Catalytic performance of ammonium salt of molybdenum heteropoly acid supported silica for oxidative desulfurization of benzothiophene, dibenzothiophene and 4,6-dimethyldibenzothiophene in model oil

Article abstract of DOI:10.14233/ajchem.2017.20589

An ammonium salt of molybdenum heteropoly acid was loaded on silica support (HPA-SiO2), used as catalyst in oxidative desulfurization for removing benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) from model oil. The structural properties of prepared catalyst were studied by using various analytical techniques; Fourier transforms infrared spectroscopy, X-ray diffraction, scanning electron microscopy and thermogravimetric analysis. The results showed that the ammonium salt of molybdenum heteropoly acid highly dispersed on SiO2. The final catalyst exhibits good catalytic activity of oxidative desulfurization. The 94.6 % of thiophene compounds of the model oil were removed under optimal conditions; temperature at 80 °C, reaction time 100 min. The oxidation is decreased in order of DBT > 4,6-DMDBT > BT.

Full text of DOI:10.14233/ajchem.2017.20589

Asian Journal of Chemistry; Vol. 29, No. 8 (2017), 1723-1727
ASIAN JOURNAL OF CHEMISTRY
https://doi.org/10.14233/ajchem.2017.20589
Catalytic Performance of Ammonium Salt of Molybdenum Heteropoly Acid Supported
Silica for Oxidative Desulfurization of Benzothiophene, Dibenzothiophene and
4,6-Dimethyldibenzothiophene in Model Oil
*
DEEPTIRAJ PANT, RAMSWAROOP SINGH THAKUR and SUNDER LAL
Department of Chemical Engineering, Maulana Azad National Institute of Technology, Bhopal-462 003, India
*Corresponding author: E-mail: sunderlalin@gmail.com
Received: 15 February 2017;
Accepted: 12 April 2017;
Published online: 12 June 2017;
AJC-18430
An ammonium salt of molybdenum heteropoly acid was loaded on silica support (HPA-SiO2), used as catalyst in oxidative desulfurization
for removing benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) from model oil. The
structural properties of prepared catalyst were studied by using various analytical techniques; Fourier transforms infrared spectroscopy ,
X-ray diffraction, scanning electron microscopy and thermogravimetric analysis. The results showed that the ammonium salt of molybdenum
heteropoly acid highly dispersed on SiO2. The final catalyst exhibits good catalytic activity of oxidative desulfurization. The 94.6 % of
thiophene compounds of the model oil were removed under optimal conditions; temperature at 80 °C, reaction time 100 min. The
oxidation is decreased in order of DBT > 4,6-DMDBT > BT.
Keywords: Benzothiophene, Dibenzothiophene, 4,6-Dimethyldibenzothiophene, Heteropoly acid, Model oil.
Therefore, many alternative non-hydrodesulfurization techno-
logies, such as oxidation [7-9], adsorption [10-12], extraction
[13], biodesulfurization [14], have been widely investigated.
Among these, oxidative desulfurization, which operates under
mild conditions without requiring hydrogen, is regarded as
the promising and economical process [15]. This process is
used to convert thiophene, benzenethiophene, dibenzothio-
phene and 4,6-dimethyldibenzothiophene into sulfones and
sulfoxides in presence of oxidizing agent and sulfones and
sulfoxides are easily separated by extraction with polar solvent
[16-19]. The ODS process widely studied in various systems,
such as organic acid [20], heteropolyoxo-metalates [21], ionic
liquid [22,23], molecular sieve [24,25] and photocatalysts [26],
heteropoly acid (HPA) catalysts [27-29], HPA catalysts, espe-
cially those having the keggin structure, have been determined
to be very effective for the oxidation of sulfur containing
compounds in a liquid-liquid two phase system. Li et al. [30]
reported mesoporous silica pillared clay incorporated with
phosphotungstic acid. Chamack et al. [31] reported a series of
cesium salts of tungsten substituted molybdo-phosphoric
acid, CsxH3-x [PMo12-yWyO40], (x = 1-3, y = 2-1) supported on
platelet SBA-15). Wu et al. [32] reported metal-modified
[PW11O39(H2O)M]5– (M = Ni2+ or Co2+) were incorporated into
the mesoporous silica pillared clays (MSPC) which shows
excellent performance in ODS. These catalysts showed the
INTRODUCTION
Environmental regulations for sulfur emission limit for
liquid fuels became more strict in last 20 years and more
stringent legislation on the specification of sulfur content in
liquid fuel have been issued over word wide. Production of
ultra low sulfur liquid fuel is a challenge for petroleum industry
because existing hydro desulfurization processes (HDS) for
reducing sulfur in gasoline and diesel require critical operating
conditions (high temperature and high pressure) for deep
desulfurization. Hydro desulfurization processes is most common
industrial process used in petroleum industry to reduce sulphur
content of liquid fuels and has received extensive attention
since its discovery in 1930’s, It is a process in which light oils
is heated, mixed with hydrogen and charged in to a reactor
packed with a catalyst. At this temperature, some or all of the
feed is convert in to vapour, depending on the boiling range of
the feed and the pressure is the unit [1,2]. Liquid fuels contain
sulfur in form of mercaptans, sulphides, disulfides and thio-
phenes. It is undesirable compound because it forms sulphur
dioxide (SO2) during fuel oil combustion [3]. The HDS has been
fail to removing aromatic organo sulfur compounds, such as
benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyl-
dibenzothiophene due to their stearic hindrance [4-6]. The
electron density of BT, DBT, 4,6-DMDBT shown in Table-1.

1724 Pant et al.
Asian J. Chem.
good performance in ODS system. In this work, an ammonium
salt of molybdenum heteropoly acid (HPA) was synthesized
and loaded on silica support. The porosity and large surface
area of silica combine with heteropoly acid which have good
redox properties and strong acidity The catalyst were system-
atically characterized by several techniques and applied on
oxidative desulfurization process for removal of BT, DBT and
4,6-DMDBT under mild condition.
according to the literature [34].Aqueous solution of ammonium
heptamolybdate, (NH4)6Mo7O24·4H2O (4.24 g, 0.0034 mol) was
prepared by dissolving it in 40 mL of water at 373 K. Similarly
an aqueous solution of diammonium hydrogen orthophosphate,
(NH4)2HPO4 (0.27 g, 0.002 mol) was prepared in 20 mL of
water. Then they were mixed and its pH was adjusted to 1.0 by
addition of nitric acid. The yellow precipitate obtained was the
ammonium salt of molybdenum heteropoly acid, (NH4)3PMo12O40
(denoted by HPA). This suspension was directly used for the
preparation of the heterogeneous catalyst.
TABLE-1
ELECTRON DENSITY OF BT, DBT, 4,6-DMDBT [[RReeff.. 3333]]
Preparation of final catalyst: The silica (50 g) was added
to 30 g of HPA solution. This was then kept under reflux con-
ditions at 70 °C for 8 h. The solid material which was filtered
and dried was the final catalyst having 5 % of the HPA. This
final catalyst is denoted by HPA-SiO2.
Sulfur
compounds
Electron
density
Structure
BT
5.716
Catalyst test
Reaction: Three model oils were prepared by dissolving
benzothiophene (BT), dibenzothiophene (DBT) or 4,6-dimethyl-
dibenzothiophene (4,6-DMDBT) separately into iso-octane to
give a desired sulfur content (sulfur-contenting compounds
concentration of 0.2 wt. %). In a typical run, 20 mL of model
oil was placed in a three-neck flask. The reaction temperature
was stabilized by oil bath. The catalyst and 30 wt. % H2O2 were
added to the flask successively to run the reaction. The samples
were collected at an interval of 20 min. Dibenzothiophene
oxidize to form corresponding sulfoxides and sulfones in
presence of oxidizing agent and catalyst is shown in Fig. 1.
Mechanism for sulfur removal from oil in the presence
of ammonium salt of molybdophosphoric acid (HPA)-SiO2
catalyst by ODS process shown in Fig. 2 [32]. (1) Firstly,
S
S
DBT
5.756
H3C
CH3
S
4,6-DMDBT
5.760
O
EXPERIMENTAL
XS
M=O
O
All the chemicals including hydroperoxide analytical
reagent grade benzothiophene (BT), dibenzothiophene (DBT),
4,6-dimethyl-dibenzothiophene (4,6-DMDBT), ammonium
heptamolybdate, (NH4)6Mo7O24·4H2O (4.24 g, 0.0034 mol),
diammonium hydrogen orthophosphate, (NH4)2HPO4 (0.27 g,
0.002 mol), iso-octane, nitric acid, SiO2 purchased from
Molychem company.
H2O2
(1)
O
XS
H2O
O
O
O
O
(2)
(2)
M
M
Preparation of model oil:Three model oils were prepared
by dissolving benzothiophene, dibenzothiophene and 4,6-
dimethyl-dibenzothiophene separately into iso-octane to give
a desired sulfur content (sulfur-contenting compounds concen-
tration of 0.2 wt. %.
XS
H2O
(1)
XS=O
H2O2
M=O
Preparation of catalyst
Preparation of (NH4)3PMo12O40 heteropoly acid (HPA):
An ammonium salt of molybdenum heteropoly acid was prepared
Fig. 2. Mechanism for the oxidation of organosulfur compound by H2O2
(oxidant) and HPA-SiO2 (catalyst) [31]
Oxidant
Catalyst
Oxidant
Catalyst
S
S
S
O
O
O
Fig. 1. Reaction of DBT in the catalytic oxidation

Vol. 29, No. 8 (2017)
Catalytic Performance of Ammonium Salt of Molybdenum Heteropoly Acid Supported Silica in Model Oil 1725
1401 cm–1 – NH4+
1024 cm–1 – P–O
930 cm–1 – Mo–O
addenda atom of POM take the oxygen from oxidant and forms
peroxo species (2), the peroxo species donate an oxygen to
organosulfur compound and oxidise it to organo sulfoxide and
recovered as POM (1).
780 cm–1 – Mo–O–Mo
1.02
1.00
0.98
0.96
0.94
0.92
0.90
0.88
0.86
Reaction monitoring: To determine the concentration of
the selected sulfur compound in the organic phase, approxi-
mately 0.5 mL of sample were withdrawn from the reactor at
fixed time intervals and after phase separation the organic phase
was analyzed by gas chromatography. GC equipped with a
flame ionization detector, using a 30 m. i.d. 0.32 mm HP-5
column. The main parameters were the following: carrier gas,
nitrogen with a flow of 2 mL/min; injector temperature. 250 °C:
detector temperature. 260 °C: temperature program, 40 °C or
the first 5 min, 40-100 °C at 10 °C/min. 100-200 °C at 15 °C/min,
200 °C for 5 min, 200-250 °C for 15 min [35].
1401 cm–1
1024 cm–1
930 cm–1
780 cm–1
1000 500
4000
3500
3000
2500
Wavenumber (cm–1)
2000
1500
Fig. 4. FTIR of HPA-SiO2 catalyst
30000
25000
20000
15000
10000
5000
0
RESULTS AND DISCUSSION
Catalyst characterization
Intensity
C
Scanning electron microscope (SEM) of HPA/SiO2: The
SEM analysis of catalyst has been taken on a JEOL JSM-840A.
The powder sample is gold coated under vacuum to make it
conducting for electrons. The SEM image (Fig. 3) confirms
the dispersion of ammonium salt of molybdophosphoric acid
(HPA) over silica surface and shows porous surface of the
catalyst with high surface area.
-5000
0
10
20
30
40
(°)
50
60
70
80
2θ
Fig. 5. X-ray diffraction of HPA-SiO2
nm) radiation. The voltage and current applied to the X-ray
tube were 40 kV and 20 mA, respectively and the sampling
step was set at 0.05° with scanning speed as 3 °/min (2θ = 10°
to 80°). 2θ = 10-20 and 25-30 correspond to the deposition of
molybdenum heteropoly acid (HPA) [34].
Thermogravimetric analysis: The TGA analysis of the
complex catalyst was carried out using a Perkin-Elemer instru-
ment in N2 atmosphere. The TGA graph of HPA/SiO2 catalyst
is given in Fig. 6. The catalyst showed a considerable weight
loss on heating till 250 °C while the corresponding catalyst
were found to be stable till 500 °C.
Fig. 3. SEM image of HPA-SiO2 catalyst
FTIR analysis of the HPA/SiO2 catalyst: The FTIR analysis
was carried out on a Bruker Vector 22 instrument in the 4000-
400 cm-1 wave number range. The FTIR spectrum of the final
heterogeneous catalyst is shown in Fig. 4. The deposition of
molybdenum heteropoly acid on the surface of the support
was shown by the bands at 1401, 1024 and 930 cm-1 which
represent the stretching vibrations of NH4+ ion, P-O and Mo-O,
respectively and the band at 780 cm-1 is attributed to stretching
vibration of Mo=O=Mo.
0.1
100
Weigth (%) vs. T
90
dW/dt vs. T
0.0
80
70
60
50
40
30
20
10
0
-0.1
-0.2
-0.3
-0.4
-0.5
X-ray diffraction of HPA/SiO2: X-ray diffraction measu-
rements of the catalyst were carried out on a Sietronics XRD
diffractometer equipped with Cu-Kα (λ = 1.54 nm) radiation.
The voltage and current applied to the X-ray tube were 40 kV
and 20 mA, respectively and the sampling step was set at 0.05°
with scanning speed as 3 °/min (2θ = 10° to 80°). The XRD
pattern of the final catalyst, HPA/SiO2 is shown in Fig. 5. X-ray
diffraction measurements of the catalyst were carried out on a
Sietronics XRD diffractometer equipped with Cu-Kα (λ = 1.54
100
200
300
400
500
600
700
Temperature (oC)
Fig. 6. TGA of HPA-SiO2

1726 Pant et al.
Asian J. Chem.
Influence of reaction temperature on the catalytic
reaction rate was. Therefore, the sulfur removal followed
order of 40 °C < 60 °C < 80 °C.After 100 °C the sulfur removal
decreased. The phenomenon attributed to decomposition of
H2O2.As the reaction progressed, the higher temperature could
lead to more decomposition of H2O2 and worse utilization of
H2O2. On the basis of results, the reaction for 100 min at 80
°C is chosen as an optimal condition for the desulfurization
process. The maximum sulfur removal at optimal condition in
presence of HPA/SiO2 catalyst is shown in Table-2.
performance of HPA/SiO2 on ODS of BT: The influence of
reaction parameters, such as the reaction temperature and O/S
molar ratio with reaction time were studied by using HPA/
SiO2 catalyst shown in Fig. 7.
Fig. 7 (a), (c), (e) shows the effect of temperature and
O/S molar ratio on oxidation of sulfur compounds under the
experimental conditions at different molar ratio of 2, 3, 4, 5,
with time at 80 °C. With increasing the ratio from 2 to 3, the
sulfur removal increase up to 91.2, 94.3 and 92.1 in 100 min
for oil containing BT, DBT and 4,6-DMDBT, respectively.
Further increase in molar ratio up to 5 leads to decreasing the
sulfur removal. The phenomenon may be attributed to the fact
that there is a competition between the decomposition of H2O2
and the oxidation of sulfur compounds by H2O2 [15]. When
the O/S molar ratio was increased from 4 to 5, the decrease of
desulfurization activity was attributed to the water produced
from oxidation reaction and thermal decomposition of H2O2,
which inhibited the oxidation reaction. Therefore, the opti-
mized H2O2/BT mole ratio as selected as 3.
TABLE-2
SULFUR REMOVAL AT OPTIMAL CONDITIONS
Compounds
Benzothiophene (BT)
Conversion (%)
91.5
94.6
92.5
Dibenzothiophene (DBT)
4,6-Dimethyldibenzothiophene (4,6-DMDBT)
Conclusion
As oxidative desulfurization ammonium salt of molybdenum
heteropoly acid were successfully impregnated on silica. FTIR
spectra and SEM confirms the deposition of HPA over silica
support. TGA shows high thermal stability of catalyst. After
100 min the sulfur compounds BT, DBT and 4,6-DMDBT are
oxidized up to 91.5, 94.6 and 92.5, respectively. The oxidation
reactivity decreased in order of DBT > 4,6-DMDBT > BT.
The solid catalyst was separated easily from the oxidation
system by centrifugation and could be recycled for seven times
without obvious decreasing in oxidation activity. The HPA/
SiO2 oxidation system was effective to remove aromatic sulfur-
Fig. 7, (b) (d) (f) shows the effect of the amount of H2O2
on oxidation of sulfur compounds under the experimental
conditions at 40, 60, 80 and 100 °C with time. With increasing
the temperature from 60 to 80, the sulfur removal increases
up to 91.5, 94.6 and 92.5 for oil containing BT, DBT, 4,6-
DMDBT, respectively. It was indicated that the sulfur removal
of BT increased with increasing time at different temperatures.
At beginning of reaction, the less H2O2 decomposed. It was
known that the higher the reaction temperature was, the faster
100
90
90
80
(a)
(b)
(c)
90
80
70
80
70
60
70
60
50
40
60
50
40
30
20
40 ºC
60 ºC
80 ºC
100 ºC
50
40
30
2
3
4
2
3
4
5
30
20
5
10
20
40
60
80
100
20
40
60
80
100
20
40
60
80
100
Time (min)
Time (min)
Time (min)
100
80
60
40
20
0
90
90
80
70
60
50
40
30
20
(d)
(e)
(f)
80
70
60
50
40
30
20
10
2
3
4
5
40 ºC
60 ºC
80 ºC
100 ºC
40 ºC
60 ºC
80 ºC
100 ºC
20
40
60
80
100
20
40
60
80
100
20
40
60
80
100
Time (min)
Time (min)
Time (min)
Fig. 7. (a) Influence of O/S molar ratio on performance of catalyst (HPA-SiO2) in ODS of BT at temperature 80 °C (b) Influence of temperature
at O/S molar ratio 3 on performance of catalyst in ODS of BT(c) Influence of O/S molar ratio on performance of catalyst in ODS of
DBT at temperature 80 °C (d) Influence of temperature at O/S molar ratio 3 on performance of catalyst in ODS of DBT (e) Influence
of O/S molar ratio on performance of catalyst in ODS of 4,6-DMDBT BT at temperature 80 °C (f) Influence of temperature at O/S
molar ratio 3 on performance of catalyst in ODS of 4,6-DMDBT

Vol. 29, No. 8 (2017)
Catalytic Performance of Ammonium Salt of Molybdenum Heteropoly Acid Supported Silica in Model Oil 1727
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