Hierarchical pore structures

The first step in the generation of hierarchical pore structured materials is the implementation of two different pore systems which build up a highly interconnected pore network in one single bead. Other morphologies, e.g. monoliths with a bimodal pore size distribution, have already been shown to have superior chromatographic performance by Nakanishi et al. [1,2,3]. 

Primary and Branched Pores. The branched pores are significantly smaller than the primary pores.The hierarchical pore structure has been found to be universal to the PS formed on all types of substrates. ... 

Wang DH, Jakobson HP, Kou R, Tang J, Fineman RZ, Yu DH, Lu YF (2006) Metal and semiconductor nanowire network thin films with hierarchical pore structures. Chem Mater 18 4231 237... 

The formation of hierarchical pore structures in the silica has also been achieved by the use of a gelator 2,3-di- -decyloxyanthracene in methanol [61]. Hollow fibers with micron-sized diameters are obtained on removal of the gelator, which contain mesopores from smaUer gelator aggregates. Changes in the mesopore diameter (5-12 nm) and shape (ink-bottle or cyHndrical) occur for different gelator concentrations. 

P-07 - The zeolitisation of diatoms to create hierarchical pore structures... 

The synthesis of a hierarchical pore structure, combining the macroporous diatomaceous earth with microporous zeolites, is reported. Diatomaceous earth is an abundant and varied source of macroporous silica which has been zeolitisatised to produce a bifunctional, hierarchical composite. A range of different zeolites have been synthesised to generate different pore architectures, hydrophobic/hydrophilic materials and ion-exchange/catalytic properties. 

Other examples of nonplanar hierarchically structured porous films are for instance the preparation of membranes for advanced filtration (also known as microsieves) composed by a hierarchical pore structure [208] or the construction of hierarchical structures by combination of electrospinning or electrospraying and breath figures [209-211]. 

Very recently, Broda and Muller [87] extended the carbon sol-gel technique first reported by Pekala [88] to synthesize Al203-stabilized, CaO-based CO2 sorbents with a hierarchical pore structure. It was argued that by nanostructuring the material, diffusive limitations of the carbonation reaction could be avoided. Here, a carbon gel acted as a template for pores in the small micrometre range. A schematic sketch of the different steps of the synthesis protocol is shown in Fig. 6.31. In the first step, formaldehyde was added to an aqueous solution of resorcinol to obtain a molar ratio of resorcinol to formaldehyde of 1 2. Subsequently, an... 

Fig. 6.31 Schematic of the synthesis steps employed to produce CaO-based, Al203-stabilized CO2 sorbent with a hierarchical pore structure. Reproduced from Ref. [87] by permission of John WUey Sons Ltd...

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... 

The fabrication process for metal silicate nanotubes is depicted in Fig. 32A. HRTEM image of the as-prepared SNT template (Fig. 32B) showed its hollow structure and hierarchical pore structure mesopores at the wall with about a 30 nm thickness and macropores at the center. This hierarchical pore structure is veiy suitable to prepare silicate materials. Under hydrothermal conditions, metal ions and other ions in water solution could easily diffuse into the pores of the SNT template and react with silica species to form metal silicates in situ (Fig. 32C). The original silica mesopores, where the reaction occurs, are uniformly dispersed in the walls, and the metal ions in water solution could easily diffuse into the pores of the SNT template. The whole silica wall with about 30 nm thickness can be readily converted to metal silicates under the reaction conditions. 

Panels, J., et al. (2008). Synthesis and characterization of magnetically active carbon nanofiber/iron oxide composites with hierarchical pore structures,... 

The preparation of free-standing mesoporous Ti02-Si02 aerogels with hierarchical pore structure, which are expected to have applications in photocatalytic degradation... 

Yao N, Cao SL, Yeung KL (2009) Mesoporous Ti02-Si02 aerogels with hierarchal pore structures. Micro-... 

Continuous mesoporous carbon thin films were fabricated by direct carbonization of sucrose-silica nanocomposite films and subsequent removal of the silica [236]. The mesoporous carbon film with uniform and interconnected pores had a surface area of 2603 mVg and a pore volume of 1.39 cmVg. Subsequently, nanoporous carbons with bimodal PSD centered at about 2 and 27 nm in diameter were prepared by using both the TEOS-derived silica network and the colloidal silica particles as templates [237]. Figure 2.33 illustrates the preparation pathway. The pore sizes of the carbon are determined by the sizes of the added silica particles and the silica network. As the colloidal silica particles are commercially available with different diameters (e.g., 20 to 500 nm), this dual template synthesis process provides an efficient route to preparing nanoporous carbons with a controllable hierarchical pore structure. 

Preparing carbons with hierarchical pore structure is done by impregnation of preformed macropo-rous structures, such as silica colloids, with the carbon precursor gels, followed by precursor carbonization and macropore template dissolution. Although in hard-templated carbons, two particle sizes of the hard template are simultaneously required for the colloidal imprinting method the soft-templating method makes this procedure much simpler and broadens the selection of templates for the larger mesopores and macropores. 

TAILORING MESOPORES TO OBTAIN HIERARCHICAL PORE STRUCTURES... 

Nakanishi K., Takahashi R, Nagakane T., Kitayama K., Koheiya N., Shikata H., Soga N. Formation of hierarchical pore structure in silica gel. J. Sol-Gel Sci. Technol. 2000 17 191-210 Poppe H. Some reflections on speed and efficiency of modern chromatographic methods. J. Chromatogr. A 1997 778 3-21... 

These three different approaches are distinguished by the type of pore formers that are introduced in each case leaving particulates, molecules or functional groups in the former, self-organized entities (mainly micelle and lamellar structure formers) in the second approach, and a continuous polymeric phase in the latter approach. The three different approaches also yield, respectively, very different gel morphologies microporous or macroporous material mesoporous materials and hierarchical pore structures with macro- or mesoporosity as well as nanoscale pores within the same material domain. 

Liu Z, Nie H, Yang Z, Zhang J, Jin Z, Lu Y, Xiao Z, Huang S (2013) Sulfirr-nitrogen co-doped three-dimensional carbon foams with hierarchical pore structures as efficient metal-ffee electrocatalysts for oxygen reduction reactions. Nanoscale 5(8) 3283-3288... 

In 2009, Yeung and coworkers reported the possibility to prepare freestanding Ti02—Si02 monoliths with ultralow densities and well-defined hierarchical pore structures [65]. For this purpose, they mixed a solution of titanium isopropoxide... 

Another family of synthesis strategies uses small solid particles as additional templates to micellar templating to create hierarchical pore structures. These small solid particles as additional templates consist of colloidal crystals, biomaterials, macroporous polymers, salts, and ice crystals. This combination of surfactant and small solid particles offers an efficient way for the generation of ordered and interconnected mesoporous-macroporous architectures, small solid particles creating macropores and surfactant micelles creating mesopores. 

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