Experimental and Theoretical Study of Propene Adsorption on K-FER Zeolites: New Evidence of Bridged Complex Formation

The interaction of propene with K-FER zeolites was investigated by a combination of IR spectroscopy, adsorption calorimetry, and theoretical study. Periodic density functional theory (DFT) calculations were performed using the DFT/CC scheme based on the Perdew–Burke–Ernzerhof density functional for the description of the interaction between propene and K-FER zeolites. On the basis of good agreement between experimental and theoretical results, three types of adsorption complexes were identified: (i) propene adsorbed on single K+ cation sites characterized by a νC═C vibrational band at 1639 cm–1, (ii) propene bridging two nearby K+ cations in dual-cation sites represented in the IR spectra by a νC═C vibrational band at 1633 cm–1, and (iii) propene molecules interacting with the zeolite framework mainly by dispersion interactions characterized by the νC═C vibrational band at 1645 cm–1. The DFT calculations show that propene binds to the potassium cation via a cation−π interaction. The propene molecule adsorbed in the dual-cation site also exhibits the cation−π binding mode, and it is stabilized by 14 kJ/mol with respect to the adsorption complex at the isolated K+ cation site. The population of such bridged complexes increases with a decreasing Si/Al ratio. The knowledge of various types of adsorption complexes and their properties and parameters influencing their population is crucial for understanding the adsorption properties of zeolites as well as their ability to separate, purify, and store various gases, especially hydrocarbons.