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
  • C25B13/00—Diaphragms; Spacing elements
  • C25B13/04—Diaphragms; Spacing elements characterised by the material
• • 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
  • C25B3/00—Electrolytic production of organic compounds
  • C25B3/20—Processes
  • C25B3/25—Reduction

Definitions

• the present invention is directed to a method for the electrochemical production of alkali alcoholates and to an electrolytic cell that can be used for this purpose.
• the invention concerns a method of production in which aqueous saline solutions can be used on the anode side.
• Alkali alcoholates (Na/KOMe/OEt, etc.) are among the standard products of the chemical industry. Compounds in which the alcohol component contains up to 4 carbon atoms are especially valuable in organic synthesis.
• the alcoholate can also be obtained by dissolving an alkali metal in the corresponding alcohol.
• sodium and potassium react so violently with lower alcohols that usage on an industrial scale would be too risky and higher or branched-chain alcohols may react too slowly.
• DE 19802013 suggests a solution to this problem. Specifically, amalgam stemming from chlor-alkali electrolysis is reacted with alcohol under the catalytic action of transition metal nitrides and transition metal carbides. This method has the disadvantage that the alcohols can be obtained more or less only contaminated with mercury which must then be removed in additional steps.
• DE 3346131 suggests an electrochemical method in which alkali-metal ions are produced from alcoholic saline solutions on the anode side using a cation exchange membrane as separator material. Alkali-metal ions migrate via the membrane into the cathode compartment, where alcohols are transformed into their anions during the development of hydrogen.
• U.S. Pat. No. 5,425,856 discloses a continuous electrochemical method for the production of alkali alcoholates starting from alcoholic saline solutions. This method has the disadvantage that chlorinated alcohol compounds arise in the anode compartment as by-products and these compounds may be pathogenic.
• DE 19603181 also suggests an electrochemical method for the production of alkali alcoholates.
• the anode compartment and the cathode compartment are totally separated by an ion-conducting solid electrolyte in order to avoid a disadvantageous mixing of the anolyte and catholyte solutions.
• PCT/EP00/08278 presents another method for the production of alkali metals.
• An aqueous solution of an alkali-metal salt is placed in the anode compartment and this compartment is separated helium-tight from the cathode compartment containing, e.g., a steel cathode.
• a saline melt is suggested as a cathode-side liquid electrolyte.
• a solid electrolyte is suggested as separator that conducts only the considered cations and which totally suppresses the diffusion of water into the cathode compartment.
• Exclusively coated, ion-conducting solid electrolytes are suggested as separators resistant to aqueous systems.
• the present invention concerns an electrochemical method for the production of alkali alcoholates that avoids many of the disadvantages of other procedures.
• the method is suitable for use on an industrial scale both from an economic and an ecological standpoint. More specifically, the method may be used for the cost-effective preparation of the desired products while avoiding the emission of pathogenic substances.
• the invention is directed to a method for the electrochemical production of alkali alcoholates starting from a saline, aqueous anolyte and an alcoholic catholyte.
• the cathode compartment and the anode compartment of the electrolytic cell are separated by a solid electrolyte that conducts only ions and is anolyte-stable and catholyte-stable.
• ⁇ -aluminum oxide is used as solid electrolyte and the solid electrolyte is coated on one side or on both sides with one or several ion-permeable, electrically conductive layers.
• These layers may be applied to the solid electrolyte by vapor deposition or they may be burned onto its surface as an immersion layer.
• the expression “ion-permeable” is to be understood to signify that ions can diffuse through the material on account of porosities and not on account of defects (as in ionic conduction) in this material.
• the cathode side and the anode side surfaces of the solid electrolyte will be in contact with the means supplying and removing current.
• the solid electrolyte is the form of a plane-parallel plate, i.e., a plate with parallel faces.
• the anode compartment and/or the cathode compartment of the electrolytic cell is/are filled with electrically conductive powders.
• the temperature of the anolyte during the electrolysis should be between ⁇ 10° C. and 0° C.
• the temperature of the catholyte should be between ⁇ 45° C. and ⁇ 5° C., calculated in each instance from the boiling point of the anolyte or of the catholyte.
• the method is carried out in a continuous manner with the anolyte at a pH of>7.
• the invention is directed to an electrolytic cell that comprises an anode compartment containing an aqueous solution of at least one alkali-metal salt as anolyte, and a cathode compartment.
• the cell has a solid electrolyte as membrane by means of which the anode compartment and the cathode compartment are separated from one another.
• the solid electrolyte does not comprise any other ion-conducting layer and the solid electrolyte is, preferably, ⁇ -aluminum oxide.
• the solid electrolyte may be coated on one side or on both sides with one or several ion-permeable, electrically conductive layers.
• the solid electrolyte can be applied to the solid electrolyte by vapor deposition or they can be burned onto its surface as immersion layer.
• the cathode side and the anode side surfaces of the solid electrolyte will usually be in contact with the means supplying and removing current.
• the anolyte has a pH of>7.
• the invention is directed to a method for the electrochemical production of alkali alcoholates starting from a saline, aqueous anolyte and an alcoholic catholyte.
• the cathode compartment and the anode compartment are separated by a solid electrolyte that conducts only ions and is anolyte-stable and catholyte-stable.
• the method may be used to manufacture water-sensitive alkali alcoholates from aqueous saline solutions and simple alcohols that are readily available using electric current without entailing the danger of a contamination of the products with mercury.
• a conducting salt such as, e.g., the alkali alcoholate to be produced, can also be added to the catholyte on the order of ⁇ 5% by wt., preferably ⁇ 4% by wt., more preferably ⁇ 3% by wt. and especially preferably ⁇ 2% by wt.
• ion-conduction solid electrolytes can be used in so far as they meet the prerequisites indicated above. Suitable solid electrolytes should, in addition, have the most selective conductivity possible for the specific alkali ion with the lowest possible specific resistance. Such solid electrolytes are mentioned, for example, in GB 1155927.
• a solid electrolyte of the ⁇ -aluminum oxide type is preferably used.
• the solid electrolyte has the approximate composition Alk 2 O 11 Al 2 O 3 in which Alk preferably stands for sodium, potassium or lithium as a function of which alkali alcoholate is to be produced.
• the solid electrolyte can optionally also be provided with the ion-conducting coatings described in PCT/EP00/08278.
• the solid electrolyte is coated on one or both sides with one or several ion-permeable, electrically conductive layers.
• the layer, or layers, are preferably applied to the solid electrolyte by vapor deposition or burned on to its surface as an immersion layer.
• Production methods for solid electrolytes coated in this manner are taught in the literature (Edelmetall-Taschenbuch, 2 nd edition, Huthig-Verlag Heidelberg; Thick Film Technology: A User's Guide. London, Andy. Cermalloy Div., Heraeus Inc., West Conshohocken, Pa., USA. Editor(s): N. H. Kordsmeier, Jr.; Charles A. Harper, et al., Electron. Mater.
• a preferred embodiment is one in which the cathode side and the anode side surfaces of the solid electrolyte coated with an electrically conductive, porous layer are in contact with the means supplying and removing current.
• This embodiment has the advantage that only the solid electrolyte is located between the primary anode and cathode, and, as a result, the transfer resistance, which can be characterized as a voltage drop of the cell, can be substantially minimized.
• the solid electrolyte can assume any form known in the art, such as casings, disks or tubes. However, a solid electrolyte in the form of a plane-parallel plate, preferably a disk, is preferred because of its two freely accessible surfaces.
• Another embodiment that can be used is one in which the anode compartment and/or cathode compartment is/are filled with electrically conductive powders in order to improve the distribution of potential in the catholyte and anolyte.
• the electrode surface is enlarged with this measure and, as a result, the overvoltage phenomena, especially due to diffusion problems, is reduced as much as possible.
• Potential electrically conductive powders are preferably those used for the decomposition of amalgam in alcohols (DE19802013) or for the anode coating in electrolyses (De Nora, et al., Chem. Ing. Tech. 47:125-128 (1975); Electrochemical Engineering: Science and Technology in Chemical and Other Industries, H. Wendt; G.
• metal powders of every type in the cathode compartment provided that they are sufficiently stable and do not result in a contamination of the alkali alcoholates. Included among preferred metal powders are nickle powders, tungsten powders, titanium powders, copper powders and steel powders. The use of Hastelloy powders is especially preferred.
• Another powder that can be used is graphite, either alone or in combination with one or more metal powders or modified in accordance with DE198020123.
• the following may be used in the anode compartment: graphite powder, titanium powder and materials that are used for dimensionally stable anodes (Ullmann's Encyclopedia of Industrial Chemistry, vol. A6, pp. 450-454, VCH Weinheim, 1996; D. L. Caldwell, in Comprehensive Treatise of Electrochemistry, vol. 12, Plenum 1981 ed. Bockris, pp. 122-126; Comninellis, et al., J. Appl. Electrochem. (1991), 21(4):335-345 (1991); Hinden, et al., Eur. Pat. Appl. 91982)).
• Graphite is preferably used.
• the alcohol that forms the alkali alcoholate is used as catholyte in the electrolysis.
• saline solutions to be used as anolyte.
• the concentration of the saline solution should be as high as possible. Therefore, saturated solutions of carbonates, chlorides, sulfates, sulfites, hydroxides, etc. are preferably used.
• the use of carbonates, sulfates and chlorides is especially preferred.
• the saline anolyte solution should have a pH at which the solid electrolyte has its maximum stability. Since the solid electrolytes are preferably oxides of aluminum that have a rather high chemical stability in the basic pH range, the anolyte should be adjusted with special preference to pH's greater than 7 and with even greater preference to pH's greater than 8.
• the conversion achieved using the methods discussed above is temperature-dependent.
• the temperature should therefore be held as high as possible for reasons of efficiency.
• boiling of the anolyte solutions and catholyte solutions should be avoided from an engineering standpoint. Nevertheless, work can be performed above the boiling point if solutions are put under pressure, the pressure resistance of the solid electrolyte setting a natural limit.
• the temperature of the anolyte during the electrolysis is preferably adjusted to a range from greater than ⁇ 10° C., preferably greater than ⁇ 5° C., to less than 0° C., and preferably less than ⁇ 1° C., calculated from the boiling point of the anolyte.
• the temperature of the catholyte during the electrolysis is adjusted to a range from greater than ⁇ 45° C., preferably greater than ⁇ 20° C., to less than ⁇ 5° C., and preferably less than ⁇ 10° C., calculated from the boiling point of the catholyte.
• the temperature of the catholyte is approximately 1-2 degrees higher than the anolyte.
• a disk as solid electrolyte with two electrolyte compartments comprising an inlet and an outlet.
• a disk with 70 mm diameter is fixed between two half shells via an appropriate sealing system.
• the system should seal both hollow compartments (anolyte compartment and catholyte compartment) in an airtight manner and thus prevent a passage of the electrolyte solutions into one another.
• Electrolyte can flow through the electrolyte compartments (approximately 25 ml) in a defined manner via hose pumps.
• the volumetric flow rate is advantageously selected for the aqueous anolyte sols in a range of 0.11 l/h to 10 l/h, preferably 1 l/h to 2 l/h.
• the anolyte can be heated via a thermostat. If the anolyte compartment is filled with powder, a separator system (filter) should be arranged in front of the outlet line.
• the catholyte solution is also pumped in this procedure through the catholyte compartment and optionally separated from entrained powder by filter insert before leaving the catholyte compartment.
• the described arrangement makes it possible to heat the anolyte substantially above the boiling temperature of the catholyte alcohol system.
• a cooling system can optionally be provided.
• the catholyte should be moved into the temperature range indicated above only at the beginning of the test in order to rapidly obtain the desired reaction rate.
• its flowthrough rate can be appropriately raised in an advantageous manner.
• the volumetric flow rate in this instance can be between 1 and 10 l/h, and preferably in a range of 2 to 5 l/h.
• the continuous circulation brings about a removal of the gaseous products from the electrolyte compartments.
• the intermediate containers used as pump receiver and for thermostating can be equipped with nitrogen-veiled reflux condensers open at the top.
• anode materials to be used in the method are well known in the art.
• the materials cited in PCT/EP00/08278 can be used for this purpose.
• Materials discussed above for filling the anode compartment are preferably also used as anode material. The corresponding situation applies to the cathode material.
• the invention concerns an electrolytic cell that comprises an anode compartment containing an aqueous solution of at least one alkali-metal salt as anolyte, a cathode compartment and a solid electrolyte as membrane by means of which the anode compartment and the cathode compartment are separated from one another, and in which the solid electrolyte does not comprise any other ion-conducting layer. It was surprisingly determined that solid electrolytes of the type cited in PCT/EP00/08278 and not coated with ion-conducting layers have sufficient stability against the solutions used so that they can be employed on an industrial scale.
• An arrangement of the solid electrolyte is designed in such a manner that an ageing (age-hardening) of a casing (tube open at the top) in aqueous solution is possible.
• the inner compartment is tightly closed by a seal and a glass headpiece.
• the glass headpiece is provided with an escape valve for pressure compensation and for taking specimens.
• the casing is fixed in a thermostatable container filled with a saturated soda solution.
• the casing itself is filled with 30% sodium methylate solution.
• the ageing test is carried out at a temperature of 60° C.
• the resistance of the solid electrolyte is checked by a weekly determination of the water content in the alcoholate solution. After 3 months the test can be halted and there should not be an elevation in the water content in the alcoholate solution.
• a disk (d 70 mm) is fixed between two half shells, one filled with saturated soda solution and other with 30% alcoholate solution.
• the half shells are fastened in a tightly sealed manner against the disk and the ambient.
• the construction is thermostated in a water bath at 60° C. After 3 months there should be no water in the alcoholate solution.
• a tube, open on one side (Ionotec Ltd., B1-100-LNZ) with the dimensions 100 ⁇ 30 ⁇ 1.3 (hxDxd) was filled with methanol and, in a second test, with 10% sodium methylate solution.
• the tube was tightly closed and aged in sodium carbonate solution at a temperature of 50° C. After 4 weeks, in each instance the water component of the alcohol solution was determined.
• the analyses yielded values below 2%, which is a reflection of the tightness of the Na-beta-A1203 membrane.
• a disk (Ionotec Ltd., D65-2-LNZ) with the dimensions 65 ⁇ 1 (Dxd) was fixed between two half shells designated as the anolyte compartment and the catholyte compartment.
• the anolyte compartment was filled with saturated sodium carbonate solution and the catholyte compartment with 10% sodium methylate solution.
• the tightly closed system was tempered in a water bath at 55° C.
• a total of 7 specimens were drawn out of the catholyte compartment at intervals of 4 weeks and analyzed for their water content. The amount of alcoholate solution removed was filled back in each instance. The analyses indicated no rise of the water content in the catholyte for the entire testing time.
• the aged tube from preliminary test 1 was used for electrochemical syntheses.
• Graphite felt was placed on the outer surface for contacting and connected via platinum wires to the anode output of a rectifier.
• the inner compartment was filled with graphite powder (particle size ⁇ 50 ⁇ m) up to 20 mm below the edge and also connected to the rectifier via a platinum wire.
• the inner compartment was filled with 40 ml pure methanol and adjusted via a thermal element to 59° C.
• the tube discharge was provided with a water cooler and the anolyte was tempered to 65° C. Using this system, a static current-voltage curve was recorded (0.5 V/5 min). The data is presented in table 1.
• Example 1 The tube used in example 1 was cleaned and filled with Hastelloy powder (Praxair, NI-544). A test was then performed analogously to example 1.
• Hastelloy powder Praxair, NI-544
• the aged disk of preliminary test part 2 was also used for electrochemical syntheses.
• the disk was clamped between two half disks and the anolyte compartment designed so that it its temperature could be controlled.
• the anolyte compartment and the catholyte compartment were filled with Hastelloy powder (NI-544) and contacted via platinum wires.
• the degree of filling of the cell with Hastelloy powder corresponded on both sides to 98% and resulted in an available surface of 2083 mm 2 .
• Current-voltage was recorded as described in example 1.
• the anolyte compartment was filled with saturated sodium carbonate solution and heated to 100° C. and the temperature in the catholyte compartment (3.5% sodium methylate solution) was held at 55° C.
• test 3 The arrangement of test 3 was used except that a voltage difference of 6 V was immediately applied and not changed for 4 hours.
• a determination of the alcohol component in the anolyte indicated a rise of 1.2% in 20.8 g anolyte.
• the water component of the solution ( ⁇ 0.2%) remained constant.

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