Study of adsorption sites heterogeneity in zeolites by means of coupled microcalorimetry with volumetry

Adsorption of CO(2) as probe molecule on alkali-metal zeolites of MFI structure was investigated by joint volumetry-calorimetry. Consideration was given to the interpretation of the heat evolved when a probe molecule is adsorbed on the surface. In particular, the number and the strength of adsorption sites are discussed as functions of zeolite structure, concentration, and nature of extra-framework cation. The adsorption heats (q (iso)) of CO(2) interaction with alkali-metal cations decrease for MFI zeolite with high Si/Al in the sequence Li(+) > Na(+) > K(+) from 54 kJ/mol to 49 and 43 kJ/mol, respectively. In addition, the adsorption heats are influenced by concentration of Al in the framework. This phenomenon is attributed to formation of bridged CO(2) adsorption complexes formed between two cations. On the base of quantitative analysis of adsorption processes, presence of geminal adsorption complexes was suggested for adsorption at higher equilibrium pressures.