Ultra-Deep Desulfurization of Diesel Fuel Using Light-Irradiated TiO2−CeO2 Adsorbents

Ultra-Deep Adsorptive Desulfurization of Light-Irradiated Diesel Fuel
over Supported TiO2−CeO2 Adsorbents
Jing Xiao,†,‡
Xiaoxing Wang,†
Yongsheng Chen,†
Mamoru Fujii,†
and Chunshan Song*,†
†
Clean Fuels and Catalysis Program, EMS Energy Institute, and Department of Energy and Mineral Engineering, Pennsylvania State
University, 209 Academic Projects Building, University Park, Pennsylvania 16802, United States
‡
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
*S Supporting Information
ABSTRACT: This study investigates ultra-deep adsorptive desulfurization (ADS) from light-irradiated diesel fuel over
supported TiO2−CeO2 adsorbents. A 30-fold higher desulfurization capacity of 95 mL of fuel per gram of adsorbent (mL-F/g-
sorb) or 1.143 mg of sulfur per gram of adsorbent (mg-S/g-sorb) was achieved from light-irradiated fuel over the original low-
sulfur fuel containing about 15 ppm by weight (ppmw) of sulfur. The sulfur species on spent TiO2−CeO2/MCM-48 adsorbent
was identified by sulfur K-edge XANES as sulfones and the adsorption selectivity to different compounds tested in a model fuel
decreases in the order of indole > dibenzothiophenesulfone ≫ dibenzothiophene > 4-methyldibenzothiophene >
benzothiophene > 4,6-dimethyldibenzothiophene > phenanthrene > 2-methylnaphthalene ∼ fluorene > naphthalene. The
results suggest that during ADS of light-irradiated fuel, the original sulfur species were chemically transformed to sulfones,
resulting in the significant increase in desulfurization capacity. For different supports for TiO2−CeO2 oxides, the ADS capacity
increases with a decrease in the point of zero charge (PZC) value; for silica-supported TiO2−CeO2 oxides (the lowest PZC value
of 2−4) with different surface areas, the ADS capacity increases monotonically with increasing surface area. The supported
TiO2−CeO2/MCM-48 adsorbent can be regenerated using oxidative air treatment. The present study provides an attractive new
path to achieve ultraclean fuel more effectively.
1. INTRODUCTION
Ultra-deep desulfurization of diesel fuel has attracted great
attention because of the stringent fuel specifications for diesel
(<15 ppm by weight (ppmw) of sulfur) to fulfill the regulations
on emissions of pollutants and the more strict requirement of
ultra deep-desulfurized diesel fuel (<1 ppmw-S) for the on-
board and on-site reforming of diesel for fuel cell
applications.1−4
For example, proton-exchange membrane fuel
cell (PEMFC) can only tolerate less than 1 ppmw-S to avoid
poisoning of the reforming catalysts, water gas shift catalysts, as
well as electrode catalysts.1
Therefore, ultra-deep desulfuriza-
tion of diesel fuel has become an increasingly important
research subject.
Removal of sulfur-containing compounds is an important
operation in petroleum refining, and is achieved by catalytic
hydrotreating processes operated at elevated temperatures
(>300 °C) and high pressures of H2 (20−100 atm)5
using
alumina supported Ni−Mo or Co−Mo sulfide catalyst.6
However, the major sulfur compounds in current commercial
ultra low sulfur diesel fuel are the refractory alkyl
dibenzothiophenes (DBTs) with two or three alkyl substitutes
at the 4- and 6-positions such as 4,6-dimethyl DBT, 4-ethyl,6-
methyl DBT, 2,4,6-trimethyl DBT, 4,6-diethyl DBT, and 6-
ethyl,2,4-dimethyl DBT.7
These refractory sulfur compounds
show low hydrotreating reactivity because of their large ring
size and alkyl substitutes at the neighboring position of sulfur
atom.3
Therefore, using current hydrotreating technology for
ultra-deep desulfurization of diesel fuel would require a much
larger catalyst bed, higher temperature and pressure, and more
hydrogen consumption,8
and thus becomes more expensive and
less efficient. Therefore, it is desired to develop alternative or
complementary technologies for ultra-deep desulfurization.
Adsorptive desulfurization (ADS), which can be conducted
under ambient conditions without using costly hydrogen and
can selectively remove organosulfur compounds in fuels to less
than 1 ppmw level, has become a promising alternative
approach for ultra-deep desulfurization. Various adsorbents
have been reported in the literature for desulfurization of liquid
fuels, including a π-complexation sorbent,5,9
an immobilized π-
acceptor sorbent,10,11
metal organic frameworks (MOFs),12
a
Ni-based sorbent,7
an oxygen-functionalized carbon sorb-
ent,13−17
and supported metal oxides.15−17
However, most of
these adsorption approaches are not efficient for the ultra-deep
ADS of diesel fuel that already has a very low sulfur content.
The remaining critical challenges include the following: (a) The
steric hindrance effect of alkylated sulfur compounds in
diesel.18,19
Alkyl groups on the neighboring positions of sulfur
atom in thiophenic compounds sterically hinder the direct
interaction between sulfur and adsorption site. (b) The
competitive adsorption between trace amount of sulfur
compounds (<15 ppmw-S) and relatively large amount of
polyaromatic hydrocarbons (can be 5−10 wt %).20
As is well-
known, diesel fuel contains a significant amount of aromatic
compounds that have aromatic skeleton structures similar to
the sulfur compounds.14
Therefore, it is desirable to develop
Received: August 19, 2013
Revised: October 9, 2013
Accepted: October 10, 2013
Published: October 10, 2013
Article
pubs.acs.org/IECR
© 2013 American Chemical Society 15746 dx.doi.org/10.1021/ie402724q | Ind. Eng. Chem. Res. 2013, 52, 15746−15755

new selective desulfurization approaches for ultraclean fuel with
sulfur content less than 1 ppmw from current commercial diesel
fuel.
In our recent communication,21
we proposed a new
approach for ultra-deep desulfurization of diesel fuel with a
superior desulfurization capacity, in which the original fuel was
treated by light irradiation before it was desulfurized by
adsorption over the TiO2−CeO2/MCM-48 adsorbent under
ambient conditions. Yet the mechanism of ultra-deep
desulfurization remains unclear. Remaining questions include:
What is the sulfur chemistry involved in ADS and how it affects
desulfurization capacity? What is the effect of support for
TiO2−CeO2? How is the regenerability of the adsorbent? What
is a feasible process configuration? As a follow-up, a detailed
study of ultra-deep adsorptive desulfurization from light-
irradiated diesel fuel over supported TiO2−CeO2 adsorbents
was carried out in this work. Sulfur K-edge X-ray absorption
near-edge structure (XANES) spectroscopy was used to
identify sulfur species on the spent supported TiO2−CeO2
adsorbent. Adsorption selectivity of different compounds in a
model fuel, including sulfone, nitrogen- and sulfur- containing
heteroatom compounds, and aromatics over the supported
TiO2−CeO2 adsorbent was studied. Dipole magnitude of
various sulfur compounds was calculated. Various supports with
different PZC values and surface areas were used to prepare
supported TiO2−CeO2 adsorbents and compared on the basis
of desulfurization performance evaluated in a fixed-bed flow
sorption system under ambient conditions using a commercial
diesel fuel with a sulfur content of 14.5 ppmw. Regenerability of
TiO2−CeO2/MCM-48 adsorbent was also examined. A
conceptual process for ultra-deep desulfurization of diesel fuel
was proposed.
2. EXPERIMENTAL SECTION
2.1. Materials. Support. Al2O3 and TiO2 were purchased
from Aldrich Chemical Co. Silica material of EH-5 was
provided by Cabot Co. AC-WPH was provided by Calgon,
Co. Mesoporous silica supports, including SBA-15, MCM-41,
and MCM-48, were synthesized in our laboratory by a
hydrothermal method.22
Supported TiO2−CeO2 Adsorbents. The supported TiO2−
CeO2 adsorbents were prepared by incipient wetness
impregnation assisted with ultrasound7
using cerium ammo-
nium nitrate (NH4)2Ce(NO3)6 (Aldrich, 99.99%), titanium
oxysulfate TiOSO4·xH2SO4·xH2O (Aldrich, Ti: 20 wt %) as
metal precursors. The molar ratio of Ti:Ce was 9:123
and the
optimized loading amount of TiO2−CeO2 was 13.3 wt % as
indicated in Supporting Information Figure S1. The sample of
bulk TiO2−CeO2 mixed oxides with a molar ratio of Ti:Ce of
9:1 prepared by urea coprecipitation was used as a reference.
The preparation procedure of TiO2−CeO2 mixed oxides was
described elsewhere.23
2.2. Fuels. Diesel Fuel. The low-sulfur diesel fuel with 14.5
ppmw sulfur was supplied by BP America. The composition
was reported elsewhere.7
Light Irradiation of Diesel Fuel. Diesel fuel was loaded and
constantly stirred in a two-necked flat-bottom flask, and
irradiated with a Hamamatsu 150 W xenon lamp. A constant
air flow at 10 mL/min was bubbled into the fuel
simultaneously. Peroxides with a concentration of 12 mmol/
kg,21
as monitored with a Milwaukee MI490 peroxide
photometer, was generated in the diesel fuel by light irradiation
that mimics sunlight with light intensity of 65.1 mW/cm2
for
9.6 h at 25 °C (controlled by a water bath), which is denoted as
light-irradiated diesel fuel in this study. It should be noted that
peroxides generation in the fuel is strongly dependent on the
irradiation time and light intensity applied but insensitive to the
photon energy (or wavelength) as reported in our recent
letter.21
Also the rate of peroxides generation can be improved
by increasing temperature, as shown in Figure 1. Neither a
catalyst nor an adsorbent was added during the light irradiation
of fuel.
Model Fuel. To study the adsorption selectivity of different
compounds over TiO2−CeO2/MCM-48 adsorbent, a model
fuel (MDF) containing the same molar concentration (3.12
μmol/g) of benzothiophene (BT), dibenzothiophene (DBT),
4-methyldibenzothiophene (4-MDBT), 4,6-dimethyldibenzo-
thiophene (4,6-DMDBT), dibenzothiophenesulfone (DBTO2),
indole, naphthalene (Nap), fluorene (Flu), 2-methylnaphtha-
lene (MNap), and phenanthrene (Phe) in a mixed solvent was
prepared. The composition of the model fuel is listed in Table
1. The model fuel also contained 8.0 wt % of tert-butyl benzene
to mimic the monoaromatics in real diesel and 2 wt % of furan
as fuel additives. All the chemicals were purchased from Sigma-
Aldrich and were used as received.
2.3. Adsorption Tests. The ADS was performed at 25 °C
in a fixed-bed flow adsorption system with a stainless steel
column (4.6 mm I.D. × 75 mm length, bed volume of 1.25
mL). A given amount (i.e., 0.25 g for TiO2−CeO2/MCM-48)
of the adsorbent was packed into the column and treated under
air flow at 120 °C for 2 h. The fuel was then fed by a HPLC
pump at a flow rate of 0.2 mL/min (i.e., a LHSV of 9.6 h−1
).
The effluent fuel was periodically collected at an interval of 15−
20 min for analysis. The total sulfur concentration of the
treated fuel samples was analyzed using ANTEK 9000 series
sulfur analyzer. The sulfur compounds in the initial and treated
fuels were analyzed using a Hewlett-Packard gas chromato-
graph equipped with a sulfur-selective, pulsed flame photo-
metric detector (GC-PFPD). A detailed analysis method was
reported in a previous study.24
For the adsorption selectivity
study, concentrations of different compounds in treated MDF
samples were analyzed by a gas chromatograph (Varian CP
3800) equipped with a flame ionization detector (FID). A
detailed analysis method has been reported previously.25
2.4. Characterization. Sulfur K-edge XANES. Sulfur K-
edge (2472.0 eV) X-ray absorption near edge structure
(XANES) spectroscopic measurements of spent adsorbents
were performed at Beamline 9-BM/XOR of the Advanced
Photon Source (APS) at Argonne National Laboratory. The
storage ring was operated with an electron beam of 7 GeV and
an electron current of 100 mA in a top-up mode. The
Figure 1. Amount of peroxides (mmol-peroxides/kg) generated in
diesel fuel under light irradiation that mimics sunlight at an air flow
rate of 10 mL/min at 25, 50, and 100 °C_dark.
Industrial & Engineering Chemistry Research Article
dx.doi.org/10.1021/ie402724q | Ind. Eng. Chem. Res. 2013, 52, 15746−1575515747

monochromator was double-crystal Si(111), and the XANES
spectra were collected in fluorescence mode with a Si DRIFT 4-
element detector (Vortex). Air absorption was controlled by
the use of helium purging in the incident light path and the
sample chamber, which were separated by a 5 μm thick
polycarbonate window. The monochromator crystals had an
energy resolution of approximately 0.3 eV at 2.5 KeV.
Harmonics were rejected by use of a Rh-coated flat mirror in
the experimental station. The beam was focused to a spot size
of approximately 1 mm in the horizontal and vertical directions
by the use of a Rh-coated toroidal mirror.26
To avoid further
oxidation, the spent adsorbent was sealed in the adsorption
column after ADS experiment and transferred to and sealed in a
sample cell with a propylene window, which allows for XANES
measurements of liquid or volatile samples. Energy calibration
was accomplished by setting the edge energy of elemental sulfur
to 2472.0 eV. Reference compounds, 4,6-DMDBT and DBTO2
were diluted in sulfur-free fuel, resulting in a concentration of
1−2 wt %. Up to 5 scans were collected and averaged to
improve the signal-to-noise ratio. XANES data were processed
using Athena.27
N2 Adsorption Test. Textural properties of adsorbents were
measured by nitrogen adsorption/desorption at −196 °C using
an ASAP2020 analyzer (Micromeritics). The surface area and
pore volume were calculated using BET method28,29
and the
amount of nitrogen adsorbed at P/P0 = 0.95, respectively. The
pore size distribution was determined according to density
functional theory (DFT) method.30
Prior to each measure-
ment, all samples were outgassed at 150 °C for 12 h.
Zeta Potential Measurement. The zeta potential distribu-
tions were obtained using a Brookhaven ZetaPALS zeta
potential analyzer, and the point of zero charge (PZC) of
various supports was calculated on the basis of the distribution
curves.
2.5. Molecular Calculation. The dipole magnitude of
different sulfur compounds examined in this study was
calculated by a semiempirical quantum chemistry method,
PM3 in Hyperchem 7.0. The PM3 method determines both the
optimum geometry and electronic properties of molecules by
solving the Schrödinger equation using the PM3 semiempirical
Hamiltonians developed by Stewart.31,32
3. RESULTS AND DISCUSSION
3.1. Promoting Effect of Light Irradiation of Diesel
Fuel on ADS. In order to examine the effect of light irradiation
of diesel fuel on ultra-deep adsorptive desulfurization (ADS),
ADS tests over the MCM-48-supported TiO2−CeO2 adsorbent
from the regular and light-irradiated diesel fuel were compared.
Figure 2 shows the breakthrough capacity of total sulfur
compounds over the MCM-48-supported TiO2−CeO2 adsorb-
ent from the two diesel fuels under ambient conditions. Over
30 times greater ADS capacity (1.143 mg-S/g-sorb or 95 mL-
F/g-sorb) of light-irradiated diesel fuel was achieved as
compared to that of the original diesel fuel (0.036 mg-S/g-
sorb or 3 mL-F/g-sorb) at a breakthrough point of 1 ppmw-S.
The same trend is noticed at a breakthrough point of 4 ppmw-
S, as ADS capacity reaches 1.432 mg-S/g-sorb (or 119 mL-F/g-
sorb) from the light-irradiated diesel fuel, whereas it is 0.170
mg-S/g-sorb (or 14 mL-F/g-sorb) for the original fuel. The
sharp contrast clearly indicates a strong promotion effect of the
light irradiation of diesel fuel on ADS over the MCM-48-
supported TiO2−CeO2 adsorbent.
To clarify the possible change in sulfur chemistry due to the
light irradiation of diesel fuel, sulfur compounds on the spent
MCM-48-supported TiO2−CeO2 adsorbent after ADS were
examined by XANES at the sulfur K-edge, which is a powerful
tool to provide detailed information of the adsorbed sulfur
species on the spent adsorbent/catalyst.26
Figure 3 shows sulfur
K-edge XANES spectra of the spent TiO2−CeO2/MCM-48
adsorbent and two reference compounds, 4,6-DMDBT and
DBTO2. Interestingly, no initial refractory sulfur compounds,
such as 4,6-DMDBT were present on the spent MCM-48-
supported TiO2−CeO2 adsorbent. Instead, the major sulfur
species were sulfone species having almost the same white line
position as the reference compound, DBTO2. The result
provides direct evidence that the initial refractory sulfur
compounds in diesel are chemically transformed to sulfone
species during ADS and adsorbed over the MCM-48-supported
TiO2−CeO2 adsorbent, which further supports our earlier
hypothesis on chemical transformation of sulfur compounds
Table 1. Composition of the Model Diesel Fuel
concentration
chemicals wt.% ppmw S or N
molar concentration
(μmol/g)
sulfur compounds
BT (99%) 0.042 100.0 3.12
DBT (98%) 0.058 100.0 3.12
4-MDBT (96%) 0.063 100.0 3.12
4,6-DMDBT (97%) 0.067 100.0 3.12
DBTO2 (97%) 0.068 100.0 3.12
nitrogen compounds
indole (≥99%) 0.038 43.8 3.12
aromatics
Nap (99%) 0.040 3.12
Flu (98%) 0.053 3.12
MNap (97%) 0.045 3.12
Phe (98%) 0.057 3.12
tert-butylbenzene
(99%)
8.0
paraffins
n-decane (≥99%) 44.7
n-hexadecane
(≥99%)
44.7
n-tetradecane (98%) 0.063 (internal
standard)
3.12
additives
furan (98%) 2.0
total 100.0
Figure 2. Desulfurization capacity of TiO2−CeO2/MCM-48 adsorb-
ent for the original and light-irradiated diesel fuel at breakthrough
(B.T.) points of 1 ppmw-S and 4 ppmw-S at 25 °C and LHSV of 9.6
h−1
.

during ADS using the indirect evidence of detected sulfone
species with GC-PFPD in acetone solvent washing of the spent
MCM-48-supported TiO2−CeO2 adsorbent.21
It should be
noted that sulfones were formed in the adsorption step rather
than the irradiation step, as no sulfones were detected in the
light-irradiated fuel from the GC-PFPD analyses.21
The
irradiation step is accompanied by air bubbling through the
fuel. Therefore, besides peroxide generation, some side
oxidation reactions may take place, which can be indicated by
the fuel color change to more yellowish or brownish.33
In our
system, the fuel color change before and after irradiation was
almost invisible, and only a trace amount of peroxides (12
mmol/kg) was generated under the current irradiation
conditions, as can also be seen in Figure 1. Thus, the change
in the fuel properties induced by light irradiation under air-
bubbling should be negligible. The in situ generated peroxides
during light irradiation of diesel fuel were suggested as the
oxygen source for the later chemical transformation of sulfur
compounds during ADS over the MCM-48-supported TiO2−
CeO2 adsorbent. Therefore, besides being an adsorbent, the
MCM-48-supported TiO2−CeO2 seems to act as a catalyst for
the oxidation of the sulfur compounds to sulfone species during
ADS under ambient conditions. It should also be mentioned
here that using supported metal oxides-based catalysts for
catalytic oxidative desulfurization (ODS) under mild reaction
conditions has been reported in the literature. Chica et al.
studied the Ti-MCM-41 catalyst using tert-butyl hydroperoxide
as oxidant for ODS of diesel fuel at 80 °C.33
Sundararaman et
al. studied silica supported MoO3 using in situ catalytically
generated hydroperoxides for ODS of diesel and jet fuels at 85
°C.34
Here, our result indicates that under ambient conditions,
MCM-48-supported TiO2−CeO2 adsorbent may serve as a
catalyst for oxidation of the sulfur compounds in diesel fuel.
In the literature, there are reported adsorbents for ADS from
model fuels and high sulfur fuels achieving high ADS capacity.
It was reported that Cu+
and Ag+
exchanged zeolite Y can
adsorb sulfur compounds from a commercial 430 ppmw-S
diesel by π complexation under ambient conditions with a
capacity of 12.1 mg-S/g-sorb (or 34 mL-diesel fuel/g-sorb).9
Microporous coordination polymers (MCPs) were reported to
be effective adsorbents for ADS from a 300 ppmw-S diesel fuel,
and UMCM-150 MCP showed a high capacity of 17.4 mg-S/g-
sorb (or 70 mL-diesel fuel/g-sorb).12
Different from high sulfur
diesel fuels, ADS from current low-sulfur diesel (<15 ppmw-S)
in this work is much more difficult due to the refractory nature
of the remaining sterically hindered thiophenic sulfur
compounds and more significant competitive adsorption
between sulfur compounds and other fuel components. Carbon
materials showing high ADS capacity for model fuels were also
tested for ADS of the real low-sulfur diesel fuel in our
laboratory, and low capacities of less than 0.120 mg-S/g-sorb
(or 10 mL-F/g-sorb) were obtained. Sentorun-Shalaby et al.
developed Ni/MCM-48 adsorbent for ADS of current low
sulfur diesel through direct Ni−S chemical interaction, and
achieved a desulfurization capacity of 1.812 mg-S/g-sorb or 160
mL-F/g-sorb,7
where H2 was used in the ADS process at an
operating temperature of 200 °C. In this study, MCM-48-
supported TiO2−CeO2 adsorbent achieved a superior ADS
capacity of 1.143 mg-S/g-sorb or 95 mL-F/g-sorb from the real
diesel fuel to produce ultraclean fuel with <1 ppmw-S under
ambient conditions without H2, which is a less energy-intensive
desulfurization approach.
The adsorption capacity of an adsorbent would be influenced
by the total number of the accessible adsorption sites and
adsorption affinity of each adsorption site on the adsorbent.8
In
addition, the competitive adsorption of the fuel molecules
should always be considered. For the adsorption of the
refractory sulfur species in low-sulfur diesel fuel over the
TiO2−CeO2/MCM-48 sorbent, a very low desulfurization
capacity of 3 mL-F/g-sorb was obtained, as shown in Figure 2.
This can be mainly attributed to the weak adsorption affinity
between the adsorbent and the original sulfur compounds (e.g.,
4, 6-DMDBT in the fresh low-sulfur diesel fuel) due to the
steric hindrance effect19,35
in diesel fuel. In addition, strong
competitive adsorption due to the other fuel components, that
is, a significant amount (5−10 wt % or higher) of polyaromatic
hydrocarbons (PAHs),20
could inhibit ultra-deep desulfuriza-
tion. With the chemical transformation of initial sulfur species
to sulfones during ADS from light-irradiated diesel fuel, MCM-
48-supported TiO2−CeO2 adsorbent showed a significantly
higher breakthrough capacity of 1.143 mg-S/g-sorb compared
to 0.036 mg-S/g-sorb from the fresh fuel, suggesting a much
stronger adsorption of transformed sulfones over the MCM-48-
supported TiO2−CeO2 adsorbent compared to that of the
unoxidized, refractory sulfur compounds in the diesel fuel.
To shed light on the adsorption mechanism, adsorption
selectivity of different compounds in fuel over the MCM-48-
supported TiO2−CeO2 adsorbent was studied. Figure 4 shows
the breakthrough curves of different compounds, including
aromatics, refractory sulfur compounds, sulfone, and nitrogen
compound in the model fuel (MDF) over the TiO2−CeO2/
MCM-48 adsorbent. The first breakthrough species are
aromatic compounds, including Nap, Flu, MNap, and Phe,
Figure 3. Sulfur K-edge XANES spectra of the spent TiO2−CeO2/
MCM-48 adsorbent and two reference compounds, 4,6-DMDBT and
DBTO2.
Figure 4. Adsorption selectivity for different compounds in a model
diesel fuel (MDF) over the MCM-48-supported TiO2−CeO2
adsorbent at 25 °C and 9.6 h−1
of LHSV.

which give adsorption capacities of <6 mL-F/g-sorb. The
second type of breakthrough species are refractory sulfur
species, including DMDBT, BT, MDBT, and DBT, which give
desulfurization capacities of <12 mL-F/g-sorb. In a sharp
contrast, the third kind of breakthrough species are DBTO2 and
indole, which give similar capacities of around 76 mL-F/g-sorb,
significantly higher than the aromatic compounds and
refractory sulfur compounds. The much higher breakthrough
capacities of sulfone (DBTO2) and indole suggest a much
stronger adsorption affinity between the TiO2−CeO2/MCM-
48 adsorbent and sulfone/indole compared to that of the other
compounds. An interesting phenomenon in Figure 4 is that
after passing through the saturation point (C/C0 = 1), the
outlet concentrations of some compounds, especially the
aromatic compounds and refractory sulfur compounds, rise
over its initial concentration in the model fuel by even more
than 25% (C/C0 > 1.25). After passing the maximum value, the
outlet concentration gradually decreases to the initial one,
whereas the concentrations of the subsequent breakthrough
compounds increase to C/C0 = 1. This phenomenon reflects
the competitive adsorption among the compounds over the
adsorbent, which resulted in at least partial replacement of the
initially adsorbed compounds with lower adsorptive affinity by
the compounds with higher adsorptive affinity.25
A significant
replacement of DMDBT after saturation was observed in
Figure 4, indicating the adsorption of subsequent compounds
(e.g., BT, MDBT, and DBT) was stronger, which further
indicates that the steric hindrance effect of 4- and 6-methyl
groups on the adsorption of DMDBT over the TiO2−CeO2/
MCM-48 adsorbent can be significant. In addition, BT showed
a weaker adsorption compared to DBT with one more aromatic
ring, which is likely due to its lower dipole moment than DBT
as shown in Table 2. To sum up, on the basis of the order of
partial replacements, the adsorptive affinity follows the order of
indole ∼ DBT sulfone > refractory sulfur compounds >
aromatic compounds.
To make a quantitative discussion of the adsorptive
selectivity, a relative selectivity factor is defined in this study as
α =− Q /Qi n i n (1)
where Qi is the adsorptive capacity of compound i
corresponding to the breakthrough point and Qn is the
adsorptive capacity of the reference compound, naphthalene
(Nap), corresponding to its breakthrough point. It should be
mentioned that in using the kinetic breakthrough capacities
instead of the equilibrium capacity in eq 1, the defined
selectivity factor is not for the equilibrium selectivity.25
The adsorption selectivity for the ten compounds over the
TiO2−CeO2/MCM-48 adsorbent decreases in the order of
indole > DBTO2 ≫ DBT > MDBT > BT > DMDBT > Phe >
MNap ∼ Flu > Nap with the relative selectivity factor (αi−n) of
27.6, 26.9, 3.7, 2.7, 2.4, 2.0, 1.5, 1.3, 1.3, and 1, respectively, as
listed in Table 2. Both the breakthrough and saturation
capacities of the ten compounds listed in Table 2 follow the
order of indole > DBTO2 ≫ DBT > MDBT > BT > DMDBT
> Phe > MNap ∼ Flu > Nap, which is consistent with the order
of adsorption selectivity. The selectivity of refractory sulfur
compounds follows the order of DBT > MDBT > DMDBT,
suggesting that the presence of −CH3 groups neighboring
sulfur atom in nonoxidized sulfur compounds sterically hinder
ADS over the TiO2−CeO2/MCM-48 adsorbent. The result
also indicates that the adsorption of nonoxidized sulfur
compounds over the TiO2−CeO2/MCM-48 sorbent is likely
through the sulfur atom, similar as the case of supported nickel
adsorbent7
but different from the case of activated carbon.13,14
In contrast, the adsorption of DBT sulfone can go through the
oxygen atom instead of sulfur atom, because of the higher
electronegativity of oxygen than sulfur and much greater dipole
moment of sulfones than nonoxidized sulfur compounds,
resulting in a significantly higher adsorption selectivity and
capacity, as indicated in Table 2. In this case, chemical
transformation of initial refractory sulfur compounds in diesel
fuel to sulfones over the TiO2−CeO2/MCM-48 adsorbent is
likely to strengthen ADS by shifting the adsorbate from the
sterically hindered sulfur atom on alkylated refractory sulfur
molecules (e.g., 4,6-DMDBT) to a more attractive oxygen atom
on the corresponding sulfone molecules. In other words, the
adsorption between sulfone and adsorbent may go through a
highly electronegative oxygen atom rather than a less
electronegative sulfur atom in sulfone, resulting in higher
ADS selectivity and capacity.
Without light irradiation to generate peroxides, the refractory
sulfur compound 4,6-DMDBT is not oxidized and, thus, is less
strongly adsorbed on the surface of TiO2−CeO2/MCM-48
than DBT or BT, whereas the oxidized DBT compounds (e.g.,
DBTO2) is much more strongly adsorbed on TiO2−CeO2/
MCM-48, showing that the oxidation of refractory sulfur
compounds to sulfones, with the help of light irradiation, is
necessary and highly beneficial for achieving high adsorption
capacity for ultra-deep adsorptive desulfurization. Moreover, in
terms of sulfone formation, 4,6-DMDBT may show higher
reactivity than DBT because of the higher electron density of
the sulfur atom.36,37
Therefore, with the combination of light
irradiation and adsorptive desulfurization, the present system is
particularly promising for ultra-deep desulfurization of low-
sulfur diesel fuel.
Additionally, a strong adsorption capacity of indole was
observed over the TiO2−CeO2/MCM-48 adsorbent as shown
in Figure 4, which is similar to DBTO2. However, indole shows
a lower dipole moment than DBTO2 as listed in Table 2,
suggesting the interaction mode for the adsorption of indole
may be different from DBTO2 over the TiO2−CeO2/MCM-48
adsorbent. As indole shows a bifunctional structure38
(i.e.,
electron acceptor from the hydrogen bonded to N and electron
donor from conjugated π system), it behaves both as a weak
acid due to the presence of the H−N bond and as a weak base
due to the basicity of the N atom.39
With available silanol group
sites and TiO2−CeO2 oxide sites on the surface of the TiO2−
Table 2. Adsorption Selectivity Factor Relative to Naphthalene (αi−n), Adsorption Breakthrough and Saturation Capacities
(QB.T. and QSat., μmol/g), and Dipole Moment for Each Compound in MDF over the TiO2−CeO2/MCM-48 Adsorbent
Nap Flu MNap Phe DMDBT BT MDBT DBT DBTO2 indole
αi−n 1 1.3 1.3 1.5 2.0 2.4 2.7 3.7 26.9 27.6
QB.T. 7 9 9 11 14 16 19 26 188 193
QSat. 10 12 12 20 20 23 25 39 225 233
dipole (D) 0.764 1.090 1.087 1.363 5.453 2.004

CeO2/MCM-48 adsorbent, both hydrogen bonding interaction
as well as acid−base interaction might play important roles in
the adsorption of indole. The strong adsorption of indole over
the TiO2−CeO2/MCM-48 adsorbent also suggests that the
presence of nitrogen compounds in the fuel might suppress
ADS over the TiO2−CeO2/MCM-48 adsorbent through
competitive adsorption. An integration of two or more
adsorbent beds may further improve the ultra-deep desulfuriza-
tion process.
3.2. Effect of Support of TiO2−CeO2 Oxides on ADS.
By comparison of the desulfurization performance of MCM-48-
supported TiO2−CeO2 and bulk TiO2−CeO2 in this study, the
desulfurization capacity of the former is over 30 times higher
than the latter, even though only 13.3 wt % of TiO2−CeO2
oxides were loaded on MCM-48. This suggests a strong effect
of the support on ADS performance of the TiO2−CeO2 oxide-
based adsorbents. In order to clarify support effect, various
types of supports, including fumed silica (EH-5), activated
carbon (AC-WPH), anatase TiO2, and gamma-Al2O3, were
examined. Table 3 lists some physical properties and
desulfurization capacities of bulk and supported TiO2−CeO2
adsorbents. Both the gravimetric and volumetric desulfurization
capacities of various supported TiO2−CeO2 oxides follow the
order of EH-5 > AC > TiO2 > Al2O3. The calculated
desulfurization capacity on the basis of BET surface area is
also listed in Table 3, which follows the order of EH-5 > TiO2 >
AC > Al2O3. Supporting Information Figure S2 shows
desulfurization capacity versus SBET of various supported
TiO2−CeO2. No correlation is found between desulfurization
capacity and SBET, suggesting desulfurization capacity is not
mainly determined by BET surface area of supported TiO2−
CeO2 adsorbents. More active sites (TiO2−CeO2) may be
exposed on silica support (EH-5) followed by those on TiO2,
AC, and Al2O3, which could be attributed to the electronic or
chemical properties of the supported materials.
The PZC value of the support affects the adsorption of
metal-containing ions in metal precursors via electrostatic
interaction, which could further influence the dispersion of
formed metal oxides,40
and thus alter its adsorption perform-
ance. Figure 5 shows desulfurization capacity of various
supported TiO2−CeO2 adsorbents versus the PZC value of
the support. It can be seen that desulfurization capacity of
supported TiO2−CeO2 adsorbents increases with decreasing
PZC value of the support. It should be mentioned that the pH
of the precursor solutions of mixed Ti−Ce salts to prepare
supported TiO2−CeO2 adsorbent was as low as 0.185.
Therefore, the surfaces of the studied supports (PZC > 2)
were all positively charged during impregnation. It is likely that
silica support had a stronger electrostatic adsorption to Ti−Ce
precursors, yielding better dispersion of the supported oxides,41
resulting in a higher desulfurization capacity. High dispersion of
TiO2−CeO2 on silica supports are also reflected by the absence
of TiO2−CeO2 peak on EH-5 and MCM-48 supports, unlike
the unsupported TiCeO in XRD patterns (Supporting
Information Figure S3), as the XRD technique has limitation
on the identification of smaller particles. The result suggests
that the PZC value of the support plays a more important role
than surface area. An exceptional case was AC, which had a
higher PZC value than TiO2 but a slightly higher desulfuriza-
tion capacity than TiO2. This may be attributed to the
significantly higher surface area of AC (930 m2
/g) than that of
TiO2 (90 m2
/g). Another possibility is that TiO2−CeO2 may
have more suitable interaction with silica support than other
supports, which makes the TiO2−CeO2 sites more active for
sulfur oxidation and/or sulfones adsorption. To clarify the
effect of support, further investigations are needed and in
progress in our laboratory by using the DFT method.
As suggested from the above results, silica is the most
effective support to load TiO2−CeO2 oxides for desulfurization
among the supports investigated. Mesoporous silica materials,
such as SBA-15, have PZC values similar to EH-5 (between 2
and 4)42,43
but differ in the surface area. In order to examine
the effect of surface area of silica supported TiO2−CeO2
adsorbents, various silica supports with different surface area,
including EH-5, SBA-15, MCM-41, and MCM-48, were
investigated for desulfurization. Mesoporous silica supports
were chosen to ensure the pore size is large enough so that
diffusion of the bulky sulfur compounds in the diesel fuel (i.e.,
4,6-DMDBT (0.6 nm44
)) would not be limited. Table 4 lists
BET surface area of the supports and supported TiO2−CeO2
adsorbents and the desulfurization capacity of various silica-
supported TiO2−CeO2 adsorbents. The adsorption capacity
per unit volume of adsorbent follows the order of MCM-48 >
MCM-41 > EH-5 > SBA-15. The higher volume-based capacity
of TiO2−CeO2/EH-5 than TiO2−CeO2/SBA-15 is due to the
higher packing density of the EH-5 supported TiO2−CeO2.
The correlation between BET surface area and desulfuriza-
tion capacity of various silica-supported TiO2−CeO2 adsorb-
ents is shown in Figure 6. It is worth noting that desulfurization
capacity increases monotonically with increasing surface area of
the silica supports. This is probably due to a better dispersion
of metal oxides with higher surface area of silica support,
resulting in a greater amount of accessible TiO2−CeO2 active
Table 3. BET Surface Area of Various Supports and Supported TiO2−CeO2 Adsorbents, and Desulfurization Capacity of the
Supported TiO2−CeO2 Adsorbents
support SBET‑support (m2
/g) SBET‑supportedTiO2−CeO2
(m2
/g) PZC value Q (mg-S/g-sorb) Q (μg-S/m2
-sorb) pack. density (mg/cm3
) Q (mg-S/mL-sorb)
Al2O3 155 104 8.9 0 0 0.80 0
TiO2 170 90 5.5 0.010 0.10 0.80 0.008
AC 1087 930 7.4 0.048 0.05 0.20 0.010
EH-5 315 196 2.4 0.354 1.80 0.40 0.142
Figure 5. Desulfurization capacity of supported TiO2−CeO2
adsorbents versus PZC value of the supports, SiO2 (EH-5), TiO2,
AC, and Al2O3.

sites or exposed surface area of the TiO2−CeO2 particles45
for
sulfur oxidation and/or sulfone adsorption and thus a higher
desulfurization capacity. The MCM-48-supported TiO2−CeO2
adsorbent shows the highest desulfurization capacity, which
may be attributed to a better dispersion of TiO2−CeO2 oxides
over MCM-48 support.7
The control experiment shows that the
desulfurization capacity of MCM-48 support itself is only 0.012
mg-S/g-sorb as shown in Figure 6, indicating that MCM-48
support alone does not contribute to the significant increase in
the desulfurization capacity.
It should be mentioned that the TiO2−CeO2/MCM-48
adsorbent plays at least two main roles in ADS; one is to
provide active sites for sulfur oxidation, which can be
determined by the supported TiO2−CeO2 oxides, and the
other is to provide adsorption sites for oxidized sulfones, which
can be influenced by not only the supported TiO2−CeO2
oxides but also the silica support itself. The adsorption of
oxidized sulfones is likely through two types of interactions, (a)
acid−base interaction between acidic sites on TiO2−CeO2
oxides and basic sulfones (electronegative oxygen in sulfones
acts as electron donor and thus it can behave as a weak Lewis
base46
) and (b) hydrogen bonding interaction between
sulfones and silanol groups47
on the surface of the silica
support (the average concentration of silanol groups on silica
materials is 5 −OH/nm−2
).48
The contribution of the two
modes to the sulfone adsorption will be further studied in the
future work.
3.3. Adsorbent Regeneration. For practical applications,
a potential adsorbent should not only possess a high adsorption
capacity and selectivity to sulfur species but also have excellent
regenerability and stability.49
The regeneration of spent TiO2−
CeO2/MCM-48 adsorbent was performed via oxidative treat-
ment at 340 °C under an air flow rate of 50 cm3
/min for 2 h.
The adsorption performance of the regenerated adsorbents was
examined and compared with that of the fresh ones. The
breakthrough curves of total sulfur compounds and the
desulfurization capacity over the regenerated TiO2−CeO2/
MCM-48 adsorbents in the first three cycles are shown in
Figure 7. The breakthrough curves for the regenerated TiO2−
CeO2/MCM-48 adsorbent coincide well with that for the fresh
one, with the breakthrough desulfurization capacities of 1.143,
1.083, and 1.095 mg-S/g-sorb, or 95, 90, and 91 mL-F/g-sorb,
respectively, for the first three regeneration cycles. The
standard deviations were 1.6%, 2.2%, and 1.1%, respectively.
The results show that the adsorption capacity can be recovered
by air oxidation. In other words, oxidative air treatment can be
an effective regeneration method for the TiO2−CeO2/MCM-
48 adsorbent.
Figure 8 shows sulfur K-edge XANES spectra of the spent
and regenerated TiO2−CeO2/MCM-48 adsorbent and three
reference compounds, 4,6-DMDBT, DBTO2, and NiSO4. The
peak positions of sulfur compounds vary with the different
oxidation states of sulfur in XANES spectra. It can be seen that
no sulfur species were detected on the regenerated TiO2−
CeO2/MCM-48 adsorbent, indicating that the adsorbed sulfone
species can be oxidatively decomposed during air treatment.
Additionally, the fact that the spent TiO2−CeO2/MCM-48
Table 4. BET Surface Area of Supports and Supported TiO2−CeO2 Adsorbents and Desulfurization Capacity of Various Silica
Supported TiO2−CeO2 Adsorbents
support for TiO2−CeO2 EH-5 SBA-15 MCM-41 MCM-48 unsupported
SBET‑support (m2
/g) 315 950 1229 1281
SBET‑supported TiO2−CeO2
(m2
/g) 196 750 850 1089 249
Q (mg-S/g-sorb) 0.354 0.636 0.778 1.143 0.035
Pack. Density(mg/cm3
) 0.40 0.20 0.20 0.20 0.46
Q (mg-S/mL-sorb) 0.142 0.127 0.156 0.229 0.016
Figure 6. Desulfurization capacity of various silica supported TiO2−
CeO2 oxides, bulk TiO2−CeO2, and MCM-48 support versus their
surface areas.
Figure 7. Breakthrough curves of total sulfur compounds over the
TiO2−CeO2/MCM-48 adsorbent in the first three regeneration cycles
via oxidative treatment using air at 340 °C. (Inset: Desulfurization
capacity versus regeneration cycles.)
Figure 8. Sulfur K-edge XANES spectra of the spent and regenerated
TiO2−CeO2/MCM-48 adsorbent and three reference compounds,
4,6-DMDBT, DBTO2, and NiSO4.

adsorbent was effectively regenerated at 340 °C also implies a
good thermal stability of the adsorbent. The negligible loss in
the adsorption capacity of the regenerated TiO2−CeO2/MCM-
48 adsorbent suggests that the TiO2−CeO2/MCM-48
adsorbent can be a promising air-regenerable adsorbent for
ultra-deep desulfurization of diesel fuel.
3.4. Proposed Two-Stage Process for Ultra-deep
Desulfurization. On the basis of the above study, a novel
two-stage process for selective and ultra-deep desulfurization of
diesel fuel is proposed for the production of ultraclean fuel, as
shown in Figure 9. In the first stage, peroxides are generated in
fuel through sunlight or visible light irradiation. In the second
stage, light-irradiated fuel is fed into an adsorption bed packed
with dual-functional adsorbent/catalyst material (e.g., TiO2−
CeO2/MCM-48) which serves as the oxidation catalyst for
refractory sulfur compounds to sulfone species by the peroxides
generated in the first step, as well as the adsorbent for
simultaneous sulfone adsorption. After the adsorption bed
reaches its working capacity, it can be switched to the
regeneration side using oxidative air treatment. With the
proposed new process, ultra-deep adsorptive desulfurization of
diesel fuels can be achieved effectively under ambient
conditions without using costly hydrogen. Moreover, this
process may incorporate a good utilization of natural sunlight,
which may lead to a more sustainable and clean process.
4. CONCLUSION
A systematic study was carried out on ultra-deep adsorptive
desulfurization from light-irradiated diesel fuel over supported
TiO2−CeO2 adsorbents. Sulfur K-edge XANES identified
sulfones as were the major sulfur species on the spent TiO2−
CeO2/MCM-48 adsorbent. Adsorption selectivity of different
compounds from a model fuel follows the order of DBTO2 ≫
DBT > MDBT > BT > DMDBT > Phe > MNap ∼ Flu > Nap,
indicating a higher adsorption selectivity of sulfones over the
original sulfur compounds and aromatics in fuel. We conclude
that the original sulfur species are chemically transformed to
sulfones that are more selectively adsorbed during ADS of light-
irradiated diesel fuel over the supported TiO2−CeO2
adsorbent, resulting in over 30-fold higher desulfurization
capacity (95 mL-F/g-sorb or 1.143 mg-S/g-sorb) as compared
to ADS of the original diesel fuel. Additionally, the adsorption
of sulfones over supported TiO2−CeO2 adsorbent is likely
through the highly electronegative oxygen atom rather than
sulfur atom.
Desulfurization capacity of light-irradiated fuel over
supported TiO2−CeO2 adsorbents is found to increase with
decreasing PZC value of the supports.
Among the different supports examined in this work, silica
material with the lowest PZC value (2−4) is the most effective
support to load TiO2−CeO2 oxides for desulfurization, possibly
through a strong electrostatic adsorption of Ti−Ce precursors
over the silica support.
For different silica supported TiO2−CeO2 oxides, the
desulfurization capacity increases monotonically with the
increasing of surface area in the order of MCM-48 > MCM-
41 > SBA-15 > EH-5, which may be attributed to a higher
dispersion of TiO2−CeO2 oxides on the higher surface area
support.
The TiO2−CeO2/MCM-48 adsorbent can be regenerated by
oxidative air treatment; no sulfur of any types (sulfones, or
sulfates, or DMDBT) was detected on the regenerated TiO2−
CeO2/MCM-48 adsorbent by sulfur K-edge XANES spectros-
copy.
On the basis of the present study, a new two-stage process
for ultra-deep adsorptive desulfurization of diesel fuel is
proposed that may provide an attractive path to achieve
ultraclean fuel more effectively and efficiently under ambient
conditions without using costly hydrogen.
■ ASSOCIATED CONTENT
*S Supporting Information
Effect of TiO2−CeO2 loading on the desulfurization capacity
over the TiO2−CeO2/MCM-48 adsorbent with TiO2−CeO2
loading of 10.0, 13.3, 15.0, and 20.0 wt % from light-irradiated
diesel at a breakthrough point of 1 ppmw-S at 25 °C and LHSV
of 9.6 h−1
(Figure S1), effect of BET surface area of adsorbents,
desulfurization capacity (mg-S/m2
-sorb) versus BET surface
area (m2
/g) of various supported TiO2−CeO2 adsorbents
(Figure S2), and XRD patterns of various supported TiO2−
CeO2 adsorbents (Figure S3). This information is available free
of charge via the Internet at http://pubs.acs.org.
■ AUTHOR INFORMATION
Corresponding Author
*C. Song. Phone: (814) 863-4466. Fax: (814) 865-3573. E-
mail: csong@psu.edu.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported in part by the U.S. Department of
Energy, National Energy Technology Laboratory, the U.S.
Office of Naval Research, and the U.S. National Science
Foundation−U.S. Environmental Protection Agency Joint TSE
program. J. Xiao gratefully acknowledges the support by the
National Natural Science Foundation of China (21306054), the
Guangdong Natural Science Foundation (S2013040014747),
and the Fundamental Research Funds for the Central
Universities (2013ZM0047). Sulfur K-edge XANES work at
the CMC Beamline is supported in part by the Office of Basic
Energy Sciences of the U.S. Department of Energy and by the
National Science Foundation Division of Materials Research.
Use of the Advanced Photon Source, an Office of Science User
Facility operated for the U.S. Department of Energy (DOE)
Office of Science by Argonne National Laboratory, was
supported by the U.S. DOE under Contract No. DE-AC02-
Figure 9. Proposed selective desulfurization process for ultra-deep desulfurization of diesel fuel.

06CH11357. We would like to thank Prof. Jim Adair at Penn
State for the help on Zeta potential measurements.
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