Abstract:
A method for the production of Mg(OH) 2  nanoparticles, by means of polyol-mediated synthesis, from an Mg precursor as well as a base. The particles produced with this method have a diameter between 10 nm to 300 nm, have a mono-disperse particle distribution, and are present in non-agglomerated form. They can be converted to MgO particles by means of calcination.

Description:
BACKGROUND OF THE INVENTION 
     This application claims priority under 35 USC 119 to German Patent Application No. 10 2006 027 915.8 filed on Jun. 17 2006, the contents of which are herein incorporated by reference. 
     1. Field of the Invention 
     The method relates to the production of Mg(OH) 2  nanoparticles, in other words particles having a diameter ≦100 nm. These Mg(OH) 2  nanoparticles can be converted into MgO nanoparticles by means of calcination. 
     2. The Prior Art 
     Mg(OH) 2  nanoparticles and MgO nanoparticles are finding increasing importance in technical applications, such as, for example, as a catalyst material, in the modification or finishing of material surfaces, such as, for example for the purpose of surface hardening or UV stabilization. Nano-scale Mg(OH) 2  particles and MgO particles are furthermore used in the modification of plastics/polymers in connection with flame protection. Another area of use relates to functionalized electrodes and so-called hopping processes. 
     Several methods are known for the production of nano-scale particles, and especially of MgO particles and Mg(OH) 2  particles, respectively. 
     A common method represents the wet grinding of MgO or Mg(OH) 2 . Disadvantages of this method are an inefficient use of energy as well as a broad particle size distribution. Another disadvantage lies in the fact that the synthesized nanoparticles must be stabilized to prevent re-agglomeration, by means of adding an additive. 
     The synthesis of MgO nanoparticles or Mg(OH) 2  nanoparticles has been essentially based on gas phase methods until now. In German Patent No. DE 101 09 892 A1, a pyrolysis method, proceeding from MgCl 2 , for the production of nano-scale MgO having a monomodal particle size distribution is described. Since gas phase methods have the result of the formation of not fully saturated, i.e. unprotected solid body surfaces, it can fundamentally be assumed that an agglomeration of the MgO particles will occur after their synthesis. 
     German Patent No. DE 102 54 567 A1 discloses a production method for ZSO 4  nanoparticles (Z=Mg, Ca, Sr, or Ba, or binary mixtures of the same) in coordinating solvents such as, for example, ethylene glycol or other polyethylene glycols. The nanoparticles synthesized according to this method have a diameter of 0.5 nm to 50 nm and are characterized by a narrow particle size distribution. 
     This production method cannot be transferred to the production of Mg(OH) 2  nanoparticles or MgO nanoparticles, respectively, because of the high basicity of Mg(OH) 2  and MgO, respectively ([OH − ]&gt;10 −4  mol 1 −l ). Because of the high concentration of hydroxide ions [OH − ], decomposition/polymerization of the polyol component is to be expected, and this prevents a targeted synthesis of nanoparticles. 
     SUMMARY OF THE INVENTION 
     It is therefore the task of the present invention to produce Mg(OH) 2  nanoparticles and MgO nanoparticles, which have a narrow grain size distribution, at an average particle size diameter ≦100 nm, produced in targeted manner. Furthermore, these particles are supposed to be present in non-agglomerated form and are supposed to be easily re-dispersible in other solvents. 
     It was found that nano-scale Mg(OH) 2  particles can be produced synthetically by means of precipitation out of an aqueous MgCl 2 , Mg(NO 3 ) 2  or Mg(CH 3 COO) 2  solution, with a base such as NaOH, for example, at relatively high OH concentrations, in the presence of polyols, without any decomposition of the polyol component occurring. 
     According to the invention, first two solutions are produced, one of which (Solution 1) consists of the Mg precursor, which is dissolved in the selected polyol, and the other solution (Solution 2) represents a component having an alkaline reaction, which is also dissolved in the selected polyol. 
     MgCl 2  or Mg(NO 3 )  2  or Mg(CH 3 COO)  2  or their hydrates or aqueous solutions of these salts can be used as Mg precursors. Preferably, MgCl 2  ×6H 2 O or a 1% to 40% aqueous MgCl 2  solution is used. 
     When using an aqueous Mg precursor, after addition of the polyol, the excess water must be removed from Solution 1 or from the mixture of Solution 1 with Solution 2, by means of a suitable method. 
     The polyol proportion in Solution 1 is supposed to amount to between 90 wt.-% and 99 wt.-%. The base in Solution 2 serves as the precipitant and is selected from the group of NaOH, KOH, LiOH, or quaternary ammonium hydroxide (NR 4 OH with R =CH 3 , C 2 H 5 , C 3 H 7 , C 4 H 9 ) or metal alcoholates. Preferably, NaOH or KOH is used. Particularly preferably, NaOH is used. 
     Preferably, ethylene glycol, diethylene glycol, glycerin, propane diol, or butane diol is used as the polyol in Solution 1 and Solution 2. However, mixtures of two or more of these polyols can also be used. Particularly preferably, diethylene glycol is used. 
     The polyol proportion in Solution 2 is supposed to amount to between 60 wt.-% and 90 wt.-%. 
     After the production of Solutions 1 and 2, as described, these are combined, whereby thorough mixing is supported by strong stirring. In this connection, the mixture ratio of Solutions 1 and 2 lies in the range of 1:1 to 1:1.5. 
     One-stage or multi-stage heating of the resulting mixtures to a temperature between 30° C. and 300° C. follows. It has been shown that it is advantageous to first initiate the precipitation of the Mg(OH) 2  at temperatures from 30° C. to 80° C., and to increase the temperature of the mixture to 50° C. to 300° C. in a further step. In this connection, the temperature should be held over a period of 5 min to 60 min during the initial phase (Step 1). After further heating to the final temperature, this temperature should be held over a period of 10 min to 30 min. 
     For the second stage of heating, the mixture can also be placed into an autoclave. 
     By means of the method of procedure as described, a suspension of nano-scale Mg(OH) 2  is formed, which is subsequently cooled to ambient temperature. The nano-scale Mg(OH) 2  solid is removed by means of a suitable solid/liquid separation method. Centrifugation represents a suitable method. After separation, a damp nano-scale Mg(OH) 2  product is obtained. This product can be further purified by means of a one-stage or multi-stage process, and adhering polyol can be removed. This generally takes place by means of re-suspension of the damp product in polar solvents such as water, ethanol, isopropanol, or acetone, or a mixture of two or more of these solvents, and subsequent centrifugation. 
     However, the polyol can also be evaporated by means of heating in air. According to the invention, the surface of the nanoparticles remains covered with the polyol being used, in each instance, even after heating, as long as drying takes place at temperatures less than 250° C. Preferably, drying takes place at temperatures of 40° C. to 100° C., particularly preferably at 50° C. to 80° C. 
     Nano-scale Mg(OH) 2  particles can be produced according to the method described above, and after calcination, these can be converted into MgO nanoparticles. Calcination is carried out at temperatures between 250° C. and 500° C., particularly preferably at 270° C. to 350° C. 
     The Mg(OH)  2  particles and/or MgO particles produced according to the method according to the invention are present in amorphous or crystalline form, and are characterized by the following properties:
         The particle diameters lie in the range of 10 nm to 300 nm, particularly preferably 20 nm to 150 nm, at a mono-disperse particle size distribution, whose deviation from the average value amounts to &lt;±20%, particularly preferably &lt;±5%.   The particle diameters of the nanoparticles produced according to the method according to the invention can be influenced by means of varying the temperature after the addition of Solution 2 to Solution 1. In this connection, the formation of smaller particles is promoted by a lower temperature. Promoting the formation of smaller particles is also possible, in a further embodiment of the method according to the invention, in that the mixture ratio of Solution 1/Solution 2 is varied so that a smaller proportion of Solution 2 leads to the formation of smaller particles.   The particles possess a spherical habitus and are present in non-agglomerated form, whereby the particle surface is covered by the polyol used, in each instance.   The nano-scale Mg(OH) 2  or MgO according to the invention can be used both as a dry powder and in the form of a suspension with 0.1 wt.-% to 50 wt.-% solid component in a multivalent alcohol, for example glycol, diethylene glycol, glycerin, propane diol, or butane diol. Suspensions having a solid component of 1 wt.-% to 20 wt.-% are particularly preferred.       

    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The method will be explained in greater detail below, using exemplary embodiments. 
     Exemplary Embodiment 1 
     Under nitrogen, 0.50 g MgCl 2 ×6H 2 O are dissolved in 25 ml diethylene glycol (Solution 1). 1.50 g NaOH are dissolved in 3 ml water, and subsequently, 25 ml diethylene glycol are added (Solution 2). Solution 2 is heated to 50° C., and Solution 2 is added while stirring strongly. The temperature is held at 50° C. for another 30 min. Subsequently, the temperature is increased to 240° C., under distillation conditions, and held for another 10 min. After cooling, a suspension of 0.15 g Mg(OH) 2  in 50 ml diethylene glycol is obtained. The suspension is colloidally stable and contains agglomerate-free spherical particles having a diameter of about 100 nm. The suspension can be concentrated to a solid content of up to 0.15 g Mg(OH)  2  in 5 ml by means of centrifugation. By means of centrifugation followed by suitable washing processes (for example repeated re-suspension and centrifugation of the particles from ethanol, isopropanol, acetone, or water), the nano-scale solid can be removed. The particle surfaces are covered with diethylene glycol even in the dry powder. Based on this surface modification, easy re-dispersion of the solid in other solvents (for example methanol, ethanol, isopropanol, butanol, butane diol, or water) can take place. Re-dispersion can be supported by means of stirring and/or the effect of ultrasound. 
     Exemplary Embodiment 2 
     Under nitrogen, 2.50 g MgCl 2 ×6H 2 O are dissolved in 50 ml diethylene glycol (Solution 1). 7.50 g NaOH are dissolved in 6 ml water, and subsequently, 50 ml diethylene glycol are added (Solution 2). Solution 2 is heated to 50° C., and Solution 1 is added while stirring strongly. The temperature is held at 50° C. for another 30 min. Subsequently, the temperature is increased to 240° C., under distillation conditions, and held for another 10 min. After cooling, a suspension of 0.72 g Mg(OH)  2  in 100 ml diethylene glycol is obtained. The suspension is colloidally stable and contains agglomerate-free spherical particles having a diameter of about 100 nm. The suspension can be concentrated to a solid content of up to 0.72 g Mg(OH) 2  in 5 ml by means of centrifugation. By means of centrifugation followed by suitable washing processes (for example repeated re-suspension and centrifugation of the particles from ethanol, isopropanol, acetone, or water), the nano-scale solid can be removed. The particle surfaces are covered with diethylene glycol even in the dry powder. Based on this surface modification, easy re-dispersion of the solid in other solvents (for example methanol, ethanol, isopropanol, butanol, or butane diol, water) can take place. Re-dispersion can be supported by means of stirring and/or the effect of ultrasound. 
     Exemplary Embodiment 3 
     Under nitrogen, 0.48 g Mg(NO 3 )2×6H 2 O are dissolved in 25 ml diethylene glycol (Solution 1). 1.50 g NaOH are dissolved in 3 ml water, and subsequently, 25 ml diethylene glycol are added (Solution 2). Solution 2 is heated to 50° C., and Solution 1 is added while stirring strongly. The temperature is held at 50° C. for another 30 min. Subsequently, the temperature is increased to 240° C., under distillation conditions, and held for another 10 min. After cooling, a suspension of 0.15 g Mg(OH) 2  in 50 ml diethylene glycol is obtained. The suspension is colloidally stable and contains agglomerate-free spherical particles having a diameter of about 100 nm. The suspension can be concentrated to a solid content of up to 0.15 g Mg(OH) 2  in 5 ml by means of centrifugation. By means of centrifugation followed by suitable washing processes (for example repeated re-suspension and centrifugation of the particles from ethanol, isopropanol, acetone, or water), the nano-scale solid can be removed. The particle surfaces are covered with diethylene glycol even in the dry powder. Based on this surface modification, easy re-dispersion of the solid in other solvents (for example methanol, ethanol, isopropanol, butanol, butane diol, or water) can take place. Re-dispersion can be supported by means of stirring and/or the effect of ultrasound. 
     Exemplary Embodiment 4 
     Under nitrogen, 0.53 g Mg(CH 3 COO) 2 ×4H 2 O are dissolved in 25 ml diethylene glycol (Solution 1). 1.50 g NaOH are dissolved in 3 ml water, and subsequently, 25 ml diethylene glycol are added (Solution 2). Solution 2 is heated to 50° C., and Solution 1 is added while stirring strongly. The temperature is held at 50° C. for another 30 min. Subsequently, the temperature is increased to 240° C., under distillation conditions, and held for another 10 min. After cooling, a suspension of 0.15 g Mg(OH) 2 in 50 ml diethylene glycol is obtained. The suspension is colloidally stable and contains agglomerate-free spherical particles having a diameter of about 100 nm. The suspension can be concentrated to a solid content of up to 0.15 g Mg(OH) 2  in 5 ml by means of centrifugation. By means of centrifugation followed by suitable washing processes (for example repeated re-suspension and centrifugation of the particles from ethanol, isopropanol, acetone, or water), the nano-scale solid can be removed. The particle surfaces are covered with diethylene glycol even in the dry powder. Based on this surface modification, easy re-dispersion of the solid in other solvents (for example methanol, ethanol, isopropanol, butanol, butane diol, or water) can take place. Re-dispersion can be supported by means of stirring and/or the effect of ultrasound. 
     Exemplary Embodiment 5 
     Under nitrogen, 0.55 ml of a 30% aqueous MgCl 2  solution are mixed with 25 ml diethylene glycol (Solution 1). 1.50 g NaOH are dissolved in 3 ml water, and subsequently, 25 ml diethylene glycol are added (Solution 2). Solution 2 is heated to 50° C., and Solution 1 is added while stirring strongly. The temperature is held at 50° C. for another 30 min. Subsequently, the temperature is increased to 240° C., under distillation conditions, and held for another 10 min. After cooling, a suspension of 0.15 g Mg(OH)  2  in 50 ml diethylene glycol is obtained. The suspension is colloidally stable and contains agglomerate-free spherical particles having a diameter of about 100 nm. The suspension can be concentrated to a solid content of up to 0.15 g Mg(OH) 2  in 5 ml by means of centrifugation. By means of centrifugation followed by suitable washing processes (for example repeated re-suspension and centrifugation of the particles from ethanol, isopropanol, acetone, or water), the nano-scale solid can be removed. The particle surfaces are covered with diethylene glycol even in the dry powder. Based on this surface modification, easy re-dispersion of the solid in other solvents (for example methanol, ethanol, isopropanol, butanol, butane diol, or water) can take place. Re-dispersion can be supported by means of stirring and/or the effect of ultrasound. 
     Exemplary Embodiment 6 
     Under nitrogen, 0.50 g MgCl 2 ×6H 2 O are dissolved in 25 ml butane diol (Solution 1). 1.50 g NaOH are dissolved in 3 ml water, and subsequently, 25 ml butane diol are added (Solution 2). Solution 2 is heated to 50° C., and Solution 1 is added while stirring strongly. The temperature is held at 50° C. for another 30 min. Subsequently, the temperature is increased to 220° C., under distillation conditions, and held for another 10 min. After cooling, a suspension of 0.15 g Mg(OH) 2  in 50 ml butane diol is obtained. The suspension is colloidally stable and contains agglomerate-free spherical particles having a diameter of about 100 nm. The suspension can be concentrated to a solid content of up to 0.15 g Mg(OH)  2  in 5 ml by means of centrifugation. By means of centrifugation followed by suitable washing processes (for example repeated re-suspension and centrifugation of the particles from ethanol, isopropanol, acetone, or water), the nano-scale solid can be removed. The particle surfaces are covered with butane diol even in the dry powder. Based on this surface modification, easy re-dispersion of the solid in other solvents (for example methanol, ethanol, isopropanol, butanol, or water) can take place. Re-dispersion can be supported by means of stirring and/or the effect of ultrasound. 
     Exemplary Embodiment 7 
     Under nitrogen, 0.50 g MgCl 2 ×6H 2 O are dissolved in 25 ml ethylene glycol (Solution 1). 1.50 g NaOH are dissolved in 3 ml water, and subsequently, 25 ml ethylene glycol are added (Solution 2). Solution 2 is heated to 50° C., and Solution 1 is added while stirring strongly. The temperature is held at 50° C. for another 30 min. Subsequently, the temperature is increased to 190° C., under distillation conditions, and held for another 10 min. After cooling, a suspension of 0.15 g Mg(OH) 2  in 50 ml ethylene glycol is obtained. The suspension is colloidally stable and contains agglomerate-free spherical particles having a diameter of about 100 nm. The suspension can be concentrated to a solid content of up to 0.15 g Mg(OH)  2  in 5 ml by means of centrifugation. By means of centrifugation followed by suitable washing processes (for example repeated re-suspension and centrifugation of the particles from ethanol, isopropanol, acetone, or water), the nano-scale solid can be removed. The particle surfaces are covered with ethylene glycol even in the dry powder. Based on this surface modification, easy re-dispersion of the solid in other solvents (for example methanol, ethanol, isopropanol, butanol, butane diol, or water) can take place. Re-dispersion can be supported by means of stirring and/or the effect of ultrasound. 
     Exemplary Embodiment 8 
     Under nitrogen, 0.50 g MgCl 2 ×6H 2 O are dissolved in 25 ml of a mixture consisting of 50% diethylene glycol and 50% propane diol (Solution 1). 1.50 g NaOH are dissolved in 3 ml water, and subsequently, 25 ml of a mixture consisting of 50% diethylene glycol and 50% propane diol are added (Solution 2). Solution 2 is heated to 50° C., and Solution 1 is added while stirring strongly. The temperature is held at 50° C. for another 30 min. Subsequently, the temperature is increased to 200° C., under distillation conditions, and held for another 10 min. After cooling, a suspension of 0.15 g Mg(OH) 2  in 50 ml diethylene glycol/propane diol mixture is obtained. The suspension is colloidally stable and contains agglomerate-free spherical particles having a diameter of about 100 nm. The suspension can be concentrated to a solid content of up to 0.15 g Mg(OH)  2  in 5 ml by means of centrifugation. By means of centrifugation followed by suitable washing processes (for example repeated re-suspension and centrifugation of the particles from ethanol, isopropanol, acetone, or water), the nano-scale solid can be removed. The particle surfaces are covered with diethylene glycol or propane diol, respectively, even in the dry powder. Based on this surface modification, easy re-dispersion of the solid in other solvents (for example methanol, ethanol, isopropanol, butanol, butane diol, or water) can take place. Re-dispersion can be supported by means of stirring and/or the effect of ultrasound. 
     Exemplary Embodiment 9 
     Under nitrogen, 0.50 g MgCl 2 ×6H 2 O are dissolved in 25 ml diethylene glycol (Solution 1). 2.1 g KOH are dissolved in 3 ml water, and subsequently, 25 ml diethylene glycol are added (Solution 2). Solution 2 is heated to 50° C., and Solution 1 is added while stirring strongly. The temperature is held at 50° C. for another 30 min. Subsequently, the temperature is increased to 240° C., under distillation conditions, and held for another 10 min. After cooling, a suspension of 0.15 g Mg(OH)  2  in 50 ml diethylene glycol is obtained. The suspension is colloidally stable and contains agglomerate-free spherical particles having a diameter of about 100 nm. The suspension can be concentrated to a solid content of up to 0.15 g Mg (OH)  2  in 5 ml by means of centrifugation. By means of centrifugation followed by suitable washing processes (for example repeated re-suspension and centrifugation of the particles from ethanol, isopropanol, acetone, or water), the nano-scale solid can be removed. The particle surfaces are covered with diethylene glycol even in the dry powder. Based on this surface modification, easy re-dispersion of the solid in other solvents (for example methanol, ethanol, isopropanol, butanol, butane diol, or water) can take place. Re-dispersion can be supported by means of stirring and/or the effect of ultrasound. 
     Exemplary Embodiment 10 
     Under nitrogen, 0.50 g MgCl 2 ×6H 2 O are dissolved in 30 ml diethylene glycol and heated to 80° C. (Solution 1). 0.25 g NaOH are dissolved in 5 ml water, and added to Solution 1 while stirring strongly. The temperature is held at 80° C. for another 10 min. After cooling, a suspension of 0.15 g Mg(OH) 2  in 30 ml diethylene glycol is obtained. The suspension is colloidally stable and contains agglomerate-free spherical particles having a diameter of about 20 nm. The suspension can be concentrated to a solid content of up to 0.15 g Mg(OH) 2  in 5 ml by means of centrifugation. By means of centrifugation followed by suitable washing processes (for example repeated re-suspension and centrifugation of the particles from ethanol, isopropanol, acetone, or water), the nano-scale solid can be removed. The particle surfaces are covered with diethylene glycol even in the dry powder. Based on this surface modification, easy re-dispersion of the solid in other solvents (for example methanol, ethanol, isopropanol, butanol, butane diol, or water) can take place. Re-dispersion can be supported by means of stirring and/or the effect of ultrasound. 
     Exemplary Embodiment 11 
     Under nitrogen, 0.50 g MgCl 2 ×6H 2 O are dissolved in 25 ml diethylene glycol (Solution 1). 1.50 g NaOH are dissolved in 3 ml water, and subsequently, 25 ml diethylene glycol are added (Solution 2). Solution 2 is heated to 50° C., and Solution 1 is added while stirring strongly. The temperature is held at 50° C. for another 30 min. Subsequently, the mixture is heated in an autoclave, to 300° C., and held at this temperature for 10 min. After cooling, a suspension of 0.15 g Mg(OH)  2  in 50 ml diethylene glycol is obtained. The suspension is colloidally stable and contains agglomerate-free spherical particles having a diameter of about 100 nm. The suspension can be concentrated to a solid content of up to 0.15 g Mg(OH)  2  in 5 ml by means of centrifugation. By means of centrifugation followed by suitable washing processes (for example repeated re-suspension and centrifugation of the particles from ethanol, isopropanol, acetone, or water), the nano-scale solid can be removed. The particle surfaces are covered with diethylene glycol even in the dry powder. Based on this surface modification, easy re-dispersion of the solid in other solvents (for example methanol, ethanol, isopropanol, butanol, butane diol, or water) can take place. Re-dispersion can be supported by means of stirring and/or the effect of ultrasound. 
     Exemplary Embodiment 12 
     Under nitrogen, 0.50 g MgCl 2 ×6H 2 O are dissolved in 30 ml diethylene glycol and heated to 80° C. (Solution 1). 0.25 g NaOH are dissolved in 5 ml water and added to Solution 1 while stirring strongly. The temperature is held at 80° C. for another 10 min. Subsequently, the mixture is heated in an autoclave, to 300° C., and held at this temperature for 10 min. After cooling, a suspension of 0.15 g Mg(OH) 2  in 30 ml diethylene glycol is obtained. The suspension is colloidally stable and contains agglomerate-free spherical particles having a diameter of about 20 nm. The suspension can be concentrated to a solid content of up to 0.15 g Mg(OH)  2  in 5 ml by means of centrifugation. By means of centrifugation followed by suitable washing processes (for example repeated re-suspension and centrifugation of the particles from ethanol, isopropanol, acetone, or water), the nano-scale solid can be removed. The particle surfaces are covered with diethylene glycol even in the dry powder. Based on this surface modification, easy re-dispersion of the solid in other solvents (for example methanol, ethanol, isopropanol, butanol, butane diol, or water) can take place. Re-dispersion can be supported by means of stirring and/or the effect of ultrasound. 
     Accordingly, while only a few embodiments of the present invention have been described, it is obvious that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.