Adsorption Kinetics and Size Exclusion Properties of Probe Molecules for the Selective Porosity in a Carbon Molecular Sieve Used for Air Separation

Abstract

The adsorption characteristics of a series of planar (ethylene, benzene, and pyridine) and tetrahedral (methane, chloromethane, dichloromethane, chloroform, and carbon tetrachloride) molecules on a carbon molecular sieve used for air separation (CMS A) were investigated over a range of temperatures as a function of pressure, to study the selective porosity. The size-exclusion characteristics of planar and tetrahedral molecules indicate that the selective porosity behaves as though it has spherical-shaped structural characteristics and, therefore, two minimum dimensions need to be considered in relation to size exclusion. The partial exclusion of the probe molecule adsorptives from the microporous structure for sizes >360 pm allowed the selective and nonselective microporosity and the meso/macroporosity to be quantified. Adsorption kinetics obey a linear-driving-force mass transfer, combined barrier resistance/diffusion, or Fickian kinetic models depending on the adsorptive and experimental conditions. Comparison of the results with previous studies of the adsorption of linear and spherical molecules on CMS A show that a decrease in the adsorption rate constant of ∼ 4 orders of magnitude was observed for an increase in molecular size over the range ∼290−420 pm. The activation energies and preexponential factors for the adsorption kinetics and the isosteric heats of adsorption are discussed in terms of the structural characteristics of the adsorptives and the adsorption mechanism.