Nanoscale graphene oxide sheets as highly efficient carbocatalysts in green oxidation of benzylic alcohols and aromatic aldehydes

Article abstract of DOI:10.1016/S1872-2067(17)62776-1

Nanoscale graphene oxide (NGO) sheets were synthesized and used as carbocatalysts for effective oxidation of benzylic alcohols and aromatic aldehydes. For oxidation of alcohols in the presence of H₂O₂ at 80 °C, the NGOs (20% mass fraction) as carbocatalysts showed selectivity toward aldehyde. The rate and yield of this reaction strongly depended on the nature of substituents on the alcohol. For 4-nitrobenzyl alcohol, <10% of it was converted into the corresponding carboxylic acid after 24 h. By contrast, 4-methoxybenzyl alcohol and diphenylmethanol were completely converted into the corresponding carboxylic acid and ketone after only 9 and 3 h, respectively. The conversion rates for oxidation of aromatic aldehydes by NGO carbocatalysts were higher than those for alcohol oxidation. For all the aldehydes, complete conversion to the corresponding carboxylic acids was achieved using 7% (mass fraction) of NGO at 70 °C within 2–3 h. Possible mechanisms for NGO carbocatalyst structure-dependent oxidation of benzyl alcohols and structure-independent oxidation of aromatic aldehydes are discussed.

Full text of DOI:10.1016/S1872-2067(17)62776-1

Chinese Journal of Catalysis 38 (2017) 745–757
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Nanoscale graphene oxide sheets as highly efficient carbocatalysts
in green oxidation of benzylic alcohols and aromatic aldehydes
Alireza Sedrpoushana,*, Masoud Heidaria, Omid Akhavanb,c
a Department of Chemical Technologies, Iranian Research Organization for Science and Technology (IROST), P.O. Box 33535‐111, Tehran, Iran
b Department of Physics, Sharif University of Technology, P.O. Box 11155‐9161, Tehran, Iran
c Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box 14588‐89694, Tehran, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
Nanoscale graphene oxide (NGO) sheets were synthesized and used as carbocatalysts for effective
oxidation of benzylic alcohols and aromatic aldehydes. For oxidation of alcohols in the presence of
H2O2 at 80 °C, the NGOs (20% mass fraction) as carbocatalysts showed selectivity toward aldehyde.
The rate and yield of this reaction strongly depended on the nature of substituents on the alcohol.
For 4‐nitrobenzyl alcohol, <10% of it was converted into the corresponding carboxylic acid after 24
h. By contrast, 4‐methoxybenzyl alcohol and diphenylmethanol were completely converted into the
corresponding carboxylic acid and ketone after only 9 and 3 h, respectively. The conversion rates
for oxidation of aromatic aldehydes by NGO carbocatalysts were higher than those for alcohol oxi‐
dation. For all the aldehydes, complete conversion to the corresponding carboxylic acids was
achieved using 7% (mass fraction) of NGO at 70 °C within 2–3 h. Possible mechanisms for NGO
carbocatalyst structure‐dependent oxidation of benzyl alcohols and structure‐independent oxida‐
tion of aromatic aldehydes are discussed.
Received 19 November 2016
Accepted 22 December 2016
Published 5 April 2017
Keywords:
Carbocatalyst
Nanoscale graphene oxide
Green chemistry
Oxidation
Metal‐free catalyst
Colloidal dispersion
© 2017, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
Different carbon allotropes have been investigated and used as
carbocatalysts [1,13,17–26]. The various carbon allotropes
Because of the mass production and utilization of various
organic materials in the modern world, effective and green
catalytic approaches are required to reduce the risks of these
chemicals to humans and the environment [1–8]. The most
widely explored catalytic nanomaterials are different forms of
transition metals, such as metal oxides [9] and hybrid materials
[10], which need ultraviolet (UV)‐visible irradiation to work
efficiently. However, these catalytic nanomaterials may induce
toxic effects, and can also be complex to remove from the envi‐
ronment [11,12]. Consequently, recent research has focused on
development of metal‐free catalysts.
have the same basic graphene‐based structure [14,23,24], and
in recent years, graphene‐based materials have attracted atten‐
tion as carbocatalysts. Graphene‐based structures such as pure
graphene, doped graphene, and graphene oxide (GO) have been
used as carbocatalysts in various reactions, but the gra‐
phene‐based carbocatalytic field is still in its infancy
[1,18,27–29]. Because of its acidic nature, the most common
uses of GO are in organic reactions such as Michael addition,
Friedel‐Crafts reaction, aza‐Michael addition, and ring opening
polymerization [30–38]. The first reported application of gra‐
phene as a carbocatalyst was of GO for the oxidation of benzyl
alcohol [39]. Since then, applications of graphene and/or re‐
duced GO (rGO) sheets to reduction of nitrobenzene [40], oxi‐
Recently, carbocatalysts have been proposed as promising
metal‐free alternatives to transition metal catalysts [13–16].
* Corresponding author. E‐mail: sedrpoushan1395@gmail.com
DOI: 10.1016/S1872‐2067(17)62776‐1 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 38, No. 4, April 2017

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Alireza Sedrpoushan et al. / Chinese Journal of Catalysis 38 (2017) 745–757
dation of 9H‐fluorene [41], and oxidation of benzene to phenol
[42] have been reported. Although graphene‐based carbocata‐
lysts are promising, they require harsh conditions, long reac‐
tion time, high catalyst concentrations, and produce low yields.
Because defects in carbonaceous skeletons act as effective ac‐
tive sites for carbocatalysis [18,43,44], research on gra‐
phene‐based carbocatalysts has concentrated on producing
defects by adding heteroatoms such as boron, nitrogen, and
sulfur [1,18,45]. These defective graphene structures have been
widely used in oxidation reactions and produce higher yields
than pure graphene or rGO [46–51]. However, synthesis of
these structures is often complex, requiring doping reagents
and specialized equipment, which makes their widespread
application in catalysis difficult [18]. Therefore, more effective
carbocatalysts need to be developed.
To date, no one has reported increasing the carbocatalytic
activity of graphene sheets by decreasing the dimensions of the
sheets. In addition, all previous reports on graphene‐based
materials have been for solvent free and/or heterogeneous
catalytic systems. There have been no reports on the applica‐
tion of graphene‐based materials as colloidal dispersion sys‐
tems, that is, systems at the boundary between homogeneous
and heterogeneous catalyst systems, to carbocatalysis. It is
expected that a reduction in the size of GO will increase the
peroxidase‐like activity of the graphene skeleton. This is be‐
cause any reduction in size will increase the density of defects,
especially vacancies at edges, and it has been determined that
lone pairs in the zigzag structure of sp2 bonds at vacancies at
edges, which have high delocalization states, are active sites for
the peroxidase‐like activity of carbonaceous structures
[42,52–58]. It is important to retain oxygen functional groups
of the graphenic sheets during any size reduction, because they
contribute to the ability of nanoscale graphene oxide (NGO) to
disperse in aqueous solutions and form a colloidal dispersion
system. Both the method for decreasing the size of GO sheets
and the magnitude of the size reduction could affect the cata‐
lytic performance of the NGO. It has been determined that son‐
ication can provide energy for oxidative cleavage of the C–C
bond of GO sheets by O2 from air [59], but does not lead to
physical breaking of the sheets. Moreover, in comparison with
thermodynamic methods, sonication results in only partial
cleavage at oxygen containing functional groups [60].
A modified Hummers' method [61] coupled with a lengthy
sonication step was used for synthesis of the NGO sheets, as
reported in another study [62]. Briefly, 0.5 g of graphite and 0.5
g of NaNO3 were stirred in 23 mL of H2SO4 for 12 h at room
temperature (RT). The mixture was then placed in an ice bath
and 3 g of KMnO4 was slowly added with stirring for 2 h. The
temperature was then increased to 40 °C with stirring for 1 h.
Next, 40 mL of deionized (DI) water was added to the suspen‐
sion, and temperature was increased to 90 °C and held at this
temperature for 1 h. Then, 100 mL of DI water was added, and
after decreasing the temperature to 30 °C, 3 mL of H2O2 was
added to the mixture. To remove sulfate and manganese ions,
the reaction product was repeatedly washed with a 5% (vol‐
ume fraction) HCl solution and DI water. The solid product was
retained and 100 mL of DI water was added, followed by soni‐
cation for 8 h. The mixture was then centrifuged at 5000 r/min
for 10 min, and the pellet was removed. The supernatant was
retained and washed two more times with 100 mL of DI water
followed by sonication for 8 h. The second and third centrifuga‐
tion steps were at 7000 rpm (10 min) and 10000 rpm (30 min),
respectively. These washing steps separated out any large par‐
ticles and/or clusters and the supernatant changed from a tur‐
bid brown solution to a clear brown solution, which was used
in the carbocatalytic reactions.
2.2. Oxidation of benzylic alcohols by NGO
To study the catalytic effect of the NGO sheets on the oxida‐
tion of benzylic alcohols, we used H2O2 as an oxidant in an
aqueous solution. Various mass fractions of the NGO sheets
((5–200)%) and different reaction temperatures (RT to 100 °C)
were investigated. In a preliminary experiment, benzyl alcohol
was heated in the presence of H2O as solvent, an excess amount
of H2O2 as oxidant, and 200% (mass fraction) of NGO as catalyst
at 100 °C for 24 h. To ensure that the only oxidant present was
H2O2, the reaction was carried out under N2 atmosphere. To
study the role of H2O2, the oxidation of benzyl alcohol was also
conducted with different concentrations of H2O2 (10, 2.2, 1, 0.5
and 0 equivalents (eq.)) in the presence of 100% (mass frac‐
tion) NGO in 2 mL of DI water at 100 °C for 24 h. To optimize
the different reaction parameters, the effects on the yield were
investigated for different benzyl alcohol derivatives (benzyl
alcohol, and 4‐chloro, 4‐methoxy, and 4‐nitro derivatives), NGO
mass fractions ((10–200)%) reaction temperatures (RT to 100
°C with (20–100)% (mass fraction) of NGO and 2.2 eq. of H2O2),
and reaction time (1–24 h with 20% (mass fraction) of NGO
and 2.2 eq. of H2O2 at 80 °C). The role of substituents on the
benzylic carbon in the oxidation process was investigated by
studying the oxidation of 1‐phenylethanol and diphenyl‐
methanol in the presence of 20% (mass fraction) of NGO and
1.1 eq. of H2O2 at 80 °C for 24 h. Finally, to highlight the catalyt‐
ic performance of the NGO, the oxidation reactions of all of the
benzyl alcohol derivatives were conducted with H2O2 but
without NGO at 100 °C for 24 h. The progress of all reactions
was monitored by thin layer chromatography (aluminum
sheets, 20 × 20 cm, Merck Millipore) and gas chromatography
(PHILIPS PU4500). After completion of the reaction the prod‐
The aims of this work were to investigate reducing the di‐
mensions of GO to NGO with minimum loss of oxygen function‐
al groups, and to prepare a colloidal dispersion of a carbocata‐
lysts for oxidation reactions. Because of the benefits of soni‐
cation discussed above, we used sonication for the synthesis
NGO from GO. After physical and chemical characterization of
NGO, its ability to catalyze oxidation of benzylic alcohols was
examined in an aqueous medium using H2O2 as an oxidant.
Further information was obtained by applying the NGO catalyst
to oxidation of aromatic aldehydes.
2. Experimental
2.1. Preparation of NGO sheets

Alireza Sedrpoushan et al. / Chinese Journal of Catalysis 38 (2017) 745–757
747
uct was extracted into CH2Cl2 (3 × 10 mL).
uously monitored by thin layer chromatography and gas chro‐
matography.
2.3. Oxidation of aromatic aldehydes by NGO
2.4. The effect of oxidation conditions on the structure of NGO
The performance of NGO sheets as carbocatalysts was also
investigated for the oxidation of aldehydes (benzaldehyde, and
its 4‐chloro, 4‐hydroxy, and 4‐nitro derivatives). To investigate
the role of NGO in the oxidation of each benzaldehyde deriva‐
tive to the corresponding carboxylic acid, the reaction was
performed with 100% (mass fraction) of NGO in the absence of
H2O2 at reaction temperatures ranging from RT to 100 °C under
the ambient atmosphere for 24 h. Then, the reaction conditions
were optimized as follows: concentration of NGO, (0–100)%
(mass fraction) of NGO with 1.1 eq. of H2O2 at 80 °C for 12 h;
reaction temperature, 7% (mass fraction) of NGO with 1.1 eq.
of H2O2 at RT–80 °C for 12 h; reaction time, 7% (mass fraction)
of NGO with 1.1 eq. of H2O2 at 70 °C. All reactions were contin‐
Because NGO is a carbonaceous structure and can undergo
oxidation, the effects of the oxidation reaction conditions on the
structure of NGO were studied. Fourier transform infrared
spectroscopy (FTIR) and UV‐visible spectroscopy were used to
analyze structure of NGO after reaction with H2O2 at 80 °C for 8
h.
2.5. Oxidation by similar catalysts
To further understand the effects of various characteristics
of the NGO catalysts on the oxidation reactions, some similar
catalysts were also used and the results were compared. The
Fig. 1. (a) Scanning electron microscopy image of synthesized NGO clusters, and a magnification (inset); (b) transmission electron microscopy image
of synthesized NGO sheets, and a magnification (inset); (c) atomic force microscopy image of the NGO structure; (d) height profile diagram of a line; (e
height profile histogram of the atomic force microscopy image.

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Alireza Sedrpoushan et al. / Chinese Journal of Catalysis 38 (2017) 745–757
2θ/( o )
Size (d/nm)
Wavenumber (cm-1)
Temperature (oC)
Fig. 2. (a) Dynamic light scattering histogram; (b) X‐ray powder diffraction pattern; (c) Raman spectrum; (d) Thermogravimetric analysis/differential
thermal analysis of NGO.
other catalysts were NGO‐amide, acetic acid, benzoic acid, and
epoxy cyclohexane. The oxidation reactions of benzyl alcohol
and benzaldehyde were examined using 20% (mass fraction) of
each catalyst in the presence of H2O2 at 100 °C for 24 h.
NGO‐amide was prepared by converting the carboxylic acid
groups to amides by addition of aniline according to an estab‐
lished method [63]. Moreover, to study the oxidation of alco‐
hols by peroxy acids, we used 1.8 eq. of meta‐chloroperbenzoic
acid (mCPBA) as an oxidant instead of H2O2 in the oxidation of
benzyl alcohol. The reaction was also investigated by adding a
stoichiometric amount of KI as a radical scavenger [64] to the
reaction medium.
ters (Fig. 1(a)). The formation of clusters could be assigned to
aggregation of the NGO sheets in the following two stages: (1)
initial aggregation of the NGO sheets, which gave particles with
diameters of approximately 500 nm, and (2) aggregation of the
particles to form clusters with dimensions of approximately
100 μm. The transmission electron microscopy, atomic force
microscopy, and DLS results showed these clusters could dis‐
integrate into sheets with nanoscale dimensions under soni‐
cation. The sheets were less than 100 nm wide (Fig. 1(b) and
(c)). The thickness of the sheets (~1 nm, Fig. 1(d) and (e)) was
consistent with the thickness of single‐layer GO sheets (~0.8
nm) [65–67]. The DLS histogram (Fig. 2(a)) showed that the
average width of the NGO sheets was approximately 60 nm.
The X‐ray powder diffraction pattern of the NGO sheets (Fig.
2(b)) showed only one diffraction peak at 2θ = 11.63°, which
corresponded to a d‐spacing of approximately 0.74 nm and was
consistent with the typical thickness of GO sheets. There was no
diffraction peak at 2θ = 26.30°, which is characteristic of the
d‐spaceing of graphite (~0.34 nm) [68], which indicated that
effective oxidation of the starting graphitic materials had oc‐
curred.
3. Results
3.1. Characterization of NGO sheets
The morphology, size, and size distribution of the NGO
sheets were studied by scanning electron microscopy, trans‐
mission electron microscopy, atomic force microscopy, and
dynamic light scattering (DLS) (Figs. 1 and 2). The scanning
electron microscopy images showed highly porous NGO clus‐
The Raman spectrum of the NGO sheets (Fig. 2(c)) showed

Alireza Sedrpoushan et al. / Chinese Journal of Catalysis 38 (2017) 745–757
749
Fig. 3. (a) Fourier transform infrared (FTIR) spectrum of unreacted NGO; (b) Ultraviolet (UV)‐visible spectrum of unreacted NGO; (c) FTIR spectrum
of NGO after oxidation; (d) UV‐visible spectrum of NGO after oxidation.
two bands for carbon at 1540 and 1317 cm–1 for the G and the
D bands, respectively. In Raman spectroscopy, the G band
(~1580 cm–1) results from E2g phonon scattering from the
graphitic structure and the D band (~1350 cm–1) originates
from the breathing mode of κ‐point phonons of A1g symmetry
of the sp3 defect bonds [69,70]. The D band can also arise from
amorphous carbon species, edge defects, grain boundaries, and
vacancies in disordered carbonaceous structures [71]. The
ratio of the intensities of these bands (IG/ID) can be used to
measure the sp2 domain size of a carbon structure with both sp2
and sp3 bonds. In this work, the IG/ID ratio was 0.637, indicating
the presence of a high level of disorder/defects in the NGO
structure, which could act as active sites in catalytic reactions.
The thermogravimetric analysis/differential thermal analy‐
sis results of NGO samples under N2 (Fig. 2(d)) showed two
major mass losses. The first mass loss stage was from 50 to 110
°C with a 21% mass reduction because of loss of water. The
second mass loss stage was from 140 to 400 °C with a 31%
mass reduction corresponding to loss of labile oxygen contain‐
ing functional groups, particularly carboxylic acid groups, to
form CO2 and CO gas [59,60,72].
carboxylic acid; 1627 cm–1 for C–C stretching in‐ring vibrations
of benzene rings; 1384 cm–1 for C–C stretching in‐ring vibra‐
tions of benzene rings, C–H bending and rocking of alkanes, and
O–H deformations in C–OH groups; and 1062 cm–1 for C–O
stretching of alcohols, carboxylic acid, and ethers. These results
confirmed the presence of a variety oxygen containing func‐
tional groups on the NGO sheets.
The UV‐visible spectrum of the NGO sheets (Fig. 3(b))
showed an absorption peak at 224 nm for the π→π* transition
of C=C bonds in the sp2 domain of the carbon skeleton [73]. In
addition, a shoulder at 295 nm was assigned to the n→π* tran‐
sition of C=O bonds [74]. It is worth noting that a photolumi‐
nescence peak was not detected for the NGO sheets with exci‐
tation at 330 nm at pH = 7. This is consistent with the nanoscale
dimensions (100 nm) of the NGO sheets [75]. These results
show that the NGO sheets do not behave as quantum dots.
To investigate if the NGO contained heavy metals, it was an‐
alyzed by inductively coupled plasma optical emission spec‐
trometry. With a detection limit of 100 ppb, no heavy metals
were detected in an aqueous suspension of the NGO.
The molecular structure of the NGO sheets was further in‐
vestigated by FTIR spectroscopy. The FTIR spectrum (Fig. 3(a))
exhibited the following characteristic peaks: 3428 cm–1 for O–H
stretching vibrations of phenols, alcohols, carboxylic acid, and
adsorbed water; 1726 cm–1 for C=O stretching vibrations of
3.2. Oxidation of benzylic alcohols by the NGO carbocatalyst
In oxidation of benzyl alcohol derivatives by the NGO cata‐
lyst, all of the alcohols were oxidized and the reaction did not
stop at the aldehyde but proceeded to the carboxylic acid

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Alireza Sedrpoushan et al. / Chinese Journal of Catalysis 38 (2017) 745–757
O
H
(b)
(a)
HN
O
O
OH
HO
NGO
100 oC, 24 h
catalytic amount
NO Reaction
200% NGO (210 mg)
100 oC, 24 h
H2O2 as oxidant
H2O as solvent
N2 (1 atm = 0.1 MPa)
Or
H2O2 as oxidant
H2O as solvent
HO
1mmol
N
2 (1 atm = 0.1 MPa)
(d)
(c)
O
H
O
H
O
O
OH
H3C
catalytic amount
100 oC, 24 h
H2O2 as oxidant
H2O as solvent
N2 (1 atm = 0.1 MPa)
OH
catalytic amount
100 oC, 24 h
H2O2 as oxidant
H2O as solvent
NO Reaction
NO Reaction
Or
Or
HO
HO
N2 (1 atm = 0.1 MPa)
(f)
(e)
O
O
Cl
OH
HO
HO
1.8 eq. mCPBA
100 oC, 24 h
H2O as solvent
N2 (1 atm = 0.1 MPa)
catalytic amount
total conversion 13%
NO Reaction
100 oC, 24 h
H2O2 as oxidant
H2O as solvent
37% aldehyde
63% carboxylic acid
N
2 (1 atm = 0.1 MPa)
(g)
O
OH
O
H
HO
80 oC, 12 h
20% NGO (21 mg)
+
stoichiometric amount of KI
2.2 eq. H2O2 as oxidant
H2O as solvent
1mmol
NO Conversion
N
2 (1 atm = 0.1 MPa)
Scheme 1. Oxidation of benzyl alcohol and/or benzaldehyde in the presence of (a) NGO, (b) NGO–amide, (c) acetic acid, (d) benzoic acid, and (e)
epoxy cyclohexane as catalysts, (f) meta‐chloroperbenzoic acid (oxidant) and (g) KI (radical scavenger).
(Scheme 1(a)). Based on the reaction optimization experi‐
ments, the optimum conditions were 2.2 eq. of H2O2 (Table 1),
20% (mass fraction) of NGO (Table 2), and a reaction tempera‐
ture of 80 °C (Table 3). The low yield for oxidation of benzyl
alcohols in the absence of NGO confirmed the NGO acted as a
catalyst (Table 2). Altering the concentration of H2O2 in the
reaction did not greatly affect the product selectivity of benzyl
alcohol oxidation, which indicates that only NGO (not H2O2)
acts as a catalyst in this reaction (Table 1). Different derivatives
of benzyl alcohol exhibited different ratios of alde‐
hyde/carboxylic acid during the oxidation process, and also
required different reaction time to reach completion. These
results could be attributed to the dependence of the reaction
rate on the nature of substituents on the benzyl alcohols, and
the rates were in the following order: ‒OMe > ‒H > ‒Cl > ‒NO2.
Because of this dependence, the reaction yield for the oxidation
of 4‐nitrobenzyl alcohol was low (Table 4). The oxidation re‐
sults for secondary and primary benzylic alcohols were com‐
parable (Table 5). The highest reaction rate was observed for
diphenylmethanol, indicating the role of the substituents
groups on the benzylic carbon was in the order ‒Ph >> –H >
‒CH3.
Table 1
Oxidation of neat benzyl alcohol using various concentrations of H2O2.
O
H
O
OH
HO
100 oC, 24 h
100 NGO (105 mg)
+
H2O2 as oxidant
H2O as solvent
A
B
X
X
X
N
2 (1 atm = 0.1 MPa)
1 mmol
Entry
X
H
H
H
H
H
H2O2 (eq) Conversion to A (%) Conversion to B (%)
1
2
3
4
5
10
2.2
0
0
100
98
63
37
0
3.3. Oxidation of aromatic aldehydes by the NGO carbocatalyst
1
0
In the oxidation of benzaldehyde derivatives, the NGO
sheets alone could not act as an oxidant (Table 6) and the reac‐
0.5
0
0
<5

Alireza Sedrpoushan et al. / Chinese Journal of Catalysis 38 (2017) 745–757
751
tion only occurred in the presence of H2O2. In fact, H2O2 could
induce the reaction even with a low mass fraction of the NGO
catalyst (7%, Tables 7 and 8). The optimum NGO catalyst mass
fraction and reaction temperature were 7% and 70 °C, respec‐
Table 2
Oxidation of benzyl alcohol derivatives using NGO catalysts with various concentrations.
O
OH
HO
100 °C , 24h
2.2 eq. H2O2 as Oxidant
H2O as Solvent
N2 (1 atm)
X
X
1 mmol
4‐Choloro benzyl alcohol
4‐Methoxy benzyl alcohol
Benzyl alcohol
4‐Nitro benzyl alcohol
Entry NGO (%)
Conversion (%) C.W. a (mg) Conversion (%) C.W. a (mg) Conversion (%) C.W. a (mg) Conversion (%) C.W. a (mg)
1
2
3
4
5
6
7
200
100
50
30
20
10
0
95
95
96
94
95
31
<5
284
142
71
43
28
14
0
99
99
99
99
99
63
<5
276
138
69
41
28
14
0
98
97
97
97
96
43
<5
210
105
52
32
21
10
0
8
9
8
8
7
306
153
76
46
31
15
0
<5
<5
a Catalyst mass (mg).
Table 3
Oxidation of benzyl alcohol derivatives at various reaction temperatures in the presence of NGO catalysts.
Temperature
NGO
(wt%)
100
100
100
100
100
100
20
20
20
20
20
4‐Chloro benzyl alcohol 4‐Methoxy benzyl alcohol
Benzyl alcohol
conversion (%)
4‐Nitro benzyl alcohol
Entry
(οC)
RT
60
70
80
conversion (%)
conversion (%)
conversion (%)
1
2
3
4
5
6
7
8
0
<5
31
94
95
95
0
0
<5
42
100
100
100
0
0
<5
38
97
97
97
0
0
0
0
8
8
9
0
0
0
7
7
9
90
100
RT
60
70
80
0
<5
0
9
18
93
95
95
39
38
96
97
97
10
11
12
100
100
100
90
100
20
Table 4
Oxidation of benzyl alcohol derivatives with various catalytic reaction time in the presence of NGO catalysts.
Conversion (%)
TON a
Entry
X
1
3
6
9
12
24
(10‒2)
A
B
A
B
A
B
A
10
0
0
8
B
86
100
6
A
0
—
0
<5
B
A
B
—
—
9
1
2
3
4
H
OMe
NO2
Cl
25
37
0
10
<5
0
19
40
0
47
40
0
16
30
0
73
70
<5
62
96
—
8
—
—
0
4.4
3.6
0.2
3.3
19
11
12
40
10
72
87
0
93
a The turnover number (TON) was calculated as a ratio of the amount of oxidized product to the mass of NGO.

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Alireza Sedrpoushan et al. / Chinese Journal of Catalysis 38 (2017) 745–757
Table 5
Table 8
Oxidation of secondary alcohol derivatives to highlight the role of sub‐
strate structure in the oxidation reaction with a NGO catalyst.
Oxidation of aromatic aldehyde derivatives at various reaction temper‐
atures in the presence of NGO catalysts.
En‐
try
1
Catalyst mass Time Conversion TON a
Derivatives
(mg)
52
24
(h)
3
12
(%)
>99
>99
(10–2)
2.7
4‐Hydroxy 4‐Nitro ben‐ 4‐Choloro
Benzaldehyde
Diphenylmethanol
1‐Phenylethanol
En‐
try
benzaldehyde zaldehyde benzaldehyde
T (°C) conversion
2
4.1
conversion
conversion conversion
(%)
a The turnover number (TON) was calculated as a ratio of the amount of
oxidized product to the mass of NGO.
(%)
7
27
(%)
14
(%)
14
1
2
3
4
RT
60
70
80
11
27
56
33
Table 6
>99
>99
>99
>99
>99
>99
>99
>99
Oxidation of aromatic aldehyde derivatives using NGO as the catalyst in
the absence of H2O2.
was studied by FTIR and UV‐visible spectroscopy. The FTIR
spectrum (Fig. 3(c)) showed the characteristic peaks of (e.g.
1062, 1627, and 3428 cm–1) and new peaks at 1415 and 3081
cm–1. The NGO peak at 1384 cm–1 shifted to 1415 cm–1 after
oxidation, and this peak was similar to that for the O–OH bond
of peroxy acid [76]. The peak at 3081 cm–1, for the OH of car‐
boxylic acid, was separated from the OH peak of phenol, alcohol
and adsorbed H2O, which occurred at 3428 cm–1. The carbonyl
peak shifted from 1726 cm–1 for unreacted NGO to 1700 cm–1
for NGO after oxidation, and this peak and the aromatic C–C
peak at 1627 cm–1 both increased in intensity. These peaks
suggest that oxidation increases the number of carboxylic acid
groups in NGO, and converts some of these groups to peroxy
acids.
The UV‐visible spectrum of the NGO after oxidation (Fig.
3(d)) showed the same absorption peak as unreacted NGO at
224 nm for the sp2 domain of the carbon skeleton (π→π* tran‐
sition of C=C bonds). However, the peak for the n→π* transition
of the C=O bonds shifted to 284 nm for the NGO after oxidation.
This red shift, which was attributed to inter‐ and intramolecu‐
lar hydrogen bonding of peroxy acids [77], confirms the for‐
mation of peroxy acids from carboxylic acids on the NGO under
the oxidation reaction conditions.
Benzalde‐
hyde
conversion conversion
4‐hydroxy
benzaldehyde zaldehyde
4‐nitro ben‐
4‐choloro ben‐
zaldehyde
conversion (%)
En‐
try
T (οC)
conversion
(%)
12
6
6
<5
(%)
12
6
<5
<5
(%)
18
12
9
1
2
3
4
100
90
80
15
9
9
RT
<5
<5
tively, for the oxidation reaction. The reaction time experi‐
ments confirmed the dependency of the reaction rate on the
substituents. Aldehyde derivatives with electron withdrawing
substituents had higher reaction rates than those with other
substituents (‒NO2 > ‒Cl > ‒H > ‒OH) (Table 9).
3.4. Characterization of NGO after oxidation
The structure of NGO after reaction with H2O2 at 80 °C for 8
h (i.e. the optimum conditions for oxidation of benzyl alcohols)
Table 7
Oxidation of aromatic aldehyde derivatives using NGO catalysts with various concentrations.
O
OH
O
H
o
80 C, 12 h
(0 to 100)% NGO
1.1 eq H O as oxidant
2
2
H O as solvent
2
X
X
1 mmol
NGO (%)
100
50
20
7
5
0
Entry
X
Con.
(%)
>99
>99
>99
>99
C.W. a
(mg)
106
122
151
Con.
(%)
>99
>99
>99
>99
C.W. a
(mg)
53
61
76
Con.
(%)
>99
>99
>99
>99
C.W. a
(mg)
21
24
30
Con.
(%)
>99
>99
>99
>99
C.W. a
(mg)
8
9
11
Con.
(%)
46
38
63
C.W. a
(mg)
5
6
8
7
Con.
(%)
18
<5
32
C.W. a
(mg)
0
0
0
0
1
2
3
4
H
OH
NO2
Cl
140
70
28
10
48
16
a Catalyst mass (mg).

Alireza Sedrpoushan et al. / Chinese Journal of Catalysis 38 (2017) 745–757
753
Catalysis with NGO occurs in two major steps, activation of
the catalyst by the oxidant, and then oxidation of the substrate
by the activated catalyst. In the activation step, the peroxi‐
dase‐like ability of NGO leads to activation by the oxidant, and
NGO is converted to the active catalyst through radical interac‐
tions. This was confirmed by results obtained for oxidation of
benzyl alcohol in the presence of KI. Based on the FTIR and
UV‐visible spectroscopy results for NGO after oxidation, the
similar product selectivity for benzyl alcohol oxidation by
mCPBA and NGO separately, and the oxidation of alcohol
and/or aldehyde by NGO‐amide, we believe the activated cata‐
lyst is NGO‐peroxy acid. This activated catalyst is formed
through oxidation of the carboxylic group of NGO to peroxy
acid. In the subsequent catalytic cycle, the activated catalyst
(NGO‐peroxy acid) acts as an oxidant in oxidation of the sub‐
strate through a non‐radical interaction, which was confirmed
by the dramatic difference between the yield of oxidation of
4‐nitrobenzyl alcohol and other alcohol derivatives. The order
of the oxidation rates of diphenylmethanol, benzyl alcohol, and
1‐phenylethanol also confirmed this interaction did not involve
radicals because these results are incompatible with radical
reactions [78]. These steps are also in accordance with previ‐
ous reports on GO [30]. Therefore, based on what the above
descriptions, we proposed mechanisms for activation of the
NGO, oxidation of alcohol to aldehyde, and oxidation of alde‐
hyde to carboxylic acid.
Table 9
The catalytic reaction time required for oxidation of aromatic aldehyde
to yield a conversion rate of >99%.
4‐Hydroxy 4‐Nitro
benzalde‐ benzalde‐ benzalde‐
4‐Choloro
En‐
try
Benzal‐
dehyde
Derivatives
hyde
hyde
hyde
1
2
3
Time (min)
Conversion (%)
TON a (10–2)
150
>99
13.5
180
>99
120
>99
132
>99
10.2
11.7
9.5
a The turnover number (TON) was calculated as a ratio of the amount of
oxidized product to the mass of NGO.
3.5. Oxidation using similar catalysts
The oxidation of benzyl alcohols and benzaldehydes was
investigated using NGO‐amide and acetic acid as catalysts to
study the effect of the acidic nature of NGO (Scheme 1(b) and
(c)), benzoic acid to study the autocatalytic effect of the reac‐
tion product (Scheme 1(d)), and epoxy cyclohexane to study
the role of epoxide groups in NGO (Scheme 1(e)). The reaction
did not proceed using these catalysts. This means that no auto‐
catalytic effect occurred during the oxidation reaction. It also
indicates that although the acidic nature of NGO is important in
the oxidation reaction, this property alone cannot catalyze the
reaction. In fact, the high performance of NGO as a catalyst in
the reactions can be assigned to the synergistic effects of its
surface functional groups (not the active sites separately). It
should be noted that the oxidation of benzyl alcohols by
mCPBA resulted in production of benzaldehyde and benzoic
acid, and the selectivity was comparable to the NGO catalyst
(Scheme 1(f)). There was no conversion of benzyl alcohols in
the presence of KI, even under the optimum conditions, indi‐
cating that radical interactions were important in the NGO ca‐
talysis (Scheme 1(g)).
4.1. Activation mechanism of NGO
According to our proposed mechanism for activation of NGO
(Scheme 2), lone pair electrons delocalized at the vacancies at
the edges of the zigzag structure of the conjugated π‐system
activate the oxygen group of H2O2. Then, activated oxygen,
through a series of radical interactions with the carboxylic acid
groups of NGO, leads to formation of NGO‐peroxy acid as the
activated catalyst. This is compatible with the traditional syn‐
thesis for peoxy acid by H2O2 in an acidic medium [79].
4.2. Mechanism for oxidation of alcohols to aldehydes
4. Discussion
Based on our proposed mechanism for oxidation of alcohol
Scheme 2. The activation mechanism of the NGO catalyst.

754
Alireza Sedrpoushan et al. / Chinese Journal of Catalysis 38 (2017) 745–757
to aldehyde (Scheme 3), after activation of NGO to NGO‐peroxy
acid, with protonation of the hydroxyl group of the alcohol by
peroxy acid of NGO, partial positive charge formation can occur
at the benzylic carbon. Then, nucleophilic attack by oxygen
from the peroxy acid to the benzylic position occurs with loss of
H2O, and this leads to formation of structure A as an intermedi‐
ate. A C–H bond at the benzylic carbon and O–O bond in inter‐
mediate A then break and reforms NGO‐carboxylic acid and the
aldehyde product. The catalytic cycle can then continue. Inter‐
mediate A, and its decomposition process, are consistent with a
Criegee intermediate, and the higher migration ability of H to
the aryl group of this intermediate in the Baeyer‐Villiger reac‐
tion with mCPBA as oxidant [80,81]. The dependence of the
reaction rate and yield for oxidation of secondary alcohols on
the substituents at the benzylic carbon, and that of primary
alcohols on the substituents on the aromatic ring, determined
the formation of a partial positive charge at benzylic carbon,
and its stability is rate‐limiting. 4‐Nitrobenzyl alcohol, which
has the strongest electron withdrawing substituent, has the
lowest density of electrons at the benzylic carbon and the low‐
est ability for formation of a positive charge at this site, which
results in the lowest reaction rate and yields among the alcohol
derivatives. Among the alcohols, diphenylmethanol, which has
two electron donating substituents, has the highest electron
density at this position and the highest ability to form a positive
charge at this site. Consequently, it had the highest reaction
rate and yield for the oxidation among the alcohol derivatives.
Scheme 4. The oxidation mechanism for conversion of aromatic alde‐
hydes to carboxylic acids using the NGO catalyst.
benzylic carbon plays an important role. In contrast with the
alcohols, aldehyde derivatives with stronger electron with‐
drawing substituents on the aromatic ring have higher reaction
rates for the oxidation. In summary, the dependency of the
oxidation reaction rates and yields for the alcohols and alde‐
hydes on the substrate substituents, and the general principles
of these mechanisms, are similar to to the oxidation of ketones
by mCPBA in the Baeyer‐Villiger reaction.
4.4. Comparison of NGO carbocatalysts with other
graphene‐based carbocatalysts for oxidation
4.3. Mechanism of oxidation of aldehydes to carboxylic acids
Compared with the protocols for GO, rGO, nitrogen‐doped
graphene, and nitrogen‐doped activated carbon, the NGO pro‐
tocol is greener with a shorter reaction time, lower tempera‐
ture, and lower carbocatalyst requirements. The results con‐
firmed that reduction of the size of GO was effective for in‐
creasing the catalytic performance. Compared with GO, the
NGO carbocatalytic reaction uses a lower temperature, shorter
reaction time, less carbocatalyst, and has a higher conversion
yield for some substrates (e.g. 1‐phenylethanol, Table 10, en‐
tries 1–3). In addition, the oxidation of alcohols has different
selectivity in some cases (e.g. benzyl alcohol, Table 10, entries
9–11). Compared with N‐doped graphene, NGO is very similar
and gives the same selectivity and conversion yield, but NGO is
considered a greener oxidant with a lower carbocatalytic load
(Table 10, entries 4, 5, 9 and 10). Nitrogen‐doped activated
carbon carbocatalysts have been studied for the oxidation of
In accordance with our proposed mechanism for oxidation
of aldehydes to carboxylic acids (Scheme 4), protonation of the
oxygen group of the carbonyl and immediate nucleophilic at‐
tack to the carbon of carbonyl groups by peroxy acid of NGO
leads to formation of structure B as an intermediate. Then, loss
of benzylic C–H and O–O bonds in intermediate B results in
formation of NGO‐carboxylic acid and the carboxylic acid
product. All aspects of this mechanism, such as the order of
steps and the structure of B, are consistent with the mechanism
of Baeyer‐Villiger reaction with mCPBA as the oxidant [80,81].
Based on the results for oxidation of aldehydes, it is clear that
as with the alcohol oxidation, the density of electrons at the
OH
O
H
O
O
OH
NGO
X
X
H2O
benzyl alcohol over 5 h using O2
as the oxidant. However, the
O
reaction has a low yield and requires a higher temperature and
higher load of carbocatalyst than if the reaction is conducted
with NGO (Table 10, entries 6 and 9). The rGO does not act as a
catalyst in the oxidation of benzylic alcohols [82] (Table 10,
entries 7 and 8).
HO
O
NGO
H
H
O
O
NGO
H2O
H2O2
O
O
HO
NGO
"A"
X
O
5. Conclusions
X
H
Scheme 3. The oxidation mechanism for conversion of benzyl alcohols
to aromatic aldehydes using the NGO catalyst.
In this study, the catalytic activity of GO was increased by
reduced the size of GO sheets to NGO. The NGO is a green oxi‐

Alireza Sedrpoushan et al. / Chinese Journal of Catalysis 38 (2017) 745–757
755
Table 10
Comparison of NGO carbocatalysts with other graphene‐based carbocatalysts.
Entry
Substrate
Catalyst
Load of catalyst Oxidant
T (°C ) t (h) Conversion yield (%)
Product
Ref.
1
GO
200%
200%
O2/GO
O2/GO
O2/GO
TBHP
TBHP
100
100
100
80
24
24
24
12
12
92
98
26
99
99
[30]
[30]
[30]
[47]
[47]
2
3
4
5
GO
GO
200%
O
N‐doped
graphene
55% (30 mg)
50% (30 mg)
OH
N‐doped
graphene
80
N‐doped
activated carbon
6
7
8
85% (100 mg)
18% (6 mg)
16% (6 mg)
O2
O2
O2
120
100
100
5
12
12
23
0
[48]
[82]
[82]
rGO
rGO
—
—
0
This
work
9
NGO
NGO
NGO
20% (21 mg)
20% (24 mg)
20% (52mg)
H2O2
H2O2
H2O2
80
80
80
12
12
3
96
99
99
This
work
10
11
This
work
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hydes in the presence of H2O2 in aqueous media at low temper‐
atures (70–80 °C). The catalytic efficiency of the NGO colloidal
dispersion is comparable with those of conventional met‐
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Graphical Abstract
Chin. J. Catal., 2017, 38: 745–757 doi: 10.1016/S1872‐2067(17)62776‐1
Nanoscale graphene oxide sheets as highly efficient carbocatalysts in green oxidation of benzylic alcohols and aromatic
aldehydes
Alireza Sedrpoushan*, Masoud Heidari, Omid Akhavan
Iranian Research Organization for Science and Technology (IROST); Sharif University of Technology
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