Relative internal volume accessible

Generally, a, reflects the effects of two-body collisions between xenon atoms. The change in slope can be interpreted as a change in free volume upon coking. One may estimate effective relative internal volume accessible to xenon gas per gram of material from the slopes ... 

The porosity of gels is usually determined by equilibrium permeation or size exclusion chromatography by measuring the internal volume, accessible to test molecules depending on their size (molecular weight MW, radius MR). The cumulative pore volume distribution, however, can also be measured optically. By microinterferometry of specimens, embedded in solutions of test molecules, the density of the matrix M, the inner volume and the relative access of defined molecules can be determined by measuring the optical path difference r between the porous specimen of thickness d and the embedding medium in the equilibrium... 

The total pore volume, Vp, sometimes called specific pore volume when referred to unit mass, is the total internal volume per unit mass of catalysts. Some of this pore volume may be completely enclosed and thus inaccessible to molecules participating in a catalytic reaction. The total accessible pore volume is often derived from the amount of vapour adsorbed at a relative pressure close to unity, by assuming that the pores are then filled with liquid adsorptive. The accessible pore volume may be different for molecules of different sizes. It may be useful to determine the dead space by means of a nonsorbable gas (normally helium) in conjuction with the determination of the bulk volume of the catalyst by means of a non-wetting liquid (mercury). 

The CO2 isotherms at 273 K for a various high-organic soils and humic acid particles (77,75) are Type I, characteristic of microporous solids (19). Furthermore, CO2 adsorption is two or more orders of magnitude greater than Nj adsorption at comparable relative pressures. Using a model that assumes liquid condensation of CO2 in the pores, porosities of up to several percent of total solid volume are indicated. Given that this porosity is not revealed in the N2 isotherms, it is reasonable to infer that the pores are small (< 1 nm in aperture), internal, and accessible only by diffusion through the solid state. 

The reversible Type I isotherm (Type I isotherms are sometimes referred to as Langmuir isotherms, but this nomenclature is not recommended) is concave to the p/pa axis and na approaches a limiting value as p/p° — 1. Type I isotherms are given by microporous solids having relatively small external surfaces (e.g. activated carbons, molecular sieve zeolites and certain porous oxides), the limiting uptake being governed by the accessible microporc volume rather than by the internal surface area. 

By making a comparison of the rates of dehydration of sec-butanol over Linde lOX and 5A zeolites at relatively high temperature and low conversion, Weisz (7) also found that the rate constant per unit volume of 5A was between two and three orders of magnitude smaller than that of lOX. These relative magnitudes were consistent with the ratio of available surface areas (0.6-3.5 m /gm for the external area of l-5/i sized crystals of shape-selective 5A and 500-700 m /gm for lOX, where the internal surface was accessible to the sorbate. 

The active surface of a zeolite is internal and intrinsic to the crystal structure. Diffraction techniques can therefore yield direct data on those structural features that control catalytic or sorptive performance. However, zeolite characteristics hamper the effective application of diffraction methods. Zeolite constituents have, generally, low atomic numbers and the normalized scattering power of a zeolite unit cell is relatively small. Zeolites have open firework structures supporting accessible void volumes which can be as much as 50% of the total crystal volume [1-3]. The void spaces are either empty (and hence contributing no scattered intensity to the measured diffraction pattern) or filled with species that are have positional or dynamic disorder and hence contribute to the diffraction peaks almost exclusively at low scattering angles. 

In a second strategy, MOFs with different crystal sizes should be prepared and tested as catalysts for large- and small-size substrates. If the reaction takes place predominantly on the external surface, an increase of the catalytic activity for samples with smaller crystallite size should be observed as a consequence of the relative increase of the external versus internal surface as the crystallite size is reduced. On the other hand, if the reaction takes place inside the pores, no influence of the crystallite dimensions should be observed as for large crystallites the internal pore volume is already accessible and the total area of small and large crystallites counting the internal surface should be very similar. Scheme 2.7 summarizes the three possible locations of catalytic sites with respect to the MOF structure. 

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