198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

132 Y. Aoyama
4 Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
4.1 Criteria of Zeolitic Catalysis . . . . . . . . . . . . . . . . . . . . . . . 154
4.2 Catalysis by a Hydrogen-Bonded Organic Solid . . . . . . . . . . . . 154
4.3 Immobilization ofSoluble Metal Complexes . . . . . . . . . . . . . . 157
5 Concluding Remarks and Future Prospects . . . . . . . . . . . . . . 158
5.1 Manipulation of Pores . . . . . . . . . . . . . . . . . . . . . . . . . . 158
5.2 Solid Catalysts in Organic Transformations . . . . . . . . . . . . . . 158
5.3 Organic Zeolite Analogues as Enzyme Mimics in Water . . . . . . . 159
6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
1
Introduction
Zeolites are a class of microporous inorganic crystals composed of aluminosilicate
tetrahedra, whose internal cavities are capable of not only reversible guest
binding but also catalysing chemical reactions [1–5]. These unique properties
challenge chemists to mimic some of the zeolitic functions by using organic or
metal-organic components [6–9]. A great advantage of molecular solid materials
is that they are spontaneously formed from molecular building blocks. The
diversity and designability of organic structures as well as intermolecular interactions
spur the research activity in this area [10–13].
There is no widely-accepted definition of organic zeolite analogues. The definition
may depend on the functions expected for such materials. An obvious
function is as selective adsorbents, which may be used in the separation, purification,
or trapping and storage of particular chemicals [6, 9]. Another potential
one is that of functional materials as either host-guest adducts or guest-free
porous solids, which may exhibit unique electronic, optical or mechanical
properties [14, 15]. Still, a third or a most important function is that of solid
catalysts, which should be readily recovered; this is highly attractive from the
environmental as well as resource utilisation viewpoints. In mimicking the key
guest-binding properties of zeolites, a variety of lattice inclusion compounds
were prepared, where guest molecules were included in well-defined host cavities
[6–13]. However,“unusual” properties and catalytic activities of solid organic
hosts remain almost completely unexplored. Heterogeneous catalysts have so
far been exclusively inorganic materials including zeolites. In addition to the
general problems in crystal engineering [16, 17], there seem to be a couple of
fundamental questions associated with functional organic zeolite analogues.
One is static or structural; is it really possible to maintain a large volume of
permanent voids in the absence of any guests in organic crystals? The other is
dynamic or kinetic; how do molecules diffuse in solid materials?
The object of this article is to review the present stage of mimicking the zeolitic
functions, to highlight the achievement, although very preliminary, in constructing
catalytic organic and metal-organic solids, and to discuss the problems