[2008年09月20日]

 

      理论和实验相结合是科学研究发展的方向。我组又一篇理论和实验相结合的工作“骨架过渡金属拉曼信号的归属的理论和实验研究”(Framework Fe Ions in Fe-ZSM-5 Zeolite Studied by UV Resonance Raman Spectroscopy and Density Functional Theory Calculations)的工作发表在Journal of Physical Chemistry C上( http://dx.doi.org/10.1021%2Fjp802955s)。
      含过渡金属的分子筛如Fe-ZSM-5,Fe-AlPO-5,TS-1,等在许多选择性催化氧化反应中表现出高活性和选择性。但是对于这些含过渡金属的分子筛的活性中心的表征中面临重重困难,这是因为分子筛中过渡金属含量一般较低,因此表征信号一般较弱。另外过渡金属在分子筛中一般有多种组分组成,对于一般的表征手段来讲,各种表征信号混合在一起,解析极其复杂。紫外共振拉曼光谱通过拉曼共振效应,将拉曼信号增强几个数量级,从而解决了弱信号的问题,此外,紫外共振拉曼技术通过调节不同的激发线,激发不同的活性物种,从而达到区分不同的过渡金属组分的目的。因此,紫外共振拉曼光谱非常适合来研究过渡金属分子筛中的活性中心的问题。以Fe-ZSM-5为例,我们在实验中观察到它有 1165, 1115,1005 and 516 cm-1 四个谱峰,但仅仅通过实验很难对其归属进行确定的研究。通过DFT理论计算及拉曼活性的理论分析,我们对这几个谱峰进行了详细的归属,并通过进一步的实验验证了我们归属的合理性。通过我们的归属,以及进一步的分析,我们可以得出一个可以推广的结论:对于硅基分子筛中的过渡金属,在1100cm-1 和 500cm-1附近存在两个共振谱峰可以作为存在骨架过渡金属的主要证据,另外两个谱峰由于受周围环境的影响较大,因此可以作为辅助证据。对分子筛中过渡金属的表征研究,对研究分子筛的合成机理,分子筛中活性中心的催化机理的研究打下了坚实的基础。

Abstract: The transition metal containing zeolites show high selectivity and activity in many catalytic reactions under mild conditions, however, detailed characterization of transition metal site ions and nature of the active sites were remained elusive. Here, we present a study of the framework Fe ions in Fe-ZSM-5 zeolite via interplay between resonance Raman spectroscopy excited by 244 and 532 nm laser lines and density functional theory (DFT) calculations. Raman bands at 1165, 1115, 1005 and 516 cm-1 from the framework iron ions in the framework of Fe-ZSM-5 were observed. Based on the DFT calculations, the bands at 1165, 1115 and 516 cm-1 were attributed to ν0(A1), ν1(A1) and ν2(A1) modes with totally symmetric vibration of FeO4 tetrahedron in Fe-ZSM-5, originated from resonant enhanced Raman scattering, while the band at 1005 cm-1 for ν6(T2) was identified as a non-totally symmetric vibration of FeO4 tetrahedron from normal Raman scattering. The present work indicates that the resonance Raman bands at 500 and 1100 cm-1 can be used as the spectral characteristics of the transition metal ions substituted in the framework of zeolites.
Key words: UV resonance Raman spectroscopy, Fe-ZSM-5, zeolites, DFT.