desirable to develop nonmetallic carbocatalysts for these re-
was characterized by chemical analysis and XPS (see the
Supporting Information). In particular, this latter technique
was very informative because the deconvolution of the C1s
and B1s peaks agrees with the binding energies previously
reported by Rao and co-workers for (B)G prepared by arc-
[28]
actions. The oxidation of tetralin (1) affords 1-tetralone,
which is a commercially important organic intermediate and
can be used as an additive to enhance the cetane number in
[29]
diesel fuels.
transition-metal oxides,
and homogeneous metal complexes
This reaction has been performed by using
[30]
[31–33]
[36]
metal–organic frameworks,
discharge of graphite electrodes.
ACHTUNGTRENNGNU( B,N)G was prepared by modification of chitosan
[34]
as catalysts with
oxygen or peroxide as the oxidant. Therefore it would be
desirable from a green point of view and also for sustaina-
bility to develop catalysts for the oxidation of 1 to the corre-
sponding alcohol and ketone with high selectivity, preferably
by using oxygen as the oxidizing reagent, in the absence of
transition-metal catalysts to minimize waste.
[(B,N)G] solutions in water with borate, followed by pyroly-
sis of the corresponding boron-containing chitosan at 9008C
and final exfoliation. Note that in contrast to previous prec-
edents, the carbocatalysts employed in this work can be con-
sidered as formed of individual or few-layer G. The catalysts
and the procedures used for their preparation are detailed
in Table 1. Chemical analysis showed the absence of transi-
tion-metal impurities except in the case of GO, as indicated
by ICP-AAS.
Furthermore, nitrogen-doped graphene (N(G)) is pre-
pared from chitosan, a nitrogen-containing biopolymer that
can form high-quality films on glass, quartz, metals, and
other hydrophilic surfaces. Pyrolysis of the chitosan films
[35]
under argon at 8008C leads to N(G). This method offers a
new alternative methodology for the synthesis of G and
N(G) from naturally occurring biomass and can be used to
develop sustainable catalysts.
Table 1. G catalysts, the corresponding precursor, preparation procedure,
and heteroatom loading.
Material Precursor Preparation method
Heteroatom
loading
[wt%]
[
a]
In this paper we wish to report that nitrogen- and boron-
doped graphene ((N)G and (B)G) are highly active catalysts
in the absence of any transition metal for the activation of
dioxygen in the aerobic oxidation of benzylic compounds,
cycloalkanes, and styrene under solvent-free conditions at a
low catalyst/substrate mass ratio (<1 wt%). The aim of the
process was to achieve high selectivity of the ketone or alco-
hol/ketone mixture at moderate or high substrate conver-
sions. It will be shown that doped G acts as a carbocatalyst
for the aerobic oxidation of styrene under solvent-free con-
ditions resulting in around 45% SO selectivity. In addition,
the positive effect of heteroatom doping on G sheets on the
generation of radicals will be discussed.
G
GO
alginate
graphite Hummers oxidation, exfoliation
alginate doping with borate, pyrolysis at
008C, exfoliation
pyrolysis at 9008C, exfoliation
–
–
[
37]
(
B)G
B 13
9
(
N)G
chitosan pyrolysis at 9008C, exfoliation
N 6.65
A
H
U
G
R
N
N
(B,N)G chitosan doping with borate, pyrolysis at
B 2.1, N 5.6
9008C, exfoliation
[
a] The heteroatom contents were determined by XPS analysis.
We will first summarize the experimental results obtained
for the aerobic oxidation of tetralin with the (co)doped Gs
as catalysts and then report on the catalytic activity of these
carbocatalysts for the aerobic oxidation of styrene to BA
and SO.
Results and Discussion
Aerobic oxidation of tetralin and cyclooctane: Initially we
selected the aerobic oxidation of tetralin to tetralinhydro-
peroxide (2), tetralol (3), and tetralone (4) as the model re-
action to optimize the conditions (see Table 2). In the ab-
sence of catalyst, 9% conversion of 1 was observed with
91% selectivity of 3 and 4 (alcohol/ketone) in 24 h due to
some degree of auto-oxidation. By using GO as the catalyst,
1 was oxidized with 4% conversion in 8 h with 90% selec-
tivity of 3 and 4 along with 10% of 2. The formation of 2 in-
creased gradually reaching a maximum and then decreased
due to the subsequent formation of 3 and 4. After 24 h,
20% conversion with 92% selectivity of 3 and 4 was ob-
served by using GO as the catalyst. When molecular oxygen
was excluded from the reaction and oxygen gas was re-
placed by nitrogen then a negligible conversion of 1 was ob-
served with GO and N(G) as catalysts, which proves the
active role of oxygen as oxidant in the reaction. Figure 1
shows the time–conversion profile for the oxidation of 1.
It is well documented in the literature that aerobic oxida-
tions with homogeneous catalysts can be favored by using
[
2]
[35]
GO and (N)G
were prepared following previously re-
ported procedures. In particular, GO was obtained by
Hummers oxidation of graphite followed by exfoliation by
sonication in water and its detailed characterization can be
found in the Supporting Information. Note that this proce-
dure produces isolated GO nanosheets that become dis-
persed in the liquid phase. (N)G was obtained by pyrolysis
[35]
of chitosan beads at 9008C under an inert atmosphere.
The nitrogen content was 6.65 wt%, as determined by com-
bustion chemical analysis. Note that chitosan as a substrate
offers the advantage of acting simultaneously as a carbon
and nitrogen source, giving rise to (N)G that, according to
X-ray photoelectron spectroscopy (XPS), contains about
5
0% of pyridinium-like nitrogen, 48% of pyridine-like ni-
[35]
trogen, and a small amount of N-oxide.
(
B)G was prepared by boration of alginate in aqueous sol-
utions followed by calcination of the borate esters of algi-
nate at 9008C under an inert atmosphere. The preparation
of this (B)G sample has not been previously reported and it
7548
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 7547 – 7554