In-situ far-infrared study of vibrations between zeolite frameworks and metallic or molecular cations

Abstract Cation vibrations of several alkaline metals and molecular cations in zeolite pores were observed by the in-situ far-IR with transmission method using a closed gas evacuation and circulation system. The peak-top frequency of a series of alkaline metal cation vibrations on FAU-type zeolites was proportional to the minus one half of the mass of cations, indicating that only cations are regarded as oscillators. In addition, the frequency of the same cation vibration differed depending on the zeolite topology. This implies the presence of other origins responsible for the vibrational energy than mass. The largest energy difference of Na+ cation vibration among different zeolites was found between FAU- and MOR-types to be about 6.0 × 10−3 kJ mol−1, possibly attributed to the “confinement effect”. The molecular (ammonium and pyridinium) cation vibrations were also observed by the dynamic observation of adsorption and desorption of molecules. In the case of NH4+-form FAU-type zeolite, a band at 176 cm−1 was observed at room temperature and disappeared by evacuation at 523 K due to the desorption ammonia. Then, the intensity of the band was gradually recovered by the successive ammonia adsorption at room temperature. This in-situ measurement gave an accurate assignment of the molecular cation vibration. Moreover, the cation vibration of pyridinium cation was also observed by in-situ adsorption of pyridine on H+-form (NH3-desorbed) zeolites. The effect of the zeolite topology on molecular cation vibration was also observed, which confirmed the presence of the confinement effect in zeolites.