Infrared spectroscopic studies on cations in zeolite pores using CH4 as a probe molecule

To clarify the nature of cation sites, such as the strength of the electric field exerted by them, their location, etc., in zeolite pores, adsorption of methane on a series of ion (Li, Na, K and Rb) exchanged zeolites (FAU, FER and MOR) with different Si/Al ratios was discussed based on IR spectroscopy and adsorption thermodynamics. The interaction energy of methane with the zeolites was strongly correlated with the micropore size of the main channel rather than the cation content in the zeolites. On the contrary, the strength of the electric field in the vicinity of Na sites increased with the increase in the Si/Al ratio of the zeolite in spite of the decrease in the amount of cations. All of the cations incorporated in the zeolites studied were not always available for methane adsorption since cations in the zeolites are distributed into deep (methane inaccessible) sites and surface (methane accessible) sites, and the amount of inaccessible sites often increased with the decrease in the size of the cation.