Adsorption of carbon monoxide by calcium-exchanged zeolite Y

Abstract

Carbon monoxide adsorption on NaY (–23 to 45°C) obeys Henry's law and the isosteric heat of adsorption is low (q∼25 kJ mol^–1). As Na⁺ ions are replaced by Ca²⁺ ions, the adsorption on the outgassed zeolite remains non-specific with little change until 50 % of the Na⁺ ions have been replaced. At 50–55 % exchange, a more specific type of CO adsorption develops. The heat of adsorption rises abruptly to 40–45 kJ mol^–1, and the entropy of adsorption shows that the CO molecules are more localized. The specific adsorption obeys the Langmuir isotherm and the uptake in this adsorption mode progressively increases as further Na⁺ ions are replaced by Ca²⁺ ions in the range 55–87 % exchange.

The results show that Ca²⁺ ions introduced into NaY by aqueous ion exchange preferentially enter internal sites on subsequent dehydration. Beyond 16 Ca²⁺ ions per unit cell (∼55 % exchange), which corresponds to 100% occupation of site I or to 50% occupation of site I′, further exchange results in Ca²⁺ ions occupying locations exposed to the supercages (site II), as shown by the onset of specific adsorption of CO. The limiting quantity of specific adsorption on zeolite outgassed at 350°C, determined from the Langmuir isotherm, corresponds closely to one molecule of CO per exposed Ca²⁺ ion. The work establishes that CO is a sensitive probe molecule for investigating the location of unshielded multivalent cations in zeolites.