Redox catalysis over metallo-zeolites: Contribution to environmental catalysis

The paper reviews development in analysis and understanding of the structure of active sites in metallo-zeolites and their function in SCR–NOx, NO–NO2 equilibration reaction, and benzene oxidation with N2O to phenol. The opportunities and limitations of various spectral techniques (FTIR, UV-Vis, ESR, EXAFS, Moessbauer, Raman spectroscopy) in connection with these studies are discussed. The attention is centered on Co and Fe species planted in pentasil ring zeolites of various topologies, specifically on Co-beta and Fe-oxo species in zeolites, exhibiting stable SCR–NOx activity under presence of high excess of water vapor existing in real exhaust gases from combustion processes. Also participation of the protonic, Al-Lewis sites and specific Fe-oxo sites is analyzed with respect to these redox reactions. It is shown that the zeolite matrix has a dramatic effect on the SCR–NOx activity of the hosted metal ion species. Main parameters controlling metallo-zeolite activity are the type and structure of metallo-species, distances between the cationic sites and concentration and distribution of Al in the framework. Specific Fe-oxo structures coordinated to zeolite framework exhibit extremely high activity in benzene oxidation with N2O to phenol in contrast to protonic and Al-Lewis sites which contribute to these reactions only by activation of hydrocarbons. Analogous (but probably not identical) Fe-oxo structures bear a potential to be applied in SCR–NOx with C3+ paraffins under high excess of water vapor. Stable SCR–NOx activity of Co-beta zeolite indicates either presence of Co-oxo species of extremely high activity or positive effect of high density of aluminium in specific local framework structure of Co-beta zeolite.