Titanosilsesquioxane anchored on mesoporous silicas: A novel approach for the preparation of heterogeneous catalysts for selective oxidations

Article abstract of DOI:10.1002/chem.200801241

Ti-polyhedral oligomeric silsesquioxanes (Ti-POSS) based heterogeneous catalysts were prepared by anchoring of a functional titanium-containing silsesquioxane on the surface of an ordered mesoporous silica (SBA-15) and a non-ordered silica (SiO₂-Dav). The anchoring process was followed by IR spectroscopy, which leads to the condensation reactions between surface silanols of silicas and ethoxy groups. The heterogeneous character of catalysts checked by removing the solid catalyst by centrifugation and testing the residual liquid mixture show no significant loss of active species. Anchored heterogeneous catalysts show lower conversions with respect to reference materials prepared by direct grafting of titanocene on the same silica supports. It is also seen that the anchored materials display slightly better results than those obtained over reference titanium-silica catalysts with comparable metal loading.

Full text of DOI:10.1002/chem.200801241

DOI: 10.1002/chem.200801241
Titanosilsesquioxane Anchored on Mesoporous Silicas: A Novel Approach
for the Preparation of Heterogeneous Catalysts for Selective Oxidations
Fabio Carniato,[a] Chiara Bisio,[a] Enrico Boccaleri,[a] Matteo Guidotti,[b]
Elena Gavrilova,[b, c] and Leonardo Marchese*[a]
Polyhedral oligomeric silsesquioxanes (POSS) are a class
of condensed three-dimensional organosiliceous compounds
with cage frameworks that have different degrees of symme-
This work is focused on the preparation of Ti–POSS-
based heterogeneous catalysts, by anchoring of a functional
titanium-containing silsesquioxane on the surface of an or-
dered mesoporous silica (SBA-15) and of a non-ordered
silica (SiO2-Dav). An innovative bifunctional POSS, bearing
in the structure both a TiIV metal centre and a triethoxy
group for grafting on the silica surfaces, was specifically syn-
thesised for this purpose and named Ti–POSS–TSIPI
(Scheme 1, 2).
Ti–POSS–TSIPI was prepared by an equimolar reaction
between Ti–NH2POSS (Scheme 1, 1), the synthesis of which
was reported by some of us,[15] and 3-isocyanatopropyl tri-
ethoxysilane (TSIPI) under basic conditions.
try. These molecular materials have a general formula
[1–3]
R8Si8O12
and display silicon atoms bound to one-and-a-
half oxygen atoms (sesqui), the remaining valence being sa-
turated by organic entities (R). Numerous metals have been
incorporated successfully into POSS cages with the aim to
obtain either specific homogeneous catalysts,[4] or models
for active sites of heterogeneous catalysts. For example,
POSS cages with titanium(IV) complexes proved to be
highly active in the epoxidation of olefins.[5–9]
Special attention was recently devoted to the heterogeni-
sation of Ti–POSS compounds onto insoluble and easily re-
coverable supports. Interesting examples encompass the in-
corporation of Ti–POSS into mesoporous MCM-41, sol–gel
matrices, polysiloxanes and the preparation of organic–inor-
ganic hybrid materials and coatings based on polystyrene
polymers containing Ti–POSS moieties.[9–14] However, the
literature on the direct anchoring, through covalent bonding
of preformed Ti–POSS cages onto silica supports is very
poor.[14] The stabilisation of Ti–POSS compounds onto silica
surfaces through covalent bonds is a relevant issue for ob-
taining heterogeneous catalysts with isolated active centres,
because it may avoid metal leaching from the catalysts.
The product of the reaction was monitored by both IR
(Figure 1) and 1H NMRspectroscopy (see Experimental
[a] Dr. F. Carniato, Dr. C. Bisio, Dr. E. Boccaleri, Prof. L. Marchese
Dipartimento di Scienze e Tecnologie Avanzate and
Nano-SISTEMI Interdisciplinary Centre
Università del Piemonte Orientale “A. Avogadro”
Via Bellini 25G, 15100, Alessandria (Italy)
Fax : (+39)0131360250
E-mail: leonardo.marchese@mfn.unipmn.it
[b] Dr. M. Guidotti, Dr. E. Gavrilova
Istituto di Scienze e Tecnologie Molecolari and
IDECAT CNRUnit, Via G. Venezian 21, Milano (Italy)
[c] Dr. E. Gavrilova
Dip. Scienze Chimiche e Ambientali, Via Valleggio 11, Como (Italy)
Scheme 1. The reaction between Ti-NH2POSS (1) and 3-isocyanatopropyl
triethoxysilane for the preparation of Ti–POSS-TSIPI (2); R=isobutyl
group.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801241.
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COMMUNICATION
product delivered by Grace and it represents an easy avail-
able and cheap mesoporous silica support.
The anchoring of 2 was carried out after evacuating both
silica samples at 773 K for 4 h. The amount of 2 used in
both syntheses was fixed to 20 wt% respect to the support.
The final solids showed a titanium concentration of 0.26 and
0.33 wt.% for 2/SBA-15 and 2/SiO2, respectively.
The anchoring process was followed by IRspectroscopy
(see supporting information), which provided clear-cut evi-
dences that condensation reactions occurred between sur-
face silanols of silicas and ethoxy groups of 2.
Relative to the parent SBA-15, the grafted sample dis-
played a decrease of the XRD peaks intensity (Figure 2,
curve b), which is assigned to the modification of the scat-
tering properties of the solid, for the presence of compound
2. Structural order and morphology of SBA-15 were pre-
served after the grafting of 2, as highlighted by the narrow
and distinct XRD reflections and by TEM micrograph
(Figure 2 inset). A clear shift to higher 2q values of the
XRD peaks was also found, which was assigned to the im-
mobilisation of 2 on the surface of the SBA-15 channels, in
agreement with IRspectroscopic results.
Figure 1. IRspectra in KBr matrix of a) Ti-NH POSS, b) Ti–POSS-TSIPI
(2) and c) 3-isocyanatopropyl triethoxysilane.
2
À1
Section). The absence of the IRabsorption at 2270 cm
,
due to the asymmetric stretching of N=C=O group, con-
firmed that the reaction between 3-isocyanatopropyl trie-
thoxysilane and 1 was complete. Moreover, the IRspectrum
of 2 (Figure 1, b) showed a wide band at about 3300 cmÀ1,
assigned to the NH stretching of ureic group. The success of
the reaction was also confirmed by the absence of the band
at 1600 cmÀ1, due to a bending mode of the NH2 group of 1,
which was replaced by two peaks at 1630 and 1575 cmÀ1 as-
A textural analysis was performed by nitrogen adsorption
(Table 1) to define specific surface area and specific pore
Table 1. Textural properties (measured by N2 adsorption-desorption iso-
therms) of the porous materials studied in this work.
À
signed to the C=O stretching and N H bending of ureic
group, respectively. It is worth noting that the Si-O-Ti
stretching band at 920 cmÀ1 of both the reactant (1) and
product (2) was not modified; this testifies the integrity of
the Ti–POSS cage after reaction with the triethoxysilane.
Thanks to the presence of three ethoxy groups, compound
2 can be easily anchored onto the surface of porous silica
SBA-15 and SiO2-Dav, which are both rich in surface sila-
Material
SBET
[m2 gÀ1
Pore
diameter [Š]
Pore
A
]
volume [cm3 gÀ1
]
SBA-15
650
547
290
265
88
85
221
206
1.04
0.91
1.3
2/SBA-15
SiO2-Dav
2/SiO2-Dav
1.2
À
nols (Si OH). SBA-15 was prepared as previously described
in the literature[16] and showed the typical X-ray diffraction
pattern (Figure 2, curve a) with three distinct reflections at
0.9, 1.53 and 1.768 2q assigned to (100), (110), (200) planes
of a hexagonal array of pores.[16] SiO2-Dav is a commercial
volume. Both SBA-15 and SiO2-Dav showed type IV iso-
therms, according to the IUPAC classification, and H1 hys-
teresis loops. In the case of SBA-15 the hysteresis loop is
representative of structural mesoporous cylindrical or rod-
like pores. The presence of 2 did not significantly modify
the textural features of both silicas (Table 1). This suggests
that 2 is evenly distributed on the surface of the silicas and
that no pore blocking or hindering occurred upon anchor-
ing.
Both titanium-containing materials, were used as catalysts
for the epoxidation of limonene (1-methyl-4-(prop-1-en-2-
yl)cyclohex-1-ene) with tert-butylhydroperoxide (TBHP;
Scheme 2), which was chosen as a test reaction to verify
whether the TiIV sites were accessible to the reactants and
possessed catalytic activity.
The anchored materials were compared to widely studied
titanium-containing heterogeneous catalysts obtained by
grafting a similar loading of titanocene dichloride precursors
onto mesoporous silica supports.[17]
The systems 2/SBA-15 and 2/SiO2 are both active in the
epoxidation of limonene and the specific activity values
(turnover numbers; TONs) obtained on them are rather
Figure 2. X-ray diffraction of SBA-15 (curve a) and 2/SBA-15 (curve b).
Inset: TEM micrograph of 2/SBA-15.
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L. Marchese et al.
ent in the reference titanocene-derived catalysts with respect
to the anchored ones.
A similar behaviour is observed in carveol (2-methyl-5-
(prop-1-en-2-yl)cyclohex-2-enol) epoxidation. Anchored Ti–
POSS-derived catalysts show TON values almost identical
to reference Ti/SBA-15 catalyst (47, 52 and 48, for 2/SBA-
15, 2/SiO2 and Ti/SBA-15, respectively), but they display a
remarkably higher selectivity to endocyclic epoxide than ti-
tanocene-derived systems (ca. 80% vs. 60%, respectively;
see Supporting Information).
Scheme 2. Limonene (1-methyl-4-(prop-1-en-2-yl)cyclohex-1-ene) epoxi-
dation with TBHP.
similar (Table 2). Furthermore, the heterogeneous character
of both catalysts was checked by removing the solid catalyst
by centrifugation and testing the residual liquid mixture for
further reaction. No significant loss of active species was de-
In conclusion, a novel approach for the preparation of
heterogeneous catalysts based on an innovative and versatile
functional titanosilsesquioxane (Ti–POSS-TSIPI; 2) and
mesoporous silicas (SBA-15 and non-ordered silica) is pro-
posed. The spectroscopic characterisation showed that 2 was
effectively anchored with good dispersion on the surface of
both ordered and non-ordered silica supports. Preliminary
catalytic tests in the epoxidation reaction of limonene and
carveol showed that both hybrid materials (2/SBA-15 and 2/
SiO2-Dav) display interesting catalytic activity that was not
sensitive to the morphology of the support. Anchored cata-
lysts showed, in terms of selectivity, good results comparable
to those obtained over the reference titanocene-derived ma-
terials (Ti/SBA-15 and Ti/SiO2-Dav). Finally, this new ap-
proach of functionalisation and immobilisation of Ti–POSS
species may generate a wider interest, especially for mecha-
nistic studies for which a thorough tuning and control of the
chemical surroundings of the Ti active sites is crucial.
Table 2. Catalytic performance of limonene epoxidation over Ti-contain-
ing catalysts.[a]
Catalyst
Ti content[b] C[c]
TON[d]
S[e]
ACHTREUNG
2/SBA-15
2/SiO2-Dav
Ti/SBA-15
Ti/SiO2-Dav 0.29
no catalyst
SiO2
0.23
0.33
0.24
25
39
48
60
4
95
108
192
198
–
88
85
78
82
–
–
n.d.[f]
n.d.
5
–
[a] Reaction conditions: glass batch reactor; 10 mL AcOEt solvent;
358 K; 24 h; 50 mg catalyst; 1.2 mmol TBHP; 1.0 mmol limonene; mesi-
tylene internal standard. [b] Obtained by ICP-AES. [c] Limonene con-
version after 24 h. [d] Turn-over Numbers after 24 h (mol converted sub-
strate/mol Ti). [e] Selectivity to endocyclic limonene monoepoxide after
24 h. [f] Not detected.
Experimental Section
tected (see Supporting Information). Thus, the immobilisa-
tion technique by covalent anchoring does not hinder the
oxidising activity of the TiIV sites and, more significantly, it
is not sensitive to the morphology of the support. When the
epoxidation of limonene is considered, in fact, the choice of
an ordered mesoporous silica with a marked confinement
effect is not essential (this is not the case when simple im-
mobilisation by impregnation is employed[10]) and non-or-
dered commercial silica with large mesopores can be used
successfully.
Anchored catalysts show lower conversions with respect
to reference materials (Ti/SBA-15 or Ti/SiO2-Dav) simply
prepared by direct grafting of titanocene on the same silica
supports. This is likely due to the optimal site isolation ach-
ieved by grafting modest amount of titanocene over high-
surface-area silica supports. However, in terms of selectivity,
the anchored materials display slightly better results than
those obtained over reference titanium–silica catalysts with
comparable metal loading. In all cases, the major product is
the endocyclic limonene epoxide (80–88%). Virtually the
only other product is the exocyclic epoxide (10–12%) when
anchored Ti–POSS-derived catalysts are used, whereas acid-
derived by-products (9–14%) are found when reference tita-
nium catalysts are employed. This behaviour suggests that a
similar reactivity takes place at the TiIV sites for the epoxi-
dation reaction, but a more marked acidic character is pres-
Preparation of Ti–POSS-TSIPI (2): Ti-NH2POSS (1 g, 1.1”10À3 mol)[15]
was dissolved in chloroform (40 mL). An equimolar amount of 3-isocya-
natopropyl triethoxysilane and triethylamine was added to the solution.
The reaction was performed under inert conditions, using nitrogen flow,
at room temperature for 20 h. Finally, the solvent was evaporated until a
white powder was obtained. 1H NMR(400 MHz CDCl 3): d=3.9 (t, 2H;
NH, ureic group), 3.7 (q, 6H; CH2, OEt) ,3.6 (m, 1H; CH, OiPr), 2.9 (m,
4H; CH2 of ureic group), 1.85 (m, 6H; CH), 1.20 (m, 15H; CH3 of OEt
and OiPr groups), 0.93 (d, 36H; CH3 of isobutyl groups), 0.59 ppm (m,
20H; CH2).
Preparation of SBA-15: Pluronic P123 (4.0 g, Sigma–Aldrich) was dis-
solved in water (30 g) and HCl (2n, 120 g) with stirring at 308 K. Tetra-
ethoxysilane (8.5 g, Sigma–Aldrich) was added to the solution and stirred
at 308 K for 24 h. The mixture was aged at 373 K in an autoclave for
24 h. The solid product was filtered and washed several times by water.
Calcination was carried out increasing the temperature at 18CminÀ1
under air flow from room temperature to 823 K and heating the material
at 823 K for 5 h.
Preparation of 2/SBA-15: SBA-15 (1 g) was evacuated at 773 K for 4 h in
order to remove the adsorbed water and to activate the surface. Then the
powder was dispersed in anhydrous THF (by Sigma–Aldrich) under
vacuum. Compound 2 (20 wt% respect to the support) was added to the
suspension. The reaction was stirred at 323 K for 24 h. The final product
was filtered through a fine sintered glass funnel and washed several times
by THF.
Preparation of 2/SiO2: SiO2-Dav (1 g, commercial silica, obtained from
Grace) was treated at 773 K for 4 h in order to remove the adsorbed
water and to activate the surface. Then, SiO2 was dispersed in anhydrous
THF (by Sigma–Aldrich) under vacuum. Compound 2 (20 wt% respect
to the support) was added to the suspension. The reaction was stirred at
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Heterogeneous Catalysis
COMMUNICATION
323 K for 24 h. The final product was filtered through a fine sintered
glass funnel and washed several times by THF.
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The authors acknowledge financial support from MIUR(PRIN Project
“Progettazione e sintesi di Silsesquiossani Poliedrici Multifunzionali per
Compositi Polimerici Innovativi Termicamente Stabili”), Dr. Claudio
Cassino (DiSAV Alessandria-Italy) for NMRcharacterisation and Dr.
Luca Bertinetti (Dipartimento di Chimica IFM and Nanostructured In-
terfaces and surfaces (NIS), Università di Torino) for TEM analysis. E.G.
thanks MIURfor a studentship.
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Keywords: heterogeneous catalysis
mesoporous materials · silicates · silsesquioxanes · titanium
·
epoxidation
·
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Received: June 23, 2008
Published online: August 4, 2008
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