CHARACTERIZATION OF TITANIA SILICALITES

Titanium-substituted silicalites, prepared from two different silicalite precursors, are characterized using Raman spectroscopy, X-ray absorption spectroscopy, pyridine adsorption, and methanol oxidation. The silicalite precursors used are tetraethyl orthosilicate and Ludox AS 40. Two different types of titania species are observed in the titania silicalites. Below 1.6 mole% Ti/(Ti + Si), a dispersed TiO(x) species is present that does not possess a terminal Ti=O bond or Bronsted acidity. The (average) structure of the TiO(x) species changes with the titania substitution into the silicalites, but this structural change, apparently, does not affect the reactivity of the catalytic active center during reactions involving liquid water. This TiO(x) species in silicalites is active toward redox reactions, and its reactivity is similar to the titania species present on the surface of amorphous SiO2 (Cab-O-Sil). Above 1.6 mole% Ti/(Ti + Si), TiO2 (anatase) particles are present in addition to the dispersed TiO(x) species. The TiO2 (anatase) particles are not effective in redox reactions and form weak Lewis acid sites. The titania silicalites produced from the different precursors were structurally similar, but differed chemically due to acidic impurities present in Ludox AS 40. The critical factor in determining the reactivities of Ti silicalite and surface titania supported on amorphous silica is the stability of the Ti-O-Si bond in the presence of liquid water.