XAFS STUDY OF TI-SILICALITE - STRUCTURE OF FRAMEWORK TI(IV) IN THE PRESENCE AND ABSENCE OF REACTIVE MOLECULES (H2O, NH3) AND COMPARISON WITH ULTRAVIOLET-VISIBLE AND IR RESULTS

X-ray absorption at the Ti K edge (both XANES and EXAFS) of a very pure Ti-silicalite containing a small fraction of Ti(IV) substituting for Si(IV) has been performed in order to Study the effect of the presence or absence of ligands such as H2O and NH3 on the Ti(IV) coordination sphere. In particular, the effect of an outgassing treatment at 400 K and of the interaction with NH3 has been studied and described in detail. It has been found that the Ti(IV) is fourfold coordinated in the samples outgassed at 400 K and expands its coordination sphere number with action of adsorbates. The process is partially reversible at room temperature. The pre-edge absorption of a well-manufactured Ti-silicalite has been compared with a Ti-silicalite containing extraframework Ti and with other reference compounds such as anatase and amorphous SiO2/TiO2. Further information has been obtained from EXAFS determination of first-shell bonding parameters.