Pore texture

Hydrothermal synthesis is a powerful method used for the fabrication of nanophase materials due to the relatively low temperature during synthesis, facile separation of nanopartides in the product, and ready availability of apparatus for such syntheses. Versatile physical and chemical properties of nanomaterials can be obtained with the use of this method that involves various techniques (e.g., control of reaction time, temperature and choice of oxidant and its concentration). Several extensive reviews are available that discuss the fundamental properties and applications of this method [2, 3]. These reviews cover the synthesis of nanomaterials with different pore textures, different types of composition [2, 4—6], and different dimensionalities in terms of morphology [6-8]. 

These macropores are not effective for adsorption of various molecules, but their presence before activation is preferable for creating micropores in the walls. The pore texture of most activated carbons is illustrated in Figure 2.17b, where macropores (>50nm width) and mesopores (2-50nm... 

In order to control the pore texture in carbon materials, blending of two kinds of carbon precursors, the one giving a relatively high carbonization yield and the other having a very low yield, was proposed and called polymer blend method [112], This idea gave certain success to prepare macroporous carbons from poly(urethane-imide) films prepared by blending poly(amide acid) and phenol-terminated polyurethane prepolymers [113]. By coupling this polymer blend method with... 

Posttreating CDC using physicochemical methods has been pursued in order to modify the pore texture and surface chemistry beyond what is achievable with the chlorination alone. As CDC... 

The fluorinated carbon-coated AAO film has an interesting adsorption characteristic that has not been reported so far. Figure 3.12 shows N2 adsorption/desorption isotherms at -196°C for the pristine carbon-coated AAO film and the films fluorinated at different temperatures [119]. The isotherm of the pristine film is characterized by the presence of a sharp rise and a hysteresis in a high relative pressure range. Such a steep increase can be ascribed to the capillary condensation of N2 gas into the nanochannels of the AAO films, that is, the inner space of the nanotubes embedded in the AAO films. The amount of N2 adsorbed by the condensation into the fluorinated channels is lower than that of the pristine one. Moreover, the amount drastically decreases with an increase in the severity of fluorination. Since TEM observation revealed that the inner structure of the fluorinated CNTs was not different from that of the pristine nanotubes, the reason why the N2 isotherm was so changed as in Figure 3.12 cannot be attributed to the alteration of the pore texture upon the... 

Physical adsorption of gases is, undoubtedly, the most widely used technique [4], Due to the considerable sensitivity of nitrogen adsorption isotherms to the pore texture in both microporous and mesoporous ranges and to its relative experimental simplicity, measurements of subcritical nitrogen adsorption at 77 K are the most used. However, this technique has some limitations, and other complementary techniques are needed for the characterization of microporous solids. 

SAXS techniques offer a number of advantages for the characterization of porous materials [79-81] (1) they are sensitive to both closed and open porosity, (2) SAXS intensity profiles are sensitive to shape and orientation of the scattering objects, (3) they can be used to investigate samples that are saturated with liquids, and (4) they can be used to investigate the pore texture of materials under operating conditions. However, the equipment required for SAXS experiments is not as available as other adsorption equipment. 

As previously discussed in this chapter, gas adsorption techniques are the most common approaches to the characterization of the pore texture, nitrogen adsorption at 77 K being the most popular technique. However, as mentioned above, the SAXS technique represents an alternative to gas adsorption methods. 

Another tanning method is mineral tanning, which involves soaking a skin in a solution of alum and salt. This is called tawing. The products of this process are white and open-pored, and become stiff and hard when dried. Due to its open-pored texture, tawed leather is often treated with additives that fill in some of the irregularities and add to the skin s strength. Historically, these fillers have included flour, grease, egg yolks, and fat. 

The accuracy of the permeability method depends on the available relationship between the permeability parameters B0 and Kt and the structural parameters of porous media. When the pore texture is not sufficiently random and uniform, the accuracy of Equations 3.3 and 3.6 (and consequently of the method) becomes poor. This is a serious disadvantage of the permeability method, compared to the adsorption method, which does not depend on the pore texture. Nevertheless, permeability measurements are indicative of the porous structure and are useful for the determination of the parameters of the transport models. Various other experiments on flow and diffusion are also indicative of the texture of the porous particles. Some are discussed in Chapter 5. 

Fig. 7 Relationship between F and d) for different pore textures, the best-fit of F-d> relationship for each texture.

In the present paper the pore textures of several porous silicas have been analyzed on the basis of liquid nitrogen data and compared with the results obtained by SAXS and PALS. Silica gels with chemically bonded hydrocarbon phases C-2, C-8 and C-18 were used in experiment. 

The above modeling study showed therefore the importance of having a proper knowledge of the pore texture and catalyst distribution of the catalytic filter, since they can seriously affect its performance. This suggests, in line with Ref. 40, the need of a proper characterization of the porous structure of the catalytic filters, concerning pore connectivity, pore size distribution, presence of deadend pores, etc., since each of these features might play a primary role in reactor performance. On the basis of such characterization work, valuable information could be drawn in order to choose or optimize the preparation routes. 

Second step preparation of silica-alumina gels. Aluminium sulfate solution (33 wt% Al2(S04)3-18H20) is added to the silica gel slurry. A further addition of an ammonia solution (20 wt% NHj) leads to the precipitation of alumina at pH 6. The obtained slurries of silica-alumina hydrogels are successively filtered under vacuum and washed several times to remove impurities, and spray-dryed in well defined conditions. Spray drying leads to solid spherical particles with reproducible physical characteristics, in particular pore size distribution, pore texture and particle size distribution. 

In thermoporometry experiments the pore radius is deduced from the measurement of the solidification temperature and the volume of these pores is calculated from the energy involved during the phase transition. The pore radius distribution and the pore surface are then calculated. The pore texture can be described from numerical values (mean pore radius, total pore volume or surface, etc...) or by curves. For example, curves of figure 1 are the cumulative pore volume vs pore radius while curves of figure 2 are the pore radius distributions. Texture modifications are conveniently depicted by the pore size distribution curves. 

Realizations of such microcompartments are obtained in normal heterogeneous catalysis by using zeolite crystals as support material, e. g., in the formation of are-nes from cycloparaffins by use of Y-zeolite crystals as catalysts [15] or the hydro-isomerization of light paraffins by Pt-doped Y-zeolite [16]. Concentration effects, resembling channeling in enzyme-catalyzed reactions, are caused by the hindrance of the transport of larger molecules through the apertures between the cavities which form the three-dimensional pore texture of zeolite crystals. 

Type of carbon Origin Surface area (m /g) Pore texture... 

The pore texture of an adsorbent is a measure of how the pore system is built. The pore texture of a monolith is a coherent macropore system with mesopores as primary pores that are highly connected or accessible through the macropores. Inorganic adsorbents often show a corpuscular structure cross-linked polymers show a network structure of inter-linked hydrocarbon chains with distinct domain sizes. Porous silicas made by agglutination or solidification of silica sols in a two-phase system are aggregates of chemically bound colloidal particles (Fig. 3.25). 

Table 3 Pore texture sol-gel-derived alumina, zirconia, and titania (calcined at 450° C for 3 hr)...

Explore entirely new sorbent synthesis routes to better control of both sorbent pore texture and surface property. 

Moreno-CastiUa, C., Carrasco-Marin, F., Utrera-Hidalgo, E., and Rivera-UtiiUa, J. (1993). Activated carbons as adsorbents of sulfur dioxide in flowing air. Effect of their pore texture and surface basicity. Langmuir, 9, 1378—83. 

The variety of mechanisms that may be involved in the sorption process of metal ions onto activated carbon induces a great number of factors that control the adsorption the surface oxygen complex content, the pH of point of zero charge, the pore texture of carbon, the solution pH and its ionic strength, the adsorption temperature, the nature of the metal ion given by its speciation diagram, its solubility, and its size in adsorption conditions. The influence of these various conditions is detailed in Section 24.2.1.4. 

Among the characteristics of the adsorbent are its pore texture, surface chemistry, and mineral matter content. The characteristics of the adsorptive are its molecular size, solubility, polarity, pIC, (for electrolytes), and nature of the substituents if it is aromatic. Finally, the solution chemistry factors are the pH and the ionic strength [5]. I shall focus in this section only on the role of the characteristics of the adsorbent, especially its carbon surface chemistry, on the adsorption processes, because although its importance has long been recognized [6, 7], the exact nature of this importance has often been controversial and misunderstood [1]. 

The recent work by Li and coworkers [18] provides a good illustration of the importance of the surface chemistry and pore texture of carbon materials on nonelectrolyte adsorption. They studied the adsorption of trichloroethene (TCE) and methyl ieri-butyl ether (MTBE) on different commercial activated carbons and activated carbon fibers with different porosity and surface chemistry. TCE is a relatively hydrophobic planar molecule. MTBE is tetrahedron-like and relatively hydrophilic. The results of the adsorption from aqueous solutions on the more hydrophobic carbons showed that TCE adsorption was controlled by a pore volume ranging from 0.7 to 1 nm width, as shown in Fig. 25.2. MTBE was primarily adsorbed in pores with widths between 0.8 and 1.1 nm. These micropore ranges were between 1.3 and 1.8 times the kinetic diameter of the adsorptives. 

All these results show the importance of the carbon surface chemistry and pore texture on the adsorption of nonelectrolytic organic solutes. Thus, for hydrophobic carbons, which generally have a low content of surface oxygen complexes, the adsorption of organic molecules is by dispersion and hydrophobic interactions, and the pores involved in the adsorption depend on the molecular size of the adsorptive. Conversely, when the adsorbent s content of surface oxygen complexes increases or its hydrophobicity decreases, there is a preferential adsorption of water on these complexes, which reduces the adsorption capacity of the adsorbent. 

Other important organic electrolytes are the dye molecules. The adsorption of dyes is of interest largely because they are pollutants frequently found in textile wastewaters and because some of them were proposed as molecular probes to characterize the pore texture of carbon adsorbents. However, this last apphcation should be viewed with caution [1] because dye adsorption is profoundly affected by the carbon surface chemistry and solution pH. Thus, Graham [40] found a good linear relationship between a decreased uptake of the anionic metanil yellow and an increased carbon surface acidity. This author concluded that acidic groups on the carbon surface tend to reduce the capacity for anionic adsorbates in general. The adsorption of dyes was subsequendy investigated by other authors [1]. For instance, Nandi and Walker [41] studied the adsorption of acid and basic dyes on different carbon materials and found that the area covered by a dye molecule depended on the nature of the solid surface. 

In many liquid-phase applications, the bacterial colonization of activated carbons can occur quite readily [67]. This colonization [68] is considered to result from (i) the adsorptive properties of carbon, which produce an increase in the concentration of nutrients and oxygen as well as the removal of disinfectant compounds (ii) the pore texture of the carbon particles, which provides the bacteria with a protective environment (iii) the presence of a large variety of functional groups on the carbon surface, which enhances the adhesion of microorganisms and (iv) the nature of the mineral matter content of the carbon, which can favor bacteria adhesion. In general, bacteria attached to carbon particles are very resistant to disinfectants. 

The series of 10 chapters that constitute Part 3 of the book deals mainly with the use of adsorption as a means of characterizing carbons. Thus, the first three chapters in this section complement each other in the use of gas-solid or liquid-solid adsorption to characterize the porous texture and/or the surface chemistry of carbons. Porous texture characterization based on gas adsorption is addressed in Chapter 11 in a very comprehensive manner and includes a description of a number of classical and advanced tools (e.g., density functional theory and Monte Carlo simulations) for the characterization of porosity in carbons. Chapter 12 illustrates the use of adsorption at the liquid-solid interface as a means to characterize both pore texture and surface chemistry. The authon propose these methods (calorimetry, adsorption from solution) to characterize carbons for use in such processes as liquid purification or liquid-solid heterogeneous catalysis, for example. Next, the surface chemical characterization of carbons is comprehensively treated in Chapter 13, which discusses topics such as hydrophilicity and functional groups in carbon as well as the amphoteric characteristics and electrokinetic phenomena on carbon surfaces. 

The XRD analysis shows that all of synthesized materials are amorphous. Table 2 reports the solid samples specific surface area and main pore texture characteristics. The solids have large specific surface area, and this value diminish as decrease of Si/Al molar ratio in both USG and VSG series. Fig. 3 displays the pore size distribution of USGl and VSGl samples, respectively. It is clear from the figure that the both samples have a narrower pore size distribution (i.e., 2- lOnm). 

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