Thermochemistry of Na-faujasites with varying Si/Al ratios

Abstract

The thermochemical stability of dehydrated sodium faujasites, Na x Al x Si 1−x O 2, 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 Si 4+→ Al 3 + Na 4 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 SiO 2, Al 2O 3, Na 2O (derived from Na 2CO 3 drop solution experiments) and NaAlO 2, standard molar enthalpies of formation of dehydrated framework structures from elements, simple oxides, and NaAlO 2 and SiO 2 were determined. Implications for the synthesis of high silica materials and the apparent thermal stability are discussed.