Experimental and Quantum Chemical Studies on the Adsorption of Carbon Dioxide on Alkali-Metal-Exchanged ZSM-5 Zeolites

The adsorption of carbon dioxide onto M-ZSM-5 zeolites (M = Li, Na, K, Cs) was studied by means of FTIR spectroscopy and adsorption microcalorimetry. Quantum chemical calculations, at the B3-LYP level, on the interaction of CO 2 with the bare alkali-metal cations were performed to assist interpretation of the experimental results. With the likely exception of Li +, CO 2 was found to undergo a two-step interaction with the metal ions. At a low equilibrium pressure linear 1:1 adducts of the type M + ···O C O (M = Na +, K +, Cs + ) are formed; upon increasing the CO 2 equilibrium pressure, the metal cation coordinates a second CO 2 molecule, forming a 2:1 adduct. Calculated (ab initio) bond lengths for the 1:1 adduct are given, as well as corresponding values of the binding energy and enthalpic term. Experimentally derived values of the main thermodynamic functions (Δ H °, Δ G °, and Δ S °) are discussed and correlated with detailed results from IR spectroscopy. The interaction cation/CO 2 alone cannot account for the body of evidence, and the contribution of nearby O 2- anions has to be invoked.