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 V­(IV) ions in AlPO-5 zeotype materials. Two EPR-active species are detected, whose spin Hamiltonian parameters are in accord with vanadyl ions (VO²⁺) experiencing slightly different local environments. Interactions of the unpaired electrons of the paramagnetic VO²⁺ species with all relevant nuclei (¹H, ³¹P, ²⁷Al, and ⁵¹V) could be resolved, allowing for the first detailed structural analysis of the VO²⁺ paramagnetic ions in AlPO materials. Dehydration treatments indicate that the observed ¹H hyperfine couplings stem from structural protons in the first coordination sphere of the VO²⁺ species, strongly pointing to charge compensating mechanisms associated with isomorphous framework substitution at Al³⁺ sites, in good agreement with the large ³¹P hyperfine couplings. Detection of fairly large ²⁷Al couplings point to the presence of VO²⁺–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 VO²⁺ species isomorphously substituted in the AlPO framework at Al³⁺ sites and extraframework VO²⁺ species docked in the center of the 6-membered rings that line up the main channel of the AFI structure

Electronic Structure of Ti³⁺–Ethylene Complexes in Microporous Aluminophosphate Materials. A Combined EPR and DFT Study Elucidating the Role of SOMO Orbitals in Metal–Olefin π Complexes

The interaction of tetrahedrally coordinated Ti³⁺ ions generated in the framework of TiAlPO-5 microporous materials with ^12,13C₂H₄ leads to the formation of side-on η² {Ti³⁺C₂H₄} complexes with a unique 5-fold coordination of titanium, supported by four oxygen donor ligands of the framework. The detailed electronic and magnetic structure of this adduct is obtained by the combination of advanced EPR techniques (HYSCORE and SMART-HYSCORE) in conjunction with periodic and cluster model DFT calculations. The binding of C₂H₄ results from the σ overlap of low lying C₂H₄ filled π orbitals with the 3<i>d</i>_<i>z</i>² empty orbital of titanium, enhanced by a small contribution due the π overlap between the semioccupied 3<i>d</i>_<i>yz</i> orbital of titanium and the empty π* orbital of ethylene. The spin density repartition over the ethylene molecule, obtained experimentally, allows probing directly the entity of the metal-to-substrate π*-back-donation, highlighting an asymmetry in the spin density delocalization. This interesting feature is supported by parallel theoretical calculations, which cast the role of the oxygen donor ligands in driving this bonding asymmetry. As a consequence, the interesting structural feature of potential and actual inequality in the electronic spin states (α,β) on the two ethylene carbon atoms of the π coordinated ethylene molecule is produced. The underlying electronic effects associated with the π coordination of ethylene to an early transition metal in paramagnetic state are thus revealed with an unprecedented accuracy for the first time

Nature of Reduced States in Titanium Dioxide as Monitored by Electron Paramagnetic Resonance. II: Rutile and Brookite Cases

We have systematically used electron paramagnetic resonance (EPR) to understand the nature of excess electron centers in titanium dioxide and to classify their spectroscopic features. Excess electrons in TiO₂ (probably the most important photoactive oxide) have been generated either by photoinduced charge separation or by reductive treatments and are stabilized in the solid by titanium ions which reduce to paramagnetic Ti³⁺. These are monitored by EPR and classified on the basis of their <b>g</b> tensor values in order to amend a certain confusion present in the literature about this subject. In the previous paper of this series (S. Livraghi et al. <i>J. Phys. Chem. C</i> <b>2011</b>, <i>115</i>, 25413–25421), excess electron centers in anatase were investigated while the present one is devoted to rutile and brookite, the two other TiO₂ polymorphs, in the aim of providing a thorough and consistent guideline to researchers working in the wide area of titanium dioxide applications

Toward Understanding the Catalytic Synergy in the Design of Bimetallic Molecular Sieves for Selective Aerobic Oxidations

Structure–property correlations and mechanistic implications are important in the design of single-site catalysts for the activation of molecular oxygen. In this study we rationalize trends in catalytic synergy to elucidate the nature of the active site through structural and spectroscopic correlations. In particular, the redox behavior and coordination geometry in isomorphously substituted, bimetallic VTiAlPO-5 catalysts are investigated with a view to specifically engineering and enhancing their reactivity and selectivity in aerobic oxidations. By using a combination of HYSCORE EPR and <i>in situ</i> FTIR studies, we show that the well-defined and isolated oxophilic tetrahedral titanium centers coupled with redox-active VO²⁺ ions at proximal framework positions provide the loci for the activation of oxidant that leads to a concomitant increase in catalytic activity compared to analogous monometallic systems

Insights into Adsorption of NH₃ on HKUST‑1 Metal–Organic Framework: A Multitechnique Approach

We report a careful characterization of the interaction of NH₃ with the Cu­(II) sites of the [Cu₂C₄O₈] paddle-wheel cornerstone of the HKUST-1 metallorganic framework, also known as Cu₃(BTC)₂. The general picture emerging from combining XRPD, EXAFS, XANES, mid- and far-IR, DRUV–vis, and EPR techniques is that the presence of traces of water has relevant consequences on the effect of ammonia on the MOF framework. NH₃ adsorption on the dry system results in a strong chemisorption on Cu­(II) sites that distorts the framework, keeping the crystallinity of the material. Perturbation observed upon NH₃ adsorption is analogous to that observed for H₂O, but noticeably enhanced. When the adsorption of ammonia occurs in humid conditions, a time-dependent, much deeper modification of the system is observed by all of the considered techniques. On a methodological ground, it is worth noticing that we used the optimization of XANES spectra to validate the bond distance obtained by EXAFS

Marked Difference in the Electronic Structure of Cyanide-Ligated Ferric Protoglobins and Myoglobin Due to Heme Ruffling

Electron paramagnetic resonance experiments reveal a significant difference between the principal <i>g</i> values (and hence ligand-field parameters) of the ferric cyanide-ligated form of different variants of the protoglobin of <i>Methanosarcina acetivorans</i> (<i>Ma</i>Pgb) and of horse heart myoglobin (hhMb). The largest principal <i>g</i> value of the ferric cyanide-ligated <i>Ma</i>Pgb variants is found to be significantly lower than for any of the other globins reported so far. This is at least partially caused by the strong heme distortions as proven by the determination of the hyperfine interaction of the heme nitrogens and mesoprotons. Furthermore, the experiments confirm recent theoretical predictions [Forti, F.; Boechi, L., Bikiel, D., Martí, M.A.; Nardini, M.; Bolognesi, M.; Viappiani, C.; Estrin, D.; Luque, F. J. <i>J. Phys. Chem</i>. <i>B</i> <b>2011</b>, <i>115</i>, 13771–13780] that Phe­(G8)­145 plays a crucial role in the ligand modulation in <i>Ma</i>Pgb. Finally, the influence of the N-terminal 20 amino-acid chain on the heme pocket in these protoglobins is also proven

Chemical Composition of an Aqueous Oxalato-/Citrato-VO²⁺ Solution as Determinant for Vanadium Oxide Phase Formation

Aqueous solutions of oxalato- and citrato-VO²⁺ complexes are prepared, and their ligand exchange reaction is investigated as a function of the amount of citrate present in the aqueous solution via continuous-wave electron paramagnetic resonance (CW EPR) and hyperfine sublevel correlation (HYSCORE) spectroscopy. With a low amount of citrate, monomeric <i>cis</i>-oxalato-VO²⁺ complexes occur with a distorted square-pyramidal geometry. As the amount of citrate increases, oxalate is gradually exchanged for citrate. This leads to (i) an intermediate situation of monomeric VO²⁺ complexes with a mix of oxalate/citrate ligands and (ii) a final situation of both monomeric and dimeric complexes with exclusively citrato ligands. The monomeric citrato-VO²⁺ complexes dominate (abundance > 80%) and are characterized by a 6-fold chelation of the vanadium­(IV) ion by 4 RCO₂^– ligands at the equatorial positions and a H₂O/R–OH ligand at the axial position. The different redox stabilities of these complexes, relative to that of dissolved O₂ in the aqueous solution, is analyzed via ⁵¹V NMR. It is shown that the oxidation rate is the highest for the oxalato-VO²⁺ complexes. In addition, the stability of the VO²⁺ complexes can be drastically improved by evacuation of the dissolved O₂ from the solution and subsequent storage in a N₂ ambient atmosphere. The vanadium oxide phase formation process, starting with the chemical solution deposition of the aqueous solutions and continuing with subsequent processing in an ambient 0.1% O₂ atmosphere, differs for the two complexes. The oxalato-VO²⁺ complexes turn into the oxygen-deficient crystalline VO₂ B at 400 °C, which then turns into crystalline V₆O₁₃ at 500 °C. In contrast, the citrato-VO²⁺ complexes form an amorphous film at 400 °C that crystallizes into VO₂ M1 and V₆O₁₃ at 500 °C