Comparison of liquid-phase olefin epoxidation over MoOx inserted within mesoporous silica (MCM-41, SBA-15) and grafted onto silica
Molybdenum(VI)-containing siliceous-MCM-41 and -SBA-15 mesoporous molecular sieves and silica-supported molybdenum(VI) complexes have been prepared according to an original pathway by the reaction between low-cost Mo(VI) peroxo species and silica precursors (tetraethylorthosilicate), silica beads (pure SiO2) or precipitated silica in an acidic and peroxidic medium (peroxo route). Hydrogen peroxide is added to avoid the formation of iso-(or hetero-)polyoxometalates in the direct one-pot synthesis of mesoporous solids, or in the case of pure SiO2. The resulting calcined materials have been characterized by chemical analysis, X-ray powder diffraction, nitrogen sorption isotherms, TEM and EDX analysis, UV–vis diffuse reflectance spectroscopy, Raman spectrometry and catalysis tests. The peroxo routes lead to inserted and/or supported molybdenum oxide zones which are restricted in size due to the porous structure of the silica-based materials and to the involment of low-condensed and even mononuclear oxoperoxo species. This allows to support MoOx species in a dispersion which cannot be achieved with the systems using heptamolybdate [Mo7O24]6− salts or heteropolyoxometalates, which favor the anchoring of MoO3 clusters. Catalysis tests show that the very dispersive structure of the MoOx-siliceous-MCM-41, -SBA-15 and MoOx/SiO2 promotes the generation of fairly active oxidation catalysts for liquid-phase olefin epoxidation (cyclooctene, R-(+)-limonene from ambient to 40 °C) using anhydrous tert-butylhydroperoxide (TBHP) as oxidant. Under specific conditions, leaching of redox-active species has been minimized (less than 2 ppm) especially with MoOx/SiO2 which is proposed as the simplest catalyst for these oxidations. MCM-41 and SBA-15 do not show clear advantages over silica in epoxidation reactions with TBHP.
Journal: Applied Catalysis A: General - Volume 300, Issue 2, 26 January 2006, Pages 91–99