Systems and methods for converting methane to one or more products

Abstract

The systems and methods convert methane to one or more products using a green rust material in spontaneous, electrocatalytic, and photochemical reactions under aerobic and anaerobic conditions. In an embodiment, a first electrode that includes a green rust material and a second electrode are contacted by an electrolyte, where the first and second electrodes are separated by an ion exchange membrane. For electrocatalytic reactions, a potential difference is applied across the electrodes. For photochemical reactions, the green rust material is illuminated. Liquid products (e.g., methanol and / or acetate) can be removed from the electrolyte in contact with the green rust material after the desired contact period.

Description

SYSTEMS AND METHODS FOR CONVERTING METHANE TO ONE OR MORE PRODUCTSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 528,155, filed July 21, 2023, which is incorporated by reference as if disclosed herein in its entirety.BACKGROUND

[0002] The conversion of greenhouse gases (GHGs) such as methane (CH4) into green fuels and valuable chemicals is of paramount interest in achieving a decarbonized future. Therefore, there is great interest in converting CH4 into high-value chemicals such as methanol and multicarbon products for both energy and environmental sustainability because they can be stored and transported relatively easily without requiring major infrastructure, such as pipelines or liquification plants. Liquid fuels such as methanol are environmentally benign due to their biodegradability and solubility in water and can be directly converted to electricity on-site on demand and have a much lower carbon footprint. Furthermore, it is also a versatile precursor chemical for the production of a number of other materials, including formaldehyde, methylamine, and acetic acid, and can also serve as a carbon source in biotechnological applications. However, conversion of CH4, the primary constituent of natural gas (NG), is highly challenging due to the high energy required to activate the highly stable C-H bonds of methane. Commercially, NG / CH4 is converted through a two-step process that involves oxidation to synthetic gas at high temperatures and pressures and then to methanol through Fischer-Tropsch synthesis. However, this indirect conversion process is highly energy-consuming and has a high carbon dioxide footprint.

[0003] As the simplest alkane, CH4 activation to multicarbon products poses a significant challenge due to the high inertness of its C-H bond (bond energy >100 kcal / mol) and the lack of appreciable solubility in water due to its nonpolar nature. Consequently, only a few catalysts, such as Ni or Co dispersed on support ZrCh can selectively convert CH4 molecules to liquid methanol. On most noble metals like Pt, non-selective CH4 activation leads to complete oxidation to CO2 gas, leading to a loss of selectivity for useful liquid products. Exploring catalysts beyond simple metals is beneficial to expand the scope of products and activities beyond what has been achieved so far.

[0004] Therefore, there remains a need in the art for materials and methods for direct, low- temperature conversion technologies for conversion of CH4 and CO2 to high-value liquid chemicals that are cleaner and more efficient without necessitating extreme process conditions.SUMMARY

[0005] Some embodiments of the present technology use green rust, a FenFenioxyhydroxide compound, as a highly active catalyst for spontaneous (unassisted), electrocatalytic or photocatalytic conversion of CF (CO2R) to multicarbon liquid products under aerobic and anaerobic conditions at ambient / low temperature and at atmospheric pressure. Green rust films can spontaneously convert CF to high-value liquid products, such as acetate (a C2 product) solutions and methanol (a Cl product), without the application of any external bias or light at a maximum production rate of 272 pM h'1for acetate and 540 pM h'1for methanol under aerobic catalysis. Some embodiments include the spontaneous activation of CFU at ambient temperature and pressure. In some embodiments, under an applied bias of -0.7 V versus the standard calomel electrode (SCE), the concentration of the products increased and reached a maximum production rate of 340 pM h'1of acetate and 630 pM h'1of methanol in the first 2 minutes of reaction. Aspects of the present disclosure present a new benchmark for efficient electrocatalytic conversion of CFU to liquid products. Therefore, in some embodiments, green rust is an effective electrocatalyst conversion of CFU to methanol and multicarbon products. Additionally, without wishing to be bound by theory, given that green rust is also a semiconductor capable of absorbing visible light, aspects of the present disclosure are directed to its use as a photocatalyst for CFU conversion to methanol and multi carb on products.

[0006] Aspects of the present disclosure are directed to systems for converting methane to one or more products, comprising: a container comprising a first volume adapted to hold a first electrolyte; a first electrode, comprising a quantity of a green rust material positioned in the container so that the green rust material is in contact with the first electrolyte; a second electrode arranged in the container to form, together with the first electrode, electrical terminals of an electrochemical cell; an ion exchange membrane disposed in the container and separating the first volume from a second volume in which the second electrode is arranged; and a source of methane configured to supply methane to the first volume.

[0007] In some embodiments, a source of potential connected to the first and second electrodes and adapted to apply a potential difference across the electrodes. In some embodiments, the source of potential is adapted to apply a potential to the first electrode to create a voltage bias compared to a reference electrode, wherein the voltage bias comprises between about -1. 5 V and about -0.01 V under aerobic conditions or voltage bias comprises between about 0 V and about +1.5 V under anaerobic conditions.

[0008] In some embodiments, the second electrode comprises platinum. In some embodiments, the first electrolyte comprises KHCO3. In some embodiments, the source of methane is adapted to provide a continuous supply of methane to the container. In some embodiments, the source of methane is adapted to supply a mixture of methane and oxygen. In some embodiments, a source of oxygen is configured to supply oxygen to the second volume. In some embodiments, the ion exchange membrane comprises a Nafion™ membrane.

[0009] In some embodiments, the green rust material comprises at least one crystalline layer and wherein the at least one crystalline layer comprises Fen-OH and Feni-OH. In some embodiments, the ratio of Fen-OH to Feni-OH in the green rust material is between about 0.5 and about 1.0. In some embodiments, as synthesized and prior to exposure to air, the at least one crystalline layer consists essentially of Fen-OH and Feni-OH. In some embodiments, a light source is configured to illuminate the green rust material.

[0010] Aspects of the present disclosure are directed to methods for converting methane to one or more products, comprising the steps of: supplying methane to an electrolyte; contacting a first electrode comprising a quantity of a green rust material with the electrolyte; contacting a second electrode with the electrolyte; separating the first electrode from the second electrode with an ion exchange membrane; and removing the electrolyte after contacting the first electrode for a contact period.

[0011] In some embodiments, the methods further comprise a step of supplying oxygen to the electrolyte on the same side of the ion exchange membrane as the first electrode. In some embodiments, the methods further comprise the step of supplying oxygen to the electrolyte on the same side of the ion exchange membrane as the second electrode.

[0012] In some embodiments, the methods further comprise the steps of: contacting a reference electrode with the electrolyte on the same side of the ion exchange membrane asthe first electrode; and applying a potential to the first electrode to create a voltage bias compared to the reference electrode. In some embodiments, the voltage bias comprises between about -1. 5 V and about -0.01 V under aerobic conditions. In some embodiments, the voltage bias comprises between about 0 V and about +1.5 V under anaerobic conditions. In some embodiments, the green rust material is illuminated.

[0013] In some embodiments, the green rust material comprises at least one crystalline layer and wherein the at least one crystalline layer comprises Fen-OH and Felll-OH. In some embodiments, the ratio of Feu-OH to Fein-OH in the green rust material is between about 0.5 and 2.5. In some embodiments, the electrolyte comprises KHCO3. In some embodiments, the second electrode comprises platinum. In some embodiments, the contact period is between about 10 minutes and 180 minutes.

[0014] In some embodiments, the methods further comprise the step of forming the green rust material, comprising the steps of supplying N2 or Ar to a second electrolyte, wherein the second electrolyte comprises NaHCCh adjusted to a pH of about 9.5 by adding NaOH; contacting the first electrode and a counter electrode with the second electrolyte; and applying a potential difference across the first electrode and the counter electrode. In some embodiments, a step of cleaning the first electrode is performed before the step of contacting the first electrode and the counter electrode with the second electrolyte.

[0015] Various embodiments of the technology will now be described with reference to the drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The drawings show embodiments of the disclosed subject matter for the purpose of illustrating the technology. However, it should be understood that the present application is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein:

[0017] FIGs. 1 A and IB show schematic views of electrocatalytic systems and certain reactions for CH4 conversion under aerobic and anaerobic conditions, respectively, using green rust (GR, FenFeinoxyhydroxide) as the working electrode electrocatalyst according to one embodiment of the present technology.

[0018] FIG. 2 shows a schematic representation of the crystallographic structure of layered green rust with variable interlayer spacing.

[0019] FIG. 3 shows a photo of a green rust film on a steel substrate according to one embodiment of the present technology.

[0020] FIG. 4 shows the Raman spectrum of green rust formed in connection with an embodiment of the present technology.

[0021] FIG. 5 shows the multiplet peak fitting analysis of the core-level Fe 2p3 / 2 XPS spectrum of green rust film using the multiplet peaks of Fe11and Fe111in connection with an embodiment of the present technology.

[0022] FIG. 6 shows a 'H-NMR spectrum of pH = 8.5 electrolyte measured after spontaneous aerobic conversion of methane in the presence of 1 : 1 O2 taken 2 minutes after placing the GR / steel sheet into the gas-purged 0.5 M KHCO3 electrolyte in connection with an embodiment of the present technology.

[0023] FIG. 7 shows a chart of the concentration analysis of methane conversion products formed in the electrolyte as a function of reaction time at the GR film in the presence of 1 : 1 O2 after placing the GR / steel sheet into the gas-purged 0.5 M KHCO3 electrolyte under aerobic conditions in connection with an embodiment of the present technology.

[0024] FIG. 8 shows a chart of the concentration of the methane conversion products obtained via aerobic CH4 electrolysis as a function of reaction time at the GR film in connection with an embodiment of the present technology.DETAILED DESCRIPTION

[0025] Aspects of the present disclosure are directed to electrocatalytic oxidation of methane to methanol by both direct and indirect pathways. Direct activation uses a catalyst that is capable of performing the reaction: CH4+ 2OH~ CH30H + 2e~ + 2H2O. Reported catalysts that can perform are scarce. Indirect activation uses ionizing UV radiation or high anodic polarization potentials to generate powerful reactive oxygen species (ROS) such as hydroxyl radical (*OH) and peroxide (H2O2) for indirect C-H bond cleavage. Typically, the *OH generation (OH~ <->* OH + e-) requires at least +1.9 V versus the standard hydrogen electrode (SHE) under standard conditions. Furthermore, hydroxyl radicals are nonselective oxidants, which frequently leads to the overoxidation of desired products to CO2 (a dominant side product) in many investigated transition metal catalysts such as Pd, and Pt.

[0026] The present technology relates to the use of green rust or Fe11Fe111oxyhydroxide compound for spontaneous as well as electrocatalytic conversion of CF to liquid products at low temperature and ambient pressure conditions under aerobic and anaerobic conditions. The reaction schemes are shown schematically in FIGs. 1 A and IB for the aerobic and anaerobic conditions, respectively. In the aerobic cell set up, CFU conversion is achieved using O2 as the electron acceptor from the GR cathode. In contrast, under anaerobic conditions, CFU is directly oxidized at the GR anode using OH“ as the source of oxygen.

[0027] Referring now to FIGs. 1 A and IB, aspects of the present disclosure are directed to systems 10 and 100, respectively, for converting methane to one or more products, comprising: a container 110 comprising a first volume 111 adapted to hold a first electrolyte 112; a first electrode 113, comprising a quantity of a green rust material 114 positioned in the container 110 so that the green rust material is in contact with the first electrolyte 112; a second electrode 123 arranged in the container to form, together with the first electrode, electrical terminals of an electrochemical cell; an ion exchange membrane 117 disposed in the container and separating the first volume from a second volume 121 in which the second electrode is arranged; and a source of methane 115 coupled to the container and adapted to supply methane to the first volume 111. In some embodiments, the container is a tank. In other embodiments, the container is in the form of a flow conduit such that first electrode is positioned in the conduit so that the green rust material is contacted by electrolyte flowing through the conduit.

[0028] In some embodiments, the first electrode 113 is formed of steel. In other embodiments, the first electrode 113 is formed of iron or other iron-containing alloys. The first electrode 113 includes a quantity of a green rust catalyst 114. The first electrode is positioned in the container so that the green rust material 114 is in contact with the first electrolyte 112. In some embodiments, substantially all of the first electrode 113 that is in contact with the first electrolyte 112 includes the green rust material 114. In other embodiments, only a portion of the first electrode 113 that is in contact with the electrolyte includes the green rust material. In other words, in some embodiments, the green rust material covers less than all of the surface of the first electrode that is in contact with the electrolyte. In other embodiments, substantially all of the surface of the first electrode that is in contact with the electrolyte comprises the green rust material.

[0029] In some embodiments, the second electrode 123 comprises platinum, gold rhodium, or graphite.

[0030] In some embodiments, a source of potential 116 connected to the first and second electrodes (113, 123) and adapted to apply a potential difference across the electrodes. In some embodiments, the source of potential is adapted to apply a potential to the first electrode to create a voltage bias compared to a reference electrode 118, wherein the voltage bias comprises between about -1. 5 V and about -0.01 V under aerobic conditions. In some embodiments, the voltage bias comprises between about -1. 5 V and about -1.4 V, between about -1.4 V and about -1.3 V, between about -1.3 V and about -1.2 V, between about -1.2 V and about -1.1 V, between about -1.1 V and about -1.0 V, between about -1.0 V and about - 0.9 V, between about -0.9 V and about -0.8 V, between about -0.8 V and about -0.7 V, between about -0.7 V and about -0.6 V, between about -0.6 V and about -0.5 V, between about -0.5 V and about -0.4 V, between about -0.4 V and about -0.3 V, between about -0.3 V and about -0.2 V, between about -0.2 V and about -0.1 V, or between about -0.1 V and about -0.01 V.

[0031] In some embodiments, the voltage bias comprises between about 0 V and about +1.5 V under anaerobic conditions. In some embodiments, the voltage bias comprises between about 0.0 V and about 0.1 V, between about 0.1 V and about 0.2 V, between about 0.2 V and about 0.3 V, between about 0.3 V and about 0.4 V, between about 0.4 V and about 0.5 V, between about 0.5 V and about 0.6 V, between about 0.6 V and about 0.7 V, between about 0.7 V and about 0.8 V, between about 0.8 V and about 0.9 V, between about 0.9 V and about 1.0 V, between about 1.0 V and about 1.1 V, between about 1.2 V and about 1.3 V, between about 1.3 V and about 1.4 V, or between about 1.4 V and about 1.5 V.

[0032] In some embodiments, the first electrolyte 112 comprises LiHCCh, NaHCCh, KHCO3, RbHCCh, NaOH, KOH, LiOH, ammonia, 2-methyl-2-amino-l -propanol, 3- (Cyclohexylamino)-2-hy droxy- 1 -propanesulfonic acid, N-cy clohexyl-2-aminoethanesulfonic acid, N-(l, l-Dimethyl-2-hy droxy ethyl)-3-amino-2-hydroxypropanesulfonic acid, N- tris(Hydroxymethyl)methyl-4-aminobutanesulfonic acid, 2-Amino-2-methyl- 1,3 -propanedi oh bis-tris propane.

[0033] In some embodiments, the first electrolyte 112 comprises 0.5 M KHCO3. In some embodiments, the first electrolyte has a pH of about 9.5. In some embodiments, the firstelectrolyte has a pH of about 7.7. In some embodiments, the first electrolyte has a pH of about 7.4. In some embodiments, the first electrolyte has a pH of about 8.2. In some embodiments, the first electrolyte has a pH of about 7.3. In some embodiments, the first electrolyte has a pH of about 8.3. In some embodiments, the first electrolyte has a pH between about 7 and about 14. In some embodiments, the first electrolyte has a pH between about 7.3 and about 9.5. In some embodiments, the first electrolyte has a pH of between about 7.4 and about 8.2. In some embodiments, the first electrolyte has a pH of between about 9.2 and about 9.8. In some embodiments, the first electrolyte has a pH of between about 9.0 and about 10.0. In some embodiments, the first electrolyte has a pH of between about 9.5 and about 10.8. In some embodiments, the first electrolyte has a pH of between about 7.0 and about 7.6. In some embodiments, the first electrolyte has a pH of between about 6.8 and about 7.8. In some embodiments, the first electrolyte has a pH of between about 7.3 and about 8.3.

[0034] In some embodiments, the source of methane 115 is adapted to provide a continuous supply of methane to the container. In some embodiments, the source of methane 115 is adapted to supply a mixture of methane and oxygen. In some embodiments, a source of oxygen 125 is configured to supply oxygen to the second volume.

[0035] In some embodiments, the green rust material 114 comprises at least one crystalline layer and wherein the at least one crystalline layer comprises Fen-OH and Feni-OH. In some embodiments, the ratio of Fen-OH to Feni-OH in the green rust material is between about 0.5 and about 2.5. In some embodiments, as synthesized and prior to exposure to air, the at least one crystalline layer consists essentially of Fen-OH and Feni-OH. In some embodiments, a light source is configured to illuminate the green rust material.

[0036] Aspects of the present disclosure are directed to methods for converting methane to one or more products, comprising the steps of: supplying methane to an electrolyte; contacting a first electrode comprising a quantity of a green rust material with the electrolyte; contacting a second electrode with the electrolyte; separating the first electrode from the second electrode with an ion exchange membrane; and removing the electrolyte after contacting the first electrode for a contact period.

[0037] In some embodiments, the methods further comprise a step of supplying oxygen to the electrolyte on the same side of the ion exchange membrane as the first electrode. In someembodiments, the methods further comprise the step of supplying oxygen to the electrolyte on the same side of the ion exchange membrane as the second electrode.

[0038] In some embodiments, the methods further comprise the steps of: contacting a reference electrode with the electrolyte on the same side of the ion exchange membrane as the first electrode; and applying a potential to the first electrode to create a voltage bias compared to the reference electrode. In some embodiments, the voltage bias comprises between about -1. 5 V and about -0.01 V under aerobic conditions. In some embodiments, the voltage bias comprises between about 0 V and about +1.5 V under anaerobic conditions. In some embodiments, the green rust material is illuminated.

[0039] In some embodiments, the green rust material comprises at least one crystalline layer and wherein the at least one crystalline layer comprises Fen-OH and Feni-OH. In some embodiments, the ratio of Fen-OH to Feni-OH in the green rust material is between about 0.5 and 2.5. In some embodiments, the electrolyte comprises KHCO3. In some embodiments, the second electrode comprises platinum. In some embodiments, the contact period is between about 10 minutes and 180 minutes. In some embodiments, the contact period is between about 10 minutes and 60 minutes. In some embodiments the contact period is about 10 minutes, at which time the electrolyte is extracted from the container for further processing. In some embodiments, the contact period is about 20 minutes. In some embodiments, the contact period is about 30 minutes. In some embodiments, the contact period is about 40 minutes. In some embodiments, the contact period is about 50 minutes. In some embodiments, the contact period is about 60 minutes. In some embodiments, the contact period is about 60 minutes. In some embodiments, the contact period is about 70 minutes. In some embodiments, the contact period is about 80 minutes. In some embodiments, the contact period is about 90 minutes. In some embodiments, the contact period is about 100 minutes. In some embodiments, the contact period is about 110 minutes. In some embodiments, the contact period is about 120 minutes. In some embodiments, the contact period is about 130 minutes. In some embodiments, the contact period is about 140 minutes. In some embodiments, the contact period is about 150 minutes. In some embodiments, the contact period is about 160 minutes. In some embodiments, the contact period is about 170 minutes. In some embodiments, the contact period is about 180 minutes.

[0040] In some embodiments, the methods further comprise the step of forming the green rust material, comprising the steps of: supplying N2 or Ar to a second electrolyte, wherein thesecond electrolyte comprises NaHCCh adjusted to a pH of about 9.5 by adding NaOH; contacting the first electrode and a counter electrode with the second electrolyte; and applying a potential difference across the first electrode and the counter electrode. In some embodiments, a step of cleaning the first electrode is performed before the step of contacting the first electrode and the counter electrode with the second electrolyte.

[0041] Aerobic Electrocatalytic Conversion. In an exemplary aerobic cell set up, CH4 conversion is achieved using O2 as the electron acceptor from the GR cathode and is given by for methanol as:CH4+ O2+ 2e~ + H2O CH30H + 2OH~

[0042] Reaction- 1 (Cathode)

[0043] The corresponding equation at the anode can be water oxidation to produce dioxygen. In this case, the ion- exchange membrane will transport OH- produced at the cathode to the anode, where it is oxidized back to dioxygen. This reaction under alkaline conditions is given by:

[0044] Reaction-2 (Anode) 40 H~ O2+ 4e~ + 2H2O

[0045] Anaerobic Electrocatalytic Conversion. In this exemplary reaction scheme, a green rust working electrode is used as an anode, where CH4 is directly oxidized to various products, such as methanol, acetate, acetone, using OH- as the source of oxygen. The reaction at the working electrode, under alkaline conditions, is given for methanol as:

[0046] Reaction-3 (Anode) CH^ + 20 H~ <-> CH30H + 2e~ + 2H2O

[0047] The corresponding reaction at the cathode counter electrode is a dioxygen reduction reaction given by:

[0048] Reaction-4 (Cathode) O2+ 4e~ + 2H2O <-> 40 H~

[0049] In both schemes, an ion-exchange membrane (typically Nafion) allows the selective transport of H+and OH' across the two compartments, but prevents the cross-over of the CH4 conversion product to the other chamber, where they can undergo further reaction.

[0050] Spontaneous Conversion of CH4 to liquid products: In this exemplary setup, no external bias is applied to the GR film. Alternatively, experiments can be done with GRpowder samples, which can be added directly into the gas-sparged electrolyte without requiring any external electrical connections.

[0051] Formation and Characterization of Green Rust (Fe"-Fe,nOxyhydroxide) Film and Powder

[0052] Green rust (GR) is a Fen-Fenioxyhydroxide compound that is known to form as a result of the pitting corrosion of iron and iron alloys under anaerobic conditions, GR is a metastable compound that protects iron once formed and is prevalent in many natural anoxic and suboxic environments, such as in hydromorphic soils, groundwater, ferruginous lakes, and mine drainage sites. It includes a layered structure with a general formula [(Fen,Mgn)i-xFexni(OH)2]'<[x / nAllmH2O]'<where interlayer anions ( A" = SO ~, CO ~, Cl~ etc) compensate the excess positive charges of structural Fe111. In the schematic illustration in FIG. 2, two crystalline layers 200 and 210 and one anion layer 205 are shown. This flexible interlayer structure can expand or contract in the c-axis direction from 8 to 40 A depending on the size of the interlayer molecule. Consequently, GR can support a variety of intercalants, such as metals, metalloids, and other anions, making it highly reactive to many chemicals and environmental pollutants, such as the reduction of nitrate, nitrite, heavy metals, and the dechlorination of chlorinated hydrocarbons. Referred to as the ‘seed of all life’, GR is one of the key catalytic minerals believed to have played a role in the emergence of life on Earth in CCh-rich alkaline hydrothermal vents by catalyzing CCER to many life-nourishing chemicals. Its hydrated interlayered structure also bears structural and compositional similarity to iron-containing metalloenzymes, which has led to some to suggest that it may have served as their ancestral percussor.

[0053] Thin Films: In one exemplary embodiment, a thin film of synthetic green rust mineral was formed directly on a steel substrate in an N2 environment by electrochemical cycling in a 0.5 M NaHCCh solution whose pH was adjusted to 9.5 using a NaOH solution. The initially silver metallic luster of the steel slowly took on a dark green hue upon continuous cycles (50 cycles), as shown in FIG. 3, providing visual confirmation of the formation of a green rust on the steel surface. This electrochemically formed film is relatively much more stable in air for up to several hours compared to the green rust powder typically produced by the co-precipitation technique, which undergoes oxidation and phase transformation to other FeOxphases within several minutes under aerobic conditions.

[0054] Referring now to FIG. 4, the Raman spectrum of this film was obtained under ambient air conditions after synthesis in a wide wavenumber range from 75 cm’1to 1000 cm’1, where most of the crystalline and amorphous FeOxphases exhibit characteristic peaks. The film showed peaks at 427 cm’1and 503 cm’1wavenumbers that are characteristic of the Fen-OH and Feni-OH stretching vibrations of mixed-valent GR. The absence of other bands in the 300-800 cm’1range confirms that the as-synthesized material is a pure phase of GR. However, prolonged exposure to air for more than 4 hours results in the transformation of green rust to other FeOxphases, predominantly hematite, as seen from the strong Raman resonance peaks at 292 cm’1, as well as the visual change in the color of the sample from dark green to yellow brown seen with increasing exposure time in air.

[0055] Green Rust Powder: In some embodiments, the powder samples used for some measurements are synthesized using the simple co-precipitation method that involves the oxidation of Fe(OH)2 in an aqueous NaOH solution containing the respective Fe11and Fe111sulfate or chloride salts.

[0056] Characterization: The composition of the film was determined by X-ray photoelectron spectroscopy. Both the core and satellite peaks of Fe 2pi / 2 and Fe 2ps / 2 as well as the Fe 3p peak are sensitive to the ionic environment and can be used for qualitative analysis of the ferrous (Fe11) and ferric (Fe111) cation composition in Fe minerals. The survey spectrum of the sample showed the presence of Fe, O, and C. Therefore, the interlayer anion is likely carbonate. Further composition analysis was performed through multiplet peak fitting analysis of Fe 2p3 / 2 using the multiplet peaks of Fe11and Fe111using the fitting parameters of Grosvenor et al. employed for FesC , another mixed-valent Fen-Fenimineral. The spectrum along with its multiplet peaks is shown in FIG. 5. A Fe11to Fe111ratio of 0.66 was obtained for carbonate GR produced in this study, which is close to the value of 1 reported in the literature for GR synthesized by electrochemical polarization but can be as high as 2.5.

[0057] Aerobic CH4 conversion: Spontaneous conversion

[0058] In one exemplary embodiment, green rust film on the steel substrate was exposed to 0.5 M KHCO3 electrolyte purged with CH4 and O2 in a volumetric ratio of 1 : 1 for varying length of time. A small amount of the electrolyte was periodically withdrawn for the analysis of products through 1H- nuclear magnetic resonance ('H-NMR) spectroscopy. To maximizethe concentration of liquid phase products in the electrolyte and allow for unambiguous sensitive detection of analytes by NMR spectroscopy, a large GR coated steel sheet was used as a working electrode that had a geometric surface area (S) of 62 cm2with respect to the electrolyte volume (V) of 40 ml, an S / V ratio of 1.55 cm.

[0059] FIG. 6 shows the representative 'H-NMR spectrum of the 0.5 M KHCO3 (pH = 8.5) electrolyte measured after spontaneous aerobic conversion of CH4 in the presence of 1 : 1 O2 taken 2 minutes after placing the GR / steel sheet in the gas-purged electrolyte.Trimethylsilylpropanoic acid (TMSP-d4) at a concentration of 58 pM was added to an electrolyte as an internal standard to allow quantitative compositional analysis and has a peak resonance at 0 ppm. The background H2O / D2O peaks at 4.80 ppm and is not shown for clarity. A strong singlet peak of dissolved CH4 can be observed at 0.187 ppm. In addition, peaks can also be seen at 1.93 ppm, and 3.36 ppm, which correspond to the peaks of acetate / acetic acid (H3CCOO / H3CCOOH), and methanol (CH3OH), respectively.

[0060] From the quantitative analysis of the 'H-NMR spectrum, the composition of the electrolytes corresponding to the peak values was determined and is shown in FIG. 7 for various reaction times. The results confirm that CH4 could be spontaneously activated to form methanol and acetate on GR without the application of any external bias or light. In some embodiments, a maximum production rate of 272 pM h'1for acetate and 540 pM h'1for methanol could be achieved under spontaneous aerobic catalysis.

[0061] Aerobic CH4 conversion: Electrocatalytic Studies

[0062] In some embodiments, the electrocatalytic activity of green rust for electrochemical CH4 conversion was evaluated using a three-electrode H-cell set-up with a Nafion 117 proton exchange membrane separating the two compartments. The electrolyte on both sides was 0.5 M KHCO3 unless otherwise noted. A platinum mesh served as a counter electrode on the anode side where the corresponding reaction was water oxidation. In the case of electrocatalytic studies, a voltage bias of -0.7 V versus standard calomel electrode (SCE) was applied to the GR film exposed to gas-purged electrolytes. The electrolyte was sampled again periodically for product analysis.

[0063] The exemplary compositional analysis, as obtained from the ' H-NMR spectrum, of the electrolyte done at various time intervals is shown in FIG. 8. Analysis shows that at pH = 8.5, the predominant reduction products were again acetate, and methanol, but were producedat a higher concentration. Under an applied bias of -0.7 V versus the standard calomel electrode (SCE), the concentration of the products increased and reached a maximum production rate of 340 pM h'1of acetate and 630 pM h'1of methanol in the first 2 minutes of reaction.

[0064] All the above exemplary results show that green rust is a highly active catalyst for CH4 conversion that uses either zero or very low overpotentials for CH4 activation and conversion to multicarbon products like acetate that typically involve the difficult C-C coupling.

[0065] This disclosure shows that green rust, a compound that can be formed both naturally and synthetically through the corrosion of iron and its alloys, as well as from precipitation from Fen-rich aqueous solution or phase transformation from other Fe111compounds, can be used for the effective conversion of CH4 to a variety of products including acetic acid, and methanol via electrocatalytic process. Given that green rust is a semiconductor capable of absorbing visible light, we propose its use as a photocatalyst for the CH4 conversion to variety of products.EXAMPLES

[0066] Synthesis of green rust film: In one exemplary embodiment, experiments were performed on GR thin films coated on steel substrates. A commercially available steel sheet of 2 cm x 15.5 cm was degreased sequentially with soapy water, acetone, ethanol, and deionized water and finally cleaned by electrochemical polarization with a cathodic bias of -1 V in 0.5 M NaHCCE electrolyte of pH=9.5 for 4000 seconds. Here, a platinum wire electrode and a standard calomel electrode (SCE) were used as the counter and reference electrodes, respectively. A thin film of synthetic green rust mineral was formed directly on the cleaned steel substrate through cyclic voltammetry scans between -1 V and -0.2 V at a scan rate of 100 mV / sec for 50 cycles. The development of a dark green hue on the steel surface provided visual confirmation for the formation of GR. This electrochemically formed film is relatively much more stable in air, up to a day, compared to GR powder that is typically produced by the co-precipitation technique, which undergoes oxidation and phase transformation to other FeOxphases within an hour under aerobic conditions. However, all GR samples are very stable (up to at least a week) under CH4 and anerobic conditions. To prevent air oxidation, GR samples are freshly prepared prior to each CH4 conversionexperiment. Powder samples used for some measurements will be synthesized using the simple co-precipitation method of Genin et al. that involves the oxidation of Fe(OH)2 in an aqueous NaOH solution containing the respective Fe11and Fe111salts. Both films and powder samples were characterized by Raman, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy.

[0067] Spontaneous Conversion of CH4 to liquid Products: In one exemplary embodiment, the ability of GR for spontaneous, unassisted activation of GR was evaluated by directly exposing a GR film deposited on steel substrate to 0.5 M KHCO3 sparged with 1 : 1 mixture of CH4 / O2. No electrical leads were connected, and the cell was kept in dark.

[0068] Aerobic and Anaerobic Electrocatalytic (assisted) CH4 Conversion: In some exemplary embodiments, the performance of GR for electrocatalytic methane conversion under aerobic and anaerobic conditions was evaluated using an air-tight three-electrode glass H-cell setup. The cathode and anode sides were separated by a proton-exchange Nafion 117 membrane (MTI corp.) to prevent intermixing of the product stream except for the generated protons. A platinum mesh electrode and SCE served as the counter and reference electrodes. Potential measured against SCE was later converted to the reversible hydrogen electrode (RHE) scale. The electrolyte on both sides was 0.5 M K2CO3.

[0069] Aerobic CH4 conversion. In one exemplary embodiment, for aerobic electrocatalytic CH4 conversion, GR / Steel substrate was used as a cathode and a mixture of CH4 / O2 (1 : 1) was bubbled through the cathode compartment, which also housed SCE reference electrode. The corresponding reaction at the Pt mesh anode was water oxidation on the anode side. Before starting the experiments, the catholyte was purged with the reaction gas (CH4 / O2) for 15 minutes and was continuously purged for the duration of the experiments. Catalytic performance was evaluated through potentiostatic measurements, in which a constant cathodic potential was applied to the WE for various time intervals. The liquid phase samples were periodically extracted to determine their composition using 'H-NMR spectroscopy. The total Faradaic efficiency (FE) of the CH4 reduction to the products was determined using the equation:Q Moles of chemical product (moles) v.No.of electrons(n) X96500 ( - — )

[0070] FE (%) = - - - - - - -Amout of charge passed, It (C)

[0071] Here n is 2, 8, 16, 6, and 12, for the reaction forming formate, acetate, acetone, methanol, and ethanol, respectively.

[0072] Raman Spectroscopy: Raman spectra were acquired with a HORIBA Scientific LabRAM HR Evolution spectrometer at room temperature using the 633nm line of a heliumneon laser at a laser power of 16 mW. Band gap PL measurements in the energy range of 3.8 to 1.2 eV were done using 325nm excitation energy of He-Cd Laser. Care was taken to avoid laser heating of the sample.

[0073] X- ray Photoelectron Spectroscopy (XPS): XPS measurements were performed using a Physical Electronics PHI 5000 VersaProbe operating with Al Ka radiation (hv= 1486.6 eV) from a monochromatic X-ray source at room temperature to characterize the lattice composition of GR. The binding energy (BE) referenced to the Fermi level of a cleaned Au reference sample.

[0074] 1H-NMR spectroscopy and GC: q1H-NMR Spectroscopy: Liquid products of CH4 conversion reaction was analyzed using quantitative 'H-NMR spectroscopy, which was performed using a Bruker 600 (14.1 Tesla) 5 channel AVIII NMR spectrometer at room temperature with pulse angle, water suppression, a relaxation time of 5s and no spinning. For all 'H-NMR run, the solution consisted of 600 pL of sample analyte, 65 pL of D2O, and 35 pL of 10 mM DMSO as the internal reference. The total concentration of the dissolved products was determined from the integrated intensity of its resonance normalized to number of H nuclei that cause that resonance and relating to the normalized signal from TMSP. The OH proton of many products is in fast exchange with the residual water (HOD) resonance at 4.78 ppm, therefore a separate resonance is not observed for this proton.

[0075] Although the technology has been described and illustrated with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the present invention.

Claims

CLAIMSWhat is claimed is:

1. A system for converting methane to one or more products, comprising: a container comprising a first volume and a second volume adapted to hold a first electrolyte; an ion exchange membrane disposed in the container and separating the first volume from the second volume; a first electrode, comprising a quantity of a green rust material positioned in the first volume of the container to contact the first electrolyte; a second electrode positioned in the second volume of the container to form, together with the first electrode, electrical terminals of an electrochemical cell; and a source of methane configured to supply methane to the first volume.

2. The system of claim 1, further comprising a source of potential connected to the first and second electrodes and adapted to apply a potential difference across the electrodes.

3. The system as in claim 1 or 2, wherein the second electrode comprises platinum.

4. The system as in any of the preceding claims, wherein the first electrolyte comprises KHCO3.

5. The system as in any of the preceding claims, wherein the source of methane is adapted to provide a continuous supply of methane to the container.

6. The system as in any of the preceding claims, wherein the source of methane is adapted to supply a mixture of methane and oxygen.

7. The system as in any of claims 1-5, further comprising a source of oxygen configured to supply oxygen to the second volume.

8. The system as in any of claims 2-6, wherein the source of potential is adapted to apply a potential to the first electrode to create a voltage bias compared to a reference electrode, wherein the voltage bias comprises between about -1. 5 V and about -0.01 V under aerobic conditions.

9. The system as in any of claims 2-5 or 7, wherein the source of potential is adapted to apply a potential to the first electrode to create a voltage bias compared to a reference electrode, wherein the voltage bias comprises between about 0.0 V and about +1.5 V under anaerobic conditions.

10. The system as in any of the preceding claims, wherein the green rust material comprises at least one crystalline layer and wherein the at least one crystalline layer comprises Fen-OH and Feni-OH.

11. The system as in any of the preceding claims, wherein the ratio of Fen-OH to Feni-OH in the green rust material is between about 0.5 and about 2.5.

12. The system as in any of the preceding claims, wherein, as synthesized and prior to exposure to air, the at least one crystalline layer consists essentially of Fen-OH and Feni-OH.

13. The system as in any of the preceding claims further comprising a light source configured to illuminate the green rust material.

14. A method for converting methane to one or more products, comprising the steps of supplying methane to an electrolyte; contacting a first electrode comprising a quantity of a green rust material with the electrolyte; contacting a second electrode with the electrolyte; separating the first electrode from the second electrode with an ion exchange membrane; and removing the electrolyte after contacting the first electrode for a contact period.

15. The method of claim 14, further comprising a step of supplying oxygen to the electrolyte on the same side of the ion exchange membrane as the first electrode.

16. The method of claim 14, further comprising the step of supplying oxygen to the electrolyte on the same side of the ion exchange membrane as the second electrode.

17. The method as in any of claims 14-16, further comprising the steps of: contacting a reference electrode with the electrolyte on the same side of the ion exchange membrane as the first electrode; and applying a potential to the first electrode to create a voltage bias compared to the reference electrode.

18. The method of claim 17, wherein the voltage bias comprises between about -1. 5 V and about -0.01 V under aerobic conditions.

19. The method of claim 17, wherein the voltage bias comprises between about 0.0 V and about +1.5 V under anaerobic conditions.

20. The method as in any of claims 14-19, wherein the green rust material comprises at least one crystalline layer and wherein the at least one crystalline layer comprises Fe11- OH and Felll-OH.

21. The method as in any of claims 14-20, wherein the ratio of Feu-OH to Fein-OH in the green rust material is between about 0.5 and 2.5.

22. The method as in any of claims 14-21, wherein the electrolyte comprises KHCO3.

23. The method as in any of claims 14-22, wherein the second electrode comprises platinum.

24. The method as in any of claims 14-23, wherein the contact period is between about 10 minutes and 60 minutes.

25. The method as in any of claims 14-24, further comprising the step of forming the green rust material, comprising the steps of:supplying N2 or Ar to a second electrolyte, wherein the second electrolyte comprises NaHCCh adjusted to a pH of about 9.5 by adding NaOH; contacting the first electrode and a counter electrode with the second electrolyte; and applying a potential difference across the first electrode and the counter electrode.

26. The method of claim 25 further comprising, before the step of contacting the first electrode and the counter electrode with the second electrolyte, a step of cleaning the first electrode.

27. The method as in any of claims 14-17 or 20-26, further comprising illuminating the green rust material.

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