Patent Publication Number: US-11643332-B2

Title: Method for preparing a high-purity AFX structural zeolite with a nitrogen-containing organic structuring agent

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national phase application filed under 35 U.S.C. § 371 of International Application No. PCT/EP2019/062556, filed May 16, 2019, designating the United States, which claims priority from French Patent Application No. 18/54.383, filed May 24, 2018, which are hereby incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a novel process for preparing an AFX-structure zeolite. This novel process can be used to synthesize an AFX-structure zeolite by converting/transforming an FAU-structure zeolite under hydrothermal conditions. In particular, said novel process can be used to synthesize an AFX-structure zeolite starting from an FAU-structure zeolite used as a source of silicon and aluminum and a specific organic molecule or structuring agent comprising two quaternary ammonium functions, chosen from the dihydroxide form of 1,5-bis(methylpiperidinium)pentane, 1,6-bis(methylpiperidinium)hexane or 1,7-bis(methylpiperidinium)heptane. Said AFX-structure zeolite obtained according to the process of the invention advantageously finds its application as a catalyst, adsorbent or separating agent. 
     PRIOR ART 
     Crystalline microporous materials, such as zeolites or silicoaluminophosphates, are solids that are extensively used in the petroleum industry as catalysts, catalytic supports, adsorbents or separating agents. Although many microporous crystalline structures have been discovered, the refining and petrochemical industry is constantly in search of novel zeolitic structures which have particular properties for applications such as the purification or separation of gases, the conversion of carbon-based species or the like. 
     The AFX-structure group includes in particular the zeolite SSZ-16 and the related solids known as zeotypes SAPO-56 and MEAPSO-56. AFX-structure zeolites have a three-dimensional system of pores delimited by eight tetrahedrons and formed by two types of cages: gmelinite (GME cage) and a large AFT cage (˜8.3×13.0 Å). Numerous methods for synthesizing AFX-structure zeolites, and in particular the zeolite SSZ-16, are known. The zeolite SSZ-16 has been synthesized using organic nitrogenous species derived from 1,4-di(1-azoniabicyclo[2.2.2]octane) lower alkane compounds (U.S. Pat. No. 4,508,837). Chevron Research and Technology Company prepared the zeolite SSZ-16 in the presence of DABCO-Cn-diquat cations, where DABCO represents 1,4-diazabicyclo[2.2.2]octane and n is 3, 4 or 5 (U.S. Pat. No. 5,194,235). S. B. Hong et al. used the diquaternary alkylammonium ion Et6-diquat-n, where Et6-diquat represents N′,N′-bis-triethylpentanediammonium and n is 5, as structuring agents for synthesizing the zeolite SSZ-16 (Micropor. Mesopor. Mat., 60 (2003) 237-249). The use of 1,3-bis(adamantyl)imidazolium cations as a structuring agent for preparing AFX-structure zeolites can also be cited (R. H. Archer et al. in Micropor. Mesopor. Mat., 130 (2010) 255-2265; Johnson Matthey Company WO2016077667A1). Inagaki Satoshi et al. (JP2016169139A) used divalent N,N,N′,N′-tetraarquirubicyclo[2.2.2]oct-7-ene-2,3:05,6-dipyrrolidium cations substituted with alkyl groups to prepare the zeolite SSZ-16. Chevron U.S.A. (WO2017/200607 A1) proposes the synthesis of an SSZ-16 zeolite using the dications 1,1′-(1,4-cyclohexylenedimethylene)bis[1-methylpiperidinium], 1,1′-(1,4-cyclohexylenedimethylene)bis[1-methylpyrrolidinium], 1,1′-(1,4-cyclohexylenedimethylene)bis[1-ethylpyrrolidinium]. H.-Y. Chen et al. (Johnson Matthey Company, US2018/0093897) used a mixture of cations containing at least 1,3-bis(adamantyl)imidazolium and a neutral amine to prepare the AFX-structure zeolite JMZ-10 in the absence of alkali cations. H.-Y. Chen et al. (Johnson Matthey Company, US2018/0093259) used a mixture of cations containing an organic molecule chosen from 1,3-bis(adamantyl)imidazolium, N,N-dimethyl-3,5-dimethylpiperidinium, N, N-diethyl-cis 2,6-dimethylpiperidinium, N, N, N-1-trimethyladamantylammonium, N,N,N-dimethylethylcyclohexylammonium and at least one alkaline-earth metal cation to obtain the AFX-structure zeolite JMZ-7, which has close Al sites as compared with the zeolite obtained by a synthesis using alkali cations. 
     DESCRIPTION OF THE INVENTION 
     Summary of the Invention 
     The invention relates to a process for preparing an AFX-structure zeolite comprising at least the following steps:
         i) mixing, in an aqueous medium, an FAU-structure zeolite having an SiO 2 (FAU) /Al 2 O 3 (FAU)  molar ratio of between 2.00 (limit included) and 6.00 (limit excluded), an organic nitrogenous compound R, R being chosen from   1,5-bis(methylpiperidinium)pentane dihydroxide,   1,6-bis(methylpiperidinium)hexane dihydroxide or   1,7-bis(methylpiperidinium)heptane dihydroxide, at least one source of at least one alkali and/or alkaline-earth metal M of valency n, n being an integer greater than or equal to 1, M being chosen from lithium, potassium, sodium, magnesium and calcium and a mixture of at least two of these metals,   the reaction mixture having the following molar composition:   (SiO 2 (FAU) )/(Al 2 O 3 (FAU) ) between 2.00 (limit included) and 6.00 (limit excluded), preferably between 3.00 (limit included) and 6.00 (limit excluded)   H 2 O/(SiO 2 (FAU) ) between 1 and 100, preferably between 5 and 60   R/(SiO 2 (FAU) ) between 0.01 and 0.6, preferably between 0.05 and 0.5   M 2/n O/(SiO 2 (FAU) ) between 0.005 and 0.7, preferably between 0.05 and 0.6, limits included,   in which SiO 2 (FAU)  denotes the amount of SiO 2  provided by the FAU zeolite and Al 2 O 3 (FAU)  denotes the amount of Al 2 O 3  provided by the FAU zeolite, until a homogeneous precursor gel is obtained;   ii) hydrothermal treatment of said precursor gel obtained on conclusion of step i) at a temperature of between 120° C. and 220° C., for a time of between 12 hours and 15 days.       

     R is preferably 1,6-bis(methylpiperidinium)hexane dihydroxide. 
     The SiO 2 /Al 2 O 3  ratio of the zeolite obtained is advantageously between 4.00 and 100, preferably between 6.00 and 80, limits included. 
     Preferably, M is sodium. 
     The source of at least one alkali and/or alkaline-earth metal M is preferably sodium hydroxide. 
     The reaction mixture of step i) may include at least one additional source of an XO 2  oxide, X being one or more tetravalent element(s) chosen from the group formed by the following elements: silicon, germanium, titanium, such that the XO 2 /SiO 2  (FAU) molar ratio is between 0.1 and 33, and preferably between 0.1 and 15, limits included, the SiO 2 (FAU)  content in said ratio being the content provided by the FAU-structure zeolite. 
     The reaction mixture of step i) advantageously has the following molar composition:
         (XO 2 +SiO 2 (FAU) )/Al 2 O 3 (FAU)  between 2 and 200, preferably between 4 and 95   H 2 O/(XO 2 +SiO 2 (FAU) ) between 1 and 100, preferably between 5 and 60   R/(XO 2 +SiO 2 (FAU) ) between 0.01 and 0.6, preferably between 0.05 and 0.5   M 2/n O/(XO 2 +SiO 2 (FAU) ) between 0.005 and 0.7, preferably between 0.05 and 0.6, limits included.       

     Preferably, X is silicon. 
     The reaction mixture of step i) may include at least one additional source of a Y 2 O 3  oxide, Y being one or more trivalent element(s) chosen from the group formed by the following elements: aluminum, boron, gallium, such that the Y 2 O 3 /Al 2 O 3 (FAU)  molar ratio is between 0.001 and 2, and preferably between 0.001 and 1.8, limits included, the Al 2 O 3 (FAU)  content in said ratio being the content provided by the FAU-structure zeolite. 
     The reaction mixture of step i) advantageously has the following molar composition:
         SiO 2 (FAU) /(Al 2 O 3 (FAU) +Y 2 O 3 ) between 2.00 (limit included) and 6.00 (limit excluded), preferably between 3.00 (limit included) and 6.00 (limit excluded)   H 2 O/(SiO 2 (FAU) ) between 1 and 100, preferably between 5 and 60   R/(SiO 2 (FAU) ) between 0.01 and 0.6, preferably between 0.05 and 0.5   M 2/n O/(SiO 2 (FAU) ) between 0.005 and 0.7, preferably between 0.05 and 0.6, limits included,   SiO 2 (FAU)  being the amount of SiO 2  provided by the FAU zeolite and Al 2 O 3 (FAU)  being the amount of Al 2 O 3  provided by the FAU zeolite.       

     Preferably, Y is aluminum. 
     In one embodiment, the reaction mixture of step i) may contain:
         at least one additional source of an XO 2  oxide   and at least one additional source of a Y 2 O 3  oxide,       

     the FAU zeolite representing between 5 and 95% by mass, preferably between 50 and 95% by mass, very preferably between 60 and 90% by mass and even more preferably between 65 and 85% by mass of an FAU-structure zeolite, relative to the total amount of the sources of the trivalent and tetravalent elements SiO 2 (FAU) , XO 2 , Al 2 O 3 (FAU)  and Y 2 O 3  in the reaction mixture, and the reaction mixture having the following molar composition:
         (XO 2 +SiO 2  (FAU))/(Al 2 O 3 (FAU) +Y 2 O 3 ) between 6.00 and 200, preferably between 6 and 95   H 2 O/(XO 2 +SiO 2 (FAU) ) between 1 and 100, preferably between 5 and 60   R/(XO 2 +SiO 2 (FAU) ) between 0.01 and 0.6, preferably between 0.05 and 0.5   M 2/n O/(XO 2 +SiO 2 (FAU) ) between 0.005 and 0.7, preferably between 0.05 and 0.6, limits included.       

     Advantageously, the precursor gel obtained on conclusion of step i) has a molar ratio of the total amount expressed as oxides of tetravalent elements to the total amount expressed as oxides of trivalent elements of between 2 and 80, limits included. 
     Preferably, the FAU-structure zeolite has an SiO 2 (FAU) /Al 2 O 3 (FAU)  molar ratio of between 3.00 (limit included) and 6.00 (limit excluded), very preferably an SiO 2 (FAU) /Al 2 O 3 (FAU)  molar ratio of between 4.00 (limit included) and 6.00 (limit excluded). 
     Seed crystals of an AFX-structure zeolite may be added to the reaction mixture of step i), preferably in an amount of between 0.01 and 10% of the total mass of the sources of said tetravalent and trivalent element(s) in anhydrous form present in the reaction mixture, said seed crystals not being taken into account in the total mass of the sources of the tetravalent and trivalent elements. 
     Step i) may include a step of maturing the reaction mixture at a temperature of between 20 and 100° C., with or without stirring, for a time of between 30 minutes and 48 hours. 
     The hydrothermal treatment of step ii) may be performed under autogenous pressure at a temperature of between 120° C. and 220° C., preferably between 150° C. and 195° C., for a time of between 12 hours and 15 days, preferably between 12 hours and 12 days, even more preferably between 12 hours and 8 days. 
     The solid phase obtained on conclusion of step ii) may be filtered off, washed, and dried at a temperature of between 20 and 150° C., preferably between 60 and 100° C., for a time of between 5 and 24 hours, to obtain a dried zeolite. 
     The dried zeolite may then be calcined at a temperature of between 450 and 700° C. for a time of between 2 and 20 hours, the calcination possibly being preceded by a gradual temperature increase. 
     The invention also relates to an AFX-structure zeolite having an SiO 2 /Al 2 O 3  ratio of between 4.00 and 100, limits included, which is capable of being obtained by the preparation process described above. 
     The invention also relates to an AFX-structure zeolite having an SiO 2 /Al 2 O 3  ratio of between 4.00 and 100, limits included, which is capable of being obtained by the preparation process described above and is calcined, and for which the mean d hkl  values and relative intensities measured on an X-ray diffraction diagram are as follows: 
     
       
         
           
               
               
               
             
               
                   
               
               
                 2 thêta 
                   
                   
               
               
                 (°) 
                 dhkl (Å) 
                 Irel 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 7.49 
                 11.79 
                 mw 
               
               
                 8.73 
                 10.12 
                 VS 
               
               
                 11.72 
                 7.55 
                 VS 
               
               
                 12.98 
                 6.82 
                 S 
               
               
                 14.98 
                 5.91 
                 vw 
               
               
                 15.66 
                 5.66 
                 w 
               
               
                 17.51 
                 5.06 
                 mw 
               
               
                 18.05 
                 4.91 
                 m 
               
               
                 19.62 
                 4.52 
                 vw 
               
               
                 19.88 
                 4.46 
                 w 
               
               
                 20.38 
                 4.35 
                 S 
               
               
                 21.85 
                 4.06 
                 VS 
               
               
                 22.48 
                 3.95 
                 w 
               
               
                 23.84 
                 3.73 
                 w 
               
               
                 26.11 
                 3.41 
                 mw 
               
               
                 27.08 
                 3.29 
                 vw 
               
               
                 27.17 
                 3.28 
                 vw 
               
               
                 27.57 
                 3.23 
                 vw 
               
               
                 28.24 
                 3.16 
                 mw 
               
               
                 28.68 
                 3.11 
                 vw 
               
               
                 30.29 
                 2.95 
                 w 
               
               
                 30.57 
                 2.92 
                 mw 
               
               
                 31.19 
                 2.87 
                 vw 
               
               
                 31.59 
                 2.83 
                 mw 
               
               
                 31.84 
                 2.81 
                 vw 
               
               
                 32.74 
                 2.73 
                 vw 
               
               
                 33.90 
                 2.64 
                 w 
               
               
                 34.28 
                 2.61 
                 vw 
               
               
                 34.75 
                 2.58 
                 w 
               
               
                 37.79 
                 2.38 
                 vw 
               
               
                 39.15 
                 2.30 
                 vw 
               
               
                 39.57 
                 2.28 
                 vw 
               
               
                   
               
            
           
         
       
     
    
    
     
       LIST OF FIGURES 
         FIG.  1    represents the chemical formulae of the organic nitrogenous compounds which may be chosen as the structuring agent used in the synthesis process according to the invention. 
         FIG.  2    represents the X-ray diffraction pattern of the AFX zeolite obtained according to Example 7. 
         FIG.  3    represents a scanning electron microscope (SEM) image of the AFX zeolite obtained according to Example 7. 
         FIG.  4    represents the X-ray diffraction pattern of the AFX zeolite obtained according to Example 8 with the analcime zeolite (structural code ANA) as impurity. 
     
    
    
     Other characteristics and advantages of the synthesis process according to the invention will become apparent on reading the following description of non-limiting exemplary embodiments with reference to the appended figures described below. 
     DETAILED DESCRIPTION OF THE INVENTION 
     One subject of the present invention is a novel process for preparing an AFX-structure zeolite, by converting/transforming an FAU-structure zeolite of specific SiO 2 /Al 2 O 3  ratio under hydrothermal conditions in the presence of an organic nitrogenous compound or specific structuring agent chosen from the dihydroxide form of the following compounds: 1,5-bis(methylpiperidinium)pentane, 1,6-bis(methylpiperidinium)hexane or 1,7-bis(methylpiperidinium)heptane. 
     In particular, the applicant has discovered that the organic nitrogenous compound or structuring agent chosen from the dihydroxide form of 1,5-bis(methylpiperidinium)pentane, 1,6-bis (methylpiperidinium)hexane or 1,7-bis(methylpiperidinium)heptane, mixed with an FAU-structure zeolite having an SiO 2 (FAU) /Al 2 O 3 (FAU)  molar ratio of between 2.00 (limit included) and 6.00 (limit excluded), used as a source of silicon and aluminum, in the presence or absence of an additional supply, within said mixture, of at least one source of at least one tetravalent XO 2  element, and/or of at least one source of at least one trivalent Y 2 O 3  element, leads to the production of a precursor gel of an AFX-structure zeolite having a molar ratio of the total amount expressed as oxides of tetravalent elements to the total amount expressed as oxides of trivalent elements of between 2 and 80, then to the production of an AFX-structure zeolite of high purity, the total amount of oxides of tetravalent element representing the sum of the SiO 2  content originating from the FAU zeolite and the XO 2  content originating from the possible additional source of an XO 2  oxide in the case where at least one additional source of an XO 2  oxide is added, the total amount of oxides of trivalent element representing the sum of the Al 2 O 3  content originating from the FAU zeolite and the Y 2 O 3  content originating from the possible additional source of a Y 2 O 3  oxide in the case where at least one additional source of a Y 2 O 3  oxide is added. Any other crystalline or amorphous phase is generally and very preferentially absent from the crystalline solid consisting of the AFX-structure zeolite obtained on conclusion of the preparation process. 
     More precisely, one subject of the present invention is a novel process for preparing an AFX-structure zeolite comprising at least the following steps:
         i) mixing, in an aqueous medium, an FAU-structure zeolite having an SiO 2 (FAU) /Al 2 O 3 (FAU)  molar ratio of greater than or equal to 2.00 and strictly less than 6.00, an organic nitrogenous compound R, also referred to as a specific structuring agent, chosen from 1,5-bis(methylpiperidinium)pentane dihydroxide,   1,6-bis(methylpiperidinium)hexane dihydroxide, or   1,7-bis(methylpiperidinium)heptane dihydroxide, at least one alkali metal and/or alkaline-earth metal M of valency n, n being an integer greater than or equal to 1, the mixture having the following molar composition:   (SiO 2 (FAU) )/(Al 2 O 3 (FAU) ) between 2.00, limit included and 6.00, limit excluded, preferably strictly between 3.00, limit included, and 6.00, limit excluded   H 2 O/(SiO 2 (FAU) ) between 1 and 100, preferably between 5 and 60   R/(SiO 2 (FAU) ) between 0.01 and 0.6, preferably between 0.05 and 0.5   M 2/n O/(SiO 2 (FAU) ) between 0.005 and 0.7, preferably between 0.05 and 0.6       

     in which SiO 2 (FAU)  is the amount of SiO 2  provided by the FAU zeolite and Al 2 O 3 (FAU)  is the amount of Al 2 O 3  provided by the FAU zeolite, H 2 O is the molar amount of water present in the reaction mixture, R is the molar amount of said organic nitrogenous compound, M 2/n O is the molar amount expressed in the form of M 2/n O oxide of the source of alkali metal and/or of alkaline-earth metal and M is one or more alkali and/or alkaline-earth metal(s) chosen from lithium, sodium, potassium, calcium, magnesium and a mixture of at least two of these metals, very preferably M is sodium, step i) being performed for a time enabling a homogeneous mixture known as a precursor gel to be obtained;
         ii) hydrothermal treatment of said precursor gel obtained on conclusion of step i), at a temperature of between 120° C. and 220° C. for a time of between 12 hours and 15 days, until said AFX-structure zeolite forms.       

     One advantage of the present invention is thus that it provides a novel preparation process for forming an AFX-structure zeolite of high purity starting from an FAU-structure zeolite, said process being performed in the presence of a specific organic structuring agent chosen from 1,5-bis(methylpiperidinium)pentane dihydroxide, 1,6-bis(methylpiperidinium)hexane dihydroxide or 1,7-bis(methylpiperidinium)heptane dihydroxide. 
     The starting FAU-structure zeolite having an SiO 2 /Al 2 O 3  molar ratio of between 2.00 (limit included) and 6.00 (limit excluded) may be obtained by any method known to a person skilled in the art and may be used in its sodium form or any other form, after partial or total exchange of the sodium cations with ammonium cations, possibly followed by a calcination step. As FAU sources having an SiO 2 /Al 2 O 3  ratio of between 2.00 (limit included) and 6.00 (limit excluded), mention may be made of the commercial zeolites CBV100, CBV300, CBV400, CBV500 and CBV600 produced by Zeolyst and the commercial zeolites HSZ-320NAA, HSZ-320HOA and HSZ-320HUA produced by TOSOH. 
     The preparation process according to the invention thus allows the SiO 2 /Al 2 O 3  ratio of the precursor gel containing an FAU-structure zeolite to be adjusted as a function of the additional supply or not, within the reaction mixture, of at least one source of at least one tetravalent XO 2  element and/or of at least one source of at least one trivalent Y 2 O 3  element. 
     Another advantage of the present invention is to allow the preparation of a precursor gel of an AFX structure zeolite exhibiting an SiO 2 /Al 2 O 3  molar ratio identical to, greater than or less than the SiO 2 (FAU) /Al 2 O 3 (FAU)  molar ratio of the starting FAU structure zeolite. 
     In the molar composition of the reaction mixture of step i) and throughout the description: 
     XO 2  denotes the molar amount of the additional tetravalent element(s), expressed in oxide form, and Y 2 O 3  denotes the molar amount of the additional trivalent element(s), expressed in oxide form, 
     SiO 2 (FAU)  denotes the amount of SiO 2  provided by the FAU zeolite, and Al 2 O 3 (FAU)  denotes the amount of Al 2 O 3  provided by the FAU zeolite, 
     H 2 O the molar amount of water present in the reaction mixture, 
     R the molar amount of said organic nitrogenous compound, 
     M 2/n O the molar amount, expressed in oxide form, of M 2/n O provided by the source of alkali metal and/or of alkaline-earth metal.
         Step i) comprises mixing, in an aqueous medium, an FAU-structure zeolite having an SiO 2 (FAU) /Al 2 O 3 (FAU)  molar ratio of between 2.00 (limit included) and 6.00 (limit excluded) an organic nitrogenous compound R, R being   1,5-bis(methylpiperidinium)pentane dihydroxide,   1,6-bis(methylpiperidinium)hexane dihydroxide, or   1,7-bis(methylpiperidinium)heptane dihydroxide, at least one alkali metal and/or alkaline-earth metal M of valency n, n being an integer greater than or equal to 1, the reaction mixture having the following molar composition:   (SiO 2 (FAU) )/(Al 2 O 3 (FAU) ) between 2.00 (limit included) and 6.00 (limit excluded), preferably between 3.00 (limit included) and 6.00 (limit excluded), H 2 O/(SiO 2 (FAU) ) between 1 and 100, preferably between 5 and 60   R/(SiO 2 (FAU) ) between 0.01 and 0.6, preferably between 0.05 and 0.5   M 2/n O/(SiO 2 (FAU) ) between 0.005 and 0.7, preferably between 0.05 and 0.6 in which SiO 2 (FAU)  is the amount of SiO 2  provided by the FAU zeolite and Al 2 O 3 (FAU)  is the amount of Al 2 O 3  provided by the FAU zeolite, H 2 O the molar amount of water present in the reaction mixture, R the molar amount of said organic nitrogenous compound, M 2/n O the molar amount, expressed in oxide form, of M 2/n O provided by the source of alkali metal and/or of alkaline-earth metal and M is one or more alkali and/or alkaline-earth metal(s) chosen from lithium, sodium, potassium, calcium, magnesium and a mixture of at least two of these metals, M very preferably being sodium.       

     In a preferred embodiment, the reaction mixture of step i) also includes at least one additional source of an XO 2  oxide, such that the XO 2 /SiO 2 (FAU)  molar ratio is between 0.1 and 33, the mixture advantageously having the following molar composition:
         (XO 2 +SiO 2 (FAU) )/Al 2 O 3 (FAU)  between 2 and 200, preferably between 4 and 95   H 2 O/(XO 2 +SiO 2 (FAU) ) between 1 and 100, preferably between 5 and 60   R/(XO 2 +SiO 2 (FAU) ) between 0.01 and 0.6, preferably between 0.05 and 0.5   M 2/n O/(XO 2 +SiO 2 (FAU) ) between 0.005 and 0.7, preferably between 0.05 and 0.6       

     in which X is one or more tetravalent element(s) chosen from the group formed by the following elements: silicon, germanium, titanium, X preferably being silicon, SiO 2 (FAU)  being the amount of SiO 2  provided by the FAU zeolite and Al 2 O 3 (FAU)  being the amount of Al 2 O 3  provided by the FAU zeolite, R being 1,5-bis(methylpiperidinium)pentane dihydroxide, 1,6-bis(methylpiperidinium)hexane dihydroxide or 1,7-bis(methylpiperidinium)heptane dihydroxide, and M is one or more alkali and/or alkaline-earth metal(s) chosen from lithium, sodium, potassium, calcium, magnesium and a mixture of at least two of these metals, M very preferably being sodium. 
     In another preferred embodiment, the reaction mixture of step i) also includes at least one additional source of a Y 2 O 3  oxide, such that the Y 2 O 3 /Al 2 O 3 (FAU)  molar ratio is between 0.001 and 2, the mixture advantageously having the following molar composition:
         SiO 2 (FAU) /(Al 2 O 3 (FAU) +Y 2 O 3 ) between 2.00 (limit included) and 6.00 (limit excluded), preferably between 3.00 (limit included) and 6.00 (limit excluded),   H 2 O/(SiO 2 (FAU) ) between 1 and 100, preferably between 5 and 60   R/(SiO 2 (FAU) ) between 0.01 and 0.6, preferably between 0.05 and 0.5   M 2/n O/(SiO 2 (FAU) ) between 0.005 and 0.7, preferably between 0.05 and 0.6       

     in which Y is one or more trivalent element(s) chosen from the group formed by the following elements: aluminum, boron, gallium, Y preferably being aluminum, SiO 2  (FAU) being the amount of SiO 2  provided by the FAU zeolite and Al 2 O 3 (FAU)  being the amount of Al 2 O 3  provided by the FAU zeolite, R being 1,5-bis(methylpiperidinium)pentane dihydroxide, 1,6-bis(methylpiperidinium)hexane dihydroxide or 1,7-bis(methylpiperidinium)heptane dihydroxide, and M is one or more alkali and/or alkaline-earth metals chosen from lithium, sodium, potassium, calcium, magnesium and a mixture of at least two of these metals, M very preferably being sodium. 
     In another preferred embodiment, the reaction mixture of step i) contains a percentage of between 5 and 95% by mass, preferably between 50 and 95% by mass, very preferably between 60 and 90% by mass and even more preferably between 65 and 85% by mass of an FAU-structure zeolite relative to the total amount of sources of trivalent and tetravalent elements SiO 2 (FAU) , XO 2 , Al 2 O 3 (FAU)  and Y 2 O 3  of the reaction mixture and also includes at least one additional source of an XO 2  oxide and at least one additional source of a Y 2 O 3  oxide, the reaction mixture having the following molar composition:
         (XO 2 +SiO 2  (FAU))/(Al 2 O 3 (FAU) +Y 2 O 3 ) between 2.00 and 200, preferably between 4.00 and 95   H 2 O/(XO 2 +SiO 2  (FAO between 1 and 100, preferably between 5 and 60   R/(XO 2 +SiO 2 (FAU) ) between 0.01 and 0.6, preferably between 0.05 and 0.5   M 2/n O/(XO 2 +SiO 2 (FAU) ) between 0.005 and 0.7, preferably between 0.05 and 0.6       

     in which X is one or more tetravalent element(s) chosen from the group formed by the following elements: silicon, germanium, titanium, X preferably being silicon, Y is one or more trivalent element(s) chosen from the group formed by the following elements: aluminum, boron, gallium, preferably aluminum, SiO 2  (FAU) being the amount of SiO 2  provided by the FAU zeolite and Al 2 O 3 (FAU)  being the amount of Al 2 O 3  provided by the FAU zeolite, R being 1,5-bis(methylpiperidinium)pentane dihydroxide, 1,6-bis(methylpiperidinium)hexane dihydroxide or 1,7-bis(methylpiperidinium)heptane dihydroxide, and M is one or more alkali and/or alkaline-earth metals chosen from lithium, sodium, potassium, calcium, magnesium and a mixture of at least two of these metals, M very preferably being sodium. 
     Step i) enables a homogeneous precursor gel to be obtained. 
     Step ii) comprises a hydrothermal treatment of said precursor gel obtained on conclusion of step i), which is carried out at a temperature of between 120° C. and 220° C. for a time of between 12 hours and 15 days, until said AFX-structure zeolite crystallizes. 
     In accordance with the invention, an FAU-structure zeolite having an SiO 2  (FAU)/Al 2 O 3 (FAU)  molar ratio of between 2.00 (lower limit included) and 6.00 (upper limit excluded), preferably between 3.00 (lower limit included) and 6.00 (upper limit excluded), and very preferably between 4.00 (lower limit included) and 6.00 (upper limit excluded), is incorporated into the reaction mixture for the performance of step (i) as a source of silicon and aluminum element. 
     In accordance with the invention, R is an organic nitrogenous compound chosen from 1,5-bis(methylpiperidinium)pentane dihydroxide, 1,6-bis(methylpiperidinium)hexane dihydroxide or 1,7-bis(methylpiperidinium)heptane dihydroxide, said compound being incorporated into the reaction mixture for the performance of step (i) as an organic structuring agent. The anion associated with the quaternary ammonium cations present in the organic structuring species for the synthesis of an AFX-structure zeolite according to the invention is the hydroxide anion. 
     In accordance with the invention, at least one source of at least one alkali and/or alkaline-earth metal M of valency n is used in the reaction mixture of step i), n being an integer greater than or equal to 1, M preferably being chosen from lithium, potassium, sodium, magnesium and calcium and a mixture of at least two of these metals. Very preferably, M is sodium. 
     The source of at least one alkali and/or alkaline-earth metal M is preferably sodium hydroxide. 
     In accordance with the invention, at least one additional source of an XO 2  oxide, X being one or more tetravalent element(s) chosen from the group formed by the following elements: silicon, germanium, titanium, and X preferably being silicon, such is that the XO 2 /SiO 2 (FAU)  molar ratio is between 0.1 and 33, and preferably between 0.1 and 15, the SiO 2 (FAU)  content in said ratio being the content provided by the FAU-structure zeolite, is advantageously used in the reaction mixture of step i). 
     In particular, adding at least one additional source of an XO 2  oxide enables the XO 2 /Y 2 O 3  ratio of the precursor gel of an AFX-structure zeolite obtained on conclusion of step i) to be adjusted. 
     The source(s) of said tetravalent element(s) may be any compound comprising the element X and which can release this element in aqueous solution in reactive form. When X is titanium, Ti(EtO) 4  is advantageously used as the source of titanium. 
     In the preferred case in which X is silicon, the source of silicon may be any one of said sources commonly used for zeolite synthesis, for example powdered silica, silicic acid, colloidal silica, dissolved silica or tetraethoxysilane (TEOS). Among the powdered silicas, use may be made of precipitated silicas, especially those obtained by precipitation from a solution of alkali metal silicate, fumed silicas, for example Cab-O-Sil or Aerosil, and silica gels. Colloidal silicas having various particle sizes, for example a mean equivalent diameter of between 10 and 15 nm or between 40 and 50 nm, may be used, such as those sold under registered brand names such as Ludox. Preferably, the source of silicon is Aerosil. 
     In accordance with the invention, at least one additional source of a Y 2 O 3  oxide, Y being one or more trivalent element(s) chosen from the group formed by the following elements: aluminum, boron, gallium, is advantageously used in the mixture of step i). Preferably, Y is aluminum, such that the Y 2 O 3 /Al 2 O 3 (FAU)  molar ratio is between 0.001 and 2, and preferably between 0.001 and 1.8, the Al 2 O 3 (FAU)  content in said ratio being the content provided by the FAU-structure zeolite. 
     Adding at least one additional source of a Y 2 O 3  oxide thus enables the XO 2 /Y 2 O 3  ratio of the precursor gel of an AFX-structure zeolite obtained on conclusion of step i) to be adjusted. 
     The source(s) of said trivalent element(s) Y may be any compound comprising the element Y and which can release this element in aqueous solution in reactive form. Element Y may be incorporated into the mixture in an oxidized form YO b  with 1≤b≤3 (b being an integer or a rational number) or in any other form. In the preferred case where Y is aluminum, the source of aluminum is preferably aluminum hydroxide or an aluminum salt, for example chloride, nitrate or sulfate, a sodium aluminate, an aluminum alkoxide, or alumina itself, preferably in hydrated or hydratable form, for instance colloidal alumina, pseudoboehmite, gamma-alumina or alpha or beta trihydrate. Use may also be made of mixtures of the sources mentioned above. 
     Step (i) of the process according to the invention consists in preparing an aqueous reaction mixture containing an FAU-structure zeolite, optionally a source of an XO 2  oxide or a source of a Y 2 O 3  oxide, at least one organic nitrogenous compound R, R being chosen from 1,5-bis(methylpiperidinium)pentane dihydroxide, 1,6-bis(methylpiperidinium)hexane dihydroxide or 1,7-bis(methylpiperidinium)heptane dihydroxide, in the presence of at least one source of one or more alkali and/or alkaline-earth metal(s), to obtain a precursor gel of an AFX-structure zeolite. The amounts of said reagents are adjusted in the manner described above so as to give this gel a composition allowing an AFX-structure zeolite to be crystallized. 
     It may be advantageous to add seeds of an AFX-structure zeolite to the reaction mixture during said step i) of the process of the invention so as to reduce the time required for the formation of the crystals of an AFX-structure zeolite and/or the total crystallization time. Said seed crystals also promote the formation of said AFX-structure zeolite to the detriment of impurities. Such seeds comprise crystalline solids, in particular crystals of an AFX-structure zeolite. The seed crystals are generally added in a proportion of between 0.01% and 10% of the total mass of the sources of said tetravalent and trivalent element(s) in anhydrous form present in the reaction mixture, said seed crystals not being taken into account in the total mass of the sources of the tetravalent and trivalent elements. Said seeds are not taken into account either for determining the composition of the reaction mixture and/or of the gel, defined above, i.e. in the determination of the various molar ratios of the composition of the reaction mixture. 
     The mixing step i) is performed until a homogeneous mixture is obtained, preferably for a time of greater than or equal to 30 minutes, preferably with stirring by any system known to those skilled in the art, at a low or high shear rate. 
     On conclusion of step i), a homogeneous precursor gel is obtained. 
     It may be advantageous to perform a maturation of the reaction mixture during said step i) of the process according to the invention, before the hydrothermal crystallization, so as to control the size of the crystals of an AFX-structure zeolite. Said maturation also promotes the formation of said AFX-structure zeolite to the detriment of impurities. Maturation of the reaction mixture during said step i) of the process of the invention may be performed at room temperature or at a temperature of between 20 and 100° C. with or without stirring, for a time advantageously of between 30 minutes and 48 hours. 
     In accordance with step (ii) of the process according to the invention, the precursor gel obtained on conclusion of step i) is subjected to a hydrothermal treatment, preferably performed at a temperature of between 120° C. and 220° C. for a time of between 12 hours and 15 days, until said AFX-structure zeolite forms. 
     The precursor gel is advantageously placed under hydrothermal conditions under an autogenous reaction pressure, optionally with addition of gas, for example nitrogen, at a temperature preferably of between 120° C. and 220° C., preferably between 150° C. and 195° C., until an AFX-structure zeolite has fully crystallized. 
     The time required to obtain crystallization ranges between 12 hours and 15 days, to preferably between 12 hours and 12 days and more preferably between 12 hours and 8 days. 
     The reaction is generally performed with or without stirring, preferably with stirring. The stirring system that may be used is any system known to those skilled in the art, for example inclined paddles with counter-blades, stirring turbomixers or endless screws. 
     At the end of the reaction, after performing said step ii) of the preparation process according to the invention, the solid phase formed from an AFX-structure zeolite is preferably filtered off, washed and then dried. The drying is generally performed at a temperature of between 20° C. and 150° C., preferably between 60° C. and 100° C., for a time of between 5 and 24 hours. 
     The dried zeolite may then advantageously be calcined. The calcined AFX-structure zeolite is generally analyzed by X-ray diffraction, this technique also making it possible to determine the purity of said zeolite obtained via the process of the invention. 
     Very advantageously, the process of the invention leads to the formation of an AFX-structure zeolite, free of any other crystalline or amorphous phase. The AFX zeolite obtained has a purity greater than 90%, preferably greater than 95%, very preferably greater than 97% and even more preferably greater than 99.8%. Said AFX-structure zeolite, after the drying step, is then ready for subsequent steps such as calcination and ion exchange. For these steps, any conventional method known to those skilled in the art may be employed. 
     The loss on ignition of said AFX-structure zeolite obtained after drying and before calcination is generally between 5 and 15% by weight. According to the invention, loss on ignition (LOI) refers to the percentage loss in mass experienced by a solid compound, a mixture of solid compounds or a paste, in the case of the present invention preferably by said prepared AFX zeolite, during a heat treatment at 1000° C. for 2 hours, in a static oven (of muffle furnace type), relative to the mass of the solid compound, of the mixture of solid compounds or of the paste in its initial form, in the case of the present invention preferably relative to the mass of the dried AFX zeolite that was tested. The loss on ignition corresponds in general to the loss of solvent (such as water) present in the solids, but also to the removal of organic compounds contained in the inorganic solid constituents. 
     The step of calcining an AFX-structure zeolite obtained according to the process of the invention is preferably performed at a temperature of between 450 and 700° C. for a time of between 2 and 20 hours. 
     The AFX-structure zeolite obtained on conclusion of the calcining step is free of any organic species and in particular of the organic structuring agent R. 
     On conclusion of said calcining step, X-ray diffraction makes it possible to confirm that the solid obtained via the process according to the invention is indeed an AFX-structure zeolite. The purity obtained is preferably greater than 99.8%. In this case, the solid obtained has the X-ray diffraction pattern which includes at least the lines recorded in Table 1. Preferably, the X-ray diffraction pattern does not contain any other lines of significant intensity (i.e. with an intensity greater than about three times the background noise) than those recorded in Table 1. 
     This diffraction pattern is obtained by radiocrystallographic analysis by means of a diffractometer using the conventional powder method with the Kai radiation of copper (λ=1.5406 Å). On the basis of the position of the diffraction peaks represented by the angle 2θ, the lattice constant distances d hkl  characteristic of the sample are calculated using the Bragg relationship. The measurement error Δ(d hkl ) on d hkl  is calculated by means of the Bragg relationship as a function of the absolute error Δ(2θ) assigned to the measurement of 2θ. An absolute error Δ(2θ) equal to ±0.02° is commonly accepted. The relative intensity I rel  assigned to each value of d hkl  is measured according to the height of the corresponding diffraction peak. The X-ray diffraction pattern of the AFX-structure crystalline solid according to the invention includes at least the lines at the values of d hkl  given in Table 1. In the column of the d hkl  values, the mean values of the inter-lattice distances are given in Angstroms (Å). Each of these values must be assigned the measurement error Δ(d hkl ) of between ±0.6 Å and ±0.01 Å. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Mean values of d hkl  and relative intensities 
               
               
                 measured on an X-ray diffraction pattern 
               
               
                 of the calcined AFX-structure crystalline solid 
               
            
           
           
               
               
               
            
               
                 2 thêta 
                   
                   
               
               
                 (°) 
                 dhkl (Å) 
                 Irel 
               
               
                   
               
            
           
           
               
               
               
            
               
                 7.49 
                 11.79 
                 mw 
               
               
                 8.73 
                 10.12 
                 VS 
               
               
                 11.72 
                 7.55 
                 VS 
               
               
                 12.98 
                 6.82 
                 S 
               
               
                 14.98 
                 5.91 
                 vw 
               
               
                 15.66 
                 5.66 
                 w 
               
               
                 17.51 
                 5.06 
                 mw 
               
               
                 18.05 
                 4.91 
                 m 
               
               
                 19.62 
                 4.52 
                 vw 
               
               
                 19.88 
                 4.46 
                 w 
               
               
                 20.38 
                 4.35 
                 S 
               
               
                 21.85 
                 4.06 
                 VS 
               
               
                 22.48 
                 3.95 
                 w 
               
               
                 23.84 
                 3.73 
                 w 
               
               
                 26.11 
                 3.41 
                 mw 
               
               
                 27.08 
                 3.29 
                 vw 
               
               
                 27.17 
                 3.28 
                 vw 
               
               
                 27.57 
                 3.23 
                 vw 
               
               
                 28.24 
                 3.16 
                 mw 
               
               
                 28.68 
                 3.11 
                 vw 
               
               
                 30.29 
                 2.95 
                 w 
               
               
                 30.57 
                 2.92 
                 mw 
               
               
                 31.19 
                 2.87 
                 vw 
               
               
                 31.59 
                 2.83 
                 mw 
               
               
                 31.84 
                 2.81 
                 vw 
               
               
                 32.74 
                 2.73 
                 vw 
               
               
                 33.90 
                 2.64 
                 w 
               
               
                 34.28 
                 2.61 
                 vw 
               
               
                 34.75 
                 2.58 
                 w 
               
               
                 37.79 
                 2.38 
                 vw 
               
               
                 39.15 
                 2.30 
                 vw 
               
               
                 39.57 
                 2.28 
                 vw 
               
               
                   
               
               
                 where VS = very strong; S = strong; m = medium; mw = moderately weak; w = weak; vw = very weak. The relative intensity Irel is given as a relative intensity scale in which a value of 100 is attributed to the most intense line in the x-ray diffraction diagram: vw &lt; 15; 15 ≤ w &lt; 30; 30 ≤ mw &lt; 50; 50 ≤ m &lt; 65; 65 ≤ S &lt; 85; VS ≥ 85. 
               
            
           
         
       
     
     X-ray fluorescence spectrometry (XFS) is a chemical analysis technique using a physical property of matter, X-ray fluorescence. It enables the analysis of the majority of the chemical elements starting from beryllium (Be) in concentration ranges ranging from a few ppm to 100%, with precise and reproducible results. X-rays are used to excite the atoms in a sample, which makes them emit X-rays having an energy characteristic of each element present. The intensity and the energy of these X-rays are then measured to determine the concentration of the elements in the material. 
     It is also advantageous to obtain the protonated form of the AFX-structure zeolite obtained via the process according to the invention. Said hydrogen form may be obtained by performing an ion exchange with an acid, in particular a strong mineral acid such as hydrochloric, sulfuric or nitric acid, or with a compound such as ammonium chloride, sulfate or nitrate. The ion exchange may be performed by placing said AFX-structure zeolite in suspension one or more times with the ion-exchange solution. Said zeolite may be calcined before or after the ion exchange or between two ion-exchange steps. The zeolite is preferably calcined before the ion exchange, so as to remove any organic substance included in the porosity of the zeolite, since the ion exchange is thereby facilitated. 
     The AFX-structure zeolite obtained via the process of the invention may be used after ion exchange as acidic solid for catalysis in the fields of refining and petrochemistry. It may also be used as an absorbent or as a molecular sieve. 
     EXAMPLES 
     The invention is illustrated by the examples that follow, which are not in any way limiting in nature. 
     Example 1: Preparation of the Organic Structuring Agent 1,6-Bis(Methylpiperidinium)Hexane Dihydroxide (Structuring Agent R) 
     50 g of 1,6-dibromohexane (0.20 mol, 99%, Alfa Aesar) are placed in a 1 L round-bottomed flask containing 50 g of N-methylpiperidine (0.51 mol, 99%, Alfa Aesar) and 200 mL of ethanol. The reaction medium is stirred at reflux for 5 hours. The mixture is then cooled to room temperature and then filtered. The mixture is poured into 300 mL of cold diethyl ether and the precipitate formed is then filtered off and washed with 100 mL of diethyl ether. The solid obtained is recrystallized from an ethanol/ether mixture. The solid obtained is dried under vacuum for 12 hours. 71 g of a white solid are obtained (i.e. a yield of 80%). 
     The product has the expected  1 H NMR spectrum.  1 H NMR (D 2 O, ppm/TMS): 1.27 (4H, m); 1.48 (4H, m); 1.61 (4H, m); 1.70 (8H, m); 2.85 (6H, 5), 3.16 (12H, m). 18.9 g of Ag 2 O (0.08 mol, 99%, Aldrich) are placed in a 250 ml Teflon beaker containing 30 g of the prepared structuring agent 1,6-bis(methylpiperidinium)hexane dibromide (0.07 mol) and 100 mL of deionized water. The reaction medium is stirred for 12 hours in the absence of light. The mixture is then filtered. The filtrate obtained is composed of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide. Assay of this species is performed by proton NMR using formic acid as standard. 
     Example 2: Preparation of an AFX-Structure Zeolite According to the Invention 
     25.72 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (21.56% by weight) prepared according to Example 1 are mixed with 46.46 g of deionized water, with stirring and at room temperature. 0.761 g of sodium hydroxide (98% by weight, Aldrich) is dissolved in the preceding mixture with stirring and at room temperature. 4.44 g of Aerosil 380 silica (100% by weight, Degussa) are subsequently poured in as small fractions with stirring. As soon as the suspension obtained is homogeneous, the addition is begun of 2.63 g of FAU structure zeolite (CBV600 Zeolyst, SiO 2 /Al 2 O 3 =5.48, LOI=12.65%) and the suspension obtained is kept stirred for 30 minutes, at room temperature. The SiO 2 (Aerosil)/Al 2 O 3 (FAU) molar ratio is 2.65. The precursor gel obtained exhibits the following molar composition: 1 SiO 2 :0.05 Al 2 O 3 :0.167 R:0.093 Na 2 O:36.73 H 2 O, i.e. an SiO 2 /Al 2 O 3  ratio of 20. The precursor gel is subsequently transferred into a 160 mL stainless steel reactor equipped with a stirring system with 4 inclined paddles. The reactor is closed and then heated at 170° C. for 34 hours with stirring at 100 rpm. The solid obtained is filtered off, washed with deionized water and then dried overnight at 100° C. The loss on ignition of the dried solid is 10.1%. The solid is then introduced into a muffle furnace where a calcination step is performed: the calcination cycle comprises an increase in temperature of 1.5° C./minute up to 200° C., a steady stage at 200° C. maintained for 2 hours, an increase in temperature of 1° C./minute up to 550° C., followed by a steady stage at 550° C. maintained for 8 hours, then return to room temperature. 
     The calcined solid product was analyzed by X-ray diffraction and identified as consisting of an AFX-structure zeolite of purity greater than 99% by weight. 
     Example 3: Preparation of an AFX-Structure Zeolite According to the Invention 
     25.72 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (21.56% by weight) prepared according to Example 1 are mixed with 46.46 g of deionized water, with stirring and at room temperature. 0.761 g of sodium hydroxide (98% by weight, Aldrich) is dissolved in the preceding mixture with stirring and at room temperature. 4.44 g of Aerosil 380 silica (100% by weight, Degussa) are to subsequently poured in as small fractions with stirring. As soon as the suspension obtained is homogeneous, the addition is begun of 2.63 g of FAU structure zeolite (CBV600 Zeolyst, SiO 2 /Al 2 O 3 =5.48, LOI=12.65%) and the suspension obtained is kept stirred for 30 minutes, at room temperature. The SiO 2 (Aerosil)/Al 2 O 3 (FAU)  molar ratio is 2.65. The precursor gel obtained exhibits the following molar composition: 1 SiO 2 :0.05 Al 2 O 3 :0.167 R:0.093 Na 2 O:36.73 H 2 O, i.e. an SiO 2 /Al 2 O 3  ratio of 20. The precursor gel is subsequently transferred into a 160 mL stainless steel autoclave. The autoclave is closed and then heated for 40 hours at 170° C. without stirring. The solid obtained is filtered off, washed with deionized water and then dried overnight at 100° C. The loss on ignition of the dried solid is 9.5%. The solid is then introduced into a muffle furnace where a calcination step is performed: the calcination cycle comprises an increase in temperature of 1.5° C./minute up to 200° C., a steady stage at 200° C. maintained for 2 hours, an increase in temperature of 1° C./minute up to 550° C., followed by a steady stage at 550° C. maintained for 8 hours, then return to room temperature. 
     The calcined solid product was analyzed by X-ray diffraction and identified as consisting of an AFX-structure zeolite of purity greater than 95% by weight. 
     Example 4: Preparation of an AFX-Structure Zeolite According to the Invention 
     19.62 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (21.56% by weight) prepared according to Example 1 are mixed with 52.4 g of deionized water, with stirring and at room temperature. 0.774 g of sodium hydroxide (98% by weight, Aldrich) is dissolved in the preceding mixture with stirring and at room temperature. 4.52 g of Aerosil 380 silica (100% by weight, Degussa) are subsequently poured in as small fractions with stirring. As soon as the suspension obtained is homogeneous, the addition is begun of 2.67 g of FAU structure zeolite (CBV600 Zeolyst, SiO 2 /Al 2 O 3 =5.48, LOI=12.65%) and the suspension obtained is kept stirred for 30 minutes, at room temperature. The SiO 2 (Aerosil)/Al 2 O 3 (FAU)  molar ratio is 2.65. The precursor gel obtained exhibits the following molar composition: 1 SiO 2 :0.05 Al 2 O 3 :0.125 R:0.093 Na 2 O:36.73 H 2 O, i.e. an SiO 2 /Al 2 O 3  ratio of 20. The precursor gel is then transferred into a 160 mL stainless-steel reactor equipped with a stirring system with 4 inclined paddles. The reactor is closed and then heated for 30 to hours at 170° C. with stirring at 100 rpm. The solid obtained is filtered off, washed with deionized water and then dried overnight at 100° C. The loss on ignition of the dried solid is 9.5%. The solid is then introduced into a muffle furnace where a calcination step is performed: the calcination cycle comprises an increase in temperature of 1.5° C./minute up to 200° C., a steady stage at 200° C. maintained for 2 hours, an is increase in temperature of 1° C./minute up to 550° C., followed by a steady stage at 550° C. maintained for 8 hours, then return to room temperature. 
     The calcined solid product was analyzed by X-ray diffraction and identified as consisting of an AFX-structure zeolite of purity greater than 95% by weight. 
     Example 5: Preparation of an AFX-Structure Zeolite According to the Invention 
     29.25 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (21.56% by weight) prepared according to Example 1 are mixed with 42.74 g of deionized water, with stirring and at room temperature. 0.760 g of sodium hydroxide (98% by weight, Aldrich) is dissolved in the preceding mixture with stirring and at room temperature. 4.10 g of Aerosil 380 silica (100% by weight, Degussa) are subsequently poured in as small fractions with stirring. As soon as the suspension obtained is homogeneous, the addition is begun of 3.15 g of FAU structure zeolite (CBV600 Zeolyst, SiO 2 /Al 2 O 3 =5.48, LOI=12.65%) and the suspension obtained is kept stirred for 30 minutes, at room temperature. The SiO 2 (Aerosil)/Al 2 O 3 (FAU)  molar ratio is 2.04. The precursor gel obtained exhibits the following molar composition: 1 SiO 2 :0.06 Al 2 O 3 :0.167 R:0.093 Na 2 O:36.73 H 2 O, i.e. an SiO 2 /Al 2 O 3  ratio of 16.67. The precursor gel is then transferred into a 160 mL stainless-steel reactor equipped with a stirring system with 4 inclined paddles. The reactor is closed and then heated for 40 hours at 170° C. with stirring at 100 rpm. The solid obtained is filtered off, washed with deionized water and then dried overnight at 100° C. The loss on ignition of the dried solid is 9.6%. The solid is then introduced into a muffle furnace where a calcination step is performed: the calcination cycle comprises an increase in temperature of 1.5° C./minute up to 200° C., a steady stage at 200° C. maintained for 2 hours, an increase in temperature of 1° C./minute up to 550° C., followed by a steady stage at 550° C. maintained for 8 hours, then return to room temperature. 
     The calcined solid product was analyzed by X-ray diffraction and identified as to consisting of an AFX-structure zeolite of purity greater than 99.8% by weight. 
     Example 6: Preparation of an AFX-Structure Zeolite According to the Invention 
     24.97 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (21.56% by weight) prepared according to Example 1 are mixed with 44.9 g of is deionized water, with stirring and at room temperature. 0.980 g of sodium hydroxide (98% by weight, Aldrich) is dissolved in the preceding mixture with stirring and at room temperature. 5.75 g of Aerosil 380 silica (100% by weight, Degussa) are subsequently poured in as small fractions with stirring. As soon as the suspension obtained is homogeneous, the addition is begun of 3.40 g of FAU structure zeolite (CBV600 Zeolyst, SiO 2 /Al 2 O 3 =5.48, LOI=12.65%) and the suspension obtained is kept stirred for 30 minutes, at room temperature. The SiO 2 (Aerosil)/Al 2 O 3 (FAU)  molar ratio is 2.65. The precursor gel obtained exhibits the following molar composition: 1 SiO 2 :0.05 Al 2 O 3 :0.125 R:0.093 Na 2 O:27.55 H 2 O, i.e. an SiO 2 /Al 2 O 3  ratio of 20. The precursor gel is subsequently transferred into a 160 mL stainless steel reactor equipped with a stirring system with 4 inclined paddles. The reactor is closed and then heated for 22 hours at 170° C. with stirring at 100 rpm. The solid obtained is filtered off, washed with deionized water and then dried overnight at 100° C. The loss on ignition of the dried solid is 9.5%. The solid is then introduced into a muffle furnace where a calcination step is performed: the calcination cycle comprises an increase in temperature of 1.5° C./minute up to 200° C., a steady stage at 200° C. maintained for 2 hours, an increase in temperature of 1° C./minute up to 550° C., followed by a steady stage at 550° C. maintained for 8 hours, then return to room temperature. 
     The calcined solid product was analyzed by X-ray diffraction and identified as consisting of an AFX-structure zeolite of purity greater than 95% by weight. 
     Example 7: Preparation of an AFX-Structure Zeolite According to the Invention 
     19.62 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (21.56% by weight) prepared according to Example 1 are mixed with 52.4 g of deionized water, with stirring and at room temperature. 0.774 g of sodium hydroxide (98% by weight, Aldrich) is dissolved in the preceding mixture with stirring and at room temperature. 4.52 g of Aerosil 380 silica (100% by weight, Degussa) are subsequently poured in as small fractions with stirring. As soon as the suspension obtained is homogeneous, the addition is begun of 2.67 g of FAU structure zeolite (CBV600 Zeolyst, SiO 2 /Al 2 O 3 =5.48, LOI=12.65%) and the suspension obtained is is kept stirred for 30 minutes, at room temperature. The SiO 2 (Aerosil)/Al 2 O 3 (FAU)  molar ratio is 2.65. The precursor gel obtained exhibits the following molar composition: 1 SiO 2 :0.05 Al 2 O 3 :0.125 R:0.093 Na 2 O:36.73 H 2 O, i.e. an SiO 2 /Al 2 O 3  ratio of 20. 0.6 g of seeds of AFX structure zeolite originating from Example 5 (8.7% relative to the mass of anhydrous CBV600 zeolite and of Aerosil 380 silica) is introduced into the precursor gel with stirring. The precursor gel containing the seeds of AFX zeolite is subsequently transferred into a 160 mL stainless steel reactor equipped with a stirring system with 4 inclined paddles. The reactor is closed and then heated for 14 hours at 170° C. with stirring at 100 rpm. The solid obtained is filtered off, washed with deionized water and then dried overnight at 100° C. The loss on ignition of the dried solid is 9.6%. The solid is then introduced into a muffle furnace where a calcination step is performed: the calcination cycle comprises an increase in temperature of 1.5° C./minute up to 200° C., a steady stage at 200° C. maintained for 2 hours, an increase in temperature of 1° C./minute up to 550° C., followed by a steady stage at 550° C. maintained for 8 hours, then return to room temperature. 
     The calcined solid product was analyzed by X-ray diffraction and identified as consisting of an AFX-structure zeolite of purity greater than 97% by weight. 
     Example 8: Preparation of an AFX-Structure Zeolite According to the Invention 
     25.72 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (21.56% by weight) prepared according to Example 1 are mixed with 46.154 g of deionized water, with stirring and at room temperature. 0.761 g of sodium hydroxide (98% by weight, Aldrich) is dissolved in the preceding mixture with stirring and at room temperature. 4.39 g of Aerosil 380 silica (100% by weight, Degussa) are subsequently poured in as small fractions with stirring. As soon as the suspension obtained is homogeneous, the addition is begun of 2.98 g of FAU structure zeolite (CBV500 Zeolyst, SiO 2 /Al 2 O 3 =5.65, LOI=20.4%) and the suspension obtained is to kept stirred for 30 minutes, at room temperature. The SiO 2 (Aerosil)/Al 2 O 3 (FAU)  molar ratio is 2.54. The precursor gel obtained exhibits the following molar composition: 1 SiO 2 :0.05 Al 2 O 3 :0.167 R:0.093 Na 2 O:36.73 H 2 O, i.e. an SiO 2 /Al 2 O 3  ratio of 20. The precursor gel is subsequently transferred into a 160 mL stainless steel reactor equipped with a stirring system with 4 inclined paddles. The reactor is closed and is then heated for 26 hours at 170° C. with stirring at 100 rpm. The solid obtained is filtered off, washed with deionized water and then dried overnight at 100° C. The loss on ignition of the dried solid is 9.5%. The solid is then introduced into a muffle furnace where a calcination step is performed: the calcination cycle comprises an increase in temperature of 1.5° C./minute up to 200° C., a steady stage at 200° C. maintained for 2 hours, an increase in temperature of 1° C./minute up to 550° C., followed by a steady stage at 550° C. maintained for 8 hours, then return to room temperature. 
     The calcined solid product was analyzed by X-ray diffraction and identified as consisting of an AFX-structure zeolite of purity greater than 90% by weight, comprising traces (less than 10% by weight) of analcime zeolite (ANA).