Adsorption and its Applications in Industry and Environmental Protection

Author : P. Samaras, ... G.P. Sakellaropoulos, in Studies i

Update time : 2021-07-09 13:27:32

P. Samaras, ... G.P. Sakellaropoulos, in Studies in Surface Science and Catalysis, 1999
CMS have had a significant impact on separation processes and they have also found applications as catalysts. In recent years, most research and development efforts concerning CMS materials have been devoted to their preparation, property characterization and use in gas separations [4]. Less attention has been paid to their catalytic properties and their use as selective catalyst support, which is currently an active field of research [5, 6].

The literature on CMS is large and growing due to the advantages of CMS over zeolites, in being more stable at higher temperatures, less hydrophilic, and inert to acidic and basic media, except to oxidation. The properties of CMS are quite variable and depend upon initial manufacturing procedures and starting material properties. Some typical properties of CMS and activated carbons are given in Table 1.

CMS have substantially uniform micropores (3 to 12 Å) while ordinary activated carbons have a wide pore size distribution (2 to 2000 Å). Narrow pores of CMS favor the diffusion of small molecules, and inhibit penetration of large ones, thus providing a sharp separation as compared to ordinary activated carbons. Another important property of CMS is the slit shaped pore structure which makes them useful for kinetic separation and adsorption equilibrium studies.

Gas separation is achieved by two alternative mechanisms: kinetic separation and selective adsorption. Kinetic separation is based on the kinetically-controlled gas diffusion caused by the constrictions of the apertures of the pores. The diameters of the bottle-necked pores are in the same range as those of the adsorbed molecules. Thus, when a CMS is used in an air separation process, the oxygen molecules, which have a smaller diameter than the nitrogen molecules, can penetrate much quicker into the pores than the nitrogen molecules. Therefore, nitrogen is recovered to a high degree whilst almost all the oxygen is adsorbed. In the second separation mechanism, the pore system is sufficiently wide to enable fast diffusion; separation is caused by selective adsorption which depends upon the van der Waals forces between the carbonaceous substrate and the gas species [1].

CMS are amorphous materials. Their pore structure below 5 Å can not be studied by X-ray diffraction, in contrast to most mineral molecular sieves. Transmission electron microscopy has also not been found suitable for determining such small pore dimensions. The most effective method for characterization is the analysis of adsorption isotherms of small probe molecules with different critical dimensions, viz. O₂, N₂, CO₂, CH₄. These adsorption isotherms are useful in determining the pore size distribution, surface area, pore volumes and separation capacity of CMS. In addition, these isotherms give information on the potential industrial applications of these materials, e.g. for the separation of nitrogen from air or of carbon dioxide and methane from flue gases.

Previous : The measurement of preventing carbon molecular sieve from powdering in the nitrogen generator

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