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 sit u calorimetric method at a heating rate of 0.1 degreesC/min. The scanning c alorimetric curve provides a coherent and consistent recording of the synth esis process. At low temperature (25-66 degreesC), the calorimetric curve d rifts slightly in the endothermic direction. Chemical analysis shows a slig ht increase in the dissolution of the amorphous gel/solid phase with increa sing temperature in this period. The onset of the exothermic peak at 66.7 d egreesC is suggested to signal the beginning of the nucleation/crystallizat ion 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 o n TO2 (T = Si or Al) or -457 +/- 12 kJ/mol based on the unit cell formula o f the FAU product, Na-91. Si105Al91O384. 277H(2)O. The integral heat for th e crystallization by calorimetry agrees well with the X-ray diffraction (XR D) or NMR crystallinity of the solid phase. At the early stages of crystall ization (integral heat <0.1% of the total) the compositional changes in sol id and solution were similar to those in the pre-crystallization period. Th e FAU phase was slightly richer in At than the gel/solid amorphous precurso r. The abrupt drop of the soluble At concentration during the rapid crystal lization period indicated the direct participation of the soluble aluminosi licate species in the crystal growth. The crystalline phase could be observ ed using XRD, NMR or IR only significantly after the onset of the calorimet ric peak because the mass fraction of the nuclei/crystals in the solid phas e was initially too low to be detected by the former methods. This demonstr ated the high sensitivity of the calorimetric method. (C) 2001 Elsevier Sci ence B.V. All rights reserved.