Manganese Catalyzed Partial Oxidation of Light Alkanes

Abstract

The catalytic partial oxidation of methane is achieved at low temperatures (<200 °C) using manganese oxides and manganese salts in mixtures of trifluoroacetic acid and trifluoroacetic anhydride. Dioxygen is used as the in situ terminal oxidant. For Mn oxides (e.g., MnO 2 , Mn 2 O 3 , and Mn 3 O 4 ), we studied stoichiometric methane partial oxidation in HTFA (TFA = trifluoroacetate). Using a Mn trifluoroacetate salt, at 180 °C and under 25 psig of methane, product selectivity for the mono-oxidized product methyl trifluoroacetate (MeTFA) is observed to be >90% at ∼35% methane conversion at approximately 6 turnovers. Under our catalytic methane oxidation reaction conditions, MeTFA is stable against overoxidation, which explains the likely high selectivity at conversions >15%. Using combined experimental studies and DFT calculations, a mechanism involving soluble and molecular Mn species in the catalytic cycle is proposed. The proposed reaction pathway involves initial activation of Mn II by dioxygen, cleavage of a methane C−H bond by a Mn III hydroxo intermediate, rebound of the methyl radical to generate MeTFA, and finally regeneration of the starting Mn II complex. Also, this process is shown to be applicable to the oxidation of ethane, favoring the mono-oxidized product ethyl trifluoroacetate (EtTFA) and reaching ∼46% conversion.

NaCl-Promoted Cobalt-Catalyzed Dioxygen-Mediated Methane Oxidation to Methylene Bis(trifluoroacetate) with a Dramatic Salt Effect

“…The relatively high yields of MeOTFA in some cases (Table , entries 10, 15; Table , entries 2, 4; the highest temperature reached 146–190 °C in these cases) are probably from the thermal decomposition of MeO 2 TFA to TFAO· and MeO·, which was further transformed into MeOTFA. It might be the reason that only MeOTFA was obtained in previous studies where a high temperature (∼180–200 °C) was needed. − The radical chain mechanism also agrees with the observation of the violent reactions, in which a high ratio of TFAA or high temperature increases the collision probability of related species, leading to a quicker and quicker reaction. A low ratio of TFAA might terminate the propagation cycle easily, causing much lower yields.…”

“…A heterogeneous Co catalyst was also demonstrated to be effective at 215 °C with up to 31 turnovers by the van Bokhoven group in 2021 . Manganese-catalyzed partial oxidation of methane in the presence of O 2 at 180 °C with 6–9 turnovers was reported by the Chen, Gunnoe, and Goddard groups . An O 2 -mediated palladium-catalyzed methane oxidation in TFA with NO 2 (NaNO 2 ) or benzoquinone as the co-oxidant was realized at 80 °C by the Bao group, but the turnovers were less than 10 .…”

The Full d3–d5 Redox Series of Mononuclear Manganese Complexes: Geometries and Electronic Structures of [Mn(dgpy)2]n+

“…Consequently, only limited information is available on molecular [MnN 6 ] complexes, while the full series of solid manganese oxides MnO (manganosite), Mn 2 O 3 (bixbyite), and MnO 2 (pyrolusite) represents standard textbook knowledge . MnO 2 is employed in batteries and is utilized for the oxidation of organic substrates. , Oxido manganese­(IV/III) clusters Mn 4 CaO 5 perform the oxidation of water to dioxygen in the oxygen-evolving complex (OEC) of photosystem II. , Two different Jahn–Teller distortions (elongations) of the dangling six-coordinate manganese­(III) site with high-spin d 4 electron configuration along two different axes explain two different EPR signatures of the OEC in its S 1 state of the Kok cycle (S 0 –S 4 ) . For the bis­(terpyridine) manganese­(III) complex [A H,4‑tolyl ] 3+ an unusual Jahn–Teller compression has been reported, which might be an effect of the rigid terpyridine .…”

“…5 MnO 2 is employed in batteries 1 and is utilized for the oxidation of organic substrates. 2,17 Oxido manganese(IV/III) clusters Mn 4 CaO 5 perform the oxidation of water to dioxygen in the oxygenevolving complex (OEC) of photosystem II. 18,19 Two different Jahn−Teller distortions (elongations) of the dangling sixcoordinate manganese(III) site with high-spin d 4 electron configuration along two different axes explain two different EPR signatures of the OEC in its S 1 state of the Kok cycle (S 0 − S 4 ).…”

Heterogeneous Mn-Based Catalysts for the Aerobic Conversion of Methane-to-Methyl Trifluoroacetate

“…In more conventional systems for the conversion of methane to methyl trifluoroacetate, many homogeneous transition-metal-based catalysts have been studied, especially those based on copper and cobalt , salts. Still, the use of manganese-based catalysts for this reaction is more limited, even despite the advantage that manganese is one of the most cost-effective and abundant 3d transition metals, especially compared to cobalt . More than a decade prior, a manganese oxide precatalyst was first shown to have high activity (36% methane-based yield, >95% selectivity) for homogeneous conversion of methane to methyl trifluoroacetate using air as the oxidant, and, recently, a manganese trifluoroacetate salt was reported to be highly active in mixtures of TFA and trifluoroacetate anhydride. , The notable turnovers of up to 8.5 of these studies on homogeneous manganese catalysts for the production of methyl esters motivates further research into exploring manganese-based catalysts for methane partial oxidation applications.…”