Method for producing hydrophobized mixed metal oxide nanoparticles and use thereof for heterogeneous catalysis

Abstract

The invention relates to methods for producing hydrophobized, doped or non-doped mixed metal oxide nanoparticles or doped metal oxide nanoparticles by flame spray pyrolysis, wherein a combustible precursor solution A, containing at least two metal alkyloates of general formula Me(OOCR).sub.x with differing metals Me, or a combustible precursor solution B containing at least one metal alkyloate of general formula Me(OOCR).sub.x and at least one metal salt containing a metal ion Me and at least one metal salt containing a metal ion Me, with Me selected from metal ions of the subgroups of the periodic system of the elements, with R=alkyl or aryl, wherein the alkyl chain is branched or straight, and wherein x corresponds to the oxidation step of the metal ion, is used in stoichiometric excess relative to a quantity of oxygen, and wherein a combustion ratio of 3.5 bis 0.4 is established, and hydrophobized nanoparticles and the use thereof.

Claims

1. A method for producing hydrophobized, hopcalite nanoparticles on or in which carbon is deposited, with variable carbon content by flame spray pyrolysis, wherein a combustible precursor solution A, containing at least two metal alkyloates of which include at least one manganese alkyloate and at least one copper alkyloate, or a combustible precursor solution B, containing a metal alkyloate/metal salt mixture, selected from copper alkyloate and manganese alkyloate, said metal salt containing a metal ion Me, with Me selected from metal ions of the subgroups of the periodic system of the elements, wherein in precursor solution B, Me (metal alkyloate)Me (metal salt), and wherein the alkyl chain in the alkyloates of precursor solution A and precursor solution B is used in stoichiometric excess based on a quantity of oxygen in a dispersion gas, and wherein a combustion ratio of from 3.5 bis 0.4 is established.

2. The method according to claim 1, characterized in that each precursor solution contains at least one organic solvent selected from n-hexane, ethanol, 2-ethylhexanoic acid, methanol, acetic acid, n-heptane, xylene, toluene, benzene, trichloromethane, ethylbenzene and mixtures of these.

3. The method according to claim 1, characterized in that a quartz tube is placed over the combustion flame.

4. The method according to claim 1, characterized in that the precursor solution A contains the two metal alkyloates with a molar ratio of 10:90 to 90:10.

5. The method according to claim 1, characterized in that in precursor solution B the molar ratio of the total quantity of the metal alkyloates to the total quantity of metal salt is 80:20 to 99:1.

6. Hydrophobized hopcalite nanoparticles, containing carbon at a content of 0.1 to 5 wt.-%, based on the total quantity of nanoparticles, and a hydrophobicity index of 0.2 to 1.

7. Hydrophobized hopcalite nanoparticles according to claim 6, characterized in that the nanoparticles have a specific surface of 10 to 250 m.sup.2/g.

8. A method of oxidation of carbon monoxide and/or formaldehyde, said method comprising: providing the hydrophobized hopcalite nanoparticles according to claim 7 and using said hydrophobized hopcalite nanoparticles for the oxidation of carbon monoxide and/or formaldehyde.

9. A method of oxidation of carbon monoxide and/or formaldehyde, said method comprising: providing the hydrophobized hopcalite nanoparticles according to claim 7 in respirator masks.

Description

EXEMPLIFIED EMBODIMENTS

(1) Based on the figures and embodiments presented, the invention will be described in detail, without being limited to these. These show the following:

(2) FIG. 1 Nitrogen physisorption isotherms at 77 K of the hopcalite nanoparticles produced from an oleate precursor with n.sub.Cu/n.sub.Mn=1:2 according to example 1b,

(3) FIG. 2 X-ray diffractogram (PXRD measurement) of the hopcalite nanoparticles produced from an oleate precursor with n.sub.Cu/n.sub.Mn=1:2 according to example 1b,

(4) FIG. 3 SEM image of the hopcalite nanoparticles produced from an oleate precursor with n.sub.Cu/n.sub.Mn=1:2 according to example 1b,

(5) FIG. 4 Conversion-temperature diagram for the hopcalite nanoparticles produced according to example 2b compared with commercially available Carulite 300,

(6) FIG. 5 Catalytic activity for the hopcalite nanoparticles produced according to example 2b compared with commercially available Carulite 300,

(7) FIG. 6 X-ray diffractogram (PXRD measurement) of the hopcalite nanoparticles produced from an ethylhexanoate precursor with n.sub.Cu/n.sub.Mn=1:2 according to example 3b,

(8) FIG. 7 Nitrogen physisorption isotherms at 77 K of the hopcalite nanoparticles produced according to example 3b,

(9) FIG. 8 H.sub.2O physisorption isotherms at 25 C. of the hopcalite nanoparticles produced according to example 3b compared with Carulite 300,

(10) FIG. 9 x-time diagram for the long-term activity of the hopcalite nanoparticles produced according to example 3b in CO oxidation compared with activated Carulite 300 at room temperature under dry conditions

(11) FIG. 10 Structure of the flame generation unit in a flame spray pyrolysis unit,

(12) FIG. 11 Structure of the flame generation unit in a flame spray pyrolysis unit with quartz tube,

EXAMPLE 1FLAME SPRAY PYROLYSIS OF OLEATE PRECURSORS WITHOUT QUARTZ TUBE

1a) Synthesis of the Precursor Solution, Manganese Oleate-Copper Oleate Precursor Solution

(13) Mixed Mn(II)/Cu(II) precursors are synthesized starting from sodium oleate and metal chlorides. For this, sufficient CuCl2.2H2O (Alfa Aesar 99%) and MnCl2.2H2O (Grssing 99%) are dissolved in a mixture of ethanol and water (1:1; V/V) so that the metal ions are present in a total concentration of 0.6 M. Then a suspension of 0.6 M sodium oleate (Sigma Aldrich82%), dissolved in n-hexane (Merck analytical grade) is added and refluxed for 4 h at 70 C. Then the organic phase is washed twice with half its volume of deionized water. Finally the separated organic phase is diluted with enough n-hexane so that a clear, brown to green 0.05 M solution, based on the total concentration of the metal ions, is formed.

1b) Flame Spray Pyrolysis

(14) The solution is conveyed at 3.0 mL min.sup.1 to a two-substance nozzle (FIG. 10) and atomized to form an aerosol with a dispersion gas flow of 4.89 slm. This is ignited with the aid of a methane flame, through 1.50 slm CH.sub.4 and 3.00 slm O.sub.2. The dispersion gas pressure drop at the outlet of the nozzle is 2.0 bar. The black particles were collected using a 15 cm diameter glass fiber filter by applying a negative pressure.

1c) Characterization of the Hopcalite Nanoparticles

(15) The hopcalite nanoparticles produced according to example 1b are characterized by nitrogen physisorption measurement (FIG. 1), PXRD measurement (FIG. 2), scanning electron imaging (SEM imaging, FIG. 3, and elemental analysis.

(16) The nitrogen physisorption measurement of the hopcalite nanoparticles produced from an oleate precursor with n.sub.Cu/n.sub.Mn=1:2 (FIG. 1) shows the typical isotherm form of nonporous particles (type II isotherms) The particle size can be calculated from the specific surface at 13.8 nm.

(17) The X-ray diffractogram (FIG. 2) shows that nanocrystalline Cu.sub.1.5Mn.sub.1.5O.sub.4 has formed. In addition to Cu.sub.1.5Mn.sub.1.5O.sub.4, Mn.sub.3O.sub.4 is already present as a minor phase.

(18) The SEM imaging, FIG. 3) shows that homogeneously distributed hopcalite nanoparticles with a uniform size distribution formed according to example 1b. The particles have a diameter of about 14 nm.

(19) Elemental analysis of Mn:Cu-2:1 from oleate precursor:

(20) TABLE-US-00001 Element Measurement 1 Measurement 2 Mean C 0.1 0.085 0.0925 N 0.363 0.366 0.3645

EXAMPLE 2FLAME SPRAY PYROLYSIS OF ETHYLHEXANOATE PRECURSORS

2a) Synthesis of the Precursormanganesebis(2-ethylhexanoate)-copperbis(2-ethylhexanoate) Precursor Solution

(21) As the precursor, the 2-ethylhexanoates of copper and manganese are used and dissolved in a mixture of 90 vol.-% methanol and 10 vol.-% acetic acid, so that a total metal ion concentration of 0.05 mol/l results. The molar ratio of copper to manganese is 1:2.4.

2b) Flame Spray Pyrolysis

(22) The solution was conveyed at 2.5 mL min.sup.1 to a two-substance nozzle (FIG. 10) and atomized to form an aerosol with a dispersion gas flow of 7.5 slm. This was ignited with the aid of a methane flame, fed through 1.50 slm CH.sub.4 and 3.00 slm O.sub.2. The dispersion gas pressure drop at the outlet of the nozzle was 2.0 bar. The black particles were collected using a 15 cm diameter glass fiber filter by applying a negative pressure.

2c) Characterization of the Hopcalite Nanoparticles

(23) The nitrogen physisorption measurement gives a specific surface of 160 m.sup.2/g, from which a particle size of approx. 7 nm can be derived. The carbon content is determined as 0.31 wt.-% in a triplicate measurement (elemental analysis). Using ICP-OES (optical emission spectrometry with inductively coupled plasma) a manganese to copper molar ratio of 1.95 is determined.

2d) CO Oxidation

(24) The CO oxidation takes place in a 6 mm wide and 40 cm long integral glass tube, 30 cm of which are surrounded by the heating zone of an oven. With the aid of this oven, the reaction temperatures of 25 to 200 C. are set and tested in the catalyst bed using a temperature sensor. Before use in catalysis, the synthesized samples are washed twice with distilled water and centrifuged.

(25) For measuring the powdered catalysts, a layer of quartz glass wool is placed on the raised floor in the glass tube, then 100 mg of the washed catalyst (approx. 0.5 cm filling depth) is placed on top of this. The gas stream is passed through the catalyst bed from the top.

(26) The reaction gas used was a mixture of 0.67 Vol % CO in 66.00 Vol % N.sub.2 and 33.33 Vol % O.sub.2. The WHSV (weight hourly space velocity) is 35760 mL h.sup.1 g.sub.Kat.sup.1. The resulting CO.sub.2 is detected online using a smartMODUL.sup.PREMIUM NDIR sensor from Pewatron. The model number is P1-212506 (0-5 Vol.-%). Using a precisely 100 resistance, the current signal (0-20 mA) emitted by the detector is converted to a voltage signal (0-2V) and recorded as a time-dependent digital signal using a Meilhaus RedLab 1208LS. The signal rate is 1 Hz and the sensor resolution, 10 mV.

(27) To remove residual gases from the system, each catalyst is purged with nitrogen at 3 L h.sup.1 for at least 10 min before beginning the series of measurements. The CO conversion is measured at room temperature and oven temperatures of 80, 110, 150 and 200 C. After a sample is measured at the highest temperature, it is activated at 300 C. for 1 h in a 1 L h.sup.1 oxygen stream without carbon monoxide being present, then cooled to room temperature and again investigated at the various oven temperatures to test the effect of activation on the catalytic activity. The measurement variable used is the mean of the signal of the last 5 respective minutes of measurement (300 measured values). The conversions are calculated by comparison with a previously recorded calibration curve.

(28) FIG. 4 shows the conversion as a function of the temperature for the hopcalite catalyst produced according to example 2b compared with commercially available Carulite 300. For measuring the catalytic activity under moist conditions, the reaction gas mixture was passed through a water bath, which was maintained at 15 C. using a thermostat. A moisture content of 75% is obtained for the gas at this temperature. The gas mixture enriched with water in this way was then passed over the catalyst. The activity was measured at room temperature for 20 min. FIG. 5 shows the catalytic activity for the hopcalite catalyst produced according to example 2b compared with commercially available Carulite 300 under humid conditions after 10 and 20 min. Advantageously, the catalyst according to the invention shows that it was possible to distinctly improve the stability against moisture. The conversion after 10 and 20 minutes in humid air is distinctly increased compared with the commercial product.

EXAMPLE 3FLAME SPRAY PYROLYSIS OF ETHYLHEXANOATE PRECURSORS WITH A QUARTZ TUBE

3a) Precursor Synthesis

(29) As the precursor, the 2-ethylhexanoates of copper and manganese are used and dissolved in a mixture of 90 vol.-% methanol and 10 vol.-% acetic acid, so that a total metal ion concentration of 0.05 mol/l results. The molar ratio of copper to manganese is 1:2.

3b) Flame Spray Pyrolysis

(30) A quartz glass tube 400 mm in length and with an external diameter of 45 mm (wall thickness 2-2.5 mm) is additionally placed on the two-substance nozzle used in 1b), enclosing the flame, as shown in FIG. 11. The ignition of the aerosol atomized by a dispersion gas flow of 4.12 slm O.sub.2 (=1.0; see Equation 1) is performed with a pilot flame fed with methane (1.5 slm) and oxygen (3 slm).

(31) Using a micro annular gear pump, the precursor from example 3a is fed to the outlet of the capillary at 5 ml/min, without pulsations, at a dispersion gas pressure drop of 2 bar. The black particles were collected using a 15 cm diameter glass fiber filter by applying a negative pressure.

3c) Characterization of the Hopcalite Nanoparticles

(32) The hopcalite nanoparticles produced according to example 3b are characterized by X-ray powder diffractometry (FIG. 6), nitrogen physisorption (FIG. 7), water physisorption (FIG. 8) and elemental analysis. The nitrogen physisorption measurement gives a specific surface of 83 m.sup.2/g, from which a particle size of approx. 13 nm can be derived. The carbon content is determined as 0.75 wt.-% in a triplicate measurement (elemental analysis).

(33) Table 1 shows the elemental analysis of the nanoparticles according to example 3c.

(34) TABLE-US-00002 TABLE 1 C 0.76 N 0.03 H 0.268 S 0.02

(35) The X-ray powder diffractogram (FIG. 6) shows the X-ray diffractogram (PXRD measurement) of the hopcalite nanoparticles produced according to example 3b with n.sub.Cu/n.sub.Mn=1:2. In addition to Cu.sub.1.5Mn.sub.1.5O.sub.4, Mn.sub.3O.sub.4 is already present as a minor phase.

(36) FIG. 8 shows the H.sub.2O physisorption isotherms at 25 C. of the hopcalite nanoparticles produced according to example 3b compared with Carulite 300. The hydrophobicity of the samples can be recognized from the slope of the curves at low p/p0 values. Hydrophilic samples such as Carulite 300 already exhibit a steep slope at low pressures, and associated with this, a high water uptake capacity. The hopcalite particles produced according to the invention exhibit only a very slight slope at low pressures, indicating a low uptake capacity of the hydrophobic particles for water

3d) CO Oxidation

(37) The CO oxidation takes place in a 6 mm wide and 40 cm long integral glass tube, 30 cm of which are surrounded by the heating zone of an oven. With the aid of this oven, the reaction temperatures of 25 to 200 C. are set and tested in the catalyst bed using a temperature sensor. Before use in catalysis, the synthesized samples are washed twice with distilled water and centrifuged.

(38) For measuring the powdered catalysts, a layer of quartz glass wool is placed on the raised floor in the glass tube, then 100 mg of the washed catalyst (approx. 0.3 cm filling depth) is placed on top of this. The gas stream is passed through the catalyst bed from the top.

(39) The reaction gas used is a mixture of 0.67 Vol % CO in 66.00 Vol % N.sub.2 and 33.33 Vol % O.sub.2. The WHSV is 42000 mL h.sup.1 g.sub.Kat.sup.1. The resulting CO.sub.2 is detected online using a smartMODUL.sup.PREMUM NDIR sensor from Pewatron. The model number is P1-212506 (0-5 Vol.-%). Using a precisely 100 resistance, the current signal (0-20 mA) emitted by the detector is converted to a voltage signal (0-2V) and recorded as a time-dependent digital signal using a Meilhaus RedLab 1208LS. The signal rate is 1 Hz and the sensor resolution, 10 mV.

(40) To remove residual gases from the system, each catalyst is purged with nitrogen at 3 L h.sup.1 for at least 10 min before beginning the series of measurements. The conversion of CO is measured at room temperature and oven temperatures of 80, 110, 150 and 200 C. After a sample is measured at the highest temperature, it is activated at 300 C. for 1 h in a 1 L h.sup.1 oxygen stream without carbon monoxide being present, then cooled to room temperature and again investigated at the various oven temperatures to test the effect of activation on the catalytic activity. The measurement variable used is the mean of the signal of the last 5 respective minutes of measurement (300 measured values). The conversions are calculated by comparison with a previously recorded calibration curve.

SYMBOLS

(41) a) Precursor solution b) Shielding gas c) Ignition gas d) Dispersion gas. e) Pilot flame f) Combustion flame g) Quartz tube