Micropore capacity

Many attempts have been made to obtain the micropore capacity by the analysis of composite isotherms. The calculation of the micropore volume, up(mic), from np(mic) is almost invariably based on the assumption that the adsorbate in the micropores has the same density as the adsorptive in the liquid state at the operational... 

In principle, a /-plot can be used to assess the micropore capacity provided that the standard multilayer thickness curve has been determined on a non-porous reference material with a similar surface structure to that of the microporous sample. In our view, it is not safe to select a standard isotherm with the same BET C value (i.e. the procedure recommended by Brunauer (1970) and Lecloux and Pirard (1979)) since this does not allow for the fact that the sub-monolayer isotherm shape is dependent on both the surface chemistry and the micropore structure. 

Once the micropores have been filled, both plots in Figure 8.3 become linear, provided that capillary condensation is absent (or only detectable at high p/p°). The low slope signifies that multilayer adsorption has occurred on a relatively small external surface. Back-extrapolation of the linear multilayer section gives the specific micropore capacity, np(mic), as the intercept on the n axis. 

The first material to be studied in this way was an activated sample of carbon black. It was found that prolonged outgassing at a temperature of 350°C was required to achieve complete removal of the pre-adsorbed nonane. All the intermediate nitrogen isotherms were found to be parallel in the multilayer range and the vertical separation between the isotherms obtained after outgassing at 20° and 350°C provided a satisfactory measure of the micropore capacity. Convincing evidence was also obtained that the nonane was removed only from the external surface at 20°C. 

Backward extrapolation of the linear multilayer section of the as-plot allows us to assess the total micropore capacity (as indicated in Chapter 8) and hence to evaluate the effective micropore volume, v(mic, S). The values of vp(mic, S) in Table 9.1 were obtained by making the usual assumption that the pores are filled with liquid nitrogen (density 0.808 g cm-3). 

The as-plots in Figures 9.18-9.22 have been constructed with the aid of standard adsorption data obtained with Elftex 120 and other non-porous carbon blacks (Carrott et al., 1987, 1988a). As noted earlier, each as-plot has two linear sections (Section 8.2.1). The first linear section (at as < 1.0 pjp° < 0.4) can be attributed to adsorption on the walls of the supermicropores and therefore its back-extrapolation to as = 0 gives the ultramicropore capacity. The second linear section is obtained at as > 1.0 and is associated with multilayer adsorption on the external surface and the intercept gives the total micropore capacity. 

The /-method of isotherm analysis adopted by Cases et al. (1992) is not entirely satisfactory and therefore the interpretation of the results is not altogether straightforward. However, the high BET C value is consistent with the conclusion that there was a small micropore filling contribution. To arrive at a more realistic quantitative assessment of the microporosity it would be desirable to obtain nitrogen isotherm data on a truly non-porous form of Na-montmorillonite. In practice, however, this may be difficult to accomplish and a more pragmatic approach would be to construct a series of comparison plots for the adsorption of N2 (and preferably also Ar) on pairs of samples of differing particle sizes and defect structures. In this way it should be possible to establish quantitative differences in the micropore capacities. 

The as-method was also used by Grange and his co-workers (Gil et al., 1995 Gil and Grange, 1996) for analysing nitrogen isotherms on a series of pillared clays prepared from Na-montmorillonite. Hysteresis loops of Type H4 were associated with the secondary porosity and high values of the Langmuir constant b (see Equation (4.38)) indicated microporosity. In the case of a sample of Al-PILC, the micropore capacity was estimated to contribute about 60% to the total uptake at p/p° = 0.99. 

Recently, Setoyama et al. (1996) have extended their earlier investigations of the properties of fluorinated activated carbons. Nitrogen isotherms were determined both before and after the fluorination of cellulose-based ACF. Analysis of the as-plots and the corresponding DR plots has indicated that although the micropore capacity and width were reduced, the micropore structure appeared to become more homogeneous as a result of fluorination. 

The difficulty of arriving at an unambiguous assessment of the micropore capacity is immediately apparent when we see the shape of the water isotherm in Figure 12.16. Since the micropore filling process is confined to the range pjp° < 0.1, there is an overestimate of 20% in vf if the molecular sieve capacity is taken as the uptake at p p° = 0.4. 

Various procedures have been used to evaluate the micropore capacity from the experimental isotherm data (e.g. the Dubinin-Radushkevich plot), but in practice these are all empirical methods. It should be kept in mind that no theoretical significance can be deduced from the fact that a particular equation gives a reasonably good fit over a certain range of an isotherm determined at only one temperature. In our view, a safer approach is to plot the amount adsorbed against standard data determined on a non-porous reference material (i.e. to construct a comparison plot or Os-plot)-... 

To convert the micropore capacity into the micropore volume, it is usually assumed that the pores are filled with liquid adsorptive - as in the case of mesopore filling. This procedure does not allow for the dependency of molecular packing on both pore size and pore shape. For this reason, we recommend that the term apparent micropore volume should be adopted and that the gas and temperature should always be specified. 

The characterization of porous materials exhibiting a composite pore structure encompassing micro-meso-and perhaps macro-pore sizes, is of particular significance for the development of separation and reaction processes. Among the characterization methods for materials exhibiting ultramicropore structures, DpDubinin-Radushkevich (DR)[2], Dubinin-Astakov (DA) [3], Dubinin-Stoeckli (DS) [4], as well as the Horvath-Kawazoe (HK), [5] methods are routinely used for the evaluation of the micropore capacity and the pore size distriburion (PSD). 

For the 2- and 3-dimensional cases the channel interconnections allow a substantial fraction of the micropore capacity to be accessible below a percolation threshold in blocking probability. Above this percolation threshold, the accessible micropore volume is limited to the perimeter region of the ciystallite [82]. 

Carrott, who suggested that there are at least two basic types of molecular sieve actions for the separation of small molecules. In the case of polymer carbons produced at low bum-off, a small micropore size is thought to be largely responsible for the molecular sieve action, but in certain others it is thought that constrictions at the micropore entrances are involved. It has also been found that there is an empirical relationship between log (r o) and log (VJ, where Po is the micropore capacity for a given adsorbate in mmol/g and in cm mol is the molar volume of the adsorbate. Carrott has shown that the observed differences in the linearity and slope of the log (Po) against log (YJ plots can be rationalized on the basis of different types of molecular sieve behavior and that the analysis of the plots should, therefore, be able to provide useful information relating not only to the micropores themselves but also to the micropore entrances. 

If the material is purely microporous, its isotherm is perfectly Type I. In this case, the horizontal plateau exactly provides the micropore capacity. There is no need for any further assumption and therefore no need for applying the Langmuir equation. 

This is fortunately offered by the calorimetric experiments which suggest that the BET monolayer content physically corresponds to an energetically strong retention. This quantity, provided by the BET equation, could therefore be called safely the BET strong retention capacity This quantity includes two parts, which are the micropore capacity and the monolayer content on the non-microporous portions of the surface. The latter, which provides the external (/.e. non-microporous) surface area is easily assessed by means of the as or t methods, without even requesting the very low part of the adsorption isotherm. The as method is to be preferred when one wishes to carry out a more detailed analysis of the micropores and when the low pressure range of the adsorption isotherm is available. Conversely, if one only wishes to assess a reliable external surface area, he will probably find it simpler to use the t method this can indeed easily be done in a software, after introducing the appropriate multilayer equation, like the Harkins and Jura t-curve equation [13]. The recommended succession of calculations is therefore ... 

It may be worth pointing out that the micropore capacity (for a given adsorptive) is highly meaningful and close to physical reality, which is not the case for the micropore volume whose calculation relies on the unknown packing of the adsorbate in the micropores)... 

The four quantities above can be determined systematically and automatically by a software, whatever the adsorbent in the presence of micropores, they should all be meaningful, whereas in their absence the derived micropore capacity should simply be negligible. It is only in the latter case that the concept of BET monolayer content can be restored and used without ambiguity. 

Finally, we suggest that instead of the absolute concepts of BET surfece area (inadequate for micropores) or microporous volume (which is never correctly assessed, because of the unknown packing of the adsorbate), one should more safely use the concepts of "BET strong retention capacity", "external surface area", micropore capacity" and saturation capacity" which are close to physied reality and therefore suitable for sound interpretation and practical application. 

It was mentioned above that micropore capacity, a j, can be converted to micropore volume, v . For the same adsorbent it might be expected that access to small micropores is limited for large adsorbate molecules (ref. 16). Accordingly, similar values of V suggest that molecules of different adsorbates are accessible to the microporous structure of the adsorbent to the same degree. In this context, comparisons of DR parameters and micropore volumes estimated from adsorption of carbon dioxide, nitrogen, n-butane... 

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