Electron Spin Resonance and Electron Spin Echo Modulation Evidence for the Isomorphous Substitution of Ti in TAPO-5 Molecular Sieve

Abstract

New evidence for the isomorphous substitution of titanium into titanium aluminophosphate molecular sieve of structure type AFI (TAPO-5) is reported by combined electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopic studies. Synthesized TAPO-5 and TiO2/SAPO-5 materials are compared. These materials were activated and then reduced by γ-irradiation at 77 K or by CO at ambient to high temperatures. γ-irradiation at 77 K of TAPO-5 produces an ESR signal characterized by g1 = 1.965, g2 = 1.920, and g3 = 1.879 best explained as arising from trivalent titanium at a framework site together with a strong signal due to V centers. γ-irradiation of TiO2/SAPO-5 gives an ESR signal with g⊥ = 1.971 and g∥ = 1.907 typical of Ti(III) in octahedral symmetry. Upon CO reduction TAPO-5 gives a spectrum with rhombic g parameters g1 = 1.969, g2 = 1.954, and g3 = 1.923, which are different from the corresponding parameters of the Ti(III) species produced by γ-irradiation. CO reduction of TiO2/SAPO-5 ...