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

Abstract

The adsorption of C1−C14 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...