OF CARBON DIOXIDE WITH WATER

PRIORITY DATA

[0001] This international patent application claims priority to U.S. Patent App.

No. 61/719,407, filed October 28, 2012, which is hereby incorporated by reference herein for all purposes.

FIELD OF THE INVENTION

[0002] This disclosure concerns production of renewable chemicals and fuels by direct, electrochemical reduction of carbon dioxide.

SUMMARY OF THE INVENTION

[0003] In some variations, the invention provides a process for

electrochemically converting carbon dioxide and water to oxalic acid, the process comprising:

(a) providing a source of carbon dioxide;

(b) providing a source of water;

(c) electrolyzing the water to produce dilute protons and oxygen in the presence of a first electrochemical catalyst;

(d) converting the dilute protons to hydrogen in the presence of a second electrochemical catalyst;

(e) oxidizing the hydrogen to produce concentrated protons in an acidic solution; (f) reacting the carbon dioxide electrolytically with a lithium salt, to generate lithium oxalate;

(g) alternately loading an acid-tolerant ion exchange material with the lithium oxalate or the acidic solution, to generate oxalic acid or the lithium salt, respectively;

(h) recovering the oxalic acid; and

(i) recycling the lithium salt to step (f).

[0004] In some embodiments, the first electrochemical catalyst is a cobalt- based catalyst system.

[0005] In some embodiments, the second electrochemical catalyst is selected from the group consisting of platinum, palladium, rhodium, ruthenium, osmium, iridium, nickel, molybdenum disulfide, tungsten disulfide, and combinations thereof.

[0006] In some embodiments, the lithium salt is selected from the group consisting of LiC10 ₄ , LiBF ₄ , LiPF ₆ , LiCF ₃ C0 ₂ , L1CF ₃ SO ₃ , and combinations thereof.

[0007] The ion-exchange material is a sulfonated polystyrene resin, in some embodiments. Step (c) may be conducted at a pH of about 6 to 8. The pH of the acidic solution may be selected from about 0 to about 1.3, for example.

[0008] In some embodiments, step (e) and step (f) are configured in separate half-cells connected by a salt bridge operating in a common solvent.

[0009] In some embodiments, during step (h), oxalic acid precipitates and is isolated by separation of oxalic acid crystals from the acidic solution. The oxalic acid may be recovered in oxalic acid dihydrate form (H ₂ C ₂ 0 ₄ -2H ₂ 0).

[0010] The source of carbon dioxide may be an industrial manufacturing process for chemical or fuel production. Alternatively, or additionally, the source of carbon dioxide may be the atmosphere. The oxalic acid is preferably produced from renewable and sustainable carbon.

[0011] The process preferably utilizes renewable electricity. In various embodiments, the process utilizes solely renewable electricity for net electron demand of the process.

[0012] In some embodiments, the process further comprises recovering at least some of the oxygen (from water electrolysis) as a co-product. All of the co- product oxygen may be recovered. [0013] In certain embodiments, the process further comprises converting the oxalic acid to ethylene glycol. The ethylene glycol may optionally be converted to ethylene and/or propylene. In some of these embodiments, the process utilizes concentrated solar power to provide electricity for net electron demand of the process, as well as to provide heat for the process.

[0014] Other variations of the invention provide a process for

electrochemically converting carbon dioxide to oxalic acid, the process comprising:

(a) providing a source of carbon dioxide;

(b) providing a source of hydrogen;

(c) oxidizing the hydrogen to concentrated protons in an acidic solution;

(d) reacting the carbon dioxide electrolytically with a metal salt, to generate an oxalate salt;

(e) alternately loading an acid-tolerant ion exchange material with the oxalate salt or the acidic solution, to generate oxalic acid or the metal salt, respectively;

(f) recovering the oxalic acid; and

(g) recycling the metal salt to step (d).

[0015] In some embodiments, the source of hydrogen is electrolysis of water.

For example, electrolysis of water may include the substeps of electrolyzing water to produce dilute protons and oxygen in the presence of a first electrochemical catalyst, converting the dilute protons to hydrogen in the presence of a second electrochemical catalyst, and optionally recovering the oxygen as a co-product.

[0016] The source of hydrogen may be syngas, such as syngas provided by gasification of a renewable carbon-containing feedstock (e.g., biomass).

[0017] The source of carbon dioxide may be an industrial manufacturing process for chemical or fuel production. In some embodiments, the source of carbon dioxide is the atmosphere, either directly or indirectly (e.g., recovered from

combustion of annually renewable biomass). The oxalic acid is preferably produced from renewable and sustainable carbon.

[0018] The first electrochemical catalyst may be a cobalt-based catalyst system, and the second electrochemical catalyst may be selected from the group consisting of platinum, palladium, rhodium, ruthenium, osmium, iridium, nickel, molybdenum disulfide, tungsten disulfide, and combinations thereof. [0019] In some embodiments, the metal salt is a lithium salt. For example, the lithium salt may be selected from the group consisting of L1CIO ₄ , L1BF ₄ , LiPF ₆ , L1CF3CO2, L1CF3SO3, and combinations thereof.

[0020] In certain embodiments, the ion-exchange material is a sulfonated polystyrene resin. The pH of the acidic solution may be selected from (but is not limited to) about 0 to about 1.3.

[0021] Step (c) and step (d) may be configured in separate half-cells connected by a salt bridge operating in a common solvent.

[0022] In some embodiments, during step (f), oxalic acid precipitates and is isolated by separation of oxalic acid crystals from the acidic solution. The oxalic acid may be recovered in oxalic acid dihydrate form.

[0023] The process preferably utilizes at least some renewable electricity.

The process preferably utilizes solely renewable electricity for net electron demand of the process.

[0024] The process may further include converting the oxalic acid to ethylene glycol, ethylene, propylene, or combinations thereof. In these embodiments, the process may utilize concentrated solar power to provide electricity for net electron demand of the process, as well as to provide heat for the process.

[0025] The present invention also provides products produced by any of the disclosed processes. In some embodiments, a composition comprising oxalate is provided, wherein the composition is produced by a process comprising at least some of the methods or steps as disclosed herein. The oxalate may be in the form of oxalic acid, oxalic acid dihydrate, an oxalate salt, hydrogen oxalate ion, or combinations thereof. In certain embodiments, ethylene glycol, ethylene, and/or propylene are produced by a process comprising at least some of the methods or steps as disclosed herein.

[0026] In some embodiments, a composition comprising oxalate is provided, wherein the oxalate is characterized by a ¹⁴ C content that is about the same as the ¹⁴ C content of atmospheric C0 ₂ , and wherein the oxalate is in the form of oxalic acid, oxalic acid dihydrate, an oxalate salt, hydrogen oxalate ion, or combinations thereof. [0027] In some embodiments, a composition comprising ethylene glycol is provided, wherein the ethylene glycol is characterized by a ¹⁴ C content that is about the same as the ¹⁴ C content of atmospheric C0 ₂ .

[0028] In some embodiments, a composition comprising ethylene is provided, wherein the ethylene is characterized by a ¹⁴ C content that is about the same as the ¹⁴ C content of atmospheric C0 ₂ .

[0029] In some embodiments, a composition comprising propylene is provided, wherein the propylene is characterized by a ¹⁴ C content that is about the same as the ¹⁴ C content of atmospheric C0 ₂ .

[0030] The ¹⁴ C content of atmospheric C0 ₂ is, generally speaking, currently about 1 part per trillion ¹⁴ C of the total carbon contained in the C0 ₂ .

BRIEF DESCRIPTION OF THE FIGURE

[0031] FIG. 1 is a graph of market prices and production of several C ₂ compounds, including oxalic acid, plotted against average carbon oxidation number.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0032] The processes, methods, systems, and apparatus of the present invention will be described in detail by reference to various non-limiting

embodiments.

[0033] This description will enable one skilled in the art to make and use the invention, and it describes several embodiments, adaptations, variations, alternatives, and uses of the invention. These and other embodiments, features, and advantages of the present invention will become more apparent to those skilled in the art when taken with reference to the following detailed description of the invention in conjunction with the accompanying drawings.

[0034] As used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly indicates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs.

[0035] Unless otherwise indicated, all numbers expressing conditions, concentrations, dimensions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending at least upon a specific analytical technique.

[0036] The term "comprising," which is synonymous with "including,"

"containing," or "characterized by" is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. "Comprising" is a term of art used in claim language which means that the named claim elements are essential, but other claim elements may be added and still form a construct within the scope of the claim.

[0037] As used herein, the phase "consisting of excludes any element, step, or ingredient not specified in the claim. When the phrase "consists of (or variations thereof) appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. As used herein, the phase "consisting essentially of limits the scope of a claim to the specified elements or method steps, plus those that do not materially affect the basis and novel characteristic(s) of the claimed subject matter.

[0038] With respect to the terms "comprising," "consisting of," and

"consisting essentially of," where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus in some embodiments not otherwise explicitly recited, any instance of "comprising" may be replaced by "consisting of or, alternatively, by "consisting essentially of."

[0039] Constraints operating on several levels encourage development of sustainable chemicals and fuels. The economic and military security of the United States of America is in jeopardy due to its reliance on petroleum from unstable regions of the world whose populace may become increasing hostile to American interests. A second constraint lies in the fact that readily recovered petroleum production likely will go through a maximum in the next few decades followed by an inexorable decline in global production. Economic disruptions on a large scale will accompany this decline in the absence of effective countermeasures. Finally, there is overwhelming scientific consensus that climate change is proceeding at an increasing rate. Engaging in the experiment of doubling tropospheric C0 ₂ levels (or increasing them beyond that) while doing nothing is reckless and may result in dire

consequences.

[0040] Unfortunately, these concerns, and others, that otherwise would drive creation and commercialization of renewable chemical and energy production technologies, do not appear in current prices for energy or carbon feedstocks. This market failure discourages development and commercialization of renewable chemical and fuel production technology. Businesses based on fossil carbon exploitation enjoy huge competitive advantages from large, historic subsidies provided by governments earlier in the development of those industries that allowed them to pay for huge amounts of accumulated capital. These businesses have tremendous advantages over new entrants who seek to commercialize sustainable technologies for production of chemicals or fuels.

[0041] The scientific and technical problems associated with development and deployment of sustainable, renewable, clean energy and chemicals are huge. It is often suggested that what is needed is a new Apollo-type program focused on overcoming these problems. This perspective mischaracterizes the problem. Nathan Lewis, George L. Argyros Professor of Chemistry at the California Institute of Technology, formulates the problem differently. According to him, the challenge is more akin to developing new technology for flying to moon, then arriving there only to find that Southwest Airlines is already there, giving away peanuts to passengers. Organizations engaged in deployment of sustainable chemicals and fuels will need to do so in the presence of entrenched incumbents, with low-cost products, who are unlikely to cede market share in their core markets without a fight. One example of resistance by entrenched incumbents is funding by oil refiners headquartered in Kansas and Texas to delay, by an initiative in the November 2010 election in

California, California's implementation of its low-carbon fuel standard. Another example are comments by Abdalla El-Badri, secretary-general of the Organisation of the Petroleum Exporting Countries (OPEC), published in the 6 June 2007 issue of the Financial Times, that OPEC members may not invest adequately in their production capabilities in the face of European and American encouragement of bio fuel production, thus leading the way to future price shocks.

[0042] There are several ways that society may approach this problem of technology development and deployment in the context of market failure and probable resistance from entrenched incumbents. Governments could opt to develop production capabilities themselves, following models such as the Institut Francais du Petrole (IFP) in France or the Tennessee Valley Authority (TV A) in the United States. This approach has the merit that national governments possess the ability to invest on the scale needed to make a difference in a short time frame. But it is unlikely that nations in central or western Europe or North America will choose such an option in the near term, given political resistance to government ownership of new industrial enterprises.

[0043] Another option involves government command. Governments worldwide command manufacturers of cars and trucks to limit the amount of hydrocarbon, carbon monoxide, and nitrogen oxide pollutants emitted by their products. This has led to a competitive global marketplace for automotive emission control catalyst systems, by companies whose product is cleaner air than would otherwise be emitted into the environment. This clean air has no price but is mandated by governments. Initially, requirements were relatively lenient but governments, led by the State of California and by the European Union, gradually increased the severity of requirements as technology improved. While government could use such a process to mandate creation of renewable chemicals and fuels businesses, such an approach may elicit substantial political opposition and faces "stroke-of-a-pen" risk. "Stroke -of-a-pen" risk is the risk that a change of political control in government could result in substantial weakening or even abolishment of command requirements demanding reductions in emissions of gases that are associated with climate change (namely, carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, among others). [0044] A third option, which has been used by governments in Europe and the

Americas for a decade or so (longer in Brazil, a leader in this area), involves creation of various price incentives and subsidies for the production and consumption of "greener" energy technology. This approach has not yet been used much to encourage production of renewable chemicals. Bioethanol enjoys subsidies in the USA and Brazil. Biodiesel enjoys subsidies in Europe and has been subsidized in the USA. Hybrid gasoline-electric light duty vehicles have been subsidized in the USA. This option, while more politically palatable currently, also suffers from stroke-of-a- pen risk. An example of stroke-of-the-pen risk involves biodiesel, where a subsidy in the USA expired and its renewal has been obstructed, successfully, by the minority in the US Senate. This risk may be rising in some G7 economies as they struggle to recover from the Great Recession. This risk is manifest in current attempts to delay implementation of California's low carbon fuel standard.

[0045] What is needed, then, is a path to develop and commercialize sustainable chemicals and fuels that does not suffer from these shortcomings.

Initially, products are preferably demanded at relatively small scale in markets that are not core to entrenched energy and chemical incumbents. Those incumbents may tend to ignore such product commercializations. The products themselves must command high margins in order to support commercialization which, especially in the absence of subsidies, will involve a level of market risk that causes investors to demand the prospect of higher return. That is, this path preferably does not involve, as an initial target of commercialization, substances that could be used currently in large volume energy markets. Those products have relatively cheap market prices and the markets themselves may be subject to aggressive defense by incumbents.

[0046] Sustainable chemicals and fuels are ultimately derived from carbon dioxide (C0 ₂ ). C0 ₂ can be reduced to various Ci compounds or reductively dimerized to C ₂ products.

[0047] C0 ₂ may be electrochemically reduced to CO in acidic solution (Beley et al., Journal of the American Chemical Society, vol. 108, 7461-7467, 1986). It may also be reduced to CO using a homogeneous copper catalyst; in this case a diorganoborane is the oxygen acceptor (Laitar et al. , Journal of the American

Chemical Society, vol. 127, 17196-17198, 2005). However, CO is not a particularly attractive reduced product of C0 ₂ . Its market utility itself is limited, yet it is already manufactured by very large chemical and petrochemical companies. CO may be used, in combination with H ₂ , to make various chemicals and fuels. The

aforementioned paths for making CO from C0 ₂ are not advantageous over established methods of making CO/H ₂ mixtures such as steam methane reforming or gasification of coal and petroleum coke. Next, an approach similar to Laitar, cited earlier in this paragraph, has the distinct disadvantage of making a B-0 compound as a

stoichiometric product. Boron-oxygen bonds are hard to reduce, even harder than C0 ₂ , and such compounds have no known utility in the market. Finally, reducing C0 ₂ to CO and then engaging in further conversion of CO to useful products does not avoid challenging large, entrenched incumbents in markets in which they already operate and which they are likely to defend.

[0048] In a scientifically interesting contribution, Riduan et al. reduce C0 ₂ to methanol using carbene catalysts and either diorganosilanes or triorganosilanes (Angewandte Chemie International Edition English, vol. 48, 3322-3325, 2009). The corresponding siloxanes are coproduced. In order to provide a practically interesting process opportunity, means need to be found to reduce the produced siloxanes back to the reagent silanes. This reduction will be difficult to accomplish, since silicon- oxygen bonds are very strong. Thus, the required reduction of siloxanes is expected to be complex and expensive, if it is feasible at all. Beyond that, methanol is cheap and is made at very large scale from stranded national gas. Methanol producers are likely to react to market entry of expensive, renewable methanol made from C0 ₂ , making successful market entry for the new technology even more problematic.

[0049] Another option for reduction of C0 ₂ involves its reductive

dimerization to oxalate. This is attractive since the product, oxalic acid commands high price in the market yet is made at small scale. Further, prior art syntheses of oxalic acid that are practiced in commerce involve dirty, polluting technology.

Reductive dimerization of C0 ₂ entails making the first C-C bond and this is in itself valuable. As will be shown below, once the first C-C bond is made, removal of oxygen as water by reaction with hydrogen is much easier. More reduced compounds may be accessed from an oxalic acid platform and very large portions of industrial organic chemistry become accessible. Oxalic acid may be the key intermediate compound to creating a sustainable chemical and fuel industry. Significantly, it is also valuable in its own right.

[0050] Successive commercialization of oxalic acid, then ethylene glycol, and then ethylene and propylene conforms to the novel path suggested here. Oxalic acid is not a core product of petrochemical energy companies, nor, arguably, is ethylene glycol. All the carbon in these compounds, when made by the methods disclosed herein, is derived from C0 ₂ . The C0 ₂ may be obtained in concentrated form, such as an emission from an existing business such as a fossil-fueled electric generation plant, or a chemical processing facility, or a brewery. Alternately, the C0 ₂ may simply be obtained from the atmosphere. The carbon in these compounds, made by the processes described herein, can be and preferably is renewable and sustainable. One can determine whether the carbon source is fossil or renewable carbon by ¹⁴ C assay. Renewable carbon is expected to have ¹⁴ C content of about 1 part per trillion of the total carbon (which is primarily ¹² C). Such an assay is readily available from commercial laboratories that make radiochemical determinations.

[0051] Angamuthu, Bouwmann and colleagues {Science, volume 327, 313-

315, 2010) disclosed a process for making lithium oxalate, Li ₂ C ₂ 0 ₄

electrocatalytically from C0 ₂ and a lithium salt (specifically lithium perchlorate) in acetonitrile. The reaction can utilize atmospheric carbon dioxide although higher rates are obtained using more concentrated C0 ₂ sources. The reaction takes advantage of the fact that Li ₂ C ₂ 0 ₄ is insoluble in the acetonitrile solvent used to run the electrocatalytic reduction of C0 ₂ . Li ₂ C ₂ 0 ₄ may be isolated by filtration from its acetonitrile mother liquor. It will be preferable to use C0 ₂ sources that have very low amounts, preferably less than 10 ppm, more preferably less than 100 ppb, and even more preferably, essentially no detectable CO or H ₂ S or other good Lewis bases, because these compounds will bind very strongly to Cu(I) intermediates in the catalytic cycle, deactivating the catalyst. Particularly interesting aspects of the catalyst disclosed by Angamuthu are its tolerance of 0 ₂ while reducing C0 ₂ at about zero volts vs NHE (Normal Hydrogen Electrode), that is, at essentially no

overvoltage. However, markets for Li ₂ C ₂ 0 ₄ are at best very small, since the lithium itself is currently rather valuable and is expected to get more expensive with time. [0052] Of course, an electrocatalytic process requires electricity in order to work. There is no guarantee than an electrocatalytic process, per se, is sustainable. The source of electricity matters. Electricity generated by combustion of coal, petroleum, or natural gas will tend to be not sustainable due to the C0 ₂ emission associated with electricity generation. It is possible that electricity made from nuclear power plants will be relatively sustainable. However, if electricity specifically from nuclear reactors is purchased at market rates, that electricity will probably be relatively expensive. Farmer and Makhijani, in comments published in Nature _(vol. 467, 392-393, 2010), note that electricity generated by nuclear power costs $0.12- $0.20/kW-hr; these prices are not expected to decreases in the near- or mid-term. Renewable electricity may be generated by wind turbines ($0.11-$0.14/kW-hr), large- scale photovoltaics ($0.16/kW-hr) or by concentrated solar power (CSP). It is expected that CSP will be less expensive than photovoltaic electricity, and perhaps cheaper than wind, for some time. Hydroelectric power also can be quite inexpensive, on the order of $0.04/kW-hr. In order to make renewable, chemically reduced compounds electrochemically from C0 ₂ , electricity should be purchased intentionally from renewable electricity producers. Alternately, a renewable electricity generation capability could be built specifically for the purpose of making reduced chemicals from C0 ₂ .

[0053] In order to make renewable oxalic acid from lithium oxalate, a renewable source of concentrated protons is needed. One could use sulfuric acid as a proton source but this undesirably leads to a sulfate by-product stream that may not be renewable. Protons may be obtained by electrochemical oxidation of elemental hydrogen; this begs the question of how to get the hydrogen.

[0054] One option is to use hydrogen obtained by gasification of a

carbonaceous feedstock. While conceivable, several factors render this option less preferable. Gasification of coal, petroleum (including petroleum coke or undesired fractions of crude oil), and natural gas all have the liability of being unsustainable. Fossil carbon is emitted into the atmosphere as C0 ₂ when these feedstocks are refined or used. Fossil fuel gasification, followed by water gas shift to make H ₂ and C0 ₂ , combined with C0 ₂ sequestration might be relatively sustainable. However, there is risk that the C0 ₂ sequestration reservoirs will leak C0 ₂ over time, and thus fail to prevent C0 ₂ emission into the atmosphere. Thus, while H ₂ production from fossil fuels coupled with C0 ₂ sequestration could be a means of H ₂ production in the context of this invention, it is not preferred. Biomass could, in principle, be gasified to synthesis gas (a mixture containing essentially CO and H ₂ ) and this synthesis gas could be used as the H ₂ source, optionally with water-gas shift to increase the H ₂ /CO ratio. However, a very effective separation of CO and H ₂ is typically necessary, since the C0 ₂ reduction catalyst is expected to be susceptible to poisoning by CO. This separation of CO and H ₂ is likely to be relatively expensive and to involve

consumption of energy.

[0055] A preferable option is to obtain the required H ₂ by electrolysis of water. Electrolysis is a relatively mature but expensive process that, in currently commercial deployments, uses electrodes based on expensive precious metals, often comprising platinum. Those electrodes are not preferred due to their expense. Water electrolysis is the sum of two electrochemical half reactions: reduction of water to H ₂ and, perhaps, hydroxide on the one hand and oxidation of water to 0 ₂ and, perhaps, protons. Of the two, the water oxidation reaction is much more demanding chemically. Recently, however, a research group led by Daniel Nocera at the

Massachusetts Institute of Technology has demonstrated the certain cobalt phosphate compositions, deposited on conductive glasses, catalyze oxidation of water to 0 ₂ and protons at essentially no overpotential. (For example, see Kanan and Nocera, Science, vol. 321, 1072-1075, 2008.) More recently, a research group led by Craig Hill at Emory University, has shown that homogeneous solutions of certain cobalt tungstophosphate complexes also efficiently catalyzes water oxidation to 0 ₂ and protons at essentially no overpotential. (See, for example, Yin, et ah, Science, vol. 342, 342, 345, 2010.) These and other cobalt-based catalysts oxidize water at lower overpotential and, in all likelihood, at lower electrode cost compared to conventional Pt-based materials. However, they work in aqueous solutions near pH 7, which is insufficiently acidic to allow synthesis of oxalic acid, since the logarithm of the first acid dissociation constant, pKi, is 1.27. When made in aqueous solution, the pH of oxalic acid solutions should be below about 1.3. These cobalt catalysts make 0 ₂ and protons at pH 7 but protons are needed at pH of 0-1.3. [0056] To solve this problem, one may use the cobalt catalyst system to make

0 ₂ and dilute protons and convert those dilute protons electrolytically to H ₂ using a variety of known electrode materials, including those containing precious metals (Pt, Pd, Rh, Ru, Os, or Ir), nickel, molybdenum disulfide, tungsten disulfide, or other electrodes known to permit production of H ₂ from dilute acid solutions.

[0057] It is beneficial, but optional, to capture the 0 ₂ made in this process and provide it to the market, since pure 0 ₂ itself is a commodity possessing significant value. However, the 0 ₂ also may be vented harmlessly to the atmosphere.

[0058] The H ₂ made by water electrolysis is transported to a second electrochemical apparatus where it is oxidized electrochemically to protons at low pH. This acidic solution preferably is in contact with an acid-tolerant ion exchange material, such as a sulfonated polystyrene resin. The anion in this system needs to be compatible with the ion exchange resin. Nitrate is inappropriate. Perchlorate may be acceptable; tetrafluoroborate, hexafluorophosphate, and triflate are acceptable. This ion exchange material shall alternately be loaded with Li ⁺ (from L1 ₂ C ₂ O ₄ ) and H ⁺ (from H ₂ electrolysis). When the resin is being loaded with Li ⁺ , oxalic acid is being produced and, when sufficiently concentrated, will precipitate and may be isolated by separation of oxalic acid crystals from mother liquor. Optionally, oxalic acid may be recrystallized from water to yield oxalic acid dihydrate, which is the form of oxalic acid commonly used in commerce. Preferably, the H ₂ will be oxidized in acetonitrile solvent, so that an electrochemical cell comprising H ₂ oxidation and C0 ₂ reduction in separate half-cells connected by a salt-bridge operate in a common solvent.

[0059] When the resin is being loaded with protons, a solution of a lithium salt, such as L1CIO ₄ , L1BF ₄ , LiPF ₆ , or L1CF ₃ CO ₂ is being produced and the salt may be isolated from a sufficiently concentrated solution that the crystalline salt separates. The salt may then be introduced into the C0 ₂ reduction half-cell, facilitating synthesis of L1 ₂ C ₂ O ₄ . Two electrochemical reactions are contemplated:

H ₂ 0 → H ₂ + 0.5 0 ₂ E° = 1.23 V vs NHE

2 C0 ₂ + H ₂ → H ₂ C ₂ 0 ₄ E° = -0.40 V vs NHE

[0060] Since both reactions, conducted by the means discussed in this disclosure, run at negligible overpotential, one might conservatively estimate the total potential required to run this process as 1.5 V. One kilowatt-hr of electricity, used to make oxalic acid dihydrate at a potential of 1.5 volts, would make 1.57 kg of product. The market price of oxalic acid is about $10/kg. The gross margin of a process is simply the value of the product(s) of a process from which is subtracted the cost of inputs. To a first approximation, we estimate the cost of water as free and the cost of C0 ₂ as free or negative. (Eventually, emitters of C0 ₂ will eventually have to pay for the privilege of emitting this pollutant to the environment. This means that producers will eventually pay consumers of C0 ₂ to take responsibility of the pollutant.) If we conservatively estimate the price of a kW-hr of electricity at $0.25, the gross margin of this process is about 6300%. Note that this estimate builds in the process costs, generally perceived to be high, of electrolyzing water to the elements yet does not capture any benefit of the 0 ₂ coproduct. The extremely high gross margin of this process is expected to be sufficient to pay for expenses to run the process and still provide a generous profit.

[0061] FIG. 1 shows market prices (in units of dollars/mole carbon on the left ordinate axis) and production (tonnes/yr on the right ordinate axis) of several C ₂ compounds plotted against average carbon oxidation number. It illustrates why oxalic acid is relatively expensive. In prior art, oxalic acid is made from more reduced compounds, usually of petrochemical origin. The closer you get to ethane, the C ₂ petrochemical, the cheaper they get and the more that is produced. It is reasonable that the molar price of carbon increases with its oxidation number since those more- oxidized compounds are made from more -reduced compounds, while the most highly reduced compound, ethane, is mined. That is, C ₂ compounds with average oxidation number greater than or equal to -2 come either from petroleum, natural gas, or coal. As the average C ₂ oxidation number increases, less and less is used in commerce. It is believed that this fact is a function of the cost of the material rather than exclusively its relative nonutility. It is believed that if inexpensive oxalic acid became available, especially if it were made renewably, much more of it would be used and new uses would be created.

[0062] Oxalic acid, in particular, is made by prior art that is practiced commercially today, involving oxidation of various precursors with nitric acid. Such a process is messy and polluting, since the product NO (or N ₂ 0) is often vented to atmosphere as waste. NO is a precursor to photochemical generation of ozone in polluted air while N ₂ 0 is a potent greenhouse gas (about 300 times as potent, on a molar basis, as C0 ₂ ). China is now the leading producer of oxalic acid, perhaps due in part to its lax environmental requirements. Currently, no oxalic acid is known to be produced in the United States. Annual consumption in the United States is about 8000 tonnes and consumption globally is about 115,000 tonnes.

[0063] A leading use of oxalic acid is in rare earth refining, since it is a quantitative precipitant of rare earth ions in aqueous solution. Given current market conditions in rare earths, where the Chinese have essentially cornered the market, there is strong interest, from economic and military security perspectives, in resuming rare earth production in the United States of America. Prior to the mid-1990s, the United States of America was the world's leading producer of these elements. The mine at Mountain Pass, California, is being put back in service and products from this mine will require oxalic acid as they are refined.

[0064] Since the oxalic acid market is rather small and chemically and technologically remote from petrochemicals, an entrant renewable chemical manufacturer may be able to bring its synthesis to market without meeting firm resistance from existing major petroleum and chemical companies. Especially if the expected resumption of domestic rare earth mining and refining in the USA proceeds, a manufacturer of oxalic acid using clean, nonpolluting chemical processes should find an easy outlet to market.

[0065] Currently, markets for oxalic acid are relatively small and, once the process for making it is practiced at moderate scale, production may become large compared to demand. The US market is worth only about $80MM/yr while the global market is worth perhaps $1100MM/yr. Producing oxalic acid at moderate scale will involve considerable learning about technology required to electrochemically reduce C0 ₂ and this learning will bring down costs. At that time, C0 ₂ reduction to compounds more reduced than oxalic acid may become desirable. As time passes, the cost of renewable electricity is also expected to decrease as production scales increase and process learning improves in that industry. A next target is ethylene glycol, which may be made from dialkyl esters of oxalic acid, such as dimethyloxalate. The methanol used in this process need not be renewable, since it is recycled in the process: HOC(0)C(0)OH + 2 CH ₃ OH CH ₃ OC(0)C(0)OCH ₃ + 2 H ₂ 0

CH ₃ OC(0)C(0)OCH ₃ + 4 H ₂ HOCH ₂ CH ₂ OH + 2 CH ₃ OH

HOC(0)C(0)OH + 2 H ₂ 0 HOCH ₂ CH ₂ OH + 2 0 ₂

[0066] Given a water electrolysis process that runs at essentially no

overvoltage, and an electricity cost of $0.10/kW-hr, ethylene glycol would be available at a gross margin of about 30%. A well-designed process, especially if coupled with electricity costs lower than $0.10/kW-hr, will be viable. If production were situated where inexpensive hydroelectric power were available (estimated to cost about $0.04/kW-hr), gross margin increases to about 340% (assuming costs of water and oxalic acid intermediate are negligible).

[0067] Thermal catalytic reduction of dimethyl oxalate, or other dialkyl oxalates, to ethylene glycol using H ₂ as the reductant, is well-described in older patent literature, such as U.S. Patent No. 4,112,245. A number of catalysts, based on noble metals (Pt, Pd, Rh, Ru, Os, Ir), can be effective but the catalysts would be costly. Supported bimetallic catalysts of copper, such as Cu-Cr or Cu-Zn catalysts, also work. While Cu-Cr is said to work well, environmental risks of using Cr probably makes Cu-Zn catalysts preferred.

[0068] Dehydration of ethylene glycol, followed by reduction with one equivalent of H ₂ , yields ethanol which, upon dehydration, yields ethylene. While this transformation is not currently known, a bifunctional Pt-acid zeolite catalyst will likely work. If propylene is desired, it may be generated from ethylene using, for example, SAPO-34 zeolite.

[0069] The current disclosure describes electrochemical (or, equivalently, electrolytic) means of converting C0 ₂ and water to oxygen and oxalic acid, ethylene glycol, ethylene, or propylene. The more reduced the carbon-containing product, the more electricity (more specifically, the more current) is required to run the process. Renewable electricity, obtained from hydroelectric plants, concentrated solar power, wind, or photovoltaics, can provide the electricity required to run the process at costs estimated to be in the range of $0.04-0.16/kW-hr. Production of oxalic acid requires relatively little electricity and therefore delivers attractive gross margin even at quite high electricity cost. Production of ethylene glycol requires much more current and therefore needs less expensive electricity to be attractive. At electricity cost of $0.10/kW-hr, gross margin is estimated to be about 30% while at $0.04/kW-hr, gross margin is over 300%. Ethylene requires only one more equivalent of H ₂ when produced from ethylene glycol and, given inexpensive electricity, will also return attractive gross margins. Electrolytic reduction of C0 ₂ to oxalic acid, ethylene glycol, ethylene and propylene opens broad perspectives for introduction of renewable, sustainable carbon into a very wide range of chemicals and, eventually, fuels.

[0070] Reduction of oxalic acid diesters to ethylene glycol, together with reduction of ethylene glycol to ethylene and conversion of ethylene to propylene, are all thermocatalytic processes. The temperatures of reaction are in the range of 200- 500°C. Preferably, the heat to drive this process shall not come from combustion. If H ₂ is burned, then precious hydrogen derived by electrolysis is used to generate low- or medium-quality heat. Of course, burning biomass or, worse, a fossil fuel is also not desired.

[0071] The present disclosure contemplates use of renewable electricity derived from wind, photovoltaics, hydroelectric plants, or concentrated solar power (CSP). Of these, CSP also generates, as a product or by-product, heat that can be in the range of 200-500°C or hotter. Thus, a CSP plant could provide both the electricity to run the electrolysis of water and the electrolytic reduction of C0 ₂ by H ₂ and also provide process heat for subsequent thermal reduction chemistries.

[0072] In this detailed description, reference has been made to multiple embodiments and to the accompanying drawings in which are shown by way of illustration specific exemplary embodiments of the invention. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that modifications to the various disclosed embodiments may be made by a skilled artisan.

[0073] Where methods and steps described above indicate certain events occurring in certain order, those of ordinary skill in the art will recognize that the ordering of certain steps may be modified and that such modifications are in accordance with the variations of the invention. Additionally, certain steps may be performed concurrently in a parallel process when possible, as well as performed sequentially.

[0074] All publications, patents, and patent applications cited in this specification are herein incorporated by reference in their entirety as if each publication, patent, or patent application were specifically and individually put forth herein.

[0075] The embodiments, variations, and figures described above should provide an indication of the utility and versatility of the present invention. Other embodiments that do not provide all of the features and advantages set forth herein may also be utilized, without departing from the spirit and scope of the present invention. Such modifications and variations are considered to be within the scope of the invention defined by the claims.