Chemistry Reference
In-Depth Information
Fig. 7.2
Control over pore size by micelle expansion and contraction.
S
o
I
o
S
+
I
-
SI
N
o
I
o
S
+
X
-
I
+
S
-
I
+
S
-
M
+
I
-
non-charged materials display lower long-range
order and slightly wider pore size distributions
(although these are still very narrow). It is unlikely
that, in the majority of cases, either of these factors
will be of major importance in catalytic applications.
However, a very important factor in the production
of catalysts based on this technology is the recovery
of template, both on economic and environmental
grounds. Here, the quaternary-derived materials
suffer from a major drawback. It is very difficult to
remove the template in such a way that it can be
recovered [38]. The traditional method to remove
Fig. 7.3
Schematic summary of the possible templating
systems used for micelle-templated assembly: I,
inorganic species; S, surfactant head group (S
+
,
typically trimethylammonium; S
-
, typically sulfonate; S
o
,
NH
2
); N
o
, a polyether; X
-
and M
+
, mediating counter-ions;
—, H-bond interactions. The right-hand structure with its
full bond between S and I is a covalently attached
system.
of template can influence the material in several
ways. The charged templates produce the materials
with the most extensive long-range order and possi-
bly the tightest pore size distribution, whereas the
