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.