of
CoFe2O4 attributed to 311 crystal plane. The EDX elemental mapping composition of the
nanocomposite is in good agreement with the synthesis of the final photocatalyst and showed the
presence of all desired elements such as C, O, Co and Fe (Fig. 6e). The weight percentage of the
elements in EDX was found to be carbon (89.1%), oxygen (8.2%), iron (1.6%) and cobalt (1.1%).
The elemental mapping of GO-CoFe2O4 showed a homogenous distribution of all elements
throughout the nanocomposite at 500 nm scale, as represented in Figure 7.
Figure 6. HR –TEM images of GO-CoFe2O4 a) at 200nm scale, b) SAED pattern, c) at 5 nm scale,
d) Interplanar d-spacing observed at 5 nm scale, e) EDX pattern.
14

Figure 7. Elemental mapping of the GO-CoFe2O4, where a) Carbon, b) Oxygen, c) Iron and d)
Cobalt
In order to understand the surface morphology of the synthesized fresh photocatalyst and recovered
one scanning electron microscopy (SEM) analysis was carried out. The SEM images of both
versions of nanocomopsite as shown in figures 8a and 8b revealed that the morphology remained
intact in the recovered photocatalyst even after the six cycles.
15

Figure 8. SEM images of GO-CoFe2O4(a fresh catalyst and (b recycled catalyst.
Disorder in the nanocomposite in terms of sp2 carbon material was characterized by Raman
spectroscopy. In the Raman spectrum of GO as shown in figure 9, the two peaks in the range of
1100-1400cm-1 and 1500-1700 cm-1are attributed to D and G band respectively. The Raman shifts
near the range of 230 - 300 cm-1, 400- 500 cm-1 and 500-750 cm-1in GO-CoFe2O4 are attributed to
the Eg, T2g and A1g shifts of CoFe2O4 respectively [29]. The ratio of ID/IG indicates the presence
of disordered carbon ratio in the graphene oxide. The ratio of disorder in GO-CoFe2O4 was found
to be 1.03 while in GO it was 0.9. This implies that the reaction of cobalt ferrite with graphene
oxide increases the total amount of defected carbon as well as a decrement in sp2 carbon. Raman
spectra showed a clear image of changes from GO to GO-CoFe2O4 nanocomposites.
16

Figure 9. Raman spectra of the GO and GO-CoFe2O4
The specific surface properties such as BET surface area and total pore volume are
examined by nitrogen adsorption-desorption isotherm. Both CoFe2O4 and GO-CoFe2O4 isotherms
show type IV isotherm characteristic hysteresis loop and are represented in Figure 10 and 11
respectively. The type IV isotherm shows the mesoporous nature and multilayer adsorption
capability of the material. The pore volume-pore diameter graph of the CoFe2O4 (Inset in Fig. 10)
shows the mesoporous (2 nm to 50 nm) nature of the material and the sharp peak indicates
uniformity in the pore size. GO-CoFe2O4 shows type IV isotherm and H4 hysteresis having internal
voids and narrow slit-like pores. A broad peak in the pore volume-pore diameter graph of the GO-
CoFe2O4 (Inset in Fig.11) indicates the variable size of the pores in the nanocomposite. From
CoFe2O4 to GO-CoFe2O4 the steepness of the isotherm decreased and the pore size of the material
increased. The SBET and the total pore volume of the CoFe2O4 are 134.18 m2/g and 2.60 cm3/g
17

while in GO-CoFe2O4 the values are 158.4 m2/g and 1.75 cm3/g respectively. After the introduction
of GO, a slight increment in the surface area and a modest drop in total pore volume was observed.
Figure 10.Adsorption-desorption isotherm of CoFe2O4
Figure 11. Adsorption-desorption isotherm of GO-CoFe2O4
Thermal behaviour patterns of GO, CoFe2O4 and GO-CoFe2O4 were examined and
o
represented in Figure 12. Graphene oxide showed an initial weight loss around at 100 C due to
18

the evaporation of adsorbed water molecules and ethanol. Another weight loss in the range of 168
to 300 oC attirbuted to the decomposition of organic moieties (COOH, OH etc) presented on the
o
surface of GO. The TG-DTA curve of CoFe2O4 showed a weight loss near 100 C due to
evaporation of water molecules. In case of GO-CoFe2O4 nanocomposites a major weight loss in
o
the range of 345 to 546 C was attributed to the decomposition of oxygen containing organic
moieties of GO.
Figure 12. DT-TGA curves of the samples a), b) and c) are the weight % and (d, (e and (f are the
derivative weight % of GO, CoFe2O4 and GO-CoFe2O4 respectively.
19

3.2 Photocatalytic activity
Aerobic oxidative coupling of benzylamine to corresponding imine was selected as a model
example to evaluate the photo-catalytic performance of the synthesized photocatalysts under the
visible light illumination (20 W white LED light) at room temperature and 1atm pressure of
molecular oxygen using acetonitrile as solvent. The results of these optimization studies are
summarized in Table 1. The reaction did not take place both in the absence of the photocatalyst as
well as in dark without light irradiation (Table 1, entries 1-2). These results confirmed that the
reaction was truly photocatalytic and essentially required a photocatalyst to proceed. Further, the
use of pure CoFe2O4 under described reaction conditions exhibited poor efficiency and afforded
lower conversion in comparison to the final photocatalyst (Table 1, entry 3). In order to screen out
the best designed photocatalyst, a series of photocatalysts prepared by varying the loading of cobalt
ferrite on graphene oxide, for example GO-0.2CoFe2O4, GO-0.1CoFe2O4, 2GO-0.05CoFe2O4. The
2
2
conversion of benzylamine to corresponding imine was found to be increased with the loading of
cobalt ferrite in the nanocomposite (Table 1, entry 4-6). Therefore, among the synthesized samples
2GO-0.2CoFe2O4 provided the highest conversion of 97% with 99% selectivity of the desired imine.
After establishing the superior photocatalytic activity of GO-0.2CoFe2O4 nanocomposite over
2
other two materials, its concentration/ amount used was optimized and on the basis of collected
experimental data, 50 mg of the photocatalyst was found to be optimum (Table1, entry 6-7).
Subsequently, among the different organic solvents studied acetonitrile afforded maximum
conversion and therefore considered as most favorable reaction media for this oxidative coupling
of benzyl amine (Table 1, entries 8-9). Moreover, in terms of reaction period, reduction in reaction
time from 12 h to 6 h affected the reaction adversely (Table 1, entry 10). Hence, the optimized
20

conditions for the reaction were framed as 50 mg of photocatalyst (2GO-0.2CoFe2O4), acetonitrile
as a reaction media under visible light irradiation for 12 h.
Table 1. Optimization of reaction conditions for photocatalytic coupling of benzylaminea
Entry Solvent
Catalyst
Reaction
time, h
12
Conv./Selec.b TOF
(%)b
(h-1)
1
Acetonitrile
-
0/0
_
2c
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Methanol
2GO-0.2CoFe2O4
CoFe2O4
12
0/0
_
3
12
56/99
34/99
57/99
97/99
79 /99
91/99
82/99
48/99
-
4
2GO-0.05CoFe2O4 12
2.8
4.7
8.1
6.5
7.5
6.8
7.5
5
2GO-0.1CoFe2O4
2GO-0.2CoFe2O4
2GO-0.2CoFe2O4
2GO-0.2CoFe2O4
2GO-0.2CoFe2O4
2GO-0.2CoFe2O4
12
12
12
12
12
06
6
7d
8
9
Toluene
10
Acetonitrile
aReaction conditions: 0.5 mmol of benzyl amine, 50 mg of catalyst (2GO-0.2CoFe2O4), 3ml of solvent, in
presence of light for 12 hour at 1 atm pressure of O2, b Conversion and selectivity were determined by GC-
MS analysis. cExperiment was conducted in the absence of light; dusing 30 mg of the photocatalyst
After the optimization of reaction conditions, we extended the scope of the reaction for a
variety of differently substituted benzylamines and the obtained data are tabulated in Table 2. The
substrates having electron donating groups such as methoxy and methyl were found to be more
reactive than the substrates with electron withdrawing groups (F, Cl, Br, etc).
21

Table 2.Photocatalytic coupling of differently substituted benzylamines[a]
Entry
Reactant
Product
Conv.[b]
(%)
Select.[b]
(%)
NH2
N
1
2
3
97
96
96
99
99
99
N
NH2
OCH3
H3CO
H3C
H3CO
H3C
N
N
NH2
CH3
NH2
4
5
94
94
99
99
Cl
Cl
F
Cl
F
NH2
N
F
aReaction conditions: 0.5mmol of benzylamine, 3ml of acetonitrile, 50 mg of photocatalyst (2GO-
0.2CoFe2O4) under the visible light irradiation with an atmospheric pressure of O2 for 12 h; bConversion and
selectivity were determined by GC-MS.
In addition, cross-coupling of benzylamine with aniline, substituted anilines and
benzylamines was explored and the results of these experiments are summarized in Table 3. In
cross-coupling reaction with aniline, the poor yield of the desired product was probably due to the
self-coupling of benzylamine and lacking of alpha hydrogen atom in the aniline. In the case of
different substrates of benzylamine the conversion was more efficient but the selectivity of the
desired product reduced due to the self-coupling of the two benzylamine substrates.
22

Table 3.Cross-coupling of benzylamine with other amines[a]
Entry Benzylamine Amine 2
Rr
Product
Conv.[b]
(%)
Select.[b]
(%)
NH2
89
95
1
NH2
N
Cl
NH2
77
95
2
NH2
N
Cl
Br
NH2
65
95
3
NH2
N
Br
NH2
NH2
NH2
96
95
92
90
90
89
4
N
NH2
OCH3
H3CO
5
N
NH2
H3C
CH3
6
N
NH2
Cl
Cl
[a]Reaction conditions: 0.5 mmol of reactant1, 1.2 mmol of reactant 2 (1 to 3 reaction), 0.7 mmol of reactant 2 (4 to 6
reaction), 3 ml of acetonitrile, 50 mg of catalyst ( GO-0.2CoFe2O4) under the visible light irradiation with an
2
atmospheric pressure of O2 for 12 h; [b]Conversion and selectivity were determined by GC-MS analysis.
23

Further, the recyclability of the photocatalyst was evaluated by performing the oxidative coupling
of benzylamine under optimized reaction conditions. The photocatalyst was isolated from the
reaction mixture with the help of an external magnet and washed with ethanol followed by
deionized water, dried and used further for next experiments with fresh reactant. The activity of
the photocatalyst was observed almost similar in terms of conversion and selectivity for six recycle
experiments as represented in Figure 13.
Figure 13.Recycling experiments of the reaction
The mechanism for the photocatalytic oxidative coupling of benzylamines with oxygen is well
documented in the literature [11,23]. In analogy to the existing reports a plausible mechanism of
the reaction is depicted in Scheme 3. Under the light irradiation, charge separation occurred on the
surface of the cobalt ferrite and transfer of the excited electrons form valance band to conduction
24

band occurs. These electrons in the CB are captured by the molecular oxygen to generate the
.-
superoxide radicals (O2 ) that is the key component in this coupling reaction. Holes are transferred
to amine to give the amine radical cation that forms an intermediate imine on reaction with
.-
superoxide radical (O2 ). Further, this imine is get activated by another hole and furnish the desired
coupling product upon interaction with second amine molecule with formation of ammonia as a
byproduct [42-43]. In case of GO supported CoFe2O4, owing to the presence of unsaturated sp2 carbon
framework in the GO/ or rGO, photogenerated electrons can flow on the surface with minimum resistance
and therefore inhibits the recombination of the electron-hole pairs [44]. Because of the diminished
recombination, the photogenerated charge carriers are readily available for the reaction in GO-
CoFe2O4 and therefore it provided higher photocatalytic efficiency as compared to the pure
CoFe2O4 that afforded very low yield of the desired product imine under identical experimental
conditions. In addition, presence of graphene enhances the surface area of the photocatalyst that
increases the adsorption of the substrate on the surface because of the interaction between the
conjugated π-electron system of GO with the substrate [45].
In order to confirm the involvement of superoxide radical, we added superoxide scavenger
(benzoquinoe, 2%) to the reaction mixture and continued the reaction under described conditions.
The product yield was found to be decreased remarkably (20%) that confirmed the generation of
superoxide radical during the photocatalytic coupling of amines under developed protocol. The
generated hydrogen peroxide might have decomposed to oxygen in the presence of cobalt ferrite
catalyst as reported in the literature [46].
25

Scheme 3: Plausible mechanism for the oxidative coupling of benzylamine using GO-CoFe2O4 as
photocatalyst
Conclusions
The present paper demonstrated the application of a magnetically separable graphene oxide–cobalt
ferrite nanocomposite, readily synthesized by a novel, rapid and energy efficient microwave
assisted approach as photocatalyst for the oxidative coupling of amines to imines using molecular
oxygen as oxidant under mild conditions. The photocatalyst was characterized by a number of
analytical tools including XRD, FT-IR, Raman etc to elucidate the crystallinity and loading of
cobalt ferrite on GO. Surface topography and morphology of the GO-CoFe2O4 was determined
using FE- SEM and HR-TEM techniques. The photo catalytic experiments of self-coupling and
cross-coupling of amines to imines showed good conversion and selectivity with GO-CoFe2O4.
The photocatalyst could easily be separated by external magnetic effect and showed consistent
activity for several recycles. The developed protocol represented an efficient photocatalyst and can
be used for the selective one-pot synthesis of imines under mild reaction conditions.
Declaration of interests
26

The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
Acknowledgment
Authors gratefully acknowledge Director IIP for granting Permission to publish these findings.
Analytical department of the institute is kindly acknowledged for analysis of the samples. DST
New Delhi is acknowledged for providing funding in the project GAP-3126.
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