THERMOCHEMISTRY OF GALLOSILICATE ZEOLITES WITH NAT FRAMEWORK TOPLOGY: AN ENERGETIC VIEW ON THE ORDERING TRANSFORMATION

Zeolites are a series of microporous materials with 3D network structures built by full corner sharing of TO₄ (T = Si, Al) tetrahedral via single T-O-T bridges. The isomorphous replacement of Al by Ga in silica-based zeolite frameworks is an efficient way to improve their physicochemical and catalytic properties; and it is also a viable approach in the search for novel topologies. Thermodynamics of zeolite structures can probe the driving force for synthesis, detect the energy differences among various structures, and provide insight into the mechanisms and interactions important in the assembly of these materials. In the present study, we have synthesized sodium and potassium gallosilicate zeolites with the same topology (NAT), very close composition (Si/Ga ≈ 1.5) but with different T-atom distributions and extent of ordering. The materials have been characterized by using powder X-ray diffraction, elemental and thermal analyses, and multinuclear MAS NMR. Their formation enthalpies were studied by high temperature solution calorimetry in molten lead borate at 700 ^oC. The formation enthalpies of sodium Ga-NAT from oxide ranges from -54.1 to -56.5 kJ/mol, while that of potassium Ga-NAT lies between -64.6 and -67.08 kJ/mol. This small energy difference provides a thermodynamic explanation for the in situ transformation between disordering (orthorhombic) and ordering (tetragonal) structures under their crystallization conditions. The ordering of the NAT structure can be controlled by just varying the crystallization temperature during synthesizing, without changing the starting mixture.