Indian Patents. 248248:GRANULES BASED ON PYROGENICALLY PREPARED SILICON DIOXIDE AND METHOD FOR THEIR PREPARATION

Title of Invention	GRANULES BASED ON PYROGENICALLY PREPARED SILICON DIOXIDE AND METHOD FOR THEIR PREPARATION
Abstract	Granules based on pyrogenically prepared silicon dioxide and having the following physicochemical properties: Average grain size: 10 to 120 ?m; BET surface area: 40 to 400 m2/g; Pore volume: 0.5 to 2.5 ml/g; Pore size distribution: less than 5 % of the total pore volume exists of pores with a diameter < 5 nm, rest meso- and macropores pH value: 3.6 to 8.5; Tapped density: 220 to 700 g/l.

Full Text	Granules based on pyrogenically prepared silicon dioxide, method for their preparation and use thereof The invention relates to granules based on pyrogenically prepared silicon dioxide, the method for their preparation and the use thereof as catalyst supports and for glass applications. The preparation of pyrogenic silicas or silicon dioxides from SiC14 by means of high-temperature-or flame hydrolysis is known (Ullmanns Enzyklopadie der technischen Chemie, 4th Edition, Volume 21, page 464 (1982)). Pyrogenic silicon dioxides are distinguished by having extremely fine particles, high specific surface (BET), very high purity, spherical particle shape and the absence of pores. On account of these properties pyrogenically prepared silicon dioxides are attracting increasing interest as supports for catalysts (Dr. Koth et al., Chem. Ing. Techn. 52, 628 (1980)). For this application the pyrogenically prepared silicon dioxide is shaped by mechanical means, for example, tabletting machines. The shaping of pyrogenically prepared silicon dioxide into sprayed granules also by means of spray drying, to obtain a starting material for sintered ceramic materials, is also known (DE-A 36 11 449). It is also known that silicon dioxide pyrogenically prepared in an electric arc may be shaped by means of spray drying into sprayed granules, which can be used as adsorption media or else as catalyst supports (DE-A 12 09 108). The subjection of pyrogenically prepared silicon dioxide to a gel process and the subsequent shaping into granules by means of spray drying is also known. These granules, after coating with chromium oxide, are used in the polymerisation of ethylene (EP-A 0 050 902, US-A 4,386,016). Furthermore, the use of precipitated silicon dioxide as a catalyst support for the catalytic polymerisation of olefins is known (WO 91/09881) . The known sprayed granules of pyrogenically prepared silicon dioxides have the disadvantage that they are not optimally suitable for use as catalyst supports, for example, in the production of polyethylene. The object was therefore the development from pyrogenically prepared silicon dioxide of sprayed- granules which can be used as catalyst supports in the production of polyethylene. The present invention provides granules based on pyrogenically prepared silicon dioxide and having the following physicochemical properties: Average grain size: 10 to 12 0 urn BET surface area: 40 to 400 m2/g Pore volume: 0.5 to 2.5 ml/g Pore size distribution: less than 5 % of the total pore volume exists of pores with a diameter meso- and macropores pH value: 3.6 to 8.5 Tapped density: 220 to 700 g/1 The granular material according to the invention can be prepared by dispersing pyrogenically prepared silicon dioxide in water, spray drying it and heating the granules obtained at a temperature of from 150 to 1,100 °C for a period of 1 to 8 h. The invention also provides granules based on pyrogenically prepared silicon dioxide and having the following physicochemical properties: Average grain size: 10 to .120 um BET surface area: 40 to 400 m2/g Pore volume: 0.5 to 2.5 ml/g Pore size distribution: less than 5 % of the total pore volume exists of pores with a diameter meso- and macropores pH value: 3.6 to 8 . 5 Tapped density: 220 to 700 g/1 The granular material according to the invention can be prepared by dispersing pyrogenically prepared silicon dioxide in water, spray drying it and silanising the granules obtained. Halosilanes, alkoxysilanes, silazanes and/or siloxanes can be used for the silanisation. The following substances in particular can be used as halosilanes: a) Organosilanes of the types (RO)3Si(CnH2n+1) and (RO)3Si(CnH2n-i) R = Alkyl, as for example Methyl-, Ethyl-, n-Propyl-, i-Propyl-, Butyl- n = 1 - 20 b) Organosilanes of the types R'x (RO) ySi (CnH2n+1) and R'x(RO)ySi(CnH2„-i) R = Alkyl, as for example Methyl-, Ethyl-, n-Propyl-, i-Propyl-, Butyl- R" = Alkyl, as for example Methyl-, Ethyl-, n-Propyl-, i-Propyl-, Butyl- R'=Cycloalkyl n =1-20 x+y = 3 x =1,2 y = 1,2 c) Halogenorganosilanes of the types X3Si (CnH2n+1) and X3Si(CnH2n-i) X = CI, Br n = 1 - 20 d) Halogenorganosilanes of the types X2(R')Si (CnH2n+1) and X2(R')Si(CnH2n-l) X = CI, Br R' = Alkyl, as for example Methyl-, Ethyl-, n-Propyl-, i-Propyl-, Butyl- R'=Cycloalkyl n =1-20 e) Halogenorganosilanes of the types X (R1 ) 2Si (CnH2n+1) and X(R')2Si(CnH2n_1) X = CI, Br R' = Alkyl, as for example Methyl-, Ethyl-, n-Propyl-, i-Propyl-, Butyl- R'=Cycloalkyl n =1-20 f) Organosilanes of the types (RO) 3Si (CH2)m-R' R = Alkyl, as Methyl-, Ethyl-, Propyl- m = 0,1 - 20 R' = Methyl-, Aryl (as for example -C6H5, substituted phenyl groups) -C4F9, OCF2-CHF-CF3, -C6F13, -0-CF2-CHF2 -NH2, -N3, -SCN, -CH=CH2, -NH-CH2-CH2-NH2, -N- (CH2-CH2-NH2)2 -OOC(CH3)C = CH2 -OCH2-CH(0)CH2 -NH-CO-N-CO-(CH2)5 -NH-COO-CH3, -NH-COO-CH2-CH3, -NH-(CH2)3Si(OR)3 -Sx-(CH2)3Si(OR)3 -SH -NR^^R'1' (R' = Alkyl, Aryl; R' ' = H, Alkyl, Aryl; R1' = H, Alkyl, Aryl, Benzyl, C2H4NR'' R''«' mit R'' = H, Alkyl und R'"" = H, Alkyl) g) Organosilanes of the types (R")x(RO)ySi(CH2)m-R' R" = Alkyl x+y = 3 = Cycloalkyl x =1,2 y = 1,2 m =0,1 bis 2 0 R' = Methyl-, Aryl (as for example -C6H5 ,substituted phenyl groups) -C4F9, -OCF2-CHF-CF3, -C6F13, -0-CF2-CHF2 -NH2, -N3, -SCN, -CH=CH2, -NH-CH2-CH2-NH2, -N-(CH2-CH2-NH2)2 -OOC(CH3)C = CH2 -OCH2-CH(0)CH2 -NH-C0-N-C0-(CH2)5 -NH-C00-CH3, -NH-COO-CH2-CH3, -NH-(CH2)3Si(OR)3 -Sx-(CH2)3Si(OR)3 -SH - NR^'R'1 (R' = Alkyl, Aryl; R' = H, Alkyl, Aryl; R'' = H, Alkyl, Aryl, Benzyl, C2H4NR' ' ' ' R* ' ' ' ' with R',,! = H, Alkyl and Ri i i » i = H, Alkyl) h) Halogenorganosilanes of the types X3Si(CH2)m-R' X = CI, Br m = 0,1 - 20 R' = Methyl-, Aryl (for example -C6H5, substituted Phenyl groups) -C4F9, -OCF2-CHF-CF3, -C6F13, -0-CF2-CHF2 -NH2, -N3, -SCN, -CH=CH2, -NH-CH2-CH2-NH2 -N-(CH2-CH2-NH2)2 -OOC(CH3)C = CH2 -OCH2-CH(0)CH2 -NH-CO-N-CO- (CH2)5 -NH-COO-CH3, -NH-COO-CH2-CH3, -NH-(CH2)3Si(OR)3 -Sx-(CH2)3Si(OR)3 -SH i) Halogenorganosilanes of the types (R) X2Si(CH2)m-R' X = CI, Br R = Alkyl, as Methyl,- Ethyl-, Propyl- m =0,1-20 R' = Methyl-, Aryl (z.B. -C6H5, substituted Phenyl groups) -C4F9, -OCF2-CHF-CF3, -C6F13, -0-CF2-CHF2 -NH2, -N3, -SCN, -CH=CH2,-NH-CH2-CH2-NH2, -N-(CH2-CH2-NH2)2 -OOC(CH3)C = CH2 -OCH2-CH(0)CH2 -NH-CO-N-CO- (CH2)5 -NH-COO-CH3, -NH-COO-CH2-CH3, -NH-(CH2)3Si(OR)3, whereby R may be Methyl-, Ethyl-, Propyl-, Butyl- -Sx-(CH2)3Si(OR)3, whereby R may be Methyl-, Ethyl-, Propyl-, Butyl- -SH ! ) Halogenorganosilanes of the types (R) 2X Si (CH2)m-R' X = CI, Br R = Alkyl m = 0,1 - 20 R' = Methyl-, Aryl (z.B. -C6H5, substituted Phenyl groups) -C4F9, -OCF2-CHF-CF3, -C6F13, -0-CF2-CHF2 -NH2, -N3, -SCN, -CH=CH2, -NH-CH2-CH2-NH2, -N-(CH2-CH2-NH2)2 -OOC(CH3)C = CH2 -OCH2-CH(0)CH2 -NH-CO-N-CO- (CH2)5 -NH-COO-CH3, -NH-COO-CH2-CH3, -NH-(CH2)3Si(OR)3 -Sx-(CH2)3Si(OR)3 -SH ) Silazane of the type R'R2Si-N-SiR2R" H R = Alkyl, Vinyl, Aryl R' = Alkyl, Vinyl, Aryl ) Cyclic polysiloxanes of the types D 3, D 4, D 5, whereby D 3, D 4 and D 5 cyclic may be polysiloxanes with 3,4 or 5 units of the types -0-Si(CH3)2- i.e. octamethylcyclotetrasiloxane = D 4 m) Polysiloxanes respectively silicon oil of the type m = 0,1,2,3,...00 n = 0,1,2,3,...°° u = 0,1,2,3, . . .co Y=CH3, H, CnH2n+1 n=l-20 Y=Si(CH3)3, Si(CH3)2H Si(CH3)2OH, Si(CH3)2(OCH3) Si(CH3)2(CnH2n+1) n=l-20 R = Alkyl, as CnH2n+1, whereby n = 1 to 20 is, Aryl, as Phenyl- and substituted phenyl groups, (CH2)n-NH2, H R' = Alkyl, as CnH2n+1, whereby n = 1 to 20 is, Aryl, as phenyl- and substituted phenyl groups, (CH2)n-NH2, H R" = Alkyl, as CnH2n+1, whereby n =- 1 to 20 is, Aryl, as phenyl- and substituted phenyl groups, (CH2)n-NH2, H R'" = Alkyl, as CnH2n+1, whereby n = 1 to 20 is, Aryl, as phenyl- and substituted phenyl groups, (CH2)n-NH2, H Preferably the silane Si 108 [(CH30) 3-Si-C8Hn], trimethoxyoctylsilane, is used as silanising agent. The invention also provides granules based on pyrogenically prepared silicon dioxide and having the following physicochemical properties: Average grain size: 10 to 120 urn BET surface area: 40 to 400 m2/g Pore volume: 0.5 to 2.5 ml/g Pore size distribution: less than 5 % of the total pore volume exists of pores with a diameter meso- and macropores Carbon content: 0.3 to 15.0 wt.% pH value: 3.6 to 8 . 5 Tapped density: 220 to 700 g/1 The granular material according to the invention preferably has rneso- and macropores, with the volume of the mesopores constituting 10 to 80% of the total pore volume. The carbon content of the granular material according to the invention may be from 0.3 to 15.0 wt.-%. The particle size distribution of the granular material according to the invention may be 80 vo.1.% larger than 8 um and 80 vol.% smaller than 96 um. In a preferred embodiment of the invention, the proportion of pores smaller than 5 um may be at most 5% referred to the total pore volume. The granular material according to the invention can be prepared by dispersing pyrogenically prepared silicon dioxide in water, spray drying it, heating the granules obtained at a temperature of from 150 to 1,100 °C for a period of 1 to 8 h and then silanising them. The same halosilanes, alkoxysilanes, silazanes and/or siloxanes described above can be used for the silanisation. The invention further provides a method for the preparation of granules based on pyrogenically prepared silicon dioxide, which is characterised in that pyrogenically prepared silicon dioxide, preferably silicon dioxide prepared from silicon tetrachloride by means of flame hydrolysis, is dispersed in water, spray dried, the granules obtained are optionally heated at a temperature of from 150 to 1,100 °C for a period of 1 to 8 h and/or silanised. The dispersion in water can have a silicon dioxide concentration of from 5 to 25 wt.-%. The spray drying can be carried oul di d. Lumperature of from 200 to 600 °C Disc atomisers or nozzle atomisers can be used for this purpose. The heating of the granules can be carried out both in fixed beds, for example chamber kilns, and in moving beds, for example rotary dryers. The silanisation can be carried out using the same halosilanes, alkoxysilanes, silazanes and/or siloxanes as described above, for which the silanising agent can be optionally dissolved in an organic solvent such as, for example, ethanol. Preferably the silane Si 108 [ (CH30) 3-Si-C8H17] , trimethoxyoctylsilane, is used as silanising agent. The silanisation can be carried out by spraying the granular material with the silanising agent and subsequently heat-treating the mixture at a temperature of from 105 to 400 °C over a period of 1 to 6 h. In an alternative method, the silanisation of the granules can be carried out by treating the granular material with the silanising agent in vapour form and subsequently heat- treating the mixture at a temperature of from 200 to 800 °C over a period of 0.5 to 6 h. The heat treatment can take place under protective gas such as, for example, nitrogen. The silanisation can be carried out continuously or batchwise in heatable mixers and dryers equipped with spraying facilities. Examples of suitable devices are ploughshare mixers, disk dryers or fluidised bed dryers. The physicochemical variables of the granules, such as the specific surface, the particle size distribution, the pore volume, the tamped density and the silanol group concentration, the pore distribution and pH value can be altered within the specified limits by varying the starting materials and the conditions during spraying, heating and silanisation. The metal oxide or metalloid oxide to be used according to the invention can be pyrogenically produced silicon dioxide powder with a BET surface area of 30 to 90 m2/g, a DBP index of 80 or less a mean aggregate area of less than 25000 nm2, a mean aggregate circumference of less than 1000 nm, wherein at least 70% of the aggregates have a circumference of less than 1300 nm. This pyrogenically produced silicon dioxide is disclosed in WO 2004/054929. The BET surface area may preferably be between 35 and 75 m2/g. Particularly preferably the values may be between 4 0 and 60 m2/g. The BET surface area is determined in accordance with DIN 66131. The DBP index may preferably be between 60 and 80. During DBP absorption, the take-up of force, or the torque (in Nm), of the rotating blades in the DBP measuring equipment is measured while defined amounts of DBP are added, comparable to a titration. A sharply defined maximum, followed by a drop, at a specific added amount of DBP is then produced for the powder according to the invention. A silicon dioxide powder with a BET surface area of 40 to 60 m2/g and a DBP index of 60 to 80 may be particularly preferred. Furthermore, the silicon dioxide powder to be used according to the invention may preferably have a mean aggregate area of at most 20000 rim2. Particularly preferably, the mean aggregate area may be between 15000 and 20000 nm2. The aggregate area can be determined, for example, by image analysis of TEM images. An aggregate is understood to consist of primary particles of similar structure and size which have intergrown with each other, the surface area of which is less than the sum of the individual isolated primary particles. Primary particles are understood to be the particles which are initially formed in the reaction and which can grow together to form aggregates as the reaction proceeds further. A silicon dioxide powder with a BET surface area of 40 to 60 m2/g, a DBP index of 60 to 80 and a mean aggregate area between 15000 and 20000 nm2 may be particularly preferred. In a preferred embodiment, the silicon dioxide powder to be used according to the invention may have a mean aggregate circumference of less than 1000 nm. Particularly preferably, the mean aggregate circumference may be between 600 and 1000 nm. The aggregate circumference can also be determined by image analysis of TEM-images. A silicon dioxide powder with a BET surface area of 40 to 60 m2/g, a DBP index of 60 to 80, a mean aggregate area between 15000 and 20000 nm2 and a mean aggregate circumference between 600 and 1000 nm may be particularly preferred. Furthermore, it may be preferable for at least 80%, particularly preferably at least 90%, of the aggregates to have a circumference of less than 1300 nm. In a preferred embodiment, the silicon dioxide powder to be used according to the invention may_ assume a degree of filling in an aqueous dispersion of up to 90 wt.%. The range between 60 and 80 wt.% may be particularly preferred. Determination of the maximum degree of filling in an aqueous dispersion is performed by the incorporation of powder, in portions, into water using a dissolver, without the addition of other additives. The maximum degree of filling is achieved when either no further powder is taken up into the dispersion, despite elevated stirring power, i.e. the powder remains in dry form on the surface of the dispersion, or the dispersion becomes solid or the dispersion starts to form lumps. Furthermore, the silicon dioxide powder to be used according to the invention may have a viscosity at a temperature of 23°C, with respect to a 30 wt.% aqueous dispersion at a rate of shear of 5 rpm, of less than 100 mPas. In particularly preferred embodiments, the viscosity may be less than 50 mPas. The pH of the silicon dioxide powder to be used according to the invention may be between 3.8 and 5, measured in a 4% aqueous dispersion. The process for preparing the silicon dioxide powder to be used according to the invention, is characterised in that at least one silicon compound in the vapour form, a free- oxygen-containing gas and a combustible gas are mixed in a burner of known construction, this gas mixture is ignited at the mouth of the burner and is burnt in the flame tube of the burner, the solid obtained is separated from the gas mixture and optionally purified, wherein the oxygen content of the free-oxygen-containing gas is adjusted so that the lambda value is greater than or equal to 1, the gamma-value is between 1.2 and 1.8, the throughput is between 0.1 and 0.3 kg SiC>2/m3 of core gas mixture, the mean normalised rate of flow of gas in the flame tube at the level of the mouth of the burner is at least 5 m/s . The oxygen content of the free-oxygen-containing gas may correspond to that of air. That is, in this case air is used as a free-oxygen-containing gas. The oxygen content may, however also take on higher values. In a preferred manner, air enriched with oxygen should have an oxygen content of not more than 4 0 vol.%. Lambda describes the ratio of oxygen supplied in the core to the stoichiometrically required amount of oxygen. In a preferred embodiment, lambda lies within the range 1 Gamma describes the ratio of hydrogen supplied in the core to the stoichiometrically required amount of hydrogen. In a preferred embodiment, gamma lies within the range 1.6 The normalised rate of flow of gas refers to the rate of flow at 273 K and 1 atm. A burner of known construction is understood to be a burner with concentric tubes. The core gases are passed through the inner tube, the core. At the end of the tube, the mouth of the burner, the gases are ignited. The inner tube is surrounded by at least one other tube, the sleeve. The reaction chamber, called the flame tube, starts at the level of the mouth of the burner. This is generally a conical tube, cooled with water, which may optionally be supplied with other gases (sleeve gases) such as hydrogen or air. The mean, normalised rate of flow of the gas in the flame tube at the level of the mouth of the burner of at least 5 m/s refers to the rate of flow immediately after the reaction mixture leaves the burner. The rate of flow is determined by means of the volume flow of the reaction products in vapour form and the geometry of the flame tube. The core gases are understood to be the gases and vapours supplied to the burner, that is the free-oxygen-containing gas, generally air or air enriched with oxygen, the combustible gas, generally hydrogen, methane or natural gas, and the silicon compound or compounds in vapour form. An essential feature of the process is that the mean normalised rate of flow of gas in the flame tube at the level of the mouth of the burner is at least 5 m/s. In a preferred embodiment, the mean normalised rate of flow of the gas in the flame tube at the level of the mouth of the burner assumes values of more than 8 m/s. The mean rate of discharge of the gas mixture (feedstocks) at the mouth of the burner is not limited. However, it has proven to be advantageous when the rate of discharge at the mouth of the burner is at least 30 m/s. In a preferred embodiment, additional air (secondary air) may be introduced into the reaction chamber, wherein the rate of flow in the reaction chamber may be raised further. In a preferred embodiment, the mean normalised rate of flow of gas in the flame tube at the level of the mouth of the burner may be 8 to 12 m/s. The type of silicon compound used in the process is not further restricted. Silicon tetrachloride and/or at least one organochlorosilicon compound may preferably be used. A particularly preferred embodiment of the process is one in which silicon tetrachloride is used, the lambda value is such that 1 the gamma-value is between 1.6 and 1.8, - the throughput is between 0.1 and 0.3 kg Si02/m of core gas mixture, in addition at least double the amount of air, with respect to the amount of free-oxygen-containing gas introduced into the burner, is introduced into the flame tube and the rate of flow of the gas of feedstocks at the mouth of the burner is 40 to 65 m/s (with respect to standard conditions) and the mean normalised rate of flow of gas in the flame tube at the level of the mouth of the burner is between 8 and 12 m/s. In general during the preparation of pyrogenic oxides, the rate of flow of gas in the water-cooled reaction chamber (flame tube) and in the subsequent cooling unit (cooling stretch) is adjusted in such a way that the best possible cooling power, that is to say rapid cooling of the reaction products, is ensured. In principle,"it is true that the cooling power increases with decreasing rate of flow of gas. The lower limit is simply based on the requirement of still being able to transport the product through the pipes with the gas stream. It was demonstrated that although a considerable increase in the rate of flow of gas in the reaction chamber resulted in a reduced cooling power, it led to a powder with unexpected properties. Whereas physical characteristics such as BET surface area and DBP absorption are substantially unchanged as compared with powders according to the prior art, the powder exhibits a much lower structure. Furtheron the metal oxide or metalloid oxide to be used according to the invention can be pyrogenically produced silicon dioxide which is characterised by a metals content of less than 9 ppm. In a preferred embodiment the high-purity pyrogenically prepared silicon dioxide to be used according to the invention, can be characterised by the following metal contents: The total metal content can then be 3252 ppb (~ 3.2 ppm) or less. In an embodiment of the invention, which is further preferred, the high-purity pyrogenically prepared silicon dioxide can be characterised by the* following metal contents: The total metal content can then be 1033 ppb (~ 1.03 ppm) or less. The process for the preparation of the high-purity pyrogenically prepared silicon dioxide is characterised in that silicon tetrachloride is in known manner reacted in a flame by means of high-temperature hydrolysis to give silicon dioxide, and a silicon tetrachloride is used here which has a metal content of less than 30 ppb. In a preferred embodiment of the invention a silicon tetrachloride can be used which has the following metal contents in addition to silicon tetrachloride: Silicon tetrachloride having this low metal content can be prepared according to DE 100 30 251 or according to DE 100 30 252. The metal content of the silicon dioxide according to the invention is within the ppm range and below (ppb range). Examples The BET surface area is determined in accordance with DIN 66131. The dibutyl phthalate absorption is measured with a RHEOCORD 90 instrument made by Haake, Karlsruhe. For this purpose, 16 g of the silicon dioxide powder, weighed out to an accuracy of 0.001 g, is placed in a mixing chamber, this is sealed with a lid and dibutyl phthalate is added at a pre-set rate of addition of 0.0667 ml/s via a hole in the lid. The mixer is operated with a motor speed of 125 revs per minute. After reaching maximum torque, the mixer and DBP addition are automatically switched off. The DBP absorption is calculated from the amount of DBP consumed and the amount of particles weighed out in accordance with: DBP index (g/100 g) = (DBP consumed in g / .initial weight of particles in g) x 100. A programmable rheometer for testing complex flow behaviour, equipped with a standard rotation spindle, was available for determining the viscosity. Rate of shear: 5 to 100 rpm Temperature of measurement: room temperature (23°C) Concentration of dispersion: 30 wt.% Procedure: 500 ml of dispersion are placed in a 600 ml glass beaker and tested at room temperature (statistical recording of temperature via a measuring sensor) under different rates of shear. Determination of the compacted bulk density is based on DIN ISO 787/XI K 5101/18 (not sieved). Determination of the pH is based on DIN ISO 7 87/IX, ASTM D 1280, JIS K 5101/24. The image analyses were performed using a TEM instrument H 7500 made by Hitachi and a CCD camera MegaView II, made by SIS. Image magnification for evaluation purposes was 30000 : 1 at a pixel density of 3.2 nm. The number of particles evaluated was greater than 1000. Preparation was in accordance with ASTM 3849-89. The lower threshold limit for detection was 50 pixels. Determining the maximum degree of filling in an aqueous dispersion: 200 g of fully deionised water were initially placed in a 1 1 vessel (diameter about 11 cm). A dissolver from VMA-Getzmann, model Dispermat®-CA-40-C with a dissolver disc, diameter about 65 mm, was used as the dispersing unit. At the start, the dissolver is operated at about 650 rpm. The powder is added in portions of about 5 g. After each addition, there is a waiting period until the powder has been completely incorporated into the suspension. Then the next portion is added. As soon as incorporation of an added amount of powder takes longer than about 10 s, the speed of the dissolver disc is increased to 1100 rpm. Then further stepwise addition is performed. As soon as incorporation of an added amount of powder takes longer than about 10 s, the speed of the dissolver disc is increased to 1700 rpm. The maximum degree of filling is achieved when either no further powder is taken up by the dispersion, despite increased stirring power, i.e. the powder remains in dry form on the surface of the dispersion, or the dispersion becomes solid or the dispersion starts to form lumps. The amount of powder added can be determined by difference weighing (preferably difference weighing of the powder stock). The maximum degree of filling is calculated as: Maximum degree of filling = amount of powder added [g]/(amount of powder added [g] + amount of water initially introduced [g]) x 100% Example 1 (comparison example): 500 kg/h SiCl4 are vaporised at about 90°C and transferred to the central tube of a burner of known construction. 145 Nm3/h of hydrogen and 207 Nm3/h of air with an oxygen content of 35 vol.% are also introduced into this tube. This gas mixture is ignited and burnt in the flame tube of the water-cooled burner. The mean normalised rate of flow of gas in the flame tube at the level of the mouth of the burner is 0.7 m/s. After cooling the reaction gases, the pyrogenic silicon dioxide powder is separated from the hydrochloric acid-containing gases using a filter and/or a cyclone. The pyrogenic silicon dioxide powder is treated with water vapour and air in a deacidification unit. Examples 2 to 4 (comparison examples) are performed in the same way as example 1. The parameters which are altered each time are given in Table 1. Example 5 (working example): 400 kg/h SiCl4 are vaporised at about 90°C and transferred to the central tube of a burner of known construction. 195 Nm3/h of hydrogen and 303 Nm3/h of air with an oxygen content of 30 vol.% are also introduced into this tube. This gas mixture is ignited and burnt in the flame tube of the water-cooled burner. The mean normalised rate of flow of gas in the flame tube at the level of the mouth of the burner is 10 m/s. After cooling the.reaction gases, the pyrogenic silicon dioxide powder is separated from the hydrochloric acid-containing gases using a filter and/or a cyclone. The pyrogenic silicon dioxide powder is treated with water vapour and air in a deacidification unit. Examples 6 to 8 are performed in the same way as described in example 1. The parameters which are altered each time are given in Table 1. The analytical data for powders 1 to 8 are given in Table 2. The powders in examples 5 to 8 exhibit much lower values for mean aggregate area, mean aggregate circumference and maximum and minimum aggregate diameter and thus much less structure than the powders in comparison examples 1 to 4. The powders have a much higher maximum degree of filling and a much lower viscosity in an aqueous dispersion. Example 9 (Comparison Example) 500 kg/h S1CI4 having a composition in accordance with Table 3 are evaporated at approx. 90°C and transferred into the central tube of a burner of known design. 190 Nm3/h hydrogen as well as 326 Nm3/h air having a 35 vol.% oxygen content are introduced additionally into this tube. This gas mixture is ignited and burns in"the flame tube of the water-cooled burner. 15 Nm3/h hydrogen are introduced additionally into a jacket nozzle surrounding the central nozzle, in order to prevent baking-on. 250 Nm3/h air of normal composition are moreover introduced additionally into the flame tube. After cooling of the reaction gases the pyrogenic silicon dioxide powder is separated by means of a filter and/or a cyclone from the hydrochloric acid- containing gases. The pyrogenic silicon dioxide powder is treated with water vapour and air in a deacidifying unit in order to remove adherent hydrochloric acid. The metal contents are reproduced in Table 5. Example 10 (Embodiment Example) 500 kg/h SiCl4 having a composition in accordance with Table 4 are evaporated at approx. 90°C and transferred into the central tube of a burner of known design. 190 Nm3/h hydrogen as well as 32 6 Nm3/h air having a 35 vol.% oxygen content are introduced additionally into this tube. This gas mixture is ignited and burns in the flame tube of the water-cooled burner. 15 Nm3/h hydrogen are introduced additionally into a jacket nozzle surrounding the central nozzle, in order to prevent baking-on. 250 Nm3/h air of normal composition are moreover introduced additionally into the flame tube. After cooling of the reaction gases the pyrogenic silicon dioxide powder is separated by means of a filter and/or a cyclone from the hydrochloric acid- containing gases. The pyrogenic silicon dioxide powder is Measuring method The pyrogenically prepared silicon dioxides which are obtained are analysed as to their metal content. The samples are dissolved in an acid solution which comprises predominantly HF. The Si02 reacts with the HF, forming SiF4 + H20. The SiF4 evaporates, leaving behind completely in the acid the metals which are to be determined. The individual samples are diluted with distilled water and analysed against an internal standard by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) in a Perkin Elmer Optima 3000 DV. The imprecision of the values is the result of sample variations, spectral interferences and the limitations of the measuring method. Larger elements have a relative imprecision of +_ 5%, while the smaller elements have a relative imprecision of + 15%. The pyrogenically prepared silicon dioxides used are silicon dioxides having the physicophemical properties given below. To prepare the silicon dioxides, a liquid silicon compound is sprayed into an oxyhydrogen flame consisting of hydrogen and air. In most cases silicon tetrachloride is used. This substance is hydrolysed to silicon dioxide and hydrochloric acid by the action of the water formed during the hydrogen- oxygen reaction. After leaving the flame, the silicon dioxide enters a so-called coagulation zone, wherein the primary Aerosil particles and primary Aerosil aggregates agglomerate. The product, which exists at this stage as a kind of aerosol, is separated from the accompanying gaseous substances in cyclones and then aftertreated with moist heated air. By this process the residual hydrochloric acid content can be lowered to 0.025%. As the silicon dioxide obtained at the end of this process has a bulk density of only about 15 g/1, there is a subsequent vacuum compaction, whereby tamped densities of about 50 g/1 and more can be established. The particle sizes of the silicon dioxides can be varied by means of the reaction conditions such as, for example, flame temperature, proportions of hydrogen and oxygen, quantity of silicon tetrachloride, residence time in the flame or length of the coagulation path. The BET surface area is determined using nitrogen in accordance with DIN 66 131. The pore volume is calculated from the sum of the micro-, meso- and macropore volumes. The micro- and mesopores are determined by recording an N2 isotherm and evaluation thereof by the methods of BET, de Boer and Barrett, Joyner and Halenda. The macropores D > 30 nm are determined by the Hg porosimetry method. For the determination of the macropores, the sample is dried for 15 h at 100 °C in the drying oven and degassed at room temperature in a vacuum. For the determination of the micro- and mesopores, the sample is dried for 15 h at 100 °C in the drying oven and degassed for 1 h at 200 °C in a vacuum. The silanol group concentration is determined by the lithium alanate method. Here the SiOH- groups are reacted with LiAlH4 and the quantity of hydrogen formed during this reaction is determined from the pressure. Principle of measurement The granular material is weighed into a four-necked flask. The flask is evacuated and an oil bath is heated to 150 °C The temperature in the flask (controlled by an internal thermometer) rises with the temperature of the oil bath to about 130 °C. The pressure during the preliminary treatment is recorded using a pressure measuring device PI2 (TM 210, from Leybold, measuring range 103 to 10"3 mbar) . The desorption of the water can be monitored from the pressure measurement. At the end of the preliminary treatment (30 min at the end temperature) a pressure of less than 10" mbar must have been achieved. After completion of the preliminary treatment, the evacuated flask is separated from the vacuum unit by closing the stop valve and is brought to normal temperature. The actual measurement is based on a measured quantity of LiAlH4 solution being introduced into the flask through the dropping funnel and the rise in pressure being measured from the hydrogen formed. If the volume of the flask is known, the quantity of H2 can be calculated from the ideal gas law. The pressure is recorded using a digital measuring device (PIJ (MKS Instruments PR-2000), having a measuring range of between 0 and 1 bar. The LiAlH4 solution used (2% LiALH4 in diethylene glycol dimethyl ether) is degassed prior to the experiment being carried out, in order to remove readily volatile constituents, which distort the pressure measurement. For this purpose the pressure above the solution in the dropping funnel is lowered by a second vacuum pump to the vapour pressure (3.7 mbar at 22 °C) , so that the liquid boils. A blank measurement without a sample is taken to test whether the solution is sufficiently degassed. In the determination of the hydrogen pressure, a correction is made using the vapour pressure of the solvent. Interpretation The apparatus is calibrated by first of all determining the volume of the dropping funnel provided with a ground-glass stopper, gauging the capacity in litres. The volume of the reaction flask inclusive of all connections as far as the stop valve is obtained by the following experiment: The dropping funnel, filled with air at atmospheric pressure, is attached to the evacuated flask. A pressure compensation between the two volumes is then brought about by opening the tap of the dropping funnel. The pressure established is indicated by the digital measuring device. The volume of the reaction vessel is obtained from the mass balance. A volume VR equal to 243.8 _ml is obtained with the present arrangement. The number of moles of hydrogen formed is obtained from the equations: p.V (corr.) n = ------------ RT corr. R solids solution p is the increase in pressure in the reaction flask. This value is corrected by an amount corresponding to the vapour pressure of the solvent (3.7 mbar at 22 °C). At room temperatures greatly differing from 22 °C the vapour pressure is taken from the vapour pressure table. It is useful so to select the weighed sample, that a value for p of between 200 and 800 mbar is obtained. In this case minor changes in the vapour pressure owing to temperature variations have hardly any effect on the result. The volume of the reaction vessel is corrected by deducting the volume of solid matter and the volume of the solution introduced. The former is given from the weighed portion and the density and the latter is read from the dropping funnel. The density of silanol groups is finally obtained from the equation: n-NL d = ----- F NL: Lohschmidt number F: Surface of the weighed solid matter The samples are treated as follows: 1 h heating at 120oC and 0.2 mbar; cooling to 60 °C; addition of LiAlH4; after 10 mins, reading the pressure difference which has arisen. The particle size distribution is determined by means of the laser optical particle size analyser Cilas Granulametre 715. The tapped volume is determined in accordance with ASTM D 4164-88. Equipment: Tapping volumeter STA V 2003 from Engelsmann, in accordance with DIN 53194, section 5.2. b-f Measuring cylinder 250 ml, graduation marks every 2 ml Balance with limit of error of max. ± 0.1 g Procedure The counter of the tapping volumometer is set to 1000 strokes. The measuring cylinder is tared. The granular material is placed in the measuring cylinder up to the 250 ml mark. The weight of the sample is recorded (± 0.1 g). The measuring cylinder is placed in-the volumeter and the apparatus is switched on. End of tapping: the apparatus automatically switches off after 1000 strokes The tapped bulk volumes are read to an accuracy of 1 ml. Calculation E: weighed portion of granular material in g V: volume read in ml W: water content in wt.% (determined in accordance with Specification P001) E x (100 - W) Tapped density = ------------- V x 100 The pH value is determined in 4% aqueous dispersion, in the case of hydrophobic catalyst supports in water:ethanol 1:1. Preparation of the granules according to the invention The pyrogenically prepared silicon dioxide is dispersed in completely demineralised water. A dispersing aggregate which operates according to the rotor/stator principle is used in the process. The suspensions formed are spray dried. The finished product is precipitated by a filter or cyclone. The sprayed granules are heated in a muffle furnace. The spray-dried and optionally heated granules are placed in a mixer for the silanisation process and sprayed with intensive mixing optionally first of all with water and then with the silane Si 108 (trimethoxyocty.lsi.lane) or HMDS (hexamethyldisilazane). After spraying has been completed, the material is mixed for 15 to 30 more minutes and then heated for 1 to 4 h at 100 to 400 °C. The water used can be acidified with an acid, for example, hydrochloric acid, to a pH value of 7 to 1. The silanising agent used can be dissolved in a solvent such as, for example, ethanol. WE CLAIM: 1. Granules based on pyrogenically prepared silicon dioxide and having the following physicochemical properties: Average grain size: 10 to 120 µm BET surface area: " 40 to 400 m2/g Pore volume: 0.5 to 2.5 ml/g Pore size distribution: less than 5 % of the total pore volume exists of pores with a diameter meso- and macropores pH value: 3.6 to 8.5 Tapped density: 220 to 700 g/1 2. Method for the preparation of granules based on pyrogenically prepared silicon dioxide as claimed in claim 1, wherein pyrogenically prepared silicon dioxide, preferably silicon dioxide prepared from silicon tetrachloride by means of flame hydrolysis, is dispersed in water, spray dried, the granules obtained are optionally heated at a temperature of from 150 to 1,100 °C for a period of 1 to 8 h and/or silanised. 3. Method as claimed in claim 2, wherein pyrogenically prepared silicon dioxide is dispersed in water and spray dried and the granules obtained are heated at a temperature of from 150 to 1,100°C for a period of 1 to 8 h. 4. Method as claimed in claim 2, wherein pyrogenically prepared silicon dioxide is dispersed in water and spray dried and the granules obtained are silanised. 5. Method as claimed in claim 2, wherein pyrogenically prepared silicon dioxide is dispersed in water and spray dried and the granules obtained are heated at a temperature of from 150 to 1,100°C for a period of 1 to 8h and subsequently silanised. 6. Method as claimed in claims 2 to 5, wherein as pyrogenically prepared silicon dioxide a pyrogenic silicon dioxide powder with - a BET surface area of 30 to 90 m2/g, a DBP index of 80 or less a mean aggregate area of less than 25000 nm2, a mean aggregate circumference of less than 1000 nm, wherein at least 70% of the aggregates have a circumference of less than 1300 nm is used. 7. Method as claimed in claims 2 to 5, wherein as pyrogenically prepared silicon dioxide a high-purity pyrogenically prepared silicon dioxide, having a metal content of less than 9 ppm is used. Granules based on pyrogenically prepared silicon dioxide and having the following physicochemical properties: Average grain size: 10 to 120 ?m; BET surface area: 40 to 400 m2/g; Pore volume: 0.5 to 2.5 ml/g; Pore size distribution: less than 5 % of the total pore volume exists of pores with a diameter rest meso- and macropores pH value: 3.6 to 8.5; Tapped density: 220 to 700 g/l.

Full Text

Granules based on pyrogenically prepared silicon dioxide,
method for their preparation and use thereof
The invention relates to granules based on pyrogenically
prepared silicon dioxide, the method for their preparation
and the use thereof as catalyst supports and for glass
applications.
The preparation of pyrogenic silicas or silicon dioxides
from SiC14 by means of high-temperature-or flame hydrolysis
is known (Ullmanns Enzyklopadie der technischen Chemie, 4th
Edition, Volume 21, page 464 (1982)).
Pyrogenic silicon dioxides are distinguished by having
extremely fine particles, high specific surface (BET), very
high purity, spherical particle shape and the absence of
pores. On account of these properties pyrogenically
prepared silicon dioxides are attracting increasing
interest as supports for catalysts (Dr. Koth et al., Chem.
Ing. Techn. 52, 628 (1980)). For this application the
pyrogenically prepared silicon dioxide is shaped by
mechanical means, for example, tabletting machines.
The shaping of pyrogenically prepared silicon dioxide into
sprayed granules also by means of spray drying, to obtain a
starting material for sintered ceramic materials, is also
known (DE-A 36 11 449).
It is also known that silicon dioxide pyrogenically
prepared in an electric arc may be shaped by means of spray
drying into sprayed granules, which can be used as
adsorption media or else as catalyst supports
(DE-A 12 09 108).
The subjection of pyrogenically prepared silicon dioxide to
a gel process and the subsequent shaping into granules by
means of spray drying is also known. These granules, after
coating with chromium oxide, are used in the polymerisation
of ethylene (EP-A 0 050 902, US-A 4,386,016).
Furthermore, the use of precipitated silicon dioxide as a
catalyst support for the catalytic polymerisation of
olefins is known (WO 91/09881) .
The known sprayed granules of pyrogenically prepared
silicon dioxides have the disadvantage that they are not
optimally suitable for use as catalyst supports, for
example, in the production of polyethylene.
The object was therefore the development from pyrogenically
prepared silicon dioxide of sprayed- granules which can be
used as catalyst supports in the production of
polyethylene.
The present invention provides granules based on
pyrogenically prepared silicon dioxide and having the
following physicochemical properties:
Average grain size: 10 to 12 0 urn
BET surface area: 40 to 400 m2/g
Pore volume: 0.5 to 2.5 ml/g
Pore size distribution: less than 5 % of the total
pore volume exists of pores
with a diameter meso- and macropores
pH value: 3.6 to 8.5
Tapped density: 220 to 700 g/1
The granular material according to the invention can be
prepared by dispersing pyrogenically prepared silicon
dioxide in water, spray drying it and heating the granules
obtained at a temperature of from 150 to 1,100 °C for a
period of 1 to 8 h.
The invention also provides granules based on pyrogenically
prepared silicon dioxide and having the following
physicochemical properties:
Average grain size: 10 to .120 um
BET surface area: 40 to 400 m2/g
Pore volume: 0.5 to 2.5 ml/g
Pore size distribution: less than 5 % of the total
pore volume exists of pores
with a diameter meso- and macropores
pH value: 3.6 to 8 . 5
Tapped density: 220 to 700 g/1
The granular material according to the invention can be
prepared by dispersing pyrogenically prepared silicon
dioxide in water, spray drying it and silanising the
granules obtained. Halosilanes, alkoxysilanes, silazanes
and/or siloxanes can be used for the silanisation.
The following substances in particular can be used as
halosilanes:
a) Organosilanes of the types (RO)3Si(CnH2n+1) and
(RO)3Si(CnH2n-i)
R = Alkyl, as for example Methyl-, Ethyl-, n-Propyl-,
i-Propyl-, Butyl-
n = 1 - 20
b) Organosilanes of the types R'x (RO) ySi (CnH2n+1) and
R'x(RO)ySi(CnH2„-i)
R = Alkyl, as for example Methyl-, Ethyl-,
n-Propyl-, i-Propyl-, Butyl-
R" = Alkyl, as for example Methyl-, Ethyl-,
n-Propyl-, i-Propyl-, Butyl-
R'=Cycloalkyl
n =1-20
x+y = 3
x =1,2
y = 1,2
c) Halogenorganosilanes of the types X3Si (CnH2n+1) and
X3Si(CnH2n-i)
X = CI, Br
n = 1 - 20
d) Halogenorganosilanes of the types X2(R')Si (CnH2n+1) and
X2(R')Si(CnH2n-l)
X = CI, Br
R' = Alkyl, as for example Methyl-, Ethyl-,
n-Propyl-, i-Propyl-, Butyl-
R'=Cycloalkyl
n =1-20
e) Halogenorganosilanes of the types X (R1 ) 2Si (CnH2n+1) and
X(R')2Si(CnH2n_1)
X = CI, Br
R' = Alkyl, as for example Methyl-, Ethyl-, n-Propyl-,
i-Propyl-, Butyl-
R'=Cycloalkyl
n =1-20
f) Organosilanes of the types (RO) 3Si (CH2)m-R'
R = Alkyl, as Methyl-, Ethyl-, Propyl-
m = 0,1 - 20
R' = Methyl-, Aryl (as for example -C6H5, substituted
phenyl groups)
-C4F9, OCF2-CHF-CF3, -C6F13, -0-CF2-CHF2
-NH2, -N3, -SCN, -CH=CH2, -NH-CH2-CH2-NH2,
-N- (CH2-CH2-NH2)2
-OOC(CH3)C = CH2
-OCH2-CH(0)CH2
-NH-CO-N-CO-(CH2)5
-NH-COO-CH3, -NH-COO-CH2-CH3, -NH-(CH2)3Si(OR)3
-Sx-(CH2)3Si(OR)3
-SH
-NR^^R'1' (R' = Alkyl, Aryl; R' ' = H,
Alkyl, Aryl; R1' = H, Alkyl, Aryl, Benzyl,
C2H4NR'' R''«' mit R'*' = H, Alkyl und
R'"" = H, Alkyl)
g) Organosilanes of the types (R")x(RO)ySi(CH2)m-R'
R" = Alkyl x+y = 3
= Cycloalkyl x =1,2
y = 1,2
m =0,1 bis 2 0
R' = Methyl-, Aryl (as for example -C6H5 ,substituted
phenyl groups)
-C4F9, -OCF2-CHF-CF3, -C6F13, -0-CF2-CHF2
-NH2, -N3, -SCN, -CH=CH2, -NH-CH2-CH2-NH2,
-N-(CH2-CH2-NH2)2
-OOC(CH3)C = CH2
-OCH2-CH(0)CH2
-NH-C0-N-C0-(CH2)5
-NH-C00-CH3, -NH-COO-CH2-CH3, -NH-(CH2)3Si(OR)3
-Sx-(CH2)3Si(OR)3
-SH
- NR^'R'*1 (R' = Alkyl, Aryl; R' = H,
Alkyl, Aryl; R'' = H, Alkyl, Aryl, Benzyl,
C2H4NR' ' ' ' R* ' ' ' ' with R',,! = H, Alkyl and
Ri i i » i = H, Alkyl)
h) Halogenorganosilanes of the types X3Si(CH2)m-R'
X = CI, Br
m = 0,1 - 20
R' = Methyl-, Aryl (for example -C6H5, substituted
Phenyl groups)
-C4F9, -OCF2-CHF-CF3, -C6F13, -0-CF2-CHF2
-NH2, -N3, -SCN, -CH=CH2,
-NH-CH2-CH2-NH2
-N-(CH2-CH2-NH2)2
-OOC(CH3)C = CH2
-OCH2-CH(0)CH2
-NH-CO-N-CO- (CH2)5
-NH-COO-CH3, -NH-COO-CH2-CH3, -NH-(CH2)3Si(OR)3
-Sx-(CH2)3Si(OR)3
-SH
i) Halogenorganosilanes of the types (R) X2Si(CH2)m-R'
X = CI, Br
R = Alkyl, as Methyl,- Ethyl-, Propyl-
m =0,1-20
R' = Methyl-, Aryl (z.B. -C6H5, substituted
Phenyl groups)
-C4F9, -OCF2-CHF-CF3, -C6F13, -0-CF2-CHF2
-NH2, -N3, -SCN, -CH=CH2,-NH-CH2-CH2-NH2,
-N-(CH2-CH2-NH2)2
-OOC(CH3)C = CH2
-OCH2-CH(0)CH2
-NH-CO-N-CO- (CH2)5
-NH-COO-CH3, -NH-COO-CH2-CH3, -NH-(CH2)3Si(OR)3,
whereby R may be Methyl-, Ethyl-, Propyl-, Butyl-
-Sx-(CH2)3Si(OR)3, whereby R may be Methyl-,
Ethyl-,
Propyl-, Butyl-
-SH
!
) Halogenorganosilanes of the types (R) 2X Si (CH2)m-R'
X = CI, Br
R = Alkyl
m = 0,1 - 20
R' = Methyl-, Aryl (z.B. -C6H5, substituted
Phenyl groups)
-C4F9, -OCF2-CHF-CF3, -C6F13, -0-CF2-CHF2
-NH2, -N3, -SCN, -CH=CH2, -NH-CH2-CH2-NH2,
-N-(CH2-CH2-NH2)2
-OOC(CH3)C = CH2
-OCH2-CH(0)CH2
-NH-CO-N-CO- (CH2)5
-NH-COO-CH3, -NH-COO-CH2-CH3, -NH-(CH2)3Si(OR)3
-Sx-(CH2)3Si(OR)3
-SH
) Silazane of the type R'R2Si-N-SiR2R"
H
R = Alkyl, Vinyl, Aryl
R' = Alkyl, Vinyl, Aryl
) Cyclic polysiloxanes of the types D 3, D 4, D 5, whereby
D 3, D 4 and D 5 cyclic may be polysiloxanes with 3,4
or 5 units of the types -0-Si(CH3)2-
i.e. octamethylcyclotetrasiloxane = D 4
m) Polysiloxanes respectively silicon oil of the type
m = 0,1,2,3,...00
n = 0,1,2,3,...°°
u = 0,1,2,3, . . .co
Y=CH3, H, CnH2n+1 n=l-20
Y=Si(CH3)3, Si(CH3)2H
Si(CH3)2OH, Si(CH3)2(OCH3)
Si(CH3)2(CnH2n+1) n=l-20
R = Alkyl, as CnH2n+1, whereby n = 1 to 20 is, Aryl, as
Phenyl- and substituted phenyl groups, (CH2)n-NH2, H
R' = Alkyl, as CnH2n+1, whereby n = 1 to 20 is, Aryl, as
phenyl- and substituted phenyl groups, (CH2)n-NH2, H
R" = Alkyl, as CnH2n+1, whereby n =- 1 to 20 is, Aryl, as
phenyl- and substituted phenyl groups, (CH2)n-NH2, H
R'" = Alkyl, as CnH2n+1, whereby n = 1 to 20 is, Aryl, as
phenyl- and substituted phenyl groups, (CH2)n-NH2, H
Preferably the silane Si 108 [(CH30) 3-Si-C8Hn],
trimethoxyoctylsilane, is used as silanising agent.
The invention also provides granules based on pyrogenically
prepared silicon dioxide and having the following
physicochemical properties:
Average grain size: 10 to 120 urn
BET surface area: 40 to 400 m2/g
Pore volume: 0.5 to 2.5 ml/g
Pore size distribution: less than 5 % of the total
pore volume exists of pores
with a diameter meso- and macropores
Carbon content: 0.3 to 15.0 wt.%
pH value: 3.6 to 8 . 5
Tapped density: 220 to 700 g/1
The granular material according to the invention preferably
has rneso- and macropores, with the volume of the mesopores
constituting 10 to 80% of the total pore volume.
The carbon content of the granular material according to
the invention may be from 0.3 to 15.0 wt.-%.
The particle size distribution of the granular material
according to the invention may be 80 vo.1.% larger than 8 um
and 80 vol.% smaller than 96 um.
In a preferred embodiment of the invention, the proportion
of pores smaller than 5 um may be at most 5% referred to
the total pore volume.
The granular material according to the invention can be
prepared by dispersing pyrogenically prepared silicon
dioxide in water, spray drying it, heating the granules
obtained at a temperature of from 150 to 1,100 °C for a
period of 1 to 8 h and then silanising them. The same
halosilanes, alkoxysilanes, silazanes and/or siloxanes
described above can be used for the silanisation.
The invention further provides a method for the preparation
of granules based on pyrogenically prepared silicon
dioxide, which is characterised in that pyrogenically
prepared silicon dioxide, preferably silicon dioxide
prepared from silicon tetrachloride by means of flame
hydrolysis, is dispersed in water, spray dried, the
granules obtained are optionally heated at a temperature of
from 150 to 1,100 °C for a period of 1 to 8 h and/or
silanised.
The dispersion in water can have a silicon dioxide
concentration of from 5 to 25 wt.-%.
The spray drying can be carried oul di d. Lumperature of
from 200 to 600 °C Disc atomisers or nozzle atomisers can
be used for this purpose.
The heating of the granules can be carried out both in
fixed beds, for example chamber kilns, and in moving beds,
for example rotary dryers.
The silanisation can be carried out using the same
halosilanes, alkoxysilanes, silazanes and/or siloxanes as
described above, for which the silanising agent can be
optionally dissolved in an organic solvent such as, for
example, ethanol.
Preferably the silane Si 108 [ (CH30) 3-Si-C8H17] ,
trimethoxyoctylsilane, is used as silanising agent.
The silanisation can be carried out by spraying the
granular material with the silanising agent and
subsequently heat-treating the mixture at a temperature of
from 105 to 400 °C over a period of 1 to 6 h.
In an alternative method, the silanisation of the granules
can be carried out by treating the granular material with
the silanising agent in vapour form and subsequently heat-
treating the mixture at a temperature of from 200 to 800 °C
over a period of 0.5 to 6 h.
The heat treatment can take place under protective gas such
as, for example, nitrogen.
The silanisation can be carried out continuously or
batchwise in heatable mixers and dryers equipped with
spraying facilities. Examples of suitable devices are
ploughshare mixers, disk dryers or fluidised bed dryers.
The physicochemical variables of the granules, such as the
specific surface, the particle size distribution, the pore
volume, the tamped density and the silanol group
concentration, the pore distribution and pH value can be
altered within the specified limits by varying the starting
materials and the conditions during spraying, heating and
silanisation.
The metal oxide or metalloid oxide to be used according to
the invention can be pyrogenically produced silicon dioxide
powder with
a BET surface area of 30 to 90 m2/g,
a DBP index of 80 or less
a mean aggregate area of less than 25000 nm2,
a mean aggregate circumference of less than 1000 nm,
wherein at least 70% of the aggregates have a
circumference of less than 1300 nm. This pyrogenically
produced silicon dioxide is disclosed in WO
2004/054929.
The BET surface area may preferably be between 35 and
75 m2/g. Particularly preferably the values may be between
4 0 and 60 m2/g. The BET surface area is determined in
accordance with DIN 66131.
The DBP index may preferably be between 60 and 80. During
DBP absorption, the take-up of force, or the torque (in
Nm), of the rotating blades in the DBP measuring equipment
is measured while defined amounts of DBP are added,
comparable to a titration. A sharply defined maximum,
followed by a drop, at a specific added amount of DBP is
then produced for the powder according to the invention.
A silicon dioxide powder with a BET surface area of 40
to 60 m2/g and a DBP index of 60 to 80 may be particularly
preferred.
Furthermore, the silicon dioxide powder to be used
according to the invention may preferably have a mean
aggregate area of at most 20000 rim2. Particularly
preferably, the mean aggregate area may be between 15000
and 20000 nm2. The aggregate area can be determined, for
example, by image analysis of TEM images. An aggregate is
understood to consist of primary particles of similar
structure and size which have intergrown with each other,
the surface area of which is less than the sum of the
individual isolated primary particles. Primary particles
are understood to be the particles which are initially
formed in the reaction and which can grow together to form
aggregates as the reaction proceeds further.
A silicon dioxide powder with a BET surface area of 40
to 60 m2/g, a DBP index of 60 to 80 and a mean aggregate
area between 15000 and 20000 nm2 may be particularly
preferred.
In a preferred embodiment, the silicon dioxide powder to be
used according to the invention may have a mean aggregate
circumference of less than 1000 nm. Particularly
preferably, the mean aggregate circumference may be between
600 and 1000 nm. The aggregate circumference can also be
determined by image analysis of TEM-images.
A silicon dioxide powder with a BET surface area of 40 to
60 m2/g, a DBP index of 60 to 80, a mean aggregate area
between 15000 and 20000 nm2 and a mean aggregate
circumference between 600 and 1000 nm may be particularly
preferred.
Furthermore, it may be preferable for at least 80%,
particularly preferably at least 90%, of the aggregates to
have a circumference of less than 1300 nm.
In a preferred embodiment, the silicon dioxide powder to be
used according to the invention may_ assume a degree of
filling in an aqueous dispersion of up to 90 wt.%. The
range between 60 and 80 wt.% may be particularly preferred.
Determination of the maximum degree of filling in an
aqueous dispersion is performed by the incorporation of
powder, in portions, into water using a dissolver, without
the addition of other additives. The maximum degree of
filling is achieved when either no further powder is taken
up into the dispersion, despite elevated stirring power,
i.e. the powder remains in dry form on the surface of the
dispersion, or the dispersion becomes solid or the
dispersion starts to form lumps.
Furthermore, the silicon dioxide powder to be used
according to the invention may have a viscosity at a
temperature of 23°C, with respect to a 30 wt.% aqueous
dispersion at a rate of shear of 5 rpm, of less than 100
mPas. In particularly preferred embodiments, the viscosity
may be less than 50 mPas.
The pH of the silicon dioxide powder to be used according
to the invention may be between 3.8 and 5, measured in a 4%
aqueous dispersion.
The process for preparing the silicon dioxide powder to be
used according to the invention, is characterised in that
at least one silicon compound in the vapour form, a free-
oxygen-containing gas and a combustible gas are mixed in a
burner of known construction, this gas mixture is ignited
at the mouth of the burner and is burnt in the flame tube
of the burner, the solid obtained is separated from the gas
mixture and optionally purified, wherein
the oxygen content of the free-oxygen-containing gas is
adjusted so that the lambda value is greater than or
equal to 1,
the gamma-value is between 1.2 and 1.8,
the throughput is between 0.1 and 0.3 kg SiC>2/m3 of core
gas mixture,
the mean normalised rate of flow of gas in the flame
tube at the level of the mouth of the burner is at least
5 m/s .
The oxygen content of the free-oxygen-containing gas may
correspond to that of air. That is, in this case air is
used as a free-oxygen-containing gas. The oxygen content
may, however also take on higher values. In a preferred
manner, air enriched with oxygen should have an oxygen
content of not more than 4 0 vol.%.
Lambda describes the ratio of oxygen supplied in the core
to the stoichiometrically required amount of oxygen. In a
preferred embodiment, lambda lies within the range
1 Gamma describes the ratio of hydrogen supplied in the core
to the stoichiometrically required amount of hydrogen. In a
preferred embodiment, gamma lies within the range
1.6 The normalised rate of flow of gas refers to the rate of
flow at 273 K and 1 atm.
A burner of known construction is understood to be a burner
with concentric tubes. The core gases are passed through
the inner tube, the core. At the end of the tube, the mouth
of the burner, the gases are ignited. The inner tube is
surrounded by at least one other tube, the sleeve. The
reaction chamber, called the flame tube, starts at the
level of the mouth of the burner. This is generally a
conical tube, cooled with water, which may optionally be
supplied with other gases (sleeve gases) such as hydrogen
or air.
The mean, normalised rate of flow of the gas in the flame
tube at the level of the mouth of the burner of at least 5
m/s refers to the rate of flow immediately after the
reaction mixture leaves the burner. The rate of flow is
determined by means of the volume flow of the reaction
products in vapour form and the geometry of the flame tube.
The core gases are understood to be the gases and vapours
supplied to the burner, that is the free-oxygen-containing
gas, generally air or air enriched with oxygen, the
combustible gas, generally hydrogen, methane or natural
gas, and the silicon compound or compounds in vapour form.
An essential feature of the process is that the mean
normalised rate of flow of gas in the flame tube at the
level of the mouth of the burner is at least 5 m/s. In a
preferred embodiment, the mean normalised rate of flow of
the gas in the flame tube at the level of the mouth of the
burner assumes values of more than 8 m/s.
The mean rate of discharge of the gas mixture (feedstocks)
at the mouth of the burner is not limited. However, it has
proven to be advantageous when the rate of discharge at the
mouth of the burner is at least 30 m/s.
In a preferred embodiment, additional air (secondary air)
may be introduced into the reaction chamber, wherein the
rate of flow in the reaction chamber may be raised further.
In a preferred embodiment, the mean normalised rate of flow
of gas in the flame tube at the level of the mouth of the
burner may be 8 to 12 m/s.
The type of silicon compound used in the process is not
further restricted. Silicon tetrachloride and/or at least
one organochlorosilicon compound may preferably be used.
A particularly preferred embodiment of the process is one
in which
silicon tetrachloride is used,
the lambda value is such that 1 the gamma-value is between 1.6 and 1.8,
- the throughput is between 0.1 and 0.3 kg Si02/m of core
gas mixture,
in addition at least double the amount of air, with
respect to the amount of free-oxygen-containing gas
introduced into the burner, is introduced into the flame
tube and
the rate of flow of the gas of feedstocks at the mouth
of the burner is 40 to 65 m/s (with respect to standard
conditions)
and the mean normalised rate of flow of gas in the flame
tube at the level of the mouth of the burner is between
8 and 12 m/s.
In general during the preparation of pyrogenic oxides, the
rate of flow of gas in the water-cooled reaction chamber
(flame tube) and in the subsequent cooling unit (cooling
stretch) is adjusted in such a way that the best possible
cooling power, that is to say rapid cooling of the reaction
products, is ensured. In principle,"it is true that the
cooling power increases with decreasing rate of flow of
gas. The lower limit is simply based on the requirement of
still being able to transport the product through the pipes
with the gas stream.
It was demonstrated that although a considerable increase
in the rate of flow of gas in the reaction chamber resulted
in a reduced cooling power, it led to a powder with
unexpected properties. Whereas physical characteristics
such as BET surface area and DBP absorption are
substantially unchanged as compared with powders according
to the prior art, the powder exhibits a much lower
structure.
Furtheron the metal oxide or metalloid oxide to be used
according to the invention can be pyrogenically produced
silicon dioxide which is characterised by a metals content
of less than 9 ppm.
In a preferred embodiment the high-purity pyrogenically
prepared silicon dioxide to be used according to the
invention, can be characterised by the following metal
contents:
The total metal content can then be 3252 ppb (~ 3.2 ppm) or
less.
In an embodiment of the invention, which is further
preferred, the high-purity pyrogenically prepared silicon
dioxide can be characterised by the* following metal
contents:
The total metal content can then be 1033 ppb (~ 1.03 ppm)
or less.
The process for the preparation of the high-purity
pyrogenically prepared silicon dioxide is characterised in
that silicon tetrachloride is in known manner reacted in a
flame by means of high-temperature hydrolysis to give
silicon dioxide, and a silicon tetrachloride is used here
which has a metal content of less than 30 ppb.
In a preferred embodiment of the invention a silicon
tetrachloride can be used which has the following metal
contents in addition to silicon tetrachloride:
Silicon tetrachloride having this low metal content can be
prepared according to DE 100 30 251 or according to DE 100
30 252.
The metal content of the silicon dioxide according to the
invention is within the ppm range and below (ppb range).
Examples
The BET surface area is determined in accordance with DIN
66131.
The dibutyl phthalate absorption is measured with a
RHEOCORD 90 instrument made by Haake, Karlsruhe. For this
purpose, 16 g of the silicon dioxide powder, weighed out to
an accuracy of 0.001 g, is placed in a mixing chamber, this
is sealed with a lid and dibutyl phthalate is added at a
pre-set rate of addition of 0.0667 ml/s via a hole in the
lid. The mixer is operated with a motor speed of 125 revs
per minute. After reaching maximum torque, the mixer and
DBP addition are automatically switched off. The DBP
absorption is calculated from the amount of DBP consumed
and the amount of particles weighed out in accordance with:
DBP index (g/100 g) = (DBP consumed in g / .initial weight
of particles in g) x 100.
A programmable rheometer for testing complex flow
behaviour, equipped with a standard rotation spindle, was
available for determining the viscosity.
Rate of shear: 5 to 100 rpm
Temperature of measurement: room temperature (23°C)
Concentration of dispersion: 30 wt.%
Procedure: 500 ml of dispersion are placed in a 600 ml
glass beaker and tested at room temperature (statistical
recording of temperature via a measuring sensor) under
different rates of shear.
Determination of the compacted bulk density is based on DIN
ISO 787/XI K 5101/18 (not sieved).
Determination of the pH is based on DIN ISO 7 87/IX, ASTM D
1280, JIS K 5101/24.
The image analyses were performed using a TEM instrument
H 7500 made by Hitachi and a CCD camera MegaView II, made
by SIS. Image magnification for evaluation purposes was
30000 : 1 at a pixel density of 3.2 nm. The number of
particles evaluated was greater than 1000. Preparation was
in accordance with ASTM 3849-89. The lower threshold limit
for detection was 50 pixels.
Determining the maximum degree of filling in an aqueous
dispersion: 200 g of fully deionised water were initially
placed in a 1 1 vessel (diameter about 11 cm). A dissolver
from VMA-Getzmann, model Dispermat®-CA-40-C with a
dissolver disc, diameter about 65 mm, was used as the
dispersing unit.
At the start, the dissolver is operated at about 650 rpm.
The powder is added in portions of about 5 g. After each
addition, there is a waiting period until the powder has
been completely incorporated into the suspension. Then the
next portion is added. As soon as incorporation of an added
amount of powder takes longer than about 10 s, the speed of
the dissolver disc is increased to 1100 rpm. Then further
stepwise addition is performed. As soon as incorporation of
an added amount of powder takes longer than about 10 s, the
speed of the dissolver disc is increased to 1700 rpm.
The maximum degree of filling is achieved when either no
further powder is taken up by the dispersion, despite
increased stirring power, i.e. the powder remains in dry
form on the surface of the dispersion, or the dispersion
becomes solid or the dispersion starts to form lumps.
The amount of powder added can be determined by difference
weighing (preferably difference weighing of the powder
stock). The maximum degree of filling is calculated as:
Maximum degree of filling = amount of powder added
[g]/(amount of powder added [g] + amount of water initially
introduced [g]) x 100%
Example 1 (comparison example):
500 kg/h SiCl4 are vaporised at about 90°C and transferred
to the central tube of a burner of known construction. 145
Nm3/h of hydrogen and 207 Nm3/h of air with an oxygen
content of 35 vol.% are also introduced into this tube.
This gas mixture is ignited and burnt in the flame tube of
the water-cooled burner. The mean normalised rate of flow
of gas in the flame tube at the level of the mouth of the
burner is 0.7 m/s. After cooling the reaction gases, the
pyrogenic silicon dioxide powder is separated from the
hydrochloric acid-containing gases using a filter and/or a
cyclone. The pyrogenic silicon dioxide powder is treated
with water vapour and air in a deacidification unit.
Examples 2 to 4 (comparison examples) are performed in the
same way as example 1. The parameters which are altered
each time are given in Table 1.
Example 5 (working example):
400 kg/h SiCl4 are vaporised at about 90°C and transferred
to the central tube of a burner of known construction.
195 Nm3/h of hydrogen and 303 Nm3/h of air with an oxygen
content of 30 vol.% are also introduced into this tube.
This gas mixture is ignited and burnt in the flame tube of
the water-cooled burner. The mean normalised rate of flow
of gas in the flame tube at the level of the mouth of the
burner is 10 m/s. After cooling the.reaction gases, the
pyrogenic silicon dioxide powder is separated from the
hydrochloric acid-containing gases using a filter and/or a
cyclone. The pyrogenic silicon dioxide powder is treated
with water vapour and air in a deacidification unit.
Examples 6 to 8 are performed in the same way as described
in example 1. The parameters which are altered each time
are given in Table 1.
The analytical data for powders 1 to 8 are given in
Table 2.
The powders in examples 5 to 8 exhibit much lower values
for mean aggregate area, mean aggregate circumference and
maximum and minimum aggregate diameter and thus much less
structure than the powders in comparison examples 1 to 4.
The powders have a much higher maximum degree of filling
and a much lower viscosity in an aqueous dispersion.
Example 9 (Comparison Example)
500 kg/h S1CI4 having a composition in accordance with
Table 3 are evaporated at approx. 90°C and transferred into
the central tube of a burner of known design. 190 Nm3/h
hydrogen as well as 326 Nm3/h air having a 35 vol.% oxygen
content are introduced additionally into this tube. This
gas mixture is ignited and burns in"the flame tube of the
water-cooled burner. 15 Nm3/h hydrogen are introduced
additionally into a jacket nozzle surrounding the central
nozzle, in order to prevent baking-on. 250 Nm3/h air of
normal composition are moreover introduced additionally
into the flame tube. After cooling of the reaction gases
the pyrogenic silicon dioxide powder is separated by means
of a filter and/or a cyclone from the hydrochloric acid-
containing gases. The pyrogenic silicon dioxide powder is
treated with water vapour and air in a deacidifying unit in
order to remove adherent hydrochloric acid. The metal
contents are reproduced in Table 5.
Example 10 (Embodiment Example)
500 kg/h SiCl4 having a composition in accordance with
Table 4 are evaporated at approx. 90°C and transferred into
the central tube of a burner of known design. 190 Nm3/h
hydrogen as well as 32 6 Nm3/h air having a 35 vol.% oxygen
content are introduced additionally into this tube. This
gas mixture is ignited and burns in the flame tube of the
water-cooled burner. 15 Nm3/h hydrogen are introduced
additionally into a jacket nozzle surrounding the central
nozzle, in order to prevent baking-on. 250 Nm3/h air of
normal composition are moreover introduced additionally
into the flame tube. After cooling of the reaction gases
the pyrogenic silicon dioxide powder is separated by means
of a filter and/or a cyclone from the hydrochloric acid-
containing gases. The pyrogenic silicon dioxide powder is
Measuring method
The pyrogenically prepared silicon dioxides which are
obtained are analysed as to their metal content. The
samples are dissolved in an acid solution which comprises
predominantly HF.
The Si02 reacts with the HF, forming SiF4 + H20. The SiF4
evaporates, leaving behind completely in the acid the
metals which are to be determined. The individual samples
are diluted with distilled water and analysed against an
internal standard by inductively coupled plasma-atomic
emission spectroscopy (ICP-AES) in a Perkin Elmer Optima
3000 DV. The imprecision of the values is the result of
sample variations, spectral interferences and the
limitations of the measuring method. Larger elements have a
relative imprecision of +_ 5%, while the smaller elements
have a relative imprecision of + 15%.
The pyrogenically prepared silicon dioxides used are
silicon dioxides having the physicophemical properties
given below.
To prepare the silicon dioxides, a liquid silicon compound
is sprayed into an oxyhydrogen flame consisting of hydrogen
and air. In most cases silicon tetrachloride is used. This
substance is hydrolysed to silicon dioxide and hydrochloric
acid by the action of the water formed during the hydrogen-
oxygen reaction. After leaving the flame, the silicon
dioxide enters a so-called coagulation zone, wherein the
primary Aerosil particles and primary Aerosil aggregates
agglomerate. The product, which exists at this stage as a
kind of aerosol, is separated from the accompanying gaseous
substances in cyclones and then aftertreated with moist
heated air.
By this process the residual hydrochloric acid content can
be lowered to 0.025%. As the silicon dioxide obtained at
the end of this process has a bulk density of only about
15 g/1, there is a subsequent vacuum compaction, whereby
tamped densities of about 50 g/1 and more can be
established.
The particle sizes of the silicon dioxides can be varied by
means of the reaction conditions such as, for example,
flame temperature, proportions of hydrogen and oxygen,
quantity of silicon tetrachloride, residence time in the
flame or length of the coagulation path.
The BET surface area is determined using nitrogen in
accordance with DIN 66 131.
The pore volume is calculated from the sum of the micro-,
meso- and macropore volumes. The micro- and mesopores are
determined by recording an N2 isotherm and evaluation
thereof by the methods of BET, de Boer and Barrett, Joyner
and Halenda. The macropores D > 30 nm are determined by the
Hg porosimetry method. For the determination of the
macropores, the sample is dried for 15 h at 100 °C in the
drying oven and degassed at room temperature in a vacuum.
For the determination of the micro- and mesopores, the
sample is dried for 15 h at 100 °C in the drying oven and
degassed for 1 h at 200 °C in a vacuum.
The silanol group concentration is determined by the
lithium alanate method. Here the SiOH- groups are reacted
with LiAlH4 and the quantity of hydrogen formed during this
reaction is determined from the pressure.
Principle of measurement
The granular material is weighed into a four-necked flask.
The flask is evacuated and an oil bath is heated to 150 °C
The temperature in the flask (controlled by an internal
thermometer) rises with the temperature of the oil bath to
about 130 °C. The pressure during the preliminary treatment
is recorded using a pressure measuring device PI2 (TM 210,
from Leybold, measuring range 103 to 10"3 mbar) . The
desorption of the water can be monitored from the pressure
measurement. At the end of the preliminary treatment (30
min at the end temperature) a pressure of less than 10"
mbar must have been achieved.
After completion of the preliminary treatment, the
evacuated flask is separated from the vacuum unit by
closing the stop valve and is brought to normal
temperature. The actual measurement is based on a measured
quantity of LiAlH4 solution being introduced into the flask
through the dropping funnel and the rise in pressure being
measured from the hydrogen formed. If the volume of the
flask is known, the quantity of H2 can be calculated from
the ideal gas law. The pressure is recorded using a digital
measuring device (PIJ (MKS Instruments PR-2000), having a
measuring range of between 0 and 1 bar.
The LiAlH4 solution used (2% LiALH4 in diethylene glycol
dimethyl ether) is degassed prior to the experiment being
carried out, in order to remove readily volatile
constituents, which distort the pressure measurement. For
this purpose the pressure above the solution in the
dropping funnel is lowered by a second vacuum pump to the
vapour pressure (3.7 mbar at 22 °C) , so that the liquid
boils. A blank measurement without a sample is taken to
test whether the solution is sufficiently degassed. In the
determination of the hydrogen pressure, a correction is
made using the vapour pressure of the solvent.
Interpretation
The apparatus is calibrated by first of all determining the
volume of the dropping funnel provided with a ground-glass
stopper, gauging the capacity in litres. The volume of the
reaction flask inclusive of all connections as far as the
stop valve is obtained by the following experiment:
The dropping funnel, filled with air at atmospheric
pressure, is attached to the evacuated flask. A pressure
compensation between the two volumes is then brought about
by opening the tap of the dropping funnel. The pressure
established is indicated by the digital measuring device.
The volume of the reaction vessel is obtained from the mass
balance. A volume VR equal to 243.8 _ml is obtained with the
present arrangement.
The number of moles of hydrogen formed is obtained from the
equations:
p.V (corr.)
n = ------------
RT
corr. R solids solution
p is the increase in pressure in the reaction flask. This
value is corrected by an amount corresponding to the vapour
pressure of the solvent (3.7 mbar at 22 °C). At room
temperatures greatly differing from 22 °C the vapour
pressure is taken from the vapour pressure table. It is
useful so to select the weighed sample, that a value for p
of between 200 and 800 mbar is obtained. In this case minor
changes in the vapour pressure owing to temperature
variations have hardly any effect on the result.
The volume of the reaction vessel is corrected by deducting
the volume of solid matter and the volume of the solution
introduced. The former is given from the weighed portion
and the density and the latter is read from the dropping
funnel.
The density of silanol groups is finally obtained from the
equation:
n-NL
d = -----
F
NL: Lohschmidt number
F: Surface of the weighed solid matter
The samples are treated as follows:
1 h heating at 120oC and 0.2 mbar; cooling to 60 °C;
addition of LiAlH4; after 10 mins, reading the pressure
difference which has arisen.
The particle size distribution is determined by means of
the laser optical particle size analyser Cilas Granulametre
715.
The tapped volume is determined in accordance with ASTM D
4164-88.
Equipment: Tapping volumeter STA V 2003 from
Engelsmann, in accordance with DIN 53194,
section 5.2. b-f
Measuring cylinder 250 ml, graduation marks
every 2 ml
Balance with limit of error of max. ± 0.1 g
Procedure
The counter of the tapping volumometer is set to 1000
strokes.
The measuring cylinder is tared.
The granular material is placed in the measuring cylinder
up to the 250 ml mark.
The weight of the sample is recorded (± 0.1 g).
The measuring cylinder is placed in-the volumeter and the
apparatus is switched on.
End of tapping: the apparatus automatically switches off
after 1000 strokes
The tapped bulk volumes are read to an accuracy of 1 ml.
Calculation
E: weighed portion of granular material in g
V: volume read in ml
W: water content in wt.% (determined in accordance with
Specification P001)
E x (100 - W)
Tapped density = -------------
V x 100
The pH value is determined in 4% aqueous dispersion, in the
case of hydrophobic catalyst supports in water:ethanol 1:1.
Preparation of the granules according to the invention
The pyrogenically prepared silicon dioxide is dispersed in
completely demineralised water. A dispersing aggregate
which operates according to the rotor/stator principle is
used in the process. The suspensions formed are spray
dried. The finished product is precipitated by a filter or
cyclone.
The sprayed granules are heated in a muffle furnace.
The spray-dried and optionally heated granules are placed
in a mixer for the silanisation process and sprayed with
intensive mixing optionally first of all with water and
then with the silane Si 108 (trimethoxyocty.lsi.lane) or HMDS
(hexamethyldisilazane). After spraying has been completed,
the material is mixed for 15 to 30 more minutes and then
heated for 1 to 4 h at 100 to 400 °C.
The water used can be acidified with an acid, for example,
hydrochloric acid, to a pH value of 7 to 1. The silanising
agent used can be dissolved in a solvent such as, for
example, ethanol.
WE CLAIM:
1. Granules based on pyrogenically prepared silicon dioxide
and having the following physicochemical properties:
Average grain size: 10 to 120 µm
BET surface area: " 40 to 400 m2/g
Pore volume: 0.5 to 2.5 ml/g
Pore size distribution: less than 5 % of the total
pore volume exists of pores
with a diameter meso- and macropores
pH value: 3.6 to 8.5
Tapped density: 220 to 700 g/1
2. Method for the preparation of granules based on
pyrogenically prepared silicon dioxide as claimed in
claim 1, wherein pyrogenically prepared silicon dioxide,
preferably silicon dioxide prepared from silicon
tetrachloride by means of flame hydrolysis, is dispersed
in water, spray dried, the granules obtained are
optionally heated at a temperature of from 150 to
1,100 °C for a period of 1 to 8 h and/or silanised.
3. Method as claimed in claim 2, wherein pyrogenically
prepared silicon dioxide is dispersed in water and spray
dried and the granules obtained are heated at a
temperature of from 150 to 1,100°C for a period of 1 to 8
h.
4. Method as claimed in claim 2, wherein pyrogenically
prepared silicon dioxide is dispersed in water and spray
dried and the granules obtained are silanised.
5. Method as claimed in claim 2, wherein pyrogenically
prepared silicon dioxide is dispersed in water and spray
dried and the granules obtained are heated at a
temperature of from 150 to 1,100°C for a period of 1 to
8h and subsequently silanised.
6. Method as claimed in claims 2 to 5, wherein as
pyrogenically prepared silicon dioxide a pyrogenic
silicon dioxide powder with
- a BET surface area of 30 to 90 m2/g,
a DBP index of 80 or less
a mean aggregate area of less than 25000 nm2,
a mean aggregate circumference of less than 1000 nm,
wherein at least 70% of the aggregates have a
circumference of less than 1300 nm is used.
7. Method as claimed in claims 2 to 5, wherein as
pyrogenically prepared silicon dioxide a high-purity
pyrogenically prepared silicon dioxide, having a metal
content of less than 9 ppm is used.

Granules based on pyrogenically prepared silicon dioxide and having the
following physicochemical properties: Average grain size: 10 to 120 ?m; BET
surface area: 40 to 400 m2/g; Pore volume: 0.5 to 2.5 ml/g; Pore size distribution:
less than 5 % of the total pore volume exists of pores with a diameter rest meso- and macropores pH value: 3.6 to 8.5; Tapped density: 220 to 700
g/l.

GRANULES BASED ON PYROGENICALLY PREPARED SILICON DIOXIDE AND METHOD FOR THEIR PREPARATION

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