EP2480500A1

EP2480500A1 - Process for surface modification of metal oxide particles

Info

Publication number: EP2480500A1
Application number: EP10745643A
Authority: EP
Inventors: Stipan Katusic; Jürgen Meyer; Peter Kress; Armin Wiegand
Original assignee: Evonik Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 2009-09-21
Filing date: 2010-08-26
Publication date: 2012-08-01
Also published as: WO2011032814A1; DE102009029640A1

Abstract

Process for producing surface-modified metal oxide particles, characterized in that a) in a first reaction step, a surface modifier is introduced into a gas stream comprising metal oxide particles, and the resulting reaction mixture is exposed to a temperature of 130 to 250° C for a minimum residence time in the range from 1 to 60 s, and b) in an immediately subsequent, second reaction step, the solid reaction products are removed from the flowing reaction mixture by means of a filter apparatus and the reaction mixture present at the filter is treated at a temperature of 110 to 230° C for a minimum residence time in the range from 1 min to 1 hour.

Description

Process for surface modification of metal oxide particles

The invention relates to a process for surface modification of metal oxide particles, and to the metal oxide particles obtainable by the process

themselves. The surface modification of metal oxide particles is an important process for altering the surface and hence the properties of particles.

In the simplest case, a surface modification can be performed by dispersing the unmodified powder in the presence of at least one organic compound which possesses at least one functional group which can react and/or interact with groups present on the surface of the powder particles in water and/or an organic solvent, and then optionally fully or partly removing the liquid phase.

Additionally known are processes with which especially pyrogenic metal oxide particles can be surface-modified. These processes proceed from metal chlorides which are evaporated and combusted with hydrogen and air to form metal oxide particles. In a next step, the metal oxide particles are sprayed with a surface-modifying reagent while mixing vigorously, and then heat-treated at a temperature of 100 to 400°C over a period of 1 to 6 hours. One variant of this process envisages mixing the metal oxide particles very substantially homogeneously with an organohalosilane while excluding oxygen, and heating the mixture together with small amounts of steam to temperatures of 200 to 800°C in an upright tubular oven in a continuous cocurrent process.

It was an object of the present invention to provide a process for surface modification of metal oxide particles, which is easy to perform and inexpensive compared to the processes known in the prior art. It has now been found that, surprisingly, the object is achieved by a process in which

a) in a first reaction step, a surface modifier is introduced into a gas stream comprising metal oxide particles, and the resulting reaction mixture is exposed to a temperature of 130 to 250°C, preferably of 150 to 230°C, more preferably of 170 to 210°C, for a minimum residence time in the range from 1 to 60 s, preferably 2 to 30 s, more preferably 5 to 10 s, and

in an immediately subsequent, second reaction step, the solid reaction products are removed from the flowing reaction mixture by means of a filter apparatus and the reaction mixture present at the filter is treated at a temperature of 1 10 to 230°C, preferably 130 to 180°C, more preferably 140 to 150°C, for a minimum residence time in the range from 1 min to 1 hour, preferably 10 to 40 min, more preferably 20 to 30 min. In the context of the present invention, minimum residence time shall be understood to mean that each particle is exposed to the conditions at least over this period.

In the context of the present invention, the mean residence time shall be determined by the quotient of the volume of the surface modification zone and the volume flow rate.

The metal oxide particles used may be subjected beforehand to a steam treatment at temperatures of 200 to 700°C. This may be advisable when impurities removable with steam, such as halides, are to be removed.

The metal oxide particles according to the invention are those which bear, on their surface, reactive groups which are suitable for entering into a covalent, ionic or coordinate bond with a surface modifier. In general, these groups are hydroxyl groups. The attachment of the surface modifier may comprise all or only some of the reactive groups available. The process according to the invention allows the surface modification of metal oxide particles irrespective of their structure. For instance, the particles may be present as isolated individual particles and/or as aggregates of joined individual particles. The metal oxide particles may be those comprising one, two or more than two metal

components. The particles comprising two or more metal components are mixed metal oxide particles wherein the particular proportions of the metal oxide components are not limited. In the context of the invention, coated or partly coated metal oxide particles are also understood to be mixed oxide particles. Mixed oxide particles are understood to mean those in which there is intimate mixing of the metal components at the atomic level.

The origin of the metal oxide particles is unimportant for the performance of the process according to the invention, provided that the metal oxide particles bear reactive groups at the surface thereof. For example, metal oxide particles obtainable by precipitation, sol-gel processes, hydrothermal synthesis or pyrogenic processes can be used. As explained later, it may be advantageous to use pyrogenic metal oxide particles. "Pyrogenic" is understood to mean hydrolysis, oxidation or a reaction sequence in which both types occur alongside one another, in which metal compounds are hydrolysed and/or oxidized in the gas phase, generally in a flame. The flame may be obtained, for example, by the reaction of hydrogen and oxygen. This firstly forms finely divided, nonporous primary particles, which become joined together later in the reaction to form aggregates. The BET surface area of these primary particles is between 5 and 600 m²/g. Pyrogenic metal oxide particles are very substantially free of inner pores and have hydroxyl groups on their surfaces.

The metal oxide particles are preferably selected from the group consisting of aluminium oxide, antimony oxide, cerium oxide, iron oxide, indium oxide, silicon dioxide, titanium dioxide, vanadium oxide, tungsten oxide, yttrium oxide, zinc oxide, tin oxide and zirconium dioxide. Silicon dioxide shall be considered to be a metal oxide in the context of the invention.

It has additionally been found that the object is also achieved by a process for producing surface-modified metal oxide particles, in which in a reactor which comprises, in each case successively, a mixing zone, a combustion zone, a cooling zone, a surface modification zone and a removal zone,

a) in the mixing zone, a stream comprising one or more hydrolysable and/or oxidizable metal compounds in the form of vapour or of an aerosol, one or more hydrogen-containing combustion gases and

an oxygen-containing gas is obtained, b) this gas is transferred into the combustion zone, ignited there and reacted at a mean residence time of 10 ms to 10 s, preferably 100 ms to 5 s, more preferably 1 s to 5 s,

c) optionally, steam is introduced to remove halide-containing impurities, d) then the stream of the reaction mixture is cooled in the cooling zone to temperatures of 150 to 400°C, preferably 200 to 240°C,

e) in the surface modification zone, a surface modifier is introduced into the stream of the reaction mixture, and the resulting reaction mixture is exposed to a temperature of 130 to 250°C, preferably of 150 to 230°C, more preferably of 170 to 210°C, for a minimum residence time in the range from 1 to 60 s, preferably 2 to 30 s, more preferably 5 to 10 s, and f) in an immediately downstream, second reaction step, the solid reaction products are removed from the flowing reaction mixture by means of a filter apparatus, and the reaction mixture present at the filter is treated at a temperature of 1 10 to 230°C, preferably 130 to 180°C, more preferably

140 to 150°C, for a minimum residence time in the range from 1 min to 1 hour, preferably 10 to 40 min, more preferably 20 to 30 min.

In this particular embodiment of the invention, the pyrogenic particles are directly surface-modified without further isolation. To this end, the gas stream comprising the metal oxide particles is obtained by thermal decomposition of at least one metal compound in the presence of one or more hydrolysing and/or oxidizing gases or vapours. In this case, the oxidizing and/or hydrolysing gases or vapours are preferably used in a stoichiometric excess based on the metal compound. The metal compound may preferably be present in vaporous form, liquid form or in the form of an aerosol.

The temperature needed for thermal decomposition can preferably be provided by a flame, preferably obtained by the ignition of a combustion gas with an oxygen-containing gas. Suitable oxygen-containing gases are in particular air and oxygen-enriched air. Suitable combustion gases are in particular hydrogen, methane, ethane, propane, butane, natural gas. Particular preference may be given to using a combination of air and hydrogen. The person skilled in the art is aware of different flame types suitable for performance of the process according to the invention, for example laminar or turbulent flames, premixed flames or diffusion flames, low-pressure or high- pressure flames, flames which spread below, at or above the speed of sound, pulsating or continuous flames, reducing or oxidizing flames, secondary flames, closed or open flames, flames from one or more burners, or a mixed form of the aforementioned flame types.

The metal component of the metal compound may preferably be selected from the group consisting of aluminium, antimony, cerium, iron, indium, silicon, titanium, vanadium, tungsten, yttrium, zinc, tin and zirconium. To prepare mixed metal oxides, two or more metal compounds with different metal components are correspondingly selected. The metal compounds used must be

hydrolysable and/or oxidizable.

The amounts of hydrogenous combustion gas and oxygen-containing gas are generally selected such that the metal compound can be very substantially quantitatively hydrolysed and/or oxidized to the metal oxide.

This embodiment is outlined in Figure 1 . In this figure,

A represents one or more metal compounds,

B represents a hydrogenous combustion gas,

C represents an oxygen-containing gas,

D represents the point of introduction of the surface modifier,

E represents the surface modification zone,

F represents the filter apparatus,

G represents the surface-modified metal oxide particles.

The filter apparatus used comprises filter types known to those skilled in the art. It may be advantageous to operate two or more filter apparatuses, optionally offset in time.

As already explained, it may be advantageous to use pyrogenic metal oxide particles in the process according to the invention. In a particular embodiment of the invention, the pyrogenic particles are directly surface-modified without further isolation. To this end, the gas stream comprising the metal oxide particles is obtained by thermal decomposition of at least one metal compound in the presence of one or more hydrolysing and/or oxidizing gases or vapours. In this case, the oxidizing and/or hydrolysing gases or vapours are preferably used in a stoichiometric excess based on the metal compound. The metal compound is generally present in vaporous form or in the form of an aerosol.

The temperature needed for thermal decomposition can preferably be provided by a flame, preferably obtained by the ignition of a combustion gas with an oxygen-containing gas. Suitable oxygen-containing gases are in particular air and oxygen-enriched air. Suitable combustion gases are in particular hydrogen, methane, ethane, propane, butane, natural gas. Particular preference may be given to using a combination of air and hydrogen.

The person skilled in the art is aware of different flame types suitable for performance of the process according to the invention, for example laminar or turbulent flames, premixed flames or diffusion flames, low-pressure or high- pressure flames, flames which spread below, at or above the speed of sound, pulsating or continuous flames, reducing or oxidizing flames, secondary flames, closed or open flames, flames from one or more burners, or a mixed form of the aforementioned flame types.

hydrolysable and/or oxidizable.

According to which surface modifier is used in which amount, it is possible to obtain predominantly hydrophobic or hydrophilic metal oxide particles. The surface modifier/metal oxide particle ratio in the process according to the invention is preferably selected such that the carbon content of the surface- modified metal oxide particles is 0.1 to 10% by weight. Particular preference may be given to a range of 0.5 to 5% by weight. The surface modifier may preferably be introduced in liquid or dissolved form. It is more preferably introduced in the form of fine droplets which are obtained by atomizing the surface modifier by means of a carrier gas. The mean diameter of the fine droplets is preferably less than 100 μιτι, more preferably 30-100 μιτι.

The surface modifier used in the process according to the invention has at least one functional group which can chemically react or interact with reactive groups present on the surface of the metal oxide particles to form bonds. The bonding may be in the form of chemical bonding, such as covalent, including

coordinate, bonds (complexes), or ionic bonds of the functional group with the surface groups of the particles, while examples of interactions include dipole- dipole interactions, polar interactions, hydrogen bonds and van der Waals interactions. Preference is given to the formation of a chemical bond. The surface modifier is preferably a substance whose boiling temperature is above the temperature in the first and/or second reaction step.

Preference is given to using, in the process according to the invention,

saturated or unsaturated alkyl- and arylmonocarboxylic acids having 1 to 20 carbon atoms, i.e. those having one CO₂H group, or polycarboxylic acids, i.e. those having more than one CO₂H group, and the

corresponding esters and acid halides of the aforementioned carboxylic acids;

amines of the general formula R_3-XNH_X,

where x = 0, 1 or 2 and R = alkyl or aryl;

- hydrolysable organosilanes;

disilazanes;

cyclic polysiloxanes and silicone oils.

Examples of preferred monocarboxylic acids are formic acid, acetic acid, propionic acid, butyric acid, pentanoic acid, hexanoic acid, lauric acid, stearic acid, palmitic acid or oleic acid, glycolic acid, lactic acid, malic acid, tartaric acid, citric acid, isocitric acid, mandelic acid, benzoic acid and pyromellitic acid. Examples of suitable amines are methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, aniline, N-methylaniline,

diphenylamine, triphenylamine, toluidine, ethylenediamine, diethylenetriamine.

Preferred hydrolysable silanes have the general formula R_xSiY_4-x (I)

in which x has the value of 1 , 2 or 3, and the R radicals are the same or different and are nonhydrolysable groups, the Y radicals are the same or different and are hydrolysable groups or hydroxyl groups.

In the general formula (I), the hydrolysable Y groups which may be the same or different from one another are, for example,

- hydrogen,

- halogen, for example F, CI, Br or I,

- alkoxy, preferably Ci-C6-alkoxy, such as methoxy, ethoxy,

n-propoxy, i-propoxy and butoxy,

- aryloxy, preferably C6-Cio-aryloxy, such as phenoxy,

- acyloxy, preferably Ci-C6-acyloxy, such as acetoxy or propionyloxy,

- alkylcarbonyl, preferably C2-C₇-alkylcarbonyl, such as acetyl.

Preferred hydrolysable radicals are halogen, alkoxy groups and acyloxy groups. Particularly preferred hydrolysable radicals are Ci-C₄-alkoxy groups, especially methoxy and ethoxy. The nonhydrolysable R radicals, which may be the same or different, are R radicals with or without a functional group.

The nonhydrolysable R radical without a functional group is, for example,

- alkyl, preferably Ci-Cs-alkyl, such as methyl, ethyl,

n-propyl, isopropyl, n-butyl, sec-butyl and tert-butyl, pentyl, hexyl, octyl or cyclohexyl;

substituted alkyl,

- alkenyl, preferably C2-C6-alkenyl, such as vinyl, 1 -propenyl,

2-propenyl and butenyl,

- alkynyl, preferably C2-C6-alkynyl, such as propargyl, - aryl, preferably C-6-Cio-aryl, such as phenyl and naphthyl, and

corresponding alkaryls such as tolyl, benzyl and phenethyl.

Preferred surface modifiers may especially be CH₃SiCl3, CH₃Si(OC2H₅)3, CH₃Si(OCH₃)₃, C₂H₅SiCI₃, C₂H₅Si(OC2H5)₃, C₂H₅Si(OCH₃)₃, C₃H₇Si(OC₂H5)₃, (C₂H₅O)₃SiC₃H₆CI, (CH₃)₂SiCI₂, (CH₃)₂Si(OC₂H₅)₂, (CH₃)₂Si(OH)₂,

C₆H₅Si(OCH₃)₃, C₆H₅Si(OC₂H5)₃, C₆H₅CH₂CH₂Si(OCH₃)₃, (C₆H₅)₂SiCI₂, (C₆H5)₂Si(OC₂H₅)₂, (i-C₃H₇)₃SiOH, CH₂=CHSi(OOCCH₃)₃, CH₂=CHSiCI₃, CH₂=CH-Si(OC₂H₅)₃, CH₂=CHSi(OC₂H₅)₃, CH₂=CH-Si(OC₂H₄OCH₃)₃,

CH₂=CH-CH₂-Si(OC₂H₅)₃, CH₂=CH-CH₂-Si(OC₂H₅)₃, CH₂=CH-CH₂- Si(OOCCH₃)₃, n-C₆Hi₃-CH₂-CH₂-Si(OC₂H₅)₃ and

n-C₈Hi₇-CH₂-CH₂-Si(OC₂H₅)₃.

A nonhydrolysable R radical with a functional group may comprise, for example, as a functional group, an epoxy (such as glycidyl or glycidyloxy), hydroxyl, ether, amino, monoalkylamino, dialkylamino, optionally substituted anilino, amide, carboxyl, acryloyi, acryloyloxy, methacryloyl, methacryloyloxy, mercapto, cyano, alkoxy, isocyanato, aldehyde, alkylcarbonyl, acid anhydride and phosphoric acid group.

Preferred examples of nonhydrolysable R radicals with functional groups are a

- glycidyl- or glycidyloxy-(Ci-C₂o)-alkylene radical, such as beta- glycidyloxyethyl, gamma-glycidyloxypropyl, delta-glycidyloxybutyl, epsilon- glycidyloxypentyl, omega-glycidyloxyhexyl and

2-(3,4-epoxycyclohexyl)ethyl,

- (meth)acryloyloxy-(Ci-C6)-alkylene radical, such as

(meth)acryloyloxymethyl, (meth)acryloyloxyethyl, (meth)acryloyloxypropyl or (meth)acryloyloxybutyl, and

- 3-isocyanatopropyl radical.

Specific surface modifiers which may be used include

gamma-glycidyloxypropyltrimethoxysilane, glycidyloxypropyltriethoxysilane, 3-isocyanatopropyltriethoxysilane, 3-isocyanatopropyldimethylchlorosilane, aminopropyltriethoxysilane, aminopropyltrimethoxysilane, aminomethyltriethoxysilane, aminomethyltrinnethoxysilane,

aminopropyltnchlorosilane, (N-cyclohexylanninonnethyl)triethoxysilane, 2-anninoethyl-3-anninopropyltrinnethoxysilane,

N-(n-butyl)-3-aminopropyltrinnethoxysilane,

2- aminoethyl-3-aminopropylmethyldimethoxysilane,

(3-aminopropyl)diethoxynnethylsilane, (3-aminopropyl)ethyldiethoxysilane, (3-methylaminopropyl)trimethoxysilane,

(aminoethylaminomethyl)phenethyltrimethoxysilane,

(N,N-diethyl-3-aminopropyl)thnnethoxysilane,

(N,N-dimethylannino)dinnethylchlorosilane,

(N,N-dimethylanninopropyl)thnnethoxysilane,

(N-acetylglycyl)-3-aminopropylthnnethoxysilane,

(N-cyclohexylaminomethyl)methyldiethoxysilane,

(N-cyclohexylanninonnethyl)triethoxysilane,

(N-phenylaminomethyl)methyldimethoxysilane,

(N-phenylanninonnethyl)trinnethoxysilane, 1 1 -aminoundecyltriethoxysilane, 3- (1 ,3-dinnethylbutylidene)anninopropyltriethoxysilane,

3- (1 -aminopropoxy)-3,3-dimethyl-1 -propenyltrimethoxysilane,

3-(2,4-dinitrophenylamino)propyltriethoxysilane,

3-(2-aminoethylamino)propylmethyldimethoxysilane,

3-(2-aminoethylamino)propyltrimethoxysilane,

3-(cyclohexylamino)propyltrimethoxysilane,

3-(aminophenoxy)propyltrimethoxysilane,

3-(N-allylamino)propyltrimethoxysilane,

3-(N-styrylmethyl-2-aminoethylamino)propyltrimethoxysilane,

3-(phenylamino)propyltrimethoxysilane, 3- aminopropyldiisopropylethoxysilane,

3-aminopropyldimethylethoxysilane,

3-aminopropylmethylbis(trimethylsiloxy)silane,

3-aminopropylmethyldiethoxysilane,

3-aminopropyltris(methoxyethoxyethoxy)silane, 3-aminopropyltris(trimethylsiloxy)silane, 4-aminobutyltriethoxysilane, aminophenyltrimethoxysilane, bis(2-hydroxyethyl)-3- aminopropyltriethoxysilane, diethylaminomethyltnethoxysilane, N,N- dimethylanninonnethylethoxysilane,

N-(2-aminoethyl)-3-aminoisobutyldimethylmethoxysilane,

N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane,

N-(2-aminoethyl)-3-aminopropyltriethoxysilane,

N-(2-aminoethyl)-3-aminopropyltrimethoxysilane,

N-(2-aminonnethyl)-1 1 -aminoundecyltrimethoxysilane,

N-(3-acryloyloxy-2-hydroxypropyl)-3-anninopropyltriethoxysilane,

N-(3-methacryloyloxy-2-hydroxypropyl)-3-anninopropyltriethoxysilane,

N-(6-aminohexyl)aminopropyltrimethoxysilane,

N-(hydroxyethyl)-N-methylaminopropyltrimethoxysilane,

N-3-[amino(polypropylenoxy)]aminopropyltrimethoxysilane,

n-butylaminopropyltrimethoxysilane, N- cyclohexylaminopropyltrinnethoxysilane,

N-ethylaminoisobutylmethyldiethoxysilane,

N-ethylaminoisobutyltrimethoxysilane,

N-methylaminopropylmethyldimethoxysilane,

N-methylaminopropyltrinnethoxysilane,

N-phenylaminomethyltriethoxysilane, phenylbis(dimethylannino)chlorosilane, tert-butylaminopropyltrimethoxysilane, aminopropylsilanetriol,

N-(2-aminoethyl)-3-aminopropylsilanetriol,

N-cyclohexylaminonnethyltnethoxysilane,

N-cyclohexylaminomethylmethyldiethoxysilane,

N-phenylaminomethyltrinnethoxysilane, 3- (nneth)acryloyloxypropyltriethoxysilane and 3- (meth)acryloyloxypropyltrinnethoxysilane.

In addition, it is also possible to use silylamines. Silylamines are understood to mean compounds which have at least one Si-N bond and can react with the Si-OH groups present on the surface of the silicon dioxide particles. Examples thereof are vinyldimethylsilylamine, octyldimethylsilylamine, phenyldimethylsilylamine, bis(dimethylanninodinnethylsilyl)ethane,

hexamethyldisilazane, (N,N-dimethylamino)trimethylsilane and

bis(thfluoropropyl)tetrannethyldisilazane. In addition, it is possible to use cyclic silazanes. Suitable surface modifiers also include the cyclic polysiloxanes D3, D4, D5 and the homologues thereof, where D3, D4 and D5 are understood to mean cyclic polysiloxanes having 3, 4 or 5 units of the -O-Si(CH₃)2 type, for example octamethylcyclotetrasiloxane = D4.

Also polysiloxanes or silicon oils of the

Y-O-[(RR'SiO)_m-(R"R"'SiO)n]u-Y type, where

m = 0,1 ,2,3,...∞, preferably 0,1 ,2,3,... 100 000,

n = 0,1 ,2,3,...∞, preferably 0,1 ,2,3,... 100 000,

u = 0,1 ,2,3,.. . .oo, preferably 0,1 ,2,3,... 100 000,

Y = CH₃, H, C_nH_2n+i , n=2-20; Si(CH₃)₃, Si(CH₃)₂H, Si(CH₃)₂OH,

Si(CH₃)₂(OCH₃), Si(CH₃)₂(C_nH_2n+i ), n=2-20

R, R', R", R'" are each independently alkyl such as C_nH_2n+i , n = 1 - 20; aryl such as phenyl radicals and substituted phenyl radicals, (CH₂)_n-NH₂, H.

Polysiloxanes or silicone oils are usually thermally activated for surface modification. Particular preference may be given to using citric acid, lauric acid, stearic acid and pyromellitic acid.

The process according to the invention allows an inexpensive production for surface modification of metal oxide particles. For instance, the apparatus expenditure is low, the reaction time is short and the conversion is high. It is particularly advantageous to link the production of the metal oxide particles by a pyrogenic process to immediately subsequent surface modification. This can be done, for example, by utilizing the heat which arises in the process of preparation of the metal oxide particles, and apparatus as would be required in any case in the production of the metal oxide particles, for example a filter apparatus, for surface modification. The reason why the reaction times are only very short in the process according to the invention even though the reaction takes place in the immobile filtercake, compared to known surface modification processes, is not known to date.

Examples Example 1

160 kg/h of TiCI₄ are evaporated in an evaporator at 140°C. The vapours are transferred into a mixing chamber by means of 15 m³ (STP)/h of nitrogen as a carrier gas with a carrier gas moisture content of 15 g/m³ of carrier gas.

Separately therefrom, 52 m³ (STP)/h of hydrogen and 525 m³ (STP)/h of primary air are introduced into the mixing chamber. In a central tube, the reaction mixture is fed to a burner and ignited. The flame burns into a water- cooled flame tube. In addition, 200 m³ (STP)/h of secondary air are introduced into the reaction chamber. The titanium dioxide particles which form are aftertreated with steam at temperatures of 500 to 600°C, and the process gas stream is cooled to 160°C. Subsequently, at this temperature, 2 kg/h of vaporous octyltrimethoxysilane are metered into the process gas stream.

Subsequently, the solid particles are removed at a filter, the temperature at the filter being 170°C.

The minimum residence time of the particles from the time of addition of the surface modifier up to the filter is 1 .2 s. In addition, the minimum residence time of the particles on the filter until the filter is cleaned is 8 min.

Surface-modified titanium dioxide particles with a carbon content of 1 .7% by weight are obtained.

Example 2

95 kg/h of silicon tetrachloride and 5 kg/h of trichlorosilane (TCS) are

evaporated and transferred by means of nitrogen into the mixing chamber of a burner. At the same time, 34 m³ (STP)/h (1 .5 kmol/h) of hydrogen and 70 m³ (STP)/h (3.1 kmol/h) of primary air are introduced into the mixing chamber. The mixture has a temperature of 90°C. It is ignited and combusted in a flame into a reaction chamber. In addition, 24 m³ (STP)/h (1 .1 kmol/h) of secondary air which surrounds the flame is introduced into the reaction chamber. The ratio of secondary air to primary air is 0.34.

The silicon dioxide particles which form are aftertreated with steam at temperatures of 500 to 600°C, and the process gas stream is cooled to 160°C. Subsequently, at this temperature, 2 kg/h of vaporous octyltrimethoxysilane are metered into the process gas stream. Subsequently, the solid particles are removed at a filter, the temperature at the filter being 170°C.

The minimum residence time of the particles from the time of addition of the surface modifier up to the filter is 1 .8 s. In addition, the minimum residence time of the particles on the filter until the filter is cleaned is 15 min.

Surface-modified silicon dioxide particles with a carbon content of 2.6% by weight are obtained.

Example 3

1200 g/h of a solution of cerium(lll) 2-ethylhexanoate (49% by weight) in 2- ethylhexanoic acid (51 % by weight) are atomized into a reaction chamber by means of air (5 m³ (STP)/h) via a nozzle with a diameter of 0.8 mm. A hydrogen/oxygen gas flame composed of hydrogen (10 m³ (STP)/h) and primary air (10 m³ (STP)/h) burns here, in which the aerosol is reacted. In addition, 20 m³/h of secondary air are introduced into the reaction chamber. The process gas stream is cooled to 185°C with quench air.

Subsequently, at this temperature, 200 g/h of an alcoholic lauric acid solution (10% by weight in ethanol) are metered into the process gas stream.

Subsequently, the solid particles are removed at a filter, the temperature at the filter being 165°C.

The minimum residence time of the particles from the time of addition of the surface modifier up to the filter is 1 .2 s. In addition, the minimum residence time of the particles on the filter until the filter is cleaned is 9 min.

Surface-modified cerium oxide particles with a carbon content of 7.8% by weight are obtained.

Example 4

900 g/h of a solution consisting of 76.20% by weight of dibutyltin diacetate,

2.77% by weight of antimony(lll) acetate, 16.03% by weight of acetic anhydride and 5.00% by weight of acetic acid are atomized with 4.0 m³ (STP)/h of atomizer air by means of a two-substance nozzle. The resulting droplets have a droplet size spectrum d3o from 5 to 15 μιτι. The droplets are combusted into a reaction chamber in a flame formed from hydrogen (2.5 m³ (STP)/h) and air (24.0 m³ (STP)/h), corresponding to a lambda of 4.1 .

The antimony-tin mixed oxide particles which form are aftertreated with steam at temperatures of 500 to 600°C, and the process gas stream is cooled to 190°C. Subsequently, at this temperature, 200 g/h of an alcoholic lauric acid solution (10% by weight in ethanol) are metered into the process gas stream. Subsequently, the solid particles are removed at a filter, the temperature at the filter being 200°C.

The minimum residence time of the particles from the time of addition of the surface modifier up to the filter is 1 .5 s. In addition, the minimum residence time of the particles on the filter until the filter is cleaned is 8 min.

Surface-modified antimony-tin mixed oxide particles with a carbon content of 5.6% by weight are obtained.

Example 5

3.86 kg/h of TiCI₄ and 0.332 kg/h of SiCI₄ are evaporated at approx. 200°C. The vapours are mixed by means of nitrogen together with 1 .45 m³ (STP)/h of hydrogen and 7.8 m³ (STP)/h of dried air in the mixing chamber of a burner of known design, and supplied via a central tube, at the end of which the reaction mixture is ignited, to a water-cooled flame tube and combusted there. In addition, an outer tube concentrically surrounding the central tube is used to supply 0.9 m³ (STP)/h of hydrogen and 25 m³ (STP)/h of air to the flame tube. The silicon-titanium mixed oxide particles which form are aftertreated with steam at temperatures of 500 to 600°C, and the process gas stream is cooled to 200°C. Subsequently, at this temperature, 200 g/h of alcoholic pyromellitic acid (10% by weight in ethanol) are metered into the process gas stream.

Subsequently, the solid particles are removed at a filter, the temperature at the filter being 200°C.

The minimum residence time of the particles from the time of addition of the surface modifier up to the filter is 10.3 s. In addition, the minimum residence time of the particles on the filter until the filter is cleaned is 17 min.

Surface-modified silicon titanium mixed oxide particles with a carbon content of 0.5% by weight are obtained.

Example 6

9 kg/h of zinc is evaporated at approx. 800°C. The vapour is introduced into a reactor via a central tube by means of 3.5 m³ (STP)/h of nitrogen at 600°C, and oxidized with 15 m³ (STP)/h of air. The addition of 40 m³ (STP)/h of quench air cools the process gas stream to 210°C.

Subsequently, at this temperature, 1000 g/h of alcoholic pyromellitic acid (10% by weight in ethanol) are metered into the process gas stream. Subsequently, the solid particles are removed at a filter, the temperature at the filter being 200°C.

The minimum residence time of the particles from the time of addition of the surface modifier up to the filter is 1 .1 s. In addition, the minimum residence time of the particles on the filter until the filter is cleaned is 5 min.

Surface-modified silicon-titanium mixed oxide particles with a carbon content of 0.3% by weight are obtained.

Claims

Process for producing surface-modified metal oxide particles,

characterized in that

a) in a first reaction step, a surface modifier is introduced into a gas stream comprising metal oxide particles, and the resulting reaction mixture is exposed to a temperature of 130 to 250°C for a minimum residence time in the range from 1 to 60 s, and

b) in an immediately subsequent, second reaction step, the solid

reaction products are removed from the flowing reaction mixture by means of a filter apparatus and the reaction mixture present at the filter is treated at a temperature of 1 10 to 230°C for a minimum residence time in the range from 1 min to 1 hour.

Process for producing surface-modified metal oxide particles,

characterized in that

the metal oxide particles are selected from the group consisting of aluminium oxide, antimony oxide, cerium oxide, iron oxide, indium oxide, silicon dioxide, titanium dioxide, vanadium oxide, tungsten oxide, yttrium oxide, zinc oxide, tin oxide and zirconium dioxide.

Process for producing surface-modified metal oxide particles,

characterized in that

in a reactor which comprises, in each case successively, a mixing zone, a combustion zone, a cooling zone, a surface modification zone and a removal zone,

a) in the mixing zone, a stream comprising one or more hydrolysable and/or oxidizable metal compounds in the form of vapour or of an aerosol,

one or more hydrogen-containing combustion gases and an oxygen-containing gas is obtained,

b) this gas is transferred into the combustion zone, ignited there and reacted at a mean residence time of 10 ms to 10 s, c) optionally, steam is introduced to remove halide-containing impurities,

d) then the stream of the reaction mixture is cooled in the cooling zone to temperatures of 400 to 150°C,

e) in the surface modification zone, a surface modifier is introduced into the stream of the reaction mixture, and the resulting reaction mixture is exposed to a temperature of 130 to 250°C for a minimum residence time in the range from 1 to 60 s, and f) in an immediately downstream, second reaction step, the solid reaction products are removed from the flowing reaction mixture by means of a filter apparatus, and the reaction mixture present at the filter is treated at a temperature of 1 10 to 230°C for a minimum residence time in the range from 1 min to 1 hour.

Process according to Claims 1 to 3,

characterized in that

the metal component of the metal compound is selected from the group consisting of aluminium, cerium, iron, indium, silicon, titanium,

vanadium, tungsten, yttrium, zinc, tin and zirconium.

Process according to Claims 1 to 4,

characterized in that

two or more filter apparatuses are used.

Process according to Claims 1 to 5,

characterized in that

the surface modifier/metal oxide particle ratio is selected such that the carbon content of the surface-modified metal oxide particles is 0.1 to 10% by weight.

Process according to Claims 1 to 6,

characterized in that

the surface modifier is selected from the group consisting of saturated or unsaturated alkyl- and arylmonocarboxylic acids having 1 to 20 carbon atoms, i.e. those having one CO₂H group, or polycarboxylic acids, i.e. those having more than one CO₂H group, and the corresponding esters and acid halides of the aforementioned carboxylic acids; amines of the general formula R_3-XNH_X, where x = 0, 1 or 2 and R = alkyl or aryl;

hydrolysable organosilanes; disilazanes; and cyclic polysiloxanes and silicone oils.