Immobilized oxo-vanadium Schiff base on graphene oxide as an efficient and recyclable catalyst for the epoxidation of fatty acids and esters

Article abstract of DOI:10.1039/c4ra03454f

Oxo-vanadium Schiff base was covalently immobilized onto chemically functionalized graphene oxide (GO) using 3-aminopropyltriethoxysilane as a coupler. The loading of vanadyl Schiff base onto GO nanosheets was confirmed by FTIR, XRD, TGA, and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The synthesized heterogeneous catalyst was found to be efficient and selective for the epoxidation of fatty acids and esters using t-butyl hydroperoxide (TBHP) as an oxidant. Interestingly, the immobilized catalyst showed a higher catalytic efficiency than the homogeneous vanadyl acetylacetonate. The recycling experiment results indicated that the catalyst was highly stable and maintained very high activity, and selectivity even after being used for six cycles. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03454f

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Immobilized oxo-vanadium Schiff base on
graphene oxide as an efficient and recyclable
Cite this: RSC Adv., 2014, 4, 30598
catalyst for the epoxidation of fatty acids and esters
a
b
b
a
Sanny Verma, Mounika Aila, Savita Kaul* and Suman L. Jain*
Oxo-vanadium Schiff base was covalently immobilized onto chemically functionalized graphene oxide (GO)
using 3-aminopropyltriethoxysilane as a coupler. The loading of vanadyl Schiff base onto GO nanosheets
was confirmed by FTIR, XRD, TGA, and inductively coupled plasma-atomic emission spectroscopy (ICP-
AES). The synthesized heterogeneous catalyst was found to be efficient and selective for the epoxidation
of fatty acids and esters using t-butyl hydroperoxide (TBHP) as an oxidant. Interestingly, the immobilized
catalyst showed a higher catalytic efficiency than the homogeneous vanadyl acetylacetonate. The
recycling experiment results indicated that the catalyst was highly stable and maintained very high
activity, and selectivity even after being used for six cycles.
Received 16th April 2014
Accepted 20th June 2014
DOI: 10.1039/c4ra03454f
www.rsc.org/advances
Owing to the on-going price increase for fossil resources, their strategies have been reported to immobilize the homogeneous
uncertain availability, and due to growing environmental catalysts on insoluble support matrixes such as organic poly-
6
7
8
9
concerns, the biorenery concept has been developed to inte- mers, mesoporous silica, zeolites and clays.
grate the processes and technologies needed to convert biomass Recently, graphene oxide (GO), due to the presence of plenty
1
into chemicals and fuels. Among the known chemicals, epox- of oxygen functionalities, such as hydroxyl and epoxide groups
idized fatty acids and their derivatives have received consider- on both sides of its surface, has been acknowledged as one of
able interest owing to their widespread application, including the most promising supports to immobilize various homoge-
10
as additives in lubricants, as stabilizers in halogen-containing neous materials, including transition metal complexes. In this
11
polymers, and in the manufacture of cosmetics, and bioactive regard, Scheuermann et al. reported that Pd nanoparticles-
2
chemicals. The conventional method for the epoxidation of supported graphene oxide exhibited higher catalytic activity and
fatty acid compounds involves the use of peracids which are low palladium leaching for the Suzuki–Miyaura coupling reac-
either pre-formed or synthesized in situ from the reaction of an tion. Moreover, gold nanoparticles supported on GrO were
3
organic acid, i.e. acetic or formic acid with hydrogen peroxide.
found to be efficient catalysts for the reduction of o-nitroaniline
However, this process for epoxidation has certain drawbacks, to 1,2-benzenediamine with signicantly higher catalytic activ-
12
such as lower selectivity to the epoxidized products due to the ities than the corresponding gold nanoparticles. Mungse
13
acid-catalyzed ring opening of the epoxide ring, a difficult et al. recently reported the graing of an oxo-vanadium Schiff
separation of acidic by-products from the reaction mixture and base onto GrO nanosheets for the oxidation of alcohols, which
tedious handling procedures due to the use of highly-concen- showed good catalytic behavior.
trated hydrogen peroxide and strong acids. Thus, the develop-
In a continuation to our work on the development of green
ment of catalytic processes using inexpensive eco-friendly oxidation methodologies, herein, we report the synthesis, char-
oxidants is considered to be an attractive approach, from both acterization of an immobilized oxo-vanadium Schiff base onto
4
environmental and economical viewpoints. Conventional graphene oxide (VO–NH
2
-GO), and its use for the epoxidation of
epoxidation catalysts, including transition metal-based cata- fatty acids and esters using t-butyl hydroperoxide as oxidant
lysts using hydrogen peroxides and alkyl hydroperoxides, have (Scheme 1). The obtained heterogeneous catalyst VO–NH -GO
2
been extensively used for the epoxidation of unsaturated exhibited higher catalytic activity than the homogeneous oxo-
5
compounds. However, high costs, difficult recovery, and non-
recyclability of the catalyst make these catalysts undesirable for
large-scale application. As a consequence, many methods and
a
Chemical Sciences Division, CSIR-Indian Institute of Petroleum, Dehradun-248005,
India. E-mail: suman@iip.res.in; Fax: +91-135-2660202; Tel: +91-135-2525788
b
Biofuel Division, CSIR-Indian Institute of Petroleum, Dehradun-248005, India.
E-mail: skaul@iip.res.in
Scheme 1 Epoxidation of fatty acids and esters.
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15,16
ꢀ1
vanadium catalyst, coupled with the additional benets of a pure GO.
In addition, a broad peak at 3392 cm is attrib-
facile recovery and recycling ability of the catalyst.
GO with a high specic surface area along with plenty of ence of many hydroxyl groups. The peak at 1629 cm
oxygen functionalities decorated on both sides of the nano- attributed to the C]C stretching of sp -bonded carbons in the
uted to the stretching mode of O–H bonds, revealing the pres-
ꢀ
1
is
2
13
sheets was used as the support for the immobilization of oxo- graphene oxide. The FTIR spectrum of NH
2
-GO exhibited a
vanadium Schiff base using 3-aminopropyltriethoxysilane doublet at 2850 cm and 2917 cm , corresponding to the
APTES) as a coupler. The amino functionalization of the symmetric CH and asymmetric CH of the alkyl chains
support matrix using APTES for the graing of metal complexes assigned to the silane moieties of APETS, and conrms the
ꢀ1
ꢀ1
(
2
2
14
is well documented in the literature. The loading of amino successful graing of aminosilane moieties onto the GO
ꢀ1
propyl groups onto GO changes the elemental composition with support. Moreover, the appearance of bands at 1108 cm
the introduction of nitrogen and silicon. The loading of the 1037 cm and 1620 cm due to Si–O–Si, Si–O–C, and C–N
,
ꢀ
1
ꢀ1
17
amino propyl groups onto the graphene oxide support was vibration, respectively, provided more evidence for the
ꢀ1
18
found to be 1.5 mmol g , as determined by the nitrogen successful chemical functionalization. The FTIR spectra of
ꢀ1
content determined by elemental analysis (2.1% N). Amino- VO–NH
functionalized graphene oxide is labeled as NH -GO and oxo- to C]N stretching due to the Schiff base, indicating that the
vanadium Schiff base immobilized graphene oxide is labeled as chemical surroundings of the nitrogen atom have been changed
VO–NH -GO. The schematic presentation of the synthesis of and almost all of the –NH groups have reacted with acetyla-
heterogeneous vanadyl complex is shown in Scheme 2.
The BET surface area of synthesized GO, NH -GO and VO– 1600–1300 cm are due to C–O and C–N vibrations, indicating
2
-GO shows a new peak at around 1610 cm , attributed
2
2
2
19
cetone to form a Schiff base. Moreover, peaks in the region of
ꢀ
1
2
ꢀ1
2
NH -GO was found to be 85, 27, and 22.8 m g , respectively. that the Schiff base was successfully immobilized onto GO.
2
The chemical changes that occurred during the immobili-
zation of vanadyl acetylacetonate complex onto GO were
examined by FTIR (Fig. 1), showing the vibrational assessments
XRD studies
2 2
of GO (Fig. 1a), NH -GO (Fig. 1b) and VO–NH
-GO (Fig. 1c). The XRD patterns are used to study the changes in structure of
ꢀ1
strong bands at 1731, 1222.9, and 1035.5 cm , were attributed graphene oxide aer the immobilization of the oxo-vanadium
to the stretching modes of C]O, C–OH, and C–O–C, respec- complex (Fig. 2). The XRD spectrum of GO, as shown in Fig. 2a,
ꢁ
tively, revealing the presence of carboxyl, and epoxy groups in shows the characteristic diffraction peak at 10.8 , correspond-
ing to (001), which is due to the oxidation of graphite powder
leading to the introduction of various oxygen containing func-
20
tional groups. Aer amino functionalization and immobili-
zation of the oxo-vanadium Schiff base, this peak disappeared
ꢁ
and another broad diffraction of graphite (002) at 2q value ꢂ22
appeared, which is closer to the reduced graphene, indicating
that the major oxygen containing groups of GO have been
21
successfully functionalized.
SEM studies
The SEM images of GO and VO–NH
and b. It can be seen that GO and VO–NH
2
-GO are shown in Fig. 3a
-GO have a similar
2
Scheme 2 Schematic presentation of the synthesis of oxo-vanadium
Schiff base immobilized to graphene oxide (VO–NH -GO).
2
Fig. 1 FTIR spectra of: (a) GO; (b) NH
2
-GO; (c) VO–NH
2
-GO.
2
Fig. 2 XRD pattern of (a) GO; (b) VO–NH -GO.
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appearance, having two-dimensional nanosheets with wave-like carrying functionalities and APTMS moieties, which have not
22
structures. Furthermore, EDAX analysis (Fig. 3c) conrmed participated in further chemical functionalization. The last
ꢁ
the presence of vanadium in the synthesized heterogeneous major mass loss observed in the range of 360–470 C is related
2
4
catalyst.
The TG curves of GO and VO–NH
The samples were heated at a constant rate of 10 C min from VO–NH
to the slow decomposition of the VO-Schiff base complex.
2
-GO are depicted in Fig. 4. Based on the weight loss during the thermal decomposition of
-GO, the loading of Schiff base ligand in the synthe-
ow. The TG curve of GO shows the sized catalyst was found to be 0.52 mmol g . However, the
ꢁ
ꢀ1
2
ꢁ
ꢁ
ꢀ1
100 C to 800 C under N
2
ꢁ

rst weight loss in the range of 100–150 C, evidently owing to vanadium content as estimated by ICP-AES was found to be
the evaporation of water molecules trapped in the material. The 0.24 mmol g , indicating that the molar ratio of the Schiff base
ꢀ
1
second and signicant weight loss was observed in the range of and vanadium in the synthesized catalyst is nearly 2 : 1. Based
ꢁ
1
50–250 C and is assigned to the thermal decomposition of on the nitrogen content (0.76% N) in the synthesized catalyst as
labile oxygen-containing functional groups. The VO–NH -GO determined by elemental analysis, the loading of oxo-vanadium
2
ꢁ
23
ꢀ1
catalyst exhibited three mass losses up to 550 C. The rst Schiff base was found to be 0.54 mmol g . This value is in good
weight loss occurred between 100 and 250 C evidently owing to agreement with the TGA results.
ꢁ
the evaporation of physically adsorbed water. The second
ꢁ
weight loss in the temperature range from 250 to 450 C is
Catalytic activity
attributed to the thermal decomposition of undigested oxygen
The synthesized material was used as a catalyst for the oxidation
of fatty acids and their esters to the corresponding epoxides
using t-butyl hydroperoxide as the oxidant without any addi-
tional organic solvent. The experiments were carried out
batchwise in a 25 ml roundbottomed ask equipped with a
magnetic stirrer and reux condenser. In order to obtain the
optimum reaction condition, various parameters (reaction
time, molar ratio of substrate to TBHP, temperature and catalyst
concentration) inuencing its catalytic property were studied
using oleic acid as the representative substrate. The results of
these optimization experiments are summarized in Table 1.
The reaction time is found to show a signicant effect on the
conversion of the oleic acid to the corresponding epoxide. It is
observed that the conversion of oleic acid gradually increases
from 2 to 5 h. However, the reaction time has little effect on the
epoxide selectivity (Table 1, entries 1–3).
The effect of the molar ratio of the ethylene unsaturation/
TBHP ratio on the conversion to epoxide was studied by varying
the ratio from 1 : 1 to 1 : 2.5 (Table 1, entries 4–6). In all cases,
we obtained very high conversion and high selectivity for the
epoxide formation. This reects that the TBHP concentration
does not affect the reaction much and that the developed
Fig. 3 SEM image of (a) GO; (b) VO–NH
NH -GO.
2
-GO; (c) EDAX of VO–
method is more highly selective for the epoxide formation than
for C]C bond cleavage.
2
Reaction temperature: to determine the optimum reaction
temperature for the epoxdiation reaction, a series of experi-
ments was performed by employing the described procedure
and varying the reaction temperature (Table 1, entries 7–9). The
temperature has a remarkable effect on the selectivity of
epoxide. Amongst the three tested temperatures of 25, 40 and
ꢁ
6
5 C, a maximum conversion of oleic acid and a maximum
ꢁ
selectivity to epoxide was obtained at 65 C. A further increase in
ꢁ
the reaction temperature (to 80 C) reduced the epoxide selec-
ꢁ
tivity signicantly. Therefore, we considered 65 C as the
optimum temperature for the present study.
Catalyst amount: to nd the optimum catalyst concentra-
tion, a series of epoxidation reactions of oleic acid was per-
formed in the presence of graphene oxide immobilized oxo-
vanadium catalyst, by varying its amount in the range of 20–
ꢁ
Fig. 4 TGA curve of (a) GO; (b) VO–NH
2
-GO.
50 mg at 65 C (Table 1, entries 10–11). The epoxide yield was
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Table 1 Epoxidation of oleic acid under various reaction conditions in the presence of VO–NH
2
-GO catalyst
Molar ratio of
oleic acid: TBHP
Catalyst conc.
(mg)
Conversion
(%)
Selectivity for
epoxide (%)
ꢁ
Entry
Time (h)
Temp. ( C)
1
2
3
4
5
6
7
8
9
2
4
5
5
5
5
5
5
5
5
5
1 : 1.5
1 : 1.5
1 : 1.5
1 : 1
1 : 2.0
1 : 2.5
1 : 1.5
1 : 1.5
1 : 1.5
1 : 1.5
1 : 1.5
65
65
65
65
65
65
25
40
80
65
65
30
30
30
30
30
30
30
30
30
20
50
48.8
80.5
98.0
58.8
98.4
98.5
42.5
60.4
99.0
88.4
99.8
99.0
99.1
99.1
98.7
99.2
97.4
99.1
99.0
55.7
99.0
99.1
1
1
0
1
found to increase as the catalyst concentration increased from functionalities, which may be facilitating the reaction via
0 to 30 mg. A further increase in the catalyst concentration interactions with the substrate and TBHP on the surface.
does not inuence the epoxide yield signicantly. This is To verify the scalability of the process, the oxidation of oleic
2
probably due to the reason that every reaction achieves its acid was performed in a higher mmol (20 mmol) under iden-
optimum catalyst concentration value. Beyond that value, tical experimental conditions (see Experimental section). The
excessive catalyst does not affect the reaction rate to any conversion of oleic acid was found to be 98% with 99% selec-
25
signicant extent. Hence, the epoxidation reactions were tivity for the epoxide. However, the isolated yield of the product
carried out by employing 30 mg of catalyst as the optimum was found to be 97.4%. These ndings suggested that the
amount.
Based on these results, the optimized reaction conditions for
developed method can be used to scale up the process.
Reusability of the heterogeneous catalyst: reusability of the
the epoxidation of fatty acid and esters should be: catalyst solid catalyst is one of the most important criteria to make it
(
6
30 mg), substrate/TBHP (1 : 1.5), time 5 h and temperature viable at an industrial scale. To study the reusability of gra-
5 C. The reaction was further generalized to a variety of mono- phene oxide immobilized vanadium catalyst, the epoxidation of
ꢁ
and poly-unsaturated fatty acids and esters. The results of these oleic acid was performed by following the described procedure.
experiments are summarized in Table 2. All the substrates were Aer the reaction, the catalyst was recovered from the reaction
efficiently converted to the corresponding epoxides with excel- mixture by ltration, washed with ethanol, and then dried for
lent selectivity under the described experimental conditions. 24 h. The recovered catalyst was used for six consecutive runs by
The formation of the epoxide product was initially characterized following the same experimental conditions. The reused cata-
by the FTIR technique. The disappearance of the band at 3005 lyst was found to be equally active and afforded almost a similar
ꢀ
1
cm because of the –C]C– of the double bond in the FTIR conversion and selectivity of the epoxide, as shown in Table 3.
spectra of epoxidized products supports the consumption of These results conrmed that the developed catalyst is highly
double bonds, and the formation of epoxides during the reac- stable and can be recycled without signicant loss in catalytic
1
tion. Furthermore, in the H NMR spectra, the peak between activity. Importantly, no leaching of the metal could be detected
5.3–5.5 ppm due to the protons attached to –C]C– dis- during the experiments, as ascertained by ICP-AES analyses,
appeared, conrming the successful formation of epoxides. In indicating that the reactions were truly heterogeneous in
addition, the appearance of a multiplet in the range of 3.1–3.2 nature.
ppm because of epoxide protons conrmed the formation of the
Oxo-vanadium Schiff base complex was immobilized onto
respective epoxidized products. Unreacted peroxide was deter- graphene oxide support previously functionalized with 3-ami-
mined by iodometric titration. Blank experiments, in the nopropyltriethoxysilane. The prepared catalyst was character-
absence of catalyst, did not produce the epoxidized product ized by FTIR, XRD, SEM, TGA and ICP-AES. The immobilized
even aer a prolonged reaction time (12 h). Similarly, the oxo-vanadium catalyst was found to be an efficient and highly
oxidation of oleic acid was performed using homogeneous selective catalyst for the epoxidation of fatty acids and esters
vanadyl acetylacetonate under the described experimental using TBHP as an oxidant. In addition, developed heteroge-
conditions. The reaction using homogeneous catalyst was neous catalyst was found to be quite stable during the reaction
found to be slow, affording a lesser conversion of oleic acid and could be reused for six subsequent runs. The leaching
(92.5% with 96.5% selectivity of the epoxide) than heteroge- studies showed that there was no loss of vanadium during the
neous vanadium catalyst. The superior catalytic activity of the reaction, and the vanadium content in fresh and recovered
heterogeneous catalyst can be attributed to the promoting effect catalyst was found to be almost similar, as ascertained by ICP
of the graphene oxide support containing plenty of oxygen analysis. These results suggested that the graphene oxide
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a
2
Table 2 Epoxidation of fatty acids and esters using VO–NH -GO catalyst
b
b
c
Entry
1
Substrate
Product
Conv. (%)
EO selec. (%)
Yield (%)
98.0
95.4
99
99.1
99.0
99
97
94
97
2
3
4
5
80
98
98.4
99.1
78
96
6
7
99
98
99.2
99.0
97
96
8
98
99.1
97
a
ꢁ
b
Reaction conditions: substrate (1 mmol), molar ratio of substrate and TBHP (1 : 1.5), catalyst (30 mg), temperature 65 C, reaction time 5 h. The
c
conversion and selectivity for the epoxide were determined by GC-MS. Isolated yield; EO ¼ epoxide.
a
Table 3 Results on recycling the VO–NH
2
-GO catalyst
potassium bromide window. Scanning electron microscopy
SEM) images of the synthesized GO and modied GO were
(
Run
Conversion (%)
EO select. (%)
1
98
99.0
2
98.1
99.1
3
97.9
99.0
4
98
99.0
5
97.8
99.1
6
97.8
99.1
b
executed using FE-SEM (Jeol Model JSM-6340F). X-ray powder
diffraction patterns were recorded on a Bruker D8 Advance
diffractometer at 40 kV and 40 mA with Cu Ka radiation (l ¼
0.15418 nm) for conrming GrO synthesis and its functionali-
zation, and samples for XRD were prepared on a glass slide by
adding well dispersed catalyst into the slot and drying properly.
The surface properties like Brunauer–Emmet–Teller (BET)
b
a
Conditions: oleic acid (1 mmol), TBHP (1.5 mmol), catalyst (30 mg) at
ꢁ
b
6
5 C. Determined by GC-MS; EO ¼ epoxide.
immobilized oxo-vanadium Schiff base catalyst was truly surface area, Barret–Joyner–Halenda (BJH) porosity, pore
heterogeneous in nature.
volume of GO and modied GO were examined by N adsorp-
2
tion–desorption isotherms at 77 K by using VP; Micromeritics
ASAP2010. Thermo gravimetric analyses (TGA) of the samples
Techniques used
were carried out using a thermal analyzer TA-SDT Q-600. Anal-
ꢁ
ysis was carried out in the temperature range of 40–800
C
Fourier Transform Infrared Spectra were recorded on a Perkin-
Elmer spectrum RX-1 FTIR spectrophotometer using
ꢁ
ꢀ1
under a nitrogen ow at a heating rate 10 C min . For
a
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determining vanadium loading in the synthesized catalyst, ICP- and reux condenser. In a typical experiment, oleic acid
AES analyses were carried out by an inductively coupled plasma- (1 mmol), anhydrous TBHP in dodecane (1.5 mmol) and catalyst
atomic emission spectrometer (ICP-AES, DRE, PS-3000UV, Lee- (30 mg) were added to the ask, and the resultant mixture was
ꢁ
man Labs Inc, USA). Metal in 0.05 g catalyst was leached out continuously stirred at 65 C. Aer reaction, the solid catalyst
using conc. HNO
by adding distilled water.
3
, and the nal volume was made up to 10 ml was separated by ltration, washed with ethanol, and then dried
ꢁ
at 50 C for 24 h. The recovered catalyst was used in order to
investigate the stability and recyclability. The products of the
epoxidation were analyzed by a gas using a Varian capillary
column CP SIL 24CB and a FID detector. The conversion and
Synthesis of graphene oxide
Graphene oxide (GO) was synthesized from graphite akes selectivity of the product was determined by GC and the identity
26
using the modied Hummers method. In a typical experi- of the product was established by comparing its spectral data
ment, 46 ml concentrated H SO was added into a ask con- with the authentic sample.
2
4
taining graphite akes (2 g) and sodium nitrate (0.75 gm) and
ꢁ
the resulting mixture was cooled down to 0 C in an ice bath
Experimental procedure for scale up
under stirring. Then, KMnO (6.0 g) was added slowly to this
4
ꢁ
mixture at 0 C and the mixture was stirred for 5 days. 100 ml the process
ꢁ
diluted H
2
SO
4
(5 wt%) was added to this and heated at 90 C for
Oleic acid (20 mmol), anhydrous TBHP in dodecane (30 mmol)
and catalyst (600 mg) were added to the 250 ml round-bottomed
2
H
2
h with continuous stirring. A subsequent addition of 30 wt%
solution (approximately 5.4 ml), followed by stirring for
h at room temperature, afforded graphene oxide, which was
2 2
O

ask, and the resultant mixture was continuously stirred at
ꢁ
65 C for 5 h. Aer completion of the reaction, the solid catalyst
collected by centrifugation. The as-synthesized material was
subsequently washed with by 5% HCl and deionized water and
was separated by ltration, washed with ethanol, and dried at
ꢁ
ꢁ
50 C for 24 h. The conversion and selectivity of the product was
dried at 70 C under vacuum.
determined by GC and the identity of the product was estab-
lished by comparing its spectral data with the authentic sample.
The conversion of oleic acid was found to be 98% with 99%
selectivity for the epoxide. However, the isolated yield of the
product was found to be 97.4%.
Synthesis of amino-functionalized
graphene oxide (NH -GO)
2
The synthesized graphene GO (1.0 g, 30 ml) was taken in
anhydrous toluene, and then 3-aminopropyltrimethoxysilane
(
3-APTES, 2.5 mmol) was added under a N atmosphere. The Acknowledgements
2
ꢁ
resultant suspension was reuxed at 110 C under a nitrogen
atmosphere for 24 h. The resultant solid was washed with
toluene three times, and then the resultant amino-functional-
We kindly acknowledge Director IIP for his kind permission to
publish these results. Analytical Science Division of the Institute
is acknowledged for providing analyses of the samples. SV is
thankful to CSIR, New Delhi for his research fellowship. MA is
also thankful to CSIR, New Delhi for working as Technical HR in
XII Five Year project.
ized grapheme oxide (NH
2
-GO) was dried under vacuum oven at
ꢁ
50 C for 24 h.
Immobilization of oxo-vanadium Schiff
base to amino functionalized graphene
Notes and references
oxide (VO–NH -GO)
2
1
C. Briens, J. Piskorz and F. Berruti, Int. J. Chem. React. Eng.,
008, 6, R2, DOI: 10.2202/1542-6580.1674.
NH
VO(acac)
2
-GO (1 g) was dispersed in ethanol (30 ml), and then
2
ꢁ
2
(200 mg) was added. The mixture was heated at 65 C
2
(a) U. Biermann, W. Friedt, S. Lang, W. Luhs, G. Machmuller,
J. O. Metzger, M. R. Klaas, H. J. Schafer and M. P. Schneider,
Angew. Chem., Int. Ed., 2000, 39, 2206–2224; (b) S. Tayde,
M. Patnaik, S. L. Bhagt and V. C. Renge, Int. J. Adv. Eng.
Tech., 2011, II, 491–501.
for 3.5 h. Aer cooling the reaction mixture to room tempera-
ture, the heterogeneous graphene oxide immobilized catalyst
was separated by ltration and subsequently washed with
ethanol to remove the unreacted VO(acac) . The resultant solid
2
ꢁ
powder was dried at 50 C under vacuum and labeled as VO–
3
(a) S. Dinda, A. V. Patwardhan, V. V. Goud and N. C. Pradhan,
Bioresour. Technol., 2008, 99, 3737–3744; (b) C. Cai, H. Dai,
R. Chen, C. Su, X. Xu, S. Zhang and L. Yang, Eur. J. Lipid
Sci. Technol., 2008, 110, 341–346; (c) P. Lathi and
B. Mattiasson, Appl. Catal., B, 2007, 69, 207–212.
NH -GO. The loading of vanadium in the synthesized catalyst
2
ꢀ
1
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