Synthesis and characterization of ZSM-5 type zeolites I. physico-chemical properties of precursors and intermediates

Several techniques have been used in combination to clarify the mechanisms by which crystallization of ZSM-5 type zeolites takes place from Na2O-(TPA)2-A12O3-SiO2-H2O synthesis mixtures. Two different mechanisms are observed depending on the source of silica and the Si/Al, Al/Na and (Si + A1)/TPA ratios in the reaction mixture. The first is a liquid phase ion transportation process in which few nuclei are formed and large crystallites are obtained. The second is a solid hydrogel phase transformation process in which numerous nuclei are formed, leading to polycrystalline aggregates. Classical sigmoid nucleation-growth crystallization kinetics characterize both types of syntheses with a higher rate of crystallization when solid hydrogel transformation takes place. A non-homogeneous distribution of aluminium may be observed in the zeolite crystallites due either to nucleation from nearly pure silicate species or because of crystallization of the latter as an isostructural outer shell. It is shown that the presence of ZSM-5 zeolite in intermediate phases may be evidenced by thermal analysis, in particular when a large number of microcrystallites which cannot be detected by X-ray diffraction is present. The clathrating-templating effect of TPA species is recognised. The latter are also incorporated in the framework as exchange cations upon heating at 500–600°C.