INSITU MOSSBAUER STUDY OF FRAMEWORK-SUBSTITUTED (FE)ZSM-5 ZEOLITES

The coordination states, stability, and reducibility of isomorphically substituted framework Fe(III) were studied in ZSM-5 zeolites (Si/Fe = 22) by in situ Mossbauer spectroscopy. Supplementary methods as X-ray diffraction and temperature-programmed reduction were used as well, and the catalytic activity of (Fe)ZSM-5 was tested in CO hydrogenation. The effects of exchange of charge neutralizing protons for Li+ were also studied. The measurements revealed that the symmetry of coordination of the framework Fe(III) depends considerably on the nature of charge compensation. It can be suggested that Bronsted acidic bridged hydrogen induces greater asymmetry around the framework Fe(III) than that generated by cationic (e.g., Li+ H3O+, or PrNH3+) compensatiion. The framework Fe(III) withstands hydrogen reduction and CO + H-2 treatment at 570 K, but a part of these substituted ions can be reduced to Fe2+ state at 670 K. Li+ ion exchange decreases the reducibility of the framework Fe(III).