Insights into Dopant-Mediated Tuning of Silica-Supported Mo Metal Centers for Enhanced Olefin Metathesis

We show that the electronic environment around active Mo centers supported on mesoporous silicates can be tuned by the addition of transition metals creating highly dispersed bimetallic catalysts that display enhanced activity for ethylene + 2-butene metathesis to propylene. The bimetallic catalysts are prepared by incorporating electrophilic Lewis acid metals (M) such as Nb, Ta, Zr, or Hf as dopant promoters into mesoporous KIT-6 supports using a one-pot sol–gel technique followed by impregnation of the Mo species. All the bimetallic Mo/M-KIT-6 catalysts display better activity than monometallic Mo/KIT-6 catalyst (28.7 ± 1.1 mmol (molMo s)−1), with (Mo/Nb-KIT-6) catalysts exhibiting maximum propylene formation rates (54.2 ± 0.5 mmol (molMo s)−1) at an identical Mo loading. Comprehensive catalyst characterization results (deploying high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), diffuse reflectance ultraviolet–visible (DR UV–vis), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS)) qualitatively show an increased population of the four-coordinated Mo sites in the promoted Mo catalysts, while Raman spectroscopy reveals the presence of Mo dioxo species (O═Mo═O) on both the monometallic and promoted bimetallic catalysts. These results suggest that the addition of transition metals alters Mo coordination, yielding isolated bimetallic precursors [(O═)2Mo(−O–M)(−O–Si)] in addition to the conventional ((O═)2Mo(−O–Si)2) species. Further, the intrinsic propylene formation rates on the bimetallic formulations follow a linear correlation with the Lewis acid strengths exhibited by the Mo dioxo species (O═Mo═O) revealing that catalyst activity can be enhanced by incorporating a second metal with increasing electrophilic character. Complementary 15N-pyridine solid-state NMR spectra display different chemical shifts associated with the metal centers suggesting that four-coordinated dioxo MoOx species with varying geometric and electronic configurations exist, depending on the added metal. A similar linear correlation was also observed between the average chemical shifts of the adsorbed 15N-pyridine and the Lewis acid strengths (ΔHads,pyridine), providing informative descriptors regarding the molecular origins of the electronic effects influencing olefin metathesis on Mo-based catalysts. Our results demonstrate that simple catalyst synthesis methods can be harnessed for tuning the electronic environment around metal centers in heterogeneous catalysts for enhancing activity and selectivity.