Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/28—Per-compounds
  • C25B1/30—Peroxides
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/14—Alkali metal compounds
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material

Definitions

• the present invention is directed to a method for one-step electrosynthesis of borohydride.
• the problem addressed by this invention is the need for an electrochemical synthesis of borohydride.
• the present invention is directed to a method for producing borohydride.
• the method comprises causing current to flow in an electrolytic cell between an anode and a cathode, wherein a solution of a boron-containing compound is in contact with the cathode, and wherein the cathode comprises a conductive material having activity as a high hydrogen overpotential electrode.
• borohydride means the tetrahydridoborate ion, BH 4 - .
• borohydride anions formed at the cathode are prevented from migrating to the anode.
• this is accomplished by providing a cation-selective ion exchange membrane to separate the anode and cathode compartments.
• the cation-selective membrane allows sodium, or other cations, to cross into the cathode compartment to balance the charge that would otherwise accumulate from production of hydroxide and borohydride at the cathode.
• the anolyte is acidic, and protons cross the membrane into the cathode compartment and maintain a relatively neutral pH therein.
• a microporous separator may be used to allow ions to cross in either direction; in this case, borohydride would cross over into the anode compartment to some extent and be oxidized.
• the electrolytic reaction occurs in a non-aqueous solvent in which borohydride is soluble, e.g., C 1 -C 4 aliphatic alcohols, e.g., methanol, ethanol; ammonia; C 1 -C 4 aliphatic amines; glycols; glycol ethers; and polar aprotic solvents, for example, dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide, hexamethyl phosphoramide (HMPA), and combinations thereof.
• the non-aqueous solvent is methanol, ethanol, DMF, HMPA, or combinations thereof.
• the amount of water present in non-aqueous solvents is less than 1%, more preferably less than 0.1%, more preferably less than 100 ppm, and most preferably the non-aqueous solvents are substantially free of water.
• the electrolytic reaction occurs in an aqueous solvent or an aqueous/organic solvent mixture having more than 1% water.
• Organic solvents used in an aqueous/organic solvent mixture are those having sufficient solubility in water to form a solution.
• alkali is present to stabilize the borohydride, preferably at least 0.1 N alkali.
• the boron-containing compound of the present invention is a salt or acid of a boron-containing ion, or a trialkyl borate, B(OR) 3 , wherein R preferably is methyl or ethyl.
• the boron-containing ions used in the present invention are complex ions containing only boron and oxygen. More preferably, the boron-containing ions are borate, tetraborate, or metaborate. Most preferably, the boron-containing ion is metaborate or tetraborate.
• a synthetic polymer used in the present invention includes, for example, polyolefins, e.g., polymers made from monomers comprising ethylene, propylene, other ethylenically unsaturated hydrocarbons or mixtures thereof; polymers made from monomers comprising halogenated olefins, e.g., halogenated ethylenes; polystyrenes; polyethers; polyvinyl alcohols; polyamides; and mixtures thereof.
• an addition polymer made from ethylenically unsaturated monomers is used.
• a hydrophobic synthetic polymer is used, e.g., an addition polymer substantially free of atoms other than carbon, hydrogen and halogen atoms.
• a hydrophobic synthetic polymer used in the present invention is an addition polymer comprising at least 50% by weight of monomer units derived from one or more fluorinated ethylene monomers, e.g., tetrafluoroethylene, 1,1-diffuoroethylene, or trifluoroethylene. More preferably, the hydrophobic synthetic polymer comprises at least 75% of monomer units derived from one or more fluorinated ethylene monomers. Most preferably, the hydrophobic synthetic polymer is poly(tetrafluoroethylene) (PTFE).
• PTFE poly(tetrafluoroethylene)
• a high hydrogen overpotential electrode is one where the reduction potential for electrolysis of water to form hydrogen under the reaction conditions is approximately equal to, or more negative than the reduction potential for borate reduction.
• the theoretical reduction potential for borate reduction is -1.24 volts vs. a standard hydrogen electrode ("SHE").
• the high hydrogen overpotential electrode comprises a metal inherently having such activity, for example, lead, zinc, cadmium, mercury and indium.
• the electrode comprises a high-surface-area electrode, preferably a carbon high-surface-area electrode. Examples of suitable carbons are carbon cloths and felts, vitreous carbon, and reticulated vitreous carbon.
• high-surface-area means having a surface area of at least 0.005 m 2 /g.
• Reticulated vitreous carbon foam having approximately 10 pores per inch typically has a surface area of about 0.01 m 2 /g.
• Carbon felt or cloth typically has a surface area of about 0.5 m 2 /g.
• Carbon black and gas diffusion electrodes fabricated with carbon black typically have a surface area of about 200 m 2 /g or more.
• a “nickel screen electrode” is an expanded nickel mesh.
• An example is the DelkerTM 416 nickel mesh, having diamond-shaped openings, 0.416 inches x 0.170 inches, with a strand thickness of 0.005 inches, and approximately 75% open space.
• a cathode that comprises a synthetic polymer and a conductive material having activity as a high hydrogen overpotential electrode comprises a mixture of the synthetic polymer and the conductive material supported on a metal or graphite base electrode.
• the conductive material is a metal.
• the cathode is formed by plating from a mixture of polymer particles suspended in water and a solution containing a salt of the metal.
• the base electrode onto which the mixture is plated preferably comprises the same metal as the metal salt which is plated.
• the cathode comprises a synthetic polymer and a conductive material having activity as a high hydrogen overpotential electrode
• the current density is no greater than 100 mA/cm 2 , more preferably no greater than 75 mA/cm 2 , and most preferably no greater than 50 mA/cm 2 .
• the cathode comprises a synthetic polymer and at least one metal on the surface of a high surface-area electrode.
• the metal has activity as a high hydrogen overpotential electrode due to the presence of the synthetic polymer.
• the metal preferably is nickel, an alloy comprising two metals, or a metal inherently having activity as a high hydrogen overpotential electrode.
• An alloy comprising two metals, A and B preferably is of the form AB 5 , AB, A 2 B or AB 2 .
• at least one of the metals is a transition metal.
• at least one of the metals is a rare earth metal.
• a and B are both transition metals.
• one of the metals is La, Ni, Ti or Zr.
• AB 5 is LaNi 5 , optionally with additional metals, e.g., Sn, Ge, Al or Cu.
• the metals are Ti and Zr, optionally with additional metals, e.g., Mn, Cr, Fe, V or Ni.
• the alloy is of form A 2 B, the alloy is Mg 2 Ni.
• the alloy is of form AB, it is FeTi.
• a gas diffusion electrode is one that enables direct electronic transfer from a gas phase to or from a solid phase.
• the GDE also provides a path for ionic transfer.
• a GDE typically comprises a conductive porous support, e.g., carbon cloth, carbon paper or metal mesh.
• the GDE often has a wet-proofing layer of carbon black, and optionally additional layers of wet-proofing.
• an electrocatalyst layer typically is applied to the surface, or is applied to carbon black prior to electrode assembly. The electrocatalyst facilitates reduction of boron compounds over reduction of water.
• the wet-proofing material may be a synthetic polymer, as described above, e.g., PTFE, which may be applied as an emulsion in water.
• the GDE comprises a highly dispersed metal electrocatalyst which can act as a very high surface area cathode. Hydrogen generated at the cathode, or alternatively, fed to the back of the electrode, may provide an activated catalyst which allows in situ hydride formation.
• the current density is greater than 120 mA/cm 2 .
• the amount of metal which is present on the surface of the GDE is less than 2 mg/cm 2 .
• the predominant anode reaction is the electrolysis of water to form oxygen and protons. If the anolyte is acidic, protons will transport across the separator and neutralize the hydroxide that is generated at the cathode along with borohydride. If the anolyte is basic, the protons will neutralize the hydroxide in the anode compartment and sodium will transport across the separator to make byproduct sodium hydroxide.
• the anode is a non-corroding material, for example, platinized titanium or iridium oxide on titanium. If the anolyte is basic, a lower-cost material would be quite stable, e.g., nickel. In a non-aqueous system, the anode could be a corrosion-resistant metal, e.g., platinum.
• the anolyte is an aqueous sodium salt, e.g., sodium hydroxide, sodium carbonate or sodium bicarbonate. Protons generated would then form stable species like water or carbon dioxide. Alternatively, any aqueous mineral acid would be suitable. In the case of a non-aqueous solvent, an organic-soluble conductive sodium salt would be suitable, e.g., a sodium alkoxide, or a lithium salt soluble in the non-aqueous solvent.
• borohydride may be used in the method of this invention to improve yield of borohydride, including additives that would improve solvation in non-aqueous systems; lithium or ammonium salts to raise hydrogen overpotential; and redox species, e.g., naphthalene or anthracene.
• a nickel sulfamate bath 225 g Ni(NH 2 SO 3 ) 2 and 20 g H 3 BO 3 in 0.5 L H 2 O
• 80 mL of PTFE solution TEFLON 30b solution - 30% TEFLON powder in H 2 O.
• the composite cathode was prepared by plating the PTFE-nickel material from the bath onto a nickel plate (5 cm 2 ) at 20 mA/cm 2 for 1400 coulombs of charge.
• the electrode made with Misch metal (LaNi 5 ) was prepared by grinding Misch metal and sieving to 100 mesh, thus providing a maximum particle size of 150 micron.
• An electrode was prepared by adding polyvinyl alcohol powder to 5% by weight and compressing onto a nickel screen and heat treating to provide a homogeneous electrode.
• the Misch metal concentration of the electrode was 425 mg/cm 2 .
• Electrode Preparation for Platinum/Palladium Alloy Plated Graphite Felt Graphite felt was washed with dilute hydrochloric acid and then water to remove any metal ion impurities present. The felt was then plated with a platinum/palladium alloy. The plating was performed using a plating bath of the following composition: (NH 4 ) 2 Pd(NO 2 ) 2 5 g/L (NH 4 ) 2 Pt(NO 2 ) 2 0.3 g/L KHPO 4 5 g/L The bath was adjusted to pH 9 using ammonium hydroxide.
• Plating was performed at 90°C using a constant current of 20 mA/cm 2 and a charge of 2000 coulombs passed. The plating was gray in appearance and concentrated at the outer surface of the felt.
• a typical electrolysis was performed in a two-compartment glass cell divided with a NAFION 417 membrane (available from DuPont Co.).
• the anolyte consisted of 1 M sodium hydroxide (80 mL) and the anode material was platinized titanium or nickel. Unless specified otherwise, the catholyte was 25% by weight sodium metaborate adjusted to pH 11-12 with sodium hydroxide. The electrolysis was carried out at constant current.
• the amount of borohydride in the catholyte was determined indirectly by allowing it to react with cyclohexanone and determining the amount of cyclohexanol formed by gas chromatography.
• a 5 mL sample of the catholyte was reacted with 5 mL of a solution containing 2% by weight cyclohexanone in methanol. After reaction with the large excess of cyclohexanone, the mixture was injected directly into a gas chromatograph. The cyclohexanol peak was compared to the cyclohexanol peaks determined by reacting aqueous borate solution containing known amounts of borohydride.

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• 2005
  • 2005-03-31 CA CA002503244A patent/CA2503244C/en not_active Expired - Fee Related
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