ABSTRACT
Almost all porous carbons contain pores in a wide range of sizes and do not show any selectivity in the adsorption of molecules of varying dimensions. However, some special carbons have been prepared that are highly microporous and have a large surface area but still do not adsorb in appreciable amounts molecules larger than a particular size. Such carbons are called
molecular sieve carbons
(MSC). These carbons may be viewed as a form of activated carbons, distinct from peat-, coconutbased, and other conventional activated carbons. The microporous structure of such carbons is unique, because the slit-like apertures or
constrictions
of their micropores are of a size similar to the molecular dimensions of the adsorbing species. In the separation of gases, molecules that are smaller than the size of the micropore constriction rapidly diffuse through them into the associated micropore volume. On the other hand, a large molecule is denied access to the volume behind the constriction. A small change in the effective size of the constriction can affect the rate of diffusion of an adsorbing gas molecule to a considerable extent so that its nonactivated diffusion through the pore constriction now becomes activated. These carbons contain pores of molecular width, sometimes only a few Angstrom in size. Usually they form entrances to pores wider than themselves, but these still may be of molecular width. Such ultrafine pores are found in natural materials such as coals and dehydrated zeolites. They can also be formed during the carbonization of certain organic materials such as polymers, sugar, and wood under certain conditions. Naturally occurring coals have small surface areas and are associated with a number of metallic impurities that may start unnecessary side reactions. Therefore, polymer carbon molecular sieves, although expensive, are preferred because they have a high adsorption capacity.
