Strength and pore structure

Alcaniz-Monge, J., Cazorla-Amoros, D., Linares-Solano, A., Yoshida, S. and Oya, A., Effect of the activating gas on tensile strength and pore structure of pitch-based carbon fibers. Carbon, 1994, 32(7), 1277 1283. 

It is demonstrated that besides ionic interactions, covalent interactions can be important in zeolitic systems. Then, besides acid strength, the concept of acid softness-hardness in zeolites needs to be considered and its importance to explain selectivity effects in orbital controlled reactions is discussed. The influence of zeolite composition and pore structure (electronic confinement) on acid softness-hardness, and orbital control, and their effects on para/ortho selectivity during alkylation of toluene by methanol in large pore zeolites is presented. 

Knowledge of the properties of the texture and pore structure of solid materials is highly important for the development of catalytic materials and catalysts. It should, however, be mentioned that the assessment of the dynamic changes of the material under reaction conditions may be even more important than the assessment under pristine conditions, as the changes in material during use will determine its useful lifetime. Note also that physisorption markedly influences specific properties of porous solids, such as acid strength and catalytic activity which are crucial material parameters for sorption and catalysis. 

In most cases, catalysis on zeolites occurs inside the intracrystalline voids. Nevertheless, a catalytic role is sometimes attributed to the external surface of the crystals, which for many crystallographic directions consists of a collection of pore mouths. The possibility of catalysis at pore mouths was first discussed by Venuto [41]. Catalytic sites at the pore mouths can have a strength and a structural environment different from those within the intracrystalline cavities. In principle, situations may occur where the concentration of reactants is totally different at the pore mouths compared to the crystal interior. When intracrystalline diffusion is slow compared to the rate of the chemical reaction, only active sites near the external surface of crystallites may be responsible for catalysis. The absence of intracrystalline diffusion restrictions doesnot necessarily imply that the pore mouths are a-selective catalytic enviromnents. They may provide a local geometry which is different from that available inside the crystals. 

Industrial carbon anodes and artificial graphites are not a single material but are rather members of a broad family of essentially pure carbon. Fortunately, artificial graphites can be tailored to vary widely in their strength, density, conductivity, pore structure, and crystalline development. These attributes contribute to their widespread applicability. Specific characteristics are imparted to the fmished product by conti ollmg the selection of precursor materials and the method of processing [19]... 

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

Specifically, the improved solidification (cementation) technology involves the use of (a) a special dry powder admixture for the generation of a nonsoluble crystalline formation deep within the pores and capillary tracts of the concrete—a crystalline structure that permanently seals the concrete against the penetration or movement of water and other hazardous liquids from any direction (b) special nonmetal reinforced bars for enhancing the concrete block s tensile and compressive strengths and (c) a unique chemical crystallization treatment for the waterproofing and protection of the concrete block s surface. 

PHEMA solubility decreases with increasing ion concentration. As a result, Mikos et al. used salt solutions of varying ionic strength to dilute the reaction mixtures (Liu et al., 2000). It was noted that increasing the ion content of the aqueous solution to 0.7M, interconnected macropores were obtained at 60 vol% water. Surfactants may also be used to control the network pore structure. However, not much work has been done in this area, since surfactants typically work to reduce the surface repulsions between the two phases and form a uniform emulsion. These smaller emulsion droplets when gelled will create a network with an even smaller porous structure. Yet, this is still a promising area of exploration, since it may be possible to form alternate phase structures such as bicontinuous phases, which would be ideal for cellular invasion. 

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