ChemInform Abstract: Elucidation of Structure and Location of V(IV) Ions in Heteropolyacid Catalysts H₄PVMo₁₁O₄₀ as Studied by Hyperfine Sublevel Correlation Spectroscopy and Pulsed Electron Nuclear Double Resonance at W‐ and X‐Band Frequencies.

Abstract

2001 magnetic resonance, nuclear quadrupole resonance magnetic resonance, nuclear quadrupole resonance (solids and liquids) D 6560 31 -010 Elucidation of Structure and Location of V(IV) Ions in Heteropolyacid Catalysts H 4 PVMo 11 O 40 as Studied by Hyperfine Sublevel Correlation Spectroscopy and Pulsed Electron Nuclear Double Resonance at W-and X-Band Frequencies. -31 P and 1 H ENDOR results show that V(IV) ions are incorporated as vanadyl pentaaqua complexes [VO(H 2 O) 5 ] 2+ in the void space between the heteropolyanions in the hydrated heteropolyacid. The paramagnetic vanadium species are not incorporated at molybdenum sites in the Keggin structure of dehydrated H 4 PVMo 11 O 40 . The VO 2+ ions are coordinated to four or three outer oxygen atoms from one PVMo 11 O 40 4− heteropolyanion in a trigonal-pyramidal or slightly distorted square-pyramidal coordination geometry, respectively. -(POEPPL

Electron paramagnetic resonance study of vanadium exchanged H-ZSM5 prepared by vapor reaction of VCl4. The role of 17O isotope labelling in the characterisation of the metal oxide interaction

“…This geometrical arrangement is consistent with T7 sites sitting at the zeolite channels intersection and with the low spin density transfer to the Al ion and a distance Al-V of the order of 0.28 nm, consistent with Zn-Al distances observed at similar sites [17]. The O=V-O angle can be derived from the orientation of O(2) with respect to the gz component leading to a bond angle of 130°±10° consistent with VO 2+ units coordinated at the surface of heteroplyacids, [36] We remark that this coordination geometry does not exclude the possible binding at other sites such as T8, placed on the wall of the straight channel, with a relatively flat local environment and potentially favouring coordination to a larger number of oxygens.…”

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

“…Two models have been considered based on the experimental findings discussed above, which are illustrated in Figure and consist of a VO 2+ group replacing for an Al 3+ framework ion, charge balanced by a Brönsted acid proton on a neighboring oxygen (VO Al , Figure a), and an extraframework VO 2+ group, docked in the center of a 6-membered ring of the AFI structure (VO EF , Figure b) where it has three P and three Al next-nearest-neighbor ions. Both situations have been previously suggested in the literature. ,,, A thorough computational analysis of the different possible V incorporation modes in the AlPO-5 structure and their relative energetics will be the object of a future work.…”

“…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.…”