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WO2001030492A1 - Catalyseurs a l'argent cristallin utilises pour l'oxydation de methanol en formaldehyde - Google Patents

Catalyseurs a l'argent cristallin utilises pour l'oxydation de methanol en formaldehyde Download PDF

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WO2001030492A1
WO2001030492A1 PCT/AU2000/001297 AU0001297W WO0130492A1 WO 2001030492 A1 WO2001030492 A1 WO 2001030492A1 AU 0001297 W AU0001297 W AU 0001297W WO 0130492 A1 WO0130492 A1 WO 0130492A1
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silver
electrolyte solution
concentration
catalyst
catalysts
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Graeme John Millar
Thien Quang Dam
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University of Queensland UQ
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University of Queensland UQ
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    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C45/00—Preparation of compounds having >C = O groups bound only to carbon or hydrogen atoms; Preparation of chelates of such compounds
    • C07C45/27—Preparation of compounds having >C = O groups bound only to carbon or hydrogen atoms; Preparation of chelates of such compounds by oxidation
    • C07C45/32—Preparation of compounds having >C = O groups bound only to carbon or hydrogen atoms; Preparation of chelates of such compounds by oxidation with molecular oxygen
    • C07C45/37—Preparation of compounds having >C = O groups bound only to carbon or hydrogen atoms; Preparation of chelates of such compounds by oxidation with molecular oxygen of >C—O—functional groups to >C=O groups
    • C07C45/38—Preparation of compounds having >C = O groups bound only to carbon or hydrogen atoms; Preparation of chelates of such compounds by oxidation with molecular oxygen of >C—O—functional groups to >C=O groups being a primary hydroxyl group
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/48—Silver or gold
    • B01J23/50—Silver
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/34—Irradiation by, or application of, electric, magnetic or wave energy, e.g. ultrasonic waves ; Ionic sputtering; Flame or plasma spraying; Particle radiation
    • B01J37/348—Electrochemical processes, e.g. electrochemical deposition or anodisation
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C1/00—Electrolytic production, recovery or refining of metals by electrolysis of solutions
    • C25C1/20—Electrolytic production, recovery or refining of metals by electrolysis of solutions of noble metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity

Definitions

  • This invention relates generally to the field of industrial catalysis and more particularly to crystalline silver catalysts for methanol oxidation to formaldehyde conversion.
  • This invention describes the formation of novel forms of crystalline silver which have catalytic properties that are superior to conventional silver materials prepared by known electrochemical methods.
  • Formaldehyde is a highly versatile chemical that finds widespread application in industry, particularly in the resins sector. Commercially, it is synthesised via either the partial oxidation and dehydrogenation of methanol over crystalline silver (US Patent 4,594,457, US Patent 4,584,412) or in a uniquely oxidative process in conditions of excess air in the presence of a mixed iron oxide-molybdenum oxide catalyst (US Patent 3,843,562 and US Patent 3,855,153).
  • the metal oxide system requires a substantial volume of gas which is 3.0 to 3.5 times greater than the gas flow of a conventional silver catalysed process (Kirk-Othmer, Encyclopedia of Chemical Technology, 4 th Edition).
  • Important aspects for formaldehyde production are; firstly, the need to convert the maximum amount of methanol in the feed per pass ; secondly, the necessity of producing formaldehyde in high selectivity thus achieving a high formaldehyde yield; thirdly, the requirement of the catalyst to minimise the amount of by-products formed; fourthly, the achievement of rapid reaction "light-off to avoid downtime costs and fifthly, the desire to operate the catalyst for a life in the industrial plant of at least several months without loss of performance and lastly, the ability of the catalyst to increase plant throughput. Quantities of both methanol and formaldehyde are lost to competing and/or consecutive reaction pathways.
  • the major by-products formed over the catalyst are carbon dioxide, carbon monoxide, formic acid and methyl formate.
  • crystalline silver can be obtained by operation of an electrochemical cell, for example the conventional Moebius, Thum or Prior cells, using a silver nitrate electrolyte in the pH range 1-4 containing between 5 and 100 g/L dissolved silver, a cell temperature of 10-80°C, a current density between 100 and 3000 A/m 2 and a cell voltage between 0.2 and 9 volts(US Patent 5,135,624).
  • an electrochemical cell for example the conventional Moebius, Thum or Prior cells
  • preparation methods specifically related to crystalline silver catalysts which are well known to those skilled in the art include the continuous electrolytic refining of silver in an aqueous solution of silver nitrate and nitric acid at 24 °C, 3.1 volts and a current density of 1.2 amp/dm 2 (120 A/m 2 ) (Graefen et al., French Patent 2,141 ,893) wherein silver grains of 0.2 to 2.5 mm in size are stripped from a slowly rotating polypropylene anode.
  • Szustakowski et al. (M. Szustakowski, J. Schroeder, A. Jakubowics, T. Kelm. I. Cieslik and E. Francman, Polish Patent PL 122783) disclosed the doping of silver catalysts by ⁇ 1% of activators such as Al, Be, Zr, Mg, Si, V, Mo, Se, Cd, Cr, As or Sb. These latter additives were typically introduced by addition of corresponding ions into the electrolytic refining procedure. These inventors demonstrated that the formaldehyde yields and methanol conversion efficiency could be enhanced by such additives. However, the efficiencies reported are significantly less than the comparable figures presented in this invention. Similarly, Szustakowski et al. (M.
  • Silver catalysts modified by the presence of other inorganic elements have also been previously disclosed such as in US Patent 4,045,369 which reveals that the addition of barium, strontium, calcium and/or indium may be beneficial for oxidation reactions.
  • Also known is the use of silver-gold alloys (EP 104,666 and EP 003,348), silver-cadmium alloys (US 3,334,143) and silver oxide (JP 46-20693).
  • Japanese patent 08117599 advocates the immersion of silver crystals in a platinic chloride solution to produce a catalyst, which consists of 5 ppm to 3 wt% platinum on silver.
  • European patent 0 486 777 A1 suggests that the addition of either magnesium oxide, zirconium oxide, silica, yttrium oxide or aluminium oxide in amounts of 4 wt% or less can be beneficial for methanol oxidation.
  • Supported catalysts have also been reported such as silver on Kellundite (US Patent 4,330,437) and silver on porcelain (US Patent 4,126,582). Additionally, the application of silver supported on pumice stone has been reported (Sacharov et al., Khimicheskaya Promyshlennost, 2, 75-76, 1991).
  • Japanese patent 60-89441 suggests that the use of catalysts comprising of silver and zinc dispersed on a silica support may be useful.
  • Previous inventions to alleviate the problem of slow reaction light-off include construction of elaborate multilayered beds of silver catalyst of different particle sizes (German patent 2,322,757). Using this latter invention it is claimed that light-off can be achieved at temperatures of 553-573 K. Very fine silver powder (0.1-1 micron) can also be sprinkled on top of the silver bed and this procedure permits reaction to start at 478-503 K (German Often. 2,520,219). In general, smaller silver grains are found to exhibit greater activity, however, due to sintering and plugging effects a bed cannot be entirely constructed of these fine particles.
  • a patent assigned to Koei Chem Co Ltd JP 06 1722478 discloses that the pressure drop over a silver catalyst bed can be minimized (and thus the useful catalyst lifetime increased) by ensuring that the uppermost 1.5 mm of a silver catalyst bed is composed of particles, at least 10% ⁇ 0.38 mm in diameter and at least 50% > than 0.38 mm in diameter, which have a packing density in the range 3.5 to 4.5 g/mL.
  • These latter inventors revealed that silver particle densities less than 3.5 g/mL resulted in a reduction in catalyst efficiency, whereas catalyst densities in excess of 4.5 g/mL resulted in pressure rises over the catalyst bed which made for inefficient use of the air compressor.
  • the "light-off" period is defined, as the reaction time following introduction of the catalyst to the industrial reactor, required for maximum formaldehyde yield to be attained.
  • Fig. 1 shows data acquired from an industrial formaldehyde synthesis plant using a commercially available silver catalyst. Notably, the maximum level of formaldehyde production is not achieved until after seven days of reaction. Therefore, the diminished formaldehyde production capacity during this latter period represents a significant financial penalty to the commercial producer. The reduced production capacity is not the only distressing feature of the current generation of commercial silver catalysts;
  • Fig. 2 displays data regarding the formation of formic acid by-product during a commercial methanol oxidation process which shows that formic acid levels are concomitantly undesirably high during the initial "light-off period.
  • the crystalline silver catalysts previously made have catalytic properties which are inferior to those of the silver catalysts produced in this invention.
  • This invention discloses methods that can be used to modify crystalline silver catalysts in a manner which, reduces detrimental effects in the industrial plant.
  • the primary object of the invention is to produce a silver catalyst, which exhibits superior formaldehyde yield during methanol oxidation conditions. Another preferred object of the invention is to achieve faster reaction light off during industrial plant start-up, thus minimising financial penalties accumulated during plant downtime. Another preferred object of the invention is to produce a silver catalyst, which exhibits minimal production of formic acid by-product that inhibits the ability of formaldehyde to polymerise in downstream applications. Yet a further preferred object of this invention is to produce a catalyst which enhances the conversion of methanol. Yet another preferred object of this invention is to increase the plant throughput. Still yet another preferred object of the invention is to provide means to achieve quality control on catalyst production.
  • a secondary criterion for optimal catalyst performance in addition to reduced packing density has been found to be associated with the surface area of the silver catalyst particle.
  • Silver crystals with a BET surface area in excess of 200 cm 2 /g and more desirably in excess of 400 cm 2 /g have been discovered to provide increased formaldehyde yield which is unexpected as those of average skill in the art will know that catalyst selectivity does not necessarily relate to increased surface area.
  • the fact that we have found that the level of formic acid by-products produced during industrial methanol oxidation conditions when using catalysts of the described relatively high surface area cannot be explained by practitioners in this area from existing knowledge.
  • a third criterion, in addition to the other features described above, for enhanced catalyst performance has been discovered to be the morphology and shape of the silver grains as identified by electron microscopy. Specifically, silver grains of a porous morphology have been surprisingly established to provide enhanced useful catalytic activity.
  • the invention also provides a method of manufacture of the catalyst of the invention which includes the steps of:
  • the concentration of copper species in the electrolyte has been found to be important with respect to synthesis of an active catalyst. Indeed, in contrast to conventional electrorefining practices employed by those of average skill in the art who routinely add copper ions to electrolyte solutions, it has been discovered that it is beneficial to have a concentration of less than 0.5 g/L and more preferably less than 0.1 g/L of copper in the electrolyte solution to aid formation of the desirable low density silver structure.
  • anode material which contains minimal amounts of copper impurities.
  • concentration of copper in the silver anode should be less than 10% and more preferably less than 1 % and even more preferably less than 0.1 %.
  • Use of anode materials comprising of less than 99.9% silver has been found to result in silver crystals of relatively high packing density, and comparatively low surface area. This discovery is surprisingly with respect to prior art which does not indicate any requirement for silver anode materials comprising of relatively low amounts of contaminants when synthesizing polycrystalline silver catalysts.
  • the pH of the electrolyte has been unexpectedly found to be of critical importance in preparing silver crystals with enhanced catalytic properties. Raising the pH of the solution by addition of a base such as, but by no means limited to, ammonium hydroxide, sodium hydroxide, potassium hydroxide, caesium hydroxide, calcium hydroxide, magnesium hydroxide, strontium hydroxide, barium hydroxide, sodium bicarbonate, sodium carbonate, disodium tetraborate and organic amines, can unexpectedly produce silver catalysts of not only preferable low packing density but also of relatively high surface area.
  • the pH of the solution should preferably be at least 4 and more preferably at least 5 and even more preferably greater than 6. This discovery is novel in that conventional wisdom indicates that silver should only be refined at a pH of less than 4.
  • One method for enhancing the production of silver catalysts of packing density less than 2.5 g/mL has been discovered to be the addition of a silver oxide material to the electrochemical bath.
  • the silver oxide material deposits itself over the cathode plate and without wishing to be bound by theory appears to inhibit the silver grain growth in a manner which favours formation of the desired low packing density crystals.
  • a complex with silver in the electrolyte solution has been discovered to play a vital role in the production of very low density silver catalyst with packing densities less than 1.8 g/mL.
  • the presence of a silver diammine complex ion in the electrolyte solution has been demonstrated to exhibit a desirable effect and the concentration of this complex is preferably at least 1 % and more preferably at least 10% and even more preferably at least 20% and more preferably less than 80% with respect to the concentration of silver in the electrolyte.
  • other soluble complexes of silver may be formed.
  • the addition of ammonia to a silver ion solution is the most preferable method due to the ease of silver di
  • the best silver catalysts in terms of low bulk density can only be synthesized by control of a multiple of the critical variables described above.
  • optimum crystals are prepared when using anode of >99.9% silver, an electrolyte with less than 0.1% copper present, at least 10 g/L of dissolved silver ions which comprise of silver in the form of a complex such as silver diammine and a solution pH > 4.
  • FIG. 1 Production level of formaldehyde as a function of time in a industrial methanol oxidation facility using a crystalline silver catalyst.
  • FIG. 2 Concentration of formic acid as a function of time in a industrial methanol oxidation facility using a crystalline silver catalyst.
  • FIG. 3 Scanning Electron Microscopy (SEM) image of silver catalyst of 0.64 g/mL bulk packing density.
  • FIG. 4 Scanning Electron Microscopy (SEM) image of silver catalyst of
  • crystalline silver catalysts can be synthesized by use of either a Balbach-Thum, Prior or Moebius electrochemical cell or modifications of each type as known to those skilled in the art.
  • the basic concept of the Moebius cell is to attach anodes of cast silver, which can be obtained from any convenient source, to hanger bars which are in turn surrounded by a woven cloth or polymer bag to catch slime.
  • the cathodes are usually made of stainless steel, which are convenient for removal of silver crystals by scraping, thus causing the silver crystals to collect on the bottom of the tank. Modifications to the standard design include incorporation of catchment trays to the cell to facilitate silver recovery.
  • a typical range of operating parameters would be ; (1 ) between four and twenty cathodes, (2) a current of 100 - 500 A, (3) a cell voltage of -1.5 to -2.8 V, (4) a temperature close to ambient, (5) a cathode current density of 20-40 mAcm "2 .
  • the Balbach-Thum cell is designed around a rectangular trough containing either a carbon plate or stainless steel cathode on the bottom of the cell and a group of silver anodes suspended in a basket in the upper portion of the cell. Again, woven cloth or polymer material may envelop the basket to contain anode slime. Due to the increased separation of the anode and cathode the cell voltage is normally significantly higher than the value found in a Moebius cell. Typical cell voltages in a Balbach-Thum cell may be from -3.5 to -5.5 V. In both cells the average silver concentration is approximately 30-150 gL "1 and the silver nitrate electrolyte often contains free nitric acid and traces of copper nitrate.
  • Claessens et a/.(US 5,100,528) describe the use of a continuous silver refining cell which comprises of a tank containing an electrolyte, and at least one vertical cathode disk mounted on a rotating horizontal shaft and a means for continuously removing silver from the rotating cathode.
  • Prior A. Prior, Precious Metals, 22, 163 (1998)) developed a variation on the Moebius Cell which featured not only automation of the silver electrolysis but also removal of the anode slime and silver crystals.
  • the essential element of construction which allowed throughput to be increased, was the creation of an anode in the form of a basket in which was placed silver grains (instead of the conventional cast anode).
  • the anode basket is composed of titanium and a non-conducting plastic material and is designed in two distinct compartments. The upper compartment contains the silver grains whereas the lower compartment allows collection of anode slime to occur. Anode slime can be removed by application of an appropriate suction system.
  • Silver crystals are scraped from the stainless steel cathodes and subsequently collected at the bottom of the cell wherein they are removed by use of spiral conveyor.
  • the conveyor itself is equipped with a washing system and subsequently transfers the washed silver crystals to a centrifugal drier and finally directly to the melting furnace if required.
  • the presence of impurities in the electrolyte solution i.e. metal ions other than silver
  • the presence of impurities in the electrolyte solution i.e. metal ions other than silver
  • the presence of copper ions in the electrolyte solution results in the production of silver crystals with inferior catalytic properties.
  • the initial preparation of the electrolyte solution should be performed as follows. Typically, silver metal is added to a solution of concentrated nitric acid to form a silver nitrate solution.
  • This initial solution normally comprises of relatively high concentrations of silver ions in the order of several hundred grams (e.g. 100 - 400) of silver per litre of electrolyte. Therefore, dilution of the initial silver solution with purified water should then occur to create an electrolyte with the desired final concentration of silver ions.
  • concentration of silver should be 10 g/L or more as when lesser concentrations of silver are used the current stability in the cell becomes problematic and in turn crystals of inferior catalytic properties are produced.
  • the silver nitrate electrolyte solution comprises of some free nitric acid, thus the initial pH is in the order of 0.1 to 2. It has been found in this invention that the best catalytic silver material is made when the electrolyte pH is higher than 4. Notably, prior art has taught practitioners to refine silver at a pH between 1 and 4, Consequently, our discovery that a pH in excess of 4 is beneficial for the synthesis of optimal silver catalyst is indeed surprising. In essence, any basic solution, as described above, may be added to the electrolyte solution to raise the pH from the initial value to a pH in excess of 4.
  • the silver oxide material may comprise of silver in more than one oxidation state such as Ag', Ag" and Ag'", the identity of the silver oxide not being particularly limited.
  • silver oxide powder may be purchased from any commercial supplier and simply weighed to a prescribed amount and added to the electrolyte solution. Stirring of the electrolyte solution aids the dispersion of the silver oxide material, and after an appropriate settling time the cathode should now comprise of a thin layer of well dispersed silver oxide deposit.
  • silver oxide may be freshly made in a container which comprises of a solution of silver ions to which a suitable quantity of base is added to raise the pH to a point where silver oxide begins to precipitate.
  • This solution can then be decanted until a silver oxide slurry remains.
  • This slurry can be directly added to the electrolyte bath or initially dried in an oven at a temperature sufficient to remove the water content and then the resultant silver oxide powder added.
  • the silver diammine complex can also be formed by addition of ammonia solution to silver oxide.
  • the fresh electrolyte material is now ready for transfer to the electrochemical cell, the identity of which is not particularly limited in this invention.
  • One important aspect of the cell is the composition of the anode material.
  • silver refiners obtain silver from either gold mines or the photographic industry. Consequently, the silver material which is cast into the required shape for the anode may comprise of gold and copper impurities. Therefore, once electrorefining is underway the electrolyte solution becomes contaminated with copper ions in particular and to a lesser extent gold ions.
  • the presence of metal ions other than silver in the electrolyte solution has a deleterious effect upon the formation of silver crystals with optimal catalytic properties.
  • anode material which is at least 99.9% silver.
  • the production of such anode compositions is best achieved by initially electrorefining the silver stock material in any type of electrochemical cell to purify the raw silver material to > 99.9% purity and then subsequently recasting this pure silver into the required anode shapes.
  • the identity of the cathode material is not particularly limited with the main criteria being that the cathode material is not only conductive but also chemically inert under the applied cell conditions. Consequently, stainless steel or carbon make good cathode materials.
  • the electrochemical cell is now full of the electrolyte composition disclosed in this invention and comprises of a precise anode composition as revealed in this patent application. It is now that the electrochemical cell can be connected to the rectifier and current supplied to initiate the electrocrystallization of silver.
  • the current density is another parameter with respect to synthesis of an active silver catalyst.
  • the value of the current density should be greater than 30 A/m 2 to enhance the yield of silver metal obtained.
  • the temperature of the electrolyte also appears to have an effect upon the silver catalytic properties. In particular, temperatures are best maintained in the range 10 to 40°C . Placement of a heating/cooling coil in the electrochemical bath provides a simple means of controlling the bath temperature, and if desired a stirrer can also be located in the electrolyte solution to circulate the fluid and maintain a more even temperature profile within the solution.
  • the time allowed for electrorefining of silver has also been found to be critical with respect to obtaining silver crystals of the desired low packing density which has been disclosed in this invention. The synthesis time can be from as little as one hour to over one hundred hours if so desired, before the current is switched off and the silver crystals removed from the electrochemical cell.
  • the run time should not exceed such a period where it is found that the silver crystal density has increased beyond the point where the values are not optimal for catalytic performance.
  • the silver crystals should be removed with a scraper and then comprehensively washed with purified water. Finally, the silver crystals should be dried in an oven at a temperature of >80°C.
  • the packing density of the crystals can then be measured, wherein the packing density is defined as the mass of silver crystals per unit volume.
  • the packing density is defined as the mass of silver crystals per unit volume.
  • a simple procedure known to those of average skill in the art would be to weigh a known mass of silver crystals (e.g. 100 g) and then to pour this amount of silver into a measuring cylinder which comprises of calibrated markings which allow the volume to be calculated.
  • a measuring cylinder which comprises of calibrated markings which allow the volume to be calculated.
  • To obtain an accurate value for the packing density it is usually necessary to tap the measuring container to ensure that optimum packing of the crystals occurs. Practically, practitioners in this area would be aware that the point of optimum packing can easily be determined by observation of the changes in the volume of the material recorded as a function of increased tapping.
  • the silver catalysts are ready for placement in an industrial reactor.
  • the silver crystals are placed in distinct layers of prescribed grain sizes on top of a copper gauze which is itself located on a base plate which provides mechanical support for the weight of the catalyst bed.
  • a silver catalyst was prepared by the following procedure.
  • a conventional Balbach-Thum electrochemical refining cell comprising of ca. 250 kg of an anode composed of a silver dore material obtained from a goldfield and a cathode made of stainless steel was used to synthesize silver catalyst crystals.
  • the area of the cathode employed was ca. 1.5 m 2 .
  • the basic procedure was to operate the electrochemical cell over a time frame of 24 hours wherein at the end of that 24-hour period the silver catalyst crystals were removed from the cathode surface. Once the silver crystals were collected they were then thoroughly washed with deionized water and then separated into distinct particle sizes, by means of pouring into a series of meshes of well- defined aperture dimensions.
  • Table 2 shows the parameters used to manufacture the silver catalysts.
  • the electrolyte used for this synthesis experiment comprised of only 0.16 g/L copper.
  • the packing densities for the resultant silver crystal mesh fractions are displayed in Table 3.
  • silver crystals were produced by electrochemical techniques. However, in this instance copper nitrate crystals were added to the electrolyte solution to give a copper concentration of 21.37 g/L, thus turning the electrolyte solution a deep blue color.
  • the conditions in this experiment would be recognizable to those of average skill in the art as being characteristic for conventional refining of silver metal which is currently practiced industrially (Table 2).
  • a silver catalyst was prepared by the following procedure.
  • a conventional Balbach-Thum electrochemical refining cell comprising a cathode made of stainless steel was used to synthesize silver catalyst crystals.
  • the area of the cathode employed was ca. 1.54 m 2 .
  • the anode material was a silver dore material which comprised of between 7 and 15% gold and 200 to 1000 ppm copper (0.02 to 0.1%).
  • An anode cloth was used to trap the gold impurity, however, this material did not prevent passage of copper into the electrolyte solution.
  • the basic procedure was to operate the electrochemical cell over a time frame of three days wherein at the end of each 24-hour period the silver catalyst crystals were removed from the cathode surface and the process restarted.
  • Table 4 shows the parameters used to manufacture the silver catalysts as recorded each day of the experiment. Once the silver crystals were collected they were then thoroughly washed with deionized water and then separated into distinct particle sizes, by means of a pouring into a series of meshes of well-defined aperture dimensions.
  • a silver catalyst was prepared by the following procedure.
  • a conventional Balbach-Thum electrochemical refining cell comprising a cathode made of stainless steel was used to synthesize silver catalyst crystals.
  • the area of the cathode employed was ca. 1.54 m 2 .
  • the anode material was a silver sponge material which comprised of between 1 and 2% gold and up to 1 % copper.
  • an anode cloth trapped the gold mud but allowed the copper to pass through into the electrolyte solution.
  • the basic procedure was to operate the electrochemical cell over a time frame of three days wherein at the end of each 24-hour period the silver catalyst crystals were removed from the cathode surface and the process restarted.
  • Table 4 shows the parameters used to manufacture the silver catalysts as recorded each day of the experiment. Once the silver crystals were collected they were then thoroughly washed with deionized water and then separated into distinct particle sizes, by means of a pouring into a series of meshes of well-defined aperture dimensions.
  • a silver catalyst was prepared by the following procedure.
  • a conventional Balbach-Thum electrochemical refining cell comprising of ca. 250 kg of an anode composed of purified silver material of purity in excess of 99.9% and a cathode made of stainless steel was used to synthesize silver catalyst crystals.
  • the area of the cathode employed was ca. 1.5 m 2 .
  • To the electrolyte which was initially at a pH of ca. 1 was added sodium hydroxide solution until the pH of the electrolyte attained a value of 4.3.
  • the electrolyte used for this synthesis experiment comprised of less than 100 ppm copper species.
  • the basic procedure was to operate the electrochemical cell over a time frame of 24 hours wherein at the end of that 24-hour period the silver catalyst crystals were removed from the cathode surface. Once the silver crystals were collected they were then thoroughly washed with deionized water and then separated into distinct particle sizes, by means of pouring into a series of meshes of well-defined aperture dimensions.
  • Table 6 shows the parameters used to manufacture the silver catalysts. The packing densities for the resultant silver crystal mesh fractions are displayed in Table 7.
  • Silver crystals were synthesized in an electrochemical cell in the same manner as described in example 3, except that the pH was raised to an initial value of 5.12 by addition of sodium carbonate to the electrolyte instead of sodium hydroxide.
  • Table 8 illustrates the packing density of the silver crystals obtained. Significantly, all the packing densities recorded are lower than the value of 2.5 g/mL discovered in this invention to be important with relevance to obtaining silver catalysts of good catalytic properties.
  • a silver catalyst was prepared by the following procedure.
  • a conventional Balbach-Thum electrochemical refining cell comprising of ca. 250 kg of an anode composed of purified silver material of purity in excess of 99.9% and a cathode made of stainless steel was used to synthesize silver catalyst crystals.
  • the area of the cathode employed was ca. 1.5 m 2 .
  • To the electrolyte which was initially at a pH of ca. 1 was added sodium hydroxide solution until the pH of the electrolyte attained a value of 4. Also, significantly, the electrolyte used for this synthesis experiment comprised of no copper species.
  • silver oxide powder was added to the electrolyte solution before commencement of the electrorefining process.
  • Silver oxide was prepared by the addition of a solution of sodium hydroxide to an aqueous solution comprising of 50 g/L silver ions. Raising the pH to a value in excess of 5 was sufficiently high to promote the precipitation of brown/black silver oxide material. After settling for a period of several hours the aqueous solution was decanted and the resultant silver oxide slurry allowed to dry at 150 °C.
  • the basic procedure was to operate the electrochemical cell over a time frame of 24 hours wherein at the end of that 24-hour period the silver catalyst crystals were removed from the cathode surface. Once the silver crystals were collected they were then thoroughly washed with deionized water and then separated into distinct particle sizes, by means of a pouring into a series of meshes of well-defined aperture dimensions.
  • Table 9 shows the parameters used to manufacture the silver catalysts. The packing densities for the resultant silver crystal mesh fractions are displayed in Table 10.
  • An alternative method of silver crystal formation involved the conversion of the silver ions in the electrolyte solution to a complex between ammonia and silver which was probably of the form [Ag(NH 3 )2J + .
  • Aqueous ammonia was carefully added to a solution comprising of ca. 50 g/L of Ag + ions until a point where theoretical calculations indicated that the a significant fraction of silver ions had been converted to the form [Ag(NH 3 )2] + .
  • This prepared solution was then used as an electrolyte in an electrochemical cell as employed in previous examples. The detailed cell conditions used are displayed in Table 11.
  • the silver crystals produced in this example were of exceptionally low packing density (Table 12). Therefore, it has been unexpectedly discovered that it is beneficial to use not only pure silver anode material, but also electrolyte solutions comprising of minimal concentrations of copper and moreover electrolyte solutions of pH in excess of four, in conjunction with the complexation of the silver ions in solution, e.g. in the form of [Ag(NH 3 ) 2 ] + species.
  • a silver catalyst synthesized according to the methodology described in Example 4 was subjected to a BET surface area measurement using Krypton as the adsorption gas.
  • the surface area was calculated to be 878 cm 2 /g.
  • Silver catalysts were obtained from three commercial suppliers who employ traditional electrochemical synthesis procedures known to those of average skill in the art. BET surface area measurements of these crystals was again performed using Krypton as the adsorption gas. The surface areas were calculated to be 70, 141 and 186 cm 2 /g, respectively. Therefore, it can be concluded that conventional refining techniques do not produce silver catalysts of surface areas of the magnitude of those measured for the novel catalysts described in this invention.
  • EXAMPLE 9 Samples of silver crystals formed using the methodology disclosed in this invention with a packing density of 0.64 g/mL were subjected to analysis by Scanning Electron Microscopy (SEM) in order to investigate the shape of the material to ascertain if there existed any surprising morphological attributes of the catalysts. Inspection of Figure 3 reveals that the silver grains were characterized by a distinctive layer structure. More remarkable was the fact that each silver grain was actually a porous network of agglomerated strands of the layer structure which is a structure which previously has not been disclosed. The term porosity in this instance is interpreted in terms of the presence of open space in a certain area of silver crystal.
  • a silver catalyst with the relatively low packing density described in this invention (catalyst A) was tested in a commercial formaldehyde plant with capacity of 31.9 tonnes per day of 100% formaldehyde.
  • the feedstock was a mixture of air and methanol in the ratio 1.25 which also contained 5 mol% water as ballast.
  • a silver catalyst with a comparatively high packing density (catalyst B, obtainable from Borden Chemicals Inc, USA) was also evaluated under similar conditions in the same formaldehyde plant to ascertain the effect of using silver crystals typified by relatively low packing densities.
  • the densities of the mesh sizes employed are described in Table 15 for both types of catalytic material.
  • Table 16 illustrates the industrial plant data obtained for both catalysts A and B. Significantly, it was surprisingly found that a correlation existed between lower silver packing density and better catalytic activity. Firstly, the light-off period was decreased from 2 hours to 0.5 hours. Notably, under the same plant conditions not only did the degree of methanol conversion increase markedly, but also the formaldehyde yield concomitantly increased by 2.5% which represents considerable financial benefit to the formaldehyde producer. Importantly, the level of formic acid by-product formation was also diminished by use of silver crystals of lower packing density. In this case the concentration of formic acid was reduced by 50%. Yet another benefit to the formaldehyde producer was the ability to operate the plant at substantially higher rates without any reduction in catalyst performance.
  • Table 18 illustrates the industrial performance data obtained for both catalysts C and D.
  • Table 18 illustrates the industrial performance data obtained for both catalysts C and D.
  • the level of formic acid by-product formation was also diminished by use of silver crystals of lower packing density. In this case the concentration of formic acid was reduced by 51%.
  • Yet another benefit to the formaldehyde producer was the ability to operate the plant at substantially higher rates without any reduction in catalyst performance.
  • Another aspect of the commercial value for the silver catalysts of comparatively low packing density described in this invention is the ability of these latter catalysts to produce larger quantities of formaldehyde per kg of catalyst used in the catalytic reactor (Table 19).
  • examples 1 to 10 have demonstrated that silver catalyst prepared according to the novel processes described in this invention does indeed exhibit superior formaldehyde yield during methanol oxidation conditions relative to catalysts typified by packing densities in excess of 2.5g/mL. Additionally, faster reaction light off during industrial plant start-up has been observed while reduce formation of formic acid has been recorded. Simultaneously, the silver catalyst of this invention enhanced the conversion of methanol and resultantly increased the plant throughput.
  • the discovery that the packing density of the silver catalyst is significant with respect to achieving good plant performance allows a means to monitor quality control on catalyst production.

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Abstract

La présente invention concerne un procédé de production d'un catalyseur à l'argent cristallin. Ce procédé consiste (a) à fournir une solution électrolytique présentant une concentration d'au moins 10g/l d'ions argent dissous, ladite solution électrolytique (i) ayant une concentration en ions cuivre minimale; (ii) ayant un pH supérieur à 4; et (iii) contenant un agent complexant qui génère des cations argent complexes dans la solution; (b) à soumettre la solution électrolytique à une électrolyse, dans une cellule d'électrolyse présentant une/des anode(s) en argent, présentant une pureté d'au moins 90 %, et ayant une concentration en cuivre inférieure à 10 %, ainsi que des cathodes, formées à partir de matériau conducteur, mais chimiquement inerte, ladite électrolyse étant conduite à une température de 10 °C - 40 °C et avec une densité de courant supérieure à 20 A/m2; puis (c) à isoler l'argent cristallin qui a une densité de tassement inférieure à 2,5 g/ml de la cellule d'électrolyse. La présente invention concerne également un catalyseur cristallin, utilisé pour la transformation efficace de méthanol en formaldéhyde, qui présente une densité de tassement inférieure à 2,5 g/ml.
PCT/AU2000/001297 1999-10-25 2000-10-20 Catalyseurs a l'argent cristallin utilises pour l'oxydation de methanol en formaldehyde Ceased WO2001030492A1 (fr)

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CA002388992A CA2388992A1 (fr) 1999-10-25 2000-10-20 Catalyseurs a l'argent cristallin utilises pour l'oxydation de methanol en formaldehyde
AU11151/01A AU767288B2 (en) 1999-10-25 2000-10-20 Crystalline silver catalysts for methanol oxidation to formaldehyde

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AUPQ3646A AUPQ364699A0 (en) 1999-10-25 1999-10-25 Polycrystalline silver catalysts for methanol oxidation to formal dehyde

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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN100575558C (zh) * 2005-09-16 2009-12-30 灵宝市金源桐辉精炼有限责任公司 一种白银电解工艺
DE102008045148A1 (de) 2008-09-01 2010-03-04 Allgemeine Gold- Und Silberscheideanstalt Aktiengesellschaft Silberkatalysator zur Formaldehyd-Herstellung
WO2015030591A1 (fr) * 2013-08-29 2015-03-05 Universiteit Leiden Processus de préparation d'un matériau d'anode, pile électrochimique et processus de conversion d'eau
WO2015135858A1 (fr) * 2014-03-11 2015-09-17 Bayer Materialscience Ag Procédé de production de poudres catalytiquement actives constituées d'argent métal ou de mélanges d'argent métal et d'oxyde d'argent destinées à fabriquer des électrodes à diffusion de gaz
WO2020099390A1 (fr) 2018-11-13 2020-05-22 Basf Se Lit de catalyseur comprenant des corps de catalyseur d'argent et processus de déshydrogénation oxydative d'alcools à insaturation oléfinique
US11089692B2 (en) * 2015-04-02 2021-08-10 Taiwan Green Point Enterprises Co., Ltd. Catalytic ink comprising metallic material made from diamminesilver hydroxide, and uses thereof

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CN111533330A (zh) * 2020-06-05 2020-08-14 北京朗新明环保科技有限公司 一种工业循环水耦合软化结晶处理系统及处理方法

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JPH06172248A (ja) * 1992-12-11 1994-06-21 Koei Chem Co Ltd ホルムアルデヒドの製造方法
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Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN100575558C (zh) * 2005-09-16 2009-12-30 灵宝市金源桐辉精炼有限责任公司 一种白银电解工艺
DE102008045148A1 (de) 2008-09-01 2010-03-04 Allgemeine Gold- Und Silberscheideanstalt Aktiengesellschaft Silberkatalysator zur Formaldehyd-Herstellung
EP3685915A1 (fr) 2008-09-01 2020-07-29 Allgemeine Gold- und Silberscheideanstalt AG Catalyseur d'argent pour la préparation de formaldéhyde
WO2015030591A1 (fr) * 2013-08-29 2015-03-05 Universiteit Leiden Processus de préparation d'un matériau d'anode, pile électrochimique et processus de conversion d'eau
WO2015135858A1 (fr) * 2014-03-11 2015-09-17 Bayer Materialscience Ag Procédé de production de poudres catalytiquement actives constituées d'argent métal ou de mélanges d'argent métal et d'oxyde d'argent destinées à fabriquer des électrodes à diffusion de gaz
CN106062256A (zh) * 2014-03-11 2016-10-26 科思创德国股份有限公司 用于制备气体扩散电极的由金属银或由金属银与银氧化物的混合物制成的催化活性粉末的制备方法
US11089692B2 (en) * 2015-04-02 2021-08-10 Taiwan Green Point Enterprises Co., Ltd. Catalytic ink comprising metallic material made from diamminesilver hydroxide, and uses thereof
WO2020099390A1 (fr) 2018-11-13 2020-05-22 Basf Se Lit de catalyseur comprenant des corps de catalyseur d'argent et processus de déshydrogénation oxydative d'alcools à insaturation oléfinique
CN113015574A (zh) * 2018-11-13 2021-06-22 巴斯夫欧洲公司 包含银催化剂体的催化剂床和烯属不饱和醇的氧化脱氢方法

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