Cage and Window Effects in the Adsorption of n-Alkanes on Chabazite and SAPO-34

The adsorption of C 1 −C 14 n -alkanes on SAPO-34 and Na-CHA zeolites was studied at low surface coverage using the pulse chromatographic method. On both isostructural materials, containing cages interconnected via small windows, the Henry adsorption constant K ′ varies in a nonmonotonous way with carbon number. K ′ increases more or less exponentially with carbon number from methane to n -hexane. On Na-CHA, K ′ decreases from n -hexane, reaches a minimum at n -octane, and then increases again. On SAPO-34, a minimum is attained at n -decane. From this point on, K ′ increases till the longest alkane studied, i.e., n -tetradecane. On SAPO-34, the increase in zero coverage adsorption enthalpy per additional methylene group in the alkane chain becomes smaller for carbon numbers between 5 and 11. A local minimum in adsorption enthalpy occurs at n -undecane. A compensation plot between adsorption entropy and enthalpy shows a pronounced discontinuity at n -undecane for SAPO-34. The present observations provide additional proof for the existence of “window effects”. Given their molecular length, C 1 −C 6 alkanes adsorb in a relatively unhindered way in the SAPO-34 cages. C 7 −C 10 alkanes adopt a coiled configuration inside the cage, which strongly affects their adsorption enthalpy and entropy. From n -undecane onward, the alkanes are no longer fitting within the cage and protrude the small windows connecting the cages. The presence of cations on Na-CHA results in stronger energetic interactions compared to SAPO-34, which shifts the window protrusion to lower chain lengths.