Interconnected micropores

Both natural clays and their alnminium oxide pillared analogues have also been tested for the catalytic cracking of polyethylene [49-51]. The clays investigated include mont-morillonite and saponite. They possess a layered structure which can be converted into a two-dimensional network of interconnected micropores by intercalation of molecular moieties. In the case of alnmininm pillared clays, these materials show a mild acidity... 

In the case of activated carbon fibers, pore size is small compared to GACs. Moreover, the external diameter of activated carbon fibers is at least two orders of magnitude smaller than GACs. A mesh of well-interconnected micropores from the core to the external surface of an activated carbon fiber reduces the intraparticulate diffusion time. This results in the superior adsorption dynamics of activated carbon fibers as compared to GACs (Hayes and Akamatsu 2000). 

Although it is difficult to formulate generic design rules for MOPs, a number of criteria are clearly important in terms of producing polymers with high physical surface areas and interconnected micropore structures. 

Zeolites are crystalline aluminosilicates materials that possess ordered and interconnected microporous channels with diameters ranging from 0.2-20 A. Their unique properties (microporosity, high surface area, acid-base character, shape) have made than a material of choice in a great number of applications. Zeohtes are intensively used in gas separation due to their ability to adsorb selectively a large variety of molecules and are also known as molecular sieves. Furthermore, these materials are also used as ion exchangers (water softeners) and catalysts in petrochonistry. Currently, the world s annual production of natural zeoUte is about 4 million tons. Of this quantity, 2.6 milUon tons are shipped to Chinese markets to be used in the concrete industry. The amount of synthetic zeohtes produced is about 1.5 miUion tons (Figure 5.1). 

The polymeric fillers exhibit a rigid rod-like, randomly contorted structure which allows them to exhibit intrinsic microporosity. These polymeric fillers of intrinsic microporosity exhibit behavior analogous to that of conventional microporous materials including large and accessible surface areas, interconnected micropores of less than 2 nm in size, as well as high chemical and thermal stability, but, in addition, possess properties of conventional polymers including good solubility and easy processability. The fillers are made from poly(ether)s that have a favorable interaction between carbon dioxide and the ethers within the chain. 

The high internal surface area of an adsorbent creates the high capacity needed for a successful separation or purification process. Adsorbents can be made with internal surface areas which range from about 100 m /g to over 3000 m /g. For practical applications, however, the range is normally restricted to about 300-1200 m /g. For most adsorbents the internal surface area is created from pores of various size. The structure of an adsorbent is shown in idealized form in Figure 2.1. Many adsorbent materials, such as carbons, silica gels and aluminas, are amorphous and contain complex networks of interconnected micropores, mesopores and macropores. In contrast, in zeolitic adsorbents the pores or channels have precise... 

Figure 2.2 Classification of different types of porous materials, (a) A purely microporous zeolite is considered as a non-hierarchical system according to the single level of porosity, (b) Fragmentation of the zeolite into nanocrystals engenders a network of mesopores constituting the intercrystalline space, leading to an interconnected hierarchical system. Intraconnected...

Microporous. Microporous membranes are characterized by interconnected pores, which are small, but large in comparison to the size of small molecules. If the pores are of the order of size of the molecules for at least some of the components in the feed mixture, the diffusion of those components will be hindered, resulting in a separation. Molecules of size larger than the pores will be prevented from diffusing through the pores by virtue of a sieving effect. 

Compared with microporous and mesoporous materials, the larger, interconnected voids in macroporous materials potentially provide easier molecule transportation through the materials. This is of particular interest for the transport of large biomolecules (e.g., proteins and cells). The pore sizes in macroporous materials are usually from tens to hundreds of nanometers, and the pores are typically... 

Ordered mesoporous silicas present mesopores of appropriate size to be evaluated and compared in their field of superposition (3-50 nm) of the methods of pore size evaluation by N2 volumetry and Hg intrusion. The usual models to evaluate pore sizes by Hg intrusion refer to cylindrical pores [1,2]. Here we evaluate the influence of some non ideal characteristics of the SBA-15 system, namely uneven pore walls, mesopore interconnection or presence of micropores [3, 4], on the mercury intrusion. 

The penetration of mercury in MCM-41, a material with smooth cylindrical pores, takes place at the pressure indicated by the Washbum-Laplace model, indicating that this model is still valid at the scale of a few nanometers. When the pore surface is pitted with micropores or when the pores are interconnected, like in the case of SBA-15, the Washbum-Laplace model underevaluates the size of the pores, due to the excess energy needed for advancement of the meniscus beyond the surface defects. 

Designing a specific material architecture. 3D hierarchical carbon [79,80], 3D aperiodic [79,81,82] or highly-ordered hierarchical carbons are representative samples with multimodal pore structure to optimize the performance of the capacitors. The micropore, mesopore and macropore structure of such three-dimensional hierarchical carbons are generally perfectly interconnected. 

Gonradsson, T., Dadachov, M.S., and Zou, X.D. (2000) Synthesis and structure of (Me3N)6[Ge32064](H20)4.5, a thermally stable novel zeotype with 3D interconnected 12-ring channels. Micropor. Mesopor. Mat., 41, 183-191. 

Figure 12.1 Structural models of various PMS materials. Note that in contrast to MCM-41 the hexagonally arranged mesopores of SBA-15 are three-dimensionally interconnected by micropores.

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