Coordination chemistry of Ca2+ sites in CaX zeolite: FTIR evidence of three coordination vacancies per cation

Abstract

Adsorption plays an important role in many processes in nature and industry. In some cases, more than one adsorbate molecule can be coordinated to a single cation site, which dramatically increases the adsorption capacity. In this study, we report on the coordination chemistry of Ca2+ sites in CaX zeolite as revealed by the IR spectra of probe molecules (CO and N2) adsorbed at low temperature. The accessible Ca2+ sites were found to be uniform and occupy SII positions. Due to the extremely high adsorption capacity of the material, the bands of adsorbed CO were too intense, which prevented the determination of their maxima. Only Ca2+−CO species were unambiguously detected by a band at 2194 cm-1. We therefore studied the adsorption of CO containing 0.5% 13C18O and tracked the 13C18O bands. A stepwise formation of mono- (2092 cm-1) di- (2080 cm-1) and tricarbonyls (2072 cm-1) of Ca2+ was established. The results were confirmed by analysis of the 2ν(12C16O) overtone region, where bands due to mono- di- and tricarbonyls were recorded at 4362, 4336 and 4319 cm-1, respectively. However, examination of the 13C16O spectra can lead to misleading conclusions due to the overlap with some low-intensity 12C16O bands. Similar results were obtained after N2 adsorption. Mono-, di, and triligand species were detected by bands at 2342, 2337 and 2333 cm-1, respectively. Thus, the results imply that the accessible Ca2+ sites occupying SII positions in CaX possess three coordinative vacancies each. Similarities and differences between the coordination chemistry of Ca2+ cations in CaX and CaY zeolites are discussed.