Oxidative Desulfurization of Dibenzothiophene Using Cobalt (II) Complexes with Substituted Salen-Type Ligands as Catalysts in Model Fuel Oil

Article abstract of DOI:10.1007/s10562-020-03343-4

Three cobalt(II)-salen complexes (CoL1, CoL2 and CoL3) were synthesized via the reaction of the three tetradentate ligands as N,N′-ethylenebis(salicylimine) L1, N,N′-ethylenebis(3-tert-butylsalicylimine) L2 and N,N′-ethylenebis(3,5-di-tert-butylsalicylimine) L3, with a stoichiometric amount of cobalt(II) acetate tetrahydrate, respectively. All the three complexes were studied as oxidative desulfurization catalyst on dibenzothiophene taken in model fuel oil n-dodecane. The acetonitrile used as an extracting solvent and H₂O₂ as an oxidant. Comparatively CoL3 proved to be the best catalyst which showed the 76% DBT removal at the optimized conditions. The nth-order kinetic model is the best way to represent oxidation kinetics of complexes. Graphic Abstract: [Figure not available: see fulltext.]This cobalt(II) Schiffs base complexes were studied as catalyst for oxidative desulfurization of dibenzothiphene (DBT) taken as model sulphur compounds in fuel model oil (n-dodecane) using H₂O₂ as oxidant and acetonitrile as extracting solvent for DBT sulfone in a batch experiment.

Full text of DOI:10.1007/s10562-020-03343-4

Catalysis Letters
https://doi.org/10.1007/s10562-020-03343-4
Oxidative Desulfurization of Dibenzothiophene Using Cobalt (II)
Complexes with Substituted Salen‑Type Ligands as Catalysts in Model
Fuel Oil
Deependra Tripathi1,2 · Raj K. Singh2
Received: 2 May 2020 / Accepted: 26 July 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Three cobalt(II)-salen complexes (CoL1, CoL2 and CoL3) were synthesized via the reaction of the three tetradentate
ligands as N,N′-ethylenebis(salicylimine) L1, N,N′-ethylenebis(3-tert-butylsalicylimine) L2 and N,N′-ethylenebis(3,5-di-tert-
butylsalicylimine) L3, with a stoichiometric amount of cobalt(II) acetate tetrahydrate, respectively. All the three complexes
were studied as oxidative desulfurization catalyst on dibenzothiophene taken in model fuel oil n-dodecane. The acetonitrile
used as an extracting solvent and H2O2 as an oxidant. Comparatively CoL3 proved to be the best catalyst which showed the
76% DBT removal at the optimized conditions. The nth-order kinetic model is the best way to represent oxidation kinetics
of complexes.
Graphic Abstract
This cobalt(II) Schifs base complexes were studied as catalyst for oxidative desulfurization of dibenzothiphene (DBT) taken
as model sulphur compounds in fuel model oil (n-dodecane) using H2O2 as oxidant and acetonitrile as extracting solvent
for DBT sulfone in a batch experiment.
Keywords Cobalt (II) complexes · Salen · Oxidative desulfurization · Kinetics · Dibenzothiophene · Model fuel
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1 3

D. Tripathi, R. K. Singh
Among these, the ODS has an advantage of simple
operating conditions without the requirement of sophis-
ticated utilities. In ODS, the sulfur species like DBT get
oxidised into respective sulphone (DBTO2) with the help
of the oxidising agents like hydrogen peroxide, molecular
oxygen and sometime aldehydes in the presence of the
catalyst [16, 17]. Several catalysts like organic acid [18],
polyoxometalates [19], polymolybdates [20] and transition
metal salts, oxides and complexes [17] have been used in
ODS. Among the transition metals, vanadium oxides and
complexes have been widely investigated e.g. VO(acac)2
to oxidize DBT by hydrogen peroxide [21]. Similarly
V2O5 [22], V–Mo oxides [23], vanadium peroxocom-
plexes [24] and polymer-bound oxidovanadium(IV) and
dioxidovanadium(V) complexes [25] have been used as
ODS catalyst. Molybdenum, tungsten and titanium metal
oxides have also been used as a catalyst with oxidants
including hydrogen peroxide [26, 27]. In other studies, it
was also noticed that the iron porphyrin and phthalocya-
nines complexes also have impressive catalytic activity in
the ODS process [28, 29].
1 Introduction
Due to the growing environmental problems and adverse
efects on the health of living organisms, many federal
agencies are imposing the stringent fuel standards with
respect to sulfur content. Considering this, deep desulfuri-
zation of liquid fuels is a subject of current interest in spite
of the fact that the crude is getting high in sulfur levels
[1]. Hydrodesulfurization (HDS) is widely used process in
the refnery for lowering the sulfur levels however, it has
limitations like inefciency to remove dibenzothiophene
(DBT) and its derivatives, and higher operational costs
[2]. The HDS removes the sulphur in the form of H2S from
the sulphur species like alkyl and aromatic mercaptans,
diphenyl sulphide, sulphur heterocyclic’s like thiophene,
dibenzothiophene etc. existing in fuel oils. This process
was less efcient for desulfurization of aromatic thiols,
and thiophenes derivatives although several eforts were
made to increase the efciency of HDS on these species
involving transition metal mediated cleavage of the C–S
bonds. Among the several other metals Mo [3–5], Co [6]
and Fe [7] were found to be studied extensively.
Cobalt (II) is an important metal that is widely utilized
for catalyzing various oxidation reactions either in the form
of its salts or complexes. It has also been utilized for the
ODS of model sulfur compounds in fuels [30]. Cobalt phth-
alocyaninetetrasulfonamide was found to be very efcient
catalyst for the oxidation of model sulfur compounds pre-
sent in diesel using aldehyde and molecular oxygen [31].
Co(II) Schif base complexes have been studied for the des-
ulfurization in the past too e.g. ultra-deep desulfurization of
fuel with Co(II) and Ni(II) complexes with chitosan Schif
base assisted by ultraviolet [32]. Recently we have also
used N-benzylated cobalt phthalocyaninetetrasulfonamide
as catalyst for oxidation of dibenzothiophene (DBT) in a
middle distillate model compound (n-dodecane) using H2O2
as an oxidant. The results showed that it has better catalytic
activity than cobalt phthalocyanine and cobalt phthalocya-
ninetetrasulfonamide [33]. Despite these eforts, there is a
need to develop the new high efciency, economical and
environmentally benign catalytic ODS technologies with
low catalyst loading requirements [17].
Attempts have been made in the recent past to develop
other processes like extractive desulfurization, adsorptive
desulfurization [8], biodesulfurization [9], photocatalytic
oxidation [10], electrochemical oxidation [11] and oxida-
tive desulfurization (ODS) [12]. Some integrated processes
were also tried like extraction coupled catalytic oxidative
desulfurization system (ECODS). An interesting ECODS
based study was done using air and isobutylaldehyde (IBA)
as metal-free photochemical oxidative systems and using
deep eutectic solvents (DESs) of choline chloride (ChCl)
and series of straight-chain monobasic acids for liquid–liq-
uid extraction. The sustainable and green process was found
to be efective for dibenzothiphene (DBT) removal up to
98.6% [13]. The 3D carbon spheres (3D-CSs) supported
phosphotungstic acid (HPW/3D-CSs) were synthesized by
carbonization of 3D printed spheres (3D-PSs) and then stud-
ied as deep oxidative desulfurization catalyst in comparison
to conventional catalyst powder form. The performance was
observed to be excellent at optimized conditions in remov-
ing almost ~ 100% DBT (dibenzothiophene), 4-MDBT
(4-methyldibenzothiophene) and 4, 6-DMDBT (4, 6-Dime-
thyldibenzothiophene) from fuel oil [14]. In another interest-
ing study, the well-dispersed Pt NPs supported catalyst on
hexagonal boron nitride (Pt/h-BN) was studies as aerobic
catalytic desulfurization activity. The charge transfer from B
atoms of h-BN makes the Pt NPs positively charged enabling
the boosted activity and the removal of 98.3%, 96.5%, 93.7%
and 85.9% was observed for dibenzothiophene, 4, 6-dime-
thyldibenzothiophene, 4-methyl-benzothiophene, and ben-
zothiophene respectively [15].
In order to develop a new type of highly active catalyst
for ODS, three cobalt(II) complexes with substituted and
unsubstituted salen-type ligands were synthesized and their
catalytic activity was studied in a batch experiments for the
oxidation of the model sulfur compound (DBT) taken in fuel
model oil using hydrogen peroxide as sacrifcial oxidant.
The formed DBT sulfone (DBTO2) was removed by in situ
extraction by acetonitrile. The optimization experiment were
done with respect to the dose of the catalyst, oxidant amount,
volume of the acetonitrile, reaction time and temperature.
Oxidation kinetics has been modelled by lumped power law
model.
1 3

Oxidative Desulfurization of Dibenzothiophene Using Cobalt (II) Complexes with Substituted…
2.3 Characterization
2 Materials and Methods
The structures of all synthesized complexes were confrmed
by various analytical techniques. Firstly, CHNS analysis was
done by Perkin Elmer Series II CHNS/O 2400 analyzer. The
Fourier transform infrared (FT-IR) spectra were recorded
on a Thermo-Nicolet 8700 research spectrophotometer with
a range between 450 and 4000 cm−1 (KBr pellets). 1H and
13C NMR have recorded on Bruker Avance 500 spectrometer,
in the proton noise-decoupling mode with a standard 5 mm
probe, with CDCl3 as the solvent for synthesized ligands. Opti-
cal absorption spectra in the region between 190 and 800 nm
were recorded in DMSO with a UV-2600 Shimadzu UV–Vis
spectrophotometer using 1 cm path length cuvettes at 25 °C.
The thermogravimetry curves were obtained on PerkinElmer
EXSTAR TG/DTA 6300 with aluminium pans under continu-
ous 200 mL min−1 nitrogen fow. The mass loss was recorded
from 30 to 800 °C. MALDI-TOF-MS spectra were recorded
by using Bruker UltrafeXtreme-TN MALDI-TOF-MS spec-
trometer and 2-(4-hydroxyphenylazo) benzoic acid (HABA)
matrix.
2.1 Chemicals and Materials
Cobalt(II) acetate tetrahydrate, ethylenediamine, dibenzo-
thiophene, n-dodecane, ethanol, acetonitrile, salicylalde-
hyde, 3-tert-butyl-2-hydroxybenzaldehyde and 3,5-di-tert-
butyl-2-hydroxybenzaldehyde were procured from Merck
Millipore. All the chemicals and solvents were of highest
available grade and used as received without any further
purifcation.
2.2 Synthesis
2.2.1 Synthesis of L1, L2 and L3 Ligands
For the synthesis of L1, 1.4 mL ethylenediamine
(20 mmol) was added to a solution of 4.5 mL salicyla-
ldehyde (41 mmol) with 50 mL ethanol in a round bot-
tom fask. The reaction mixture was thoroughly stirred
at refuxing temperature for 3–4 h, or until faky yellow
crystals were formed, and the solution was then left to cool
in an ice-bath. The crystals were fltered and washed, then
air-dried. Yield of L1 was found to be 5.2 g. For the syn-
thesis of L2, 7.31 g 3-tert-butyl-2-hydroxybenzaldehyde;
and for L3, 9.6 g 3,5-di-tert-butyl-2-hydroxybenzaldehyde
was taken in place of salicylaldehyde. The yield of L2 and
L3 were obtained 8.23 g and 10.12 g, respectively.
2.4 Catalytic Activity Study
The catalytic activity of the synthesized complexes was tested
for the oxidation of DBT, a heterocyclic model sulfur com-
pound. A liquid phase glass batch reactor operated at atmos-
pheric pressure under stirring was employed. The H2O2 was
used as the oxidant in diferent concentrations combining with
diferent mixtures of acetonitrile and the metal complexes in
the reactor containing 100 mL of n-dodecane that was taken
as model fuel for the optimization studies. Acetonitrile works
as the extractant for the polar sulfoxide and sulfone formed. At
frst, the dose of the catalysts was optimized by taking 100 mL
of acetonitrile keeping the 0.05 mmol concentration of DBT
in model fuel. Thereafter, the sulfur mass proportion to oxi-
dant and the extractant amount were studied. Volumes of ace-
tonitrile ranging from 100 to 200 mL were investigated for a
mixture of 100 mL of model fuel and the previously optimized
mass of metal complexes. The efect of diferent temperatures
and time was studied under the selected conditions. All the
experiments were carried out at ambient atmospheric pres-
sure. Using the optimized conditions, the comparative catalytic
potential was estimated for the three synthesized complexes
CoL1, CoL2 and CoL3. Total sulfur content in the model oil
was estimated using the XRF analysis with the Lab-X3500,
Oxford Instruments, UK as per ASTM D4294 [35].
2.2.2 Synthesis of CoL1, CoL2 and CoL3 Complexes
Three cobalt(II)-salen complexes (CoL1, CoL2 and CoL3)
have been synthesized using ethanol as solvent for ligands
and water for metal salt then refuxing this mixture [34]. In
this typical procedure, 4.02 g (15 mmol) L1 was taken in a
round-bottom fask containing a stirring-bar, and ftted with
a refux condenser and 100 mL of ethanol was added under
nitrogen. In a separate fask, 3.73 g (15 mmol) cobalt(II)
acetate tetrahydrate was dissolved in 20 mL water with stir-
ring at room temperature under nitrogen. The ligand solu-
tion was added with the help of a syringe, and swirled at the
refuxing temperature for 3 h. The dark colour precipitate
formed which was fltered and dried in an oven at 70 °C.
The yield of N,N-bis(salicylidene)ethylenediaminocobalt(II)
(CoL1) obtained was 7.54 g. Similarly, the N,N-bis(3-tert-
butyl-2-hydroxybenzaldene) ethylenediaminocobalt(II)
(CoL2) and N,N-bis (3,5-di-tert-butyl-2-hydroxybenzal-
dene) ethylenediaminocobalt(II) (CoL3) were synthesized
by taking 5.71 g of L2 and 7.39 g of L3 respectively with
obtained yield as 7.83 g and 10.04 g, respectively.
1 3

D. Tripathi, R. K. Singh
by the CoL2 and CoL1 showing the 71, 56 and 53% DBT
removal, respectively. Therefore, CoL3 was chosen for fur-
ther optimization studies.
3 Results and Discussion
Since the metal like Cu complexes with salen (N,N′-
bis(salicylidene)ethylenediamine) and substituted salen
ligands have been reported to be used as oxidation catalysts
[36] so this study was undertaken using the cobalt salen and
substituted salen complexes as the ODS catalysts. Unsubsti-
tuted salen complexes are poorly soluble in organic solvents.
Side chains attached to the ethylene bridge or the benzene
rings may increase the solubility. The presence of bulky
groups near the coordination site is generally desirable, as it
enhances catalytic activity and prevents dimerization. Salen
ligands derived from 3,5-di-tert-butylsalicylaldehyde will
be useful because they fulfll both criteria, and tend to be
soluble even in non-polar solvents like dodecane.
Now the catalyst to sulfur and the oxidant (H2O2) to
sulfur mass proportion was optimized by doing the oxida-
tion experiments with diferent catalyst:DBT molar ratio
(Fig. 1A). It was observed that catalyst:DBT molar ratio
of 1:15 gave the best result with 74% DBT oxidation while
other parameters were kept constant (DBT:H2O2 molar ratio
was kept at 1:10 and reaction temperature, 40 °C; reaction
time, 2 h; acetonitrile:n-dodecane volume ratio = 1:1).
Agglomeration and clogging of catalyst at higher catalyst
dose didn’t allow DBT molecules to reach the active sites
which decreased the DBT oxidation efciency [37].
In the catalytic oxidative desulfurization, the employed
oxidant H2O2 produces the strong pro-oxidant species like
The complexes, cobalt(II)-N,N′-ethylenebis(salicylimine)
CoL1, cobalt(II)-N,N′-ethylenebis(3-tert-butylsalicylimine)
CoL2 and cobalt(II)-N,N′-ethylenebis(3,5-di-tert-
butylsalicylimine) CoL3 were synthesized via reaction of the
three tetradentate ligands L1, L2 and L3 with the cobalt(II)
acetate tetrahydrate. All the ligands and complexes were
characterized by using the elemental analysis, FT-IR, NMR,
UV–Vis, MALDI-TOF-MS and thermogravimetric analysis
and details are given in the supplementary information.
Cobalt complexes were reported to be employed as oxi-
dative desulfurization catalyst in several papers for DBT
oxidation [30–33] while acetonitrile was reported to be the
best solvent for extracting the polar sulfoxide and sulfone
from the model fuel oil [17]. Therefore, the same reaction
(Scheme 1) was used in order to perform the comparative
catalytic activity study of the synthesized complexes. The
experiments were performed on a DBT solution in n-dode-
cane (a model fuel compound). The H2O2 was taken as oxi-
dant. First, in order to screen the complex for the optimiza-
tion experiments among the three CoL1, CoL2 and CoL3,
n-dodecane solutions (100 mL) containing the 0.05 mmol
complexes, 5.0 mmol H2O2 and 0.5 mmol DBT was stirred
at 40 °C for 2 h under the ambient atmospheric pressure.
The 100 mL acetonitrile was added insitu as extractant too.
It was found that the complex CoL3 performed best followed
O
Cobalt(II)-salen
Cobalt(II)-salen
Fig. 1 Efect of various parameters on DBT oxidation efciency in
presence of CoL3 as catalyst. A catalyst:DBT molar ratio; DBT:H2O2
molar ratio=1:10, acetonitrile:n-dodecane volume ratio=1:1,
temperature=40 °C, time=120 min. B DBT:H2O2 molar ratio;
catalyst:DBT molar ratio=1:15; acetonitrile:n-dodecane volume
ratio=1:1, temperature=40 °C, time=120 min. C Acetonitrile:n-
dodecane volume ratio; catalyst:DBT molar ratio=1:15; DBT:H2O2
molar ratio=1:5, temperature=40 °C, time=120 min. D Tempera-
ture; catalyst:DBT molar ratio=1:15; DBT:H2O2 molar ratio=1:5,
acetonitrile:n-dodecane volume ratio=1:1, time=120 min
S
O
S
S
H2O2
H2O2
O
DBTO2
DBT
DBTO
Scheme 1 Oxidation of DBT by H2O2 in the presence of synthesized
cobalt complexes
1 3

Oxidative Desulfurization of Dibenzothiophene Using Cobalt (II) Complexes with Substituted…
hydroxyl (·OH) and peroxyl (HOO·) radicals. These radi-
cals oxidizes the DBT and convert it in to the polar DBT-
sulfoxide or DBT-sulfone species. Further, the increased
solubility of these oxidized species in the used polar sol-
vent like acetonitrile, they get extracted and removed.
Some undesirable reaction also exists like decomposition
of H2O2 to form oxygen and water. But these reactions
become favourable only at high catalyst dose so must be
avoided.
80
60
40
20
0
Without Catalyst
Cobalt acetate
CoL1
CoL2
CoL3
So further, keeping the catalyst:DBT molar ratio of
1:15, the optimization of the oxidant to sulfur propor-
tion was done by performing the experiments varying the
DBT:H2O2 molar ratios as 1:3, 1:5; 1:10 and 1:15 keeping
the other parameters same as above (Fig. 1B). It was found
that DBT:H2O2 molar ratio of 1:5 gave the best results and
no further increase was observed on increasing the H2O2
amount at DBT:H2O2 molar ratios of 1:10 and 1:15. At
higher H2O2 dose, recombination of H2O2 with hydroxyl
radical (·OH) is a high possibility by following sequence
of reaction to form peroxyl radical (HOO·), water and oxy-
gen [28]:
0
30
60
90
120
Time (min)
Fig. 2 Efect of time on DBT removal efciency by diferent cata-
lysts for DBT present in n-dodecane along with H2O2 at optimized
conditions. Experimental data shown by points and lines repre-
sent ftting of the nth order kinetic model. Experimental condi-
tions: catalyst:DBT molar ratio=1:15, DBT:H2O2 molar ratio=1:5,
acetonitrile:n-dodecane volume ratio=1:1, temperature=50 °C,
time=30 min
+∙HO2
H2O2 +∙ OH�������������������→�� HOO∙ ���������������������→�� O2 + H2O
efciency of 76, 66 and 63% for CoL3, CoL2 and CoL1
respectively. The reference metal salt (cobalt acetate)
shows very low activity as the DBT oxidation was found
to be very low. The DBT removal rate was also found to
be high for CoL3, as 64% DBT oxidation occurs in frst
15 min only which reached to 76% in 30 min and does
not exceeds further from this maximum limit. Therefore,
30 min is the optimum time to get the maximum oxidation.
During the DBT oxidation, catalyst is not lost and also
excess of H2O2 was used. Therefore the kinetic expression can
be represented by a simple power law rate equation avoiding
the catalyst and H2O2 dependence [38, 39]:
(1)
−H2O
Now keeping the catalyst:DBT molar ratio at 1:15 and
DBT:H2O2 molar ratio at 1:5, diferent volume ratio of
extractant (acetonirile) relative to the n-dodecane was also
tried while the temperature was maintained at 40 °C and
stirring was done for 2 h. The best results were observed
by using the 1:1 volume ration of acetonitrile:n-dodecane.
At higher extractant ration (as 2:1) the increase in the DBT
removal was insignifcant while at low extractant volume
ratios (as 1:2 and 1:1.5) the DBT oxidation was reduced
substantially (Fig. 1C).
Further experiments were performed at different
reaction temperatures ranging from 30 to 60 °C. It was
observed that the maximum DBT removal was achieved at
50 °C (Fig. 1D). At higher temperatures>50 °C, H2O2 gets
decomposed to form oxygen in place of hydroxyl radicals
[37], thus, reducing the availability of hydroxyl radical
(·OH) to oxidize DBT to DBT-sulfoxide or DBT-sulfone.
After optimization of various parameters like
catalyst:DBT molar ratio, DBT:H2O2 molar ratio,
acetonitrile:n-dodecane volume ratio and temperature,
the comparative catalytic potential for DBT oxidation in
n-dodecane was evaluated for all the synthesized com-
plexes CoL1, CoL2, and CoL3 with respect to time. The
experiments were also conducted without catalyst and
using cobalt acetate as reference compound. Results are
shown in Fig. 2 by data points. Under the optimized con-
ditions, the order of the catalytic activity was observed to
be CoL3 > CoL2 > CoL1 with a maximum DBT oxidation
dCt
−
= knCtn
(2)
dt
where, Ct is the residual concentration of DBT at time t
(mmol/L), n is the order of reaction and kn is reaction con-
stant for the nth order reaction. Considering, Co as the initial
concentration of DBT (mmol/L) at time t=0 min, Eq. 2 can
be integrated to obtain the following equation [40]:
1−n
Ct1−n C0
−
= (1 − n)
t
kn
(3)
Many authors reported ODS reaction to follow the pseudo-
frst-order kinetic model. Equation 2 can be integrated for
n=1 to yield:
(
)
C = C exp −
t
t
k1
(4)
o
1 3

D. Tripathi, R. K. Singh
Table 1 Percentage DBT oxidation and kinetics by diferent catalysts for DBT present in n-dodecane along with H2O2 at optimized conditions
Catalyst
DBT removal
nth order kinetics
Reaction Order
1st order kinetics
efciency (%)
kn
R2
ARE (%)
k1 (min−1
)
R2
ARE (%)
((mmol/L)(1−n)
/min))
Without Catalyst
Cobalt acetate
CoL1
CoL2
CoL3
38
51
64
66
76
6.8
7.1
5.1
3.7
3.3
1.760
13.441
13.068
0.946
0.930
0.968
0.763
0.962
0.977
3.91
4.73
14.37
7.63
0.0053
0.0118
0.0170
0.0179
0.0464
0.686
0.653
0.844
0.776
0.867
12.92
25.29
19.45
28.43
38.83
1.982
9.56
Experimental conditions: catalyst:DBT molar ratio=1:15, DBT:H2O2 molar ratio=1:5, acetonitrile:n-dodecane volume ratio=1:1, tempera-
ture=50 °C, time=30 min
ARE average relative error
where, k1 is the frst-order rate constant. Average relative
error function given by Eq. 5 was used to fnd the values of
n, kn and k1 by non-linear regression method.
and diesel samples respectively [32]. The maximum DBT
removal was also observed to be 85% from n-dodecane
(model fuel oil) using an oil soluble N-benzylated cobalt
phthalocyaninetetrasulfonamide but the catalyst’s multi-
step synthesis is complicated [33].
(
)
n
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢀ
i=1
−
Ct,exp,i Ct,cal,i
100
ARE =
(5)
n
Ct,exp,i
where, Ct,exp and Ct,cal are the experimental and calculated
value of DBT concentrations in mmol/L. Values of n, k1 and
kn along with coefcient of determination (R2) and ARE
values for both nth order and 1st order reaction models for
diferent catalysts are shown in Table 1. Values of R2 are
found to be closer to 1 and that the ARE values are lower for
nth-order kinetic model in comparison to 1st order kinetic
model. Thus, the nth-order kinetic model better represented
the kinetic data. The frst-order rate constant was highest
for CoL3 catalyst in comparison to other catalyst indicating
its efectiveness over other tested catalysts. The higher dis-
pensability of the complex CoL3 in the n-dodecane due to
the presence of four tertiary butyl substituent’s is the reason
for the higher activity along with the formation of suitable
electronic density around the active catalytic metal centre.
Since the synthesized cobalt complexes CoL1, CoL2 and
CoL3 were used as homogeneous ODS catalyst so catalyst
recovery and recyclability was not studied.
4 Conclusions
Three cobalt(II)-salen complexes (CoL1, CoL2 and CoL3)
were synthesized and studied as catalyst for the oxida-
tive desulfurization (ODS) reaction performed on DBT
(dibenzothiophene) taken in the fuel reference oil i.e.
n-dodecane using H2O2 as oxidant. The acetonitrile was
taken as in-situ extractant for oxidised DBT compounds
i.e. sulfoxides and sulfone. The reaction conditions were
optimized and it was found that best oxidation occurs at
catalyst:DBT molar ratio of 1:15, DBT:H2O2 molar ratio
of 1:5, acetonitrile:n-dodecane volume ratio of 1:1, reac-
tion temperature of 50 °C and reaction time of 30 min.
Among the three, CoL3 was found to be the most active
catalyst as at the optimized conditions with 76% DBT
removal. The presence of bulky tertiary butyl groups
near the coordination site enhances catalytic activity and
prevents dimerization and also increases solubility in the
organic solvent like dodecane. Kinetics of the removal pro-
cess was found to follow power law kinetic model. The
study indicated that this new ODS process with the help of
simple cobalt(II) complex having tertiary butyl substituted
salen type ligand has the potential to meet a future direc-
tives of sulfur content in the fuel.
As far as the comparison of our CoL3 catalyst with
the other oxidation system is concerned, although high
catalytic activity is reported to be achieved but with com-
plex systems e.g. high sulfur removal of 97% was achieved
from diesel fuel oil using cobalt acetate or chloride salt
and molecular oxygen as oxidant but aldehydes were also
used as sacrifcial materials [30]. In a similar system,
91.5% sulfur removal was achieved using cobalt phthalo-
cyaninetetrasulfonamide as catalyst [31]. The ultraviolet-
assisted oxidative desulfurization (UODS) using metal
complex of chitosan salicylal Schifs base, sulfur removal
of 98.34% and 97.17% was achieved from real gasoline
Author contributions DT carried out the experiment and drafted the
manuscript; RKS helped in conceiving the idea and data interpretation.
1 3

Oxidative Desulfurization of Dibenzothiophene Using Cobalt (II) Complexes with Substituted…
21. Xu D, Zhu W, Li H, Zhang J, Zou F, Shi H, Yan Y (2009) Energy
Compliance with Ethical Standards
Fuels 23:5929–5933
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148:42–48
Conflict of interest All authors declare that they have no confict of
interest.
23. Anisimov AV, Fedorova EV, Lesnugin AZ, Senyavin VM,
Aslanov LA, Rybakov VB, Tarakanova AV (2003) Catal Today
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Afliations
Deependra Tripathi1,2 · Raj K. Singh2
2
* Deependra Tripathi
Analytical Science Division, CSIR-Indian Institute
dtripathichem@gmail.com; dtripathi@cy.iitr.ac.in
of Petroleum, Dehradun 248005, Uttarakhand, India
1
Department of Chemistry, Indian Institute of Technology
Roorkee, Roorkee 247667, Uttarakhand, India
1 3