Abraham Soffer

Study of molecular sieve carbons. Part 1.—Pore structure, gradual pore opening and mechanism of molecular sieving

The pore structure of a fibrous carbon molecular sieve has been studied by adsorption of CO2, O2, C2H2, H2, N2, CO, Xe and SF6 as molecular probes. Apart from the negligible outer surface of the fibres, all adsorption sites posses molecular sieving properties. Mild activation steps enable the graduated opening of critical pore dimensions in the range 3.1–5.6 A, which keeps adsorption selectivity between molecules varying by merely 0.2 A in cross-section 100:1. The pore opening is effected by removing surface groups as CO2 and CO due to degassing at temperatures from 100 to 700°C and by burning off skeletal carbon atoms in air at 400–450°C. Degassing at temperatures > 800°C leads to pore closure due to sintering. Removal of surface atoms must result in pore widening by steps as large as a few Angstroms, in contradistinction to the observed graduated pore opening.It is anticipated, therefore, that the fine discrimination between molecules of similar dimensions is of kinetic–statistical origin, so that molecular sieving by pores, substantially greater than the molecules considered, is possible. The detailed model is based on the existence of a few rate-determining constrictions close to the outer surface of the fibres and of wider pores composing the major part of the pore volume. Adsorption behaviour of the flat benzene molecule suggests slit-like pores. However, constriction structure allows only a finite width of slits, still within a few Angstroms.High adsorption stereospecificity over a wide pore dimension range has enabled the studied adsorbates to be ordered in a sequence of increasing critical molecular dimensions, which does not always correspond with estimates based on gas-phase collision diameter or on bond length and Van der Waals radii.

Study of molecular sieve carbons. Part 2.—Estimation of cross-sectional diameters of non-spherical molecules

The estimation of molecular dimensions by various experimental methods is discussed and their relevance to non-spherical molecules is evaluated. Liquid molar densities provide an average dimension which is adequate for spherical molecules but completely insensitive to molecular shape. The combination of bond lengths and van der Waals radii enables one to estimate satisfactorily the length of linear molecules but not their width. The kinetic diameters calculated from different physical properties of gases diverge significantly and are insensitive to molecular shape. Adsorption in molecular sieve(MS) solids exhibit high sensitivity to the width or smallest dimension of the molecule. Molecular sieve carbons (MSC) seem promising in this respect since their average pore diameter can be tailored to the exact critical dimension of any molecule in the range 3–5.5 A studied so far. The combination of adsorption stereospecificity data and liquid molar volumes provides reasonable numerical estimates of the width of non-spherical molecules. Polar molecules may have different dimensions depending on whether the carbon surface is polar (oxidised) or non-polar. Hydrogen acquires a surprisingly large width which is in accordance with its high liquid molar volume. Adsorbent–adsorbate interactions play a crucial role in determining molecular dimensions and serve to elucidate the unusual behaviour of both hydrogen and polar molecules. An overview of the concept molecular dimension is given in terms of the effect of intermolecular forces.

Application of the two-site Langmuir isotherm to microporous adsorbents

Abstract Adsorption on activated charcoals and zeolites can be accurately represented by two-site Langmuir isotherms. The Henrys law constants of the two sites are quite distinct. The ratio of contributions of adsorption at the second site to that at the first site increases upon increasing pore opening and may serve as a qualitative measure of the ratio of pore volume to surface area of the fine-pore adsorbent.

Molecular sieving range of pore diameters of adsorbents

The very sensitive molecular-dimension criterion of adsorption selectivity of molecular sieves changes rapidly to a molecular-mass criterion upon a slight enlargement of the adsorbent-pore dimension. This resulted in the inversion of the sequence of adsorbability of hydrogen and oxygen on carbons of increasingly wider pores. Considerable care must be taken in relating molecular-sieve effects on adsorption to molecular dimensions of adsorbates.

Molecular sieve carbons. Part 3.—Adsorption kinetics according to a surface-barrier model

The kinetics of the adsorption of gases on molecular sieve solids can be rate-determined by a surface barrier constituted of a thin layer of adsorbate at high coverage. The layer is not necessarily associated with the morphology of the adsorbent.The surface-barrier mechanism applies in cases of a sharp pressure increment which corresponds to a high degree of saturation. In the other cases, a bulk diffusion or mixed surface-barrier and bulk-diffusion mechanisms would prevail.The applicability and the importance of the surface-barrier mechanism are discussed.

The effect of surface polarity and pore dimension on the adsorption of polar molecules on activated carbon cloth

Abstract The adsorption of cyanogen chloride (CK) on carbon cloth activated to various degrees was studied. At intermediate activation time, maximum adsorbent-adsorbate interaction is manifested corresponding to a fit of the pore size to the dimension of the adsorbate molecule. Comparison of the maximum interaction effect of this highly polar molecule with that of nonpolar molecules and with water shows that, unlike the polar and hydrogen-bonding water molecule, only minute clustering occurs with CK at the pore size that corresponds to maximum interaction. Carbons enriched with surface oxygen groups bring about slightly smaller pore size of maximum interaction (PSMI), compared to oxygen-depleted carbon. The effect is attributed to greater dipoledipole interaction with the polar surface. The possibilities of optimizing adsorption isotherms and estimating the average ultramicropore size by utilizing the maximum interaction effect are discussed. It is stated that the pore size of maximum interaction provides an estimate of the average pore size of the molecular sieve carbon, which, quite probably, cannot be obtained otherwise, and the molecular sieving effect discussed in previous papers may serve to assess the pore constriction or aperture dimensions of the molecular sieve.