Chemistry Reference
In-Depth Information
templates from such materials is to calcine the ma-
terial and burn the template out. It has been shown
that solvent extraction methods are poorly suited to
remove quaternary ammonium templates from
these materials, not least because the template can
exist as a counter-ion if there is Al in the framework
(this imparts a negative charge on the inorganic
matrix, which is counterbalanced by the quaternary
cation). This can be ion exchanged but the process is
relatively difficult and leads to salt waste being
produced. It is likely that more recent advances, such
as supercritical fluid extraction using CO
2
doped with
methanol [39,40] or just CO
2
(for the case of amine
templates), may prove to be effective routes to
template removal and recovery [41]. Jaroniec's
recent discovery that the template can be removed
by reaction with silanes (Fig. 7.4) is a major and
remarkable step towards the efficient and clean
preparation of organically modified MCMs (see
Section 2.3) [42].
In contrast, the removal of neutral surfactants
(amines and polyethers) is very simple. The rela-
tively weak interactions between the template and
the wall of the material (H-bonding) can be broken
easily by heating in a H-bonding solvent such as
water or ethanol, and the template can be recovered
completely, leaving behind the inorganic frame-
work and a solution of template [43,44]. These
methods are currently the greenest way to prepare
such materials.
Thus, several routes exist to form a host of
materials that all display high surface area, control-
lable and well-defined porosity (both shape and
size of pores) and excellent stability. In terms of cat-
alytic applications, the vast majority of cases involve
silicas or materials that are predominantly silica with
small amounts of other ions (typically tri- or tetrava-
lent). The subsequent modification of these materi-
als constitutes a further elaboration of these
materials.
2.2 Post-functionalisation of
micelle-templated materials
Analogously to silicas, these materials can be reacted
with a range of other compounds, both inorganic
and organic, to give a range of modified materials.
Examples include post-modification with AlCl
3
[45,46], organic functions such as simple amines
[47-50], transition metal complexes [51-57] and
chiral catalytic centres [58-63] all leading to catalytic
materials with a range of active centres. The post-
functionalisation relies on the same methodology as
for the analogous reaction on amorphous silicas
[64,65] (Fig. 7.5), although the organic modification
of these materials does allow for one novel route.
Typically, the functionalisation of silicas with
organic groups takes place via an organically modi-
fied silane (RO)
3
SiR¢, where R is usually Me or Et
and R¢ contains the active group. These molecules
react with the surface of the silica by one (or both)
of two routes: the hydrolysis-condensation reaction
(when the reaction is carried out in protic solvents);
or a metathetical route involving siloxane bridges,
which predominates in non-hydroxylic solvents
such as toluene [66] (Fig. 7.6).
2.3 Direct preparation of organically
modified micelle-templated silicas
The hydrolysis-condensation reaction is the reaction
that takes place during the synthesis of silicas (and
micelle-templated silicas) when TEOS is used as the
source of silica (a similar reaction occurs with the
anionic silicate route, where SiO
-
units are proto-
nated to give SiOH, which then condenses in a
similar way). This has prompted researchers to
develop routes to organically modified materials
where TEOS is not condensed alone, but rather in
conjunction with one or even two additional
organosilanes, in the presence of a template, leading
RSi(OMe)
3
R
R
R
R
R
R
R
Fig. 7.4
Direct template removal and
functionalisation.
