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 DeltaHdegrees congruent to -44 kJ mol(-1), while for LiB(CO)(+) the corresponding value is evaluated to be DeltaHdegrees congruent to -18 kJ mol(-1). The large difference between these DeltaHdegrees 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.