Method for production of stable cerium oxide organic colloids

Abstract

An improved process for producing substantially non-polar doped or un-doped cerium oxide nanoparticle dispersions is disclosed. The cerium-containing oxide nanoparticles of an aqueous colloid are transferred to a substantially non-polar liquid comprising one or more amphiphilic materials, one or more low-polarity solvents, and one or more glycol ether promoter materials. The transfer is achieved by mixing the aqueous and substantially non-polar materials, forming an emulsion, followed by a phase separation into a remnant polar solution phase and a substantially non-polar organic colloid phase. The organic colloid phase is then collected. The promoter functions to speed the transfer of nanoparticles to the low-polarity phase. The promoter accelerates the phase separation, and also provides improved colloidal stability of the final substantially non-polar colloidal dispersion. Importantly, the glycol ether promoter reduces the temperature necessary to achieve the phase separation, while providing high extraction yield of nanoparticles into the low-polarity organic phase.

Claims

1. A process for preparing a colloidal dispersion, comprising: (a) preparing an aqueous colloidal dispersion of cerium-containing oxide nanoparticles; (b) adding a substantially non-polar solvent and an amphiphilic material; (c) mixing the liquid mixture of step (b) to form an emulsion; (d) heating the emulsion to a temperature ranging from about 20 C. to less than 60 C. for a predetermined time, whereafter the emulsion separates into a substantially non-polar colloidal phase and a remnant aqueous phase; and, (e) collecting the separated substantially nonpolar colloidal dispersion of cerium-containing oxide nanoparticles wherein the process further comprises adding at least one glycol ether prior to step (e) in step (b), after step (c), or during step (d), and adding at least one glycol ether to the collected substantially non-polar colloidal dispersion of cerium-containing oxide nanoparticles after step (e), wherein said glycol ether is selected from the group consisting of diethylene glycol monomethyl ether, propylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, dipropylene glycol methyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol rnonopropyl ether, and combinations thereof.

2. The process of claim 1, wherein said time ranges from 0 to 8 hours.

3. The process of claim herein the glycol ether added prior to step (e) is added in its entirety during step (d).

4. The process of claim 3, wherein the glycol ether added prior to step (e) is added 0 to 4 hours after the end of step (c).

5. The process of claim 1, wherein said glycol ether is selected from the group consisting of diethylene glycol monomethyl ether, propylene glycol monomethyl ether, and a mixture thereof.

6. The process of claim 1, wherein said glycol ether comprises about 5-25 wt. % of the substantially non-polar solvent, the amphiphilic material, and the glycol ether.

7. The process of claim 1, wherein said amphiphilic material is a monocarboxylic acid having from 6 to 22 carbon atoms.

8. The process of claim 7, wherein said rnonocarboxylic acid is oleic acid.

9. The process of claim 8, wherein the amount of said carboxylic acid comprises about 25-33 wt. % of the total amount of substantially nonpolar solvent, amphiphilic material, and glycol ether.

10. The process of claim 1, wherein the amount of said substantially nonpolar solvent comprises about 50-63 wt. % of the total amount of substantially nonpolar solvent, amphiphilic material, and glycol ether.

11. The process of claim 1, wherein said cerium-contain oxide nanoparticles have a nominal composition of Ce.sub.(1-x)Fe.sub.xO.sub.(2-), wherein x ranges from about 0,01 to 0.8 and ranges from about 1 to 2.

12. The process of claim 1, wherein said aqueous colloidal dispersion of cerium-containing oxide nanoparticles is prepared without a conventional nanoparticle isolation step, thereby directly using the aqueous colloid resulting from the narioparticle synthesis reaction mixture in step (a).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The FIGURE is a ternary phase diagram representing combinations of a set of exemplary non-polar solvent, amphiphilic agent, and glycol ether promoter, of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) For effectiveness in many end-use applications, nanoparticle size distributions with mean diameters ranging from below about 100 nm to below about 3 nm are useful.

(3) As used herein, the terms dispersion, colloid, suspension, sol, colloid dispersion, and colloidal dispersion are used interchangeably to mean a stable biphasic mixture of a discontinuous phase (e.g., nanoparticles) within a continuous phase (e.g., liquid or other solvent medium).

(4) As used herein, the term cerium-containing oxide includes doped and un-doped cerium oxides. Doped cerium oxide compounds include those with the formula Ce.sub.(1-x)M.sub.xO.sub.(2-) where M is a divalent or trivalent metal and is indicative of oxygen vacancies. It should be recognized by one skilled in the chemical art that dopant metal M, in addition to being either substitutionally or interstitially doped into the cerium oxide crystal structure, could be present as oxides of metal M, either as separate nanoparticles or nanocrystals, or as nanoparticles or nanocrystals in agglomeration (composite) with other doped or un-doped cerium oxide nanocrystals. In various embodiments, nanoparticles comprised of crystalline substitutionally doped or un-doped cerium oxide phases, are present. In other embodiments, nanoparticles comprised of non-crystalline metal oxide phases, such as amorphous iron oxide phases, are present. In various embodiments, dopant metal M is Fe, Zr, Pd, Pt, Ag, Co, Cu, and Ni. In particular embodiments, nanoparticles of a nominal composition of Ce.sub.(1-x)Fe.sub.xO.sub.(2-) wherein x ranges from about 0.01 to 0.8, or from about 0.5 to 0.7, and ranges from about 1 to 2, such as, for example, from about 1.5 to 2, are employed in the inventive process.

(5) The invention relies in part, on the discovery of the effectiveness of certain glycol ethers in aiding the extraction or transfer of doped or un-doped cerium oxide nanoparticles or mixtures thereof from aqueous to substantially non-polar solvents, at low process temperatures. In particular, the choice of a glycol ether, such as diethylene glycol monomethyl ether (DEGME), has been discovered by the inventors to accelerate the phase separation of aqueous and substantially non-polar colloid phases formed by the mixing of aqueous colloidal solutions with substantially non-polar materials (liquids) including a low-polarity solvent or mix of low-polarity solvents, one or more amphiphilic materials, and one or more specific glycol ethers. The mixing of the aqueous colloid and the substantially non-polar materials (liquids) provides an emulsion. In the presence of certain particular glycol ethers, the emulsion separates at room temperature or modestly elevated temperatures into an aqueous solution phase and a substantially non-polar colloid containing substantially all of the nanoparticles from the aqueous colloid, the amphiphilic material, and a portion of the glycol ether. In particular embodiments, wherein the nanoparticles exhibit substantial coloration, the efficiency or degree of transfer of the nanoparticles from the aqueous phase to the non-polar phase, may be qualitatively assessed by visual observation.

(6) In particular embodiments, additional glycol ether materials may be added to the substantially non-polar colloid to enhance colloidal stability, to enhance low temperature flow properties, and/or to raise the flashpoint temperature of the substantially non-polar colloid. In other embodiments, materials useful for modifying the low temperature flow characteristics and flash points the substantially non-polar colloid include low molecular weight organic liquids such as alcohols and diols.

(7) In particular embodiments, the glycol ether promoter may reduce the temperature necessary to achieve phase separation while providing high extraction yield of nanoparticles to the organic phase. Low temperatures and lower time at temperature during the processing have benefits of lower process energy costs and, moreover, reduced risk of hazard in managing the organic combustible materials during processing, as well as simplifying equipment and facility requirements.

(8) As mentioned previously, U.S. Pat. No. 6,271,269 to Chane-Ching et al. discloses direct transfer of cerium oxide or doped cerium oxide colloidal particles from a counterpart aqueous dispersion. The range of temperatures disclosed for the transfer reaction is from higher than 60 C. to 150 C., with a preferred range of from 80-100 C. Disclosed examples were carried out at 90 C.

(9) In particular embodiments, substantially non-polar (low-polarity) solvents include, alone or in combination, aliphatic hydrocarbons and mixtures thereof, and alicyclic hydrocarbons and their mixtures. In other embodiments, non-polar solvents include diesel fuel, biodiesel fuel, naphtha, kerosene, gasoline, and commercially available petroleum derivatives such as isoparafin distillates (e.g., Isopar), hydrotreated petroleum distillates (e.g., Kensol 4811 and Kensol 50H available from American Refining Group, Ltd of Bradford, Pa. (USA); or Calumet 420-460 available from Calumet Lubricants Co. of Cotton Valley, La. (USA)). Kensol 48H and 50H are used in particular embodiments as components of fuel-additive applications of the invention because of their low sulfur content, high flashpoint, and low concentration of components having unsaturated bonds. Solvents having some concentration of aromatics, for example, Solvesso type solvents, may be useful for the purposes of the invention. Low cost may be another driver for the choice of a particularly preferred substantially non-polar solvent. In various embodiments, the substantially non-polar solvent comprises from about 50-65 wt. % of the total substantially non-polar liquid used to form the emulsion mixture.

(10) In particular embodiments, amphiphilic materials include monocarboxylic acids having from 6 to 22 carbon atoms, dicarboxylic acids, polycarboxylic acids, and combinations thereof In particular embodiments, monocarboxylic acid materials include, for example, oleic acid, stearic acid, linoleic acid, linolenic acid, and isomers thereof, alone or in combination. In particular embodiments, dicarboxylic acids include, for example, derivatives of succinic acid, such as polyisobutylene succinic acid, and anhydrides thereof The amphiphilic materials may also characterized in that they are soluble in non-polar hydrocarbon diluents, such as kerosene, isoparafin and hydrotreated petroleum distillates, which in turn are compatible with most hydrocarbon fuels, such as gasoline, diesel and biodiesel, and lubricating oils. In various embodiments, the amphiphilic materials comprise from about 25-33 wt. % of the total substantially non-polar liquid used to form the emulsion mixture.

(11) In particular embodiments, glycol ether promoters include, for example, diethylene glycol monomethyl ether (DEGME), propylene glycol monomethyl ether (PGME), diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, dipropylene glycol methyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, and mixtures thereof Choice of particular glycol ether promoters may be based in part on efficacy of low temperature acceleration of extraction of nanoparticles from aqueous to a substantially non-polar phase. It has been found that the level of glycol ether present may be sensitive, there being a threshold for the beneficial acceleration of the separation of the emulsion to give a stable high yield substantially non-polar colloid. Consideration of the miscibility and stability, as will be discussed further below, of the ternary combination of non-polar solvent, amphiphilic agent, and the glycol ether may also be a factor in the determination of the appropriate level of glycol ether in the process and in the final product. Other considerations for the specific choice of and relative amount of glycol ether include satisfying product requirements regarding cost, low temperature flow, flashpoint, and health/environmental considerations. In various embodiments, the glycol ether promoters comprise from about 5-25 wt. % of the total substantially non-polar liquid used to form the emulsion mixture.

(12) The aqueous doped or un-doped cerium oxide colloid that is to be directly transferred or extracted into a non-polar phase could be formed according to a number of known approaches. For example those described in copending U.S. application Ser. No. 12/779,602 now published as US2010/0242342, to Reed et al, incorporated herein by reference, are applicable. In some embodiments of the invention, such an aqueous colloid as formed in its reaction vessel is directly useful for transfer to substantially non-polar colloid phase, even though the aqueous colloids have constituent components comprising reactant remnants and addenda. In other embodiments, nanoparticles formed as aqueous colloids using other well-known processes can be isolated and washed and then re-dispersed in water to form another aqueous colloid that can be used as a starting material for the inventive transfer process discussed herein.

(13) In particular embodiments, the temperature range for the formation of the emulsion, transfer of the nanoparticles between aqueous and substantially non-polar phases, and separation of the emulsion, may range from about 20 C. to 60 C. In a particular embodiment, a temperature of about 40 C. is used because an aqueous colloid in which the nanoparticles are faulted directly, will often be substantially above 40 C. at the conclusion of the aqueous nanoparticle synthesis in order to impart high yield and crystallinity in a short amount of time. An aqueous colloid so formed, when combined with the other materials that comprise the non-polar constituents, conveniently at room temperature, will yield an emulsion with a temperature near 40 C. Such low temperatures compared to prior art process temperatures are a significant advantage afforded by the inventive approach. And near this temperature, in particular embodiments, the presence of glycol ether promoters of the invention cause the emulsion to separate into two phases within about 1 to 4 hours, with substantially complete extraction of the nanoparticles from the aqueous phase into the non-polar phase.

(14) In a particular embodiment, it has been found that it may be beneficial to age (hold) for a predetermined period of time, the emulsion formed from the mixing of the aqueous cerium-containing oxide colloid, the substantially non-polar solvent, and the amphiphilic material (e.g. organic acid), prior to the addition of the glycol ether promoter. In various embodiments, the aging (holding) temperature is in the range of 20-60 C., and the aging (holding) time is in the range of 0 to 8 hours, 0 to 4 hours, or 0 to 2 hours.

(15) The inventors have explored the ternary phase diagram of a combination liquid comprising a non-polar solventKensol 50H, an amphiphilic materialoleic acid, and the promoterDEGME. FIG. 1 depicts the ternary phase diagram for the ternary system at room temperature. Note that there are 2 regions: Region A is characterized by a single-phase liquid in which all three of the components are miscible. Region B is characterized by a separation into 2 liquid phases. It may be preferable to choose the ratio of the 3 constituents to be in the single-phase region, while at the same time optimizing other desirable characteristics of the product, for example, long-term colloidal stability of the organic colloid (sol) product. Colloidal stability over the manufacturing process temperatures and product exposure temperatures, both high and low, may need to be considered. Product characteristics of concern may include flowability at low operating temperatures (cold outdoor ambient temperatures) and flash-point at higher potential exposure temperatures. Conveniently and unexpectedly, in some embodiments, product ratios of the three materials of the ternary diagram also provide for an ideal composition for the extraction of nanoparticles from the aqueous colloid to the substantially non-polar colloid.

(16) In some embodiments, it has been found that the low temperature extraction of nanoparticles from the aqueous phase to the substantially non-polar phase is accelerated by forming the emulsion with high shear mixing.

(17) In some embodiments, analysis of the final organic colloid material produced by the inventive process reveals that it is substantially free of constituents of the aqueous reaction mixture in which the nanoparticles were initially formed. Levels of water, nitrates, and nanoparticle stabilizer (e.g. methoxyacetic acid) were all lower than in the comparative process disclosed in the commonly assigned U.S. application Ser. No. 12/549,776 now US Publication 2010/0152077 A1 to Alston et al. Analysis also revealed that a portion of the glycol ether promoter material or materials may be retained in the aqueous phase after phase separation. Optionally, an additional amount of glycol ether is added to the separated substantially non-polar colloid, according to considerations previously stated.

(18) To further illustrate the invention and its advantages, the following examples are given, it being understood that the specific examples are not limiting.

EXPERIMENTAL SECTION

(19) Preparation of Ce.sub.0.6Fe.sub.0.4O.sub.(2-) Aqueous Nanoparticle Dispersion

(20) To an 11 liter round bottom Type-316 stainless steel kettle or reactor with 3 mixing baffles, was added distilled water (Kettle Water), which was maintained at 70 C. Using an impeller, the water was stirred at sufficient speed to provide good mixing. Then 98% methoxyacetic acid was added to the reactor. Two solution introduction jets directed to the impeller blades were put into the reactor and secured. An ammonium hydroxide solution was pumped through one jet at a rate of 69.3 ml/minute. A cerium-iron containing solution (334.5 gram of Ce(NO.sub.3).sub.3.6H.sub.2O and 207.5 gram of Fe(NO.sub.3).sub.3.9H2O with distilled water to make 625 ml) was pumped through the other jet at a delivery rate of 125 ml/minute. The cerium-iron solution was purged from the delivery line with a 15 ml distilled water chase. Then a 50% H.sub.2O.sub.2 solution was pumped into the reactor at 9.38 ml/minute using a third jet and was followed by a brief distilled water flush. The reaction mixture was held at 70 C. for an additional sixty minutes, after which time it was cooled to 20 C., providing a stable Ce.sub.0.6Fe.sub.0.4O.sub.2- aqueous nanoparticle colloidal dispersion, wherein is between about 1.5 to 2.

(21) Transmission electron microscopy (TEM) grain sizing revealed a particle size of 2.50.5 nm. Electron diffraction revealed a distinct CeO.sub.2 cubic fluorite electron diffraction pattern. No electron diffraction peaks characteristic of a crystalline iron oxide phase were detected. Ultra-high resolution TEM and electron energy loss spectroscopy revealed a plurality of composite nanoparticles comprised of crystalline cerium oxide rich regions and amorphous iron oxide rich regions.

Example 1

(22) A 100 ml aliquot of Ce.sub.0.6Fe.sub.0.40O.sub.(2-) aqueous nanoparticle dispersion prepared as described above, was added to a 500 ml reaction vessel and heated to a temperature of about 60 C. A 74.0 ml aliquot of Kensol 50H and 37.6 g of oleic acid were then added, these two materials being at room temperature at the time of addition. The mixture was stirred by manual shaking of the vessel for a period of 1 minute, forming an emulsion. The emulsion mixture was then held at 40 C. to age for 2 hours. Then, 30 ml of DEGME was added to the emulsion and it separated in about 4 hours to yield a stable dark brown non-turbid substantially non-polar colloid phase, above a nearly colorless aqueous phase. The non-polar colloid phase was separated out by pipette. To 100 ml of the separated organic colloid phase was added 13.9 ml of DEGME and 7.2 ml of PGME. Long-term stability observations of samples of the above non-polar colloid were carried out while samples were held in separate 10 ml vials. One was held at room temperature (about 20 C.) and the other at 40 C. At the conclusion of 6 months, the non-polar colloids, remained essentially non-turbid and free of settled precipitates.

Example 2

(23) A 100 ml aliquot of Ce.sub.0.6Fe.sub.0.4O.sub.(2-) aqueous nanoparticle dispersion prepared as described above, was added to a 500 ml reaction vessel and heated to a temperature of about 60 C. A 75.0 ml aliquot Kensol 50H and 35.9 g of oleic acid were then added, these two materials being at room temperature at the time of addition. The mixture was stirred by manual shaking of the vessel for a period of 1 minute. The emulsion mixture was then held at 40 C. to age for 2 hours. Then, 30 ml of DEGME was added to the emulsion and it was returned to 40 C., thereafter it completely separated in about 4 hours to yield a stable dark brown non-turbid substantially non-polar colloid phase, above a nearly colorless aqueous phase. The non-polar colloid phase was separated out by pipette. To 100 ml of the separated organic colloid phase were added 12.2 ml of DEGME and 9.1 ml of PGME.

(24) Long-term stability observations of samples of the above non-polar colloid were carried out while samples were held in separate 10 ml vials, one at room temperature (about 20 C.) and the other at 40 C. At the conclusion of 6 months, the non-polar colloids, remained essentially non-turbid and free of settled precipitates. Cold temperature stability was also checked at 17 C. and it was found that the sample remained a non-turbid liquid, free of precipitates.

Example 3

(25) A 500 ml aliquot of Ce.sub.0.6Fe.sub.0.4O.sub.(2-) aqueous nanoparticle dispersion prepared as described above, was heated to a temperature of about 60 C. and transferred to a 2 L reaction vessel. The liquid was stirred with a 1 9/16 R100 (Rushton) impeller that was lowered into the reactor vessel. The mixer head was positioned slightly above the bottom of the reactor vessel. The mixer was set to 1690 rpm. A mixture of 370 ml of Kensol 50H and 188 g of oleic acid, at room temperature, was added to the vessel over a 30 second period. The whole mix was then stirred at 1750 rpm for 2 minutes resulting in the formation of an emulsion. The reaction vessel was then moved to hot plate with magnetic stirrer and stirred using a 2 magnetic bar at high speed setting. 50 ml of DEGME was then added over 15 seconds. The vessel was then held without stirring at a temperature of about 45 C. After about 4 hours, the emulsion separated completely to yield about 600 ml of dark brown non-turbid organic colloid above an aqueous remnant phase.

(26) Analysis of the organic colloid by Gas Chromatography Mass Spectrometry revealed no detectable amount of methoxyacetic acid, the nanoparticle stabilizer present in the Ce.sub.0.6Fe.sub.0.4O.sub.(2-) aqueous nanoparticle dispersion prepared as described above. This reduction in methoxyacetic acid in the final organic colloid was accompanied by an improvement in long-term stability relative to organic dispersions of similar nanoparticles prepared by the solvent shifting process described by Alston et al. in US Pat. Publication 2010/0152077.

(27) The invention has been described in detail, with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as described above, by a person of ordinary skill in the art, without departing from the scope of the invention.