cessful to date. An obvious exception to this generalization, however, is the case of
MnO 2 , which is known to exhibit small channels up to about 0.7 nm in diameter
in the minerals hollandite, cryptomelane, and todorokite. Synthetic analogues of
these minerals have been made [108, 109] in the presence of metallic cations (M)
such as Mn
2þ , Ni
2þ , Cu
2þ and Zn
2þ . They are based upon a structural principle
in which octahedral MnO 6 units share edges and the metallic cations occupy
the channels, and correspond to a stoichiometry of M x MnO 2 ÁnH 2 O in the assynthesized form. As a consequence, these are mixed valence materials with a
manganese oxidation state typically in the range 3.5–3.8, and they show a variety of
interesting electronic, magnetic, and catalytic properties. A small concentration of
manganese vacancies is often observed, and a limited amount of manganese substitution is also possible. The water of hydration can be removed at temperatures
up to about 500
C, yielding nanoporous materials which will adsorb small organics such as cyclohexane.
The existence of these stable, open-frameworks in the specific case of manganese is presumably related to the high ligand field stabilization energy associated
with the t 2g
3 electronic configuration of Mn
4þ . This should render the materials
kinetically stable with respect to their collapse into more condensed phases. It is
also, no doubt, the reason why it has been possible to make stable mesoporous
phases based upon MnO 2 , and indeed to remove the surfactant templates in order
to access their microporosity [110].
18.5.5
Sulfates
Given the many examples of open-framework metal phosphates, it is not surprising that sulfates are now being explored as possible open-framework materials.
Synthesis strategies suitable for preparing open-framework materials are still being
proposed [111], but hydro/solvothermal conditions similar to those used to make
phosphates are currently being employed. Although open-framework materials
with three-dimensional architectures have not yet been prepared, one- and twodimensional examples of V [112], Fe [113, 114], Cd [115], La [116], and U [117]
sulfates have now been synthesized. Each of these examples incorporates the same
types of amine molecules commonly used to template metal phosphates, suggesting that porous materials may be achievable. Interesting polyhedral networks are
also possible. Rao and coworkers, for example, have discovered an iron fluorosulfate [114] exhibiting a distorted Kagome lattice. This relatively rare example exhibits magnetic hysteresis at low temperatures, which has not previously been
observed for undistorted examples.
18.6
Hybrid Nanoporous Materials
The rapid and eye-catching developments in the area of nanoporous inorganic
materials over the last decade have been paralleled by work on systems in which
18 Nanoporous Materials
608
MnO 2 , which is known to exhibit small channels up to about 0.7 nm in diameter
in the minerals hollandite, cryptomelane, and todorokite. Synthetic analogues of
these minerals have been made [108, 109] in the presence of metallic cations (M)
such as Mn
2þ , Ni
2þ , Cu
2þ and Zn
2þ . They are based upon a structural principle
in which octahedral MnO 6 units share edges and the metallic cations occupy
the channels, and correspond to a stoichiometry of M x MnO 2 ÁnH 2 O in the assynthesized form. As a consequence, these are mixed valence materials with a
manganese oxidation state typically in the range 3.5–3.8, and they show a variety of
interesting electronic, magnetic, and catalytic properties. A small concentration of
manganese vacancies is often observed, and a limited amount of manganese substitution is also possible. The water of hydration can be removed at temperatures
up to about 500
C, yielding nanoporous materials which will adsorb small organics such as cyclohexane.
The existence of these stable, open-frameworks in the specific case of manganese is presumably related to the high ligand field stabilization energy associated
with the t 2g
3 electronic configuration of Mn
4þ . This should render the materials
kinetically stable with respect to their collapse into more condensed phases. It is
also, no doubt, the reason why it has been possible to make stable mesoporous
phases based upon MnO 2 , and indeed to remove the surfactant templates in order
to access their microporosity [110].
18.5.5
Sulfates
Given the many examples of open-framework metal phosphates, it is not surprising that sulfates are now being explored as possible open-framework materials.
Synthesis strategies suitable for preparing open-framework materials are still being
proposed [111], but hydro/solvothermal conditions similar to those used to make
phosphates are currently being employed. Although open-framework materials
with three-dimensional architectures have not yet been prepared, one- and twodimensional examples of V [112], Fe [113, 114], Cd [115], La [116], and U [117]
sulfates have now been synthesized. Each of these examples incorporates the same
types of amine molecules commonly used to template metal phosphates, suggesting that porous materials may be achievable. Interesting polyhedral networks are
also possible. Rao and coworkers, for example, have discovered an iron fluorosulfate [114] exhibiting a distorted Kagome lattice. This relatively rare example exhibits magnetic hysteresis at low temperatures, which has not previously been
observed for undistorted examples.
18.6
Hybrid Nanoporous Materials
The rapid and eye-catching developments in the area of nanoporous inorganic
materials over the last decade have been paralleled by work on systems in which
18 Nanoporous Materials
608