Bimetallic Dispersed Catalysts from Molecular Precursors Containing Mo–Co–S for Coal Liquefaction

This work was undertaken with the goal of exploring novel bimetallic dispersed sulfide catalysts for coal liquefaction. Several heterometallic complexes consisting of two transition metals, Mo and Co, and sulfur in a single molecule were synthesized and tested as precursors of dispersed catalysts for liquefaction of a Montana subbituminous coal (0.5 wt % Mo on dmmf coal). It was found that the structure of the precursors, in particular the ligands to the metal species, affect the activity of the resulting catalyst significantly. Among the M-M′ type precursors tested, Mo-Co thiocubane, Mo₂Co₂S₄(Cp)₂(CO)₂ [Cp = cyclopentadiene], designated as MoCo-TC₂, produced in situ the best catalyst. The performance of the MoCo-TC2 was further enhanced by using temperature-programmed (TPL) conditions consisting of a low-temperature soaking at 200 °C, programmed heat-up to 400 or 425 °C followed by a 30-min hold. The programmed heat-up serves as an in situ activation of catalyst and coal pretreatment, which contributes to more effective hydrogenation of reactive fragments at high temperature. The method of loading precursor also affects the catalytic performance. Impregnating MoCo-TC2 onto coal from a swelling solvent system, a mixture of toluene-tetrahydrofuran (THF), affords the highest conversion (92 wt %) and highest oil yield for rims at 400 °C under TPL conditions. The bimetallic catalyst generated from MoCo-TC2 is substantially more active than those from mixed metal carbonyls, Mo(CO)₆ + Co₂(CO)₈, and the mixture plus sulfur. These results indicate that proper binding of Mo, Co, and S in a single molecule could result in a bimetallic sulfide catalyst that has high activity.