Texture macropores

Pore. size and surface area distribution. Pore sizes and pore volume distributions may be calculated from the relative pressures at which pores are filled (in the adsorption mode) or emptied (in the desorption mode). Fig. 3.45 shows the pore size distribution of a commercial y-alumina. The distribution is very broad both meso- and macropores are present. In practice this is usually a desired situation a texture consisting of a network of large pores (main roads) and small pores (side roads) is ideal. 

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. 

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... 

The Concept of Local Molecular Orientation (LMO). When observed by the naked eye a piece of semi-coke has a macroporous texture. When broken into fragments the pore walls appear as lamellae. After polishing, the pore walls appear as isochromatic areas when observed by optical microscopy. 

Scanning electron micrographs given in Fig. 3 clearly show a homogeneous texture of the corresponding meso- or macroporous glass membranes. 

Textural characterization was performed by N2 adsorption-desorption at 77 K using a Micromeritics ASAP 2010 analyzer. The samples were preheated under vacuum in three steps of Ih at 423 K, Ih at 513 K, and finally 4 h at 623 K. BET specific surface area, Sbet, was calculated using adsorption data in the relative pressure range, P/Po, from 0.05 to 0.2. Total pore volume, Vp , was estimated by Gurvitsch rule on the basis of the amount adsorbed at P/Po of about 0.95. The primary mesopore diameter, Dp, was evaluated using the BJH method from the desorption data of the isotherm. The primary mesopore volume, Vp, and the external surface area, Sext were determined using the t-plot method with the statistical film thickness curve of a macroporous silica gel [5]. 

Important trends in N2 isotherm when the PS beads are used as a physical template are shown in Table 1 and Fig. 2. In Table 1, PI is the alumina prepared without any templates, P2 is prepared without ]4iysical template (PS bead), P3 is prepared without chemical template (stearic acid), and P4 is prepared with all templates. For above 10 nm of pore size and spherical pore system, the Barrett-Joyner-Halenda (BJH) method underestimates the characteristics for spherical pores, while the Broekhoff-de Boer-Frenkel-Halsey-Hill (BdB-FHH) model is more accurate than the BJH model at the range 10-100 nm [13]. Therefore, the pore size distribution between 1 and 10 nm and between 10 and 100 nm obtained from the BJH model and BdB-FHH model on the desorption branch of nitrogen isotherm, respectively. N2 isotherm of P2 has typical type IV and hysteresis loop, while that of P3 shows reduced hysteresis loop at P/Po ca. 0.5 and sharp lifting-up hysteresis loop at P/Po > 0.8. This sharp inflection implies a change in the texture, namely, textural macro-porosity [4,14]. It should be noted that P3 shows only macropore due to the PS bead-free from alumina framework. 

The highly anisotropic porous texture of alumina membranes containing monodispersed cylindrical pores has been characterised using a combination of three techniques SEM, mercury porosimetry and SANS. The SANS technique is a promising and very sensitive method for the analysis of such anisotropic pore structures. Further quantitative analysis of these membranes, which contain uniform macropores, will require SANS measurements at much lower Q. 

The first term in the right hand side of (24) is related to the fraction 7 of internal surface area (or active sites) located in the mesopores which can be blocked and, therefore, contains the textural par uneters. The second term is related to the surface area in the macropores, which are too laxge to be blocked and in which deactivation only occurs by site coverage. Equation (25) is derived in a straightforward way from (2) or (3) and was already encountered in (10) It is valid for constant gas phase composition only. 

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). 

The pore texture of carbon xerogels is controllable at the meso- or macroporous level through the choice of the pH of the precursors solution (Fig. la). The final carbon material is composed of intercoimected microporous carbon nodules. 

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