Role of graphene oxide as a heterogeneous acid catalyst and benign oxidant for synthesis of benzimidazoles and benzothiazoles

Article abstract of DOI:10.1039/c5ra19066e

We report the synthesis of benzothiazoles and benzimidazoles using graphene oxide as an effective catalyst with good yields and easy recyclability. Graphene oxide plays the dual role of a metal-free acid catalyst and an oxidizing agent. The mechanism of a

Full text of DOI:10.1039/c5ra19066e

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Role of graphene oxide as heterogeneous acid catalyst and benign oxidant forDsOyI:n10t.h10e3s9i/Cs5RA19066E
of benzimidazoles and benzothiazoles
Dhopte, Kiran B.,1 Zambare, Rahul S.,1 Patwardhan, Anand V.,1 Nemade, Parag R. 1,2,
*
1
Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh
Marg, Matunga, Mumbai Maharashtra, 400019, India.
2
Department of Oils, Oleochemicals and Surfactant Technology, Institute of Chemical
Technology, Nathalal Parekh Marg, Mumbai, Maharashtra, 400 019, India.
Corresponding Author*
Tel.: + 91- 22 3361 2027;
Fax: +91- 22 3361 1020;
E-mail address: pr.nemade@ictmumbai.edu.in.
1

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DOI: 10.1039/C5RA19066E
Abstract
We report the synthesis of benzothiazoles and benzimidazoles using graphene oxide as an
effective catalyst with good yield and easy recyclability. Graphene oxide plays dual role of
metal-free acid catalyst as well as an oxidizing agent. The mechanism of action of graphene
oxide as a catalyst was confirmed using X-ray diffraction, Fourier transform infrared
spectroscopy, Raman spectroscopy and transmission electron microscopy. Good yields were
obtained at 60 °C as well as under ultrasonic irradiation at 35 °C with methanol as solvent.
Additionally, this is also the first report in which the regeneration of partially-reduced spent
graphene oxide has been carried out to restore the oxygen containing functional groups and
provide a path for reuse of graphene oxide as catalyst and oxidizing agent.
Keywords: graphene oxide, acid catalyst, oxidizing agent, benzothiazole, benzimidazole,
ultrasonication, regenerated graphene oxide
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DOI: 10.1039/C5RA19066E
Introduction
Synthesis of fused five membered heterocyclic ring systems elicit interest due to their
applicability in cell metabolism, active pharmaceutical ingredients, drug intermediates, etc. 1–3
Benzothiazole and benzimidazole moieties are particularly relevant due to their potential
activity and as a key intermediate in the synthesis of antiviral, antibacterial and other drugs.4–7
Oxidative cyclization pathway is one of the facile routes for the synthesis of these fused
heterocyclic rings. Numerous metal-based heterogeneous catalysts have been reported over the
years for benzothiazole and benzimidazole synthesis using oxidative cyclization pathway.8–12
Heavy metal toxicity and leaching concerns associated with metal oxidants, especially for
synthesis of drug intermediates have led researchers to search for metal-free pathways.
Oxidizing agents such as hydrogen peroxide, cyanuric chloride13 in conjunction with an acid
catalyst such as ion exchange resins,14–16 ultrasound mediated pathways,17,18 have been
reported with good yield, selectivity and short reaction times. However, homogeneous nature
of most oxidants and their application in stoichiometric amount is an area of concern.19–22
Among metal free catalysts, graphene oxide (GO) also termed as “carbocatalyst”, has been
reported to facilitate several organic transformations replacing hazardous chemical reagents.23–
29 Graphene oxide presents large surface area, abundant functional sites, low toxicity, ease of
synthesis and reuse potential. On account of presence of large number of oxygen containing
functional groups such as carboxylic acid and hydroxyl groups, graphene oxide exhibits highly
acidic character. GO has also been shown to act as benign oxidizing agent. The emphasis of
current work is to develop a metal free cyclization pathway to reduce environmental hazards
and overcome the shortcomings of reported pathways. We report use of graphene oxide as a
metal free catalyst for synthesis of benzimidazoles and benzothiazoles. Our present
methodology expands the role of graphene oxide as an acid catalyst utilizing the surface bound
functional groups as well as an oxidizing agent for synthesis of benzothiazoles and
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benzimidazoles under ultrasonic irradiation and heating conditions. Studies on recyclabiliVtiyewoArfticle Online
DOI: 10.1039/C5RA19066E
graphene oxide and the effect on the GO morphology has also been carried out. Further, we
report re-oxidization of spent graphene oxide that gets partially reduced to restore oxygenated
functional groups and the activity of GO catalyst for the first time.
Experimental
Materials
Natural graphite powder (325 mesh, Alfa Aesar), sulphuric acid (98% assay with 99% purity,
Merck), nitric acid (68-70% assay with 99.99% purity, Merck), hydrogen peroxide (30%, S D
Fine Chem. Ltd.), o-phenylenediamine (99% Sigma Aldrich), o-aminothiophenol (99%, Sigma
Aldrich), aromatic aldehydes (Sigma Aldrich) were procured and used without any further
purification.
General procedure for synthesis of graphene oxide
Graphene oxide (GO) was prepared from thermal exfoliation of natural graphite powder by
modified Hummers method.26 Natural graphite particles were intercalated in a mixture of
concentrated sulphuric acid and concentrated nitric acid followed by thermal exfoliation at 800
°C. Powdered exfoliated graphite was oxidized using KMnO4 in conc. H2SO4 followed by
several cycles of washing with DI water and bleaching with 30% H2O2 giving crude GO, which
was then purified for the removal of MnO2 impurities by sonication in HCl followed by DI
water wash. GO suspension was dried under vacuum to give GO flakes that were used as
catalyst. Functional sites present on graphene oxide surface were identified using FTIR spectra
on Bruker-VERTEX 80V instrument aligned with Ultra-Scan interferometer with a resolution
of 1 cm-1. The lamellar structure of graphene oxide was investigated using X-ray diffraction
in a wide angle (2θ = 5° to 80°, Bruker X- ray Diffractometer, D8-Adavance) having
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monochromatized Cu Kα radiation (λ = 1.5406 Å) with a data scanning rate of 0.0D1O6I: s10e.1c0/3Vs9iet/weCpA5rR.tiAcl1e9O0n6li6nEe
The surface morphology of graphene oxide was determined using high resolution transmission
microscopy (HR-TEM, JEOL), where images were collected with an operating voltage of 200
kV. Raman spectra (HR-800, Horiba Scientific) of the GO catalysts were analysed to identify
any structural changes. Elemental composition of graphene oxide was determined using
Thermo Finnigan, (Italy) FLASH EA 1112 analyser.
General procedure for synthesis of benzothiazole and benzimidazole
A mixture of aromatic aldehyde (1 mmol), o-phenylenediamine (1 mmol)/o-aminothio-phenol
(1 mmol) and graphene oxide (20 mg) in methanol (3 cm3) was either heated to 60 °C, or placed
in an ultrasonic irradiation chamber (33 kHz, 150 W output) at 35 °C under constant stirring
for appropriate time. The progress of reaction was monitored using thin layer chromatography
using a mixture hexane and ethyl acetate (7:3) as mobile phase. During work up, the catalyst
was separated using filtration. 10 ml of water and 10 ml of dichloromethane was added. The
organic layer was separated and solvent evaporated to afford crude product. Pure product was
isolated by column chromatography using silica gel (mesh 60-120) and a mixture of hexane
and ethyl acetate (7:3) as mobile phase. Structure of pure compound was confirmed using 1H-
NMR, 13C-NMR, mass spectroscopy, FTIR and elemental analysis, melting point, etc.
General procedure for GO regeneration
40 mg of recovered GO (recycle-5) catalyst was gradually added to 1 mL H2SO4 maintained at
0 °C under continuous stirring. 128 mg of KMnO4 was added to this mixture under continuous
stirring with the temperature being maintained below 10 °C. The reaction mixture was allowed
to warm to 30 °C and stirred for 3 h. The reaction mixture was then cooled below 10 °C and
diluted by adding 7 mL deionized water and 6 mL H2O2 and stirred for 24 h. Subsequently, the
reaction mixture was washed with deionized water, centrifuged and supernatant discarded.
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Resulting crude GO was washed with 1 M HCl, 6 times to remove any tracDeOsI:o10f.10m39e/Cta5RlA19066E
impurities. This brown coloured mass (Supporting information Figure S1) was washed with DI
water until neutral pH was obtained. This mass was dried at ambient temperature under reduced
pressure to give regenerated reoxidized GO. Reoxidized GO was characterized by FTIR
spectroscopy (Figure 2) to confirm the changes in functional groups after reoxidation compared
with GO recycled after 5th recycle.
Results and Discussion
Graphene oxide synthesized by oxidation of thermally exfoliated natural graphite powder,
contains hydroxyl, epoxide as well as carboxyl functional groups on graphene sheets.
Elemental analysis indicated 50.8 % carbon, 3.8 % hydrogen and 45.3 % oxygen in graphene
oxide, indicating large number of oxygen containing groups.25 Activity of GO for cyclization
of benzaldehydes and o-phenylenediamine/o-aminothiophenols to give benzimidazoles and
benzothiazoles was studied.
Table 1 lists the catalytic activity of graphene oxide in comparison with catalysts reported
earlier. No conversion was observed after 4 h at room temperature and in absence of catalyst,
while low conversion (30 %) was observed at 60 °C after 4 h. However in presence of GO,
hetero-cyclization proceeds with 81% yield at 60 °C within 4 h. Higher activity was observed
for reaction between benzaldehyde and o-aminothiophenol giving 82 % yield after only 3 h.
On the other hand, reduced graphene oxide, which does not contain as many carboxylic or
hydroxyl groups led to only marginal increase in the yield, indicating role of acidic oxygen
groups present in graphene oxide for catalysing cyclization. The activity of graphene oxide
compares favourably with concentrated HCl, 70% sulphuric acid a homogeneous acid catalysts.
Moreover, charring was observed during reaction containing concentrated sulphuric acid. Yield
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obtained with GO is higher than that obtained using much higher quantity of Indion-652 cation
DOI: 10.1039/C5RA19066E
exchange resin with carboxylic acid functional groups. Yield reported for strongly acidic cation
exchange resins Dowex 50W,14 Indion-190,15 and Amberlite-120 containing sulphonic acid
functional groups were similar to the yield obtained with GO but at substantially higher catalyst
loading.
High-intensity sonication has proven to be an important tool for various organic
transformations, reducing reaction time without need for high temperatures or pressures. In
order to cover scope of current methodology, cyclization reactions were carried out at 60 °C as
well as under ultrasonic irradiation (Table 2.). Graphene oxide shows excellent dispersion
under ultrasonic irradiation within short time at room temperature. Higher yield was noted for
all reactions carried out under ultrasonic irradiation at 35 °C in just 1 h compared to that
obtained at 60 °C under stirring after 3 h. Therefore, ultrasonic irradiation leads to shorter
reaction time with better yield compared to yield obtained under elevated temperature
conditions. Heterocyclization proceeds well under solvent free conditions, however, the
reaction mass becomes progressively viscous as conversion rises, leading to difficulty in
keeping the catalyst and reaction mass well mixed. The reaction mass was diluted with solvents
such as acetonitrile, dimethylformamide (DMF), ethanol, water, toluene, 1-4 dioxane etc. to
overcome demixing due to higher viscosity and the effect on yield studied. The yield of reaction
in non-polar solvents was poor as GO does not disperse well in non-polar solvents. Higher
yield was obtained for reaction in polar solvents with the exception of water. Low yield is
perhaps due to lower solubility of reactants in water. The yield of reaction in polar protic
solvents was higher than in polar aprotic solvents.
The effect of electron withdrawing and electron donating groups on the yield of reaction was
studied and is shown in Table 3. Aldehydes bearing electron withdrawing groups (Table 3,
entry 2-5, 17-19) as well as electron donating groups (Table 3, entry 7-11, 20) and amine with
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electron donating group (Table 3, entry 21) react cleanly with excellent yieldDuOnI:d10e.1r03Vb9ie/owCtA5hrRtiAcl1e9O0n6li6nEe
heating and ultrasonic irradiation. The yield of the reaction does not appear to be substantially
influenced by either electron withdrawing or electron donating groups on aldehyde.
Unactivated aliphatic aldehyde shows poor reactivity (Table 3, entry 13). Cinnamaldehyde
gives good yield under both refluxing conditions as well as under ultrasonic irradiation (Table
3, entry 14). However, aromatic aldehydes substituted with nitro group and hydroxyl group
(Entry 6, 12 and 16) exhibit low yield with prolonged reaction time possibly due to interaction
or adsorption of the compounds with graphene oxide surface during reaction, however the exact
reasons are unknown.
Graphene oxide, being heterogeneous catalyst, can easily be separated from reaction mixture
and reused. Catalytic activity of graphene oxide upon recycle was studied for 5 reaction cycles
after initial reaction to ascertain reuse potential of GO for model reaction, synthesis of 2‐(4‐
methoxyphenyl)benzothiazole from p-methoxy benzaldehyde and o-aminothiophenol in
methanol under ultrasonic irradiation at 35 °C for 1 h. The catalyst was isolated after each
recycle, washed with methanol and re-used. A marginal decrease in yield is observed with each
recycle indicating slight loss of activity (Figure 1).
Structural changes in GO arising due to the catalytic activity were analysed using Raman
spectroscopy, XRD, FTIR, elemental analysis and HR-TEM. Raman spectroscopic analysis
(Figure 2) was carried out to determine changes in structural configuration of the catalyst.
Graphene oxide shows a peak, characteristic of D band, at (1326 cm-1)from defects caused due
to sp3 hybridized carbon atoms and an another peak, characteristic of G band, at (1587 cm-1)
associated with in plane vibration of sp2 carbon atom.30,31 After 5th recycle slight shift is
observed in G band from (1587 cm-1) to (1576 cm-1) strongly indicates partial reduction of
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graphene oxide after reuse. Moreover, ID/IG ratio of the catalyst increased from 1.31 for priVsiteiwnAerticle Online
DOI: 10.1039/C5RA19066E
GO to 1.63 for GO-R5 which confirms that partial reduction of graphene oxide occurs during
reaction.32 Figure 3 shows the XRD spectra of pristine GO, and GO catalyst after 5th recycle
(GO-R5). XRD spectra of graphite is also added for reference. A comparison of pristine and
recycled catalyst indicates a reduction in the intensity of first peak (2θ = 11, d spacing = 9.1),
a characteristic peak of graphene oxide, and appearance of a new slightly broad peak at 2θ =
25 (d spacing = 3.4, crystallite size = 3.8 nm), which is a characteristic of reduced graphene
oxide (rGO) upon reuse. These results show that reduction of functional groups on GO has
occurred during reaction giving the catalyst rGO character.
Further investigation of catalyst using FTIR analysis revealed that peak at 1720 cm-1,
corresponding to carbonyl groups, in GO had disappeared completely in GO-R5 (Figure 4).
Moreover, on recycle, overall peak intensity of broad peak at 3400 cm-1, corresponding to
hydroxyl groups, decreased when compared to the intensity of peak at 1030 cm-1 corresponding
to C-O groups. Ultrasonic irradiation33 and amino groups have tendency to reduce GO giving
rGO and as GO is reduced, cyclization reaction is enhanced as observed by comparing entry 1
and entry 5 in Table 2. Contribution of oxygen containing functional groups during cyclization
was confirmed by elemental analysis after subsequent reuse. The carbon content was increased
from 50.8 % (pure GO) to 74.9 % (recovered after 5th recycle), whereas oxygen content
decreased from 45.3 % (pure GO) to 17.4 % (recovered after 5th recycle). Thus O/C ratio
decreased from 0.893 in GO to 0.232 in GO-R5. This decrease in oxygen content and detection
of 5.6 % nitrogen confirm the role of graphene oxide as an oxidizing agent during the
cyclization reaction. Morphological study of GO and GO-R5 was carried out using HR-TEM
microscopy to investigate disintegration of GO-sheets due to reactions (Figure 5). After reuse,
GO sheets appear to have disintegrated into smaller sheets along with slight aggregation.
Therefore, as GO catalyses the reaction, its reduction to rGO as well as continuous exposure to
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ultrasonic irradiation leads to its disintegration into smaller sheets, possibly explaining the
DOI: 10.1039/C5RA19066E
broad nature of rGO peak observed in GO-R5.
Surface bound oxygen functional groups on GO play important role during benzimidazole and
benzothiazole formation. The reaction proceeds through formation of imine intermediate by
reaction between aldehyde and amine group under suitable experimental conditions (Figure 6).
Acid groups in GO have been reported to efficiently catalyse formation of imine from aromatic
aldehyde and aromatic amine.25 In order to confirm our mechanistic approach, we monitored
the reaction using gas chromatography. Under ultrasonic irradiation, almost all of aldehyde,
the limiting reactant, was consumed within first 15 min coinciding with appearance of peak for
imine intermediate in the gas chromatogram. The presence of imine intermediate confirms role
of graphene oxide as an acid catalyst. Imine further undergoes oxidative cyclization mediated
by graphene oxide, which itself is partially reduced to give reduced graphene oxide. Good yield
of cyclization obtained under ultrasonic irradiation in total absence of oxygen and under N2
atmosphere confirms the oxidizing potential of GO. Thus functional groups present on
graphene oxide play a key role in product formation even in absence of atmospheric oxygen.
Oxygen containing functional groups are consumed as the reaction is catalysed. Therefore, GO
activity goes on decreasing. For long term usage as a catalyst, facile regeneration must be
possible. We performed regeneration of spent graphene oxide and the yield obtained with
regenerated graphene oxide was almost similar to that obtained with pristine GO (Table 4).
Therefore GO, which initially acts as proton donor catalysing imine formation, later on
mediates cyclization of imine as an oxidizing agent to give benzothiazole and benzimidazole
derivatives.
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DOI: 10.1039/C5RA19066E
Conclusion
In conclusion, graphene oxide as metal free acid catalyst facilitates synthesis of benzothiazole
and benzimidazole with good yield, short reaction time and easy recovery. Good yields are
obtained under both heating and ultrasonic irradiation conditions. To the best of our knowledge,
this is the first report establishing concurrent use of graphene oxide as acid catalyst and as
oxidizing agent for the cyclization reaction. The reaction time was reduced drastically even at
lower temperature with good yield of desired product under ultrasonic irradiation. Graphene
oxide is readily recovered and was used effectively for 6 times with only marginal decrease in
yield demonstrating reuse potential. We successfully carried out the re-oxidation of partially
reduced GO, in order to further increase the reuse potential of catalyst, which gives nearly the
same yield as fresh catalyst. Thus we have demonstrated carbocatalyst based metal-free
heterogeneous catalytic pathway for cyclization for synthesis of benzimidazoles and
benzothiazoles with potential for large scale application replacing metal catalysts.
Acknowledgement
KBD is thankful to the University Grants Commission of India for support under Special
Assistance Program. Authors would also like to thank the Department of Science and
Technology (DST), India for financial support under FIST program. Authors would like to
acknowledge the Sophisticated Analytical Instrumentation Facility at Indian Institute of
Technology Bombay for assistance with sample analysis.
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Table 1. Synthesis of benzimidazole in presence of various catalysts
Sr. No. Catalyst
Time
(h)
Catalyst loading
Yield b
(%)
1
2
3
4
5
No catalyst
4
4
3
4
4
̶
30
81
82 c
Graphene oxide
Graphene oxide
Graphene oxide
20 mg
20 mg
5 mg
20 mg
40
Reduced Graphene
oxide
45
6
7
Indion-652
4
12
4
50 mg
53
Dowex-20
53 mg
85 d, 15
89 16
80
9
Indion-190
50 mg
9
Amberlite IR -120
Conc. HCl
4
50 mg
10
4
109.5 mg (3 mmole)
30
11
H2SO4 (70%)
4
98 mg (1 mmole)
52
a Reaction conditions: Benzaldehyde (1 mmole), o-phenylenediamine (1 mmole), methanol (3 mL),
temperature: 60 °C
b Isolated yield
c Reaction conditions: Benzaldehyde (1 mmole), o-aminothiophenol (1 mmole), methanol (3 mL),
temperature: 60 °C
d temperature: 70 °C
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Table 2. Synthesis of benzothiazole in presence of different solvents under heating and ultrasonic iDrrOaId: 1ia0t.1i0o3n9a/C5RA19066E
Sr. No. Catalyst Solvent
Heating
Ultrasonic Irradiation
Temp °C Yieldb (%)
Yieldc (%)
1
2
3
4
5
6
7
8
9
-
Methanol
-
35
60
-
25
70
60d
-
GO
GO
GO
GO
GO
GO
GO
GO
45
35
35
82
81
-
-
60
Methanol
Methanol
Ethanol
Water
30 (RT)
60
89
85
-
60
60
Acetonitrile
Dichloromethane
60
71
62
75
80
39.6
(Reflux)
10
11
GO
GO
Toluene
60
60
40
50
50
63
1,4-dioxane
a Reaction conditions: Benzaldehyde (1 mmole), o-aminothiophenol (1 mmole), methanol: 3 cm3, graphene
oxide: 20 mg
b Isolated yield, time: 3 h
c Isolated yield, ultrasonic irradiation output of 150W at (33 kHz), temperature: 35 °C, time: 1 h
d Reaction conditions: Benzaldehyde (1 mmole), o-phenelenediamine (1 mmole), graphene oxide: 20 mg
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Table 3. Synthesis of various derivatives of benzothiazole and benzimidazole in presence of heating as welDl OasI: u10lt.1r0a3s9o/nCi5cRA19066E
irradiation.a
Sr.
-R (aldehyde)
-X (amine)
Product
Timeb (h)
Yieldd
(%)
Timec
(min)
Yieldd
(%)
No.
1
2
3
3
82
81
80
60
60
70
89
88
84
1a
2a
3.5
4
3a
4a
5a
6a
4
5
6
3.5
3
80
81
70
60
60
70
85
85
79
5.5
7
4.5
78
65
80
7a
8
9
3.5
3.5
4
80
81
78
76
60
60
75
70
82
84
81
81
8a
9a
10
11
10a
4
11a
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12
13
5
70
78
DOI:8100.1039/C5RA7195066E
12a
4.5
90
80
13a
14a
14
15
6
4
-
120
70
42
86
81
15a
16a
16
17
4.5
4
80
78
70
75
83
84
17a
18a
19a
18
19
20
4
4
5
81
80
70
70
70
80
84
83
77
20a
21a
21
4
81
80
83
a Reaction conditions: aldehyde (1 mmole), amine (1 mmole) methanol (3 cm3), graphene oxide (20 mg).
b Reaction temperature: 60 °C.
c Ultrasonic irradiation, reaction temperature: 35 °C
d Isolated yield.
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Table 4. Comparison of performance of pristine and re-oxidized graphene oxide catalysing synthesis of 2-
Phenylbenzimidazole
Sr. No. Catalyst
Time
(h)
Catalyst loading
Yield a
(%)
1
2
Graphene oxide
1
1
20 mg
10 mg
86 b
83 c
Re-oxidized
Graphene oxide
a Isolated yield
b Reaction conditions: Benzaldehyde (1 mmole), o-phenylenediamine (1 mmole), methanol (3 mL),
Ultrasonic irradiation, reaction temperature: 35 °C
c Reaction conditions: Benzaldehyde (0.5 mmole), o-phenylenediamine (0.5 mmole), methanol (2 mL),
Ultrasonic irradiation, reaction temperature: 35 °C, catalyst: graphene oxide regenerated after five recycles
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100
80
60
40
20
0
Fresh
R-1
R-2
R-3
R-4
R-5
Figure 1. Recyclability study of GO for synthesis of 2‐(4‐methoxyphenyl)benzothiazole
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Figure 2. Raman spectra of GO and GO after 5th recycle
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Figure 3. XRD spectra of graphite, GO and GO after 5th recycle
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GO
GO-R5
Re-oxidized GO
3700
3200
2700
2200
1700
1200
700
Wavenumber (cm-1)
Figure 4. FTIR spectra of GO, GO after 5th recycle and re-oxidized GO
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b
a
Figure 5. HR-TEM images of (a) GO and (b) GO after 5th recycle
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Figure 6. Proposed mechanism for graphene oxide catalysed synthesis of benzimidazole/benzothiazole
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Document: Graphical Abstract
Title: Role of graphene oxide as heterogeneous acid catalyst and benign oxidant for
synthesis of benzimidazoles and benzothiazoles
Dhopte, Kiran B.,1 Zambare, Rahul S.,1 Patwardhan, Anand V.,1 Nemade, Parag R. 1,2,
*
1
Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh
Marg, Matunga, Mumbai Maharashtra, 400019, India.
2
Department of Oils, Oleochemicals and Surfactant Technology, Institute of Chemical
Technology, Nathalal Parekh Marg, Mumbai, Maharashtra, 400 019, India.
* Corresponding Author
Tel.: + 91- 22 3361 2027;
Fax: +91- 22 3361 1020;
E-mail address: pr.nemade@ictmumbai.edu.in.
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Contents
Graphic ............................................................................................................................. 3
Text................................................................................................................................... 4
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Graphic
Re-oxidation
Partially reduced GO
GO
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Text
We report dual role of graphene oxide as active metal-free acid catalyst and as non-toxic
oxidant for benzothiazole and benzimidazole synthesis under mild reaction conditions, with
ease of recycle. Partially reduced graphene oxide was re-oxidized to regenerate the catalyst
and restore its activity.
4