An in situ calorimetric study of the synthesis of FAU zeolite

Synthesis of FAU zeolite from a mixture of silicate and aluminate solutions (5.15Na(2)O-1.00Al(2)O(3)-3.28SiO(2)-165H(2)O) was studied using an in situ calorimetric method at a heating rate of 0.1 degreesC/min. The scanning calorimetric curve provides a coherent and consistent recording of the synthesis process. At low temperature (25-66 degreesC), the calorimetric 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 degreesC 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 +/- 0.06 kJ/mol based on TO2 (T = Si or Al) or -457 +/- 12 kJ/mol based on the unit cell formula of the FAU product, Na-91. Si105Al91O384. 277H(2)O. The integral heat for the crystallization by calorimetry agrees well with the X-ray diffraction (XRD) or NMR crystallinity of the solid phase. At the early stages of 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 At than the gel/solid amorphous precursor. The abrupt drop of the soluble At 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 nuclei/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. (C) 2001 Elsevier Science B.V. All rights reserved.