ja046921h_si_001.pdf (115 kB)
In Situ Disorder−Order Transformation in Synthetic Gallosilicate Zeolites with the NAT Topology
journal contribution
posted on 2004-10-27, 00:00 authored by Suk Bong Hong, Song-Ho Lee, Chae-Ho Shin, Ae Ja Woo, Luis J. Alvarez, Claudio M. Zicovich-Wilson, Miguel A. CamblorHere, we report that synthetic gallosilicate molecular sieves with the NAT topology and Si/Ga
ratios close to but slightly higher than 1.50 undergo an in situ transformation under their crystallization
conditions. The materials have been studied ex situ by using powder X-ray diffraction, elemental and thermal
analyses, and multinuclear MAS NMR. The transformation is characterized by a change in the distribution
of Si and Ga of the NAT framework, from a quite (but not completely) disordered phase to a very highly
(but not completely) ordered one, accompanied by a change from tetragonal to orthorhombic symmetry.
During most of the solution-mediated transformation, no noticeable signs of fresh precipitation, phase
segregation, or changes in the chemical composition are detected. Intermediate materials show variations
in the degree of Si−Ga ordering and orthorhombic distortion and are not physical mixtures of the disordered
and ordered phases. Ab initio calculations strongly suggest a preferential siting of Si in the tetrahedral
sites involved in a smaller number of 4-rings in the NAT topology (i.e., the low multiplicity site). The cost
of violations of Loewenstein's rule has also been calculated. For this topology and chemical composition
the preferential siting and Loewenstein's rule drive together the system to the ordered configuration. A
Monte Carlo sampling procedure affords a reasonable model for the initial, mainly disordered state, which
fits well within the experimental disorder−order series.