Meso-macroporous structures

Yuan Z-Y, Su B-L (2006) Insights into hierarchically meso-macroporous structured materials. J Mater Chem 16 663-667... 

Yang, S.B., Wang, J., Tan, H.L., Zeng, F.Y., Liu, C.S., 2012c. Mechanically robust PEGDA-MSNs-OH nanocomposite hydrogel with hierarchical meso-macroporous structure for tissue engineering. Soft Matter 8, 8981—8989. 

Subsequently, it was found that the surfactant in the synthesis system can modify the mesoporous textural properties but does not affect the self-formation of the meso-macroporous hierarchy. Zirconium oxides with the similar meso-macropo-rous structure could also be synthesized via the self-formation phenomenon in the absence of a surfactant [131]. The resulting hierarchically meso-macroporous zir-coniiun oxides are mainly tens of micrometers in size with a regular array of parallel or funnel-like macrochannek of 300-800 nm in diameter. The macropore walls are constmcted from mesochannels (around 2.0 nm) with a wormhole-like array. These materials obtained via this self-formation phenomenon present high surface area and pure crystalline phases in meso-macroporous structures. 

Physisorption measurements showed that carbon nanomaterials exhibit rather meso- and macroporous structures (maximum micropore fraction, 15% see Table 2.1). The lowest specific surface area was measured with the platelet fiber catalyst exhibiting slightly more than 100 m2/g. The Co/HB material offers 120 m2/g of surface area, and the highest BET value was determined with the Co/ MW catalyst featuring nearly 290 m2/g. Carbon nanomaterials, though, are not really porous, as the space between the graphene layers is too small for nitrogen molecules to enter. The only location of adsorption is the external surface of the nanomaterials and the inner surface of the nanotubes. 

While for macroporous structures the inner surface can be calculated from the geometry, meso and micro PS layers require other methods of measurement First evidence that some PS structures do approach the microporous size regime was provided by gas absorption techniques (Brunauer-Emmet-Teller gas desorption method, BET). Nitrogen desorption isotherms showed the smallest pore diameters and the largest internal surface to be present in PS grown on low doped p-type substrates. Depending on formation conditions, pore diameters close to, or in, the microporous regime are reported, while the internal surface was found to... 

The pore size distribution in highly activated carbon HSGD measured with low temperature nitrogen adsorption shows absence of the curve maximum in the range of 75-900 A (Fig. 29.6). In comparison with the pore distribution of SCN hemosorbent, HSGD has predominantly meso- and small macroporous structure, with some... 

In an experiment carried out with 100 mg/1 methylene blue concentration the behaviour was the same as described before, but, there was a time in which SC-155 reached the saturation and the material stopped the adsorption the AC-ref instead continued the adsorption at longer times due to its higher carbon contents. Then, the great difference in adsorption kinetics observed between SC-155 and AC-ref is justified by the more expanded structure of carbon microdomains of SC-155 than the reference, and by the higher radius of meso-macropores observed for the SC-155 the last point provides an easy access of molecules to be adsorbed into the grains of the material, minimizing diffusional problems. 

Modification of NbMCM-41 sieves with Ni or Cu via a cation-exchange procedure causes the transformation of meso- to non porous or macroporous structure. 

The physical and chemical activation processes have been generally employed to prepare the porous carbons.18"35 However, the pore structures are not easily controlled by the activation processes and the size of the pores generated by the activation processes is limited to the micropore range only. Recently, much attention has been paid to the synthesis of meso/macroporous carbons with various pore structures and pore size distributions (PSD) by using various types of such inorganic templates as silica materials and zeolites.17,36 55... 

The meso/macroporous carbons have attracted much attention in their application as electrode materials in EDLCs, since the meso/macropores promote the formation of an effective doublelayer or the transfer of ions into the pores, resulting in the increases in the electrolyte wettability and the rate capability.67,68 In this regard, there has been considerable research targeted towards developing the synthetic methods of novel meso/macroporous carbons.17,36"55,69 72 Various types of such inorganic templates as silica materials and zeolites are widely used for the synthesis of the meso/macroporous carbons, since it was revealed17,36"55 that these inorganic templates contribute to the formation of the meso/macropores with various pore structures and broad PSD. 

The Pore Structures and Surface Fractal Characteristics of Meso/Macroporous Carbon Specimens I, n, and III Calculated from the Analyses of the N2 Gas Adsorption Isotherms. Reprinted with permission from G. -J. Lee and S. -I. Pyun, Carbon, 43 (2005) 1804. Copyright 2005, with permission from Elsevier. 

The present article first provided the brief overview of the synthetic methods of the porous carbons. In order to prepare the microporous carbons with high surface area, the physical/chemical activation methods have been widely used for a long time.18"35 Recently, the meso/macroporous carbons with various pore structures are prepared by templating methods by using various templates and changing sol-gel reaction conditions, e.g., pH, amount of template, and gelation temperature.17,36 55... 

They consist of a thin layer (<10 fxm) of a nanoporous (3-1OA) carbon film supported on a meso-macroporous inorganic solid (alumina) or on a carbonized polymeric structure [15]. They are produced by pyrolysis of polymeric films. The following two types of membranes are produced ... 

The nanoporous carbon membrane consists of a thin layer (<10pm) of a nanoporous (3-7 A) carbon film supported on a meso-macroporous solid such as alumina or a carbonized polymeric structure. They are produced by judicious pyrolysis of polymeric films. Two types of membranes can be produced. A molecular sieve carbon (MSC) membrane contains pores (3-5 A diameters), which permits the smaller molecules of a gas mixture to enter the pores at the high-pressure side. These molecules adsorb on the pore walls and then they diffuse to the low-pressure side of the membrane where they desorb to the gas phase. Thus, separation is primarily based on differences in the size of the feed gas molecules. Table 7 gives a few examples of separation performance of MSC membranes. ° Component 1 is the smaller component of the feed gas mixture. 

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. ... 

As a rule, ACF not only presents a higher adsorption capacity than conventional GAC, but the pore network is also different due to the fibril structure, which ensures a much higher adsorption kinetics. The reason is that in GAC, the adsorbate must diffuse throughout the macro and mesopores before reaching the micropore or adsorption sites, whereas the micropores are directly accessible ftom the external surface in the ACF (Fig. 23). Consequently, there is no resistance to the diffusion of adsorbates through to the adsorption pores because there is no meso/macropore network. 

A hierarchically macro-/meso-/microporous structured catalyst, Hp-ZSM, has been recently reported by. Li et al. [173]. The catalyst was synthesized using cetyltrimethylammonium bromide (CTAB) and TBAOH as the meso- and micropore templates. Ethanol was used to generate macropores, probably via an ethanol-in-water microemulsion mechanism. The hierarchical porous zeolite shows higher hydrothermal stability and sustained higher catalytic activity than either the amorphous aluminosilicate ZSM-5 or meso-ZSM-5 catalysts used in the reactions involving large molecules. 

Soboleva T, Zhao X, Malek K, Xie Z, Navessin T, Holdcroft S (2010) On the micro-, meso-, and macroporous structures of polymer electrolyte membrane fuel cell catalyst layers. ACS Appl Mater Interfaces 2 375-384... 

The diversified porous patterns of diatomaceous silicas are on the nano- to submicrometer scale (< 10-300 nm) and these meso- and macropores cannot be mediated by single macromolecules, not even proteins. To mimic these meso- and macroporous structures, a different approach can be applied based on a phase separation process as in the vesicle-mediated macromorphogenesis processes extensivily reviewed in Pickett-Heaps et al.I 1 In this case oil-in-water (0/W) emulsions are applied as a model system. 0/W emulsions are isotropic and thermodynamically stable liquid media with a continuous water domain and an oil domain, which are thermodynamically stabilized by a surfactant as micrometer-sized liquid entities. 

---