Grafting of oxo-vanadium Schiff base on graphene nanosheets and its catalytic activity for the oxidation of alcohols

Article abstract of DOI:10.1039/c2jm15644j

Graphene oxide was found to be a convenient and efficient supporting material for grafting of oxo-vanadium Schiff base via covalent attachment. The low dimensionality and rich surface chemistry of graphene oxide play critical roles in order to achieve a good degree of such grafting. Catalytic potential of the so prepared graphene-bound oxo-vanadium Schiff base and comparison with its homogeneous analogue was studied for the oxidation of various alcohols to carbonyl compounds using tert-butylhydroperoxide as oxidant. The structural and chemical nature of the catalyst was characterized by a variety of techniques including XRD, FTIR, TGA, TEM, and ICP-AES. The immobilized complex was found to be highly efficient and showed comparable catalytic reactivity as its homogenous analogue with the added benefits of facile recovery and recycling of the heterogeneous catalyst. The graphene-bound oxo-vanadium Schiff base was successfully reused for several runs without significant loss in its catalytic activity. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm15644j

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PAPER
Grafting of oxo-vanadium Schiff base on graphene nanosheets and its
catalytic activity for the oxidation of alcohols†
Harshal P. Mungse, Sanny Verma, Neeraj Kumar, Bir Sain and Om P. Khatri*
Received 3rd November 2011, Accepted 5th January 2012
DOI: 10.1039/c2jm15644j
Graphene oxide was found to be a convenient and efficient supporting material for grafting of oxo-
vanadium Schiff base via covalent attachment. The low dimensionality and rich surface chemistry of
graphene oxide play critical roles in order to achieve a good degree of such grafting. Catalytic potential
of the so prepared graphene-bound oxo-vanadium Schiff base and comparison with its homogeneous
analogue was studied for the oxidation of various alcohols to carbonyl compounds using tert-
butylhydroperoxide as oxidant. The structural and chemical nature of the catalyst was characterized by
a variety of techniques including XRD, FTIR, TGA, TEM, and ICP-AES. The immobilized complex
was found to be highly efficient and showed comparable catalytic reactivity as its homogenous
analogue with the added benefits of facile recovery and recycling of the heterogeneous catalyst. The
graphene-bound oxo-vanadium Schiff base was successfully reused for several runs without significant
loss in its catalytic activity.
Introduction
Despite the economical synthesis route and rich chemical
functionality, graphene oxide has not been extensively explored
for catalytic applications in facilitating useful chemical trans-
formations even though this area has immense potential.
Recently, some studies have demonstrated that transition metal
1
Since the invention of a two-dimensional carbon nanostructure,
known as ‘graphene’, a lot of developments have been made in
2–4
the area of materials science and engineering. Concurrently,
graphene oxide, an oxidized form of graphene, has emerged as an
important material due to its unique nanostructure and a variety
of fascinating characteristics such as very high specific surface
area with intrinsic low mass, ample oxygen carrying functional-
nanoparticles supported on graphene oxide nanosheets work as
17–20
efficient catalysts.
In most of these reports, graphene oxide
performs as an excellent supporting material and the presence of
various defects on nanosheets improves the catalytic activities. A
gold nanoparticle–graphene oxide composite exhibits signifi-
cantly higher catalytic activities in the reduction of nitroaniline
5–7
ities, and excellent mechanical strength. Graphene oxide based
materials carry immense potential not only for the fundamental
aspects but also for the prospect of various applications devel-
opment including composite materials, sensors, solar cells,
17
than the corresponding gold nanoparticles. Graphene oxide
impregnated with palladium nanoparticles works as a highly
18
active catalyst for the Suzuki–Miyaura coupling reaction. Zhou
8
–12
catalysis, and gas storage.
Among the known strategies to
prepare graphene-based materials, the use of graphene oxide
prepared by harsh oxidation of graphite is the most versatile and
et al. have reported that Au and Pt clusters on defective graphene
greatly enhanced the catalytic activity for CO oxidation by
19
reducing the reaction barrier potential. Iron nanoparticles
6
,13
easily scalable method.
Notably, graphene oxide exhibits
a hydrophilic nature owing to the wide range of oxygen carrying
supported on chemically derived graphene have been identified
as an effective catalyst for the hydrogenation of alkenes and
20
6,7
functional groups on its basal planes and edges. These func-
tional groups and the presence of the defects allow graphene
oxide to be readily functionalized with various surfactants,
polymeric materials, and nanoparticles in order to provide
enormous potential for applications in materials science and
alkynes. Harnessing the inherent catalytic activity of graphene
oxide has also become the focus of recent development. Bie-
lawski et al. have revealed the use of graphene oxide as an effi-
cient carbon catalyst for the synthesis of aldehydes or ketones
14–16
engineering.
21
from various alcohols, alkenes, and alkynes. They have also
found that graphene oxide works as an auto-tandem oxidation–
hydration–aldol coupling catalyst for the formation of chal-
2
2
Chemical Sciences Division, Indian Institute of Petroleum (CSIR - IIP),
Mohkampur, Dehradun, 248005, India. E-mail: opkhatri@iip.res.in; Fax:
cones. Recently, we have established that graphene oxide as
a catalyst facilitates the aza-Michael addition reaction very effi-
23
ciently in order to synthesize b-amino compounds.
Oxidation of alcohols to their corresponding carbonyl
compounds is one of the important reactions in synthetic organic
+
91 135 2660202; Tel: +91 135 2525767
Electronic Supplementary Information (ESI) available: FTIR and
†
TGA/DTA figures for the developed graphene immobilized catalyst
and intermediate products. See DOI: 10.1039/c2jm15644j
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2
4
ꢀ
chemistry. Apart from the conventional chromium(VI) and
mixture was raised to 98 C using an oil bath. After another 24 h,
the reaction mixture was diluted with another addition of water,
followed by addition of 30% H O . Finally, the brown oxidation
25
manganese oxidants, which produce copious amounts of
undesirable waste, a number of catalytic methods based on the
use of transition metals in conjunction with enviro-economic
oxidants such as molecular oxygen and hydrogen peroxide have
2
2
product was filtered off and purified by rinsing with 5% HCl
solution. The filtrate cake was repeatedly washed with copious
amounts of water to get rid of an excess acid. This processed dark
brown oxidized material, known as graphite oxide, was dried in
26
been reported in the literature. However, most of the existing
methods suffer from drawbacks such as effectiveness towards
activated and benzylic alcohols, difficult recovery and recycling
of the catalyst, lower catalytic activity, tedious synthesis of
catalysts, stringent reaction conditions, and lower yields of the
products. Thus, heterogenization of an active metal complex on
a high surface area material is receiving great interest for
ꢀ
an oven at 80 C. The dried product was ground to a fine powder
for further processing and characterization.
APTMS coated graphene oxide nanosheets
27
3-Aminopropyltrimethoxysilane (APTMS) was grafted on gra-
oxidation of various alcohols. In this context, a number of
effective metal complexes have been immobilized onto different
solid supports such as silica, polymers, clays, mesoporous
materials, activated carbon, zeolites, etc. and have been used as
recyclable catalysts for oxidation of various organic
36
phene oxide targeting their hydroxyl and carboxyl groups. Here,
000 mg of graphite oxide was adequately dispersed in 250 ml
1
toluene by using an ultrasonic probe. Subsequently, 3.0 ml of
APTMS was added to the graphene oxide dispersion and then the
reaction vessel was refluxed for 24 h under a nitrogen atmosphere.
As the reactionproceeded, the browncolourof the graphene oxide
dispersion eventually turned black. After reaction completion,
functionalized graphene nanosheets were washed with toluene to
remove excess/non-reacted APTMS. Toluene washing was
carried out four times and then ethanol was used for the final
washing. The black cake of APTMS coated graphene oxide
2
8–32
substrates.
A hydroxyl functionalized cobalt(II) Schiff base
complex immobilized on carbon nanotubes catalyzes the oxida-
tion of aliphatic and aromatic alcohols into the corresponding
33
carboxylic acids and ketones. An immobilized mixture of
a palladium–guanidine complex and guanidine on the meso-
porous cage-like material SBA-16 was found to work well for the
34
oxidation of benzylic alcohols and cinnamyl alcohol. However,
to the best of our knowledge there is no report on the use of
graphene oxide as a support for the grafting of a metal complex
in order to facilitate its recovery and recycling during an organic
transformation. Herein, we report the successful grafting of an
oxo-vanadium Schiff base on graphene nanosheets and then its
catalytic activity for the oxidation of various alcohols using
anhydrous tert-butylhydroperoxide (TBHP) as oxidant under
mild reaction conditions (Scheme 1).
ꢀ
nanosheets was dried in an oven at 70 C.
Grafting of oxo-vanadium Schiff base
The schematic model illustrating individual steps on the grafting
of oxo-vanadium Schiff base on graphene oxide nanosheets is
shown as Scheme 2. 1200 mg of APTMS coated graphene
nanosheets was dispersed in 250 ml ethanol using an ultrasonic
bath. This is followed by addition of 2400 mg of salicylaldehyde.
The reaction mixture was refluxed for 8 h under a nitrogen
atmosphere in order to execute the reaction between amino
groups of APTMS moieties and salicylaldehyde. After reaction
completion, four consecutive ethanol washings were performed
to remove the non-reacted salicylaldehyde. In the final step, 1000
mg of Schiff base immobilized graphene nanosheets was
dispersed in methanol and refluxed for 24 h in the presence of
2000 mg vanadium sulphate under a nitrogen atmosphere to
obtain the oxo-vanadium Schiff base on graphene nanosheets.
Then, the product was washed with methanol to remove the
undigested vanadium sulphate. Washing of the final product was
repeated four times and then it dried in an oven. This black
powder, graphene-immobilized oxo-vanadium Schiff base, was
used as a catalyst for the oxidation of alcohols, diols, and a-
hydroxyketones.
Experimental section
Synthesis of graphene oxide
Graphite powder, which is abundantly and economically avail-
able in nature, was used as a precursor to prepare a graphene
immobilized oxo-vanadium Schiff base catalyst through
a systematic chemical approach. First, graphene oxide was
35
prepared by harsh oxidation of graphite powder. In a typical
experiment, graphite powder, NaNO and H SO were mixed in
3
2
4
a reaction vessel on an ice bath. This was followed by gradual
addition of KMnO under a constant temperature and then the
4
reaction mixture was allowed to continue stirring at room
temperature for 24 h. Subsequently, a measured quantity of
water was slowly added and then the temperature of reaction
Oxidation of alcohols to carbonyl compounds
A round bottom flask containing a mixture of alcohol (1 mmol)
and graphene-bound oxo-vanadium Schiff base (0.01 mmol,
0
.2 g) in acetonitrile was kept on an oil bath under continuous
stirring. TBHP solution (1.8 M in decane, 1.5 mmol) was added
drop-wise to above mentioned reaction vessel and the mixture
ꢀ
was heated to 65 C. Progress of the reaction was monitored by
2
TLC (SiO ). At the end of the reaction, the catalyst was removed
Scheme 1 Oxidation of various alcohols in the presence of a graphene
by filtration and reused for subsequent recycling runs. The
filtrate was concentrated under reduced pressure. The residue
immobilized oxo-vanadium Schiff base catalyst.
5
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3
1
000UV, Leeman Labs). 10 mg of each sample was dissolved in
ml of concentrated HNO , heated and diluted to 10 ml. The
3
resulting solutions were filtered and subjected to analysis for V
content by ICP-AES analysis. Loading of vanadium in the
ꢁ1
prepared catalyst was found to be 0.05 mmol g .
Results & discussion
Preparation of the graphene immobilized oxo-vanadium Schiff
base catalyst and the intermediate steps were examined and
monitored by XRD, FTIR, TGA, and TEM analysis. Fig. 1
displays the XRD patterns of graphene oxide, APTMS coated
graphene nanosheets, and the graphene immobilized oxo-vana-
dium Schiff base catalyst. The exfoliated graphene oxide shows
ꢀ
a diffraction peak at 2q ¼ 11.62 with a corresponding d-spacing
ꢀ
of 7.65 A. The presence of various oxygen carrying functionalities
on the basal planes of graphene oxide layers and the trapped water
ꢀ
molecules between these layers increased the d-spacing from 3.4 A
5,13,38
ꢀ
in the pristine graphite powder to 7.65 A in graphene oxide.
Loading of APTMS on graphene oxide leads to disappearance of
ꢀ
the diffraction peak at 2q ¼ 11.62 and a new peak with a very
ꢀ
broad pattern around 2q ¼ 22 is observed, which is a little closer
ꢀ
38b
to reduced graphene (ꢂ2q ¼ 24 ). A similar pattern was found
for the graphene immobilized oxo-vanadium Schiff base catalyst.
The XRD pattern of graphene immobilized oxo-vanadium Schiff
base indicates that the developed catalytic material is almost
amorphous in nature with a very small fraction of multilayer,
which is revealed by a very weak and broad diffraction peak at
ꢀ
around 2q ¼ 22 as shown in Fig. 1.
Further, the chemical nature of graphene immobilized oxo-
vanadium Schiff base catalyst and its intermediate products were
revealed by FTIR. Their vibrational assignments are shown in
Table 1 (FTIR spectra are given as ESI†). An intense and broad
Scheme 2 (a) Schematic model of graphene oxide nanosheet and (b)
individual steps for synthesis of graphene immobilized oxo-vanadium
Schiff base.
ꢁ1
peak appeared at 3392.6 cm in the spectrum of graphene oxide,
attributed to the stretching mode of O–H bonds, revealing the
presence of many hydroxyl groups. These are major anchoring
2
thus obtained was purified by column chromatography (SiO ,
hexane/ethyl acetate, 6 : 4). Conversion of the reaction product
was determined by GC analysis. Identities of the products were
established by comparing their physical and spectral data with
ꢁ1
sites for APTMS. The strong band at 1724.1 cm (nC]O)
ꢁ1
37
represents carboxylic acid, and bands at 1222.0 cm and 1052.9
those of authentic samples.
ꢁ1
cm are attributed to the C–OH and C–O(epoxy) groups in
graphene oxide, respectively. The FTIR spectrum of APTMS
coated graphene oxide shows many new absorptions. In
Characterization of catalyst
X-Ray powder diffraction (XRD) analyses were carried out
using a Bruker D8 Advance diffractometer at 40 kV and 40 mA
with Cu-Ka radiation (l ¼ 0.15418 nm). The diffraction data
ꢀ
ꢀ
ꢀ
were recorded for 2q angles between 2 and 60 (step size: 0.02 ,
step time: 1 s). Inter-planar distances were calculated using the
Bragg equation. Vibrational spectra were recorded using
a Thermo Scientific Nicolet 8700 Research FT-IR spectropho-
ꢁ1
tometer with a 4 cm resolution. The graphene oxide powder,
the APTMS coated graphene nanosheets, and the final catalyst
powder were each mixed with potassium bromide (KBr) powder
to form pellets in order to record the FTIR spectra. Thermog-
ravimetric analyses (TGA) and differential thermal analyses
(
DTA) of these samples were performed using a thermal analyzer
TA-SDT Q-600. All samples were analyzed in the temperature
ꢀ
ꢀ
ꢁ1
range of 30 to 600 C at a heating rate of 10 C min under
nitrogen flow. Loading of vanadium in the graphene-bound oxo-
vanadium Schiff base was determined using an inductively
coupled plasma atomic emission spectrometer (ICP-AES, PS-
Fig. 1 XRD patterns of (a) graphene oxide, (b) APTMS coated gra-
phene nanosheets, and (c) graphene immobilized oxo-vanadium Schiff
base catalyst.
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ꢀ
Table 1 Infrared vibration frequencies of graphene oxide, APTMS
coated graphene nanosheets, and graphene immobilized oxo-vanadium
Schiff base catalyst with their vibrational assignments
Graphene oxide shows ꢂ18% weight loss near 100 C, evidently
owing to evaporation of water molecules which are held in the
material. The second significant weight loss is observed in the
37,39–42
ꢀ
range of 180–240 C, due to thermal decomposition of oxygen
APTMS
Graphene coated
Oxo-vanadium Schiff
base grafted on
graphene
13,43
carrying functionalities.
The APTMS grafted graphene oxide
oxide
graphene
Assignment
and the graphene immobilized oxo-vanadium Schiff base catalyst
ꢀ
both show a small weight loss around 100 C owing to the loss of
3
—
—
392.6
—
3442.7
—
—
3423.2
3036.6
O–H stretch
N–H stretch
C–H stretch
adsorbed water molecules, however, this mass loss is significantly
low compared to that of graphene oxide. The APTMS coated
graphene had two more significant mass losses. The first loss is in
(
aromatic)
C–H asym stretch
CH
C–H sym stretch
CH
C–H asym stretch
CH
—
—
—
2925.5
2854.1
2956.9
2933.0
—
ꢀ
the range of 180–240 C attributed to the decomposition of
(
2
)
undigested oxygen carrying functionalities, which have not
participated in interaction with APTMS. Then, another major
(
2
)
—
ꢀ
mass loss starts at 280 C owing to decomposition of APTMS
(
3
)
moieties. The thermogram of the graphene immobilized oxo-
vanadium Schiff base catalyst illustrates an exothermic major
1
1
—
—
1222.0
—
1
724.1
616.2
—
1631.5
—
1578.5
1200.3
1129.8
—
—
C]O stretch
C]C stretch
C]N stretch
N–H bending
C–O stretch
Si–O–Si stretch
C–O–C stretch
(epoxy)
Si–O–C stretch
C–H bending (ortho)
1655.6
1603.0
1511.0
1223.3
1111.6
—
ꢀ
weight loss over a wide range of temperature (180–600 C) on
44
account of slow decomposition of the Schiff base complex. The
transmission electron microscopy (TEM) image shown in Fig. 3
reveals the nanoscopic features of the graphene immobilized oxo-
vanadium Schiff base. The crumpled nanosheets are in an
agglomerated phase, however, the catalyst with very few layers of
graphene is seen in Fig. 3, as indicated by arrows. This truly
reflects the nanostructural features of the as-prepared graphene
based catalyst. The amorphous nature of the as-prepared catalyst
is corroborated by the roughened surface of these nanosheets.
The BET surface area of the graphene immobilized oxo-vana-
dium Schiff base catalyst, determined by nitrogen physisorption
052.9
—
—
1046.9
—
1036.4
760.4
ꢁ1
ꢁ1
particular, peaks at 3442.7 cm and 1578.5 cm reveal the
2
presence of –NH groups. The absorption peaks in the range of
ꢁ
1
2
800–3000 cm represent methylene and methyl groups of
ꢁ1
ꢁ1
APTMS. Peaks at 1129.8 cm and 1046.9 cm illustrate the
presence of Si–O–Si and Si–O–C bonds, respectively. The
appearance of these new peaks indicates the successful grafting of
APTMS onto graphene oxide nanosheets through chemical
bonding. The FTIR spectrum of the graphene immobilized oxo-
2
ꢁ1
at 77 K, was about 17 m g .
Catalytic oxidation
ꢁ1
The catalytic efficiency of the prepared graphene-bound oxo-
vanadium Schiff base and comparison with its homogenous
analogue was studied for oxidation of alcohols, both primary
and secondary, to the corresponding aldehydes and ketones
using anhydrous TBHP as oxidant (Scheme 1). The protocol
developed for the present reaction comprises the drop-wise
addition of TBHP solution to a stirred mixture of alcohol and
graphene-bound oxo-vanadium Schiff base in acetonitrile. After
completion of the reaction as monitored by TLC analysis, the
catalyst was separated by filtration and reused as such for
vanadium Schiff base catalyst shows a strong peak at 1603 cm
ꢁ1
attributing to C]N stretching. A peak at 760.4 cm represents
the ortho-substituted aromatic rings of the Schiff base. These
additional vibrational signatures confirm the successful grafting
of the oxo-vanadium Schiff base onto APTMS coated graphene
nanosheets by covalent interaction. The loading of Schiff base on
ꢁ1
the graphene oxide support (0.24 mmol g ) was estimated by the
nitrogen content determined by elemental analysis.
Fig. 2 illustrates TGA patterns for the graphene immobilized
oxo-vanadium Schiff base catalyst and intermediate products.
Fig. 2 TGA patterns of (a) graphene oxide, (b) APTMS coated gra-
phene nanosheets, and (c) graphene immobilized oxo-vanadium Schiff
base catalyst.
Fig. 3 TEM image of the graphene immobilized oxo-vanadium Schiff
base catalyst.
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subsequent runs. The filtrate was subjected to usual workup to
give the corresponding carbonyl compounds. Importantly, no
metal or ligand was detected in this course, indicating that no
leaching had occurred. Especially, selected filtrate samples were
subjected to ICP-AES, confirming that there was no leaching and
the reaction was truly heterogeneous in nature.
Table 2 Oxidation of benzhydrol under variable controlled experimental conditions
ꢀ
a
Entry
Catalyst (mol %)
T/ C
Oxidant
Time (min)
Yield (%)
1
2
3
4
5
—
1
2
1
1
65
65
65
RT
65
Anhyd.TBHP
Anhyd.TBHP
Anhyd. TBHP
Anhyd. TBHP
Aq. TBHP
120
20
15
120
20
—
94
96
50
80
a
Isolated yields.
a
Table 3 Oxidation of alcohols, diols and a-hydroxyketones
b
,
c
ꢁ1
Entry
Substrate
Product
Time (min)
20
20
20
Conv./Yield (%)
94
95
83/79
80
TOF (min
)
1
2
3
d
415
30
4
5
6
35
20
20
75
98/94
97/93
490
485
7
20
90
8
9
20
92/90
82/78
460
3
2
0
0
273.3
94
95
1
1
0
1
20
1
1
2
3
20
20
98/95
95
1
1
4
5
20
30
89
80/75
266.6
1
6
20
25
90
80
17
a
ꢀ
b
Reaction conditions: substrate (1 mmol), TBHP solution (1.8 M in decane, 1.5 mmol), catalyst (0.01 mmol, 0.2 g) in acetonitrile at 65 C. Conversion
c
d
was determined by GC. Isolated yield. By using homogeneous oxo-vanadium Schiff base as catalyst.
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Further, to optimize the reaction conditions we performed
some control experiments by using benzhydrol as a representa-
tive example. Results of these control experiments are summa-
rized in Table 2. The oxidation of benzhydrol with anhydrous
TBHP did not occur in the absence of catalyst even after pro-
longed reaction time (Table 2, entry 1). The effect of catalyst
concentration on the reaction is shown in Table 2, entries 2–3.
With increasing the catalyst concentration from 1 mol% to 2 mol%,
no significant change in the reaction rate was observed, indicating
the potential of the developed catalyst even at lower concentration.
The reaction was found to be slow at room temperature and
affordeddecreasedproductyieldwithlongerreactiontime(Table2,
entry 4). Similarly, the oxidation of benzhydrol was found to be less
efficient while using aqueous TBHP (70 wt% aq. solution) in terms
of the yield of the desired product (Table 2, entry 5). This is prob-
ably due to the non-selective decomposition of the oxo-vanadium
catalyst in the presence of water. We also compared the catalytic
efficiency of graphene-bound oxo-vanadium Schiff base with
a homogeneous analogue for the oxidation of benzhydrol under
optimized reaction conditions (Table 3, entry 1). The graphene-
bound oxo-vanadium Schiff base was found to be equally as effi-
cient as the homogeneous analogue. However, facile recovery and
efficient recyclingofthe heterogeneous catalystmake the developed
catalyst more effectual and valuable from both environmental and
economical viewpoints.
completion of reaction, the catalyst could easily be recovered by
filtration, dried and used for subsequent runs. Recyclability of
the catalyst was checked for six runs. In all the experiments, the
reaction time and yield of desired product remained almost the
same as shown in Fig. 4, establishing the efficient recycling of
the catalyst. Moreover, there was no signature of leaching of the
catalyst as revealed from the following reaction. A reaction
mixture containing catalyst and oxidant was refluxed for three
hours. After being cooled to room temperature, the catalyst was
recovered by filtration and the filtrate was charged with fresh
substrate and oxidant and the mixture was stirred under reflux
for three hours. Oxidation did not take place, indicating that
there was no leaching during the reaction and the reaction was
truly heterogeneous. This was further supported by ICP-AES
analysis of the recovered catalyst. The percentage of vanadium in
the recovered catalyst was found to be same as in the fresh
catalyst.
Conclusions
The rich surface functionalities on graphene oxide provide
enormous scope in order to develop this emerging material for
various applications including composites materials, heteroge-
neous catalysts, sensors, solar cells etc. The present work shows
excellent flexibility of graphene oxide as a support material for
the development of heterogeneous catalysts for chemical trans-
formations. The APTMS grafted graphene nanosheets were
prepared by chemical interaction between amino groups of
APTMS and oxygen functionalities available on graphene oxide.
Subsequent immobilization of an oxo-vanadium Schiff base on
the APTMS grafted graphene nanosheets was afforded via
covalent interaction as revealed by FTIR. The nano-structured
graphene oxide along with an excellent amount of hydroxyl
moieties allows the loading of the oxo-vanadium Schiff base.
XRD, FTIR, TGA, and elemental analysis revealed very good
loading of the oxo-vanadium Schiff base. The as-developed
graphene based heterogeneous catalyst was found to be inex-
pensive, convenient, and stable in ambient conditions. The
developed catalyst was noted as highly efficient for the oxidation
of various alcohols, diols, and a-hydroxyketones to carbonyl
compounds using TBHP as an oxidant. Moreover, the developed
catalyst was found to be easily recoverable and recyclable
without significant loss in the catalytic activity. This work, with
added benefits of flexibility in immobilization of various chem-
ical moieties and/or nanomaterials on graphene nanosheets,
offers new opportunities for designing highly active heteroge-
neous catalysts with environmental and economical benefits.
Furthermore, the oxidation of various alcohols, both primary
and secondary, has been examined using the described reaction
conditions. Results of these experiments are summarized in
Table 3. Aromatic substituted alcohols were found to be more
reactive as compared to the aliphatic/alicyclic secondary alco-
hols. Similarly 1,2-diols gave the corresponding 1,2-diketones in
high yields under the developed protocol. In the case of 1,2 diols,
we used 3.0 mmol of TBHP for better yield of the oxidized
products. It is worth mentioning that non-activated alcohols
such as borneol and l-menthol were also smoothly oxidized
under the described reaction conditions and afforded high yields
of the oxidized products (Table 3, entries 3 and 4).
Recycling of the graphene-bound oxo-vanadium Schiff base
was examined by selecting the oxidation of benzhydrol as
a model reaction under similar reaction conditions. After
Acknowledgements
We kindly acknowledge Director IIP for his kind permission to
publish these results. We are thankful to Dr Suman L. Jain for
kind technical input. Authors HPM and SV are thankful to UGC
and CSIR, respectively, for their research fellowships. We are
thankful to analytical division of IIP for providing the FTIR,
XRD, and ICP-AES analysis of samples. For the TGA and TEM
analysis, IACS, Kolkata and IICT, Hyderabad, respectively, are
kindly acknowledged.
Fig. 4 The reaction completion time and product yield for the oxidation
of benzhydrol in the presence of the oxidant and graphene bound oxo-
vanadium Schiff base for their different runs. After each reaction, the
catalyst was recovered and used for the subsequent run in order to
examine the heterogeneous nature of the developed catalyst.
5
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