Synthesis of amorphous silica-supported titanium catalysts with triethanolamine titanate for alkene epoxidation with dilute hydrogen peroxide

M. C. Capel-Sanchez, J.M. Campos-Martín, J. L. G. Fierro Instituto de Catálisis y Petroleoquímica, CSIC, Campus Cantoblanco, 28049 Madrid, Spain. http://www.icp.csic.es/eac/index.htm

Introduction Despite numerous reports in the literature, the epoxidation of terminal alkenes remains a challenge in petrochemistry. Many different methods have been developed for the preparation of epoxides. Among the non-zeolitic substrates, Ti–SiO2- supported catalysts remain prominent for the effective in the epoxidation of alkenes with organic hydroperoxides, though it is generally believed that they do not effectively epoxidize alkenes with hydrogen peroxide. Nevertheless, we have reported a very simple route for the preparation titanium-silica-supported catalysts which are very active and selective in the epoxidation of alkenes with hydrogen peroxide [1,2]. Along the preparation, the tetra-isopropyl titanate precursor reacts with moisture in air that difficult catalyst synthesis. In this work, we describe the preparation of highly active and selective catalysts employing triethanolamine titanate as titanium precursor.

Results and Discussion Catalysts were prepared as follows: 0.65 g of commercial triethanolamine titanate solution (80 % in 2-propanol) was added to 25 ml of 2-propanol heated at 353 K; then 5 g of silica (Grace Davison, XPO-2407) were added and the suspension stirred for 2 h. The solid was filtered and washed three times with 25 ml of 2-propanol. Two different replicas were prepared. To understand the mechanism by which the titanium precursor is anchored to the silica surface, these solids were characterized by elemental analysis, FT-IR of self-supported wafers, DRS UV-Vis and X-ray photoelectron spectroscopy (XPS) techniques. Aliquots of the resulting materials were calcined at 773 K, and then used in the epoxidation of 1-octene. In a typical run, a suspension of alkene (0.2 mol), tert- butanol (11 g) and 1 g of catalyst was heated at 333 K, and then 4 g of an organic solution of 5 wt % of H2O2 (in 1-phenylethanol) were added to the reaction vessel. The organic compounds were analysed by GC-FID (Hewlett Packard 6890-plus, equipped with a HP-WAX capillary column). The hydrogen peroxide was measured by standard iodometric titration. Elemental analyses showed that composition of replica samples was almost identical, indicating reproducibility of the synthesis. On the other hand, the atomic ratio Ti/N, in two samples, was unity, as it is in the titanium precursor. However, carbon contents were higher than the estequiometric value probably due to remaining solvent adsorbed on the surface of the solid. FT-IR spectra of the outgassed specimens under high vacuum at room temperature showed differences with that of the calcined counterpart (Fig. 1). Calcined sample showed a band at 3700 cm-1 due to isolated silanol groups, which was absent in the non-calcined one, this effect could be due to an interaction between titanium precursor species and surface OHs. Non-calcined 723 K samples showed several overlapping peaks between 3000 to 2800 cm-1 typical of C-H vibrations in organic 623 K compounds, and a peak centred at 1500 cm-1 attributed to titanium bonded with alkoxide species. To 523 K evaluate the stability of these compounds the sample was

Absorbance (AU) outgassed at 523, 623 and 723 K. FT-IR spectra of outgassed samples RT at 523 and 623 K resulted quite similar to sample outgassed at room temperature. This is clear indication Calcined of the strong interaction of adsorbed species. The peaks due to adsorbed

4000 3500 3000 2500 2000 1500 titanium species disappeared upon

-1 outgassing at 773 K. Wavenumber (cm ) DRS UV-Vis spectra of both Figure 1 FT-IR spectra of solids outgassed calcined and non-calcined samples under high vacuum were clearly different. While calcined sample showed a band centred at 225 nm, typical of titanium in a tetrahedrally hydrtated environment, the non-calcined counterpart exhibited a band at 300 nm typical of titanium in octahedral coordination. The binding energy of Ti2p core-levels of samples outgassed “in situ” exhibited only one peak at 459.0 eV, associated to titanium in a distorted octahedral- (or even penta-) coordination. The calcines sample exhibited, however, a component at 460.0 eV, which is typical of titanium in a tetrahedral environment. Quantitative surface analysis of the non-calcined sample showed a C/N atom ratio of 6 and a N/Ti ratio slightly higher than 1. Characterization data clearly indicated that titanium precursor reacts with the support surface and triethanolamine remains coordinated with Ti until sample calcination. These catalysts exhibited high hydrogen peroxide conversion and high selectivity to epoxide, a behaviour similar to that of the Ti/SiO2 ex-tetra-isopropyl titanate [1,2].

References 1. M. C. Capel-Sanchez J. M. Campos-Martin, J. L. G. Fierro, M. P. de Frutos, A. Padilla Polo, Chem. Commun., (2000) 855-856 2. M. C. Capel-Sanchez J. M. Campos-Martin, J. L. G. Fierro, M. P. de Frutos, A. Padilla Polo, XVIII Simp. Iber. Catal. (2002)