Mononuclear iron hydroxyl group activated methane to methanol at room temperature

On June 30, 2022, Nature Materials, a top international academic journal, published online the research results of Yang Sihai and Martin Schröder’s team from the School of Chemistry of the University of Manchester, UK, entitled “Direct photo-oxidation of methane to methanol over a mono-iron hydroxyl site”.

The research group found that the mononuclear iron hydroxyl site is encapsulated in a functional metal-organic framework containing photosensitizers and polymetallic oxidates, which can use H2O and O2 as oxidants at room temperature and atmospheric pressure to effectively directly oxidize CH4 to CH3OH. Dr An Bing of the University of Manchester is the first author of the paper, and Professor Yang Sihai and Professor Martin Schröde of the University of Manchester are the corresponding authors of the paper.

Direct oxidation of methane to methanol is seen as a “Dream reaction”. This is because methane (CH4) has a relatively low energy density (~36 kJ L-1) at room temperature and pressure, while methanol (CH3OH) made by selective oxidation of methane can increase its energy density to ~17 MJ L-1; At the same time, methanol, as the basic chemical raw material, can also be easily converted into important chemicals and fuels such as olefins and aromatics. At present, methane conversion in industry often needs to be carried out under harsh conditions such as high temperature and high pressure. However, the dissociation energy of the C-H bond of CH4 is much higher than that of CH3OH, and methane or methanol produced under high temperature and pressure conditions is very easy to over-oxidize to form CO or CO2. Photocatalytic direct conversion of methane, i.e. the use of light energy instead of heat energy to achieve methane activation under mild conditions, has achieved rapid development in the past two decades. Despite this, the current photocatalytic methane technology is still immature, including short life caused by photocorroding under oxygen conditions, low selectivity due to the lack of co-catalysts, especially limited to the batch reactor, making the reaction kinetic process difficult to control, and the yield and selectivity of the product are very low.

Recently, the team of Yang Sihai and Professor Martin Schröder of the University of Manchester in the United Kingdom encapsulated the mononuclear iron hydroxyl site in a single-core iron hydroxyl site containing a photosensitizer ([Ru(bpy)2(bpydc)]) and polymetallic oxide (PW9V3) in a functional metal-organic framework (PMOF-Ru), self-assembling to form PMOF-RuFe (OH). Unlike the double iron center of the monooxygenase, the single iron hydroxyl sites enclosed in PMOF-RuFe (OH) can effectively directly photooxidize CH4 to CH3OH using H2O and O2 as oxidants at room temperature and atmospheric pressure. Through the designed flow catalytic system, CH4 is converted to CH3OH with 100% selectivity, with a time yield of up to 8.81±0.34 mmol gcat-1 h-1, and even higher than methane monooxygenase (5.05 mmol gcat-1 h-1). The research team also used a series of in situ techniques to make accurate experimental characterization of the catalyst structure, CH4 adsorption site, activation process and catalytic mechanism, combined with theoretical calculations, in-depth disclosure of oxygen and methane activation mechanism in the selective oxidation process of methane, providing new insights for the active center of methane catalytic conversion.

Figure 1: PMOF-RuFe(OH) photocatalytic activity of methane oxidation to methanol

The researchers first evaluated the catalyst through a kettle system. PMOF-RuFe (OH) showed excellent CH3OH selectivity (98%) and time yield (3145±340 μmol gcat-1 h-1) under visible light (λ=400-780 nm) under visible light (λ=400-780 nm) with CH4/O2 (v/v=1/1,1atm) as the feed gas. This activity exceeds all photocatalysts and most thermal catalysts under ambient conditions, and even includes many catalytic systems operating under high and/or high pressure conditions. Isotopic labeling experiments show that CH4 is the only source of carbon, and both O2 and H2O are oxidants of the system. Through the study of Fe2O3 nanoparticles and Fe2O3/PMOF-Ru (no single iron hydroxyl locus only), the possibility of Fe2O3 nanoparticles as active species was excluded; At the same time, a series of controlled experiments have also proved that the high activity of this photocatalysis comes from the position anchoring of the three active ingredients in the MOF and their synergistic effect in PMOF-RuFe (OH).

The research team used a series of in situ characterization techniques, including inelastic neutron diffraction spectroscopy and other binding theoretical calculations to reveal the mechanism of action of monomer hydroxyl groups on methane adsorption and activation. The study found that the limiting area in the MOF in the single iron hydroxyl site by forming one[Fe−OH···CH4]Intermediates to stabilize the adsorbed CH4, this intermediate greatly reduces the activation barrier of the C−H bond. At the same time, combined with theoretical calculations, it is shown that the free radicals formed are not completely “free” states, but are stabilized by the single iron hydroxyl site and formed[Fe−OH2····CH3], inhibiting the C−C coupling reaction. In addition, free radical capture experiments observed the signal of DMPO-OH, demonstrating that the signal of DMPO-OH was observed. OH is an intermediate of reactive oxygen species, not 1O2, · O2-or· OOH; This has also been demonstrated by corresponding quenching experiments. The photocatalytic mechanism was further studied by in situ EPR, fluorescence experiment, time-resolved spectroscopy, electrochemistry and quenching experiments, indicating that the photocatalytic cycle is light-induced by electrons from photosensitizers[Ru(bpy)2(bpydc)]*The excited state shifts to PW9V3 to form a reduced prototype of PW9VIV/V3 to begin. The electron-rich PW9VIV/V3 drives the reduction of O2 light to H2O2 by proton (from H2O) coupled electrons (from PW9VIV/V3). Under light, the {Fe-OH} molecule rapidly converts the in situ generated H2O2 into hydroxyl radical OH radicals and methyl radicals formed by activating methane at single iron hydroxyl sites to form methane, and is extracted from MOF by water in the reaction system to prevent further oxidation.

Figure 2: Study of activation of C−H bonds at single iron hydroxyl sites

Based on the observed ultra-high activity of PMOF-RuFe (OH), and taking into account the importance of water in methanol desorption, we designed a continuous flowing photocatalytic unit for methane oxidation to methanol. Under irradiation, the water-saturated CH4/O2 continuously passes through the PMOF-RuFe(OH) catalyst bed, creating a dynamic gas/solid/liquid interface that maximizes the contact between CH4, O2, H2O and the catalyst. Significantly, unlike the traditional non-porous photocatalyst, in addition to the water-dissolved CH4, the MOF catalyst also has a strong adsorption effect on gaseous CH4, breaking the limitation of CH4 solubility, so, using this simple device, under normal temperature and pressure conditions, methane is 100% converted to CH3OH in 120 hours, and the CH3OH time yield reaches an unprecedented 8.81±0.34 mmol gcat-1 h-1. The authors also conducted an economic analysis of the method and the use of syngas (CO/CO2/H2) to produce methanol in industrial processes, indicating that the mononuclear iron hydroxyl catalyst has great prospects for future industrial applications. (Source: Science Network)

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