Fe-ZSM-11 zeolites, prepared by novel sol-gel process with Fe2+ and/or Fe3+ as active sites incorporated by reproducible post-synthesis methods were obtained. Investigations of the electron-donor-acceptor (EDA) sites from Fen+-containing zeolite by in situ infrared spectroscopy using pyridine as a probe molecule lead to the conclusion that the NOx conversion is a function of Fe2+ species. Fen+ as counter ions generates new Lewis sites. We differentiated it from the well-known Lewis sites from aluminum in the zeolitic framework. Thus, lower unoccupied molecular orbital (LUMO) of Fen+ and its interaction with the reactant through EDA adduct leading to a coordinative activity with Bronsted acid sites, being successfully applicable to selective catalytic reduction (SCR) of NOx. Moreover, according FTIR in situ analysis of NO adsorbed onto the catalyst, Fe2O3 as isolated species was not detected in the samples, although this specie is found in low proportion in the sample with Fe3+ as active site by XRD analysis. (C) 2004 Elsevier B.V. All rights reserved.