AND WATER

FIELD

[0001] The present disclosure generally relates to the field of electrochemical reactions, and more particularly to methods and/or systems for electrochemical production of butanol from carbon dioxide and water. BACKGROUND

[0002] The combustion of fossil fuels in activities such as electricity generation, transportation, and manufacturing produces billions of tons of carbon dioxide annually. Research since the 1970s indicates increasing concentrations of carbon dioxide in the atmosphere may be responsible for altering the Earth's climate, changing the pH of the ocean and other potentially damaging effects. Countries around the world, including the United States, are seeking ways to mitigate emissions of carbon dioxide.

[0003] A mechanism for mitigating emissions is to convert carbon dioxide into economically valuable materials such as fuels and industrial chemicals. If the carbon dioxide is converted using energy from renewable sources, both mitigation of carbon dioxide emissions and conversion of renewable energy into a chemical form that can be stored for later use will be possible.

SUMMARY OF THE PREFERRED EMBODIMENTS

[0004] A method for electrochemical reduction of carbon dioxide to produce butanol may include, but is not limited to, steps (A) to (D). Step (A) may introduce water to a first compartment of an electrochemical cell. The first compartment may include an anode. Step (B) may introduce carbon dioxide to a second compartment of the electrochemical cell. The second compartment may include a solution of an electrolyte, a catalyst, and a cathode. Step (C) may apply an electrical potential between the anode and the cathode in the electrochemical cell sufficient for the cathode to reduce the carbon dioxide to at least butanol. Step (D) may remove butanol from the second cell compartment. [0005] Another method for electrochemical reduction of carbon dioxide to produce butanol may include, but is not limited to, steps (A) to (F). Step (A) may introduce water to a first compartment of a first electrochemical cell. The first compartment may include an anode. Step (B) may introduce carbon dioxide to a second compartment of the first electrochemical cell. The second compartment may include a solution of an electrolyte, a catalyst, and a cathode. Step (C) may apply an electrical potential between the anode and the cathode in the first electrochemical cell sufficient for the cathode to reduce the carbon dioxide to at least a two- carbon intermediate. Step (D) may remove the two-carbon intermediate from the first electrochemical cell. Step (E) may introduce the two-carbon intermediate to a second electrochemical cell. The second electrochemical cell may include an anode in a first cell compartment and a cathode in a second cell compartment. The cathode may reduce the two-carbon intermediate to at least butanol. Step (F) may remove butanol from the second electrochemical cell.

[0006] A system for electrochemical reduction of carbon dioxide to produce butanol may include, but is not limited to, a first electrochemical cell including a first cell compartment, an anode associated with the first cell compartment, a second cell compartment, a separator configured to separate the first cell compartment and the second cell compartment, and a cathode and a catalyst associated with the second cell compartment. The system may also include a carbon dioxide source, where the carbon dioxide source is coupled with the second cell compartment and is configured to supply carbon dioxide to the cathode for reduction of the carbon dioxide to at least a two-carbon intermediate. The system may further include a second electrochemical cell configured to receive the two-carbon intermediate. The second electrochemical cell may include a first cell compartment, an anode associated with the first cell compartment, a second cell compartment, a separator configured to separate the first cell compartment and the second cell compartment, and a cathode positioned within the second cell compartment of the second electrochemical cell. The cathode of the second electrochemical cell may be configured to reduce the two-carbon intermediate to butanol.

[0007] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the disclosure as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the disclosure and together with the general description, serve to explain the principles of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The numerous advantages of the present disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which: FIG. 1 is a block diagram of a system in accordance with an embodiment of the present disclosure;

FIG. 2 is a block diagram of a system in accordance with another embodiment of the present disclosure;

FIG. 3 is a flow diagram of an example method of electrochemical production of butanol; and

FIG. 4 is a flow diagram of another example method of electrochemical production of butanol. DETAILED DESCRIPTION

[0009] Reference will now be made in detail to the presently preferred embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. [0010] In accordance with some embodiments of the present disclosure, an electrochemical system is provided that generally allows carbon dioxide and water to be converted to butanol. In some embodiments, the production of butanol from carbon dioxide and water may occur in a one-stage or a two- stage process. In the one-stage process, butanol may be produced with low yields and low selectivity. In the two-stage process, butanol may be produced with improved reaction rates, yield, and selectivity as compared to the direct conversion of carbon dioxide and water to butanol in the one- stage process. [0011 ] Butanol (which includes the isomer 2-butanol, also called sec- butanol, and the isomer 1 -butanol, also called n-butanol) is an industrial chemical used around the world. Industrially, butanol is produced via gas phase chemistry, using oil and natural gas as feedstocks. 2-butanol may be produced via the acid -catalyzed hydration of 1-butene or 2-butene, where 1-butene and 2-butene may be obtained via catalytic cracking of petroleum. 1-butanol may be produced via the hydroformylation of propylene to butryaldehyde, where the butyraldehyde is subsequently hydrogenated to 1 - butanol. Propylene itself may be derived from catalytic cracking of petroleum, whereas the carboxyl group introduced via hydroformylation may be from syngas derived from natural gas. In addition to using nonrenewable oil and natural gas as feedstocks, the overall process of industrially synthesizing butanol using current techniques requires a large amount of energy, which generally comes from natural gas. The combustion of natural gas contributes to the concentration of carbon dioxide in the atmosphere and thus, global climate change.

[0012] Additional production techniques for butanol include production of butanol via biological pathways. However, such biological processes can be resource intensive due to the large amounts of land, fertilizer, and water necessary to grow the crops used to sustain fermentation processes.

[0013] In some embodiments of the present disclosure, the energy used by the system may be generated from an alternative energy source to avoid generation of additional carbon dioxide through combustion of fossil fuels. In general, the embodiments for the production of butanol from carbon dioxide and water do not require oil or natural gas as feedstocks. Some embodiments of the present invention thus relate to environmentally beneficial methods and systems for reducing carbon dioxide, a major greenhouse gas, in the atmosphere thereby leading to the mitigation of global warming. Moreover, certain processes herein are preferred over existing electrochemical processes due to being stable, efficient, having scalable reaction rates, occurring in water, and having selectivity of butanol. [0014] For electrochemical reductions, the electrode may be a suitable conductive electrode, such as Al, Au, Ag, C, Cd, Co, Cr, Cu, Cu alloys (e.g., brass and bronze), Ga, Hg, In, Mo, Nb, Ni, Ni alloys, Ni-Fe alloys, Sn, Sn alloys, Ti, V, W, Zn, stainless steel (SS), austenitic steel, ferritic steel, duplex steel, martensitic steel, Nichrome, elgiloy (e.g., Co-Ni-Cr), degenerately doped n-Si, degenerately doped n-Si:As and degenerately doped n-Si:B. Other conductive electrodes may be implemented to meet the criteria of a particular application. For photoelectrochemical reductions, the electrode may be a p-type semiconductor, such as p-GaAs, p-GaP, p-lnN, p-lnP, p-CdTe, p-GalnP ₂ and p-Si. Other semiconductor electrodes may be implemented to meet the criteria of a particular application. [0015] Before any embodiments of the invention are explained in detail, it is to be understood that the embodiments may not be limited in application per the details of the structure or the function as set forth in the following descriptions or illustrated in the figures of the drawing. Different embodiments may be capable of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of terms such as "including," "comprising," or "having" and variations thereof herein are generally meant to encompass the item listed thereafter and equivalents thereof as well as additional items. Further, unless otherwise noted, technical terms may be used according to conventional usage.

[0016] A use of electrochemical or photoelectrochemical reduction of carbon dioxide and water, tailored with certain elect rocatalysts, may produce butanol in a yield of approximately less than 10% as a relative percentage of carbon-containing products, particularly when metallic cathode materials are employed. The reduction of the carbon dioxide may be suitably achieved efficiently in a divided electrochemical or photoelectrochemical cell in which (i) a compartment contains an anode suitable to oxidize or split the water, and (ii) another compartment contains a working cathode electrode and a catalyst. The compartments may be separated by a porous glass frit, microporous separator, ion exchange membrane, or other ion conducting bridge. Both compartments generally contain an aqueous solution of an electrolyte. Carbon dioxide gas may be continuously bubbled through the cathodic electrolyte solution to saturate the solution or the solution may be pre-saturated with carbon dioxide. [0017] Advantageously, the carbon dioxide may be obtained from any source (e.g., an exhaust stream from fossil-fuel burning power or industrial plants, from geothermal or natural gas wells or the atmosphere itself). Most suitably, the carbon dioxide may be obtained from concentrated point sources of generation prior to being released into the atmosphere. For example, high concentration carbon dioxide sources may frequently accompany natural gas in amounts of 5% to 50%, exist in flue gases of fossil fuel (e.g., coal, natural gas, oil, etc.) burning power plants, and high purity carbon dioxide may be exhausted from cement factories, from fermenters used for industrial fermentation of ethanol, and from the manufacture of fertilizers and refined oil products. Certain geothermal steams may also contain significant amounts of carbon dioxide. The carbon dioxide emissions from varied industries, including geothermal wells, may be captured on-site. Separation of the carbon dioxide from such exhausts is known. Thus, the capture and use of existing atmospheric carbon dioxide in accordance with some embodiments of the present invention generally allow the carbon dioxide to be a renewable and unlimited source of carbon.

[0018] Referring to FIG. 1 , a block diagram of a system 100 is shown in accordance with a specific embodiment of the present invention. System 100 may be utilized for the one-stage process for the production of butanol from carbon dioxide and water. The system (or apparatus) 100 generally comprises a cell (or container) 102, a liquid source 104, a power source 106, a gas source 108, a first extractor 110 and a second extractor 112. A product or product mixture may be presented from the first extractor 110. An output gas may be presented from the second extractor 112.

[0019] The cell 102 may be implemented as a divided cell. The divided cell may be a divided electrochemical cell and/or a divided photochemical cell. The cell 102 is generally operational to reduce carbon dioxide (C0 ₂ ) into butanol. The reduction generally takes place by bubbling carbon dioxide and an aqueous solution of an electrolyte in the cell 102. A cathode 120 in the cell 102 may reduce the carbon dioxide into a product mixture that may include one or more compounds. For instance, the product mixture may include at least one of butanol, formic acid, methanol, glycolic acid, glyoxal, acetic acid, ethanol, acetone, or isopropanol. In particular implementations, butanol may account for less than approximately 10% of the total yield of organic compounds in the product mixture. [0020] The cell 102 generally comprises two or more compartments (or chambers) 114a-114b, a separator (or membrane) 116, an anode 118, and a cathode 120. The anode 118 may be disposed in a given compartment (e.g., 114a). The cathode 120 may be disposed in another compartment (e.g., 114b) on an opposite side of the separator 116 as the anode 118. An aqueous solution 122 may fill both compartments 114a-114b. The aqueous solution 122 may include water as a solvent and water soluble salts (e.g., potassium chloride (KG)). A catalyst 124 may be added to the compartment 114b containing the cathode 120.

[0021 ] The liquid source 104 may implement a water source. The liquid source 104 may be operational to provide pure water to the cell 102.

[0022] The power source 106 may implement a variable voltage source. The power source 106 may be operational to generate an electrical potential between the anode 118 and the cathode 120. The electrical potential may be a DC voltage.

[0023] The gas source 108 may implement a carbon dioxide source. The source 108 is generally operational to provide carbon dioxide to the cell 102. In some embodiments, the carbon dioxide is bubbled directly into the compartment 114b containing the cathode 120.

[0024] The first extractor 110 may implement an organic product and/or inorganic product extractor. The extractor 110 is generally operational to extract (separate) one or products of the product mixture (e.g., butanol) from the electrolyte 122. The extracted products may be presented through a port 126 of the system 100 for subsequent storage and/or consumption by other devices and /or processes.

[0025] The second extractor 112 may implement an oxygen extractor. The second extractor 112 is generally operational to extract oxygen (e.g., O ₂ ) byproducts created by the reduction of the carbon dioxide and/or the oxidation of water. The extracted oxygen may be presented through a port devices and/or processes. Chlorine and/or oxidatively evolved chemicals may also be byproducts in some configurations, such as in an embodiment of processes other than oxygen evolution occurring at the anode 118. Such processes may include chlorine evolution, oxidation of organics to other saleable products, waste water cleanup, and corrosion of a sacrificial anode. Any other excess gases (e.g., hydrogen) created by the reduction of the carbon dioxide and water may be vented from the cell 102 via a port 130.

[0026] In the reduction of carbon dioxide to butanol, water may be oxidized (or split) to protons and oxygen at the anode 118 while the carbon dioxide is reduced to the product mixture at the cathode 120. The electrolyte 122 in the cell 102 may use water as a solvent with any salts that are water soluble, including potassium chloride (KCl) and with a suitable catalyst 124, such as an imidazole catalyst, a pyridine catalyst, or a substituted variant of imidazole or pyridine. Cathode materials generally include any conductor. However, efficiency of the process may be selectively increased by employing a catalyst/cathode combination selective for reduction of carbon dioxide to butanol (and/or other compounds included in the product mixture). For catalytic reduction of carbon dioxide, the cathode materials may include Sn, Ag, Cu, steel (e.g., 316 stainless steel), and alloys of Cu and Ni. The materials may be in bulk form. Additionally and/or alternatively, the materials may be present as particles or nanoparticles loaded onto a substrate, such as graphite, carbon fiber, or other conductor.

[0027] An anode material sufficient to oxidize or split water may be used. The overall process may be generally driven by the power source 106. Combinations of cathodes 120, electrolytes 122, and catalysts 124 may be used to control the reaction products of the cell 102.

[0028] In one implementation of the one-stage process of producing butanol from carbon dioxide and water, a low yield, low selectivity for butanol may be obtained using an approximately 400mM concentration of imidazole catalyst, KCl electrolyte, and a 316 stainless steel cathode. The process may proceed via the following reactions, with the heterocyclic catalyst facilitating the reaction similar to NADPH/NADP ⁺ in the Calvin Cycle:

Cathode: 4CO ₂ + 24H ⁺ + 24e → C ₄ Η ₉ ΟΗ + 7H ₂ O (E° = -0.41V vs. SCE at pH 6) Anode: 12H ₂ O→ 24H ⁺ + 24e- + 6O ₂ (E° = 0.63V vs. SCE at pH 6)

Cell: 4CO ₂ + 5H ₂ O→ C ₄ H9OH + 6O ₂ (E° = -1.04V at 25°C) [0029] The one-stage process of producing butanol from carbon dioxide and water may yield additional organic products, including formic acid and acetic acid, which were observed by gas chromatography (GC) and nuclear magnetic resonance (NMR) with greater relative yields than butanol. Products other than butanol in the product mixture (e.g., formic acid, acetic acid, methanol, ethanol, acetone, and/or propanol) may be reaction intermediates. For instance, because the reaction to produce butanol requires a transfer of 24 electrons and protons, butanol production may be likely to be kinetically limited relative to reaction intermediates that require fewer electron and proton transfers. For greater selectivity, yield, and reaction rates, the two-stage process for producing butanol from carbon dioxide and water may be employed. The two-stage process includes two cells with the following reactions:

Cell 1 : 2C0 ₂ + H ₂ 0 -» OCHCHO + 1 ¹ / ₂ 0 ₂ (E° = -1.44V at 25°C)

Cell 2: 2(0CHCH0) + 3H ₂ 0 -» C4H9OH + 30 ₂ (E° = -1.76V at 25°C)

[0030] The reaction in each of cell 1 and cell 2 requires six electrons per glyoxal molecule (OCHCHO). Although the total energy requirement for the two-stage process may be higher than the one-stage process for producing butanol from carbon dioxide and water, much higher selectivity and faradaic yield (current efficiency) may be provided via the two-stage process. For instance, experiments were conducted wherein a greater than 25% faradaic yield for glyoxal with greater than 90% selectivity were possible. Moreover, glyoxal was converted to 2-butanol in the second cell with greater than 99% selectivity.

[0031] Referring to FIG. 2, a block diagram of a system 200 is shown in accordance with a specific embodiment of the present invention. System 200 may be utilized for the two-stage process for the production of butanol from carbon dioxide and water. The system (or apparatus) 200 generally comprises a first cell 202, a first extractor 204, a second cell 206, and a second extractor 208. The first cell 202 and the second cell 206 may each utilize the divided cell structure as disclosed with reference to cell 102 of FIG. 1.

[0032] The first cell 202 is generally operational to reduce carbon dioxide into a glyoxal rich mixture. In a particular implementation, the first cell 202 incorporates in the cathode compartment a type 430 stainless steel cathode, a 60mM concentration of imidazole catalyst, and a 0.5M KCl electrolyte. The cathode compartment may be pH adjusted to between approximately 5 and approximately 8 by using, for example, sodium hydroxide (NaOH) or potassium hydroxide (KOH). Carbon dioxide may be bubbled through the cathode compartment, where the cathode potential may be approximately -1V vs. SCE (saturated calomel electrode). Pyrrole and other chemicals that react to convert aldehydes to imines or acetals may be added to the catholyte of the first cell 202 to drive the kinetics of the reaction in the cell toward greater glyoxal production. A solid sorbent may serve the same role and also simultaneously extract glyoxal for use in the second cell 206. The anolyte in the first cell 202 may consist of water with an electrolyte to permit water oxidation at the anode. Water may be added to the anode compartment as it is consumed for the process. Glyoxal may be extracted from the product mixture of the first cell 202 with the first extractor 204 which may incorporate any combination of derivitization, liquid-liquid extraction, and/or solid sorbents. While FIG. 2 depicts the first extractor 204 separated from the first cell 202, it may be appreciated that various extraction processes and instrumentation may be part of, implemented with, and/or coupled to the first cell 202 in order to extract a particular product (e.g., glyoxal) of the product mixture.

[0033] Glyoxal formation in the cathode compartment of the first cell 202 may be aided through various combinations of cathode materials, catalysts, and cell conditions. For instance, the cathode material may include indium, tin, molybdenum, 316 stainless steel, nickel 625, nickel 600, nickel- chromium, elgiloy (cobalt-nickel-chromium), and copper-nickel. Iron, steel, cobalt, chromium, and alloys thereof may also be utilized as cathode material in the cathode compartment of the first cell 202. Catalysts in the first cell 202 may be include pyridine, quinoline, 1 -methyl imidazole, 4,4' bipyridine, and other heterocycles to convert carbon dioxide to glyoxal under the appropriate conditions. Such conditions may include lower pHs and differing electrolytes, such as one or more of Na ₂ SO ₄ , KCl, NaNO ₃ , NaCl, NaF, NaClO ₄ , KClO ₄ , K ₂ SiO ₃ , CaCl ₂ , a H cation, a Li cation, a Na cation, a K cation, a Rb cation, a Cs cation, a Ca cation, an ammonium cation, an alkylammonium cation, a F anion, a CI anion, a Br anion, an I anion, an At anion, an alkyl amine, a borate, a carbonate, a nitrite, a nitrate, a phosphate, a polyphosphate, a perchlorate, a silicate, a sulfate, and a tetraalkyl ammonium salt. The combination of cathode, catalyst, and cell conditions sufficient for the reaction in the cathode compartment of the first cell 202 may be disclosed in U.S. Patent Application No. 12/846,221, entitled "Reducing Carbon Dioxide to Products," which is hereby incorporated by reference.

[0034] The product mixture of the first cell 202 may include one or more two-carbon intermediates including glyoxal, oxalic acid, glyoxylic acid, glycolic acid, acetic acid, and acetaldehyde. One or more of the components of the product mixture may be utilized as an intermediate in the two-stage process (i.e., may be used as an input to the second cell 206). Glyoxal may include beneficial characteristics for use as the intermediate, including, but not limited to, being non-corrosive, being stable in water, and requiring six electrons for its formation from carbon dioxide and water. Generally, the first extractor 204 is sufficient to provide a component-rich portion 210 as an input to the second cell 206, and a component-lean portion 212 (e.g., catholyte rich portion) that may be utilized for additional reactions in the first cell 202.

[0035] In the second cell 206, a two-carbon intermediate, such as glyoxal, may be converted to 2-butanol via electrohydrodimerization, as disclosed in U.S. Patent Application No. 12/846,011, "Heterocycle Catalyzed Electrochemical Process," which is hereby incorporated by reference, in a particular implementation, aqueous glyoxal is introduced as a reactant to the second cell 206 with concentrations of up to approximately 40%. The catholyte in the second cell 206 may include water and KCl, or other suitable electrolyte. The cathode compartment in the second cell 206 may include a catalyst, including a heterocyclic catalyst, such as 4,4' bipridine. However, in some instances, no catalyst or no heterocyclic catalyst is provided in the cathode compartment in the second cell 206, whereby the cathode itself facilitates the two-carbon intermediate to butanol reaction. The anolyte in the anode compartment of the second cell 206 may include water with an electrolyte sufficient for water oxidation at the anode.

[0036] The second cell 206 may include a butanol rich output 214 as a product of the second cell reactions. The output 214 may also include a portion of catholyte. Generally, the second extractor 208 is sufficient to provide a butanol product 216, i.e., the product of the two-stage process of system 200, and a butanol-lean portion 218 (i.e., a butanol lean/catholyte rich portion) from the second extractor 208 which may be utilized for additional reactions in the second cell 204.

[0037] As described herein, the present disclosure may be implemented via a one-stage or a two-stage process. The one-stage process may result in a product stream including butanol with relatively larger amounts of one-, two-, and three-carbon products. The one-stage process may be an electrochemical process (e.g., driven by any electric power source) or a photochemical process, which may occur on a photovoltaic solar panel. The two-stage process generally produces butanol with high efficiency. [0038] Referring to FIG. 3, a flow diagram of an example method 300 for producing butanol from carbon dioxide and water in a one-stage process is shown. The method (or process) 300 generally comprises a step (or block) 302, a step (or block) 304, a step (or block) 306, and a step (or block) 308. The method 300 may be implemented using the system 100.

[0039] In the step 302, water may be introduced to a first compartment of an electrochemical cell. The first compartment may include an anode. Introducing carbon dioxide to a second compartment of the electrochemical cell may be performed in the step 304. The second compartment may include a solution of an electrolyte, a catalyst, and a cathode. In the step 306, an electric potential may be applied between the anode and the cathode in the electrochemical cell sufficient for the cathode to reduce the carbon dioxide to at least butanol. Removing butanol from the second cell compartment may be performed in the step 308.

[0040] Referring to FIG. 4, a flow diagram of an example method 400 for producing butanol from carbon dioxide and water in a two-stage process is shown. The method (or process) 400 generally comprises a step (or block) 402, a step (or block) 404, a step (or block) 406, a step (or block) 408, a step (or block) 410, and a step (or block) 412. The method 400 may be implemented using the system 200.

[0041 ] In the step 402, water may be introduced to a first compartment of a first electrochemical cell. The first compartment may include an anode. Introducing carbon dioxide to a second compartment of the first electrochemical cell may be performed in the step 404. The second compartment may include a solution of an electrolyte, a catalyst, and a cathode. In the step 406, an electric potential may be applied between the anode and the cathode in the first electrochemical cell sufficient for the cathode to reduce the carbon dioxide to at least a two-carbon intermediate. Removing the two-carbon intermediate from the first electrochemical cell may be performed in the step 408. In the step 410, the two-carbon intermediate may be introduced to a second electrochemical cell. The second electrochemical cell may include an anode in a first cell compartment and a cathode in a second cell compartment. The cathode may reduce the two-carbon intermediate to at least butanol. In the step 412, butanol may be removed from the second electrochemical cell. [0042] It is believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the disclosure or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.