e.g. SiO 4 , AlO 4 etc., many of the newer architectures involve other types of polyhedra such as octahedral XO 6 , pentacoordinated XO 5 , or pyramidal XO 4 or XO 3
units. These exciting developments will gradually have an impact on the applications of such materials. The traditional applications of aluminosilicate zeolites lie
in the areas of ion-exchange, separations and catalysis [7], but the new generation
of materials offers a wider range of chemical and physical properties that are only
just beginning to be explored.
Two other developments have further enhanced the scope of crystalline nanoporous materials during the last decade. One is the discovery of the so-called
mesoporous materials at Mobil in 1992 [8]. The essence of Mobil’s discovery is that
self-assembling organic molecules, such as surfactants or block copolymers, can be
used to create silicas with porosity on the 2–30 nm scale. This class of materials is
often referred to as the M41S family. Their porosity is periodic, but the silica walls
of the materials remain amorphous. The concept has been extended to other types
of materials, such as metals [9], oxides [10], and phosphates [11], and other classes
of self-assembling organics, such as block copolymers [12], have also been used.
These fascinating materials have interesting properties and are beginning to be
exploited for a diverse range of application, including catalysis, low-k dielectrics,
and lasers. We shall not cover this area in the present chapter since it has recently
been reviewed in some detail [13].
The second area that has exploded during the last decade concerns materials that
involve open-frameworks that are constructed from both inorganic and organic
components; we shall refer to them collectively as hybrid materials and will cover
them in two separate sections. The first will examine the so-called coordination
polymers [14] in which molecular coordination compounds are connected by organic linkers to form chains, sheets or 3-D networks. The second class involves
extended metal–oxygen–metal networks that are decorated by organic ligands; we
shall refer to these as hybrid metal oxides.
18.2
Stability of Open-Framework Materials
The open frameworks of zeolites are slightly less stable than the corresponding
condensed structures [15, 16] into which they will transform during severe thermal
treatment. Nevertheless, the difference in energy between a-quartz, the most stable
polymorph of silica, and siliceous faujasite, one of the most open and least stable,
is only about 15 kJ mol
À1 . The extensive occurrence of aluminosilicate zeolites
and their widespread utility in industry therefore depend heavily upon both the
strengths of their TaO bonds (e.g. SiaO @ 466 kJ mol
À1 ), which render them
stable with respect to framework rearrangement. The challenge with many of the
newer materials is that their stability with respect to transformation into alternative
condensed structures is considerably lower and they frequently collapse on dehydration or other means of activation. It is for this reason that only a small subset of
the many open-framework families of materials can be rendered truly nanoporous,
18 Nanoporous Materials
590
units. These exciting developments will gradually have an impact on the applications of such materials. The traditional applications of aluminosilicate zeolites lie
in the areas of ion-exchange, separations and catalysis [7], but the new generation
of materials offers a wider range of chemical and physical properties that are only
just beginning to be explored.
Two other developments have further enhanced the scope of crystalline nanoporous materials during the last decade. One is the discovery of the so-called
mesoporous materials at Mobil in 1992 [8]. The essence of Mobil’s discovery is that
self-assembling organic molecules, such as surfactants or block copolymers, can be
used to create silicas with porosity on the 2–30 nm scale. This class of materials is
often referred to as the M41S family. Their porosity is periodic, but the silica walls
of the materials remain amorphous. The concept has been extended to other types
of materials, such as metals [9], oxides [10], and phosphates [11], and other classes
of self-assembling organics, such as block copolymers [12], have also been used.
These fascinating materials have interesting properties and are beginning to be
exploited for a diverse range of application, including catalysis, low-k dielectrics,
and lasers. We shall not cover this area in the present chapter since it has recently
been reviewed in some detail [13].
The second area that has exploded during the last decade concerns materials that
involve open-frameworks that are constructed from both inorganic and organic
components; we shall refer to them collectively as hybrid materials and will cover
them in two separate sections. The first will examine the so-called coordination
polymers [14] in which molecular coordination compounds are connected by organic linkers to form chains, sheets or 3-D networks. The second class involves
extended metal–oxygen–metal networks that are decorated by organic ligands; we
shall refer to these as hybrid metal oxides.
18.2
Stability of Open-Framework Materials
The open frameworks of zeolites are slightly less stable than the corresponding
condensed structures [15, 16] into which they will transform during severe thermal
treatment. Nevertheless, the difference in energy between a-quartz, the most stable
polymorph of silica, and siliceous faujasite, one of the most open and least stable,
is only about 15 kJ mol
À1 . The extensive occurrence of aluminosilicate zeolites
and their widespread utility in industry therefore depend heavily upon both the
strengths of their TaO bonds (e.g. SiaO @ 466 kJ mol
À1 ), which render them
stable with respect to framework rearrangement. The challenge with many of the
newer materials is that their stability with respect to transformation into alternative
condensed structures is considerably lower and they frequently collapse on dehydration or other means of activation. It is for this reason that only a small subset of
the many open-framework families of materials can be rendered truly nanoporous,
18 Nanoporous Materials
590