Hierarchical macroporous—mesoporous

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. 

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 mesoporous silica display a high-quality surface pattern ( 1000nm), which is made up of a macroporous ( 100nm) framework of cubic mesoporous silica ( 11 nm), as shown in TEM image (e). (Reprinted with permission from P. Yang et al. Science, 1998, 282, 2244)...

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

Cell assemblies. As macroporous templates, these provide a facile bioinspired method for the synthesis of hierarchical macro-mesoporous titania with tunable macroporous morphology and enhanced photocatalytic activity [134]. This is also a simple and facile technique that can be used to prepare many types of metal oxide porous materials with good control over the pore size and morphology. 

Hierarchically ordered mesoporous carbons (HOMC) are attractive as a support for fuel cell applications because of their interconnected bimodal pore-size distribution. Both pore systems can be mesoporous or one can be mesoporous while other can be macroporous. While a mesoporous pore structure imparts high surface area and uniform distribution of catalyst particles, macropores provide efficient mass transfer. Of course, the interconnectivity between pores has a significant role in realizing the advantages of both pore stmctures. Also, a novel feature about these structures is that the two pore structures can be adjusted independently, allowing for good control over their porosity [73, 74]. Like OMC, controllable pore structure, and carbon microstracture and surface chemistry, makes them an attractive support for fuel cell catalysis. Fang et al. have shown that Pt on hollow... 

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). 

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. 

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. ... 

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. 

As outlined in the Introduction, porous materials can be classified into materials which consist of microporous (below 2 nm), mesoporous (2-50 nm), and macroporous (larger than 50 nm) channels and voids. Networks, which comprise various classes of pores with characteristically different dimensions, are termed hierarchical according to the discussion above. The controlled synthesis of hierarchically ordered porous materials is highly desirable and has attracted much attention during the last decade. The search for such syn-... 

Ren TZ, Yuan ZY, Su BL (2004) Microwave-assisted preparation of hierarchical mesoporous-macroporous boehmite AlOOH and gamma-Al203. Langmuir 20 1531... 

Sen T, Tiddy GJT, Casci JL, Anderson MW (2004) Synthesis and characterization of hierarchically ordered porous silica materials. Chem Mater 16 2044 Deng Y, Liu C, Yu T, Liu F, Zhang F, Wan Y, Zhang L, Wang C, Tu B, Webley PA, Wang H, Zhao D (2007) Facile synthesis of hierarchically porous carbons from dual colloidal crystal/block copolymer template approach. Chem Mater 19 3271 Luo Q, Li L, Yang B, Zhao D (2000) Three-dimensional ordered macroporous structures with mesoporous silica walls. Chem Lett 29 378... 

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