IR Spectroscopic Study on Adsorption of Nitrogen and Methane upon ZSM-5 Zeolites at Room Temperature under High Pressure

Abstract

IR spectra of nitrogen and methane adsorbed on NaZSM-5 zeolites with different compositions (SiO₂/Al₂O₃ mole ratio=23.3, 40, 4, and 49.8) and on HZSM-5 (SiO₂/Al₂O₃ mole ratio=23.3)were measured at 300 K under the pressures ranging from O.05 to 1.0 MPa.
The infrared forbidden bands of nitrogen and methane were induced by the adsorption on ZSM-5 zeolites, indicating that, even at room temperature, the adsorbates were perturbed by the electric fields of cationic adsorption sites located on the ZSM-5 zeolite surfaces. The magnitude of the electric fields (E) were estimated to be 1.5-1.8×10⁵ esu, which were slightly smaller than those obtained from an earlier study carried out at a low temperature (226 K). The E values were found to increase with an increase in the SiO₂/Al₂O₃ mole ratios of the ZSM-5 zeolite.
The analyses of the induced absorption band (peak position, peak width, and peak intensity)showed that the motional freedoms (libration, rotation and translation) of adrnolecules increased with the increase in temperature.