Lithium Promotes Acetylide Formation on MgO During Methane Coupling Under Non-Oxidative Conditions.

A prototypical material for the oxidative coupling of methane (OCM) is Li/MgO, for which Li is known to be essential as a dopant to obtain high C2 selectivities. Herein, Li/MgO is demonstrated to be an effective catalyst for non-oxidative coupling of methane (NOCM). Moreover, the presence of Li is shown to favor the formation of magnesium acetylide (MgC2 ), while pure MgO promotes coke formation as evidenced by solid-state 13 C NMR, thus indicating that Li promotes C-C bond formation. Metadynamic simulations of the carbon mobility in MgC2 and Li2 C2 at the density functional theory (DFT) level show that carbon easily diffuses as a C2 unit at 1000 °C. These insights suggest that the enhanced C2 selectivity for Li-doped MgO is related to the formation of Li and Mg acetylides.

Role and dynamics of transition metal carbides in methane coupling.

Transition metal carbides have numerous applications and are known to excel in terms of hardness, thermal stability and conductivity. In particular, the Pt-like behavior of Mo and W carbides has led to the popularization of metal carbides in catalysis, ranging from electrochemically-driven reactions to thermal methane coupling. Herein, we show the active participation of carbidic carbon in the formation of C2 products during methane coupling at high temperature that is associated with the dynamics of Mo and W carbides. A detailed mechanistic study reveals that the catalyst performance of these metal carbides can be traced back to its carbon diffusivity and exchange capability upon interaction with methane (gas phase carbon). A stable C2 selectivity over time on stream for Mo carbide (Mo2C) can be rationalized by fast carbon diffusion dynamics, while W carbide (WC) shows loss of selectivity due to slow diffusion leading to surface carbon depletion. This finding showcases that the bulk carbidic carbon of the catalyst plays a crucial role and that the metal carbide is not only responsible for methyl radical formation. Overall, this study evidences the presence of a carbon equivalent to the Mars-Van Krevelen type mechanism for non-oxidative coupling of methane.

Methane-to-Methanol on Mononuclear Copper(II) Sites Supported on Al2 O3 : Structure of Active Sites from Electron Paramagnetic Resonance*.

The selective conversion of methane to methanol remains one of the holy grails of chemistry, where Cu-exchanged zeolites have been shown promote this reaction under stepwise conditions. Over the years, several active sites have been proposed, ranging from mono-, di- to trimeric CuII . Herein, we report the formation of well-dispersed monomeric CuII species supported on alumina using surface organometallic chemistry and their reactivity towards the selective and stepwise conversion of methane to methanol. Extensive studies using various transition alumina supports combined with spectroscopic characterization, in particular electron paramagnetic resonance (EPR), show that the active sites are associated with specific facets, which are typically found in γ- and η-alumina phase, and that their EPR signature can be attributed to species having a tri-coordinated [(Al2 O)CuIIO(OH)]- T-shape geometry. Overall, the selective conversion of methane to methanol, a two-electron process, involves two monomeric CuII sites that play in concert.

Non-oxidative Methane Coupling over Silica versus Silica-Supported Iron(II) Single Sites.

Non-oxidative CH4 coupling is promoted by silica with incorporated iron sites, but the role of these sites and their speciation under reaction conditions are poorly understood. Here, silica-supported iron(II) single sites, prepared via surface organometallic chemistry and stable at 1020 °C in vacuum, are shown to rapidly initiate CH4 coupling at 1000 °C, leading to 15-22 % hydrocarbons selectivity at 3-4 % conversion. During this process, iron reduces and forms carburized iron(0) nanoparticles. This reactivity contrasts with what is observed for (iron-free) partially dehydroxylated silica, that readily converts methane, albeit with low hydrocarbon selectivity and after an induction period. This study supports that iron sites facilitate faster initiation of radical reactions and tame the surface reactivity.

Monomeric Copper(II) Sites Supported on Alumina Selectively Convert Methane to Methanol.

Monomeric CuII sites supported on alumina, prepared using surface organometallic chemistry, convert CH4 to CH3 OH selectively. This reaction takes place by formation of CH3 O surface species with the concomitant reduction of two monomeric CuII sites to CuI , according to mass balance analysis, infrared, solid-state nuclear magnetic resonance, X-ray absorption, and electron paramagnetic resonance spectroscopy studies. This material contains a significant fraction of Cu active sites (22 %) and displays a selectivity for CH3 OH exceeding 83 %, based on the number of electrons involved in the transformation. These alumina-supported CuII sites reveal that C-H bond activation, along with the formation of CH3 O- surface species, can occur on pairs of proximal monomeric CuII sites in a short reaction time.