TITANIUM SITES IN TITANIUM SILICALITES - AN XPS, XANES AND EXAFS STUDY

Three titanium silicalite samples with MEL structure and different Ti loadings have been examined by Near-Edge and Extended X-ray Absorption Fine Structure techniques. Titanium species are present in these zeolites as monomeric and dimeric species and have three types of coordination: tetrahedral, square pyramidal, and octahedral. The relative content of these species depends on the overall Ti loading. The most striking finding of this investigation is that the TiOx units and SiO4 tetrahedra are linked through an edge than via the usual comer-sharing mode. Framework Ti species are thus better described as structural defects rather than T atoms isomorphously substituted for Si in the zeolite lattice.