Characterisation of Al-rich microporous micelle-templated silicates. Part II: Spectroscopic and microcalorimetric study of the accessibility of exchanged alkali-cations to carbon dioxide

Two micelle-templated materials with Si/Al ratio = 2.5, exchanged at 43% with Li+ ions and at 82% with Na+ ions, respectively, have been characterized by means of CO2 adsorption followed by FT-IR spectroscopy and microcalorimetry, their texture and acidic properties having been studied in a previous paper [B. Bonelli, B. Onida, J.D. Chen, A. Galarneau, F. Di Renzo, F. Fajula, E. Garrone, Micropor. Mesopor. Mater. 67 (2004) 95]. CO2 adsorption shows that no strong basic sites (oxygen anions able to form carbonate-like species) occur at the surface, in contrast to findings with zeolites of comparable composition. CO2 interacts instead in a linear form with Lewis acidic sites, and reveals that only a part of Na or Li sites are accessible to the probe. The amorphous nature of the walls allows exchangeable alkali-cations to be partially “sunk” among surrounding oxygen anions: the majority of Na+ ions are not accessible to CO2, whereas only about one half of Li+ ions are so. A dramatic decrease in the differential heats of adsorption with coverage indicates that interaction with the CO2 ligand probably implies some extraction of the cations from the amorphous matrix. Na cations only form 1:1 adducts with CO2, whereas Li cations may form 1:2 species.