The thermochemical stability of dehydrated sodium faujasites, NaxAlxSi(1-x)O2, with Si/Al ratios of 1.25 to 360 (x = 0 to 0.44) has been studied by high temperature solution, drop solution, and transposed temperature drop calorimetry near 977 K. Enthalpies of solution become more endothermic as the Si/Al ratio decreases, implying an exothermic (stabilizing) enthalpy of the charge coupled substitution Si4+→Al3 + Na4 in these structures. Deviation of the sample with Si/Al = 1.25 (13X) from the linear trend suggests that there may be a maximum stabilization for the composition with Si/Al = 1. These observations are consistent with previous studies of charge coupled substitutions in aluminosilicate glasses and dense sodium aluminosilicates. Using enthalpies of solution of SiO2, Al2O3, Na2O (derived from Na2CO3 drop solution experiments) and NaAlO2, standard molar enthalpies of formation of dehydrated framework structures from elements, simple oxides, and NaAlO2 and SiO2 were determined. Implications for the synthesis of high silica materials and the apparent thermal stability are discussed.
ASJC Scopus subject areas
- Materials Science(all)