Probing ZnAPO-34 self-assembly using simultaneous multiple in situ techniques

Abstract

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 predominantly separate 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, Zn2+ species appear heterogeneously distributed throughout the sample, but in the final crystalline CHA phase Zn2+, it is much more homogeneously distributed. One template molecule is found per CHA cage. (iv) Zn2+ is found to promote nano particle growth and that results in the production of increasing amounts of crystalline material. (v) The structure-directing effect of Zn2+ ions leading to CHA formation is most likely initiated via an electrostatic interaction between Zn2+ in a Zn−O−P−O−Al−O−P matrix and the TEA template.