Thermochemistry of gallosilicate zeolites with the NAT topology: An energetic view on their in situ disorder–order transformation and thermal stability

The thermochemistry of eight gallosilicate zeolites with the NAT topology, six of which are characterized by similar Ga contents (Si/Ga ∼1.6) but different T-atom distributions and the other two materials by an unusual higher Ga content (Si/Ga ∼1.3), is described. The formation enthalpies of the sodium form of gallosilicate natrolites with lower Ga contents (Na-NAT-I, Na-NAT-II and Na-NAT-III) from oxides range from −50.3 to −57.0 kJ mol−1 of TO2 (T = Si or Ga), while those of the potassium form (K-NAT-I, K-NAT-II and K-NAT-III) lie between −65.5 and −68.4 kJ mol−1 of TO2. These small energy differences provide a thermodynamic explanation for the in situ transformation between disordered and ordered structures in the crystallization medium. While the formation enthalpy of another potassium natrolite with a high Ga content (K-PST-1) is highly exothermic, consistent with its high thermal stability, its sodium counterpart (Na-PST-1) has a considerable less exothermic formation enthalpy, as well as lower thermal stability.