Formation and dehydration enthalpy of gallosilicate zeolites

Gallosilicate zeolites Ga-NaSOD, Ga-NaFAU, Ga-NaNAT, Ga-KNAT* (a mixture of Ga-KNAT and Ga-KTNU-2), Ga-KLTL and Ga-KTNU-1 have been synthesized and characterized by X-ray diffraction (XRD), microprobe analysis, thermogravimetric analysis and differential scanning calorimetry (TG-DSC). Generally, the lattice parameters increase after Ga substitution for Al, except for Ga-KNAT*. The formation and dehydration enthalpies of these zeolites were measured by high temperature oxide melt solution calorimetry. Compared to the analogous aluminosilicate zeolite of similar Si/T ³⁺, the gallosilicate zeolite has similar dehydration enthalpy per mole of tetrahedra, but has less endothermic dehydration enthalpy per mole of water due to the larger number of H₂O molecules in the enlarged unit cell of the Ga zeolite. The dehydration enthalpy per mole of water is a monotonic function of framework density while that per mole of tetrahedra is mainly influenced by cation type. The gallosilicate zeolites have less exothermic formation enthalpies from oxide components than the analogous aluminosilicate zeolites, confirming their lower stability. The formation enthalpies of hydrated and dehydrated gallosilicate zeolites are correlated with Ga/(Ga + Si) ratio and framework density (FD).