WO2009013187A1

WO2009013187A1 - Microwave-induced process for preparing nanoparticulate metal oxides

Info

Publication number: WO2009013187A1
Application number: PCT/EP2008/059259
Authority: WO
Inventors: Andrey Karpov; Hartmut Hibst
Original assignee: Basf Se
Priority date: 2007-07-24
Filing date: 2008-07-16
Publication date: 2009-01-29

Abstract

The present invention relates to a process for preparing nanoparticles comprising at least one metal oxide, said process comprising the following steps: (A) providing a reaction mixture comprising at least one precursor compound of said at least one metal oxide and at least one oxygen source in a solvent which comprises at least one monohydric alcohol, at a temperature T1 of 0 to 50°C, (B) heating the reaction mixture provided in step (A) to a temperature T2 of 50 to 135°C by microwave radiation, giving the nanoparticles comprising at least one metal oxide, and (C) cooling the reaction mixture from step (B), comprising the nanoparticles, to a temperature T3 of 0 to 50°C.

Description

Microwave-induced process for the production of nanoparticulate metal oxides

DESCRIPTION The present invention relates to a process for the preparation of nanoparticles containing at least one metal oxide, wherein the nanoparticles produced in this way are characterized in that they have a narrow particle size distribution and can be prepared in large quantities on an industrial scale.

Metal oxides are used for a variety of purposes, such. As a white pigment, as a catalyst, as part of antibacterial skin protection creams and as an activator for the rubber vulcanization. In cosmetic sunscreens there are finely divided zinc oxide or titanium dioxide as UV-absorbing pigments. Nanoparticles are particles of the order of nanometers. Their size is in the transition region between atomic or monomolecular systems and continuous macroscopic structures. In addition to their usually very large surfaces, nanoparticles are distinguished by special physical and chemical properties, which differ significantly from those of larger particles. For example, nanoparticles often have a lower melting point, absorb light only at shorter wavelengths, and have different mechanical, electrical, and magnetic properties than macroscopic particles of the same material. By using nanoparticles as building blocks, many of these special properties can also be used for macroscopic materials (Winnacker / Küchler, Chemische Technik: Processes and Products, (Ed .: R. Dittmayer, W. Keim, G. Kreysa, A. Oberholz ), Volume 2: New Technologies, Chapter 9, Wiley-VCH Verlag 2004).

In the context of the present invention, the term "nanoparticles" refers to particles having a mean diameter of from 1 to 500 nm, determined by means of electron microscopy methods.

Methods for producing nanoparticles comprising at least one metal oxide are already known from the prior art.

The production of metal oxides, for example of zinc oxide, by wet and dry processes is known. The classical method of burning zinc, which is known as a dry process, eg Gmelin Volume 32, 8th Edition, Supplementary Volume p. 772 et seq., Produces aggregated particles with a broad size distribution. Although it is fundamentally possible to produce particle sizes in the submicrometer range by means of grinding methods, dispersions having average particle sizes in the lower nanometer range can not be achieved from such powders because of the shearing forces which can be achieved to a small extent. Particularly finely divided zinc oxide is mainly wet-chemically precipitated by Processes produced. The precipitation in aqueous solution generally yields hydroxide and / or carbonate-containing materials which must be thermally converted to zinc oxide. The thermal aftertreatment has a negative effect on fineness, since the particles are subjected to sintering processes which lead to the formation of micromaster-sized aggregates which can only be broken down incompletely onto the primary particles by grinding.

JP 2003-342007 A discloses a method for producing crystalline metal oxides having a particle diameter in the nanometer range. The metal compounds used can be selected from hydrates or other salts of titanium, silicon, tin and zinc. For this purpose, the corresponding precursor compounds are dispersed in a polyol-containing solution and heated by microwave irradiation to a temperature of 140 or 240 ⁰ C. The method disclosed in JP 2003-342007 A is carried out in polyol-containing solvents. A process for producing nanoparticles containing at least one metal oxide in a solvent containing at least one monohydric alcohol is not disclosed in this document.

DE 103 24 305 A1 discloses a process for the production of zinc oxide particles, in which a methanolic solution containing zinc acetate and potassium hydroxide is heated to a temperature of 40 to 65 ^° C., so that the desired zinc oxide nanoparticles precipitate. The zinc oxide nanoparticles must mature for a period of 5 to 50 minutes according to this document. DE 103 24 305 A1 does not disclose how the tempering is carried out.

In Span Span, MA Anderson, J. Am. Chem. Soc. 1991, 113, 2826-2833 discloses a method for producing ZnO wurtzite clusters. For this purpose, a solution of zinc acetate in ethanol and a solution of LiOH ^* H ₂ O are combined in ethanol and stirred for 10 min at 0 ⁰ C, wherein ZnO colloids precipitate. EP 1 157 064 B1 discloses a process for preparing nanoparticulate redispersible zinc oxide gels by basic hydrolysis of a suitable zinc compound in an alcohol-water mixture. According to this document, the zinc oxide particles must ripen for over one hour to ensure complete flocculation of the ZnO.

This prior art does not disclose that in the production of nanoparticulate metal oxides, rapid heating of the solution containing the substrates containing at least one alcohol is important. Furthermore, the prior art documents do not disclose methods of making nanoparticles containing metal oxides using microwaves to heat the reaction solution. The prior art discloses methods in which the precipitated particles ripen for a period of time to ensure complete precipitation. The object of the present invention is to provide a process for producing nanoparticles containing at least one metal oxide having a narrow particle size distribution in large quantities, which is suitable for use on an industrial scale. Furthermore, it is an object of the present invention to provide a method by which nanoparticles containing at least one metal oxide can be produced in consistent quality and a narrow particle size distribution as possible. It is also an object to provide a process in which it is possible to dispense with a ripening of the metal oxide particles.

These objects are achieved by a method for producing nanoparticles containing at least one metal oxide, comprising the steps:

(A) providing a reaction mixture comprising at least one precursor compound of the at least one metal oxide and at least one oxygen source in a solvent which contains at least one monohydric alcohol, at a temperature T1 of 0 to 50 ^° C.,

(B) heating the reaction mixture provided in step (A) to a temperature T2 of 50 to 135 ⁰ C by microwave radiation, wherein the nanoparticles containing at least one metal oxide are obtained and

(C) cooling the reaction mixture from step (B) containing the nanoparticles to a temperature T3 of 0 to 50 ⁰ C.

The process according to the invention is carried out continuously in a preferred embodiment. The process according to the invention is carried out batchwise in another embodiment. It is also possible according to the invention for individual steps of the method to be carried out continuously and other steps to be discontinuous. The individual steps of the method according to the invention are explained in more detail below:

Step (A): Step (A) of the process of the invention comprises providing a reaction mixture containing at least one precursor compound of the at least one metal oxide and at least one oxygen source in a solvent containing at least one monohydric alcohol at a temperature T1 of 0 to 50 ^° C In the process according to the invention, nanoparticles containing at least one metal oxide are prepared.

Suitable metal cations which are present in the nanoparticles produced according to the invention are those of metals of the main groups or subgroups of the Periodic Table of the chemical elements, as well as lanthanides and actinides and mixtures thereof. In a preferred embodiment, the metals are selected from groups 1 to 15 of the Periodic Table of the LUPAC nomenclature chemical elements, as well as the groups of lanthanides and actinides and mixtures thereof. In a particularly preferred embodiment, the metal present in the nanoparticle according to the invention is selected from the groups 2, 4, 5, 6, 7, 8, 9, 10, 12, 13 and 15 of the Periodic Table of the Elements and the groups of lanthanides and mixtures from that. Examples of most preferred metals are selected from the group consisting of nickel, copper, zinc, cadmium, aluminum, gallium, indium, tin, lead, antimony, bismuth, cerium, and mixtures thereof.

With the method according to the invention it is possible to produce nanoparticles containing one or more metal oxides. Nanoparticles containing one or two different metal oxides are preferably produced by the process according to the invention. It is also possible according to the invention that a metal oxide which contains two or more different metals, for example a mixed oxide, is present in the nanoparticle produced.

Very particularly preferred metal cations present in the nanoparticles prepared according to the invention are cerium, vanadium, bismuth, zinc, copper, nickel and mixtures thereof, for example a mixture of bismuth and vanadium.

In step (A) of the process according to the invention, a reaction mixture containing at least one precursor compound of the at least one metal oxide is provided.

Suitable precursor compounds of the at least one metal oxide are all compounds known to those skilled in the art, which can be converted into the corresponding oxides by a hydrolysis reaction. In a preferred embodiment, organic or inorganic salts of the metal oxides present in the nanoparticles according to the invention are used. Examples of preferably used inorganic metal salts are the salts of the abovementioned metals with an anion selected from the group consisting of halide, sulfate, nitrate and mixtures thereof. Examples of preferably used organic metal salts are salts of the abovementioned metals with mono- or polyvalent anions of organic carboxylic acids.

Suitable anions are derived, for example, from organic monocarboxylic acids such as formic acid (formate), acetic acid (acetate), propionic acid, isobutyric acid, caproic acid, caprylic acid, lauric acid, myristic acid, palmitic acid and stearic acid, unsaturated fatty acids such as acrylic acid, methacrylic acid, crotonic acid, oleic acid and linolenic acid, saturated polybasic carboxylic acids, such as oxalic acid, malonic acid, succinic acid, adipic acid, suberic acid and β, β-dimethylglutaric acid, unsaturated polybasic carboxylic acids, such as maleic acid and fumaric acid, saturated alicyclic acid. cical acids, such as cyclohexanecarboxylic acid, aromatic carboxylic acids, such as the aromatic monocarboxylic acids, in particular phenylacetic acid and toluic acid and unsaturated polybasic carboxylic acids, such as phthalic acid, isophthalic acid, terephthalic acid, pyromellitic and trimellitic acid, compounds having functional groups, such as OH groups, amino groups, nitro groups , Alkoxy groups, sulfonic groups, cyano groups and halogen atoms in the molecule besides a carboxyl group such as trifluoroacetic acid, orthochlorobenzoic acid, orthonitrobenzoic acid, anthranilic acid, para-aminobenzoic acid, para-chlorobenzoic acid, toluic acid, lactic acid, salicylic acid, and polymers containing at least one of the aforementioned unsaturated acids as a polymerizable compound such as acrylic acid homopolymers and acrylic acid / methyl methacrylate copolymers.

Suitable anions are furthermore alcoholates derived from aliphatic or aromatic alcohols having one or more hydroxyl functions by cleavage of at least one proton of at least one hydroxyl function. Examples of alcoholates which can be used according to the invention are methanolate, ethanolate, propoxides such as n-propoxide and isopropoxide, butoxides and others.

The metal salts used can optionally contain water of crystallization or alcohol molecules. Suitable alcohols are selected from the group consisting of methanol, ethanol, propanols such as n-propanol and isopropanol, butanols and mixtures thereof. The amount of water of crystallization optionally present in the metal salts depends on the stoichiometry of the compound used specifically, its crystal structure and / or its pretreatment. For example, it is possible to lower the amount of water of crystallization by heating the compounds.

In a very particularly preferred embodiment, nitrates, alkoxylates or acetates are used as precursor compounds. In a particularly preferred embodiment of the method according to the invention, zinc acetate dihydrate Zn (OOCCH ₃₎ ₂ ^* 2 H ₂ O is used as the precursor compound.

Thus, nanoparticles which contain the oxides of the abovementioned metals are preferably formed in the process according to the invention. Nanoparticles of ZnO are particularly preferably obtained by the process according to the invention.

In step (A) of the process according to the invention, a reaction mixture is provided which contains at least one oxygen source in addition to the said at least one precursor compound.

In the context of the present invention, the term "oxygen source" is to be understood as meaning a compound which in the process according to the invention at least one precursor compound of the at least one metal oxide can be converted into the corresponding at least one metal oxide. In a preferred embodiment, the oxygen sources used in step (A) are selected from compounds selected from the group consisting of water, water of crystallization of the precursor compounds used, bases and mixtures thereof.

In a further preferred embodiment, an OH ^" -containing compound, ie a base, is added to the reaction mixture in step (A) Examples of such compounds are metal hydroxides, for example alkali and / or alkaline earth metal hydroxides, or ammonium hydroxides containing an ammonium cation Formula NR ₄ ⁺ , wherein R is independently selected from the group consisting of hydrogen, linear or branched carbon radicals having 1 to 8 carbon atoms, preferably hydrogen, methyl or ethyl In a preferred embodiment, an oxygen source is used in the inventive method from water or water of crystallization of the precursor compound used.

The solvent used in step (A) of the process according to the invention contains at least one monohydric alcohol. According to the invention, a single alcohol may be present in the reaction mixture, in a further embodiment it is also possible to use a mixture of two or more alcohols. In the process according to the invention, it is generally possible to use all monohydric alcohols which are liquid under the given reaction conditions. Examples of monohydric alcohols which are suitable according to the invention are monohydric aliphatic or aromatic alcohols.

Suitable monohydric aliphatic alcohols are those of the general formula (I) R-OH (I) where R is linear or branched, saturated or unsaturated, linear or cyclic alkyl radical having 1 to 12 carbon atoms or mixtures thereof. Preferred aliphatic alcohols are, for example, methanol, ethanol, propanols such as n-propanol, isopropanol, butanols such as n-butanol, isobutanol, pentanols, such as n-pentanol, isopentanols, tert-pentanols, cyclohexanol and mixtures thereof. A most preferred monohydric alcohol is iso-propanol.

In a further embodiment, the solvent used according to the invention may contain further organic solvents, which are preferably selected from the group consisting of amines, for example n-butylamine, tert-butylamine, n-pentylamine, n-hexylamine, n-heptylamine, n-butylamine. Octylamine, n-dodecylamine, benzylamine, pyridine, 4-dimethylaminopiridyne, thiols, eg 1-butanethiol, tert-butanethiol, 1-pentanethiol, 1-hexanethiol, 1-heptanethiol, 1-octanethiol, tert-octanethiol, 1-Dodecanethiol, benzylthiol, phosphonic acids, for example n-butylphosphonic acid, tert-butylphosphonic acid, n-pentylphosphonic acid, n-hexylphosphonic acid, n-heptylphosphonic acid, n-octylphosphonic acid, n-decylphosphonic acid, n-dodecylphosphonic acid, 3-aminopropylphosphonic acid, 4 -Aminobutylphosphonic acid, phenylphosphonic acid, p-tolylphosphonic acid, 4-methoxyphenylphosphonic acid, polymers, for example polyethylene glycols, polyvinylpyrrolidones, polyacrylates, polyvinyl ethers, polyvinyl acetates.

Preferably, one of the abovementioned monohydric alcohols is used as the solvent in step (A) of the process according to the invention, without further solvents being present in a mixture.

As a solvent, a mixture of monohydric alcohol and water can be used.

In step (A) of the process according to the invention, the reaction mixture in a

Temperature T1 from 0 to 50 ⁰ C, preferably 15 to 40 ⁰ C, most preferably provided at room temperature. The temperature T1 present in step (A) of the process according to the invention depends on the solubility of the at least one precursor compound used. Preferably, the temperature T1 should be selected so that the precursor compound is only dissolved and is not completely or partially converted into an oxide. If this is readily soluble in the solvent used, then step (A) is very particularly preferably carried out at room temperature. If the precursor compound is not dissolved at T1, it may be ground prior to step (A) to provide a finely divided suspension in step (A). In preferred form, T1 is chosen to be just below the reaction temperature of the precursor compound in the metal oxide to reduce the residence time in step (B). If the precursor compound used in step (A) of the process according to the invention is sparingly soluble in the solvent at room temperature, step (A) is carried out in a particularly preferred further embodiment at a temperature T1 of 35 to 50 ^° C.

In step (A) of the process according to the invention, in a preferred embodiment, the at least one metal salt acting as precursor compound is in one

Co-nitrate titration of 0.01 to 1 mol ^* 1 ^" , more preferably 0.05 to 0.5 mol ^* l ^{" 1} , used.

In step (A) of the process of the invention, the at least one oxygen source in a preferred embodiment in a concentration of 0.01 to 5 mol ^* r ¹ , more preferably 0.05 to 3 mol ^* 1 ^" , most preferably 0 , 0.05 to 2.5 mol ^* I ^"1 used. These concentration data refer to the entire reaction mixture. Step (A) of the process according to the invention can be carried out in any reactor known to those skilled in the art which is suitable for mixing the said components. If the process according to the invention is carried out continuously, in a preferred embodiment the reactor has corresponding devices in order to be able to continuously feed starting compounds and solvents. Suitable reactors are known to the person skilled in the art. The mixing of the reaction mixture in step (A) of the process according to the invention is carried out by devices known to the person skilled in the art.

Preferably, the provision of the reaction mixture in step (A) of the process according to the invention is carried out so that a homogeneous mixture is obtained. In the context of the present invention, "homogeneous" means that there are no density and / or concentration differences within the mixture.

In a preferred embodiment of the method according to the invention are in

Step (A) the above components mixed together, for example at room temperature, and at a temperature of 35 to 50 ⁰ tempered C. This tempering step within step (A) of the process according to the invention is carried out in particular when the at least one precursor compound of the at least one metal oxide used in step (A) in the solvent used is not completely soluble at room temperature. This tempering is preferably carried out until the at least one precursor compound is completely dissolved in the solvent used. According to the invention, it is of particular advantage if all components are completely dissolved in the reaction mixture provided in step (A) of the process according to the invention. The optionally carried out tempering of the reaction mixture in step (A) of the method according to the invention can be carried out by any of the skilled person known methods, for example by electric heating, heating with a heated medium in a heat exchanger and / or microwaves. In a preferred embodiment, the optional tempering in step (A) is effected by microwave radiation. In principle, microwaves in the frequency range from 0.2 GHz to 100 GHz can be used for this dielectric radiation. For industrial practice, frequencies of 0.915, 2.45 and 5.8 GHz are available, with 2.45 GHz being particularly preferred.

Radiation source for dielectric radiation is the magnetron, which can be irradiated simultaneously with several magnetrons. It must be ensured that the field distribution during irradiation is as homogeneous as possible in order to obtain a uniform heating of the reaction mixture.

In a further preferred embodiment of the method according to the invention, two solutions are provided, one solution comprising at least one runner compound of the at least one metal oxide in a solvent containing at least one monohydric alcohol, and the second solution containing the at least one source of oxygen in the same or a different solvent. Both solutions are heated independently in this preferred embodiment to a temperature of preferably 15 to 40 ⁰ C, more preferably 25 to 40 ⁰ C. The person skilled in the art for this purpose suitable methods and devices are known. The hot solutions are then mixed together, forming a solution in a preferred embodiment. If, after combining the two original solutions, a suspension is formed, the mixing in a preferred embodiment should take place as rapidly as possible, in a particularly preferred embodiment the addition takes place within 1 minute, very preferably within 30 seconds, particularly preferably within 10 seconds.

Step (B):

Step (B) of the method according to the invention comprises heating the reaction mixture provided in step (A) to a temperature T2 of 50 to 135 ^° C. by microwave radiation, the nanoparticles containing at least one metal oxide being obtained.

In step (B), the reaction mixture provided in step (A) is heated to a temperature T2 of 50 to 135 ⁰ C, preferably 60 to 120 ⁰ C, particularly preferably 70 to 100 ⁰ C heated. The process according to the invention is characterized in that the reaction mixture provided in step (A) is heated particularly rapidly to the reaction temperature in step (B), so that nanoparticles are produced which are distinguished by a particularly small and particularly uniform particle size. In a preferred embodiment, the heating in step (B) is carried out at a heating rate of at least 10 K ^* min ^"1 , more preferably at least 20 K ^* min ^{" 1} , very particularly preferably at least 50 K ^* min ^"1 , particularly preferably at least 100 K. ^* min ^"1 . In the process according to the invention, step (B) is preferably carried out such that in a corresponding vessel the reaction mixture according to step (A) is preferably heated to at least 20, 50 or 100 K ^* min ^-1 . according to the process of the invention, and conveyed by means of a suitable pump, for example a membrane pump, rotary pump, rotary vane pump, gear pump or HPLC pump, into a reactor suitable for continuous processes, for example a tubular reactor heated to a certain distance by means of a heater to the present in step (B) temperature T2 from 50 to 135 ⁰ C. This heating is carried out by microwave radiation.With the use of microwaves, it is inventively according to preferred that the tubular reactor, in the step (B) of the inventive method is preferably carried out, consists of a material which weakly or not at all interferes with the microwaves, ie with a penetration depth of> 100 cm, preferably> 500 cm, especially preferably> 1000 cm, each at 2.45 GHz. Examples of suitable materials are borosilicate glass, quartz, plastics such as polyethylenes, polytetrafluoroethylene, ceramics based on silicate raw materials, on oxidic raw materials, eg Al ₂ O ₃ or on non-oxidic raw materials.

In step (B) of the method according to the invention, it is generally possible to work with microwaves in the frequency range from 0.2 GHz to 100 GHz. For industrial practice frequencies of 0.915, 2.45 and 5.8 GHz are available, with 2.45 GHz being particularly preferred.

Radiation source for dielectric radiation is the magnetron, which can be irradiated simultaneously with several magnetrons. Care must be taken to ensure that the field distribution during irradiation is as homogeneous as possible in order to obtain a uniform heating of the reaction mixture and thus a uniform particle size distribution. Furthermore, it is possible with microwaves to achieve very high heating rates, so that short residence times of the reaction solution in the individual stages of the process according to the invention are possible. As a result, the process according to the invention can be carried out in a particularly cost-effective manner and is particularly suitable for carrying out on an industrial scale. The process according to the invention furthermore makes it possible to obtain metal oxide nanoparticles without the need for a time-consuming and thus cost-intensive ripening step.

The process according to the invention can be carried out in all devices known to the person skilled in the art, for example in a tubular reactor. The preferably used tubular reactor can be installed in any spatial orientation so that the reaction mixture flows horizontally, vertically or diagonally. Furthermore, it is advantageous if the residence time of the reaction mixture in the reactor is as uniform as possible in order to avoid widening of the particle size distribution and deterioration of the quality features attributable to a uniform residence time. Therefore, in a preferred embodiment, the reactor is designed to avoid partial stagnation of the flow of the reaction mixture and / or disadvantageously nonuniform distribution of residence times.

The shape of the tube of the tube reactor preferably used in cross-section is not subject to any restrictions. In a preferred embodiment, the cross-section is circular or concentric annular to avoid inconsistent flow, stagnation, turbulence or inconsistent heating of the reaction mixture. It is inventively necessary that the cross-sectional area of the tube reactor preferably used is not excessively large, to ensure that the flowing reaction mixture is heated as uniformly as possible. Furthermore, the diameter of the tube reactor which is preferably used is chosen such that, in combination with the flow rate of the reaction mixture in step (B) of the process according to the invention, a residence time of the reaction mixture in the hot zone results, which ensures that as complete a conversion as possible, for example, at least 90%, preferably at least 95%, takes place. The diameter of the tube reactor is preferably 0.01 cm to 10 cm, particularly preferably 0.1 cm to 5 cm.

The residence time of the reaction mixture in the reaction zone in step (B) of the process according to the invention is preferably <30 minutes, more preferably <10 minutes, most preferably <5 minutes.

It is advantageous according to the invention if the best possible thorough mixing takes place in the region of the reactor in which the reaction of the at least one precursor compound takes place with the at least one metal oxide. This mixing can be carried out by all methods known to those skilled in the art. In a preferred embodiment, a static mixer is used in step (B), i. In the tube reactor preferably used, devices known to those skilled in the art, for example baffles, are incorporated, which mix the flowing reaction mixture during the flow.

In step (B) of the process according to the invention, the at least one metal oxide in the form of nanoparticles is obtained from the at least one precursor compound used in step (A) and the at least one oxygen source by the thermal energy introduced. These nanoparticles are present after carrying out step (B) as a suspension in the solvent used in step (A). Step (C):

Step (C) of the process according to the invention comprises cooling the reaction mixture from step (B) containing the nanoparticles at a temperature T3 of 0 to 50 ^° C.

In a preferred embodiment of the continuous process according to the invention, the reaction mixture obtained is then cooled to the abovementioned temperature at step (B) by means known to those skilled in the art for cooling a liquid reaction mixture. In a preferred embodiment is cooled to a temperature T3 of 10 to 25 ⁰ C, most preferably cooled to room temperature. T3 in step (C) of the process of the invention is generally chosen so that the solvent used in steps (A) and (B) is not frozen. In a further preferred embodiment, step (C) also in a tubular reactor, for example a heat exchanger. It is also possible according to the invention that several heat exchangers are connected in series. In step (C) of the process according to the invention, the reaction solution is cooled in order to suppress further, possibly uncontrolled, growth of the nanoparticles.

According to the invention, it is preferred that the cooling in step (C) of the process according to the invention is particularly rapid. The cooling in step (C) is preferably carried out at a cooling rate of at least 10 K ^* min ^"1, more preferably at least 20 K ^* ^{min" 1,} very particularly preferably at least 50 K ^* min ^"1, particularly preferably at least 100 K ^* ^{min" 1} . The high cooling rates of at least 20, 50 or 100 K ^* min ^-1 are preferably achieved by continuously carrying out the process according to the invention. This very fast cooling makes it possible according to the invention to use nanoparticles with a particularly uniform particle size distribution and smaller particle sizes than in a process in which is cooled at a lower cooling rate to obtain.

In step (C) of the process according to the invention is a set to a temperature T3 of 0 to 50 ⁰ C cooled suspension in step (B) from the runner connecting a pre least and the nanoparticles formed of at least one metal oxide in the step (A) Solvent received.

In a preferred embodiment of the method according to the invention, the nanoparticles are functionalized in or after step (B) and / or in or after step (C). The reagents can also be added already in step (A) of the method according to the invention.

Methods and reagents for functionalizing nanoparticles obtained by the process according to the invention during the synthesis, preferably on the surface thereof, are known to the person skilled in the art.

Suitable reagents are, for example, phosphonic acids or salts / esters of phosphonic acids R-PO (OH) ₂ , see WO 2006/124670, sulfonic acids or salts / esters of sulfonic acids R-SO ₂ (OH), see DE 10 2005 047 807 A1, Organosilanes, see WO 2005/071002, DE 10 2005 010 320 A1, organic acids, see WO 2004/052327, polyacrylates, see EP 1 630 136 A1, amphiphilic molecules, see DE 10 2004 009 287 A1, polyethylene glycols, polyvinylpyrrolidone, fatty acids, Alkylamines, alkanethiols and others. The content of said documents is hereby expressly incorporated into the present application.

In general, the nanoparticles prepared in step (B) of the process according to the invention can be isolated by all methods known to the person skilled in the art from the suspension obtained in step (C) of the process according to the invention. In one possible embodiment, step (C) is followed by step (D):

(D) Concentrating the reaction mixture from step (C).

According to the invention, it is possible for step (C) of the process according to the invention to be followed by a step (D) which comprises concentrating the mixture obtained in step (C). The concentration in step (D) can be carried out by methods known to the person skilled in the art, for example filtration, such as nano-, ultra-, microfiltration and / or centrifugation, for example ultracentrifugation.

A step (D) according to the invention is then preferably used when the process is carried out in high dilution, which takes place, for example, when small particles are to be obtained.

In a further preferred embodiment, the method according to the invention comprises a step (E):

(E) separating the nanoparticles present in the reaction mixture from step (C) or (D) by filtration, for example nano-, ultra-, microfiltration and / or centrifugation, for example ultracentrifugation.

It is possible according to the invention that step (E) is followed directly by step (C). In another embodiment, step (C) is followed by step (D), followed by step (E).

In a preferred embodiment, the residue obtained from the filtration or centrifugation in step (E) is washed with a suitable solvent, for example water or organic solvents such as ethanol, isopropanol or mixtures thereof and again filtered or centrifuged. Washing may also be by means of a membrane process such as nano, ultra, micro or crossflow filtration. This washing process can be repeated until a desired degree of purity is reached. The resulting filter cake or Zentrifugierrückstand can be dried in a conventional manner, for example in a drying oven at temperatures of 40 to 100 ⁰ C, preferably 50 to 70 ⁰ C under atmospheric pressure to constant weight.

The steps (A), (B), (C) and optionally (D) and (E) are carried out independently of one another in a possible embodiment under an inert protective gas atmosphere. In another possible embodiment, steps (A), (B), (C) and optionally (D) and (E) are not carried out independently of one another in an inert atmosphere, for example in air. All combinations of steps in inert and non-inert atmosphere steps are possible. Suitable inert gases are noble gases, for example helium or argon, nitrogen or mixtures thereof.

The method according to the invention is carried out continuously in one embodiment. In another preferred embodiment, the process according to the invention is carried out batchwise.

The nanoparticles obtained by the process according to the invention have an average particle size of about 5 to 100 nm, preferably 7 to 50 nm, in each case determined by dynamic light scattering (DLS) (on suspensions) and scanning or transmission electron microscopic investigations (SEM or TEM) (on powders).

Furthermore, the nanoparticles produced by the process according to the invention are characterized by a particularly narrow particle size distribution. In a preferred embodiment, at least 90% of the resulting particle sizes are in the size range described by the average particle size ± 15% of this average particle size.

Another advantage of the method according to the invention is that the nanoparticles produced therewith are present without agglomerates. This can be demonstrated by the fact that both the determination of the particle size by dynamic light scattering, as well as the determination of the particle size by scanning electron microscopic investigations give the same value for the particle size. Figure:

FIG. 1 shows the X-ray diffraction of a zinc oxide powder obtained according to the invention.

The process of the invention is further illustrated by the following example.

Embodiment:

Preparation of nanoscale ZnO in isopropanol using microwaves A mixture of 37.67 g (0.172 mol) of zinc acetate dihydrate (Chemetall) and 1720 ml of isopropanol is initially introduced. 16.55 g (0.295 mol) of potassium hydroxide (from Riedel de Haen) are added to the mixture. The mixture is heated with stirring (325 rpm) to 75 ° C within 4.5 minutes by means of microwaves (Ethos 1800, MLS) and held at this temperature for 5 minutes. The resulting solid is separated from the solvent and washed twice with 1 L of isopropanol. Subsequently, isopropanol is completely removed on a rotary evaporator. The X-ray diffraction of the obtained powder shows exclusively the diffraction reflections of zinc oxide (Fig. 1). The number of reflections is plotted on the x-axis. 2-theta is plotted on the y-axis. The step time is 3.6 s. The measurement is carried out at room temperature. A Cu anode is used. The X-ray recovery corresponds to that of hexagonal ZuO.

From the half-width of the X-ray reflexes, an average crystallite size of about 8 nm can be calculated.

Claims

claims

1. A process for the preparation of nanoparticles comprising at least one metal oxide, comprising the steps:

(A) providing a reaction mixture containing at least one precursor compound of the at least one metal oxide and at least one oxygen source in a solvent containing at least one monohydric alcohol at a temperature T1 of 0 to 50 ^° C,

(C) cooling the reaction mixture from step (B) containing the nanoparticles to a temperature T3 of 0 to 50 ^° C.

2. Method according to claim 1, characterized in that step (D) follows at step (C):

(D) Concentrating the reaction mixture from step (C).

3. The method according to claim 1 or 2, characterized in that at step (C) or (D) step (E) follows:

(E) separating the nanoparticles present in the reaction mixture from step (C) or (D) by filtration and / or centrifugation.

4. The method according to any one of claims 1 to 3, characterized in that it is carried out continuously.

5. The method according to any one of claims 1 to 3, characterized in that it is carried out batchwise.

6. The method according to any one of claims 1 to 5, characterized in that the nanoparticles are functionalized in or after step (B) and / or in or after step (C).

7. The method according to any one of claims 1 to 6, characterized in that the oxygen source is selected from water or water of crystallization of the precursor compounds used.

8. The method according to any one of claims 1 to 7, characterized in that the heating in step (B) at a heating rate of at least 10 K ^* min ^"1 takes place.

9. The method according to any one of claims 1 to 8, characterized in that the heating in step (B) at a heating rate of at least 20 K ^* min ^"1 takes place.

10. The method according to any one of claims 1 to 9, characterized in that the cooling in step (C) with a cooling rate of at least 10 K ^* min ^"1 takes place.

1 1. A method according to any one of claims 1 to 10, characterized in that ddaass P cooling in step (C) with a cooling rate of at least 20 K ^* min ^"1 follows.