Electron Spin Resonance Evidence for Isomorphous Substitution of Titanium into Titanosilicate TiMCM-41 Mesoporous Molecular Sieve

Abstract

New evidence for the framework substitution of titanium into TiMCM-41 mesoporous molecular sieve is found from electron spin resonance (ESR) studies. TiMCM-41 samples with varying Si/Ti ratios have been synthesized as have titanosilicate TS-1, siliceous MCM-41, AlMCM-41, and Ti−AlMCM-41 in the last of which titanium ions are incorporated by ion exchange. These materials were examined by ESR spectroscopy after a thermal activation treatment followed by γ-irradiation at 77 K. γ-Irradiation at 77 K of TiMCM-41 having a high Si/Ti ratio produces a strong orthorhombic ESR signal due to V centers and an axial signal with g ∥ = 1.971 and g ⊥ = 1.901 best explained as arising from trivalent titanium situated at a framework tetrahedral site. An upper limit for the substitution of titanium into a framework tetrahedral site is observed. As the titanium concentration in the synthesis gel increases, the resulting material contains both tetrahedral and octahedral titanium. γ-Irradiation of ion-exchanged Ti−AlMCM-41 gives an axial ESR signal characterized by reverse g values g ∥ = 1.898 and g ⊥ = 1.967 typical of Ti(III) in distorted octahedral symmetry. Both TiMCM-41 and Ti−AlMCM-41 differ in their behavior toward interaction with various adsorbate molecules. The Ti(III) species observed in TiMCM-41 after adsorption of D2O, CO, NH3, and CD3OH changes its ESR characteristics from that of the original Ti(III) species observed in the absence of these adsorbates. The ESR characteristics of the Ti(III) species in Ti−AlMCM-41 after adsorption of the above adsorbates are different from those of TiMCM-41, indicating that the Ti in these two materials are in different coordination environments. This independently supports the different g tensors for Ti(III) in TiMCM-41 versus Ti−AlMCM-41.

Effect of preparation parameters on SrTiO3±δ catalyst for the flameless combustion of methane

“…3), characterised by S = 1/2, g // < g ⊥ < g e = 2.002319, can be attributed to Ti 3+ paramagnetic ions, investigated since long time by EPR in zeolites, silicalites, titanosilicates and other systems [6][7][8][9][10][11][12][13]. Indeed, it has been calculated that the Ti 3+ g values, though strongly dependent on the crystal field, are always lower than g e , the only exception being, possibly, Ti 3+ in either a tetragonally compressed tetrahedral or in a trigonally distorted octahedral symmetry (unpaired electron in the d 2 Z orbital, g // ≈ g e ).…”

“…This assignment is supported also by the fact that the intensity of the B line reduces during the sample oxidation, presumably because of the oxidation of Ti 3+ to Ti 4+ . Usually Ti 3+ ions present anisotropic EPR signals in solid state centres [9,11], their g values strongly depending on the crystal field parameters. Both g // > g ⊥ and g // < g ⊥ cases have been observed [12] with different octahedrally co-ordinated Ti 3+ complexes in frozen solution, in the presence of different crystal fields, and a nearly isotropic single EPR feature has been reported [7] with titanium-exchanged zeolites.…”

Titanium species in titanium silicalite TS-1 prepared by hydrothermal method

“…Tuel et al [8] [9] use ESR to study Ti-MCM-41 sample after a thermal activation treatment followed by ␥-irradiation at 77 K. But, it has not been reported to use ESR to directly study the interaction of titanium silicalite and H 2 O 2 .…”

Mesoporous gadolinium doped titania photocatalyst through an aqueous sol–gel method

“…8. All the samples showed bands around 300-350 nm, due to presence of segregated anatase crystallites [56]. The undoped titania and 1 mol% Gd doped titania shows a 'red shift' due to the presence of rutile in the system which have a band gap of 3 eV.…”