Zeolite-Stabilized Di- and Tetranuclear Molybdenum Sulfide Clusters Form Stable Catalytic Hydrogenation Sites

Engineering surface framework TiO6 single sites for unprecedented deep oxidative desulfurization

Catalytic oxidative desulfurization (ODS) using titanium silicate catalysts has emerged as an efficient technique for the complete removal of organosulfur compounds from automotive fuels. However, the precise control of highly accessible and stable-framework Ti active sites remains highly challenging. Here we reveal for the first time by using density functional theory calculations that framework hexa-coordinated Ti (TiO6) species of mesoporous titanium silicates are the most active sites for ODS and lead to a lower-energy pathway of ODS. A novel method to achieve highly accessible and homogeneously distributed framework TiO6 active single sites at the mesoporous surface has been developed. Such surface framework TiO6 species exhibit an exceptional ODS performance. A removal of 920 ppm of benzothiophene is achieved at 60°C in 60 min, which is 1.67 times that of the best catalyst reported so far. For bulky molecules such as 4,6-dimethyldibenzothiophene (DMDBT), it takes only 3 min to remove 500 ppm of DMDBT at 60°C with our catalyst, which is five times faster than that with the current best catalyst. Such a catalyst can be easily upscaled and could be used for concrete industrial application in the ODS of bulky organosulfur compounds with minimized energy consumption and high reaction efficiency.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

Zeolites with ordered microporous systems, distinct framework topologies, good spatial nanoconfinement effects, and superior (hydro)thermal stability are an ideal scaffold for planting diverse active metal species, including single sites, clusters, and nanoparticles in the framework and framework-associated sites and extra-framework positions, thus affording the metal-in-zeolite catalysts outstanding activity, unique shape selectivity, and enhanced stability and recyclability in the processes of Brønsted acid-, Lewis acid-, and extra-framework metal-catalyzed reactions. Especially, thanks to the advances in zeolite synthesis and characterization techniques in recent years, zeolite-confined extra-framework metal catalysts (denoted as metal@zeolite composites) have experienced rapid development in heterogeneous catalysis, owing to the combination of the merits of both active metal sites and zeolite intrinsic properties. In this review, we will present the recent developments of synthesis strategies for incorporating and tailoring of active metal sites in zeolites and advanced characterization techniques for identification of the location, distribution, and coordination environment of metal species in zeolites. Furthermore, the catalytic applications of metal-in-zeolite catalysts are demonstrated, with an emphasis on the metal@zeolite composites in hydrogenation, dehydrogenation, and oxidation reactions. Finally, we point out the current challenges and future perspectives on precise synthesis, atomic level identification, and practical application of the metal-in-zeolite catalyst system.

Di- and Tetrameric Molybdenum Sulfide Clusters Activate and Stabilize Dihydrogen as Hydrides

NaY zeolite-encapsulated dimeric (Mo₂S₄) and tetrameric (Mo₄S₄) molybdenum sulfide clusters stabilize hydrogen as hydride binding to Mo atoms. Density functional theory (DFT) calculations and adsorption measurements suggest that stabilization of hydrogen as sulfhydryl (SH) groups, as typical for layered MoS₂, is thermodynamically disfavored. Competitive adsorption of H₂ and ethene on Mo was probed by quantifying adsorbed CO on partly hydrogen and/or ethene covered samples with IR spectroscopy. During hydrogenation, experiment and theory suggest that Mo is covered predominately with ethene and sparsely with hydride. DFT calculations further predict that, under reaction conditions, each Mo _x S _y cluster can activate only one H₂, suggesting that the entire cluster (irrespective of its nuclearity) acts as one active site for hydrogenation. The nearly identical turnover frequencies (24.7 ± 3.3 mol_ethane·h^-1·mol_cluster ^-1), apparent activation energies (31-32 kJ·mol^-1), and reaction orders (∼0.5 in ethene and ∼1.0 in H₂) show that the active sites in both clusters are catalytically indistinguishable.

Construction of stable MoxSy/CeO2 heterostructures for the electrocatalytic hydrogen evolution reaction

The unique structures of polynuclear Mo_xS_y clusters make it possible to maximize the number of their active sites and for them to be good candidates for HER catalysts. An appropriate support is highly necessary not only to avoid the desorption of Mo_xS_y clusters in a working environment, but also to improve their HER activity. Our work here shows that the CeO₂ support could provide strong support for interaction with various Mo_xS_y clusters and the formed Mo_xS_y/CeO₂ hetero-structures also have modest ΔG_H* for the HER. The electronic features of Mo_xS_y clusters are regulated by the CeO₂ support, which leads to charge redistribution on edge atoms and plays a key role in H adsorption. Our studies provide instructive predictions on efficient candidates of molybdenum-sulfur based catalysts for the HER.

Structure, reactivity, and spectroscopy of nitrogenase-related synthetic and biological clusters

The reduction of dinitrogen (N2) is essential for its incorporation into nucleic acids and amino acids, which are vital to life on earth. Nitrogenases convert atmospheric dinitrogen to two ammonia molecules (NH3) under ambient conditions. The catalytic active sites of these enzymes (known as FeM-cofactor clusters, where M = Mo, V, Fe) are the sites of N2 binding and activation and have been a source of great interest for chemists for decades. In this review, recent studies on nitrogenase-related synthetic molecular complexes and biological clusters are discussed, with a focus on their reactivity and spectroscopic characterization. The molecular models that are discussed span from simple mononuclear iron complexes to multinuclear iron complexes and heterometallic iron complexes. In addition, recent work on the extracted biological cofactors is discussed. An emphasis is placed on how these studies have contributed towards our understanding of the electronic structure and mechanism of nitrogenases.

Zeolite-Stabilized Di- and Tetranuclear Molybdenum Sulfide Clusters Form Stable Catalytic Hydrogenation Sites

Supercages of faujasite (FAU)‐type zeolites serve as a robust scaffold for stabilizing dinuclear (Mo₂S₄) and tetranuclear (Mo₄S₄) molybdenum sulfide clusters. The FAU‐encaged Mo₄S₄ clusters have a distorted cubane structure similar to the FeMo‐cofactor in nitrogenase. Both clusters possess unpaired electrons on Mo atoms. Additionally, they show identical catalytic activity per sulfide cluster. Their catalytic activity is stable (>150 h) for ethene hydrogenation, while layered MoS₂ structures deactivate significantly under the same reaction conditions.