Microporosity measuring

Comment. Gas sorption analysis is a well-established tool for the characterization of open porous solids. For aerogels the method provides reliable information on the surface area. However, care has to be taken in case of microporous aerogels here a well-equilibrated isotherm in combinatiOTi with the right choice of the evaluation range will still yield reliable values for the microporosity and the specific surface area. For detailed analysis of microporosity measurements with CO2 at 273 K are recommended. 

The development of microporosity during steam activation was examined by Burchell et al [23] in their studies of CFCMS monoliths. A series of CFCMS cylinders, 2.5 cm in diameter and 7.5 cm in length, were machined from a 5- cm thick plate of CFCMS manufactured from P200 fibers. The axis of the cylinders was machined perpendicular to the molding direction ( to the fibers). The cylinders were activated to bum-offs ranging from 9 to 36 % and the BET surface area and micropore size and volume determined from the Nj adsorption isotherms measured at 77 K. Samples were taken from the top and bottom of each cylinder for pore sfructure characterization. 

Since these materials have significant microporosity, we expect their bulk densities to be low. For example, the tap density (100 taps) of BrlOOO was measured to be 0.81 g/cc, compared to 1.34 g/cc for the synthetic graphitic carbon powder, MCMB2700, measui ed by the same method. 

Various noncellulosic thln-film-composlte membranes were examined by scanning electron microscopy (SEM). Figure 3 illustrates the type of surface structure and cross-sections that exist in these membranes. Figure 3a shows the surface microporosity of polysulfone support films. Micropores in the film were measured by both SEM and TEM typically pore radii averaged 330 A. Figure 3b is a photomicrograph of a cross-section of a NS-lOO membrane. 

Another promising approach to the study of microporosity of zeolites involves the measurement of the isotropic 13C NMR chemical shift which, as has been shown in the studies of the tacticity of polymers, is highly sensitive to the environment of the nucleus. In the first study of this kind, Boxhoorn et al. (329) observed that the C-3 carbon resonance from the tetrapropylam-monium cation enclathrated in the framework of zeolite ZSM-5 in the course of synthesis is split into two components of equal intensity. The reason for this is that the cation is located at the cross-section of the two nonequivalent... 

The fact that all the fibers adsorb water in excess of the expected monolayer amounts suggests that the water adsorption is multilayer in nature, or that there is pore space which is accessible to water molecules—but not accessible to the Kr used to measure the specific surface area. The XPS analyses showed that the silane overlayers increased in thickness in the 4% and 6% B,03 fibers. But the increase in water adsorptivity with % B,03 is not in direct proportion to the increase in silane overlayer thickness it is considerably larger. This suggests that B,0, has influenced the chemical and physical structure of the adsorbed silane overlayer. It is likely that there is microporosity, free volume, and/or reactive sites within the silane overlayer, in general. 

It should be kept in mind that any change in surface area, surface chemistry, or microporosity will result in a change in the energy of immersion. Because immersion calorimetry is quantitative and sensitive, and because the technique is not too difficult to apply in its simplest form, it can be used for quality testing. The preliminary outgassing requires the same care as for a BET measurement, but, from an operational standpoint, energy of immersion measurements are probably less demanding than gas adsorption measurements. 

Optimization of the pore size distribution is important for the control of both the equilibria and the dynamics of physisorption (see Ruthven, 1984 Do et al., 1993). Most activated carbons are highly microporous, but for some purposes it is desirable to extend the range of pore size into the mesopore or macropore range - or even eliminate the microporosity. Progress in this direction has been made by the use of special pre-treatment procedures and the careful control of the conditions of carbonization and activation. In this connection, physisorption measurements have an important role to play in characterizing the material at various stages of manufacture. 

The latter approach was adopted by Trillo et al. (1993) in their study of the effect of thermal and hydrothermal treatment on the accessible microporosity of alumina-pillared montmorillonite. This work revealed that X-ray measurements of the dmi spacing taken alone may give a misleading impression of the thermal stability of the PILC micropore structure. For example, after heat treatment of the Al-PILC at 300°C, it was found that the apparent micropore volume available for nitrogen adsorption amounted to only c. 30% of that indicated by the dm spacing. 

Values of apparent surface area can be derived only if the solute isotherm exhibits a long saturation plateau. Unfortunately, the derived values are often of questionable significance since the exact structure of the monolayer (containing both solute and solvent) is rarely known. The study of microporosity by adsorption from solution measurements is in its infancy, but the use of comparison plots appears to be a promising approach. 

In Figure 1 nitrogen sorption isotherms at 77 K for the pristine B and impregnated B-Fc203 MCM-48 silica materials are shown. Both sorption isotherms exhibit similar shape, i.e reversible pore condensation at p/po < 0.4. The MCM-48 silica phases exhibit no microporosity as revealed by measurements in the low pressure region [13]. Different methods were used to analyze the nitrogen sorption isotherms to obtain surface and pore size... 

Figure 2 presents the CO2 adsorption isotherms obtained at 273 K for samples of series CS (a) and CW (b). The amount of carbon dioxide adsorbed increases, for both series, with bum-off. Isotherms are rather similar for samples with low burn-off (CS-2 to CS-8 on one hand, CW-1 and CW-2 on the other) what makes it difficult to distinguish them only with these measurements. When plotted in Dubinin-Radushkevich (D-R) coordinates, these isotherms become straight lines as corresponds to samples with a narrow and uniform homogeneous microporosity. The micropore volumes of the different samples, obtained by application of the D-R equation to the CO2 adsorption data are reported in Table 1. Micropore volumes increase from 0.22-0.23 cm g- in the less activated CMS to 0.32 cm g- in samples with the highest burn-off in each series although, due to the different reactivity of the starting materials, they are obtained after very different activation times (16 h for CW series and 70 h for CS series). 

Porous texture characterisation of LTA, FAU and MFI type zeolites has been carried out by N2 adsorption at 77K and CO2 adsorption at 273K. These results have been compared to the data obtained from crystallographic estimations. In this way, the micropore volumes obtained by CO2 adsorption at 273K are, in all the cases, similar to the expected from the crystal framework of the zeolite. However, the micropore volumes obtained by N2 adsorption at 77K are significantly smaller. These results confirm the limitations of the N2 adsorption at 77K in measuring narrow microporosity and make evident the usefulness of CO2 adsorption at 273K to characterise porous solids with narrow microporosity. 

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