B. Bonelli et al., Experimental and quantum chemical studies on the adsorption of carbon dioxide on alkali-metal-exchanged ZSM-5 zeolites, J PHYS CH B, 104(47), 2000, pp. 10978-10988
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 C
O2 with the bare alkali-metal cations were performed to assist interpretati
on of the experimental results. With the likely exception of Li+, CO2 was f
ound to undergo a two-step interaction with the metal ions. At a low equili
brium pressure linear 1:1 adducts of the type M+. . .O=C=O (M = Na+, K+, Cs
+) are formed; upon increasing the CO2 equilibrium pressure, the metal cati
on coordinates a second CO2 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 va
lues of the main thermodynamic functions (DeltaH(degrees), DeltaG degrees,
and DeltaS degrees) are discussed and correlated with detailed results from
IR spectroscopy. The interaction cation/CO2 alone cannot account for the b
ody of evidence, and the contribution of nearby O2- anions has to be invoke
d.