A solid solution model for Si-Al substitution in disordered FAU and LTA zeolites

Configurational entropies (S-con) arising from disordering of Si and Al in the frameworks of FAU and LTA zeolites were calculated from published Si-29 MAS NMR spectra using the cluster variation method (CVM). The calculations indicate that S-con is a systematic function of aluminum mole fraction (X- Al) and predict that S-con becomes zero at X-Al = 0 and 0.417. The latter v alue corresponds to the compositions above which FAU and LTA zeolites exhib it long range order, as observed previously from structure determinations a nd Monte Carlo modeling. This change in ordering state requires that FAU an d LTA zeolites with X-Al < 0.417 be treated as solid solutions that are the rmodynamically distinct from the long range ordered samples with X-Al > 0.4 17. The compositions X-Al = 0 (siliceous Y or ZK-4) and X-Al = 0.417 (alumi nous Y or ZK-4) represent stoichiometric endmembers for these solid solutio ns. The negligible excess enthalpy of mixing observed for Si-Al substitutio n in zeolites permits these solid solutions to be described by an athermal solution model, which is derived explicitly from excess entropies calculate d through the CVM. Activity composition relationships consistent with this model may be used to describe the energetic consequences of Si-Al solid sol ution in highly symmetrical zeolites like FAU and LTA. (C) 2001 Elsevier Sc ience B.V. All rights reserved.