(177g) Towards Enhancing the Reactivity in Olefin Metathesis over Bimetallic Monb Single Site Catalyst

Propylene production via the cross-metathesis of ethylene and 2-butene is receiving significant interest. Supported WO₃/SiO₂ catalysts are used commercially however, the activity is often limited due to low dispersion of the active W species. Supported molybdenum oxide catalysts have also been widely investigated for olefin metathesis reaction. 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 as well as DFT calculations. In this work, we demonstrate significant enhancements in metathesis activity are observed in Mo-based catalysts when doped with metals such as Nb or Ta

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 a simple wet impregnation technique. The catalytic performance investigated in a continuous FBR under differential and steady-state conditions demonstrated that the propylene formation rates on the Nb doped catalyst (54.2 ± 0.5 mmol (mol_Mo s)^-1) are approximately two-fold greater than for the Mo/KIT-6 (28.7 ± 1.1 mmol (mol_Mo s)^-1. Further, the deconvoluted DR UV-Vis shows a higher ratio of tetrahedral to octahedral MoO_x species suggesting an increase in the population of the isolated active sites (Figure 1). These results suggest that the addition of Nb alters Mo coordination yielding additional bimetallic single sites precursors (MoNbO_x) that are more active than their monometallic (Mo-O-Si) counterparts, most likely by varying the O=Mo=O bond angle.

The selective enhancement of metathesis activity over bimetallic Mo-based catalysts is a significant result. It 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 (EXAFS, TPR-H₂, Raman, and ¹⁵N NMR) guided by DFT calculations, will be presented to explain the structure-activity relationships.