Electron spin resonance evidence for isomorphous substitution of titanium into titanosilicate TiMCM-48 mesoporous materials

Abstract

Hydrothermal synthesis of TiMCM-48 with varying Si/Ti ratios has been achieved. Comparative electron spin resonance (ESR) studies between synthesized TiMCM-48 and ion-exchanged Ti–AlMCM-48 show significant differences with respect to location and adsorbate interactions. Gamma-irradiation at 77 K of TiMCM-48 after a thermal activation treatment produces a strong ESR signal due to V centers and an axial signal with g_∥ = 1.976 and g_⊥ = 1.903 for isolated trivalent titanium in framework tetrahedral sites. An axial ESR signal with reversed g values of g_∥ = 1.900 < g_⊥ = 1.977 typical of Ti(III) with distorted octahedral symmetry is observed after γ-irradiation at 77 K of ion-exchanged Ti–AlMCM-48. Both TiMCM-48 and Ti–AlMCM-48 also differ in their behavior towards various adsorbates. The Ti(III) species observed in TiMCM-48 and Ti–AlMCM-48 after adsorption of D₂O, CO, NH₃ and CH₃OD change their ESR characteristics indicating significant interaction with adsorbates. The ESR characteristics of the Ti(III) species in TiMCM-48 after adsorption of these adsorbates are different from those of Ti(III) in Ti–AlMCM-48. The contrasting characteristics of Ti(III) in TiMCM-48 and Ti–AlMCM-48 suggest that the titanium ion is in a framework site in TiMCM-48.