STRUCTURAL STUDY OF TITANIUM-MODIFIED SILICA CATALYSTS BY EXAFS AND XANES

Silica modified by Lewis acids TiCl4, TiCl2((OPr)-Pr-i)(2) and Ti((OPr )-Pr-i)(4) have been studied by means of EXAFS and XANES at the Ti K e dge. These solids are efficient catalysts in the Diels-Alder reactions , but their catalytic activity depends on the Lewis acid used to modif y the silica. The treatment of silica with TiCl4 gives rise to a very active catalysts of Diels-Alder cycloaddition. The XAS results show ho w the Ti environment, and hence the catalytic actvity of the solid, de pend on the Ti compound used as precursor. Titanium is bonded to four oxygens in the case of Ti((OPr)-Pr-i)(4) while chlorine is present in the first coordination sphere when TiCl4 is used. The catalyst prepare d from TiCl2((OPr)-Pr-i)(2) gives rise instead to the formation of sma ll size anatase particles.