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Mechanism of CIT‐6 and VPI‐8 Crystallization from Zincosilicate Gels
Author(s) -
Serrano David P.,
van Grieken Rafael,
Davis Mark E.,
Melero Juan A.,
Garcia A.,
Morales G.
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20021115)8:22<5153::aid-chem5153>3.0.co;2-e
Subject(s) - crystallization , nucleation , crystallography , materials science , phase (matter) , crystal (programming language) , amorphous solid , scanning electron microscope , crystal growth , chemical engineering , population , chemistry , composite material , organic chemistry , demography , sociology , computer science , engineering , programming language
The crystallisation of CIT‐6, a large‐pore zincosilicate with the framework topology of zeolite Beta and synthesised from clear hydrogels that contain, tetraethylammonium (TEA + ), Li + and Zn 2+ cations, proceeds initially through the formation of an amorphous solid that incorporates all the initial Zn species. Nucleation of the *BEA phase is effected by reorganisation of the amorphous phase, whereas crystal growth involves the incorporation of soluble species also. A highly crystalline CIT‐6 material is obtained after 164 h of synthesis at 140 °C. Scanning electron microscopy (SEM) shows that this sample exhibits two different types of crystals: well‐defined pseudo‐cubic crystals and rounded crystals. The latter has a broad crystal‐size distribution. If crystallisation is continued with longer synthesis times, the VPI‐8 crystalline phase appears, and a new population of needle‐shaped crystals is detected in the SEM images. This new crystalline phase is nucleated on the surface of the rounded CIT‐6 crystals, which disappear as the crystallisation progresses, while no changes are observed in the population of pseudo‐cubic CIT‐6 crystals. At higher crystallisation temperatures these phase transformations are accelerated, and the formation of VPI‐8 is favoured over that of CIT‐6.