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 molethane·h-1·molcluster -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.

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 (Mo2 S4 ) and tetranuclear (Mo4 S4 ) molybdenum sulfide clusters. The FAU-encaged Mo4 S4 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.