1 research outputs found
Structure of the Catalytic Active Sites in Vanadium-Doped Aluminophosphate Microporous Materials. New Evidence from Spin Density Studies
Electron
spin resonance and hyperfine sublevel correlation (HYSCORE)
spectroscopy at X- and Q-band frequencies have been employed, in conjunction
with DFT modeling, to determine the location of VÂ(IV) ions in AlPO-5
zeotype materials. Two EPR-active species are detected, whose spin
Hamiltonian parameters are in accord with vanadyl ions (VO<sup>2+</sup>) experiencing slightly different local environments. Interactions
of the unpaired electrons of the paramagnetic VO<sup>2+</sup> species
with all relevant nuclei (<sup>1</sup>H, <sup>31</sup>P, <sup>27</sup>Al, and <sup>51</sup>V) could be resolved, allowing for the first
detailed structural analysis of the VO<sup>2+</sup> paramagnetic ions
in AlPO materials. Dehydration treatments indicate that the observed <sup>1</sup>H hyperfine couplings stem from structural protons in the
first coordination sphere of the VO<sup>2+</sup> species, strongly
pointing to charge compensating mechanisms associated with isomorphous
framework substitution at Al<sup>3+</sup> sites, in good agreement
with the large <sup>31</sup>P hyperfine couplings. Detection of fairly
large <sup>27</sup>Al couplings point to the presence of VO<sup>2+</sup>–O–Al linkages associated with a different structural
arrangement, in agreement with the presence of two EPR-active species.
The interpretation of the experimental results is corroborated by
DFT modeling, which affords a microscopic description of the system
investigated. The two EPR-active species are found to be consistent
with isolated VO<sup>2+</sup> species isomorphously substituted in
the AlPO framework at Al<sup>3+</sup> sites and extraframework VO<sup>2+</sup> species docked in the center of the 6-membered rings that
line up the main channel of the AFI structure