Non-hydrolytic synthesis of mesoporous silica-titania catalysts for the mild oxidation of sulfur compounds with hydrogen peroxide

Article abstract of DOI:10.1039/b811668g

A SiO₂-TiO₂ mesoporous xerogel prepared in one-step by a non-hydrolytic route shows excellent performance in the mild oxidation of sulfides, sulfoxides and thiophenes with aqueous solutions of H ₂O₂. The Royal Society of Chemistry.

Full text of DOI:10.1039/b811668g

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Non-hydrolytic synthesis of mesoporous silica–titania catalysts for the
mild oxidation of sulfur compounds with hydrogen peroxide
Ana Mihaela Cojocariu,ab P. Hubert Mutin,*a Emil Dumitriu,b Francois Fajula,a
¸
a
a
´
Andre Vioux and Vasile Hulea*
Received (in Cambridge, UK) 10th July 2008, Accepted 12th August 2008
First published as an Advance Article on the web 18th September 2008
DOI: 10.1039/b811668g
A SiO2–TiO2 mesoporous xerogel prepared in one-step by a
non-hydrolytic route shows excellent performance in the mild
oxidation of sulfides, sulfoxides and thiophenes with aqueous
solutions of H2O2.
oxidation of organic molecules with hydrogen peroxide than
crystalline Ti-containing silicates. This behavior has been
ascribed to the large number of surface silanol groups, leading
to extensive water adsorption hindering the access to the active
Ti sites. Amorphous SiO2–TiO2 mixed oxides with well-dis-
persed Ti sites have been synthesized by both hydrolytic14–16
and non-hydrolytic17,18 sol–gel procedures. The latter method
offers a very simple and efficient way to synthesize mesoporous
titania–silica xerogels with controlled texture,18 avoiding the
use of templates or supercritical drying. These non-hydrolytic
SiO2–TiO2 mixed oxides, with good homogeneity and textural
properties, proved to be efficient catalysts for olefin epo-
xidations with alkylhydroperoxides as the oxidizing agent.19
In this communication we report the one-step non-hydro-
lytic synthesis from cheap chloride precursors of a SiO2–TiO2
mesoporous xerogel (hereafter designated as NH SiO2–TiO2)
with a high dispersion of Ti and unprecedented textural
properties. This xerogel was used for the first time as a
heterogeneous catalyst in the oxidation of sulfur-containing
compounds with hydrogen peroxide. Under mild conditions
(20–60 1C), very high conversions of the sulfur-containing
compounds to sulfones and excellent hydrogen peroxide
selectivities were obtained.
Catalytic sulfoxidation reactions are important reactions
notably in the synthesis of molecules of pharmaceutical
interest.1,2 Over the past ten years, they have also been attract-
ing increasing attention for the removal of sulfur-containing
pollutants. Thus, catalytic oxidative desulfurization is a promis-
ing alternative technology for ultra-deep desulfurization of
fuel3–5 by conversion of sulfides and thiophene derivatives into
the corresponding sulfoxides and sulfones, which are easily
extracted due to their increased polarity. Selective catalytic
sulfoxidation was also proposed as an effective method for
treating wastewater contaminated with dimethyl sulfoxide,6 by
converting it into biodegradable dimethyl sulfone.
Typically, sulfoxidation reactions are carried out in the
liquid phase in the presence of oxidizing agents such as organic
peroxyacids, peroxides, and hydroperoxides, or hydrogen
peroxide. Hydrogen peroxide is particularly attractive owing
to its high effective-oxygen content, cleanliness (water being
the only by-product), and acceptable safety in storage and
operation. Therefore, over the past few years the importance
of hydrogen peroxide as a ‘‘green’’ oxidizing agent for liquid-
phase organic reactions has grown considerably7,8 and the
catalytic oxidation of sulfur-containing compounds by H2O2
has been extensively studied.
NH SiO2–TiO2 was prepared by reaction of SiCl4 and TiCl4
i
(molar ratio Si/Ti = 16) with Pr2O (diisopropyl ether) as
shown in Scheme 1. The reaction temperature was fixed at
150 1C and CH2Cl2 was added in order to increase the pore
volume.17w
Powder X-ray diffraction data indicated that the solid was
amorphous. The Ti and Si content, measured by energy
dispersive X-ray analysis (EDX), were 3.8 wt% and 46.9
wt%, respectively.
Microporous Ti-containing crystalline silicates (TS-1, Ti-b,
etc.), with strong oxidation capacity and high stability, are
particularly effective heterogeneous catalysts for the sulfoxi-
dation with H2O2.9–11 However, their application is limited to
reactants with molecular sizes smaller than 0.7 nm due to their
small micropore openings. The synthesis of Ti-containing
mesoporous templated silicates (Ti-MCM-41, Ti-MCM-48)
having uniform mesopores ranging from 2 to 10 nm in
diameter opened new possibilities for the oxidation of bulky
molecules.12,13 Unfortunately, these materials exhibited a
lower catalytic activity and lower H2O2 selectivity in the
The textural properties of NH SiO2–TiO2, derived from the
nitrogen physisorption isotherm (Fig. 1), were remarkable.
The specific surface area and pore volume (1215 m2 gꢀ1 and
2.4 cm3
SiO2–TiO2 xerogels (525 m2 gꢀ1 and 0.17 cm3
MCM41 (936 m2 gꢀ1 and 0.49 cm3 gꢀ1),12 and even aerogels
(518 m2 gꢀ1 and 0.99 cm3 gꢀ1 14
with similar or lower Ti
g
ꢀ1) were much higher than those reported for
g
ꢀ1),15 Ti-
)
contents. A t-plot analysis showed that the surface area
located in the micropores was negligible (about 50 m2 ꢀ1).
g
a Institut Charles Gerhardt, UMR 5253, CNRS-UM2-ENSCM-UM1,
´
`
Universite Montpellier 2, Place Eugene Bataillon, 34095 Montpellier
Cedex 5, France. E-mail: mutin@univ-montp2.fr;
vasile.hulea@enscm.fr
b Laboratory of Catalysis, Faculty of Chemical Engineering and
Environment Protection, Technical University of Iasi,
71 D. Mangeron Ave., 700050 Iasi, Romania
Scheme 1 Synthetic route to NH SiO2–TiO2.
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Scheme 2 Sulfoxidation reactions investigated.
could be oxidized after 360 min at 60 1C (Fig. 3). The
conversions measured under the same conditions but in the
absence of catalyst were 8%, 0.5% and 12% for MPS, DBT
and DMSO, respectively. The intermediate formation of sulf-
oxide was not observed with DBT (Fig. 3).
Fig. 1 N2 adsorption–desorption isotherm at 77 K and pore size
distribution (inset) of NH SiO2–TiO2 (open and filled symbols corre-
spond to the adsorption and desorption branches, respectively).
Barrett–Joyner–Halenda (BJH) analysis of the desorption
isotherm indicated the presence of a broad distribution of
pores in the 3–40 nm range, with two maxima centered at
about 10 and 20 nm, and an average pore diameter of 8.2 nm.z
The coordination environment of Ti atoms in NH
SiO2–TiO2 was investigated using diffuse reflectance UV-Vis
spectroscopy (Fig. 2).z The major absorption band in the
range of 210–240 nm, may be ascribed to the isolated four-
coordinate Ti atoms20 that are considered responsible for the
peroxo species activation. The shoulder in the 250–320 nm
region would indicate the presence of higher-coordinated Ti
atoms resulting from the oligomerization of Ti species, or the
presence of very small TiO2 nano-domains (quantum size
effect).20,21 The absence of a band at 330 nm shows that
anatase is not present in significant quantities.
The selectivity to H2O2 (corresponding to the ratio between
the H2O2 consumed in the sulfoxidation and the H2O2 con-
verted) was 100% for MPS and DMSO, and 95% for DBT,
indicating that H2O2 was mainly consumed in the sulfoxida-
tion reaction and that direct decomposition of the oxidizing
agent into water and oxygen was negligible.
In order to demonstrate that the activity of our catalyst was
not due to the leaching of Ti species, an additional catalytic
test was performed with DBT: the catalyst was filtered off at
60 1C after 15 min reaction (30% conversion) then the solution
was further reacted for 60 min at 60 1C. Only a very small
increase in conversion of DBT (from 30% to 32%) was
observed (Fig. 3).
The recycling of the catalyst in the oxidation of DBT was also
investigated. After each run (60 min, 60 1C), the catalyst was
separated by filtration, washed with acetonitrile, and then placed
into a fresh reagent mixture. The conversion of DBT for the four
runs was 61, 59, 58, and 57%, respectively, indicating that our
catalyst was stable under operating conditions.
Three representative molecules were chosen in order to
evaluate the potential of NH SiO2–TiO2 as a catalyst for the
sulfoxidation reaction with H2O2: methyl-phenyl sulfide
(MPS), dibenzothiophene (DBT) (a model compound for
organic sulfur in fossil fuels) and dimethyl sulfoxide (DMSO)
(Scheme 2).y
Thus, using a stoichiometric amount of H2O2, complete
oxidation required only 60 min for MPS and 45 min for
DMSO. In the case of DBT, the less reactive molecule, 92%
For the sake of comparison, the oxidation of MPS was
carried out with a Ti-MCM41 catalyst (795 m2 gꢀ1, 0.7 wt%
Ti) prepared as described in the literature.12 After 15 min, the
conversion of MPS with Ti-MCM41 was 9% instead of 82%
with NH SiO2–TiO2. After 60 min, the conversion of MPS was
36% with Ti-MCM41 and the H2O2 selectivity was 60%.
Accordingly, our catalyst exhibited significantly higher activity
(even taking into account the higher Ti content) and H2O2
selectivity than Ti-MCM 41. This behavior might be related to
the high degree of condensation typical of non-hydrolytic
SiO2–TiO2 xerogels,18 that would lead to a lower amount of
surface silanols and a lower hydrophilicity than in Ti-MCM 41
samples. Indeed, the low activity and H2O2 selectivity of Ti-
MCM 41 in the oxidation of alkenes (in comparison to
titanosilicalites) has been ascribed to the high density of
silanols in Ti-MCM 41.12 In order to test this hypothesis,
the hydrophilicities of the surface of NH SiO2–TiO2 and of our
Ti-MCM 41 sample were characterized by water adsorption.
After dehydration for 8 h at 350 1C under dry nitrogen and
equilibration over a saturated NH4Cl solution (79% relative
humidity) for 24 h, NH SiO2–TiO2 adsorbed 0.26 mg H2O
Fig. 2 Diffuse reflectance UV-Vis spectrum of NH SiO2–TiO2.
5358 | Chem. Commun., 2008, 5357–5359
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Fig. 3 Sulfoxidation with 30 wt% H2O2 aqueous solution catalyzed by NH TiO2–SiO2 of: (A) methyl-phenyl sulfide in acetonitrile at 40 1C, (B)
dibenzothiophene in acetonitrile at 60 1C, and (C) dimethyl sulfoxide in water at 20 1C. Conditions: 50 ml batch reactor, 50 mg catalyst, 1.5 mmol
substrate, 15 ml solvent, H2O2/MPS = H2O2/DBT = 2, H2O2/DMSO = 1.
per m2, whereas under the same conditions the Ti-MCM 41
absence of catalyst. Samples of the reaction mixture were withdrawn
periodically and analyzed on a Varian 3900 chromatograph equipped
sample adsorbed 0.42 mg H2O per m2.
with a capillary column (DB-1, 60 m, 0.20 mm id, 0.25 mm film
thickness). The hydrogen peroxide concentration was measured by
standard iodometric titration.
The unique catalytic performances of NH SiO2–TiO2 thus
likely resulted from its relatively high amount of well-dis-
persed Ti species, its outstanding texture, and its high degree
of condensation.
1 K. Kaczorowska, Z. Kolarska, K. Mitka and P. Kowalski, Tetra-
hedron, 2005, 61, 8315.
In conclusion, the non-hydrolytic sol–gel route described
here provided a simple one-pot method for the synthesis of a
mesoporous SiO2–TiO2 xerogel with outstanding textural
properties. After calcination, this xerogel proved to be an
excellent sulfoxidation catalyst with H2O2 under mild condi-
tions, exhibiting high activity and selectivity to H2O2, even in
the case of dibenzothiophene, which is considered a refractory
substrate. Such materials open new possibilities for the oxida-
tion with high H2O2 efficiency of bulky substrates for fine
chemicals synthesis and environmental applications.
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Notes and references
w NH SiO2–TiO2 was prepared by adding, in a glovebox, SiCl4
(38.8 mmol), TiCl4 (2.4 mmol), Pr2O (80 mmol) and CH2Cl2 (10 ml)
i
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into a 150 ml, Teflon-lined, stainless steel autoclave. After heating
under autogenous pressure for 4 days at 150 1C, the gel was filtered off,
washed with dry CH2Cl2, dried under vacuum at 110 1C for 6 h and
calcined in dry air at 500 1C for 5 h.
z N2 physisorption experiments were done at ꢀ196 1C on a Micro-
meritics ASAP 2010 sorptometer. The samples were desorbed under
vacuum at 200 1C for 15 h. Diffuse reflectance UV-Vis spectra were
recorded under ambient conditions on a Perkin-Elmer Lambda 14
spectrometer equipped with a BaSO4 integration sphere; the samples
were diluted in BaSO4; the spectra were plotted using the Kubelka–
Munk function.
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y Catalytic sulfoxidation reactions were carried out at atmospheric
pressure in a 50 ml glass batch reactor, equipped with magnetic stirrer,
thermometer, and condenser, and placed in a thermostated bath.
50 mg of catalyst (NH SiO2–TiO2 or Ti-MCM 41) was suspended
under stirring (800 rpm) in a mixture containing 1.5 mmol of organic
substrate and 15 ml solvent. The adequate volume of hydrogen
peroxide (30 wt% aqueous solution) was added at the beginning of
experiment. Blank runs were carried out mixing the reactants in the
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This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 5357–5359 | 5359