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

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.