Electron spin resonance and electron spin echo modulation evidence for the isomorphous substitution of Ti in TAPO-5 molecular sieve

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 gamma-irradiation at 77 K or by CO at ambient to high temperatures. gamma-irradiation at 77 K of TAPO-5 produces an ESR signal characterized by g(1) = 1.965, g(2) = 1.920, and g(3) = 1.879 best explained as arising from trivalent titanium at a framework site together with a strong signal due to V centers. gamma-irradiation of TiO2/SAPO-5 gives an ESR signal with g(perpendicular to) = 1.971 and g(parallel to) = 1.907 typical of Ti(III) in octahedral symmetry. Upon CO reduction TAPO-5 gives a spectrum with rhombic g parameters g(1) = 1.969, g(2) = 1.954, and g(3) = 1.923, which are different from the corresponding parameters of the Ti(LII) species produced by gamma-irradiation. CO reduction of TiO2/SAPO-5 produces an ESR signal with similar axial g parameters as observed for the Ti(LII) species produced after gamma-irradiation. These differences in behavior between the two materials are best explained in terms of additional coordination of tetrahedral framework Ti by CO in TAPO-5, while octahedral Ti in TiO2/SAPO-5 is incapable of further coordination with CO. P-31 ESEM for the tetrahedral Ti(III) species in TAPO-5 obtained by gamma-reduction shows one phosphorus atom at 3.9 Angstrom and two phosphorus atoms at 4.8 Angstrom, which supports that titanium substitutes for a framework phosphorus site. Strong Al-27 modulation is observed for this Ti(III) signal, which further confirms the closer proximity of the Ti ions to framework aluminum than to phosphorus.