EPR EVIDENCE FOR THE ISOMORPHOUS SUBSTITUTION OF TITANIUM IN SILICALITE STRUCTURE

TS-1 zeolite has been synthesized following the literature procedure and characterized via X-ray diffraction, IR spectrometry and 29Si NMR and shown to match previous findings. In addition titanium-exchanged HZSM-5 was prepared. Both solids were examined by EPR spectroscopy following CO reduction. Whereas the titanium-exchanged ZSM-5 exhibited the usual EPR signal consistent with trivalent titanium in tetragonally distorted octahedral symmetry with g⊥ = 1.97 and g∥ = 1.94, the Ti-substituted silicalite exhibited an axially symmetric signal with a split g∥ component (g∥)1 = 1.98, (g∥)2 = 1.97 and g⊥ = 1.913 best interpreted as arising from a trivalent titanium in a tetrahedral coordination, indicating that the precursor tetravalent ion was indeed in a T site. Reactivity towards O2 was also monitored and differences in the gzz values for O2- adsorbed on titanium confirm the different crystal fields prevailing at each titanium siting. It is also concluded that reduction of framework titanium with CO does not result in its removal from the lattice. © 1990.