Impregnation treatments of TS-1 catalysts and their relevance in alkene epoxidation with hydrogen peroxide

Ti-containing MFI (TS-1) samples were synthesized and then modified with small amounts (0.3-1.7 wt.%) of alkali, alkaline-earth and lanthanide ions. UV-Vis reflectance spectra for all the samples revealed absorption at 215 nm, due to the O2- --> Ti4+ charge transfer transition of tetrahedrally coordinated titanium incorporated into the silicalite framework. Similarly, the binding energy of the Ti 2p(3/2) Core-level for all the M/TS-1 samples appeared at ca. 460.0 eV, which is also indicative of Ti(IV) coordinated to oxide anions in a tetrahedral symmetry. The catalysts were tested in the epoxidation reaction of 1-octene, using aqueous hydrogen peroxide as an oxidant agent at 353 K. The conversion of hydrogen peroxide was slightly lower for the M/TS-1 samples than for the titanium-silicalite-1 (TS-1) reference and selectivity to epoxide was higher when the metal cation (Mn+) was added. A good correlation between the formation of secondary reaction products in the 1-octene epoxidation and Bronsted acid sites (measured by the infrared spectra of adsorbed deuterated acetonitrile) was found. This finding is explained as being due to the neutralization of the surface acidity of the TS-1 zeolite by the metal oxide (MO) incorporated, with the subsequent inhibition of the solvolysis reactions of the epoxide on the acid sites. (C) 2002 Elsevier Science B.V. All rights reserved.