Framework Fe Ions in Fe-ZSM-5 Zeolite Studied by UV Resonance Raman Spectroscopy and Density Functional Theory Calculations

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 the nature of the active sites remain 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. On the basis of 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, originating from resonant enhanced Raman scattering, while the band at 1005 cm−1 for ν6(T2) was identified as a nontotally symmetric vibration of the 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.