Selective oxidation of bulky organic sulphides over layered titanosilicate catalysts

Article abstract of DOI:10.1039/c5cy02083b

Selective oxidation of sulphides is a straightforward method of preparation of organic sulphoxides and sulphones, which are important chemical intermediates and building blocks of pharmaceuticals and agrochemicals. Oxidation of methylphenyl sulphide (MPS), diphenyl sulphide (Ph₂S), and dibenzothiophene (DBTH) over lamellar titanosilicate catalysts with the MFI and UTL-derived topology was investigated with hydrogen peroxide as the oxidant. Lamellar titanosilicates combine the advantages of crystalline zeolites and mesoporous molecular sieves due to accessible active sites located on the external surface of their layers. The selectivity of the MPS oxidation to methylphenyl sulphoxide is driven by the diffusion restrictions in the catalyst. A methylphenyl sulphoxide selectivity of 95% at 40% conversion was achieved using the Ti-IPC-1-PI catalyst together with an outstanding TON_tot = 1418 after 30 min. The selectivity can be adjusted also by dosing of the oxidant to keep its concentration low during the reaction. The silica-titania pillared TS-1-PITi catalyst showed the highest potential of the tested catalysts in oxidative desulphuration, easily oxidising the DBTH to dibenzothiothene sulphone.

Full text of DOI:10.1039/c5cy02083b

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Page 1 of 34
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Selective oxidation of bulky organic sulphides over layered
titanosilicate catalysts
Jan Přech1*, Russell E. Morris2, Jiří Čejka1
J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,
v.v.i., Dolejškova 3, CZ-182 23 Prague 8, Czech Republic
2EaStCHEM School of Chemistry, University of St Andrews, St Andrews KY16 9ST, UK
*Corresponding author, e-mail: jan.prech@jh-inst.cas.cz, phone: +420 266 053 856
Abstract
Selective oxidation of sulphides is a straightforward method of preparation of organic sulphoxides
and sulphones, which are important chemical intermediates and building blocks of pharmaceuticals
and agrochemicals. Oxidation of methylphenyl sulphide (MPS), diphenyl sulphide (Ph2S), and
dibenzothiophene (DBTH) over lamellar titanosilicate catalysts with the MFI and UTL-derived
topology was investigated with hydrogen peroxide as the oxidant. Lamellar titanosilicates combine
the advantages of crystalline zeolites and mesoporous molecular sieves due to accessible active sites
located on the external surface of their layers. The selectivity of the MPS oxidation to methylphenyl
sulphoxide is driven by the diffusion restrictions in the catalyst. A methylphenyl sulphoxide selectivity
of 95% at 40% conversion was achieved using the Ti-IPC-1-PI catalyst together with an outstanding
TONtot = 1418 after 30 min. The selectivity can be adjusted also by dosing of the oxidant to keep its
concentration low during the reaction. The silica-titania pillared TS-1-PITi catalyst showed the highest
potential of the tested catalysts in oxidative desulphuration, easily oxidising the DBTH to
dibenzothiothene sulphone.
Keywords
Oxidation, catalysis, sulphide, lamellar titanosilicate, layered TS-1, hydrogen peroxide
1. Introduction
Selective oxidation is one of the important reactions in organic synthesis and selective oxidation of
sulphides is a straightforward method of preparation of organic sulphoxides and sulphones. These
compounds are known to be valuable intermediates1 and building blocks of pharmaceuticals2 and
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agrochemicals3. Furthermore, oxidative desulphuration can represent an alternative route to
hydrodesulphuration (HDS); especially, where the sulphur compounds are difficult to remove using
the conventional HDS4.
5
Although there is a palette of different oxidants (e.g. peroxides, NaOCl, H5IO6, KMnO4 , peroxynitrous
acid6) and homogenous catalysts (e.g. complexes of titanium7, vanadium8, tungsten9 and zinc10,
borax11), which are able to selectively oxidize sulphides to sulphoxides and sulphones, from the
engineering point of view it is always convenient to work with a heterogeneous catalyst. Considering
heterogeneous oxidation catalysts, titanium based catalysts are one of the first choices. Titanosilicate
zeolites are well established catalysts of olefin epoxidation12, 13, 14, 15, oxidation of alkanes to alcohols
and ketones16, oxidation of aromatic hydrocarbons to phenols17, oxidation of phenols to quinones18
ammoxidation19, 20 and oxidation of amines to hydroxylamines21 with hydrogen peroxide and organic
hydroperoxides. Last but not least, the titanosilicate ability to catalyse the oxidation of thioethers to
sulfoxides is also known22. The use of H2O2 as the oxidant is highly convenient from both the
economic and environmental point of view because it has high content of the active oxygen and the
only by-product is water.
Hulea et al. demonstrated that titanosilicates Ti-BEA and Ti-MFI (TS-1) are active catalysts in the
sulphoxidation of dialkyl thioesters with hydrogen peroxide22. Thioesters bearing short linear
substituents (methyl, ethyl, n-butyl) were oxidised rapidly (conversion over 75% after 2 h at 30°C).
However, the diffusion limitations of the conventional titanosilicates manifested in the case of
diphenyl sulphide (conversion 5 – 38 % after 2h at 30°C, depending on the solvent and catalyst) and
methyl-tert-butyl sulphide (conversion over TS-1 46% after 2h at 30°C). Corma et al. reported
oxidation of methylphenyl sulphide and methyl-iso-pentyl sulphide over Ti-BEA and Ti-MCM-41 (a
mesoporous catalyst) with both hydrogen peroxide and tert-butylhydroperoxide23. The Ti-MCM-41
provided higher conversion (30% after 2h) of methyl-iso-pentyl sulphide then the Ti-BEA (18%)
although for methylphenyl sulphide it was vice versa (80% vs. 95% after 0.5 h), confirming the
importance of diffusion limitations influence on the reaction. Trukhan et al. compared Ti-MCM-41
with other mesoporous titanosilicate Ti-SBA-15 demonstrating the largest disadvantage of the
mesoporous titanosilicates: low titanium dispersion and high thickness of the wall, which leads to
part of the titanium centres remaining inaccessible24. Recently, Kon et al. investigated dialkyl and
alkyl-aryl sulphides over Ti-MWW (titanosilicate analog of MCM-22) material and Ti-IEZ-MWW
(interlayer expanded zeolite). Especially for the bulkiest diphenyl sulphide, the decrease in the
diffusion restrictions provided by the interlayer expansion was clearly revealed (conversion over Ti-
MWW 36% vs. Ti-IEZ-MWW 64% after 18 h at 40°C)25. Encouraged by these findings, we turned our
2

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attention towards the use of lamellar titanosilicate catalysts with MFI (layered and pillared TS-1) and
UTL-derived (denoted Ti-IPC-1-PI) topology as catalysts of the bulky sulphides oxidation.
In general, lamellar materials combine the advantages of crystalline zeolites and mesoporous
molecular sieves. They possess well-defined active centres, characteristic of highly crystalline
materials, on one hand and they provide easy accessibility of the active sites located on the outer
surface of the layers on the other hand. There are three basic approaches to the preparation of
lamellar zeolitic materials26: (i) Some zeolites (e.g. MWW, FER) form lamellar precursors, which
condense into fully connected zeolitic framework upon calcination.27, 28 (ii) Lamellar zeolites (e.g.
MFI) can be prepared using specially designed templates, which restrict the crystal growth in one of
the crystallographic directions.29 (iii) Some germanosilicate zeolites (e.g. UTL, IWW) have the form of
dense silica layers connected by double 4-ring (D4R) units. Germanium is preferentially located in
these D4R units making centres of instability in the framework. The germanium D4R can be
hydrolysed under mild acidic medium leaving the crystalline silica layers of a unit cell thickness
intact.30, 31
In this paper, we report on oxidation of various bulky sulphides (methylphenyl sulphide, diphenyl
sulphide, dioctyl sulphide and dibenzothiophene) over lamellar crystalline titanosilicate catalysts
under mild conditions with hydrogen peroxide as the oxidant. The discussed catalysts are synthesized
by the latter two methods of lamellar zeolite preparation i.e. surfactant templated synthesis32
(layered and pillared TS-1) and D4R units hydrolysis33 (Ti-IPC-1-PI). The results are compared with
standard TS-1 and Ti-UTL catalyst.
2. Experimental
2.1. Synthesis of conventional TS-1
Conventional titanosilicates TS-1 (TS-1) were prepared from a gel with initial Si/Ti ratio 43 using the
procedure originally described in Ref. 34 with slight modifications. Tetrabutyl orthotitanate (TBOTi,
Aldrich, 97%) and tetraethyl orthosilicate (TEOS, Aldrich, 98%) were used as the titanium and silica
source, respectively. Tetrapropylammonium hydroxide (Aldrich, 40 wt. % in water) served as a
structure directing agent (SDA). The initial gel composition was 100 TEOS: 2.32 TBOTi : 35 SDA : 4000
H2O. Materials with two different crystal sizes were obtained by adjusting the hydrothermal
crystallization time to 70 h (TS-1 (200)) or 94 h (TS-1 (600)) in a 90 ml Teflon-lined autoclave at 175°C
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under agitation. The final zeolite was centrifuged, washed with water, dried at 80°C and finally
calcined in air, at 550°C for 8 h, using a temperature ramp of 1°C/min.
2.2. Synthesis of layered and pillared TS-1
The synthesis of parent layered TS-1 was carried out according to the Na et al. procedure32 from
TEOS and TBOTi, using surfactant SDA C18H37 -N+(CH3)2 -C6H12 -N+(CH3)2 -C6H13 in hydroxide form (C18-6-
6OH2; prepared as described in the literature32). The TBOTi was added dropwise into TEOS and stirred
for 30 minutes. Then an aqueous solution of the SDA was added to the mixture. The synthesis
mixture with initial composition 100 TEOS: 2.5 TBOTi : 6 C18-6-6OH2 : 5000 H2O was hydrolysed at 60°C
for 3 h. The evaporated mass of ethanol and water were replaced with the same mass amount of
water at the end. The final gel hydrothermally crystallized in a 90 ml Teflon-lined autoclave at 160°C
for 236 h under agitation. After the given time, the zeolite was filtered off, washed with water, dried
at 80°C and finally calcined or subjected to the pillaring treatment. Calcination was carried out at
570°C for 8 h, using a temperature ramp of 1°C/min.
The silica and silica-titania pillaring procedures for the layered TS-1 were performed using our
previously reported procedures35. Dry as-synthetized layered TS-1 was dispersed in TEOS (TS-1-PISi)
or a mixture of TEOS and TBOTi (TS-1-PITi) in Si/Ti molar ratio 60 (10 g of the TEOS/1 g of the zeolite)
and stirred at 65°C for 24 h. Then the mixture was centrifuged and the solid material was dried for 48
h at room temperature. The dry product was hydrolysed in water with 5% of ethanol (100 ml/1 g) at
ambient temperature for 24 h under vigorous stirring. At the end, the zeolite was centrifuged again,
dried at 65°C and calcined in air, at 550° C for 10 h, using the temperature ramp of 2° C/min.
2.3. Synthesis of Ti-UTL and related Ti-IPC-1-PI and Ti-IPC-2
The preparation procedures for the Ti-UTL, Ti-IPC-1-PI and Ti-IPC-2 are described and discussed in
our previous paper36. The initial gel for the Ti-UTL was prepared from Cab-O-Sil M5 silicon oxide
(Havel Composites, Czech Republic), TBOTi, germanium oxide (Alfa Aesar, Germany, 99.999%),
distilled water, and (6R,10S)-6,10-dimethyl-5-azoniaspiro[4.5]decane hydroxide in the role of an SDA
(the SDA preparation is described in Ref. 37). Cab-O-Sil M-5 and GeO2 were added to an aqueous
solution of the SDA under stirring. After 30 minutes of homogenization, TBOTi,dilluted 1:3 with 1-
butanol, was added dropwise and the gel was stirred for another 30 minutes. The initial molar
composition of the synthesis gel was 2 TBOTi : 50 GeO2 : 50 SDA : 100 SiO2 : 3750 H2O. The zeolite
crystallized in a 90-ml Teflon-lined autoclave at 175°C for 420 h under agitation. The final product
was filtered, washed with water, dried at 85°C and calcined in air. The temperature program of the
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calcination was 200°C for 2 h, then 350°C for 4 h and finally 550°C for 8h, using a temperature ramp
of 2°C/min.
The Ti-IPC-1-PI33 and Ti-IPC-230 materials were prepared by the post-synthesis modification of the
calcined Ti-UTL. The Ti-UTL was converted into lamellar precursor Ti-IPC-1P by hydrolysis of the
germanium D4R units with 0.01 M HCl (250ml/g) at 75°C for 16 h under agitation. The resulting
lamellar precursor Ti-IPC-1P was collected by filtration, washed with water, dried at 65°C and
subjected to the pillaring to form Ti-IPC-1PI or silylation to form Ti-IPC-2.
To prepare the Ti-IPC-1PI, the precursor Ti-IPC-1P was swollen with a solution of
cetyltrimethylammonium hydroxide (CTMA OH, 25 wt. % in water, 30 g/g of the zeolite). The swelling
occurred at room temperature for 24 h under stirring. The swollen product was centrifuged, washed
with water and dried at 65°C. The pillaring was done in the same way as for the layered TS-1. Pure
TEOS was used only (10ml/g of the swollen Ti-IPC-1P, vide supra).
The Ti-IPC-2 catalyst was prepared by silylation of the Ti-IPC-1P with diethoxydimethylsilane in 1M
HNO3 in a 25-ml Teflon-lined autoclave at 175°C for 16 h without agitation. Typically, 10 ml of the
HNO3 solution and 0.5 g of the diethoxydimethylsilane were used per 1 g of the Ti-IPC-1P. A strongly
hydrophobic product was collected by filtration, dried and calcination at 550°C for 8 h using a
temperature ramp of 2°C/min.
2.4. Characterization techniques
X-ray powder diffraction (XRD) patterns were collected using a Bruker AXS D8 Advance
diffractometer equipped with a graphite monochromator and a position sensitive detector Våntec-1
using CuKα radiation in Bragg–Brentano geometry. Data were collected in continuous mode over the
2θ range of 1-40° for the lamellar materials and 2θ range of 5-40° for the conventional zeolites with a
step size of 0.00853° and time per step 0.25 s.
The textural properties were determined from nitrogen sorption isotherms. The isotherms were
measured at liquid nitrogen temperature (-196°C) with Micromeritics Gemini volumetric instrument.
Prior to the sorption measurements, individual zeolites were degassed in a stream of helium at 300°C
for 3 h.
BET area was evaluated using adsorption data in the range of a relative pressure from p/p0 = 0.05 to
p/p0 = 0.20. The t-plot method38 was applied to determine the volume of micropores (Vmicro) and
external surface area (Sext). The adsorbed amount of nitrogen at p/p0 = 0.95 reflects the total
adsorption capacity (Vtotal) of the material.
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DR-UV/Vis absorption spectra were collected using Perkin-Elmer Lambda 950 Spectrometer with
2mm quartz tube and large 8 x 16 mm slit. The data were collected in the wavelength range of 190-
500 nm. All the samples were analyzed after calcination.
Chemical composition of the materials (expressed hereafter as a Si/Ti ratio) was determined by ICP-
OES ThermoScientific iCAP 7000 instrument.
2.5. Catalytic tests
The oxidations of methylphenyl sulphide (MPS, Aldrich, 99 %) were carried out in a 25 ml
magnetically stirred glass three-necked round bottom flask equipped with a Dimroth condenser at
30°C. Typically, 8 mmol of the sulphide was dissolved in 10 ml of acetonitrile (Fisher chemical, HPLC
grade) together with 250 µl of 1,3-diisopropylbenzene (internal standard, Fluka, 95%) and 50 mg of
the catalyst were introduced into the mixture. For MPS, this setup corresponds to substrate/catalyst
mass ratio S/C = 20. The mixture was heated to the reaction temperature and the reaction was
started by addition of H2O2 aqueous solution (Aldrich, 35 wt. %). Typically, the sulphide/H2O2 molar
ratio was 2. In the experiment including the isolation of the product, the reaction was performed
without the internal standard. When the reaction was terminated, the catalyst was removed by
centrifugation and the solvent was evaporated. The mixture was transferred into ethyl acetate, dried
over MgSO4 and purified by column chromatography on Silicagel 60 (Merck) using ethyl
acetate/hexane 1:3 as an eluent.
Experiments under the solvent free conditions followed a procedure used in ref. 25. The MPS
oxidation was carried out in a 10 ml glass reactor with magnetic stirring (1000 rpm) at 25°C. Typically,
50 mg of the catalyst, 5 mmol of MPS and 5-6 mmol of the H2O2 aqueous solution were used. 1,3-
diisopropylbenzene served as the internal standard.
The oxidations of diphenyl sulphide (Ph2S, Aldrich, 98 %) and dibenzothiophene (DBTH, Aldrich, 98 %)
were carried in a similar setup at 40°C. In the case of Ph2S, 4 mmol of Ph2S were mixed with 10 ml of
acetonitrile, 250 µl of 1,3-diisopropylbenzene and 50 mg of the catalyst (S/C = 15) and the reaction
was started by the addition of 4 mmol of H2O2 aqueous solution (sulphide/H2O2 = 1). In case of DBTH,
2 mmol of the substrate were dissolved in 15 ml of acetonitrile together with 125 µl of 1,3-
diisopropylbenzene and 25 mg of the catalyst (S/C = 15) and the reaction was started by the addition
of 2 mmol of H2O2 aqueous solution (sulphide/H2O2 = 1). The higher dilution in the case of DBTH was
used to prevent crystallisation of the DBTH and products out of the samples while cooled.
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Oxidation of dioctylsuphide (Aldrich, 96%) to dioctyl sulphone was performed at 50°C. 4 mmol of
dioctylsulphide were mixed with 10 ml of acetonitrile, 10 mmol of H2O2 and 50 mg of the layered TS-
1 catalyst. 2 mmol of H2O2 were added after 12 h. The reaction mixture was stirred intensively for 16
h. Then, it was diluted with additional 10 ml of acetonitrile the catalyst was removed by
centrifugation and the remaining solution was evaporated. The crude product was dissolved in 10 ml
ethyl acetate (Penta, Czech republic, p.a.), dried over MgSO4 and finally the product crystallised after
addition of 2 ml of methanol at 4°C.
Samples of the reaction mixture were taken in regular intervals, immediately centrifuged, cooled and
analysed using an Agilent 6850 GC system with 50 m long DB-5 column, an autosampler and a FID or
an MS detector. Helium was used as a carrier gas. The internal standard calibration method was used
for the evaluation of the kinetic data.
Conversion was calculated as the ratio between consumed and initial molar amount of the substrate
(eqn 1). Yield of sulphoxide resp. sulphone was calculated as the ratio between produced sulphoxide
resp. sulphone and consumed sulphide (eqn 2). Sulphoxide selectivity at certain conversion was
calculated as the ratio between sulphoxide yield and sulphide conversion. Hydrogen peroxide
efficiency at 100% conversion of the H2O2 was calculated as the ratio between sum of oxidation
product yields and initial amount of the hydrogen peroxide (eqn 3). TON of the primary oxidation of
sulphide was calculated as the molar amount of the converted sulphide per the molar amount
titanium (eqn 4). Overall TON (including the oxidation of sulphoxide to sulphone) was calculated as a
sum of converted sulphide and produced sulphone molar amount divided by molar amount of
titanium (eqn 5).
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3. Results and discussion
3.1. Catalyst description and characterization
The layered and pillared TS-1 catalysts preparation is based on the surfactant templated synthesis
approach, where the growth of the crystals is restricted by long hydrophobic chains of the
template32. As a result, lamellar material consisting of MFI layers approximately 2 nm thick is
formed32. The layers are not perfectly uniform (do not fit perfectly one on another) because each of
them is, in fact, a single crystal. Therefore, the layered TS-1 preserves some slit-shaped mesopores
(in contrast to the D4R hydrolysed lamellar materials33) even after calcination.
The XRD patterns of the TS-1 based catalysts are presented in Figure 1. The conventional TS-1
catalysts (denoted TS-1 (200) and TS-1 (600) according to their typical crystal size determined form
SEM images) possess the XRD patterns, which are characteristic for the materials with MFI topology.
The pattern of the layered TS-1 is consistent with that for nanosheet TS-1 observed by Na et al.32 In
the low 2θ region, a diffraction line on 2θ = 1.5° can be observed because the layers did not collapse
perfectly one on another and enhanced interlamellar distance was partially preserved. To keep the
layer apart and to enhance the active sites accessibility, the so-called pillaring treatment35 is used. A
part of the interlayer space is filled with amorphous mesoporous silica (TS-1-PISi) or silica-titania (TS-
1-PITi) and the material keeps its original morphology even after calcination. For the TS-1-PISi and TS-
1-PITi, a strong diffraction line is present on 2θ = 1.7°. This line characterizes the regular organization
of the layers in the pillared material. The position of the line corresponds to d-spacing of 4.5 nm and
interlayer distance of 2.5 nm. The titanium source (1.7 %) was added into the pillaring medium to
impregnate the layers with additional Ti sites to suppress the active phase dilution by the amorphous
pillars35.
Figure 1: Powder XRD patterns of the TS-1 based catalysts.
The synthesis and post-synthesis modifications of the Ti-UTL are thoroughly discussed in our previous
paper36. The XRD patterns of the Ti-UTL, Ti-IPC-1-PI and Ti-IPC-2 are presented in Figure 2. The Ti-IPC-
2 is a zeolite with OKO topology39, which was prepared by stabilization of the Ti-IPC-1P (lamellar
precursor formed by the germanium D4R hydrolysis) with diethoxydimethylsilane. The process of
the UTL to OKO transformation, so-called ADOR process, is intensively studied by our group30. A shift
of the most intensive 200 diffraction line of the UTL towards lower angles can be observed after the
transformation to Ti-IPC-1-PI, indicating the extension of the d-spacing form 2.9 nm to 3.7 nm (note
that the intensity of 200 line decreases in Ti-IPC-1-PI pattern and the 100 line is the most intense). On
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the other hand, the 200 line is shifted towards higher angles for Ti-IPC-2 material, where the
interlayer distance decreases in comparison with Ti-UTL (d-spacing 2.3 nm).
Figure 2: Powder XRD patterns of the UTL based catalysts.
The textural properties obtained by N2 sorption isotherms analysis at -196°C together with the
titanium content (expressed as Si/Ti molar ratio) are summarized in Table 1. The conventional TS-1
(200), TS-1 (600) and layered TS-1 catalysts exhibit BET areas between 450 and 528 m2/g. The total
adsorption capacity (Vtot) increased from 0.16 cm3/g of the TS-1 (600) to 0.30 cm3/g by the decrease
in the crystal size due to increase in the interparticle volume. The lamellar material possesses lower
micropore volume (Vmic = 0.09 cm3/g) in comparison with the conventional TS-1 (Vmic = 0.15 resp. 0.13
for TS-1 (200) and TS-1 (600)). Therefore, the Ti sites are expected to be better accessible in the
lamellar material.
The pillaring treatment led to an increase in both BET area (TS-1-PISi: 575 m2/g, TS-1-PITi: 685 m2/g)
and total adsorption capacity (TS-1-PISi: 0.39 cm3/g, TS-1-PITi: 0.37 cm3/g). The decrease in the Ti
content in the TS-1-PISi material (Si/Ti=55 vs. layered TS-1 Si/Ti=36) can be compensated by the
silica-titania pillaring (TS-1-PITi Si/Ti=31). A typical shape of the nitrogen sorption isotherm (except
the filling of micropores up to p/p0 = 0.05, exhibiting considerable uptake in the range of p/p0 =0.05-
0.3 due to filling of the pores between 2.5 and 3 nm and then being flat above p/p0 =0.5) confirms
the pillared character of the material 40. The isotherms are presented in the supporting information
(Figure S1, S2).
Similarly, the hydrolysis and transformation of the Ti-UTL into the Ti-IPC-1-PI material led to a strong
increase in both BET area (Ti-UTL: 539 m2/g, Ti-IPC-1-PI: 1001 m2/g) and adsorption capacity (Ti-UTL:
0.29 cm3/g, Ti-IPC-1-PI: 0.67 cm3/g). The Ti-IPC-1-PI has no micropores because there are no channels
running through the crystalline layers. Contrary to Ti-IPC-2, where the BET area (383 m2/g),
micropore volume (0.15 cm3/g), and adsorption capacity (0.29 cm3/g) decreased in comparison with
Ti-UTL due to the reduction of the size of channels (from 14x12-ring (UTL) to 12x10-ring (OKO)). The
decrease in the titanium content form Si/Ti=138 (Ti-UTL) to Si/Ti=210 (Ti-IPC-2) and Si/Ti=480 (Ti-IPC-
1-PI) suggests that the titanium is homogenously distributed through the Ti-UTL. In the case of Ti-IPC-
2, the D4R units (apparently containing some Ti atoms in addition to germanium) are removed and
replaced by the silica S4R units. In the case of Ti-IPC-1-PI, the D4R are removed and the crystalline
material is diluted with the silica pillars. Nevertheless, both materials are catalytically active (vide
infra).
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Table 1: Textural properties and Ti content of the lamellar and conventional titanosilicate catalysts under study.
BET
Sext
Vmeso
Titanosilicate
TS-1 (600)a
TS-1 (200)a
lam TS-1
(m2/g)
(m2/g)
Vmic (cm3/g)
0.13
(cm3/g)
Vtot (cm3/g)
0.16
Si/Ti
39
450
528
473
575
685
539
1001
383
151
204
292
337
523
33
0.03
0.16
0.21
0.29
0.30
0.05
0.67
0.07
0.15
0.31
36
0.09
0.30
36
TS-1-PISi
0.10
0.39
55
TS-1-PITi
0.07
0.37
31
Ti-UTL
0.24
0
0.29
0.67
0.22
139
480
210
Ti-IPC-1-PI
Ti-IPC-2
n.d.b
49
0.15
a The number in brackets expresses a typical crystal size in nm determined from SEM.
b The external surface area could not be determined by the t-plot method.
To characterise the titanium sites, the DR-UV-Vis spectra were recorded for all the catalysts. The
spectra of the TS-1 based materials are presented in Figure 3 and the spectra of the UTL-based
catalysts are shown in Figure 4. The main feature in all the spectra is a band centred around 210 nm.
Furthermore, some absorption between 260 and 300 nm is observed in the spectra of TS-1-PISi, TS-1-
PITi, Ti-UTL and Ti-IPC-2. No absorption around 330 nm is observed except of the TS-1 (200) sample.
Zecchina et al.41 and Wu et al.42 ascribed the band at 210 nm to tetrahedrally coordinated framework
Ti(OSi)4 species. The absorption between 260-290 nm results from the presence of 5- and 6-
coordinated extra-framework titanium species43. The absorption above 300 nm is characteristic for
the anatase TiO2 phase, which is known to ineffectively decompose the hydrogen peroxide; however,
with the exception of some traces in the TS-1 (200), it is not present in the discussed catalysts.
Figure 3: DR-UV-Vis spectra of the TS-1 (200) (a), TS-1 (600) (b), layered TS-1 (c), TS-1-PISi (d), TS-1-PITi (e).
Figure 4: DR-UV-Vis spectra of the Ti-UTL (a), Ti-IPC-1-PI (b) and Ti-IPC-2 (c).
3.2. Oxidation of methyl phenyl sulphide
The TS-1 based catalysts and the Ti-UTL derived catalysts were used in oxidation of methylphenyl
sulphide (MPS) with aqueous hydrogen peroxide as the oxidant in acetonitrile at 30°C. The
conversion curves using the TS-1 based catalysts are given in Figure 5 and the product distribution,
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selectivity and TON are summarized in Table 2. The MPS was chosen being one of the most common
model substrates to study oxidation of organic sulphides. A blank experiment revealed that the
reaction without catalyst occurs only slowly (conversion of 2.5% after 120 min) under the
experimental conditions. Therefore, the observed kinetic profiles can be ascribed to the catalysts.
The maximum theoretical conversion is 50% under the used conditions. In all the cases, the reaction
started rapidly and slowed down as the hydrogen peroxide was running out until it stops at the 100%
conversion of H2O2. When fresh H2O2 was added, the reaction was restored.
The TS-1 (600) catalyst gave the lowest conversion (11% after 30 min) and the least selective (75 % at
25% conversion) of the TS-1 based catalysts. The use of a catalyst with smaller crystals resulted in an
increase in the conversion (21% after 30 min) due to shorter diffusion paths. The use of lamellar
catalysts led to another strong increase in the conversion: TS-1-PISi 26% < layered TS-1 36% < TS-1-
PiTi 41% after 30 min. Among the lamellar TS-1 catalysts, it appears that the conversion is more
dependent on the titanium content than on the textural properties. When a titanium containing
amorphous molecular sieve Ti-HMS (BET area 893 m2/g, Vtot = 0.61, average pore diameter 3.5 nm,
Si/Ti = 84) was used, conversion reached 5.3 % after 30 min with 87% selectivity.
To compare the intrinsic activity of the catalysts, TON values were calculated after 30 min of the
reaction (Table 2). There are two different TON presented. The primary TON (TONprim) includes only
the oxidation of MPS to methylphenyl sulphoxide (MPSO); however, the consecutive oxidation of
MPSO to methylphenyl sulphone (MPSO2) is also catalysed and therefore is considered. The total
TON (TONtot) includes both reactions. The highest TONtot = 151 is observed for the TS-1-PISi
possessing the most open structure (Vtot = 0.39 cm3/g, Vmicro = 0.10 cm3/g). Then the both TONprim and
TONtot decrease with decreasing mesopore volume (Vmeso, estimated as a difference between Vtot and
Vmic): layered TS-1 (Vmeso=0.21 cm3/g, TONtot=136) > TS-1 (200) (Vmeso=0.16 cm3/g, TONtot=87) > TS-1
(600) (Vmeso=0.03 cm3/g, TONtot=60). The TS-1-PITi does not fit into this trend (Vmeso=0.30 cm3/g,
TONtot=132). This is most probably due to the inaccessibility of a part of the titanium sites located
inside the silica pillars.
Figure 5: Catalytic oxidation of methylphenyl sulphide (MPS) over the TS-1 based catalysts at 30°C. Reaction conditions: 8
mmol of MPS, 4 mmol of H2O2 (35 wt.%) , 50 mg of catalyst, 250 μl of internal standard, 10 ml acetonitrile, 900 rpm.
The selectivity was determined at 40% conversion. The order of the catalyst selectivity to sulphoxide
is the same as in the case of TON: TS-1 (600) (61%) < TS-1 (200) (78%) < layered TS-1 (87%) < TS-1-PISi
(91%). The selectivity of the reaction to the sulphoxide is mostly dependent on the diffusion
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restrictions in the catalyst and partially on the titanium content. The lower diffusion restrictions, the
higher sulphoxide selectivity was observed. The assumption of the diffusion driven selectivity is also
in accordance with the data reported by Corma et al.23 and Kon et al.25 on MPS oxidation. The TS-1-
PITi stands again out of the trend (S = 85%). Based on the fact that it is the catalyst providing the
highest conversion before the H2O2 is consumed, we expect that it has an increased amount of easily
accessible active sites in comparison with TS-1-PISi, which were formed post-synthetically during the
silica-titania pillaring and thus the TS-1-PITi is less selective than it should be according to its textural
properties.
Finally, the efficiency of the hydrogen peroxide used was estimated from the yield of sulphoxide and
sulphone at 100% conversion of the H2O2 (in the case it was reached during the experiment). For the
catalysts TS-1 (200), layered TS-1 and TS-1-PISi (and also for the Ti-UTL) it was the same (94-95%). In
the case of TS-1-PITi, the efficiency was lower (91%) due to decomposition on the hexacoordinated
titanium sites (see the DR-UV-Vis discussion) whose structure might by close to the anatase structure
(titanium atoms in anatase are also 6-coordinated).
Table 2: Oxidation of methylphenyl sulphide at 30°C.
Product composition (120 min)a
b
c
d
e
S(40%)
(%)
E(100%)
(%)
n.a.
94
TONprim
TONtot
MPSO (%)
MPSO2 (%)
(30 min)
43
(30 min)
60
Catalyst
TS-1 (600)
TS-1 (200)
layered TS-1
TS-1-PISi
TS-1-PITi
Ti-UTL
60
74
94
91
93
96
94
84
40
26
6
61 (25%)
78
74
87
87
94
127
136
9
91
95
137
151
7
85
91
124
132
4
98
95
200
208
Ti-IPC-1-PI
Ti-IPC-2
6
95
100
n.a.
1338
101
1418
115
16
69 (8%)
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a Ratio between methylphenyl sulphoxide (MPSO) and methylphenyl sulphone (MPSO2) after 120 min
of the reaction.
b Methylphenyl sulphoxide selectivity at 40 % conversion. Where the conversion of 40 % was not
reached during the experiment, the selectivity is given at maximum conversion.
c Hydrogen peroxide efficiency at 100% conversion of the H2O2.
d TON of the primary oxidation of MPS.
e Overall TON (encounting the oxidation of MPSO to MPSO2.
The conversion curves obtained using the Ti-UTL and derived catalysts are presented in Figure 6
together with the curve of TS-1 (600) as a benchmark. All the catalysts (Ti-UTL, Ti-IPC-1-PI and Ti-IPC-
2) have low titanium content (Si/Ti = 139, 480 and 210 respectively) in comparison with the TS-1
based catalysts (Si/Ti = 31 – 55); however, they possess extra-large and large pores (Ti-UTL possess
14x12 ring 2D-channel system and Ti-IPC-2 12x10 ring channels) and the Ti-IPC-1-PI is purely
mesoporous. Therefore, they are expected to have lowed diffusion restrictions in comparison with
the TS-1 based catalysts. The Ti-UTL gave conversion of 15%, the Ti-IPC-1-PI 29% and Ti-IPC-2 only 5%
after 30 min of the reaction. However, when the activity is expressed as the TON, the intrinsic activity
of Ti-IPC-2 (TONtot = 115 after 30 min) corresponds to the TS-1 based catalysts (from TONtot = 60 to
151). The Ti-UTL and Ti-IPC-1-PI catalysts own an outstanding intrinsic activity, providing TONtot of
208 and 1418, respectively. Furthermore, the Ti-UTL is the most selective of the discussed catalysts
(98% at 40% conversion) and the Ti-IPC-1-PI is most efficiently using the H2O2 (E = 100%) together
with the selectivity of 95%. One could say, that high TON is not important while the catalyst possess
only very little of the active centres, but this is not the case of the Ti-IPC-1-PI catalyst. The MPS
conversion over this catalyst (29% after 30 min) is fully comparable with the lamellar TS-1 catalysts
(e.g. TS-1-PISi: 26%, layered TS-1 36%) at the same conditions.
On the other hand the selectivity of the Ti-IPC-2 is only 69% at 8% conversion and after initial 20 min
the conversion increases only slowly (from 5% to 8% between 35 and 180 min of the reaction), this is
most probably because the pores are not wide enough and the diffusion restrictions are too severe.
Figure 6: Catalytic oxidation of methylphenyl sulphide (MPS) over the Ti-UTL based catalysts at 30°C. TS-1 (600) is shown
as a benchmark. Reaction conditions: 8 mmol of MPS, 4 mmol of H2O2 (35 wt.%) , 50 mg of catalyst, 250 μl of internal
standard, 10 ml acetonitrile, 900 rpm.
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When solvent free conditions and 1 molar equivalent of the H2O2 were used at 25°C (Table 3), the TS-
1-TiPI catalyst provided the highest conversion after 30 min (79%), which remained unchanged from
that moment on because of total conversion of the oxidant. The Ti-IPC-1-PI gave conversion of 80%
after 120 min being the same as for the Ti-IEZ-MWW used in the ref. 25. It should be noted, that the
Ti content in the Ti-IPC-1-PI is more than 5 times lower (Si/Ti = 480) than in Ti-IEZ-MWW (Si/Ti = 86
25).The Ti-IEZ-MWW is an interlayer expanded zeolite containing 12-ring windows44 and its
architecture (2D channel system, d-spacing of 2.7 nm44, BET area 521 m2/g 25) is close to the Ti-UTL
(2D, channel system, d-spacing 2.9 nm, BET area 539 m2/g). Both Ti-UTL and Ti-IEZ-MWW provided
similar MPS conversion after 120 min (89 % vs. 79%) and MPSO selectivity (92% vs. 89%). The layered
TS-1 catalyst was tested with 1.2 molar eq of the oxidant providing conversion of 94% after 30 min
with the 92% selectivity. Total conversion of MPS was observed after 60 min. In the case of Ti-IEZ-
MWW, total conversion of MPS is reported after 120 min with the 94% selectivity.
Table 3: Oxidation of MPS with 1 molar eq of H2O2 at 25°C under solvent free conditions.
X (30 min)a X (120 min)a
S (X=80%)b
(%)
Yield (120 min)
(%)
20
(%)
47
MPSO (%) MPSO2 (%)
Catalyst
TS-1 (600)
38
65
73
82
66
70
94
8
14
27
7
82
82
73
92
83
89
94
TS-1-PITi
79
80
layered TS-1c
Ti-UTL
94
>99
89
37
Ti-IPC-1-PI
Ti-IEZ-MWWd
Ti-IEZ-MWW c,d
42
80
13
6
n.a.
79
n.a.
99
6
a MPS conversion after 30 resp. 120 min.
b MPSO selectivity at 80% convesion.
c 1.2 molar eq based on MPS of H2O2 was used.
d Results are adopted from ref. 25.
3.3. Driving the selectivity
In the previous section, we have shown that the selectivity of the MPS oxidation depends mainly on
the diffusion properties of the catalyst. In this section, we report on the influence of other
parameters on the selectivity i.e. the substrate/catalyst ratio (S/C) and the influence of H2O2
concentration in the reaction mixture.
The influence of the S/C ratio was investigated using the layered TS-1 catalyst. Based on the high
selectivity obtained using the Ti-UTL and Ti-IPC-1-PI catalysts, which have low concentration of the
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active sites, we expected that the decrease in the amount of the catalyst might influence the
selectivity. The Figure 7 presents the obtained conversion curves. When the TONprim in 30 min are
calculated, some increase in the intrinsic activity can be observed (S/C=20: 127, S/C=40: 136,
S/C=100: 170). The blank experiment showed that the oxidation of MPS occurs also without the
presence of the catalyst (Figure 5). Assuming the rate of the non-catalysed oxidation is independent
on the amount of catalyst; we conclude that the observed differences result from different
proportion between the amounts of MPS converted non-catalytically and total conversion. The
selectivity curves of the experiments with different S/C ratio are presented in Figure 8. We can
observe that they correspond perfectly to one another, showing the selectivity of 86% at 25%
conversion and therefore we conclude the selectivity is not influenced by the used amount of
catalyst (S/C ratio).
Figure 7: Conversion curves of MPS oxidation over layered TS-1 using different S/C ratio.
Figure 8: Selectivity curves of MPS oxidation over layered TS-1 using different S/C ratio.
The concentration of the oxidant is another parameter, which can be expected to influence the
selectivity in a system of two consecutive oxidation reactions. To investigate this influence, we have
chosen two catalysts i.e. TS-1 (600) and layered TS-1 and we conducted the catalytic experiment with
0.5 molar equivalent of H2O2 based on the MPS (a standard experiment), with 1 molar equivalent of
H2O2 and with 5 doses of 0.1 equivalent of H2O2 during 5 h. The obtained conversion curves resp.
curves showing the development of conversion in the case of the dosing experiment are presented in
Figure 9. For the layered TS-1 it can be observed, that in the initial 20 min the reaction rate is the
same for 0.5 and 1 eq and at higher conversion, the 0.5 eq reaction starts to fade out. When the
hydrogen peroxide is dosed, after each addition, the total conversion is reached within first 20 min
and the reaction is restored only after addition of fresh H2O2.
Using the TS-1 (600) catalyst, the reaction is faster with the 1 eq of H2O2 since the very beginning
(conversion 20% after 30 min) of the experiment than in the standard 0.5 eq run (11% after 30 min).
In the case of the dosed experiment some steps can be observed on the curve of the increase of the
conversion at the beginning (as in the case of layered TS-1); however, total conversion is not reached
during the interval between the doses and the curve is smoothing.
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The selectivity curves are shown in Figure 10. An increase in the selectivity in the case of 0.1 eq
dosing experiment is clearly visible. For the layered TS-1 the selectivity (calculated at 40%
conversion) increased upon dosing of the oxidant from 87% (standard experiment) to 91%.
Furthermore, considering the rapid conversion of the H2O2 dose, we believe that the dosing of the
oxidant can be tuned in such way that the selectivity is increased without or only with a slight
decrease in the reaction rate. Using 1 eq of H2O2 at the beginning of the reaction, the selectivity
decreased only slightly to 85%.
For the TS-1 (600) catalyst similar behaviour was observed. The selectivity of the reactions is
calculated at 25% conversion; however, the selectivity curves are linear, therefore, the selectivity
does not change much over the studied range of conversion. When the oxidant was dosed, the
selectivity increased from 61 % (standard experiment) to 65%. When the 1 eq of H2O2 was used, it
decreased to 56%.
An experiment starting with 16 mmol of MPS was conducted using layered TS-1 catalyst at otherwise
typical conditions aiming to isolate the MPSO in a maximum yield. The oxidant was added in 0.1
molar eq doses in 10 min interval first 40 min and then in 20 min interval following 140 min (total 1.1
molar eq of H2O2,, 17.6 mmol). After total 140 min of the reaction, the conversion was 67% with 97%
selectivity. When the reaction was terminated after total 220 min, the conversion was 82% with 93%
selectivity. After workup and column chromatography purification, MPSO was isolated in 70% yield
(1.58 g).
Figure 9: Development of conversion in time using different amount of H2O2 and dosing of the oxidant over TS-1 (600)
(gray) and layered TS-1 (black).
Figure 10: Selectivity curves of the experiments using different amount of H2O2 and dosing of the oxidant over TS-1 (600)
(gray) and layered TS-1 (black).
3.4. Oxidation of bulky sulphides
To further demonstrate the potential of the lamellar titanosilicate catalysts, we studied oxidation of
two substrates bulkier than the MPS. Table 4 summarises the results of dibenzothiophene (DBTH and
diphenyl sulphide (Ph2S) oxidation. The experiments were conducted at 40°C using molar 1 eq. of the
oxidant. The order of the catalysts in terms of conversions and TON is similar to the results obtained
in the MPS oxidation.
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In the DBTH oxidation, the major product was the dibenzothiophene sulphone (DBTHSO2) contrary to
the other substrates. The highest conversion (28% after 240 min) and yield of the DBTHSO2 (24 %)
was obtained over the TS-1-PITi thanks to its well accessible titanium sites formed during the silica-
titania pillaring. The layered TS-1 provided conversion of 20% because the DBTF cannot access the
micropores in the layers (evidenced by the zero conversion over the TS-1 (600) catalyst) and
therefore only a part of the titanium sites located on the outer surface of the layers is active. The Ti-
IPC-1-PI catalyst provided conversion of 9.5 % and yield of 7.9 % after 240 min. Among the
conventional zeolites, the Ti-UTL was the only catalyst, which provided some conversion (3.3 % after
240 min). The TS-1 (600) and Ti-IPC-2 were not active at all (zero conversion after 240 min) because
their channels are not wide enough for the DBTH molecule. We conclude, that especially the silica-
titania pillared catalysts like TS-1-PITi have the potential to oxidise bulky and rigid molecules like
DBTF in the desulphuration processes and the advantages of lamellar titanosilicates in oxidation of
bulky sulphides were clearly demonstrated.
The oxidation of diphenyl sulphide (Ph2S) occurred over all the examined catalysts. Even the TS-1
(600) provided conversion of 22% after 240 min being the most selective (y(Ph2SO) = 20%, selectivity
93% at 25% conversion). It appears that the Ph2S is flexible enough to enter the TS-1 micropores;
however it is not the case of the diphenylsulphoxide (Ph2SO) and therefore high selectivity is gained
over the catalysts with higher diffusion restrictions. The layered TS-1 provided conversion of 34% at
the same time and TS-1-PITi 59%; however, the selectivity over TS-1-PITi was only 85%. The Ti-UTL
provided conversion similar to the TS-1 (600) with slightly lower selectivity (87% at 25% conversion)
and the Ti-IPC-1-PI showed once more an outstanding intrinsic activity (TONprim = 1818 in 240 min) in
comparison with the other catalysts (TONprim from 81 (TS-1 (600)) to 287 (Ti-UTL)).
Table 4: Oxidation of dibenzothiophene (DBTF) and diphenyl sulphide (Ph2S).
Oxidation of DBTF
Oxidation of Ph2S
d
X
(240min)a
(%)
y (Ph2SO,
TONprim
X (240 min)a
y (DBTHSO2
TONprim
240 min) d (240
(%)
0
240min)b (%) (240 min)
(%)
20
30
46
19
31
min)
81
Catalyst
TS-1 (600)
layered TS-1
TS-1-PITi
0
0
69
83
44
438
22
20
28
3.3
9.5
18
24
2.6
7.9
34
117
175
59
Ti-UTL
23
287
Ti-IPC-1-PI
39
1818
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Ti-IPC-2
0
0
0
6.3
4.2
127
a Conversion of DBTH resp. Ph2S after 240 min of the reaction.
b Yield of DBTHSO2 after 240 min of the reaction.
c Yield of Ph2SO after 240 min of the reaction.
d TON of the primary oxidation of Ph2S
To further investigate the oxidation of Ph2S to Ph2SO, a series of experiments over the TS-1-PITi
(giving the highest conversion) at different temperatures was conducted. The observed conversion
curves are presented in Figure 11. When the temperature was lower from 40 °C being a standard to
30 °C, the conversion decreased nearly to one half after 120 min (47% vs. 25%) on the other hand
upon rise of the temperature to 50 °C, the conversion after 120 min increased only to 54%. The
Figure 12 shows the selectivity curves obtained in this experiment. The selectivity was determined as
the slope of the linear part of the curve up to the conversion of 60%. The selectivity of the
experiments conducted at 30 °C and 40 °C was 85%. When the temperature was elevated to 50°C,
the selectivity increased to 89%. This suggests that the selectivity of Ph2S oxidation might be also
diffusion driven (as in the case of MPS), only this effect manifests itself at higher temperature.
Figure 11: Conversion curves of oxidation of Ph2S over TS-1-PITi at different temperatures.
Figure 12: Selectivity curves for Ph2SO in oxidation of Ph2S over TS-1-PITi at different temperature.
To examine the possibility of the catalyst reusing, the layered TS-1 and Ti-IPC-1-PI were evaluated in
4 consecutive runs of Ph2S oxidation. After each run, the catalyst was recovered by centrifugation,
washed once with acetonitrile and dried overnight. Catalyst loses (about 10% of each run) were
compensated by the reduction of the reaction mixture amount. For the layered TS-1 the conversion
after 180 min was decreasing run by run from 26% > 14 % > 7% > 7%. For the Ti-IPC-1-PI, the
decrease in conversion after 180 min was slightly lower starting from 31% > 20% > 13% > 12%.
Stabilisation of the conversion at about 1/3 of the initial value can be observed.
An oxidation of dioctylsulphide to dioctylsulphone was performed with 2.5 molar equivalent of the
oxidant at 50°C. A 4 mmol scale was used with the intention to isolate the product. After 12 h, 0.5
mol eq of the H2O2 was added and after total 16 h, complete conversion of dioctylsuphide was
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observed. The product was isolated in 46% yield (516 mg) after recrystalisation what is similar to the
result repoted by Kon et al. (isolated yield 37%)25.
4. Conclusions
In conclusion, the oxidation of methylphenyl sulphide (MPS), diphenyl sulphide (Ph2S),
dibenzothiophene (DBTF) and dioctylsulphide over lamellar crystalline titanosilicate catalysts under
mild conditions with hydrogen peroxide as the oxidant was investigated. The lamellar titanosilicates
provided enhanced activity thanks to easily accessible active sites located on the outer surface of
their layers. The higher mesopore volume, the higher activity was observed at similar titanium
content in the catalysts. The selectivity of the MPS oxidation to methylphenyl sulphoxide is driven by
the diffusion restrictions in the catalyst. The lower are the diffusion restrictions the higher is the
selectivity. The selectivity might be further increased by dosing of the oxidant to keep its
concentration low during all the reaction.
The Ti-IPC-1-PI prepared by the postsynthesis modification of Ti-UTL provided an outstanding
intrinsic activity (TONtot = 1418) together with conversion (e.g. MPS conversion 48% after 120 min)
similar to other lamellar titanosilicates (e.g. MPS conversion over layered TS-1 46% after 120 min) in
both MPS and Ph2S oxidations. The silica-titania pillared TS-1-PITi catalyst showed the highest
potential of the tested catalysts to act as an oxo-desulphuration catalyst, easilly oxidising the DBTH
to dibenzothiothene sulphone (DBTH conversion 28% after 240 min). On the other hand,
conventional zeolites TS-1 and Ti-IPC-2 catalysts did not provide any conversion of DBTH.
Acknowledgement
The authors acknowledge the Czech Science Foundation (P106/12/G015) for the financial support.
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21

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Figure 1: Powder XRD patterns of the TS-1 based catalysts.
74x68mm (600 x 600 DPI)

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Figure 2: Powder XRD patterns of the UTL based catalysts.
64x50mm (600 x 600 DPI)

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Figure 3: DR-UV-Vis spectra of the TS-1 (200) (a), TS-1 (600) (b), layered TS-1 (c), TS-1-PISi (d), TS-1-
PITi (e).
69x57mm (600 x 600 DPI)

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Figure 4: DR-UV-Vis spectra of the Ti-UTL (a), Ti-IPC-1-PI (b) and Ti-IPC-2 (c).
70x60mm (600 x 600 DPI)

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Figure 5: Catalytic oxidation of methylphenyl sulphide (MPS) over the TS-1 based catalysts at 30°C.
Reaction conditions: 8 mmol of MPS, 4 mmol of H2O2 (35 wt.%) , 50 mg of catalyst, 250 µl of internal
standard, 10 ml acetonitrile, 900 rpm.
69x57mm (600 x 600 DPI)

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Figure 6: Catalytic oxidation of methylphenyl sulphide (MPS) over the Ti-UTL based catalysts at 30°C. TS-1
(600) is shown as a benchmark. Reaction conditions: 8 mmol of MPS, 4 mmol of H2O2 (35 wt.%) , 50 mg of
catalyst, 250 µl of internal standard, 10 ml acetonitrile, 900 rpm.
68x56mm (600 x 600 DPI)

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Figure 7: Conversion curves of MPS oxidation over layered TS-1 using different S/C ratio.
69x57mm (600 x 600 DPI)

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Figure 8: Selectivity curves of MPS oxidation over layered TS-1 using different S/C ratio.
68x57mm (600 x 600 DPI)