Mesoporous frameworks, thermal stability

The thermal stability of mesoporous frameworks substantially increases with an increase in the wall thickness and pore size, which can be varied even for the same template by changing the processing conditions. Ozin et al.55 developed a way to prepare crystalline titania films with a 2D-hexagonal architecture by replacement of ethanol in the Pluronic-containing precursor solution with more hydrophobic butanol-1. The latter promotes phase separation at low surfactant-to-titania ratios, resulting in thicker pore walls, which are more compatible with the crystal growth during subsequent calcination. 

Silica materials have been studied extensively because of the structural flexibility of silica (through Si04 tetrahedral connections), easy control of hydrolysis and polymerization of silica species, high thermal stability of silica framework, easy modification of the silica surface, and well known silica and zeolite chemistry. Amorphous silica is also the main inorganic component for certain natural materials obtained from bioassembly, such as diatoms. Various mesoporous silica materials have been reported, which are very important for both fundamental research and applications. 

Oxides Compared to silica-based networks, nonsiliceous ordered meso-poious materials have attracted less attention, due to the relative difficulty of applying the same synthesis principles to non-sihcate species and their lower stability (227). Nonsiliceous framework compositions are more susceptible to redox reactions, hydrolysis, or phase transformations to the thermodynamically preferred denser crystalline phases. Template removal has been a major issue and calcination often resulted in the collapse of the mesostracture. This was the case for mesostractured surfactant composites of mngsten oxide, molybdenum oxide, and antimony oxide, and meso-structured materials based on vanadia that were obtained at early stages. Because of their poor thermal stability, none of these mesostructures were obtained as template-free mesoporous solids (85, 228, 229). 

The definition for catalytic purposes of a zeolite reads as follows a crystalline material with micropores and cation-exchange capacity that is insoluble in water and common organic solvents and has sufficient thermal stability that allows removal of all pore-filling agents present in the as-synthesized materials. This definition is narrower than that of the IZA Constitution, which includes mesoporous solids, metal organic frameworks (MOFs), cationic and anionic clays [3]. 

Metal-modified aluminophosphates show great potential in the oxidation reactions under mild conditions. However, the low thermal stability and the metal leaching from the framework reduce their wider applications. Some of the metal-modified mesoporous catalyst and their catalytic applications are listed in Table 15. 

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). 

Schlichte, K., Kratzke, T, and Kaskel, S. Improved synthesis, thermal stability and catalytic properties of the metal-organic framework compound CU3(BTCX2). Micro-por Mesopor Mat 13, 81-88 (2004). 

Ni [182], V [183], and A1 [184]. SU-M [185] is a mesoporous germanium oxide with crystalline pore walls, possessing one of the largest primitive cells and the lowest framework density of any inorganic material. The channels are defined by 30-rings. Structural and thermal information show that there exists a mismatch between framework stability and template decomposition. The latter requires temperatures higher than 450 °C, while the structure is preserved only until 300 °C. 

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