Effect of alkali-metal ion addition to silica-supported molybdenum oxide on photocatalysis Photooxidation of propane and propene, and photo-assisted metathesis of propene

Abstract

The effect of adding alkali-metal ions (Li ⁺, Na ⁺, K ⁺, and Rb ⁺) to silica-supported molybdenum oxide (MoO ₃/SiO ₂) has been investigated by photooxidation of propane and propene with gaseous oxygen, and photo-assisted metathesis of propene at room temperature. Over alkali-metal ion modified MoO ₃/SiO ₂, propanone has been mainly produced from propane, and acrolein from propene, whereas over MoO ₃/SiO ₂ a variety of products have been formed, such as ethanal, propene, propanone and propanal from propane, and ethanal and acrolein from propene. The photooxidation selectivities change upon the addition of alkali-metal ions because the electrophilicity of the oxygen atom in the molybdenum species in the excited state ( i. e., it is active for photooxidation) is weakened by the interaction with the alkali-metal ions. In the photo-assisted metathesis reaction of propene, the conversion is lower over alkali-metal ion modified MoO ₃/SiO ₂ than over MoO ₃/SiO ₂. The activity for the metathesis reaction has been related to the reducibility of the surface molybdenum species by CO under UV light irradiation. In addition, the Raman spectra suggest that the surface molybdenum species in alkali-metal ion modified MoO ₃/SiO ₂ is like Mo ₂O ₇^2- besides a small amount of MoO ₄^2-, whereas the molybdenum species in MoO ₃/SiO ₂ exists as a mononuclear species.