UV Raman Spectroscopic Studies on Active Sites and Synthesis Mechanisms of Transition Metal-Containing Microporous and Mesoporous Materials

摘要

Microporous and mesoporous materialsnare widely used as catalysts and cata-nlyst supports. Although the incorporation ofntransition metal ions into the framework ofnthese materials (by isomorphous substitution ofnAl and Si) is an effective means of creatingnnovel catalytic activity, the characterization ofnthe transition metal species within these mate-nrials is difficult. Both the low concentration ofnthe highly dispersed transition metal and thencoexistence of extraframework transition metal species present clear challenges. Moreover, the synthetic mechanisms thatnoperate under the highly inhomogeneous conditions of hydrothermal synthesis are far from well understood.nA useful technique for addressing these challenges is UV Raman spectroscopy, which is a powerful technique for cata-nlyst characterization and particularly for transition metal-containing microporous and mesoporous materials. ConventionalnRaman spectroscopy, using visible and IR wavelengths, often fails to provide the information needed for proper character-nization as a result of fluorescence interference. But shifting the excitation source to the UV range addresses this difficulty:ninterference from fluorescence (which typically occurs at 300 700 nm or greater) is greatly diminished. Moreover, signalnintensity is enhanced because Raman intensity is proportional to the fourth power of the scattered light frequency.nIn this Account, we review recent advances in UV Raman spectroscopic characterization of (i) highly dispersed transi-ntion metal oxides on supports, (ii) transition metal ions in the framework of microporous and mesoporous materials, andn(iii) the synthetic mechanisms involved in making microporous materials. By taking advantage of the strong UV resonancenRaman effect, researchers have made tremendous progress in the identification of isolated transition metal ions incorpo-nrated in the framework of microporous and mesoporous materials such as TS-1, Ti-MCM-41, Fe-ZSM-5, and Fe-SBA-15. Thensynthetic mechanisms involved in creating microporous materials (such as Fe-ZSM-5 and zeolite X) have been investigatednwith resonance and in situ UV Raman spectroscopy. The precursors and intermediates evolved in the synthesis solution andngels can be sensitively detected and followed during the course of zeolite synthesis. This work has resulted in a greater under-nstanding of the structure of transition metal-containing microporous and mesoporous materials, providing a basis for thenrational design and synthesis of microporous and mesoporous catalysts.