Self-formation phenomenon

Self-Formation Phenomenon to Target Porous Hierarchy... 

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. 

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. 

With the development of the self-formation phenomenon, hierarchically porous materials can be synthesized even without the need for any macro/mesotemplate. In 2004, Davis and coworkers [127] reported the preparation of ordered macroporous titania by dropwise addition of various titanium alkoxides to aqueous ammonia in the absence of siufactants. This work claimed that macroporosity is only dependent on the Ti alkoxides and solutions used. A microphase-separated domain mechanism was therefore proposed. Simultaneously, Su and coworkers [128,129] demonstrated that spontaneous synthesis of macroporous structures can be carried out at different starting pH values in the absence of... 

Hierarchically Porous Materials Based on the Self-Formation Phenomenon... 

Subsequently, it was found that the surfactant in the synthesis system can modify the mesoporous textural properties but does not affect the self-formation of the meso-macroporous hierarchy. Zirconium oxides with the similar meso-macropo-rous structure could also be synthesized via the self-formation phenomenon in the absence of a surfactant [131]. The resulting hierarchically meso-macroporous zir-coniiun oxides are mainly tens of micrometers in size with a regular array of parallel or funnel-like macrochannek of 300-800 nm in diameter. The macropore walls are constmcted from mesochannels (around 2.0 nm) with a wormhole-like array. These materials obtained via this self-formation phenomenon present high surface area and pure crystalline phases in meso-macroporous structures. 

In comparison with the materials synthesized in the presence of surfactants, the surface area of the hierarchically porous zirconium oxides obtained via the self-formation phenomenon is relatively lower. Furthermore, the synthesis temperature can affect the crystallinity of the final product [124,139]. Upon increase of the hydrothermal treatment temperature to 130 °C (generally reported to be 60-80 °C), thermally stable meso-macroporous zirconias with a nanocrystalline framework were prepared by using a mixture of amphiphilic block copolymer P123 and poly(ethylene oxide) surfactant Brij 56. 

Titanium oxides with hierarchically meso(micro)-macroporous structure could also be prepared via the spontaneous self-formation phenomenon [137,141,142]. The hierarchically porous titanias can be prepared in acidic or alkaline aqueous solution [137]. Different macroporous structures and macropore sizes were obtained when using different alkoxide precursors and under different pH values. The ethoxide, -propoxide, and n-butoxide alkoxides produced powders with limited and localized macroporosity with irregularity and wide pore size distributions (Figure 32.11e-0 [137]. 

Recently, the self-formation phenomenon used for the formation of hierarchically porous metal oxides was ako applied to the formation of hierarchically micro-macroporous niobium oxides [12,141,144]. These synthesized niobium oxide particles with amorphous nature are mainly 100 in size with a regular array of parallel macropores (Figure 32.13a). The macropores with the pore size in the range of 0.3-10 pm extend through almost the whole particle. The macropore walls are constructed from accessible micropores. The hierarchically porous niobium oxides can be obtained via a self-formation phenomenon under different pH values (2, 7, and 12) [141,144]. 

Meso-Macroporous Yttrium Oxides The self-formation phenomenon was also used for the preparation of hierarchically porous yttriiun oxides by a controlled polymerization of yttrium butoxide in aqueous media [12,141,144], The synthesized yttrium oxides are 0.5-2 pm in size and are covered by a smooth surface. The fissure particles with funnel-hke and parallel macrochannels below the smooth surfeice were observed by higher resolution SEM observations (Figure 32.13b). The yield of the synthesis as well as the amount of macropores per particle continuously decreases with increasing initial synthesis pH values. The macropore diameters are 1-5, 2-8, and 5-10 pm for syntheses carried out in acidic, neutral, and basic media, respectively. The macropore walls are formed by a regation of mes-ostructured nanoparticles giving a supplementary interparticle porosity centered at 30 nm. A third level of porosity is demonstrated by the inhomogeneous pores centered at 3-7 nm for syntheses in acidic and neutral media and 5-15 nm in an alkaline medium. As for all the previously described compositions, the meso-macroporous yttria structures are amorphous at the atomic scale. 

Meso-Macroporous Mixed Oxides Multicomponent oxides play a central role in chemical and petrochemical processing as catalysts and as supports for catalytically active species [145]. It is known that the catalytic efficiency of metal oxides can be improved by doping them with a metal or combining them with another metal oxide [145]. The strategy based on the self-formation phenomenon to fabricate the porous hierarchy demonstrated its simplicity and superiority in the synthesis of hierarchically meso-macroporous metal oxides with multiple compositions. 

Figure 32.14i-l), alumina-zirconia (Figure 32.15a-b), and zirconia-silica (Figure 32.15c-d), can be prepared by the use of mixed alkoxide solutions via the self-formation phenomenon in the presence or absence of surfactants [125]. 

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. 

Meso-macroporous titanium phosphates with multiscaled and tunable porosity can also be prepared either by the use of a neutral surfectant or by the template-free method, but always via the self-formation phenomenon (Figure 32.16e-h) [126]. The formation of well-defined small macropore (50-160 nm in size) systems constructed from mesopores with a wormhole-like array could be spontaneously achieved in the absence of any surfactant molecule and/or external template (such as colloid crystals and emulsion droplets), while... 

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 proved that the assisted conditions, such as surfactant molecules, pH values, organic solvent molecules, reaction temperature, and hydrothermal treatment, do not play a direct role in the creation of macroporosity. However, these assisted conditions can affect the resulting morphology of macroporous structure, meso-porosity, surface area, and so on. In this section, we will review the effects of general synthesis conditions on the self-formation phenomenon, including metal alkoxide, surfactant, solvent, pH values, and hydrothermal synthesis. 

It is proved that the surfactant molecules do not play a role in the creation of macroporosity via the self-formation phenomenon. However, some properties of the resulting hierarchically porous materials, such as the surface area, meso-porosity, morphology of the macroporous structure, degree of crystallinity of the nanoparticle, and stability of the porous structure, are tunable by using surfec-tant in the synthesis system. 

Keywords homogeneous mixed-oxides, self-formation phenomenon, aluminosilicates, zirconosilicates... 

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