Chemistry Reference
In-Depth Information
O
Si
SO
3
H
O
(EtO)
4
Si
+
(MeO)
3
Si(CH
2
)
3
SH
OR
Me
H
2
O
2
+
(MeO)
3
SiCH
3
catalyst
7
6
OH
OCOR
5
RCO
2
H
4
3
OH
OH
2
catalyst
1
OH
0
OH
0
2
4
6
Methyl groups (meq/g)
Fig. 7.14
Influence of hydrophobisation of the surface on esterification activity.
Fig. 7.15
Formation of
difurylmethanes using sulfonic acid
catalysts.
varying size and bulkiness, many attempts have been
made to mimic TS-1 using mesoporous analogues.
The work has met with some success but it has also
highlighted the dangers of assuming that the newer
mesoporous materials are simply zeolites of larger
pore size. As was seen for the acid catalysts described
earlier, the activities can be very different. This is true
for the Ti materials too, where the much more
hydrophilic nature of the mesoporous materials
causes a significant reduction in activity and selec-
tivity compared with that of the hydrophobic TS-1.
The following section details the attempts to produce
an effective Ti catalyst that can be used in epoxida-
tion reactions, especially with the green oxidant
hydrogen peroxide [86]. Following on from this is a
discussion of alternative catalytic systems that can be
used in epoxidation.
Early attempts to prepare and use Ti-containing
mesoporous materials met with enough success to
encourage several groups to modify the catalysts and
to enhance their activity significantly. Initial work
was done by the groups of Pinnavaia [87] and Corma
[88,89], both of whom showed that Ti-containing
materials could be synthesised readily in a straight-
forward extension of the all-Si procedures available
using tetraalkyl titanates in addition to the silica pre-
cursor, and that the materials were active in epoxi-
dations using hydrogen peroxide. As might be
expected, bulkier substrates could be reacted within
the pores of the catalyst than is possible with TS-1,
