Nanocast oxides

Synthesis and Catalytic Applications of Nanocast Oxide-Type Perovskites... 

Figure 17.9 TEM images of C03O4 nanocasted oxides. (Reprinted with permission from Ref. [31]. Copyright 2010, Elsevier.)...

The performance of various nanocasted oxides v/as studied by Vasconcelos et al. in the gas-phase dehydration of glycerol to produce hydroxyacetone (Table 13.7). The CeZr catalyst exhibited the best catalytic performance, CeOs being the worst. The binary oxide CeOj-ZrOg, with moderate basicity, proved to have a cooperative acid-base character in the Ceo.8Zro202 phase, resulting in a higher production of hydroxyacetone. 

Two kinds of template, viz. hard template and soft template, are usually available for nanocasting processes. The true liquid crystal templating synthesis can be considered a soft-template process. In general, the hard template means an inorganic solid. For example, mesoporous silica as a template to replicate other materials, such as carbon or metal oxides, by which the pore structure of the parent can be transferred to the generated porous materials. A 3-D pore network in the template is necessary to create a stable replica. Mesoporous silica and carbon are commonly used templates for nanocasting synthesis. 

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. 

The nonionic ABC templates used for nanocasting commonly consist of a hydrophobic soft block (T below or around room temperature in order to warrant sufficient solubility at room temperature), such as poly(butadiene) [34], poly(ethylene-co-butylene) (Kraton Liquid) or relatively short poly(styrene) and a poly(ethylene oxide) block as the hydrophilic moiety [35]. 

Besides cooperative pathways, also tme liquid crystal templating (TLCT) and the hard template route (Section 9.3.7) have been developed for the synthesis of ordered mesoporous materials. In the case of the TLCT, a preformed surfactant liquid crystalline mesophase is loaded with the precursor for the inorganic materials (140). The nanocasting route, on the other hand, is a clearly distinct method (141). Here, no soft surfactant template is used but, instead, the pore system of an ordered mesoporous solid is used as the hard template serving as a mold for preparing varieties of new mesostructured materials, for example, metals, carbons, or transition metal oxides. 

EISA. The mechanism in each case has been the subject of much research and the current understanding of each mechanism is described below. The other two methods to produce mesoporous templated materials are electrodeposition,which has been successfully used to produce surfactant templated porous metal films from high concentration surfactant solutions, and nanocasting, where a surfactant templated silicate is used as a sacrificial template to generate further porous materials by coating the silica structure in another oxide or carbon precursors. The second material is sintered or solidified, and the silicate removed by HE or high pH solvent wash. This is discussed further in Section 2.10. 

Many experiments were designed to obtain ordered mesoporous materials with completely different compositions of the network no longer correlated to silica. Also here, nanocasting is beneficial. Due to their high relevance in many areas of catalysis and their variable redox- and magnetic properties, much work was devoted to the creation of stable ordered mesoporous transition-metal oxides. In the meantime, many compositions with Ti, Zr, V, Ta, Mo, W, Mn, and Y, as the central element were introduced." ... 

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