DRIFTS and Raman Investigation of N₂ and O₂ Adsorption on Zeolites at Ambient Temperature

Abstract

Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and Raman spectroscopy were used to examine N2 and O2 adsorption on cation-exchanged (K, Na, Sr, Ca, and Li) low silica X (LSX) zeolites. IR and Raman absorption bands were observed for the molecular vibration of adsorbed N2 and O2 at room temperature and atmospheric pressure. The intensity (in Kubelka-Munk units) of the IR band increased with N2 pressure and mirrored the adsorption isotherm for N2. Both O2 and N2 displayed a similar dependence of the molecular vibrational frequency on cation charge density, which suggests that both gases are interacting directly with the cations. The vibrational frequencies for adsorbed N2 and O2 were more sensitive to the cation charge density than to framework Al content. Infrared studies of N2 and O2 on mixed cation forms of LSX show that N2 interaction was localized near individual cations within the sorption cavity of the zeolite. Thus, adsorbed N2 can be used to probe accessibility of specific cations within the zeolite framework. The spectroscopic data are consistent with the theory that the stronger interaction of N2 over O2 is caused by the stronger influence of the electric field with the larger quadrupole of N2.

Time-Resolved FT-Infrared Spectroscopy of Visible Light-Induced Alkene Oxidation by O2 in a Zeolite

“…The electric shielding of the cations in the supercage by the wall oxygens is poor, resulting in high electrostatic fields around the cations. There is extensive experimental evidence for these electrostatic fields in cation-exchanged zeolites from infrared − and ESR spectroscopy , of small guest molecules, from electron densities determined by X-ray measurements, and from heat of adsorptions of rare gases . Model and ab initio calculations confirm these results. − The interaction of the high electrostatic field (NaY, 0.3 V Å -1 ; BaY, 0.9 V Å -1 ) with the large dipole generated upon excitation of the hydrocarbon·O 2 pair to the charge-transfer state (about 15 D) leads to a stabilization of the state by 1.5−3 eV for pairs oriented parallel to the field.…”

“…The electric shielding of the cations in the supercage by the wall oxygens is poor, resulting in high electrostatic fields around the cations. There is extensive experimental evidence for these electrostatic fields in cationexchanged zeolites from infrared [8][9][10][11] and ESR spectroscopy 12,13 of small guest molecules, from electron densities determined by X-ray measurements, 14 and from heat of adsorptions of rare gases. 15 Model and ab initio calculations confirm these results.…”

Vibrational and thermodynamic properties of Ar, N2, O2, H2 and CO adsorbed and condensed into (H,Na)-Y zeolite cages as studied by variable temperature IR spectroscopy

“…The adsorption of N 2 at 100-70 K on a variety of zeolitic systems has already been extensively studied. 24,25,[32][33][34][35][36][37][38][39] Here we report and discuss the spectra of this molecule adsorbed on (H,Na)-Y at temperature as low as 60 K.…”

Multivariate Analysis of Infrared Spectra for Monitoring and Understanding the Kinetics and Mechanisms of Adsorption Processes

“…Infrared (IR) spectroscopy has also been employed to study adsorption of species onto zeolites. [2][3][4][5][6] When compared to traditional gravimetric, volumetric, and chromatographic methods, IR spectroscopy has the advantage of providing structural information about both the adsorbed species and the adsorbent. Most of the studies that have employed IR spectroscopy to study adsorption have used the technique only to identify the structure of the adsorbed molecules and the adsorption sites on the zeolite.…”