JOURNAL OF CATALYSIS 173, 423–432 (1998) ARTICLE NO. CA 971918 Titania–Silica Mixed Oxides: Influence of Reaction Additives on the Epoxidation of (E )-2-Hexen-1-ol M. Dusi, T. Mallat, and A. Baiker 1 Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, ETH-Z entrum, CH-8092 Z ¨ urich, Switzerland Received June 16, 1997; revised August 18, 1997; accepted September 11, 1997 The epoxidation of the allylic alcohol (E)-2-hexen-1-ol has been investigated using an amorphous titania–silica aerogel with a TiO 2 content of 20 wt%. The catalyst was prepared by the alkoxide–sol– gel route and the solvent was removed by semicontinuous extraction with supercritical CO 2 at 313 K. The epoxidation was carried out with tert .-butylhydroperoxide as oxygen source. Epoxide ring open- ing by the acid-catalyzed attack of (E)-2-hexen-1-ol and formation of high-molecular-weight by-products were observed as the main side reactions underoptimized conditions the competitive oxidation of the alcoholic function was negligible. The rate and selectivity of the epoxidation reaction could significantly be improved by careful drying of the aerogel in situ at 473 K, by azeotropic distillation of toluene from the reaction mixture, and/or by addition of solid bases (NaHCO 3 , zeolite 4A) to the reaction mixture. High epoxide selec- tivities up to 90–98%were obtained at 50–90%peroxide conversion within 5–60 min, using a catalyst/hexenol ratio of 3.3 wt%. The ac- tivity and selectivity of the aerogel is comparable to that of TS-1. The positive influence of solid bases was found to be due to their minor solubility in the weakly polar medium and to the neutralization of the acidic sites on the aerogel. Contrary to earlierobservations, bases can increase or decrease the rate, depending on the water content of the system. Zeolite 4A acts as a base and a drying agent. c  1998 Academic Press INTRODUCTION The development ofTi-and Si-containingheterogeneous epoxidation catalysts has received considerable attention. The first truly heterogeneous and active catalyst was an amorphous silica-supported titania (0.5–5 wt%), which has been used for the manufacture of propylene oxide by Shell (1). The invention of Ti-substituted molecular sieves (TS-1, TS-2), which can use the environmentally friendly H 2 O 2 as oxygen source, opened a new field in heterogeneously catalyzed oxidation reactions (2). The application of TS-1 is limited to relatively small reactants that are able to reach the active sites located in the microporous channels. This limitation has been overcome with large and ultralarge- 1 To whom correspondence should be addressed. Fax: +41-1-632 11 63. E-mail: baiker@tech.chem.ethz.ch. pore Ti-containing zeolites isomorphous to zeolite β or MCM-41 (3), though their activity and selectivity to epox- ide are not as outstanding as those of TS-1 (4). Titania–silica mixed oxides derived by the solution sol–gel method have also been proposed for epoxidation. Xerogels with microporous structure revealed only low to moderate activity in the epoxidation of unfunctionalized olefins (5, 6). Amorphous microporous silicates with dif- ferent Ti loadings were tested in 1-hexene and 1-octene epoxidation and the turnover frequency was found to be highest for silicates with low Ti content (4% TiO 2 ) (7). Ti- containing mesoporous silica was more active and selective than Ti-β and TS-1 in the epoxidation of cyclohexene using aqueous H 2 O 2 as oxidant (8). A catalyst, selective and active with aqueous H 2 O 2 ,a peroxide that normally cannot be used with the rather hy- drophilic titania–silica mixed oxides, was obtained by graft- ing TiF 4 on silica (9). On the other hand, incorporation of B 3+ and Al 3+ resulted in loss of activity and in diol forma- tion (10). Titania–silica mixed oxides with a Ti loading of 10–20 wt% , derived by the sol–gel method, provided good results in the epoxidation of 1-octene and cyclohexene us- ing tert.-butylhydroperoxide (11). The best epoxide yield reported was 73.6% along with a peroxide selectivity of 76.3%. A titania–silica mixed oxide catalyst prepared by coprecipitation was also tested in the epoxidation of 1-hexene,usingalkylhydroperoxidesaswellasH 2 O 2 asoxy- gen source, but both reaction rate and selectivity were very low (12). Recently, highly active and selective titania–silica has been prepared by the sol–gel method using tetramethoxy- silicon and tetraisopropoxytitanium stabilized with acetyl acetone (13). Drying of the gel was found to be a crucial parameter for the catalytic activity of these mixed oxides. Removal of the solvent by semicontinuous extraction with supercritical CO 2 at low temperature preserved the high Ti dispersion in the silica matrix and the mesoporous struc- ture ofthe aerogels.These catalystsproved to be superior to TiO 2 -on-SiO 2 , TS-1, and titania–silica xerogels in the epox- idation of cyclic olefins (14–17). Epoxide selectivities up to 423 0021-9517/98 $25.00 Copyright c  1998 by Academic Press All rights of reproduction in any form reserved.