Deep oxidative desulfurization with task-specific ionic liquids: An experimental and computational study

Article abstract of DOI:10.1016/j.molcata.2010.08.003

A series of task-specific acidic ionic liquids (TSILs), immiscible with oil, halogen-free and containing -COOH group in the cations, were used for oxidative desulfurization as both the catalyst and extractant. The removal of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) from model diesel at 298 K could reach 96.7% and 95.1%, respectively. The TSIL could be recycled 5 times without any apparent loss of the catalytic activity. Meanwhile, the structures, acidities and interactions between the cation and the anion of TSILs have been investigated by density functional theory (DFT) method, and found that catalytic properties of TSILs are close related to the structures, acidities and extraction capabilities. Furthermore, an oxidative desulfurization mechanism has been proposed.

Full text of DOI:10.1016/j.molcata.2010.08.003

Journal of Molecular Catalysis A: Chemical 331 (2010) 64–70
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Deep oxidative desulfurization with task-specific ionic liquids: An experimental
and computational study
Jianzhou Gui , Dan Liua,b, Zhaolin Sun , Daosheng Liu , Dayoung Min , Busub Song , Xilai Peng
a,∗
a
a
b
b
a
a
College of Chemistry and Materials Science, Liaoning Shihua University, Fushun 113001, Liaoning, PR China
Division of Green Chemistry, Korea Research Institute of Chemical Technology (KRICT), Sinseongno 19, Yuseong-gu, Daejeon 305-600, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of task-specific acidic ionic liquids (TSILs), immiscible with oil, halogen-free and containing
COOH group in the cations, were used for oxidative desulfurization as both the catalyst and extractant.
Received 17 March 2010
Received in revised form 18 June 2010
Accepted 4 August 2010
–
The removal of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) from model
diesel at 298 K could reach 96.7% and 95.1%, respectively. The TSIL could be recycled 5 times without
any apparent loss of the catalytic activity. Meanwhile, the structures, acidities and interactions between
the cation and the anion of TSILs have been investigated by density functional theory (DFT) method,
and found that catalytic properties of TSILs are close related to the structures, acidities and extraction
capabilities. Furthermore, an oxidative desulfurization mechanism has been proposed.
Available online 11 August 2010
Keywords:
Task-specific ionic liquid
Oxidative desulfurization
Model diesel
©
2010 Elsevier B.V. All rights reserved.
DFT
1
. Introduction
Ionic liquids (ILs) have been regarded as “green solvent” in sep-
arations, chemical synthesis, electrochemistry, catalysis and so on.
Recently, ODS method combined with H O as the oxidant in IL was
Deep removal of sulfur from fuels, particularly from gasoline and
2
2
diesel, has been an important and challenging issue in worldwide
petroleum refining industry, which is not only due to increas-
ingly stringent environmental regulations on sulfur concentration
of transport fuel, but also because of the great importance for mak-
ing ultra-low-sulfur fuels for fuel cell applications [1–3].
developed to be effective [4–11]. Wei and coworkers reported an
one-pot desulfurization of light oils by chemical oxidation and sol-
vent extraction in ILs, and found that [BMIm]PF was more effective
6
than [BMIm]BF4 for increasing the rate of chemical oxidation [4].
Zhao et al. applied the coordinated IL in ODS process for the removal
of thiophene using acetic acid as the catalyst [5]. Yen et al. reported
ultrasound-assisted oxygenative desulfurization in halogen-free
ILs together with trifluoroacetic acid, which turns to be a multi-
ple phase-transfer catalysis process [6]. Li and his coworker carried
out oxidative desulfurization of model diesel in ILs, combined with
Nowadays hydrodesulfurization (HDS),
a
conventional
hydrotreating method, can be effective to remove sulfides,
disulfides, mercaptans and light thiophenic sulfur compounds,
however, difficult in the removal of the most refractory sulfur
molecules such as alkyl derivatives of DBT because of steric
hindrance. To achieve low sulfur level by HDS, problems including
high operating cost, shorter lifetime of catalyst, higher hydrogen
consumption and lower lubricating properties of treated diesel
would be brought. Alternative desulfurization processes that
the catalysts decatungstates [7], phosphotungstic acid [8], V O5
2
[9], peroxophosphomolybdate [10], and peroxotungsten or perox-
omolybdenum complexes [11]. Generally, the ILs work as reaction
media and extractant in the above reports, and additional catalyst
was needed.
operate under mild conditions without H consumption have been
2
extensively investigated, among which oxidative desulfurization
Recent advance in ILs research provided another route for
achieving TSILs in which a functional group is covalently tethered
to the cation or anion of the ILs, especially to the two N atoms
of the imidazole ring. It was expected that these TSILs may fur-
ther enlarge the application scope of ILs in chemistry [12]. Until
now, only a few reports have been published on ODS with TSILs
working as both a catalyst and extractant. Gao and his coworkers
use [HMIm]BF4 as a catalyst and solvent to removal 60–93% sul-
fur from DBT-containing model oil, however, the exact catalytic
role of IL was not proposed, and the amount of IL is quite large
(
ODS) has received wide attention. Organic S compounds can be
selectively oxidized to sulfoxides or sulfones, and then removed
by polar extractant for achieving ultra-low-sulfur fuel. However,
the extractant is usually volatile organic compound, which is
flammable and finally leads to further environmental and safety
concerns.
∗ Corresponding author. Tel.: +86 413 6865161.
E-mail addresses: guijz@lnpu.edu.cn, jzgui@hotmail.com (J. Gui).
(Vmodel oil (1000 ␮g/mL S)/VIL = 3.2:5) [13]. Zhao et al. reported effective
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.08.003

J. Gui et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 64–70
65
Fig. 1. Structures of five TSILs.
10 K min 1 heating rate under nitrogen). Densities were measured
−
oxidative desulfurization of model diesel by Brönsted acid ionic
liquid (N-methyl-pyrrolidonium tetrafluoroborate) both as extrac-
tant and catalyst in the presence of H O , and the catalytic role of
using a U-shape vibrating-tube densimeter (Model DA-500) oper-
ating in a static mode. The rheometer used (Brookfield, RVDV-III )
+
2
2
IL is to decompose H O to form hydroxyl radicals that were strong
oxidizing agents for DBT. The drawback of this method is the large
allows measurements from 293 to 393 K at atmospheric pressure
and in a wide viscosity range (from 100 to 3 M cP).
2
2
amount of IL (Vmodel oil/VIL = 1:1) [14]. At the same time, the anions
IL1: density: 1.377 g/mL (298 K); viscosity: 1000.6 MPa s (353 K).
−
−
1
of these ILs are usually BF4 or PF6 which yields white fumes of
HF or hydrate precipitates easily. So there is still much room for
the development of more effective and halogen-free TSILs for ODS
process.
H NMR (300 MHz, DMSO-d , TMS); 3.87 (s, 3H), 5.11 (s, 2H), 7.70
6
(s, 2H), 9.10 (s, 1H). 13C NMR (75 MHz, DMSO-d , TMS); ı 36.52,
6
−
1
50.26, 123.89, 124.38, 138.35, 168.89. IR (cm ): 3158, 2509, 1741,
1572, 1364, 1168, 1176, 1050, 852, 639,588. C H O N S (237.13)
6
10
6
2
In order to design TSILs for oxidative desulfurization, i.e. ILs can
works both as a catalyst and extractant, several requirements for IL
should be taken into consideration: (1) the ILs should be immiscible
with fuels, then two phases will appear. In this case, water and the
sulfones produced can be easily extracted into the IL phase, which
will provide a convenient way for the separation of product fuel
from the reaction mixture; (2) some functional group for catalytic
oxidation should be introduced; (3) the ILs have good extraction
capacity to S-compounds, which is a requirement for TSIL working
as a phase transfer catalyst.
calcd: C, 34.96; H, 4.85; N, 13.58. Found: C, 34.03; H, 4.90; N, 13.62.
The thermal decomposition point of IL1: 578.5 K.
IL2: density: 1.340 g/mL (298 K); viscosity: 9896.9 MPa s (353 K).
1
H NMR (300 MHz, D O, TMS); 3.22 (s, 3H), 4.44 (s, 2H), 6.81 (t,
2
2H), 8.10 (s, 1H). 13C NMR (75 MHz, D O, TMS); ı 35.74, 49.55,
2
−
1
123.17, 124.11, 136.90, 169.54. IR (cm ): 3158, 2488, 1734, 1572,
1364,1212, 1191, 1168, 995, 639, 621. C H O N P (237.04) calcd:
6
11
6
2
C, 34.98; H, 5.34; N, 13.59. Found: C, 34.88; H, 5.39; N, 13.60. The
thermal decomposition point of IL2: 572.5 K.
IL3: density: 1.115 g/mL (298 K); viscosity: 1175.4 MPa s (353 K).
1
Our group had synthesized several TSILs and applied them
in Beckmann rearrangement, alkylation, esterification, oxidation,
polymerization reactions and so on [15–23]. Recently, the effect of
different cations or anions on extractive desulfurization has been
investigated in our group [22]. With our continuous research in
TSILs and desulfurization, a series of ILs with two Brönsted acid
sites have been designed and firstly synthesized in our group [23].
The carboxylic acid (−COOH) group with catalytic oxidative func-
tion was introduced into the cations of several acidic ILs, and
H NMR (300 MHz, DMSO-d , TMS); 2.84 (t, 2H), 3.82 (s, 3H), 4.31 (s,
6
13
2H), 7.71 (s, 1H), 7.79 (s, 1H), 9.33 (s, 1H). C NMR (75 MHz, DMSO-
d , TMS); ı 34.92, 36.36, 45.53, 123.02, 124.05, 137.60, 172.45. IR
6
−
1
(cm ): 3158, 2586, 1743, 1572, 1364, 1168, 639. C H O N Cl
7
11
2
2
(190.53) calcd: C, 44.13; H, 5.77; N, 14.70. Found: C, 44.23; H, 5.59;
N, 14.88. The thermal decomposition point of IL3: 516.4 K.
IL4: density: 1.195 g/mL (298 K); viscosity: 9677.1 MPa s (353 K).
1
H NMR (300 MHz, D O, TMS); 2.28 (t, 2H), 3.16 (s, 3H), 3.74 (t, 2H),
2
13
6.70 (s, 1H), 6.79 (s, 1H), 8.01 (s, 1H). C NMR (75 MHz, D O, TMS); ı
33.84, 35.65, 44.71, 122.21, 123.59, 136.29, 173.87. IR (cm ): 3158,
2
−
−
−1
their anions are [HSO4] and [H PO ] (see Fig. 1). In this study,
2
4
those TSILs were used as both catalyst and extractant for oxidative
desulfurization of model diesel, which have proven to be effective
and combined the oxidation and extraction process in one step.
Meanwhile, the structures, interactions between cation and anion,
acidities of these ILs were studied by DFT method. To the best of our
knowledge, it is the first report on oxidation desulfurization with
halogen-free TSILs without adding additional catalyst.
3098, 2602, 2486, 1749, 1572, 1364, 1287, 1168, 1069, 1006, 885,
850, 639, 614, 454. C H O N S (252.14) calcd: C, 38.19; H, 5.45;
N, 12.12. Found: C, 38.23; H, 5.49; N, 12.37. The thermal decompo-
sition point of IL4: 524.3 K.
7
12
6
2
IL5: density: 1.233 g/mL (298 K); viscosity: 9539.6 MPa s (353 K).
1
H NMR (300 MHz, D O, TMS); 2.47 (t, 2H), 3.36 (s, 3H), 3.94 (t,
2
13
2H), 6.90 (s, 1H), 6.97 (s, 1H), 8.22 (s, 1H). C NMR (75 MHz,
D O, TMS); ı 33.89, 35.64, 44.60, 122.20, 123.52, 136.30, 174.05.
2
−
1
IR (cm ): 3158, 1726, 1572, 1364, 1167, 1109, 997, 639, 621.
C7H13O N P (252.05) calcd: C, 38.21; H, 5.91; N, 12.72. Found: C,
2
. Experimental
6
2
2
.1. Preparation of TSILs
38.43; H, 5.79; N, 12.58. The thermal decomposition point of IL5:
06.1 K.
5
The TSILs were prepared by two-step synthesis through
-methylimidazole combined first with chloroacetic acid or 3-
1
2.2. Catalytic oxidative desulfurization
chloropropionic acid to form zwitterions salts, followed by addition
of concentrated sulphuric acid (97%) or o-phosphoric acid (85%).
The detailed preparations are in our previous work [23]. The
C, N and H elemental analyses were performed on an Elemen-
tar Vario EL element analyzer. IR spectra (FTIR) were recorded
Model diesel was prepared by dissolving DBT or 4,6-DMDBT in
n-tetradecane to form solutions with sulfur content of 500 ␮g/g.
The oxidative desulfurization experiments were carried out in a
25-mL round-bottom flask. The mixture containing 10 mL model
TM
1
on a PE Spectrum
GX FTIR spectrometer using liquid film.
H
diesel, 2.5 mmol IL and 1 mL 30 wt% H O2 was stirred vigorously at
2
NMR (300 MHz) and 13C NMR (75 MHz) were obtained on Var-
ian Mercury-plus 300BB instruments as solutions in deuterium
substituted reagent. Chemical shifts were reported in parts per
million (ppm, d). The thermal decomposition point of ionic liq-
uids was determined by TGA (Perkin-Elmer TGA Pyris1instrument,
room temperature (298 K). Upon standing, the upper phase (model
diesel) was separated easily from the IL phase by decantation at
room temperature and analyzed the sulfur content of diesel phase
by gas chromatography coupled with an atomic emission detector
(GC-AED) and microcoulometry.

6
6
J. Gui et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 64–70
Fig. 2. ODS results of model diesel catalyzed by difference TSILs.
Fig. 3. Mole concentration of peroxycarboxylic acids versus time at 298 K in differ-
ent TSILs.
2.3. Analysis of peroxycarboxylic acids
As shown in Fig. 3, the formations of peroxycarboxylic acids
In order to demonstrate the existence of peroxycarboxylic acid,
the reaction of IL and oxidant were carried out in 50 mL clean
in different TSILs (IL3–5) were investigated without adding model
diesel in the system. The efficiencies of forming peroxycarboxylic
acid in these three TSILs decreased as follows: IL4 > IL5 > IL3, and
the same trend can be found on their desulfurization capacity (see
Fig. 2). Therefore, the formation rate of peroxycarboxylic acid was
close related to the oxidation rate of DBT. Meanwhile, the acidity of
ground-glass flasks. 75 mmol IL and 30 mL 30% H O2 were added
2
in the flask, and well mixed by stirring (500 rpm) at room tempera-
ture (298 K). Samples were withdrawn from the mixture by a plastic
syringe (to avoid contamination of the solution by trace of metals)
and analyzed by the Greenspan and Mackellar method [24]. Each
datum was the average result of at least double tests.
−
−
−
their anions has the order of [HSO4] > [H PO ] > Cl , suggesting
2
4
that the formation rate depends on the acidity of the ILs. It can be
concluded that the stronger the acidity of IL is, the larger the rate
of producing the peroxy acid groups (–COOOH) is, and the better
the desulfurization performance is. This result agrees well with the
previous studies that the existence of stronger acid will acceler-
ate the formation of the peroxycarboxylic acid [25,26]. As for IL3,
the anion has no acidity, while some conversion was noticeable.
2
.4. Calculation methods
All calculations were performed by applying the generalized
gradient-corrected density functional theory using the DMol3
module in Materials Studio package of Accelrys. BLYP exchange
correlation function, double-numeric quality basis set with polar-
ization functions were both employed and all electrons are included
throughout the calculations. A thermal smearing of 0.005 Hartree
was used to improve the computational performance. All SCF tol-
+
The reason is that the carboxylic acid itself can also dissociate H ,
which is able to act as a catalyst to accelerate the forming rate of
peroxycarboxylic acid [26].
The preferred TSIL is IL4 (i.e. [(CH ) COOHmim][HSO ]), which
2
2
4
erances were set to fine, i.e., the tolerance of energy, gradient and
has good extraction capability and stronger acidity. It could be
concluded that the ODS results depends upon both the extraction
ability and the acidity of TSILs.
displacement are converged to 1.0e−5 Ha., 0.002 Ha./Å and 0.005 A˚ ,
respectively.
The structures of the cations were optimized and related elec-
tric properties were also calculated, and finally equilibrium existing
modes of the cation and anion in five TSILs were investigated.
3.2. ODS of model diesel (containing DBT or 4,6-DMDBT) by IL4
Figs. 4 and 5 show the sulfur-specific GC-AED chromatograph of
3
. Results and discussion
model diesel (containing DBT and 4,6-DMDBT, respectively) before
and after oxidation. All the DBT in model diesel were oxidized into
3.1. Effect of different TSILs
its corresponding sulfone (DBTO ) within 1.5 h, and almost all the
2
sulfones produced were extracted into the ionic liquid phase (see
Fig. 4). Because the TISLs were immiscible with model diesel, the
model diesel can be easily separated by decantation after reaction.
Dual functions of both oxidation and extraction were combined in
one step for the desulfurization of model diesel. Therefore, this IL
is quite efficient and effective for deep removal of DBT.
The effect of different TISLs on oxidative desulfurization capac-
ity of model diesel is shown in Fig. 2. Before the oxidant H O2 was
2
added (reaction time t = 0), the model sulfur compound (DBT) had
partially extracted by ILs. Their extraction desulfurization capa-
bility decreased in the following order: IL4 > IL5 > IL3 > IL1 > IL2. It
shows that TSILs with cation [(CH ) COOHmim] (including IL4,
As for the removal of 4,6-DMDBT, the oxidation reaction would
almost complete in 7 h and the oxidation product (see Fig. 5),
i.e. the corresponding sulfone, remains little in the model diesel
phase. This demonstrates that this ODS method is also suitable
for the 4,6-DMDBT containing system. Fig. 6 indicates that the
removal rate of DBT is much quicker than that of 4,6-DMDBT,
while the relative order of oxidation desulfurization rate is 4,6-
DMDBT > DBT when using acetic acid as the catalyst in earlier
literature [27]. Our interesting result is found to be consistent
with that of oxidative desulfurization in the catalytic system
(peroxophosphomolybdate/H O /ILs), which has been owing to
2
2
IL5 and IL3) has better extraction capability than those with
CH COOHmim] as cation. This may be caused by increasing the
[
2
branch length of cations.
As for the TSILs with the same cation, the desulfurization capa-
bility of ILs decreased as follows: IL4 > IL5 > IL3; IL1 > IL2. This can be
explained by the rate of forming peroxycarboxylic acid by the reac-
tion of these TSILs with hydrogen peroxide. The peroxycarboxylic
acid generated in situ is expected to be the active species for DBT
oxidation. Thus, it was of interest to explore the influence of per-
oxycarboxylic acid formation in different TSILs on DBT conversion.
2
2

J. Gui et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 64–70
67
Table 1
Deprotonation energies of different Brönsted acid sites in cation A, B and C.
Deprotonation
H1
H2
H3
H atom in
energies (kcal/mol)
COOH group
A
B
C
291.1
292.1
293.7
287.5
287.7
293.1
270.7
271.4
273.6
253.8
278.1
287.4
the steric hindrance of the methyl group of 4,6-DMDBT, i.e. the
obstacle for the approach of the sulfur atom to the catalytic active
species in IL [10]. In our experiments, it is proposed that the result is
mainly caused by better extraction capability of [(CH ) COOHmim]
2
2
[
HSO ] for DBT than 4,6-DMDBT. As shown in Fig. 6, when the
4
oxidant H O2 was not added in the system, the model sulfur com-
2
pounds (DBT and 4,6-DMDBT) had partially extracted by IL4. The
extraction of DBT in IL4 is better than that of 4,6-DMDBT, and their
sulfur content in model diesel changes from the initial 500 ␮g/g to
3
04.3 ␮g/g and 347 ␮g/g, respectively.
In this desulfurization process, as for the model diesels contain-
Fig. 4. Sulfur-specific GC-AED chromatographs of model diesel (500 ␮g/g S of DBT
in n-tetradecane) before and after oxidation.
ing DBT and 4,6-DMDBT, the sulfur content can be decreased from
the initial 500 ␮g/g to 16.35 and 24.43 ␮g/g after reacting for 7 h,
respectively (see Fig. 6).
Because water and sulfones produced in the ODS process exists
in the ionic liquid phase, the product diesel can be separated from
the IL phase by simple decantation after reaction, which provided
a simple way for separation. The ionic liquid phase separated was
extracted three times with equal volume of 1,4-butyrolactone to
remove the sulfones produced, and then dried under vacuum at
3
73 K for 2 h in order to remove the oxidant and water. Fresh H O2
2
and model diesel then were added to the ionic liquid for the next
run. The ionic liquid could be recycled for 5 times without any
obvious change on the desulfurization capability.
3.3. Molecular modeling of TSILs
In this study, DFT method is also included to study the structures
of TSILs, in order to find the relationships among the structures,
interactions between the anion and cation, acidity and catalytic
oxidation properties.
3.3.1. Effect of different cations
In order to study the effect of branch length on acidity, the
Fig. 5. Sulfur-specific GC-AED chromatograms of model diesel (500 ␮g/g S of 4,6-
DMDBT in n-tetradecane) before and after oxidation.
imidazole-based cations with (–(CH )xCOOH) (x = 1, 2, 3) as a
branch were optimized, and their structures were listed in Fig. 7.
2
Each of the three different cations, i.e. [CH COOHmim],
2
[
(CH ) COOHmim] and [(CH ) COOHmim], has four different
2
2
2 3
active H atoms as the possible acidic sites, namely, H1, H2, H3, and
H in –COOH group. The deprontonation energy for each site was
calculated using the following equation:
EDP = Ecation − Ecation-H
EDP is the deprotonation energy; Ecation is the total energy of
cation with the charge of +1; Ecation-H is the total energy of cation
when removing one active H atom. Bigger deprotonation energy
will lead to weaker acidity of the Brönsted acid site.
The deprotonation energies of different Brönsted acid sites in
cation A, B and C were combined in Table 1. It shows that the depro-
tonation energy of each protonic acid site in the imidazole ring
would not change obviously when increasing chain length and H3
site is strongest in acidity. As for the –COOH group, the deprotona-
tion energy increases and the acidity decrease with the increase of
the chain length. This could be explained by the inductive effect of
electron-withdrawing group (methyl imidazole-1-yl) on acidity of
carboxylic acids (–COOH). Inductive effects operate through bonds
by successive bond polarizations. As such, they diminish rapidly
Fig. 6. Different desulfurization results of model diesel containing 4,6-DMDBT and
DBT during oxidation catalyzed by IL4 at 298 K.

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J. Gui et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 64–70
Fig. 7. Optimized structures of cations in TSILs and their atom labeling. (A) Cation with branch chain –CH2COOH. (B) Cation with branch chain –(CH2)2COOH. (C) Cation with
branch chain –(CH2)3COOH.
with distance so that very little effect results if an inductive effect
must be transferred through more than four bonds. So the longer
the distances between them, the weaker the inductive effect is and
the weaker the acidity of carboxylic acids is. The strongest acid site
is –COOH group in cation A, while H3 turns to be the major con-
tribution to the acidity in cation B and C. As for the acidity of ILs,
there are other factors on acidity exists, such as the acidity of anion,
interaction between cation and anion.
(C–)H10· · ·Cl (2.246 A˚ ), formed, and the bond lengths of C − H3,
O–H11 and C–H10 are 1.120 A˚ , 0.997 A˚ and 1.100 A˚ , respectively.
The interaction energy of cation and anion in A, B and C is −93.2,
−111.3 and −113.4 kcal/mol as shown in Fig. 8. It can be seen that
more hydrogen bonds forming will lead to more stable existing
modes of IL. For IL3, the existing mode C is the most energetically
stable one.
−
When the anion is oxy-acid group (including HSO4 and
−
H PO ), the interaction between the cation and anion of IL is more
2
4
3
.3.2. Equilibrium existing mode of TSILs
The interaction modes between the cation and anion of each TSIL
complex. The hydrogen bonding is also the major contribution to
the interactions between cation and anion. Various different hydro-
gen bonding modes are calculated, Results show that the terminal
oxygen atom of anion should be more easily to form strong hydro-
gen bond, and more hydrogen bonds formed will lead to stronger
interaction. As for anion HSO4 and H2PO4 , when the two termi-
nating oxygen atom forms hydrogen bonds with H3 and H atom
in –COOH group, respectively, the interaction energy tends to be
more negative, and the existing mode of IL is more energetically
stable. The optimized structures of most stable existing modes for
four different ILs are shown in Fig. 9. Geometry parameters of most
stable configurations of ILs are listed in Table 2.
were investigated. The structures of their cation and anion were
optimized, and then the configurations of every TSIL with various
modes of interaction were geometrically optimized to find the most
stable existing mode. The interaction energy between the cation
and anion was calculated using this expression:
−
−
Eint = Eanion + Ecation − Eanion+cation
Among the five TSILs, it is easy to study the existing modes of IL3
−
on account of its simple anion Cl . Three types of interactions were
investigated and shown as A, B and C in Fig. 8. The results show
that: (A) When H11 forms one hydrogen bond with the anion Cl
−
As for ionic liquids (IL3–IL5) containing the same cation, the
interaction energies between cation and anion are −113 kcal/mol,
(
see Fig. 8A), only one hydrogen bond (O–)H11· · ·Cl (1.598 A˚ ) exists;
and O–H11 bond length is 1.209 A˚ , which is much longer than nor-
−94.0 kcal/mol and −110.0 kcal/mol, respectively. It means that
the strength of the interaction between anion and cation
increases in the following order: [(CH ) COOHmim] [HSO4]
mal O–H single bond. It indicates that forming hydrogen bond leads
to weaker chemical bond between H and its adjacent O atom. (B)
2
2
(
(
IL4) < [(CH ) COOHmim] [H PO ] (IL5) < [(CH ) COOHmim] Cl
IL3). When the interaction between cation and anion is stronger,
When H3 forms hydrogen bond with Cl as shown in Fig. 8B, two
2 2 2 4 2 2
hydrogen bonds, (C–)H3· · ·Cl (2.081 A˚ ) and (C–)H10· · ·Cl (2.601 A˚ ),
that’s to say, the hydrogen in cation is more difficult to remove
because of hydrogen bonding, indicating that the acidity of TSIL
will depends on the anion to a larger extent. According to above
discussions, it is evident that the acidity of TSILs should decrease in
the order of IL4 > IL5 > IL3, whose trend is the same to that of their
desulfurization capability (see Fig. 2). Therefore, the desulfurization
capability is close related to their acidity.
formed. This was contributed to the strong inductive effect of their
adjacent electron withdrawing group, which makes H3 and H10
have more positive charges. Bond strength of C–H3 (1.108 A˚ ) and
C–H10 (1.110 A˚ ) are weaker than normal C–H single bond. (C) The
H3 and H11 interacted simultaneously with Cl (See Fig. 8C) and
the structure parameters were combined in Table 2. Three hydro-
gen bonds, i.e. (C–)H3· · ·Cl (2.115 A˚ ) and (O–)H11· · ·Cl (2.561 A˚ ),

J. Gui et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 64–70
69
Fig. 8. Three different existing modes of [(CH2)2COOHmim] Cl (IL3). (A) One hydrogen bond formed (Cl−· · ·H11). (B) Two hydrogen bonds formed (Cl· · ·H3; Cl· · ·H10). (C)
−
Three hydrogen bonds formed (Cl · · ·H3; Cl· · ·H11; Cl · · ·H10).
Fig. 9. Optimized structures of most stable hydrogen-bond modes for four different TSILs.
−
As for the IL1 and IL4 with the same anion [HSO ] , the active H
the same with the acidity of IL2 and IL5. However, catalytic oxida-
tive desulfurization capability is in the order of IL5 > >IL2, IL6 > >IL3
(see Fig. 2), which indicated acidity is not the only factor for the
desulfurization, and the extraction capability to sulfur compounds
caused by different chain length is also the important factor.
4
atoms in their cations will form hydrogen bonds with the terminal
O atom of the anion and are difficult to remove, suggesting that
the acidity of IL is largely determined by that of anion. So it can be
concluded that IL1 and IL4 have similar acidity strength, and it is
Table 2
Geometry parameters of most stable existing modes of five TSILs.
Ionic liquids
IL1
IL2
IL3
−113.4
IL4
IL5
Interaction energy between cation and anion
O−H bond lengths in –COOH group
Bond length between H3 and adjected C
H bond between (COO)H and anion (Å)
H bond between H3 and anion (Å)
Other H bond interaction
−100.1
−116.2
−94.0
−110.0
1.023
1.048
0.997
1.003
1.021
1.091
1.608
1.992
–
1.112
1.506
1.766
1.120
2.561
2.115
1.097
1.780
1.912
–
1.114
1.614
1.703
–
H8· · ·O
H10· · ·Cl
Bond length (Å)
2.248
2.246

7
0
J. Gui et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 64–70
Fig. 10. Oxidation mechanism of sulfur compounds catalyzed by TSILs.
3.4. The reaction mechanism of oxidative desulfurization
Department of China (No. 209031), the Education Department of
predicted
Liaoning Province (Nos. 2009A430 and 2010243), the Program for
Liaoning Excellent Talents in University (No. LR201024), Natural
Science Foundation of Liaoning Province and the Korea Foundation
for Advanced Studies.
According to the discussions above, the reaction mechanism
of catalytic oxidation by TSILs can be predicted to experience
three steps (see Fig. 10): firstly, the TSILs can extract the sulfur-
compounds from the diesel; Secondly, the carboxylic acid group
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9
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2
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We are grateful for the financial support from the National Nat-
ural Science Foundation of China (No. 20706027), the Education