When does molybdenum oxide equal molybdenum hexacarbonyl? Relations between heterogeneous and homogeneous molybdenum catalysts in syngas catalysis

Molybdenum provides one of the most versatile sets of materials for synthesis gas catalysis. In its reduced states (Mo⁰-Mo⁴⁺), molybdenum can act as a "metal" system yielding mainly hydrocarbon products. In its oxidized state, as the trioxide or as a metal molybdate, molybdenum may serve as an alcohol synthesis catalyst or as an alcohol to hydrocarbon conversion system. The oxides can also be used as a support for more typical syngas catalysts. Molybdenum hexacarbonyl undergoes chemistry which can be related to these reactions. These relationships offer a frontier between molybdenum homogeneous and heterogeneous catalysis. Literature and mechanistic studies indicate that the formate and formyl intermediates represent the link between homogeneous catalysis by molybdenum carbonyl and oxide catalysis of reactions of carbon monoxide. The formate approach centers on the fact that virtually all catalysts that have a water-gas shift activity will promote, or are themselves, methanol catalysts. It is more than coincidental that these two embodied reactions have been shown to involve a similar type of intermediate, metal formates. This moiety, found in base-promoted molybdenum carbonyl and molybdenum trioxide catalyzed water-gas shift reactions, symbolizes the link between these chemistries and the potential route to developing new methanol catalyst systems. The use of carbon monoxide and water, or low-H₂:CO ratio syngas, for the direct production of methanol and/or hydrocarbons with heterogeneous molybdenum systems can be explained. The formyl approach is based on a stoichiometric reaction of some main-group hydrides with molybdenum carbonyl to yield methanol. This chemistry is related to metal oxide catalyzed methanol synthesis from high-ratio (2:1 ≤ H₂:CO) syngas. The molybdenum carbonyl or oxide is used to coordinate or fix the carbon monoxide while another metal system functions by activating the hydrogen. This can be pictured as the reduction of a carbonyl group by a complex metal hydride. Initial demonstration of the conceptual basis of each of these approaches has been accomplished at Brookhaven National Laboratory. Concepts for the extension of homogeneous catalysis into these regions of overlap with some examples will be presented.