MXPA01001813A - Process for preparing nanosize metal oxide powders - Google Patents

Abstract

Disclosed is a process for preparing nanoscale metal-based powders from a metal salt and an amphiphilic copolymer containing ethylene oxide. The copolymer and metal salt are mixed to form a metal salt/copolymer paste which is then calcined at a temperature sufficient to remove water and organics and to form a metal oxide.

Description

PROCESS FOR PREPARING NANOTAMAUM METALLIC OXIDE POWDERS This invention relates to a process for producing powders based on metal or nanoscale metals. In particular, the invention relates to a process for preparing nanoscale powders of a metal salt solution and an amphiphilic material. Metal oxide or metal particles of nanotamaño size and submicron are a valuable industrial item that finds use in many applications, including the manufacture of industrial catalysts as they should be used in the chemical industry, in the manufacture of ceramics, of electronic components, coatings, manufacture of catalysts, condensers, mechanical-chemical polishing mixtures, magnetic tapes and as fillers, for example, for plastics, paints and cosmetics. A wide variety of techniques are available for the manufacture of metal oxide or metal powders having a very fine particle size. Such techniques include high temperature gas phase and solution processes and condensed phase synthesis. For a comprehensive summary of the general technique available for producing nanomaterials, see for example, "Chemical Engineering Aspects of Advanced Ceramic Materials" by V. Hlavacek and J.A. Puszynski published in the Journal of Chemical Research and Industrial Engineering, pages 349-377, Volume 35, 1996. A review of the sol-gel processing is given in Controlled Particle, Droplet and Bubble Formation, ^^ > ^^ fa ^^ i? ^ edited by David J. Wedlock, Butterworth-Heinemann Ltd., 1994, pages 1-38. Despite the numerous processes available, nanoscale powders are generally expensive and difficult to prepare in large quantities, thus limiting their applications, for example, to high-tech ceramics. A simplified procedure for producing particle of nanometer size is described in the U.S. Patent. 5,240,493. The process described requires the calcination of a polyurethane foam containing a metal cation. In a related procedure, the U.S. Patent. No. 5,698,483 describes mixing an aqueous continuous phase containing a metal salt with a hydrophilic organic polymer, forming a gel, and then heat treating the gel to remove water and organic, leaving as a residue a nanoscale powder. The metal oxide product produced from the polymer solution by the method described is very low and is improved by using an intermediate drying step. Therefore, it would be desirable to develop a cost effective process that leads to the production of metal oxide or metal powders having a consistently fine particle size. It is also desirable to have a process that could be operated using a high level of metal in proportion to a polymer. It would be advantageous if such a procedure were able to be provided for the production of metallic powders in a high product. The present invention is a process for preparing metal-based or nanoscale metal-based powder by calcining at a temperature sufficient to remove the organics of a composition comprising (a) a solution containing at least one metal salt (b) a copolymer containing amphiphilic ethylene oxide wherein the copolymer has an average molecular weight of more than 400, a content of ethylene oxide from 1 to 90 percent and a hydrophilic-lipophilic balance (HLB) from -15 to 1 5 and (c) optionally a coagulating agent, provided that when aluminum is the only metal, a coagulating agent is present. The process produces metal-based powders of high purity and uniform size. The paste formed in the present process by the mixing of the metal salt and copolymer contains a high concentration of meta in comparison with other known processes. The formation of a paste with a high concentration of metal is advantageous since it reduces the amount of water that needs to be removed from the paste before or during calcination and reduces the cost against existing technologies. According to the process of the present invention, it has been unexpectedly found that by mixing at least one metal salt with an amphiphilic copolymer containing ethylene oxide, the higher concentrations of metal salt and higher salt for copolymer proportions can be used in comparison to known processes using hydrophilic polymers. The use of a high salt to copolymer ratio minimizes the reduction in activity of the salt solution in the addition of the copolymer. The activity is defined in the present context as metallic oxide in grams obtained after the calcination of 100 grams of metal salt solution or metal salt paste / copolymer. Additionally, the present process gives a substantial increase in the surface area of the nanoscale sized particles compared to the surface area of particles prepared in the absence of the copolymer. Nanoscale particle means the main particle or crystalline size is about 200 manometers or less, preferably in the range of from 5 to 100 manometers. The mixing of a copolymer with a metal salt according to the present invention produces a metal salt / copolymer paste. The term "paste" used herein means a smooth, smooth semi-solid or solid. When the copolymer is added to the metal solution, a paste is formed. Copolymers suitable for use in the present invention are amphiphilic copolymers containing ethylene oxide wherein the content of ethylene oxide is between 1 and 90 percent. The ethylene oxide is the weight percent of ethylene oxide units in the total weight of the copolymer. Preferably the ethylene oxide content is greater than about 5 percent of the copolymer. More preferred are copolymers wherein the ethylene oxide content is about 8 percent or more. More preferred are copolymers wherein the content of ethylene oxide is about 10 percent or more. Preferably, the ethylene oxide is less than about 80 percent of the copolymer. More preferred are copolymers wherein the content of ethylene oxide is less than about 75 percent. In a preferred embodiment of the present process, the copolymers are block copolymers containing ethylene oxide. The term "amphiphilic" as used herein means a compound having an HLB of between -1 5 and 1 5 as calculated Pro Davis, Proc. Intern. Congr. Surface Activity, Vol. 1 London 1957, p. 426. The procedure assigns numerical values to several groups, for example, hydrophilic groups -SO "Na +, -COO" K + and -COOH are assigned values of +38.7, +21 .1 and +2.1 respectively; to the hydrophobic groups > CH-, -CH2- and -CH3 are assigned a value of -0.475. For a given structure, the HLB number is calculated by substituting the group numbers in the following equation: HLB =? (Hydrophilic group numbers) +? (numbers of the lipophilic group) +7. Preferably, the copolymers used in the present process have an HLB of greater than -10 and preferably less than 1 3.

More preferred are copolymers having a HLB balance of -5 to 10. Hydrophilic compounds as defined by the above HLB show a tendency to be completely miscible with water in all proportions under environmental conditions, or in the case of materials solids, at some elevated temperature slightly above its melting point (eg, about 60 ° C for high molecular weight linear polyethylene oxide polymers). In contrast, lipophilic compounds show a tendency to be completely immiscible with water, even at elevated temperatures. The range of HLB values for copolymers of the present invention represents an intermediate case comprising materials that form two-phase liquid systems in the mixing with water (or for solids, after moderate heating, so that at least one of the two phases 5 contains more than indicative amounts of the opposite phase For the purposes of the present invention, this intermediate class is designated as amphiphilic, in contrast to the hydrophilic and lipophilic classes which represent, respectively the upper and lower segment of the value range. HLB In summary, as discussed in this HLB > 15 represents hydrophilic compounds; an HLB of -15 to 15 represents amphiphilic compounds; and an HLB < -15 represents lipophilic compounds. In addition to the ethylene oxide content of the copolymer, to obtain the desired metal oxide product, the copolymers for use in the present invention have an average molecular weight greater than 400. Preferably, the copolymers have an average molecular weight greater than 500. More preferred are copolymers having an average molecular weight greater than 750. More preferred are copolymers having an average molecular weight greater than 1000. Generally the The average molecular weight of the copolymer is less than approximately 100,000. Preferably the average molecular weight of the copolymer is less than 80,000. More preferred are copolymers with an average molecular weight of less than 50,000. The ethylene oxide copolymers having the ethylene oxide content in percent and average molecular weight as described hereby can be produced by standard procedures in the M ®mmmmM «j & m ^? rj ¡& m¡? m, - ~. The material is produced to produce ethylene oxide copolymers. As used herein, the term "metal" refers to metalloid or metallic elements as defined in the Periodic Table of Selected Elements of Groups 2a, 3a, 4a, 5a, 6a; 2b, 3b, 4b, 5b, 6b, 7b, 8, 1b and 2b; the elements of lanthanide; and the actinide elements. The metal can in principle be any element that is desired to obtain a powder, however those that currently have higher industrial value and suitable for use in the present invention include, lanthanum, barium, strontium, chromium, zirconium, yttrium, aluminum, lithium , iron, antimony, bismuth, lead, calcium, magnesium, copper, boron, cadmium, cesium, cerium, dysprosium, erbium, europium, gold, hafnium, holmium, lutetium, mercury, molybdenum, niobium, osmium, palladium, platinum, praseodymium rhenium, rhodium, rubidium, ruthenium, samarium, scandium, sodium, tantalum, thorium, thulium, tin, zinc, nickel, titanium, tungsten, uranium, vanadium, ytterbium, magnesium, cobalt, gadolinium or a mixture of two or more of the same. When the metal salt, aluminum nitrate, is used to obtain nanoscale particles having an increased surface area above the metal salt alone, it has been found that aluminum needs to be used in combination with a coagulating agent as described herein. . The metals used can vary based on the application. For example, for electrical applications, such as electric capacitors, the metals Bi, Ba, Cu, La, Mg, Nb, Sn, Ti, Zr or a mixture thereof are preferred. For use in autocatalysts, Al, Ce, La, Ng, Nb, Y, Zr or a mixture thereof are preferred.

The metals are generally used in the form of a salt which is dissolved in a solvent system such as water, alcohol, acetone, tetrahydrofuran, dimethylformamide or other solvent system selected according to its ability to dissolve the metal salts and for its compatibility with the copolymer. Preferably the solvent is water. The concentration of metal salt present in the solvent is as high as practically possible in consideration of its solubility limit. Where possible it is preferred to use aqueous compositions which are essentially saturated solutions at room temperature. The activity of the salt solution will depend on the solubility of the metal salt and the ratio of the molecular weight of the metal or metal-based compound to the molecular weight of the metal salt. For metal salts that are readily soluble in water, such as nitrates, the metal solution generally has an activity of more than 5 percent. Preferably, such solutions have an activity of 7 percent or greater. More preferred are solutions with an activity of 10 percent or greater. More preferred are solutions with an activity of 15 percent or greater. Generally, the metal salt solution of commercial interest has an activity of less than 50 percent. The amount of copolymer added to the metal salt is generally an amount that does not reduce the activity of the initial salt solution by more than 50 percent (oxide in grams obtained after the calcination of a metal salt solution compared to the in grams obtained after the calcination of a metal salt / copolymer mixture. Preferably, the amount of copolymer - Ewftjfe ». added to the metallic salt results in less than 45 percent reduction in activity. More preferably, the amount of copolymer added to the metal salt results in less than 40 percent reduction in activity. More preferred is a metal salt for polymer ratio so that the reduction in activity is 30 percent or less. Although not preferred, a coagulating agent may be added to the metal salt / amphiphilic copolymer. Such coagulating agents are described in WO 99/03629 published on January 28, 1999. In general, the coagulating agent is any substance that is capable of inducing coagulation, i.e. inducing a change from a fluid state to a solid state or semi-solid, that is, paste. In addition to the aid in the formation of a paste, it is observed that with some metals the addition of a coagulating agent plus copolymer will increase the surface area of the nanoscale particles above that observed with the copolymer alone, eg, titanium or zirconium. For some metallic salts, such as cerium, the addition of a coagulating agent will reduce the surface area of the nanoscale particles when compared to the use of a copolymer alone. A determination of whether the coagulating agent will increase the surface area above that obtained with the use of a copolymer can only be determined based on the teachings herein. The addition of a coagulating agent and any water or solvent vehicle with the coagulating agent will obviously reduce the activity.

J-a¡-- -. -. fk »< m ír ** a * »íÍV? > The coagulating agent can be a prganic or inorganic substance. Advantageously, the substance should not leave any residue after pyrolysis / calcination. When the coagulating agent is an organic substance, the alkanolamines or amides, primary or secondary amines are suitable. When the coagulating agent is a suitable inorganic substance, the basic substances include, for example, ammonium hydroxide, ammonium hydrogen carbonate, ammonium carbonate. Examples of inorganic, acidic coagulating agents include hydrogen sulfide. Examples of organic coagulating agents include citric acid, ethylene diamine tetraacetic acid and other carboxylic compounds. When it's used, preferably the coagulating agent is a hydroxide such as an ammonium solution or an alkaline hydroxide solution, such as sodium or potassium. Ammonium hydroxide is preferred because of its basic basicity, attractive water solubility which leads to a rapid coagulation result, and abseof an additional metal. Ammonium hydroxide will also volatilize on heating. The ammonium hydroxide can be introduced as an aqueous solution, bubbling NH gas, or alternatively generated in situ by the use of a precursor. Examples of precursors include ammonium and urea gas. Urea in thermal energy exposure undergoes decomposition leading to the generation of nascent ammonium, which in the aqueous environment provides immediate formation of ammonium hydroxide. The formation of ammonium hydroxide by means of urea provides an extremely efficient distribution of the coagulating agent through the composition and . «. *? - t? A-8? -t. «• m-S & £? T t > 4m? Tia? AieA in any case superior to that which can be achieved by direct introduction and mechanical mixing. The amount of coagulating agent to be added is preferably at least the amount required to coagulate off the metal under consideration. A process of the present invention results in metal-based powders having an increase in surface area above the powders produced in the absence of the copolymer. Generally, when conditions are selected as described above, the increase in surface area is greater than 30 percent. Preferably, the amount of copolymer added to the metal salt gives a powder having more than 50 percent increase in surface area. More preferably, the copolymer for the metal salt is selected to give a powder with a 75 percent increase in surface area. Any equipment commonly used to mix viscous liquids can be used to produce the composition of this invention. Such equipment provides efficient mixing, under high lateral movement conditions, of controlled amounts of aqueous base solution with the aqueous composition comprising both the metal salt and the copolymer composition. It is currently believed that high lateral movement during mixing is desired, so that a fine dispersion of the salt in the paste is obtained. In contrast, it is believed that lower lateral movement speeds during mixing provide an undesired opportunity for the growth of metallic salt crystals during the process. Preferred methods are those capable of efficiently mixing the components, such as the techniques described in U.S. Patent No. 5,688,842. The composition described when calcined under controlled conditions, which provide for the removal of all organic substances, results in the formation of a metal-containing powder, substantially uniformly sized. Typically, the calcination conditions require exposing the composition at a temperature of from 300 ° C to 3000 ° C, and preferably from 400 ° C to 1000 ° C for a period of a few minutes to many hours.

Optionally, the metal salt paste / copolymer formed can be dried before calcination. The drying of the paste formed prior to calcination may increase the surface area of the nanoscale particles, particularly when a coagulating agent is used. The described metal-containing powders having a nanoscale sizes are of value in the manufacture of ceramic articles, industrial catalysts, electronic components, and as fillers for plastics, paints or cosmetics. When used as a filler the metal-containing powder will be present, based on the total weight of the volume matrix and powder, typically in an amount of from 0.1 to 50, and more usually in an amount of from 1 to 25 weight percent. The volume matrix can be, for example, a plastic, which includes a thermoplastic or thermoset polymer, a paint, or a cosmetic, cream or oil composition. The nanoscale particles can also be used in the chemical-mechanical polishing as described in the Patent of E.U. 4,057,939.

The invention is particularly useful for providing catalysts in catalyst supports such as those used to reduce aspiration emissions. For example, the composition before calcining may first be deposited on at least a portion of a surface of a catalyst support suitable for control of the aspiration emission (eg, metal, ceramics or combinations thereof). Preferably, the catalyst substrate is a ceramic selected from cordierite, mulita and combinations thereof. More preferably, the substrate is cordierite, acicular mulita or combinations thereof. The invention is illustrated by means of the following Examples. Unless stated otherwise, all quantities are expressed as parts by weight. Examples 1-9 A series of sodium nitrate solutions (Ce (NO3) 3 »6H2O) were prepared by adding 0.5, 1, 2 and 3 kg of cerium nitrate to one liter of water. This represents a calculated activity of 13.2, 19.8, 26.4 and 29.7, respectively. To 80 parts of the various salt solutions was added 20 parts by weight of a copolymer as listed in Table 1 giving the name of the initiator and the formula, molecular weight, percent of ethylene oxide, propylene oxide and / or butylene and HLB content of the copolymers. The addition of 20 parts by weight of copolymer to the solutions of 0.5, 1, 2 and 3 kg plus 1 liter of water gives an activity calculated for the initial mixture of metal salt / copolymer mixture of 10.6, 15.9, 21 .1 and 23.8, respectively. The copolymers A-1 are examples of the present invention and the J-Q copolymers are shown for comparative purposes.

Table Description of Copolymers Ol Not an example of this invention After the addition of a copolymer to the metal salt, the mixture, the mixture was mixed rapidly using a rotary mixer (Servis Heidolph Model RGL 500). The formation of a paste occurs within thirty seconds. The obtained pulps were then calcined at 500 ° C for two hours to fire the organic material (temperature increase of 25 ° C / minute until the calcination temperature is reached). The surface area of the resulting powders was measured by the N2 BET absorption technology using a Pulse Cehemisorb Model 2700 from Micrometrics Instruments Corporation. The product and surface area of the powders obtained at the various levels of metal salt concentration are given in Table II. The particle surface area obtained without the addition of any polymer was 54, 62, 65 and 65 m2 / g of initial solutions of 0.5, 1, and 3 kg of metallic salt plus 1 liter of water (marked I, II, III and IV in Table IV), respectively. The results show a substantial increase in the surface area of the nanoscale particles produced using amphiphilic copolymers compared to the absence of a copolymer or in comparison to the use of a hydrophilic or lipophilic copolymer.

Table I! Results in terms of surface area (S.A.) and activity, the activity defined as oxide in grams / 100 grams of initial copolymer gel Example Copolymer Activity S.A. of I Activity S.A. of II Activity S.A of Activity S.A of I of III of III. III of IV IV. 1 A * 12.8 160 17.0 150 24.0 97 24.3 51 2 B * 12.5 175 18.4 159 23.2 97 23.5 60 3 C * 12.7 115 18.1 121 23.2 74 26.6 68 4 D * 11.6 169 17.1 156 22.8 150 25.8 155 E 11.9 143 17.3 133 23.6 140 26.5 146 6 F 11.7 143 17.1 118 22.7 121 25.9 153 7 G 11.2 137 16.8 110 22.0 74 24.8 21 8 H * 12.1 132 17.9 117 22.3 101 25.3 102 9 I * 12.9 159 18.3 129 23.4 110 26.9 123 1C J * a 14.1 116 17.1 56 16.1 15 16.0 14 2C K * a 14.2 105 16.1 78 21.4 75 21.5 47 3C L * a 11.4 137 16.3 24 20.6 3 23.9 4 4C M * a 12.1 103 20.6 84 23.8 81 23.1 44 5c N * a 12.9 78 17.3 61 17.5 37 20.0 42 6C 0a 14.5 80 17.7 60 14.9 31 9.9 28 7c p * a 13.0 84 17.5 72 20.1 33 15.2 31 8C Qa 14.2 75 17.0 56 21.5 30 18.5 17 * Commercially available from The Dow Chemical Company to Not an example of this invention Examples 10-13 A 27 percent active solution of titanium chloride in water was made by slowly mixing 64.4 parts of TiCl4 with 35.6 parts of water (by weight). The pastes were prepared by mixing, under vigorous stirring, predetermined amounts of the TiCl solution with copolymer D in variable solution / copolymer proportions as indicated by Table III (Pbw = parts by weight). The obtained pastes were calcined at 500 ° C for two hours. The surface area and activity of the metal powders formed with or without the addition of NH OH as a coagulating agent are given in Table III. The ammonium hydroxide was added after the addition of the copolymer. Table III Nanoscale particles of titanium chloride 15 of Zr / Ce gives, after calcination, approximately 80/20 weight percent ZrO / CeO2. The gels were prepared by mixing, under vigorous stirring, different amounts of the metal solution, copolymer D, and NH4OH as the coagulating agent as given in Table IV. The ammonium hydroxide was added after the addition of the copolymer. The gels obtained were calcined at 500 ° C for two hours. The surface area and activity of the metal powders formed are given in Table IV Table IV Nanoscale Powder obtained using Zirconium / Cerium r o Examples 18-37 A solution of cerium acetate was prepared by mixing 20 grams of cerium acetate H2O to 100 grams of water. The pastes were prepared by mixing, under vigorous stirring, different amounts of the metal solution, copolymer D, and NH4OH as the coagulating agent are given in Table V. The ammonium hydroxide was added after the addition of the copolymer. The obtained pastes were calcined at 500 ° C for two years. The surface area and activity of the metal powders formed are given in Table V.

Table V Nanoscale powders obtained cerium acetate ro ro Table V (continued) Nanoscale powders obtained cerium acetate ro

Claims (15)

1. CLAIMS 1. A process for preparing metal-based or nanoscale metal-based powder by calcining at a temperature sufficient to remove the organics of a composition comprising (a) a solution containing at least one metal salt (b) a copolymer containing amphiphilic ethylene wherein the copolymer has an average molecular weight greater than 400, an ethylene oxide content of 1 to 90 percent and an HLB of between -15 and 15 and (c) optionally a coagulating agent; with the proviso that when aluminum is the only metal, a coagulating agent is present.
2. 2. The process according to claim 1, characterized in that the polymer has an ethylene oxide content of 5 to 80 percent, preferably an ethylene oxide content of 8 to 75 percent and more preferably an ethylene oxide content of 1 0 to 75 percent.
3. 3. The process according to claim 1 or 2, characterized in that the copolymer has an average molecular weight of between 500 and 100,000 and preferably between 750 and 80,000.
4. The process according to any of the preceding claims, characterized in that the copolymer has an HLB of -10 to 13 and preferably 0 to 13.
5. 5. The process according to claim 4, characterized in that the metal salt is selected from one or more metals of the Periodic Table of Elements as listed in Group 2a to 6a, Group 1 to 8 lanthanides and actinides.
6. 6. The process according to claim 5, characterized in that the metal salt is selected from one to more metals of lanthanum, barium, strontium, chromium, zirconium, yttrium, aluminum, lithium, iron, antimony, bismuth, lead, calcium, magnesium, copper, boron, cadmium, cesium, cerium, dysprosium, erbium, europium, gold, hafnium, holmium, neodymium, lutetium, mercury, molybdenum, niobium, osmium, palladium, platinum, praseodymium, rhenium, rhodium, rubidium, ruthenium, samarium, scandium, sodium, tantalum, thorium, thulium, tin, zinc, nickel, titanium, tungsten, uranium, vanadium and ytterbium.
7. The process according to claim 6, characterized in that the metal is zirconium, yttrium, cerium, lanthanum, fiancé, magnesium, aluminum or a mixture thereof.
8. The process according to any of the preceding claims, characterized in that the composition is heat treated at a temperature in the range of 300 ° C to 1000 ° C.
9. 9. The process according to any of the preceding claims, characterized in that the metal salt solution is an aqueous salt solution.
10. The process according to claim 1, characterized in that the composition further contains (c) as a coagulating agent. eleven .
11. The process according to claim 10, characterized in that the coagulating agent comprises ammonium hydroxide or an alkanolamine.
12. The process according to any of the preceding claims, characterized in that it further comprises depositing the composition, prior to calcination, on at least a portion of a surface of a catalyst substrate.
13. The process according to claim 12, characterized in that the catalyst substrate is comprised of a metal, ceramic or combinations thereof.
14. The process according to claim 13, characterized in that the catalyst substrate is a selected cordierite ceramic, mulite and combination thereof.
15. 15. A catalyst comprising a catalyst substrate having a surface having the metal or metal nanoscale powder of any of claims 1 to 1 deposited on at least a portion of said surface.