An in situ calorimetric study of the synthesis if FAU zeolite

Synthesis of FAU zeolite from a mixture of silicate and aluminate solutions (5.15Na₂O-1.00Al₂O₃-3.28SiO₂- 165H₂O) was studied using an in situ calorimetric method at a heating rate of 0.1°C/min. The scanning calorimetric curve provides a coherent and consistent recording of the synthesis process. At low temperature (25-66°C), the calometric curve drifts slightly in the endothermic direction. Chemical analysis shows a slight increase in the dissolution of the amorphous gel/solid phase with increasing temperature in this period. The onset of the exothermic peak at 66.7°C is suggested to signal the beginning of the nucleation/crystallization process. Formation of the FAU structure is an exothermic event and the associated heat effect was directly measured, -2.38±0.06kJ/mol based on TO₂ (T=Si or Al) or -457±12 kJ/mol based on the unit cell formula of the FAU product, crystallization (integral heat <0.1% of the total) the compositional changes in solid and solution were similar to those in the pre-crystallization period. The FAU phase was slightly richer in A1 than the gel/solid amorphous precursor. The abrupt drop of the soluble A1 concentration during the rapid crystallization period indicated the direct participation of the soluble aluminosilicate species in the crystal growth. The crystalline phase could be observed using XRD, NMR or IR only significantly after the onset of the calorimetric peak because the mass fraction of the nuclie/crystals in the solid phase was initially too low to be detected by the former methods. This demonstrated the high sensitivity of the calorimetric method.