Hierarchically using metal alkoxides

It is found that hierarchically meso/macroporous metal oxides can be synthesized even without the use of any external macrotemplate. In fact, great efforts have been made by scientists to promote development of hierarchically porous materials via the spontaneous self-formation phenomenon from metal alkoxides during the past decade. In this section, we will review the history of self-formation phenomenon to target hierarchically porous materials based on metal alkoxides. 

As mentioned earlier, at first metal alkoxides were used as inorganic sources for the formation of hierarchically porous materials combined with templating... 

More complex in preparation and expensive, hierarchically nanostructured porous group V metal oxides synthesized via a spontaneous autoformation process from alkoxide precursors were used as supports for palladium in total oxidation of toluene (Figure 18.8) [40]. In addition to the effect of the support cation, multimodal porosities of these supports facilitate both the dispersion of the noble metal nanoparticles and the internal transport of the substrates. 

Tunable chemical composition Hierarchically porous materials with single or multiple chemical compositions can be prepared via this spontaneous selfformation procedure by using single or multiple alkoxide precursors. Pure oxide materials can be obtained by starting with just the alkoxide or alkylme-tal precursors in aqueous solutions, as opposed to other synthesis strategies where the resulting products are often contaminated by residual species. Furthermore, metal phosphates with hierarchically porous structures can also be prepared by this self-formation process. 

Using the self-formation phenomenon to fabricate hierarchically meso-mac-roporous binary mixed oxides, the mesoporosity and macroporosity can be tailored by tuning contents of the chemical components. Furthermore, the use of mixed alkoxide solutions and surfactants allows for the formation of binary metal oxide materials with structural properties of large multiporosity and high specific siuface area. Such modulable monolithic materials with multiscaled pore sizes and a wide variety of chemical compositions should be promising for multiple apphcations in catalysis and separation technology. 

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