Identification of α-Fe in High-Silica Zeolites on the Basis of ab Initio Electronic Structure Calculations

Abstract

α-Fe is the precursor of the reactive Fe═O core responsible for methane oxidation in Fe-containing zeolites. To get more insight into the nature and stability of α-Fe in different zeolites, the binding of Fe(II) at six-membered-ring cation exchange sites (6MR) in ZSM-5, zeolite beta, and ferrierite was investigated using DFT and multireference ab initio methods (CASSCF/CASPT2). CASPT2 ligand field (LF) excitation energies of all sites were compared with the experimental DR-UV-vis spectra reported by Snyder et al. From this comparison it is concluded that the 16000 cm band of α-Fe, observed in all three zeolites, can uniquely be assigned to a high-spin square-planar (SP) Fe(II) located at a 6MR with an Al-Si-Si-Al sequence, where the Al atoms are positioned opposite in the ring and as close to each other as possible. The stability of such conformations is also confirmed by the binding energies obtained from DFT. The bands at 10000 cm in the experimental spectra, assigned to spectator Fe(II), are attributed to six-coordinated trigonal-prismatic Fe(II) species, as calculated for the γ-site in ZSM-5. The entatic effect of the zeolite lattice on the stability of the SP sites was investigated by making use of the unconstrained Fe(II) model complex FeL (with L = [Al(OH)]). The SP conformer is approximately 2 kcal/mol more stable than the tetrahedral form, indicating that the SP coordination environment of α-Fe is not imposed by the zeolite lattice but rather electronically preferred by Fe(II) in the environment of four O ligands. A significant contribution to the stability of the SP conformer is provided by mixing of the doubly occupied 3d orbital with the higher lying 4s.

Advances in the synthesis, characterisation, and mechanistic understanding of active sites in Fe-zeolites for redox catalysts

“…Moving up in Fe loading from the single-site -Fe materials, Fe spectators appear in the UV-Vis and Mössbauer spectra of Fe-*BEA. 31 These are likely monomers in the other exchange sites of the zeolite, as calculated by Hallaert et al 44 Their exact coordination environment should be confirmed with site selective techniques (VTVH-MCD, rR). While these may be less relevant for catalysis, taking a step back to figure out the less glamorous spectroscopy will pay off eventually as they will likely cloud the spectroscopy of dimers that should start forming upwards a certain Fe/Al ratio.…”

“…With a limited number of boundary conditions set by experiment, modelling at sufficiently high levels of theory can provide quite accurate predictions on active site geometry and electronic structure. 44 Between a restricted number of possible configurations, the condition dependence of different Fe species' stability in zeolites can also be predicted with reasonable confidence. 45 The information from these studies, whether relevant to -Fe/-O or not, may give useful insight in Fe sites for catalysis when they become experimentally defined in the future.…”

“…The bottom right box of scheme 1 suggests some other paths to valorise the work on characterising the -Fe/-O active sites. Given the site's extraordinary reactivity and its preferential formation on many 6MR zeolites 31,44,50 it may well have played a role in a lot of the Fe-zeolite catalysis studied in the past, even if -Fe/-O was not detected. Fezeolites are intensively investigated for SCR, [8][9][10][11][12][13][14][15][16][17][18][19][20][21]59 and the activity of isolated Fe and -Fe on ZSM-5 zeolite has often been proposed.…”

Advanced X‐ray Absorption Spectroscopy Analysis to Determine Structure‐Activity Relationships for Cu‐Zeolites in the Direct Conversion of Methane to Methanol

“…The α‐site is a planar 6MR along the straight channel with an O−T−O bridge, while the β‐site is a planar 6MR in the perpendicular channel. These sites have also been identified in the MFI framework . The γ‐site is boat‐shaped with one O−T−O bridge and is found in the intersection of the straight and perpendicular channels.…”

Synthesis–Structure–Activity Relations in Fe-CHA for C–H Activation: Control of Al Distribution by Interzeolite Conversion

“…27 Moreover, the near exclusive presence of only the active α-Fe II species was noted at low iron loadings using Mössbauer spectroscopy while at higher loadings, other undesired iron species (spectators) are formed. 26,28 As a consequence, further improvement in the productivity in MPO will benefit from control over the Al-distribution of the host zeolite (Scheme 1). Scheme 1.…”