Monoliths with Hierarchical Porosity

As described above, molecular and supramolecular templating methods have been very successfully applied for the syntheses of a large variety of microporous systems (e.g., zeolites, AlPOs) and mesoporous systems, respectively. Such approaches allow the design of the pore structure inside a crystal or particle. The simultaneous control about the pore structure at the micro and the macro scale during the formation process can be realized by the combination of at least two strategies. The combination of molecular templating with a sol-gel processing approach results in porous spheres [204]. 

Multimodal porous systems in the form of monolithic-shaped forms (bodies) were obtained from a process where a (true) liquid-crystal templating is combined with a sol-gel process [205]. One characteristic of such a process is the incompatibility of lyotropic surfactants with an alcohol, mostly an inherently produced alcohol, which is responsible for the phase separation during the sol-gel processing, stabilizing the resulting monolithic architecture of the monolith [206]. Different approaches use various components, namely, the alcohol source, the silicate sources (both are sometimes combined in one molecule), and molecular or supramolecular templating agents [207]. 

Other attempts follow the dual micellular templating approach involving a combination of block copolymers, surfactants, and alcohols [208]. The colloidal templating method produces three-dimensionally ordered macrostructures (3DOM-structures) [208]. However, even for the colloidal templating, the routes include besides the formation of the three-dimensionally arranged colloidal crystal always a second 

Very recently, hierarchically arranged aerogel monoliths [209], silica-based monoliths from swollen liquid crystals [210] or heptane/water/ethanol/sodium dodecylsulfate microemulsion [211], monolith-based hybrid components such as the PMOs [212], and silicon oxycarbide monoliths with better thermal properties than the pure (organo) silica monoliths [213] have been reported. These approaches have been extended also to alumina or aluminumsilicates [214, 215], titania [216, 217], zirconia [218], or ceramic-based [219] systems. 

Such silica-based monoliths are characterized by a multimodal pore system with macropores in the range of about 200-800 nm, with mesopores around 8-10 nm, surface areas of about 800-1000 m g and an exceptionally high macropore volume (volume of transport pores) of about 3-5 cm with diameters in the range of 0.45/0.8 to 6 to 11 pm [207]. 

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Brandhuber, D., Torma, V., Raab, C., Peterlik, H., Kulak, A. and Husing, N. (2005) Glycol-modified silanes in the synthesis of mesoscopically organized silica monoliths with hierarchical porosity. Chemistry of Materials, 17, 4262 1271. 

Figure 3.5.14 Carbon monolith with hierarchical porosity. Reprinted from [69] with permission, copyright 2007 Wiley-VCH.

Huang Y, Cai H, Feng D, Gu D, Deng Y, Tu B, Wang H, Webley PA, Zhao D (2008) One-step hydrothermal synthesis of ordered mesostructured carbonaceous monoliths with hierarchical porosities. Chem Commun 23 2641-2643... 

Baumann T, Worsley M, Han T, Satcher J (2008) High surface area carbon aerogel monoliths with hierarchical porosity. J Non-Cryst Solids 354 3513-3515. 

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-... 

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