Theoretical studies of HNCO adsorption at stabilized iron complexes in the ZSM-5 framework

Abstract

The stabilization of monomeric and dimeric iron complexes, such as FeOH, HOFeOFeOH and FeOFe, which are representative of the iron-exchange in the ZSM-5 structure, has been investigated using a DFT approach with a cluster model. A Si20O53H28 cluster was chosen to represent part of a single channel in the ZSM-5 structure. In this basic cluster, two Si atoms were replaced by two aluminum atoms to simulate the exchange sites in ZSM-5 and to determine energetically favorable locations for AlAl couples. The distance of the aluminum atoms varied from 4.8 to 7.5Å. Inside the clusters of the ZSM-5 structure, iron oxohydroxide and oxide species formed stable monomer and dimer configurations, which were bound to the oxygen atoms of AlO4- due the more negative charge and the stronger ion attraction compared to SiO4. The stabilization energy per iron site was about 3.5eV for the monomeric iron complex and 1.0–2.5eV for the different dimeric iron sites. These similar energy levels, in combination with a detailed study of the geometric structure of the iron and oxygen centers in Fe-ZSM-5, support our hypotheses that the distribution of iron sites follows the distribution of the Al atoms and that no preferred configurations exist for these interactions. However, when aluminum centers are present with a distance compatible to the FeFe distance the formation of a dimer is slightly favored. In addition, the molecular and dissociative adsorption of isocyanic acid on Fe-ZSM-5 has been investigated due to its importance for the selective catalytic reduction of NOx with urea.