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.