Graphene oxide as an acid catalyst for the room temperature ring opening of epoxides

Article abstract of DOI:10.1039/c2cc31385e

The minute amount of hydrogen sulfate groups introduced into the graphene oxide (GO) obtained by Hummers oxidation of graphite renders this material as a highly efficient, recyclable acid catalyst for the ring opening of epoxides with methanol and other primary alcohols as nucleophile and solvent.

Full text of DOI:10.1039/c2cc31385e

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COMMUNICATION
Graphene oxide as an acid catalyst for the room temperature ring
opening of epoxidesw
´ ´
Amarajothi Dhakshinamoorthy, Mercedes Alvaro, Patricia Concepcion, Vicente Fornes and
Hermenegildo Garcia*
Received 24th February 2012, Accepted 10th April 2012
DOI: 10.1039/c2cc31385e
The minute amount of hydrogen sulfate groups introduced into
the graphene oxide (GO) obtained by Hummers oxidation of
graphite renders this material as a highly efficient, recyclable
acid catalyst for the ring opening of epoxides with methanol and
other primary alcohols as nucleophile and solvent.
with fuming sulfuric acid at 180 1C was tested as catalyst for
the esterification of acetic acid, the Pechmann condensation
and hydration of propylene oxide.18 The difference between
the material used in this prior work and the present one is that
we are using here directly the GO samples obtained by the
conventional Hummers oxidation without any subsequent
treatment. Our GO is, therefore, more conveniently prepared
and does not require additional and hazardous treatments.
The sample of GO used in the present study was obtained by
Hummers oxidation of graphite followed by exfoliation in
aqueous solution. The resulting GO was recovered by collecting
the non-sedimented supernatant solution after centrifugation at
15000 rpm and freeze drying.
Graphene and related materials have attracted much interest in
nanoelectronics due to the fast charge mobility, high mechanical
resistance and the fact that they represent the thickness limit for
an electrically conductive material.1,2 Triggered by these remark-
able properties,3 the interest in graphene-like materials has
expanded to other areas in chemistry including adsorption and
photocatalysis,4–6 heterogeneous catalysis7 and biosensors.8 The
high solubility, high surface area9 and the fact that there are no
barriers to mass transfer for substrates reaching the graphene
surface make these materials also very suitable in catalysis as a
new form of carbon.10,11
The activity of GO as an acid catalyst was tested for the ring
opening of epoxides using 1a as a substrate in methanol as
solvent and nucleophile at room temperature as a model
reaction. The observed results are given in Table 1. In the
absence of the catalyst, unreacted 1a was recovered. When the
One of the most convenient ways to obtain graphene in large
quantities is by graphite oxidation followed by exfoliation of the
resulting graphene oxide to obtain highly persistent aqueous
suspensions of graphene oxide (GO). The most widely applied
oxidation conditions, the Hummers method12 is based on the
exhaustive oxidation of graphite using strong acid conditions by
permanganate and hydrogen peroxide. In the present manuscript,
we report that the GO obtained by the conventional Hummers
method is a highly efficient, recyclable acid catalyst for the ring
opening of epoxides with methanol. The available data suggest
that the minor sulfate groups present in the GO are sufficient to
Table 1 Ring opening of styrene oxide with methanol catalyzed by
GO and other catalystsa
Selectivityb
(%)
b
Time/min Conversion (%)
2a
3a
Run Catalyst
act as recoverable and recyclable Bronsted acid sites.
¨
Recently, GOs have been used as catalysts13 in the oxidation
of sulfides and thiols,14 oxidation of benzylic positions15 and
benzyl alcohol.16 In addition, reduced graphene oxide has also
been used as catalyst for the room temperature reduction of
nitrobenzene.17 In a recent precedent, sulfated graphene
obtained by hydrothermal sulfation of reduced graphene oxide
1
2
3
4
5
6
7
8
9
GO
GO
GO
GO
GO
GO
GO
GO
10
20
60
60
60
60
240
360
60
5
91
96
99
99
1
4
7
—
3
9
10
10
—
—
—
—
96
93
100
97
91
90
90
99
99
99
—
2c
99d
69e
97e
98e
5
99
99
—
Norit A
10 H2SO4
11 p-CH3–C6H4–SO3H
12 CH3COOH
5
60
Instituto Universitario de Tecnologı´a Quı´mica CSIC-UPV and
´
Departamento de Quımica, Universidad Politecnica de Valencia,
Av. De los Naranjos s/n, 46022 Valencia, Spain.
´
a
Reaction conditions: 1a (1 mL), methanol (10 mL), catalyst (5 mg),
b
room temperature. Determined by GC. With 0.1 mL of pyridine.
c
E-mail: hgarcia@qim.upv.es; Fax: +34 9638 7807
w Electronic supplementary information (ESI) available: Details of
general experimental procedure and characterization of GO. See DOI:
10.1039/c2cc31385e
d
e
After third reuse. Reaction conditions: 1a (3 mL), methanol
(15 mL), catalyst (2 mg), room temperature.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5443–5445 5443

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of the partially reconstituted GO is negligible. The catalytic
activity for the ring opening of this partially reconstituted GO
was also negligible, supporting the hypothesis that the catalytic
activity for this reaction is associated with the presence of
sulfur atoms.
To verify whether or not the catalysis is truly heterogeneous
or is due to some leached active species present in the liquid
phase, the reaction was carried out under the optimized
conditions described in Table 1 and the GO solid catalyst
was filtered from the reaction mixture at 55% formation of 2a.
After removal of the GO catalyst, the solution in the absence
of the solid was again stirred at room temperature. After 1 h,
in the absence of the solid no further product formation was
observed. Hence, it can be concluded that catalysis occurs on
the surface of GO and the process is truly heterogeneous. The
reusability of GO was investigated for the ring opening
reaction of 1a with methanol under identical conditions as
described in Table 1. After the required time, the solid GO was
separated from the reaction mixture by centrifugation at
15 000 rpm and the recovered solid was used without any
further treatment. After three consecutive reuses, GO exhibited
almost identical catalytic activity. We notice, however, that
recycled GO exhibits a little bit higher selectivity towards 2a due
to the decrease in the formation percentage of 3a. Since 3a is a
secondary product derived from 2a by acid promoted exchange
of the primary hydroxyl group by a methoxy substituent, we
suggest that a minor decrease in the acid strength of GO upon
reuse is beneficial for 2a selectivity by minimizing the nucleo-
philic substitution of the primary hydroxyl group that should
require stronger acidity than the epoxide ring opening. Also, no
noticeable changes were observed in Raman and XPS analyses
after catalysis compared to fresh GO (Fig. S2 and S3, ESIw).
Information about the nature of acid sites and their inter-
action with probe molecules can be obtained by IR spectro-
scopy. After activation of the sample at 100 1C in vacuum
(10À6 mbar) (Fig. 2a) functional groups such as carbonyl
groups (1733 cmÀ1) and sulfate groups (1054 cmÀ1, SO3–H
stretching) are observed.19 The band at 1577 cmÀ1 is associated
with the C sp2 vibration of the graphene lattice. Methanol
adsorption on the sample at 100 1C leads to a shift of the
Fig. 1 Time-conversion plot for the ring opening of 1a with methanol
(squares) and filtration test (circles). Triangles represent the time-conversion
plot for a different sample of GO obtained in an independent batch.
reaction of 1a and methanol was carried out at room tempera-
ture in the presence of GO, almost 499% conversion with 93%
selectivity towards 2-methoxy-2-phenylethanol (2a) was achieved
in 1 h. Product 3a was also observed in 7% selectivity and its
yield grows along the reaction time indicating that 3a is a
secondary product derived from 2a. When the reaction was
performed in the presence of 0.1 mL of pyridine, only 2%
conversion of 1a was observed indicating the poisoning of acid
sites present in GO (Table 1, entry 4). It has to be stressed here
that the substrate to catalyst ratio used in the present study is
5 mg of GO versus 1 mL of 1a that corresponds to a significantly
lower amount of catalyst than that employed for the oxidation of
benzyl alcohol (200 wt%)16 and even hydration of propylene
oxide (0.1 g for 50 mmol).18 The temporal conversion profile of
the ring epoxides methanolysis of 1a is given in Fig. 1. These
encouraging results allowed us to reduce the catalyst–substrate
ratio further to determine the minimum productivity of GO for
the conversion of 1a by carrying out the reaction in methanol
with 3 mL of 1a and 2 mg of GO. At 1 h reaction time, 69% of 1a
was converted to 2a with 490% selectivity. After a prolonged
time (6 h), the conversion of 1a was almost complete without
much decrease in the selectivity of product 2a.
Reproducibility of the catalytic activity was checked by
performing the same reaction with a different GO batch and
its temporal profile is also shown in Fig. 1 giving a visual
indication of the data reproducibility. Although the catalyst
exhibited slightly higher initial reaction rate, the final conversion
and selectivity towards 2a remained the same. These minor
differences in the catalytic activity should be related to differences
in the sulfur content (see ESIw). The catalytic activity of GO was
much higher than that of Norit A activated carbon that resulted
in only 5% conversion of 1a. In order to assess the active sites
present in GO, some control experiments were performed
comparing the activity of GO with those of concentrated
sulfuric acid, p-toluenesulfonic acid and glacial acetic acid.
The ring opening of 1a with methanol was more facile with
the former two acids while the carboxylic acid was unreactive
and only the unreacted starting material was isolated. This
clearly indicates that the active sites of GO are not carboxylic
groups, but most probably some hydrogen sulfate acid groups
introduced on the GO scaffold during its synthesis. This is
further evidenced from the elemental analysis that shows
1.16 wt% of sulfur atoms. To support this proposal, we
submitted the as prepared GO to thermal treatment at 200 1C.
It is known that mild heat tends to reconstitute graphene from
GO and this effects the removal of some groups. Chemical
analysis after this treatment indicated that now the sulfur content
1054 cmÀ1 band towards higher wavenumber (1094 cmÀ1
)
+
(Fig. 2b). This shift may be related to the formation of CH3
ions from CH3OH interacting with the sulfate group
(SO3ÀCH3+). Indeed, the band at 1054 cmÀ1 is restored after
Fig. 2 IR spectra of the sample after 100 1C vacuum activation (a);
after evacuation at 100 1C, and admission of methanol at 36 mbar for
5 min (b); followed by evacuation under vacuum at 100 1C (c).
c
5444 Chem. Commun., 2012, 48, 5443–5445
This journal is The Royal Society of Chemistry 2012

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Table 2 Ring opening of epoxides with various substrates catalyzed
by GO with different nucleophilesa
Epichlorohydrin, 1d, gave only one isomer with low
conversion while glycidol, 1e, resulted in both isomers in
42% conversion. Then, 1-hexene oxide, 1f, was also subjected
to ring opening of epoxides using methanol as solvent and it
was found that both isomers were formed in equal amounts in
28 h. Norbornene oxide, 1h, also exhibited low conversion
even after a prolonged time.
Product selectivityb (%)
To differentiate between SN1 and SN2 reaction mechanisms,
1,2-epoxy-2-methylpropane, 1g, was selected as a substrate
and the nucleophilic ring opening of this epoxide was carried
out with methanol. Formation of the less-substituted alcohol
as the major product over the more-substituted regioisomer
clearly indicates that the reaction is likely to occur through a
SN1 mechanism.
3a
4a
Run Substrate
Time/h Conv.b (%)
2a
1
2
3
4
5
6
7
8
1
1
3
1
3
17
1
3
18
24
1
99
70c
97c
60d
96d
12e
43
50
60
62
5
93
99
97
92
89
99
97
97
96
95
14
15
17
21
7
—
3
—
—
—
—
—
—
—
—
—
—
86
85
83
79
8
11
—
3
3
4
In conclusion, we report a simple and efficient general procedure
for the room temperature epoxide ring opening using GO resulting
from Hummers oxidation of graphite. Steric encumbrance on the
alcohol or impeded diffusion plays an unfavorable role in the
conversion of epoxides. The regioselectivity in most of the cases
was high, rendering the product expected from an acid catalyzed
SN1 ring-opening mechanism. The catalyst could be easily
recovered after the reaction and reused for additional runs before
deactivation of the catalyst. The available analytical data and
control experiments suggest that the small sulfation occurring
during the oxidation protocol introduces sufficient acidity, which
combined with the high solubility of GO in water and alcohols
and the availability of the active sites located on a 2D surface
make the material highly active as an acid catalyst.
9
10
11
12
13
14
5
—
—
—
—
3
18
24
12
35
41
15
24
7
—
—
99
16
24
42
24
—
76
17
18
19
1
5
28
14
26
34
43
44
45
—
—
—
57
56
55
20
1
95
12
97
99
—
—
3
21
28
—
Notes and references
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a
Reaction conditions: substrate (1 mL), alcohol (10 mL), GO (5 mg),
b
c
room temperature. Determined by GC. Using ethanol as a nucleo-
d
phile. Using 1-propanol as a nucleophile. Using t-butanol as a
e
nucleophile.
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removal of the methoxy group (Fig. 2c). On the other hand, bands
at 1006 cmÀ1, associated with a C–O stretching, and at 2946 and
2838 cmÀ1, associated with C–H stretching vibration of the
methoxy group, are also observed upon adsorption of CH3OH
(Fig. 2b). These spectroscopic changes prove the acidity of the GO
obtained by the conventional Hummers oxidation of graphite.
After optimizing the reaction of ring opening of epoxides
with 1a using methanol as a nucleophile, we wanted to study
the effect of other alcohols such as ethanol, 1-propanol and
t-butanol as nucleophiles. As shown in Table 2, methanol
exhibited higher reaction rate than ethanol and 1-propanol.
Particularly 1a showed very low conversion with t-butanol,
this may be due to the steric hindrance caused by methyl
groups and a similar reactivity has been reported earlier with
other solid acids including MOFs as heterogeneous catalysts.20
It has to be mentioned here that all the alcohols resulted
selectively in a single regioisomer, 1b, in the presence of GO as
catalyst. The methanolysis of cyclohexene oxide resulted in
62% conversion in 24 h to 2-methoxycyclohexanol, 2b, in 95%
selectivity. Interestingly, 2-benzyloxirane showed both isomers,
2c and 4c, although the formation of the secondary alcohol is
favoured over the primary alcohol.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5443–5445 5445