WO2009007369A2 - Method for the continuous production of nanoparticulate metal oxides in solvents containing polyol - Google Patents

Abstract

The present invention relates to a method for the continuous production of at least one metal oxide containing nanoparticles, consisting of the following steps: a) preparation of a reaction mixture containing at least one precursor compound of the at least one metal oxide and at least one oxygen donor in a solvent which contains at least one polyol, at a temperature T1 from 0°C to 150°C; b) heating of the reaction mixture prepared in step a) to a temperature T2 of 130°C to 350°C, wherein the at least one metal oxide containing nanoparticles is obtained; and c) cooling down of the reaction mixture from step b) containing the nanoparticles to a temperature T3 of 0°C to 70°C.

Description

 Continuous process for the preparation of nanoparticulate metal oxides in polyol-containing solvents

description

The present invention is a continuous process for the production of nanoparticles containing at least one metal oxide, wherein the nanoparticles thus produced are characterized in that they have a narrow particle size distribution and can be prepared in large quantities on an industrial scale.

Metal oxides find use for a variety of purposes, e.g. as a white pigment, as a catalyst, as a component of antibacterial skin protection creams and as an activator for 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 characterized by particular 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 / Kuchler, 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 containing 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 in principle possible to produce particle sizes in the submicrometer range by means of grinding processes, dispersions having average particle sizes in the lower nanometer range are not available from such powders due to the shearing forces which can be achieved to a small extent achievable. Particularly finely divided zinc oxide is mainly produced wet-chemically by precipitation processes. 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 micrometer-sized aggregates, which can only be broken down to the primary particles by grinding in an incomplete manner.

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 process disclosed in JP 2003-342007 A is carried out batchwise, ie, batchwise. A continuous process for the production of nanoparticles containing at least one metal oxide on an industrial scale and in a consistently high quality is not disclosed in this document.

D. Jezequel, J. Guenot, N. Jouini and F. Fievet, Materials Science Forum 1994, 152-153, 339-342 disclose a method of making monodisperse zinc oxide having submicron particle sizes. For this purpose, zinc acetate dihydrate is dissolved in diethylene glycol and heated to 180 ⁰ C, wherein zinc oxide precipitates. This document also discloses that heating rates of 6 ⁰ C ^* min ^"1 or 14 ⁰ C ^* min ^{" 1 have} a positive influence on the particle size of the nanoparticles produced. The preparation of the nanoparticles according to this document is carried out on a laboratory scale and is carried out batchwise.

D. Yamamoto, Y. Wada and S. Yanagida, Proceedings of the World Congress on Microwave and Radio Frequency Applications, September 22-26, 2002, Sidney, Austra-Na, disclose a process for producing TiC> ₂ or ZnO nanoparticles with particle sizes of <10 nm from corresponding precursors by dissolving these compounds in alkanediols at 240 ⁰ C. the cited document discloses a batch process, wherein the reaction solution with heating rates of 3-14 ⁰ C ^* min ^"1 is heated. also this Method is performed on a laboratory scale of about 50 ml and discontinuous.

US 2006/0222586 A1 discloses a method for producing a zinc oxide nanoparticle sol by neutralizing an inorganic zinc salt with an inorganic base, both of which are dissolved in ethylene glycol. In this method, the precipitated nanoparticles are aged for 1 to 6 hours at 40 to 100 ⁰ C to obtain highly transparent zinc oxide particles. DE 103 24 305 A1 discloses a continuous 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 methods disclosed in the prior art for the production of nanoparticles are carried out batchwise. A disadvantage of the discontinuous procedure is that when treating larger reaction volumes, for example on an industrial scale, only low heating or cooling rates are achieved by the large reaction volumes, which in turn leads to relatively large particles and a broad particle size distribution.

The prior art also discloses a method for producing nanoparticles containing zinc oxide, which is carried out continuously, but is carried out in a solvent having a low boiling point, and thus does not allow high reaction temperatures.

The object of the present invention is to provide a continuous process for the preparation of nanoscale metal oxides with narrow particle size distribution in large quantities, which is suitable to be used on an industrial scale. Furthermore, it is an object of the present invention to provide a method by which nanoscale metal oxides can be produced in consistent quality.

These objects are achieved by a continuous process for the production 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 polyol at a temperature T1 of from 0 to 150 ^° C.,

(B) heating the reaction mixture provided in step (A) to a temperature T2 of 130 to 350 ⁰ C, wherein the nanoparticles containing at least one

Metal oxide can be obtained and

(C) cooling the reaction mixture from step (B) containing the nanoparticles to a temperature T3 of 0 to 70 ⁰ C.

The individual steps of the continuous process according to the invention are described in

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 polyol at a temperature T1 of 0 to 150 ^° 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 oxide (s). 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 used in the inventively prepared

Nanoparticles 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 those prepared in accordance with the invention Nanoparticles present metal oxides used. Examples of preferred inorganic metal salts are the salts of the abovementioned metals having 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 ß, ß-dimethylglutaric acid, unsaturated polybasic carboxylic acids such as maleic acid and fumaric acid, saturated alicyclic acids such as cyclohexanecarboxylic acid, aromatic carboxylic acids such as the aromatic monocarboxylic acids, especially 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 group n, sulfonic groups, cyano groups and halogen atoms in the molecule in addition to a carboxyl group such as trifluoroacetic acid, orthochlorobenzoic acid, orthonitrebenzoic 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 elimination of at least one proton of at least one hydroxyl function. Examples of alcoholates which can be used according to the invention are methoxide, ethanolate, propoxides such as n-propoxide and isopropoxide, butoxides and others.

The metal salts used may 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 process according to the invention metal salts are used, selected from the group consisting of Ce (NOs) ₃ ^* 6 H ₂ O, VO (OiPr) ₃ , Bi (NO ₃ ) ₃ ^* 5 H ₂ O, the Acetates of zinc, copper or cobalt, CeCl ₃ and mixtures thereof.

Thus, nanoparticles which contain the oxides of the abovementioned metals are preferably formed in the process according to the invention. The process according to the invention particularly preferably produces metal oxides selected from among

Group consisting of ZnO, CeO _x (1, 5 <x <2), VO _x (1, 5 <x <2.5), BiVO _x (1, 5 <x <

3.0), TiO ₂ , CuO _x (O <x <1), CoO _x (O <x <1), NiO _x (O <x <1), where x is from each of the

Reaction conditions such as type of polyol used, precursor compounds and / or reaction time depends.

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, can convert the at least one precursor compound of the at least one metal oxide into the corresponding at least one metal oxide. A) used oxygen sources selected from compounds selected from the group consisting of water, water of crystallization of the precursor compounds used, bases and mixtures thereof It is also possible according to the invention that the anion of the at least one metal salt, which is used as a precursor compound in step (A) reacted with the solvent of the reaction mixture with elimination of one molecule of water or an OH ^" -Anion. Examples of such anions are anions of carboxylic acids such as formate, acetate or propionate. These anions may react with the polyols contained in the solvent, see below. In this reaction at the hydroxy functionality of the at least one polyol is built up with elimination of an H ₂ O molecule of an ester function. The OH ^" anion obtained therefrom can then function as an oxygen source in the reaction according to the invention.

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 of the formula NR ₄ ⁺ , where R is selected independently of one another 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 process according to the invention, selected from water or water of crystallization of the precursor compound used.

The solvent used in step (A) of the process of the invention contains at least one polyol. In the process according to the invention, it is generally possible to use all polyols which are liquid under the given reaction conditions. According to the invention, "polyol" is understood to mean a compound which has at least two hydroxyl functions In accordance with the invention, a single polyol may be present in the reaction mixture, in a further embodiment it is also possible to use a mixture of two or more polyols.

Suitable polyols are, for example, diols. These are preferably selected from

1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol, 1,4-butanediol, but-2-en-1, 4-diol, 1, 2-pentanediol, 1, 5-pentanediol, 2,4-pentanediol, 2-methyl-2,4-pentanediol, 1, 2-hexanediol, 1, 6-hexanediol, 2,5-hexanediol, 2,4-heptanediol, 2-ethyl-1,3-hexanediol, octanediol, 1, 10-decanediol, 1, 2-dodecanediol, 1, 12-dodecanediol, neopentyl glycol, 3-methylpentan-1, 5-diol, 2, 5-dimethyl-1,3-hexanediol, 2,2,4-trimethyl-1,3-pentanediol, 1,2-cyclohexanediol, 1,4-cyclohexanediol, 1,4-bis (hydroxymethyl) cyclohexane, hydroxypivalic acid neopentyl glycol monoester, 2,2-bis (4-hydroxyphenyl) propane, 2,2-bis [4- (2-hydroxypropyl) phenyl] propane, diethylene glycol, dipropylene glycol, triethylene glycol, tetraethylene glycol, tripropylene glycol, tetrapropylene glycol, 3-thio pentane-1, 5-diol, polyethylene glycols, polypropylene glycols and polytetrahydrofurans with molecular weights of 200 to 10,000, diols based on block copolymers of ethylene oxide or propylene xid or copolymers incorporating ethylene oxide and propylene oxide groups incorporated.

Suitable diols are also OH-terminated polyether homopolymers such as polyethylene glycol, polypropylene glycol and polybutylene glycol, binary copolymers such as ethylene glycol / propylene glycol and ethylene glycol / butylene glycol copolymers, straight-chain tertiary copolymers such as ternary ethylene glycol / propylene glycol / ethylene glycol, propylene glycol / Ethylene glycol / propylene glycol and ethylene glycol / butylene glycol / ethylene glycol copolymers.

Suitable diols are also OH-terminated polyether block copolymers such as binary

Block copolymers such as polyethylene glycol / polypropylene glycol and polyethylene glycol / polybutylene glycol, straight-chain, ternary block copolymers such as polyethylene glycol / polypropylene glycol / polyethylene glycol, polypropylene glycol / polyethylene glycol / Polypropylene glycol and polyethylene glycol / polybutylene glycol / polyethylene glycol terpolymers.

Particularly preferred polyhydric alcohols are those with 10 or fewer carbon atoms. Of these, preference is given to those alcohols which are in the liquid state at 25 ^° C. and 1013 mbar and have such low viscosity that they can be used as the sole dissolving and dispersing medium as part of the reaction mixture without the aid of a further liquid phase. Examples of such polyhydric alcohols are ethylene glycol, diethylene glycol, 1, 2-propanediol, 1, 3-propanediol, 1, 2-butanediol, 1, 3-butanediol, 1, 4-butanediol, 2,3-butanediol, pentanediol, hexanediol and octanediol wherein ethylene glycol (1, 2-ethanediol) and 1, 2-propanediol are particularly preferred.

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, e.g. n-butylamine, tert-butylamine, n-

Pentylamine, n-hexylamine, n-heptylamine, n-octylamine, n-dodecylamine, benzylamine,

Pyridine, 4-dimethylaminopyridine, thiols, e.g. 1-butanethiol, tert-butanethiol, 1

Pentanethiol, 1-hexanethiol, 1-heptanethiol, 1-octanethiol, tert-octanethiol, 1-dodecanethiol, benzylthiol, phosphonic acids, e.g. 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, e.g. Polyethylene glycols, polyvinylpyrrolidones, polyacrylates, polyvinyl ethers, polyvinyl acetates.

In a preferred embodiment, the solvent used is a polyol selected from the abovementioned group, which has no further solvents. If water is used as the oxygen source in step (A), a mixture of polyol with the appropriate amount of water can be used as the solvent.

In step (A) of the process according to the invention, the reaction mixture at a temperature T1 of 0 to 150 ⁰ C, preferably 15 to 125 ⁰ C, most preferably provided at room temperature or a temperature of 80 to 125 ⁰ C.

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 a temperature of 15 to 40 ^° C. If the provisional ferric compound is not dissolved at T1, it may be ground before 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). Is used in step (A) of the inventive method used precursor compound in the solvent at room temperature sparingly soluble, as step (A) in a particularly preferred further embodiment at a temperature T1 is performed of 80 to 125 ⁰ C.

In step (A) of the process according to the invention is approximate shape in a preferred execution the at least one metal salt in a concentration of 0.01 to 1 mol ^* I acting as a precursor compound ^"1, more preferably 0.05 to 0.5 mol ^{* I" 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 generally be carried out at any pressure at which the reaction mixture is liquid under the present temperatures. In a preferred embodiment, step (A) of the process according to the invention is carried out at a pressure of 1 mbar to 100 bar, more preferably 200 mbar to 50 bar, most preferably 0.8 bar to 30 bar.

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. Since the process according to the invention is a continuous process, 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 differences in density and / or concentration within the mixture In a preferred embodiment of the process according to the invention, in step (A) the abovementioned components are mixed with one another, for example at room temperature, and tempered to a temperature of 50 to 125 ⁰ C. This Tempering step within step (A) of the method 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 optional temperature control of the reaction mixture in step (A) of the process according to the invention can be carried out by all methods known to the person skilled in the art, 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. 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.

In a further preferred embodiment of the method according to the invention, two solutions are provided, one solution comprising the at least one precursor compound of the at least one metal oxide in a solvent containing at least one polyol and the second solution containing at least one oxygen source in the same or another Solvent contains. Both solutions are heated in this preferred embodiment, independently of each other to a temperature of preferably 50 to 125 ⁰ C, more preferably 80 to 125 ⁰ C. The skilled person is known to suitable methods and devices. 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, 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 process according to the invention comprises heating the reaction mixture provided in step (A) to a temperature T2 of 130 to 350 ^° C., 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 130 to 350 ^° C., preferably 130 to 220 ^° C., particularly preferably 130 to 200 ^° C.

The inventive method is characterized in that the provided in step (A) reaction mixture particularly fast on the reaction temperature in

Step (B) is heated so that nanoparticles are generated, which are characterized by a particularly small and particularly uniform particle size. In a preferred embodiment, the heating in step (B) takes place at a heating rate of at least 20 K ^* min ^-1 , more preferably at least 50 K ^* min ^-1 , very particularly preferably at least 100 K ^* min ^-1 .

In the method according to the invention, step (B) is preferably carried out such that the reaction mixture according to step (A) of the method according to the invention is provided in a corresponding container, and by means of a suitable pump, for example a diaphragm pump, rotary piston pump, rotary vane pump , Gear pump or HPLC pump in a suitable for continuous process reactor, such as a tubular reactor is conveyed. This tubular reactor is preferably heated to a certain distance by means of a heater to the present in step (B) temperature T2 of 130 to 350 ⁰ C. This heating can be carried out by all methods known to the person skilled in the art, heating by microwave radiation preferably takes place. When microwaves are used, it is preferred according to the invention for the tube reactor in which step (B) of the method according to the invention is preferably carried out to consist of a material which weakly or not interferes with the microwaves, ie with a penetration depth of> 100 cm , preferably> 500 cm, more preferably> 1000 cm, in each case 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.

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, so as to broaden the particle size distribution and to worsen the quality features attributable to a uniform residence time. avoid. Therefore, in a preferred embodiment, the reactor is designed to avoid partial stagnation of the reaction mixture flow and / or unfavorably uneven 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 according to step (B) of the process according to the invention is preferably <30 minutes, more preferably <15 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 a preferred embodiment, the heating in step (B) of the method according to the invention is carried out 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 ensure a uniform heating the reaction mixture and thus to obtain a uniform particle size distribution.

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 70 ^° 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 35 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) is also carried out in a tube reactor, for example a heat exchanger. It is also possible according to the invention that several heat exchangers are connected in series.

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) preferably takes place at a cooling rate of at least 20 K ^* min ^-1 , particularly preferably at least 50 K ^* min ^-1 , very particularly preferably 100 K ^* min ^-1 To obtain nanoparticles with a particularly uniform particle size distribution and smaller particle sizes than in a method in which is cooled at a lower cooling rate.

In step (C) of the process according to the invention, a suspension of the nanoparticles formed in step (B) from the at least one precursor compound and the at least one metal oxide in the solvent used in step (A) is cooled to a temperature T3 of 0 to 70 ^° C. ,

In a preferred embodiment of the method according to the invention, the

Nanoparticles 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 their surface, 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 can also be carried out 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 nanoparticles obtained by the continuous process according to the invention have an average particle size of about 10 to 100 nm, preferably 20 to 80 nm, in each case determined by dynamic light scattering (DLS) (on suspensions) and scanning electron microscopy (SEM) (on powders ).

Furthermore, the products produced by the process according to the invention are characterized

Nanoparticles 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. The process of the invention is further illustrated by the following examples.

EXAMPLES

example 1

Polyol-mediated production of nanoparticles from CeO _x with x = 1.5 to 2.0. Two diethylene glycol solutions are first prepared. Solution 1 contains 108.5 g (0.25 mol) of Ce (NO _{3) 3} ^* 6 H ₂ O (Fa. Aldrich) in 1000 ml of DEG. Solution 2 contains 90.6 g (0.5 mol) of [Me ₄ N] OH ^* 5 H ₂ O (Aldrich) in 400 ml of DEG. Solution 2 is placed in a glass reactor and heated to 120 ⁰ C. In the stirred solution 2 which is also heated to 120 ⁰ C Solution 1 is abruptly ben zugege- under nitrogen stream. This forms a white suspension in the glass reactor. Immediately after the addition of solution 1, a suspension stream of 50 ml / min is pumped out of the suspension obtained via a riser pipe by means of a gear pump (Gather, type d / GFK / LAB22-120PP) and into a microwave (Ethos 1800, Fa MLS) to 170 ⁰ C heated heat exchanger. The heat exchanger has a volume of 300 ml. The heat exchanger is preheated to the desired temperature with pure DEG before use. Thereafter, the suspension flows through a second and third heat exchanger in succession, in which the suspension is cooled to room temperature within one minute. The resulting product is centrifuged and washed three times by repeated centrifugation and resuspension with ethanol and then dried in air at 70 ⁰ C.

X-ray diffraction of the obtained powder shows exclusively the diffraction reflections of ceria. Dynamic light scattering (DLS) and scanning electron microscopy (SEM) provide a mean particle size of about 40 nm.

Example 2

Polyol-mediated presentation of nanoparticles from VO _x

1000 ml of DEG and 40 ml of deionized water are placed in a glass reactor. With constant stirring and nitrogen 48.8 g (0.2 mol) of VO (OiPr) ₃ (Aldrich) are added abruptly. The reaction solution is then (LAB22-120PP Fa. Gather, type d / GRP /) pumped via a riser pipe by means of gear pump with a flow rate of 50 ml / min and a means of microwaves (Ethos 1800, Fa. MLS), heated to 180 ⁰ C. Heat exchanger transferred. The heat exchanger has a volume of 300 ml. The heat exchanger is preheated to the desired temperature with pure DEG before use. Thereafter, the suspension flows through a second and third heat exchanger, in which the suspension within a Minute is cooled to room temperature. The result is a black precipitate of VO _x . The product is washed three times by repeated centrifugation and resuspension with ethanol and then dried in air at 70 ⁰ C. DLS and REM provide an average particle size of about 30 nm.

Example 3

The polyol-mediated production of nanoparticles from BiVO _x In a glass reactor, a solution of 19.4 g (0.04 mol) Bi (NOs) ₃ ^* 5 are presented H ₂ O (Fa. Aldrich) in 1000 ml of DEG. With constant stirring and nitrogen, 9.8 g (0.04 mol) of VO (OiPr) ₃ (Aldrich) and 4 ml of deionized H ₂ O are added. The reaction solution is then (LAB22-120PP Fa. Gather, type d / GRP /) pumped via a riser pipe by means of gear pump with a flow rate of 50 ml / min and a means of microwaves (Ethos 1800, Fa. MLS), heated to 135 ⁰ C. Heat exchanger transferred. The heat exchanger has a volume of 300 ml. The heat exchanger is preheated to the desired temperature with pure DEG before use. Thereafter, the suspension flows through a second and third heat exchanger in succession, in which the suspension is cooled to room temperature within one minute. The result is yellow BiVO _x . The resulting product is washed three times by repeated centrifugation and resuspension with ethanol and then dried in air.

DLS and REM provide an average particle size of about 50 nm.

Claims

claims 1. A continuous process for the preparation of nanoparticles containing 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 polyol at a temperature of from T1 0 to 150 ^° C, (B) heating the reaction mixture provided in step (A) to a temperature T2 of 130 to 350 ⁰ C, wherein the nanoparticles are obtained containing at least one metal oxide, and (C) cooling the reaction mixture from step (B) containing the nanoparticles to a temperature T3 of 0 to 70 ⁰ 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 the nanoparticles are functionalized in or after step (B) and / or in or after step (C). 5. The method according to any one of claims 1 to 4, characterized in that the heating in step (B) is effected by microwave radiation. 6. The method according to any one of claims 1 to 5, characterized in that the oxygen source is selected from water or water of crystallization of the precursor compounds used. 7. The method according to any one of claims 1 to 6, characterized in that the heating in step (B) at a heating rate of at least 20 K ^* min ^"1 takes place. 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 50 K ^* min ^"1 takes place. 9. The method according to any one of claims 1 to 8, characterized in that the Cooling in step (C) with a cooling 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 50 K ^* min ^"1 takes place.