Pore types

FIG. 11 Pore-size distributions, (a) Spherical molecules in various pore types, (b) thin rods in various pore types. (Reprinted with permission from Ref. 135, Copyright 1968, American Chemical Society.)... 

It should be noted that the decomposition shown in Eq. 3.7.2 is not necessarily a subdivision of separate sets of spins, as all spins in general are subject to both relaxation and diffusion. Rather, it is a classification of different components of the overall decay according to their time constant. In particular cases, the spectrum of amplitudes an represents the populations of a set of pore types, each encoded with a modulation determined by its internal gradient. However, in the case of stronger encoding, the initial magnetization distribution within a single pore type may contain multiple modes (j)n. In this case the interpretation could become more complex [49]. 

The pore types FAU, MFI, MOR, and LTA discussed above contain basically all micropore types possible in zeolites. There are accessible and nonaccessible cages (FAU and LTA) and straight or meandering channels in one, two, or three dimensions, which may be either isolated or connected to each other (MFI and MOR). 

Hint Use an approach of enclosed partitioning. At each level of partitioning divide a system into the particles (porous or nonporous) and pores between these particles. For a mathematical consideration it is convenient to substitute a real multilevel PS with a set of systems of particle-pore type in the sense of the previous sentence. 

Eikerling et al. ° used a similar approach except that they focus mainly on convective transport. As mentioned above, they use a pore-size distribution for Nafion and percolation phenomena to describe water flow through two different pore types in the membrane. Their model is also more microscopic and statistically rigorous than that of Weber and Newman. Overall, only through combination models can a physically based description of transport in membranes be accomplished that takes into account all of the experimental findings. 

The shape of the hysteresis loop in the adsorption/desorption isotherms provides information about the nature of the pores. The loops have been classified according to shape as A, B and E (De Boer, 1958) or as HI - H4 by lUPAC (Sing et al, 1985). Ideally, the different loop shapes correspond to cylindrical, slit shaped and ink-bottle pores the loops in the isotherm IV and V of Figure 5.3 correspond to cylindrical pores. Wide loops indicate a broad pore size distribution (for an example see Fig. 14.9). The absence of such a loop may mean that the sample is either nonporous or microporous. These generalizations provide some initial assistance in assessing the porosity of a sample. In fact the adsorption/desorption isotherms are often more complicated than those shown in Figure 5.3 owing to a mixture of pore types and/or to a wide pore size distribution. 

Transport mechanisms of the pore type (Fig. 3) seem to be induced (e.g., by amphotericin B) in bilayer membranes and there is very good... 

This difference between H O and N adsorption data has been attributed to either the accessibility of water to interlayer spaces in the tobermorite gel or to the presence of ink bottle pores with narrow necks and wide bodies. The considerable increase (4-5 times) in pore surface and pore volume available to nitrogen in pastes containing calcium chloride suggests that the crumpled pore type of morphology is more open than the spicular type [20],... 

Fig. 3 a Model for the binding of cyclostreptin at the proposed initial MSA binding site at pore type I. Taken from [21]. b Scheme of the route of paclitaxel to its lumenal site in the microtubules. Paclitaxel binds to the external site at the pores of the microtubules, being later transported to its lumenal site while the external site gets blocked. In the presence of cyclostreptin the external site gets irreversibly blocked thus, paclitaxel cannot reach the lumenal site... 

Since the lumenal site of microtubules was well supported by the structural data and also by the fact that mutations in the lumenal site confer resistance to taxanes [45], an alternative mechanism with binding to an initial exposed binding site located in pore type I of the microtubule wall and later transportation of the ligand to the lumenal site was proposed in [22] (Fig. 9). 

From these results a structural binding pathway consistent with that observed kinetically was proposed [21], Paclitaxel binds fast to a site located in the surface of the microtubules, into the pore type I. Then it has to be transferred to the second luminal, final location. The transfer involves probably the switch of some of the elements of the first site, since only one molecule of paclitaxel can bind to each molecule of P-tubulin (Fig. 3b). 

Fig. 9 Insight of the outer (a) and (b) inner surface of a high resolution microtubule model, showing two different types of pores, I and II (see text). Green beads, polar residues yellow beads, hydrophobic residues red beads, acid residues blue beads, basic residues white beads, paclitaxel bound at its site grey beads, nucleotide. Detail of a pore type I viewed from above (c). Ribbon representation of two neighbour P-tubulin subunits as seen from the plus end of the microtubule, paclitaxel, GDP and the four residues forming a putative taxoid binding site are shown in Van der Waals representation. Taken from [22]...

Various commonly used nitrogen adsorption-desorption isotherms analysis methods have been applied on model samples containing micro and mesopores in known proportions. The very observation of the isotherms, as well as of the corresponding /-plots, allows a qualitative conclusion to be drawn about pore type and shape. It has been checked that even a very low amount of micropores leads to a visible downward deviation in the /-plot, such as in the X(13) sample. This conclusion is also true for mesopores that are already visible in the /-plot of sample X(85). These two observations confirm the usefulness and reliability of the /-plot as a tool for qualitative texture analysis. 

SAMPLE DATABASE, TEXTURES AND PORE-TYPES USED... 

Samples number Formation Mineralogy Texture Pore-types F m Chargeability M=Ri/(Rr.+Rn) n Water porosity Permeability (mD)... 

Chargeability factor M depends on the brine/gas saturation of porous solids. Figure 3 gives the relationship between the chargeability and brine saturation for two samples. We noted that the M decreases hardly with the decrease of the brine saturation. The presence of vugs and karsts pore types (sample 9-LS8) seems to speed up the decrease of the M Chargeability factor M can be explained by a multi-linear model composed of different structures parameters such as the formation resistivity factor, water porosity, Hg-specific surface area and water permeability, e.g.. Fig. 5. 

FIGURE 4.16 Schematic illustration of pore types in porous solid with open pores (c,d), locked-in (a) and dead-end pores (b,e,t). (From Rouquerol J., Avnir D., Fairbridge C.W., Everett D.H., Haynes J.H., Pemicone N., Ramsey J.D.F., Sing K.S.W., Unger K.K., Pure Appl. Chem., 66, 1739, 1994. With permission.)... 

Figure 11 Disttibulioo of pore ndworic complexity in Four main carbonate texture, relationship with their primary pore-types and disttihution of pore throat size.

FIG. 172. Drying of a system containing two pore sizes (o) the region of evaporation from both pore types, ( >) evaporation from the smaller pores (Keey, 1972). 

According to the porosity data of Uchida et al. [102] the matrix of carbon grains (20-40 nm) forms an agglomerated structure with a bimodal psd. Primary pores (micropores, 5-40 nm) exist within agglomerates, between the carbon grains. Larger, secondary pores (macropores, 40-200 nm) form the pore spaces between agglomerates. The relation between the relative pore volume fractions of the two pore types depends on the contents of PFSI and PTFE. Due to their molecular size these components are not able to penetrate micropores. They affect only the macropore volume. The experimental study revealed that an increased PFSI content leads to a decrease of the macropore volume fraction. The opposite effect was found for PTFE. 

Hydrocarbons of different nature having low octane numbers can be converted into gasoline with the properties of motor fuels using middle-and wde-pore-type zeolites. The yield and composition of gasoline obtained are determined by the composition of the initial feed as well as the process conditions. 

Fig. 2.3. Schematic picture of pore types in a porous membrane, a Isolated pore b,f dead end pore c,d tortuous and/or rough pores (d) with constrictions (c) e conical pore.

A schematic picture of different t5q)es of pores is given in Fig. 9.1 and of main types of pore shapes in Fig. 9.2. In single crystal zeolites the pore characteristics are an intrinsic property of the crystalline lattice [3] but in zeolite membranes other pore types also occur. As can be seen from Fig. 9.1, isolated pores and dead ends do not contribute to the permeation under steady conditions. With adsorbing gases, dead end pores can contribute however in transient measurements [1,2,3]. Dead ends do also contribute to the porosity as measured by adsorption techniques but do not contribute to the effective porosity in permeation. Pore shapes are channel-like or slit-shaped. Pore constrictions are important for flow resistance, especially when capillary condensation and surface diffusion phenomena occur in systems with a relatively large internal surface area. 

Table 2. Point-count analyses of pore types...

Classification by pore form is also possible. Typical pore types are shown in Fig. 

Nuclear track-etch membranes (capillary-pore type). 

Ringrose-Voase, A.J., and P. Bullock. 1984. The automatic recognition and measurement of soil pore types by image analysis and computer programs. J. Soil Sci. 35 673-684. 

Different solid acid catalysts like zeolite Y [2-6], beta [7-9], MCM-22 [10], solid superacids [11-13], sulphonic acid resins [14], etc. have been proposed as potential alkylation catalysts and some of them are being tested at a pilot plant scale. Zeolites and solid superacids of sulfated zirconia type were found to be the most active but they suffer rapid deactivation after an initial period. Among different zeolites studied large-pore zeolites are prefered over medium-pore type because the former favors the formation and diffusion of bulkier tri-methylpentane isomers. Beside pore size and zeolite structure, the fiamework composition (Si/Al ratio) and acid strength distribution also play an important role on the activity, selectivity and deactivation of the catalysts. It is known that the adsorption behavior of the zeolite and the extent of hydrogen transfer capacity (a crucial factor of alkylation activity) both depend on the aluminium concentration in the framework [15-16]. 

Zeolite Pore type Dimension (A) Channel dimensionality... 

---