Determination of the environment of titanium atoms in TS-1 silicalite by Ti K-edge X-ray absorption spectroscopy, 29Si and 1H nuclear magnetic resonance

Abstract

This chapter describes determination of the environment of titanium atoms in TS-1 silicalite by Ti Kedge X-ray absorption apectroscopy, Si and H Nuclear Magnetic Resonance. Microporous titanium silicalites of MFI or MEL structures, TS-1 and TS-2 respectively, have attracted many scientists owing to their unique capability to catalyze the oxyfunctionalization reactions of alkanes using H202 as oxidant. Despite the number of new titanium silicalites with larger and larger pore sizes such as Ti-beta, titanium mesoporous materials such as Ti-MCM-41, Ti-MCM-48 or high-surface area amorphous TiO2-SiO2, no other materials were active for this demanding reaction. The presence of micropores might be necessary for oxyfunctionalization to take place, though a specific local environment around titanium might as well be required for these reactions. These materials exhibit an IR band at 960 cm 1. It is assigned to a local vibration mode of the asymmetric stretching of a SiO4 unit linked to a Ti 4+, a V 4+ or a Cr 3+ ion in amorphous or crystallyzed systems. Consequently, this band is not an intrinsic characteristic of titanium silicalite materials. The Ti isomorphous substitution for Si in these systems is a widely shared belief, based on a conjunction of facts obtained from a panel of techniques such as XRD, IR, UV-visible, XPS, EXAFS, and XANES rather than on a direct proof. Recently, the multiple scattering (MS) theory is used to analyze the second shell of neighbors around the titanium.