Two distinguishable lithium sites in the zeolite Li-ZSM-5 as revealed by adsorption of CO: an infrared spectroscopic and thermodynamic characterisation

Abstract

By combining FTIR spectroscopic and microcalorimetric measurements on adsorbed CO, two lithium sites were found to be present in the zeolite Li-ZSM-5. Both of these sites, Li_A and Li_B, form Li(CO)⁺ monocarbonyl species with adsorbed CO; the corresponding C–O stretching frequencies are 2193 and 2187 cm⁻¹, respectively. Formation of Li_A(CO)⁺ involves an estimated adsorption enthalpy of ΔH° ≅ − 44 kJ mol⁻¹, while for Li_B(CO)⁺ the corresponding value is evaluated to be ΔH° ≅ −18 kJ mol⁻¹. The large difference between these ΔH° values is explained in terms of an endothermic process occurring during formation of the Li_B(CO)⁺ species, which involves a slight movement of the corresponding Li⁺ ion (away from its equilibrium position among oxygen anions of the zeolite framework). Formation of dicarbonyls is also discussed, as well as formation of O-bonded Li(OC)⁺ species.