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

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 D 2 O, CO, NH 3, and CD 3 OH 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.