Fine Micropores Research Articles

Adsorption isotherm equations associated with the gamma micropore-size distribution and their application for characterizing microporous solids

The gamma micropore-size distribution generates a four-parameter equation for the overall adsorption isotherm. For structurally-heterogeneous solids with very fine micropores, this equation reduces to a simpler three-parameter isotherm equation that gives a good representation of experimental adsorption isotherms. Model studies reveal that this simplified equation permits evaluation of the parameters that characterize the microporous structure. These model studies show also that the simplified equation may be used for describing adsorption on heterogeneous solids with large micropores if their size-distribution resembles a symmetrical distribution function.

Surface characteristics and surface behaviour of polymer carbons—IV: Adsorption isotherms of organic vapours

The adsorption isotherms of several organic vapours, such as methanol, ethanol, benzene, n-hexane, n-heptane and carbon tetrachloride, varying in size and polar character have been studied on five different polymer charcoals before and after activation. The shape of the isotherms is generally Type I and indicates that the charcoals are highly microporous with pores only a few molecular diameters in width. The adsorption of some vapours to varying extents and the little or no adsorption of others indicates that different charcoals have pores of different diameters. PF, UF and PVC charcoals adsorb only very small amounts of methanol, ethanol and benzene (only in PF charcoal) but do not adsorb any of the larger molecules such as n-hexane, n-heptane and carbon tetrachloride. These charcoals, therefore, have ultra fine micropores which are accessible only to smaller molecules. PVDC and Saran charcoals, on the other hand, have larger pores since they could also adsorb appreciable amounts of even larger molecules. The adsorption of these larger molecules, although appreciable, is smaller than those of the smaller molecules. The activation of Saran charcoal in steam enhances the adsorption capacity of the charcoal. The adsorption of polar molecules such as those of ethanol and methanol is also enhanced by the presence of associated oxygen, while the adsorption of benzene and others is enhanced by the elimination of the associated oxygen. The associated oxygen, which is evolved as CO 2 on evacuation, suppresses the adsorption of nonpolar benzene but its elimination and emergence of the CO-complex enhances the adsorption of benzene. The enhanced adsorption of benzene is attributed to the interaction of π electrons of the benzene ring with the partial positive charge on the carbonyl carbon atom. Thus while the presence of associated oxygen makes charcoals hydrophillic, its elimination renders charcoal organophillic in character.

Physical properties and surface characteristics of asphaltenes

The pore structure of asphaltenes is investigated by N 2 adsorption at 77 K, density measurements in helium at 298 K and determination of the penetration of mercury at high pressures. The results indicate that pore sizes fall into two groups, those > 200 nm (wide macropores) and those < 1.5 nm (fine micropores) which may be considered as being inherent to the molecular structure of asphaltenes. Paramagnetic resonance data show a relation between spin numbers and carbon content and the various atomic ratios.

Microporous structures of carbonaceous adsorbents

The microporous structure of coals (anthracite and lean coal) carbonized and activated with water vapour at 900°C was investigated by the small-angle X-ray scattering and adsorption methods. As distinct from the previously studied active carbons from sucrose possessing uniform microporosity (inertia radii R = 6–7 A ̊ ), coals are characterized by a considerably greater polydispersity of micropores. Small-angle X-ray scattering data, which were in agreement with the results of the theoretical analysis of benzene adsorption isotherms, revealed coarser micropores of R = 6–7 A ̊ along with fine micropores of R = 6–7 A ̊ , in active coals. At low degrees of activation, fine micropores are predominant in the active carbons obtained. As burn-out increases the fraction of coarser micropores grows. It is believed that the size polydispersity of active coal micropores, which roughly corresponds to the radius range R = 4–15 A ̊ , stems from the non-homogeneity of the structure of the initial products used for activation, i.e. anthracite and lean coal.