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From Molecular Fragments to Crystals: A UV Raman Spectroscopic Study on the Mechanism of Fe‐ZSM‐5 Synthesis
Author(s) -
Fan Fengtao,
Sun Keju,
Feng Zhaochi,
Xia Haian,
Han Bo,
Lian Yuxiang,
Ying Pinliang,
Li Can
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200801916
Subject(s) - raman spectroscopy , nucleation , amorphous solid , high resolution transmission electron microscopy , silicate , crystallization , materials science , zsm 5 , zeolite , spectroscopy , resonance raman spectroscopy , crystallography , analytical chemistry (journal) , chemistry , nanotechnology , transmission electron microscopy , catalysis , organic chemistry , optics , physics , quantum mechanics
The nucleation process of iron‐exchanged zeolite Fe‐ZSM‐5, from the assembly of distorted tetrahedrally coordinated iron species and silicate rings in the precursor to the final Fe‐ZSM‐5 crystals, as well as variations in the coordination environment of iron, were studied by UV resonance Raman spectroscopy and complementary techniques.The entire sequence of crystallization events of Fe‐ZSM‐5 was monitored by UV Raman spectroscopy in combination with HRTEM, UV/Vis spectroscopy, X‐ray diffraction patterns, and periodic DFT calculations. Fe‐ZSM‐5 was synthesized by an organic‐free method to avoid signal interference from the organic template in Raman spectra. Framework iron atoms with resonance Raman bands at 516, 1115, and 1165 cm −1 , and a Raman band at 1016 cm −1 are detected for Fe‐ZSM‐5. In the early stage of Fe‐ZSM‐5 synthesis, the precursor contains iron atoms in distorted tetrahedral coordination and five‐ and six‐membered silicate rings. Nucleation by aggregation of the precursor species was monitored by UV Raman spectroscopy based on the resonance Raman effect, and confirmed by periodic DFT calculations. Evolution of iron species on the surface and in the bulk phase was monitored by UV Raman spectroscopy with excitation at 244 and 325 nm, as well as HRTEM. Nucleation takes place first in the core of the amorphous particles, and crystalline nuclei with Fe‐ZSM‐5 structure are formed in the core by consuming the amorphous shell. Finally the amorphous particles are completely transformed into Fe‐ZSM‐5 crystals.