Using inorganic silicate precursor/molybdenum peroxo complexes/onium salt interfaces in aqueous acidic media to design mesoporous silica with high molybdenum content and high dispersion

Abstract

Mesoporous molybdosilicate, [Mo]-MCM-41, molecular sieves with variable amounts of molybdenum (VI) have been synthesized in acidic media from MoO 3, aqueous hydrogen peroxide and tetraethyl orthosilicate as the silicon source. This procedure avoids the formation of iso-(or hetero-)polyooxometalates and eliminates the need for careful control of the rate of hydrolysis of the metal precursors, e.g. alkoxides, sometimes involved in the syntheses of transition metal-containing mesoporous materials. The parent materials are calcined in air (60°C h −1, isothermal at 920 K for 4 h) to decompose the organic structure-directing agent. The materials have been characterized by chemical analysis, X-ray diffraction and EDS analysis, TEM, 29 Si MAS NMR, UV-visible diffuse reflectance spectroscopy and nitrogen sorption isotherms. In contrast to previous preparations, such syntheses give an increase in the Mo/Si (mol/mol) ratio of up to 0.04 and a greater dispersion of the surface species. For chemical characterization, ( R)-(+)-limonene epoxidation with anhydrous tert-butylhydroperoxide/decane/pentane mixtures was used as a test; high conversions and selectivities can be obtained at 20°C. Different leaching behaviors were observed as a consequence of the different catalyst preparation methods. A comparative IR spectroscopic study was performed to obtain evidence for the formation of [Mo]-OOt-Bu surface species.