Elucidating the Nature and Reactivity of Ti Ions Incorporated in the Framework of AlPO-5 Molecular Sieves. New Evidence from³¹P HYSCORE Spectroscopy

Abstract

The incorporation of Ti ions within the framework of aluminophosphate zeotype AlPO-5 and their chemical reactivity is studied by means of CW-EPR, HYSCORE, and UV-vis spectroscopies. Upon reduction, Ti(3+) ions are formed, which exhibit large (31)P hyperfine couplings, providing direct evidence for framework substitution of reducible Ti ions at Al sites.

The interaction of H2O2 with TiAlPO-5 molecular sieves: probing the catalytic potential of framework substituted Ti ions

“…Moreover, when UHP is used instead of 20 hydrated H 2 O 2 , the presence of a residual H + on an oxygen framework in the first coordination shell is observed. These observations together with previous findings reported by some of us 16,17 allow proposing a mechanistic pathway for this reaction, casting the role of Ti ions localized at Al 3+ sites as prominent actors in the redox activity of TiAlPO materials. It seems natural to associate the redox activity of this specific Ti ion with the fact that the Ti 3+ reduced state has the natural formal charge for this specific site.…”

“…This situation holds for the three different cases, thus regardless to the chemical pathway followed in the generation of the superoxide anion, this is stabilized on the same site, showing that the reactivity of the system is intimately linked to the Ti 4+ /Ti 3+ redox activity of Ti ions at Al 3+ sites, in good agreement 15 with previous evidences. 16,17 While the 31 P hyperfine interactions do not change for the three different systems, the 1 H hyperfine interaction depends on the generation method. In particular a distinct difference is observed in the maximum proton coupling for the O 2 -generated via reaction of the sample with anhydrous H 2 O 2 (≈10 MHz) and 30% aqueous solution H 2 O 2 (≈3.5 MHz).…”

“…15 Advanced EPR methods, in particular hyperfine techniques (HYSCORE and ENDOR) provide 35 invaluable information about both structural and chemical aspects of the catalytically active paramagnetic centers, such as their geometry, localization and electronic structure, which ultimately governs their chemical reactivity and their catalytic potential. We recently reported a series of papers [16][17][18] devoted to the detailed spectroscopic characterization of the active sites of TiAlPO-5 systems using a combination of advanced EPR methods and DRUV-Vis spectroscopy to monitor the nature of reduced titanium species. In particular we focused on reduced Ti 3+ species and their coordinating chemistry, providing firm evidence for Ti incorporation into the ) while Ti ions at P sites (where P has a formal +5 charge) are much harder to reduce, but can act as coordinating centers because of its unsaturated nature.…”

Predictive design of engineered multifunctional solid catalysts

“…72 EPR is also a potent technique to probe the local structural environment of paramagnetic transition-metal ions. 73 In partnership with our collaborators, 3,74 we have employed continuous wave (CW) and electron spin echo measurements to elucidate the oxidation states and coordination geometries of our CoTiAlPO-5 and VTiAlPO-5 catalysts, that were particularly active in selective oxidation reactions. In order to quantitatively probe the local environment of the individual metal ions, with a view to studying the nature of the transitionmetal substitution mechanism (see Scheme 2), an array of hyperfine techniques ( Figure 7D) associated with EPR can be utilised; namely Electron Spin Echo Envelop Modulation (ESEEM), Electron Nuclear Double Resonance (ENDOR), Hyperfine Sublevel Correlation (HYSCORE).…”

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

“…[19][20][21][22][23] However, recent work from some of us gave clear-cut evidence for the presence of reducible Ti ions at Al sites in TiAPO-5, suggesting that the framework neutrality can be obtained by replacement of Al and P couples by two Ti ions. 24,25 to form Me 4+ clusters reminiscent of the Si islands in SAPOs.…”