US20090090638A1 - Processes and reactors for alkali metal production - Google Patents

Abstract

Electrochemical processes and apparatus for obtaining metals from metal salts, including for separating alkali metal and alcohols from alkali metal alkoxide compounds, are disclosed. Aqueous solutions of metal alkoxides or metal carbonates are converted to metals by electrochemical processes which may also be integrated into processes for the production of borohydrides, such as sodium borohydride.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
• The invention was made with Government support under Cooperative Agreement No. DE-FC36-04G014008 awarded by the Department of Energy. The United States Government has certain rights in this invention.
FIELD OF THE INVENTION
• The invention is directed to electrochemical reduction of metal compounds with applications in, for example, elemental metal and metal borohydride production.
BACKGROUND OF THE INVENTION
• Sodium borohydride is a very versatile chemical and is used in organic synthesis, waste water treatment, and pulp and paper bleaching.
• Several processes exist for making sodium borohydride, all of which depend on some form of sodium borate to supply the boron. Traditionally, the source of boron is the mineral borax. One process which is currently used to supply commercial sodium borohydride is known as the Schlesinger process. During the Schlesinger process, the sodium and boron contents of the mineral must be separated. This separation is achieved by reaction with an acid, thereby producing boric acid and the sodium salt of the acid. This process generates large quantities of a sodium salt by-product, which is typically valueless. The sodium needed to make sodium borohydride is reintroduced from another source; the process makes no use of the sodium content of the sodium borate mineral.
• In the manufacture of sodium borohydride, metallic sodium or sodium hydride is used as a starting material. The largest single form of consumption of sodium metal in the United States is the process for making sodium borohydride. Essentially all of such sodium in the marketplace is obtained from an energy inefficient electrolysis processes, such as electrolysis of sodium chloride. As a result, the market price of sodium is high, which in turn raises the cost of making sodium borohydride. Therefore, it is desirable to achieve improved processes for making sodium.
SUMMARY OF THE INVENTION
• The present invention is directed to electrochemical processes and apparatus for obtaining a metal from a metal salt. In accordance with one embodiment of the present invention, aqueous solutions of metal salts are converted to elemental metal and water or an alcohol by an electrochemical process. The present invention also provides electrochemical reactors for the transformation of metal salts to metal. The invention further provides processes for the production of sodium borohydride.
• These and other features and advantages of the invention will become apparent from the following detailed description that is provided in connection with the accompanying drawings and illustrated exemplary embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is an exploded view of an exemplary electrolytic cell for synthesis of an alkali metal from an alkali metal salt, according to an embodiment of the invention.
• FIG. 2 is a schematic view of an exemplary three compartment electrolytic cell for synthesis of an alkali metal from an alkali metal salt, according to an embodiment of the invention.
• FIG. 3 is a flow diagram of a typical process for producing sodium borohydride.
• FIG. 4 is a diagram depicting a typical process for producing sodium borohydride in industrial practice.
• FIGS. 5A and 5B are diagrams of improved processes for producing sodium borohydride, in accordance with embodiments of the invention.
DETAILED DESCRIPTION OF THE INVENTION
• In accordance with one embodiment of the present invention, aqueous solutions of metal salts are converted into elemental metal and water or an alcohol by an electrochemical process.
• In accordance with another embodiment of the invention, a metal carbonate is converted to elemental metal.
• One embodiment of the present invention provides an electrochemical reactor comprising a ceramic membrane, suitable for the transformation of metal salts to metals.
• Another embodiment of the present invention provides a three chamber electrochemical reactor comprising a ceramic membrane suitable for the transformation of metal salts to metals.
• In another embodiment of the invention, the invention provides a process for the production of sodium borohydride by: (i) electrolyzing a solution of sodium methoxide to produce sodium metal and methanol; (ii) reacting the sodium metal with hydrogen to produce sodium hydride; (iii) reacting the sodium hydride and trimethylborate to produce sodium borohydride and sodium methoxide; and (iv) optionally recycling the methanol in the process.
• Another embodiment of the present invention provides a process for the production of sodium borohydride by: (i) electrolyzing a solution of sodium methoxide to produce sodium metal and methanol; (ii) electrolyzing an aqueous solution of sodium borates and an alcohol to produce sodium metal and trialkylborate; (iii) reacting the sodium metal with hydrogen to produce sodium hydride; (iv) reacting the sodium hydride and trialkylborate to produce sodium borohydride and sodium methoxide; and (v) optionally recycling the methanol in the process.
• In another embodiment, the present invention provides a process for the production of sodium borohydride comprising the steps of: (i) reacting sodium hydride and trialkylborate to produce borohydride and sodium alkoxide, and hydrolyzing the sodium alkoxide to sodium hydroxide and methanol; (ii) optionally recycling the alcohol in the process; and (iii) electrolyzing the sodium hydroxide to sodium metal which may be used to make sodium borohydride.
• In yet another embodiment, the present invention provides a process for the production of sodium borohydride comprising the steps of: (i) electrolyzing a solution of sodium carbonate and methanol to produce sodium metal and a methanol; (ii) electrolyzing a solution of sodium methoxide to produce sodium metal and methanol; (iii) reacting the sodium metal with hydrogen to produce sodium hydride; (iv) reacting the sodium hydride and trialkylborate to produce borohydride and sodium alkoxide; and (v) optionally recycling the alcohol in the process.
• The present invention relates to electrochemical processes for obtaining alkali metals and alkaline earth metals from alkali metal and alkaline earth metal salts. The alkali metals are the Group I metals, and preferably used are lithium, sodium, and potassium. The alkaline earth metals are the Group II metals, and preferably used are calcium and magnesium. The electrochemical process for obtaining an elemental metal may further be combined with additional electrochemical and/or chemical steps to produce metal borohydride compounds.
• In accordance with one embodiment of the present invention, a metal salt represented by the formula M_nOR, wherein M is an alkali metal ion such as sodium, potassium, or lithium (wherein n=2) or an alkaline earth metal such as calcium or magnesium (wherein n=1) and preferably is sodium; and R is selected from the group consisting of H and straight- or branched-chain or cyclic alkyl groups containing from 1 to 6, preferably from 1 to 4, carbon atoms, can be converted into elemental metal and a product of formula R-OH, which includes water (for those salts where R=H) or alcohols (for those salts where R is an alkyl group), through electrolysis. The electrolysis may be conducted with a gas stream comprising hydrogen being supplied to the anode compartment via a gas inlet. Suitable gas inlets for supplying a hydrogen or hydrogen-containing gas stream may include a pipe, a sparger, a hose, or a hydrogen gas diffusion material, among others.
• In a preferred embodiment, the metal salt is sodium methoxide which can be converted to sodium metal and methanol in accordance with Equation (1):
• 
  2NaOCH₃+H₂→2Na⁰+2HOCH₃  (1)
• This reduction is carried out by supplying a metal salt to the anode compartment of an electrolytic cell, for instance, such as that illustrated byFIG. 1. The reaction may be carried out at ambient temperature or at temperatures above ambient. In preferred embodiments, one or both of the anolyte and catholyte are heated. This heating can occur within the electrochemical cell or the anolyte and/or catholyte are circulated and heated externally from the cell. Preferably, the catholyte is maintained at a temperature such that the metal in the cathode compartment is in a liquid or molten state, preferably at about 95° C. to about 150° C.
• The anode compartment need not be heated and may be maintained at ambient temperature, or the anode compartment may be maintained at the same elevated temperature as the cathode for ease and convenience. If the anode compartment is heated to temperatures greater than the boiling point of the solvent, the cell may be pressurized or the solvent in the anolyte may be refluxed and condensed. The applied voltage may be, for example, about 1.0 to 7 volts, preferably greater than about 1.6 volts, and more preferably greater than about 3.25 volts.
• The overall process provided in Equation (1) can be summarized by the following individual reactions of Equations (2)-(4). At the hydrogen assisted-anode, the alkali metal methoxide is converted to methanol as shown in Equations (2) and (3). The alkali metal ions are transported from the anolyte solution through the membrane where they are reduced at the cathode to the metal as shown in Equation (4).
• 
  Anode Reaction: H₂→2H⁺+2e⁻  (2)
• 
  Anode Compartment: 2H⁺+2OCH₃⁻→2HOCH₃  (3)
• 
  Cathode Reaction: 2M⁺+2e⁻→2M   (4)
• FIG. 1 illustrates an exemplary two-compartmentelectrochemical reactor100 suitable for the transformation of metal salts to metals, which allows for the safe generation of reactive materials (e.g., sodium metal and an alcohol or water) in close proximity.Cell100 comprises a cathode compartment5, acathode3, ananode compartment8, an anode9, a membrane6 and membrane holder7 that separates the anode andcathode compartments8,5. Each of the anode andcathode compartments8,5 has its own inlet and outlet (10 and11 for the anode compartment,12 and13 for the cathode compartment) respectively in a concurrent flow (a counter current flow may also be used). The anode9 andcathode3 may be typical electrodes in electrical communication, such as, but not limited to, electrodes comprised of nickel or nickel alloys and its commercial variations such as Raney nickel ornickel100,200 or300; stainless steel of any grade and variety; zirconium; platinum; palladium; gold; graphite; or carbon. Dimensionally stable anodes such as those offered by Industrie De Nora (Milano, Italy) and comprising a metal base at least partially covered with an electrically conductive coating comprising a platinum group metal, and which are configured to not substantially change dimensions during the electrolysis process, can also be used.
• The anode andcathode compartments8,5 are separated by an ion-conducting membrane, which is permeable to alkali metal ions but is not permeable to water and water vapor. Suitable membrane materials include, for example, ceramics such as lithium-β-aluminum oxide, lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide, lithium analogs of NaSICON ceramics, LiSICONs, and lithium ion conductors with perovskite structure, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, NaSICON ceramics, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, potassium-β/β″-aluminum oxide, and potassium analogs of NaSICON ceramics, KSICONs, among others.
• The membrane6 used in theelectrochemical reactor100 can be any convenient shape, and is preferably either round or square, and is held by a frame7 constructed of a polymer that is substantially impervious to water such as polyphenylene sulfide or polyetheretherketone (PEEK). Preferably the reactor is also nonconductive and substantially impervious to the anolyte and to sodium metal.
• Theelectrochemical reactor100 is suitable for use with other metal salts such as a metal borate salt as described in U.S. Patent Application 2006/0102491 A1; entitled “Processes for Separating Metals from Metal Salts,” the disclosure of which is hereby incorporated by reference herein in its entirety. The metal salt is represented by the formula zM_nO.xB₂O₃.yH₂O,
• wherein z is ½ to 5; x is 0.1 to 5; y is 0 to 10; and M is an alkali metal ion such as sodium, potassium, or lithium wherein n=2, or an alkaline earth metal such as calcium or magnesium wherein n=1, and preferably is sodium, and can be converted into a boron compound and elemental metal through electrolysis. The overall reaction is shown in Equation (5), where M is selected from the group of alkali metals; the borate salt: zM_nO.xB₂O₃.yH₂O is shown as MBO₂in Equation (5):
• 
  4MBO₂+6H₂O→4M+4B(OH)₃+O₂  (5)
• In another embodiment of the present invention, an aqueous solution of an alkali metal carbonate salt represented by formula M₂CO₃is converted to an alkali metal in a three compartment electrochemical reactor such as those illustrated inFIG. 2 in accordance with Equation (6):
• 
  2M₂CO₃→4M+O₂+2CO₂  (6)
• An exemplary three compartment reactor comprises a series of compartments separated by an ion-conducting membrane, which is permeable to alkali metal ions but is not permeable to water and water vapor. Suitable membrane materials include, for example, ceramics such as lithium-β-aluminum oxide, lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide, lithium analogs of NaSICON ceramics, LISICONs, lithium ion conductors with perovskite structure, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, NaSICON ceramics, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, potassium-β/β″-aluminum oxide, potassium analogs of NaSICON ceramics, and KSICONs, among others. The membranes do not have to be the same in both cases; i.e., the membrane separating chamber1 from chamber2 does not have to the same material as the membrane separating chamber2 fromchamber3. Electrodes are present in compartments one and three; the electrodes may be typical electrodes in electrical communication, such as, but not limited to, electrodes comprised of nickel or nickel alloys and its commercial variations such as Raney nickel ornickel100,200, or300; stainless steel of any grade and variety; zirconium; platinum; palladium; gold; graphite; or carbon. Dimensionally stable anodes such as those offered by Industrie De Nora (Milano, Italy) and comprising a metal base at least partially convered with an electrically conductive coating comprising a platinum group metal, and which do not change dimensions during the electrolysis process, can also be used as well.
• Compartment three is preferably maintained at a temperature such that the metal in the cathode compartment is in a liquid or molten state, preferably at about 95° C. to about 150° C. for sodium. Compartment three can be heated, for example, by the use of a heating bath or heating coils within the compartment. The other compartments need not be heated and may be maintained at ambient temperatures, or the compartment may be maintained at the same elevated temperature as the third compartment for ease and convenience. If any of the compartments are heated to temperatures greater than the boiling point of water, the cell may be pressurized or the solvent may be refluxed and condensed.
• The conversion of alkali metal carbonate salt to an alkali metal is carried out by providing an aqueous solution of a carbonate salt in the first compartment; an alkoxide salt, preferably sodium methoxide, in the second compartment; and a liquid alkali metal in the third compartment. The aqueous solution of a carbonate salt preferably comprises an aqueous solution of sodium carbonate with a concentration range between 0.1 M and 10.0 M, and more preferably, a saturated aqueous sodium carbonate solution. The alkoxide salt in the second compartment is provided in an alcoholic solution of methanol, ethanol, propanol or butanol. The alkali metal carbonate salt is converted in the first compartment to alkali metal ions through a combination of reactions, as shown in Equations (7) and (8):
• 
  2H₂O→O₂+4H⁺+4e⁻  (7)
• 
  4H⁺+2Na₂CO₃→2H₂O+2CO₂+4Na⁺  (8)
• Water is converted to oxygen and protons at the anode in the first compartment as shown in Equation (7). The alkali metal carbonate salt can react with the protons in the anolyte of the first compartment, as shown for Na₂CO₃, for example, in Equation (8). The resultant alkali metal ions are transported into the second compartment through the first ion exchange membrane.
• The reactions may be carried out at room temperature or at a temperature higher than room temperature. The temperature range of the cell can be between 5° C. and 110° C. Preferably, the temperature range is between 15° C. and 80° C. In the most preferred embodiment, the temperature of the cell is between 25° C. and 35° C. The applied potential range may be, for example, between 1.0 volts and 15.0 volts. Preferably, the range is between 2.0 volts and 10.0 volts. In the most preferred embodiment, the applied potential range is between 3.0 volts and 3.5 volts.
• Sodium cations are transported through the second membrane where they are reduced to sodium metal in the third compartment in accordance with Equation (9):
• 
  4Na⁺+4e⁻→4Na   (9)
• The liquid sodium metal in compartment three is in intimate contact with the surface of the membrane between compartments two and three and the surface of the cathode; the reduction of sodium ions to sodium metal reduction reaction occurs at the interface of the membrane and the liquid metal.
• The transport of sodium ions out of the second compartment and into the third will result in a scarcity of positively charged species in the second compartment. To relieve the imbalance of charge, sodium ions or protons will migrate from the first compartment and into the second compartment through a membrane. The sodium ions will react with any alcoholic solvent to form a sodium alkoxide. It is preferred that the membrane separating the first and second compartments favors the transport of sodium ions over the transport of protons. The net effect is that sodium ions exit from compartment two into compartment three and enter compartment two from compartment one at the same rate, such that sodium methoxide is neither created nor consumed. The overall conversion of sodium carbonate to sodium metal is shown in Equation (10):
• 
  2Na₂CO₃→4Na+O₂+2CO₂  (10)
• In reference toFIG. 2B, a gas stream comprising hydrogen can be supplied via a gas inlet to the first compartment to lower the cell voltage. Suitable gas inlets for supplying a hydrogen or hydrogen-containing gas stream include a pipe, a sparger, a hose or a hydrogen gas diffusion material, among others. In such case, hydrogen is reduced in accordance with Equation (11) rather than water as shown in Equation (7).
• 
  2H₂→4H⁺+4e⁻  (11)
• 
  4H⁺+2Na₂CO₃→2H₂O+2CO₂+4Na⁺  (8)
• The alkali metal carbonate salt can react with the protons in the compartment one anolyte, as shown for Na₂CO₃in Equation (8). The resultant alkali metal ions are transported into the second compartment through the first ion exchange membrane and the conversion to sodium metal occurs as described above. The overall conversion of sodium carbonate to sodium metal using the hydrogen in compartment one is shown in Equation (12):
• 
  2Na₂CO₃+2H₂→4Na+2H₂O+2CO₂  (12)
• In another embodiment of the present invention, the electrochemical processes and reactors of the invention are incorporated into a process for producing sodium borohydride. Sodium borohydride may be produced by a typical multi-step synthetic process, as illustrated inFIG. 3 and Table 1. The general steps include production of hydrogen by steam methane reforming atStep302; electrolysis of sodium chloride to produce sodium metal atStep304; preparation of sodium hydride by reaction of sodium and hydrogen atStep306; refining of borax to generate boric acid atStep308; conversion of boric acid to trimethylborate atStep310; and reaction of sodium hydride and trimethylborate to produce sodium borohydride along with sodium methoxide atStep312. The generated sodium methoxide is reacted with water as shown at Step314 (not illustrated inFIG. 3) to yield methanol and caustic soda (NaOH). Methanol is typically recycled from this stream in order to lower overall manufacturing costs. Sodium metal, the most costly raw material associated with the process, is not recycled in this process.

• TABLE 1
  4 NaCl	→	4 Na + 2Cl2	304
  4 Na + 2 H2	→	4NaH	306
  ¼ Na2B4O7•5 H2O + ¼ H2SO4	→	¼ Na2SO4+ B(OH)3	308
  B(OH)3+ 3 HOCH3	→	B(OCH3)3+ 3 H2O	310
  4 NaH + B(OCH3)3	→	NaBH4+ 3NaOCH3	312
  3 NaOCH3+ 3 H2O	→	3 NaOH + 3 HOCH3	314
  ¼ Na2B4O7•5 H2O + ¼ H2SO4+ 4 NaCl + 2 H2	→	NaBH4+ ¼ Na2SO4+ 2 Cl2+ 3 NaOH

• FIG. 4 illustrates the process operations for the process described in Table 1 andFIG. 3. The actual preparation of sodium borohydride (e.g., steps306,310 and312) is conducted in the first four vessels inFIG. 4. The remainder of the process operations illustrated inFIG. 4 are for the separation and isolation of the sodium borohydride from the reaction mixture. When the reaction slurry is added to water, the oil is separated from the aqueous layer and the methanol (CH₃OH or MeOH) is stripped from the solution to yield a solution containing12% sodium borohydride in aqueous caustic soda (NaOH). These steps are conducted in the lower row of equipment shown inFIG. 4.
• The raw material input forStep308 is generally the mineral borax, which is refined to produce boric acid through treatment with sulfuric acid. Though this conversion proceeds with good yield, it results in a substantial quantity of sodium sulfate as a byproduct. In essence, the process separates the boron and sodium values present in borax and discards the sodium from the borax. In order to make sodium borohydride, sodium must be re-introduced into the manufacturing process.
• Commercially, sodium metal is prepared by the electrolysis of sodium chloride atstep304. Much of the energy inefficiency and related cost of the process derives from this means of manufacturing sodium metal. SupplementingStep304 with the more efficient process for making sodium described in the present application provides significant improvements in both energy utilization and cost. Alternatively,Step304 could be replaced with the hydrogen-assisted processes for producing sodium disclosed in U.S. patent application Ser. No. 10/388,197, now U.S. Pat. No. 7,108,777 entitled “Hydrogen-Assisted Electrolysis Process,” the disclosure of which is incorporated by reference herein in its entirety.
• As disclosed in U.S. Pat. No. 7,108,777 and in accordance with an embodiment of the present invention, the sodium methoxide produced during the production of sodium borohydride atStep312 is removed from the process stream with an extraction process, and then electrolyzed in the presence of hydrogen gas to generate sodium metal and methanol (shown in Table 2). Both compounds are raw materials for the overall process and thus can be recycled. By incorporating the electrochemical step402, it is possible to eliminate the water processing at Step314, to recycle 100% of the methanol produced in402 atStep310, to recycle 75% of the sodium produced atSteps402 and304 at Step306 (the remaining 25% of sodium is “removed” from the process as sodium borohydride), and to arrive at an overall reaction with fewer consumable raw materials, fewer side products and fewer reactions than the process of Table 1.

• TABLE 2
  NaCl	→	Na +½ Cl2	304
  3 NaOCH3+ 3/2 H2	→	3 Na + 3 HOCH3	402
  4 Na + 2 H2	→	4NaH	306
  ¼ Na2B4O7•5 H2O + ¼ H2SO4	→	¼ Na2SO4+ B(OH)3	308
  B(OH)3+ 3 HOCH3	→	B(OCH3)3+ 3 H2O	310
  4 NaH + B(OCH3)3	→	NaBH4+ 3NaOCH3	312
  ¼ Na2B4O7•5 H2O + ¼ H2SO4+ NaCl + 7/2 H2	→	NaBH4+ ¼ Na2SO4+ ½ Cl2+ 3 H2O

• Alternatively, sodium carbonate could be used as the sodium source, and Step304 replaced with Step404 while utilizing the electrochemical step402′ as shown in Table 3.

• TABLE 3
  Na2CO3+ 2 HOCH3	→	2 NaOCH3+ CO2+ ½ O2+ H2	404
  4 NaOCH3+ 2 H2	→	4 Na + 4 HOCH3	402′
  4 Na + 2 H2	→	4NaH	306
  ¼ Na2B4O7•5 H2O + ¼ H2SO4	→	¼ Na2SO4+ B(OH)3	308
  B(OH)3+ 3 HOCH3	→	B(OCH3)3+ 3 H2O	310
  4 NaH + B(OCH3)3	→	NaBH4+ 3NaOCH3	312
  ¼ Na2B4O7•5 H2O + ¼ H2SO4+ ½ Na2CO3+ 4 H2	→	NaBH4+ ¼ Na2SO4+ ½ CO2+ 7/2 H2O

• The process of Table 2 can be performed in accordance with embodiments of the invention using the unit operations illustrated inFIGS. 5A and 5B, and comprises a separator to isolate sodium borohydride from sodium methoxide which is subsequently converted into sodium metal. In a first variation illustrated inFIG. 5A, the sodium methoxide is the extract while the sodium borohydride is the raffinate. In a second variation illustrated inFIG. 5B, the sodium borohydride is the extract and the sodium methoxide is the raffinate. The process shown inFIG. 4 is thus improved. For example, operators that have to mix water are replaced by operations to separate the components from the reaction section using equipment such as extractors (for example, a liquid-liquid extraction column), or strippers (for example, a liquid-liquid extraction column or a distillation column). Sodium methoxide and sodium borohydride can be separated, for example, from the product stream ofStep312 by a liquid-liquid extraction process using a solvent chosen from Table 4.

• TABLE 4
  Item	Solvents

  1	1,1,3,3-Tetramethyl butyl amine
  2	1-Methyldiethyl amine
  3	1-Methyldodecyl amine
  4	1-Methyloctyl amine
  5	1-Pentanol
  6	2,2,4-Trimethylpentane
  7	2,6-Dimethyl-4-heptanol
  8	2-Ethyl-1-butanol
  9	2-methoxyethyl ether
  10	2-ethylhexanol
  11	2-Ethylhexyl amine
  12	2-Methyl furan
  13	2-Methyl-1-butanol
  14	2-Methyl-1-pentanol
  15	2-Methyl-2-pentanol
  16	3-Heptanol
  17	Acetonitrile
  18	Amyl acetate
  19	Aniline
  20	β,β′-Iminodipropionitrile
  21	β,β′-Oxydipropionitrile
  22	β,β′-thiodipropionitrile
  23	Benzyl alcohol
  24	Butylene glycol diacetate
  25	butyronitrile
  26	Caproic acid
  27	Carbon disulfide
  28	Creosote oil
  29	Cyclohexanol
  30	cyclohexylamine
  31	Diallyl ether
  32	Dibutyl ether
  33	Diisobutylene
  34	Diisopropyl carbinol
  35	Diisopropyl ether
  36	Dimethyl acetamide
  37	Dimethyl sulfoxide
  38	Dipropylene glycol
  39	ethyl amine
  40	Ethyl butyrate
  41	Ethyl isovalerate
  42	Ethyl propionate
  43	Ethylbenzene
  44	Ethylene glycol
  45	Ethylene glycol dimethyl ether
  46	Ethylenediamine
  47	Fenchone
  48	Furfural
  49	iso-Amyl alcohol
  50	iso-Butanol
  51	iso-butyl amine
  52	Isophorone
  53	iso-propyl amine
  54	Isopropyl ether
  55	methyl amine
  56	Methyl butyrate
  57	Mineral oil
  58	Monochlorobenzene
  59	Monoethanolamine
  60	Morpholine
  61	n-Amyl alcohol
  62	Naphtha
  63	n-Butanol
  64	n-butyl amine
  65	n-Ethyl-sec-butyl amine
  66	n-Ethyl-tert-butyl amine
  67	Nitrobenzene
  68	Nitroethane
  69	Nitromethane
  70	n-propyl amine
  71	p-Cresol
  72	Phenol
  73	Pyridine
  74	sec-Butanol
  75	Styrene
  76	tert-butanol
  77	tert-butyl amine
  78	tert-Butyl hypochlorite
  79	tert-Nonyl amine
  80	Tetrachloroethylene
  81	Tetraethylene glycol dimethyl ether
  82	Tetrahydrofuran
  83	Tetrahydrofurfuryl alcohol
  84	Trichloroethylene
  85	Triethylene glycol
  86	Triethylene glycol dimethyl ether
  87	Xylene

• For sodium borohydride, the preferred extraction solvents would be any of the glycols, amines, amides, ethers, nitriles and nitro-compounds. More specifically, the most preferred solvents would be: ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, methyl amine, ethylamine, n-propylamine, iso-propylamine, n-butylamine, iso-butylamine, cyclohexylamine, morpholine, aniline, pyridine, monoethanolamine, ethylenediamine, dimethylacetamide, dimethylsulfoxide, acetonitrile and tetrahydrofuran. For sodium methoxide, the preferred extraction solvents are alcohols, and most preferably, methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol, sec-butanol, tert-butanol, pentanol, hexanol and phenol. The sodium methoxide separated by the extraction is converted to sodium and an alcohol in an electrochemical reactor at Step402. The electrochemical reactor may be the reactor ofFIG. 1.
• In another embodiment of the present invention, an improved process for the production of sodium borohydride utilizing the electrochemical reactor of the present invention is provided in Table 5, in which the sodium hydroxide produced at Step314 is recycled to sodium metal. For example, aqueous sodium hydroxide can be provided to the anodic compartment of the filter press electrochemical cell and reduced to sodium metal as at step502 of Table 5.
• Alternatively, hydrogen can be passed at the anode during the conversion of sodium hydroxide into sodium metal at Step502 using, for example, the hydrogen-assisted processes disclosed in U.S. application Ser. No. 10/388,197, now U.S. Pat. No. 7,108,777 entitled “Hydrogen-Assisted Electrolysis Process,” the disclosure of which is incorporated by reference herein in its entirety, thus resulting in modified Step502′ as shown below:
• 
  3NaOH+1.5H₂→3Na+3H₂O   502′

• TABLE 5
  NaCl	→	Na +½ Cl2	304
  3 NaOH	→	3 Na + 3/2 H2O + ¾ O2	502
  4 Na + 2 H2	→	4NaH	306
  ¼ Na2B4O7•5 H2O + ¼ H2SO4	→	¼ Na2SO4+ B(OH)3	308
  B(OH)3+ 3 HOCH3	→	B(OCH3)3+ 3 H2O	310
  4 NaH + B(OCH3)3	→	NaBH4+ 3NaOCH3	312
  3 NaOCH3+ 3 H2O	→	3 NaOH + 3 HOCH3	314
  ¼ Na2B4O7•3 ½ H2O + ¼ H2SO4+ NaCl + 2 H2	→	NaBH4+ ¼ Na2SO4+ ½ Cl2+ ¾ O2

• Reference is now made to Tables 6 and 7, which illustrate another embodiment for an improved process for the production of sodium borohydride utilizing the electrochemical process of the present invention which utilizes an aqueous solution of a borate salt, which may illustratively be prepared from the product from a hydrogen generation reaction but may comprise any borate salt according to the teachings in U.S. Patent Application 2006/0102491 A1 entitled “Processes for Separating Metals from Metal Salts,” the disclosure of which is hereby incorporated by reference herein in its entirety. The use of a sodium borate salt rather than sodium chloride as the “sodium source” allows the elimination ofStep306 from the process used in Table 1.

• TABLE 6
  NaBO2+ 3/2 H2O	→	Na + B(OH)3+ ¼ O2	602
  3 NaOCH3+ 3/2 H2	→	3 Na + 3 HOCH3	402
  4 Na + 2 H2	→	4 NaH	306
  B(OH)3+ 3 HOCH3	→	B(OCH3)3+ 3 H2O	308
  4 NaH + B(OCH3)3	→	NaBH4+ 3NaOCH3	310
  NaBO2+ 7/2 H2	→	NaBH4+ 3/2 H2O + ¼ O2

• TABLE 7
  NaBO2+ H2O + ½ H2	→	Na + B(OH)3	602′
  3 NaOCH3+ 3/2 H2	→	3 Na + 3 HOCH3	402
  4 Na + 2 H2	→	4 NaH	306
  B(OH)3+ 3 HOCH3	→	B(OCH3)3+ 3 H2O	308
  4 NaH + B(OCH3)3	→	NaBH4+ 3NaOCH3	310
  NaBO2+ 4 H2	→	NaBH4+ 2 H2O

• At Step602, the aqueous solution of alkali metal borate, preferably a solution comprising sodium metaborate, and more preferably a product solution comprising sodium metaborate and sodium hydroxide, is introduced to the anode compartment and subjected to an electrochemical reduction in an electrochemical cell comprising anode and cathode compartments separated by an ion-conducting membrane. A supporting electrolyte to enhance ionic conductivity in the anolyte may be included. Aqueous sodium ions are transported from the anode compartment through the membrane to the cathode chamber and reduced to sodium metal at the cathode. In the anode compartment, the borate solution is acidified to form boric acid. Boric acid can be converted to trimethyl borate through reaction with methanol in a separate reactor, as shown inStep308. Alternatively, an alcohol, preferably methanol, can be introduced to the anode compartment along with the aqueous solution of alkali metal borate at Step602 to form a trialkyl borate in situ, which can be removed from the anode compartment through distillation. This would eliminateStep308.
• Whether trialkyl borate is prepared in situ or not, the process may optionally be carried out with hydrogen gas supplied to the anode as shown at Step602′ in Table 7. When hydrogen is supplied to the anode, the oxidation occurring at the anode is represented by Equation (7):
• 
  ½H₂→H⁺+e⁻  (7)
• With no hydrogen, the anode product will be oxygen as shown at Step602 instead of water at Step602′. In both cases, the rate of boric acid production as a function of current passed remains the same.
• As used in the above embodiments, a metal borate salt is preferably of the formula zM_nO.xB₂O₃.yH₂O, wherein z is ½ to 5; x is 0.1 to 5; y is 0 to 10; and M is an alkali metal ion such as sodium, potassium, or lithium wherein n=2, or an alkaline earth metal such as calcium or magnesium wherein n=1, and preferably is sodium. The aqueous solution of a borate salt may be prepared from the product from a hydrogen generation apparatus, such as one used to supply a hydrogen fuel cell and as described in U.S. Pat. No. 6,534,033, entitled “A System for Hydrogen Generation,” the disclosure of which is incorporated by reference herein in its entirety. The aqueous solution comprises an aqueous solution of alkali metal hydroxide and alkali metal borate, represented by the formula zM_nO.xB₂O₃.yH₂O, wherein z is ½ to 5; x is 0.1 to 5; and y is 0 to 10. Preferably, the alkali metal ion in both the alkali metal hydroxide and alkali metal borate is typically sodium, although other alkali metal ions, such as potassium, may be utilized. The alkali metals of the alkali metal hydroxide and alkali metal borate need not be the same. Typically, the fuel solution that is introduced into a hydrogen generator comprises from about 15% to 100% by weight sodium borohydride to about 0% to 15% by weight sodium hydroxide as a stabilizer. The product comprises sodium metaborate and sodium hydroxide in a molar ratio corresponding to the fuel concentration, but the percent by weight of sodium metaborate is typically from about 27% to 100% by weight as a result of the higher molecular weight thereof in comparison to sodium borohydride, and the reduced amount of water present. The borate product may be a solution, a heterogeneous mixture, a solid or a slurry depending on the concentration of the ingredients and the temperature. The term “about” as used herein refers to ±10% of the stated value.
• The following examples further describe and demonstrate features of the present invention. The examples are given solely for illustration purposes and are not to be construed as limitations of the present invention.
EXAMPLE 1
• A reaction flask was charged with a 30 weight % sodium methoxide in methanol solution. A tube with a ceramic membrane bottom was inserted into the solution. The tube bottom comprised the membrane or separator. The volume inside the tube is the cathode compartment and the volume outside the tube, but inside the reaction flask, is the anode compartment. The anode was a Ni-frit with flowing hydrogen gas immersed in the methanolic anolyte. The cathode was a Ni-wire that was immersed within the catholyte which consisted of molten sodium metal.
• The reactor was operated between 110° C. and 120° C. under 10 psig H₂while the sodium metal compartment inside the membrane tube was placed under 10 psig N₂. Under these conditions, the sodium in the cathode compartment was molten. After a potential was applied to the cell and the desired amount of current passed through the cell, the cell was cooled to room temperature and the amount of sodium metal that was generated was measured. Sodium was generated with current efficiencies of at least 80%, wherein 80% of the electrons passing through the cell resulted in conversion of sodium ions from the sodium methoxide solution into sodium metal. The results of four runs with different membrane separators are presented in Table 8.

• TABLE 8
  		Current	Current
  	Applied	density,	efficiency,	Sodium metal
  Membrane	Potential, V	mA/cm2	%	production, g
  zirconium	5.0	65.6	99.79	1.8121
  stabilized sodium	5.0	67.6	89.87	1.5020
  β″ alumina
  sodium β″	5.0	46.1	93.86	1.2705
  alumina	4.0	31.8	84.20	1.1935

• While the present invention has been described with respect to particular disclosed embodiments, it should be understood that numerous other embodiments are within the scope of the present invention.

Claims (25)

1. An electrochemical cell comprising:

an anode compartment containing an anode, the anode compartment having a first inlet and a first outlet;

a cathode compartment containing a, cathode, the cathode compartment having a second inlet and a second outlet; and

a separator located between the anode and cathode compartments, wherein the separator is permeable to ions, and wherein the separator is held in a polymer frame that is substantially impervious to water.

2. The electrochemical cell ofclaim 1, wherein the polymer is selected from the group consisting of polyphenylene sulfide and polyetheretherketone.

3. The electrochemical cell ofclaim 1, wherein the separator comprises a material selected from the group consisting of lithium-β-aluminum oxide, lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, potassium-β-aluminum oxide, potassium-β″-aluminum oxide and potassium-β/β″-aluminum oxide.

4. The electrochemical cell ofclaim 1, wherein the separator is a NaSICON, a LiSICON or a KSICON membrane.

5. The electrochemical cell ofclaim 1, wherein the separator is round.

6. A reactor comprising:

a first compartment containing a first electrode;

a second compartment;

a first separator located between the first and second compartments, wherein the separator is permeable to ions;

a third compartment containing a second electrode; and

a second separator located between the second and third compartments, wherein the separator is permeable to ions.

7. The reactor ofclaim 6, further comprising a gas inlet in the first compartment.

8. The reactor ofclaim 6, wherein the reactor is configured to maintain the third compartment at a temperature of about 5° C. to about 110° C.

9. The reactor ofclaim 6, wherein at least one of the first and second separators comprises a material selected from the group consisting of lithium-β-aluminum oxide, lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, potassium-β-aluminum oxide, potassium-β″-aluminum oxide and potassium-β/β″-aluminum oxide.

10. The reactor ofclaim 6, wherein at least one of the first and second separators is a NaSICON, a LiSICON or KSICON membrane.

11. A process for reducing a metal salt of formula M_nOR wherein M is a metal cation selected from the group consisting of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺and Ba²⁺; R is selected from the group consisting of H and straight- or branched-chain or cyclic alkyl groups containing from 1 to 6; and n is the valence of the active metal cation compound, comprising:

providing an electrolytic cell containing anode and cathode compartments separated by a separator which is permeable to ions;

supplying at least one metal salt to the anode compartment;

applying an electric potential to the cell; and

providing hydrogen to the anode compartment.

12. The process ofclaim 11, wherein the electrical potential is from about 1 volt to about 7 volts.

13. The process ofclaim 11, further comprising electrooxidizing hydrogen at the anode.

14. The process ofclaim 11, further comprising maintaining the cathode compartment at a temperature of about 95° C. to about 150° C.

15. A process for reducing a metal carbonate, comprising:

providing a reactor comprising a first compartment containing a first electrode, a second compartment, a first separator between the first and second compartments, a third compartment containing a second electrode, and a second separator between the second and third compartments, wherein the first and second separators are permeable to ions;

supplying a metal carbonate compound to the first compartment;

supplying a metal alkoxide salt to the second compartment; and

applying an electric potential to the cell.

16. The process ofclaim 15, wherein the electrical potential is from about 1 volt to about 15 volts.

17. The process ofclaim 16, wherein the electrical potential is from about 2 volts to about 10 volts.

18. The process ofclaim 17, wherein the electrical potential is from about 3 volts to about 3.5 volts.

19. The process ofclaim 15, further comprising supplying hydrogen to the first compartment and electrooxidizing hydrogen.

20. A process for producing sodium borohydride, comprising:

reacting sodium hydride and trialkylborate to produce sodium borohydride and sodium alkoxide;

separating the sodium borohydride and the sodium alkoxide by extraction with a solvent;

electrolyzing the sodium alkoxide to sodium metal and an alcohol; and

reacting the sodium metal with hydrogen to produce sodium hydride.

21. The process ofclaim 20, further comprising electrolyzing sodium carbonate to produce sodium and reacting the sodium with hydrogen to form sodium hydride.

22. The process ofclaim 20, further comprising:

reacting the alcohol with boric acid to prepare a triaklyborate; and

electrolyzing a mixture comprising an alcohol and sodium borate to produce sodium and trialkylborate, and reacting the sodium with hydrogen to form sodium hydride.

23. The process ofclaim 20, wherein the solvent is selected from the group consisting of an alcohol, an esther, an amine, dimethylacetamide, dimethylsulfoxide and acetonitrile.

24. The process ofclaim 23, wherein the alcohol is selected from the group consisting of methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol, sec-butanol, tert-butanol, pentanol, hexanol and phenol.

25. The process ofclaim 23, wherein the ether is selected from the group consisting of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether and tetrahydrofuran.

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