Diffusion in ion-exchanged clinoptilolites



The two-dimensional channel structure of clinoptilolite has been altered systematically by ion exchange to study the effects of cation type, size, location, and distribution on the diffusion of N2 and CH4 probe molecules. Concentration-dependent diffusion time constants (D/L2) were determined from gravimetric uptake measurements for fully-exchanged K+, Na+, and H+ clinoptilolites, and highly-exchanged Ca2+ (89%) and Mg2+ (72%) clinoptilolites. Both plane sheet and parallel channel diffusion models were developed from the one-dimensional plane sheet diffusion equation and fit to the uptake data. Resulting values of D/L2 varied by a factor of more than 1,000 for both N2 and CH4, while kinetic selectivity spanned nearly two orders of magnitude for this group of modified clinoptilolites. Achieving this range in performance for the difficult N2/CH4 separation demonstrates the excellent potential for tailoring clinoptilolite by cation manipulation for the kinetic separation of other gas mixtures.