Hierarchically micro-macroporous

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]. 

As the further step in the synthesis of this hierarchic materials, micro/macroporous composites were synthesized although it is not clear at the moment whether such materials can be used in catalysis [103,104]. 

The first successful preparation of micro/mesoporous or micro/macroporous molecular sieves as well as mesoporous zeolite single crystals started an intensive search of optimization procedures for their synthesis, to increase their thermal stability and to tailor their acid, base and redox properties for possible applications in heterogeneous catalysis. There is no doubt that mastering of synthesis of these hierarchic materials is an important challenge in the area of porous materials. 

Nb-based catalysts are among the investigated systems in total removal of -butanol due to the capacity of niobium to adopt variable oxidation states. Thus, Nb-doped hierarchically micro(meso) macroporous Ti02 (anatase phase) (Figure 17.10) synthesized via a self-formation procedure showed close performances with catalysts in which the same materials served as supports for noble metals [46]. The efficiency of these catalytic systems in the total oxidation of butan-l-ol was enhanced by the improved diffusion through the intrinsic macro-porous network. An effect of the niobium content was evidenced as welL... 

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]. 

Many practical applications of hierarchically micro-meso-macroporous materials have recently been emerging, including photocatalysis, catalysis, separation, and... 

The strategy of this method is to utilize the inherent porosity of bulky substrates in the construction of hierarchical stractures by incorporating additional pore systems. Diatoms are unicellular algae whose walls are composed of silica with an internal pore diameter at submicron to micron scales. Zeolitization of diatoms, in which zeolite nanoparticles are dispersed on the surface of diatoms followed by a hydrothermal conversation of a portion of the diatom silicas into zeolites, resulted in the formation of a micro/mesoporous composite material. Similarly, wood has also been used as a substrate to prepare meso/macroporous composites and meso/macroporous zeolites. After the synthesis, wood is removed by calcination. ... 

Besides the biomaterials mentioned above, the cuttlebone [49] and chito-san [50] with unique structure have also been used as templates for the formation of hierarchically porous materials, which maintained the biological structure. Starch gel and dextran were also used to produce hierarchically sponge-like micro-, meso/macroporous monoliths of silicalite and meso/macro-porous metal oxides [51,52]. 

Three hierarchy levels of the channel structure of the mesoporous zeolite X nanosheet sample, 200 nm = macropores 7 nm = mesopores 0.74 nm = micropores possible flow paths in the hierarchical pore system are shown by the arrows on the right route (a) macro-meso-micro, route (b) macro-micro. Reprinted with permission from InayatA, Knoke I, Spiecker E, Schwieger W. Assemblies of mesoporous FAU-type zeolite nanosheets. Angew Chem Int Ed 2012 51 1962 1965. Copyright... 

---