Preparation of Overexchanged Fe–ZSM-5 Zeolites Using the Ferrous Oxalate Method: Why Does It Fail?

A plausible explanation of why the ferrous oxalate method is unsuitable for the preparation of overexchanged Fe–ZSM-5 is proposed on the basis of TG–DSC, FTIR, and 27Al MAS NMR results. Iron is precipitated onto the zeolite mainly as an FeC2O4 complex, which blocks the pores and impedes a complete exchange of the Na+ cations. Most of the precipitate is removed by extensively washing the zeolite, whereas most of the remaining iron species is transformed into iron oxide during the subsequent thermal treatment. A small number of iron species may also occupy ion-exchange positions (Fe/Al⪡0.5). IR spectroscopy revealed the formation of Brønsted acid sites, both during the exchange and upon drying of the zeolite. These results suggest that overexchanged iron zeolites cannot be prepared easily by conventional ion exchange in aqueous solution. The effectiveness of this method does not seem to depend on the source of the parent ZSM-5 used.