Post on 31-Jan-2023
A novel oxidative desulfurization process to remove refractory
sulfur compounds from diesel fuel
Jeyagowry T. Sampanthar *, Huang Xiao, Jian Dou, Teo Yin Nah,Xu Rong, Wong Pui Kwan
Applied Catalysis Technology Group, Institute of Chemical and Engineering Sciences, Agency for Science,
Technology and Research (A*STAR), No. 1, Pesek Road, Jurong Island, Singapore 627833, Singapore
Received 17 June 2005; received in revised form 12 September 2005; accepted 12 September 2005
Available online 25 October 2005
Abstract
Manganese and cobalt oxide catalysts supported on g-Al2O3 have been found to be effective in catalyzing air oxidation of the sulfur impurities in
diesel to corresponding sulfones at a temperature range of 130–200 8C and atmospheric pressure. The sulfones were removed by extraction with polar
solvent to reduce the sulfur level in diesel to as low as 40–60 ppm. Oxidation of model compounds showed that the most refractory sulfur compounds in
hydrodesulfurization of diesel were more reactive in oxidation. The oxidative reactivity of model impurities in diesel follows the order: trialkyl-
substituted dibenzothiophene > dialkyl-substituted dibenzothiophene > monoalkyl-substituted dibenzothiophene > dibenzothiophene.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Oxidative desulfurization; Diesel; Sulfur; Catalyst; MnO2/g-Al2O3; Co3O4/g-Al2O3; Solvent extraction
www.elsevier.com/locate/apcatb
Applied Catalysis B: Environmental 63 (2006) 85–93
1. Introduction
Deep desulfurization of diesel fuel has become an important
research subject due to the upcoming legislative regulations to
reduce sulfur content in most western countries. The US Clean
Air Act Amendments of 1990 and the new regulations by the
US Environmental Protection Agency (EPA) and government
regulations in many countries call for the production and use of
more environment-friendly transportation fuels with lower
contents of sulfur and aromatics. The demand for transportation
fuels has been increasing in most countries for past two
decades. For example, US Environmental Protection Agency
has set up guidelines to limit the sulfur content of diesel fuel to
15 ppm by 2006 [1]. Conventional hydrodesulfurization (HDS)
process has been employed by refineries to remove organic
sulfur from fuels for several decades and the lowest sulfur
content achieved by such process in the fuels is around
500 ppm. However, to meet the challenges of producing ultra-
clean fuels, especially with sulfur content lower than 15 ppm,
both capital investment and operational costs would be rather
high due to more severe operating conditions [2]. Consequently,
* Corresponding author. Tel.: +65 67963819; fax: +65 63166182.
E-mail address: T_Jeyagowry@ices.a-star.edu.sg (J.T. Sampanthar).
0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2005.09.007
several alternative approaches have been used, such as bio-
desulfurization [3], selective adsorption [4], extraction by
ionic-liquid [5] and oxidative desulfurization (ODS) [6–14].
Various studies on the ODS process have reported the use of
differing oxidizing agents and catalysts, such as H2O2/acetic
acid [7] and H2O2/formic acid [8], H2O2/heteropolyacids [9],
H2O2/inorganic solid acids [10], NO2/heterogeneous catalysts
[11], ozone/heterogeneous catalysts [12], tert-butylperoxides/
heterogeneous catalysts [13] and O2/aldehyde/cobalt catalysts
[14]. The ODS process is usually carried out under mild
conditions which present competitiveness over the conven-
tional HDS process [15]. In this process, the sulfur compounds
present in diesel are oxidized by the oxidizing agent to give rise
to the corresponding sulfones. These sulfones are highly
polarized compounds, such that they are removed from the
diesel by subsequent solvent extraction using water-soluble
polar solvents, such as NMP, DMF, DMSO and MeOH, etc.
[15]. By combination of the processes, the sulfur content of the
diesel can be reduced to �50 ppm [16]. Scheme 1 shows the
oxidation of organic sulfur compounds. The resulting sulfones
can be removed by either extraction and/or adsorption.
Here, we report the effective use of air as an environmentally
benign and low-cost oxidant to oxidize the sulfur compounds in
diesel at ambient pressure and moderate temperature in the
J.T. Sampanthar et al. / Applied Catalysis B: Environmental 63 (2006) 85–9386
Scheme 1.
presence of heterogeneous based simple transition metal oxides
loaded on g-Al2O3.
2. Experimental
2.1. Materials
Untreated diesel with a sulfur content in the range of 430–
465 ppm was obtained from Shell Petroleum Corporation,
Singapore. Co(NO3)�6H2O, Mn(CH3COO)2�4H2O, diben-
zothiophene (DBT), 4-methyl-dibenzothiophene (4-MDBT),
4,6-dimethyl-dibenzothiophene (4,6-DMDBT), 4,6-diethyl-
dibenzothiophene (4,6-DEDBT) and n-tetradecane were purc-
hased from Sigma–Aldrich, Singapore and used without further
purification. Model diesel was prepared by adding equimolar
amounts of dibenzothiophene, 4-methyl-dibenzothiophene,
4,6-dimethyl-dibenzothiophene and 4,6-diethyl-dibenzothio-
phene to n-tetradecane to make up a solution with a total sulfur
content of 400 ppm.
2.2. Catalyst preparation
A 10 g of g-Al2O3 pellet (obtained from Singapore Catalyst
Technology Center, diameter �3–4 mm, length �6–10 mm
with a specific surface area and a total pore volume of
370 m2 g�1 and 0.87 ml g�1, respectively) was impregnated
with cobalt nitrate and/or manganese acetate aqueous solutions
by an incipient wetness method. The total metal oxide loading
with respect to g-Al2O3 ranged from 2 to 13 wt%. The
impregnated samples were left on a roller which was set at
25 rpm for 18 h to obtain better dispersion. The water content of
the samples were removed and dried at 120 8C in the oven for
18 h followed by calcination in a static furnace at 550 8C for 5 h
with a ramp of 5 8C min�1. Other catalysts, such as W, Ni, Fe
and Cu, were also prepared in a similar manner.
2.3. Catalyst characterization
The physical and chemical properties of the prepared
catalysts were characterized by various analytical techniques.
The monolayer deposition of metal oxides on g-Al2O3 was
confirmed using powder X-ray diffraction (XRD) technique
using a Bruker AXS D8 Advance instrument with Cu Ka
radiation at 40 kV and 40 mA. The N2 adsorption–desorption
isotherm of the catalysts were studied using an Autosorb-1 at
77 K. Prior to the measurement, the calcined catalysts were
degassed at 300 8C. The thermal behavior of uncalcined
catalysts were studied by thermo gravimetric analysis (TGA)
using Universal V2.5H TA, Model SDT 2690 instrument under
laminar flow of air with a flow rate of 90 ml min�1. X-ray
photoelectron spectroscopy (XPS) investigation was conducted
on a VGESCALAB 250 spectrometer using monochromatic Al
Ka, X-ray source (1486.6 eV) at a constant analyzer pass
energy of 20.0 eV. All binding energies were referenced to the
C 1s peak arising from adventitious carbon (BE 285.0 eV). The
metal oxides composition and loading in the catalysts were also
characterized and confirmed by ICP (Model Vista-MPX) and
SEM-EDX (Model Jeol JSM-6700F).
2.4. Analysis of S—content in the model and real diesel
Sulfur content of the model and real diesel samples were
analyzed by XRF and GC-AED. The total sulfur content in the
model and real diesel samples were analyzed using XRF
(Bruker AXS S4 Exporer), which was calibrated with six liquid
calibration standards (obtained from AccuStandard), and
concentration ranging from 0 to 500 ppm sulfur by wt%.
The 10 ml of samples were placed into 40 mm diameter plastic
cells equipped with 2.5 mm MYLAR polyester film window.
Each cell was vented to prevent the polyester film window from
bulging during the analysis. The samples were then placed in
the automatic sample chamber and the optical path of the XRF
was flushed with helium gas prior to the measurement.
A 6890 GC coupled with atomic emission detector (JAS-
AED) was used to identify the various sulfur compounds and
their concentration. The GC was equipped with a split/splitless
injection port and operated in split mode. A 30 m � 0.32 mm
i.d. � 1 mm film thickness HP-1 MS capillary column was used
for separation as it has a lower specified rate of column bleed
than conventional methyl silicone capillary columns. Hydrogen
and oxygen gases were used as reagent gases for both carbon
(179 nm) and sulfur (181 nm). In order to improve the sulfur
selectivity over carbon, the AED gas flows (hydrogen, oxygen
and helium makeup) were optimized to minimize interferences
from hydrocarbons. The samples volume of 1 ml was injected
without any solvent dilution. The GC-AED instrument was
calibrated with sulfur in diesel fuel SRM 2724 obtained from
National Institute of Standards and Technology (NIST),
Reference Material Department, US. The SRM 2724 is a
commercial no. 2-distillate fuel oil as defined by ASTM with
J.T. Sampanthar et al. / Applied Catalysis B: Environmental 63 (2006) 85–93 87
Table 1
BET surface areas and total pore volume of the prepared catalysts after
calcinations at 550 8C
Catalysta Surface area (m2 g�1) TPV (ml)
g-Al2O3 377 0.87
�2%MnO2/g-Al2O3 361 0.86
�5%MnO2/g-Al2O3 350 0.86
�8%MnO2/g-Al2O3 331 0.78
�11%MnO2/g-Al2O3 317 0.80
�13%MnO2/g-Al2O3 305 0.77
�2%Co3O4//g-Al2O3 368 0.79
�5%Co3O4//g-Al2O3 350 0.76
�8%Co3O4//g-Al2O3 323 0.76
�5%MnO2/3%Co3O4//g-Al2O3 322 0.72
�3%MnO2/3%Co3O4//g-Al2O3 331 0.75
�3%MnO2/5%Co3O4//g-Al2O3 310 0.72
a All the above samples were calcined at 550 8C under static air and degas at
300 8C for 5 h before measurements.
the certified total sulfur content of 425 ppm. In addition to the
SRM 2724, the AccuStandard (also obtained from NIST) with
sulfur content in diesel fuel 0, 100, 200, 300, 400 and 500 ppm
samples were also used for the calibration.
2.5. Solvent extraction on real diesel without oxidative
treatment
Solvent extraction studies for the removal of sulfur
compounds in untreated diesel (obtained from Shell Petroleum
Corporation, Singapore with a sulfur content in the range of 430–
465 ppm) were carried out with four different organic solvents of
different polarities, such as acetonitrile (AcN), dimethylfor-
amide (DMF), 1-methyl-2-pyrrolidinone (NMP) and methonal
(MeOH). A 25.0 ml of untreated diesel were mixed with the
known volume of polar organic solvents to determine the
efficiency of solvent extraction. The diesel–solvent mixture was
stirred for 30 min before separating the two layers. After
extraction by the respective polar solvents, the sulfur content in
the diesel was measured by XRF and GC-AED.
2.6. Catalytic oxidation followed by solvent extraction on
model diesel
The oxidation experiments in this study were carried out with
20.0 ml of model diesel in a refluxed round bottom flask.
Approximately, 20–30 mg of g-Al2O3 supported Mn and/or Co
oxides in the form of pellets were used as catalysts. The reactions
were carried out at a temperature range of 90–180 8C, during
which air was introduced via a gas disperser at a constant flow
rate of 100 ml min�1 while the reaction mixture was stirred
throughout the experiment. Awater-cooled reflux condenser was
mounted on top of the reaction flask to prevent solvent loss and
Fig. 1. X-ray photoelectron spectroscopy analysis of metals oxides loaded on gamma
�3%MnO2/�5%Co3O4/g-Al2O3; (D) �8%Co3O4/g-Al2O3.
serve as an air outlet. The progress of the reaction was monitored
periodically withdrawing 0.5 ml aliquots of the reaction mixture
for GC-AED analysis. Blank experiments were carried out with
model diesel and pure support of g-Al2O3 under exactly similar
experimental conditions.
The oxidized products in the model diesel were extracted
with NMP or methanol after the completion of the reaction.
The reacted model diesel was mixed with one of these polar
solvents at different volume ratio (e.g. diesel:polar solvent = 4:1
when NMP as a solvent, 1:1 when MeOH as solvent) and was
magnetically stirred for 30 min. The mixture was then transferred
into a separating funnel and separated into two layers of model
diesel and polar solvent which were analyzed for sulfur
compounds using GC-AED. When MeOH was used as solvent,
it was removed from solvent–sulfone mixture using rotary
-alumina: (A) �11%MnO2/g-Al2O3; (B) �5%MnO2/�3%Co3O4/g-Al2O3; (C)
J.T. Sampanthar et al. / Applied Catalysis B: Environmental 63 (2006) 85–9388
Fig. 2. Sulfur-specific gas chromatograms of model diesel (air oxidation of model diesel (400 ppm sulfur) catalyzed MnO2 (11%) loaded on g-alumina support;
solvent extraction was carried out using NMP:oxidized diesel = 1:1, single extraction).
evaporator and the product of sulfones mixture was precipitated
at the bottom of the flask.
2.7. Catalytic oxidation by Mn and/or Co oxides supported
on g-Al2O3 followed by solvent extraction on real diesel
A 150.0 ml real diesel underwent oxidative desulfurization
reaction in the presence of about 100 mg of catalyst at
temperature range of 130–200 8C in a two-necked round
bottom flask. The reaction mixture was magnetically stirred to
ensure a good mixing and bubbled with purified air at constant
flow of 100 ml min�1. The reaction mixture was periodically
Fig. 3. Conversion % of the thiophenic compounds to the corresponding sulfone
in model diesel (catalyst: 11%MnO2 loaded in g-Al2O3, temperature 150 8C).
sampled and analyzed using GC-AED and reaction was ceased
after about 18 h. The oxidized diesel was then cooled to room
temperature and 25.0 ml of reacted diesel was treated with
varying volume of different solvents for solvent extraction.
Sulfur content of the extracted oxidized real diesel was
measured by XRF and GC-AED. Similar reaction and solvent
extraction method were carried out with different loading of
both Mn and/or Co oxide catalysts.
3. Results and discussion
3.1. Characterization of catalyst
The TGA studies showed that the most of the metal salts
loaded on the g-Al2O3 converted into their corresponding
oxides at below 500 8C under laminar flow of air. Table 1
summarizes the specific surface area and total pore volume of
the prepared catalysts. It shows that the specific surface area in
the series considerably lowered from 377 m2 g�1 for pure
g-Al2O3 support to the lowest value of 305 m2 g�1 for the
sample loaded with the maximum amount of transition metal
oxide (�13%MnO2/g-Al2O3). The total pore volume of the
calcined samples also decreases as the loading of the transition
metal oxides increases. The decreasing behavior of both surface
area and total pore volume with the increasing loading of the
metal oxides are consistent due to the possible blockage of the
inner pores, especially the smaller ones, and dilution of the
J.T. Sampanthar et al. / Applied Catalysis B: Environmental 63 (2006) 85–93 89
Fig. 4. Sulfur-specific gas chromatograms of real diesel (air oxidation of real diesel diesel (�450 ppm sulfur) catalyzed MnO2 (11%) loaded on g-alumina support;
solvent extraction was carried out using NMP:oxidized diesel = 1:1, single extraction).
Fig. 5. Conversion % thiophenic compounds to corresponding sulfone in real
diesel (catalyst: 11%MnO2 loaded in g-Al2O3, temperature 150 8C).
initial support material, g-Al2O3, by the uniformly dispersed
and dense metal oxide, MnO2 and Co3O4, phase.
The absence of characteristic diffraction peaks in XRD
patterns confirmed that the deposition of metal oxides on the
support g-Al2O3 were in the form of amorphous layer. The data
obtained for the actual loadings of metal oxides from ICP
analysis, XRF and SEM coupled with EDX were almost equal
to the initial calculated values. This confirmed the calcinations
process and its process conditions were optimum and virtually
all the metal oxides coated on the support material.
XPS spectra with binding energies (eV) for metal elements are
shown in Fig. 1. The binding energies of Mn 2p (641.5 eV) and
Co 2p (780.4 eV) for manganese oxide sample A and cobalt
oxide sample D are consistent with the formation of MnO2 and
Co3O4 in these samples. However, for samples B and C which
contains binary Mn–Co oxides coatings, there is a pronounced
shift from +0.7 to +1.2 eV for Mn 2p which could be attributed to
the interaction of manganese with alumina support. The positive
shift of Al 2p binding energies in sample B and C also suggests
that existence of interaction between the coated metal oxides and
the g-Al2O3 support when both Mn and Co oxides are present.
3.2. Selective catalytic sulfur oxidation followed by solvent
extraction on model diesel
As shown by the sulfur-specific gas chromatograms in Fig. 2
and % of conversion versus time in Fig. 3, the thiophenes
conversion increased with time and it reached its maximum
conversion of �80–90% at 8 h. Fig. 3 also shows that the
oxidative reactivity of the model thiophene compounds follows
the order of 4,6-dEDBT > 4,6-dMDBT > 4-MDBT > DBT.
The observed order of reactivity is opposite to that observed in
the hydrodesulfurization process where the most sterically
hindered thiophenes, 4,6-dEDBT and 4,6-dMDBT, are the least
reactive. Apparently, the increased electron density of the sulfur
atoms in disubstituted thiophenes can overcompensate for the
steric hindrance of the C4 and C6 alkyl groups in the oxidative
process.
J.T. Sampanthar et al. / Applied Catalysis B: Environmental 63 (2006) 85–9390
Table 2
Sulfur content analysis results after solvent extraction of diesel with and without
oxidation
Catalysta Extraction
solvent (vol)
S content (ppm)b
(treated diesel)
Diesel, no oxidation No extraction 430
Diesel, no oxidation AcN (10 ml) 310
Diesel, no oxidation DMF (10 ml) 226
Diesel, no oxidation NMP (10 ml) 219
Diesel, no oxidation MeOH (25 ml) 314
�2%Co3O4/g-Al2O3 AcN (10 ml) 237
�2%Co3O4/g-Al2O3 DMF (10 ml) 146
�2%Co3O4/g-Al2O3 NMP (10 ml) 129
�2%Co3O4/g-Al2O3 MeOH (25 ml) 215
�5%Co3O4/g-Al2O3 AcN (10 ml) 236
�5%Co3O4/g-Al2O3 DMF (10 ml) 145
�5%Co3O4/g-Al2O3 NMP (10 ml) 134
�5%Co3O4/g-Al2O3 MeOH (25 ml) 215
�8%MnO2/g-Al2O3 AcN (10 ml) 198
�8%MnO2/g-Al2O3 DMF (10 ml) 117
�8%MnO2/g-Al2O3 NMP (10 ml) 108
�8%MnO2/g-Al2O3 MeOH (25 ml) 172
a Oxidation reaction carried out at 130 8C; 25.0 ml oxidized diesel extracted
with solvent.b S content was measured by XRF and GC-AED.
3.3. Selective catalytic sulfur oxidiation followed by
solvent extraction on real diesel
Similar results were obtained with real diesel containing
approximately 450 ppm sulfur as shown by the sulfur-specific
GC-AED chromatograms in Fig. 4. The conversion of the
substituted thiophenes (Fig. 5) to corresponding sulfones was in
the range of 65–75% depending on the type of catalysts and
operating temperatures in the range of 130–200 8C. The
selectivity was about 90–100%. The total sulfur content of the
diesel before and after was same in most of the cases and when
the operating temperature increases, some of the sulfur
compounds were over oxidized and converted (see Scheme
1) into SO2 (gas). The elimination of SO2 was confirmed by
scrubbing the outlet gas with a AgNO3 solution to form AgSO3
precipitate.
Table 2 summarizes the results of extracting real diesel
before and after oxidation. Among the polar solvents tested,
NMP was found to be the most efficient in extracting sulfur
compounds from both diesel and oxidized diesel. While both
thiophenes and sulfones can be extracted from diesel, the
sulfones are significantly easier to be removed from diesel by
polar solvents due to higher polarity. The results also show that
the extraction efficiency for both thiophenes and sulfones with
the polarity of the extraction solvent.
The GC-AED carbon chromatogram shows there were no
significant changes in the product distribution before and
after oxidation of the real diesel samples. The trisubstitued
dibenzothiophenes compounds were easier to be oxidized than
the monosubstituted dibenzothiophene, such as 4-methyl-
dibenzothiophene is difficult to oxidize compared to 4,6-
diethyl-dibenzothiophene (Table 3).
Table 3
Sulfur content analysis results after solvent extraction of oxidized diesel at variou
Catalysta Reaction temperature (8C)
�2%MnO2/g-Al2O3 130
�2%MnO2/g-Al2O3 130
�2%MnO2/g-Al2O3 130
�2%MnO2/g-Al2O3 130
�2%MnO2/g-Al2O3 130
�5%MnO2/g-Al2O3 130
�5%MnO2/g-Al2O3 130
�5%MnO2//g-Al2O3 130
�5%MnO2/g-Al2O3 130
�5%MnO2/g-Al2O3 130
�8%MnO2/g-Al2O3 150
�8%MnO2/g-Al2O3 150
�8%MnO2/g-Al2O3 150
�8%MnO2/g-Al2O3 150
�11%MnO2/g-Al2O3 150
�11%MnO2/g-Al2O3 180
�13%MnO2/g-Al2O3 150
�13%MnO2/g-Al2O3 180
�5%MnO2/3%Co3O4//g-Al2O3 180
�3%MnO2/5%Co3O4//g-Al2O3 180
Oxidation reaction temperature in 8C.a 30.0 ml oxidized diesel extracted with different amount of different solvent (sib S content was measured by XRF and GC-AED.
3.4. Properties of oxydesulfurized real diesel
The treated diesel (oxidized followed with solvent extrac-
tion) was analyzed for diesel specification parameters, such as
density, cetane index, pour point, kinematic viscosity, etc. and
the results are given in Table 4. The studies show that the olefin
content of the diesel was increased and aromatic content of
the diesel was reduced substantially. Cetane index increased
s temperatures and various catalysts system
Extraction solvent (vol) S (ppm)b
NMP (10 ml � 3) 64
NMP (10 ml) 188
NMP (20 ml) 126
NMP (30 ml) 96
DMF (10 ml) 193
NMP (10 ml � 3) 51
NMP (10 ml) 168
DMF (10 ml) 187
DMF (20 ml) 145
DMF (30 ml) 115
NMP (10 ml) 143
NMP (20 ml) 119
NMP (30 ml) 103
NMP (40 ml) 61
NMP (10 ml) 104
NMP (30 ml) 66
NMP (10 ml) 84
NMP (30 ml) 44
NMP (10 ml) 109
NMP (10 ml) 123
ngle or multiple extraction).
J.T. Sampanthar et al. / Applied Catalysis B: Environmental 63 (2006) 85–93 91
Table 4
Some diesel specification analysis for the untreated and treated diesel samples
Testa Method Real diesel Treated diesel
S content (ppmw) (wt%) ASTM D3120-96 0.043 0.001
Kinematic viscosity @ 40 8C (cSt) ASTM D445-01 4.376 4.982
Density @ 15 8C (kg l�1) ASTM D4052-96 0.8541 0.8286
Olefins (vol%) ASTM D1319-99 2.4 3.6
Aromatics (vol%) ASTM D1319-99 46.4 12.5
Water content (ppm) Karl Fischer 120 139
Pour point (8C) ASTM D97-96a +6 +12
Lubricity (mm) CEC F06-A-96 175 474
Cetane index ASTM D976-91(00) 53 62.8
a Analysis carried out by Intertek Testing Services (S) Pvt. Ltd., Singapore Technical Centre.
Fig. 6. Conversion % thiophenic compounds to corresponding sulfone in real
diesel (catalyst: (A) 8%MnO2 loaded in g-Al2O3, temperature 150 8C; (B)
5%MnO2/3%Co3O4 loaded in g-Al2O3, temperature 180 8C; (C) 3%MnO2/
5%Co3O4 loaded in g-Al2O3, temperature 180 8C).
approximately by 20%. Density and other parameters were
within the required limits. Lubricity of the treated diesel was
increased by substantial amount.
3.5. Effect of the catalyst composition
Metal oxide catalysts derived from Mn, Co, W, Ni, V, Fe and
Cu, respectively, were tested for their ability to oxidize sulfur
impurities in real diesel and a model diesel mixture composed
of n-tetradecane and various substituted dibenzothiophene
compounds in air at a temperature of 110–180 8C. Among these
catalysts, only manganese and cobalt oxides were found to be
effective in catalyzing the oxidation of the thiophenes to
sulfones. The unmodified support, g-Al2O3, was also ineffec-
tive.
The effects of metal loading and reaction temperature were
investigated. Below 110 8C, the oxidation reaction was not
observed. There was no significant difference in conversions for
the oxidation of model diesel catalyzed by either 2 or 5%
Co3O4/g-Al2O3 at 130 or 150 8C. However, in the case of
MnO2/g-Al2O3 catalysts, higher metal loading led to higher
conversion at all test temperatures: 130, 150, 180 and 200 8C.
The best results were obtained with catalysts containing the
highest MnO2 loadings (11 and 13%) at 180 8C. Similar effect
was observed for the oxidation of real diesel (Table 2).
For binary mixed metal (Mn and Co) oxide catalysts, higher
activity was observed with higher Mn/Co ratio. Thus,
5%MnO2/3%Co3O4//g-Al2O3 showed better oxidation activity
than 5%Co3O4/3%MnO2//g-Al2O3 (Table 3 and Fig. 6).
3.6. Simplified process diagram for the treatment of
production S-free diesel
Fig. 7 shows a possible process diagram for the integration
of oxidative desulfurization process into an existing refinery
process unit for desulfurization of diesel. The ODS reactor unit
may be installed as downstream of a conventional hydro-
desulfurization reactor unit. Oxidative desulfurization reaction
can be carried out as a secondary desulfurization process
for diesel that have been treated by conventional HDS process.
The treated/oxidized diesel is channeled to a stirred/mixing
tank containing polar solvent for removing the oxidized sulfur
compounds. The diesel/solvent mixture is then channeled to a
J.T. Sampanthar et al. / Applied Catalysis B: Environmental 63 (2006) 85–9392
Fig. 7. Simplified process diagram for the treatment and production of S-free diesel.
settler where the treated diesel is separated from the solvent.
The solvent can be recycled by distillation and can be reused.
The treated diesel is further passed through a basic adsorbent-
bed unit for further removal of remaining sulfur-containing
compounds in the diesel. The remaining sulfones in the treated
diesel could be easily removed by adsorption compare to
thiophinic compounds.
4. Conclusion
It has been demonstrated that Mn- and Co-containing oxide
catalysts are highly effective for selective oxidation of the
refractory sulfur compounds in diesel fuel using molecular
oxygen in air at atmospheric pressure and the sulfur content can
be easily reduced to 40–60 ppm after coupled with extraction
by polar solvent. During our research studies, Murata et al. [14]
have reported recently oxidation of sulfur compounds using
molecular oxygen in the presence of cobalt catalysts and
aldehydes in monophasic system. In our system, the catalyst
(heterogeneous) can be easily reactivated and reused. The polar
solvent used for extraction can be recycled by vacuum
distillation. The low-sulfur (�10–15 ppm sulfur) diesel was
obtained by simply pass through treated diesel (oxidized and
solvent extracted diesel) into the activated basic g-Al2O3
adsorbent-bed at room temperature.
This oxidative desulfurization process has several advan-
tages over other oxidative desulfurization processes which were
reported. One advantage of this process that the reaction can be
carried out using inexpensive oxygen found air compare to
costly oxidants, such as H2O2 or ozone, which were reported in
the literature for the oxidative desuflurisation processes. In
addition, the use of air as oxidant also eliminates the need to
carry out any oxidant recovery process that is usually required if
liquid oxidants (tert-butylperoxide or H2O2) are used. Another
advantage of this process is the mild operating conditions
compared to hydrodesulfurization process which more severe
conditions are needed. Yet another advantage of this process is
the ease of integration into any existing refinery for the
production of diesel, as afforded by the mild process conditions
of liquid phase contacting and the use of air. Furthermore, the
use of a selective oxidation catalyst also permits the tuning of
experimental parameters, such as temperature and contacting
time, to achieve optimal conversion and selectivity. The
simplified process flow sheet of the oxidative desulfurization
process which can be adopted for the refineries without major
changes in the infrastructure is shown in Fig. 7.
Acknowledgements
This work was financially supported by the Agency for
Science, Technology and Research (Project No. ICES/04-
112001). J.T. Sampanthar wish to thank Prof. Hua Chun Zeng
(National University of Singapore, Singapore) and Mr. Sam
Mylvaganam (ICES) for their valuable comments and useful
discussion.
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