(368ar) Strategies to Tune Catalytic Properties of Metal-Incorporated Mesoporous Silicates for Enhanced Activity and Selectivity in Acid-Catalyzed Reactions

Research Interests: Heterogeneous Catalysis, Reaction Engineering, Olefins Chemistry, Sustainable Chemistry and Process Development

Synthesis of propylene from ethylene and 2-butene via olefin metathesis is an attractive and atom economical technology to augment the increasing propylene demand. In this study, we investigate the electronic and structural effects of Mo sites in mesoporous silicates by incorporating transition metals such as Nb, Ta, Zr, or Hf. These metals were selected for their varying electronegativity and Lewis acid strength, influencing the catalytic properties of the resulting bimetallic materials. Using a one-pot sol–gel synthesis technique, we prepared highly dispersed bimetallic Mo catalysts supported on KIT-6 and evaluated their performance in olefin metathesis reaction. Our results show that these bimetallic catalysts exhibit significantly enhanced activity and selectivity compared to their monometallic counterparts. For instance, Mo/Nb-KIT-6 catalysts demonstrated the highest propylene formation rates, nearly doubling the activity of the monometallic Mo/KIT-6 catalysts. This improvement is attributed to the increased population of isolated four-coordinated Mo sites and the presence of Mo dioxo species (O═Mo═O), confirmed by advanced characterization techniques. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), diffuse reflectance ultraviolet–visible (DR UV–vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption near-edge structure (XANES) further revealed significant structural changes in the bimetallic catalysts, indicating an increased abundance of four-coordinated Mo sites and the formation of bimetallic precursors [(O═)2Mo(−O–M)(−O–Si)]. Raman spectroscopy confirmed the presence of structurally similar Mo dioxo species on both monometallic and bimetallic catalysts. Additionally, ¹⁵N-pyridine solid-state NMR spectroscopy provided crucial insights into the electronic environments around the Mo sites. The NMR spectra displayed distinct chemical shifts correlated with Lewis acid strengths, suggesting that the addition of transition metals tunes the electronic properties of the Mo centers. These findings demonstrate that the addition of transition metals tunes the electronic properties of Mo centers, enhancing catalytic performance. The correlation between intrinsic propylene formation rates and Lewis acid strengths underscores the importance of electronic tuning. This study provides valuable design principles for developing more efficient heterogeneous catalysts. By leveraging simple synthesis methods and advanced spectroscopic tools, we can effectively tune the electronic environment around metal centers to enhance catalytic activity and selectivity. Our results highlight the significance of solid-state NMR in understanding molecular interactions and electronic effects, offering new avenues for optimizing catalyst performance in industrial applications.

Epoxides are valuable precursors for producing polymers and pharmaceutical additives. Supported transition metal oxide catalysts (Nb, Ti, Zr, Ta, W, Mo) have been studied for the selective epoxidation of cyclohexene using aqueous H₂O₂. However, many of these catalysts are poisoned by water coordination, reducing epoxide selectivity. Robust catalysts that maximize H₂O₂ utilization and epoxide selectivity are in high demand. This study explores the intrinsic properties of Ta(V) oxide, which is less prone to water coordination due to an extra anionic coordination site and demonstrates its effectiveness when incorporated into mesoporous silicates like KIT-6 and TUD-1 via one-pot synthesis methods. Our results show that Ta-incorporated catalysts achieve over 95% epoxide selectivity and higher catalytic activity, with turnover frequencies (TOFs) surpassing those of other metals. Specifically, Ta (2 wt%) loadings display higher and stable catalytic activity (96% selectivity) and high H₂O₂ utilization efficiency (80%) over 10 hours. Comprehensive characterization using DR UV-Vis, XPS, Raman, and EXAFS reveals that isolated Ta(V) species are dominant at lower loadings, while mixed oligomeric species (Ta-O-Ta) appear with increasing metal loadings. Additionally, Py-IR experiments identify dominant Lewis acid sites, which favorably tune the electronic environment to stabilize epoxide intermediates and selectively catalyze the epoxidation pathway. This study highlights the importance of synthesis techniques that maximize Ta utilization by forming isolated sites, essential for maximizing epoxidation TOFs. The influence of metal loadings, reaction conditions, and H₂O₂ utilization, along with detailed catalyst characterization (TEM, DR UV-Vis, XPS, in situ Py-IR, EXAFS, TPR-H2, Raman, and ¹⁵N Pyridine NMR), are presented to elucidate structure-activity relationships. Our findings provide significant insights into designing efficient catalysts for selective epoxidation, emphasizing the crucial role of solid-state NMR in understanding molecular interactions and electronic effects.