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.

No abstract

Coprocessing of biocrudes from hydrothermal liquefaction (HTL) of biomass and wastes with petroleum streams in refinery hydroprocessing has significant potential to accelerate the production of renewable transportation fuels in the near term. However, HTL biocrudes with some problematic characteristics can potentially cause faster deactivation of the catalysts, which is one of the biggest barriers to the adoption of biocrude in the current refinery process. In this work, we investigated the deactivation modes of a sulfide NiMo/Al2O3 hydrotreating catalyst used for coprocessing HTL biocrude (from wastewater sludge) with straight-run diesel. Spent catalysts were collected after coprocessing diesel with different biocrudes after >300 h time on stream and characterized by various techniques. Their catalytic activities were evaluated by model compound testing. Loss of catalyst activity was observed after coprocessing biocrudes. No structural change of the catalyst after coprocessing biocrudes was observed, and catalyst deactivation was largely due to catalyst fouling by carbonaceous species and metal contaminants (such as Fe and K) from biocrudes. Inorganic contaminant deposition leads to the external coating of the catalyst pellets on the top of the reactor, whereas the carbonaceous species potentially bind near the sulfide active phase, thereby blocking access to the active edge sites. Pretreatment of the HTL biocrude can reduce problematic species and alleviate catalyst deactivation during coprocessing.

NaY zeolite-encapsulated dimeric (Mo 2 S 4 ) and tetrameric (Mo 4 S 4 ) 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 MoS 2 , is thermodynamically disfavored. Competitive adsorption of H 2 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 H 2 , 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 H 2 ) show that the active sites in both clusters are catalytically indistinguishable.

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.