(532bj) Enhancing Reactivity in Olefin Metathesis over Mo-Based Bimetallic Catalyst

To bridge the so-called propylene gap, propylene production via the cross-metathesis of ethylene and 2-butene is receiving renewed interest. Supported WO₃/SiO₂ catalysts are used commercially to produce propylene however, the activity is often limited due to low dispersion of the active W species. Recently, our group found that doping W-based catalysts with Nb significantly enhances the metathesis activity by creating new and superior active sites, as confirmed by EXAFS, TPR, and ToF-SIMS. In this work, we demonstrate that similar enhancements in metathesis activity are observed in Mo-based catalysts when doped with metals such as Nb or Zr.

A series of bimetallic (Mo_x/M_y-KIT-6) catalysts were successfully synthesized with variable Mo and dopant metal (M=Nb/Zr/Ta/Hf) loadings using a simple wet impregnation technique. The bimetallic Mo₂/Nb_1.5-KIT-6 and Mo₂/Zr_1.5-KIT-6 catalysts displayed enhanced propylene yield (45.5 ± 0.5% and 42.0 ± 0.8%) when compared to Mo₂/KIT-6 (37.2 ± 0.3%) under identical reaction conditions (Fig 1a). Further, the deconvoluted DR UV-Vis spectra show that compared to Mo₂/KIT-6 and Mo₂/Zr_1.5-KIT-6 catalysts, the Mo₂/Nb_1.5-KIT-6 catalyst shows a higher ratio of tetrahedral to octahedral MoO_x species suggesting an increase in the population of the isolated active site precursors (Fig 1b). These results suggest that the addition of Nb or Zr alters Mo coordination yielding additional active sites (MoNbO_x) that are more active than their monometallic counterparts, most likely by varying the O=Mo=O bond angle. The influence of metal loadings along with detailed catalyst characterization results (XRD, EXAFS, In situ Py-IR, Raman, TPD, TPR, XPS, MAS NMR), as well as DFT calculations, will be presented to explain the structure-activity relationships.