Organic–Inorganic Hybrid Saponites
[30,31]
widely studied titanium-containing silicate materials.
Different arrangements of Ti-NH POSS molecules in the
3
With TBHP, no formation of epoxide was detected and the
conversion of cyclohexene was negligible. The solids were
active, on the contrary, in the presence of aqueous hydrogen
peroxide and, in this case, the formation of cyclohexene ep-
oxide was recorded as the main product. Such behavior is a
further confirmation that Ti centers are located in the inter-
layer spaces and are reachable only by small and sterically
unencumbered oxidants. Actually, the formation of cyclo-
hexene epoxide was low in both hybrid materials (around
saponite interlayer space were suggested.
DR UV/Vis data showed that the tetrahedral coordination
of titanium centers of Ti-NH POSS were preserved during
3
the intercalation into H-SAP, while it was drastically modi-
fied in the case of the Ti-NHM-1 hybrid material, where ag-
gregation of molecular complexes occurred. It is relevant to
underline that the embedding of Ti-NH POSS in saponite
3
clays leads to a remarkable thermal stabilization of the mo-
lecular compound and that, depending on the chemical
nature of the clay interlayer, hybrid materials with signifi-
cantly different chemical properties can be prepared.
6
% for Ti-NHM-1 and 10% for Ti-NHM-2), but noteworthy
if one considers that, under the same conditions, a maximum
epoxide yield of about 30% is obtained over the highly ac-
cessible Ti-SBA-15 mesoporous silica.
The catalytic dehydrogenation performances of Ti-
NH POSS embedded in sodium saponite (Ti-NHM-1)
3
No epoxide formation was detected without catalyst (that
is, no epoxide resulting from noncatalyzed auto-oxidation)
or with the saponite material without titanium (that is, no
activity owing to the sole action of the acid matrix). Thus,
the presence of titanium in the catalysts is essential for ep-
oxide formation. For the same reason, the in situ formation
of peroxoimidic acid (owing to the copresence of acetoni-
trile and H O via Payne-type reactions) as an active oxidiz-
should be exploited for applications in polymer science, for
example, in the preparation of polymer composites with en-
hanced thermal properties, which have already been report-
ed in the literature in the case of composite material based
[20]
on polystyrene.
In this case, the catalytic activity of Ti
centers in combination with the physical barrier effects of-
fered by the clay should be relevant for the formation of
charring products, which have flame-retardant effects,
during combustion. Furthermore, the intercalation of Ti-
2
2
ing species can be excluded.
In terms of selectivity, Ti-NHM-1 and Ti-NHM-2 solids
showed a selectivity toward epoxide after 3 h that was com-
parable to that of Ti-SBA-15, and about 40% of the ob-
served secondary products were due to allylic oxidation to
NH POSS into protonic acid saponite leads to a bifunctional
3
Ti-NHM-2 material which may be used as a heterogeneous
catalyst in which acid sites and isolated metal redox-active
centers are simultaneously present and necessary for two-
step bifunctional catalysis.
2
-cyclohexen-1-ol and 2-cyclohexen-1-one. Such compounds
[32,33]
are formed by free-radical homolytic attack by H O .
In
2
2
addition, the unselective formation of epoxide cleavage
products (short-chain dicarboxylic acids) was detected over
Ti-NHM-2 in minor, but non-negligible, amounts. This
means that the copresence of acid sites (H ) in the interlay-
er space can promote the cleavage of the epoxidized cycloal-
kene.
These results, obtained using a model reaction, demon-
strate that: 1) titanium atoms in Ti-NHM-1 and Ti-NHM-2
are accessible to small reactants and have coordination sites
that are available for interaction with oxidizing species and
suitable for redox reactions, despite the different organiza-
Experimental Section
Materials Preparation
+
Ti-NH
pared following the procedeure reported in the literature.
Na-SAP and H-SAP: Synthetic saponite clays were prepared by hydro-
thermal synthesis of a gel formed by mixing Al[OCH(CH , NaOH,
O (with a gel composition of 1SiO ; 0.834
[OCH(CH ; 0.113NaOH; 18.6 H O) and ex-
2 2
POSS: Ti-aminopropyl-isobutyl POSS (Ti-NH POSS) was pre-
[
19]
A
H
U
G
E
N
N
A
H
U
G
R
N
U
G
3 2 3
) ]
Mg
Mg
A
H
N
T
E
N
N
3
COO)
2
, SiO
2
, and H
2
2
A
H
N
T
E
N
G
3
CO ; 0.113Al
2
)
2
A
H
U
G
R
N
U
G
A
H
U
G
R
N
N
3
)
2
]
3
2
+
+
changed by Na (in the case of Na-SAP sample) and H ions (H-SAP)
according to procedures already described in the literature
[
22–23]
and opti-
[
21, 24]
mized in our laboratories.
(CEC) of the produced solids was 59.7 meq/100 g.
clays were prepared by hydrothermal synthesis of a gel formed by mixing
Al[OCH(CH , NaOH, Mg(CH COO) , SiO , and H O (with a gel
composition of 1SiO 0.834Mg(CH CO 0.113Al[OCH(CH
.113NaOH; 18.6H O) and exchanged by Na and H ions, according to
procedures already described in the literature
The measured cation exchange capacity
tion of Ti-NH POSS molecules in the protonic and sodic
[25,26]
3
Synthetic saponite
clays, and 2) protonic acid centers in Ti-NHM-2 are avail-
able in the catalyst and can be exploited for acid-catalyzed
reactions. By an appropriate choice of the reactants and of
the catalytic reaction, Ti-NHM-2 could be thus used as a
catalyst for reactions where redox and acid-catalyzed steps
are simultaneously required over substrates of medium size.
A
H
U
G
E
N
G
A
H
U
G
R
N
N
3
)
2
]
3
A
H
U
G
R
N
U
G
3
2
2
2
2
;
A
H
U
G
R
N
N
3
2
)
2
;
A
H
U
G
R
N
N
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3 2 3
) ] ;
+
+
0
2
[34,35]
and optimized in our
[
24,25]
laboratories.
Intercalation of Ti-NH
2
POSS in Na-SAP and H-SAP: Suspensions of
both Na-SAP and H-SAP were prepared by adding 1 g of powders to
0 mL deionized water and stirred for 4 h. In parallel, Ti-NH POSS
0.538 g) was mixed with 5 mL THF (Sigma Aldrich) and 2 mL HCl
5
2
(
(
(
Conclusions
[
24]
10%) for 1 h, adapting the procedure reported in literature, to form
Ti-NH POSS)Cl. Finally, the solution containing (Ti-NH POSS)Cl and
3
3
A titano-silsesquioxane bearing in the same molecular struc-
the clay suspensions were mixed under stirring at 508C for 24 h, which
led to Ti-NH POSS/Na-SAP and Ti-NH POSS/H-SAP composite materi-
als, named Ti-NHM-1 and Ti-NHM-2, respectively. Subsequently, the
products were filtered and washed several times with water and dried in
an oven at 808C.
+
ture a functional organic group (-NH3 ) and a metal center
3
3
IV
(
Ti ) was intercalated into two synthetic saponite clays con-
+
taining sodium and H , respectively. XRD and HRTEM
clearly showed that organomodified synthetic clays with dif-
ferent degrees of Ti-NH POSS intercalation were produced.
3
Chem. Asian J. 2011, 6, 914 – 921
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
919
Carniato, Fabio
