Mesoporous hierarchical structure

Zeolite materials with tunable size and volume of mesopores can be prepared by using dispersed carbon black particles with narrow distribution of their sizes as inert mesoporous matrix or as secondary template. In such confined space for synthesis the crystallization of zeolite gel occurs inside the interparticle voids of carbon matrix [10,11,12]. In the case of generation of mesopores by secondary templating by means of addition of carbon black into the reaction mixture, zeolite crystals are formed around carbon particles [13]. After burning off a carbon matrix or carbon particles, zeolite crystals with a controlled pore size distribution and a crystalline micro-mesoporous hierarchical structure are prepared. 

One route to fabricating mesoporous hierarchical structures using an aerosol spray method, and photos of the structures produced, are given in Fig. 3.14. 

G.J. de A. Soler-Illia, C. Sanchez, B. Lebeau, J. Patari, Chemical strategies to design textured materials from microporous and mesoporous oxides to nanonetworks and hierarchical structures. Chem. Rev. 102 (2002) 4093. 

The addition of water-soluble polymers such as polyethylene oxide (PEO) or polyvinyl alcohol (PVA) into the synthetic mixture of the C TMAX-HN03-TE0S-H20 system (n = 16 or 18 X = Br or Cl) under shear flow is found to promote uniformity and elongation of rope-like mesoporous silica. The millimeter-scaled mesoporous silica ropes are found to possess a three-level hierarchical structure. The addition of water-soluble polymer does not affect the physical properties of the silica ropes. Moreover, further hydrothermal treatment of the acid-made material under basic ammonia conditions effectively promotes reconstruction of the silica nanochannels while maintaining the rope-like morphology. As a result, a notable enhancement in both thermal and hydrothermal stability is found. 

In principle the bicontinuous 3-dimensional network structure of MCM-48 would act as a good catalytic support.[7] However, its lower hydrothermal and thermal stability has led to much less application of MCM-48 in catalysis. Recently, a family of mesoporous molecular sieves (denoted as MSU-G) with vesicle-like hierarchical structure, worm-like mesoporous structure and bicontinuous nano-porous silica had been synthesized.[8-10] It was proposed that highly accessible mesoporous materials could be obtained through different synthetic procedure and composition. 

Soler-Illia G., Sanchez C., Lebeau B., Patarin J. (2002) Chemical Strategies To Desing Textured Materials from Microporous and Mesoporous Oxides to Nanonetworks and Hierarchical Structures, Chem. Rev., 102(11), 4093-4138. 

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 structures 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 materials which have been mentioned here so far are predominantly shaped in planar films of hierarchical order. However, the synthesis of hierarchically structured particles is also highly desirable, as they might be further processed and used for the preparation of composite porous materials. Wu et al. showed the synthesis of raspberry-like hollow silica spheres with a hierarchically structured, porous shell, using individual PS particles as sacrificial template [134]. In another intriguing approach by Li et al. [135], mesoporous cubes and near-spherical particles (Fig. 10) were formed by controlled disassembly of a hierarchically structured colloidal crystal, which itself was fabricated via PMMA latex and nonionic surfactant templating. The two different particle types concurrently generated by this method derive from the shape of the octahedral and tetrahedral voids, which are present in the template crystal with fee lattice symmetry. 

Monolithic carbons are easier to handle than powdered materials. Direct shaping of monolithic mesoporous carbons during their preparation is highly desirable. Mesoporous carbon monoliths may be fabricated by using mesoporous silica monoliths as template. Carbon monoliths with well-developed and accessible porosity have been produced using silica monoliths with a hierarchical structure containing macropores and meso-pores as templates and furfuryl alcohol or sucrose as a carbon precur-... 

As well as the crystal structure of the microporous catalysts, the secondary mesoporosity is also important, because molecular transport to and from the active sites is favoured in these materials. In steamed Y the mesoporosity and extra-framework aluminium results in a very active catalyst for cracking. Designed hierarchical structures, in which nanoparticles of zeolites are joined together to and connected by a secondary mesopore system for the same reason are discussed further in Chapter 10. 

Recently, Song et al. fabricated silica nanotubes with mesoporous walls (SNT) of about 30 nm thickness. The SNT material is regarded as being hierarchically structured. It possesses two levels of pores mesopores at the wall and macropores at the center. This hierarchical pore structure showed faster mass transportation in catalysis. So, in their studies, SNT... 

Choi H, Sofranko A C and Dionysiou D D (2006b), Nanocrystalline TiOj photocatalytic membranes with a hierarchical mesoporous multilayer structure synthesis, characterization, and multifunction , Adv Funct Mater, 16,1067-1074. 

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... 

Larsen, G., S. Noriega, R. Spretz, and R. Velarde-Ortiz (2004a). Electrohydrodynamics and hierarchical structure control submicron-thick silica ribbons with an ordered hexagonal mesoporous structure. Journal of Materials Chemistry 14(15) 2372-2373. 

In nanomaterials with a hierarchical structure of pores the phase transformation occurs simultaneously in the pores of all types up to the complete filling of micropores. In micropores potential adsorption increased due to the addition of dispersive potentials of the closely set pore walls which leads to increase of temperature and the filling of such time using nanotubes at a small relative pressures. Phase transformations can be managed in meso- and macropores mechanisms in the form of nanotubes (Fig. 16.1). After the limit completion of mesopores temperature fields can be controlled. 

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