Characterization of Metal Centers in Zeolites for Partial Oxidation Reactions

This chapter addresses the power and possibilities provided by an appropriate combination of advanced characterization techniques in understanding the environment of metal ions in different metal-zeolites, as well as in the role played by them in different catalytic reactions. Three different classes of materials are considered as case studies: (1) Cu-zeolites, where CuI and CuII ions are mainly present as counterions; (2) Fe-zeolites, often containing a wide variety of isolated, oligonuclear, and aggregated (oxide/hydroxide) FeII/FeIII species; and (3) TS-1, which is the closest to a “single-site” catalyst, mainly containing framework TiIV sites. TS-1 is studied in relation to its activity in the propene epoxidation reaction in the presence of aqueous H2O2, while Cu- and Fe-zeolites are here considered in relation to the direct conversion of methane to methanol (MTM) (a so-called dream reaction) with O2, N2O, or H2O2 as oxidizing agents. Main focus is into the nature of the active site precursors (i.e., mono or di-/trinuclear ions, oxidation state, and local environment) and into the electronic and geometric structure of the oxo species formed upon interaction with the oxidizing agents. Moreover, examples about in situ or operando experiments following changes during the reaction are reviewed. The main considered techniques are X-ray absorption spectroscopy (XAS) and resonance Raman (rR) and diffuse reflectance UV-Vis spectroscopies, often coupled to density functional theory (DFT) modeling. Depending on the studied system, results obtained with infrared, Mössbauer, X-ray emission (XES) and electron paramagnetic resonance (EPR) spectroscopies are also described. The discussion includes the open debates, the main drawbacks and potentialities of the techniques, and the related characterization challenges.