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

Abstract

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 Å, which keeps adsorption selectivity between molecules varying by merely 0.2 Å 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 Ångstroms, 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 Ångstroms.

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.