SORPTION PROPERTIES OF ZSM-5 AND ITS GALLIUM ISOMORPHS

HAl-ZSM-5 (R = 36) and HGa-ZSM-5 (R = 40,97,289) were characterized by equilibrium sorption capacities of different probe molecules like wat er, n-hexane, cyclohexane, n-butylamine (nBA), tributylamine (TBA), an d xylene isomers at 298 K, P/P0 = 0.8 (up to 2 h). The specific surfac e area (BET) was evaluated by low-temperature (78 K) nitrogen sorption . The void volume evaluated from nitrogen sorption data by applying th e Dubinin-Radushkevich equation decreases from 0.19 to 0.1 7 mL g-I wi th the decrease in the framework gallium content in the MFI zeolites. The isotherms for nBA sorption in HAl-ZSM-5(36) and a series of HGa-ZS M-5 zeolites were measured in the temperature range 323-523 K. The sor ption data were attempted to be analyzed in terms of different sorptio n isotherm models such as the Langmuir, BET, Freundlich, Dubinin-Radus hkevich, and Sips and Koble-Corrigan models. The coefficients of the s tatistical models of Langmuir (K(L)) and Volmer (K(V)) failed to yield meaningful conclusions. Chemical affinity curves obtained from the nB A sorption data also helped to characterize the nature of the sorption centers (acidic sites) in the zeolites. Isosteric heats (Q(st)) were evaluated by applying the Claussius-Clapeyron equation to the sorption equilibrium (sorption isosteres). HAl-ZSM-5(36) showed higher Q(st) v alues than the corresponding gallium analogue. nBA sorption energetics was found to be useful in studying the acidic nature of the HAl-ZSM-5 and HGa-ZSM-5 zeolites.