Location of Vanadium(IV) in VAPO-5 as Studied by Hyperfine Sublevel Correlation Spectroscopy

Abstract

Vanadium(IV) framework substitution in aluminophosphate molecular sieves has been explored by electron spin resonance (ESR) and hyperfine sublevel correlation (HYSCORE) spectroscopy. In both as-synthesized and hydrated materials V(IV) exists as VO2+ species that coordinates to two framework oxygen atoms and presumably three water molecules in a distorted octahedral geometry.

Structure of the Catalytic Active Sites in Vanadium-Doped Aluminophosphate Microporous Materials. New Evidence from Spin Density Studies

“…Careful inspection of the X-band spectrum reveals that two species with slightly different spin-Hamiltonian parameters contribute to the overall spectral pattern. The spin Hamiltonian parameters of the two species deduced from computer simulation of the X- and Q-band spectra are reported in Table and agree with previous literature data. ,,, In Table the signs of the experimental hfi tensor elements were chosen such that agreement with the signs of the calculated A values was maintained (vide infra), i.e., assuming the sign of the experimental A values to be all negative, in agreement with previous experimental and theoretical work. , The abundance ratio between the two species was found, based on computer simulation, to be of the order of 1:0.7 in fair agreement with the results of ref .…”

“…The spin Hamiltonian parameters of the two species deduced from computer simulation of the X-and Q-band spectra are reported in Table 1 and agree with previous literature data. 19,20,45,62 In Table 1 the signs of the experimental hfi tensor elements were chosen such that agreement with the signs of the calculated A values was maintained (vide infra), i.e., assuming the sign of the experimental A values to be all negative, in agreement with previous experimental and theoretical work. 40,59 The abundance ratio between the two species was found, based on computer simulation, to be of the order of 1:0.7 in fair agreement with the results of ref 45.…”

“…EPR is one of the most powerful methods to obtain a detailed description of the local structure of vanadium centers in their +4 (paramagnetic) oxidation state (3d 1 , S = 1/2). In particular, so-called hyperfine techniques (HYSCORE and ENDOR) have been proven extremely useful to elucidate the local topology of vanadium-containing heterogeneous catalysts in general − and the nature of paramagnetic transition metal ions such as Ti, V, and Mn in AlPOs in particular. ,,− In the case of AlPOs, both aluminum and phosphorus have suitable magnetic properties for the investigation of long-range magnetic interactions by high-resolution EPR techniques, which allow discriminating between different substitutional sites. In particular, in the present work, the major spectroscopic information obtained are the 1 H, 31 P, and 27 Al ligand hyperfine interactions (hfi), which are measured by means of HYSCORE spectroscopy at X- and Q-band frequencies.…”

Synthesis, characterization and catalysis of (Co, V)-, (Co, Cr)- and (Cr, V)APO-5 molecular sieves

“…The spectral similarity of (Co, V)APO-5-0.05-0.03 to VAPO-5 indicates that V sites in the (Co, V)APO-5 and VAPO-5 molecular sieves had a similar local geometry [27], viz. part of them was present in the state of isolated square-pyramidal and/or distorted octahedral vanadyl-like (VO 2+ ) species [3,12,15,22,33,37]. After calcination, no ESR signal was detected, showing that all V 4+ ions were oxidized to the EPR-inactive V 5+ .…”

“…Some authors suggested that V 4+ substitutes for Al 3+ [38], while other research groups believe it replaces P 5+ [22,33,34]. Nagarajan et al very recently deduced from ESR and hyperfine sublevel correlation (HYSCORE) spectroscopy that VO 2+ species is present in the as-synthesized VAPO-5 by equatorially coordinating to two framework oxygen atoms and presumably three water molecules in a distorted octahedral geometry with phosphorus and aluminum framework atoms in the second coordination shell [37]. Despite of the controversy, the common point in all the well-documented literature reports is that vanadyl-like species is anchored to the framework via coordination to two framework oxygen atoms.…”