Selective Catalytic Oxidation of Organic Sulfides to Sulfoxides without Forming Sulfones over Solid Molybdenum Blue: Kinetic and Thermodynamic Studies

Article abstract of DOI:10.14233/ajchem.2020.22789

The present investigation reports studies on the selective catalytic oxidation of organic sulfide substrates over molybdenum blue catalyst supported on boron phosphate. The catalyst was synthesized through partial precipitation method and characterized by

Full text of DOI:10.14233/ajchem.2020.22789

Asian Journal of Chemistry; Vol. 32, No. 9 (2020), 2267-2274
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2020.22789
Selective Catalytic Oxidation of Organic Sulfides to Sulfoxides without Forming
Sulfones over Solid Molybdenum Blue: Kinetic and Thermodynamic Studies
*,
P. RATHESHKUMAR, S. INDUJA and P.S. RAGHAVAN
Department of Chemistry, Hindustan Institute of Technology and Science, Padur, Kelambakkam, Chennai-603103, India
*Corresponding author: Fax: +91 44 27476208; Tel: +91 44 27474262; E-mail: raghavan@hindustanuniv.ac.in
Received: 21 April 2020;
Accepted: 4 June 2020;
Published online: 20 August 2020;
AJC-20024
The present investigation reports studies on the selective catalytic oxidation of organic sulfide substrates over molybdenum blue catalyst
supported on boron phosphate. The catalyst was synthesized through partial precipitation method and characterized by XRD, FTIR and
SEM techniques. The sulfoxidation was carried out in a batch reactor using benzyl phenyl sulfide as the substrate over the present catalyst
and the reaction parameters were varied and optimized. The results were compared with the MoO3 impregnated boron phosphate. The
catalyst was also studied for its performance over other sulfide substrates and the results were compared with available studies in literature.
The reaction followed pseudo first-order kinetics and rate of the reaction under optimized condition was 10.1 × 10-3 min-1, with energy of
activation of 29.3 kJ/mol. The Mo-O-Mo bridging and -Mo=O bonds present in molybdenum blue were participating in the reaction and
possible mechanism has been proposed. The 100% selectivity of the product towards sulfoxide has been attributed to the big-wheel
structure of molybdenum blue as it sterically hinders further reaction of sulfoxides formed to sulfones.
Keywords: Molybdenum blue catalyst, Boron phosphate, Sulfoxidation, Thermodynamic studies.
1.5 equivalent of catalyst leading to the formation of sulfoxide
and by taking 4 equivalents of oxidant and 15 equivalents of
the catalyst, the process resulted in the formation of sulfones
[20]. Gamelas et al. [21] got sulfoxides by using 1 equivalent
of H2O2 and sulfones by using 2 equivalents of H2O2 [21]. Mo(VI)
complex anchored on ferrous ferric oxide supported silica exhi-
bited controlled oxidation to form sulfoxides from phenyl methyl
sulfide with substrate to catalyst ratio of 1:1.8 using urea-H2O2
oxidant in dichloromethane-methanol solvent [22]. The yield
obtained was 79% within 30 min of reaction time.A recent report
on using Mo(VI) Schiff base complex supported on ferrous
ferric oxide nanoparticles under solvent free conditions showed
100% conversion of sulfides within 3 min of reaction time with
nearly 80% selectivity of sulfoxide [23]. In the present work,
molybdenum blue is made up of Mo154 big-wheel with Mo-
O-Mo bridging and terminal -Mo=O bonds containing Mo6+
and Mo5+ in the ratio of 5:1 [24]. The uniform distribution of
bridging molybdenum allows flipping of oxidation states, which
makes it as an efficient catalyst. Thus, the oxidation behavior
of molybdenum blue catalyst over various organic sulfides has
been studied and the results were discussed. The reaction para-
INTRODUCTION
Oxidation of organic sulfides to respective sulfoxides is
an industrially important reaction [1]. Sulfoxides are used in
fundamental research [2-8], active ingredients in the preparation
of drugs [9], valuable intermediates in the preparation of
chemicals and medication [10,11], antifungals [12,13], food and
cosmetics [14]. Conventionally, the oxidation of sulfides is
catalyzed by peracids as catalyst and halogen derivatives [15].
However, these processes are always associated with the formation
of byproducts, which are not environmentally safe. Selective
oxidation of sulfides to sulfoxides is always a challenge, as any
oxidation catalyst, generally lead to complete oxidation of sub-
strate forming sulfones.Various transition metals were evaluated
for the oxidation of sulfides [16-18]. Among the transition
metals, molybdenum based catalysts were widely used by different
research groups. MoO3 was used as the catalyst for oxidation
of various sulfides using H2O2 as oxidant in ethanol, which resulted
in formation of both sulfoxides and sulfones with nearly 98%
conversion in 12 min of reaction time [19]. Subsequently, Mo(VI)
salt was used for the oxidation with 1.05 equivalent of oxidant,
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2268 Ratheshkumar et al.
Asian J. Chem.
meters were varied and studied in order to find the kinetics and
thermodynamics of the reaction.
BPO4 (PDF:741169)
MoO3 (PDF:895108)
EXPERIMENTAL
(b) MoO3-BP
Molybdenum blue (MB) catalyst was prepared by adopting
the procedure reported in literature [25]. The typical procedure
involves mixing calcium carbonate and ammonium hepta moly-
bdate in the ratio of CaO:MoO3 = 1:0.08, followed by addition
of phosphoric acid in the ratio of CaO:P2O5 = 1:1.1 and mixed
thoroughly. The contents were dried, calcined at 500 ºC for 4 h
and labeled as MB-BP. MoO3 impregnated with borophosphate
was prepared by taking ammonium heptamolybdate and boron
phosphate in the weight ratio MoO3:BPO4 of 1:10, mixing thor-
oughly in presence of acetone, followed by drying, calcination
at 500 ºC for 4 h and labeled as MoO3-BP.
(a) MB-BP
5
15
25
35
45
55
The PANalytical X-PERT3 instrument was used to collect
XRD patterns for powders, the SEM images were captured
using JEOL JSM-7401F instrument. The FTIR spectra were
recorded in Perkin-Elmer, Spectrum RXi instrument.
The reaction was carried out in a batch reactor by taking
substrate (benzyl phenyl sulfide, 50 mg) in 2 mL of a suitable
solvent. A 5 mg of catalyst was added along with hydrogen
peroxide as oxidant (30%, 1 equivalence) and stirred at room
temperature. The reaction was monitored by analyzing the form-
ation of product (Scheme-I) as well as the decrease of substrate
in the reaction mixture using TLC and HPLC . The formation
of product (sulfoxide) was further confirmed by recording mass
spectra.
2θ (°)
Fig. 1. XRD patterns of (a) MB-BP catalyst and (b) MoO3-BP impregnated
catalyst
4000
3500
3000
2500
Wavenumber (cm–1)
Fig. 2. FTIR spectra of MB-BP catalyst
2000
1500
1000
500
S
S
O
Benzyl(phenyl)sulfide
(Benzylsulfinyl)benzene
C13H12OS
C13H12
S
200.30
216.30
Scheme-I: Oxidation of sulfide to sulfoxide
RESULTS AND DISCUSSION
The XRD patterns of molybdenum blue and molybdenum
oxide impregnated samples are given in Fig.1. The molybdenum
blue has been reported as amorphous [26] and hence, no charact-
eristic phases were observed in the XRD analysis. The MB-BP
showed presence of pure boron phosphate phase (PDF No.
741169, Fig.1a) and the XRD pattern of MoO3-BP showed
presence of molybdenum oxide (PDF No. 895108, Fig. 1b) and
boron phosphate phases in the sample. FTIR spectra of MB-BP
showed absorptions at around 720 and 900 cm-1 corresponding
to -Mo-O and Mo-O-Mo vibrations, respectively [25] (Fig.2).
The SEM image of MB-BP showed the deposition of molyb-
denum blue particles over boron phosphate matrix (Fig. 3).
The oxidation of sulfoxide involves activation of hydrogen
peroxide as the first step. The cleavage of peroxide linkage in
H2O2 can take place via two types; either by formation of HO•
radicals through homolytic fission or by formation of water
Fig. 3. SEM image of MB-BP catalyst
oxide by elimination of proton, which reacts with sulfide to
form sulfoxide [27]. Efficiency of the reaction depends on the
activation of peroxide.
Table-1 presents the initial studies over different catalyst,
by varying solvent and reaction time. The conversion of the
substrate is presented along with selectivity towards sulfoxide.

Vol. 32, No. 9 (2020)
Solvent
Selective Catalytic Oxidation of Organic Sulfides to Sulfoxides 2269
TABLE-1
INITIAL STUDIES ON THE CATALYST FOR OPTIMIZING THE REACTION CONDITIONS
Selectivity
(sulfoxide %)
Catalyst (mg)
H2O2 equivalence
Time (min)
Conversion (%)
Methanol
Methanol
Ethanol
MB-BP
MB-BP
MB-BP
MB-BP
MB-BP
MB-BP
MB-BP
MB-BP
MB-BP
1
1
1
1
1
1
1
1
1
1
1
1
30
120
30
120
30
120
120
120
120
30
95.36
98.00
94.96
4.50
94.74
9.60
11.20
0.80
1.40
100
100
100
100
100
100
100
100
100
80
Dichloromethane
Dichloromethane/methanol (1:1)
Acetonitrile
Acetone
Hexane
Toluene
Methanol
Methanol
MoO3-BP
MoO3-BP
Boron phosphate (blank)
40.00
58.00
5.00
120
180
55
90
Methanol
Substrate - Benzyl phenyl sulfide – 50 mg; Solvent – 2 mL; Catalyst – 5 mg.
The MB-BP catalyst exhibited 95% conversion within 30 min
of reaction time using methanol and ethanol as solvent. On using
dichloromethane, acetonitrile, hexane, toluene and acetone as
the solvents, the oxidation was poor with very low conversion
even after 120 min of reaction time.When, a mixture of dichloro-
methane and methanol was used as the solvent, again it showed
good activity. This proves that the reaction requires protonated
solvent, which favours sulfoxidation through water oxide mech-
anism. The plausible mechanism for the oxidation of sulfide
by MB-BP catalyst is shown in Scheme-II.
performance of the above two catalysts were studied by varying
reaction parameters.
Influence of catalyst content: Catalyst content with respect
to the substrate plays an important role in accelerating the
reaction. In certain cases, it decides the nature of product formed,
as reported by Kandasamy et al. [20] that with 10-fold increase
of catalyst leads to formation of 95% selectivity of sulfones,
which was 96% sulfoxide otherwise. In the present MB-BP
catalyst, when the catalyst content was changed from 0.5 mg
to 5 mg, the conversion was found to increase, with 100% selec-
tivity towards sulfoxide (Fig. 4). However, beyond 2 mg of
catalyst content, the reactivity was found to be nearly same.
The reaction reached steady state at 30 min, beyond which time;
the conversion did not change appreciably. The reactivity of
impregnated catalyst was found to be very low as compared to
that of MB-BP catalysts.
Oxidant content in the reaction mixture plays a vital role.
Relatively, less quantity of oxidant lead to incomplete reaction
and higher quantity lead to pollution to the environment, if
unreacted oxidant is released as such without treatment. In
addition, there are reports available that the oxidant content
alters the selectivity of the product. Generally, the conversion
increases with the increase in the oxidant concentration. In
order to study the influence of oxidant concentration, the catalyst
content was fixed as 1 mg; since, 5 mg catalyst generally leads
to completion of reaction within few minutes of reaction time.
The conversion was very low for 0.25 equivalence of H2O2
over MB-BP catalyst, which increased with the increase in
oxidant and reached steady state at 60 min of reaction time
(Fig. 5). The important observation made in the study was the
selectivity of sulfoxide was retained in spite of increasing oxidant
concentration. The impregnated sample showed low activity.
Kinetics studies: The rate of the reaction is given by decre-
ase in concentration of sulfide or increase in concentration of
sulfoxide with respect to time. The reaction takes place between
sulfide and H2O2 and hence, expected to follow second order
kinetics.
H
O
O
H
O
O
H
Mo
O
O
H
OH
O
O
O
Mo
O
Mo
Catalyst
H
O
O
S
-H2O
Sulphoxide
O
O
Mo
O
O
O
S
O
S
Sulfide
Scheme-II: Mechanism of sulfoxidation by MB-BP catalyst
In the literature, the formation of sulfones was observed
with the increase in oxidant equivalence, where sulfoxide formed
in the Step-I reacted again with the catalyst to form sulfone in
Step-II. In the presence catalyst, cluster of molybdenum blue
does not favour further reaction of sulfoxide due to steric hind-
rance. This can be inferred by the fact that irrespective of oxidant
equivalence or with reaction time, no sulfone was formed (entries
1 & 2, Table-1), while the impregnated sample (MoO3-BP)
showed 80% selectivity towards sulfoxide, which decreased with
the increase in reaction time (entries 10 & 11, Table-1). The
The second order rate equation can be given as:
d[sulfide]
−
= k[sulfide][oxidant]
dt

2270 Ratheshkumar et al.
Asian J. Chem.
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
(a)
(b)
0.5 mg
1 mg
2 mg
3 mg
5 mg
0.5 mg
1 mg
2 mg
3 mg
5 mg
0
50
100
150
0
50
100
150
Time (min)
Time (min)
Fig. 4. Oxidation of benzyl phenyl sulfide: influence of catalyst content over (a) MB-BP catalyst (b) MoO3-BP catalyst
100
90
80
70
60
50
40
30
20
10
0
100
90
(b)
(a)
80
70
60
50
40
30
20
10
0
0.25 eq
0.50 eq
0.75 eq
1.0 eq
0.25 eq
0.50 eq
0.75 eq
1.0 eq
1.5 eq
1.5 eq
0
50
100
150
0
50
100
150
Time (min)
Time (min)
Fig. 5. Oxidation of benzyl phenyl sulfide: influence of oxidant content over (a) MB-BP catalyst and (b) MoO3-BP catalyst
which yields
pseudo first-order kinetics and the equation can be given as
follows:
1
1
=
+ kt
d[sulfide]
[sulfide] [sulfide]o
−
= k[sulfide]
dt
A plot of 1/[sulfide]t vs. time 't' should give a straight line.
But, when the data were fitted, straight line was not obtained,
which infers that the reaction do not proceed through second
order kinetics, which might be due to fact that activation of
H2O2 being the rate determining step, the reaction may follow
which yields
ln [sulfide]t = ln [sulfide]0 – kt
A plot of ln[sulfide]t vs. 't' should give a straight line and the
same was obtained for MB-BP catalyst. Fig. 6 shows the first
5
5
0.5 mg; y = -0.000x + 4.602
0.5 mg; y = -0.002x + 4.412
4
4
1.0 mg; y = -0.001x + 4.341
1.0 mg; y = -0.006x + 4.218
2.0 mg; y = -0.001x + 4.613
3
3
2
1
0
2.0 mg; y = -0.007x + 3.662
3.0 mg; y = -0.009x + 1.839
5.0 mg; y = -0.010x + 1.747
(a)
(b)
3.0 mg; y = -0.002x + 4.573
5.0 mg; y = -0.003x + 4.181
2
1
0
0
20
40
60
80
100 120 140
0
50
100
150
Time (min)
Time (min)
Fig. 6. Pseudo first order kinetics graph at different catalyst content for (a) MB-BP catalyst (b) MoO3-BP catalyst

Vol. 32, No. 9 (2020)
Selective Catalytic Oxidation of Organic Sulfides to Sulfoxides 2271
TABLE-3
order kinetics plot of the catalyst with different concentrations
and the rate was calculated from the slope.
RATE OF REACTION WITH VARYING
OXIDANT (H2O2) EQUIVALENCE
The rate of the reaction was calculated from slope of the
straight line for different catalyst content and is given in Table-
2. The rate of the reaction was found to increase with the increase
in MB-BP and MoO3-BP catalysts (Fig.7). However, the rate
of the reaction was found to be linear beyond 1 mg of catalyst
content for MB-BP and the performance was around 4 times
faster than the impregnated catalyst.
Rate × 10-3 (min-1)
H2O2 (equivalence)
MB-BP
0.1
MoO3-BP
0.09
0.25
0.50
0.75
1.00
1.50
1.1
1.6
6.2
24.1
0.09
1.30
1.40
2.10
TABLE-2
0.030
RATE OF REACTION WITH VARYING CATALYST CONTENT
Rate × 10-3 (min-1)
MB-BP
MoO3-BP
Cat. wt. (mg)
0.025
0.020
0.015
0.010
0.005
0
MB-BP
2.1
MoO3-BP
0.11
0.5
1.0
2.0
3.0
5.0
6.2
7.1
9.0
10.1
1.20
1.50
2.40
3.50
0.012
0.010
0.008
0.006
0.004
0.002
0
MB-BP
MoO3-BP
0
0.5
1.0
1.5
Oxidant equivalence
Fig. 9. Variation of rate with respect to oxidation equivalence over (a) MB-
BP catalyst and (b) MoO3 catalyst
Thermodynamic studies: In general, with the increase
in temperature, the rate of the reaction and conversion tends
to increase. The influence of temperature on the oxidation of
sulfide over MB-BP catalyst was studied between 5 ºC and 60
ºC (catalyst wt. = 1 mg; H2O2: 1 eq.). With an increase in the
temperature, the conversion increased and steady state was
attained at 60 min of reaction time, except of the reaction that
was carried out at 60 ºC, where the conversion was found to
increase linearly (Fig. 10a).
The rate of the reaction at different temperatures was calcu-
lated by substituting the data in first order rate equation (Table-
4) (Fig. 10b), to find the thermodynamic parameters of catalyst.
A linear increase in the rate of the reaction with temperature
was observed (Fig. 11).
0
1
2
3
4
5
6
Catalyst weight (mg)
Fig. 7. Variation of rate of the reaction with catalyst content
Influence of oxidant content over the rate of the reaction
was studied by varying oxidant equivalents to substrate from
0.25 to 1.5 over MB-BP catalyst and the plot of substrate content
with respect to time allows us to calculate the rate of the reac-
tion (Fig. 8) and the same is presented in Table-3. A plot of rate
vs. oxidant content shows that upto 0.75 equivalence of H2O2
content, the rate did not change appreciably, but on further
increase in H2O2 content, an exponential increase in the rate
was observed (Fig. 9). The impregnated catalyst showed a liner
increase in the rate with respect to oxidant content.
5
4.7
0.25 eq; y = -0.000x + 4.600
0.25 eq; y = -0.000x + 4.37
4
0.50 eq; y = -0.000x + 4.609
4.6
0.50 eq; y = -0.001x + 4.149
0.75 eq; y = -0.001x + 4.606
0.75 eq; y = -0.001x + 4.193
1.0 eq; y = -0.006x + 4.218
1.5 eq; y = -0.024x + 4.334
3
2
1
0
4.5
1.0 eq; y = -0.001x + 4.613
1.5 eq; y = -0.002x + 4.652
4.4
4.3
4.2
(a)
(b)
0
20
40
60
80
100 120
0
20
40
60
80
100 120
Time (min)
Time (min)
Fig. 8. First order kinetics: variation of oxidant over (a) MB-BP catalyst and (b) MoO3-BP catalyst

2272 Ratheshkumar et al.
Asian J. Chem.
5
4
3
2
1
0
100
90
80
70
60
50
40
30
20
10
0
5 °C
15 °C
30 °C
45 °C
60 °C
5 °C; y = -0.002x + 4.486
15 °C; y = -0.005x + 4.477
30 °C; y = -0.006x + 4.218
45 °C; y = -0.011x + 4.350
60 °C; y = -0.020x + 4.717
(a)
(b)
0
50
100
150
0
50
100
Time (min)
Time (min)
Fig. 10. Oxidation of sulfides over MB-BP catalyst (a) influence of temperature (b) first order kinetics plot
TABLE-4
The energy of activation for the reaction was obtained by
plotting rate of the reaction with 1/T according to the following
Arrhenius equation:
RATE OF THE SULFOXIDATION OVER MB-BP
CATALYST AT DIFFERENT TEMPERATURES
Temp. (°C)
Rate × 10-3 (min-1) MB-BP
Ea
ln(k) = ln(A) −
RT
5
2.2
5.3
6.2
11.1
20.2
15
30
45
60
A slope of the straight line of the activation energy (∆G+)
of catalyst for reaction was determined (Fig.12a). By plotting
ln (k/T) vs. 1/T, a straight line was obtained and from the slope,
H+ for the reaction was determined [28] (Fig.12b). By substi-
tuting values of ∆G+ and ∆H+, the value of ∆G+ for the reaction
was obtained.
0.025
0.020
0.015
0.010
0.005
0
The energy of activation was found to be 29.3 kJ/mol, the
H+ for the catalyst was 26.77 kJ/mol and the entropy change
∆S+ of the catalyst was 8.48 J K-1. For the oxidation of various
sulfides to sulfoxdes, the activation energy was reported in the
range of around 40-80 kJ/mol [27]. The presence catalyst was
very much efficient in bringing down the activation energy to
around 29.3 kJ/mol.
Finally, the reactivity of MB-BP catalyst was evaluated
for various substrates and the results were compared with other
catalyst reported in literature. A report on the reaction condi-
tions along with the conversions are presented in Table-5. It is
evident from the above substrate studies that MB-BP catalyst,
effectively converts most of the organic sulfides to their
respective sulfoxides, without forming sulfones.
0
20
40
Temperature (°C)
60
80
Fig. 11. Oxidation of sulfides over MB-BP catalyst: variation of rate with
temperature
0
0
0.0028
-1
0.0030
0.0032
0.0034
0.0036
0.0038
0.0028
-2
0.0030
0.0032
0.0034
0.0036
0.0038
-4
-6
-2
-3
-4
-5
-6
-7
y = -3524.x + 6.632
y = -3220.x – 0.085
-8
(a)
(b)
-10
-12
-14
1/T
1/T
Fig. 12. Oxidation of sulfide over MB-BP catalyst, (a) Arrhenius plot and (b) ln (k/T) vs. 1/T plot

Vol. 32, No. 9 (2020)
Selective Catalytic Oxidation of Organic Sulfides to Sulfoxides 2273
TABLE-5
OXIDATION OF VARIOUS SULFIDE SUBSTRATES OVER DIFFERENT CATALYSTS
Oxidant
equivalence
Time
(min)
Selectivity of
sulfoxde (%)
Reaction
Catalyst
MB-BP
Conv. (%)
99
Ref.
–
O
S
S
1
1
30
100
>80
CH3
CH3
TaC/NbC
MB-BP
165
99
99
[[2299]]
–
Thioanisole
COOH
COOH
S
S
1
30
100
O
3-(Benzylthio)propanoic acid
O
S
S
MB-BP
1
1
60
99
99
100
99
–
CH3
CH3
Br
Br
Rh(II) complex
>180
[[3300]]
(4-Bomophenyl)(methyl)sulfide
O
S
MB-BP
1.1
1
60
99
75
100
75
–
CH3
S
CH3
O2N
O2N
Mo complex
>180
[21]
[21]
Methyl(4-nitrophenyl)sulfide
O
S
MB-BP
1.2
3
60
99
75
100
75
–
S
WO4-SiO2
>150
[31]
[31]
iso-Propyl(phenyl)sulfide
O
S
MB-BP
MoO3
1.1
1
60
45
99
94
100
6
–
S
[[1199]]
Allyl(phenyl)sulfide
5. N. Iranpoor and B. Zeynizadeh, Synthesis, 49 (1999);
Conclusion
https://doi.org/10.1055/s-1999-3693
Oxidation of benzyl phenyl sulfide was studied over solid
molybdenum blue deposited over boron phosphate. The Mo-
O-Mo bridging bonds and terminal Mo=O bonds bought the
reaction. It is found that the presence of Mo6+ and Mo5+ oxid-
ation states in molybdenum blue favours the oxidation process.
Moreover, the activation of H2O2 is the crucial step for the oxid-
ation process. The selectivity of the product namely, sulfoxide
was 100%, irrespective of the oxidant and catalyst concentration,
which was mainly due to the structure of Mo clusters in molyb-
denum blue. The catalyst could oxidize various other sulfide
substrates to respective sulfoxides in a maximum time of 60
min at room temperature.
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