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 (mol_Mo s)⁻¹), with (Mo/Nb-KIT-6) catalysts exhibiting maximum propylene formation rates (54.2 ± 0.5 mmol (mol_Mo 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 MoO_x 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 (ΔH_ads,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.

中文翻译：

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深入了解二氧化硅支撑的钼金属中心的掺杂剂介导调节以增强烯烃复分解


我们表明，介孔硅酸盐负载的活性Mo中心周围的电子环境可以通过添加过渡金属来调节，从而产生高度分散的双金属催化剂，该催化剂表现出增强的乙烯+2-丁烯复分解成丙烯的活性。双金属催化剂是通过使用一锅溶胶-凝胶技术将亲电路易斯酸金属（M）（例如 Nb、Ta、Zr 或 Hf）作为掺杂剂促进剂掺入介孔 KIT-6 载体中，然后浸渍 Mo 物质来制备的。所有双金属 Mo/M-KIT-6 催化剂均表现出比单金属 Mo/KIT-6 催化剂更好的活性 (28.7 ± 1.1 mmol (mol _Mo s) ⁻¹ )，其中 (Mo /Nb-KIT-6) 催化剂在相同的 Mo 负载量下表现出最大丙烯形成速率 (54.2 ± 0.5 mmol (mol _Mo s) ⁻¹ )。全面的催化剂表征结果（采用高角度环形暗场扫描透射电子显微镜 (HAADF-STEM)、漫反射紫外可见光 (DR UV-vis)、X 射线光电子能谱 (XPS) 和 X 射线吸收光谱(XAS)) 定性地显示了促进的 Mo 催化剂中四配位 Mo 位点数量的增加，而拉曼光谱则揭示了单金属和促进的双金属催化剂上都存在 Mo 二氧化合物 (O=Mo=O)。这些结果表明，过渡金属的添加改变了Mo配位，除了传统的（（ O═) ₂ Mo(−O–Si) ₂ ) 物种。 此外，双金属配方上的固有丙烯形成速率与Mo二氧化合物(O=Mo=O)表现出的路易斯酸强度呈线性相关，这表明通过掺入具有增加的亲电特性的第二金属可以增强催化剂活性。互补的 ¹⁵ N-吡啶固态NMR谱显示与金属中心相关的不同化学位移，表明存在具有不同几何和电子构型的四配位二氧代MoO _x 物种，具体取决于添加的金属。在吸附的 ¹⁵ N-吡啶的平均化学位移和路易斯酸强度（ΔH _ads,pyridine ）之间也观察到类似的线性相关性，提供了有关分子起源的信息描述符。影响钼基催化剂上烯烃复分解的电子效应。我们的结果表明，可以利用简单的催化剂合成方法来调整多相催化剂中金属中心周围的电子环境，以提高活性和选择性。