Mesoporous materials other than metal oxides

MESOPOROUS MATERIALS OTHER THAN METAL OXIDES... 

The key property required of the inorganic species is ability to build up (polymerize) around the template molecules into a stable framework. As is already evident in this article, the most commonly used inorganic species are silicate ions, which yield a silica framework. The silica can be doped with a wide variety of other elements (heteroatoms), which are able to occupy positions within the framework. For example, addition of an aluminium source to the synthesis gel provides aluminosilicate ions and ultimately an aluminosilicate mesoporous molecular sieve. Other nonsilica metal oxides can also be used to construct stable mesoporous materials. These include alumina, zirconia, and titania. Metal oxide mesophases, of varying stability, have also been obtained from metals such as antimony (Sb), iron (Fe), zinc (Zn), lead (Pb), tungsten (W), molybdenum (M), niobium (Nb), tantalum (Ta), and manganese (Mn). The thermal stability, after template removal, and structural ordering of these mesostructured metal oxides, is far lower, however, than that of mesoporous silica. Other compositions that are possible include mesostructured metal sulfides (though these are unstable to template removal) and mesoporous metals (e.g., platinum, Pt). 

Ordered mesoporous materials of compositions other than silica or silica-alumina are also accessible. Employing the micelle templating route, several oxidic mesostructures have been made. Unfortunately, the pores of many such materials collapse upon template removal by calcination. The oxides in the pore walls are often not very well condensed or suffer from reciystallization of the oxides. In some cases, even changes of the oxidation state of the metals may play a role. Stabilization of the pore walls in post-synthesis results in a material that is rather stable toward calcination. By post-synthetic treatment with phosphoric acid, stable alumina, titania, and zirconia mesophases were obtained (see [27] and references therein). The phosphoric acid results in further condensation of the pore walls and the materials can be calcined with preservation of the pore system. Not only mesoporous oxidic materials but also phosphates, sulfides, and selenides can be obtained by surfactant templating. These materials have pore systems similar to OMS materials. 

Ordered mesoporous silica seems to be an ideal hard template, which can be used as a mold for other mesostructures with various compositions, such as ordered mesoporous carbon and metal oxides. Mesoporous silicas with various different structures are available, and silica is relatively easily dissolved in HF or NaOH. Alternatively, mesoporous carbons with a solid skeleton structure are also suitable choices as hard templates due to their excellent structural stability on thermal or hydrothermal and chemical treatment. A pronounced advantage of carbon is the fact that it is much easier to remove than silica by simple combustion. The nanocasting synthesis of mesoporous carbon by using mesoporous silica as template will be discussed in detail in the section on mesoporous carbon. In many cases, silica is unsuitable for synthesizing framework compositions other than carbon, since the leaching of the silica typically affects the material which is filled into the silica pore system. 

Compared to sihca-based networks, nonsiliceous ordered mesoporous materials have attracted less attention, due to the relative difficulty to apply the principles employed to create mesoporous silica to nonsilica compositions. Other framework compositions are much more sensitive than silica to redox reactions, hydrolysis, or phase transformations. The reactivity of the inorganic precursors is much more difficult to control in the case of transition metal oxides, the reaction kinetics being much faster. Also, crystalline nonsiliceous frameworks are less prone to adapt the curvature of micellar aggregates, whereas the amorphous nature of silica allows for certain flexibility. 

Mesoporous oxides from elements other than sihca have been reported as early as 1994. Cieslaetal. [169] found that metals such as Sb, Fe, Zn, Pb,W, and Mo also form mesoporous oxides. However, many of the mesophases obtained were lamellar and were not porous after template removal (calcination). Antonelli and Ying reported the transformation of titanium, niobium, and tantalum alkoxides into stable mesophases [170]. Subsequently, mesoporous oxides based on zirconium, hafnium, and manganese have been synthesized (for a recent review on these materials see [171]). Bagshaw and Pinnavaia [172] prepared mesoporous alumina with worm-like pores and a specific surface area of more than 400 m g . Mesoporous alumina with surface areas above 700 m g have been reported by Vaudry et al. [173]. 

Today, even after more than 20 years, almost all developed methods related to the synthesis of mesoporous materials by surfactant soft templates still use knowledge based on mesoporous silica materials. The synthesis of mesoporous oxides of TMs (i.e., Ti, Zr, and Mn), metalloids (i.e., Ge), posttransition metals (i.e., A1 and Ga), and lanthanides (i.e., Ce) has been adapted from the methods developed in mesoporous silica synthesis [44-49]. In other words, one can easily find a silica analog of any procedure for the synthesis of non-silicious mesoporous oxides. Flexible Si—O bonds made via well-known and easily manageable sol-gel chemistry, allow one to use various solvents or solvent mixtures (i.e., aqueous or alcoholic), pH (1-7), temperatures, and pressures to synthesize numerous mesoporous silica materials [50]. However, sol-gel chemistry of other elements especially TMs requires more controlled reaction conditions. The sol-gel chemistry (hydrolysis and condensation) of early (group I-IV) TMs can be controlled in alcoholic solutions with proper pH, temperature, and humidity adjustments [2,4,10,46,47,50]. Typical TM sources are either commercially available alkoxides (i.e., titanium isopropox-ide) or can be formed in situ by the reaction between anhydrous TM chloride salts and alcohols (i.e., WClg + EtOH W(OCH2CH3)6). 

---