METHOD FOR PRODUCING NANOPARTICLES FROM A LIQUID MIXTURE

Abstract

A process for the production of nanoparticles from a liquid mixture comprising at least one precursor and at least one solvent in a reactor with continuous through-flow comprises the steps of feeding at least one oxygen-containing gas inflow stream having a temperature into the at least one reactor, adding at least one fuel having a temperature to the oxygen-containing gas inflow stream, wherein the fuel and the oxygen-containing gas inflow stream form a homogeneous ignitable mixture having a temperature, wherein the temperature of the homogeneous ignitable mixture is above the autoignition temperature of the homogeneous ignitable mixture, introducing at least one precursor-solvent mixture into the homogeneous ignitable mixture; autoignition of the ignitable mixture of oxygen-containing gas and fuel after an ignition delay time to form a stabilized flame and reacting the precursor-solvent mixture in the stabilized flame to form nanoparticles from the metal salt precursor, removing the formed nanoparticles.

Claims

1. A process for the production of nanoparticles from a liquid mixture comprising at least one precursor and at least one solvent in a reactor with continuous through-flow, comprising: a) feeding at least one oxygen-containing gas inflow stream having a temperature T.sub.I into the at least one reactor, b) adding at least one fuel having a temperature T.sub.F to the oxygen-containing gas inflow stream, wherein the fuel and the oxygen-containing gas inflow stream form a homogeneous ignitable mixture having a temperature T.sub.IM, wherein the temperature of the homogeneous ignitable mixture T.sub.IM is above the autoignition temperature T.sub.AIM of the homogeneous ignitable mixture, c) introducing at least one precursor-solvent mixture into the homogeneous ignitable mixture, d) autoignition of the ignitable mixture of oxygen-containing gas and fuel after an ignition delay time tip to form a stabilized flame and reacting the precursor-solvent mixture in the stabilized flame to form nanoparticles from the metal salt precursor, and e) removing the nanoparticles formed from the reactor.

2. The process as claimed in claim 1, wherein the homogeneous ignitable mixture has a flow velocity v.sub.IM in the reactor that is greater than the turbulent flame velocity v.sub.F of the flame formed from the ignitable mixture in step d) through autoignition.

3. The process as claimed in claim 1, wherein the flow velocity v.sub.IM of the oxygen-containing gas inflow stream is in a range between 5 and 200 m/s, preferably between 10 and 100 m/s.

4. The process as claimed in claim 1, wherein the oxygen-containing gas used is air or a mixture of oxygen with at least one inert gas, in particular nitrogen, carbon dioxide, argon.

5. The process as claimed in claim 1, wherein the temperature T.sub.I of the oxygen-containing gas inflow stream is in a range between 500-1500 K, preferably between 900-1400 K.

6. The process as claimed in claim 1, wherein the at least one fuel is a gaseous fuel and/or a liquid fuel.

7. The process as claimed in claim 1, wherein the air ratio of the ignitable mixture is in a range between 0.1 and 25, preferably in a range between 0.5 to 10, especially preferably in a range between 1 to 3.

8. The process as claimed in claim 1, wherein at least one precursor is a metal salt selected from the group of aluminum, barium, bismuth, calcium, cerium, iron, magnesium, platinum, palladium, strontium, titanium, zirconium, manganese, chromium, zinc, copper, nickel, cobalt, yttrium, silver, vanadium, molybdenum or other metals or metalloids.

9. The process as claimed in claim 1, wherein the precursor-solvent mixture is injected/sprayed into the homogeneous ignition mixture through at least one nozzle or atomizer.

10. The process as claimed in claim 9, wherein the precursor-solvent mixture is injected using an ultrasonic atomizer or a pressure-controlled injection nozzle.

11. The process as claimed in claim 1, wherein the solvent for the metal salt-precursor-solvent mixture is selected from a group including water or an organic solvent.

12. The process as claimed in claim 1, wherein the ignition delay time t.sub.ID is in a range between 1 s to 1 s, preferably 1 ms to 200 ms, in particular 10 ms to 100 ms.

13. The process as claimed in claim 1, the nanoparticles produced have a particle diameter with a d95 value of less than 1000 nm, preferably less than 800 nm, especially preferably less than 500 nm.

14. A reactor for the execution of a process as claimed in claim 1, comprising: a first section A having at least one means of introducing an oxygen-containing gas stream into the reactor; a second section B provided downstream of the first section A, having at least one means for introducing at least one fuel into the reactor; a third section C provided downstream of the first section A, having at least one means for injecting the at least one precursor-solvent mixture into the reactor; a fourth section D provided downstream of the second section B and third section C, for autoignition of the ignitable mixture of oxygen-containing gas and fuel; and a fifth section E provided downstream of section D, for the formation and removal from the reactor of the nanoparticles formed.

15. The reactor as claimed in claim 14, wherein the distance x between section B, in which the fuel is introduced, and section D, in which the autoignition of the ignitable mixture takes place, is given by the equation x=v.sub.IM*t.sub.ID.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The proposed solution will be explained in more detail below on the basis of working examples with reference to the figure.

[0075] FIG. 1 shows a schematic representation of a first embodiment of the present process.

DETAILED DESCRIPTION

[0076] FIG. 1 shows a schematic structure for an autoignition-stabilized flame for nanoparticle synthesis from a liquid spray.

[0077] In a first section A, a heated, preconditioned, oxygen-containing gas inflow stream having the temperature T.sub.I is provided in step 1. Preconditioning of the inflow stream can be achieved through, for example, an electric heater. Other options are to use a lean preflame, for example a lean, swirl-stabilized natural gas flame, or a plasma generated by high voltage. It is not just the resulting temperature of the inflow stream that plays a role here. Likewise, changing the concentration of oxygen through a preflame can cause an increase in the ignition delay time, whereas radicals present in the stream as a consequence of combustion or the plasma can drastically reduce the ignition delay time.

[0078] In section B, a preferably gaseous fuel is added to the hot gas stream in step 2, this being introduced such that the fuel mixes very rapidly with the gas to form a homogeneous mixture. Suitable fuels are hydrogen, natural gas, methane, propane, butane but also liquid fuels. Here it is essential that, after a short time or short travel distance in the reactor, there is always a homogeneous mixture present.

[0079] In step 3, an ignitable mixture is formed from the homogeneous mixture of oxygen-containing gas and fuel. Although the temperature of the resulting mixture is above the autoignition temperature of the mixture, combustion of the mixture does not occur instantaneously. This commences only once the ignition delay time to has elapsed, i.e. on reaching section D.

[0080] In section C of the reactor, the liquid precursor-solvent mixture is introduced through an atomization nozzle in step 4. The distance to the flame in section D of the reactor is chosen such that the liquid is not yet fully evaporated before reaching the flame. In the figure, section C is shown downstream of section B; depending on the process design, section C may also be upstream of section B. The solvent used does not necessarily need to be flammable.

[0081] In section D, i.e. once the characteristic ignition delay time has elapsed, autoignition of the ignitable mixture of oxygen-containing gas and fuel takes place (step 5). In section D, the fuel and, if combustible, the solvent burn. The precursor for the nanoparticles preferably undergoes transition into the gas phase. This allows the precursor to undergo reaction (oxidation, reduction, pyrolysis, hydrolysis) and the solvent, if combustible, to release additional thermal energy through burning, as a result of which very high temperatures are locally achieved. Supercritical heating of liquid droplets and consequent explosive evaporation is also possible (Rosebrock et al., AlChE Journal, 2016, Vol. 62, 381-391). This mechanism also allows the formation of droplets of the precursor-solvent mixture that are smaller than one micrometer and thus the formation of nanoparticles without prior transition into the gas phase.

[0082] To ensure this process flow, precise preconditioning of the inflow stream is necessary. The temperatures and mass flows of the supplied substances must be controlled very stably to one value in order to keep the flame stable in one position. Although the theoretical ignition delay times can serve as a guide when setting the flame position under defined process conditions, optical access to the reactor to adjust and control the flame position has been found to be helpful in laboratory implementation.

[0083] In section E of the reactor, nanoparticles are formed in step 6, preferably by condensation from the gaseous phase during and after combustion, which are subsequently removed from the reactor in an appropriate manner. However, formation of particles directly from the liquid phase is also possible.

WORKING EXAMPLE 1

[0084] An air inflow stream having a temperature of 1000 K is fed into the reactor. Hydrogen is added at an air ratio of 2 and a temperature of 300 K. This gives rise in the reactor to an air-hydrogen mixture having a temperature of 890 K. This mixture has an ignition delay of approx. 10 ms and a turbulent flame velocity of approx. 40 m/s. The flow velocity in the reactor is now given by the overall mass flow and the cross-section area and is set hereinafter at 80 m/s. This gives a travel distance x of 800 mm from the fuel injection (2) to the position of the flame (5). At a distance of 100 mm upstream of the flame, a 0.1 molar solution of iron(III) nitrate in ethanol is sprayed in at 300 K (4). This already reaches the flame after approx. 1 ms, which means that the drops will not yet have evaporated. In the flame, the liquid solution evaporates and the solvent burns, with the high temperatures resulting in the formation of Fe.sub.2O.sub.3 nanoparticles.

WORKING EXAMPLE 2

[0085] An air inflow stream having a temperature of 1000 K is fed into the reactor. Hydrogen is added at an air ratio of 1.7 and a temperature of 300 K. This gives rise in the reactor to an air-hydrogen mixture having a temperature of 870 K. This mixture has an ignition delay of approx. 11 ms and a turbulent flame velocity of approx. 56 m/s. The flow velocity in the reactor is now given by the overall mass flow and the cross-section area and is set hereinafter at 100 m/s. This gives a travel distance x of 1100 mm from the fuel injection (2) to the position of the flame (5). At a distance of 100 mm upstream of the flame, a 0.1 molar solution of manganese(II) nitrate in isopropanol is sprayed in at 300 K (4). This already reaches the flame after approx. 1 ms, which means that the drops will not yet have evaporated. In the flame, the liquid solution evaporates and the solvent burns, with the high temperatures resulting in the formation of Mn.sub.2O.sub.3 nanoparticles.

WORKING EXAMPLE 3

[0086] An air inflow stream having a temperature of 1000 K is fed into the reactor. Hydrogen is added at an air ratio of 2.5 and a temperature of 300 K. This gives rise in the reactor to an air-hydrogen mixture having a temperature of 910 K. This mixture has an ignition delay of approx. 8 ms and a turbulent flame velocity of approx. 24 m/s. The flow velocity in the reactor is now given by the overall mass flow and the cross-section area and is set hereinafter at 50 m/s. This gives a travel distance x of 400 mm from the fuel injection (2) to the position of the flame (5). At a distance of 100 mm upstream of the flame, a 0.1 molar solution of zinc naphthenate in ethanol is sprayed in at 300 K (4). This already reaches the flame after approx. 2 ms, which means that the drops will not yet have evaporated. In the flame, the liquid solution evaporates and the solvent burns, with the high temperatures resulting in the formation of ZnO nanoparticles.

WORKING EXAMPLE 4

[0087] An air inflow stream having a temperature of 900 K is fed into the reactor. Hydrogen is added at an air ratio of 2.5 and a temperature of 300 K. This gives rise in the reactor to an air-hydrogen mixture having a temperature of 830 K. This mixture has an ignition delay of approx. 48 ms and a turbulent flame velocity of approx. 23 m/s. The flow velocity in the reactor is now given by the overall mass flow and the cross-section area and is set hereinafter at 50 m/s. This gives a travel distance x of 2400 mm from the fuel injection (2) to the position of the flame (5). At a distance of 100 mm upstream of the flame, a 0.1 molar solution of aluminum triisopropoxide in isopropanol is sprayed in at 300 K (4). This already reaches the flame after approx. 2 ms, which means that the drops will not yet have evaporated. In the flame, the liquid solution evaporates and the solvent burns, with the high temperatures resulting in the formation of Al.sub.2O.sub.3 nanoparticles.

WORKING EXAMPLE 5

[0088] An air inflow stream having a temperature of 1400 K is fed into the reactor. Methane is added at an air ratio of 1.7 and a temperature of 300 K. This gives rise in the reactor to an air-methane mixture having a temperature of 1330 K. This mixture has an ignition delay of approx. 25 ms and a turbulent flame velocity of approx. 16 m/s. The flow velocity in the reactor is now given by the overall mass flow and the cross-section area and is set hereinafter at 30 m/s. This gives a travel distance x of 750 mm from the fuel injection (2) to the position of the flame (5). At a distance of 100 mm upstream of the flame, a 0.1 molar solution of tetraisopropyl orthotitanate in xylene is sprayed in at 300 K (4). This already reaches the flame after approx. 3 ms, which means that the drops will not yet have evaporated. In the flame, the liquid solution evaporates and the solvent burns, with the high temperatures resulting in the formation of TiO.sub.2 nanoparticles.

WORKING EXAMPLE 6

[0089] An air inflow stream having a temperature of 1300 K is fed into the reactor. Methane is added at an air ratio of 1.7 and a temperature of 300 K. This gives rise in the reactor to an air-methane mixture having a temperature of 1230 K. This mixture has an ignition delay of approx. 70 ms and a turbulent flame velocity of approx. 14 m/s. The flow velocity in the reactor is now given by the overall mass flow and the cross-section area and is set hereinafter at 30 m/s. This gives a travel distance x of 2100 mm from the fuel injection (2) to the position of the flame (5). At a distance of 100 mm upstream of the flame, a 0.1 molar solution of tetraethyl orthosilicate in isopropanol is sprayed in at 300 K (4). This already reaches the flame after approx. 3 ms, which means that the drops will not yet have evaporated. In the flame, the liquid solution evaporates and the solvent burns, with the high temperatures resulting in the formation of SiO.sub.2 nanoparticles.