Structural study of titanium-modified silica catalysts by EXAFS and XANES

Silica modified by Lewis acids TiCl 4 , TiCl 2 (O i Pr) 2 and Ti(O i Pr) 4 have been studied by means of EXAFS and XANES at the Ti K edge. These solids are efficient catalysts in the Diels-Alder reactions, but their catalytic activity depends on the Lewis acid used to modify the silica The treatment of silica with TiCl 4 gives rise to a very active catalysts of Diels-Alder cycloaddition. The XAS results show how the Ti environment, and hence the catalytic actvity of the solid, depend on the Ti compound used as precursor. Titanium is bonded to four oxygens in the case of Ti(O i Pr) 4 while chlorine is present in the first coordination sphere when TiCl 4 is used. The catalyst prepared from TiCl 2 (O i Pr) 2 gives rise instead to the formation of small size anatase particles.