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PublicationJournal of Physical Chemistry C
Volume115 (14)

Probing ZnAPO-34 Self-Assembly Using Simultaneous Multiple in Situ Techniques

Authors:A.M. Beale , M.G. O?Brien , M. Kasunic , A. Golobic , Manuel Sánchez Sánchez, A.J.W. Lobo , D.W. Lewis , D.S. Wragg , S. Nikitenko , W. Bras , B.M. Weckhuysen
Groups of research:Molecular Sieves

The hydrothermal crystallization of ZnAPO-34 (CHA) molecular sieves has been studied for the first time using a combined in situ four technique setup utilizing SAXS/WAXS/XAFS/Raman to follow the various steps that occur during the complex transformation process of an amorphous precursor gel into a crystalline microporous material. These data are also supported by a detailed characterization of both the precursor gel (using Raman, NMR, XAFS, and TEM) and the final crystalline material (NMR, XRD, XAFS, TEM, and energy minimization calculations). Thus, all components during the various stages

of reaction have been studied allowing for fundamental insight from the atomic/molecular level up to the bulk scale. On the basis of this multitechnique approach, the following observations are made:

(i) The initial formation of a heterogeneous gel containing predominantlyseparate Al-O-P and Zn-O-P containing species as well as the presence of particles with a broad size distribution were noted.

(ii) During sample heating, the SAXS data reveal a second population (14-16 nm) at the onset of crystallization, which were also accompanied by changes in both the ZnO3-O-O3P environment (XAFS) and the template conformer state (Raman)

(iii) Before crystallization, Zn2þ species appear heterogeneously distributed throughout the sample, but in the final crystallineCHAphase Zn2þ, it is much more homogeneously distributed. One template molecule is found per CHA cage.

(iv) Zn2þ is found to promote nanoparticle growth and that results in the production of increasing amounts of crystalline material.

(v) The structure-directing effect of Zn2þ ions leading to CHA formation is most likely initiated via an electrostatic interaction between Zn2þ in a Zn-O-P-O-Al-O-P matrix and the TEA template.


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