Mechanism of H₂O Vapor Adsorption in A Type Zeolites: A Model Based on Adsorption Calorimetry

In this study, the energy characteristics of water vapor adsorption in NaA and NaA(NaBO 2 ) zeolites were measured by adsorption calorimetry. Information of differential heats, isotherms, entropy and kinetic of adsorption in the matrix of NaA and NaA(NaBO 2 ) zeolites were obtained. Adsorption isotherms (a) and differential heats of adsorption (Qd) of water vapor by NaA and NaA(NaBO 2 ) zeolites were measured at 303 K. The heats of adsorption are stepwise and each step corresponds to the stoichiometric formation of adsorption complexes of H 2 O molecules with Na + or H + ions, (H 2 O) 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(NaBO 2 ). Hydration mechanism of NaA and NaA(NaBO 2 ) are complex and at saturation, the zeolites are occupied by the next water zeolitic host/guest systems: NaA – 2.2[(H 2 O) 4 /Na + III +H + ], 3[(H 2 O) 2 /Na + II ] and 6.6[(H 2 O)2/Na + I ]; NaA(NaBO 2 ) - 2[(H 2 O) 4 /Na + III ], 3[(H 2 O) 4 /Na + II ] and 6.86[(H 2 O)/Na + I ]. The complete hydration of the α- and β cages of NaA amounts thus to 21,8 and 6,2 respectively H 2 O/u.c. and α-cage of NaA(NaBO 2 ) – 26,3 H 2 O/u.c. 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.

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