title: Mechanism of CIT-6 and VPI-8 crystallization from zincosilicate gels.
abstract: 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 Zn2+ 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 degrees 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.
authors.affiliation: Environmental and Chemical Engineering Group, School of Experimental Sciences and Technology, Rey Juan Carlos University, 28933 Mostoles, Madrid, Spain. d.serrano@escet.urjc.es
authors.forename: David P
authors.initials: DP
authors.lastname: Serrano
authors.forename: Rafael
authors.initials: R
authors.lastname: van Grieken
authors.forename: Mark E
authors.initials: ME
authors.lastname: Davis
authors.forename: Juan A
authors.initials: JA
authors.lastname: Melero
authors.forename: A
authors.initials: A
authors.lastname: Garcia
authors.forename: G
authors.initials: G
authors.lastname: Morales