Porous microporous materials

Separator s a physical barrier between the positive and negative electrodes incorporated into most cell designs to prevent electrical shorting. The separator can be a gelled electrolyte or a microporous plastic film or other porous inert material filled with electrolyte. Separators must be permeable to ions and inert in the battery environment. 

ITie BET method is the most widely used procedure for determining the surface area of porous materials. In this chapter, BET results were obtained from single point measurements using a Micromeritics Flowsorb II 2300 surface area analyzer. A mixture of nitrogen in helium (30 70 mole percentage) was used. Although this simple method is not quantitative for the microporous materials studied in section 5, it still allows qualitative comparisons to be made. 

Recent reports describe the use of various porous carbon materials for protein adsorption. For example, Hyeon and coworkers summarized the recent development of porous carbon materials in their review [163], where the successful use of mesoporous carbons as adsorbents for bulky pollutants, as electrodes for supercapacitors and fuel cells, and as hosts for protein immobilization are described. Gogotsi and coworkers synthesized novel mesoporous carbon materials using ternary MAX-phase carbides that can be optimized for efficient adsorption of large inflammatory proteins [164]. The synthesized carbons possess tunable pore size with a large volume of slit-shaped mesopores. They demonstrated that not only micropores (0.4—2 nm) but also mesopores (2-50 nm) can be tuned in a controlled way by extraction of metals from carbides, providing a mechanism for the optimization of adsorption systems for selective adsorption of a large variety of biomolecules. Furthermore, Vinu and coworkers have successfully developed the synthesis of... 

Nitrogen adsorption/desorption isotherms on Zeolite and V-Mo-zeolite are very similar and close to a type I characteristic of microporous materials, although the V-Mo-catalysts show small hysterisis loop at higher partial pressures, which reveals some intergranular mesoporosity. Table 1 shows that BET surface area, microporous and porous volumes, decrease after the introduction of Molybdenum and vanadium in zeolite indicating a textural alteration probably because of pore blocking by vanadium or molybdenum species either dispersed in the channels or deposited at the outer surface of the zeolite. The effect is far less important for the catalysts issued from ZSM-5. 

Pores are found in many solids and the term porosity is often used quite arbitrarily to describe many different properties of such materials. Occasionally, it is used to indicate the mere presence of pores in a material, sometimes as a measure for the size of the pores, and often as a measure for the amount of pores present in a material. The latter is closest to its physical definition. The porosity of a material is defined as the ratio between the pore volume of a particle and its total volume (pore volume + volume of solid) [1]. A certain porosity is a common feature of most heterogeneous catalysts. The pores are either formed by voids between small aggregated particles (textural porosity) or they are intrinsic structural features of the materials (structural porosity). According to the IUPAC notation, porous materials are classified with respect to their sizes into three groups microporous, mesoporous, and macroporous materials [2], Microporous materials have pores with diameters < 2 nm, mesoporous materials have pore diameters between 2 and 50 nm, and macroporous materials have pore diameters > 50 nm. Nowadays, some authors use the term nanoporosity which, however, has no clear definition but is typically used in combination with nanotechnology and nanochemistry for materials with pore sizes in the nanometer range, i.e., 0.1 to 100 nm. Nanoporous could thus mean everything from microporous to macroporous. 

Fig. 9 Schematic representation of three approaches to generate nanoporous and meso-porous materials with block copolymers, a Block copolymer micelle templating for mesoporous inorganic materials. Block copolymer micelles form a hexagonal array. Silicate species then occupy the spaces between the cylinders. The final removal of micelle template leaves hollow cylinders, b Block copolymer matrix for nanoporous materials. Block copolymers form hexagonal cylinder phase in bulk or thin film state. Subsequent crosslinking fixes the matrix hollow channels are generated by removing the minor phase, c Rod-coil block copolymer for microporous materials. Solution-cast micellar films consisted of multilayers of hexagonally ordered arrays of spherical holes. (Adapted from [33])...

Details about the porous texture properties of the studied materials can by found in our previous papers 4 18. In general, all activated carbons, activated carbon fibers and activated carbon monoliths are essentially microporous materials with a negligible contribution of meso- and macroporosity. 

Among various preparation techniques of metal/alloy particles on the nanometre scale, the templating synthesis using microporous materials as microreactors is one of the promising methods [7, 10, 11]. Uniform void spaces of porous hosts work to synthesize nanostructured metal/aUoy particles as a guest, which are... 

Studies of metal compound diffusion in porous media have consistently demonstrated that the rate of diffusion within the microporous material is less than would be observed in an unrestricted medium. This discrepancy, observed for all liquid diffusion processes in pores of small diameter is related to hydrodynamic phenomena. The proximity of the molecule to the pore wall increases the frictional drag on the diffusing species when the... 

Substrate Accessibility Porous materials are divided according to their pore size. This can be measured using adsorption techniques (the principles of adsorption are outlined below). According to the International Union of Pure and Applied Chemistry (IUPAC), microporous materials have pores under 2 nm in diameter, mesoporous materials have pores of 2-50 nm diameter, and any material with an average pore diameter above 50 nm is macroporous [74]. The pores must be sufficiently large for substrates to enter and products to exit. Pores come in different shapes and sizes (Figure 4.14). Some materials have large pores with narrow pore mouths, known as... 

The sol-gel process involves the transition of a system from a liquid "sol" (mostly colloidal) into a solid "gel" phase (11). By applying this methodology, it is possible to fabricate ceramic or glass materials in a wide variety of forms ultrafine or spherical-shaped powders, thin film coatings, ceramic fibers, microporous inorganic membranes, monolithic ceramics and glasses, or extremely porous aerogel materials. 

Porous oxide catalytic materials are commonly subdivided into microporous (pore diameter <2nm) and mesoporous (2-50 nm) materials. Zeolites are aluminosilicates with pore sizes in the range of 0.3-1.2 nm. Their high acidic strength, which is the consequence of the presence of aluminium atoms in the framework, combined with a high surface area and small pore-size distribution, has made them valuable in applications such as shape-selective catalysis and separation technology. The introduction of redox-active heteroatoms has broadened the applicability of crystalline microporous materials towards reactions other than acid-catalysed ones. 

Porous carbon materials mostly consist of carbon and exhibit appreciable apparent surface area and micropore volume (MPV) [1-3], They are solids with a wide variety of pore size distributions (PSDs), which can be prepared in different forms, such as powders, granules, pellets, fibers, cloths,... 

Silica is one of the most abundant chemical substances on earth. It can be both crystalline or amorphous. The crystalline forms of silica are quartz, cristobalite, and tridymite [51,52]. The amorphous forms, which are normally porous [149] are precipitated silica, silica gel, colloidal silica sols, and pyrogenic silica [150-156], According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be classified as follows microporous materials are those with pore diameters from 3 to 20 A mesoporous materials are those that have pore diameters between 20 and 500 A and macroporous materials are those with pores bigger than 500 A [149],... 

An experimental facility was described in Section 4.5.9 (see Figure 4.22) that was used to carry out the characterization of the groups present on the surface of a porous material or the channels and/or cavities of a microporous material applying the FTIR methodology. With this methodology, it is also possible to measure different diffusion coefficients in microporous materials with the help of the FTIR method [87-92], Here, a laboratory-assembled facility similar to that reported in Section 4.5.9 that has two manifolds (Figure 5.31) instead of one, for the introduction of the diffusing molecules, and thus has the capability to deliver two different hydrocarbons to the IR cell, is described [90],... 

Porous materials are of huge practical significance for applications in industry, energy production, and pollution abatement. In this regard, microporous materials, such as zeolites and related materials,... 

In some cases, the rate-controlling polymeric membrane is not compact but porous. Microporous membranes can be prepared by making hydrophobic polymer membranes in the presence of water-soluble materials such as polyethylene glycol), which can be subsequently removed from the polymer matrix by dissolving in aqueous solution. Cellulose esters, loosely cross-linked hydrogels and other polymers given in Table 4.2 also give rise to porous membranes. 

The fractal concept is based on the assumption of reproduction of the general elements of structure of porous materials at all levels—from microscopic to macroscopic ones. This assumption is valid for numerous macroporous materials, while it is too difficult to check its validity for microporous ones. However, based on general thermodynamic considerations, one may assume that fractal concepts also apply to some of microporous materials. As it is shown below, the main condition of the applicability of the fractal approach to microporous materials consists in their homogeneity. However, one has to take into account that this strict analysis does not allow the assumption of homogeneity of any microporous system, not least, because the subsystem micropore-wall of micropore is obviously heterogeneous. Therefore, the fractal concept is probably not applicable to very narrow micropores (ultramicropores, according to Dubinin s classification). 

In principle, a /-plot can be used to assess the micropore capacity provided that the standard multilayer thickness curve has been determined on a non-porous reference material with a similar surface structure to that of the microporous sample. In our view, it is not safe to select a standard isotherm with the same BET C value (i.e. the procedure recommended by Brunauer (1970) and Lecloux and Pirard (1979)) since this does not allow for the fact that the sub-monolayer isotherm shape is dependent on both the surface chemistry and the micropore structure. 

Various procedures have been used to evaluate the micropore capacity from the experimental isotherm data (e.g. the Dubinin-Radushkevich plot), but in practice these are all empirical methods. It should be kept in mind that no theoretical significance can be deduced from the fact that a particular equation gives a reasonably good fit over a certain range of an isotherm determined at only one temperature. In our view, a safer approach is to plot the amount adsorbed against standard data determined on a non-porous reference material (i.e. to construct a comparison plot or Os-plot)-... 

The described treatment of mass transport presumes a simple, relatively uniform (monomodal) pore size distribution. As previously mentioned, many catalyst particles are formed by tableting or extruding finely powdered microporous materials and have a bidisperse porous structure. Mass transport in such catalysts is usually described in terms of two coefficients, a effective macropore diffusivity and an effective micropore diffusivity. 

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