Graphene oxide-catalysed oxidation reaction of unsaturated compounds under microwave irradiation

Article abstract of DOI:10.1016/j.catcom.2015.09.021

Graphene oxide (GO) was investigated as a catalyst in the oxidation of alkenes and alkynes in combination with microwave heating. Styrene and phenylacetylene were selected as model compounds for the study. Moderate catalyst loadings (25 wt.%) and short re

Full text of DOI:10.1016/j.catcom.2015.09.021

Catalysis Communications 72 (2015) 133–137
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Graphene oxide-catalysed oxidation reaction of unsaturated compounds
under microwave irradiation
Jose M. Bermudez a,b, J. Angel Menendez c, Ana Arenillas c, Rafael Martínez-Palou d,
Antonio A. Romero a, Rafael Luque a,
⁎
a
Departamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie, Ctra Nnal IV-A, Km 396, E14014 Córdoba, Spain
Chemical Engineering Department, Imperial College London, SW7 2AZ London, United Kingdom
Instituto Nacional del Carbón-CSIC, Apdo. 73, 33080 Oviedo, Spain
b
c
d
Dirección de Investigación en Transformación de Hidrocarburos, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, 07730 Mexico, D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 June 2015
Received in revised form 22 September 2015
Accepted 23 September 2015
Available online 28 September 2015
Graphene oxide (GO) was investigated as a catalyst in the oxidation of alkenes and alkynes in combination with
microwave heating. Styrene and phenylacetylene were selected as model compounds for the study. Moderate
catalyst loadings (25 wt.%) and short reaction times (1 h) were employed, demonstrating that the combination
of GO and H2O2 under microwave irradiation possessed an optimum oxidation capacity to oxidize double bonds
to different alcohols and aldehydes.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Graphene oxide
Catalysis
Oxidation
Microwaves
1. Introduction
GO is a readily available and inexpensive material that has been
proven catalytically active for a number of organic transformations
During the last years, graphene and chemically modified graphenes
(CMGs) have attracted much attention due to their outstanding
physico-chemical properties [1–2]. Moreover, CMGs are expected to
have a significant impact in different fields such as chemistry, physics,
engineering and materials science [3–8]. Although in the early stages
of research the synthesis of these materials was complex, the advances
on this matter have already allowed access to high quality materials [3,
4,6,7].
The use of these materials in catalysis is gaining increasing interest
[8,9,10], being a relatively new area with a huge potential but low devel-
opments. The application of graphene and CMGs in catalysis has been
mainly focused on their use as supports for transition metal with cata-
lytic activity [9,10,11]. However, the diminishing supplies of metals
used in industrial processes as catalyst and the progress achieved in
the use of metal-free carbons in synthetic chemistry drove a more direct
application of CMGs as catalyst instead of their use as supports [9,10].
Graphene oxide (GO) accounts as most relevant CMG as it has been ob-
served that functionalized carbons are comparatively more effective in
catalysis as compared to unfunctionalized counterparts (e.g. graphene)
[8,9,10,12,13].
including oxidation of alcohols, alkenes and alkynes [10,12,13,14]. The
main advantages of the use of GO as catalyst include simplicity and
inexpensive nature, metal-free reactivity and easy recovery via simple
filtration from the reaction media. However, the interesting results pub-
lished to date have been obtained under extreme conditions, especially
in terms of catalyst loading (400 wt.%) and reaction times (24 h+) [12,
13]. These conditions are an important drawback which needs to be
overcome for a future application of GO as catalyst in the chemical
industry. For this reason, recent research efforts have been focused on
the improvement of the catalytic activity of GO to improve these
extreme conditions of catalyst loading and reaction time.
Microwave heating is gaining interest due to its several advantages
as compared to conventional heating [15–17]. Among these advantages
it can be highlighted the rapid and homogeneous heating rates, selec-
tive heating, non-contact heating, quick start/stop and heating from
within the material (i.e., energy conversion instead of heat transfer)
[15,17,18]. In addition, its use in combination with carbon materials
has given rise to a new generation of processes and materials that
cannot be obtained under conventional heating [18,19,20] including
catalytic applications [18,21]. The use of microwave heating has been
reported to provide higher conversions and occasionally better selectiv-
ities in chemical reactions as compared to those obtained using conven-
tional heating under otherwise similar reaction conditions. Moreover, it
is possible to obtain higher reaction rates which can result in lower
⁎
Corresponding author.
E-mail address: q62alsor@uco.es (R. Luque).
http://dx.doi.org/10.1016/j.catcom.2015.09.021
1566-7367/© 2015 Elsevier B.V. All rights reserved.

134
J.M. Bermudez et al. / Catalysis Communications 72 (2015) 133–137
reaction times [21–24]. Particularly, microwaves as non-conventional
heating source had a significant impact in the field of organic chemistry
in recent years. A countless number of studies have reported reduced
reaction times and improved yields and selectivity obtained in chemical
processes due to the previously mentioned rapid and homogeneous as
well as selective heating achieved under microwave irradiation [22–24].
However, no studies can be found about the use of microwave
heating to improve the performance of unmodified GO as catalyst in
oxidation reactions of alkenes and alkynes.
For these reasons we envisaged the use of microwave heating in
combination with GO as a novel possibility to achieve high conversions
in oxidation reactions using lower catalyst loadings and milder opera-
tion conditions. In this work, we present a study of the catalytic activity
of graphene oxide in different oxidation reactions of styrene and
phenylacetylene under microwave heating, with the aim of reducing
reaction times and catalyst loading as well as working under milder
conditions to those previously reported.
aqueous solution) and 50 mg of GO as catalyst were mixed. All the reac-
tions were carried out at 150 °C during 1 h. The influence of the oxidant
species was also studied, by carrying out an experiment with H2O2 in a
lower concentration than the reference test (using 0.2 mL, 35% v/v
aqueous solution). Three additional blank experiments were per-
formed, one in absence of catalyst and two of them in the absence of
oxidant (one with GO as catalysts and the other one with rGO). Addi-
tionally, a conventionally heated experiment using identical quantities
of reagents, namely 0.2 mL styrene, 2 mL acetonitrile, 0.3 mL H2O2
(50% v/v aqueous solution) and 50 mg of GO was conducted at 85 °C
(boiling acetonitrile/water solution) for 12–24 h.
After the microwave experiments, the reacting mixture was filtered
and the filtrate was analysed by means of GC and GC/MS using an Agilent
6890 N fitted capillary column HP-5 (30 m × 0.32 mm × 0.25 μm) and a
flame ionisation detector (FID). The recovered GO was then reused in
subsequent tests (denoted as runs rS4-1 and rS4-2) under identical
conditions to those of run S3 in order to check GO deactivation.
2. Experimental
3. Results and discussion
2.1. Graphene oxide synthesis
Characterisation studies of GO were conducted in NanoInnova
(http://www.nanoinnova.com/Product) using several techniques in-
cluding XRD, TGA XPS and IR spectroscopy. Fig. 1 depicts XRD patterns
of as-prepared GO with respect to graphite (used as starting material),
evidencing a complete oxidation of graphite and the formation of the
characteristic diffraction lines of GO [10–14]. The clear XRD pattern in-
dicates a high purity of GO achieved after graphite powder treatment.
The presence of the high purity GO was also confirmed by TGA and IR
results (Fig. 2) in which a number of distinctive bands corresponding
to C_O (carbonyl/carboxylic acid groups, 1714 cm−1), C_C from aro-
matics (1615 cm−1), C–O (carboxylic acid groups, 1317 cm−1; epoxy
and alkoxy 1221, 1028 cm−1, respectively) could be clearly observed.
The TGA trace shows three distinctive peaks, a first mass loss at ca.
100 °C (10.8%) due to the removal of water molecules from GO, followed
by a peak at 300 °C corresponding to GO decarboxylation (32–35% mass
loss). Further decomposition (20–22% mass loss) takes place up to 800–
850 °C.
XPS measurements were also conducted to support IR data, clearly
showing the presence of C–O and C_O groups (deconvoluted bands
at 286.7 and 288.4 eV, respectively, for C1s as well as 532.2 and
531.9 eV for O1s) both clearly visualised in C1s and O1s spectra (Fig. 3).
Two model reactions were selected to investigate the activity and
selectivity of characterised GO in oxidation reactions of unsaturated
systems under microwave irradiation, starting from styrene and
phenylacetylene, and employing H2O2 as oxidant (Fig. 1).
A series of experiments under different conditions was performed in
order to study the catalytic properties of graphene oxide (GO). GO was
provided by NanoInnova Technologies (Madrid, Spain) and it was syn-
thesized by using a modified Hummers' method [25]. Briefly, graphite
powder (b150 μm Sigma-Aldrich) was chemically oxidized in a solution
containing NaNO3, H2SO4 and KMnO4. Reduced GO (rGO) was also
kindly provided by NanoInnova. rGO has a surface area of around
100 m2 g−1 and different structural features as compared to GO [26].
Full details, including characterisation of the materials, can be found
at the website of the company http://www.nanoinnova.com/Product.
2.2. Characterisation of GO
X-ray diffraction was performed using a Panalytical X'Pert PRO θ/θ
system, using CuKa-radiation, and X'Celerator detector. Samples were
scanned between 5 and 100° (2θ) using a step size of 0.0167° (2θ) and
a count time of 100 s.
Thermogravimetric analysis was performed with a TGA Q-500 (TA
Instruments) at 1 °C/min (temperature range 20–900 °C) under nitro-
gen atmosphere.
FT-IR spectra were collected on a “Jasco FT/IR-410”, using KBr
pressed pellet technique. Spectra were recorded in the 3600–
500 cm−1 range as well as in the 1800–600 cm−1 range to identify
the different functional groups in GO.
Photoelectron spectra (XPS) were recorded in a VG Escalab 200R
spectrometer equipped with a hemispherical electron analyser and a
Mg Ka (hν = 1254.6 eV) X-ray source, powered at 120 W. Binding
energies were calibrated relative to the C1s peak at 284.8 eV. High-
resolution spectra envelopes were obtained by curve fitting synthet-
ic peak components using the software “XPS peak”. Symmetric
Gaussian–Lorentzian curves were used to approximate the line
shapes of the fitting components.
Table 1 summarise the results achieved in the performed experi-
ments. Blank runs (in absence of catalysts as well as in the absence of
oxidant) were also performed, with a negligible conversion in the
systems. However, a comparatively high conversion (N80%) can be
achieved when the process is carried out in the presence of both GO
as catalyst and H2O2 as oxidant. This high conversion demonstrates
that the combination of GO and H2O2 has an optimum oxidative
2.3. Catalytic experiments
Oxidation tests were performed under microwave heating in a
pressure-controlled CEM-discover microwave reactor (www.cem.
com). The reactor uses cylindrical vials of 10 mL. The reactor is a multi-
mode microwave reactor which power varies in the range of 0–300 W
with a frequency of 2450 MHz. An infrared sensor is used to monitor
temperature during the experiment. Two different substrates were
used: styrene (S) and phenylacetylene (PA).
In a typical experiment, in a sealed tube (10 mL) containing stirring
bar acetonitrile (2 mL), the substrate (0.2 mL), H2O2 (0.3 mL, 50% v/v
Fig. 1. XRD patterns of the GO prepared and the graphite used as precursor.

J.M. Bermudez et al. / Catalysis Communications 72 (2015) 133–137
135
Fig. 2. IR spectra (top images) and TGA trace of graphene oxide (GO, sample GO.Z.10–4 from NanoInnova).
capacity to oxidize double bonds under microwave irradiation. A com-
parative experiment under conventional heating provided low catalytic
activities (b50%) and selectivities to benzaldehyde (ca. 25%) of the com-
bination GO/H2O2 even after 24 h reaction. Reactions run under micro-
wave irradiation could provide high yields to products with improved
selectivities (less number of byproducts generated) while decreasing
reaction time (1 h vs 24 h) and catalyst loadings (25 wt.% vs
400 wt.%) as compared to previously reported results using convention-
al heating for the oxidation of alkenes [27,28]. The oxidative capacity of
the catalytic system was observed to decrease when using diluted H2O2
or when reduced GO was employed as catalyst instead of GO. Under
these conditions, conversion decreased dramatically (b25%). This
decrease in conversion pointed out the combination of H2O2 and GO
as an efficient catalytic system in the oxidation of double bonds.
In terms of selectivity, benzaldehyde, styrene glycol and
phenylethanol were observed as main products, with benzaldehyde
and styrene glycol obtained as major products (phenylethanol was
only observed in low quantities). Other products found were phenol
and benzyl alcohol but together account for less than 10% of selectivity
in all cases, except for the conventionally heated reaction (entry S7) for
which larger quantities of these and other side products were obtained.
Over-oxidation to benzoic acid was not observed under any investigat-
ed conditions as opposed to previous reports when high temperatures
and more extreme conditions (longer reaction times and/or higher cat-
alyst loadings) were utilised [12,13]. Apart from speeding up times of
reaction, the use of microwave heating could provide a slight improve-
ment in benzaldehyde selectivity (ca. 46–50 vs 20–25%) with respect to
conventionally heated reactions. The effect of microwave irradiation in
the proposed system is clearly a rapid ad homogeneous heating of the
bulk solution (as well as the GO material) which leads to an effective re-
action under the investigated conditions.
The ratio between benzaldehyde and styrene glycol was observed to
be constant, regardless of the concentration of H2O2. The concentration
of H2O2 does not therefore seem to play a key role in the selectivity of
the process. Comparatively, the use of GO or rGO influenced both con-
version and selectivity in the process, since the ratio between benzalde-
hyde and styrene glycol was changed. This ratio (1.3:1,
benzaldehyde:styrene glycol) observed for GO changed to 3:1 when
rGO was used. These results pointed to an increase in benzaldehyde pro-
duction as a consequence of the lower oxidative capacity of rGO. rGO
also gave rise to values of conversion and selectivity very similar to
those achieved with reused GO. These results pointed out that GO
loses oxidative capacity upon reuse as well as activity.
However, characterisation studies of GO before and after reaction
could not provide strongly supportive and conclusive insights for such
activity decrease as shown in the comparative IR profiles of fresh and
reused catalyst from Fig. 4. These characterisation experiments might
Fig. 3. XPS C1s (A) and O1s (B) spectra of graphene oxide (GO, sample GO.Z.10-4 from
NanoInnova).

136
J.M. Bermudez et al. / Catalysis Communications 72 (2015) 133–137
Table 1
Conditions used and conversions and selectivities achieved in the GO-catalysed oxidation of styrene.
Run
Catalyst
Oxidant
Conversion
Selectivity
Others
S1
S2
None
GO
rGO
GO
GO
H2O2
–
–
H2O2
H2O2 diluted
H2O2
H2O2
H2O2
H2O2
–
–
–
83
22
23
38
24
17
–
–
–
46
42
71
24
63
71
–
–
–
8
6
4
8
6
4
–
–
–
36
46
23
28
26
21
–
–
–
10
7
2
40
5
S3
S4a
S5
4
3
1
3
2
S6
rGO
GO
S7b
rS4-1
rS4-2
Spent GOc
Spent GOd
3
a
Run S4 was repeated 3 times to determine the experimental error.
Reaction run under conventional heating (85 °C, 24 h reaction).
The catalyst used in this run was GO recovered from run S4.
The catalyst used in this run was GO recovered from run rS4-1.
b
c
d
point to a reduced intensity in C–O and C_O bands in the reused mate-
rials, which may be the main reason of the observed GO deactivation,
but the contribution of surface C_O and C–O groups as measured by
XPS (results not shown) did not significantly change after the oxidation
reaction.
consecutive reactions. This mechanism may also explain the decrease
in conversion observed in runs rS4-1 and rS4-2. Upon catalyst reusing,
less C–O and C_O groups seemed to be present in GO (although only
qualitatively as shown in IR results, Fig. 4) to interact with the reactants
and give rise to the products, thus a decrease in conversion is observed.
Table 2 shows the results achieved in experiments performed to
study the oxidation capacity of the GO in alkynes using phenylacetylene
(PA) as model compound. In the case of PA, a very low conversion was
obtained (b15%) even when both GO and H2O2 were employed. This
means that this combination has not enough oxidative capacity to
quantitatively oxidize triple bonds under the investigated reaction
conditions and the selected catalysts loading and short times of re-
action even under microwave irradiation. Longer reaction times
and/or catalyst loadings are needed to oxidize triple bonds in
order to achieve similar results to those reported under convention-
al heating.
A proposed mechanism for the selective oxidation process can be
shown in Fig. 1.
The oxygenated groups present on the surface of GO activate the for-
mation of radicals •OH that are then able to oxidize the double bound of
the alkene species. Three different reaction paths may give rise to the
main reaction products. The interaction of OOH− radicals with surface
groups of GO may give rise to a superoxide which interacts with the
double bond of styrene to form the proposed transition state shown in
Fig. 1. Finally, an electronic rearrangement takes place to produce benz-
aldehyde. Other possibility is a nucleophilic addition to give rise to
phenylethanol. In the case of the styrene glycol, it is necessary to have
an intermediate state in which the alkene is epoxidized. The epoxide
is then opened to styrene glycol due to a nucleophilic addition. This
second reaction of the mechanism also needs the presence of active
sites from GO [29]. For this reason, the selectivity to this compounds de-
creases when rGO or reused GO were employed as catalysts since the
oxidative capacity of these materials is not enough to catalyse the two
4. Conclusions
Graphene oxide can be a very efficient and selective catalyst for the
oxidation of unsaturated compounds under microwave irradiation, par-
ticularly alkenes. Quantitative conversion of styrene could be achieved
Fig. 4. IR spectra of fresh GO (top images) and double reused GO (entry rS4-2, bottom images) in the microwave-assisted oxidation of styrene.

J.M. Bermudez et al. / Catalysis Communications 72 (2015) 133–137
137
Table 2
Conditions used and conversions and selectivities achieved in the GO-catalysed oxidation of phenylacetylene.
Run
Catalyst
Oxidant
Conversion
Selectivity
Others
P1
P2
P3
P4
P5
None
GO
GO
GO
rGO
H2O2
None
H2O2
H2O2 diluted
H2O2
–
–
15
5
b5
–
–
59
38
83
–
–
41
62
17
–
–
–
–
–
–
–
–
–
–
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