(730d) Investigating Promoter Effects in Bimetallic Mo-Nb Silicates for Enhanced Olefin Metathesis

To bridge the so-called propylene gap, propylene production via the cross-metathesis of ethylene and 2-butene is receiving significant 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 and Raman spectroscopy as well as DFT calculations. 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) were successfully synthesized with variable Mo and dopant metal (M=Nb/Zr/Ta/Hf) loadings using simple wet impregnation technique. The catalytic performance investigated in a continuous FBR demonstrated that 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 steady-state and identical reaction conditions. 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 sites. These results suggest that the addition of Nb or Zr alters Mo coordination yielding more additional active sites (MoNbO_x) as inferred from the metathesis activity results.

The selective enhancement of metathesis activity over bimetallic Mo-based catalysts is a significant result and opens up a relatively simple technique for creating more active catalytic sites and tuning catalyst activity, most likely by varying the O=M=O bond angle. The influence of metal loadings along with detailed catalyst characterization results (XRD, EXAFS, IR, TPD, TPR, Raman, XPS, TOF-SIMS, MAS NMR) as well as DFT calculations will be presented to explain the structure-activity relationships.