Patent Application
20080085412
As used herein, the terms “colloid” and other like terms including “colloidal,” “sol,” “acid sol,” and the like refer to a two-phase system having a dispersed phase and a continuous phase. The colloids of the invention have a solid phase dispersed or suspended in a continuous or substantially continuous liquid phase, typically an aqueous solution. Thus, the terms “colloid” or “colloidal composition” encompasses both phases, whereas the terms “colloidal particles” or “particles” refer to the dispersed or solid phase.
In one embodiment, the invention provides a metal-rich siliceous composition. The siliceous composition includes a metal oxide dispersion having one or more metal oxides and one or more colloidal silica particles. In an embodiment, the metal is dispersed in a controlled manner within the colloidal silica particles. In another embodiment, at least a portion of the metal oxide dispersion is coated with at least a portion of the colloidal silica particles.
In an embodiment, the invention provides a colloidal composition including a metal oxide dispersion that is at least partially or fully coated with at least one layer of a siliceous material. It is contemplated that the siliceous material may include monomers, such as [Si(OH)4]8, one or more colloidal silica particles being made up of silicic acid monomers having a general molecular formula of [SiOX(OH)4-2X]N (where X is from 0 to about 4 and N is from 1 to about 16), such as [Si(OH)4]8, the like, and combinations thereof. In an embodiment, the colloidal silica particles include nanoparticles having a diameter from about 2 nanometers to about 1000 nanometers. In a preferred embodiment, the nanoparticles have a diameter from about 4 nanometers to about 250 nanometers.
In another embodiment, the siliceous material includes one or more colloidal silica particles having an inner volume wherein at least a portion of the metal oxide dispersion resides completely within the inner volume. That is, the colloidal silica particles comprise a “shell” with a metal oxide “core.” In alternative embodiments, the colloidal particles may be spherical, amorphous, or have any other suitable shape. Alternatively, the metal oxide dispersion is partially coated with one or more layers of the siliceous material. Surface functionalization of pure metal oxides is very difficult. The shell/core design of this embodiment and the reactivity of the silica provide surface morphology characteristics that are amenable to functionalization. In alternative embodiments, the shell need not completely cover the metal oxide (i.e., partial coating of siliceous material on the metal oxide is sufficient) to enhance surface functionalization capacity.
Further, the metal oxide sol may be used in certain coating applications. For example, a metal part may be coated with the metal oxide sol and then heated to form a layer of refractory material on the part. The refractivity may be adjusted according to an aspect of the invention by, for instance, choosing different metal oxides. In this example, 3:1 Al2O4:SiO2 may be used to coat the metal part. Heating converts this coating into mullite or Al6SiO2, which provides, for instance, high temperature stability, thermal shock resistance, a low coefficient of thermal expansion, and resistance to many corrosive environments.
It should be appreciated that the colloidal composition may include a variety of metal oxides. In an embodiment, the metal oxide dispersion includes only one species of metal oxide. Alternatively, the metal oxide dispersion includes a plurality of different metal oxides. Representative metal oxides include aluminum oxide, aluminum oxide hydroxide, boehmite crystals, or oxides of cesium, titanium, zirconium, iron, strontium, zinc, cerium, nickel, molybdenum, boron, rhenium, vanadium, copper, the like, and combinations thereof. In an embodiment, the metal oxide has the formula MN+OA(OH)B, where “M” is an alkali metal, an alkaline earth metal, a first row transition metal, a second row transition metal, or a lanthanide; “N” is from 1 to about 4; “A” is from 1 to about 3; and “B” is from 0 to about 3. In a preferred embodiment, M is aluminum, cesium, titanium, zirconium, iron, strontium, zinc, or combinations thereof. In another preferred embodiment, M is aluminum or zirconium.
The invention also provides a method of controlling a silica to metal ratio within a metal-rich siliceous material. The synthesis procedure used to implement this method enables controlling a comprehensive range of metal oxide to silica ratios in the metal-rich siliceous material. In a preferred embodiment, siliceous colloidal particles include from about 0.01 percent to about 99.99 percent metal oxide (i.e., about 0.01:99.99 to about 99.99:0.01 metal oxide to silica ratio), based on silica. More preferably, the particles include 0.1 percent to about 99.9 percent metal oxide (i.e., about 0.1:99.9 to about 99.9:0.1 metal oxide to silica ratio), based on silica. Combining various types and concentrations of metal oxides with known types and concentrations of siliceous material yields a highly controllable and variable metal to silica ratio, as shown in the examples below.
In one embodiment, the method includes preparing a silicic acid. It is contemplated that the silicic acid may be prepared using any suitable method. A representative method includes deionizing a sodium silicate, such as sodium orthosilicate (Na4SiO4), sodium metasilicate (Na2SiO3), sodium polysilicate (Na2SiO3)n, sodium pyrosilicate Na6Si2O7, the like, an any combination thereof with an ion exchange resin. Preferably, the sodium silicate is deionized with a strong acid ion exchange resin to produce the silicic acid or acid sol. An alternative method includes using the well-known Stöber process to produce the silicic acid. The preferred method is deionization.
In an embodiment, the method includes preparing a metal oxide dispersion. The metal oxide dispersion may include a variety of different metals, as described in more detail herein. The metal oxide dispersion is prepared using any suitable method. A preferred method is to prepare an acidulated solution using a suitable acid, such as nitric acid, and adding to the solution an effective amount of a metal oxide. For example, to prepare a 10 percent dispersion of Al2O3, 12.5 grams of bochmite would be added to a nitric acid solution having a pH from about 3 to about 4. Further detailed examples are provided below.
The type and amount (in relation to silica) of metal oxide chosen determines several factors of the colloidal silica-coated metal oxide including surface porosity, surface area, and composition. It is contemplated (and exemplified in the examples below) that controlling these factors and the metal oxide to silica ratio in the metal oxide sol throughout a comprehensive range is possible.
In an embodiment, the method includes preparing a basic heel solution typically in the range from about 10 milliequivalents (“meq”) to about 200 meq. This heel solution acts as a catalyst for forming the colloidal silica-coated metal oxide particles and can alternatively include various types of bases. Representative bases include sodium hydroxide, lithium hydroxide, potassium hydroxide, ammonium hydroxide, primary amines, secondary amines, tertiary amines, quaternary amines, quaternary compounds, the like, and combinations thereof. Representative quaternary compounds include tetraethyl ammonium hydroxide, tetra-n-butyl ammonium hydroxide, tetra-n-propyl ammonium hydroxide, tetramethyl ammonium hydroxide, NNN-trimethyl-2-butyl ammonium hydroxide, NNN-trimethyl-propyl ammonium hydroxide, the like, and combinations thereof.
In one embodiment, the method includes mixing a known proportion of the silicic acid and the metal oxide dispersion to form a blend. Such mixing may include adjusting the reaction conditions, such as temperature, time, agitation, and/or stirring. Detailed examples of such conditions are provided below. In a preferred embodiment, this mixing step is performed prior to introducing the silicic acid or the metal oxide dispersion to the basic heel solution. This order, in an embodiment, allows a higher degree of control over the metal oxide to silica ratio in the metal-rich siliceous colloidal particles.
In another embodiment, the method includes combining the silicic acid and metal oxide dispersion blend with the basic heel solution. Such combining forms one or more silica-coated metal oxide particles with a highly controllable metal oxide content ranging from about 0.01 percent to about 99.99 percent, based on silica. The metal oxide to silica ratio of the particles is dependent upon factors such as silicic acid type and concentration, metal oxide type and concentration, rate of mixing the silicic acid and the metal oxide to form a blend, rate of combining the blend with the heel solution, temperature, time, pH, stirring, and other reaction conditions. This embodiment includes determining and adjusting such reaction conditions to yield the desired metal oxide to silica ratio. Other colloidal particle properties, such as surface area and porosity, are likewise affected by such conditions. Detailed examples of representative reaction conditions are provided below.
In an embodiment, the method includes optionally further processing the colloidal silica-coated metal oxide composition. It is contemplated that further processing, such as heating, ultra-filtration, deionization, surface functionalization, combining with other compositions, the like, and combinations thereof may be used. Such surface modifications provide a means to further determine and adjust the metal oxide sol properties, such as thermal stability, expansion, and contraction; refractivity; reactivity; and the like.
The foregoing may be better understood by reference to the following examples, which are intended to be illustrative and are not intended to limit the scope of the invention.
A 10 weight percent Al2O3 dispersion was prepared by adding a few drops of concentrated nitric acid to 75 ml of deionized water to bring the pH of the water to between 3 and 4. 12.5 grams of aluminum oxide-hydroxide (sometimes referred to as boehmite and available from Sasol, Johannesburg, South Africa, under the tradename “Dispal 23N4-80”) was then slowly added to the acidulated water to produce the dispersion. More deionized water was added to bring the final volume of the dispersion to 100 ml.
Sodium silicate was deionized with a strong acid ion exchange resin to produce silicic acid or acid sol. Both the dispersion and the silicic acid are naturally acidic and are compatibly mixed in any ratio. In this example, the Al2O3 dispersion was combined with the silicic acid at various concentrations, as illustrated in Table 1. Upon mixing, the resulting pH was acidic and is listed as “Initial pH” in Table 1. The pH of the mixes was adjusted to be from about 9 to about 10 to enhance stability of the metal-rich silica colloid by adding 0.1 N NaOH to achieve the “Stable pH.” Table 1 indicates the resulting pH at which greatest stability was achieved. All samples remained stable for at least several months.
A typical synthesis for 5 weight percent Al2O3 and 95 weight percent SiO2 includes preparing an acid sol (specific gravity of 1.0436 g/ml and 7.15 weight percent SiO2) by deionizing 3717 grams of sodium silicate and preparing a 10 weight percent alumina dispersion (Dispal 23N4-80) with 139.8 grams of Al2O3, as above. The acid sol and the alumina dispersion were mixed with constant stirring on ice (i.e., about 0° C.) to form a blend. The blend was then added to a heel containing 200 ml of deionized water and 50 meq NaOH over the course of three hours at 80° C. The temperature was held at 80° C. for an additional one hour after addition was complete. The silica-coated aluminum oxide colloid was further processed via ultrafiltration. Properties of the colloidal silica-coated aluminum oxide particles are illustrated in Table II.
A typical synthesis for 28 weight percent Al2O3 and 72 weight percent SiO2 includes deionizing 1630 grams of sodium silicate (specific gravity of 1.038 g/ml and 6.23 wt % SiO2) to form acid sol and preparing a 10 weight percent alumina dispersion with 360 grams of alumina (Catapal 200, available from available from Sasol, Johannesburg, South Africa), as above. The acid sol and the alumina dispersion were mixed with constant stirring on ice (i.e., about 0° C.) to form a blend. The blend was then added to a heel containing 300 ml of deionized water and 10 grams of AMP-95® (2-amino-2-methyl-1-propanol with 5 percent water: available from The Dow Chemical Company®, Midland, Md. over the course of three hours at 70° C. The temperature was held at 70° C. for an additional one hour after addition was complete. The silica-coated aluminum oxide colloid was further processed via ultrafiltration. Properties of the final colloidal silica-coated aluminum oxide particles are illustrated in Table III.
A typical synthesis for 85 weight percent Al2O3 and 15 weight percent SiO2 includes deionizing 143 grams of sodium silicate (specific gravity of 1.038 g/ml and 6.23 weight percent SiO2) to form an acid sol and preparing a 12.66 weight percent alumina dispersion with 399 grams of alumina (Dispal 14N4-80, available from Sasol, Johannesburg, South Africa), as above. The acid sol and the alumina dispersion were mixed with constant stirring on ice (i.e., about 0° C.) to form a blend. The blend was then added to a heel containing 200 ml of deionized water and 2 grams of AMP-95® over the course of three hours at 57° C. The temperature was held at 57° C. for an additional two hours after addition was complete. The silica-coated aluminum oxide colloid was further processed via ultrafiltration. The properties of the colloidal silica-coated aluminum oxide are illustrated in Table IV.
A typical synthesis for 79 weight percent Al2O3 and 21 weight percent SiO2 includes preparing a 10 weight percent alumina dispersion with 581 grams of alumina (Dispal 14N4-80, available from Sasol, Johannesburg, South Africa), as above. An acid sol comprising 150 grams of 6.85 weight percent silicic acid and the alumina dispersion were mixed with constant stirring on ice (i.e., about 0° C.) fro about 3 hours to form a blend. To the blend was added 23 grams of tetramethyl ammonium hydroxide. The silica-coated aluminum oxide colloid was further processed via ultrafiltration. The properties of the colloidal silica-coated aluminum oxide are illustrated in Table V.
A typical synthesis for 50 weight percent ZrO2 and 50 weight percent SiO2 includes deionizing 111 grams of sodium silicate (specific gravity of 1.044 g/ml and 7.7 weight percent SiO2) to form an acid sol and preparing a 10 weight percent zirconia dispersion with 620 grams of fumed zirconia (available from Degussa Corporation®, Parsippany, N.J.), as above. The acid sol and zirconia dispersion were mixed with constant stirring on ice (i.e., about 0° C.) to produce a blend. The blend was then added to a heel containing 300 ml of deionized water and 50 meq NaOH over the course of three hours at 70° C. The temperature was held at 70° C. for an additional one hour after addition was complete. The silica-coated aluminum oxide colloid was further processed via ultrafiltration. The final colloidal silica-coated zirconium oxide properties are illustrated in Table VI.
Table VII indicates zeta potentials measured for various Al2O3 to SiO2 ratios. These measurements confirm that the alumina is actually coated with a layer of silica.
Table VIII illustrates several variations of Al2O3 to SiO2 ratios. Column one gives the intended final aluminum oxide concentration. Column two illustrates the various bases that can be used to obtain the silica-coated metal oxides of the invention. Column six gives the actual final aluminum oxide concentration as measured by X-Ray Fluorescence Spectroscopy (“XRF”). Column seven provides the surface area as obtained by surface titration using the Sears method. This method is affected by the presence of alumina and thus gives artificially high numbers. Column eight lists the surface area as measured by the BET method, which is unaffected by composition and shows consistently lower surface area figures. The porosity values in columns nine and ten vary with alumina crystal and silica packing differences.
It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the invention and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.
