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

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, LiA and LiB, form Li(CO)+ monocarbonyl species with adsorbed CO; the corresponding C-O stretching frequencies are 2193 and 2187 cm-1, respectively. Formation of LiA(CO)+ involves an estimated adsorption enthalpy of ΔH° ≅ - 44 kJ mol-1, while for LiB(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 LiB(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.