Chemistry Reference
In-Depth Information
Chapter 7: Micelle-templated Silicas as
Catalysts in Green Chemistry
DUNCAN MACQUARRIE
1 Introduction
such as Al (acid zeolite) or a transition metal (redox-
active zeolites). The most well-known and success-
ful example of the latter class is the titanium silicalite
zeolite TS-1, which is a very effective oxidation cata-
lyst that functions by the activation of hydrogen
peroxide [9-13].
Although these microporous materials are excep-
tionally good at the transformation of small mol-
ecules and are used extensively in industry for the
conversion of small-molecule hydrocarbon feed-
stocks, their small pore size precludes them from
wider use, in particular limiting their application in
synthetic organic chemistry.
The advent of the first of the micelle-templated
materials brought with it the promise of an ex-
tension of the remarkable chemistry associated
with zeolites to such larger molecule chemistry.
Although the extension to such systems has not
been quite as direct as initially expected, these
materials are nonetheless displaying great promise in
a range of applications, including many for which
zeolites are not suitable. Reviews on the synthe-
sis and properties of these systems are available
[14-21], as are reviews on catalytic applications
[22,23].
The subject of this chapter is the use of micelle-
templated silicas as catalysts for green chemistry.
Micelle-templated silicas (and related materials) are
mesoporous inorganic solids that serve as supports
for catalysts or, with appropriate framework substi-
tution, as catalysts in their own right. Limiting the
chapter to these structured mesoporous systems
leaves out the large body of work carried out on
amorphous, irregular mesoporous solids such as
modified chromatographic silicas. However, the
potential of the newer, structured materials parallels
that of the amorphous materials in the majority of
cases, with the additional benefit of a more regular
structure capable of shape selectivity in some appli-
cations and a more regular environment for the cat-
alytic centre. Reviews on functional mesoporous,
amorphous systems are available [1-5].
2 Structured Mesoporous Materials
The ability to produce mesoporous materials with
highly controllable structural features has been
enhanced dramatically by the discovery of the
micelle-templated silicas [6-8] and, less directly rel-
evant so far to green chemistry, many other metal
oxides. These materials can be produced relatively
easily using sol-gel techniques with micellar tem-
plates. In this respect, the synthesis of such materi-
als can be considered as being broadly analogous to
those of the microporous zeolites discussed else-
where in the topic. The synthesis of zeolites involves
the (hydrolysis and) condensation of inorganic
monomeric species around a single molecule
template (typically a quaternary ammonium salt),
leading to a regular inorganic material with occluded
template molecules. This very regular material then
is treated thermally to remove the template mol-
ecules, leaving behind a highly structured
microp-
orous
solid. Within the pores are found catalytically
active sites, usually associated with a heteroatom
2.1 Synthesis of micelle-templated materials
One of the most positive aspects of these materials is
that the synthesis of the materials is generally rela-
tively simple (manipulatively speaking; the detailed
nature of the chemistry involved is very complex).
Coupled with a great deal of flexibility, the range of
materials accessible is remarkably high. Already, a
wide range of materials can be prepared, including
silicas (either purely silica-containing or doped with
a wide range of other transition and non-transition
elements), alumina [24-26], zirconia [27-29], tin
oxide, niobium and tantalum oxides [30-32]. Addi-
tionally, it is possible to incorporate organic groups
during the synthesis as well as post-synthetically
[33-35].
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