Abstract
Propene adsorption on Li- and Na-FER zeolites was investigated combining IR spectroscopy and calorimetric measurements of adsorption heats with DFT calculations using a DFT/CC scheme based on the PBE density functional. Considering the good agreement between experimental and theoretical results, the following adsorption complexes of propene in the M-FER zeolites investigated in this study can be distinguished: (i) propene interacting with the zeolitic framework via dispersion interactions mainly populated in zeolites with a high Si/Al ratio and with a characteristic nu(C=C) vibrational band at 1646 cm(-1) and adsorption heat of approximately 48 kJ/mol, (ii) propene interacting with cations coordinated in 6-rings characterized by IR bands 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 bands at 1630 cm(-1) (Li-FER) and 1633 cm(-1) (Na-FER) and (iv) propene bridging two nearby sodium cations in dual-cation sites characterized by a vibrational band at 1626 cm(-1) and with an adsorption heat of 85 kJ/mol, which is 6 kJ/mol higher than that of the strongest interaction with a single Na+ cation (79 kJ/mol).
The population of bridged complexes in Na-FER was significantly lower than those in previously studied K-FER zeolites due to the preference of potassium cations for 8-rings, which is more suitable for the creation of dual-cation sites than 6-rings, wherein sodium cations are preferentially coordinated. No bridged complexes were found in the case of Li-FER because Li+ cations are closer to the framework oxygen atoms and thus relatively long distance from each other.