US20140155257A1 - Catalyst from flame-spray pyrolysis and catalyst for autothermal propane dehydrogenation - Google Patents

Catalyst from flame-spray pyrolysis and catalyst for autothermal propane dehydrogenation Download PDF

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US20140155257A1
US20140155257A1 US14/171,868 US201414171868A US2014155257A1 US 20140155257 A1 US20140155257 A1 US 20140155257A1 US 201414171868 A US201414171868 A US 201414171868A US 2014155257 A1 US2014155257 A1 US 2014155257A1
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catalyst
pyrolysis
zirconium
lanthanum
precursor compounds
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Stefan Hannemann
Dieter Stützer
Goetz-Peter Schindler
Peter Pfab
Frank Kleine Jäger
Dirk Grossschmidt
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BASF SE
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/54—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
    • B01J23/56—Platinum group metals
    • B01J23/63—Platinum group metals with rare earths or actinides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/54—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
    • B01J23/56—Platinum group metals
    • B01J23/58—Platinum group metals with alkali- or alkaline earth metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B01J35/391—Physical properties of the active metal ingredient
    • B01J35/393—Metal or metal oxide crystallite size
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/40—Catalysts, in general, characterised by their form or physical properties characterised by dimensions, e.g. grain size
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J35/61—Surface area
    • B01J35/613—10-100 m2/g
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/0072—Preparation of particles, e.g. dispersion of droplets in an oil bath
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/34—Irradiation by, or application of, electric, magnetic or wave energy, e.g. ultrasonic waves ; Ionic sputtering; Flame or plasma spraying; Particle radiation
    • B01J37/349—Irradiation by, or application of, electric, magnetic or wave energy, e.g. ultrasonic waves ; Ionic sputtering; Flame or plasma spraying; Particle radiation making use of flames, plasmas or lasers

Definitions

  • the invention relates to catalyst particles, a method of production thereof and the use of the catalyst particles as dehydrogenation catalyst.
  • DE-A 196 54 391 describes the production of a dehydrogenation catalyst by impregnation of essentially monoclinic ZrO 2 with a solution of Pt(NO 3 ) 2 and Sn(OAc) 2 or by impregnation of ZrO 2 with a first solution of Pt(NO 3 ) 2 and then a second solution of La(NO 3 ) 3 .
  • the impregnated supports are dried and then calcined.
  • the catalysts thus obtained are used as dehydrogenation catalysts for the dehydrogenation of propane to propene.
  • the problem to be solved by the present invention is to provide an inexpensive and time-saving method of production of dehydrogenation catalysts, wherein the dehydrogenation catalysts obtained should be comparable in activity and selectivity to the catalysts of the prior art, produced by impregnation processes or spray drying.
  • catalyst particles comprising platinum and tin and also at least one further element, selected from lanthanum and cesium, on a support comprising zirconium dioxide, comprising the steps
  • FIG. 1 illustrates activities and selectivities for the flame-synthesized catalysts ( ⁇ example 13, ⁇ example 17) and for the reference catalyst ( ⁇ ) in the autothermal dehydrogenation of propane to propene.
  • the metal compounds and oxide-forming precursor compounds are fed as aerosol to the pyrolysis zone. It is preferable if the aerosol fed to the pyrolysis zone is obtained by nebulization of just one solution, which contains all the metal compounds and oxide-forming precursor compounds. In this way it is always ensured that the composition of the particles produced is homogeneous and constant.
  • the individual components are thus preferably selected so that the oxide-forming precursors and the precious metal compounds used contained in the solution are dissolved uniformly alongside one another until nebulization of the solution.
  • the solution or solutions can contain both polar and apolar solvents or solvent mixtures.
  • the temperature in the pyrolysis zone is above the decomposition temperature of the precious metal compounds at sufficient temperature for oxide formation, usually between 500 and 2000° C. Pyrolysis is preferably carried out at a temperature from 900 to 1500° C.
  • the pyrolysis reactor can be heated indirectly from outside, for example by means of an electric furnace. Owing to the temperature gradient from outside to inside that is required in indirect heating, the furnace must be much hotter than corresponds to the temperature required for pyrolysis. Indirect heating requires a thermally stable furnace material and an expensive reactor construction, but the total amount of gas required is less than in the case of a flame reactor.
  • the pyrolysis zone is heated by a flame (flame-spray pyrolysis).
  • the pyrolysis zone then comprises an ignition device.
  • usual combustible gases are used, although preferably hydrogen, methane or ethylene is used.
  • the temperature in the pyrolysis zone can be adjusted as required by means of the ratio of the amount of combustible gas to the total amount of gas.
  • the pyrolysis zone can also be supplied with pure oxygen instead of air as the O 2 source for combustion of the combustible gases.
  • the total amount of gas also comprises the carrier gas for the aerosol and the evaporated solvent of the aerosol.
  • the aerosol or aerosols supplied to the pyrolysis zone are preferably fed directly into the flame.
  • air is generally preferred as carrier gas for the aerosol, it is also possible to use nitrogen, CO 2 , O 2 or a combustible gas, for example hydrogen, methane, ethylene, propane or butane.
  • the pyrolysis zone is heated by an electric plasma or an inductive plasma.
  • the catalytically active precious metal particles are deposited on the surface of the support particles and are fixed firmly thereon.
  • a flame-spray pyrolysis device generally comprises a storage container for the liquid to be nebulized, feed pipes for carrier gas, combustible gas and oxygen-containing gas, a central aerosol nozzle, and an annular burner arranged around this, a device for gas-solid separation comprising a filter element and a discharging device for the solid and an outlet for the exhaust gas.
  • the particles are cooled by means of a quench gas, e.g. nitrogen or air.
  • the pyrolysis zone preferably comprises a so-called pre-drier, which subjects the aerosol to preliminary drying before its entry into the pyrolysis reactor, this preliminary drying taking place, for example, in a flow tube with a heating assembly disposed around it. Where preliminary drying is not carried out, the risk exists of obtaining a product with a relatively broad particle size spectrum, and more particularly an excessive fine fraction.
  • the temperature of the pre-drier is dependent on the nature of the dissolved precursors and on the concentration thereof.
  • the temperature in the pre-drier is typically above the boiling point of the solvent, up to 250° C.; in the case of water as a solvent, the temperature in the pre-drier is preferably between 120 and 250° C., more particularly between 150 and 200° C.
  • the combustion space which is preferably tube-shaped, is heat-insulated.
  • a pyrolysis gas which contains spherical particles with varying specific surface.
  • the size distribution of the pigment particles obtained results essentially directly from the droplet spectrum of the aerosol fed into the pyrolysis zone and the concentration of the solution or solutions used.
  • the pyrolysis gas is cooled so that sintering together of the particles is excluded.
  • the pyrolysis zone preferably comprises a cooling zone, which adjoins the combustion space of the pyrolysis reactor. Cooling of the pyrolysis gas and of the catalyst particles contained therein to a temperature of about 100-500° C. is generally required, depending on the filter element used. Cooling to approx. 100-150° C. preferably takes place.
  • the pyrolysis gas, containing catalyst particles, and partially cooled enters a device for separating the particles from the pyrolysis gas, which comprises a filter element.
  • a quench gas for example nitrogen, air or water-moistened air, is fed in.
  • Suitable zirconium dioxide—forming precursor compounds are alcoholates, such as zirconium(IV) ethanolate, zirconium(IV) n-propanolate, zirconium(IV) isopropanolate, zirconium(IV) n-butanolate and zirconium(IV) tert-butanolate.
  • zirconium(IV) propanolate preferably as solution in n-propanol, is used as ZrO 2 precursor compound.
  • zirconium dioxide—forming precursor compounds are carboxylates, such as zirconium acetate, zirconium propionate, zirconium oxalate, zirconium octoate, zirconium 2-ethyl-hexanoate, zirconium acetate, zirconium propionate, zirconium oxalate, zirconium octanoate, zirconium 2-ethylhexanoate, zirconium neodecanoate zirconium stearate and zirconium propionate.
  • zirconium(IV) acetylacetonate is used as precursor compound.
  • the precursor compounds additionally comprise a silicon dioxide precursor compound.
  • Possible precursors for silicon dioxide are organosilanes and reaction products of SiCl 4 with lower alcohols or lower carboxylic acids. It is also possible to use condensates of the aforementioned organosilanes and/or -silanols with Si—O—Si units. Siloxanes are preferably used. It is also possible to use SiO 2 .
  • the precursor compounds comprise hexamethyldisiloxane as silica-forming precursor compound.
  • the catalyst particles according to the invention further comprise platinum and tin and also at least one further element, selected from lanthanum and cesium.
  • the Pt loading is 0.05 to 1 wt. % and the Sn loading is 0.05 to 2 wt. %.
  • Preferred precursor compounds for lanthanum and cesium, respectively are carboxylates and nitrates, corresponding for example to the carboxylates identified above in connection with zirconium.
  • the precursor compounds comprise lanthanum(III) acetylacetonate and/or cesium acetate.
  • the precursor compounds comprise lanthanum(III) 2-ethylhexanoate.
  • Preferred precursor compounds for tin are carboxylates and nitrates, corresponding for example to the carboxylates identified above in connection with zirconium.
  • the precursor compounds comprise tin 2-ethylhexanoate.
  • Preferred precursor compounds for platinum are carboxylates and nitrates, corresponding for example to the carboxylates identified above in connection with zirconium, and ammonium platinates.
  • the precursor compounds comprise platinum acetylacetonate.
  • Both polar and apolar solvents or solvent mixtures can be used for production of the solution or solutions required for aerosol formation.
  • Preferred polar solvents are water, methanol, ethanol, n-propanol, iso-propanol, n-butanol, tert-butanol, n-propanone, n-butanone, diethyl ether, tert-butyl-methyl ether, tetrahydrofuran, C 1 —C 8 carboxylic acids, ethyl acetate and mixtures thereof.
  • one or more of the precursor compounds are dissolved in a mixture of acetic acid, ethanol and water.
  • this mixture contains 30 to 75 wt. % acetic acid, 30 to 75 wt. % ethanol and 0 to 20 wt. % water.
  • zirconium(IV) acetylacetonate, hexamethyldisiloxane tin 2-ethylhexanoate, platinum acetylacetonate, lanthanum(II) acetylacetonate and cesium acetate are dissolved in a mixture of acetic acid, ethanol and water.
  • Preferred apolar solvents are toluene, xylene, n-heptane, n-pentane, octane, isooctane, cyclohexane, methyl, ethyl or butyl acetate or mixtures thereof. Hydrocarbons or mixtures of hydrocarbons with 5 to 15 carbon atoms are also suitable. Xylene is especially preferable.
  • Zr(IV) propylate, hexamethyldisiloxane tin 2-ethylhexanoate, platinum acetylacetonate and lanthanum(III) acetylacetonate are dissolved in xylene.
  • the present invention also relates to the supports and catalyst particles obtainable by the method according to the invention. These preferably have a specific surface of 20 to 70 m 2 /g.
  • the catalyst particles have the following percentage composition: 30 to 99.5 wt. % ZrO 2 and, 0.5 to 25 wt. % SiO 2 as support, 0.1 to 1 wt. % Pt, 0.1 to 10 wt. % Sn, La and/or Cs, relative to the mass of the support, wherein at least Sn and La or Cs are contained.
  • the present invention also relates to the use of the catalyst particles as hydrogenation catalysts or dehydrogenation catalysts.
  • Alkanes such as butane and propane, but also ethylbenzene, are preferably dehydrogenated.
  • HMDSO Hexamethyldisiloxane
  • the solvent is HoAc: EtOH: H 2 O in the proportions by weight 4.6 to 4.6 to 1.
  • the acetic acid-ethanol mixture is freshly prepared.
  • the precursor compounds for Sn, Cs, La, Si, Pt and Zr are dissolved therein.
  • composition of the polar solutions of the precursor compounds for the examples 1, 2, 3, 9 and 10 is shown in Table 1.
  • the solution containing the precursor compounds was supplied by means of a piston pump via a two-component nozzle and atomized with a corresponding amount of air. To reach the corresponding temperatures, sometimes a support flame from an ethylene-air mixture was used, which was supplied via an annular burner located around the nozzle. The pressure drop was kept constant at 1.1 bar.
  • a baghouse filter was used for separating the particles. These filters could be cleaned by applying 5 bar pressure surges of nitrogen to the filter bags.
  • the BET surface area was investigated as a function of the temperature of the combustion chamber. This involved a comparison of the solutions comprising the precursor compounds, in terms of their solvent (acetic acid versus xylene). In the case of the acetic acid mixtures, there was no clear trend apparent.
  • the xylene mixtures exhibited an increasing BET surface area with increasing temperature, and this may be attributed to a shorter residence time, thereby restricting particle growth.
  • Propane dehydrogenation was carried out at approx. 600° C. (Flows at 20 ml cat. volume, mass see Table 5): 21 Nl/h total gas (20 Nl/h propane, 1 Nl/h nitrogen as internal standard), 5 g/h water. Regeneration is carried out at 400° C. as follows: 2 hours 21 Nl/h N 2 +4 Nl/h air; 2 hours 25 Nl/h air; 1 hour 25 Nl/h hydrogen.
  • the support of the reference catalyst from hydrothermal synthesis (ZrO 2 ) with subsequent spray drying is composed of 95% ZrO 2 and 5% SiO 2 .
  • the active/promoter metals are 0.5% Pt, 1% Sn, 3% La, 0.5% Cs and 0.2% K, and were applied to the support wet-chemically by impregnation in accordance with the instructions of EP 1 074 301, example 4.
  • FIG. 1 shows activities and selectivities for the flame-synthesized catalysts ( ⁇ example 13, ⁇ example 17) and for the reference catalyst ( ⁇ ) in the autothermal dehydrogenation of propane to propene.
  • the catalyst ( ⁇ ) only the support was produced by pyrolysis, and the support was subsequently subjected to wet-chemical impregnation as for the reference catalyst. The time in hours is plotted on the abscissa, and the conversions (40 to 50%) and selectivities (>80%) are plotted on the ordinate.
  • the three catalysts have comparable performance.
  • the reference catalyst has lower initial selectivities. However, over the test cycles of a few weeks it equalizes to the catalysts according to the invention. Thus, the flame-synthesized catalyst behaves like an aged catalyst, which was produced by a conventional wet-chemical process.

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Abstract

The invention relates to a method of production of catalyst particles, comprising platinum and tin and also at least one further element, selected from lanthanum and cesium, on zirconium dioxide as support, comprising the steps: preparation of one or more solutions containing precursor compounds of Pt, Sn and at least one further element of La or Cs and also ZrO2, converting the solution(s) to an aerosol, bringing the aerosol into a directly or indirectly heated pyrolysis zone, carrying out pyrolysis, and separation of the particles formed from the pyrolysis gas.
Suitable precursor compounds comprise zirconium(IV) acetylacetonate, lanthanum(II) acetylacetonate and cesium acetate, hexamethyldisiloxane, tin 2-ethylhexanoate, platinum acetylacetonate, zirconium(IV) propylate in n-propanol and lanthanum(II) acetylacetonate.
The invention also relates to the catalyst particles obtainable using the method according to the invention, and to the use thereof as dehydrogenation catalysts.

Description

    CROSS-REFERENCE TO RELATED APPLICATIONS
  • This application is a divisional application of U.S. Non-Provisional Application No. 13/356,787, filed Jan. 24, 2012, which claims the benefit (under 35 USC 119(e)) of U.S. Provisional Application 61/435,797, filed Jan. 25, 2011, the contents of each of which are incorporated herein by reference in their entireties.
    • BACKGROUND OF THE INVENTION
  • The invention relates to catalyst particles, a method of production thereof and the use of the catalyst particles as dehydrogenation catalyst.
  • Production of dehydrogenation catalysts by impregnation processes or spray drying is known. In these methods the catalytically active metals are applied on an oxide support or a silicate support by impregnation processes or the catalyst is produced by spray drying of coprecipitated oxide precursors.
  • DE-A 196 54 391 describes the production of a dehydrogenation catalyst by impregnation of essentially monoclinic ZrO2 with a solution of Pt(NO3)2 and Sn(OAc)2 or by impregnation of ZrO2 with a first solution of Pt(NO3)2 and then a second solution of La(NO3)3. The impregnated supports are dried and then calcined. The catalysts thus obtained are used as dehydrogenation catalysts for the dehydrogenation of propane to propene.
  • A known method of production of metal catalysts by flame-spray pyrolysis is described in Pisduangnawakij et al., Applied Catalysis A: General 370 1-6, 2009. In this, a solution containing precursor compounds of platinum and tin and of aluminum oxide as support in xylene is converted to an aerosol, this is treated in an inert carrier gas in a pyrolysis reactor at a temperature above the decomposition temperature of the precursor compounds and then the finely-divided metal that has formed is separated from the carrier gas.
  • The known synthesis of precious metal powder catalysts by wet-chemical preparation is time-consuming and costly.
  • The methods for the production of dehydrogenation catalysts are therefore still in need of improvement in terms of the time and costs they involve.
    • A SUMMARY OF THE INVENTION
  • The problem to be solved by the present invention is to provide an inexpensive and time-saving method of production of dehydrogenation catalysts, wherein the dehydrogenation catalysts obtained should be comparable in activity and selectivity to the catalysts of the prior art, produced by impregnation processes or spray drying.
  • This problem is solved by a method of production of catalyst particles, comprising platinum and tin and also at least one further element, selected from lanthanum and cesium, on a support comprising zirconium dioxide, comprising the steps
  • (i) preparation of one or more solutions containing precursor compounds of platinum, tin and the at least one further element, selected from lanthanum and cesium, and also of zirconium dioxide,
    (ii) converting the solution(s) to an aerosol,
    (iii) bringing the aerosol into a directly or indirectly heated pyrolysis zone,
    (iv) carrying out pyrolysis, and
    (v) separation of the particles formed from the pyrolysis gas.
    • A BRIEF DESCRIPTION OF THE FIGURE
  • FIG. 1 illustrates activities and selectivities for the flame-synthesized catalysts (▴ example 13, ▪ example 17) and for the reference catalyst (−) in the autothermal dehydrogenation of propane to propene.
    •  A DETAILED DESCRIPTION OF THE INVENTION
  • The metal compounds and oxide-forming precursor compounds are fed as aerosol to the pyrolysis zone. It is preferable if the aerosol fed to the pyrolysis zone is obtained by nebulization of just one solution, which contains all the metal compounds and oxide-forming precursor compounds. In this way it is always ensured that the composition of the particles produced is homogeneous and constant. During preparation of the solution that is to be converted to an aerosol, the individual components are thus preferably selected so that the oxide-forming precursors and the precious metal compounds used contained in the solution are dissolved uniformly alongside one another until nebulization of the solution. Alternatively it is also possible to use several different solutions, which, on the one hand, contain the oxide-forming precursors and, on the other hand, contain the active or promoter metal compounds. The solution or solutions can contain both polar and apolar solvents or solvent mixtures.
  • In the pyrolysis zone, decomposition of the precious metal compound to form the precious metal and decomposition and/or oxidation of the oxide precursors, with formation of the oxide, take place. It may also happen that some of the precious metal evaporates and then redeposits in cooler zones on support particles already formed. Pyrolysis generally results in spherical particles with varying specific surface.
  • The temperature in the pyrolysis zone is above the decomposition temperature of the precious metal compounds at sufficient temperature for oxide formation, usually between 500 and 2000° C. Pyrolysis is preferably carried out at a temperature from 900 to 1500° C.
  • The pyrolysis reactor can be heated indirectly from outside, for example by means of an electric furnace. Owing to the temperature gradient from outside to inside that is required in indirect heating, the furnace must be much hotter than corresponds to the temperature required for pyrolysis. Indirect heating requires a thermally stable furnace material and an expensive reactor construction, but the total amount of gas required is less than in the case of a flame reactor.
  • In a preferred embodiment the pyrolysis zone is heated by a flame (flame-spray pyrolysis). The pyrolysis zone then comprises an ignition device. For direct heating, usual combustible gases are used, although preferably hydrogen, methane or ethylene is used. The temperature in the pyrolysis zone can be adjusted as required by means of the ratio of the amount of combustible gas to the total amount of gas. To keep the total amount of gas low but nevertheless achieve a temperature as high as possible, the pyrolysis zone can also be supplied with pure oxygen instead of air as the O2 source for combustion of the combustible gases. The total amount of gas also comprises the carrier gas for the aerosol and the evaporated solvent of the aerosol. The aerosol or aerosols supplied to the pyrolysis zone are preferably fed directly into the flame. Although air is generally preferred as carrier gas for the aerosol, it is also possible to use nitrogen, CO2, O2 or a combustible gas, for example hydrogen, methane, ethylene, propane or butane.
  • In another embodiment of the method according to the invention, the pyrolysis zone is heated by an electric plasma or an inductive plasma. In this embodiment, the catalytically active precious metal particles are deposited on the surface of the support particles and are fixed firmly thereon.
  • A flame-spray pyrolysis device generally comprises a storage container for the liquid to be nebulized, feed pipes for carrier gas, combustible gas and oxygen-containing gas, a central aerosol nozzle, and an annular burner arranged around this, a device for gas-solid separation comprising a filter element and a discharging device for the solid and an outlet for the exhaust gas. The particles are cooled by means of a quench gas, e.g. nitrogen or air.
  • The pyrolysis zone preferably comprises a so-called pre-drier, which subjects the aerosol to preliminary drying before its entry into the pyrolysis reactor, this preliminary drying taking place, for example, in a flow tube with a heating assembly disposed around it. Where preliminary drying is not carried out, the risk exists of obtaining a product with a relatively broad particle size spectrum, and more particularly an excessive fine fraction. The temperature of the pre-drier is dependent on the nature of the dissolved precursors and on the concentration thereof. The temperature in the pre-drier is typically above the boiling point of the solvent, up to 250° C.; in the case of water as a solvent, the temperature in the pre-drier is preferably between 120 and 250° C., more particularly between 150 and 200° C. The pre-dried aerosol, supplied to the pyrolysis reactor via a line, then enters the reactor via an exit nozzle.
  • To produce a balanced temperature profile, the combustion space, which is preferably tube-shaped, is heat-insulated.
  • As the pyrolysis result, a pyrolysis gas is obtained, which contains spherical particles with varying specific surface. The size distribution of the pigment particles obtained results essentially directly from the droplet spectrum of the aerosol fed into the pyrolysis zone and the concentration of the solution or solutions used.
  • Preferably, prior to separation of the particles formed from the pyrolysis gas, the pyrolysis gas is cooled so that sintering together of the particles is excluded. For this reason the pyrolysis zone preferably comprises a cooling zone, which adjoins the combustion space of the pyrolysis reactor. Cooling of the pyrolysis gas and of the catalyst particles contained therein to a temperature of about 100-500° C. is generally required, depending on the filter element used. Cooling to approx. 100-150° C. preferably takes place. After leaving the pyrolysis zone, the pyrolysis gas, containing catalyst particles, and partially cooled, enters a device for separating the particles from the pyrolysis gas, which comprises a filter element. For cooling, a quench gas, for example nitrogen, air or water-moistened air, is fed in.
  • Suitable zirconium dioxide—forming precursor compounds are alcoholates, such as zirconium(IV) ethanolate, zirconium(IV) n-propanolate, zirconium(IV) isopropanolate, zirconium(IV) n-butanolate and zirconium(IV) tert-butanolate. In a preferred embodiment of the method according to the invention, zirconium(IV) propanolate, preferably as solution in n-propanol, is used as ZrO2 precursor compound.
  • Other suitable zirconium dioxide—forming precursor compounds are carboxylates, such as zirconium acetate, zirconium propionate, zirconium oxalate, zirconium octoate, zirconium 2-ethyl-hexanoate, zirconium acetate, zirconium propionate, zirconium oxalate, zirconium octanoate, zirconium 2-ethylhexanoate, zirconium neodecanoate zirconium stearate and zirconium propionate. In another preferred embodiment of the method according to the invention, zirconium(IV) acetylacetonate is used as precursor compound.
  • In one embodiment, the precursor compounds additionally comprise a silicon dioxide precursor compound. Possible precursors for silicon dioxide are organosilanes and reaction products of SiCl4 with lower alcohols or lower carboxylic acids. It is also possible to use condensates of the aforementioned organosilanes and/or -silanols with Si—O—Si units. Siloxanes are preferably used. It is also possible to use SiO2. In a preferred embodiment of the method according to the invention, the precursor compounds comprise hexamethyldisiloxane as silica-forming precursor compound.
  • Besides zirconium dioxide and optionally silicon dioxide as supports, the catalyst particles according to the invention further comprise platinum and tin and also at least one further element, selected from lanthanum and cesium.
  • In one preferred embodiment of the invention, the Pt loading is 0.05 to 1 wt. % and the Sn loading is 0.05 to 2 wt. %.
  • Preferred precursor compounds for lanthanum and cesium, respectively, are carboxylates and nitrates, corresponding for example to the carboxylates identified above in connection with zirconium. In one preferred embodiment of the method according to the invention, the precursor compounds comprise lanthanum(III) acetylacetonate and/or cesium acetate.
  • In a further, preferred embodiment of the method according to the invention, the precursor compounds comprise lanthanum(III) 2-ethylhexanoate.
  • Preferred precursor compounds for tin are carboxylates and nitrates, corresponding for example to the carboxylates identified above in connection with zirconium. In a further, preferred embodiment of the method according to the invention, the precursor compounds comprise tin 2-ethylhexanoate.
  • Preferred precursor compounds for platinum are carboxylates and nitrates, corresponding for example to the carboxylates identified above in connection with zirconium, and ammonium platinates. In a preferred embodiment of the method according to the invention, the precursor compounds comprise platinum acetylacetonate.
  • Both polar and apolar solvents or solvent mixtures can be used for production of the solution or solutions required for aerosol formation.
  • Preferred polar solvents are water, methanol, ethanol, n-propanol, iso-propanol, n-butanol, tert-butanol, n-propanone, n-butanone, diethyl ether, tert-butyl-methyl ether, tetrahydrofuran, C1—C8 carboxylic acids, ethyl acetate and mixtures thereof.
  • In a preferred embodiment of the method according to the invention, one or more of the precursor compounds, preferably all the precursor compounds are dissolved in a mixture of acetic acid, ethanol and water. Preferably this mixture contains 30 to 75 wt. % acetic acid, 30 to 75 wt. % ethanol and 0 to 20 wt. % water. In particular, zirconium(IV) acetylacetonate, hexamethyldisiloxane tin 2-ethylhexanoate, platinum acetylacetonate, lanthanum(II) acetylacetonate and cesium acetate are dissolved in a mixture of acetic acid, ethanol and water.
  • Preferred apolar solvents are toluene, xylene, n-heptane, n-pentane, octane, isooctane, cyclohexane, methyl, ethyl or butyl acetate or mixtures thereof. Hydrocarbons or mixtures of hydrocarbons with 5 to 15 carbon atoms are also suitable. Xylene is especially preferable.
  • In particular, Zr(IV) propylate, hexamethyldisiloxane tin 2-ethylhexanoate, platinum acetylacetonate and lanthanum(III) acetylacetonate are dissolved in xylene.
  • The present invention also relates to the supports and catalyst particles obtainable by the method according to the invention. These preferably have a specific surface of 20 to 70 m2/g.
  • In a preferred embodiment the catalyst particles have the following percentage composition: 30 to 99.5 wt. % ZrO2 and, 0.5 to 25 wt. % SiO2 as support, 0.1 to 1 wt. % Pt, 0.1 to 10 wt. % Sn, La and/or Cs, relative to the mass of the support, wherein at least Sn and La or Cs are contained.
  • The present invention also relates to the use of the catalyst particles as hydrogenation catalysts or dehydrogenation catalysts. Alkanes, such as butane and propane, but also ethylbenzene, are preferably dehydrogenated.
  • The use of the catalysts according to the invention for the dehydrogenation of propane to propene is especially preferred.
  • The invention is explained in more detail with the following examples.
    • Examples
  • Chemicals used
  • Zirconium acetylacetonate Zr(acac)2 (98%)
  • Zirconium(IV) propoxide Zr(OPr)4 (70% in 1-propanol)
  • Hexamethyldisiloxane (HMDSO) (98%)
  • Tin(II) 2-ethylhexanoate (approx. 95%)
  • Platinum(II) acetylacetate (98%)
  • Lanthanum(III) 2-ethylhexanoate (10% w/v)
  • Lanthanum(III) acetylacetonate (99.99%)
  • Cesium acetate (99.99%)
  • Mixture of acetic acid (100%), ethanol (96%) and water (deionized)
  • Xylene (BASF, mixture of isomers)
    • Preparation of the Solutions of the Precursor Compounds
  • The solvent is HoAc: EtOH: H2O in the proportions by weight 4.6 to 4.6 to 1. The acetic acid-ethanol mixture is freshly prepared. The precursor compounds for Sn, Cs, La, Si, Pt and Zr are dissolved therein.
  • The composition of the polar solutions of the precursor compounds for the examples 1, 2, 3, 9 and 10 is shown in Table 1.
  • TABLE 1
    Compositions of the solutions of the precursor
    compounds for polar mixtures (EtOH:HoAc:H2O)
    [g] Substance Purity [wt. %]
    99.52 Zirconium(IV) acetylacetonate 98
    1.77 Hexamethyldisiloxane 99
    0.93 Tin 2-ethylhexanoate 95
    0.27 Platinum acetylacetonate 98
    2.45 Lanthanum(III) acetylacetonate 99.9
    0.38 Cesium acetate 99.99
  • For preparing the solution of the precursor compound for example 4, the following substances were dissolved in xylene. The composition is shown in Table 2.
  • TABLE 2
    Compositions of the solutions of the precursor
    compounds for apolar mixtures (xylene)
    [g] Substance Purity [wt. %]
    374.40 Zr(IV) propylate in n-propanol 70
    10.11 Hexamethyldisiloxane 99
    5.32 Tin 2-ethylhexanoate 95
    1.52 Platinum acetylacetonate 98
    103.47 Lanthanum(III) 2-ethylhexanoate 10
  • In the case of the preparation of the solutions of the precursor compounds for examples 5, 6 and 8, an additional 2.14 g of cesium acetate are used as well.
    • Examples 1 to 10
  • Production of the catalyst particles by flame-spray pyrolysis
  • The solution containing the precursor compounds was supplied by means of a piston pump via a two-component nozzle and atomized with a corresponding amount of air. To reach the corresponding temperatures, sometimes a support flame from an ethylene-air mixture was used, which was supplied via an annular burner located around the nozzle. The pressure drop was kept constant at 1.1 bar.
  • The flame synthesis conditions are summarized in Table 3.
  • TABLE 3
    Test parameters relating to the production of flame-spray pyrolysis catalysts
    cZr Flow rate Total Dispersion
    [mol/kg of precursor gas flow gas flow Ethylene
    Example Solvent solution] compound [l/h] [l/h] [l/h] GLMR1
    1 HoAc, EtOH, 0.5 500 3500 1200 40  3
    H2O
    2 HoAc, EtOH, 0.5 510 3500 1200 20-40 3
    H2O
    3 HoAc, EtOH, 0.2 515 3500 1200 10-50 3
    H2O
     4* Xylene 1 280 3500 1200 0 5
    5 Xylene 1 290 3500 1200 0 5
    6 Xylene 1 310 3500 1200 0 4
     7** Xylene 1 310 3500 1200 0 4
    8 Xylene 1 255 3500 1200 20-40 5
    9 HoAc, H2O 0.2 520 3500 1200 130-120 3
    10  HoAc, H2O 0.25 385 4140 1740 190-230 5
    Solution without cesium precursor compound
    **Only Si and Zr precursors present
    1GLMR = Gas to Liquid Mass Ratio.
  • A baghouse filter was used for separating the particles. These filters could be cleaned by applying 5 bar pressure surges of nitrogen to the filter bags.
  • Particle characterization was carried out by means of X-ray diffractometry (XRD) and BET measurement, and an element analysis was carried out as well. The crystallite size of the catalyst particles formed using the solution of the precursor compounds 3 and 4 is set out in Table 4.
  • TABLE 4
    X-ray powder diffractometry for the characterization of the ZrO2
    Crystal- Crystal- Average
    Precursor Tetragonal Monoclinic lite size, lite size crystal-
    compound ZrO2 ZrO2 tetragonal monoclinic lite size
    used [%] [%] [nm] [nm] [nm]
    3 82 18 19 13 18
    4 90 10 28 9 26
  • The syntheses of the catalysts from the above solutions comprising precursor compounds with the settings specified above produced particles having a specific surface area of 36-70 m2/g (see Table 5).
  • In a further experiment, the BET surface area was investigated as a function of the temperature of the combustion chamber. This involved a comparison of the solutions comprising the precursor compounds, in terms of their solvent (acetic acid versus xylene). In the case of the acetic acid mixtures, there was no clear trend apparent.
  • The xylene mixtures exhibited an increasing BET surface area with increasing temperature, and this may be attributed to a shorter residence time, thereby restricting particle growth.
    • Examples 11 to 17 Catalytic Measurements
  • Propane dehydrogenation was carried out at approx. 600° C. (Flows at 20 ml cat. volume, mass see Table 5): 21 Nl/h total gas (20 Nl/h propane, 1 Nl/h nitrogen as internal standard), 5 g/h water. Regeneration is carried out at 400° C. as follows: 2 hours 21 Nl/h N2+4 Nl/h air; 2 hours 25 Nl/h air; 1 hour 25 Nl/h hydrogen.
  • The support of the reference catalyst from hydrothermal synthesis (ZrO2) with subsequent spray drying is composed of 95% ZrO2 and 5% SiO2. The active/promoter metals are 0.5% Pt, 1% Sn, 3% La, 0.5% Cs and 0.2% K, and were applied to the support wet-chemically by impregnation in accordance with the instructions of EP 1 074 301, example 4.
  • The conversion, the long-term stability and the selectivity of propene formation were investigated in the catalytic tests. The results are summarized in Table 5. The activities and selectivities relate to an optimum operating state.
  • TABLE 5
    Catalyst results for the flame-synthesized catalyst
    particles in autothermal propane dehydrogenation
    Mass of Catalyst used BET Activity Selectivity
    Example catalyst/g from example [m2/g] % %
    11 15.24 1 66 17 83
    12 16.22 2 50 38 94
    13 16.41 3 51 47 96
    14 22.26 4 36 46 95
    15 16.90 5 59 35 94
    16 17.75 6 52 31 92
    17 16.80 7 23 48 95
  • FIG. 1 shows activities and selectivities for the flame-synthesized catalysts (▴ example 13, ▪ example 17) and for the reference catalyst (−) in the autothermal dehydrogenation of propane to propene. In the case of the catalyst (▪), only the support was produced by pyrolysis, and the support was subsequently subjected to wet-chemical impregnation as for the reference catalyst. The time in hours is plotted on the abscissa, and the conversions (40 to 50%) and selectivities (>80%) are plotted on the ordinate.
  • It can be seen that the three catalysts have comparable performance. The reference catalyst has lower initial selectivities. However, over the test cycles of a few weeks it equalizes to the catalysts according to the invention. Thus, the flame-synthesized catalyst behaves like an aged catalyst, which was produced by a conventional wet-chemical process.

Claims (5)

1-17. (canceled)
18. A catalyst particle, comprising platinum and tin and also at least one further element, selected from lanthanum and cesium, on a support comprising zirconium dioxide and optionally silicon oxide, wherein said catalyst particle is obtained by a method comprising the steps
(i) preparing one or more solutions containing precursor compounds of platinum, tin and the at least one further element, selected from lanthanum and cesium, and also of zirconium dioxide and optionally silicon dioxide,
(ii) converting the solution(s) to an aerosol,
(iii) bringing the aerosol into a directly or indirectly heated pyrolysis zone,
(iv) carrying out pyrolysis with a gas, and
(v) separating the catalyst particles formed from the pyrolysis gas.
19. The catalyst particle of claim 18, wherein said catalyst particle contains 0.05 to 1 wt. % Pt and 0.05 to 2 wt. % Sn.
20. The catalyst particle of claim 18, wherein said catalyst particle has a specific surface of 36 to 70 m2/g.
21. The catalyst particle of claim 18, wherein said catalyst particles comprises 30 to 99.5 wt. % ZrO2 and 0.5 to 25 wt. % SiO2 as support, and 0.1 to 1 wt. % Pt, 0.1 to 10 wt. % Sn, relative to the mass of the support, La and/or Cs, wherein at least Sn and at least La or Cs are contained.
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