Hierarchic, hierarchical porosity templating

The resulting mesoporous layers don t usually exhibit extraporosity at a larger scale, but as smaller pores in the oxide walls (Figure 25.24). ° Due to limitations associated with intrinsic mesostructure characteristics, anisotropy resulting from preferential orientations, or boundaries between ordered domains, the tem-plated mesoporosity is usually not directly interconnected. " In such situations, it does directly define the selectivity of the membrane, which depends on the pore size of the oxide walls. However, the resulting hierarchical porosity (templated mesopores and smaller pores of the oxide walls) favors a decrease in layer permeability. The templated mesoporosity can also be used to functionalize the membrane. 

Petkovich, N.D. and Stein, A. (2013) Controlling macro- and mesostructures with hierarchical porosity through combined hard and soft templating. Chem. Soc. Rev., 42, 3721-3739. 

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. 

Examples of dense silica, hybrid silica, metal oxides, solid-state metal oxide solutions, or colloidal self-assembly are unlimited. However, the recent developments to accurately control processing conditions (e.g., atmosphere, temperature, and motion) led to films with unique properties (see Figure 9.6) [52,53]. These progresses concern mesoporous coatings with controlled pore size and structure [26], hard template infiltration and/or replication [54-58], nanostructured epitaxial low-quartz thin films [59], ultrathin nanostructured supported networks [60,61], ultrathick porous Ti02 layer prepared from aqueous solutions [51], coatings with hierarchical porosity [62], multilayer porous stacks [63], colloidal MOF layers [64,65], pillar planar nanochannels (PPNs) for nanofluidics [66], and so on. 

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-... 

Very recently, Dai and coworkers reported a template-free synthesis of hierarchical mesoMOFs of Zn-MOF-74 of mesopores exceeding 15nm by fast reaction of Zn salts and H2-DHBDC at room temperature. The surface morphology and porosity of the bimodal materials can be modified by etching the pore walls with various synthesis solvents for different reaction times. This template-free strategy broadens the preparation strategies of mesoMOFs, which was previously unattainable in the synthesis of mesoMOFs. 

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