CHARACTERIZATION OF TITANIA SILICALITES

Titanium-substituted silicalites, prepared from two different silicali te precursors, are characterized using Raman spectroscopy, X-ray absor ption spectroscopy, pyridine adsorption, and methanol oxidation. The s ilicalite precursors used are tetraethyl orthosilicate and Ludox AS 40 . Two different types of titania species are observed in the titania s ilicalites. Below 1.6 mole% Ti/(Ti + Si), a dispersed TiO(x) species i s present that does not possess a terminal Ti=O bond or Bronsted acidi ty. The (average) structure of the TiO(x) species changes with the tit ania substitution into the silicalites, but this structural change, ap parently, does not affect the reactivity of the catalytic active cente r during reactions involving liquid water. This TiO(x) species in sili calites is active toward redox reactions, and its reactivity is simila r 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 pr esent in addition to the dispersed TiO(x) species. The TiO2 (anatase) particles are not effective in redox reactions and form weak Lewis aci d sites. The titania silicalites produced from the different precursor s were structurally similar, but differed chemically due to acidic imp urities present in Ludox AS 40. The critical factor in determining the reactivities of Ti silicalite and surface titania supported on amorph ous silica is the stability of the Ti-O-Si bond in the presence of liq uid water.