Catalytic properties of molybdenum-containing zeolites in epoxidation reactions: I. Catalysts preparation and characterization

Abstract

Molybdenum ions have been introduced into a Y-type zeolite by using a solid-solid exchange method, in which the ultrastable zeolite or normal hydrogen zeolite was mixed with MoOCl 4 and heated to 400 °C. The physicochemical properties of the resulting MoHY u, MoHY, and MoCoHY zeolites were investigated by infrared, EPR and photoelectron spectrscopy and by X-ray diffraction techniques. Comparison of the XPS signals of Mo in an impregnated sample with those of an ionexchanged sample confirms that the molybdenum ions were indeed exchanged into zeolites. The binding energies of the Mo 3 d doublet at 232.7 and 235.8 eV show that the Mo ion was mainly present as Mo(VI); however, EPR spectra of the fresh, dehydrated molybdenum-exchanged zeolites reveal that about 3.5% of the molybdenum was present as the Mo(V) ion. After being used as a catalyst for the epoxidation of cyclohexene, all of the low-valent Mo ions were oxidized to Mo(VI), whereas there was no change in the oxidation state of Co(II) ions. A band at 900 cm −1 in the infrared spectra of MoHY and MoCoHY zeolites is attributed to the MoO bond vibration. Based on the spectroscopic data, two structures of molybdenum ions in Mo-exchanged zeolites are postulated. Structural analysis of the zeolites by X-ray diffraction and infrared spectrscopy indicates that the solid-phase exchange reaction resulted in some loss in crystallinity for all of the Mo-exchanged zeolites.