Metal-oxide frameworks molecular materials

The sol-gel technique has attracted enormous interest over 3 decades in the area of material science because this method facilitates the preparation of novel materials that are based on metal-oxide clusters. This process begins with molecular precursor at ambient temperature and then forms a metal or metal-oxide framework by hydrolysis and condensation. The formation of interpenetrating networks between inorganic and organic moieties at a milder temperature improves compatibility and builds strong interfacial interactions between two phases. This process has been used successfully to prepare different nanocomposites (Mallakpour and Dinari, 2012). 

Since mesoporous materials contain pores from 2 nm upwards, these materials are not restricted to the catalysis of small molecules only, as is the case for zeolites. Therefore, mesoporous materials have great potential in catalytic/separation technology applications in the fine chemical and pharmaceutical industries. The first mesoporous materials were pure silicates and aluminosilicates. More recently, the addition of key metallic or molecular species into or onto the siliceous mesoporous framework, and the synthesis of various other mesoporous transition metal oxide materials, has extended their applications to very diverse areas of technology. Potential uses for mesoporous smart materials in sensors, solar cells, nanoelectrodes, optical devices, batteries, fuel cells and electrochromic devices, amongst other applications, have been suggested in the literature.11 51... 

Nanoporous materials like zeolites and related materials, mesoporous molecular sieves, clays, pillared clays, the majority of silica, alumina, active carbons, titanium dioxides, magnesium oxides, carbon nanotubes and metal-organic frameworks are the most widely studied and applied adsorbents. In the case of crystalline and ordered nanoporous materials such as zeolites and related materials, and mesoporous molecular sieves, their categorization as nanoporous materials are not debated. However, in the case of amorphous porous materials, they possess bigger pores together with pores sized less than 100 nm. Nevertheless, in the majority of cases, the nanoporous component is the most important part of the porosity. 

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 wUl 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. 

Isomorphous substitution of T element in a molecular sieve material is very interesting in order to modify its acidic or redox catalytic and shape selective properties. Different ways to perform such a substitution are now well established either during synthesis or post synthesis in( luding solid-solid reaction between the zeolite and another oxide. The substituted eliiment may be strongly or weakly bound to the framework i.e. may remain stable or may give rise to well dispersed metallic oxide particles entrapped in the cavities. This results in different catalytic properties and may even lead to bifunctional catalysis as for Ga-ZSM-5 material. 

Zeolite catalysts incorporated or encapsulated with transition metal cations such as Mo, or Ti into the frameworks or cavities of various microporous and mesoporous molecular sieves were synthesized by a hydrothermal synthesis method. A combination of various spectroscopic techniques and analyses of the photocatalytic reaction products has revealed that these transition metal cations constitute highly dispersed tetrahedrally coordinated oxide species which enable the zeolite catalysts to act as efficient and effective photocatalysts for the various reactions such as the decomposition of NO into N2 and O2 and the reduction of CO2 with H2O into CH3OH and CH4. Investigations on the photochemical reactivities of these oxide species with reactant molecules such as NOx, hydrocarbonds, CO2 and H2O showed that the charge transfer excited triplet state of the oxides, i.e., (Mo - O ), - O ), and (Ti - O ), plays a significant role in the photocatalytic reactions. Thus, the present results have clearly demonstrated the unique and high photocatalytic reactivities of various microporous and mesoporous zeolitic materials incorporated with Mo, V, or Ti oxide species as well as the close relationship between the local structures of these transition metal oxide species and their photocatalytic reactivities. 

It has thus been elucidated that well-ordered micro- or mesopores of zeolites or mesoporous materials can accommodate transition metal oxides or ions in an isolated state as single-site photocatalysts to realize unique and selective photocatalytic reactions essentially different from those on semiconducting photocatalysts such as Ti02. It was observed that zeolite or mesoporous frameworks offer one of the most promising molecular reaction fields and approaches in the development of effective new photocatalytic systems that can contribute to the reduction of global air pollution and utilize solar energy as a clean, safe and abundant resource. 

Sulfide Molecular Sieves. All crystalline molecular sieves and microporous crystals have so far been based on oxide frameworks. As discussed previously, the oxide-based molecular sieve family shows rich compositional and structural diversity, and the number of new species is still growing at a rapid rate. An important new direction for molecular sieves is provided by the recent discovery of sulfide-based molecular sieves. The first publication on these materials already described a whole family of sulfide molecular sieves containing germanium (Ge) and tin (Sn) or several other metals. The crystal structures are all new and include 12 unique framework structures. These materials may have potential applications using sulfur-containing feedstocks in which the sulfur present may stabilize the composition of the sulfide molecular sieve. 

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). 

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