Energetics and structural evolution of Na-Ca exchanged zeolite A during heating

Area of Science:

• Materials Science
• Solid-State Chemistry
• Geochemistry

Background:

• Zeolite A properties are significantly altered by sodium-calcium ion exchange.
• Understanding thermal stability and phase transformations is crucial for zeolite applications.

Purpose of the Study:

• To systematically investigate the impact of cation composition on temperature-induced phase transformations.
• To determine the energetics of sodium-calcium exchanged zeolite A during heating.

Main Methods:

• Powder X-ray diffraction (XRD) for structural analysis.
• Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) for thermal events.
• High-temperature oxide melt solution calorimetry for formation enthalpies.

Main Results:

• Na-Ca zeolite A exhibits three stages upon heating: dehydration, amorphization, and densification/recrystallization.
• Amorphization and recrystallization temperatures increase with higher calcium content.
• Enthalpies of formation for amorphous and dense phases are linearly dependent on calcium content, decreasing with increased Ca.

Conclusions:

• Framework degradation occurs at higher temperatures, forming new aluminosilicate phases.
• The dense phase is energetically more stable than the amorphous phase for the same chemical composition.
• Na-A shows the most exothermic enthalpy of formation, while CaNa-A shows the least.