Applications hierarchical porosity

There are various recent developments in this area. Forlin et al. (i84), Goebbel et al. (i85), and Hofen and Thiele ( 86) developed continuous epoxidation processes with hydrogen peroxide. The incorporation of an additional metallic element improved the activity of TS-1, as demonstrated with doubly-substituted sdicalites containing the combinations titanium-vanadium (131) or titanium-tin (187). The hydrophobicity of the catalysts was found to be important for the epoxidation activity (188). The preparation of extruded catalysts based on TS-1 for application in propylene epoxidation (26a) was described, and titanosilicate beads with hierarchical porosity (99a) were investigated. 

Polymerization of the continuous phase and removal of the dispersed one, used as a soft template, lead to solid microcellular foams emulsions are powerful tools to generate hierarchical porosity when combined with lyotropic meso-phases, and various synthetic routes have been reviewed by Zhang and Cooper [104]. If silica precursors such as tetraethyl-orthosilane (TEOS) are used, the materials synthesized are called Si-(HIPE). They can possess very high porosity and very low bulk density. The void size is usually situated in the microcellular range (1-100 pm). These materials will be used as supports for a wide range of applications. 

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

Hierarchical porous materials are solids that are ordered at different length scales. Materials with multiple porosities are of high interest for applications in catalysis and separation, because these applications can take advantages of different pore structures. For example, microporous-mesoporous composites have shown superior catalytic activities by the combination of strong acidity from zeolites with high reactant or product mobility due to large uniform mesopores. Several approaches have been reported on the design and synthesis of hierarchical porous materials, as discussed below. 

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... 

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