(500c) Oxidative Coupling of Methane: The Role of the Tungstate Promoter in Mn-Na2WO4

The recent increase in recoverable natural gas resources has renewed interest in using this resource beyond combustion via conversion to higher value chemicals. One reaction route of particular interest is the conversion of methane to ethylene via oxidative coupling (OCM). This reaction has been studied for many decades but is still missing an efficient catalyst to-date. In the present work, we hence revisit the mechanism of oxidative coupling of methane over Mn-Na₂WO₄, one of the most promising catalysts studied for this reaction, with specific focus on elucidating the role of the tungstate promoter in this catalyst system.

Through careful fixed bed reactor studies using both Mn-Na₂WO₄ mixed oxide catalysts as well as individual manganese oxide and tungstate catalysts, we found—in agreement with previous studies—that lattice oxygen of unpromoted Mn-oxide catalysts is effective for methane activation, but results in poor C₂ yields since the undesired total oxidation dominates the surface reaction. The addition of the well-known promoter Na₂WO₄ improved reaction selectivity strongly by both suppressing CO₂ formation and increasing C₂H₄ yield. Interestingly, however, we found the same improvement in C₂ selectivity when using a simple physical mixture of separate Mn-oxide and Na₂WO₄ catalysts: Product distribution with the single, mixed oxide catalyst and the mixture of two pure metal oxides was experimentally indistinguishable. This contradicts the prevailing mechanism in the published literature that concludes that the promotion by tungstate is based on an exchange of lattice oxygen between the two metal phases.

A series of further kinetic tests and catalyst characterization performed to elucidate the role of Na₂WO₄ revealed that methane activation occurs exclusively on Mn-oxide, while Na₂WO₄ is essentially inactive towards all hydrocarbon species in the system but can be easily reduced by H₂. The promotion of Mn-oxides by Na₂WO₄ hence does not rely on a lattice oxygen sharing mechanism but instead seems to be based on a concerted reaction mechanism in which the tungstate selectively removes H₂ during the C₂H₆ dehydrogenation step in OCM. Thus, the combination of Mn-oxide and tungstate catalysts—either in a single mixed oxide or a via physical mixture of two individual catalysts—synergistically combines the methane activation over the Mn oxide, resulting in the formation of ethane, and the oxidative dehydrogenation of ethane to ethylene over the tungstate phase. Our results hence show that the currently prevailing understanding of the reaction mechanism over this catalyst is incomplete and opens a new direction for rational catalyst design for oxidative coupling of methane.