Complex sorption kinetics of carbon dioxide in NaX-zeolite crystals

Ad-/desorption-kinetic measurements were performed for carbon dioxide on a mono-disperse crystal sample of NaX-type zeolite (mean crystal diameter: 2R congruent to (100 +/- 6) mum) by a constant-volume-variable-pressure method in a "differential" concentration mode over ranges of temperature, (193-293) K, and pressure, (20-26,700) Pa. They revealed a complex sorption process in sorbate uptake and release during experiments of ad- and desorption, respectively, which is comprised of a slow, hitherto unknown rate mechanism superimposed on very fast intracrystalline diffusion of carbon dioxide. IR-spectroscopic data that evidence formation of chemisorption species in CO2-NaX sorption systems may allow for a tentative interpretation of this finding. Circumstantial evidence for the correctness of this conclusion follows from sorption-kinetic experiments for carbon dioxide on BaKX-type zeolite crystals (mean crystal diameter: 2R congruent to (120 +/- 6) mum).