Photochemical Oxidative Coupling of 2-Naphthols using a Hybrid Reduced Graphene Oxide/Manganese Dioxide Nanocomposite under Visible-Light Irradiation

Article abstract of DOI:10.1002/cctc.201701470

We describe a simple, cost-effective, efficient, and high-yielding photocatalytic approach for the oxidative self-dimerization of 2-naphthols using a semiconductor–metal hybrid that consists of intercalated manganese dioxide nanoparticles in reduced graphene oxide (rGO/MnO₂) under visible-light irradiation. The desired photocatalyst was synthesized in a single step by mixing MnO₂ nanoparticles with rGO ultrasonically. The hybrid photocatalyst exhibited a significantly higher activity than neat MnO₂ nanoparticles and rGO, which is believed to be because of the synergistic effect of its components. To our knowledge, this hybrid rGO/MnO₂ nanocomposite is the first heterogeneous, green photocatalyst for the oxidative coupling of 2-naphthols under mild conditions.

Full text of DOI:10.1002/cctc.201701470

Accepted Article
Title: Photochemical Oxidative Coupling of 2-Naphthols using Hybrid
rGO/MnO2 Nanocomposite under Visible Light Irradiation
Authors: Suman Lata Jain, Anurag Kumar, Aathira MS, and Ujjwal Pal
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Photochemical Oxidative Coupling of 2-Naphthols using Hybrid
rGO/MnO2 Nanocomposite under Visible Light Irradiation
Anurag Kumar,[a,b] Aathira M. S.,[a] Ujjwal Pal,[c] and Suman L. Jain*[a]
Abstract: The present paper describes a simple, cost effective,
efficient and high yielding photocatalytic approach for the oxidative
self-dimerization of 2-naphthols using a semiconductor-metal hybrid
consisting of intercalated manganese dioxide nanoparticles to
reduced graphene oxide (rGO/MnO2) under visible light irradiation.
The desired photocatalyst was synthesized in a single step by
mixing of MnO2 nanoparticles with reduced graphene oxide under
ultrasonication. The hybrid photocatalyst exhibited significantly
higher activity as compared to neat MnO2 NPs and rGO, which is
believed to be due to the synergistic effect of its components. To our
knowledge, hybrid rGO/MnO2 nanocomposite is the first
heterogeneous green photocatalyst for the oxidative coupling of 2-
naphthols under mild conditions.
Manganese dioxide (MnO2) is a well known semiconductor
material which has seevral fascinating properties like easy
preparation, inexpensive and low toxicity.[9,10] In addition the
lower band gap (0.9-1.2 eV) and larger surface area of MnO2
leads to its higher photocatalytic activity.[11] However the bare
MnO2 has a narrow band gap which tends to show absorption in
the infrared region. The composites prepared by the
combination of MnO2 with other semiconductors such as TiO2,
ZnO etc. has sufficient band gap to provide activity in the visible
light region.[12-13] Recently our group has established a concept
to immobilize the metal nanoparticles/complexes/ salts or oxides
to photoactive semiconductor supports either via in situ or post
grafting approach to develop high-performance photocatalysts
with the additional benefits of facile recovery and recycling of the
catalyst for various transformations.[14-16] Graphene oxide/
reduced graphene oxide, one of fascinating carbon
nanomaterials due to their extraordinary chemical, mechanical
Introduction
BINOL is optically active, and its derivatives have been
extensively used as chiral auxiliaries and ligands in asymmetric
synthesis, and shown extremely high stereo-control property in a
wide range of asymmetric transformations.[1,2] There are several
methods known for the oxidative coupling of 2-naphthols to give
BINOL using stoichiometric oxidants, as well as transition metal
based catalysts in conjunction with terminal oxidants.[3,4] Also,
very few photocatalytic asymmetric oxidative coupling reactions
of 2-naphthols to BINOL derivatives under visible light irradiation
are reported. Jana et al. reported β–MnO2 as a photocatalyst for
the oxidative coupling of 2-Naphthol under visible light
irradiations.[5] Irie and co-worker have reported the asymmetric
synthesis of chiral BINOL using chiral (NO)Ru(II)-salen complex
as catalyst under irradiation of visible light.[6] However, the
known methods suffer from certain drawbacks such as
expensive transition metal such as ruthenium, tedious recovery
and inefficient recycling due to homogeneous nature and poor
catalytic activity.[7,8]
and optical properties, has become widely used as
a
semiconductor for immobilizing various sensitizers including
metal NPs and metal complexes to develop improved
photocatalysts resulting from the synergistic combination of the
individual properties of the component materials.[17,18]
The present paper report a simple, cost effective, efficient
and high yielding photocatalytic approach for the oxidative self
dimerization 2-naphthols using
a
hybrid nanocomposite,
synthesized via in situ grafting of MnO2 nanoparticles to reduced
graphene oxide (rGO/MnO2) under visible light irradiation
(Scheme 1). Importantly, the chemical interaction between the
metal center of MnO2 and semiconductor support produces a
synergistic effect which provided higher activity, selectivity, and
durability to the hybrid photocatalyst.
[a]Anurag Kumar, Aathira M. S., Dr. Suman L. Jain
Chemical Sciences Division, CSIR-Indian Institute of
Petroleum, Dehradun India 248005. *Email:
suman@iip.res.in
[b]Anurag kumar,
Scheme 1. Oxidative naphthol coupling by using rGO/MnO2 as a photocatalyst
Academy of Scientific and Industrial Research (AcSIR), New
Delhi India 110001.
[c]Dr. Ujjwal Pal
Inorganic and Physical Chemistry Division, CSIR-Indian
Results and Discussion
Institute of Chemical Technology, Hyderabad-500007, India.
Synthesis and characterization of catalyst
The desired hybrid nanocomposite was obtained by grafting of
MnO2 NPs produced from KMnO4 on rGO support as shown in
Scheme 2.
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Scheme 2. Synthesis of rGO/MnO2 hybrid nanocomposite
The surface electronic state of elements and compositional
analysis of rGO/MnO2 was explained by X-ray photoelectron
spectroscopy. XPS analysis indicated the presence of C, O and
Mn elements in the composite material (Fig. 1a). High-resolution
XPS spectrum of rGO/MnO2 in the C1s region exhibited three
characteristic peak components at binding energies 284.1, 285
and 287.5 eV due to the C–C, C–O and O–C–O carbons,
respectively (Fig. 1b).[19] For the O1s spectrum (Fig. 1c), the main
peak was divided into two peaks located at 529.6 and 532.4 eV
related to Mn-O-Mn and Mn-O-H of MnO2, respectively.[20] Figure
1d represented the high-resolution spectrum of Mn 2p, in which
two peaks appeared at 653.6 and 642.3 eV with spin energy
Fig. 2 HR-TEM images of rGO and rGO/MnO2 a) and b) at 200 nm, c) at 100
nm, d) 50 nm and e) at 5 nm showing fringes, f) at 5 nm scale bar showing
interplaner d-spacing.
HR-TEM elemental mapping of rGO/MnO2 photocatalyst showed
that all the elements namely, carbon, oxygen, and Mn were
homogeneously distributed in the hybrid nanocomposite
rGO/MnO2. Furthermore, EDX pattern of rGO/MnO2 showed the
presence of all the constituent elements like C, O, Mn (Fig. 3).
SAED pattern of rGO/MnO2 confirmed the presence of MnO2
nanoparticles in the rGO/MnO2 hybrid nanocomposite (Fig. 3d).
The peaks for the copper were observed due to copper present
in carbon coated copper TEM grid.
separation of 11.38 eV attributed to Mn 2p3/2 and Mn 2p1/2
,
respectively.[21,22] These results were found to be in good
agreement with Mn(IV) characters in the phase of MnO2.
Fig. 1 High-resolution XPS spectra of rGO/MnO2 in a) survey spectrum, b)
C1s, c) O1s and d) Mn2p region
The morphology of the prepared samples was determined by
HR-TEM analysis (Fig. 2). The TEM images of rGO and
rGO/MnO2 at 200 nm scale showed the presence of layered and
crystallite fringes corresponded to 0.24 nm interplaner d-spacing
of 100 plane due to the presence of MnO2 nanoparticles in
graphitic sheets (Fig. 2a-f). The HR-TEM image of bare MnO2
NPs revealed nearly spherical shaped particles of size in the
range 16-20 nm (Fig. S1).
Fig. 3 HR-TEM Elemental mapping of rGO/MnO2 a) Carbon, b) Oxygen, c)
Manganese, d) SAED pattern and e) EDX Pattern showing elemental
composition
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Fig. 4 illustrated the FTIR spectra of rGO and rGO/MnO2. The
spectrum of rGO showed the absorption peak at 1570 cm-1 due
to the C=C of the graphene sheets and the absorption peak at
1223 cm-1 was attributed to C-OH stretching (Fig. 4a).[23] After
the intercalation of MnO2 in between the rGO sheets, a new
intense peak appeared at 558 cm-1 related to the Mn-O of MnO2
which confirmed the successful grafting of MnO2 nanoparticles in
between the rGO sheets (Fig. 4b).[24]
Figure 6 showed the UV-visible spectra of rGO and rGO/MnO2.
The UV spectrum of rGO exhibited a characteristic band at 260
nm due to π-π* transitions of aromatic C-C bonds indicating the
restoration of the extensive conjugated sp2 carbon network in
rGO. Also, a small hump around 350 nm due to n-π* transitions
of C=O bonds was observed. For the comparison, UV-Vis
spectrum of bare MnO2 NPs is shown in Fig. S2. As shown a
broad band in the range of 400 - 620 nm was observed which
attributed to the d-d transition of Mn (Fig. S2).[28] After the
intercalation of MnO2 NPs in between the rGO sheets in
rGO/MnO2, a new peak in the range 350-400 nm appeared.
Hence, the deduction in the characteristic peak of rGO and the
emergence of a new peak originating from MnO2 NPs clearly
indicated the interaction of MnO2 nanoparticles with graphene
surface.[29]
Fig 4. FTIR Spectra of a) rGO, b) rGO/MnO2.
The crystalline and phase structure of the synthesized material
was described by X-ray diffraction pattern (Fig. 5). XRD
diffraction pattern of rGO revealed an intense broad peak at
25.4° and a small peak at 43.6°, 2θ corresponding to (002) and
(111) planes confirmed the formation of rGO from graphene
oxide (Fig. 5a).[25] XRD pattern of MnO2 nanoparticles showed
two characteristic peaks at 17.3° (001) and 36.8° indexed to 220
plane.[26] In the case of rGO/MnO2 (JCPDS No. 42-1317), the
peak observed at 25.4° indexed to 002 planes with 0.24 nm
interlayer distance was attributed to the presence of rGO in the
nanocomposite (Fig. 5c). Other major peaks observed at 2θ =
36.8° and 66° (020) were due to the manganese oxide, which
was indexed to inter-atomic distances in materials consisted of
edge-sharing MnO6 octahedra.[27]
Fig. 6 UV/Vis absorption spectra of a) rGO, b) rGO/MnO2.
To evaluate the light absorption capacity of the materials, Tauc
plot was constructed for optical band gap determination (Fig. 7).
The band gap of MnO2 NPs was found to be 1.0 eV (Fig. 7a),
which is unsuitable for the absorption in visible light. However in
nanocomposite rGO/MnO2 the band gap 1.3 eV (Fig.7b)
indicated its higher activity in visible light.
Fig. 7 Tauc plot for band gap determination of a) MnO2; b) rGO/MnO2.
Raman spectroscopy is a powerful tool to characterize the
structure and electronic properties of the synthesized material.
The Raman spectrum of reduced graphene oxide (rGO) shows
two characteristic peaks, one at 1335 cm-1 for D band and
another at 1594 cm-1 for G bands (Fig. 8a). The D band is
associated with first-order Raman scattering of the E2g
Fig. 5 XRD Pattern of a) rGO, b) MnO2 NPs and c) rGO/MnO2.
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vibrational mode, and the G band corresponds to sp2 bonded
carbon atoms.[30] Hence the integral intensity ratio of D to G
bands (ID/IG) for rGO was found to be 0.98. In the case of
rGO/MnO2, both the D and G bands are found to be in slightly
reduced intensity. The peaks arise at 484, 579, 620 cm-1 shows
the presence of MnO2 NPs in the composite material (Fig. 8b).[31]
The characteristic peak arises at 620 cm-1 due to the Mn-O
lattice vibrations. The increased value of ID/IG in composite i.e.
1.2 indicated the presence of large number of defects and a
decrease in the average size of the in-plane sp2 domains of C-C
in the composite.[32,33]
Fig. 9 Adsorption desorption isotherm of a) rGO, b) rGO/MnO2 and c) MnO2
The thermal stability and degradation pattern of the samples
was determined by thermo-gravimetric analysis (TGA). Under
the air flow, TGA thermogram of rGO showed a weight loss in
between 100-280 °C due to the evaporation of adsorbed water
molecules and degradation of remaining oxygen-containing
groups on rGO sheets (Fig. 10a).[38] The thermogram of
rGO/MnO2 demonstrated two main weight loss regions; the first
weight loss of about 6.5% below 230 °C, was corresponding to
the interlayer water, while the second weight loss observed at
450 °C was corresponding to the phase transformation of MnO2
due to the loss of oxygen.[25]
Fig. 8 Raman spectra of a) rGO, b) rGO/MnO2.
The specific surface properties of synthesized samples were
determined by the adsorption and desorption of the nitrogen gas
on the surface of the material with the help of the BET theory of
multilayer adsorption and desorption (Fig. 9). The nitrogen
adsorption-desorption isotherm for rGO was of type-(IV) which
confirmed the mesoporous nature of the material. The higher
absorption capacity of rGO could be attributed to the larger
surface area of the material. The SBET and pore volume for rGO
were found to be 110.2 m2g-1 and 1.50 cm3g−1, and for
rGO/MnO2 the values were obtained as 165.2 m2g-1 and 1.17
cm3g-1, respectively.[27] The large surface area and lower pore
volume of composite rGO/MnO2 was believed due to the
intercalation of MnO2 NPs in between the sheets of reduced
graphene oxide, which led to the creation of enlarged gallery
spaces in between the rGO sheets.[34] In bare MnO2 NPs the
BET surface area was found to be 196.57 m2g-1, which is
significantly higher than the commercial MnO2
nanostructures reported in the literature [36,37]
[35]
and MnO2
Fig. 10 TGA diagram of a) rGO, b) rGO/MnO2.
The photo-catalytic activity
The photocatalytic activity of the synthesized rGO and hybrid
rGO/MnO2 was tested for the oxidative coupling of naphthols
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using toluene as a solvent under visible light irradiation at room
temperature under an oxygen atmosphere. The reaction mixture
was irradiated by using a 20W LED for 12 h under stirring
condition. The results of these experiments are summarized in
Table 1. In the absence of photocatalyst, there was no reaction
occurred even after prolonged time (24 h) of visible light
irradiation (Table 1, entry 1). The reaction was found to be very
slow and afforded only 30% conversion of 2-naphthol to 1,1’-
BINOL using rGO as a photocatalyst. However in case of
rGO/MnO2 composite, the reaction was occurred efficiently and
afforded almost quantitative yield of the desired product. Higher
activity of the composite suggested a synergistic effect of the
components on photo activity. In order to check the synergistic
effect of photocatalyst components on activity, we synthesized
three rGO/MnO2 samples using different amount of rGO (0.25,
0.5 and 1 g). The obtained composites were marked as
0.25rGO/MnO2, 0.5rGO/MnO2 and 1.0rGO/MnO2 and tested for the
oxidative coupling of 2-naphthol under identical conditions. The
activity of the composites was found to be in order
0.25rGO/MnO2<0.5rGO/MnO2>1.0rGO/MnO2 (Table 1, entry 3). The
poor activity of 0.25rGO/MnO2 attributed to the inefficient charge
mobility and fast recombination. The composite 0.5rGO/MnO2
showed maximum activity, which was assumed due to the
intimate contact between rGO and MnO2 that accelerates the
transfer of photogenerated electrons from MnO2 to rGO and
provides better charge mobility, effective charge anti-
recombination and higher photocatalytic activity. In 1.0rGO/MnO2
the slightly decreased activity was resulted to the less synergy
between photocatalyst components due to the presence of
excess rGO and isolated MnO2 NPs on the surface. Based on
these results we have considered 0.5rGO/MnO2 as the optimum
photocatalyst for further studies. In the absence of light, the
reaction gave only trace yield of the product using 0.5rGO/MnO2
photocatalyst, which indicated that the reaction is truly
photochemical in nature (Table 1, entry 3). The presence of
oxygen was found to be vital, and in its absence, no reaction
occurred even in the presence of 0.5rGO/MnO2 photocatalyst
(Table 1, entry 3). Furthermore, among the different solvents
such as acetonitrile, dichloromethane, and toluene studied,
toluene was found to be optimum reaction medium for this
transformation. For comparison, the reaction was also
Table 1. Effect of various parameters on the reaction [a]
S.No
.
catalyst
Visible
light
Oxidant
Conv
(%)[b]
[%]/t
1
2
-
Yes
O2
-
-
rGO
rGO + BQ
Yes
Yes
O2
O2
30
Trac
e
2.5
0.25rGO/MnO2
0.5rGO/MnO2
1.0rGO/MnO2
Yes
O2
72.8
97.2,
90.5
22.5
6.1
d
3
4
Yes, -c
Yes
O2,
O2
O2
-
8.1, -c, -d
7.5
rGO/MnO2
BQ
+
Yes
1.9
MnO2 NPs
MnO2 + BQ
Yes
Yes
O2
O2
42.5
8
3.5
[a]Reaction conditions: 2-naphthol 1.0 mmol, toluene 5 ml, room temperature,
time 12h visible light (λ >400 nm); [b]determined by GC-MS; [c]In the absence of
visible light; [d]in the absence of O2 under N2 atmosphere
BQ=p-benzoquinone (2%) was added as a radical scavenger
[%]/t= conversion vs time
Further, the reaction was generalized to various
substituted
naphthol
derivatives
using
0.5rGO/MnO2
photocatalyst under described optimized reaction conditions and
the results of these experiments are summarized in Table 2.
Among the various substrates, those bearing electron donating
groups were found to be more reactive towards the coupling
reaction and afforded excellent yields of corresponding coupling
products (Table 2, entry 1-4). In all substrates corresponding
1,1’-BINOL was obtained selectively without any evidence for
the formation of any by-product. In case of lower yield of the
desired product, unreacted substrate was recovered at the end
of the reaction (Table 2, entry 5-7).
performed using bare MnO2 NPs; however,
a very poor
conversion observed which is most likely due to the
agglomeration of NPs under the reaction conditions (Table 1,
entry 4).
Table 2: rGO/MnO2 catalyzed photochemical oxidative coupling of 2-
naphthols.[a]
Entry
Reactant
Product
Yield
(%)[b]
1
OH
OH
OH
97.0
92.0
OCH3
OCH3
OH
2
OH
OH
OCH3
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CH3
3
The exact mechanism for this transformation is not clear at this
stage, however, a plausible mechanism is shown in Scheme 3. It
is believed that reduced graphene oxide (rGO) due to the sp2
and sp3 carbon network acts as a semiconductor, and also
provide better charge separation and higher mobility to suppress
the recombination.[39] After the absorption of visible light
irradiation, the photogenerated electrons transfer from the
valence band to the conduction band of MnO2 NPs. These
electrons passed through the conjugated network of rGO
OH
OH
CH3
OH
96.0
98.0
CH3
4
H3CO
H3CO
OH
OH
H3CO
OH
Br
Br
5
•-
nanosheets and used for the conversion of O2 to O2
(superoxide radical).[40] The superoxide radical abstracts
hydrogen atom from naphthol radical cation to produce
hydrogen peroxide (H2O + ½ O2 = H2O2) as a byproduct. Self-
dimerization of the naphthol radicals produces corresponding
BINOL (Scheme 3). To confirm that a superoxide radical was
involved in the photocatalyzed oxidative coupling of 2-naphthol
OH
OH
OH
76.2
82.0
75.0
Br
COOCH3
OH
COOCH3
6
7
OH
OH
COOCH3
COOPh
reaction, p-benzoquinone (BQ), a well-known superoxide
COOPh
OH
scavenger was added to the reaction mixtures containing MnO2,
rGO and composite rGO/MnO2 under described conditions
(Table 1, entry 2-4). The desired product was obtained in
OH
OH
COOPh
significantly poor yield, indicating that the formation of a
[a]Reaction conditions: 2-naphthol (1.0 mmol), toluene (5 ml), 0.5rGO/MnO2
superoxide radical is an essential step in the present study.[41,42]
(100 mg), room temp., time 12h, visible light (λ >400 nm); [b]Isolated yield.
Furthermore, we evaluated the recycling of the photocatalyst by
considering the oxidative coupling of 2-naphthol under optimized
conditions as a representative example. After completion of the
reaction, the photocatalyst was separated by simple filtration,
washed with methanol, dried and reused in subsequent
experiments for five runs (Fig. 11). No significant loss was
observed in the activity of the recycled photocatalyst, and the
product yield remained almost unchanged, which confirmed that
the developed rGO/MnO2 photocatalyst is highly stable, leach-
proof and truly heterogeneous in nature. Furthermore, the used
photocatalyst recovered after fifth recycling experiment was
characterized by FTIR, XRD and HR-TEM analyses (Fig. S3-S5).
The almost similar data of recovered photocatalyst with fresh
one further confirmed the higher stability of the photocatalyst
under the described conditions.
Scheme 3. (a) Plausible mechanism of aerobic oxidative coupling of 2-
naphthol using rGO/MnO2 photocatalyst, (b) band position of the components
in the photocatalyst
Conclusions
An efficient semiconductor-metal hybrid photocatalyst consisting
of manganese dioxide nanoparticles intercalated in reduced
graphene oxide nanosheets was synthesized. The synthesized
photocatalyst has been successfully exploited as an efficient
photocatalyst for oxidative dimerization of 2-naphthols using
Fig. 11. Recycling experiment for oxidative coupling of 2-naphthol
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so obtained was filtered through a membrane filter (2 mm dia). The
resulting black reduced graphene oxide was dried in vacuum oven for
overnight.
molecular oxygen as oxidant. The hybrid photocatalyst exhibited
excellent synergistic effect due to its components and therefore
showed much higher activity as compared to neat MnO2 NPs
and rGO. The intercalation of metal nanoparticles in between the
rGO sheets provides a facile approach to synthesize economical
and active heterogeneous catalysts for photochemical
transformations.
Synthesis of in situ rGO/MnO2 hybrid nanocomposite[44]
After some modifications in situ rGO/MnO2 composite was synthesized.
0.5 g of reduced graphene oxide was dissolved in 20 ml solution of 1 M
KMnO4, and the volume of the mixture was adjusted to 50 mL with
ultrapure water. The pH of the reaction medium was controlled by buffer
tablet (pH=7.0), the mixture was sonicated for 30 min. After that, the
separated light black, brown colloid (rGO/MnO2) was collected by
centrifuge and washed 3-4 times with ultrapure water for the removal of
K+ ions present in the reaction mixture. Similarly, other rGO/MnO2
samples having different rGO (0.25 and 1 g) were prepared for the
comparative study.
Experimental Section
Materials
Graphite flakes, potassium permanganate (99%), sodium nitrate (99%)
and 2-naphthol was purchased from Sigma-Aldrich were of analytical
grade and used without further purification. Buffer tablets pH 7.0 was
purchased from Qualigens fine chemicals. Hydrazine mono hydrate,
hydrochloric acid, ethanol, toluene, hydrogen peroxide (30%), sulphuric
acid (H2SO4), water was of HPLC grade and procured from Alfa Aesar.
All other chemicals were of A.R. grade and used without further
purification.
Synthesis of pure MnO2 nanoparticles [26]
The synthesis of MnO2 nanoparticles occurred by a redox reaction of
KMnO4 and Mn(AC)2. First 0.01 mol KMnO4 and 0.015 mol Mn(AC)2 were
separately dissolved in two beakers that contained 50 mL distilled water.
Then the Mn(AC)2 solution stay at 60 oC under continuously and add the
prepared solution of KMnO4 dropwise. The temprature of mixture was
maintained at 60 oC under continuously stirring for 24 h and pH of
solution was adjusted to 7. After that, the solution was filtered and
washed with deionized water until the filtrate was colorless and at 100 oC
for 12 h.
Techniques used
X-ray photoelectron spectroscopy (XPS) measurement of photocatalyst
was carried out using ESCA+ (omicron nanotechnology, Oxford
instruments Germany) XPS system having an angle between analyzer to
the source is 90 o. All of the binding energies were referenced to the C1s
peak at 284.6 eV of the surface adventitious carbon. High-resolution
transmission electron microscopy (HR-TEM) images were recorded with
JEM 2100, Japan operated at an accelerating voltage of 200 kV and
lanthanum hexaboride was used as a source of electrons. A very dilute
suspension of the sample was deposited on the copper grid by sonication
method to collect HRTEM images. The images were processed by using
GATAN digital micrograph software. Fourier Transform Infrared (FT-IR)
spectra were recorded on Perkin-Elmer spectrum RX-1 IR
spectrophotometer using Potassium bromide window. The crystal
structure of the material was characterized by Bruker D8 Advance
diffractometer with Cu Ka radiation, (λ= 1.5418 Å) in the range of 2θ =
20-80 and a scan rate of 40 min-1. The sample for XRD was prepared on
a glass slide by adding well-dispersed catalyst in slot and drying properly.
UV-visible spectra of samples were collected on Perkin Elmer lambda-19
Photocatalytic oxidative 2-naphthol coupling experiment
For the photocatalytic oxidative coupling reaction, 2-naphthol (1.0 mmol),
toluene (10 ml) and photocatalyst (100 mg) were taken in a round
bottomed flask, and the resulting mixture was irradiated with visible light
(20 W LED) at room temperature with stirring for 12 h under oxygen
atmosphere. After completion of the reaction, the catalyst was separated
from reaction mixture via syringe filter. The obtained crude product was
purified by column chromatography on the SiO2 gel using ethyl acetate-
hexane (4:6) as eluent. The pure products were characterized by 1H
NMR and 13C NMR.
Acknowledgements
UV-VIS-NIR spectrophotometer using
a 10 mm quartz cell while
Authors would like to thanks, Director IIP for granting Permission to
publish these findings. AK is thankful to CSIR for providing research
fellowship for conducting research. Analytical department of Institute
is acknowledged for analysis of samples. DST New Delhi is kindly
acknowledged for providing financial assistance under the project
GAP-3125. The analytical department is kindly acknowledged for the
analysis of samples.
absorption spectra of solid samples were recorded by using fine powder
of BaSO4 as a reflectance standard. Raman spectra of samples were
collected at room temperature using a Raman Microprobe (HR-800
Jobin-Yvon) with 532 nm Nd-YAG excitation source. The specific surface
area of materials was analyzed by nitrogen adsorption desorption
isotherm at 77 K by using VP; Micromeritics ASAP2010. TG-DTA
analysis for calculating the thermal stability of samples was carried out
using a thermal analyzer TA-SDT Q-600 in the temperature range of 40
to 800 °C under nitrogen flow with a heating rate 10 °C/min. 1H NMR and
13C NMR of the reaction products were collected at 500 MHz by using a
Bruker Advance-II 500 MHz instrument.
Keywords: photocatalysis • 2-naphthol • hybrid nanocomposite •
oxidative coupling • manganese dioxide • visible light
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Entry for the Table of Contents
Full Paper
An economical and easily accessible
semiconductor-metal hybrid,
Anurag Kumar, Aathira, MS, Ujjwal Pal
and Suman L. Jain,
synthesized by grafting of MnO2 NPs
to reduced graphene oxide, has
been reported for the self-
dimerization of 2-naphthols using
oxygen under visible light irradiation.
The excellent synergy between
MnO2 with rGO support through
metal-π interaction make the
developed photocatalyst much more
active as compared to the individual
components.
Page No. – Page No.
Photochemical Oxidative Coupling
of 2-Naphthols using hybrid
rGO/MnO2 Nanocomposite under
Visible Light Irradiation
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