Abstract:
The present invention relates to a catalytic composition for the synthesis of carbon nanotubes, comprising an active catalyst and a catalytic support, the active catalyst comprising a mixture of iron and cobalt in any oxidation form and the catalytic support comprising exfoliated vermiculite.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    This patent application is a divisional of co-pending U.S. patent application Ser. No. 14/005,043, filed Nov. 7, 2013, which claims priority to PCT/EP2012/054029, filed Mar. 8, 2012, which claims priority to European Application No. 11158890.1, filed Mar. 18, 2011, the entire teachings and disclosure of which are incorporated herein by reference thereto. 
     
    
     OBJECT OF THE INVENTION 
       [0002]    The present invention relates to a catalytic composition for the synthesis of carbon nanotubes and more particularly to a catalytic composition comprising a mixture of active iron and cobalt sites on an exfoliated vermiculite support. The invention also relates to a method for the synthesis of the catalytic composition and of the carbon nanotubes obtained by means of this catalytic composition. The invention is also directed to polymeric composites comprising the thereby obtained carbon nanotubes with their improved electric features. 
       STATE OF THE ART 
       [0003]    Supported catalysts on exfoliated vermiculite for the synthesis of carbon nanotubes are known from the state of the art. Qiang Zhang, in his article &lt;&lt; Mass production of aligned carbon nano tube arrays by fluidized bed catalytic chemical vapor deposition &gt;&gt; in CARBON 48 (2010)1196-1209 discloses an iron-molybdenum-based supported catalyst on vermiculite. In this document, the exfoliated vermiculite had a particle size comprised between 100 and 250 μm. For the synthesis of the supported catalyst, this vermiculite was suspended in an aqueous solution of Fe(NO 3 ) 3 .9H 2 O and of (NH 4 ) 6 Mo 7 O 24 .4H 2 O. 
         [0004]    The synthesis of carbon nanotubes is carried out according to a fluidized bed method and the yields which are relatively low, are located between 0.224 and 1.167 grams of carbon nanotubes per gram of catalyst for a variable synthesis time ranging up to 30 minutes. 
         [0005]    Moura et al. in their article &lt;&lt; Catalytic growth of carbon nanotubes and nanofibers on vermiculite to produce floatable hydrophobic &lt;&lt;nanosponges&gt;&gt; for oil spill remediation ” in Applied catalysis B: Environmental, 2009, vol. 90, pages 436-440, also discloses a supported catalyst based on iron, on molybdenum or on an iron-molybdenum mixture on exfoliated vermiculite. For this purpose, the vermiculite was exfoliated at 1,000° C. for 60 seconds and impregnated with a solution of Fe(NO 3 ) 3  and of MoO 2 (acac) 2  by using water or methanol as a solvent. Only the iron-molybdenum combinations give rise to a synthesis of carbon nanotubes. 
         [0006]    Vermiculite is a clayey hydrated mineral with a lamellar structure of aluminum, iron and magnesium silicates resembling muscovites like mica and when they are subject to heat, they exfoliate depending on the steam generated between the lamellas. Vermiculite is inert and non-combustible, it has a melting temperature around 1,300° C. and a unit weight between 40 and 80 g/l when it is exfoliated. 
         [0007]    Document U.S. Pat. No. 3,062,753 A and document U.S. Pat. No. 5,879,600 disclose methods for exfoliating vermiculite and document U.S. Pat. No. 7,541,311 discloses a catalyst using vermiculite. The information on vermiculite contained in these three documents is representative of the knowledge of the person skilled in the art on vermiculite. 
       OBJECTS OF THE INVENTION 
       [0008]    The present invention aims at providing an improved catalytic composition allowing the synthesis of carbon nanotubes with particular features. 
         [0009]    It also aims at providing a method for synthesizing carbon nanotubes with said catalyst. 
         [0010]    The invention finally aims at disclosing polymeric composites with improved physical properties and obtained by means of the carbon nanotubes synthesized by means of the catalytic composition according to the invention. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention discloses a catalytic composition for the synthesis of carbon nanotubes comprising an active catalyst and a catalytic support, the active catalyst comprising a mixture of iron and cobalt in any oxidation form and the catalytic support comprising exfoliated vermiculite. 
         [0012]    The particular embodiments of the present invention comprise at least one or a suitable combination of the following features:
       in the catalytic composition, the molar proportion of cobalt and iron (Co/Fe) is comprised between 0.1 and 2, preferably between 0.25 and 1.5;   in the catalytic composition, weight the percentage of the active catalyst relatively to the catalytic composition is comprised between 1.5 and 20% and preferably between 2.2 and 12%, even more preferably between 2.2 and 8%;   in the catalytic composition, the exfoliated vermiculite has a particle size comprised between 50 and 1,000 μm, preferably between 500 and 1,000 μm;       
 
         [0016]    The present invention also discloses a method for synthesizing the catalytic composition according to the invention comprising the following steps:
       Exfoliation of the vermiculite by treatment of a vermiculite ore at a temperature above 800° C.;   Contacting the exfoliated vermiculite with a solution of a cobalt and iron salt;   Calcination of the vermiculite contacted with a solution of a cobalt and iron salt at a temperature above 350° C.       
 
         [0020]    According to particular embodiments, the method comprises at least one or a suitable combination of the following features:
       in the method, the iron salt is Fe(NO 3 ) 3 .9H 2 O;   in the method, the cobalt salt is Co(OAc) 2 .4H 2 O;   in the method, contacting the exfoliated vermiculite with a solution of a cobalt and iron salt is accomplished by impregnation with an aqueous solution.       
 
         [0024]    The present invention also discloses a method for synthesizing carbon nanotubes by decomposing a gaseous hydrocarbon on a catalytic composition according to the invention comprising the following steps:
       Conditioning the catalytic composition under an inert atmosphere;   Contacting the catalytic composition with a gaseous carbon source at a temperature between 600 and 800° C. for at least 5 minutes.       
 
         [0027]    According to a particular embodiment of the method for synthesizing carbon nanotubes, the supply of catalytic composition and the extraction of synthesized carbon nanotubes is continuous. 
         [0028]    The present invention also discloses polymeric composites comprising carbon nanotubes obtained according to the method of the invention. 
     
    
     
       SHORT DESCRIPTION OF THE FIGURES 
         [0029]      FIG. 1  shows the influence of the water impregnation volume on the yield of the catalytic composition while keeping the molar ratio Co/Fe and the metal percentage constant. 
           [0030]      FIG. 2  shows the influence of the Co/Fe ratio on the yield of the catalyst. A series of 10 catalysts was prepared while keeping a level of 4% of metals. 
           [0031]      FIG. 3  shows the influence of the percentage of metals on the yield of the catalyst. 
           [0032]      FIG. 4  shows the influence of the synthesis time on the yield of the catalyst (a plateau is reached after 25 minutes). 
           [0033]      FIG. 5  shows the influence of the vermiculite type on the yield of the catalyst. 
           [0034]      FIG. 6  shows the influence of the type of solvent used for dissolving the metal salts on the yield of the catalyst. 
           [0035]      FIG. 7  shows the bulk resistivity of a polycarbonate composite (Makrolon 2205-Bayer) using different concentrations of carbon nanotubes obtained by various synthesis methods comprising the carbon nanotubes obtained by catalytic synthesis on a vermiculite support according to the invention. 
           [0036]      FIG. 8  shows the surface resistivity of a polycarbonate composite comprising different concentrations of carbon nanotubes obtained by various synthesis methods including the carbon nanotubes obtained by catalytic synthesis on a vermiculite support according to the invention. 
           [0037]      FIG. 9  shows the surface resistivity and  FIG. 10  shows the bulk resistivity of a polysiloxane composite (VQM 809+crosslinker 100+Catalyst 510+inhibitor 600-Hanse Chemie) comprising different concentrations of the carbon nanotubes obtained by various synthesis methods including the carbon nanotubes obtained by catalytic synthesis on a vermiculite support according to the invention. 
           [0038]      FIG. 11  shows the surface resistivity of a composite of an epoxy resin (Epikote 828—Hexion) comprising different concentrations of carbon nanotubes obtained by various synthesis methods comprising the carbon nanotubes obtained by catalytic synthesis on a vermiculite support according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0039]    The invention consists in producing a catalytic composition with an active catalyst and a catalytic support, the active catalyst comprising a mixture of cobalt and iron on a catalytic support comprising exfoliated vermiculite. This catalytic composition allows to obtain carbon nanotubes which, when they are dispersed in polymeric matrices, provide a high electric conductivity level with low concentrations of carbon nanotubes. The invention also describes a method for the synthesis of the catalytic composition. 
         [0040]    The method for preparing the catalytic composition comprises the following steps:
       Exfoliation of the vermiculite by treatment of a vermiculite ore at a temperature above 800° C.;   Impregnation of the exfoliated vermiculite with a solution of a cobalt and iron salt.   Calcination of the impregnated vermiculite under an inert atmosphere at a temperature above 350° C.       
 
       Example of Synthesis of the Catalytic Composition 
       [0000]    
       
         
           
             1) Exfoliation of the vermiculite by treatment of a vermiculite ore (Imerys, Shawa mine Zimbabwe) at 900° C. for 2 minutes under nitrogen and then letting it cool down to room temperature. 
             2) Preparation of a solution of metal salts: 1.14 g of Fe(NO 3 ) 3 .9H 2 O and 0.35 g of Co(OAc) 2 .4H 2 O are weighed, and then dissolved in 10 ml of water. 
             3) Impregnation: The solution is poured into 6 g of exfoliated vermiculite. Mixing is performed with a spatula for homogenization. Impregnation is left to act for about 1 night. 
             4) Drying/calcination: The mixture is calcined under nitrogen according to a temperature program consisting of a rapid rise in temperature up to 400° C., followed by a plateau for 1 hour and then by cooling. 
           
         
       
     
       Example of Synthesis of Carbon Nanotubes 
       [0000]    
       
         
           
             1) 1 g of catalyst is weighed and then scattered on a container. 
             2) The container is placed in the cold area of the reactor. The plug of the quartz tube is placed and a flow of 2 l/min of nitrogen is let through for 6 mins. 
             3) A nitrogen, ethylene and hydrogen mixture is let through with flows of 0.857 l/min, 1.744 l/min and 0.286 l/min respectively. One waits for 6 mins so that the concentrations in the reactor stabilize. 
             4) The container is introduced into the hot area of the reactor, adjusted beforehand to 700° C. It is left to react for 20 mins. 
             5) The gases are stopped and a flow of 2 l/min of nitrogen is introduced. The container is placed in the cold area. The container is left to cool for 6 mins. The plug is opened and the nanotubes are collected.
 
After having synthesized the carbon nanotubes by means of the catalytic composition according to the invention, the latter are dispersed by conventional means into various polymeric matrices.
 
           
         
       
     
       Example of Dispersion of Carbon Nanotubes in a Polycarbonate Matrix 
       [0053]    12.5 g mixtures are prepared containing 0.75%, 1%, 1.5%, 2%, 3%, 4% respectively by mass fraction of carbon nanotubes in a polycarbonate matrix PC2205.
 
The different mixtures are passed into a co-rotary twin-screw micro-extruder of the 15 cm 3  DSM Xplore type. Mixing is performed at 280° C., at 50 rpm for 5 minutes.
 
The molten mixture is then injected with a micro-injector of the 12 cm 3  DSM Xplore type, the piston chamber of which is heated to 280° C. and the mold to 100° C. The injection is achieved with a pressure of 8 bars for 2 seconds, followed by a rise up to 12 bars within 8 seconds and finally 12 bars are maintained for 4 seconds. The mold is a 2-bar IZOD mold.
 
The ends of the bars are sawed over 3 mm.
 
Silver paint is applied on the ends of the bars and the 2-point measurement of bulk resistivity is carried out with a multimeter of the Keithley 2700 type.
 
Silver paint is applied in 2 strips separated by 1 cm and the 2-point measurement of surface resistivity is carried out with a multimeter of the Keithley 2700 type. The results of these measurements are illustrated in  FIGS. 7 to 11 .
 
         [0054]    Influence of Different Parameters on the Yield of the Catalytic Composition According to the Invention 
       Examples 
       [0055]    A) The Co/Fe Ratio 
         [0000]      FIG. 2  shows different Co/Fe ratios. Maximum productivity is reached with a ratio of about 0.5 to 0.66. 
         [0056]    Preparation Conditions 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
               
                 Support 
                 Metals 
                 Impregnation 
                 calcination 
               
               
                   
               
             
             
               
                 6 g Imerys Vermiculite, Shawa 
                 Fe(NO 3 ) 3 •9H 2 O 
                 Water 
                 400° C.  
               
               
                 Mine Zimbabwe, Micron 
                 Co(OAc) 2 •4H 2 O 
                 volume: 8 ml 
                 1 h 
               
               
                 Sifted (&gt;500 μm) 
                 Variable Co/Fe molar ratio 
                 Rest: 16 h 
                     1 m 3 /h N 2   
               
               
                 Exfoliated for 2 mins at 900° C. 
                 Mass % of metal = 4.0% 
               
               
                 under 2 l/min of N 2   
               
               
                   
               
             
          
         
       
     
       Results 
       [0057]      
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Mass of 
                 Mass of 
                 Co/Fe 
                 Yield 
               
               
                 Catalyst 
                 Co(OAc) 2 •4H 2 O 
                 Fe(NO 3 ) 3 •9H 2 O 
                 molar ratio 
                 (g/g) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 137 
                 0.35 
                 1.14 
                 1/2 
                 11.7 
               
               
                 138 
                 0.26 
                 1.28 
                 1/3 
                 7.9 
               
               
                 139 
                 0.53 
                 0.86 
                 1/1 
                 10.7 
               
               
                 185 
                 0.63 
                 0.69 
                 3/2 
                 5.04 
               
               
                 143 
                 0.42 
                 1.03 
                 2/3 
                 11.7 
               
               
                 144 
                 0.48 
                 0.94 
                 5/6 
                 10.9 
               
               
                 177 
                 0.34 
                 0.28 
                 2/1 
                 3.7 
               
               
                 178 
                 0.11 
                 0.69 
                 1/4 
                 6.5 
               
               
                 235 
                 0.1 
                 1.57 
                  1/10 
                 3.99 
               
               
                 193 
                 0 
                 1.74 
                 0/1 
                 2.0 
               
               
                   
               
             
          
         
       
     
         [0058]    B) Percentage of Metal in the Catalytic Composition 
         [0000]    Three series of catalysts were prepared. The series with the constant ratio Co/Fe=0.333 and Co/Fe=1.5 comprises 4 points. The series with the constant ratio Co/Fe=0.5 comprises 9 points. The curves of  FIG. 3  show that a metal percentage around 5% gives the best yield in the catalytic composition regardless of the Co/Fe ratio. 
       Preparation Conditions 
       [0059]      
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
               
                 Support 
                 Metal 
                 Impregnation 
                 calcination 
               
               
                   
               
             
             
               
                 6 g Imerys Vermiculite, 
                 Fe(NO 3 ) 3 •9H 2 O 
                 Water 
                 400° C.  
               
               
                 Shawa Mine Zimbabwe, 
                 Co(OAc) 2 •4H 2 O 
                 volume: 8 ml 
                 1 h 
               
               
                 Micron 
                 Co/Fe molar ratio: 0.33; 
                 Rest: 16 h 
                     1 m 3 /h N 2   
               
               
                 Sifted (&gt;500 μm) 
                 0.5 and 1.5 
               
               
                 Exfoliated for 2 mins at 
                 Mass % of metal = variable 
               
               
                 900° C. under 2 l/min of N 2   
                 from 0.74 to 20. 
               
               
                   
               
             
          
         
       
     
       Results 
       [0060]      
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 Mass % 
                 Co/Fe 
                   
               
               
                   
                 Mass of 
                 Mass of 
                 of 
                 molar 
                 Yield 
               
               
                 Catalyst 
                 Co(OAc) 2 •4H 2 O 
                 Fe(NO 3 ) 3 •9H 2 O 
                 metal 
                 ratio 
                 (g/g) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 146 
                 0.064 
                 0.21 
                 0.74 
                 0.5 
                 0.64 
               
               
                 195 
                 0.086 
                 0.284 
                 1 
                 0.5 
                 1.2 
               
               
                 194 
                 0.132 
                 0.426 
                 1.5 
                 0.5 
                 4.31 
               
               
                 142 
                 0.19 
                 0.62 
                 2.2 
                 0.5 
                 9 
               
               
                 137 
                 0.35 
                 1.14 
                 4 
                 0.5 
                 11.7 
               
               
                 141 
                 0.51 
                 1.66 
                 5.8 
                 0.5 
                 12.1 
               
               
                 182 
                 0.7 
                 2.274 
                 8 
                 0.5 
                 10.9 
               
               
                 183 
                 1.052 
                 3.41 
                 12 
                 0.5 
                 9.3 
               
               
                 196 
                 1.752 
                 5.682 
                 20 
                 0.5 
                 5.31 
               
               
                 184 
                 0.31 
                 0.336 
                 2 
                 1.5 
                 1.72 
               
               
                 185 
                 0.622 
                 0.672 
                 4 
                 1.5 
                 5.04 
               
               
                 186 
                 0.932 
                 1.008 
                 6 
                 1.5 
                 5.13 
               
               
                 187 
                 1.864 
                 2.016 
                 12 
                 1.5 
                 3.07 
               
               
                 234 
                 3.10 
                 3.66 
                 20 
                 1.54 
                 2.76 
               
               
                 188 
                 0.132 
                 0.642 
                 2 
                 0.33 
                 7.01 
               
               
                 189 
                 0.264 
                 1.284 
                 4 
                 0.33 
                 9.26 
               
               
                 190 
                 0.396 
                 1.926 
                 6 
                 0.33 
                 9.24 
               
               
                 191 
                 0.792 
                 3.854 
                 12 
                 0.33 
                 7.44 
               
               
                 233 
                 1.32 
                 6.42 
                 20 
                 0.33 
                 3.74 
               
               
                   
               
             
          
         
       
     
         [0061]    C) The Vermiculite Type 
         [0000]    A series of 3 catalysts was prepared from 3 vermiculites of different origin. The Imerys and Nestaan vermiculites were thermally exfoliated at about 900° C. under nitrogen. Sifting was also required for removing the impurities (&gt;500 μm). The results show that the origin of the vermiculite has little influence on the yield of the catalyst. 
       Preparation Conditions 
       [0062]      
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Metals 
                 Impregnation 
                 calcination 
               
               
                   
                   
               
             
             
               
                   
                 1.14 g Fe(NO 3 ) 3 •9H 2 O 
                 Water 
                 400° C.  
               
               
                   
                 0.35 g Co(OAc) 2 •4H 2 O 
                 volume: 5 ml 
                 1 h 
               
               
                   
                 Fe/Co molar ratio = 2/1 
                 Rest: 16 h 
                     1 m 3 /h N 2   
               
               
                   
                 Mass % of metal = 4.0% 
               
               
                   
                   
               
             
          
         
       
     
       Results 
       [0063]      
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                   
                 Yield 
               
               
                 Catalyst 
                 Provider 
                 Grade 
                 Origin 
                 Preparation 
                 (g/g) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 173 
                 Imerys 
                 M8 
                 Mud tank 
                 sifted (&gt;500 μm), 
                 9.7 
               
               
                   
                   
                   
                 (Australia) 
                 exfoliated at 
               
               
                   
                   
                   
                   
                 900° C. under N 2   
               
               
                 137 
                 Imerys 
                 Micron 
                 Shawa mine 
                 sifted (&gt;500 μm), 
                 11.7 
               
               
                   
                   
                   
                 (Zimbabwe) 
                 exfoliated at 
               
               
                   
                   
                   
                   
                 900° C. under N 2   
               
               
                 169 
                 Nestaan 
                 Micron 
                 (China) 
                 Already exfoliated 
                 10.5 
               
               
                   
               
             
          
         
       
     
         [0064]    D) Synthesis Time 
         [0000]    In  FIG. 4 , it is clearly shown that after 25 minutes a yield plateau is reached. 
       Preparation Conditions 
       [0065]      
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
               
                 Support 
                 Metals 
                 Impregnation 
                 calcination 
               
               
                   
               
             
             
               
                 6 g Imerys Vermiculite, 
                 1.03 g Fe(NO 3 ) 3 •9H 2 O 
                 Water volume: 
                 400° C.  
               
               
                 Shawa Mine Zimbabwe, 
                 0.42 g Co(OAc) 2 •4H 2 O 
                 8 ml 
                 1 h 
               
               
                 Micron 
                 Co/Fe molar ratio: 
                 Rest: 16 h 
                     1 m 3 /h N 2   
               
               
                 Sifted (&gt;500 μm) 
                 0.66 
               
               
                 Exfoliated for 2 mins at 
                 Mass % of metal = 4.0% 
               
               
                 900° C. under 2 l/min de 
               
               
                 N 2   
               
               
                   
               
             
          
         
       
     
       Results 
       [0066]      
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
                 Synthesis 
                   
               
               
                 Catalyst 
                 time (min) 
                 Yield (g/g) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 143 
                 10 
                 7.13 
               
               
                   
                 15 
                 9.61 
               
               
                   
                 20 
                 11.69 
               
               
                   
                 25 
                 11.98 
               
               
                   
               
             
          
         
       
     
         [0067]    E) Influence of the Type of Solvents 
         [0000]    5 catalysts were prepared from 5 different solvents. The vermiculite used is the one from Australia. 
       Preparation Conditions 
       [0068]      
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
               
                 Support 
                 Metals 
                 Impregnation 
                 calcination 
               
               
                   
               
             
             
               
                 6 g Imerys Vermiculite, Mud 
                 1.14 g Fe(NO 3 ) 3 •9H 2 O 
                 Solvent volume: 
                 400° C.  
               
               
                 Tank Mine (Australia) 
                 0.35 g Co(OAc) 2 •4H 2 O 
                 5 ml 
                 1 h 
               
               
                 Sifted (&gt;500 μm) 
                 Fe/Co molar ratio = 
                 Rest: 16 h 
                     1 m 3 /h N 2   
               
               
                 Exfoliated for 2 mins at 
                 2/1 
               
               
                 900° C. under 2 l/min of N 2   
                 Mass % of metal = 4.0% 
               
               
                   
               
             
          
         
       
     
       Results 
       [0069]      
         [0000]                                                  Catalyst   Solvent   Yield (g/g)                                173   Water   9.7       174   Acetone   8.37       200   Ethylene glycol   3.37       231   Ethanol   2.94       232   Methanol   3.58                    
The results show that the solvent has an influence on the yield of the catalyst. Although water in this specific case is the best solvent, an organic solvent such as acetone may also give a high-yield catalyst.
 
Furthermore, an additional catalyst was prepared with 1.14 g of Fe(NO 3 ) 3 .9H 2 O and 0.41 g of Co(NO 3 ) 2 .6H 2 O (Co/Fe ratio=0.5) in 5 ml of acetone. The latter gave a yield of 5.83 g/g. This shows that a good solvent-salts combination may sometimes be necessary for having a high-yield catalyst.
 
         [0070]    F) The Water Impregnation Volume 
         [0000]    In order to determine the limit between an impregnation and a suspension method, different volumes of water were tested (see  FIG. 1 ). 
       Preparation Conditions for the Catalytic Composition 
       [0071]      
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
               
                 Support 
                 Metals 
                 Impregnation 
                 calcination 
               
               
                   
               
             
             
               
                 6 g Imerys Vermiculite, 
                 1.14 g of Fe(NO 3 ) 3 •9H 2 O 
                 Water volume: 
                 400° C.  
               
               
                 Shawa Mine Zimbabwe, 
                 0.35 g of Co(OAc) 2 •4H 2 O 
                 variable from 4 to 30 ml 
                 1 h 
               
               
                 Micron, sifted (&gt;500 μm) 
                 Fe/Co molar ratio = 2/1 
                 Rest: 16 h 
                     1 m 3 /h N 2   
               
               
                 Exfoliated for 2 mins at 
                 Mass % of metal = 4.0% 
               
               
                 900° C. under 2 l/min of N 2   
               
               
                   
               
             
          
         
       
     
       Results 
       [0072]      
         [0000]                                                  Catalyst   Water           No.   volume (ml)   Yield (g/g)                                114   10   9.2       134   8   11.3       135   12   10.1       137   5   11.7       138   8   7.9       145   4   11.0       179   15   8.3       180   20   7.6       181   30   7.6                    
The impregnation may be considered as &lt;&lt;dry&gt;&gt; when the water volume is less than 10 ml for 6 g of vermiculite. Beyond this volume, the vermiculite is immersed in the metal solution and during drying, a deposition of metals may occur on the walls of the container. This loss of metals is expressed by a reduction in the yield. The water volume threshold may however depend on the grain size of the vermiculite used.
 
Method by Filtration as Compared with the Method by Infiltration
 
A series of 4 catalysts was also prepared by the method of the articles of Zhang et al. i.e. by suspension-filtration, by using the best metal salts (Fe(NO 3 ) 3 .9H 2 O and Co(OAc) 2 .4H 2 O) and the best Co/Fe ratio (1/2).
 
       Preparation Conditions 
       [0073]    6 g of Nestaan micron vermiculite are placed in an Erlenmeyer. Water and a magnetic stirrer are added therein. The mixture is stirred at 80° C. for 6 hours in order to generate a suspension. An aqueous solution of Fe(NO 3 ).9H 2 O and of Co(OAc) 2 .4H 2 O is added therein. The mixture is filtered by means of a frit in vacuo. The obtained paste is calcined at 400° C. under nitrogen for 1 hour.
 
2 volumes of water as well as 2 amounts of metal salts are used.
 
         [0000]                                                          Yield       Mass of metal salts   H 2 O (ml)   (g/g)                                1.14 g Fe(NO 3 ) 3 •9H 2 O + 0.35 g Co(OAc) 2 •4H 2 O   50   1.0       1.14 g Fe(NO 3 ) 3 •9H 2 O + 0.35 g Co(OAc) 2 •4H 2 O   100   0.16       4.54 g Fe(NO 3 ) 3 •9H 2 O + 1.40 g Co(OAc) 2 •4H 2 O   50   11.9       4.54 g Fe(NO 3 ) 3 •9H 2 O + 1.40 g Co(OAc) 2 •4H 2 O   100   7.2                    
It can be seen that this preparation method may give rise to active catalysts with high yield provided that sufficiently concentrated metal solutions are available, nevertheless it shows a few difficulties as compared with the method by dry impregnation:
       the level of metals deposited on the vermiculite is difficult to control, it depends on the filtration quality;   the preparation time is very long, notably for suspension and filtration;   this method requires a larger amount of metal salts, a good portion is lost in the filtrate.