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In situ disorder-order transformation in synthetic gallosilicate zeolites with the NAT topology

Title
In situ disorder-order transformation in synthetic gallosilicate zeolites with the NAT topology
Authors
Suk B.H.Lee S.-H.Shin C.-H.Ae J.W.Alvarez L.J.Zicovich-Wilson C.M.Camblor M.A.
Ewha Authors
우애자
SCOPUS Author ID
우애자scopus
Issue Date
2004
Journal Title
Journal of the American Chemical Society
ISSN
0002-7863JCR Link
Citation
Journal of the American Chemical Society vol. 126, no. 42, pp. 13742 - 13751
Indexed
SCI; SCIE; SCOPUS WOS scopus
Document Type
Article
Abstract
Here, 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.
DOI
10.1021/ja046921h
Appears in Collections:
사범대학 > 과학교육과 > Journal papers
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