Title of Invention

A PROCEES FOR THE PREPARATION OF PYROGENIC SILICON-TITANIUM MIXED OXIDE POWDER

Abstract

Process for the preparation of a pyrogenic silicon-titanium mixed oxide powder having a BET surface area is 200 to 400 m2 /g, silicon dioxide content of 97.0 ± 1.5% by weight, titanium dioxide content of 3.5 ± 1.0% by weight and the sum of silicon dioxide content and titanium dioxide content being greater than 99.7% by weight, all percentages by weight relating to the total amount of the powder, said process is characterized in that 97.0 ±1.5 parts by weight calculated as SiO2 of silicon halide and 3.5 ±1.5 parts by weight calculated as TiO2 of titanium halide are evaporated, the vapours are taken to a mixing chamber, hydrogen and primary air are taken to the mixing chamber separately therefrom, the mixture of the vapours of silicon halide and titanium halide, hydrogen- containing combustible gas and primary air is subsequently ignited in a burner and the flame is burned into a reaction chamber, secondary air is additionally introduced into the reaction chamber, the solid is subsequently separated from gaseous substances, and the solid is subsequently freed as far as possible from halide-containing substances by treatment with steam at temperatures of 250 to 700°C the amount of the required substances consisting of silicon chloride, titanium chloride, combustible gas, primary air and secondary air being chosen such that an adiabatic flame temperature Tad results, for which the following is true: 900°C < Tad < 1200°C, with Tad = temperature of required substances + sum of the reaction enthalpies of the partial reactionslhead capacity of the substances which leave the reaction chamber, comprising silicon dioxide, water, hydrogen chloride, if appropriate carbon dioxide, oxygen, nitrogen, and if appropriate of the carrier gas if this is not air or nitrogen, the specific heat capacity of these substances at 1000°C being used as a basis.

Full Text

Silicon-titanium mixed oxide powder, dispersion thereof and titanium-containing zeolite prepared therefrom The invention relates to a pyrogenic silicon-titanium mixed oxide powder and its preparation. The invention furthermore relates to a dispersion comprising the pyrogenic silicon-titanium mixed oxide powder. The invention furthermore relates to processes for the preparation of a titanium-containing zeolite by means of the pyrogenic silicon-titanium mixed oxide powder or of a dispersion comprising this powder. The invention furthermore relates to the titanium-containing zeolites obtainable by these processes and to their use as a catalyst. The use of silicon-titanium mixed oxide powders for the preparation of titanium-containing zeolites is known from EP-A-814058. Titanium-containing zeolites are efficient catalysts for the oxidation of olefins using hydrogen peroxide. They are obtained by hydrothermal synthesis starting from silicon-titanium mixed oxide powders in the presence of a template. In EP-A-814058, it is disclosed that pyrogenic silicon-titanium mixed oxides having a silicon dioxide content of 75 to 99.9% by weight and a titanium dioxide content of 0.1 to 25% by weight can be employed for this. A composition which contains from 90 to 99.5% by weight of silicon dioxide and 0.5 to 5% by weight of titanium dioxide is particularly advantageous. As templates, amines, ammonium compounds or alkali/alkaline earth metal hydroxides can be employed. A disadvantage of the process disclosed in EP-A-814058 is the long reaction time which is necessary for the reaction of the silicon-titanium mixed oxide in the presence of the template. Furthermore, not all titanium-containing zeolites obtained according to EP-A-814058 show adequate catalytic activity. An object of the invention was therefore to make available a silicon-titanium mixed oxide, with which the reaction times in the preparation of the titanium-containing zeolite can be reduced. A further object of the invention was to make available a titanium-containing zeolite having a high catalytic activity. The invention relates to a pyrogenic silicon-titanium mixed oxide powder, which has - a BET surface area of 200 to 400 m2/g, - a silicon dioxide content of 97.0 ± 1.5% by weight, - a titanium dioxide content of 3.5 ± 1.0% by weight and - in which the sum of silicon dioxide content and titanium dioxide content is greater than 99.7% by weight, all percentages by weight relating to the total amount of the powder. Pyrogenic is to be understood as meaning metal mixed oxide particles obtained by flame oxidation and/or flame hydrolysis. In this process, oxidizable and/or hydrolysable starting, substances are as a rule oxidized or hydrolysed in a hydrogen-oxygen flame. The metal mixed oxide particles according to the invention are as far as possible pore-free and have free hydroxyl groups on the surface. They are present in the form of aggregated primary particles. It has been shown that a high BET surface area markedly reduces the period of time for the preparation of a titanium-containing zeolite from the silicon-titanium mixed oxide powder according to the invention. A silicon-titanium mixed oxide powder according to the invention having a BET surface area of 250 to 350 m2/g is preferred and particularly preferably one of 300 ± 30 m2/g. Furthermore, a silicon-titanium mixed oxide powder having a silicon dioxide content of 97.0 ± 1.0% by weight and a titanium dioxide content of 3.5 ± 0.75% by weight is preferred where the sum of silicon dioxide content and titanium dioxide content is greater than 99.9% by weight. A silicon-titanium mixed oxide powder having a silicon dioxide content of 97.0 ± 0.5% by weight and a titanium dioxide content of 3.5 ± 0.5% by weight is particularly preferred where the sum of silicon dioxide content and titanium dioxide content is greater than 99.9% by weight. The sum of silicon dioxide content and titanium dioxide content in the powder according to the invention is greater than 99.7% by weight and preferably greater than 99.9% by weight. The content of the metals Al, Ca, Co, Fe, K, Na, Ni and Zn is preferably less than 50 ppm each and particularly preferably less than 25 ppm each. The content of chloride is preferably less than 7 00 ppm. It has proved advantageous for the preparation of titanium-containing zeolites if the contents of these metals and chloride do not exceed these values. These impurities can originate from the required substances and/or can be caused due to the process. A further subject of the invention is a process for the preparation of the silicon-titanium mixed oxide powder according to the invention in which - 97.0 ± 1.5 parts by weight calculated as SiO2 of a silicon chloride and 3.5 ± 1.0 parts by weight calculated as TiO2 of a titanium chloride are evaporated, the vapours are taken to a mixing chamber, hydrogen and primary air are taken to the mixing chamber separately therefrom, - the mixture of the vapours of silicon chloride and titanium chloride, hydrogen-containing combustible gas and primary air is subsequently ignited in a burner and the flame is burned into a reaction chamber, - secondary air is additionally introduced into the reaction chamber, the solid is subsequently separated from gaseous substances, and - the solid is subsequently freed as far as possible from halide-containing substances by treatment with steam at temperatures of 250 to 700°C - the amount of the required substances consisting of silicon chloride, titanium chloride, combustible gas, primary air and secondary air being chosen such that an adiabatic flame temperature Tad results, for which the following is true: 900°C with Tad = temperature of required substances + sum of the reaction enthalpies of the partial reactions/heat capacity of the substances which leave the reaction chamber, comprising silicon-titanium mixed oxide, water, hydrogen chloride, if appropriate carbon dioxide, oxygen, nitrogen, and if appropriate of the carrier gas if this is not air or nitrogen, the specific heat capacity of these substances at 1000°C being used as a basis. The specific heat capacities can be determined, for example, with the aid of the VDI Warmeatlas [VDI heat atlas] (Chapter 7.1 to 7.3 and 3.7, 8th Edition). The reaction of the silicon chlorides and titanium chlorides in the presence of oxygen and of a combustible gas yields silicon-titanium mixed oxide, water, hydrochloric acid and, in the case of carbon-containing silicon and/or titanium compounds and/or carbon-containing combustible gases, carbon dioxide. The reaction enthalpies of these reactions can be calculated by means of standard works known to the person skilled in the art. In Table 1, some selected values of reaction enthalpies of the reaction of silicon halides and titanium tetrachloride in the presence of hydrogen and oxygen are given. Methyltrichlorosilane (MTCS, CH3SiCl3) , trichlorosilane (TCS, SiHCl3) and/or dichlorosilane (DCS, SiH2Cl2) and titanium tetrachloride can particularly preferably be employed. Suitable combustible gases are hydrogen, methane, ethane, propane and/or natural gas, hydrogen being preferred. It can further be advantageous if the exit velocity of the reaction mixture from the mixing chamber to the reaction space is 10 to 80 m/s. The vapours of the silicon chloride and of the titanium chloride can also be taken to the mixing chamber, in mixed or separate form, by means of a carrier gas. The required substances combustible gas, primary air and/or secondary air can be introduced in preheated form. A suitable temperature range is 50 to 400°C. Furthermore, primary and/or secondary air can be enriched with oxygen. Preferably, the process according to the invention can be carried out such that SiCl4 is employed as silicon halide, TiCl4 is employed as titanium halide and the adiabatic flame temperature Tad = 1050 ± 50°C. A further subject of the invention is a dispersion which comprises the silicon-titanium mixed oxide powder according to the invention and water. The average aggregate diameter of the silicon-titanium mixed oxide particles in the dispersion is preferably less than 200 nm and particularly preferably less than 100 nm. Preferably, the following is true for the dispersion according to the invention: 10 silicon-titanium mixed oxide the range is 12 mixed oxide Furthermore, a dispersion can be preferred which additionally contains a basic, quaternary ammonium compound. Dispersions are particularly preferred which contain tetraalkylammonium hydroxides such as, for example, tetraethylammonium hydroxide, tetra-n-propylammonium hydroxide and/or tetra-n-butylammonium hydroxide. The content of quaternary, basic ammonium compound in the dispersion according to the invention is not limited. If the dispersion is to be stored for a relatively long time, it can be advantageous to add to it only a part of the amount of the dispersion necessary for the preparation of a titanium-containing zeolite. Preferably, the quaternary, basic ammonium compound can be added in such an amount that a pH of 9 to 11, in particular 9.5 to 10.5, results. The dispersion shows good stability in this pH range. If the dispersion is to be employed, for example, immediately after its preparation for the preparation of a titanium-containing zeolite, the dispersion can already also contain the total amount of quaternary, basic ammonium compound. Preferably, the following is then true: 0.12 mol of ammonium compound/mol of silicon-titanium mixed oxide silicon-titanium mixed oxide preferred. A further subject of the invention is a process for the preparation of the dispersion according to the invention, comprising the steps: - water, which, if the silicon-titanium mixed oxide powder introduced later leads to a pH of the aqueous phase of 4, is adjusted by addition of acids or bases to pHs of 2 to 4, is recycled from a receiver by means of a rotor/stator machine, and - an amount of the silicon-titanium mixed oxide powder according to the invention is introduced continuously or batchwise by means of a filling device and with the rotor/stator machine running into the shear zone between the slots of the rotor teeth and of the stator slots such that a predispersion having a solids content of 20 to 40% by weight results, and - after all the silicon-titanium mixed oxide powder has been added, the filling device is closed and the predispersion is sheared further such that the shear rate lies in the range between 10 000 and 40 000 s-1, and - if appropriate water and a basic, quaternary ammonium compound are subsequently added with retention of the dispersion conditions. A further subject of the invention is a process for the preparation of a titanium-containing zeolite, in which the silicon-titanium mixed oxide powder according to the invention and a basic, quaternary ammonium compound are treated in an aqueous medium at a temperature of 150 to 220°C for a period of less than 12 hours. Preferably, the process is carried out such that the following is true: 10 titanium mixed oxide range is 12 oxide It is furthermore advantageous to carry out the process such that the following is true: 0.12 compound/mol of silicon-titanium mixed oxide Particularly preferably, the range is 0.13 ammonium compound/mol of silicon-titanium mixed oxide As basic, quaternary ammonium compounds, tetraalkylammonium hydroxides such as, for example, tetraethylammonium hydroxide, tetra-n-propylammonium hydroxide and/or tetra-n- butylammonium hydroxide are particularly preferred. Basic, quaternary ammonium compounds are used as templates which determine the crystal structure by incorporation into the crystal lattice. Tetra-n-propylammonium hydroxide is preferably employed for the preparation of titanium silicalite-1 (MFI structure), tetra-n-butylammonium hydroxide for the preparation of titanium silicalite-2 (MEL structure) and tetraethylammonium hydroxide for the preparation of titanium (3-zeolites (BEA crystal structure) . A further subject of the invention is a process for the preparation of a titanium-containing zeolite, in which the dispersion according to the invention, if appropriate with further addition of a basic, quaternary ammonium compound, is treated at a temperature of 150 to 220 °C for a period of less than 12 hours. Under the specified conditions of the process according to the invention, the crystallization time is conventionally less than 12 hours. The crystals are separated by filtering, centrifuging or decanting and washed with a suitable washing liquid, preferably water. The crystals are then dried if needed and calcined at a temperature between 400°C and 1000°C, preferably between 500°C and 750°C in order to remove the template. The particle fineness of less than 200 nm in the dispersion leads to rapid dissolution of the particles and formation of the titanium-containing zeolite. A further subject of the invention is a titanium-containing zeolite which is obtainable by the process according to the invention starting from silicon-titanium mixed oxide powder. A further subject of the invention is a titanium-containing zeolite which is obtainable by the process according to the invention starting from the dispersion comprising silicon- titanium mixed oxide powder. Both titanium-containing zeolites are obtained in powder form. For their use as an oxidation catalyst, they are converted if needed to a form suitable for use, e.g. to micropellets, spheres, tablets, solid cylinders, hollow cylinders or honeycombs, using known methods for the creation of pulverulent catalysts, such as, for example, pelletization, spray drying, spray pelletization or extrusion. The titanium-containing zeolites according to the invention can be used as catalysts in oxidation reactions with hydrogen peroxide. In particular, they can be used as catalysts in the epoxidation of olefins with the aid of aqueous hydrogen peroxide in a water-miscible solvent. Examples: Required materials: The required materials silicon tetrachloride and titanium tetrachloride of Examples 1 to 5 have contents of Na, K, Fe, Co, Ni, Al, Ca and Zn of Examples 1 to 4: Titanium-silicon mixed oxide powder according to invention Example 1: 5.15 kg/h of silicon tetrachloride and 0.15 kg/h of titanium tetrachloride are evaporated. The vapours are taken to a mixing chamber by means of 15 Nm3/h of nitrogen as a carrier gas. Separately therefrom, 2 Nm3/h of hydrogen and 8 Nm3/h of primary air are introduced into the mixing chamber. The reaction mixture is fed to a burner and ignited in a central tube. The flame burns here in a water- cooled flame tube. 15 Nm3/h of secondary air are additionally introduced into the reaction space. The resulting powder is separated in a filter connected in series and subsequently treated with water vapour at 520°C in countercurrent. Examples 2-4 are carried out analogously to Example 1 using the amounts listed in the table. Example 5 is a comparative example whose composition lies in the range claimed, but has a markedly lower BET surface area than the claimed powders. The substance parameters of the powders obtained are summarized in the table. In all examples, the content of Na is Fe £1 ppm, Co Example 6: Preparation of a dispersion (according to the invention) 32.5 kg of completely demineralized water are initially introduced into a 100 1 stainless steel make-up vessel. Subsequently, with the aid of the suction nozzle of the Ystral Conti-TDS 4 (stator slots: 6 mm ring and 1 mm ring, rotor/stator distance about 1 mm), 17.5 kg of the silicon- titanium mixed oxide powder from Example 1 are drawn in under shear conditions. After completion of the drawing-in, the suction nozzle is closed and the 35 per cent by weight pre-dispersion is subsequently additionally sheared at 3000 rpm for 10 min. Undesired warming of the dispersion due to the high energy input is countered by a heat exchanger and the temperature increase is restricted to a maximum of 40°C. Due to the acidic character of the pyrogenically prepared silicon-titanium mixed oxide powder, the pH of the dispersion is about 3.6. WE CLAIM: 1. Process for the preparation of a pyrogenic silicon-titanium mixed oxide powder having a BET surface area is 200 to 400 m2 /g, silicon dioxide content of 97.0 ± 1.5% by weight, titanium dioxide content of 3.5 + 1.0% by weight and the sum of silicon dioxide content and titanium dioxide content being greater than 99.7% by weight, all percentages by weight relating to the total amount of the powder, said process is characterized in that - 97.0 ±1.5 parts by weight calculated as SiO2 of silicon halide and 3.5 ± 1.5 parts by weight calculated as TIO2 of titanium halide are evaporated, the vapours are taken to a mixing chamber, - hydrogen and primary air are taken to the mixing chamber separately therefrom, - the mixture of the vapours of silicon halide and titanium halide, hydrogen- containing combustible gas and primary air is subsequently ignited in a burner and the flame is burned into a reaction chamber, - secondary air is additionally introduced into the reaction chamber, the solid is subsequently separated from gaseous substances, and - the solid is subsequently freed as far as possible from halide-containing substances by treatment with steam at temperatures of 250 to 700°C - the amount of the required substances consisting of silicon chloride, titanium chloride, combustible gas, primary air and secondary air being chosen such that an adiabatic flame temperature Tad results, for which the following is true: 900°C required substances + sum of the reaction enthalpies of the partial reactionslhead capacity of the substances which leave the reaction chamber, comprising silicon dioxide, water, hydrogen chloride, if appropriate carbon dioxide, oxygen, nitrogen, and if appropriate of the carrier gas if this is not air or nitrogen, the specific heat capacity of these substances at 1000°C being used as a basis. 2. Process as claimed in claim 1, wherein SiCl4 is employed as silicon halide, TiCl4 is employed as titanium halide and Tad = 1050 ± 50°C. 3. Process as claimed in claim 1 or 2, wherein the exit velocity VB of the gases employed from the burner into the reaction space is 10 to 80 m/s. 4. Process for the preparation of a dispersion comprising pyrogenic silicon-titanium mixed oxide powder having a BET surface area is 200 to 400 m2/g, silicon dioxide content of 97.0 ± 1.5% by weight, titanium dioxide content of 3.5 ± 1.0% by weight and the sum of silicon dioxide content and titanium dioxide content being greater than 99.7% by weight, all percentages by weight relating to the total amount of the powder, comprising the steps: - water, which, if the silicon-titanium mixed oxide powder introduced later leads to a pH of the aqueous phase of 4, is adjusted by addition of acids or bases to pHs of 2 to 4, is recycled from a receiver by means of a rotor/stator machine, and 1 an amount of said silicon-titanium mixed oxide powder is introduced continuously or batchwise by means of a filling device and with the rotor /stator machine running into the shear zone between the slots of the rotor teeth and of the stator slots such that a pre- dispersion having a solids content of 20 to 40% by weight results, and - after all the silicon-titanium mixed oxide powder has been added, the filling device is closed and the predispersion is sheared further such that the shear rate lies in the range between 10 000 and 40 000 s-1, and - if appropriate water and a basic, quaternary ammonium compound are subsequently added with retention of the dispersion conditions. ABSTRACT Title: A process for the preparation of pyrogenic silicon-titanium mixed oxide powder. Process for the preparation of a pyrogenic silicon-titanium mixed oxide powder having a BET surface area is 200 to 400 m2 /g, silicon dioxide content of 97.0 ± 1.5% by weight, titanium dioxide content of 3.5 ± 1.0% by weight and the sum of silicon dioxide content and titanium dioxide content being greater than 99.7% by weight, all percentages by weight relating to the total amount of the powder, said process is characterized in that 97.0 ±1.5 parts by weight calculated as SiO2 of silicon halide and 3.5 ±1.5 parts by weight calculated as TiO2 of titanium halide are evaporated, the vapours are taken to a mixing chamber, hydrogen and primary air are taken to the mixing chamber separately therefrom, the mixture of the vapours of silicon halide and titanium halide, hydrogen- containing combustible gas and primary air is subsequently ignited in a burner and the flame is burned into a reaction chamber, secondary air is additionally introduced into the reaction chamber, the solid is subsequently separated from gaseous substances, and the solid is subsequently freed as far as possible from halide-containing substances by treatment with steam at temperatures of 250 to 700°C the amount of the required substances consisting of silicon chloride, titanium chloride, combustible gas, primary air and secondary air being chosen such that an adiabatic flame temperature Tad results, for which the following is true: 900°C substances + sum of the reaction enthalpies of the partial reactionslhead capacity of the substances which leave the reaction chamber, comprising silicon dioxide, water, hydrogen chloride, if appropriate carbon dioxide, oxygen, nitrogen, and if appropriate of the carrier gas if this is not air or nitrogen, the specific heat capacity of these substances at 1000°C being used as a basis.

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