UV Raman Spectroscopic Studies on the Synthesis Mechanism of FeAlPO4-5

FeAlPO4-5 samples with various iron contents were investigated by UV resonance Raman spectroscopy and UV-Vis spectroscopy. The Raman spectra of FeAlPO4-5 show four feature bands related to the framework iron at 630, 1060, 1140, and 1210 cm−1 when excited by 266 nm laser. It is found that only part of ferric ions could enter into the framework, while the others exist in the extra-framework in the form of six-coordinated state. This kind of iron gives characteristic Raman band at 285 cm−1. Combined with UV resonance Raman spectroscopy, UV-Vis spectroscopy, and X-ray diffraction, the crystallization process of FeAlPO4-5 (Al/Fe = 760) was studied. It is found that, in the early stages of the crystallization, the iron species exist in the form of six-coordinated state while attached to the end of the 1-dimensional AlPO4 chain. The Fe–O bonds in the six-coordinated state hamper the crystallization of AlPO4. The crystallization of the FeAlPO4-5 began when the reaction between the 1-dimensional AlPO4 chains occurred. At the same time, the six-coordinated ferric ions converted to the tetrahedral iron.

摘要利用紫外共振拉曼光谱和紫外-可见漫反射光谱对不同铁含量的 FeAlPO4-5 分子筛进行了研究. 结果表明, FeAlPO4-5 分子筛的骨架铁有 4 个特征的拉曼谱峰, 分别位于 630, 1060, 1140 和 1210 cm-1. 当凝胶中 Al/Fe 比小于 380 时, 只有一部分铁离子可以进入分子筛形成四配位的骨架铁物种; 而另一部分则以骨架外六配位的铁物种存在, 其特征拉曼谱峰位于 285 cm-1. 结合紫外拉曼光谱、紫外-可见漫反射光谱和 X 射线衍射研究了 Al/Fe 比为 760 时 FeAlPO4-5 分子筛的晶化过程. 结果发现, 在分子筛晶化前, 铁物种以六配位的形式存在, 它为分子筛前体中的一维链状磷酸铝添加了一个惰性端基, 六配位的 Fe-O 键不利于它与其它磷酸铝物种发生反应; 当分子筛开始晶化时, 带有铁离子端基的磷酸铝链与其它磷酸铝链进一步反应形成分子筛骨架. 同时, 六配位的铁离子和邻近的磷酸铝物种反应转化为四配位的骨架铁物种.

The mechanism for FeAlPO4-5 synthesis was investigated by UV resonance Raman spectroscopy. Some of the six-coordinated ferric ions in the precursor gel converted to tetrahedral iron atoms during crystallization and the balance existed in the octahedral state.Figure optionsDownload as PowerPoint slide