Chemistry Reference
In-Depth Information
peroxide utilisation [91] were noted. In one case
(hexane oxidation with Ti-MCM-41/H
2
O
2
) silylation
caused a reduction in hydrogen peroxide decompo-
sition from 74.7% to 0%, albeit at low conversion
levels. Similar results were obtained with epoxida-
tions (ca. 60% decomposition to 0% upon silylation)
(Fig. 7.17). Similar results have been noted with
other systems such as guanidines [93] in the base-
catalysed epoxidation using hydrogen peroxide (see
Section 3.4).
A related paper by Kochkar & Figueras [94]
describes an approach where the surface polarity
was reduced by incorporating phenyl groups during
the synthesis of the material. This approach has
proved successful also in base catalysis [68] (see
Section 3.4).
A further enhancement in activity has been found
by the incorporation of Ge into the MCM-41 frame-
work. Post-incorporation of Ti centres led to a
40% increase in catalyst lifetime in epoxidation
reactions, using organic peroxides such as cumene
hydroperoxide and
t
-BuOOH [95]. Abbenhuis
et al
.
have demonstrated the efficiency and reusability of
a novel supported Ti silsesquioxane (Fig. 7.18) in the
epoxidation of alkenes with
t
-BuOOH in hexane
[96].
The paper by Abbenhuis
et al
. [96] contains some
very interesting information on some aspects of the
catalyst system that were not discussed in many of
the previous papers and that may have some gener-
ality and help to develop further this family of cata-
lysts. Firstly, the Ti species was supported on a
variety of MCM-41 support materials, with varying
quantities of Al in the framework. It is clear from the
catalytic results that the presence of Al is very dam-
aging to both the activity of the catalyst and its sta-
bility towards leaching. Leaching of catalytically
active material from the all-Si MCM-41 catalysts was
not observed, and the leaching in other cases could
be stopped by silylation of the surface, something
that is not found on amorphous silica-supported Ti
silsesquioxane. Further insights into the stability of
Fig. 7.17
Enhanced utilisation of
hydrogen peroxide via surface
treatment.
Fig. 7.1
8
Epoxidation of
cyclooctene with supported
titanopolysilsesquioxanes.
