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

Abstract

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.

A DFT-D study of the interaction of methane, carbon monoxide, and nitrogen with cation-exchanged SAPO-34

“…However, the heats of methane adsorption obtained for other systems, such as Na-, Mg-, and Ca-SAPO-34, are much lower, all of them lying below 20 kJ mol -1 . A comparably low heat of methane adsorption has been reported for Na-SAPO-34 by Denayer et al [71]. In the same work, the authors also studied Na-chabazite: Here, the heat of adsorption amounts to 22 kJ mol -1 .…”

Reactions in Zeolites

“…61 As long as the alkane can fi t inside a single zeolite cage, they have observed the well known linear increase of enthalpy of adsorption with increased number of carbons. Remarkable contributions have shown that the adsorption enthalpy of alkanes is strongly dependent on both the topology of the zeolite and the branching and geometry of the alkanes 12 .…”

Study of carbon dioxide and methane equilibrium adsorption on silicoaluminophosphate-34 zeotype and T-type zeolite as adsorbent

“…SAPO-34, as a kind of zeotype material, is a cage-type molecular sieve with framework structure similar to the naturally occurring chabazite zeolite (Lee et al 2007). Chabazite structure has a three-dimensional pore system with ellipsoidal cages interconnected via 8-membered ring windows with pore apertures of 0.38 nm 9 0.38 nm (Djieugoue et al 2000;Denayer et al 2008;Yan et al 2009). This geometry allows molecules, such as CO 2 , with kinetic diameter of 0.33 nm to easily diffuse through the crystal structure (Rivera-Ramos et al 2008).…”