Hierarchical macroporous

Figure 10.3 Exo-templating the method of Stein can be used to create hierarchical macroporous/microporous (or macroporous/mesoporous) silicas.

Kim and coworkers reported the application of breath figure method to a small photo-responsive molecule, i.e., a organogelator self-assembled into supramolecu-lar fibrillar networks and further ordered in a hierarchically honeycomb structure [194]. Recently, a new organogelator was synthesized and large-scale ordered honeycomb patterns were also observed [195]. Moreover, Babu et al. also reported the formation of hierarchical macroporous structures from an amino acid linked p-conjugated organogelator [196]. 

Zhang, Y.L., Ding, H., Wei, S., Liu, S., Wang, Y.P., Xiao, F.S. Hierarchical macroporous epoxy resin templated from single semi-fluorinated surfactant. J. Por. Mater 17, 693-698 (2010)... 

Babu, S.S., Mahesh, S., Kartha, K.K., Ajayaghosh, A. Solvent-directed self-assembly of pi gelators to hierarchical macroporous structures and aligned fiber bundles. Chem. Asian J. 4, 824 (2009)... 

Hierarchical macroporous—mesoporous solid acid and base materials... 

Figure 6.17 Dual templating route approaches toward hierarchical macroporous-mesoporous silicas.

Using an identical hierarchical macroporous-mesoporous SBA-15 support method has been employed to produce an alumina grafted Al-MM-SBA-15 support framework for alkali and nitrate-free synthesis of HT coatings from Mg(OMe)2 deposition. XRD revealed that HT/MM-SBA-15 exhibit smaller crystallite size compared to ConvHT with similar diffraction pattern while basicity is found to be similar (Fig. 6.18). Limiting conversions of 34 and 64% occurred after the hrst-hour reaction subjected to HT/MM-SBA-15 only composes a thin hydrotalcite coating and the majority of this catalyst is deposited in inert silica (Creasey et al., 2015). 

Dhainaut, J., et al., 2010. Hierarchical macroporous—mesoporous SBA-15 sulfonic acid catalysts for biodiesel s3mthesis. Green Chemistry 12 (2), 296. Available at http //xlink.rsc.org/ DOI=b919341c. 

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

A novel zeolite material possessing an inherent hierarchical structure with good mechanical and chemical strength has been prepared by the LbL assembly of zeolite nanocrystals and PDDA on the diatomite substrates [129]. The diatomite used has a disk-like morphology (Figure 7.12A) and exhibits abundant and uniform macropores (about 300-500 nm) in the diatomite plates (Figure 7.12B). The zeolite-diatomite (ZD)... 

Designing a specific material architecture. 3D hierarchical carbon [79,80], 3D aperiodic [79,81,82] or highly-ordered hierarchical carbons are representative samples with multimodal pore structure to optimize the performance of the capacitors. The micropore, mesopore and macropore structure of such three-dimensional hierarchical carbons are generally perfectly interconnected. 

So far, we have discussed various self-assembly and templating mechanisms geared towards the synthesis of porous, ordered materials at different length scales. As was mentioned previously, hierarchically ordered materials that simultaneously exhibit order over all length scales are very attractive novel additions whose synthesis usually requires a combination of all of the techniques mentioned previously. Patterning of mesopores and macropores simultaneously achieves structures with order on several length scales. 

One of the first examples of mesoscopic-macroscopic two-dimensional ordering within a structure involved a bacterial superstructure formed from the co-aligned multicellular filaments of Bacillus subtilis that was used to template macroporous fibers of either amorphous or ordered mesoporous silica [82], The interfilament space was mineralized with mesoporous silica and, following removal of the organic, a macroporous framework with 0.5 pm wide channels remained. Mesoporous silica channel walls in this hierarchical structure were curved and approximately 100 nm in thickness. Dense, amorphous walls were obtained by replacing the surfactant-silicate synthesis mixture with a silica sol solution. The difference in the mode of formation between porous and non-porous wall structures was explained in terms of assembly from close-packed mesoporous silica coated bacterial filaments in the former compared to consolidation of silica nanoparticles within interfilament voids in the latter. 

Several approaches towards the synthesis of hierarchical meso- and macro-porous materials have been described. For instance, a mixture that comprised a block co-polymer and polymer latex spheres was utilized to obtain large pore silicas with a bimodal pore size distribution [84]. Rather than pre-organizing latex spheres into an ordered structure they were instead mixed with block-copolymer precursor sols and the resulting structures were disordered. A similar approach that utilized a latex colloidal crystal template was used to assemble a macroporous crystal with amesoporous silica framework [67]. 

Figure 8 Schematic of the fabrication of hierarchical ordered oxides (a) (Reprinted from Ref. 179, 2001, with permission from Elsevier) scanning electron microscopy (SEM) images (b, c, d), at different magnifications, of hierarchical ordered mesoporons sdica display a high-quality surface pattern ( 1000nm), which is made up of a macroporous ( 100nm) framework of cubic mesoporous silica ( 11 run), as shown in TEM image (e). (Reprinted with permission from P. Yang et al., Science, 1998, 282, 2244)...

Colloidal zeolites have been used as building blocks to fabricate hierarchical porous materials. Infiltrating ethanol sol of zeolite nanoparticles into an ordered array of polystyrene spheres resulted in macroporous zeolites, which involves a self-assembly process. After ethanol evaporation, zeolite nanoparticles were aggregated by capillary forces. High concentration of external silanol groups favored the formation of hydrogen bonds between particles and eventually Si-O-Si bonds after calcination. The method has been further developed to produce transparent and self-standing zeolite membranes with controlled mesoporosity. Concurrently, the preformed zeolite-coated polystyrene spheres have been... 

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. ... 

Complex templates combine soft and hard template techniques. This methodology is used for synthesizing hierarchically bimodal and trimodal meso-macroporous materials with interconnected pore channels combining a surfactant template with a colloidal crystal template (Yuan and Su, 2004). 

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. 

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