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

Mesoporous molybdosilicate, [Mo]-MCM-41, molecular sieves with variable amo unts of molybdenum (VI) have been synthesized in acidic media from MoO3, aq ueous hydrogen peroxide and tetraethyl orthosilicate as the silicon source. This procedure avoids the formation of iso-(or hetero-)polyooxometalates a nd eliminates the need for careful control of the rate of hydrolysis of the metal precursors, e.g. alkoxides, Sometimes involved in the syntheses of t ransition metal-containing mesoporous materials. The parent materials are c alcined in air (60 degrees C h(-1), isothermal at 920 K for 4 h) to decompo se the organic structure-directing agent. The materials have been character ized by chemical analysis, X-ray diffraction and EDS analysis, TEM, Si-29 M AS NMR, W-visible diffuse reflectance spectroscopy and nitrogen sorption is otherms. In contrast to previous preparations, such syntheses give an incre ase 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 epoxid ation with anhydrous tert-butylhydroperoxide/decane/pentane mixtures was us ed as a test; high conversions and selectivities can be obtained at 20 degr ees C. Different leaching behaviors were observed as a consequence of the d ifferent catalyst preparation methods. A comparative IR spectroscopic study was performed to obtain evidence for the formation of [Mo]-OOt-Bu surface species. (C) 1999 Elsevier Science B.V. All rights reserved.