FR2979842A1 - PROCESS FOR THE CONTINUOUS SYNTHESIS OF METAL OXIDE NANOPARTICLES BY HYDROTHERMAL REACTION IN A SUPERCRITICAL ENVIRONMENT - Google Patents

Abstract

The invention relates to a process for continuously synthesizing inorganic particles of nanometric metal oxides by hydrothermal reaction in a supercritical medium. It comprises: a) an introduction of pressurized water into an annular zone and a first end of a substantially tubular reactor comprising an outer wall and an inner tube; b) heating of the water in the annular zone; c) introducing heated and pressurized water into the inner tube and simultaneously introducing an aqueous and / or alcoholic solution of metal or organometallic precursors into said inner tube at the second end of the reactor; d) a mixture of the water and the aqueous and / or alcoholic solution in a first part of the inner tube, followed by a cooling of the mixture obtained in a second part of the inner tube; and e) isobaric evacuation of the cooled reactor mixture from the inner tube. The water and the aqueous and / or alcoholic solution flow continuously or almost continuously through the inner tube.

Description

The present invention relates to the synthesis of inorganic particles of nanometric metal oxides in a supercritical medium by hydrothermal reaction. BACKGROUND OF THE INVENTION The present invention relates to the synthesis of inorganic particles of nanometric metal oxides in supercritical medium by hydrothermal reaction. Nanoscale particles are particles with dimensions between 5 and 100 nm. STATE OF THE PRIOR ART Nowadays, particles of nanometric size (also called nanoparticles) in metal oxide are commonly used in many fields as raw material or as finished material. By way of examples, they are used for the production of tires, paints, catalysts, ceramic materials, cosmetics, etc. Typically, the nanoparticles made of metal oxide are obtained by hydrothermal reaction in a supercritical medium. The hydrothermal reaction operates according to a two-step mechanism, namely the hydrolysis and then the dehydration of a metal salt or precursor, which leads to the oxidation of said salt or said metal precursor: hydrated metal ions in the form of acetates, nitrates or sulphates are hydrolysed and precipitate in crystallized forms of solid oxides after dehydration at high temperature. In the supercritical state, the high pressure and temperature conditions cause the dielectric constant of the water to drop, which has the effect of improving the hydrolysis and dehydration steps. It is recalled that a supercritical fluid (the fluid being water here) is a fluid for which the pressure and temperature conditions are greater than the critical pressure and temperature (Pc and Tc) of the fluid in question. Numerous hydrothermal syntheses have been carried out in different solvents at a temperature of between 400 and 490 ° C. and a pressure of between 300 and 400 bars. As examples, Cabanas et al. studied the hydrolysis of various metal acetates in supercritical water (document [1]); Sakaki et al. (document [2]) and Aimable et al. (document [3]) have, in turn, described the continuous synthesis of MgFe2O4 nanoparticles and the synthesis of LiFePO4 nanoparticles, respectively. The synthesis of nanoparticles by hydrothermal reaction can be carried out batchwise (in "batch" mode in English) in a rudimentary reaction zone consisting of a simple pressure reactor (autoclave). It is however preferable to carry out such a continuous synthesis in a flow reactor suitable for the production of nanoparticles. For this, it is known to introduce an aqueous solution of metal salts or metal hydroxide, on the one hand, and a supercritical fluid, on the other hand, within a T-shaped element, in which the solution and the fluid are mixed, and then introduce the mixture thus formed into a simple tubular reactor at the desired pressure and temperature. The major disadvantage of this type of assembly is the lack of control of the precipitation of the nanoparticles, which induces a recurrent clogging at the T-shaped element where the mixing takes place and which, in the end, limits the obtaining a reliable and continuous synthesis device. It is also known to introduce the aqueous solution of metal salts or of metal hydroxide and the supercritical fluid into a countercurrent mixing reactor (document [4]), which has the advantage of reducing these capping effects. due to the rapid precipitation of metal species.

Nevertheless, this type of reactor is subject to corrosion attacks, which cause not only the destruction of the reactor, but also a contamination of the particles synthesized by the corrosion products. Such a reactor therefore does not allow continuous and continuous operation. In view of the above disadvantages, the inventors have set themselves the goal of developing a process for synthesizing nanoparticles by hydrothermal reaction in a supercritical fluid medium, in a particular reactor, which not only eliminates problems of plugging the reactor, but also to address the corrosion problem encountered in the prior art. DISCLOSURE OF THE INVENTION These and other objects are achieved by the invention which proposes a process for the continuous synthesis of inorganic particles of nanometric metal oxides by hydrothermal reaction in a supercritical medium, characterized in that it comprises the following steps: a) the introduction of pressurized water into an annular zone and a first end of a substantially tubular reactor comprising an outer wall and an inner tube, the annular zone of the reactor being defined by the wall; external and the inner tube, and the pressure of the water being greater than its critical pressure (P> 221 bar) b) heating the water under pressure in said annular zone at a temperature above its critical temperature (T> 374 ° C) to obtain supercritical water; c) the introduction of the supercritical water obtained at the end of step b) into the inner tube of the reactor at a second end of the reactor, and the simultaneous introduction of an aqueous solution and / or an alcoholic one or more metal or organometallic precursors in said inner tube at said second end of the reactor; d) mixing the supercritical water and the aqueous and / or alcoholic solution in a first portion of said inner tube to oxidize the metal or organometallic precursor or precursors present in the aqueous and / or alcoholic solution, followed by cooling the oxidized mixture in a second part of the inner tube; and e) the isobaric evacuation of the cooled mixture, obtained at the end of step d), from the reactor directly from the inner tube at the first end of the reactor, said cooled mixture comprising inorganic oxide particles nanometer-sized metal suspended in a liquid; the water and the aqueous and / or alcoholic solution running continuously or almost continuously through the inner tube. The inorganic particles of metal oxides that are produced can be, for example, oxides of titanium, zinc, aluminum, magnesium, zirconium ... It is specified that the metals of the metal oxides can be any metal among the known metals (alkali metals, alkaline earth metals, transition metals and poor metals), and especially transition metals.

The metal or organometallic precursor (s) may be chosen from metal salts and metal hydroxides (such as, for example, nitrates, sulphates, acetates, etc.).

The organometallic precursor (s) can comprise, as ligands, acetonate, alkyl or allyl ligands. At the end of step e) of the process, the nanoparticles are in suspension in the mixture recovered at the outlet of the reactor. According to the invention, the method may further comprise, after step e), a step f) of separating the metal oxide particles from the liquid in which they are suspended and, optionally, drying the particles thus separated. . This is particularly useful when it is desired to obtain a nanoparticle powder. Advantageously, the mixture of the supercritical water and the aqueous and / or alcoholic solution during step d) can be carried out by mechanical stirring. The mechanical agitation makes it possible on the one hand to guarantee a good heat transfer between the flows and the heating and cooling zones of the reactor, and on the other hand to avoid the sedimentation or the accumulation of the synthesized metal oxides which precipitate in the heart of the synthesis reactor. The mixture of the supercritical water and the aqueous and / or alcoholic solution in the inner tube 25 may be made for example by means of a subsequent agitation, or which tends towards a regime equivalent to that of a perfectly stirred reactor . This mixing can also be achieved, for example, by means of stirring which confines the stirring to successive volumes so as to maintain a substantially quasi-piston flow regime of the mixture in the inner tube. It should be noted that the hydrothermal reaction may be an exothermic reaction; in this case, there is a release of heat during the mixing of the supercritical water and the aqueous and / or alcoholic solution in step d), and this heat can be used to heat against the step b), the pressurized water present in the annular zone. Thus, the pressurized water can reach a supercritical state by being heated in the annular zone, for example between 374 ° C. and 600 ° C., on the one hand by the supercritical water / aqueous solution and / or alcoholic mixture (reaction medium ) circulating against the current in the inner tube, and secondly by a heating means disposed at the second end of the reactor. In step e), the cooling of the water / aqueous and / or alcoholic solution mixture in the inner tube is preferably carried out with vigorous stirring. The method according to the invention allows the continuous synthesis by hydrothermal reaction of inorganic particles of nanoscale metal oxides in a supercritical medium. This process thus makes it possible to synthesize metal nanoparticles of the single oxide type (TiO2, ZrO2, CeO2, ZnO, Al2O3, Fe2O3, Co304, NiO, etc.) or mixed (BaFe12019, LiCoO2, BaTO3, LiFePO4, etc.). The pressurized fluid used in the context of this invention is water in the supercritical state (T> 374 ° C., P> 221 bar), the supercritical water being the reaction medium for the hydrothermal synthesis of nanoparticles. According to the invention, it is possible for at least one oxidizing or reducing additive to be added to the pressurized water which is introduced into the annular zone of the reactor during step a), in order to oxidize or reducing the metal or organometallic precursors present in the aqueous and / or alcoholic solution, thereby initiating new reactions. These additives / reagents can be introduced into the first end or the second end of the main body. Ideally, a synthesis device for continuous hydrothermal synthesis of inorganic nanoparticles of metal oxides in a supercritical medium should have the following characteristics: - inertness to acids, bases and oxidants; - ease of assembly and disassembly for ease of maintenance; a length sufficient to obtain a homogeneous reaction temperature; - be watertight within the required range of temperature and pressure; be robust enough to withstand the high pressure temperature required for the use of a supercritical fluid and to operate for long periods without damage, so that no machining or surface treatment is required after each stoppage of operation ; to be able to treat solid suspensions without risk of accumulation, deposition or plugging; - Present turbulence conditions in the reaction zone, independent of the flow conditions induced by the supply of reagents and allowing the control of the particle size of synthesized nanoparticles. In the course of their work, the inventors have found that the reactor described in document [5] and designed to carry out the hydrothermal oxidation of organic matter in supercritical water, in particular for the destruction of waste, is also particularly suitable for producing continuous hydrothermal synthesis of nanoparticles of metal oxides in supercritical medium. In particular, the inventors have found that the original design of this reactor makes it possible to solve the problems of rapid corrosion, as well as the problems of accumulation of the nanoparticles that can cause a plug during a continuous synthesis. The invention will be better understood and other advantages and features will appear on reading the following description, given by way of non-limiting example, accompanied by the appended figure.

BRIEF DESCRIPTION OF THE FIGURE The figure shows, in longitudinal section, a diagram of an embodiment of a stirred double jacketed reactor used in the synthetic process according to the invention for the production of inorganic nanoparticles of metal oxides. DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS The synthesis device 1, which is implemented in the method according to the invention, comprises a main body 3 extending in a longitudinal direction between a first and a second end. Preferably, the main body has a substantially tubular shape. The main body 3 is provided at a first end thereof with a first inlet 9 for introducing a pressurized fluid (F) into the main body and a discharge outlet 11 for discharging a mixture (Containing nanoparticles (P)) outside the main body, and at a second end thereof, a second inlet 17 for introducing an aqueous and / or alcoholic solution of metal salts or metal hydroxide (S) into the main body. The synthesis device further comprises an inner tube 19, which is placed inside the main body 3 so as to form an annular zone 21 along said main body, the inner tube comprising a lumen 23, a first end 25 and a second end 27.

The first end 25 of the inner tube 19 is reversibly and sealingly attached to the first end of the main body and the inner tube 19 is dimensioned so that the second end 27 of the inner tube leaves a passage between the annular zone 21 and the light 23. Thus, because of the particular positioning of the inner tube in the main body and its sealingly attachment to one of the ends of the main body, the fluid communication between the first inlet 9 and the exhaust outlet It can be done only through the annular zone and the light of the inner tube. The first end of the main body is further provided with a flange 5 and sealing means 7, which allows access if necessary to the inside of the main body and in particular to replace the inner tube with another tube internal when the latter is too deteriorated. The inner tube is thus easy to access and its change involves little manpower and a low cost. The main body, the flange and the sealing means are made of materials resistant to the pressures and temperatures of the supercritical media. The second inlet 17 may optionally comprise an injector 35, which makes it possible to introduce the aqueous and / or alcoholic solution directly into the lumen of the inner tube, starting from the second inlet, preferably at a substantial distance from the end 27 of the inner tube to prevent the reflux of material in a supercritical medium. The injector may for example be a tube of a diameter smaller than that of the inner tube. The synthesis device also comprises a stirring means 29 placed in the lumen 23 of the inner tube 19, which here comprises a rotating shaft 15 provided with a plurality of elements 37 rotatably mounted about the axis of the shaft and spaced apart. others along the axis of the shaft (the shaft being actuated in rotation by the motor 13). The rotating elements 37 mounted on the rotating axis of the shaft may be of different geometries (propellers, turbines, blades, etc.). Their number can be defined according to the turbulence to be imposed on the mixture.

The materials constituting the injector (single or coaxial nozzle), the inner tube and the stirring means are chosen according to their ability to withstand chemical attack. They can possibly be in the same material. It is not necessary that they have a good resistance to pressure. Indeed, these parts do not undergo significant mechanical stress, since they are equipressure during operation of the synthesis device. However, it is preferable that they have good behavior at temperature and corrosion. The materials may be selected from stainless steel, heat and oxidation resistant nickel alloy, [Ni58, Fe20 Mo2 O] alloy, titanium, and ceramic. Preferably, these different constituent elements are designed so that they can easily be changed within the synthesis device. The synthesis device further comprises a refrigeration means 31 placed around the main body and a first portion thereof close to the first end of the main body. It makes it possible to cool the fluid mixture / aqueous and / or alcoholic solution in the inner tube 19 before it is discharged from the device 1 via the outlet 11.

It also comprises a heating means 33 placed around the main body and a second portion thereof close to the second end of the main body. It is used to heat the fluid, which has been introduced under supercritical pressure, so that it reaches a temperature above the critical temperature and forms a supercritical fluid before it enters the lumen of the inner tube 19 at its level. second end 27.

To achieve the synthesis of nanoparticles according to the process of the invention, it is first introduced water under pressure (at a pressure greater than the critical pressure of water, which is 221 bars) in the first end of the device at the level of the annular zone of the synthesis device via the first inlet 9. The water, which is introduced under pressure into the main body, circulates in the annular zone 21 and is heated there to a higher temperature. at its critical temperature, so that when it arrives at the entrance of the inner tube, it is in a supercritical state. The water is heated by heating means 33, the water being further preheated by a portion of the calories from the reaction mixture (supercritical water mixture / aqueous solution and / or alcoholic) flowing against the current in the tube light. The water may optionally comprise gases or additives (oxidizing or reducing), for example O 2, H 2, H 2 O 2, NaOH, HNO 3, in order to lead to new reactions and the formation of new compounds (oxidation or reduction reaction organometallic salts or precursors).

When the water is in a supercritical state and arrives at the second end of the main body, introducing an aqueous solution comprising one or more metal salts or acetates or an aqueous or alcoholic solution of metal hydroxide directly into the inner tube. It should be noted that the metal hydroxides are soluble in alcoholic medium and can be injected in the form of alcoholic solution (isopropanol type). Other precursors may be organometallic compounds having, as ligands, acetonate ligands, alkyl ligands or allyl ligands. This introduction is preferably by injection using a coaxial nozzle located at the second end of the main body.

Thus, the supercritical water and the aqueous and / or alcoholic solution penetrate at the same time inside the inner tube. The point of injection of the aqueous solution and / or alcoholic via the injector 35 is preferably located at a distance from the end 27 of the inner tube 19 greater than 5 times at least the inner diameter of the inner tube 19, so to guarantee the non reflux of the aqueous and / or alcoholic solution towards the end 27, thus protecting the main body 3 from any contact with the reaction mixture. When they penetrate into the inner tube, the supercritical fluid and the aqueous and / or alcoholic solution are mixed in the inner tube lumen and hydrothermal synthesis takes place (hydrolysis and dehydration of the metal salts or metal hydroxide contained in the solution ), leading to the formation of a reaction mixture. The mixture of the supercritical water and the aqueous solution and / or alcoholic can be achieved by mechanical stirring through the rotating shaft provided with blades 37. The blades 37 are also used to maintain suspended the synthesized nanoparticles; this prevents the sedimentation or accumulation of the nanoparticles in certain parts of the lumen of the inner tube. The hydrothermal reaction, which takes place in the inner tube, releases heat which is partially used to heat the water in the annular zone 21 against the current.

The reaction mixture stream is then cooled in the inner tube 19 by being strongly agitated by the axis 29 provided with the blades 37 to ensure good heat transfer between the streams and the heating zones (reaction zone) and cooling the inner tube .

The reaction mixture stream is also cooled by the refrigeration means 31 located at the first end of the main body. The annular zone 21 is thus found to be the seat of a transfer of heat between the fluid inside the inner tube 19 and the refrigerating means 31, which contributes to the heating of the fluid located in the annular zone. Preferably, the mixture is made with vigorous stirring, that is to say for a rotation of the rotary elements of between 300 and 2000 revolutions per minute, so as to promote a quasi-piston flow and to promote the cooling of the mixture in the second end of the light of the inner tube.

Finally, the cold effluent is directed isobarically to the outlet 11. The continuous hydrothermal synthesis device of the present invention differs from those of the prior art by the implementation of a stirring system in steady state. turbulent in an inner envelope in equipression. This original configuration makes it possible: to facilitate heat transfers in order to increase the hydrothermal synthesis yields at flow rates higher than those obtained with the devices of the prior art; to ensure a very good homogeneity of the temperature and the composition of the reaction mixture on the radial axis of the synthesis device; optimizing the residence time to obtain syntheses with a high yield at lower temperatures and pressures than those of the prior art; to maintain in suspension all the nanoparticles synthesized in order to avoid the formation of plugs in the reaction zone and in the cooling zone; - to operate over long periods with concentrated reagents without requiring long and expensive maintenance operations, unlike the devices of the prior art. EXAMPLE 1 Synthesis of TiO 2 Nanoparticles A hydrothermal reaction is carried out using the synthesis method according to the invention, which decomposes into a hydrolysis reaction (1), followed by a dehydration reaction (2). ): Ti (SO 4) 2 + 4H 2 O Ti (OH) 4 + 2H 2 SO 4 (1) Ti (OH) 4 - TiO 2 + 2H 2 O (2) For this purpose, a fluid under pressure (for example water) is introduced into the first end of the main body of the synthesis device. In this example, the fluid to be the supercritical fluid is water; the pressure of the water introduced into the first end of the main body is therefore greater than the supercritical pressure of the water.

Simultaneously or immediately after the introduction of the fluid under pressure into the first end, an aqueous solution of Ti (SO4) 2 is introduced into the synthesis device via the nozzle located at the second end of the body. principal of the synthesis device. The concentration of the aqueous solution in the reactor corresponds to a molarity of between 0.01 and 0.25 mol per kg of water.

In order for the pressurized fluid to reach a supercritical state, the temperature at the second end of the main body is maintained at a temperature above the critical temperature of the water (T> 374 ° C). walk. A high pressure pump and a pressure regulating device located downstream of the synthesis device make it possible to maintain a constant pressure of 300 bars during the synthesis of the nanoparticles.

Once they are synthesized, the nanoparticles are collected, for example in a liquid / gas separator or in a solid / liquid separator placed downstream of the pressure regulation system. EXAMPLE 2 Synthesis of ZnO Nanoparticles As in In the preceding example, a hydrothermal reaction is carried out using the synthesis method according to the invention.

This hydrothermal reaction decomposes into a hydrolysis reaction (3), followed by a dehydration reaction (4): Zn (NO3) 2 + 2H2O - ÷ Zn (OH) 2 + 2HNO3 (3) Zn (OH) 2 - ÷ ZnO + H20 (4) For this, a fluid under pressure (for example water) is introduced into the first end of the main body of the synthesis device, the fluid being at a pressure greater than the critical pressure of said fluid .

An aqueous solution of Zn (NO3) 2.6H20 is introduced into the synthesis device via a nozzle located at the second end of the main body of the synthesis device. The concentration of the aqueous solution in the reactor corresponds to a molarity of between 0.01 and 0.05 mol per kg of water. The synthesis device is maintained at a temperature above the critical temperature of the water (T> 374 ° C) at the second end of the main body and the mechanical agitation is turned on. A high pressure pump and a pressure regulator, located downstream of the synthesis device, allow to maintain a constant pressure of 300 bars during the synthesis.

The second end of the main body of the synthesis device, which is cooled, makes it possible to easily collect the nanoparticles, for example in a liquid / gas separator or in a solid / liquid separator placed downstream of the pressure regulation system.

BIBLIOGRAPHY [1] A. Cabanas et al. Journal of Supercritical Fluids, 40 (2007), pp. 284-292 [2] T. Sakaki et al. Journal of Supercriticals Fluids, Article in press (2009) [3] A. Aimable et al. Solid state ionics, 180 (2009), 861-866 [4] E. Lester et al.

WO 2005/077505 A2 [5] C. Joussot Dubien et al. FR 2,814,967

Claims (6)

REVENDICATIONS1. Process for the continuous synthesis of inorganic particles of nanometric metal oxides by hydrothermal reaction in a supercritical medium, characterized in that it comprises the following steps: a) the introduction of water under pressure into an annular zone and by a first end of a reactor of substantially tubular shape comprising an outer wall and an inner tube, the annular zone of the reactor being defined by the outer wall and the inner tube, and the pressure of the water being greater than its critical pressure (P > 221 bars); b) heating the water under pressure in said annular zone at a temperature above its critical temperature (T> 374 ° C) to obtain supercritical water; c) introducing the supercritical water obtained at the end of step b) into the inner tube of the reactor at a second end of the reactor, and the simultaneous introduction of an aqueous solution and / or an alcoholic one or more metal or organometallic precursors in said inner tube at said second end of the reactor; d) mixing the supercritical water and the aqueous and / or alcoholic solution in a first part of said inner tube so as to oxidize the metal or organometallic precursor (s) of the aqueous and / or alcoholic solution, followed by cooling the oxidized mixture in a second part of the inner tube; and e) the isobaric evacuation of the cooled mixture, obtained at the end of step d), from the reactor directly from the inner tube at the first end of the reactor, said cooled mixture comprising inorganic oxide particles nanometer-sized metal suspended in a liquid; supercritical water and the aqueous solution and / or alcoholic running continuously or almost continuously the inner tube. 2. Synthesis process according to claim 1, wherein the one or more metal precursors or organometallic are selected from metal salts and metal hydroxides. The synthesis method according to claim 1, wherein the one or more organometallic precursors comprise acetonate, alkyl or allyl ligands. 4. Synthesis process according to claim 1, further comprising, after step e), a step f) of separating the metal oxide particles from the liquid in which they are in suspension and, optionally, the drying of the particles and séparées.30 5. Synthesis process according to claim 1, wherein at least one oxidizing or reducing additive is added to the pressurized water introduced into the annular zone of the reactor during step a). 6. Synthesis process according to claim 1, wherein the mixture of supercritical water and the aqueous solution and / or alcoholic in step d) is carried out by mechanical stirring.