Selective oxidation of 1-butane over silica-supported Cr(VI), Mo(VI), and W(VI) oxides

The role of surface structure and cation red-ox nature was investigated for the selective oxidation of 1-butene over SiO2-supported Mo(VI), Cr(VI), and W(VI) oxide catalysts. X-ray diffraction and Raman spectroscopy were used to characterize the surface structures over the different catalysts. The surface structure of Mo(VI)/SiO2 was controlled by using different preparation methods. An aqueous route using (NH4)(6)Mo7O24 led to a catalyst that had dispersed Mo along with small crystallites of MoO3. Organometallic routes using (C5H5)(2)Mo-2(CO)(6) and Mo(C3H5)(4) led to completely dispersed Mo(VI)/SiO2. Cr(NO3)(3) was used to prepare completely dispersed Cr(VI)/SiO2. In the case of W(VI)/SiO2, the dispersion depended on the type of SiO2 employed. The activity to formation of 1,3-butadiene revealed a structure effect for the Mo(VI)/SiO2 catalysts, with crystalline Mo(VI) having a higher activity than dispersed Mo(VI). The red-ox ability of the supported cation was seen to have a dominating effect for this reaction. The turnover number for the formation of 1,3-butadiene followed the order Cr(VI)/SiO2 > Mo(VI)/SiO2 >> W(VI)/SiO2, which is in agreement with the red-ox ability Cr > Mo > W. The importance of red-ox ability is discussed in terms of the allylic abstraction mechanism that is involved in selective oxidation. (C) 1998 Academic Press.