Fourier-transform infrared study on the encapsulation of carbon monoxide in cation-exchanged X-zeolites

Abstract

Adsorption of CO at ambient temperature on NaX zeolite produces four distinct infrared absorption bands at around 2207, 2192, 2178 and 2165 cm^–1. The relative intensities of these bands depend on sample crystallinity and vary considerably when sodium is exchanged with other Group 1 or Group 2 cations. These bands are ascribed to weakly held (CO)_n molecular clusters trapped in supercages and stabilized under cation fields. In addition, two pairs of bands are also observed in the 1200–1800 cm^–1 region, the frequencies and the relative intensities of which depend on the cation, CO exposure temperature and the sample crystallinity. These bands are stable under ambient-temperature evacuation and show a uniform isotopic shift corresponding to a frequency ratio ν(¹³C/¹²C) of ca. 0.973 on using ¹³CO as adsorbate. The possible origins of these bands are discussed in detail.