Adsorption Characteristics of High-Exchange Clinoptilolites

Clinoptiiolite, the most abundant natural zeolite, has been modified by ion exchange to fully exchanged forms of the monovalent cations K⁺, Na⁺, and H⁺ and highly exchanged forms of the bivalent cations Ca²⁺ (89%) and Mg²⁺ (72%). The Dubinin-Astakhov volume filling model was applied to the supercritical isotherm data to predict pore volume at the normal boiling temperatures of N₂ and CH₄—temperatures too low for practical isotherm measurement due to the slow diffusion of these gases. Adsorption isotherms and energetic heterogeneity have been discussed in terms of the various cations and their locations in the clinoptiiolite channels. Adsorption characteristics have been greatly altered through cation manipulation to produce a range of CH₄:N₂ selectivity ratios exceeding 1 order of magnitude. In view of the difficulty of the CH₄/N₂ separation, these results suggest excellent potential for tailoring clinoptiiolite for separation of other specific gas mixtures.