Results and Discussion
Since the intention was to study theoretically the intrinsic activity of the iron species
located in the frameworks of different zeolites, we selectively placed the iron species
on top of the highly accessible ion-exchange site of each zeolite, as explained above.
The structure of the iron species at the active sites inside the different zeolite morphologies
is presented in Figure 1. In ferrierite, the iron oxide cation [FeO]þ is coordinated
with the two oxygen bridging atoms located on the 10-membered ring pore channel
with the pore opening size of 0.420.54 nm. In ZSM-5, the iron cation is placed at
the ion exchange site on the 10-membered ring of the zigzag pore channel that intersects
with the straight pore channel. This iron cation is located here in a slightly larger
pore opening than in the FER zeolite of 0.540.56nm and, moreover, the iron cation
also has the intersection cavity adjacent to it. In BEA and FAU zeolites, the iron cation
is located at the ion-exchange site of the 12-membered ring pore channel with ellipsoidal
and circular openings of 0.660.67 and 0.74 nm for BEA and FAU zeolites,
respectively. Moreover, the Fe-FAU site is accessible from the super cage, which is
a large cavity with an approximate size of 1.121.22 nm. Despite the different pore
sizes and geometries, the local structures around the iron are not much different. In all
cases, the iron cation is coordinated by two oxygen bridging atoms of the zeolite. The
bond distances are in the range of 2.00–2.07A ˚ , and the Fe-Al distances are in the range
of 2.85–2.87A ˚ . The iron atom (FeII) is covalently bonded to the O- radical anion (O1)
with a bond distance of 1.65–1.66A ˚ (see Table 1).