Synthesis and characterization of titanium aluminosilicate zeolites

Abstract

Titanium containing molecular sieves are important catalytic materials with remarkable activity for partial oxidation of organic compounds. Since, Taramasso et al. reported the discovery of TS-1 in 1983, various efforts have been made to understand the origin of the catalytic activity and to obtain new molecular sieves containing titanium in the framework. However, only a few other molecular sieves with silica frameworks have been found to have the catalytic activity so far. Previous works have revealed that the loss of the catalytic activity with other synthesis attempt was due to the formation of a separate $TiO_2$ domain during the synthesis under experimental conditions, high levels of alkali metal cations. In the present work, a new synthesis procedure for titanium-containing aluminosilicate zeolites has been obtained using a clear colloidal dispersion (sol) which was prepared with titanium isopropoxide, water, hydrochloric acid and colloidal silica, Ludox HS-40. Various titanium aluminosilicates such as NaY, KL, offretite, mordenite, ZSM-5 and silicalite have been synthesized from the titania-silica sol thus obtained, following conventional hydrothermal crystallization procedures. All the Ti-zeolites were characterized by X-ray diffraction patterns, infra-red spectroscopy, X-ray absorption spectroscopy and partial oxidation reaction. Infrared spectra of the Ti-ZSM-5 and Ti-silicalite showed a $960cm^{-1}$ band upon ion exchange with $NH_4^+$ and subsequent evacuation at 673 K. The X-ray absorption near edge structure (XANES) obtained at the Ti K-edge of Ti-zeolites showed a pre-edge peak at 4967 eV, which was characteristic of tetra- or penta-coordinated titanium in the framework. The curve fitting for extended X-ray absorption fine structure (EXAFS) gave the Ti-O coordination number of 4.5±0.4 with a distance of 0.190±0.005nm. The Ti-zeolites in the present work showed remarkable catalytic activity for partial oxidation of cyclohexene and 2-butanol usin...