The mechanism of formation of the Fe species in Fe/ZSM-5 prepared by CVD

Abstract

The formation of the iron species in Fe/ZSM-5 prepared by chemical vapour deposition (CVD) of FeCl₃ onto H/ZSM-5 was monitored by characterising the catalyst after each step of its preparation by X-ray absorption, UV–VIS and atomic absorption spectroscopy (AAS), and by magnetic susceptibility measurements. Upon the CVD treatment, the iron ions are exchanged in the zeolite pores in the form of tetrahedrally coordinated [FeCl₂]⁺ species. During the subsequent washing of the zeolite most of these ions (ca. 80%) aggregate into small binuclear hydroxo-iron clusters with a structure similar to that of the binuclear units in α-FeOOH. The remaining ions are present as isolated species, while a minor fraction of multinuclear clusters may be present as well. Calcination in oxygen after the washing results mainly in the removal of the water molecules coordinatively bound to the iron species. Both the washing of the zeolite and its calcination in oxygen were found to be of fundamental importance for the formation of small clusters. When the washing is omitted, the calcination yields relatively large Fe₂O₃ particles. If the thermal treatment after the washing is carried out in helium rather than in oxygen, partial autoreduction of the iron from Fe³⁺ to Fe²⁺ takes place together with the formation of Fe₃O₄ clusters. The possible structure of the binuclear (hydr)oxo-iron clusters is discussed.