Pochen Chu

ZSM-4 crystallization via faujasite metamorphosis

The crystallization of ZSM-4 via faujasite metamorphosis has been studied by X-ray diffraction and scanning electron microscopy. By the addition of tetramethylammonium (TMA) ions, the well-known transition from metastable faujasite to “P” zeolite was directed to ZSM-4 zeolite instead. The following reaction sequence was observed for these crystallizations: There is no indication of a simultaneous or competitive crystallization since no ZSM-4 is formed until the faujasite crystallization is complete. Also, there is no evidence to indicate that the faujasite reverts to an amorphous material which might serve as a nutrient source for the recrystallization to ZSM-4. Scanning electron microscopic observations are consistent with a mechanism that would include a dissolution of surface faujasite crystals that supplies nutrient for the ZSM-4 crystallization which, in turn, is nucleated by the surface of the same faujasite crystals.

The deammoniation reaction of ammonium Y zeolite

The deammoniation reaction of ammonium exchanged zeolite Y has been studied using differential thermal analysis. The results show deammoniation occurs in two endothermic steps: the first is dissociation of NH 3 from the small pore structure at 250 °C, the second from large pore structure in the 300–400 °C region. It implicates that the acid sites generated by this dissociation are weaker in the small pores than in the large pores. The results also demonstrate that a highly acidic solid such as Y zeolite need not give an exothermic peak upon removal of chemisorbed ammonia in oxygen-containing atmospheres if the ammonia is rapidly removed from the reaction zone.