Adsorption pore sizes

Figure 6.4 Features of beta zeolite after Fenton treatment, (a) Saito-Foley adsorption pore-size distribution from Ar-physisorption for (O) parent zeolite containing the template (no porosity) ( ) Fenton-detemplated and (V) commercial NH4-form BEA.

The spectrum of adsorption pore sizes and pore volumes and the hydrophilic surface selectivity of the MeAPOs are similar to those described for the SAPOs. The observed catalytic properties vary from weakly to strongly acidic and are both metal- and structure-dependent. The thermal and hydrothermal stability of the MeAPO materials is somewhat less than that of the AIPO4 and SAPO molecular sieves. 

Some of the discrepancies in the reported recoveries of different solutes from various waters by different investigators who have used the same functional polymers from different manufacturers can be rationalized by considering the discussions of pore size and surface area. Even when the pore sizes and surface areas are specified, awareness of the uncertainty in their determination is needed. Two polymers having the same listed pore size and surface area can behave quite differently as accumulators of organic solutes surface area does not specify surface orientation for adsorption pore size is not uniform, so the quoted value is an average and experimentally uncertain number. 

Figure 3. (Left) Saito-Foley adsorption pore-size distribution based on Ar-physisorption ( ) parent BEA zeolite, ( ) one-pot catalyst, ( ) calcined zeolite at 923 K (H-form), (O) BEA zeolite NH4-form. The pore size distribution of BEA is schematically illustrated. (Right) N2O decomposition performance. Conditions 4.5 mbar N2O in He (pressure, 3 bara) and W/F N20= 900 kgxsxmoF. Two reference catalysts are included for comparison, based on NH4-form commercial samples prepared by conventional Fe ion-exchange.

This property characterizes the pore structure, which has a great influence on both, eqtiilibrium and rate of adsorption. Pore size distributions in the size range of 20 to... 

Tomanova, Zbuzek, Jerakek and Schneider (7) compared mercury and nitrogen adsorption pore size distributions, calculated from the Broekhoff and de Boer equations (14), obtained from a series of controlled pore glasses (CPG). There was always an overestimation of the average pore size irrespective of the cylindrical model used or the branch of the low temperature isotherm (adsorption or desorption) chosen. Nor did the isotherm comport to a slit-shaped pore model. All the adsorption average... 

The specific surface area of a solid is one of the first things that must be determined if any detailed physical chemical interpretation of its behavior as an adsorbent is to be possible. Such a determination can be made through adsorption studies themselves, and this aspect is taken up in the next chapter there are a number of other methods, however, that are summarized in the following material. Space does not permit a full discussion, and, in particular, the methods that really amount to a particle or pore size determination, such as optical and electron microscopy, x-ray or neutron diffraction, and permeability studies are largely omitted. 

Below the critical temperature of the adsorbate, adsorption is generally multilayer in type, and the presence of pores may have the effect not only of limiting the possible number of layers of adsorbate (see Eq. XVII-65) but also of introducing capillary condensation phenomena. A wide range of porous adsorbents is now involved and usually having a broad distribution of pore sizes and shapes, unlike the zeolites. The most general characteristic of such adsorption systems is that of hysteresis as illustrated in Fig. XVII-27 and, more gener-... 

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

A Type II isotherm indicates that the solid is non-porous, whilst the Type IV isotherm is characteristic of a mesoporous solid. From both types of isotherm it is possible, provided certain complications are absent, to calculate the specific surface of the solid, as is explained in Chapter 2. Indeed, the method most widely used at the present time for the determination of the surface area of finely divided solids is based on the adsorption of nitrogen at its boiling point. From the Type IV isotherm the pore size distribution may also be evaluated, using procedures outlined in Chapter 3. 

In the pioneer work of Foster the correction due to film thinning had to be neglected, but with the coming of the BET and related methods for the evaluation of specific surface, it became possible to estimate the thickness of the adsorbed film on the walls. A number of procedures have been devised for the calculation of pore size distribution, in which the adsorption contribution is allowed for. All of them are necessarily somewhat tedious and require close attention to detail, and at some stage or another involve the assumption of a pore model. The model-less method of Brunauer and his colleagues represents an attempt to postpone the introduction of a model to a late stage in the calculations. 

Various methods have been devised for incorporating the bv correction into calculations of pore size distribution. Some of them involve the length of the pores and the area of their walls others the area of the walls only and yet others avoid the direct involvement of either the length or the area. Up to the present, virtually all the procedures have been restricted to nitrogen as the adsorptive. 

Everett concludes that in systems where pore blocking can occur, pore size distribution curves derived from the desorption branch of the isotherm are likely to give a misleading picture of the pore structure in particular the size distribution will appear to be much narrower than it actually is. Thus the adsorption branch is to be preferred unless network effects are known to be absent. 

Fig. 3.19 Contrast between the pore size distribution curves based on the adsorption and the desorption branch of the hysteresis loop respectively.

Fig. 3.20 Pore size distributions (calculated by the Roberts method) for silica powder compacted at (A) Ibtonin" (B) 64tonin (C) 130 ton in". The distributions in (a) were calculated from the desorption brunch of the isotherms of nitrogen, and in (h) from the adsorption branch.

At the upper end of the pore size range there is no theoretical limit to the applicability of the Kelvin equation to adsorption isotherms so long as 9 < 90°. There is however a practical limitation, the nature of which may be gathered from Table 3.8 which gives the relative pressures corresponding to... 

The evaluation of pore size distribution by application of the Kelvin equation to Type IV isotherms has hitherto been almost entirely restricted to nitrogen as adsorptive. This is largely a reflection of the widespread use of nitrogen for surface area determination, which has meant that both the pore size distribution and the specific surface can be derived from the same isotherm. 

A factor militating against the use of other adsorptives for pore size determination at the present time is the lack of reliable r-curves. The number of published isotherms of vapours such as benzene, carbon tetrachloride or the lower alkanes, or even such simple inorganic substances as carbon dioxide, on a reasonable number of well-defined non-porous adsorbents, is very small. 

Section 3.7, the gas adsorption method breaks down for practical reasons. Since the angle of contact of mercury with solids is 140° (see later), and therefore more than 90°, an excess pressure Ap is required to force liquid mercury into the pores of a soh d. The idea of using mercury intrusion to measure pore size appears to have been first suggested by Washburn who put forward the basic equation... 

Pore size distribution—comparison of results by mercury porosimetry and by adsorption of nitrogen... 

Whereas at the lower end of its range mercury porosimetry overlaps with the gas adsorption method, at its upper end it overlaps with photomicrography. An instructive example is provided by the work of Dullien and his associates on samples of sandstone. By stereological measurements they were able to arrive at a curve of pore size distribution, which was extremely broad and extended to very coarse macropores the size distribution from mercury porosimetry on the other hand was quite narrow and showed a sharp peak at a much lower figure, 10nm (Fig. 3.31). The apparent contradiction is readily explained in terms of wide cavities which are revealed by photomicrography, and are entered through narrower constrictions which are shown up by mercury porosimetry. 

Mercury porosimetry is generally regarded as the best method available for the routine determination of pore size in the macropore and upper mesopore range. The apparatus is relatively simple in principle (though not inexpensive) and the experimental procedure is less demanding than gas adsorption measurements, in either time or skill. Perhaps on account of the simplicity of the method there is some temptation to overlook the assumptions, often tacit, that are involved, and also the potential sources of error. 

A major difficulty in testing the validity of predictions from the DR equation is that independent estimates of the relevant parameters—the total micropore volume and the pore size distribution—are so often lacking. However, Marsh and Rand compared the extrapolated value for from DR plots of CO2 on a series of activated carbons, with the micropore volume estimated by the pre-adsorption of nonane. They found that except in one case, the value from the DR plot was below, often much below, the nonane figure (Table 4.9). 

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