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

Abstract

Configurational entropies ( S con) arising from disordering of Si and Al in the frameworks of FAU and LTA zeolites were calculated from published 29 Si 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 value corresponds to the compositions above which FAU and LTA zeolites exhibit long range order, as observed previously from structure determinations and Monte Carlo modeling. This change in ordering state requires that FAU and LTA zeolites with X Al<0.417 be treated as solid solutions that are thermodynamically distinct from the long range ordered samples with X Al>0.417. The compositions X Al=0 (siliceous Y or ZK-4) and X Al=0.417 (aluminous Y or ZK-4) represent stoichiometric endmembers for these solid solutions. The negligible excess enthalpy of mixing observed for Si–Al substitution in zeolites permits these solid solutions to be described by an athermal solution model, which is derived explicitly from excess entropies calculated through the CVM. Activity composition relationships consistent with this model may be used to describe the energetic consequences of Si–Al solid solution in highly symmetrical zeolites like FAU and LTA.