Abstract
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 CO 2 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]. CO 2 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. CO 2 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 CO 2, 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 CO 2 ligand probably implies some extraction of the cations from the amorphous matrix. Na cations only form 1:1 adducts with CO 2, whereas Li cations may form 1:2 species.
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