544-971 MECHANISM OF H2 O VAPOR ADSORPTION IN A TYPE ZEOLITES. THE ADSORPTION ISOTHERMS

Abstract

In this study, the energy characteristics of water vapor adsorption in NaA and NaA(NaBO₂) zeolites  were measured by adsorption calorimetry. Information of differential heats, isotherms, entropy and kinetic of adsorption in the matrix of NaA and NaA(NaBO₂) zeolites were obtained. Adsorption isotherms (a) and differential heats of adsorption (Q_d) of water vapor by NaA and NaA(NaBO₂) zeolites were measured at 303 K. The heats of adsorption are stepwise and each step corresponds to the stoichiometric formation of adsorption complexes of H₂O molecules with Na⁺ or H⁺ ions, (H₂O)_n/Na⁺ or H⁺, (n = 1-4), which are located on the S_III, S_II and S_I crystallographic positions of NaA and NaA(NaBO₂). Hydration mechanism of NaA and NaA(NaBO₂) are complex and at saturation, the zeolites are occupied by the next water zeolitic host/guest systems: NaA – 2.2[(H₂O)₄/Na⁺_III+H⁺], 3[(H₂O)₂/Na⁺_II] and 6.6[(H₂O)₂/Na⁺_I]; NaA(NaBO₂) - 2[(H₂O)₄/Na⁺_III], 3[(H₂O)₄/Na⁺_II] and 6.86[(H₂O)/Na⁺_I].  The complete hydration of the α- and β cages of NaA amounts thus to 21,8 and 6,2 respectively H₂O/u.c. and α-cage of NaA(NaBO₂) – 26,3 H₂O/uc. The mean molar integral adsorption entropy of water is ~-30,5 J/mol K less than the entropy of the bulk liquid. This value nearly the same as entropy of crystalline water so the mobility of water in the zeolitic matrix is solidlike.