Method for the selective production of ordered carbon nanotubes in a fluidised bed

Abstract

A method for the selective production of ordered carbon nanotubes includes decomposition of a carbon source in the gaseous state in contact with at least one solid catalyst, taking the form of metallic particles borne by carrier grains. The catalyst grains are adapted so as to be able to form a fluidised bed containing between 1% and 5% by weight of metallic particles having average dimensions of between 1 nm and 10 nm. The decomposition takes place in a fluidised bed of catalyst grains. The method can be used to obtain pure nanotubes with predetermined dimensions in a high yield.

1 . A process for the selective production of ordered carbon nanotubes by decomposition of a source of carbon in the gaseous state in contact with at least one solid catalyst in the form of metallic particles comprising at least one transition metal carried on granules of solid support, so-called catalyst granules, capable of being able to form a fluidised bed, the metallic particles having a mean dimension between 1 nm and 10 nm as measured after activation by heating to 750° C., in which a fluidised bed of the catalyst granules is formed in a reactor, the so-called growth reactor ( 30 ), and the carbon source is added continuously to the growth reactor ( 30 ) in contact with the catalyst granules under conditions capable of ensuring the fluidisation of the bed of catalyst granules, the decomposition reaction and the formation of nanotubes, wherein: 
 the catalyst granules of each catalyst are produced beforehand by deposition of metallic particles on support granules in a fluidised bed of the support granules formed in a reactor, the so-called deposition reactor ( 20 ), fed with at least one precursor capable of forming the metallic particles, and so as to obtain catalyst granules comprising a proportion by weight of the metallic particles of between 1% and 5%,  
 the catalyst granules are then placed in the growth reactor ( 30 ) without contact with the external atmosphere, followed by the formation of the fluidised bed of the catalyst granules and the formation of nanotubes in the growth reactor ( 30 ).  
 
     
     
         2 . A process as claimed in  claim 1 , wherein the catalyst granules are produced having a mean dimension of the metallic particles of between 2 nm and 8 nm, and in which, for at least 97% by number of the metallic particles, the difference between their dimension and the mean dimension of the metallic particles is less than or equal to 5 nm.  
     
     
         3 . A process as claimed in  claim 1 , wherein the catalyst granules are produced with a mean dimension of the particles of the order of 4 nm to 5 nm, and in which, for at least 97% by number of the metallic particles, the difference between their dimension and the mean dimension of the metallic particles is of the order of 3 nm.  
     
     
         4 . A process as claimed in  claim 1 , wherein the catalyst granules are produced with a dimension of the metallic particles of less than 50 nm.  
     
     
         5 . A process as claimed in  claim 1 , wherein the fluidised bed is situated in the growth reactor ( 30 ) at a temperature between 600° C. and 800° C.  
     
     
         6 . A process as claimed in  claim 1 , wherein the metallic particles consist in an amount of at least 98% by weight of at least one transition metal and are substantially free of non-metallic elements apart from traces of carbon and/or oxygen and/or hydrogen and/or nitrogen.  
     
     
         7 . A process as claimed in  claim 1 , wherein the metallic particles consist of a pure metallic deposit of at least one transition metal.  
     
     
         8 . A process as claimed in  claim 1 , wherein the catalyst granules are produced with a mean dimension between 10μ and 1000μ.  
     
     
         9 . A process as claimed in  claim 1 , wherein the difference between the dimension of the catalyst granules and the mean dimension of the produced catalyst granules is less than 50% of the value of the said mean dimension.  
     
     
         10 . A process as claimed in  claim 1 , wherein the support has a specific surface greater than 10 m 2 /g.  
     
     
         11 . A process as claimed in  claim 1 , wherein the support is a porous material having a mean pore size greater than the mean dimension of the metallic particles.  
     
     
         12 . A process as claimed in  claim 1 , wherein the support is chosen from alumina, an activated carbon, silica, a silicate, magnesia, titanium dioxide, zirconia, a zeolite or a mixture of granules of several of these materials.  
     
     
         13 . A process as claimed in  claim 1 , wherein the metallic particles consist of pure iron deposited in the dispersed state on alumina granules.  
     
     
         14 . A process as claimed in  claim 1 , wherein the deposition reactor ( 20 ) and the growth reactor ( 30 ) are different.  
     
     
         15 . A process as claimed in  claim 14 , wherein the deposition reactor ( 20 ) and the growth reactor ( 30 ) are joined by at least one gas-tight line ( 25   a ,  26 ,  25   b ) and wherein the growth reactor ( 30 ) is fed with catalyst granules through this line ( 25 ).  
     
     
         16 . A process as claimed in  claim 1 , wherein the catalyst granules are produced by chemical deposition in the vapour phase of the metallic particles on the support granules in a fluidised bed of the support granules in the deposition reactor ( 20 ).  
     
     
         17 . A process as claimed in  claim 1 , wherein the deposition of the particles on the support granules is carried out at a temperature between 200° C. and 300° C.  
     
     
         18 . A process as claimed in  claim 1 , wherein the fluidised bed of the support granules in the deposition reactor ( 20 ) is fed with at least one organometallic precursor.  
     
     
         19 . A process as claimed in  claim 18 , wherein Fe(CO) 5  is used as organometallic precursor.  
     
     
         20 . A process as claimed in  claim 1 , wherein the precursor(s) is continuously diluted in the vapour phase in a gaseous mixture that is continuously fed to the deposition reactor ( 20 ) under conditions suitable for ensuring the fluidisation of the support granules.  
     
     
         21 . A process as claimed in  claim 20 , wherein the gaseous mixture comprises a neutral gas and at least one reactive gas.  
     
     
         22 . A process as claimed in  claim 21 , wherein steam is used as reactive gas.  
     
     
         23 . A process as claimed in  claim 1 , wherein the fluidised bed of the catalyst granules is formed in a cylindrical growth reactor ( 30 ) of diameter greater than 2 cm and having a wall height capable of containing 10 to 20 times the volume of the initial, non-fluidised bed of the catalyst granules as measured in the absence of any gaseous feed.  
     
     
         24 . A process as claimed in  claim 1 , wherein a fluidised bed of the catalyst granules is formed in the growth reactor ( 30 ) under a bubbling regime that is at least substantially free of leakage.  
     
     
         25 . A process as claimed in  claim 1 , wherein in order to form the fluidised bed of catalyst granules in the growth reactor ( 30 ): 
 a bed of catalyst granules is formed in the bottom of the growth reactor ( 30 ),    the growth reactor ( 30 ) is fed from underneath the bed of catalyst granules with at least one gas whose velocity is greater than the minimum velocity of fluidisation of the bed of catalyst granules and less than the minimum velocity for the occurrence of a plunger-type régime.    
     
     
         26 . A process as claimed in  claim 1 , wherein in order to form the fluidised bed of the catalyst granules in the growth reactor ( 30 ), the growth reactor ( 30 ) is fed from underneath the catalyst granules with the carbon source in the gaseous state and with at least one neutral carrier gas.  
     
     
         27 . A process as claimed in  claim 1 , wherein the growth reactor is fed with at least one carbon-containing precursor forming the carbon source, with at least one reactive gas and with at least one neutral gas, which are mixed before being introduced into the growth reactor ( 30 ).  
     
     
         28 . A process as claimed in  claim 1 , wherein the carbon source comprises at least one carbon-containing precursor chosen from hydrocarbons.  
     
     
         29 . A process as claimed in  claim 1 , wherein the growth reactor ( 30 ) is fed with hydrogen as reactive gas.  
     
     
         30 . A process as claimed in  claim 27 , wherein the molar ratio of the reactive gas(es) to the carbon-containing precursor(s) is greater than 0.5 and less than 10, and in particular is of the order of 3.  
     
     
         31 . A process as claimed in  claim 27 , wherein the growth reactor ( 30 ) is fed at a flow rate of carbon-containing precursor(s) of between 5% and 80%, in particular of the order of 25%, of the overall gaseous flow rate.  
     
     
         32 . A process for the preparation of a catalytic granular composition comprising metallic particles containing at least one transition metal carried on solid support granules, so-called catalyst granules, in which there is effected a chemical deposition in the vapour phase of the metallic particles on the support granules, wherein the deposition of the metallic particles on the support granules is carried out in a fluidised bed of the support granules fed with at least one precursor capable of forming the metallic particles, and wherein the support granules are chosen and the parameters of the deposition are adjusted so that: 
 the catalyst granules are capable of being able to form a fluidised bed,    the proportion by weight of the metallic particles is between 1% and 5%,    the metallic particles have a mean particle dimension between 1 nm and 10 nm as measured after activation by heating to 750° C.    
     
     
         33 . A process as claimed in  claim 32 , wherein the deposition is carried out in the form of a chemical deposition in the vapour phase.  
     
     
         34 . A process as claimed in  claim 32 , wherein the deposition is carried out at a temperature between 200° C. and 300° C.  
     
     
         35 . A process as claimed in  claim 32 , wherein the fluidised bed of the support granules is fed with at least one organometallic precursor.  
     
     
         36 . A process as claimed in  claim 32 , wherein Fe(CO) 5  is used as organometallic precursor.  
     
     
         37 . A process as claimed in  claim 32 , wherein the precursor(s) is continuously diluted in the vapour state in a gaseous mixture that is continuously fed to a deposition reactor ( 20 ) under conditions capable of ensuring the fluidisation of the support granules.  
     
     
         38 . A process as claimed in  claim 37 , wherein the gaseous mixture comprises a neutral gas and at least one reactive gas.  
     
     
         39 . A process as claimed in  claim 38 , wherein steam is used as reactive gas.