Chemistry Reference
In-Depth Information
Fig. 7.5
Representative mechanisms for the functionalisation of micelle-templated silicas [64,65]. Group R can be functionalised
further to elaborate the group if required.
R
Si
OMe
OMe
MeO
H
H
R
R
O
OMe
O
O
MeO
O
Si
MeO
Si
Si
O
O
Si
Si
O
O
O
MeO
MeO
O
O
O
O
O
Si
Si
O
O
O
Si
Si
O
O
Si
O
Si
O
O
O
O
O
O
O
O
Si
O
O
Si
Si
are active basic catalysts [67,68]. Furthermore, much
higher loadings can be achieved and the silanes are
incorporated more robustly into the framework,
leading to more thermally and solvolytically stable
materials. This latter point has been demonstrated by
Macquarrie
et al
., who have published details of the
stability of such materials using
29
Si magic-angle
spinning nuclear magnetic resonance (
29
Si MAS
NMR) to quantify the extent of binding of the silane
to the material [69] and thermal analysis, which
indicated that in one material organic groups were
not lost until >500°C [34]. The different synthetic
procedure may lead to materials that have different
distributions of sites on the surface than those found
in post-modification of (calcined) silicas and micelle-
templated silicas, where grafting (in toluene) is
thought to take place only on hydrophobic patches
[66].
These materials are characterised by a very high
surface area, controllable and regular pore size and
geometry (which is very well suited to diffusion) and
excellent thermal stability. In some cases, particu-
larly where in-pore chemistry is vital to the success
Fig. 7.6
Proposed mechanisms for the functionalisation of
silica surfaces with silanes.
directly to the preparation of organically modified
micelle-templated silicas (Fig. 7.7). This aspect of
these materials has been reviewed recently from the
materials perspective [21].
Examples of materials prepared with quaternary
salts, neutral amines and polyethers are known
[33-35], the latter two groups being more accessible
due to the ease of template removal, which is espe-
cially important in the preparation of solids contain-
ing organic functions. This route has the advantages
of simplicity—one step fewer is required, complete
incorporation of both silanes is often the case (rarely
achieved with post-functionalisation, where excess
silane is required to drive the reaction) and complete
recovery of template using one of the reaction sol-
vents makes the process simple, mild and atom ef-
ficient. This has been demonstrated in the case of
aminopropyl-functionalised HMS materials (those
prepared with a long-chain amine template), which
