[2009年01月05日]

 
    继我组成功地将原位紫外拉曼技术应用于分子筛合成机理研究之后(范峰滔、冯兆池、李灿等Chem. Eur. J., 14: 5125-5129, 2008)。最近,我组再次将紫外共振拉曼技术应用到研究掺杂过渡金属杂原子分子筛Fe-ZSM-5的合成机理中,首次利用光谱的手段同时从分子层次、结晶行为上研究了分子筛的成环、成核以及晶体生长行为。(Fengtao Fan, Keju Sun, Zhaochi Feng, Haian Xia, Bo Han, Yuxiang Lian, Pinliang Ying, and Can Li, 2009, Chem. Eur. J.)。
    过渡金属元素掺杂的分子筛在选择性氧化、氮氧化合物消除以及仿生催化方面表现出了非常优异的性能。但是,由于分子筛骨架中可以引入的杂原子的量非常低(<2%),这就使得表征分子筛中的杂原子变得极具挑战性。紫外拉曼光谱可以避开分子筛合成中中间物种产生荧光的干扰而增加灵敏度。更重要地是,利用紫外共振拉曼光谱,可以选择性的激发出杂原子分子筛中骨架杂原子的信息。这些优点为紫外共振拉曼光谱研究分子筛,特别是杂原子分子筛的合成机理提供了非常有利的条件。在本工作中,范峰滔等人利用原位共振拉曼技术结合非有机模板剂合成分子筛体系,研究了含铁分子筛的晶化机理。该工作不仅从分子水平上研究了分子筛从五、六元环碎片、四面体杂原子中心成核到最终形成骨架的过程,而且利用含铁分子筛样品对不同激发光源的吸收能力不同,选择性地研究了粒子内部以及表面不同的晶化过程,结合高分辨电镜技术明确地指出Fe-ZSM-5分子筛成核发生在凝胶粒子的中央,并且随着晶化时间的延长逐渐向外表面生长。
 

From molecular fragments to crystals: A UV Raman spectroscopic study on Fe-ZSM-5 synthesis mechanism

Fengtao Fan, Keju Sun, Zhaochi Feng, Haian Xia, Bo Han, Yuxiang Lian, Pinliang Ying, and Can Li*
 Chem. Eur. J., Full paper.
DOI: 10.1002/chem.200801916

 
Abstract:The entire sequence of crystallization events of Fe-ZSM-5 was monitored by UV Raman spectroscopy combined with HRTEM, UV-vis spectra, X-ray diffraction patterns and Periodic DFT calculations. Fe-ZSM-5 was synthesized using an organic-free method to avoid signal interference from the organic template in Raman spectra. The framework irons with the Resonance Raman bands at 516, 1115, and 1165 cm-1 and Raman band at 1016 cm-1 are detected for Fe-ZSM-5. In the early synthesis stage of Fe-ZSM-5, the precursor contains iron atoms in distorted tetrahedral coordination and five- and six-membered silicate rings. Nucleation via aggregation of the precursor species was monitored by UV Raman spectroscopy based on the Resonance Raman effect, and confirmed by Periodic DFT calculations. The unparalleled evolutions of the iron species on the surface and in the bulk phase are detected by UV Raman spectroscopy excited with 244 and 325 nm, confirmed by HRTEM. It is found that the nucleation takes place first in the core of the amorphous particles, and the 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.

Keywords:synthesis mechanism • hydrothermal synthesis • Fe-ZSM-5• zeolite • UV Raman spectroscopy