Alkene Epoxidation with Ethylbenzene Hydroperoxides Using Molybdenum Heterogeneous Catalysts Laura Barrio, † Jose M. Campos-Martín,* ,† M. Pilar de Frutos, ‡ and Jose L. G. Fierro* ,† Instituto de Cata ´lisis y Petroleoquímica, CSIC, Marie Curie 2, Cantoblanco, 28049 Madrid, Spain, and Centro de Tecnología Repsol YPF, A-5, Km. 18, 28930 Mo ´stoles, Madrid, Spain Molybdenum-containing catalysts were prepared by anchoring Mo(VI) groups onto different amino- functionalized silica surfaces and then tested in the epoxidation reaction of 1-octene with ethylbenzene hydroperoxide (EBHP) as an oxidant. Catalytic performance was found to depend on the chemical structure and nature of the functional group. EBHP conversion-time profiles exhibited an S-shaped behavior, suggesting that an induction period is required to develop the active site. Selectivity-conversion plots indicated that selectivities as high as 80% could be obtained at EBHP conversions of about 80% when the Mo complexes were immobilized on a silica substrate functionalized with a diamino-containing moiety. In contrast, surface functionalization with sCOOH was found to be detrimental, as acid groups catalyze EBHP decomposition. Experiments designed to determine possible leaching of molybdenum into the reaction medium during the tests indicated that the catalyst prepared on a triamine-containing substrate exhibited good stability with almost no change in performance after three consecutive runs. The high stability of the Mo(VI) complex in this catalyst was shown by UV-vis and photoelectron spectroscopic techniques. Introduction Oxidation is a core technology for converting petroleum-based materials to useful chemicals of higher oxidation states. Among these products, epoxides are chemical compounds of great importance as intermediate products in the preparation of a wide variety of chemical products ranging from petrochemical compounds to fine chemicals. 1 Although 1,2-epoxiethane can be obtained by a direct reaction in the gas phase between ethylene and oxygen in the presence of silver catalysts, the epoxides of a larger carbon chain, such as propylene oxide (PO), must be obtained by reaction in the liquid phase. A universal method of epoxide production relies on the dehydrochlorination of chlorohydrins by aqueous solution of alkalis. 1,2 This method yields an equimolar quantity of waste aqueous solution of alkali metal chlorides, as well as considerable amounts of waste chlororganic derivatives, which are the products of chlorine addition to the double bond. A safer environmental approach is the oxidation of olefin hydroperoxides in the presence of catalysts. Current worldwide PO production is approximately 6 million metric tons per year, and both technologies are used: hydroperoxides (employed in various forms by Lyondell, Shell, Sumitomo, and Repsol) and chlorohydrins (Dow). 2–4 The core of the hydroperoxide-based PO process comprises the catalyzed epoxidation reaction of propylene with ethylben- zene hydroperoxide (EBHP) to propylene oxide and 1-phenyle- thanol. The active centers of the catalysts used in this process are transition metals in high oxidation states with Lewis acidity, such as Mo(IV), Ti(IV), V(V), and W(IV). 2–6 Epoxide selectivity depends on the Lewis acidity and the oxidation state of the metals. 7 Thus, molybdenum-containing catalysts are best for this process, 8 and most of them are synthesized from dioxomolyb- denum(VI) acetylacetonate, which itself shows good catalytic properties in oxidation reactions. 9 Some studies have addressed molybdenum complexes whose ligands contain donor atoms such as oxygen, sulfur, and/or nitrogen. 10–13 Since the early 1990s and the advent of mesoporous silicas, the development of new solid catalysts and the heterogenization of homogeneous systems for oxidation processes have become very attractive topics. A major drawback of chromium, vana- dium, and molybdenum catalysts coordinated to monodentate ligands is that they usually undergo leaching of the active species into solution, especially in the presence of protic agents such as alcohols or organoperoxides. 14 A simple way to minimize the leaching of the active ingredient is to anchor polydentate ligands onto the support surface, as this type of chelating system offers high coordinative stability for the catalytic ingredient. This general idea has been explored following two different approaches: (i) polymer-supported catalysts and (ii) tethering of molybdenum complexes onto inorganic solids, mainly silica. On the one hand, the immobilization of Mo(VI) catalysts on polymer substrates has been achieved employing boronic acid, aminated polystyrene, polymethacrylate, polybenzimidazole, and polysiloxane resins and then tested in the epoxidation of alkenes in the presence of tert-butyl hydroperoxide. 15–21 On the other hand, Mo(VI) complexes have been tethered onto inorganic substrates such as sol-gel-derived silicates, 22,23 and organo- functionalized MCM-41 24–29 materials have been employed. Among these catalysts, polydentate aminated compounds per- formed best in the reaction and were found to be highly reusable. Against this background, this work was undertaken to investigate both the incorporation of molybdenum into a silica substrate functionalized with polydentate ligands and the effect of the nature of ligands on activity for the epoxidation of 1-octene with EBHP as an oxidant. Certain coordination and surface characteristics of the catalyst were revealed by UV-vis and photoelectron spectroscopy, respectively, and were related to catalyst performance in the target reaction. Experimental Section Catalyst Preparation. Several commercial functionalized silicas were employed as support (SiliaBond from SiliCycle). * To whom correspondence should be addressed. E-mail: jlgfierro@ icp.csic.es (J.L.G.F.), j.m.campos@icp.csic.es (J.M.C.-M.). Tel.: +34915854796 (J.L.G.F.), +345854948 (J.M.C.-M.). Fax: +34 915854760 (J.M.C.-M.). † Instituto de Cata ´lisis y Petroleoquímica. ‡ Centro de Tecnología Repsol YPF. Ind. Eng. Chem. Res. 2008, 47, 8016–8024 8016 10.1021/ie800262x CCC: $40.75  2008 American Chemical Society Published on Web 06/14/2008