Synthesis of framework Ti-substituted, 3-D hexagonal, mesoporous Ti-SBA-12 for selective catalytic oxidation

Article abstract of DOI:10.1039/b915324a

Framework Ti-substituted, 3-D hexagonal mesoporous, Ti-SBA-12, obtained for the first time by direct hydrothermal synthesis, oxidises bulky molecules such as cyclohexene and cyclooctene to the epoxides with >96% selectivity at high conversion levels (>60%).

Full text of DOI:10.1039/b915324a

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Synthesis of framework Ti-substituted, 3-D hexagonal,
mesoporous Ti-SBA-12 for selective catalytic oxidationw
Anuj Kumar, Darbha Srinivas* and Paul Ratnasamy
Received (in Cambridge, UK) 28th July 2009, Accepted 26th August 2009
First published as an Advance Article on the web 14th September 2009
DOI: 10.1039/b915324a
Framework Ti-substituted, 3-D hexagonal mesoporous, Ti-SBA-12,
obtained for the first time by direct hydrothermal synthesis,
oxidises bulky molecules such as cyclohexene and cyclooctene to
the epoxides with 496% selectivity at high conversion levels
(460%).
these peaks occurred at lower 2y values. With increasing Ti
content, the unit cell parameters (a and c) and unit cell volume
(V) of the hexagonal Ti-SBA-12 lattice increased linearly up to
a Ti/(Si + Ti) molar ratio of 0.033 (Fig. 1). To the best of
our knowledge, this is the first report wherein such lattice
expansion on Ti substitution is reported for
dimensional mesoporous titanosilicate structure.
a three-
Synthesis of mesoporous, titanosilicate molecular sieves con-
taining Ti ions in lattice framework positions and capable
(unlike TS-1) of selective oxidation of bulky molecules is
of current interest.1 SBA-12 has outstanding hydrothermal
stability and a three-dimensional mesoporous structure.2
Incorporation of Ti in its framework is difficult because, under
the conventional, highly acidic conditions of its synthesis, Ti
exists only in the cationic form as oxy/hydroxy species. The
former cannot be introduced easily into the framework via
condensation processes with silica species.3 While there are
reports of incorporation of heteroatoms in SBA-15, a material
with a two-dimensional hexagonal structure,4–6 there are no
such reports in the case of the three-dimensional, mesoporous
SBA-12. We report here, for the first time, the direct
hydrothermal synthesis of Ti-SBA-12 containing Ti ions in
framework positions.
Similar such correlations had been reported as evidence for
Ti substitution for Si in the framework of microporous TS-1
and large pore Ti-b molecular sieves (note: the ionic radius of
Ti (0.0606 nm) is larger than that of Si (0.04 nm)).7 The
materials showed characteristic type-IV nitrogen adsorption–
desorption isotherms with H1 hysteresis typical of an ordered
mesoporous structure2 (ESI, S1w). High-resolution transmission
electron microscopic (HRTEM) images (Fig. 2) showed a
well-ordered, 3-D, hexagonal structure consistent with the
space group, P63/mmc.2 The pore diameters (dpore) determined
from HRTEM images agree well with those from the N2
adsorption–desorption measurements. While SBA-12 crystals
have elliptical morphology, those of Ti-SBA-12 have a
spherical morphology (SEM). The particle size of the latter
is larger (B5 mm) than that of the former (2.2 mm).
SBA-12, as expected, showed no absorption bands in the
spectral region 200–850 nm. Ti-SBA-12 (Fig. 3) showed an
absorption maximum at 206 nm and a shoulder band at
217 nm. Based on earlier reports on other titanosilicates,8
these bands are attributed to Ti with tetrahedral Ti(OSi)4 and
Ti(OH)(OSi)3 structures, respectively. A small amount of Ti is
also present as anatase-like titania as revealed by the weak
absorption band at 315 nm (Fig. 3). The area of the bands due
to tetrahedral Ti increased linearly with Ti/(Si + Ti) ratio up
In a typical preparation, 8 g of Brij 76 (C18EO10; mol. wt. 711)
was dissolved in 40 g of distilled water and 160 g of 2 M HCl at
313 K with stirring for 2 h. To it, 17.6 g of tetraethyl
orthosilicate was added over 30 min. Ti-isopropoxide
dissolved in 10 ml of isopropanol was then added. Stirring
was continued for 20 h. The gel formed was transferred to a
Teflon-lined stainless steel autoclave. It was crystallized at
373 K for 24 h. The solid formed was recovered, washed with
water, dried at 373 K and calcined in air at 823 K for 8 h.
Materials were prepared varying the concentration of HCl
(0.05–2 M), temperature of synthesis (298–313 K) and Si/Ti gel
ratio (80–20).
The structural and textural properties of SBA-12 and
Ti-SBA-12 are listed in Table 1. The structural integrity with
3-D hexagonal packing was retained even after altering the
concentration of HCl from 2 to 0.05 M in the synthesis gel.
Low-angle X-ray diffraction (XRD) profiles (ESI, S1w)
showed an intense (002) reflection along with resolved (112)
and (300) reflections in the 2y range of 1.5–3.51. For Ti-SBA-12,
National Chemical Laboratory, Pune—411 008, India.
E-mail: d.srinivas@ncl.res.in; Fax: +91 20 2590 2633;
Tel: +91 20 2590 2018
w Electronic supplementary information (ESI) available: XRD, N2
adsorption–desorption profiles, structure–activity correlation plots,
scheme showing the reaction products of cyclohexene, experimental
procedure for carrying out catalytic reactions and catalyst character-
ization procedures. See DOI: 10.1039/b915324a
Fig. 1 Variation of unit cell parameters (a and c) and unit cell volume
(V) as a function of Ti/(Si + Ti) molar ratio in Ti-SBA-12 prepared at
298 K using 2 M HCl.
ꢀc
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Table 1 Composition and textural properties of Ti-SBA-12
Sample
Si/Ti (gel)
Conc. HCl/M
S
BET/m2 gÀ1
dpore/nm
Vpore/cm3 gÀ1
SBA-12
N
80
30
30
30
30
30
30
2
2
2
1
0.5
0.2
0.1
0.05
769
767
765
759
980
775
1015
1085
4.1
5.0
5.3
6.9
6.6
4.9
5.9
5.6
0.79
0.96
1.01
1.31
1.61
0.95
1.49
1.53
Ti-SBA-12
Ti-SBA-12
Ti-SBA-12
Ti-SBA-12
Ti-SBA-12
Ti-SBA-12
Ti-SBA-12
Ti(OSi)4 and Ti(OH)(OSi)3 sites are observed with gzz at 2.038
and 2.024, respectively. The former are more abundant than
the latter. In contrast, Ti-SBA-12 and Ti-MCM-41 contain
mainly the latter type superoxo-Ti species (Fig. 4). It may be
noted that such superoxo species could not be generated on
neat SBA-12 and bulk titanium oxide. The spectroscopic
investigations, thus, confirm that Ti is indeed present in
Ti-SBA-12 as isolated, tetrahedral Ti4+ ions.
Oxidation of cyclohexene was chosen as the model reaction.
In cyclohexene, oxidation of the carbon–carbon double bond
by aq. H2O2 yields cyclohexene oxide (epoxide) which upon
further reaction with water, produces 1,2-cyclohexanediol
(diol). Oxidation of the allylic C–H bond results in 2-cyclo-
hexene-1-ol (-ol) which is further oxidized to 2-cyclohexene-1-
one (-one).10 Selective epoxidations over titanosilicates occur
via heterolytic cleavage of the O–O bond by hydroperoxo/
superoxo-Ti species. Allylic C–H bond oxidations proceed
via homolytic O–O bond cleavage (ESI, S3w).8–10 Due to
differences in electronic structure, tetrapodal Ti in titano-
silicates (Ti(OSi)4) facilitate heterolytic O–O bond cleavage
while the tripodal Ti sites (Ti(OH)(OSi)3) facilitate homolytic
cleavage of the O–O bond. When both these sites co-exist, all
the four oxidation products of cyclohexene will be formed.
Indeed, we have observed all of them in the oxidation reactions
over TS-1, Ti-MCM-41 and Ti-SBA-12 (Table 2).
Fig. 2 High-resolution transmission electron microscopic images of
(a) SBA-12 and (b) Ti-SBA-12 (Si/Ti = 30) prepared at 298 K and
using 2 M HCl.
to a value of 0.033 (ESI, S2w). The concentration of tetra-
hedral Ti ions is higher (UV-visible and EPR spectroscopy)
when the materials were prepared using 0.1–0.2 M HCl.
Ti-SBA-12 showed overlapping vibrational bands in its
FTIR spectrum at 948 and 969 cmÀ1. While the former had
been attributed to Si–O–Ti vibrations of framework substituted
Ti, the latter band is due to Si–O–Si vibrations.7,8 SBA-12
showed only the latter band. Ti framework-substitution was
confirmed also from FT-Raman spectroscopy wherein a new
band at 955 cmÀ1 was observed in Ti-containing materials.
Titanosilicates on contact with aq. H2O2 generate peroxo
and superoxo titaÀnium species. While the former are diamagnetic,
the latter (Ti(O2 ꢁ)) are paramagnetic and can be detected by
Ti-SBA-12 with 3-D hexagonal mesoporous structure
resulted in 93.2% conversion of cyclohexene with 30%
aqueous H2O2 as an oxidant. Conversions of only 19.2 and
À
EPR spectroscopy.9 The spectrum of Ti(O2 ꢁ) generated
on Ti-SBA-12 along with those on Ti-MCM-41 and TS-1
(prepared by known procedures9) are depicted in Fig. 4. In
À
the case of TS-1, two types of Ti(O2 ꢁ) species arising from
Fig. 3 Diffuse reflectance UV-visible spectra of SBA-12 and Ti-SBA-12
Fig. 4 EPR spectra of superoxo-Ti on Ti-SBA-12 and other titano-
prepared at 313 K using 2 M HCl.
silicates.
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Table 2 Catalytic activity: oxidation of cyclohexenea
Product selectivity (%)
Conv. (%)
(TOF)
epoxide
diol
-ol
-one
Catalyst (Si/Ti)
TS-1 (33)b
19.2 (3)
34.2 (7)
93.2 (14)
100 (15)
100 (15)
5.9
12.9
17.3
29.7
96.2
69.3
60.3
33.9
0
9.7
15.1
6.8
14.2
20.3
1.1
Ti-MCM-41 (40)b
Ti-SBA-12 (30)b
Ti-SBA-12 (30)c
Ti-SBA-12 (30)d
20.0
34.6
49.9
2.6
0
a
Fig. 6 Tentative reaction mechanism for the oxidation of cyclohexene
Reaction conditions: catalyst, 0.1 g; cyclohexene, 0.82 g; substrate :
oxidant = 2 : 1.2; acetone, 5 ml; temperature, 333 K; reaction time, 12 h.
over Ti-SBA-12.
b
c
Oxidant = 30% aq. H2O2; solvent = acetone. Oxidant = 5.5 M
d
catalytic activity for oxidation of bulkier molecules. A tentative
reaction mechanism over tetrahedral Ti in titanosilicates is
presented in Fig. 6.
TBHP in decane; solvent = acetonitrile. Oxidant = 5.5 M TBHP in
decane; solvent = dichloromethane. TOF = mol of cyclohexene
converted per mol of titanium per hour.
In conclusion, we report here, for the first time, the synthesis
of framework Ti-substituted Ti-SBA-12 molecular sieves by
controlling the concentration of HCl used in the synthesis.
By an appropriate choice of oxidant and solvent, high
conversions and, importantly, selectivity (495%) in epoxi-
dation reactions of bulky molecules over these catalysts could
be obtained. The mesoporous structure of Ti-SBA-12 enabled
a better accessibility of Ti active sites to bulky substrate
molecules which is reflected in the high conversions of
cyclohexene and cyclooctene and B100% selectivity to the
epoxide over these novel catalysts. Such high selectivities at
high conversions for oxidation of cyclooctene over titano-
silicates have not been reported, so far.8
34.2% were obtained over microporous TS-1 and 2-D
hexagonal mesoporous Ti-MCM-41, respectively (Table 2).
The selectivity for (epoxide + diol) is lower over Ti-SBA-12
(51.2%) than on TS-1 (75.2%). Ti-SBA-12, containing more
tripodal Ti sites (Ti(OH)(OSi)3) than TS-1 (Fig. 3 and 4),
facilitates a radical mechanism and homolytic cleavage of the
O–O bond of reactive oxo-Ti species. A linear correlation was
observed between catalytic activity and the content of frame-
work tetrahedral Ti ions (Fig. 5). The epoxide selectivity also
correlated with the concentration of such Ti sites (estimated
from XRD and UV-visible spectroscopy) (ESI, S4w).
Oxidation reactions were also conducted with 5.5 M tert-
butyl hydroperoxide (TBHP) in decane instead of aqueous
H2O2 as oxidant. A major influence of solvents on product
selectivity was observed (Table 2). Epoxide selectivities of only
29.7% were observed in acetonitrile and acetone solvents.
However, reactions in dichloromethane yielded 100% cyclo-
hexene conversion and 96.2% epoxide selectivity, the highest
so far reported for cyclohexene oxidation over titanosilicate
molecular sieves. Cyclooctene, an even bulkier substrate, was
converted (61.9%) to epoxide with 100% selectivity, over
Ti-SBA-12 (Si/Ti = 30). Under similar conditions, TS-1,
using aq. H2O2 as the oxidant, as expected, exhibited negligible
A. K. acknowledges the Council of Scientific and Industrial
Research (CSIR), New Delhi for the Research Fellowship.
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Fig. 5 Correlation of catalytic activity (cyclohexene conversion) with
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ꢀc
This journal is The Royal Society of Chemistry 2009
6486 | Chem. Commun., 2009, 6484–6486