Fractional distribution of graphene oxide and its potential as an efficient and reusable solid catalyst for esterification reactions

Article abstract of DOI:10.1002/poc.3375

Graphene oxide (GrO) prepared by the Hummers method was separated into three different fractions (GrO₅₀₀₀, GrO₂₀₀₀, and GrO_res) on the basis of their dispersion stability in the water. Infrared, nuclear magnetic resonance, X-ray photoelectron spectroscopy, and elemental analyses revealed that GrO₅₀₀₀ possesses a high degree of oxygen functionalities including phenolic, carboxylic, and -OSO₂H groups, compared with the other fractions. The GrO₅₀₀₀ was found to be a highly efficient and reusable solid catalyst for the esterification of various carboxylic acids with a variety of alcohols to furnish corresponding esters in high to excellent yields. The catalytic activity of the GrO₅₀₀₀ was attributed to the ability of highly polar GrO₅₀₀₀ scaffold to adsorb/attract reactants, where the acid functionalities of GrO₅₀₀₀ facilitated the esterification process efficiently. The chemical and structural features of GrO₅₀₀₀ were discussed to understand the improved catalytic activity compared with GrO₂₀₀₀ and conventional solid acid catalysts.

Full text of DOI:10.1002/poc.3375

Research Article
Received: 30 May 2014,
Revised: 17 July 2014,
Accepted: 27 August 2014,
Published online in Wiley Online Library: 6 October 2014
(wileyonlinelibrary.com) DOI: 10.1002/poc.3375
Fractional distribution of graphene oxide and
its potential as an efficient and reusable solid
catalyst for esterification reactions
a
a
a
Harshal P. Mungse , Niharika Bhakuni , Deependra Tripathi ,
a
a
a
Om P. Sharma , Bir Sain and Om P. Khatri *
Graphene oxide (GrO) prepared by the Hummers method was separated into three different fractions (GrO5000, GrO2000, and
GrOres) on the basis of their dispersion stability in the water. Infrared, nuclear magnetic resonance, X-ray photoelectron
spectroscopy, and elemental analyses revealed that GrO5000 possesses a high degree of oxygen functionalities including
phenolic, carboxylic, and ꢀOSO
2
H groups, compared with the other fractions. The GrO5000 was found to be a highly efficient
and reusable solid catalyst for the esterification of various carboxylic acids with a variety of alcohols to furnish corresponding
esters in high to excellent yields. The catalytic activity of the GrO5000 was attributed to the ability of highly polar GrO5000
scaffold to adsorb/attract reactants, where the acid functionalities of GrO5000 facilitated the esterification process efficiently.
The chemical and structural features of GrO5000 were discussed to understand the improved catalytic activity compared with
GrO2000 and conventional solid acid catalysts. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: catalyst; esterification; fractional distribution; graphene oxide
Oxygen functionalities in GrO provide strong oxidizing
INTRODUCTION
properties and catalyze the oxidation of alcohols, thiols, sulfides,
[
12–14]
Over the past few years, catalytic activities of carbon-based
materials have been exploited as economically viable and
sustainable alternative to homogeneous acid catalysts for esteri-
and olefines.
The inherent features of GrO associated to
distorted scaffold having various oxygen functionalities were
found to function as a carbocatalyst for aza-Michael addition of
amines to activated alkenes, Friedel–Crafts addition of indoles
to α,β-unsaturated ketones, and ring opening polymerization of
[
1,2]
fications, trans-esterifications, hydrolysis reactions, and so on.
However, most of these catalysts face challenges of low catalytic
activity, little affinity toward reactants, and poor recyclability. The
large surface area, fast and easy access to acid sites by the
reactants, and excellent recyclability with long-term sustainabil-
ity are important parameters for an efficient heterogeneous solid
acid catalyst. In this context, graphene oxide (GrO), an oxidized
and exfoliated product of graphite, shows immense potential
[
15]
various cyclic lactones and lactams. Recently, GrO has shown
very good catalytic activity for the oxidative coupling of amines
to imines, attributed to the synergistic effect of carboxylic acid
[
7]
groups and unpaired electrons in the GrO. Garcia et al. have re-
vealed GrO as an acid catalyst for the room temperature ring
[
16]
opening of epoxide. Sulfonated GrO prepared by sulfonation
with fuming sulfuric acid and sulfated GrO prepared by reacting
with 4-benzenediazoniumsulfonate exhibited good degree of
[
3,4]
as a versatile catalyst.
GrO, a distorted planarity structure of
hexagonal carbon, is a nonstoichiometric and hygroscopic
material and exhibits plentiful oxygen-containing functional
groups such as hydroxyl, phenolic, epoxide, carboxyl, carbonyl,
ꢀOSO
2
H groups. These materials function as efficient solid acid
catalysts for the hydrolysis of esters and the esterification
[5]
[17,18]
and lactol. Recently, high-resolution microscopy revealed that
GrO exhibits holes throughout a distorted scaffold, which are
reactions.
Herein, we address the catalytic activity of GrO
for esterification reactions. The GrO prepared by the Hummers
2
[6,7]
[19]
found to be in the range of 5 nm .
These holes accommodate
method
was fractionally distributed on the basis of their
oxygen functional groups. The degree, type, and distribution of
oxygen functional groups in GrO/graphite oxide (GO) are
monitored by the type of graphite precursor being used,
oxidizing conditions, reaction parameters, and purification
dispersion stability in water, and then, the catalytic activities of
GrO fractions were explored for esterification reaction. The
chemical and structural features of GrO have been discussed to
understand the catalytic activity.
[5,8–10]
procedure.
Therefore, depending on the degree and
distribution of these groups, the GrO shows little variations in its
behavior, when used for the chemical modification, reduction
process or as specific applications such as catalysts and polymeric
* Correspondence to: Om P. Khatri, Chemical Science Division, CSIR – Indian
Institute of Petroleum, Mohkampur, Dehradun 248005, India.
E-mail: opkhatri@iip.res.in
[
11,12]
composition formation.
In general, GrO is a complex structure
of distorted ultrathin sheets of carbon having a variety of oxygen
functionalities and void domains; such carbon scaffold as a whole
promises great potential as a heterogeneous catalyst for diversified
range of chemical transformations.
a H. P. Mungse, N. Bhakuni, D. Tripathi, O. P. Sharma, B. Sain, O. P. Khatri
Chemical Science Division, CSIR – Indian Institute of Petroleum, Mohkampur,
Dehradun 248005, India
J. Phys. Org. Chem. 2014, 27 944–951
Copyright © 2014 John Wiley & Sons, Ltd.

GRAPHENE OXIDE: A SOLID CATALYST FOR ESTERIFICATION
mixture was raised to 100 °C. An overhead condenser was used to pre-
vent the evaporative loss of water during the esterification process. Con-
version of 1-heptanol into 1-heptyl acetate ester was determined by Gas
Chromatography-Mass Spectrometry (GC-MS, HP5890GC with 5972 mass
selective detector). The GC oven was programmed with the following
temperatures: held at 50 °C for 2 min and then heated up to 300 °C with
the thermal rate of 12 °C/min. A 0.2 μL of each sample was loaded for MS
characterization under the helium flow of 1 mL/min. Progress of the reac-
tion as a function of time was monitored by analyzing the intermediate
product by GC-MS. The recyclability of GrO5000 catalyst was examined
by esterification of acetic acid with 1-heptanol as a model reaction. After
the completion of the reaction, the catalyst remained in the polar me-
dium (mixture of excess acetic acid and water generated during the es-
terification), which was recovered by the membrane filtration. The
recovered catalyst was then reused for the subsequent experiments. This
exercise was carried out for five subsequent runs.
EXPERIMENTAL SECTION
Preparation and fractional distribution of GrO
The GO, a precursor to GrO, was prepared by harsh oxidation of graphite
powder using a mixture of NaNO
reagents. The oxidized product, known as GO, was washed with 30%
and then with 5% HCl solution to remove the undigested and
3 2 4 4
, H SO , and KMnO as strong oxidizing
2 2
H O
excess content of oxidizing reagents. Subsequently, the GO was repeat-
edly washed with pure water. This was followed by sonication of the GO
in water using an ultrasound processor (power: 500W, intensity: 60%, and
diameter of ultrasonic probe tip: 13mm) for exfoliating into the GrO. Then,
the dispersion of GrO was centrifuged at 5000 rpm for 30 min, which leads
to two distinct phases: upper phase containing dispersible fine sheets of
GrO and lower deposition constituting the semi-solid phase of GrO. The fine
fraction (upper phase) of GrO is named as GrO5000 throughout the manu-
script. In the next step, the deposited GrO materials were re-sonicated for
5
min, and then, this dispersion was centrifuged at 2000rpm for 30 min.
This leads to the upper phase having dispersed content of GrO (hereafter
known as GrO2000) and deposited material (hereafter known as GrOres) at
the bottom of centrifuge tubes. All fractions of GrO were repeatedly washed
with copious amount of water for removing acid traces and oxidized debris.
This was confirmed by the reaction of the filtrate with standard solution of
RESULTS AND DISCUSSION
Degree and distribution of oxygen functionalities, particularly
acid sites in GrO, play important roles in acid catalytic reactions.
Hence, the chemical structure of GrO was thoroughly analyzed
barium chloride salt. Finally, three different fractions of GrO (GrO5000
GrO2000, and GrOres) were dried in an oven prior to their characterization.
The three components of graphene oxide, GrO5000, GrO2000, and GrOres
,
13
by FTIR, C SSNMR, and XPS. The transmission FTIR spectra of
GrO5000, as shown in Fig. 1(a), exhibit a strong and broad vibra-
,
ꢀ
1
tion peak centered at 3416 cm attributing to the O–H stretch,
which revealed the presence of hydroxyl and phenolic groups
in the materials. The other characteristic vibrational signatures
were found in 6.875:3.875:1.000 ratio.
Characterizations of GrO
ꢀ1
ꢀ1
at 1726 cm (attributed to C¼O stretch), 1355 cm (ascribed
Fourier transform infrared (FTIR) spectra for all the samples were
recorded using a Nicolet 8700 Research spectrophotometer (Thermo
to an overlapped peak of O–H bending and S¼O asymmetric
ꢀ
1
stretch), 1265–1220 cm (attributed to the C–O stretch owing
ꢀ1
13
ꢀ1
Scientific, USA) with a resolution of 4 cm . A C solid-state nuclear mag-
netic resonance (SSNMR) spectrum of GrO5000 was measured at room
temperature using a Bruker DSX 300 SSNMR spectrometer. X-ray photo-
electron spectroscopic (XPS) measurement of GrO5000 was conducted
on ESCA + (Omicron NanoTechnology, Germany) spectrometer using
Mg Kα beam as an X-ray source. High-resolution transmission electron
microscopic images of GrO5000 were captured on a JEM 3010 (JEOL,
Japan) electron microscope with 300 kV acceleration voltage. X-ray pow-
der diffraction analysis of GrO5000 was carried out using a Bruker D8 Ad-
vance diffractometer (Bruker AXE GmbH, Germany) at 40 kV and 40 mA
with Cu Kα radiation (λ = 0.15418 nm). Thermogravimetric analyses of
GrO5000 were carried out in the temperature range of 30 to 550 °C under
nitrogen flow using a thermal analyzer (Diamond, PerkinElmer, USA). The
C, H, N, S, and O contents of all samples were determined using a
PerkinElmer CHNS/O system. The elemental analyses were carried out
using 4 mg of grounded powder of each sample. The given results are
an average of three different measurements for each sample. The
Brunauer-Emmett-Teller (BET) surface area of GrO5000 was measured
using the ASAP 2010 surface area and porosity analyzer, Micromeritics
Instrument Corporation, USA. The surface area measurement was
conducted at ꢀ203 °C, where the sample was degassed at 250 °C prior
to measurement. The total acidity of all catalysts was determined using
a back-titration method. In a typical procedure, 0.1 g of sample was
added in a vessel containing 60 mL of 0.008 M NaOH, and the mixture
was stirred for 30 min. An excessive amount of NaOH was used in order
to completely neutralize all acidic functional groups in the sample. The
excess content of NaOH was then titrated back using 0.02 M HCl as the
titrant and phenolphthalein as an indicator.
to phenols, epoxy, and ether groups), and 1064 cm (associated
to a combined signature of the C–O stretch of hydroxyl group
and S¼O symmetric stretch) revealed the presence of hydroxyl,
[7,20]
phenolic, carboxyl, and ꢀOSO H groups in the GrO
.
The
2
5000
harsh oxidation of graphite powder in the presence of H SO
2
4
introduces the ꢀOSO
2
H groups at different sites of GrO5000 as
[
21]
revealed by the presence of νas and ν
s
S¼O signatures.
The
ꢀ
1
intensity of the carboxyl group peak (at 1726 cm ) was reduced
by ~23% in GrO2000, compared with that of GrO5000 (Fig. 2). This re-
vealed that GrO5000 exhibits more carboxyl groups, which are
mostly located at the edges and in the void domains of GrO5000
nanosheets. Besides that, GrO5000 (Fig. 2) showed slightly higher
intensities for other characteristic oxygen functional groups
(
ꢀOH, C–O, C–O–C, ꢀOSO
2
H). These results were further supported
by the elemental analyses of GrO5000 and GrO2000. The compara-
tively higher weight percent of oxygen and sulfur elements in
GrO5000, as shown in Table 1, revealed the presence of more acidic
sites. Therefore, herein, GrO5000 has been selected for further study.
The characteristic oxygen functionalities, as elucidated by
13
FTIR, were further corroborated by the C SSNMR spectrum
(Fig. 1(b)), which shows strong nuclear resonance shifts at 60.2
(
C–O–C) and 71.1 ppm (C–OH), attributable to the high degree
[7,22]
of epoxide and hydroxyl groups, respectively.
A broad reso-
nance shift at 131 ppm and a shoulder at 145 nm are assignable
2
to the sp carbon skeleton and phenolic (C–OH) groups, respec-
tively. The resonance shift owing to C–OSO H (approximately
2
1
40 ppm) could not be distinguished because of obstruction by
2
Catalytic potential of GrO
a broad peak of sp carbon skeleton and a shoulder of phenolic
groups. Hence, the presence of ꢀOSO H group was confirmed
2
The potential of different fractions of the GrO as solid catalysts was
examined for esterification reaction. In a typical experiment, a 100-mL
round-bottom flask was filled with acetic acid (0.11 mol) and 1-heptanol
by the appearance of S 2p peak at 168 eV in the XPS spectrum
(Fig. 1(d)) of GrO5000. The C 1s XPS spectrum of GrO5000 showed
(
0.021 mol). Twenty milligrams (0.8 wt %) of GrO5000 as a catalyst was
an overlapped double peak structure with small tail toward
higher binding energy, revealing that the GrO5000 scaffold is
added to the reaction mixture, and then, the temperature of the reaction
J. Phys. Org. Chem. 2014, 27 944–951
Copyright © 2014 John Wiley & Sons, Ltd.
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H. P. MUNGSE ET AL.
13
Figure 1. Chemical and structural characterization: (a) Fourier transform infrared and (b) C solid-state nuclear magnetic resonance spectra of GrO5000
X-ray photoelectron spectroscopic spectra of GrO5000 in the range of (c) 293–280 and (d) 180–158eV, illustrating the C 1s and S 2p peaks, respectively.
e)-(f) Transmission electron microscopic image of GrO5000 at low and high resolutions, revealing the wrinkled and crumpled features of nanosheets
.
(
bonded to different types of oxygen functionalities. The C 1s spec-
trum (Fig. 1(c)) could be deconvoluted into four chemically shifted
components at 284.6, 286.5, 288.0, and 289.5 eV, which are
assigned to C–C/C¼C, C–OH/C–O–C, C¼O, and O¼C–OH groups,
wrinkled and crumpled features in the transmission electron
microscopic image of GrO5000 nanosheets. The sp carbon
3
centers associated to the oxygen functionalities and various
structural defects in the basal plane of GrO5000 nanosheets
disturbed the two-dimensional structure, resulting in a rough-
ened surface with many folded and crumpled features. Recently,
aberration-corrected high-resolution transmission electron
microscopic images of GrO have shown a highly inhomoge-
neous structure, having three major regions: holes, graphitic
[23]
13
respectively. Collectively, the FTIR, C SSNMR, and XPS results
revealed the presence of hydroxyl, phenolic, carboxylic acid,
ꢀ
OSO H, ketonic, and epoxides functionalities in the GrO5000
The X-ray powder diffraction pattern of GrO5000 shows a broad
.
2
diffraction peak at 2θ angle of 11.5°, attributed to the 002 plane
Supporting Information, Fig. S1) corresponding interlayer
2
3
(
domains (sp domain), and disordered regions (a blend of sp
2
distance of 7.7 Å. The higher interlayer distance is associated to
the presence of oxygen functionalities and adsorbed water
molecules in the basal plane of GrO5000. Figure 1(e) shows the
and sp carbons, indicating areas of high oxidation) with approx-
imate areas of 2%, 16%, and 82%, respectively. Scanning
tunneling microscope images of GrO have also revealed the
[
6]
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J. Phys. Org. Chem. 2014, 27 944–951

GRAPHENE OXIDE: A SOLID CATALYST FOR ESTERIFICATION
Figure 3. A schematic model of GrO5000 revealing the distribution of ox-
ygen functionalities
The potential of GrO5000 as a solid catalyst was examined for
the esterification reaction. In a typical experiment (Scheme 1),
acetic acid (0.11 mol) was treated with 1-heptanol (0.021 mol)
in the presence of GrO5000 (20 mg) as a catalyst at 100 °C for
2 h. In the subsequent step, the catalyst was recovered, and the
crude reaction mixture was washed with water to remove the
excess content of acetic acid. The extracted product afforded
81% 1-heptyl acetate, as determined by GC-MS, and no by-
product was detected. Further, efforts were made to optimize
the reaction time, reaction temperature, and catalyst quantity
(Table 2). Figure 4 shows the changes in the conversion of
1-heptanol into 1-heptyl acetate over the reaction time at 100 °C.
The product yield has three distinguishable steps over the reaction
time. During the first hour, the reaction proceeded fast and
Figure 2. Fourier transform infrared spectra of GrO5000 (black line) and
GrO2000 (red line), exhibiting characteristic oxygen functionalities
presence of hole sites along with oxygen-carrying groups, which
function as active catalyst for the oxidative coupling of amines to
[7]
imines. On the basis of spectroscopic and microscopic results
and literature evidences, we depict a structural model (Fig. 3) for
GrO5000, where acidic functionalities such as ꢀOSO H, carboxylic,
2
and phenolic groups are placed throughout the nanosheets.
Prior to catalytic reactions, the acidic sites in GrO5000 were
examined by back-titration method and estimated as 4.3 mmol/g,
owing to the presence of ꢀOSO H, carboxylic, and phenolic
2
groups. An amorphous carbon-based solid acid catalyst prepared
by sulfonation of microcrystalline cellulose has shown good
[
2a]
degree of acidic sites (4.3 mmol/g),
which is equal to the acidic
sites in the GrO5000. The amorphous carbon-based solid acid
catalyst possessed 4.8% sulfur and had an acidity of 2.0, 1.9,
and 0.4 mmol/g, associated to the phenolic OH, ꢀOSO H, and
2
ꢀ
COOH groups, respectively. In contrast, GrO5000 prepared
herein has comparatively low sulfur content (0.66%), as revealed
by elemental analysis; however, it still exhibits high acidity. An
excellent hydrophilic nature and a very high surface area
2
(
103.7 m /g) of GrO5000 nanosheets facilitate their dispersion in
aqueous and polar solvents, and provide homogeneous disper-
sion. The exposure of acidic sites to reactants becomes easier
and faster in such homogeneous dispersion, and as a result,
GrO5000 can function as an efficient solid catalyst for chemical
transformation. Moreover, the presence of nonbonded π electron
2
owing to the discrete units of sp carbons in GrO5000 synergisti-
[
7]
cally supports the catalytic activation by spin-flip process.
Scheme 1. Schematic view of GrO5000-catalyzed esterification reactions
Table 1. Elemental distribution of GrO5000 and GrO2000 along with calculated empirical chemical formula for each sample
Element (wt %)
Sample
C
H
O
N
S
Chemical formula
Reference
GrO5000
GrO2000
GO
GO
GO
44.31
46.26
47.16
61.04
—
3.29
3.17
2.68
1.85
—
51.70
49.92
40.57
37.11
—
0.04
0.03
<0.5
—
—
0.0
0.66
0.62
1.2
—
—
2.0
C1.00H0.89O0.87
C1.00H0.82O0.81
—
—
22
24
5a
10
C
C
C
C
1.00
1.00
1.00
1.00
H
0.68
H
0.36
H
0.20
H
0.55
O
0.65
O
0.45
O
0.49
O
0.75
GO
48.0
2.2
47.8
Entries 3–6 are extracted from the published reports.
J. Phys. Org. Chem. 2014, 27 944–951
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H. P. MUNGSE ET AL.
a
Table 2. Esterification of acetic acid with 1-heptanol using GrO as a solid acid catalyst under various reaction conditions
b
Reaction variations
Type of catalyst
Entry Catalyst Catalyst quantity (mg) Temperature(°C) Reaction time(h) Conversion (%)
1
2
3
4
5
6
7
8
9
0
1
GrO
GrOres
GrO2000
GrO5000
Nil
GrO5000
GrO5000
GrO5000
GrO5000
GrO5000
GrO5000
GrO5000
GrO5000
GrO5000
GrO5000
20
20
20
20
—
10
20
20
20
20
20
20
20
20
20
100
100
100
100
100
100
100
100
100
100
100
21
4
4
4
4
4
4
4
1
2
6
8
4
4
4
4
65
37
74
98
18
54
98
62
81
98
99
3
Effect of catalyst quantity
Effect of reaction time
1
1
Effect of reaction temperature
12
1
1
1
3
4
5
60
80
100
13
41
98
a
All reactions were conducted using 0.021 mol of 1-heptanol and 0.11 mol of acetic acid in the presence of the GrO catalyst. The
catalyst loading, reaction temperature, and reaction time are indicated for each entry.
b
Conversion of 1-heptanol was monitored by GC-MS.
the quantity of catalyst (Table 2, entries 6 and 7). At low quantity
(
10 mg) of the catalyst, the reaction could be completed by 15 h
with 98% conversion, whereas at high loading of the catalyst
20 mg), the reaction completed within 4 h affording 98% conver-
(
sion (Fig. 4). These results indicate that GrO5000 as a solid catalyst
functions efficiently for esterifications. Importantly, the reaction
between 1-heptanol and acetic acid without using GrO5000 as the
catalyst was found to be very slow and afforded 18% conversion
(Table 2, entry 5). The temperature also plays a significant role in
the esterification of 1-heptanol (Table 2, entries 12–15). At low
temperature (21 °C), only 3% of the product was yielded, which
increases with increasing reaction temperature, and maximum
conversion (98%) was obtained at 100 °C. It is worth to mention
that GrO, a blend of all fractions, showed low catalytic activities
with a conversion rate of 65%, whereas GrO5000, GrO2000, and
GrOres afforded 98%, 74%, and 37% conversions, respectively,
under identical reaction conditions (Table 2, entries 1–4).
Figure 4. Changes in conversion of 1-heptanol over the reaction time
using (a) 10 and (b) 20 mg of GrO5000 as a solid acid catalyst
The catalytic activities of GrO5000 and GrO2000 were compared
(
(
Table 3) with conventional solid acid catalysts Amberlite-150
polystyrene-based cation-exchangeable resin having –OSO
2
H
afforded 62% conversion. In the next stage (1–4 h), the reaction
rate gradually decreased as the product yield increases, and
then, no more significant changes over the reaction time occurred.
Esterification of acetic acid is found to be markedly influenced by
groups) and Amberlyst-15. The acidities of these catalysts were
determined by acid–base titration method. Table 3 illustrates
that Amberlite-150 exhibited low acidity than Amberlyst-15
and GrO5000. However, GrO5000 showed the highest catalytic
a
Table 3. Catalytic activities of various catalysts for the esterification of acetic acid with 1-heptanol
Catalytic activity
ꢀ
1
Entry
Catalyst
Acidity (mmol/g) Rate of heptyl acetate (mmol/g.min) ToF (min
)
Conversion (%)
1
2
3
4
GrO5000
GrO2000
Amberlite-150
Amberlyst-15
4.30
3.84
4.04
4.42
22.46
16.96
21.54
20.85
5.2
4.4
5.3
4.7
98
74
94
91
a
Esterification conditions: 1-heptanol (0.021 mol), acetic acid (0.11 mol), catalyst (20 mg), and temperature 100 °C.
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J. Phys. Org. Chem. 2014, 27 944–951

GRAPHENE OXIDE: A SOLID CATALYST FOR ESTERIFICATION
conversion among these catalysts and also showed marginal
differences with Amberlite-150 and Amberlyst-15. The efficient
catalytic activity of GrO5000 could be associated to its unique
structure and having acidic functional groups tolerable to ester-
ification reactions. The turn over frequency (ToF) of the GrO5000
was found to be nearly equal to that of Amberlite-150. However,
considering the environment perspective, GrO5000 could be a very
good alternative to the conventional solid acid catalysts. The acidic
sites in GrO2000 were noted to be ~10% lower than that in GrO5000
alcohols under the optimized conditions to afford the correspond-
ing esters. As revealed in Table 4 (entries 1–7), a wide range of
alcohols (C : 5 to 12, both linear and branched) showed excellent
n
conversion to their respective esters when treated with acetic acid
in the presence of GrO5000. The reactivity of short-chain alcohols
appeared to be better than that of long-chain alcohols. It is also
noted that branched alcohols require more time for esterification
than linear alcohols (Table 4, entries 2 and 3). Besides that, the
catalytic role of GrO5000 was expanded for esterification of various
carboxylic acids having variable chain length. All carboxylic acids
(Table 3). The GrO5000 is a highly hydrophilic and possesses ample
polar functionalities, which make it highly dispersible in the polar
reaction media. In contrast, GrO2000 exhibits comparably low
n
(C : 2 to 6) afforded excellent conversion to corresponding esters
(Table 4, entries 8–11). However, we noticed that the reaction rate
degree of phenolic, carboxylic, and ꢀOSO
by their FTIR and elemental analysis. As a result, GrO2000 exhibited
a comparatively poor catalytic activity than GrO5000
Furthermore, the scope of GrO5000 as a solid catalyst was
expanded by using a variety of carboxylic acids with various
2
H groups as revealed
gradually slows down as the acid chain length grows, which is
consistent with earlier study.
Sustainability and recyclability of the GrO5000 catalyst were
examined by the esterification of acetic acid with 1-heptanol as
a model reaction. After the completion of the reaction, the
[
25]
.
a
Table 4. Esterification of various carboxylic acids with a variety of alcohols using GrO5000 as a catalyst
Entry
Acid
Alcohol
Time (h)
4
Conversion (%)
1
2
97
b
c
c
4
2
98, 94 , 91
b
81, 79 , 72
b
c
0
4
8
.5
45, 45 , 32
3
4
42
80
8
2
86
94
1
5
6
4
86
4
8
2
4
8
2
70
88
93
62
86
94
1
1
7
0
2
4
.5
45
81
98
8
9
4
4
4
96
87
84
10
11
a
Esterification conditions: 0.021 mol of alcohol, 0.11 mol of carboxylic acid, 20 mg of GrO5000 catalyst, and
reaction temperature of 100 °C.
b
With Amberlite-150 catalyst.
c
With Amberlyst-15 catalyst.
J. Phys. Org. Chem. 2014, 27 944–951
Copyright © 2014 John Wiley & Sons, Ltd.
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H. P. MUNGSE ET AL.
for the catalytic reactions and the processes such as esterifi-
cation, hydrolysis, and trans-esterifications.
Acknowledgements
This work was generously supported by the CSIR (CSC-0118/04).
Authors kindly acknowledge the Analytical Science Division and
Refinery Technology Division of IIP, IISc Bangalore, ICMS-JNCASR,
Bangalore, and IIT Chennai for providing help in the analyses
of the samples. Author HPM is thankful to UGC for financial
support.
REFERENCES
[
1] a)M. Toda, A. Takagaki, M. Okamura, J. N. Kondo, S. Hayashi, K.
Domen, M. Hara, Nature 2005, 438, 178. b)M. Hara, Energy & Environ-
ment Science 2010, 3, 601–607. c)K. Nakajima, M. Hara, ACS Catalysis
Figure 5. Recyclability of GrO5000 catalyst in esterification of acetic acid
with 1-heptanol
2
012, 2, 1296–1304.
[
2] a) S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, H. Kato,
S. Hayashi, M. Hara, Journal of the American Chemical Society
2008, 130, 12787–12793. b) M. Kitano, K. Arai, A. Kodama, T.
Kousaka, K. Nakajima, S. Hayashi, M. Hara, Catalysis Letter 2009,
catalyst was recovered from the reaction mixture and then
used for FTIR characterization. The FTIR spectrum of residual
catalyst GrO5000 exhibited all characteristic vibrational peaks
1
31, 242–249.
[
3] a) J. Pyun, Angewandte Chemie International Edition 2011, 50, 46–48.
b) B. F. Machado, P. Serp, Catalysis Science and Technology 2012,
2, 54–75.
(
ꢀOH, C¼O, C¼C, ꢀCOOH, C–O–C, ꢀOSO H, and C–O) and is
2
indistinguishable to the spectrum of fresh GrO5
(Fig. S2),
000
indicating that the chemical nature of GrO5000 remains intact
during the esterification reaction, unlike the reported graphene
oxide catalyst that underwent partial reduction during the
[4] C. Su, K. P. Loh, Accounts of Chemical Research 2013, 46, 2275–2285.
[
5] a) T. Szabo, O. Berkesi, P. Forgo, K. Josepovits, Y. Sanakis, D.
Petridis, I. Dekany, Chemistry of Materials 2006, 18, 2740–2749.
b) D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii, Z. Sun,
A. Slesarev, L. B. Alemany, W. Lu, J. M. Tour, ACS Nano 2010,
4, 4806–4814.
[
12]
oxidation of benzyl alcohol.
This is further corroborated by
the thermogravimetric analyses pattern of GrO5000, which shows
that oxygen functionalities in GrO5000 are thermally stable upto
[6] a) K. Erickson, R. Erni, Z. Lee, N. Alem, W. Gannett, A. Zettl, Advanced
Materials 2010, 22, 4467–4472. b) K. P. Loh, Q. Bao, G. Eda, M.
Chhowalla, Nature Chemistry 2010, 2, 1015–1024.
1
90 °C (Fig. S3). Herein, all esterification reactions were executed
at 100 °C; hence, no chemical changes were observed in the
residual GrO5000 catalyst. The recovered GrO5000 catalyst was
reused for subsequent runs. This exercise was conducted for
five subsequent runs. In all the experiments, a yield of 1-heptyl
acetate remained almost the same as shown in Fig. 5.
Collectively, these results revealed the excellent sustainability
and recyclability of GrO5000 as a solid catalyst for esterification
reactions.
[
7] C. Su, M. Acik, K. Takai, J. Lu, S. Hao, Y. Zheng, P. Wu, Q. Bao, T. Enoki,
Y. J. Chabal, K. P. Loh, Nature Communications 2012, 3, 1298.
[8] D. R. Dreyer, S. Park, C. W. Bielawski, R. S. Ruoff, Chemical Society
Reviews 2010, 39, 228–240.
[
9] C. Botas, P. Alvarez, C. Blanco, R. Santamaria, M. Granda, P. Ares, F.
Rodriguez-Reinoso, R. Menendez, Carbon 2012, 50, 275–282.
[
10] C. Botas, P. Alvarez, P. Blanco, M. Granda, C. Blanco, R. Santamaria, L.
J. Romasanta, R. Verdejo, M. A. Lopez-Manchado, R. Menendez,
Carbon 2013, 65, 156–164.
11] O. C. Compton, S. T. Nguyen, Small 2010, 6, 711–723.
12] D. R. Dreyer, H. P. Jai, C. W. Bielawski, Angewandte Chemie 2010,
[
[
1
22, 6965–6968.
[
13] a)D. W. Boukhvalov, D. R. Dreyer, C. W. Bielawski, Y.-W. Son, Chem.
Cat. Chem. 2012, 4, 1844–1849. b) H. P. Jia, D. R. Dreyer, C. W.
Bielawski, Advance Synthesis and Catalysis 2011, 353, 528–532.
14] a) D. R. Dreyer, H. P. Jia, A. D. Todd, J. Geng, C. W. Bielawski, Organic
and Biomolecular Chemistry 2011, 9, 7292–7295. b) H. P. Jia, D. R.
Dreyer, C. W. Bielawski, Tetrahedron 2011, 67, 4431–4434.
CONCLUSIONS
[
In conclusion, the GrO prepared by the Hummers method
was separated into three fractions (GrO5000, GrO2000, and
GrOres) on the basis of their dispersibility in water. Among
all fractions, GrO5000 exhibited high degree of oxygen func-
tionalities as revealed by FTIR and elemental analyses. The
[15] a) S. Verma, H. P. Mungse, N. Kumar, S. Choudhary, S. L. Jain, B. Sain,
O. P. Khatri, Chemical Communications 2011, 47, 12673–12675. b) A.
V. Kumar, K. R. Rao, Tetrahedron Letter 2011, 52, 5188–5191. c) D. R.
Dreyer, K. A. Jarvis, P. J. Ferreira, C. W. Bielawski, Polymer Chemistry
distorted ultrathin sheet of GrO5000 bearing ꢀOSO
2
H, pheno-
2
012, 3, 757–766.
lic, and carboxylic groups was discovered as a solid acid cat-
alyst to facilitate the esterification of various carboxylic acids
with a variety of alcohols to their corresponding esters in
high to excellent yields. It was noted that acidic functionali-
ties are placed throughout the (including basal plane, edges,
and holes) GrO5000 nanosheets, where reactant molecules
could easily approach and facilitate the esterification process.
Furthermore, good dispersibility in reaction media and high
surface area of GrO5000 are complementing the catalytic
performance. Hence, GrO5000 could be an efficient and green
substitute to the amorphous carbon-based solid acid catalysts
[
[
[
16] A. Dhakshinamoorthy, M. Alvaro, P. Conception, V. Fornes, H. Garcia,
Chemical Communications 2012, 48, 5443–5445.
17] J. Ji, G. Zhang, H. Chen, S. Wang, G. Zhang, F. Zhang, X. Fan,
Chemical Science 2011, 2, 484–487.
18] F. Liu, J. Sun, L. Zhu, X. Meng, C. Qi, F.-S. Xiao, Journal of Materials
Chemistry 2012, 22, 5495–5502.
[19] W. S. Hummers Jr, R. E. Offeman, Journal of the American Chemical
Society 1958, 80, 1339.
[
20] a) T. Szabo, O. Berkesi, I. Dekany, Carbon 2005, 43, 3186–3189. b)
H. P. Mungse, S. Verma, N. Kumar, B. Sain, O. P. Khatri, Journal of
Materials Chemistry 2012, 22, 5427–5433. c) S. Choudhary, H. P.
Mungse, O. P. Khatri, Journal of Materials Chemistry 2012,
22, 21032–21039.
wileyonlinelibrary.com/journal/poc
Copyright © 2014 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2014, 27 944–951

GRAPHENE OXIDE: A SOLID CATALYST FOR ESTERIFICATION
[21] A. Dhakshinamoorthy, M. Alvaro, M. Puche, V. Fornes, H. Garcia,
Chem. Cat. Chem. 2012, 4, 2026–2030.
[25] Y. Liu, E. Lotero, J. G. Goodwin Jr, Journal of Catalysis 2006,
243, 221–228.
[22] a) W. Gao, L. B. Alemany, L. Ci, P. M. Ajayan, Nature Chemistry 2009,
1, 403–408. b) H. R. Thomas, S. P. Day, W. E. Woodruff, C. Valles, R. J.
Young, I. A. Kinloch, G. W. Morley, J. V. Hanna, N. R. Wilson, J. P.
Rourke, Chemistry of Materials 2013, 25, 3580–3588.
23] S. Choudhary, H. P. Mungse, O. P. Khatri, Chemistry – An Asian
Journal 2013, 8, 2070–2078.
SUPPORTING INFORMATION
[
Additional supporting information may be found in the online
[
24] C. Brodie, Philosophical Transactionsof the Royal Society of London,
1
859, 149, 249–259.
version of this article at the publisher’s website.
J. Phys. Org. Chem. 2014, 27 944–951
Copyright © 2014 John Wiley & Sons, Ltd.
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