A controlled post-synthesis route to well-defined and active titanium Beta epoxidation catalysts

Abstract

This paper describes the successful indirect synthesis of a titanium Beta zeolite. The post-synthesis modification, consisting of separate dealumination and titanation steps, is extensively studied by X-ray diffraction, Fourier-transformed infrared (FT-IR), X-ray photoelectron spectroscopy, diffuse reflectance UV–Vis and Raman spectroscopy. Dealumination degrees up to 90% could be obtained by using oxalic acid as well as nitric acid without any loss in crystallinity or micropore volume. FT-IR shows the formation of silanol nests upon removal of aluminum from the zeolite framework. Subsequently, these silanol nests react with titanium(IV) chloride in the titanation step, resulting in tetrahedral titanium sites. The post-synthesis procedure described allows the preparation of titanium zeolites that are free from TiO 2. The results of the titanation experiments suggest that the vacant sites are first filled with titanium, resulting in tetrahedral, isolated titanium sites, before the formation of TiO 2 starts to occur. As such, the titanium loading can be tuned to about 2 wt.% titanium and, at the same time, the formation of TiO 2 species is prevented. The titanium Beta materials obtained are active and truly heterogeneous epoxidation catalysts. Both with tert-butyl hydroperoxide (under dry conditions) and with aqueous H 2O 2, they effectively catalyze the epoxidation of alkenes under mild conditions. It is demonstrated that TiO 2-free catalysts are highly preferable, since traces of TiO 2 present on the titanium zeolite impair the catalyst performance. In the epoxidation of 1-octene, the Ti–Beta catalysts prepared via the controlled post-synthesis route can easily compete with TS-1 under the same conditions.