RU2804511C2 - Production of powder-like porous crystalline silicates of metals using flame spray pyrolysis - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to a method for producing powdered porous crystalline metal silicates. The method for producing powdered porous crystalline titanium silicalite-1 zeolite includes the steps of: a) hydrothermal synthesis in an aqueous mixture containing (A) amorphous silicon dioxide, (B) an aqueous solution of titanium tetrapropylammonium hydroxide and (C) an aqueous solution of tetrapropylammonium hydroxide, while obtaining an aqueous a suspension containing a porous crystalline metal silicate as a reaction product, and b) calcining the reaction product, wherein the calcination is carried out using flame spray pyrolysis at an adiabatic combustion temperature in the range of 450-2200°C. Next, a suspension containing ≤70 wt.% solids obtained in step a) is sprayed into a flame generated by burning fuel in the presence of oxygen, whereby a porous crystalline titanium silicalite-1 powder is obtained.

EFFECT: development of a method for obtaining powdered porous crystalline metal silicates, which includes a smaller number of technological stages compared to the standard process and solves the problem of industrial waste disposal.

9 cl, 4 dwg, 2 tbl, 10 ex

Description

The present invention relates to a method for producing powdered porous crystalline metal silicates.

Silicates are compounds formed by SiO ₄ tetrahedra, which can be connected to each other in various ways. Silicate structures in this class may include other elements, such as metals, and are then called metal silicates. An important example of these metal silicates are zeolites.

Zeolites are crystalline silicates, for example, aluminosilicates, in which the three-dimensional combination of silicate tetrahedra (SiO ₄ ^- ) and other structural units (for example, AlO ₄ ^- tetrahedra) leads to the formation of regular structures characterized by cavities and pores. There are different types of zeolites, which are named according to the type of structure they have. General information relating to zeolites, especially the types of crystal structure of known zeolites, can be found in Ullmann's Encyclopedia of Industrial Chemistry, in the chapter "Zeolites", published online 04/15/2012, DOI: 10.1002 /14356007.a28_475.pub2.

Due to their unique pore structure, zeolites have interesting catalytic properties and can be used, for example, as oxidation catalysts.

Synthetic zeolites are in most cases obtained by hydrothermal synthesis in the presence of a matrix that forms a porous structure. For example, patent CN 101348263 A describes a method for producing zeolites characterized by a Si/Al ratio from 50 to 5000 and a particle size from 30 to 200 μm, which includes the following technological steps: (1) obtaining a reaction mixture including sources of silicon and sources of aluminum, as well as metal hydroxide, (2) hydrolysis reaction, (3) subsequent spray drying of the mixture to obtain aluminosilicate microspheres, (4) hydrothermal reaction of the microspheres obtained as described above in the presence of water and an organic amine at a temperature of 160 to 200°C and crystallization of the resulting zeolite, and (5) washing, (6) drying and (7) calcination of the product at a temperature of 350 to 800°C.

US Pat. No. 4,410,501 A describes a method for producing titanium silicalite. Titanium silicalite is obtained by (1) forming a synthetic gel from a hydrolyzable silicon compound, for example, tetraethylorthosilicate, and a hydrolyzable titanium compound in the presence of tetra-n-propylammonium hydroxide at 175°C, (2) subsequent hydrothermal synthesis, hydrolysis and crystallization of the said reaction mixture. After crystallization is complete, the crystals are (3) separated by filtration, (4) washed, (5) dried, and finally (6) calcined at 550°C for 6 hours.

Patent EP 814058 A1 describes the preparation of various zeolites from corresponding pyrogenic mixed oxides of silicon and metal. Mixed oxides of silicon and metal are prepared by (1) hydrothermal synthesis at temperatures from 100 to 220°C in the presence of a matrix selected from amines, ammonium compounds and alkali metal/alkaline earth metal hydroxides, followed by (2) filtration, (3) washing with water and (4) calcination, for example, at a temperature of 550°C for 4 hours. In a particular embodiment, a preformed granular mixed oxide material containing a matrix is obtained by spray drying, which is then subjected to hydrothermal synthesis, separated by filtration, washed and calcined.

The patent CN 1482062 describes a method for producing titanium silicalite-1, according to which a hydrothermal reaction of solid silica gel with an inorganic source of titanium is carried out. The method includes the following stages: (1) impregnation of solid silica gel with Ti(SO ₄ ) ₂ salt, (2) calcination, (3) hydrothermal synthesis using silica gel and a mixture of Ti(SO ₄ ) ₂ + TPAOH + water, (4) filtration, (5) washing, (6) drying, (7) calcination.

All of the methods described above involve time-consuming, multi-step techniques. More specifically, filtering and washing the resulting zeolite is necessary to remove impurities from accompanying reagents or unreacted reagents and the like. This leads to the formation of significant quantities of contaminated water, which in most cases contains substances harmful to the organisms present in the water that are difficult to dispose of, such as tetraalkylammonium salts. In this regard, it is urgent to develop a process that, firstly, includes a smaller number of technological stages compared to the standard process, and, secondly, solves the problem of recycling industrial waste.

These problems are solved by a method for producing powdered porous crystalline metal silicate, which includes the following stages:

a) hydrothermal synthesis in an aqueous mixture comprising (A) at least one source of silicon, (B) at least one source of metal and (C) at least one mineralizing additive, thereby obtaining an aqueous suspension containing a porous crystalline metal silicate as a reaction product, and

b) calcination of the reaction product, characterized in that

calcination is carried out using flame spray pyrolysis at an adiabatic combustion temperature in the range of 450-2200°C, with a suspension containing ≤70 wt. % solids obtained in step a) is atomized in a flame created by burning fuel in the presence of oxygen, thereby forming a powdery porous crystalline metal silicate.

Surprisingly, it has been found that the structure of the metal silicate obtained in step a) of the method of the present invention is preserved during the heat treatment at relatively high temperatures in step b), this is especially true for the porous structure of zeolites. It is known that the ordered porous structure of, for example, zeolites is destroyed at elevated temperatures. For example, according to the article Advanced Materials Research, vol. 287-290, pp. 317-321 (2011), titanium silicalite-1 undergoes irreversible structural changes even at 650°C, which can negatively affect the catalytic properties of such materials.

The powdery porous crystalline metal silicate produced by the process of the present invention preferably has a zeolite structure.

Zeolites are crystalline silicates, for example, aluminosilicates, in which the three-dimensional attachment of silicate tetrahedra (SiO ₄ ^- ) and other structural units (for example, AlO ₄ ^- tetrahedra) through oxygen atoms leads to the formation of regular structures containing cavities and pores. There are different types of zeolites, which are named according to their type of structure. General information relating to zeolites, especially the types of crystal structure of known zeolites, can be found in Ullmann's Encyclopedia of Industrial Chemistry, in the chapter "Zeolites", published online 04/15/2012, DOI: 10.1002 /14356007.a28_475.pub2.

The powdered porous crystalline metal silicate produced by the method of the present invention is preferably characterized by a zeolite structure with a crystal structure of the LTA, MFI, FAU, MOR, MEL or MWW type. More preferably, the metal silicates are characterized by MFI and MEL type crystal structures. The appropriate type of crystal structure can be determined by X-ray diffraction (XRD) using X-ray diffraction. The types of structure of micro- and mesoporous materials are established by the International Zeolite Association (IZA, www.iza-online.org).

The powdered porous crystalline metal silicate produced by the process of the present invention preferably contains micro- and mesopores. According to the definition of GOPAC (International Union of Pure and Applied Chemistry), micropores are characterized by a diameter of less than 2 nm, and the diameter of mesopores ranges from 2 nm to 50 nm.

The general composition of powdered porous crystalline metal silicates is usually described by the following formula:

(SiO ₂ ) _1-x (A _m O _n ) _x ,

A means a metal of valency p selected from the group consisting of Ti, Al, Zr, Fe, Sn, Ge, In and B, the type subscripts mean the number of atoms, where m×p=2n, x means a number from 0.0001 to 0.25, preferably from 0.001 to 0.2 and especially from 0.005 to 0.1. In the case of several different metals A, x respectively refers to the total sum of all metal oxides. A preferably means the elements titanium (Ti), aluminum (Al), zirconium (Zr), iron (Fe), tin (Sn) and boron (B), Ge (germanium), In (indium), preferably titanium (Ti) .

The powdery porous crystalline metal silicate obtained by the method of the present invention may preferably be titanium silicate, borosilicate, zirconium silicate, aluminosilicate or iron silicate. First of all, titanium silicate is preferred, in particular titanium silicalite-1 (MFI structure) or titanium silicalite-2 (MEL structure).

The source of silicon used in the method of the present invention can in principle be any compound which contains silica or a mixed oxide containing silica, or from which silica can be formed by oxidation or thermal and/or hydrolytic decomposition. However, preferred are compounds that contain amorphous silica or an amorphous mixed oxide containing silica, or from which amorphous silica can be formed by oxidation or thermal and/or hydrolytic decomposition. Such a silica source may, for example, be selected from the group consisting of pyrogenic silica, precipitated silica, sol-gel silica, and mixtures thereof.

Pyrogenic silica, also called fumed silica, is produced using flame hydrolysis or flame oxidation. This includes the oxidation or hydrolysis of hydrolyzable or oxidizable starting materials, typically in a hydrogen-oxygen flame. Feed materials that can be used for pyrogenic processes include organic and inorganic substances. First of all, silicon tetrachloride is suitable. The hydrophilic silica thus obtained is amorphous. Highly dispersed silica is usually found in aggregated form. The term “aggregated” should be understood in such a way that the initially formed so-called primary particles form strong internal bonds with each other in the further course of the reaction with the formation of a three-dimensional network. The primary particles are largely free of pores and have free hydroxyl groups on their surface.

Silicon dioxide obtained by precipitation (precipitated silicon dioxide) is formed, for example, during the reaction of solutions of liquid glass (sodium silicates) with inorganic acids.

The sol-gel process is a process for producing non-metallic inorganic or hybrid polymeric materials from colloidal dispersions called sols.

The starting materials for the production of sols in most cases are metal or silicon alkoxides. Hydrolysis of such starting materials and condensation of the resulting reactive compounds are important basic reactions of the sol-gel process. Suitable sources of silicon for sol-gel processes are primarily tetraalkyl orthosilicates, where the alkyl is preferably selected from the group consisting of methyl, ethyl, propyl and butyl. The most preferred tetraalkyl orthosilicate is tetraethyl orthosilicate.

The metal source used in the method of the present compound may be any compound that contains a metal oxide or a metal-containing mixed oxide, or from which a corresponding metal oxide or mixed oxide can be formed by oxidation or thermal and/or hydrolytic decomposition. The metal sources used in the context of the present invention are sources of the following elements: titanium (Ti), aluminum (Al), zirconium (Zr), iron (Fe), tin (Sn), germanium (Ge), indium (In) and boron (B), titanium is primarily preferred.

One skilled in the art will select suitable sources of silicon and metal at his or her discretion. In principle, one skilled in the art selects from the following combinations: a) the silicon source and the metal source are in liquid form, b) the silicon source is in solid form and the metal source is in liquid form, c) the silicon source and the metal source are both are part of the same substance.

As used herein, the term "in liquid form" means that the silicon source and/or metal source is present in the form of a liquid or solution.

Sources of silicon in solid form can, for example, be selected from the group consisting of pyrogenic silica, precipitated silica, sol-gel silica, and mixtures thereof. Preferred is highly purified precipitation silica or pyrogenic silica.

High purity precipitation silica is a precipitation silica that contains

a) aluminum not more than 1 ppm,

b) boron not more than 0.1 ppm,

c) calcium not more than 0.3 ppm,

d) iron not more than 0.6 ppm,

e) nickel not more than 0.5 ppm,

g) phosphorus not more than 0.1 ppm,

h) titanium not more than 1 ppm,

i) zinc not more than 0.3 ppm,

where the total of the above elements plus sodium and potassium is less than 5 ppm. Such highly purified silica can be obtained by the method described in WO 2010/037702. The contents of WO 2010/037702 are incorporated herein by reference in their entirety.

The silicon source and the metal source may both be present in the same substance in different ways. Examples of such substances are metal-silicon mixed oxides, metal oxide-doped silica, metal-impregnated silica, metal silicate, metal-doped tetraalkyl orthosilicate, and mixtures thereof. Substances of this type are preferably amorphous. Preferably, the material is a metal oxide doped amorphous silica, a metal impregnated amorphous silica, or an amorphous metal-silicon mixed oxide.

"Mixed oxide of metal and silicon" contains, along with SiO _2, one or more metal oxides, preferably selected from the group consisting of GeO ₂ , In ₂ O ₃ , Al ₂ O ₃ , TiO ₂ , B ₂ O ₃ , SnO ₂ , ZrO ₂ or Fe ₂ O ₃ . Mixed oxides of metal and silicon can be prepared by any suitable method, for example, flame pyrolysis, co-precipitation, sol-gel process. Mixed oxides of metal and silicon are described, for example, in patents EP 0814058 or DE 102007049742.

"Metal oxide doped silica" can be produced by any known method, such as flame pyrolysis or impregnation followed by calcination.

"Metal-impregnated silica" can be produced by any known impregnation method, for example, "moisture impregnation" methods.

In a preferred embodiment of the method of the present invention in step a), component (A) and component (B) are both present in a single substance, and said substance is selected from the group consisting of amorphous mixed silica and metal oxide, amorphous silica doped with metal oxide , metal-impregnated amorphous silica, metal silicate, metal-doped tetraalkyl orthosilicate, and mixtures thereof. More preferably, component (A) in this context is a metal oxide doped amorphous silica, a metal impregnated amorphous silica, or an amorphous silica-metal mixed oxide.

In another preferred embodiment of the method of the present invention, in step a), component (A) is present in solid form and component (B) is in liquid form. More preferably, component (A) in this context is selected from the group consisting of fumed silica, precipitated silica, sol-gel silica, and mixtures thereof. Most preferably, component (A) in this context is highly purified precipitation silica or pyrogenic silica.

Hydrothermal synthesis of powdered porous crystalline metal silicates is itself known. The same applies to the hydrothermal synthesis of zeolites, especially metal zeolites. Therefore, in principle, step a) of the method of the present invention is not limited to specific parameters. On the contrary, all reagents and parameters that are known to those skilled in the art for the hydrothermal synthesis of porous crystalline metal silicates are also suitable.

In step a) of the method of the present invention, when carrying out hydrothermal synthesis in an aqueous mixture comprising (A) at least one source of silicon, (B) at least one source of metal and (C) at least one mineralizing additive, as a product the reaction produces an aqueous suspension containing porous crystalline metal silicate. Hydrothermal synthesis, also called hydrothermal crystal growth, is crystallization from aqueous mixtures at temperatures above 100°C to approximately above 300°C and elevated pressures up to approximately 100 bar. This means that growth from aqueous mixtures is suitable for substances that are moderately soluble under normal conditions. Preferably, step a) of the method of the present invention is carried out at a temperature of from 100 to 250°C, more preferably at a temperature of from 100 to 200°C, under autogenous pressure created in a high pressure reactor, for example, in an autoclave. The pressure set during the hydrothermal synthesis in step a) of the process of the present invention can be in the range from 1.05 to 50 bar. Preferably the pressure is in the range from 1.5 to 30 bar, more preferably in the range from 2 to 20 bar.

Under the above conditions for carrying out the method of the present invention, the duration of step a) of the method of the present invention is usually less than 12 hours. Preferably, the duration of step a) is in the range of 0.1 to 6 hours, more preferably in the range of 0.5 to 4 hours

The mineralizing additives used in most cases are HF or organic or inorganic bases such as quaternary ammonium hydroxides, such as tetrapropylammonium hydroxide, or inorganic hydroxides such as sodium hydroxide, potassium hydroxide, ammonium hydroxide or calcium hydroxide. These additives serve to increase the solubility of silicon sources and metal sources, as well as to achieve a pH value that is optimal for crystal formation. In addition, it is also possible to use compounds which, as mineralizing additives, can form complexes with metal sources and silicon sources. Hydrothermal synthesis is generally carried out in an alkaline environment at a pH above 7. Preferably the pH is in the range of 8 to 14, more preferably in the range of 9 to 13.

To optimally carry out step a) of the method of the present invention, the aqueous mixture may further include suitable seed crystals. Seed crystals of this type are known to one skilled in the art.

In the method of the present invention, the aqueous mixture in step a) may further comprise a matrix selected from the group consisting of amines, quaternary ammonium compounds, alcohols and mixtures of these compounds. The matrix is a compound that, when incorporated into the crystal lattice of a product during hydrothermal synthesis, determines the crystal structure of the resulting metal silicate. Examples of suitable amines include: tri(n-propyl)amine, di(n-propyl)amine, n-propylamine, di(n-butyl)amine, ethylenediamine, propanolamine, ethanolamine, quinuclidine and substituted derivatives thereof, ammonium, ammonium salts, morpholine, 1,5-diaminopentane, 1,6-diaminohexane, dipropylenetriamine, dihexamethylenetriamine, triethylenetetramine, diethylenetriamine, 1-alkyl-4-azobicyclo[2.2.2]octane-4-oxide halides. Examples of suitable alcohols include: glycerin, hexanediol, pentanediol.

First of all, it is preferred to use tetraalkylammonium compounds such as tetraalkylammonium hydroxides, especially tetra-n-propylammonium hydroxide (TPAOH), for example, to obtain titanium silicalite-1 (MFI structure), tetra-n-butylammonium hydroxide, for example, to obtain silicalite -2 titanium (MEL structure), as well as tetraethylammonium hydroxide. Quaternary ammonium compounds are preferably used in the form of aqueous solutions.

The molar ratio of the amount of matrix used in step a) of the method of the present invention and the amount of silicon used is in principle not limited. Preferably, the ratio is ≥0.12 moles of matrix and less than <0.20 moles of silicon.

The term "flame spray pyrolysis" is known to one skilled in the art and refers to the process of oxidative thermal conversion of a liquid starting material dispersed as fine particles in an atomized gas stream or suspension in a flame generated by combustion of a fuel in the presence of oxygen. Flame spray pyrolysis is a common method for the production of metal oxides, which is described, for example, in application WO 2017/001366 A1 and US patent 2002/0041963 A1. For example, WO 2017/001366 A1 describes a process of this type for the production of powdered metal oxides using flame spray pyrolysis, in which a siloxane-containing aerosol is fed directly into a flame in a reactor where it is converted to silica.

Examples of fuels used in step b) of the method of the present invention include hydrogen, methane, ethane, propane, butane, natural gas and mixtures thereof. Said fuel is preferably supplied in a gaseous state to a reactor suitable for carrying out step b).

Oxygen can be supplied in the form of any oxygen-containing gas. Air is preferred.

Adiabatic combustion temperature is a standard parameter in the art for characterizing the combustion process of at least one fuel and oxidizer, the state of which is known before combustion begins. Accordingly, the adiabatic combustion temperature, for example, in a suitable reactor, can be calculated in a manner known to one skilled in the art, based on known process parameters such as preheat temperatures, specific mass flow rates, etc.

The adiabatic combustion temperature during stage b) of the method according to the present invention is in the range from 450 to 2200°C. First of all, preferably the adiabatic combustion temperature is in the range from 450 to 1600°C, more preferably in the range from 500 to 1400°C, even more preferably in the range from 500 to 1200°C, even more preferably in the range from 550 to 1000°C , most preferably in the range from 600 to 900°C.

Depending on the measurement site, the actual temperature generated in the flame varies over a relatively wide range. For example, in step b) of the method of the present invention, the temperature measured 1.5 m below the ignition site may be at least 300°C, preferably the temperature 1.5 m below the ignition site is in the range of 400 to 800°C.

The average residence time of the suspension obtained at stage a) in the reactor during stage b) can range from 1 ms to 100 s. Preferably, the average residence time is in the range of 0.1 s to 10 s, more preferably in the range of 0.5 s to 5 s. The calculation of the above-mentioned average residence time in the reactor (<t>, [s]) is carried out using the total volume of gas supplied to the reactor per unit time (V _t , [m ³ /s (STP)]) and the volume of the reactor (V _R , [m ³ ]).

<t>=V _R /v _t

The above-mentioned temperatures and the average residence time in stage b) of the method of the present invention are preferably chosen in such a way that the oxidative decomposition of the organic matrix occurs at the specified stage, while excluding damage to the porous structure of the resulting product under the influence of excessively high temperatures. Thus, if a relatively high flame temperature is established, which in the case of prolonged use can cause irreversible changes in the porous structure of the metal silicate, it is advisable to choose a relatively short residence time in the flame.

The solids content w _FT (wt %) in the aqueous suspension obtained in step a) can be calculated from the total mass of said suspension ( _MS ) and the mass of water in said suspension (M _H2O ):

W _FT =(M _S -M _H2O )/M _S *100%

The solids content of the suspension obtained in step a) is ≤70 wt. %. Preferably, the solids content is in the range of 10 wt. % up to 70 wt. %, more preferably in the range of 10 wt. % up to 60 wt. %, most preferably in the range of 20 wt. % up to 50 wt. %. Solid content more than 70 wt. % may cause technical difficulties during atomization and flame spray pyrolysis in step b) of the method of the present invention, while the solids content is less than 10 wt. % can have a negative effect on the economic efficiency of the process due to the excess of large quantities of water to be evaporated. Methods for controlling the solids content will be known to one skilled in the art, for example, the reagents can be used at a suitable concentration or the suspension can be suitably diluted.

The average diameter (d50) of the metal silicate particles in the aqueous dispersion that is obtained in step a) of the process of the present invention is preferably less than 500 nm and more preferably less than 400 nm. The average diameter of the metal silicate particles can be determined, for example, using dynamic laser light scattering (DLS).

Loss on ignition (in wt.%) is defined by the German industrial standard DIN 18128:2002-12 as the proportion of organic compounds in the sample. Calcination removes the organic component from the sample, for example, the carbon present is oxidized and eliminated as carbon dioxide. According to DIN 18128:2002-12, the loss on ignition of the porous crystalline metal silicate powder obtained by the process of the present invention is preferably less than 5 wt. %, more preferably less than 3 wt. %, most preferably less than 2 wt. %.

If a matrix is used, at least partial thermal and/or oxidative decomposition of the matrix preferably occurs in step b) of the method of the present invention. More preferably, according to the present invention, more than 70 wt. % of the matrix used, most preferably more than 90 wt. %.

The specific surface area of the powdered porous crystalline metal silicates produced by the process of the present invention may be ≥20 m ² /g, preferably 30 to 800 m ² /g, more preferably 50 to 700 m ² /g, most preferably 70 to 600 m ² /g. The specific surface area, also simply called surface area BET, is determined according to DIN 9277:2014 by nitrogen adsorption according to the Brunauer-Emmett-Teller (BET) method.

The total volume of nitrogen desorbed from the pores is calculated by the BJH method described in Barret, Joyner and Halenda, Journal of the American Chemical Society, 73, cc. 373-380 (1951).

The method of the present invention produces crystalline metal silicates in powder form. Accordingly, for use as oxidation catalysts, if necessary, they can be converted into a suitable form, for example, microgranules, spheres, tablets, cast cylinders, hollow cylinders or honeycomb bubbles by known methods for forming powdered catalysts, for example, compression, granulation, spray drying or extrusion.

In another embodiment of the method of the present invention, step b) is followed by step c), comprising the following steps:

(1) adding water to obtain an aqueous suspension of porous crystalline metal silicate,

(2) mixing the suspension obtained in step (1) with granulating auxiliaries,

(3) compression, granulation, spray drying or extrusion, which produces porous crystalline metal silicate in the form of microgranules, spheres, tablets, cast cylinders, hollow cylinders or honeycomb bubbles.

The particle size of such molded articles is preferably in the range from 0.1 mm to 10 mm.

For mixing and shaping, all known mixing and shaping devices and methods can be used, and all standard granulating agents can be used. Known forming devices of this type are described, for example, in Ullmann's Encyclopedia of Industrial Chemistry der Technischen Chemie, Ullmann's Encyclopedia of Industrial Chemistry, 4th ed., vol. 2, p. 295 ff. (1972)). It is preferable to use single and twin screw extruders or extrusion presses. In this case, it is possible to obtain a variety of known geometric shapes, for example, cast cylinders, hollow cylinders, stars, etc. However, it is also possible to obtain honeycomb bubbles.

The aqueous suspension obtained in step a) of the method of the present invention is atomized during step b), that is, atomized as fine particles in the surrounding gas, and thus forms an aerosol, a three-phase solid/liquid/gas mixture consisting of gas and liquid droplets distributed in it, which in turn contain solid particles. The gas used to atomize the aqueous suspension may contain oxygen and/or at least one of the above-mentioned fuels and/or at least one inert gas, for example nitrogen. For spraying, it is preferable to use N ₂ , H ₂ or air, air being particularly preferred.

The aerosol produced in step a) by spraying an aqueous suspension preferably comprises liquid droplets having a number average diameter of no more than 2 mm, more preferably no more than 1 mm, most preferably no more than 0.5 mm. Such number-average size of liquid droplets in an aerosol can be calculated, for example, by a person skilled in the art based on the results of determining the sizes recorded by the instruments used, the corresponding flow rates, the properties of the liquid and gas, and other parameters. Alternatively, the number-average size of the liquid droplets in the aerosol obtained in step a) can be measured directly using laser diffraction. The resulting measured droplet size distribution is used to determine the average diameter _d50 , which, as a number-average particle size, corresponds to a droplet size not exceeding 50% of all particles.

The spraying of the aqueous suspension, which is carried out in step b) of the method of the present invention, can be carried out using various installations and devices that are known to the person skilled in the art for this purpose. For example, in this context it is possible to use centrifugal atomizers, rotary atomizers, ultrasonic atomizers, single-phase, two-phase or multi-phase nozzles, as well as various injection systems or similar systems. Preferably, according to the method of the present invention, the aqueous suspension in step b) is sprayed into a flame through at least one nozzle.

The oxygen used in step b) of the process of the present invention can be supplied to the flame spray pyrolysis reactor in several sections. For example, the suspension used may be sprayed into a first gas stream comprising air, while the bulk of the air (primary air) is supplied to the flame as a second gas stream parallel to the flow direction of the supplied spray suspension, and a third gas stream (secondary air) may be supplied tangentially (at right angles to the direction of flow of the supplied spray suspension), for example, to prevent material deposition. Likewise, it is advantageous to introduce fuel into the reactor in several areas, for example, in the main stream (primary fuel stream) along with the primary air stream and the secondary stream (secondary fuel stream, external fuel), for example, to stabilize the generated flame.

First of all, it is advisable when carrying out stage b) of the method according to the present invention, the amount of oxygen exceeds the total amount of all flammable components of the reaction mixture. In this context, the reaction mixture means the suspension converted in step b), along with the gaseous components used in step b). The combustible components of said reaction mixture include, for example, the fuels and matrices used. The λ index (lambda) is the ratio obtained by dividing the amount of oxygen present in the reaction mixture by the amount of oxygen required for complete combustion of all combustible components of the reaction mixture, each amount being expressed in mol/h. Preferably, λ is set at a level from 1 to 10, more preferably, λ is selected at a level from 2 to 6.

The oxygen and fuel used in step b) of the process of the present invention can be supplied in preheated form. In this context, the range of suitable temperatures is from 50 to 400°C. The suspension obtained in step a) of the method of the present invention can also be fed into a flame preheated to a temperature of from 50 to 300°C. More preferably, said suspension can be used in step b) directly after being obtained in step a) without cooling.

The ratio of the total volume of gas used in step b), expressed in standard cubic meters, and the amount of aqueous suspension used, expressed in kg, is preferably from 0.1 to 100 m ³ /kg (normal conditions), more preferably from 0.5 up to 50 m ³ /kg (normal conditions), most preferably from 1 to 10 m ³ /kg (normal conditions).

Using the method of the present invention more preferably produces titanium-containing zeolites such as titanium silicalite-1 and titanium silicalite-2, which can be used as catalysts in oxidative reactions with hydrogen peroxide. More specifically, such titanium-containing zeolites can be used as catalysts in the epoxidation of olefins using an aqueous solution of hydrogen peroxide.

Examples

Example 1

Preparation of crude suspension by hydrothermal synthesis

The synthesis of titanium silicalite-1 zeolite (TS-1, MFI type structure) was carried out in a high-pressure reactor (volume 3 m ³ ) according to the method described in example 1 of patent EP 0814058 B1. Highly purified amorphous silicon dioxide (manufacturer Evonik Industries) was used as a source of silicon, and an aqueous solution of tetrapropylammonium titanium hydroxide (Ti-TPA solution), where the TiO ₂ content was 19.0 wt., was used as a titanium source. %. The Ti-TPA solution was prepared as follows.

Deionized water (90.1 kg), 40% aqueous tetrapropylammonium hydroxide (167.3 kg, manufactured by Sachem) and tetraethyl orthotitanate (141.6 kg, manufactured by Connect Chemicals GmbH) were stirred for 1 hour at 40°C in a closed container. The exothermic reaction resulted in an increase in temperature of approximately 25°C. Then ethanol was distilled off at 80°C at a speed of 30 l/h. The required TiO2 content in the resulting Ti-TPA solution was 19.0 wt. %. After cooling, the Ti-TPA solution was used in the synthesis of TS-1.

The high-pressure reactor was first loaded with highly purified silica (500 kg, Evonik Industries), a 40% aqueous solution of tetrapropylammonium hydroxide (382 kg, Sachem), Ti-TPA solution (193 kg), and silicalite-1 seed crystals ( 10 kg) and deionized water (1800 kg). The mixture was stirred in a closed high-pressure reactor at a stirring speed of 50 rpm for 3 hours at 170°C. The heating time to 170°C was 180 min; after cooling for 150 min, the synthesis was stopped. Stirring at a stirring speed of 50 rpm was continued from the beginning of the synthesis until its completion.

Silicalite-1 seed crystals were prepared by hydrothermal synthesis in a high-pressure reactor using highly purified silica (500 kg, Evonik Industries), 40% aqueous tetrapropylammonium hydroxide (400 kg, Sachem) and deionized water (1800 kg). The mixture was stirred in a closed high-pressure reactor at a stirring speed of 50 rpm for 3 hours at 160°C. The heating time to 160°C was 180 min; after cooling for 150 min, the synthesis was stopped. Stirring at a stirring speed of 50 rpm was continued from the beginning of the synthesis until its completion.

Comparison example 1

Standard post-hydrothermal synthesis treatment

Acetic acid (60 wt.%) was added to the crude suspension described in example 1 to pH=7, the resulting precipitate was separated by filtration on a filter press and washed with distilled water. The resulting solid was dried using a spray drying method at an inlet temperature of 420°C and a spray speed of 1700 min ^-1 (outlet temperature was 110°C). The partially dried powder was then calcined at a maximum temperature of 650°C in a tube furnace for 2 hours. The product thus obtained had a BET surface area of 470 m ² /g, with a loss on ignition (measured at 550°C) of 0.65% . According to the results of X-ray diffraction analysis (XRD, Fig. 1), the product was characterized by the crystal structure of TS-1 (identification code in the International Center for Diffraction Data ICDD: 01-089-8099). According to pore structure analysis using nitrogen using the BJH pore size distribution method, the pore volume was 0.23 ml/g.

Example 2

Spray firing after hydrothermal synthesis (600°C)

The crude suspension (16 kg/h) described in example 1 was sprayed in a pilot plant in a nitrogen stream (flow rate 18 m ³ /h) for spraying through a two-phase nozzle with an internal diameter of 2 mm and a slot of 1 mm. The hydrogen/air flame was maintained in a hydrogen flow (10 m ³ /h) and a primary air flow (45 m ³ /h). To eliminate deposits of materials, secondary air was supplied in the perpendicular direction (flow rate 25 m ³ /h). The temperature, measured 1.5 m below the ignition site, was maintained at 600°C by slightly changing the hydrogen flow. The adiabatic combustion temperature in the reactor was approximately 680°C. The residence time in the reactor was approximately 1.1 s. The off-gases containing the calcined zeolite were directed through a water-cooled cooling zone (coolant temperature: 25°C) with a diameter of 100 mm and a length of 6 m and then collected in cartridge filters at a maximum of 250°C. When subsequently cleaning the cartridge filters, you can collect the finished calcined product (4.35 kg/h). The BET surface area of the product thus obtained was 499 m ₂ /g and the loss on ignition (measured at 550° C.) was 1.35 wt. %. The XRD results (Figure 2) indicated that the product had a TS-1 crystal structure (ICDD identification code: 01-089-8099). According to pore structure analysis using nitrogen using the BJH method, the pore volume was 0.3 ml/g.

Comparison example 2 (negative example)

Spray firing after hydrothermal synthesis (400°C)

The crude suspension (15 kg/h) described in example 1 was sprayed in a pilot plant in a nitrogen stream (flow rate 18 m ³ /h) for spraying through a two-phase nozzle with an internal diameter of 2 mm and a slot of 1 mm. The hydrogen/air flame was maintained in a hydrogen flow (8 m ³ /h) and a primary air flow (45 m ³ /h). The temperature, measured 1.5 m below the ignition site, was maintained at 400°C by slightly changing the hydrogen flow. The adiabatic combustion temperature in the reactor was approximately 544°C. The residence time in the reactor was approximately 1.35 s. The off-gases containing the calcined zeolite were directed through a water-cooled cooling zone (coolant temperature: 25°C) with a diameter of 100 mm and a length of 6 m and then collected in cartridge filters at a maximum of 250°C. When subsequently cleaning the cartridge filters, you can collect the finished calcined product (4.4 kg/h). The BET surface area of the product thus obtained was 240 m ² /g and the loss on ignition (measured at 550° C.) was 9.0 wt. %. Due to significant losses on ignition, the resulting product was unsuitable for further processing to obtain the final product and use it in the control NRPO reaction.

Example 3

Spray firing after hydrothermal synthesis (700°C)

The crude suspension (30 kg/h) described in example 1 was sprayed in a pilot plant in a nitrogen stream (flow rate 18 m ³ /h) for spraying through a two-phase nozzle with an internal diameter of 2 mm and a slot of 1 mm. The hydrogen/air flame was maintained in a hydrogen flow (9.1 m ³ /h) and a primary air flow (27 m ³ /h). The temperature, measured 1.5 m below the ignition site, was maintained at 700°C by slightly changing the hydrogen flow. The adiabatic combustion temperature in the reactor was approximately 750°C. The residence time in the reactor was approximately 1.1 s. Exhaust gases containing calcined zeolite were directed through a water-cooled cooling zone (H ₂ O flow at a speed of 10 l/h, air flow at a speed of 4 m ³ / h) with a diameter of 100 mm and a length of 6 m and then collected in cartridge filters at maximum 250°C. When subsequently cleaning the cartridge filters, you can collect the finished calcined product (8.7 kg/h). The BET surface area of the product thus obtained was 506 m ² /g and the loss on ignition (measured at 550° C.) was 1.1 wt. %. The XRD results (Figure 3) indicated that the product had a TS-1 crystal structure (ICDD identification code: 01-089-8099). According to pore structure analysis using nitrogen using the BJH method, the pore volume was 0.3 ml/g.

Example 4

Spray firing after hydrothermal synthesis (800°C)

The crude suspension (14 kg/h) described in example 1 was sprayed in a pilot plant in a nitrogen stream (flow rate 18 m ³ /h) for spraying through a two-phase nozzle with an internal diameter of 2 mm and a slot of 1 mm. The hydrogen/air flame was maintained in a hydrogen flow (12.2 m ³ /h) and a primary air flow (40 m ³ /h). The temperature, measured 1.5 m below the ignition site, was maintained at 800°C by slightly changing the hydrogen flow. The adiabatic combustion temperature in the reactor was approximately 830°C. The residence time in the reactor was approximately 0.85 s. The off-gases containing the calcined zeolite were directed through a water-cooled cooling zone (coolant temperature: 25°C) with a diameter of 100 mm and a length of 6 m and then collected in cartridge filters at a maximum of 250°C. When subsequently cleaning the cartridge filters, you can collect the finished calcined product (4.2 kg/h). The BET surface area of the product thus obtained was 477 m ² /g and the loss on ignition (measured at 550° C.) was 0.87 wt. %. The XRD results (FIG. 4) indicated that the product had a TS-1 crystal structure (ICDD identification code: 01-089-8099). According to pore structure analysis using nitrogen using the BJH method, the pore volume was 0.3 ml/g.

Comparison example 3

Molding of the zeolite powder described in Comparative Example 1

The powder (1200 g) obtained in Comparative Example 1 was mixed with methylhydroxyethylcellulose (75 g, Tylose MH1000), Licowax C (75 g), colloidal silicic acid solution (1000 g, Koestrosol 0830 AS) and deionized water (350 g ) on a high-speed mixer from Eirich. The resulting mass was extruded using an extruder (HB-Feinmechanik LTW 63) through a perforated plate with a diameter of 3.2 mm. The extrudates were then dried in an oven at 80°C for 1 hour and calcined in a muffle furnace at 570°C for 12 hours.

Example 5

Molding of the powdered zeolite described in example 2

The powder (1200 g) obtained in example 2 was mixed with methylhydroxyethylcellulose (75 g, Tylose MH1000), Licowax C product (75 g), colloidal silicic acid solution (1000 g, Koestrosol 0830 AS) and deionized water (350 g) on a high-speed mixer from Eirich. The resulting mass was extruded using an extruder (HB-Feinmechanik LTW 63) through a perforated plate with a diameter of 3.2 mm. The extrudates were then dried in an oven at 80°C for 1 hour and calcined in a muffle furnace at 570°C for 12 hours.

Example 6

Catalytic test using the catalyst described in Comparative Example 3

Epoxidation of propene using hydrogen peroxide (60%) was carried out in two fluidized bed reactors, each containing the catalyst (9 g) described in Comparative Example 3 in the form of extrudates. The reactors were installed in series (reactor 1 → reactor 2) and operated in an upward flow. A first feed stream with a total flow rate of 20 g/h, containing methanol, hydrogen peroxide and water, and a second feed stream with a total flow rate of 20 g/h, containing propene, were both fed to the first reactor. The reactor pressure was maintained at 25 bar using a downstream valve to maintain the pressure in the second reactor. The volume of the reaction mixture coming from the second fluidized bed reactor was increased at normal pressure. In the resulting gas phase, the content of propene, propylene oxide and oxygen was determined, and in the resulting liquid phase, the content of propylene oxide and hydrogen peroxide was determined. The initial selectivity for propylene oxide after completion of the 23-hour reaction cycle was 91.1%. After 480 h, the selectivity for propylene oxide was 97.7%.

Example 7

Catalytic test using the catalyst described in example 5

The catalytic epoxidation of propene was carried out as described in example 6, but using the catalyst obtained in example 5.

The initial selectivity for propylene oxide after completion of the 25-hour reaction cycle was 93.5%. After 480 h, the selectivity for propylene oxide was 98.6%.

As shown in Examples 2-4 in comparison with Comparative Example 1, the method of the present invention includes significantly fewer process steps compared to the standard process. In addition, this method solves the problems of wastewater disposal, which, as a rule, are formed during filtering and purification of the product after hydrothermal synthesis. Unexpectedly, it was found that the resulting titanium silicalites, after flame spray pyrolysis, are characterized by porosity comparable to the porosity of titanium silicalite obtained by the standard method.

As follows from examples 6 and 7, as well as from the data presented in table 1, the catalyst obtained by the standard method (comparative example 3) and the catalyst that is obtained from the metal silicate according to the present invention (example 5), after operation for 480 hours during the epoxidation of propylene to propylene oxide (PO), both are highly active and selective. The catalyst which is prepared from the metal silicate of the present invention actually exhibits a 0.9% increase in propylene oxide selectivity over a conventionally prepared catalyst at comparable volumetric productivity. Thus, by using the catalyst which is obtained from metal silicalite according to the present invention, it is definitely possible to significantly increase the yield of the product propylene oxide per unit time and reactor volume.

Data from crystallographic analysis of silicalite-1 titanium (source: ICDD International Center for Diffraction Database)

Identification code: 01-089-8099

Compound Name: Titanium Silicalite

ICSD code (Inorganic Crystal Structure Database, database of inorganic crystalline substances): 88413

Reference: Lamberti S., Bordiga S., Zecchina A., Carati A., Fitch A.N., Artioli G., Petrini G., Salvalaggio M., Marra G.L., J. Catal., 183, p. 222 (1999).

List of reflexes

Claims (16)

1. A method for producing powdered porous crystalline zeolite silicalite-1 titanium, including the following stages: a) hydrothermal synthesis in an aqueous mixture comprising (A) amorphous silica, (B) an aqueous solution of tetrapropylammonium titanium hydroxide, and (C) an aqueous solution of tetrapropylammonium hydroxide, thereby obtaining an aqueous suspension containing a porous crystalline metal silicate as a reaction product, and b) calcination of the reaction product, characterized in that calcination is carried out using flame spray pyrolysis at an adiabatic combustion temperature in the range of 450-2200°C, with a suspension containing ≤70 wt. % solids obtained in step a) is atomized in a flame created by combustion of fuel in the presence of oxygen, thereby obtaining powdery porous crystalline titanium silicalite-1. 2. The method according to claim 1, characterized in that the porous crystalline metal silicate is characterized by a zeolite structure with a crystal structure of the LTA, MFI, FAU, MOR, MEL or MWW type. 3. The method according to claim 1 or 2, characterized in that the fuel is selected from the group consisting of hydrogen, methane, ethane, propane, butane, natural gas and mixtures thereof. 4. Method according to any one of paragraphs. 1-3, characterized in that the average residence time of the suspension obtained at stage a) during the conversion of said suspension at stage b) is in the range from 0.1 s to 10 s. 5. Method according to any one of paragraphs. 1-4, characterized in that, according to DIN 18128:2002-12, the loss on ignition of porous crystalline metal silicate is less than 5 wt. %. 6. Method according to any one of paragraphs. 1-5, characterized in that the aqueous mixture obtained in step a) further includes suitable seed crystals. 7. Method according to any one of paragraphs. 1-6, characterized in that the aqueous mixture obtained in step a) additionally contains a matrix selected from the group consisting of amines, quaternary ammonium compounds, alcohols and mixtures of these compounds. 8. Method according to any one of paragraphs. 1-7, characterized in that stage a) is carried out at a temperature from 100 to 250°C at autogenous pressure created in a high-pressure reactor. 9. Method according to any one of paragraphs. 1-8, characterized in that after stage b) molding is carried out at stage c), including the stages: (1) adding water to obtain an aqueous suspension of porous crystalline metal silicate powder, (2) mixing the suspension obtained in step (1) with granulating auxiliaries, (3) compression, granulation, spray drying, spray granulation or extrusion, thereby obtaining porous crystalline metal silicate in the form of microgranules, spheres, tablets, cast cylinders, hollow cylinders or honeycomb bubbles.

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