(758a) Mesoporous Silicates with Highly Dispersed Molybdenum (Mo-TUD-1) for Enhanced Olefin Metathesis

Synthesis of propylene from ethylene and 2-butene via olefin metathesis is receiving significant interest in the context of on demand propylene production. Hence, efficient catalyst development and research are required for improved performance that shows commercial potential. 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 °C) revealed the dominant presence of highly dispersed tetrahedral di-oxo (Si-O)₂Mo(=O)₂ species. Neither bulk nor crystalline MoO₃ was evident at Mo loadings from 2-7 wt.% from complementary analytical techniques suggesting that a high dispersion of MoO_x 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 °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, confirming 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 Brønsted 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.