Oxidation of dibenzothiophene to dibenzothiophene-sulfone using silica gel

Article abstract of DOI:10.1016/j.jcat.2009.10.002

The oxidation of dibenzothiophene (DBT) to dibenzothiophene-sulfone (DBT-sulfone) has been a topic of particular interest in the past few years, as a possible technique to remove sulfur from crude oil. In this manuscript we show that the oxidation of DBT

Full text of DOI:10.1016/j.jcat.2009.10.002

Journal of Catalysis 268 (2009) 329–334
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Oxidation of dibenzothiophene to dibenzothiophene-sulfone using silica gel
b
a
a
Karina Castillo a, , J.G. Parsons , David Chavez , Russell R. Chianelli
*
a Department of Chemistry, University of Texas at El Paso, 500 W University Ave., El Paso, TX 79968, United States
b Department of Chemistry, The University of Texas-Pan American, 1201 W University Drive Edinburg, TX 78539-2999, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidation of dibenzothiophene (DBT) to dibenzothiophene-sulfone (DBT-sulfone) has been a topic of
particular interest in the past few years, as a possible technique to remove sulfur from crude oil. In this
manuscript we show that the oxidation of DBT to DBT-sulfone occurs using silica and does not require the
addition of an external oxidizing agent. The effect of the synthesis pH and the calcination temperature on
the ability of the silica to oxidize DBT to DBT-sulfone was investigated. The oxidation reactions were per-
formed by refluxing silica with DBT in the following solvents: decahydronaphthalene, tetrahydronaph-
thalene, dodecane, heptane, and toluene at different temperatures. The reaction was found to work
only in hydrogen-donating solvents: tetralin and decalin. HPLC results show that 80% of the original
10,000 ppm DBT was oxidized when the reaction was carried out at 160 °C in decahydronaphthalene.
FTIR was used to show that the product of the reaction was DBT-sulfone, which confirmed 80% conver-
sion of the DBT to DBT-sulfone. The product of the oxidation reaction was further identified using powder
XRD.
Received 20 August 2009
Revised 30 September 2009
Accepted 1 October 2009
Available online 29 October 2009
Keywords:
DBT oxidation
DBT-sulfone
Oxidative desulfurization
Oxidation using silica
Published by Elsevier Inc.
1. Introduction
toxic compound. Another important reason to find an alternative
method to remove sulfur is that substituted sulfur compounds
Developed countries are setting new standards for the allow-
able amount of sulfur in diesel; in the United States, by 2014 sul-
fur levels in all diesel fuel must be below 15 ppm [1]. To meet
these specifications at the pump, refiners must produce diesel
containing one-half the amount of sulfur before it enters the dis-
tribution system, because the low-sulfur product is expected to
pick up trace amounts of sulfur as it moves through pipelines
and other distribution channels. Extra heavy oils, such as those
from the Orinoco region in Venezuela or the Alberta tar sands in
Canada, are typically upgraded in a process that is both capital-
and energy-intensive.
Hydrodesulfurization processes are currently used to remove
sulfur from crude oils. These processes use hydrogen which reacts
with the sulfur compounds to generate H2S gas. These processes
involve high temperatures of about 350 °C and high pressures of
H2 gas from 3 to 5 MPa [2]. From an economic and environmental
point of view, it would be more suitable if low-temperature and
low-pressure systems could be developed to remove sulfur from
fuels. High-temperature and high-pressure processes decrease
the lifetime of the catalyst and consumes larger amounts of H2,
which results in higher costs. In addition, high-temperature and
high-pressure processes result in the generation of H2S, a highly
such as methyl ethyl dibenzothiophene, 4-methyl dibenzothio-
phene, 3 methyl dibenzothiophene, and others are non-reactive
under hydrodesulfurization process [3–8].
An alternative method to remove sulfur from fuels is oxidative
desulfurization. There have been many catalysts investigated to
oxidize sulfur-containing molecules in organic solvents. For exam-
ple, Mo/Al2O3 catalyst has been studied for the oxidative desulfur-
ization (OD) process in conjunction with hydrogen peroxide as the
oxidizing agent [9]. Another example of an oxidation desulfuriza-
tion catalyst is Ti3(PW12O40)4, which also requires the use of
H2O2 as an oxidizing agent [10]. Potassium superoxide has shown
sulfur removal as high as 99% with samples of benzothiophene,
dibenzothiophene, and a number of selected diesel oil samples
[11].
As mentioned before these oxidative desulfurization techniques
utilize oxidizing agents such as NO2, H2O2, and tert-butyl-hydro-
peroxide. The advantage of using these techniques is that sulfur
can be removed at relatively low temperatures and atmospheric
pressure. The major drawback to ODS is that current catalytic sys-
tems require the use of an oxidizing agent such as H2O2. The con-
stant use of strong oxidizing agents such as hydrogen peroxide can
become expensive and makes these catalysts cost prohibitive on
the large scale.
In the present study, the oxidative catalytic properties of SiO2
were investigated. First, silica gel grade 22 was obtained from
Alfa Aesar and tested in the dibenzothiophene (DBT) model reac-
tion. Then, silica gel was synthesized at different pHs and tested
* Corresponding author.
E-mail address: kcastillo2@miners.utep.edu (K. Castillo).
0021-9517/$ - see front matter Published by Elsevier Inc.
doi:10.1016/j.jcat.2009.10.002

330
K. Castillo et al. / Journal of Catalysis 268 (2009) 329–334
in the model reaction. Fourier-Transformed Infrared spectroscopy
was used to follow the oxidation of DBT (dibenzothiophene) to
DBT-sulfone at the different pHs and different reaction tempera-
tures. X-ray diffraction was used to characterize the catalysts be-
fore and after the reactions. In addition, X-ray diffraction was
used to confirm that the product was DBT-sulfone. HPLC (high
performance liquid chromatography) was used to determine the
percentage removal of DBT after each reaction.
2.4. FT-IR infrared spectroscopy
Infrared spectroscopy was carried out with a Thermo Nicolet
Nexus 470 FT-IR. The samples for IR measurements were prepared
as pellets by embedding the sample in a polycrystalline KBr matrix.
The pressure applied to make the pellets was 5 MPa. In addition, a
background spectrum was collected before each sample spectrum
collection.
2.5. X-ray diffraction (XRD)
2. Experimental
The X-ray diffraction data were collected using a D5000 X-ray
2.1. Synthesis of silica gel at different pH
diffractometer with Cu K
a radiation with a wavelength corre-
sponding to 1.540 Å. The start angle was 5° and the stop angle
was 45°.
Silica was synthesized at different pHs. Silica gel was prepared
with slight modifications to previously reported methodologies
[12–15]. Sodium metasilicate (Na2SiO3) and hydrochloric acid
(HCl) were obtained from Alfa Aesar. Na2SiO3 was dissolved in
water, which produced a basic solution (between pH 12 and 14).
HCl was added to the solution under vigorous stirring to reduce
the pH to 10, which resulted in a gel. Subsequent to formation
the gel was then washed several times with water to remove any
NaCl and other byproducts of the synthesis. In addition, the gel
was then washed with alcohol and filtered under vacuum to re-
move any residual water. The resulting gel was then dried at three
temperatures: 100 °C (24 h), 140 °C (24 h), and 380 °C (24 h). Sim-
ilarly, SiO2 gels were synthesized at pHs 9, 8, and 7 using the same
precipitation procedure, washing, and drying conditions. The syn-
thesis of SiO2gel at pHs 6, 5, and 4 consisted of bringing the pH
down to approximately 2 and subsequently raising the pH to either
4, 5, or 6 using sodium hydroxide. SiO2 was also synthesized at
very acidic pHs 3, 2, and 1. The acidic synthesis technique was sim-
ilar as previously mentioned. HCl was added quickly to a solution
of Na2SiO3 in water under vigorous stirring to achieve a pH of 1.
The solution was then heated to form a gel, which was subse-
quently washed with water and alcohol and dried as mentioned
earlier.
3. Results
3.1. High performance liquid chromatography results
SiO2 synthesized at different pHs in the range 3–10 showed no
reactivity toward the DBT. However, when SiO2 was synthesized in
the pH range pH 1–2 the oxidation of DBT to DBT-sulfone was ob-
served. In addition, it was also observed that the percentage of DBT
removal increased as the calcination temperature of the gel was in-
creased from 100 °C to 380 °C as shown in Fig. 1A. As it can be seen
in Fig. 1A when the gel is dried at 100 °C only a 20% reduction of
the DBT concentration was observed. The concentration of DBT re-
moved when the SiO2 was calcinated at 140 °C was 40%. However,
at a calcination temperature of 380 °C (above the critical point of
water), a reduction of 80% of the DBT concentration was observed.
A comparison between commercially available silica and the in-
house-synthesized silica for the oxidation of DBT was performed.
The reactions of both the in-house-synthesized silica and the com-
mercial silica with DBT were performed at 100 °C, 140 °C, and
160 °C and are shown in Fig. 1B. From the three temperatures used
100 °C, 140 °C and 160 °C, the reaction run at 160 °C was the one
that showed the largest removal of DBT. Furthermore, it was noted,
as shown in Table 1, that the oxidation of DBT to DBT-sulfone oc-
curs only in the presence of a hydrogen-donating solvent such as
decalin or tetralin. No oxidation was observed when non-hydrogen
donor solvents such as heptane and toluene were used. The con-
version using either tetralin or decalin is approximately the same.
For simplicity, only data using decalin as a solvent are shown in
Fig. 1B. According to the data shown in Fig. 1B, the highest removal,
85%, of DBT occurred at 160 °C. Finally, Fig. 1C shows the time
requirements for the removal of DBT at 160 °C, using sampling
times of 60, 120, and 240 min: the reaction requires 240 min to
reach 80–85% removal of DBT.
2.2. DBT oxidation reaction
The oxidative properties of SiO2 synthesized at different pHs
were tested using the DBT model reaction. DBT (0.5 g) was dis-
solved in decalin (50 mL), 0.5 g of SiO2 was added, and the reaction
was refluxed for 4 h under vigorous stirring. The reactions were
tested at six different temperatures: 50 °C, 100 °C, 120 °C,
140 °C, and 160 °C. Different temperatures were tested to deter-
mine the optimum temperature of the reaction. In addition, differ-
ent solvents were tested which included decalin, tetralin, toluene,
heptane, and dodecane. Different solvents were used to determine
if the solvent and the temperature are important factors control-
ling the oxidation of DBT to DBT-sulfone.
3.2. FT-IR results
2.3. High performance liquid chromatography (HPLC)
The results of the reaction using DBT as the model compound
are shown in Fig. 2. These results correspond to the products of
the reactions carried out at 160 °C using silica grade 22 and the lab-
oratory-synthesized silica (pH 1) as catalysts (Fig. 2A and B). Fig. 2C
and D shows the infrared spectra corresponding to DBT-sulfone
standard and DBT, respectively.
In Fig. 3, the infrared spectrum is shown only from 800 to
400 cmÀ1 wavenumbers where the C–S and S–O ring vibrations oc-
cur. It is important to emphasize the difference in the infrared
spectrum after the oxidation reaction using silica at 160 °C
(Fig. 3A). The shift in the peak at 744 cmÀ1 occurs in the treated
samples and the absorption bands corresponding to S–O vibrations
appear in the 600–500 cmÀ1 range (Fig. 3A). Comparing the treated
High performance liquid chromatography (HPLC) was per-
formed with a SPHERI-5 (5 lm) Silica-based column, a Spectra-
Physics Spectra System P1500 gradient pump, UV2000 detector,
and Winner for Windows Software. The mobile phase will be 95%
hexanes, 5% isopropanol by volume with helium solvent de-gas-
sing. The flow rate was set to 1 mL/min. and the detector was set
to a 256-nm wavelength, all samples were run at room tempera-
ture. A 1 ll aliquot of analyte was used and in some samples it
was necessary to increase the amount to enhance the signal. A cal-
ibration curve was obtained by using four different concentrations
of DBT. The correlation coefficient of the calibration curves used in
this study was R2 = 0.99 or better.

K. Castillo et al. / Journal of Catalysis 268 (2009) 329–334
331
Table 1
100
80
60
40
20
0
Results of reactions of DBT with different SiO2: commercially available SiO2 grade 22,
SiO2 basic, SiO2 neutral, and SiO2 acid in house synthesis, using different solvents, and
reaction temperatures.
A
Catalyst
Reaction temperature (°C)
Solvent
%DBT removal
SiO2 grade 22
SiO2 grade 22
SiO2 grade 22
SiO2 grade 22
SiO2 grade 22
SiO2 neutral pH
SiO2 basic pH
SiO2 acidic pH
SiO2 neutral pH
SiO2 basic pH
SiO2 acidic pH
160
160
100
110
160
160
160
160
160
160
160
Decalin
Tetralin
Heptane
Toluene
Dodecane
Decalin
Decalin
Decalin
Tetralin
Tetralin
Tetralin
80
85
0
0
0
0
0
80
0
0
84
100
140
380
Calcination Temperature (°C)
Fig. 4A and B shows DBT after reaction with laboratory-synthesized
silica calcinated at 380 °C and 160 °C, respectively. Fig. 4C shows
DBT after reaction with laboratory-synthesized silica calcinated
at 100 °C. Absorption bands of Fig. 4A and B coincide with the
absorption bands of DBT-sulfone standard (refer to Fig. 3D); how-
ever, when the laboratory-synthesized silica is calcinated at 100 °C,
conversion of DBT to DBT-sulfone is not clearly shown. As can be
seen in Fig. 4 the appearance of the DBT-sulfone vibrations be-
comes more defined as the calcination temperature of the silica
increases.
The main differences that were observed when the reaction is
run at different temperatures are in the range of 1000–
1500 cmÀ1. The absorption bands corresponding to DBT-sulfone
start appearing when the reaction is run at 100 °C (data not
shown). However, DBT-sulfone vibrations between 1000 cmÀ1
and 1500 cmÀ1 are masked by the presence of the silica, which ab-
sorbs in the range of 1000–1500 cmÀ1 and thus not visible in the
FT-IR spectra. As the reaction temperature increased, the absorp-
tion bands of DBT-sulfone standard in the range of 1000–
1500 cmÀ1, began to become visible in the samples treated with
SiO2. Based on these data, it is suggested that a lower temperature
DBT is absorbed onto the silica first and is subsequently oxidized.
As the temperature at which the reaction is run increases
(160 °C), the silica releases the oxidized DBT (DBT-sulfone) back
to the solution. The oxidation reaction was studied at the boiling
temperature of the decalin (190 °C). At boiling point of the decalin
no oxidation was observed, not even partial oxidation was ob-
served (data not shown) (see Fig. 5).
100
80
60
40
20
0
B
100
140
160
Reaction Temperature (°C)
SiO2 Grade 22
Synthesized SiO2
C
100
80
60
40
20
0
3.3. XRD results
The X-ray diffraction patterns of DBT-sulfone obtained and of
DBT-sulfone standard are shown in Fig. 6. Cerius software was
used to assign the h k l planes. DBT-sulfone was modeled using
the cell parameters and atomic positions as previously reported
[16]. The unit cell parameters used were a = 10.09 Å, b = 13.89 Å,
c = 7.22 Å, and b = 93.5°. The space group used was C2/c. The X-
ray diffraction patterns showed a difference in the intensities
(Fig. 5). Reflections at 13.30 and 22.14 2h correspond to the 020,
and 220 planes and are in the same intensity in DBT-sulfone stan-
dard and SiO2-reacted DBT. The treated sample may have adopted
60
120
Time (min)
SiO2 Grade 22
240
Synthesized SiO2
Fig. 1. (A) Effect of calcination temperature of acidic SiO2 on the removal of DBT
from a decalin solution at 160 °C. (B) Effect of reaction temperature on the removal
of DBT in the presence of SiO2 grade 22 and synthesized acidic SiO2 calcinated at
380 °C. (C) Comparison of time of the removal of DBT using SiO2 grade 22 and
synthesized acidic SiO2 (calcinated at 380 °C) at a reaction temperature of 160 °C.
ꢀ
preferred orientation in the 110, 111, 021, 040, 221, 041, and
ꢀ
sample to DBT standard (Fig. 3D) there is only a fraction of DBT left
in the sample (approximately 20%). Comparing the treated sample
to DBT-sulfone standard (Fig. 3C), all the vibrations of the DBT-sul-
fone standard are present in the treated samples.
Fig. 4 shows DBT after reaction with laboratory-synthesized sil-
ica calcinated at different temperatures: 100 °C, 160 °C, and 380 °C.
113 planes since they are the predominant peaks in the treated
sample. It is possible that the height peaks are different because
the treated sample was more crystalline than the purchased sul-
fone. The way the treated sample was extracted from the solvent
was by slow evaporation which led to the formation of large
DBT-sulfone crystals.

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K. Castillo et al. / Journal of Catalysis 268 (2009) 329–334
Fig. 2. Mid-IR of DBT after reaction with silica grade 22 and laboratory-synthesized silica. DBT after reaction with silica grade 22 run at 160 °C (A). DBT after reaction with
laboratory-synthesized silica at pH 1 (B). DBT-sulfone standard (C). DBT standard (D).
Fig. 3. Far IR of DBT after reaction with silica gel grade 22 at 160 °C (A). DBT after reaction with laboratory-synthesized silica at pH 1 (B). DBT-sulfone standard (C). DBT
standard (D).
Only when the reaction was run at higher temperatures at
160 °C DBT-sulfone crystals could be extracted and observed in
the X-ray diffraction pattern. When the reaction was run at a lower
temperature between 100 and 140 °C DBT-sulfone could not be
completely isolated and the X-ray diffraction pattern showed silica
(a broad diffraction peak at about 20 in 2h) and DBT-sulfone dif-
fraction peaks.
only the silica synthesized at 100 °C is shown). X-ray diffraction re-
sults showed that silica grade 22 as well as silica calcinated at the
different temperatures were amorphous, showing only a broad
weak diffraction peak centered around 20 ° in 2h.
4. Discussion
In addition, silica gel grade 22 and the acidified silica synthe-
sized and calcinated at 100 °C, 160 °C, and 380 °C were also charac-
terized using X-ray diffraction as shown in Fig. 6 (for simplicity
Table 1 shows a summary of the results of the reactions run
using silica and different solvents. As can be seen in Table 1, both
the reaction temperature and solvent directly affect the oxidation

K. Castillo et al. / Journal of Catalysis 268 (2009) 329–334
333
Fig. 4. Far-IR of DBT after reaction with laboratory-synthesized SiO2 calcinated at different temperatures. DBT after laboratory-synthesized silica calcinated at 380 °C (A). DBT
after laboratory-synthesized silica calcinated at 160 °C (B). DBT after laboratory-synthesized silica calcinated at 100 °C (C). DBT-sulfone standard (D). DBT standard (E).
100
80
60
40
20
0
0
10
20
30
40
50
60
2 theta
Fig. 6. X-ray diffraction pattern of SiO2 synthesized at 100 °C.
Fig. 5. X-ray diffraction pattern of DBT-sulfone standard (dotted line) and DBT after
reaction with silica gel grade 22 at 160 °C (solid line).
of the solvent all the dissolved gases are released and thus there
is no gas left in the solution for the conversion of DBT to DBT-sul-
fone. In addition, the reactions were also studied in non-hydrogen-
donating solvents: heptane, dodecane and toluene at different
temperatures. The oxidation of DBT to DBT-sulfone did not occur
in either of these solvents at any tested temperature. This result
suggests that in addition to needing a dissolved gas in the solvent,
a hydrogen-donating molecule is also necessary to oxidize DBT.
In addition to the reaction temperature and solvent type, the pH
of the SiO2 synthesized plays an important role in the oxidation
reaction. SiO2 (silica gel) synthesized in the pH range of 3–10
had no observable reaction with the DBT molecule. The oxidation
of DBT occurred only with SiO2 synthesized at a pH between 1
and 2. The effect of the synthesis pH on the reaction may be linked
of DBT to DBT-sulfone. The oxidation reaction was also carried out
in tetralin (1,2,3,4 tetrahydronaphthalene) at the same tempera-
tures at which the decalin reaction was carried out, 100 °C,
120 °C, and 160 °C. The reactions carried out in tetralin had the
same behavior as the reactions carried out in decalin. At lower
temperatures, the DBT-sulfone was stuck on the silica and at high-
er temperatures was released back to the solution. At the boiling
temperature of tetralin and decalin, oxidation did not occur. This
suggests that dissolved gases (such as O2) in the solvent may be
partially responsible for the oxidation; as the temperature in-
creases the solubility of the gases decreases. At the boiling point

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K. Castillo et al. / Journal of Catalysis 268 (2009) 329–334
to the structure of the SiO2, which is dependent on the synthesis
pH of the SiO2. A SiO2 synthesized at acidic pHs will have H+
embedded in the matrix, whereas a silica gel synthesized under ba-
sic pH will have OHÀ embedded in the matrix [17]. Synthesis under
acidic pH conditions causes condensation to occur mainly between
the neutral silanol groups (Si–OH) and protonated silanol groups
(Si–OH+), which are monomers and at the ends of polymeric chains
of SiOH lead to the formation of linear polymers [17]. However, the
synthesis under basic pH conditions causes the condensation of Si–
OH groups through reaction of a deprotonated silanol (Si–OÀ), in
the middle of the chains, and a neutral silanol (Si–OH) or Si–OR
group at the end of chains [17]. Comparatively, the acid-catalyzed
or acidic SiO2 compounds contain higher concentrations of ad-
sorbed water, silanol groups, and unreacted alkoxy (depending
on the source of the Si in the synthesis) groups than base-catalyzed
precipitates [17]. From the results of the current study the oxida-
tion of DBT occurs only in the presence of acidic silica, which sug-
gests that the SiOH+ groups are important in the reaction
mechanism. It is hypothesized that the H+ protons from the acidic
SiO2 are the necessary driving force for the oxidation reaction. It
has been reported that sulfides can be oxidized to sulfoxides using
HNO3 supported on silica gel and polyvinylpyrrolidone (PVP) [18].
The oxidation of DBT to DBT-sulfone only in the presence of acidic
SiO2 suggest that the reaction is at least partially acid catalyzed,
however, a hydrogen-donating solvent is necessary for the reaction
to occur.
ature range of 100–160 °C with a maximum removal, 80–85%, of
the DBT occurring at 160 °C. According to the results obtained in
this study sulfur compounds in heavy feeds could be oxidized
and removed using a combination of acidified silica and a hydro-
gen-donating solvent. Further studies are needed to determine
the mechanism of this reaction if it is either acid catalyzed or
H2O2 is being made in situ.
Acknowledgment
The authors would like to thank the Robert A. Welch foundation
for financial support in this project. The authors also acknowledge
financial support from NASA through the UNEEC program.
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The current study addressed the use of acidified silica gel for the
oxidation of DBT in the presence of a hydrogen donor solvent, at
relatively low temperature and atmospheric pressure, without
the addition of an oxidizing agent. Amorphous acidified silica (syn-
thesized at an acidic pH) is capable of acting as an oxidizing agent
in the oxidation of DBT to DBT-sulfone in the presence of a hydro-
gen donor solvent. In addition, the reaction occurs over a temper-