Effective micropore volume

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 second linear section extends over the multilayer range of each as-plot in Figure 9.12b. Backward extrapolation of this branch gives the total effective micropore volume, up(mic), from the intercept on the v axis. It follows that the effective supermicropore volume, up(sup, mic), can be regarded as the difference vp(mic) -vp(u, mic). It is of interest that after an initial small change in t>p(u, mic), this has remained constant during further activation while the magnitude of t>p(sup, mic) has increased steadily. 

In a study of the porosity of alumina-pillared montmorillonites (Al-PILCs), Zhu et al. (1995) have obtained values of the mean slit-width of 0.8-0.9 nm from the volume/surface ratio. In this case, the nitrogen adsorption values were in agreement with the corresponding dm values of c. 0.8 nm. However, effective micropore volumes obtained from the nitrogen isotherms and from water sorption data were significantly different and it was suggested that the density of the sorbed water was lower than that of liquid water. 

By assuming that the density of the adsorbed nitrogen is the same as that of the liquid at 77K, we can arrive at an assessment of the effective micropore volume, Vp(mic), the effective ultramicropore voliune, Vp(u,mic), and the effective supermicropore volume, Vp(s, mic) - the latter being the difference between Vp(mic) and Vp(u, mic). If we assume that surface coverage of the walls of the supermicropores takes place before the onset of the cooperative filling process [1], we can also evaluate the internal area of the supermicropores, a(s,mic). 

The results in Tables 1 and 2 reveal that the total effective micropore volume, is almost independent of the choice of carbon black and also differences of only a few percent are obtained in the corresponding values of effective ultramicropore volume. However, mueh larger differences are observed if a graphitized carbon is employed as the nonporous reference material. 

Ultramicroporous carbons generally give linear DR plots [7, 11] over wide ranges of p/p°, but the extent of the linear region is much more restricted with most nanoporous carbons [11]. Also, DR plots on many nonporous and mesoporous carbons exhibit similar ranges of linearity [7]. Since D, E, and Mp are all empirical parameters, their significance is questionable. In consequence, it must be emphasized that the simple DR plot cannot always give a true assessment of the effective micropore volume [11]. 

Representative nitrogen adsorption isotherms for the TH(400) series of hydrous oxides and the corresponding as plots are shown in figure 1. as is defined as (amount adsorbed)/(amount adsorbed at P/P = 0.4) [17]. Details of the construction of the as plots, as well as that of the calculation of the surface areas Sbet and Ss, the pore volumes and the effective micropore volumes are given elsewhere [17,18]. Table 1 presents these values for the TH and TH(400) series of gels. 

More often, however, microporosity is associated with an appreciable external surface, or with mesoporosity, or with both. The effect of microporosity on the isotherm will be seen from Fig. 4.11(a) and Fig. 4.12(a). In Fig. 4.11(a) curve (i) refers to a powder made up of nonporous particles and curve (ii) to a solid which is wholly microporous. However, if the particles of the powder are microporous (the total micropore volume being given by the plateau of curve (ii)), the isotherm will assume the form of curve (iii), obtained by summing curves (i) and (ii). Like isotherm (i), the composite isotherm is of Type II, but because of the contribution from the Type 1 isotherm, it has a steep initial portion the relative enhancement of adsorption in the low-pressure region will be reflected in a significantly increased value of the BET c-constant and a shortened linear branch of the BET plot. 

A high value of the BET constant c is a useful preliminary indication of the presence of microporosity, but it does not enable one to estimate the micropore volume itself, that is in effect to break down the composite isotherm (iii) into its components (i) and (ii). 

The BET sur ce area of CNF-R and CNF-HT are 152.5 and 141.6 m /g respectively. After oxidation, the surface areas increase a little (160-170 m /g). All the samples have small micropore volumes (<0.008 cm /g). XRD results imply a 0.339 nm doo2 spacing for CNF-HT, which is smaller than that of CNF-R (0.341 nm). The onset weight loss tenperature (temperature nc ed to reach 5 % hum-ofrf) of the CNF-HT is 660 U while the CNF-R is only 540 U. These results indicate the graphitizaton extent of CNF increases after heat treatment. The ash content of CNF-HT is 0.06 wt%. So, the side effects of the metal on catalytic performances can be ruled out... 

It is observed (Table VI) that a reduction in micropore volume of more than 27% occurs as a result of steam deactivation at 815°C for five hours. In the discussion which follows, it is shown how destruction of crystalline zeolite with the concomitant loss in micropore volume leads to increased accessibility by the solvent, which manifests itself as an effective increase in skeletal density. 

The estimated Ad s given in Table IX indicate that coke on catalyst dominates the density separation only in the case of the separation of Fraction A from Fraction B. For separation of Fraction B from Fraction C, the effect of loss of micropore volume is comparable to that of decreased coke make, while for succeeding fractions, density increase is due primarily to the combined effects of loss of micropore volume and increasing metals levels. 

The estimates in Table IX represent the minimum effect of the loss of micropore volume. Pore-mouth constrictions which lead to partial or complete micropore plugging would serve to increase the contribution of loss of micropore volume to density change. No such pore plugging is expected to occur in the (mesoporous) catalyst matrix or in the mesoporous material that is generated during crystalline zeolite destruction. 

Nitrogen physisorption of the Ge-ZSM-5 sample revealed a considerable contribution of mesopores to the total pore volume, accompanied by a drop in micropore volume of 20%. In a study of the catalytic activity of these materials it was found that the increased mesoporosity of Ge-ZSM-5 had a beneficial effect on the catalytic activity in a series of acid-catalysed reactions.1771 It was observed that the presence of germanium in the framework does not change the strength of the acid sites but, instead, decreases the extent of deactivation from coke residues formed upon reaction. The microporous domains only have short diffusional lengths, but the shape selectivity ascribed to the zeolitic channels is still fully... 

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