Catalysis Today, Vol.343, 215-225, 2020
Highly dispersed molybdenum containing mesoporous silicate (Mo-TUD-1) for olefin metathesis
Employing a one-pot sol-gel synthesis technique, molybdenum with variable loadings (2-14 wt%), was successfully incorporated into a mesoporous TUD-1 silicate framework. Diffuse reflectance UV-vis and Raman spectra of samples dehydrated at reaction temperatures (450 degrees C) revealed the dominant presence of highly dispersed tetrahedral di-oxo (Si-O)(2)Mo(= O)(2) species. Neither bulk nor crystalline MoO3 was evident at Mo loadings from 2 to 7 wt.% from complementary analytical techniques suggesting that a high dispersion of MoOx species is achieved by this synthesis method. Mo L-edge XANES of catalyst samples at ambient conditions and Mo K-edge XAS of catalyst samples dehydrated at the reaction temperature (450 degrees C) further confirm the existence of mixed tetrahedral and octahedral Mo sites. The relative population of dioxo Mo species observed in Mo-TUD-1 catalysts at various Mo loadings correlate well with the Lewis acidity and apparent TOFs (mmol propene/mol Mo/s) observed with these samples. These observations confirm that the dioxo Mo species are the active site precursors, as also reported previously with other Mo-based catalyst formulations. Compared to Mo-impregnated SiO2(Mo/SiO2) catalyst, the Mo-TUD-1 catalysts (2-7 wt% Mo loadings) provide higher apparent TOFs. At higher Mo loadings, the fraction of MoO3 species that are Bronsted acidic in nature increases, resulting in decreased TOFs and increased catalyst deactivation due to the formation of heavier hydrocarbons that deposit on the catalyst surface. However, the metathesis activity is fully restored after simple air regeneration of the catalyst.