Oxidative coupling of aniline and desulfurization over nitrogen rich mesoporous carbon

Article abstract of DOI:10.1039/c5cy00484e

Tungsten loaded mesoporous nitrogen rich carbon (WO_xMCN_x) materials were synthesized using SBA-15 as a hard template. With these new multifunctional materials, we performed a one-pot oxidative coupling of aniline to azo-benzene followed by desulfurization of dibenzothiophene (DBT) to dibenzothiophene sulfone (DBTSO). It was observed that the nature of the support for the catalyst has a strong influence on the activity of the WO_x nanoparticle. Whilst WO_x on MCN_x proved to be a very active and selective catalyst for the formation of azo-benzene via oxidation of aniline as well as dibenzothiophene sulfone from dibenzothiophene, WO_x on activated carbon or SBA-15 did not show comparable activity. These multifunctional hybrid catalysts retain their structural framework even after the reaction, and they were recovered easily from the reaction mixture through filtration and reused several times without a significant degradation in activity. Moreover, there was no contribution from leached active species in the activity and conversion was possible only in the presence of the multifunctional catalyst.

Full text of DOI:10.1039/c5cy00484e

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Oxidative coupling of aniline and desulfurization
over nitrogen rich mesoporous carbon†
Cite this: DOI: 10.1039/c5cy00484e
a
b
a
a
Reena Goyal, Deepa Dumbre, L. N. Sivakumar Konathala, Monica Pandey and
a
Ankur Bordoloi*
x x
Tungsten loaded mesoporous nitrogen rich carbon IJWO MCN ) materials were synthesized using SBA-15
as a hard template. With these new multifunctional materials, we performed a one-pot oxidative coupling
of aniline to azo-benzene followed by desulfurization of dibenzothiophene (DBT) to dibenzothiophene sul-
fone (DBTSO). It was observed that the nature of the support for the catalyst has a strong influence on the
x x x
activity of the WO nanoparticle. Whilst WO on MCN proved to be a very active and selective catalyst for
the formation of azo-benzene via oxidation of aniline as well as dibenzothiophene sulfone from dibenzo-
Received 1st April 2015,
Accepted 3rd May 2015
thiophene, WO on activated carbon or SBA-15 did not show comparable activity. These multifunctional
x
hybrid catalysts retain their structural framework even after the reaction, and they were recovered easily
from the reaction mixture through filtration and reused several times without a significant degradation in
activity. Moreover, there was no contribution from leached active species in the activity and conversion
was possible only in the presence of the multifunctional catalyst.
DOI: 10.1039/c5cy00484e
www.rsc.org/catalysis
and stable heterogeneous catalyst system for industrial appli-
cations. Mesoporous carbon materials are important potential
Introduction
Synthesis of important oxygen containing chemicals and
intermediates via ecological and economical route is one of
the key scientific challenges in the current industrial sce-
nario. Most of the bulk chemicals and fine speciality
chemicals are synthesised by using homogenous catalysts and
oxygen reduction reaction catalysts as they are environmen-
tally sustainable, cost-effective, and highly durable, and owing
to their unique structural properties, such as high surface
area, tunable pore sizes, chemical and mechanical stability.
A variety of carbon materials, e.g. carbon nanotubes, activated
4
1
traditional processes. These established processes suffer
carbon, have been prepared and have been successfully used
5
from the use of hazardous reagents in stoichiometric
amounts and generate a large amount of waste for the envi-
ronment. Heterogeneous catalysis offers a distinct advantage
over homogeneous catalysis for facile catalyst separation and
recycling, which further meets the requirement of green and
in heterogeneous catalysis. Recently, nitrogen functionalized
carbon materials have been shown to have high catalytic
bifunctional activity in photocatalysis, oxygen reduction reac-
6
tion, and organic transformations, and as semiconductors. It
has also been reported that carbon nitride materials are
promising metal free catalysts and catalyst supports in differ-
2
sustainable chemistry. Many efforts have been made in the
7
last few decades to use green oxidants, such as O
2
and H
2
O
2
,
ent hydrogenation and oxidation reactions.
along with heterogeneous catalysts to convert conventional
processes to environmentally friendly technologies. Neverthe-
less, very limited success has been achieved to date due to
the poor efficiency and stability of heterogeneous catalyst sys-
tems in the reaction medium in the presence of various oxi-
The basicity of the material was achieved by pyridine type
8
N, which resides within the CN framework. For low temper-
x
ature catalytic applications, the acid–base property of the
material needs to be tuned further by the addition of transi-
tion metals like tungstate to the CNx framework. In this
work, our idea was to develop a heterogeneous carbon based
tungstate catalyst that can display high activity for a variety
of oxidant process, like epoxidation of alkenes, alcohol oxida-
tion, oxidative coupling of amines and oxidative desulfuriza-
3
dants. Therefore, there is ample scope to develop an efficient
a
Refinery Technology Division, CSIR-Indian Institute of Petroleum, Dehradun-
9
248005, UK, India. E-mail: ankurb@iip.res.in; Fax: +91 135 2660202;
tion. An example can be found in our previous report, where
Tel: +91 135 2525898
we have shown acid–base cooperative catalysis using WOx
b
Centre for Advanced Material and Industrial Chemistry, School of Applied
x
embalmed CN material.
Sciences, University of Melbourne, Australia 3000
Selective formation of aromatic azo compounds through
oxidative coupling of aniline is an industrially important
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy00484e
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process because aromatic azo compounds are high value
chemicals that are widely used in industry as dyes, pigments,
food additive and drugs. Previously, a variety of oxidizing
was kept at 100 °C for 48 h without any further stirring.
After cooling to room temperature, the solid product was
recovered by filtering, washing, drying, and calcining at
550 °C in order to decompose the triblock copolymer.
1
0
1
1
12
agents, like MnO, Hg (OAc)2, BaMnO4 (ref. 13) and
1
4
peracetic acid, have been used for aryl amine oxidation.
Nevertheless, this process generates a large amount of inor-
ganic waste. Nowadays, different metal based (Pd,
Hexagonally ordered mesoporous WO /SBA-15 was synthe-
x
sized by using TEOS as a silica source and poly(ethylene
oxide)-block-polyIJpropylene oxide)-block-polyIJethylene oxide)
triblock copolymer (Aldrich, MW avg. 5800, EO PO EO ,
1
5
1
6
CuCr O , Ag/WO3 (ref. 17)) heterogeneous catalysts have
2
3
20
70
20
been applied for azo compound formation. Even Au/TiO
2
has
P123) as a structure-directing agent. In a typical synthesis,
4.0 g of P123 block copolymer was dissolved with stirring in
30.0 g of water and the required amounts (20 mL, 10 mL,
also been reported to carry out aromatic azo compound for-
1
8
mation from aryl amines. Oxidative desulfurization is an
industrially important and alternative process to hydro-
5 mL, 2.5 mL) of aqueous sodium tungstate solution IJNaWO
4
1
9
desulfurization. Selective oxidation of organic sulfides to
the corresponding sulfide and sulfone is used to decrease the
·2H O, 0.5 M) were simultaneously and quickly added into
2
the mixture under vigorous stirring. After one hour, 120.0 g
of HCl (2 M) and then 9.1 g of TEOS were added with stirring
at 40 °C. After being stirred for 24 h at 40 °C, the gel compo-
sition was kept at 100 °C for 48 h without stirring. After being
cooled to room temperature, the solid product was recovered
by filtering, washing, drying and calcining at 550 °C.
2
0
sulfur content of fuels and industrial effluents. Recently,
Zhang et al. reported carbon nanotubes as an efficient cata-
lyst for the oxidative desulfurization of dibenzothiophene.
2
1
The corresponding sulfides or sulfoxides are used as fine
chemicals, pharmaceuticals and as valuable intermediates for
the construction of chemically and biologically active mole-
cules. Oxidative desulfurization reactions are mainly carried
Mesoporous nitrogen rich carbon IJWO
were synthesized using WO /SBA-15 as the hard template
using the following procedures. 1.0 of dehydrated
WO /SBA-15 was treated with a mixture of 4.5 g of ethylene
diamine IJNH NH ) and 11 g of carbon tetrachloride
x x
/CN ) materials
x
2
2
out over a variety of tungsten based complexes. Zhang et al.
have also reported the oxidation of organic sulfides over
mesoporous graphitic carbon nitride material under visible
g
x
2
C
2
H
4
2
23
light.
In this report, we have prepared a highly active, selective
reusable heterogeneous WO nano cluster supported on
(CCl ). The mixture was refluxed at 90 °C for 6 h. Then, the
4
obtained solid mixture (polymer silica composite) was dried
and pyrolyzed at two different temperatures (600 and 800 °C)
for 6 h under an inert gas atmosphere. The pyrolyzed silica/
nitrogen/carbon composite was washed with 2.5% wt of
NaOH solution in ethanol water (1 : 1) mixture with vigorous
stirring at 90 °C for 3 h to remove the silica framework. The
process was repeated two times. Then, the product was fil-
tered and washed with a water/ethanol mixture until the fil-
trate had a pH of 7.0, and then dried.
x
mesoporous nitrogen enriched carbon material. The catalyst
has been explored for oxidation of aromatic amines and
2 2
dibenzothiophene oxidation using H O as the mild oxidizing
agent.
Experimental
Materials
Tetraethylorthosilicate (TEOS), poly(ethylene oxide)-block-
polyIJpropylene oxide)-block-polyIJethylene oxide) triblock
copolymer (Aldrich, MW avg. 5800, EO PO EO , P123), eth-
Catalyst characterization
The prepared mesoporous nitrogen rich carbon materials
were characterized by N physisorption measurements at 77
2
K using an Autosorb 1C setup (Quanta chrome) adsorption
analyzer. Prior to the measurements, the samples were
degassed under vacuum (1 × 10 Torr) for 2 h at 200 °C. The
2
0
70
20
ylene diamine IJNH
2H O) were purchased from Sigma-Aldrich Co. Ethanol, car-
bon tetrachloride (CCl ), NaOH, HCl, and NH solution were
2 2 4 2 4
C H NH ), and sodium tungstate IJNaWO
·
2
4
3
−5
purchased from Merck KGaA, Darmstadt, Germany. All the
chemicals were used without further purification. Double dis-
tilled water was prepared with a BOROSIL® water distillation
unit.
BET specific surface areas were determined from the adsorp-
tion data in the relative pressure IJP/P ) range from 0.06 to
0
0.2. The pore size distributions (PSDs) were calculated from
the nitrogen adsorption branch using the Barrett–Joyner–
Halenda (BJH) method and the maximum of the PSD was
considered as the average pore size. The pore volume was
considered as the volume of liquid nitrogen adsorbed at
Catalyst preparation
SBA-15 was synthesized using tetraethylorthosilicate (TEOS)
as the silica source and poly(ethylene oxide)-block-polyIJpropylene
oxide)-block-polyIJethylene oxide) triblock copolymer (Aldrich,
MW avg. 5800, EO PO EO , P123) as a structure-directing
P/P = ca. 1.
0
x x
The morphologies of the WO /MNC materials synthesised
at different temperatures and different loadings of tungsten
were investigated using SEM (FEI Quanta 3D) and HRTEM.
Scanning electron microscopy (SEM) images were taken on a
FEI Quanta 200 F instrument, using a tungsten filament
20
70
20
agent. In a typical synthesis, 4.0 g of P123 block copolymer
was dissolved under stirring in 30.0 g of water. Then, 120.0 g
of HCl (2 M) and 9.1 g of TEOS were added under stirring at
40 °C. After 24 h of constant stirring, the gel composition
doped with lanthanum hexaboride (LaB ) as an X-ray source,
6
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fitted with an ETD detector with high vacuum mode using
secondary electrons and an acceleration tension of 10 or 30
kV. Images were recorded at various magnifications. All the
samples were analyzed by spreading them on a carbon tape
and coating with gold to increase the electrical conductivity.
Energy dispersive X-ray spectroscopy (EDX) was used in con-
nection with SEM for the elemental analysis. The elemental
mapping was also collected with the same spectrophotome-
ter. A JEOL JEM 2100 high-resolution transmission electron
microscope (HRTEM) was employed to observe the porous
nature of material. Samples were mounted by dispersing on
ethanol on a lacey carbon Formvar coated Cu grid.
the internal standard followed by their individual calibration
curve. The activity of the catalyst was calculated as:
Moles of reactant reacted

C%

Conversion

C%


 100
Moles of reactant initially used C%


Moles of product

C%

Selectivity

C%


100
Moles of reactant reacted

C%

Low and wide angle powder X-ray diffractograms (XRD)
were obtained with a D8-Advance-Bruker-AXS diffractometer
Results and discussion
(Cu-Kα radiation; λ = 1.5418 Å) in θ–2θ geometry and a posi-
tion sensitive detector (capillary technique, thickness: 1 mm).
X-ray photoelectron spectroscopy (XPS) measurements
were carried out in an ultra-high vacuum (UHV) set-up
equipped with a monochromatic AlKα X-ray source (hν =
The low angle XRD patterns of the nitrogen rich carbon mate-
rials are shown in Fig. 1a. The XRD pattern of MCN reflects
x
one prominent peak at 0.75° corresponding to the (100)
plane of the hexagonal p6mm space group. Notably, the XRD
pattern coincides well with the XRD pattern of the ordered
1
486.6 eV), operated at 14.5 kV and 35 mA, and a high resolu-
2
5
tion Gammadata-Scienta SES 2002 analyzer. The base pres-
sure in the measurement chamber was maintained at about
hexagonal mesoporous SBA-15 material. Whereas the wide
angle XRD of WO /MCN catalyst shows a peak at around 25°
x
x
−
10
7
× 10
mbar. The measurements were carried out in the
assigned for graphitic carbon, no peak assigned for any WOx
species is observed, which implies the presence of WO spe-
fixed transmission mode with a pass energy of 200 eV,
resulting in an overall energy resolution of 0.25 eV. A flood
gun was applied to compensate for the charging effects. Res-
olution spectra for C 1s, O 1s, and N 1s were recorded. The
x
cies in nanocluster form (Fig. S1 ESI†). The 2D hexagonal
structure of the prepared samples was confirmed by HRTEM
(see Fig. 2a–d). In the case of the sample prepared using eth-
ylene diamine as a source of carbon and nitrogen, and SBA-
15 as the template with 5% W loading, the TEM micrograph
indicated that the pores were channel-like and their arrange-
ment was similar to that for a 2D hexagonalIJhoneycomb)
structure, however with some imperfections. The homo-
geneity of the distribution pattern of tungsten has been
2
binding energy scales were re-calibrated based on the sp
hybridized C 1s line from graphitic carbon at 284.5 eV. The
2
4
Casa XPS software with a 70 : 30 Gaussian-Lorentzian prod-
uct function and Shirley background subtraction was used
for peak deconvolution. The obtained spectra from different
elements were plotted using the same intensity scale for all
the analyzed samples to facilitate comparison. Temperature
programmed desorption (TPD) experiments were carried out
in a Micromeritics Auto Chem II 2920 instrument connected
with a thermal conductivity detector (TCD). The amount and
strength of the acid sites were measured by ammonia adsorp-
tion–desorption technique using the same Micromeritics
Auto Chem II 2920 instrument. About 0.1 g of sample was
saturated with NH
the physically adsorbed NH
3
at 100 °C and flashed with He to remove
, finally the decomposition of
3
−
1
NH was carried out at a heating rate of 10 °C min under
3
He flow.
Multifunctional activity
The oxidation reaction was carried out in a 100 ml two
necked round bottom glass reactor fitted with two con-
densers on a hot plate with magnetic stirring (SI Analytical).
The reaction product was identified by GC-MS (HP 5890 GC
coupled with 5972 MSD) equipped with a CP-SIL-5 capillary
column. The identified product was analysed in an Agilent
Fig. 1 (a) Small-angle X-ray diffraction patterns; (b) N
desorption isotherm; (c) pore size distribution of (A) MCN
WO /MCN (5) material. (d) Normalized Raman spectra of WO
material with a 488 nm laser.
2
adsorption–
and (B)
/MCN
x
7
890, fitted with a HP-5 (30 m × 0.28 nm i.d., 0.25 μm film
x
thickness) capillary column and a FID detector. The conver-
sion and selectivity was measured using chlorobenzene as
x
x
x
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−
1
−1
cc g to 0.36 cc g , indicating the incorporation of WOx
inside the mesoporous MCN network.
x
Fig. 1d shows the Raman spectra characteristics of our
prepared WO /CN material. The existence of an amorphous
x
x
type MCN network can be confirmed by the low peak area
x
ratio of the G-band to the D-band. The vibration frequencies
of the MCNx network are expected to come close to the
modes of analogous unsaturated CN molecules, which are
−
1
−1
1
500–1600 cm for chain like molecules and 1300–1600 cm
2
6
−1
for ring like molecules. The signal at about 1598 cm is
attributed to the G-band of the tangential mode of graphite-
like materials while the peak at around 1365 cm is the
D-band representing defects in the graphite structures. It
has been observed that the position of the G-band can vary
between 1500 cm and 1600 cm ; and in the case of sp
−
1
2
7
−
1
−1
2
−
1
bonded amorphous C, the G-mode is around 1520 cm
(
lower than graphite, mainly due to bond angle distortions).
3
However, the G-band moves up with increasing sp bonding
x x
Fig. 2 HRTEM images (a–d) of WO /MCN (5) material synthesized
−
1 28
using ethylene diamine as a source of carbon and nitrogen, and 5%
WO /SBA-15 as the solid template.
in tetrahedral amorphous –C (≥1595 cm ). This indicates
small distortions in G–D region between the bands because
of the special arrangement of the C or N atoms.
x
The X-ray photoelectron (XP) spectra for C 1s have been
obtained in order to determine the local environment of C in
the MCN
x
network. The deconvoluted C 1s spectra (Fig. 3a)
show five individual peaks, of which the major two peaks at
2
2
83.9 eV and 284.8 eV are assigned to sp hybridized
2
3
3
graphite-like carbon (C–C sp ) and sp hybridized C–C sp
carbon shouldered with sp hybridized C–N atom. The rest
2
9
of the peaks can be assigned to the surface oxygen groups as
–CO (286.4 eV), –CO (288.2 eV) and –C–OO (289.5 eV). The
local environment of the nitrogen atoms also been deter-
mined by N 1s XP-spectra (Fig. 3b), which on deconvolution
shows three individual peaks. The first N 1s peak at 398.5 eV
is assigned pyridine nitrogen (N-py), while the peak at around
401.2 eV can be assigned to the N of the C–N–C network of
the graphitic ring. However, the shouldered peak at 399.9 eV
2
has been assigned to nitrogen bonded sp hybridized (Nb)
and nitrogen bonded to a carbon atom in a terminal –CN
group (Nt). The total acidity of the samples was measured
2
9
Fig. 3 X-ray photoelectron spectra of (a) C 1s and (b) N 1s of
WO /MCN material. (c–d) HRTEM images of WO /MCN (2.5).
x
x
x
x
using NH -TPD (Fig. S3 ESI†). The TPD pattern shows a
3
strong band in the temperature region between 150–500 °C
−
1
with the total number of acidic sites of 0.405 mmol g (5%
−
1
confirmed by the elemental analysis facility associated with
WO /MCN ) and 0.260 mmol g (2.5% WO /MCN ).
x x x x
the SEM instrument and is shown in Fig. S2, ESI.† Finally,
TEM images for the sample prepared with 2.5% WO
x
loaded
C–C coupling of aniline
SBA-15, i.e. WO /MCN (2.5), showed (Fig. 3c, d) regular chan-
x
x
nels and homogenously distributed pores with a wide distribu-
tion of sizes. Both MCN and WO /MCN (5) show type IV N
adsorption/desorption isotherms with a hysteresis loop H1 at
The bi-functional reactivity of the catalyst has been tested in
oxidative desulfurization as well as aniline coupling reac-
tions. Selective oxidation of aniline to aromatic azo-
compounds has been carried out with the WO /MCN cata-
x
x
x
2
relative pressure in the range of 0.40–0.92, as shown in Fig. 1b.
x
x
The pore size distribution for the MCN
x
shows the meso-
lyst, as shown in Table 1. Experimental results showed that
pores at a diameter of 3.86 nm (Fig. 1c) along with a pore vol-
with MCN , oxidation of aniline is negligible (entry 2) and
x
−
1
ume of 0.61 cc g . The specific surface area (Table S1†) esti-
the WO supported catalyst (entries 4–6) shows a significant
x
2
−1
mated for MCN
x
was 457 m g while for the WO
x
/CN
x
(5) a
amount of conversion. It is assumed that during the reaction,
2
−1
−
decrease in the surface area (427 m g ) as well as in pore
volume were noticed. The pore volume decreased from 0.61
single e oxidation of aniline took place to form a radical cat-
ion, which is exclusively mediated by the WO /MCN catalyst
x
x
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Table 1 Selective oxidation of aniline to aromatics azo-compounds over
various carbon based catalysts
Selectivity
Sl no.
Catalyst
Blank
C
Aniline
S
2b
S
2c
S
2d
E
o
1
2
3
4
5
6
7
8
—
~1.9
—
33.6
57.8
86.1
89.7
81.9
—
13.5
—
67.4
73.7
83.7
79.5
83.9
—
16.2
—
11.7
8.7
9.3
14.5
9.4
—
70.3
—
20.9
17.6
6.9
7.0
6.7
—
~0.1
—
MCN
x
WO
WO
WO
WO
WO
WO
x
x
x
x
x
x
/C
/CN
/CN
/CN
/CN
/CN
x
x
x
x
x
(1)
(2.5)
(5)
7.5
14.2
24.0
23.7
22.9
(10)
a
(5)
0
9
.93 g (10 mmol) aniline, 10 ml CH
0 °C, and time 12 h, E : H efficiency calculated by (100 × moles
added. Caniline = conversion of
3
CN, 0.05 g catalyst, temperature
Fig. 4 Effect of temperature (a) and time-on-stream (b) as a function
of aniline conversion and product selectivity. (■) Conversion of aniline;
( ) selectivity of azo-benzene; ( ) selectivity of nitroso-benzene and
O
2 2
O
of 2b formed)/total moles of H O
aniline, S2b = selectivity of 2b. After 3 reuse, S2c = selectivity of 2c
and S2d = selectivity of 2d.
2 2
a
(
) selectivity of azoxy-benzene. Effect of temperature (c) and time-
on-stream (d) as a function of DBT conversion and product selectivity.
■) Conversion of DBT; ( ) selectivity of DBTS and ( ) selectivity of
(
DBTSO.
through providing unsaturated W sites. This abstraction is
then followed by the coupling of the aniline radical cation
1
8
with neutral aniline to form azo-benzene.
clearly demonstrate that tungstate plays an important role in
the catalytic process. As observed with the WO supported
catalyst, the nature of support has a strong influence on the
activity of the WO nanoparticle. Whilst WO on MCN
The results
amount of H O (2 ml, 30 wt.% in H O). Moreover, the selec-
2
2
2
tivity of azo-benzene was also found to be doubled (from
31.1% to 67.4%) whereas the selectivity of nitroso-benzene
has been decreased. The observation is well concomitant
with our previous results that the nitroso-benzene is formed
during the reaction (intermediate) and is consumed to form
x
x
x
x
proved to be active and selective catalyst for the formation of
azo-benzene (2b), WOx on pure carbon (entry 2) or SBA-15
azo-benzene. The WO /MCNx catalyst shows the maximum
x
(not shown) was not found to be active under the present
conversion with the highest azo-benzene selectivity of 83.7%
reaction conditions. It is also observed that upon loading of
WO in the MCN network, the activity and selectivity took a
with 3 ml of H
2 2
O . Under the optimized reaction conditions
with 3 ml of oxidant, the highest H O efficiency (24.0) can
x
x
2 2
positive shift, however, selectivity goes down as loading goes
beyond 5%, which may be because of the higher particle size
due to higher loading.
be achieved for the formation of azo-benzene (Table 1, entry 6).
Oxidative desulfurization of DBT
The effect of reaction temperature has been studied and
we found that the catalyst can effectively convert 70.1% ani-
line within 12 h with 74.2% azo-benzene selectivity (Fig. 4a)
at a reaction temperature of 70 °C. Increasing the tempera-
ture from 80 °C to 90 °C leads to a decrease in azo-benzene
selectivity from 86.5% to 83.7%, with the formation of
nitroso-benzene and azooxy benzene as by-products (Fig. 4a).
A longer reaction time of 18 h at 90 °C (Fig. 4b) leads to a
decrease in azo-benzene selectivity to 20%, as 75.1% of the
excessive oxidation product (azoxy-benzene) was formed with
Oxidative desulfurization of dibenzothiophene has been car-
ried out with the WO /MCNx catalyst and the results are
x
shown in Table 2. With WO /MCN (5) catalyst the conversion
x
x
of DBT was 100% within 2 h and the selectivity of DBTSO
(confirmed by GC-MS data shown in Fig. S5, ESI†) was 99.8%
at a reaction temperature of 100 °C, with small amounts of
DBTS (0.2%) remaining as unconverted intermediate product
(entry 7), indicating the effectiveness of the WO /MCN cata-
x
x
lyst at the oxidative desulfurization of DBT. Comparing
WO /MCN with other catalysts (entry 2–5) shows nominal
8
9% aniline conversion. This trend is decreased for 6 and 9 h
x
x
reaction times, but selectivity toward azo-benzene goes down,
whereas nitroso-benzene was the major product with 66.3%
and 21.1%. Hence, it can be assumed that nitrobenzene is
the key intermediate for the formation of azo-benzene, which
is converted to azoxy-benzene (over oxidation product) with
prolonged reaction time.
activity with poor DBTSO selectivity. With the increase of
tungsten loading from 1 to 5 wt%, the conversion of DBT
reaches 100% from 26.0% for WO /MCN (1) with a steady
x x
increase in DBTSO selectivity (entry 6–8), indicating that the
acidic side provided by tungstic oxide is essential for better
conversion and DBTSO selectivity. The catalyst has been
reused; no loss in activity and selectivity was noticed, even
after 3 successive reuses (entry 10).
Aniline conversion increased almost two fold (in Fig. S4,
ESI†) form 43.7% to 81.5% with the addition of double the
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Table 2 Oxidative desulfurization of DBT over carbon based catalysts
active and has a very short life span. Finally, it transfers oxy-
gen to the activated reactant which then goes to the product
through an anionic intermediate, as shown in Scheme S1
ESI.† This anionic intermediate gets stabilized by the dona-
tion of a proton from the polar solvent. Along with this, the
rate of oxygen transfer from metal-peroxo complex to the acti-
vated species may be increased with the surface acidity.
Selectivity
Sl no.
Catalyst
C
DBT
S
1b
S
1c
E
o
1
2
3
4
5
6
7
8
9
Blank
SBA-15
2.7
98.5
79.8
46.6
41.2
75.9
21.1
13.8
0.2
1.5
0.003
0.5
2.2
1.2
0.9
1.8
5.1
9.0
8.9
Conclusions
31.0
47.9
22.1
41.2
26.0
66.1
100
20.2
51.6
58.8
24.1
78.9
86.2
99.8
99.3
x
WO /SBA-15
2D hexagonal mesoporous nitrogen-rich carbon materials
MCN
x
embedded with highly dispersed WO nanoclusters were pre-
pared through a novel approach taking WO
x
WO
WO
WO
WO
WO
x
x
x
x
x
/C
/CN
/CN
/CN
/CN
x
x
x
x
(1)
x
supported SBA-
(2.5)
(5)
15 as a hard template. The newly synthesized materials were
found to be excellent bifunctional carbon nitride-based cata-
lyst systems for selective oxidation of aniline to aromatic azo-
compounds as well as for the oxidative desulfurization of
dibenzothiophene.
From the results of the present study, it should be possi-
x
ble to develop a wide array of metal embedded MCN mate-
a
(5)
~99.9
0.7
0
.5 g (2.7 mmol) Dibenzothiophene (DBT), 10 ml CH
3
CN, 0.05 g
a
catalyst, temperature 100 °C, time 2 h. After 3 reuse, E
efficiency calculated by (100 × moles of DBTSO formed)/total moles
of H added. S1b = selectivity of DBTS and S1c = selectivity of
DBTSO.
O 2 2
: H O
2 2
O
rials for selective oxidation, provided that enough metal
dispersion and defect sites are generated on the surface.
x x
However, WO /MCN not only gives the highest selectivity of
The effect of reaction temperature on oxidative desulfuri-
zation is shown in Fig. 4c; the conversion gradually increases
up-to 100% with increasing reaction temperature from 60 °C
to 100 °C. However, selectivity towards DBTSO was found to
be 95.5% at 60 °C, which increase to a maximum of 99.8% at
the materials tested here but should also allow the efficient
one-pot conversion of aniline into azo-compounds as well as
dibenzothiophene into dibenzothiophene sulfone.
Acknowledgements
1
00 °C after 2 h time-on-stream, although no change was
observed in the conversion level with the increase of H at
00 °C (in Fig. S6, ESI†). The selectivity of DBTS gradually
goes down to <8% with 3 ml of (1 : 3 reactant to H O ratio)
2 2
O
R.G. acknowledges the Council for Scientific and Industrial
Research (CSIR), India. AB gratefully acknowledges DST for
fast track grant. We also acknowledge the Director CSIR-IIP
for his support. The authors thank the Analytical Science Divi-
sion, Indian Institute of Petroleum, for analytical services.
1
2
2
2 2 2 2
H O from 68.1% with 2 ml H O . Therefore, sufficient oxi-
dant was necessary for the second step, oxidation of DBTS to
DBTSO. With a higher amount (1 : 5) of H O , not much
2
2
change in product distribution was observed. When the reac-
tion time is 2 h (Fig. 4d), the selectivity of DBTSO was found
to be almost 100%.
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