Hierarchically metal alkoxide

Hierarchically porous metal oxide networks can be formed via a spontaneous self-formation phenomenon from metal alkoxides in aqueous solution [113]. Two chemical processes, hydrolysis and condensation, are involved in this spontaneous self-formation procedure to target hierarchically porous structures [114,115]. In fact, the hydrolysis and condensation rates are generally comparable for metal alkoxides [116]. The condensation rate is directly proportional to the rapid hydrolysis rate of reactive metal alkoxides [117,118]. It is well known that the rapid reaction rate of metal alkoxides plays the key role in the formation of hierarchically porous metal oxides [119,120]. The self-formation procedure to form hierarchically porous materials can be achieved by dropping liquid metal alkoxide precursors into an aqueous solution. In this section, the features of self-formation procediu-e and the resulting hierarchically porous materials are summarized. 

Very simple method An important benefit of this spontaneous self-formation procediu-e is its simplicity. Hierarchically porous materials can be obtained by just dropping the liquid metal alkoxide precursors into an aqueous solution, without any posttreatment such as calcination or solvent extraction to remove the exotemplate. Fiuthermore, the spontaneous self-formation pro-cediu-e generally takes place at room temperature in an open system. This feature allows the scaling up of this method, which is quite desirable in industry. 

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... 

On the basis of a series of comprehensive studies, it is found that the key point of this novel synthesis process is the very high rate at which the hydrolyzed metal alkoxides undergo condensation reactions in aqueous solution. A poro-gen mechanism has been proposed to explain the formation of porous hierarchy via the spontaneous self-formation phenomenon [124,139]. The alcohol molecules released suddenly during the hydrolysis and condensation reactions can be considered as porogens in the generation of the funnel-like macrochannels with hierarchically mesostructured porous walls. 

It is believed that the macroporosity is exactly dependent on the hydrolysis and condensation rates of the metal alkoxides. However, in sol-gel chemistry, the rates of hydrolysis and condensation strongly depend on the pH values as well. It was found that the hierarchically porous materials with different macroporosities could be obtained via a spontaneous self-formation process carried out at different pH values. As a result of these opposing effects, acid and base catalysts... 

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. 

Rooke, J.C., Barakat, T., Brunet, Li, Y Finol, M.F., Lamonier, J.-F., Giraudon, J.-M., Cousin, R., Siffert, S., and Su, B.L. (2015) Hierarchically nanostructured porous group Vb metal oxides from alkoxide precursors and their role in the catalytic remediation of VOCs. Appl Catal B Environ., 162, 300-309. 

Unique porous hierarchy The hierarchically porous materials obtained via this spontaneous self-formation procediu-e possess well-ordered tubular or funnel-like macrochannels with micro/mesoporous walls. The spontaneous self-formation procedure can be optimized by carefully adjusting the synthesis conditions relative to various parameters, such as the central metal atom of the alkoxide, the alkyl group in the alkoxide, hydrolysis ratio, pH, and temperature of the solution. 

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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