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Adsorption of propene on Li-, Na- and K-FER zeolites was investigated using a combination of IR spectroscopy, calorimetric measurements of adsorption heats and DFT calculations employing DFT/CC scheme based on PBE density functional. On the basis of good agreement between experimental and theoretical results, following adsorption complexes of propene in investigated M-FER zeolites can be distinguished: (i) propene interacting with zeolitic framework via dispersion interactions populated mainly in the zeolites with high Si/Al ratio and characterized by νC=C vibrational band at 1646 cm-1 and adsorption heat around 48 kJ/mol, (ii) propene interacting with cations coordinated in 6- rings characterized by the IR band at 1637 cm-1 (Li-FER) and 1636 cm-1 (Na-FER), (iii) propene adsorbed on remaining cationic positions excluding cationic positions in 6-rings characterized by IR band at 1630 cm-1 (Li-FER), 1633 cm-1 (Na-FER) and 1639 cm-1 (K-FER) (iv) propene bridging two nearby sodium or potassium cations in dual-cation sites characterized by vibrational band at 1626 cm-1 (Na-FER) and 1633 cm-1 (K-FER). Population of bridged complexes in Na-FER was significantly lower compared with K-FER zeolites due to preference of potassium cations for 8-rings, which is more suitable for creation of dual-cation sites then 6-rings where sodium cations are preferentially coordinated. No bridged complexes were found in the case of Li-FER due to the fact that Li+ is too close to the framework oxygen atoms and thus relatively long distance from each other.
