Switching between classical/nonclassical crystallization pathways of TS-1 zeolite: implication on titanium distribution and catalysis

Abstract

In the MFI zeolite crystallization process, the classical crystallization mechanism based upon the addition of silica species is often concomitant with the nonclassical route that is characteristic of the attachment...

Formation and Stability of μ2-Peroxo on Titanosilicates, Anatase, and Rutile: Implications for Zeotype Catalysts

“…TS-1 B and TiMWW S6) and rutile-like in the TiMWW B (peak at 4992 eV). So far, the μ 2 -peroxo has only been invoked on framework or framework-associated sites, 27,28 but could in principle also form on the surface of (extra-framework) TiO 2 . We thus investigated the formation and stability of μ 2 -peroxos on anatase and rutile nanoparticle references (for details, see ESI S1.4).…”

“…Indeed, closer inspection of the Ti K edge XANES white-line region and comparison with anatase and rutile nanoparticle references indicates that the extra-framework TiO 2 present in TS-1 B and TiMWW A is anatase-like (peak at 4987 eV, see ESI Figures S5 and S6) and rutile-like in the TiMWW B (peak at 4992 eV). So far, the μ 2 -peroxo has only been invoked on framework or framework-associated sites, 27,28 but could in principle also form on the surface of (extra-framework) TiO 2 . We thus investigated the formation and stability of μ 2 -peroxos on anatase and rutile nanoparticle references (for details, see ESI S1.4).…”

“…DFT calculations suggest that these peroxo sites correspond to off-cycle reaction intermediates, but that upon activation with a second H 2 O 2 molecule highly reactive hydroperoxo species are generated due to H-bonding with the zeolitic framework. 27,28 At present, little is known regarding the possible formation and stability of the corresponding peroxo species on extraframework TiO 2 , even if peroxotitanium complexes 29 have been invoked as precursors in both anatase and rutile formations. 30,31 We thus decided to investigate the formation and the stability of μ 2 -peroxo groups on TiO themselves and a family of titanosilicates with different content of extra-framework TiO 2 using 17 O ssNMR spectroscopy as a methodology to identify peroxo species.…”

The Current Understanding of Mechanistic Pathways in Zeolite Crystallization

“…313,652−654 Similar to the effects of Si and Al source selection, the choice of heteroatom reagent has the potential to direct growth pathways from classical to nonclassical (or vice versa). 655 Investigation of these systems has made it clear that the effects of heteroatoms at a molecular level are manifold, influencing electrostatics, oligomerization, nanoparticle stability, 656 metallosilicate speciation, 619,632,657−660 bond angles, 657,661 OH − consumption, and precipitation of impurity phases, 87,652,662,663 among others. Several groups have suggested many fundamental similarities between the mechanisms underlying aluminosilicate syntheses and those containing heteroatoms, and it is believed that only specific pathways are altered by substituting Al with an alternative element.…”

“…The addition or substitution of heteroatoms into zeolite growth mixtures has led to disparate effects on synthesis mixtures, which include altering crystallization kinetics, ,, crystal size, ,, phase selection, ,, and other phenomena such as IZT. These effects are in addition to that which is generally of greatest interest and the reason why heteroatoms are typically added to growth mixtures: the ability of heteroatoms to alter acid properties, including density, strength, zoning, or siting. ,− Similar to the effects of Si and Al source selection, the choice of heteroatom reagent has the potential to direct growth pathways from classical to nonclassical (or vice versa) . Investigation of these systems has made it clear that the effects of heteroatoms at a molecular level are manifold, influencing electrostatics, oligomerization, nanoparticle stability, metallosilicate speciation, ,,− bond angles, , OH – consumption, and precipitation of impurity phases, ,,, among others.…”