Adducts of alkali-metal ions with the CC triple bond: an experimental and ab initio study

Abstract

Acetylenes may behave either as acidic or basic probes, by virtue of the C–H group, which is acidic, and of the CC triple bond, which can interact with positively charged centers, such as acidic hydroxyls or metal cations. The interaction of methylacetylene and acetylene with alkali-metal cations, acting as Lewis sites in MCM-41 materials, is studied through combined spectroscopic and ab initio techniques. On the experimental side, MCM-41 materials with low Al content, exchanged with aqueous solutions of alkali-metal nitrates, have been selected as they provide exposed alkali-metal cations with few basic adjacent oxygens. On the computational side, ab initio density functional techniques have been applied to minimal cluster models, consisting one cation (Li, Na, K, Cs) and one molecule (either acetylene or methylacetylene). A thorough characterization of the adsorption of acetylenes on Lewis acidic centers is provided, enlightening the presence of two contributions to the interaction, namely that with alkali-cations, through the CC triple bond, and the one with basic oxygen species near the Lewis acidic sites, through H-bonding.