Eco-efficient preparation of a N-doped graphene equivalent and its application to metal free selective oxidation reaction

Article abstract of DOI:10.1039/c4gc00049h

Here, we demonstrate that graphene oxide (GO) can be converted to N-doped reduced GO (rGO) that could become a substitute for N-doped graphene. Simultaneous doping and reduction can be accomplished for this purpose by simply mixing GO with hydrazine and then continuously sonicating the solution at 65 °C. A high level of reduction is realized, as evidenced by a carbon to oxygen ratio of 20.7 that compares with the highest value of 15.3 ever reported in solution (water + hydrazine) methods. Nitrogen doping is possible up to 6.3 wt% and the extent of doping can be increased with increasing sonication time. Notably, the simple tuning process of N-doping in GO greatly enhanced the efficiency of the carbocatalyst for various kinds of metal free oxidation reactions and hence is proposed as a suitable candidate for future industrial applications. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00049h

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Cite this: DOI: 10.1039/x0xx00000x
Eco-efficient preparation of N-doped graphene
equivalent and its application to metal free selective
oxidation reaction
Received 00th January 2014,
Accepted 00th January 2014
DOI: 10.1039/x0xx00000x
Ajay K. Singh, K.C Basavaraju, Siddharth Sharma, Seungwook Jang, Chan Pil Park,
Dong-Pyo Kim*
www.rsc.org/
Here, we demonstrate that graphene oxide (GO) can be converted to Nꢀdoped reduced GO (rGO) that
could become a substitute for Nꢀdoped graphene. Simultaneous doping and reduction can be
accomplished for the purpose by simply mixing GO with hydrazine and then continuously sonicating the
o
solution at 65 C. A high level of reduction is realized as evidenced by a carbon to oxygen ratio of 20.7
that compares with the highest value of 15.3 ever reported in solution (water + hydrazine) methods.
Nitrogen doping is possible up to 6.3 wt% and the extent of doping can be increased with increasing
sonication time. Notably, the simple tuning process of Nꢀdoping in GO greatly enhanced the efficiency of
the carbocatalyst for various kinds of metal free oxidation reactions, and henceforth is proposed as a
suitable candidate for future industrial application.
lattice of graphite sheets through substitutional doping is thereby a direct
way to control the steric and electronic properties of graphite.
Furthermore, the process remarkably expands its applications in frontiers
Introduction
Highly selective catalysts that are readily recycled, cheaper and metal
free are vital for the development of sustainable chemical process,1
moreover, in certain cases, reaction parameters can be carefully tuned to
improve process performance. Generally, in case of homogeneous
catalysts, the steric and electronic features of novel metals (palladium,
rhodium, ruthenium, and iridium) are varied to attain high selectivity.2
However, the high price and limited availability of these precious metals
have spurred interest in metal free catalytic system among the scientific
community, especially carbocatalyst, due to more earthꢀabundant
inexpensive alternatives.3
of catalysis and electronic devices. Bielawski et al. introduced only O
heteroatom on graphite layer to make it a suitable candidate for the
oxidation reaction.3 The only drawbacks were low catalytic efficiency,
requirement of high loading (60ꢀ400 wt%) and low thermal stability
during oxidation reaction.4c Wang et al. introduced only N atom on
graphene layer at high temperature chemical vapour deposition (CVD)
process to make carbocatalyst, but the tuning of N and O contents on
graphite layer was very limited.4a In light of everꢀexpanding applications
of tuned Nꢀdoped reduced graphene oxide (rGO) and high throughput
availability of GO, it is highly desirable to have a direct and yet efficient
approach for simultaneous reduction and Nꢀdoping on GO.
Unintentional nitrogen doping was comprehended in an initial
attempt to reduce graphite oxide with hydrazine.5 Hydrazine is highly
toxic and dangerously unstable.6,7 Microfluidic chemical technology is
one of the most suitable tools to minimize such toxic and explosive
substances owing to their extremely small internal volume and
continuous consumption capability.8 And it can be considered as an
greener and sustainable approach towards hydrazine involved
chemistry.9
By introducing the oxygen, and nitrogen atoms in the carbon network,
the steric and electronic features of graphite (semiꢀmetal) could be tuned,
which can subsequently be used as equivalent to metal or sometimes as a
better alternative to metal catalyst.4 Engineering of hexagonal crystalꢀ
National Center of Applied Microfluidic Chemistry
Department of Chemical Engineering, POSTECH (Pohang University of
Science and Technology), Pohang, Korea 790-784
Email: dpkim@postech.ac.kr
Electronic Supplementary Information (ESI) available: Detailed
To minimize the hydrazine exposure, we present an ecoꢀefficient
and simultaneous approach for reduction and nitrogen doping on GO,
which yields the highest carbon to oxygen ratio and the highest nitrogen
content ever reported in solution methods. We also utilized tuned Nꢀ
e
xperimental
procedure,
catalysts
preparation,
catalysts
characterization, catalytic result, comparative study, catalyst
proposed mechanism and the supplementary results. See
DOI: 10.1039/c000000x/
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doped rGO to expand the horizon of applications to metalꢀfree catalysis
This hydrazine reduction of GO resulted in a carbon to oxygen
for selective oxidation of hydrocarbons, alcohols, bromoalkanes, and atom ratio of 10.3, which was 2.7 before the reduction, and at the same
oxidative coupling reaction of aminoalkanes to produce the time led to incorporation of nitrogen, yielding a carbon to nitrogen ratio
functionalized compounds having exceptional selective ketones, acid, of 16.1. Intentional nitrogen doping has invariably been accomplished in
ester, ether and amides using mild oxidants (peroxide) under evergreen the process of producing rGO from GO. The methods include thermal
water solvent.
annealing with a nitrogen compound,10 hydrothermal or solvothermal
reduction with a nitrogen compound,11 and simple melamine reduction.12
The highest nitrogen content achieved by simple hydrazine reduction
methods11d was 5.2 wt % with a carbon to oxygen atom ratio of 8.4. The
effect of retention time on extent of tuned Nꢀdoping by MR was
investigated by elemental analyses. As shown in Table 1, the nitrogen
wt% in rGO exhibited a continuous increase with extended retention
time of up to 60 min (MRꢀ60), and eventually reached a plateau of 6.3 %,
which is the highest value ever reported in simple solution methods.11d
Not much difference was observed between the nitrogen contents of
traditionally treated rGOꢀb that was prepared by the usual hydrazine
reduction (24 h) method (detailed method in the ESI) and that of MRꢀ30,
indicating significant influence of the retention time on Nꢀdoping by the
microsonochemical reduction. Brunauer–Emmett–Teller (BET) surface
area was decreased with increasing Nꢀcontent in graphene surface may
be the cause of agglomeration.
Result and Discussion
To overcome the safety issue and ecoꢀefficient simultaneous reduction
and Nꢀdoping of GO, we have devised an approach that involves
introducing aqueous solutions of GO and hydrazine hydrate into a
capillary microreactor and then subjecting the microreactor to sonication
(mechanical energy) to induce microsonochemical reduction (MR)
process. In addition, as shown in Fig. 1, both Tꢀmixer and Teflon
(PTFA) capillary microreactor were placed in the ultrasonic bath (power
= 330 W, frequency = 40 KHz, temperature = 65 oC) to circumvent tubeꢀ
blocking problem, which was encountered initially. The reduced
graphene oxide samples in the form of black precipitate were collected at
controlled retention times in a range between 30 and 70 min by varying
the capillary length with a constant flow rate of 30 ꢁL/min (see
electronic supplementary information (ESI) Table S1). The doped rGO
samples thus obtained were designated as MRꢀX, where X denotes the
retention time in minutes. It should be noted that mass producibility is
assured of Nꢀdoped rGO because the process involves sonication of a
GO solution in a continuous flow manner at 65oC. A microreactor
system is utilized here for demonstration purpose to take advantage of
better heat and mass transfer properties of the system, which facilitates
tuning of carbon, oxygen and nitrogen contents by controlling retention
time.
Whilst considering the production of tuned Nꢀdoped rGO by
microsonochemical method (MRꢀ60), it is of interest to expand the
application of Nꢀdoped rGO in areas of effective metalꢀfree catalyst for
CꢀH bond activation and comparing the outcomes with reported studies
on metal based catalysts and other carbocatalysts.4a Oxidation of 1ꢀ
Methylindolineꢀ2ꢀone, which involved reaction in aqueous phase with
tertꢀbutyl hydro peroxide (TBHP) as an oxidant4a (detailed method in
ESI) was selected as a model reaction to evaluate the effect of tuned
carbon, oxygen and nitrogen content and also the degree of influence of
exfoliation/surface area on the conversion and selectivity. An increase in
conversion with higher nitrogen content and with lower oxygen content
was observed; however, the surface area (Table 1) had no noticeable
effect on the catalytic efficiency. Oxidation of indolineꢀ2ꢀone was also
carried out to test the efficiency of Nꢀdoped rGO (MRꢀ60) as a catalyst
for CꢀH bond activation. From Table 1, in general, it is obvious that an
increased N amount in graphene oxide became less hydrophilic, then the
highly Nꢀdoped rGO layers tended to agglomerate and decrease the
surface area. Therefore, MRꢀ60 revealed the highest nitrogen (6.3%), the
lowest oxygen content (4.2%) and the lowest surface area (67 m2gꢀ1)
(Fig. S1 in the ESI), which enabled accomplishment of 96% conversion
with 92% selectivity, and formation of 1ꢀmethyl indoleꢀ2, 3ꢀdione as a
major product in 12 h (Table S2). It indicated that the catalytic CꢀH bond
oxidation reactions were dominated by nitrogen content of Nꢀdoped rGO,
rather than the surface area (Table S2).
Fig. 1. Schematic presentation for synthesis of Nꢀdoped rGO (MRꢀX
sample) by continous microsonochemical process.
Table 1. Comparative elemental analysis and XRD of graphite, graphite oxide and reduced graphene oxides powders.
Materials
C
O
H
N
C/O
C/N
C/(O+N)
d(100) (Ao)
BET surface area
[m2gꢀ1]
2θ
Graphite
GO
99.25
41.48
78.72
78.54
82.51
84.96
87.01
87.02
0.009
52.32
13.95
13.86
10.31
7.01
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
26.54
9.82
23.60
ꢀ
3.36
9.0
3.81
ꢀ
7.8
680
2.41
1.21
0.95
0.99
1.02
1.09
1.09
0.79
5.64
5.66
8.02
12.1
20.7
20.7
0.79
4.55
4.51
5.54
7.06
8.25
8.25
rGOꢀb
MRꢀ30
MRꢀ40
MRꢀ50
MRꢀ60
MRꢀ70
3.41
3.54
4.56
5.02
6.34
6.35
23.08
22.18
18.09
16.92
13.72
13.70
85.7
130.6
90.5
78.3
67.1
66.5
ꢀ
ꢀ
ꢀ
24.70
ꢀ
ꢀ
3.62
ꢀ
4.20
4.21
2 | Green Chem., 2014, 00, 1-6
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As shown in Fig. S2a (in the ESI), the intensity of N1s peak of MRꢀ of MRꢀ60 in Fig. 2h, showed crystalline structure. The first ring came
60 is higher than that of rGOꢀb, which is produced by the conventional from the (1100) plane and the bright spots corresponding to the (1100)
hydrazine batch process, indicating higher N content in MRꢀ60 platelets. reflections retained the hexagonal symmetry of the [0001] diffraction
The resolution of deconvoluted N1s peak was achieved into three patterns. The MRꢀ60 platelets with more than six bright hexagonal spots
components centred at 398.1ꢀ399.3 eV for pyridinic N, at 399.8ꢀ401.2 might contain clustered defect arising from oxidation.16
eV for pyrolic N, and at 401.1ꢀ402.7 eV for graphitic or quaternary N
(Fig. S3a in the ESI).13 Fig. S3a also reveals the nature of nitrogen in
rGOꢀb as mostly pyridinic (~50%) or pyrrolic (~49%) and only a
negligible amount of nitrogen (~1%) as graphitic or quaternary (Fig. S3b
in the ESI), whereas nitrogen in MRꢀ60 as more substantially graphitic
(~20%) (pyridinic:~51% and pyrrolic:~29%). At the same time it is
apparent that most of the nitrogen (~ 99%) was introduced into the
graphene edges and only 1% into the basal plane when GO was reduced
by the conventional hydrazine method, whereas a substantial amount of
nitrogen (~ 20%) was incorporated into graphene basal plane when GO
was reduced by the microsonochemical reduction (MRꢀ60). In the
traditional hydrazine treatment, it is known that adjacent layers of GO
platelets snap together due to π− π interaction5 as the basal planes near
the edges become reduced. As a result, there is narrowing of interlayer
distance, which prevents further penetration of hydrazine into the interior,
leading to a limited extent of nitrogen doping. However, it is well known
that the ultrasound functions by sonochemical process that involves
sequential formation, growth and collapse of millions of microscopic
vapour bubbles (voids) in the liquid. These rapid and violent implosions
generate shortꢀlived regions with highly intensified energetic
environment (~5000 oC and ~1000 atm, ~109 oC/sec as heating and
cooling rates).14 Such localized hot spots must be visualized as a
microreactor in which the energy of sound is transformed into a useful
chemical form.15 In the microsonochemical process, continuous
Fig. 2. Microscopic analysis of GO, rGOꢀb and MRꢀ60; (a, b&c) SEM
images of GO, rGOꢀb and MRꢀ60 (d, e&f) TEM image of MRꢀ60 in
surface view; crossꢀsection of a stacked graphitic layers of MRꢀ60; (g) dꢀ
spacing from stacked MRꢀ60 layers; (h) selected area electron diffraction
pattern of MRꢀ60; (i) mixed C and N elemental mapping in nanometer
range.
sonication creates hot spots due to bursting microscopic bubbles, which
along with the high surface area to volume ratio of microreactor could
possibly lead to activation of hydrazine, aiding in dispersion of GO
platelets, and enhancement of mass and heat transfer. Consequently, it is
hypothesized that combination of these factors must have facilitated
diffusion of hydrazine into the interior of GO platelets.
The platelets in MRꢀ60 samples were well dispersed in DMF solvent
and remained stable even after 1 month (Fig. S4, in the ESI), which is an
additional evidence for the presence of highly deoxygenated graphene
platelets. TGA (Fig. S5 a&b, in the ESI) shows that the weight loss by
MRꢀ60 is much less (13.2 wt%) when compared to both rGOꢀb (22 wt%)
and GO (80 wt%) when heated to 800oC in nitrogen atmosphere; the
analysis reveals high thermal stability of MRꢀ60 presumably due to the
higher level of deoxygenation. The scanning electron microscopy (SEM)
images of GO, rGOꢀb and MRꢀ60, were as shown in Fig. S6 aꢀc,
respectively. The layers of rGOꢀb was relatively ill arranged with the
outer layers slightly delaminated, presumably due to stirring, whereas
those of MRꢀ60 was well aligned. As these clusters were not fully
restored into the hexagonal graphene framework, the MRꢀ60 platelets
have inclusions containing a periodic decoration of functional groups.
Furthermore, the graphitic laminar structure of stacks of MRꢀ60 platelets
could be resolved in the ordered region. The nitrogen and carbon in MRꢀ
60 graphene was found to be uniformly distributed not only at the edge
but also in the plane of graphene, as evidenced by the TEM elemental
mapping in Fig. 2i. At here, it should be pointed out that the
microsonochemical method presented here is quite simple to carry out at
The
elemental
composition
tuning
capability
of
the
microsonochemical method is also apparent with respect to carbon to
oxygen (C/O) weight ratio as given in Table 1. In the case of the
traditional hydrazine treatment (rGOꢀb), the C/O ratio reached 5.64,
when GO with a C/O ratio of 0.79 was treated with hydrazine for 24 h.
Whereas, in case of continuous microsonochemical process, the C/O
ratio was attained as 5.66, when GO with C/O ratio of 0.79 was treated
with hydrazine only for 30 min. Subsequently, the C/O ratio reached
20.7 after 60 min, which far exceeds the highest ever reported C/O ratio
of 15.3, that was obtained by treating GO with a mixture of HI and acetic
acid for 40 h.16 XRD (Table 1, Fig. S2b, in the ESI) reveals the interlayer
distance for MRꢀ60 as 3.62 Å (2θ = 24.7o), which proves better recovery
of the layered structure as further indicated by lowering dꢀspacing than
3.81 Å of rGOꢀb. C1s and O1s, XPS spectra further confirm that the
delocalized πꢀconjugation was restored in MRꢀ60 sample (Fig. S2c&d,
Fig. S3cꢀh in the ESI).
HRꢀTEM image of MRꢀ60 sample (Fig. 2 dꢀf) shows well oriented
graphitic layers (up to 22 layers) that was ~7.8 nm thick (Fig. 2 g), which
corresponds to 3.63 Å interlayer distance, consistent with 3.62 Å
obtained by XRD measurements. This low interlayer distance of MRꢀ60
was comparable to a dꢀspacing of 3.64 Å for the reduced GO with no Nꢀ
doping, obtained by a mixture of HI and acetic acid.16 From these MRꢀ
60 platelets, the selected area electron diffractions pattern (SAED) was
performed on this region along [001] zone axis. The diffraction pattern
o
low temperature (65 C) and the doping level can simply be controlled
by extending retention time.
Electrical conductivity of MRꢀ60 (~9035 S/m) was enhanced by ~5
fold when compared to rGOꢀb (~1630 S/m) samples (details in ESI).
Eventually, the aforeꢀmentioned analytical results demonstrate excellent
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tunning capability of microsonochemical process. To the best of our
knowledge, until date, the tuned GO based material has not been
reported as an efficient oxidation catalyst under mild condition except
for the available report on layered Nꢀdoped graphite for oxidation of
benzyl alcohol.4
oxidation presumably due to πꢀπ interactions between aromatic reactant
and graphene layer.4a
Entry
Reactant
Product
% Conversion
(% Selectivity)
1
2
3
99 (99)
99 (97)
99 (98)
Entry
1
Reactant
Product
% Conversion
(% Selectivity)
96 (92)
4
5
6
7
96 (98)
97 (98)
99 (98)
99 (99)
2
3
73 (74)
O
99 (92)
99 (95)
99 (97)
93 (92)
4
5
Scheme 2: Carbocatalytic selective oxidation for syntheses of aromatic
acid products. General conditions: substrate (0.5 mmol), MRꢀ60 catalyst
(0.03 g), TBHP (5.0 mmol, 65 % in water), water (1 mL), time 12 h at 80
oC, conversion and selectivity based on GC analysis by using anisol as
an internal standard.
6
A distinct advantage of the MRꢀ60 catalyst against the supported
metal catalysts is durable catalytic activity because there is no
component to be leached out in the course of a reaction. To test this
hypothesis, MRꢀ60 catalyst was recycled six times for ethyl benzene
oxidation. There was nearly no change in selectivity only with little
decreased conversion, presumably due to incomplete sample recovery in
filtration and washing step (Fig. S6 & Table S4 in the ESI). No alteration
in Nꢀdoping was observed after recycling for six times as confirmed by
elemental analysis.
To insure the reaction mechanism for selective CꢀH bond oxidation,
ethyl benzene was chosen as a reactant without oxidant (TBHP), and no
reaction was observed, indicating that the catalyst and water oxygen
didn’t participate in the reaction. On the other hand, when a freeꢀradical
7
8
70 (48)
99 (99)
O
Scheme 1: Carbocatalytic selective oxidation for syntheses of aromatic
and aliphatic ketone products. General conditions: substrate (1.0 mmol),
MRꢀ60 catalyst (0.03 g), TBHP (3.0 mmol, 65 % in water), water (3 mL),
time 12 h at 80 C, conversion and selectivity based on GC analysis by
using anisol as an internal standard.
o
After having demonstrated the excellent activity of MRꢀ60 catalyst in scavenger (TEMPO, 10 mmol %) was used along with the reactants
the model reaction, we investigated selective oxidation of a series of including TBHP, the reaction did not occur, which is a direct evidence
structurally diverse benzyl and alkyl substituted compounds (Scheme 1).
for the progress of a reaction through free radical mechanism. Moreover,
As shown in Scheme 1, excellent yields of corresponding aromatic TBHP needs to be activated directly on the surface catalyst for the
ketones were typically obtained and compared with the previous reports oxidation reactions, with formation of radicals in the solution (Fig. S7, in
on metal based catalysts (Table S3, in the ESI). Apart from simple
the ESI). Similar results were obtained when TBHP was used to oxidize
aromatic CꢀH bond oxidation (Scheme 1, Entry 1ꢀ6); the most difficult the same substrate in the presence of Nꢀdoped graphite, where the
aliphatic CꢀH bonds were also selectively oxidized to aliphatic ketone reaction is known to proceed via radical mechanism.4a After having
(Scheme 1, Entry 7). The results of selective oxidation demonstrate that
demonstrated the reaction mechanism that the freeꢀradical intermediate
MRꢀ60 delivers a much better performance than the best known generated peroxide radical, we further investigated selective oxidation of
manganeseꢀcontaining molecular sieve catalyst (MnꢀALPOꢀ18).17 benzyl alcohol and a series of structurally diverse benzyl bromides for
Alcohol functionalized CꢀH bond was selectivity converted to ketones
synthesis of aromatic acids to understand the substitutional effect. As
(Scheme 1, Entry 8) and results were comparatively good with other can be seen in scheme 2, various acids are obtained with excellent yields
types of previously reported carbocatalysts and Pd based metal catalyst and selectivity. Apart from simple benzyl alcohol (Scheme 2, Entry 1),
(Table S3, Entry 1ꢀ8, in the ESI).18 Aromatic CꢀH bond oxidation
pꢀmethyl, pꢀfluoro, pꢀchloro, pꢀbromo benzyl bromide, and styrene were
showed better selectivity as compared to nonꢀaromatic CꢀH bond selectively converted into the corresponding aromatic acid products
4 | Green Chem., 2014, 00, 1-6
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(Scheme 2, Entry 2ꢀ7) and these results were comparable with previous functional group in peptides, polymers, and many natural products and
reports on metal catalysts (Table S3, Entry 9, in the ESI).19
pharmaceuticals.22 Our MRꢀ60 carbocatalyst also offered reasonable
conversion and selectivity for amide bond formation reactions through
dihydrogen extrusion (Scheme 4) and the results were comparable with
that of precious Ru assisted homogenous metal catalysis (Table S3,
Entry 13, in the ESI).22
Entry
1
Reactant
Product
% Conversion
(% Selectivity)
99 (85)
99 (97)
99 (99)
96 (97)
Scheme 4: Carbocatalytic selective amide synthesis. General conditions:
substrate (1.0 mmol), MRꢀ60 catalyst (0.03 g), H2O (3 mL), time 12 h at
80 oC, conversion (75 %) and selectivity (68 %) based on GC analysis by
using anisol as an internal standard.
2
3
4
Biphasic medium system (water and organic) with a solid catalyst
usually requires a long reaction time, especially when the reaction is
carried out in a batch system such as a flask. In the present work, this
timeꢀconsuming problem was overcome by performing the reaction in a
continuous flow capillary microreactor (details are provided in ESI and
Fig. S8); the high surface area to volume ratio afforded by microreactor
enhanced mass and heat transfer, thereby accelerating the reaction.23 It is
apparent from entry 3 in Table S3 (oxidation of ethylbenzene) that a
remarkable reduction in reaction time (40 min from 12 h) was achieved
with the use of capillary microreactor system (MRꢀ60 vs. MRꢀ60 in
microreactor).
5
6
97 (95)
99 (95)
7a
8a
99 (99)
99 (99)
Scheme 3: Carbocatalytic selective oxidation for syntheses of aromatic
ester or ether products. General conditions: substrate (0.5 mmol), MRꢀ60
catalyst (0.03 g), TBHP (5.0 mmol, 65 % in water), MeOH (0.5 mL),
time 12 h at 80 C, (a) without TBHP (R = Phenyl, pꢀmethylphenyl, pꢀ
fluorophenyl, pꢀchlorophenyl, pꢀbromophenyl), conversion and
Conclusion
We have developed a facile technique for C, N and O tuned rGO catalyst
by using simple microsonochemical method and propose that rGO
material can catalyze selective oxidation. The microsonochemical
approach yielded Nꢀdoped rGO catalyst (MRꢀ60) with the highest
carbon to oxygen ratio (20.7) and the highest nitrogen content (6.3 wt%)
during a short reaction time (60 min) ever reported by solution methods.
The MRꢀ60 carbocatalyst showed excellent catalytic activity and
reusability in the CꢀH bond activation for selective oxidation reaction
under mild reaction conditions. Furthermore, MRꢀ60 is functional group
tolerant, inexpensive (novel metal free catalyst) and environmentally
gentle catalyst for industrial prospective known until date. The tuned
o
selectivity based on GC analysis by using anisol as an internal standard.
After the successful synthesis of acid, we also checked the solvent
effect on the oxidation reaction, where we just replaced water solvent
with methanol to ensure better dispersion of biphasic reaction.
Nevertheless, completely surprising results were obtained due to
conversion of aromatic acid into unexpected aromatic ester.
Subsequently, we evaluated other substituted benzyl halides under
optimized conditions, and the corresponding esters were similarly
produced with nearly complete conversion and excellent selectivity
(Scheme 3, Entry 1ꢀ6). Synthesis of ester from alcohol has been mostly
catalyzed by the metal catalyst (Pd, Au, Ru, Ir, Co) but till to date, there
are no available reports on functioning of Nꢀdoped graphene oxide as an
efficient oxidation catalyst under mild conditions (Table S3, Entry 10, in
the ESI). Furthermore, there are no literature reports on direct synthesis
of aromatic ester from benzyl bromide.20 After the successful cross
esterification of methanol and various benzyl bromides and alcohol, we
were interested in demonstrating the effect of oxidant (TBHP) on
esterification reaction. As shown in Scheme 3 (Entry 7&8), aromatic
ether was selectively produced from treatment of benzyl alcohol or
bromides with methanol in the absence of TBHP and additive for 12 h at
80 oC under optimized conditions; no byproducts were observed in
crude reaction mixture. Furthermore, direct synthesis of aromatic ether
from benzyl alcohol or bromides using carbocatalyst has not been
reported except for the published reports on metal based catalyst with
base and additives (Table S3, Entry 11, 12, in the ESI).21
MRꢀ60 possesses the potential of becoming
a massꢀproducible,
substitute material for precious metal catalysts. Moreover, the immense
potentials of MRꢀ60 could also be extended to diverse applications as
electrochemical catalysts, electrode materials, bioꢀmedical applications,
green energy and water purification.
Acknowledgements
This work was supported by a National Research Foundation of Korea
(NRF) grant funded by the Korean government (MEST) (2008ꢀ
0061983).
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