STABLE METAL OXIDE DISPERSIONS

Stable dispersions of small particle metal oxides, such as magnesium oxide and calcium oxide, with an average particle size of one micron or less, often from 1-100 nm, are prepared by microwave irradiation of a composition prepared from a mixture comprising an oxide, hydroxide or carbonate of the metal, a carrier, a sulfonic or carboxylic acid dispersant, and typically a low MW carboxylic acid and water.

BACKGROUND OF THE INVENTION

Fine particle metal oxides have a variety of commercial uses. For example, overbased detergents, e.g., compositions comprising overbased alkaline metal or alkaline-earth metal compounds such as metal oxides and often complexed with an organic dispersant, are well known additives for lubricating oil compositions and petroleum fuels. In such cases, the metal oxide is most conveniently added and used as a dispersion in a carrier.

Petroleum fuels such as residual fuel oils contain large amounts of impurities which result in corrosive deposits in the equipment. For example, crude oil usually contains 1- 500 ppm of vanadium in the form of a porphyrin complex depending on the source. Because of its origin as a concentrate from the refining process, residual oil contains several times more vanadium than the crude from which it was derived. The combustion of these vanadium-containing fuels produces very corrosive deposits which can destroy a metal part, such as a gas turbine blade, in a matter of hours.

The presence of sodium in fuel can also have catastrophic consequences. For example, in maritime use the sodium level can be increased because of the introduction of sodium chloride through the air intake and contamination of the fuel by sea water. During combustion, the sodium can react with sulfur in the fuel to form a sulfate which is deposited in turbine parts. Overbased detergents, perform a variety of functions including anti-corrosion, deposit control, acid scavenger functions For example, overbased magnesium compounds complexed with sulfonate and carboxylate dispersants, have long been used as anti- corrosion and acid neutralization additives for lubricating oils and greases, anti-corrosion and acidic neutralization additives during the combustion of fuels such as residual fuel, pulverized sulfur-containing coal, corrosion inhibitors in fuels containing vanadium etc. The addition of overbased magnesium detergents to, for example, boiler fuels or gas turbine fuels, is known to reduce corrosion, presumably by forming magnesium complexes with the vanadium or sodium.

Overbased metal detergents are also added to lubricating oils to prevent or remove deposits of oil-insoluble sludge, varnish, carbon and lead compounds which otherwise form on internal combustion engine parts and for combating severe rust conditions which may be encountered during shipping or storage of machinery or exposure to out-door weather. Detergent additives for automotive and diesel engine oils also chemically react with the highly acidic by-products of combustion that find their way into the lubricating oil system.

Obviously a useful dispersion of metal oxide must be stable during storage and the overbased metal must stay well dispersed in the lubricant or fuel. A variety of parameters will affect the stability and activity of these dispersions such as the dispersants and carriers employed, particle size of the solid components, and the relationship between metal and dispersant. The process by which the overbased metal compounds and complexes are prepared will greatly influence the actual physical make up and properties of the overbased metal dispersion, impacting particle size and distribution of the metal compound throughout the dispersion, the viscosity and stability of the dispersion, the amount of the metal within the dispersion etc.

Overbased metal additives, for example, overbased MgO dispersions are often added as a dispersion in a high boiling liquid hydrocarbon. Part of the rationale for supplying MgO dispersions in high boiling carriers, i.e., carriers with boiling points over 200 °C, often greater than 250 °C or 280 °C, and even much greater than 300 °C is due to the manner in which the dispersions are made. For example, overbased stable MgO dispersions with fine particle sizes and good flowcharacteristics are typically produced, even when starting with MgO as a starting material, through thermal decomposition of Mg(OH) ₂ or Mg(OH) ₂ derived intermediates which require high temperature (300-350°C). Thus, the use of high boiling point solvents as carriers is dictated by practical processing considerations.

US Pat. 4,163,728, discloses magnesium-containing dispersions prepared by high temperature decomposition of magnesium salts of carboxylic acids to MgO in a dispersant-containing fluid. In the process, Mg(OH) ₂ , an organic carboxylic acid or sulfonic acid surfactant such as naphthenic acid, acetic acid and water are heated in a high boiling hydrocarbon to temperatures up to 350 °C, which is above the

decomposition point of magnesium acetate, 323 °C. It is believed that magnesium acetate is formed in situ and decomposes at the high temperatures used. Water is also removed at the elevated temperatures.

US Pat. 4,293,429, discloses a variation of US Pat. 4,163,728 which begins with MgO instead of Mg(OH) ₂ . In the process, the bulk MgO is converted to magnesium acetate which forms suspended MgO particles of less than 5 microns, and preferably less that 1 micron. Thus, the coarse MgO particles are converted into a dispersion of stabilized MgO microparticulates. Lower temperatures fail to provide the fine particle size MgO.

US 4,129,589, discloses a process for preparing an over-based oil-soluble magnesium salt of a sulfonic acid by contacting carbon dioxide gas with a mixture comprising an oil- soluble magnesium salt of a sulfonic acid, magnesium oxide, water, and a promoter system comprising a carboxylic acid of 1 to 5 carbons in an inert solvent for lowering the viscosity of said mixture to facilitate mixing. The products of US 4,129,589 had acceptably low viscosity but the magnesium content was typically 9-10% and no more than 14%.

US 6,197,075, discloses an overbased magnesium sulfonate, carboxylate or phenate product containing at least 14% and up to about 18% by weight of magnesium, useful as a deposit control additive for residual fuel oils and turbine fuels prepared by contacting a mixture of i) a sulfonic acid, phenol or carboxylic acid or salt thereof, ii) a magnesium oxide, iii) a co-promoter comprising a lower carboxylic acid, a lower alcohol, a succinic anhydride and water, and iv) a solvent and/or oil, with an acidic gas such as carbon dioxide at 50 °F up to the reflux temperature of the mixture to overbase the reaction mixture.

The overbased metal compositions described above and elsewhere are best described as products by process as there is typically no simple chemical formula which adequately correlates to the essential material makeup and the physical properties of the product. Often, the molecular structures of the metal complexes are not fully known and are not a critical aspect of the invention. For example, two compositions containing compounds with the same chemical formula in the same amounts and differing only by the manner in which they were prepared can have very different physical properties.

While the use of high boiling solvents or carriers in the above processes can provide useful dispersions, there is the need for improved products and methods. For example, MgO dispersions with a higher magnesium content are desirable. However simple modification of known procedures to prepare overbased detergents with high metal content can lead to unforeseen drawbacks including unacceptably high viscosities and gelling. Also, attempts to concentrate the dispersion by distillation to get higher Mg content must be carried out at very high temperatures or reduced pressure.

Co-pending application US 13/167,127, incorporated herein by reference, describes a method of obtaining free flowing MgO dispersions with high Mg content in high boiling carriers using a mixture of solvents and defined quantities of lower carboxylic acid and water. Co-pending provisional application US 61/502,914, incorporated herein by reference, discloses a process for preparing similar MgO dispersions in lower boiling solvents by performing some of the processing steps under increased pressure.

Along with heat, light etc, microwave radiation is a source of energy that can be used to promote chemical reactions. In some cases, the products produced from a microwave induced reaction are different than those produced by thermal or UV initiated reactions; in some cases the products are the same but the chemical conversion is more efficient. Makhluf, et. al, "Microwave-Assisted Synthesis of Nanocrystalline MgO and Its Use as a Bacteriocide" Advanced Functional Materials 2005, 15, 1708-1715; Sugiyama, et. al, Matsuda, T. "Synthesis of Magnesium oxide catalyst in a Microwave plasma reactor" 9th Int. Symp. Plasma Chem. 1989, 2, 820-824; and Nakano, et. al, "Surface Characterization of Metal Oxide fine Powders Prepared by Microwave Cold Plasma: AFM Observations of Silica Surface" 11th Int. Symp. Plasma Chem. 1993, 1 , 1422-1427 disclose the use of microwave radiation in the production of inorganic particles.

It has been discovered that microwave irradiation can be used to advantage in the preparation of stable, small particle size metal oxide dispersions, such as those used as overbased metal oxide lubricant and fuel additives. Stable dispersions of sub-micron particles are obtained, typically while avoiding the very high temperatures that are otherwise required.

SUMMARY OF THE INVENTION

The present invention provides a method for preparing stable dispersions of small particle size metal oxides, for example, overbased metal oxide dispersions, said method comprising subjecting a preliminary composition or preliminary dispersion formed from a mixture comprising a metal hydroxide, oxide or carbonate, a sulfonic or carboxylic acid dispersant, and an organic carrier, e.g., a high boiling hydrocarbon carrier to microwave irradiation resulting in a dispersion of metal oxide particles having an average particle size of one 1 micron or less, often having an average particle size of from 1-100 nm.

In one embodiment, the preliminary composition which is subjected to microwave irradiation is a preliminary dispersion formed by heating to a temperature of 85 °C, typically higher, e.g., from 100 to 220 °C, a mixture comprising a metal hydroxide, oxide or carbonate, a sulfonic or carboxylic acid dispersant, a high boiling hydrocarbon carrier, a low MW carboxylic acid, i.e., a C _1-5 carboxylic acid and water, optionally in the presence of an organic solvent. Often, water and other volatiles are removed, e.g., by distillation, before subjecting the resulting preliminary dispersion to microwave irradiation.

No additional solubilizing or dispersing agents, promoters or reactants such as carbon dioxide, amines, alcohols etc are needed to obtain the desired metal oxide dispersions and metal content of up to 40 or 50 weight %, and in certain embodiments higher, based on the total weight of the dispersion can be prepared. For example, metal contents of 10%, 15%, 20%, 30%, 35%, 40% and higher can be prepared. A specific chemical formula for the composition of the dispersion is not fully descriptive of the product, and the molecular structures of the metal oxide complexes of the invention are not fully known, however, the product obtained is a free flowing dispersion of predominately submicron metal oxide particles engulfed by and complexed to a sulfonate or carboxylate dispersant. Other metal compounds such as traces of metal hydroxide are also believed to be present.

The dispersions of the invention can be used as formed or may be further modified and are effective additives in fuels, lubricating oils, such as petroleum based fuels and lubricants, anti corrosive paints, and as part of any formulation containing similar materials. For example, the stable metal oxide dispersion of the invention is added to fuels and lubricants used in gas turbine, boiler, cracking and engine operations etc.

DESCRIPTION OF THE INVENTION

According to the invention, a preliminary composition, e.g., a preliminary dispersion is prepared from a mixture comprising

a) 5-80%, e.g., 5-50%, of a metal hydroxide, metal oxide or metal carbonate

b) 2-15% of a sulfonate or carboxylate dispersant

c) 10-70% of a high boiling hydrocarbon carrier selected from mineral oils, alkylated benzenes, oligomers or polymers of alpha olefins, polycyclic aromatics, alkylated derivatives of polycyclic aromatics and waxes,

d) 0-10% of a C _1-5 carboxylic acid,

e) 0%-30% water

and

f) 0 - 60% of an organic solvent,

wherein the percentages are by weight based on the total weight of the mixture, and then subjected to microwave irradiation to yield a metal oxide dispersion with average particle size of less than 1 micron.

The microwave irradiation may be performed with or without external heating, however it is likely that even without additional heating the temperature of the reaction mixture will increase. There is no specific limitation on the type of reaction vessel used, however, if a sealed vessel is used, common provisions must be made to account for the build up in pressure, whereas if the vessel is "open", either exposed to the atmosphere or under an inert atmosphere at ambient pressure, common provisions must be made to account for any byproducts, starting materials or solvents that may evaporate.

In one embodiment, the preliminary dispersion is prepared by subjecting the mixture of a) through f) to a preliminary heating step. In another embodiment, the mixture of a) through f) is subjected to a preliminary heating step and distillation to remove volatiles. The distillation may be a separate from the preliminary heating step, may occur during the preliminary heating step, or partial distillation may occur during preliminary heating followed by a second step to complete distillation, for example, a separate vacuum distillation step or distillation under higher temperatures or while using certain distillation equipment such as a thin film evaporator, distillation column etc may be used.

For example, a mixture comprising a), b), c) and any optional d), e) and f) above is heated, to temperatures of from about 85 °C to about 220 °C, for example from about

85 °C to about 180 °C, often from about 90 °C to about 150 °C, typically the mixture is heated at reflux, with stirring or other agitation for about 0.25 to 5 hours, generally 0.5 to

4 hours, for example 1 to 3 hours, during which time volatiles may be distilled and collected;

the mixture is then distilled, or further distilled, at a temperature of from about 125 °C to about 240 °C, for example from about 150 °C to about 220 °C, or from about 165 °C to about 240 °C, optionally under vacuum;

cooled and then subjected to microwave irradiation.

The time required for microwave irradiation is subject to various parameters including the intensity and wavelength of irradiation.

The metal of the hydroxide, oxide or carbonate is an alkaline metal or alkaline-earth metal, i.e., Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba or Ra, typically selected from Li, Na, K, Rb, Cs, Mg, Ca, Sr and Ba. In many embodiments the metal is an alkaline-earth metal selected from Mg, Ca and Ba, for example, Mg.

Excellent results are achieved when the sulfonate or carboxylate dispersant is an alkylarylsulfonate or alkylaryl carboxylate dispersant, for example, an alky! benzene sulfonate. In the process, there is less than a molar equivalent of the dispersant present relative to metal hydroxide, oxide or carbonate.

In most embodiments of the invention both water and a C _1-5 carboxlic acid are added to the mixture from which the preliminary composition is formed. Less than a molar equivalent of the C _1-5 carboxlic acid is added, relative to metal hydroxide, oxide or carbonate, often much less than a molar equivalent, but there can be significantly more than a molar equivalent of water added. Typically, when water and C _1-5 carboxlic acid are added, the mixture is subjected to the preliminary heating step above and volatile distillates are removed, i.e., distilled, prior to microwave irradiation. Reaction, or complex formation, involving water, C _1-5 carboxylic acetic acid, metal compound and / or dispersant may occur during this heating step and volatiles, including water, may be removed.

Even if a distillation is performed prior to microwave irradiation, it may still be necessary to remove water upon microwave irradiation as it is possible to form water during the this part of the process. Of course it is desirable to remove most or all of any added or generated water at some point as the final product dispersion will contain significantly less than 8% water, in many cases the final product dispersion will contain less than 2% water, for example, less than 1 % water.

The organic solvent f) is optional and in general is removed or greatly reduced by distillation before microwave irradiation. The organic solvent is described in more detail below, has a with a boiling point below 280 °C, generally has a with a boiling point below 220 °C, for example a boiling point ranging from about 80 °C to about 210 °C, and is not the same as the high boiling hydrocarbon carrier.

For example, a mixture comprising

a) 5-50% of a metal hydroxide, metal oxide or metal carbonate

b) 2-15% of a sulfonate or carboxylate dispersant

c) 10-70% of a high boiling hydrocarbon carrier selected from mineral oils, alkylated benzenes, oligomers or polymers of alpha olefins, polycyclic aromatics, alkylated derivatives of polycyclic aromatics and waxes,

d) 1-10% of a C1.5 carboxylic acid, e) 8%-30% water

and

f) 0 - 60% of an organic solvent with a boiling point below 220 °C, for example a boiling point ranging from about 80 °C to about 210 °C, for example xylene or mesitylene, is stirred and heated under reflux for about 0.5 to about 4 hours, for example from about 0.5 to about 2 hours, for example about 1 hour, and then heated from about 165 °C to about 240 °C to distill off volatiles, resulting in a reaction mixture which is cooled to below 100 °C, for example cooled to approximately room temperature, and subjected to microwave irradiation in a commercial microwave ( e.g., 2.45 GHz) to a yield MgO oxide nanoparticle dispersion in excellent yield, with average particle size expected to be from about 1 to about 100 nm.

For example, a mixture comprising

a) 5-40%, typically 10-40%, for example 10-25, of a metal hydroxide, metal oxide or metal carbonate

b) 2-10%, for example 3-10%, for example 3-7% of a dispersant, for example an

alkyl benzene sulfonic acid

c) 10-70%, for example, 10-50%, for example 15-40%, of a high boiling hydrocarbon carrier,

d) 1-7%, for example, 1-4% or 2-5% of a C _1-5 carboxylic acid,

e) 8%-30%, for example 10%-20%, for example, 12-18% water

and

f) 10— 60%, for example 20-60%, for example 30-50% of an organic solvent with a boiling point ranging from about 80 °C to about 210 °C, is stirred and heated under reflux for about 0.5 to about 2 hours, for example about 1 hour, and then heated from about 165 °C to about 220 °C to distill off volatiles;

cooled to approximately room temperature, and then subjected to microwave irradiation in a commercial microwave. The dispersion obtained may be used as is, or further purified, concentrated, diluted or otherwise modified.

Most or all of the added water is removed from the preliminary dispersion by distillation prior to microwave irradiation, however, the original reaction mixture from which the preliminary dispersion is prepared typically contains at least 8%, often at least 10% by weight of water, based on the total weight of the mixture, and in many embodiments 12% or more. In certain embodiments, the amount of water is comparable by weight to the amount of metal hydroxide, oxide or carbonate, and in some particular embodiments, the weight of water is higher. In terms of molar equivalents relative to, e.g., metal oxide or metal carbonate, the reaction mixture contains from about a 5:1 to 1 :1 molar ratio of water to metal compound, for example, from about 3:1 to 1 :1. Ratios of from 2.5:1 to 1 :1 , or from 2:1 to 1 :1 are common, such as 1.5, 1.8, 2, 2.2 and 3 molar equivalents of water relative to metal compound can be employed. The process can also be used to prepare metal oxide dispersions starting with metal hydroxides, but in that case, less water is typically added due to the hydroxy groups present in the starting metal compound.

The C _1-5 carboxylic acid can be any such acid, for example, acetic acid, propionic acid, butyric acid, pentanoic acid; excellent results have been obtained using acetic acid. A small amount of this acid relative to MgO is generally employed in the reaction, for example, the molar ratio of MgO to C _1-5 carboxylic acid is from about 100:1 to 2:1 , for example, from about 50:1 to about 5:1 , or from about 30 to 1 to 10:1 , such as a molar ratio of MgO to C _1-5 carboxylic acid of about 20:1.

The dispersant is a sulfonic acid or carboxylic acid. Mixtures of dispersants may be used including mixtures of sulfonic acids, mixtures of carboxylic acids or mixtures including both sulfonic and carboxylic acids. Excellent results have been obtained using sulfonic acid dispersants widely known by those skilled in the art as oil-soluble sulfonic acids.

For example, sulfonic acid dispersants be derived from natural petroleum fractions or various synthetically prepared sulfonated compounds. Typical oil-soluble sulfonic acids which may be used include: alkane sulfonic acids, aromatic sulfonic acids, alkaryl sulfonic acids, aralkyl sulfonic acids, petroleum sulfonic acids such as mahogany sulfonic acid, petroleum sulfonic acid, paraffin wax sulfonic acid, petroleum naphthene sulfonic acid, polyalkylated sulfonic acid, and other types of sulfonic acids which may be obtained by fuming sulfuric acid treatment of petroleum fractions. In one embodiment, an alkaryl sulfonic acid, i.e., an alkylbenzene sulfonic acid, is used as dispersant with excellent results. Carboxylic acid dispersants which may be used are also well known in the art. The carboxylic add dispersants are not the same as the C _1-5 carboxylic acid required for the invention, as the dispersants have more than 5 carbon atoms, typically much more than 5 carbon atoms. Some examples include, lauric, myristic, palmitic, stearic, isostearic, archidic, behenic and lignoceric acids; aromatic acids such as alkyl salicylic acids.

Mixtures of carboxylic acids include commercial grades containing a range of acids, including both saturated and unsaturated acids. Such mixtures may be obtained synthetically or may be derived from natural products, for example, tall, cotton, ground nut, coconut, linseed, palm kernel, olive, corn, palm, castor, soybean, sunflower, herring and sardine oils and tallow.

In many embodiments of the invention, the dispersant is a naturally occurring or synthetic sulfonic acid. Excellent results have been obtained using, for example, alkyated arylsulfonic acids, for example, alkylated benzenesulfonic acids. In general, the sulfonic acid dispersant will have a MW of 300 or higher, often 350 or higher, for example 400 or higher. Mixtures of sulfonic acids may be used, for example, alkylated benzene sulfonic acids may be mono-alkylated, di-alkylated or mixtures of mono- and di- alkylated compounds may be used and in some embodiments, benzene sulfonic acid may be alkylated by alkyl chains of varying lengths. In such cases, the MW is the number average molecular weight. For example, excellent results have been obtained using alkyated benzene sulfonic adds with an average MW of from about 350 to 1000.

In general, a molar ratio of MgO to dispersant of from about 10:1 to 200:1 is employed in the reaction, frequently the ratio is from about 20:1 to 200:1. In certain embodiments the molar ratio of MgO to surfactant is from about 20:1 to 100:1 or from about 25:1 to 50:1.

In many embodiments, the molar ratio of MgO to Ci _-5 carboxylic acid, for example acetic acid, is from about 50:1 to about 5:1 or from 30:1 to10:1 and the molar ratio of MgO to dispersant, for example, an alkylated sulfonic acid, is from about 20:1 to 100:1 or from about 25:1 to 50:1.

The high boiling hydrocarbon carrier is a material or mixture of materials well known in the art with a boiling point of 280 °C or higher, often much higher, for example, mineral oils, oligomers or polymers of alpha olefins, aromatic systems such as polycyclic aromatics and alkylated derivatives thereof, long chain alkanes including waxes and other similar natural or synthetic materials. Obviously, part of the reasoning for choosing a high boiling carrier is that part of the process requires temperatures of 280 °C and higher.

The optional organic solvent has a boiling point below 280 °C, typically 210 °C or lower, and it is generally desirable to remove it by distillation prior to microwave irradiation. The solvent can be used to make the initial reaction mixture more fluid and stirrable during a preliminary heating step, especially if very low amounts of carrier hydrocarbon are used. The solvent is chosen so that it does not interfere with the overall process. For example, well known aliphatic or aromatic hydrocarbons with boiling points ranging from about 80 °C to about 240 °C, for example, boiling points ranging from about 80 °C to about 220 °C and mixtures thereof are conveniently used, including linear and cycloaliphatic compounds such as octanes, decanes etc, and aromatic hydrocarbons such as xylene, mesitylene, ethylbenzene, butyl benzenes, tetralin and the like. Lower boiling solvents are optional and are readily removed, if desired, by distillation once the process reactions are complete.

The product of the process and the process itself represent embodiments of the invention.

EXAMPLE

In a 2000 mL 3-neck round bottom flask, was charged 298 grams of MgO (98%), 120 grams of an alkyl benzene sulfonic acid dispersant, 500 grams of high boiling hydrocarbon ALCHISOR DE, 1 ,000 grams of Xylene, 300 grams of water, and 62 grams of glacial acetic acid under ambient conditions. The mixture was stirred and heated to reflux for 1 hr. The mixture was further heated to 180 °C to remove all volatiles. The resulting product was cooled down to room temperature to provide a gray preliminary dispersion with a small amount of sediment. The preliminary dispersion was subjected to microwave irradiation in a commercial microwave (2.45 GHz) to yield MgO oxide nanoparticte dispersion with average particle size expected to be between 1-100 nm.