(506e) Adsorptive Cage Effects of n-Alkanes and 1-Alkenes in CHA Zeolites

Adsorption in zeolites is a complex process that involves multiple factors such as pore size, pore shape, and the interactions between the adsorbate and the zeolite framework. Especially, in small-pore zeolites, the close matching size of the molecules and the zeolite’s pore window strongly impacts adsorption, diffusion and catalytic conversion of molecules. This makes small-pore zeolites attractive adsorbents for industrial application such as methanol-to-olefins (MTO) conversion, selective catalytic reduction (SCR) of NOx, or light gas separations [1].

Generally, adsorption and diffusion properties of alkanes are monotonically chain length dependent [2]. However, for small-pore zeolites in which the pore system consists of cages connected through narrow windows, a non-monotonic chain-length dependence behavior is reported, and is often referred to as cage- and window effects [2]. Here, increasing the chain length forces the molecule to coil up, or, once the chain-length becomes too long, protrude the pore window into the adjacent cage. This incommensurate fit between adsorbate and adsorbent lowers their adsorption strength and increases their diffusion rate throughout the zeolite. These effects are mostly computationally studied on all-silica zeolites using n-alkanes as probe molecules [2-4]. Such fundamental adsorption effects have been speculated for other, more reactive, chemical groups as alkenes [4], yet have not been reported experimentally.

Our experimental study examines the zero coverage adsorption properties of n-alkanes and 1-alkenes in chabazite zeolites. By using inverse gas chromatograph (IGC), the zero coverage adsorption properties of adsorbates can be probed, allowing to deduce their fundamental adsorption behavior with the zeolite framework. Aluminosilicate and all-silica variants of zeolite CHA were selected for characterization due to their industrial relevance. Using IGC, the Henry constants, adsorption enthalpy and entropy at zero coverage of hydrocarbons with up to 12 carbon atoms are determined between 303-593 K.

Adsorptive cage effects of 1-alkenes are experimentally observed in both zeolites variants for the first time, to the best of our knowledge. A local minimum in Henry constants is observed for carbon number 9, with the adsorption enthalpy lowering with 14.1 kJ/mol between 1-octene and 1-nonene, compared to 4.1 kJ/mol between n-octane and n-nonane. In addition, in the case of Si-CHA, we qualitatively demonstrate the kinetic hindrance effect within the zeolite cage for non-linearized adsorbates up to carbon number 5. Subtle changes in adsorbate’s bond angle and bond length influences the diffusion through the narrow pore windows. The experimental results of this study are compared with the modelling predictions made in the literature, and contribute to the enhanced understanding of adsorption properties in small-pore zeolites.

References
[1] Dusselier, M., Davis M.E., Chem. Rev. 118.11 (2018): 5265-5329.
[2] Dubbeldam, D., Berend S., J. Phys. Chem. B 107.44 (2003): 12138-12152.
[3] Ter Horst, J. H., et al., Microporous Mesoporous Mater. 53.1-3 (2002): 45-57.
[4] Luna-Triguero, A., et al., J. Phys. Chem. C 119.33 (2015): 19236-19243.