State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, 457 Zhongshan Road, Dalian 116023, China.
Acc Chem Res. 2010 Mar 16;43(3):378-87. doi: 10.1021/ar900210g.
Microporous and mesoporous materials are widely used as catalysts and catalyst supports. Although the incorporation of transition metal ions into the framework of these materials (by isomorphous substitution of Al and Si) is an effective means of creating novel catalytic activity, the characterization of the transition metal species within these materials is difficult. Both the low concentration of the highly dispersed transition metal and the coexistence of extraframework transition metal species present clear challenges. Moreover, the synthetic mechanisms that operate under the highly inhomogeneous conditions of hydrothermal synthesis are far from well understood. A useful technique for addressing these challenges is UV Raman spectroscopy, which is a powerful technique for catalyst characterization and particularly for transition metal-containing microporous and mesoporous materials. Conventional Raman spectroscopy, using visible and IR wavelengths, often fails to provide the information needed for proper characterization as a result of fluorescence interference. But shifting the excitation source to the UV range addresses this difficulty: interference from fluorescence (which typically occurs at 300-700 nm or greater) is greatly diminished. Moreover, signal intensity is enhanced because Raman intensity is proportional to the fourth power of the scattered light frequency. In this Account, we review recent advances in UV Raman spectroscopic characterization of (i) highly dispersed transition metal oxides on supports, (ii) transition metal ions in the framework of microporous and mesoporous materials, and (iii) the synthetic mechanisms involved in making microporous materials. By taking advantage of the strong UV resonance Raman effect, researchers have made tremendous progress in the identification of isolated transition metal ions incorporated in the framework of microporous and mesoporous materials such as TS-1, Ti-MCM-41, Fe-ZSM-5, and Fe-SBA-15. The synthetic mechanisms involved in creating microporous materials (such as Fe-ZSM-5 and zeolite X) have been investigated with resonance and in situ UV Raman spectroscopy. The precursors and intermediates evolved in the synthesis solution and gels can be sensitively detected and followed during the course of zeolite synthesis. This work has resulted in a greater understanding of the structure of transition metal-containing microporous and mesoporous materials, providing a basis for the rational design and synthesis of microporous and mesoporous catalysts.
微孔和介孔材料被广泛用作催化剂和催化剂载体。尽管将过渡金属离子掺入这些材料的骨架中（通过 Al 和 Si 的同晶取代）是创造新型催化活性的有效手段，但这些材料中过渡金属物种的表征却很困难。高度分散的过渡金属的低浓度和外加骨架过渡金属物种的共存都带来了明显的挑战。此外，水热合成条件下的合成机制远未得到很好的理解。一种有用的技术是紫外拉曼光谱，它是一种用于催化剂表征的强大技术，特别是对于含有过渡金属的微孔和介孔材料。使用可见和红外波长的常规拉曼光谱通常由于荧光干扰而无法提供适当表征所需的信息。但是，将激发源转移到紫外范围可以解决这个困难：荧光的干扰（通常发生在 300-700nm 或更大）大大减少。此外，信号强度得到增强，因为拉曼强度与散射光频率的四次方成正比。在本综述中，我们回顾了紫外拉曼光谱在（i）负载上高度分散的过渡金属氧化物、（ii）微孔和介孔材料骨架中的过渡金属离子和（iii）制备微孔材料所涉及的合成机制的表征方面的最新进展。通过利用强紫外共振拉曼效应，研究人员在识别孤立的过渡金属离子掺入微孔和介孔材料（如 TS-1、Ti-MCM-41、Fe-ZSM-5 和 Fe-SBA-15）的框架方面取得了巨大进展。利用共振和原位紫外拉曼光谱研究了制备微孔材料（如 Fe-ZSM-5 和沸石 X）的合成机制。在沸石合成过程中，可以敏感地检测和跟踪合成溶液和凝胶中演变的前体和中间体。这项工作加深了对含过渡金属的微孔和介孔材料结构的理解，为微孔和介孔催化剂的合理设计和合成提供了基础。
