Discontinuities In the Low Coverage Adsorption of C1-C14 Linear Alkanes on Na-Cha and Sapo-34

The isostructural materials Chabazite (CHA) and SAPO-34 are two pertinent examples of small pore or eight membered ring zeolites, offering a highly selective and spatially constrained environment for adsorption, diffusion and reaction. Chabazite has a three dimensional pore system with ellipsoidal shaped cages of 6.7 x 10 Å2 interconnected via 8 membered (8MR) ring windows with pore apertures of 3.8 x 3.8 Å2 for the dehydrated form of CHA. Each cage is connected to 6 neighboring cavities. SAPO-34 is a silicon, aluminum and phosphorous based molecular sieve with the CHA topology. In contrast to the extended literature on adsorption of small gaseous molecules on CHA and SAPO-34, much less attention has been paid to the adsorption of larger molecules, although their pore diameter allows adsorption of e.g. n-alkanes.

In this work, 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 non-monotonous way with carbon number. K' increases more or less exponentially with carbon number from methane to n-hexane. K' decreases from n-hexane, and reaches a minimum at n-octane or n-decane depending on the material. On SAPO-34, the increase in zero coverage adsorption enthalpy per additional methyl 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. The present observations provide additional proof for the existence of “window effects”, in correspondence to recent molecular simulations [1]. C1-C6 alkanes adorb in a linear configuration in the SAPO-34 cages. C7-C10 alkanes adopt a coiled configuration inside the cage, which strongly affects their adsorption enthalpy and entropy. From C11 on, the alkanes are protruding 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.

[1] Dubbeldam, D., Smit, B., J. Phys. Chem. B., 107, 12138-12152, 2003.