RAPID SYNTHESIS OF A CATALYST COMPRISING A ZEOLITE HAVING AN AFX STRUCTURE AND AT LEAST ONE TRANSITION METAL FOR SELECTIVE NOX REDUCTION

Abstract

A catalyst based on a zeolite of AFX structural type and on at least one transition metal, can be prepared by a process comprising at least the following steps: i) mixing, in an aqueous medium, of at least one source of silicon in oxide form SiO2, of at least one source of aluminium in oxide form Al2O3, of an organic nitrogen-comprising compound R, of at least one source of at least one alkali metal and/or alkaline-earth metal M until a homogeneous precursor gel is obtained; ii) hydrothermal treatment of said precursor gel to obtain a crystallized solid phase, iii) at least one ion exchange with a transition metal; iv) heat treatment. The catalyst can be used for the selective reduction of NOx employing the catalyst, and can achieve an NOx conversion (conversion=(NOx inletNOx outlet)/NOx inlet) of 100% at a temperature of 430 C. or lower.

Claims

1. A catalyst based on an AFX zeolite and on at least one transition metal directly obtained by a preparation process comprising at least the following steps: i) the mixing, in an aqueous medium, of at least one source of at least one silicon oxide SiO.sub.2, of at least one source of at least one aluminium oxide Al.sub.2O.sub.3, of an organic nitrogen-comprising compound R, also referred to as specific structuring agent, 1,6-bis(methylpiperidinium)hexane dihydroxide, of at least one alkali metal and/or one alkaline-earth metal M with a valency n, n being an integer greater than or equal to 1, the reaction mixture having the following molar composition: SiO.sub.2/Al.sub.2O.sub.3 between 2 and 100, H.sub.2O/SiO.sub.2 between 5 and 60, R/SiO.sub.2 between 0.05 and 0.50, M.sub.2/nO/SiO.sub.2 between 0.05 and 0.40, wherein 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, step i) being performed for a time enabling a homogeneous mixture known as a precursor gel to be obtained; ii) the hydrothermal treatment of the precursor gel obtained at the end of step i) under autogenous pressure at a temperature of between 120 C. and 250 C. for a time of between 2 and 12 hours until the zeolite of AFX structural type is formed, iii) at least one ion exchange comprising bringing the solid obtained at the end of the previous step into contact with a solution comprising at least one species capable of releasing a transition metal in solution in reactive form, with stirring at ambient temperature for a time of between 1 hour and 2 days; iv) a heat treatment comprising drying of the solid obtained at the end of the previous step at a temperature of between 2 and 150 C. for a time of between 2 and 24 hours, followed by at least one calcination underoptionally dryair at a temperature of between 45 and 700 C. for a time of between 2 and 20 hours.

2. The catalyst as claimed in claim 1, wherein the transition metal or metals is/are selected from the group made up of the following elements: Ti, V, Mn, Mo, Fe, Co, Cu, Cr, Zn, Nb, Ce, Zr, Rh, Pd, Pt, Au, W, Ag.

3. The catalyst as claimed in claim 1, wherein the total content of the transition metals is between 0.5% and 6% by mass relative to the total mass of the anhydrous final catalyst.

4. The catalyst as claimed in claim 1, which comprises copper, alone, at a content of between 0.5% and 6% by weight relative to the total mass of the anhydrous final catalyst.

5. The catalyst as claimed in claim 1, which comprises copper in combination with at least one other transition metal chosen from the group made up of Fe, Nb, Ce, Mn, the content of copper in the catalyst being between 0.05% and 2% by mass, the content of said at least one other transition metal being between 1% and 4% by mass, relative to the total mass of the anhydrous final catalyst.

6. The catalyst as claimed in claim 1, which comprises iron in combination with another metal chosen from the group made up of Cu, Nb, Ce, Mn, the iron content being between 0.05% and 2% by mass, the content of said other transition metal being between 1% and 4% by mass, relative to the total mass of the anhydrous final catalyst.

7. A catalyst for NOx conversion comprising a CuAFX zeolite having a property that, when 200 mg of the catalyst in powder form are placed in a quartz reactor and 145 L/h of a feed having a molar composition of 400 ppm NO, 400 ppm NH.sub.3, 8.5% O.sub.2, 9% CO.sub.2, 10% H.sub.2O, remainder N.sub.2 are fed through the quartz reactor, an NOx conversion (conversion=(NOx inletNOx outlet)/NOx inlet) measured by an FTIR analyser at an outlet of the reactor shows a light-off temperature T80, corresponding to the temperature at which 80% of the NOx in the gas mixture are converted by the catalyst, of 212 C. or less.

8. The catalyst as claimed in claim 7, having a property that, when 200 mg of the catalyst in powder form are placed in a quartz reactor and 145 L/h of a feed having a molar composition of 400 ppm NO, 400 ppm NH.sub.3, 8.5% O.sub.2, 9% CO.sub.2, 10% H.sub.2O, remainder N.sub.2 are fed through the quartz reactor, an NOx conversion (conversion=(NOx inletNOx outlet)/NOx inlet) measured by an FTIR analyser at an outlet of the reactor shows a light-off temperature T90, corresponding to the temperature at which 90% of the NOx in the gas mixture are converted by the catalyst, of 228 C. or less.

9. The catalyst as claimed in claim 7, having a property that, when 200 mg of the catalyst in powder form are placed in a quartz reactor and 145 L/h of a feed having a molar composition of 400 ppm NO, 400 ppm NH.sub.3, 8.5% O.sub.2, 9% CO.sub.2, 10% H.sub.2O, remainder N.sub.2 are fed through the quartz reactor, an NOx conversion (conversion=(NOx inletNOx outlet)/NOx inlet) measured by an FTIR analyser at an outlet of the reactor shows a light-off temperature T100, corresponding to the temperature at which 100% of the NOx in the gas mixture are converted by the catalyst, of 310 C. or less.

10. A catalyst for NOx conversion comprising a zeolite of AFX structural type containing copper, wherein the catalyst has a property that, when 200 mg of the catalyst in powder form are placed in a quartz reactor and 145 L/h of a feed having a molar composition of 400 ppm NO, 400 ppm NH.sub.3, 8.5% O.sub.2, 9% CO.sub.2, 10% H.sub.2O, remainder N.sub.2 are fed through the quartz reactor, an NOx conversion (conversion=(NOx inletNOx outlet)/NOx inlet) of 100%, measured by an FTIR analyser at an outlet of the reactor, is achieved at a temperature of 430 C. or lower.

11. The catalyst as claimed in claim 10, wherein the catalyst has a molar SiO.sub.2/Al.sub.2O.sub.3 ratio between 10.2 and 14.05 as determined by X-ray fluorescence spectrometry.

12. The catalyst as claimed in claim 10, wherein a content of copper is between 1% and 4% by weight, relative to the total mass of the catalyst.

13. The catalyst as claimed in claim 10, having a property that, when 200 mg of the catalyst in powder form are placed in a quartz reactor and 145 L/h of a feed having a molar composition of 400 ppm NO, 400 ppm NH.sub.3, 8.5% O.sub.2, 9% CO.sub.2, 10% H.sub.2O, remainder N.sub.2 are fed through the quartz reactor, an NOx conversion (conversion=(NOx inletNOx outlet)/NOx inlet) measured by an FTIR analyser at an outlet of the reactor shows a light-off temperature T100, corresponding to the temperature at which 100% of the NOx in the gas mixture are converted by the catalyst, of 310 C. or less.

14. A process for the selective reduction of NOx by a reducing agent, employing a catalyst as claimed in claim 1.

15. A process for the selective reduction of NOx by a reducing agent, employing a catalyst as claimed in claim 7.

16. A process for the selective reduction of NOx by a reducing agent, employing a catalyst as claimed in claim 10.

Description

LIST OF FIGURES

[0040] FIG. 1 shows the chemical formula of the organic nitrogen-comprising compound R which is the structuring agent used in the synthesis process according to the invention.

[0041] FIGS. 2A, 2B, 2C and 2D the X-ray diffraction patterns for the copper-containing zeolites of AFX structural type obtained according to Examples 2 to 5, respectively.

[0042] FIG. 3 shows the conversion C in % obtained during a catalytic test of the reduction of nitrogen oxides (NOx) by ammonia (NH.sub.3) in the presence of oxygen (O.sub.2) under standard SCR conditions as a function of temperature T in C. for a catalyst according to example 2 (CuAFX, according to the invention, curve symbolized by diamonds), a catalyst according to example 3 (CuAFX780, according to the invention, curve symbolized by triangles), a catalyst according to example 4 (CuAFX720, according to the invention, curve symbolized by squares), a catalyst according to example 5 (CuAFX600, according to the invention, curve symbolized by circles) and a catalyst according to example 6 (CuSSZ16, comparative, curve symbolized by crosses).

[0043] Other characteristics and advantages of the synthesis process according to the invention, the catalyst according to the invention and the use 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.

DESCRIPTION OF EMBODIMENTS

[0044] The present invention relates to a process for preparing a catalyst comprising a zeolite of AFX structural type and at least one transition metal, comprising at least the following steps: [0045] i) the mixing, in an aqueous medium, of at least one source of at least one silicon oxide SiO.sub.2, of at least one source of at least one aluminium oxide Al.sub.2O.sub.3, or of at least one zeolite of FAU structural type having a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of between 2.00 and 100, of an organic nitrogen-comprising compound R, also referred to as specific structuring agent, 1,6-bis(methylpiperidinium)hexane dihydroxide, of at least one alkali metal and/or one alkaline-earth metal M with a valency n, n being an integer greater than or equal to 1, the reaction mixture having the following molar composition: SiO.sub.2/Al.sub.2O.sub.3 between 2 and 100, preferably between 12 and 40 H.sub.2O/SiO.sub.2 between 5 and 60, preferably between 10 and 40 R/SiO.sub.2 between 0.05 and 0.50, preferably between 0.10 and 0.40 M.sub.2/nO/SiO.sub.2 between 0.05 and 0.40, preferably between 0.15 and 0.30, wherein 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; [0046] ii) the hydrothermal treatment of said precursor gel obtained at the end of step i) under autogenous pressure at a temperature of between 120 C. and 250 C., preferably of between 150 C. and 230 C., for a time of between 2 and 12 hours, preferably of between 2 and 10 hours, until said zeolite of AFX structural type forms, [0047] iii) at least one ion exchange comprising bringing said solid obtained at the end of the step ii) into contact with a solution comprising at least one species capable of releasing a transition metal, in particular copper, in solution in reactive form, with stirring at ambient temperature for a time of between 1 hour and 2 days; [0048] iv) a heat treatment advantageously comprising drying of the solid obtained in the previous step at a temperature of between 2 and 150 C., preferably of between 6 and 100 C., for a time of between 2 and 24 hours, followed by at least one calcination underoptionally dryair at a temperature of between 45 and 700 C., preferably of between 50 and 600 C., for a time of between 2 and 20 hours, preferably of between 6 and 16 hours, more preferably of between 8 and 13 hours, the flow rate of optionally dry air being preferably between 0.5 and 1.5 L/h/g of solid to be treated, more preferably between 0.7 and 1.2 L/h/g of solid to be treated.

[0049] Steps iii) and iv) may be reversed and/or optionally repeated.

[0050] The present invention also relates to the catalyst comprising a zeolite of AFX structural type and at least one transition metal capable of being obtained or directly obtained by the preparation process described above.

[0051] The invention lastly relates to the use of a catalyst according to the invention in a process for the selective catalytic reduction of NOx in the presence of a reducing agent.

The Catalyst

[0052] The catalyst according to the invention comprises at least one zeolite of AFX type and at least one additional transition metal, preferably copper.

[0053] According to the invention, the transition metal or metals included in the catalyst is/are selected from the elements of the group made up of the elements of groups 3 to 12 of the periodic table of the elements, including the lanthanides. In particular, the transition metal or metals included in the catalyst is/are selected from the group formed by the following elements: Ti, V, Mn, Mo, Fe, Co, Cu, Cr, Zn, Nb, Ce, Zr, Rh, Pd, Pt, Au, W, Ag.

[0054] Preferably, the catalyst according to the invention comprises copper, alone or in combination with at least one other transition metal chosen from the group of elements listed above, in particular Fe, Nb, Ce, Mn.

[0055] The total content of the transition metals is advantageously between 0.5% and 6% by mass, preferably between 0.5% and 5% by mass, and even more preferably between 1% and 4% by mass, relative to the total mass of the final catalyst in its anhydrous form.

[0056] In the case of catalysts containing only copper as transition metal, the content is advantageously between 0.5% and 6%, preferably between 0.5% and 5%, and more preferably between 1% and 4% by weight, relative to the total mass of the anhydrous final catalyst.

[0057] In the case of catalysts comprising copper and another element such as, preferably, Fe, Nb, Ce, Mn, the content of copper in the catalyst is between 0.05% and 2% by mass, preferably between 0.5% and 2% by mass, while the content of the other transition metal is preferably between 1% and 4% by mass, the contents of transition metals being given as percentages by mass relative to the total mass of the final dry catalyst.

[0058] In the case of catalysts containing only iron as transition metal, the content of iron is between 0.5% and 4% and even more preferably between 1.5% and 3.5%, relative to the total mass of the anhydrous final catalyst.

[0059] In the case of catalysts comprising iron and another element such as, preferably, Cu, Nb, Ce, Mn, the content of iron in the catalyst is between 0.05% and 2% by mass, preferably between 0.5% and 2% by mass, while the content of the other transition metal is preferably between 1% and 4% by mass, the contents of transition metals being given as percentages by mass relative to the total mass of the final dry catalyst.

[0060] The catalyst according to the invention may also comprise other elements, such as for example alkali and/or alkaline-earth metals, for example sodium, originating in particular from the synthesis, in particular of the compounds of the reaction medium of step i) of the process for preparing said catalyst.

Process for Preparing the Catalyst

Mixing Step i)

[0061] This step implements the mixing, in an aqueous medium, of at least one source of at least one silicon oxide SiO.sub.2, of at least one source of at least one aluminium oxide Al.sub.2O.sub.3, and/or of at least one zeolite of FAU structural type having a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of between 2.00 and 100, of an organic nitrogen-comprising compound R, also referred to as specific structuring agent, [0062] 1,6-bis(methylpiperidinium)hexane dihydroxide, of at least one alkali metal and/or one alkaline-earth metal M with a valency n, n being an integer greater than or equal to 1, the reaction mixture having the following molar composition: [0063] SiO.sub.2/Al.sub.2O.sub.3 between 2 and 100, preferably between 12 and 40 [0064] H.sub.2O/SiO.sub.2 between 5 and 60, preferably between 10 and 40 [0065] R/SiO.sub.2 between 0.05 and 0.50, preferably between 0.10 and 0.40 [0066] M.sub.2/nO/SiO.sub.2 between 0.05 and 0.40, preferably between 0.15 and 0.30, wherein 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.

[0067] In the molar composition of the reaction mixture above and throughout the description: [0068] SiO.sub.2 denotes the molar amount of the tetravalent element silicon (Si), expressed in oxide form, and Al.sub.2O.sub.3 denotes the molar amount of the trivalent element aluminium (Al), expressed in oxide form, [0069] H.sub.2O is the molar amount of water present in the reaction mixture, R is the molar amount of said organic nitrogen-comprising compound, [0070] M.sub.2/nO is the molar amount expressed in oxide form M.sub.2/nO by the source of alkali metal and/or alkaline-earth metal.

[0071] In accordance with the invention, at least one source of oxide SiO.sub.2 is incorporated into the mixture for carrying out step (i) of the preparation process. 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, 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 trademarks such as Ludox. Preferably, the source of silicon is Ludox HS-40. It is also possible to use, as source of oxide SiO.sub.2, at least one zeolite of FAU structural type having a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of between 2.00 and 100, alone or in a mixture with other sources of SiO.sub.2.

[0072] The source of aluminium is preferably aluminium hydroxide or an aluminium salt, for example chloride, nitrate or sulfate, a sodium aluminate, an aluminium alkoxide, or alumina itself, preferably in hydrated or hydratable form, for instance colloidal alumina, pseudoboehmite, gamma-alumina or alpha or beta alumina trihydrate. Use may also be made of mixtures of the sources mentioned above. It is also possible to use, as source of oxide Al.sub.2O.sub.3, at least one zeolite of FAU structural type having a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of between 2.00 and 100, alone or in a mixture with other sources of Al.sub.2O.sub.3.

[0073] In accordance with the invention, at least one source of silica and/or at least one source of aluminium may also be at least one zeolite of FAU structural type having a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of between 2.00 and 100.

[0074] In accordance with the invention, R is a nitrogen-comprising organic compound, 1,6-bis(methylpiperidinium)hexane dihydroxide, said compound being incorporated into the reaction mixture for the implementation of step (i), as organic structuring agent.

[0075] In accordance with the invention, at least one source of at least one alkali metal and/or alkaline-earth metal M with a 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.

[0076] The source of at least one alkali metal and/or alkaline-earth metal M is preferably sodium hydroxide.

[0077] It may be advantageous to add seeds of a zeolite of AFX structural type to the reaction mixture during said step i) of the process of the invention so as to reduce the time needed for the formation of the crystals of a zeolite of AFX structural type and/or the total crystallization time. Said seed crystals also promote the formation of said zeolite of AFX structural type to the detriment of impurities. Such seeds comprise crystalline solids, in particular crystals of a zeolite of AFX structural type. 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 (silicon) and trivalent (aluminium) elements in oxide form which are used 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.

[0078] The mixing step i) is performed until a homogeneous mixture is obtained, preferably for a time of greater than or equal to 10 minutes, preferably with stirring by any system known to those skilled in the art, at a low or high shear rate.

[0079] At the end of step i), a homogeneous precursor gel is obtained.

[0080] It may be advantageous to perform a maturation of the reaction mixture during said step i) of the process of the invention, before the hydrothermal crystallization, so as to control the size of the crystals of a zeolite of AFX structural type. Said maturation also promotes the formation of said zeolite of AFX structural type to the detriment of impurities. The maturation of the reaction mixture during said step i) of the process of the invention may be performed at ambient temperature or at a temperature of between 2 and 80 C., with or without stirring, for a time of between 30 minutes and 24 hours.

Hydrothermal Treatment Step ii)

[0081] In accordance with step ii) of the process according to the invention, the precursor gel obtained at the end of step i) is subjected to a hydrothermal treatment, preferentially carried out at a temperature of between 120 C. and 250 C. for a time of between 2 and 12 hours, until said zeolite of AFX structural type (or crystallized solid) forms.

[0082] 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 250 C., preferably between 150 C. and 230 C., until a zeolite of AFX structural type has fully crystallized.

[0083] The time required to achieve crystallization varies between 2 and 12 hours, preferably between 2 and 10 hours, and more preferably between 2 and 8 hours.

[0084] 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.

Exchange Step iii)

[0085] The process for preparing the catalyst according to the invention comprises at least one step of ion exchange, comprising bringing the crystallized solid obtained at the end of the previous step, that is to say the AFX zeolite obtained at the end of step ii) or the dried and calcined AFX zeolite obtained at the end of step iv) in the preferred case where steps iii) and iv) are reversed, into contact with at least one solution comprising at least one species capable of releasing a transition metal, in particular copper, in solution in reactive form, with stirring at ambient temperature for a time of between 1 hour and 2 days, advantageously for a time of between 6 and 12 hours, the concentration of said species capable of releasing the transition metal in said solution depending on the amount of transition metal that is intended to be incorporated into said crystallized solid.

[0086] It is also advantageous to obtain the protonated form of the zeolite of AFX structural type after step ii). 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, before the ion exchange with the transition metal(s).

[0087] The transition metal released in the exchange solution is selected from the group made up of the following elements: Ti, V, Mn, Mo, Fe, Co, Cu, Cr, Zn, Nb, Ce, Zr, Rh, Pd, Pt, Au, W, Ag. The transition metal is preferably Fe, Cu, Nb, Ce or Mn, preferably Cu.

[0088] According to the invention, species capable of releasing a transition metal is understood to mean a species which is capable of dissociating in an aqueous medium, such as for example sulfates, nitrates, chlorides, oxalates, or organometallic complexes of a transition metal, or mixtures thereof. Preferably, the species capable of releasing a transition metal is a sulfate or a nitrate of said transition metal.

[0089] According to the invention, the solution with which the crystallized solid or dried and calcined crystallized solid is brought into contact comprises at least one species capable of releasing a transition metal, preferably a single species capable of releasing a transition metal, preferably iron or copper, preferentially copper.

[0090] Advantageously, the process for preparing the catalyst according to the invention comprises a step iii) of ion exchanges by bringing the crystallized solid into contact with a solution comprising a species capable of releasing a transition metal or by successively bringing the solid into contact with a plurality of solutions each comprising a species capable of releasing a transition metal, the various solutions comprising different species capable of releasing a transition metal.

[0091] At the end of the exchange, the solid obtained is advantageously filtered off, washed and then dried to obtain said catalyst in powder form.

[0092] The total amount of transition metal, preferably copper, contained in said final catalyst is between 0.5% and 6% by mass relative to the total mass of the catalyst in its anhydrous form.

[0093] According to one embodiment, the catalyst according to the invention is prepared by a process comprising a step iii) of ion exchange, the solid or the dried and calcined solid being brought into contact with a solution comprising a species capable of releasing copper in solution in reactive form. Advantageously, the total amount of copper contained in said final catalyst, that is to say at the end of the preparation process according to the invention, is between 0.5% and 6%, preferably between 1% and 4% by mass, all the percentages being percentages by mass relative to the total mass of the final catalyst according to the invention in its anhydrous form, obtained at the end of the preparation process.

Heat Treatment Step iv)

[0094] The preparation process according to the invention comprises a step iv) of heat treatment performed at the end of the previous step, i.e. at the end of the hydrothermal treatment step ii) or at the end of the ion-exchange step iii), preferably at the end of the ion-exchange step iii). Step iii) and step iv) of the preparation process may advantageously be reversed. Each of steps iii) and iv) may also optionally be repeated.

[0095] Said step iv) of heat treatment comprises drying of the solid at a temperature of between 2 and 150 C., preferably of between 6 and 100 C., advantageously for a time of between 2 and 24 hours, followed by at least one calcination underoptionally dryair at a temperature advantageously of between 45 and 700 C., preferably of between 50 and 600 C., for a time of between 2 and 20 hours, preferably of between 6 and 16 hours, more preferably of between 8 and 13 hours, the flow rate of optionally dry air being preferably between 0.5 and 1.5 L/h/g of solid to be treated, more preferably between 0.7 and 1.2 L/h/g of solid to be treated. The calcination may be preceded by a gradual temperature increase.

[0096] The catalyst obtained at the end of the heat treatment step iv) is devoid of any organic species, in particular devoid of the organic structuring agent R.

[0097] In particular, the catalyst obtained by a process comprising at least steps i), ii), iii) and iv) as described above has improved NOx conversion properties.

Characterization of the Catalyst Prepared According to the Invention

[0098] The catalyst comprises a zeolite of AFX structure in accordance with the classification of the International Zeolite Association (IZA), exchanged by at least one transition metal. This structure is characterized by X-ray diffraction (XRD).

[0099] The X-ray diffraction (XRD) pattern is obtained by radiocrystallographic analysis by means of a diffractometer using the conventional powder method with copper K.sub.1 radiation (=1.5406 ). On the basis of the position of the diffraction peaks represented by the angle 2, the lattice constant distances d.sub.hkl characteristic of the sample are calculated using the Bragg relationship. The measurement error (d.sub.hkl) on d.sub.hkl is calculated by means of the Bragg relationship as a function of the absolute error (2) assigned to the measurement of 2. An absolute error (2) equal to 0.02 is commonly accepted. The relative intensity I.sub.rel assigned to each value of d.sub.hkl is measured according to the height of the corresponding diffraction peak. Comparison of the diffractogram with the ICDD (International Centre for Diffraction Data) database records using software such as for example DIFFRACT.SUITE also makes it possible to identify the crystalline phases present in the material obtained.

[0100] The qualitative and quantitative analysis of the chemical species present in the materials obtained is carried out by X-ray fluorescence (XRF) spectrometry. This is a technique of chemical analysis using a physical property of matter, the X-ray fluorescence. The spectrum of X-rays emitted by the material is characteristic of the composition of the sample; by analyzing this spectrum, it is possible to deduce therefrom the elemental composition, that is to say the mass concentrations of elements.

[0101] The loss on ignition (LOI) of the catalyst obtained after the drying step (and before calcination) or after the calcination step of step iv) of the process according to the invention is generally between 5% and 18% by weight. The loss on ignition of a sample, referred to by the acronym LOI, corresponds to the difference in the mass of the sample before and after a heat treatment at 1000 C. for 2 hours. It is expressed in % corresponding to the percentage loss of mass. The loss on ignition corresponds in general to the loss of solvent (such as water) contained in the solid, but also to the removal of organic compounds contained in the inorganic solid constituents.

Process for the Selective Reduction of NO.sub.x by a Reducing Agent Such as NH.sub.3 or H.sub.2 Employing the Catalyst According to the Invention

[0102] The invention also relates to the use of the catalyst according to the invention, directly prepared or capable of being prepared by the process described above, for the selective reduction of NO.sub.x by a reducing agent such as NH.sub.3 or H.sub.2, advantageously formed by deposition in the form of a coating (or washcoat) on a honeycomb structure, primarily for mobile applications, or on a plate structure, as found in particular for stationary applications.

[0103] The honeycomb structure is formed of parallel channels which are open at both ends (flow-through channels) or comprises porous filtering walls, in which case the adjacent parallel channels are alternately blocked at either side of the channels to force the gas flow to pass through the wall (wall-flow monolith). Said honeycomb structure thus coated constitutes a catalytic block. Said structure may be composed of cordierite, silicon carbide (SiC), aluminium titanate (AITi), alpha-alumina, mullite, or any other material having a porosity of between 30% and 70%. Said structure may be formed in metal sheet, in stainless steel containing chromium and aluminium, FeCrAl steel.

[0104] The amount of catalyst according to the invention deposited on said structure is between 50 and 180 g/L for the filtering structures and between 80 and 200 g/L for the structures with open channels.

[0105] The actual coating (washcoat) comprises the catalyst according to the invention, advantageously in combination with a binder such as cerine, zirconium oxide, alumina, non-zeolitic silica-alumina, titanium oxide, a mixed oxide of cerine-zirconia type, a tungsten oxide, a spinel. Said coating is advantageously applied to said structure by a deposition method known as washcoating, which consists in soaking the monolith in a suspension (or slurry) of powdered catalyst according to the invention in a solvent, preferably water, and optionally binders, metal oxides, stabilizers or other promoters. This soaking step may be repeated until the desired amount of coating is obtained. In certain cases the slurry may also be sprayed inside the monolith. Once the coating has been deposited, the monolith is calcined at a temperature of 300 to 600 C. for 1 to 10 hours.

[0106] Said structure may be coated with one or more coatings. The coating comprising the catalyst according to the invention is advantageously combined with, i.e. covers or is covered by, another coating having capacities for adsorbing pollutants, in particular NOx, for reducing pollutants, in particular NOx, or promoting the oxidation of pollutants, in particular that of ammonia.

[0107] Another possibility is for the catalyst to be in the form of an extrudate. In this case, the structure obtained may contain up to 100% of catalyst according to the invention.

[0108] Said structure coated with the catalyst according to the invention is advantageously integrated into an exhaust line of an internal combustion engine operating mainly in lean-mixture mode, that is to say with excess air relative to the stoichiometry of the combustion reaction, as is the case with diesel engines for example. Under these engine operating conditions, the exhaust gases contain in particular the following pollutants: soot, unburned hydrocarbons (HCs), carbon monoxide (CO), nitrogen oxides (NOx). Upstream of said structure coated with the catalyst according to the invention may be placed an oxidation catalyst, the function of which is to oxidize the HCs and CO, and a filter for removing soot from the exhaust gases, the function of said coated structure being to remove the NOx, its operating range being between 10 and 900 C. and preferably between 200 C. and 500 C.

Advantages of the Invention

[0109] The catalyst according to the invention, based on a zeolite of AFX structural type obtained by rapid synthesis and on at least one transition metal, in particular copper, has improved properties compared to the prior art catalysts. In particular, the use of the catalyst according to the invention makes it possible to obtain lower light-off temperatures for the NOx conversion reaction and an improved NOx conversion over the entire operating temperature range (150-600 C.), all while maintaining a good selectivity for N.sub.2O. It also has a better resistance to hydrothermal aging, ensuring high performance even after such aging.

EXAMPLES

Example 1: Preparation of 1,6-Bis(Methylpiperidinium)Hexane Dihydroxide (Structuring Agent R)

[0110] 50 g of 1,6-dibromohexane (0.20 mol, 99%, Alfa Aesar) are placed in a 1 L round-bottom flask containing 50 g of N-methylpiperidine (0.51 mol, 99%, Alfa Aesar) and 200 mL of ethanol. The reaction medium is stirred and refluxed for 5 hours. The mixture is then cooled to ambient 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 in 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%).

[0111] The product has the expected .sup.1H NMR spectrum. .sup.1H NMR (D.sub.2O, ppm/TMS): 1.27 (4H, m); 1.48 (4H, m); 1.61 (4H, m); 1.70 (8H, m); 2.85 (6H, s); 3.16 (12H, m). 18.9 g of Ag.sub.2O (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. Assaying of this species is performed by proton NMR using formic acid as standard.

Example 2: Preparation of a Zeolite of AFX Structural Type According to the Invention with 3% Cu

Preparation of the AFX Zeolite

[0112] 467.1 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (18.36% by weight) prepared according to example 1 were mixed with 4.3 g of deionized water. 19.72 g of sodium hydroxide (solid, 98% by weight purity, Aldrich) are added to the above mixture, and the preparation obtained is kept stirring for 10 minutes. Subsequently, 15.56 g of sodium aluminate (53.17% Al.sub.2O.sub.3, Strem Chemicals) are incorporated and the synthesis gel is kept stirring for 15 minutes. Lastly, 243.38 g of colloidal silica (Ludox HS40, 40% SiO.sub.2 by weight, Grace) and 9.76 g of seeds of an AFX zeolite obtained by a method known to those skilled in the art were incorporated into the synthesis mixture with stirring. The molar composition of the mixture, without taking into account the AFX zeolite seeds, is as follows: 100 SiO.sub.2: 5 Al.sub.2O.sub.3: 16.7 R: 22.4 Na.sub.2O: 1836 H.sub.2O, i.e. an SiO.sub.2/Al.sub.2O.sub.3 ratio of 20. The precursor gel is then transferred, after homogenization, into an autoclave. The autoclave is closed and then heated with an increase in temperature of 5 C./min up to 150 C. for 4 hours under autogenous pressure and with stirring at 200 rpm using a system with 4 inclined paddles. The crystallized product obtained is filtered off, washed with deionized water and then dried overnight at 100 C. The loss on ignition is 14%. 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./min up to 200 C., a steady stage at 200 C. maintained for 2 hours, an increase in temperature of 1 C./min up to 550 C., followed by a steady stage at 550 C. maintained for 12 hours, then a return to ambient temperature.

[0113] The calcined solid product was analysed by X-ray diffraction and identified as consisting of a zeolite of AFX structural type (ICDD file, PDF 04-011-1869) with a purity of greater than 99.8%. The product has a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of 10.2 as determined by XRF.

[0114] The calcined AFX zeolite is then brought into contact with a 3 M NH.sub.4NO.sub.3 solution for 1 hour with stirring at 80 C. The ratio between the volume of NH.sub.4NO.sub.3 solution and the mass of solid is 10. The solid obtained is filtered off and washed and the exchange procedure is repeated twice more under the same conditions. The final solid is separated off, washed and dried for 12 hours at 100 C. XRD analysis shows that the product obtained is a pure zeolite of AFX structural type.

[0115] The AFX zeolite in ammoniacal form is treated under a stream of air at 550 C. for 8 hours with a temperature increase gradient of 1 C./min. The product obtained is an AFX zeolite in protonated form.

Cu Ion Exchange

[0116] The calcined AFX zeolite in protonated form is brought into contact with a [Cu(NH.sub.3).sub.4](NO.sub.3).sub.2 solution for 12 hours with stirring at ambient temperature. The final solid is separated off, washed and dried for 12 hours at a temperature of 100 C.

[0117] The exchanged Cu-AFX solid obtained after the contacting with the [Cu(NH.sub.3).sub.4](NO.sub.3).sub.2 solution is calcined under a stream of air at 550 C. for 8 hours.

[0118] The calcined solid product is analyzed by X-ray diffraction and identified as a zeolite of AFX structural type (ICDD file, PDF 04-011-1869). The diffraction pattern created by this solid is given in FIG. 2A.

[0119] The product has a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of 10.2 and a percentage by mass of Cu of 3%, as determined by XRF.

[0120] The catalyst obtained is denoted CuAFX.

Example 3: Preparation of a Zeolite of AFX Structural Type According to the Invention with 3% Cu

Preparation of the AFX Zeolite

[0121] 29.3 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (18.36% by weight) prepared according to example 1 are mixed with 41.73 g of deionized water with stirring and at ambient temperature. 0.764 g of sodium hydroxide (98% by weight, Aldrich) are dissolved in the above mixture with stirring and at ambient temperature. Subsequently, 0.675 g of amorphous aluminium hydroxide gel (Al(OH).sub.3 amorphous gel, 58.55% by mass of Al.sub.2O.sub.3, Merck), are incorporated into the synthesis mixture, this being kept stirring for half an hour at ambient temperature. As soon as a homogeneous suspension is obtained, pouring in of 7.56 g of a zeolite of FAU structural type (CBV780, SiO.sub.2/Al.sub.2O.sub.3=98.22, Zeolyst, LOI=8.52%) is commenced and the suspension obtained is kept stirring for 30 minutes at ambient temperature. In order to promote the formation of a zeolite of AFX structural type, 0.614 g of seeds (10% relative to the mass of CBV780 zeolite) of a zeolite of AFX structural type are added to the synthesis mixture, which is kept stirring for 5 minutes. The reaction mixture then undergoes a step of maturation for 24 hours at ambient temperature with stirring (200 rpm). The molar composition of the precursor gel is as follows: 1 SiO.sub.2: 0.05 Al.sub.2O.sub.3: 0.167 R: 0.093 Na.sub.2O: 36.73 H.sub.2O, i.e. an SiO.sub.2/Al.sub.2O.sub.3 ratio of 20. The precursor gel is then transferred into a 160 mL stainless steel reactor fitted with a stirring system with four inclined paddles. The reactor is closed and then heated for 5 hours under autogenous pressure with an increase in temperature of 5 C./min up to 180 C. and with stirring at 200 rpm to allow the crystallization of the zeolite of AFX structural type. The crystallized product obtained is filtered off, washed with deionized water and then dried overnight at 100 C. The loss on ignition of the dried solid is 14.69%.

[0122] 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./min up to 200 C., a steady stage at 200 C. maintained for 2 hours, an increase of 1 C./min up to 550 C., followed by a steady stage at 550 C. maintained for 12 hours, then a return to ambient temperature.

[0123] The calcined solid product was analyzed by X-ray diffraction and identified as consisting of a zeolite of AFX structural type (ICDD file, PDF 04-011-1869) with a purity of greater than 99% by weight. The product has a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of 14.05 as determined by XRF.

[0124] The calcined AFX zeolite is then brought into contact with a 3 M NH.sub.4NO.sub.3 solution for 1 hour with stirring at 80 C. The ratio between the volume of NH.sub.4NO.sub.3 solution and the mass of solid is 10. The solid obtained is filtered off and washed and the exchange procedure is repeated twice more under the same conditions. The final solid is separated off, washed and dried for 12 hours at 100 C. XRD analysis shows that the product obtained is a pure zeolite of AFX structural type.

[0125] The AFX zeolite in ammoniacal form is treated under a stream of air at 550 C. for 8 hours with a temperature increase gradient of 1 C./min. The product obtained is an AFX zeolite in protonated form.

Cu Ion Exchange

[0126] The calcined AFX zeolite in protonated form is brought into contact with a [Cu(NH.sub.3).sub.4](NO.sub.3).sub.2 solution for 12 hours with stirring at ambient temperature. The final solid is separated off, washed and dried for 12 hours at a temperature of 100 C.

[0127] The exchanged Cu-AFX solid obtained after the contacting with the [Cu(NH.sub.3).sub.4](NO.sub.3).sub.2 solution is calcined under a stream of air at 550 C. for 8 hours.

[0128] The calcined solid product is analyzed by X-ray diffraction and identified as a zeolite of AFX structural type (ICDD file, PDF 04-011-1869). The diffraction pattern created by this solid is given in FIG. 2B.

[0129] The product has a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of 14.05 and a percentage by mass of Cu of 3%, as determined by XRF.

[0130] The catalyst obtained is denoted CuAFX780.

Example 4: Preparation of a Zeolite of AFX Structural Type According to the Invention with 3% Cu

Preparation of the AFX Zeolite

[0131] 33.37 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (18.36% by weight) prepared according to example 1 are mixed with 37.15 g of deionized water with stirring and at ambient temperature. 1.72 g of sodium hydroxide (98% by weight, Aldrich) are dissolved in the above mixture with stirring and at ambient temperature. As soon as a homogeneous suspension is obtained, pouring in of 7.79 g of a zeolite of FAU structural type (CBV720, SiO.sub.2/Al.sub.2O.sub.3=33.52, Zeolyst, LOI=6.63%) is commenced and the suspension obtained is kept stirring for 30 minutes at ambient temperature. In order to promote the formation of a zeolite of AFX structural type, 0.646 g of seeds (10% relative to the mass of CBV720 zeolite) of a zeolite of AFX structural type are added to the synthesis mixture and kept stirring for 5 minutes. The reaction mixture then undergoes a step of maturation for 24 hours at ambient temperature with stirring (200 rpm). The molar composition of the precursor gel is as follows: 1 SiO.sub.2: 0.0298 Al.sub.2O.sub.3: 0.18 R: 0.20 Na.sub.2O: 34 H.sub.2O, i.e. an SiO.sub.2/Al.sub.2O.sub.3 ratio of 33.55. The precursor gel is then transferred, after homogenization, into a 160 mL stainless steel reactor fitted with a stirring system with four inclined paddles. The reactor is closed and then heated for 5 hours under autogenous pressure with an increase in temperature of 5 C./min up to 180 C. and with stirring at 200 rpm to allow the crystallization of the zeolite of AFX structural type. The crystallized product obtained is filtered off, washed with deionized water and then dried overnight at 100 C. The loss on ignition of the dried solid is 14.82%. 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./min up to 200 C., a steady stage at 200 C. maintained for 2 hours, an increase of 1 C./min up to 550 C., followed by a steady stage at 550 C. maintained for 12 hours, then a return to ambient temperature.

[0132] The calcined solid product was analyzed by X-ray diffraction and identified as consisting of a zeolite of AFX structural type (ICDD file, PDF 04-011-1869) with a purity of greater than 99% by weight. The product has a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of 11.42 as determined by XRF.

[0133] The calcined AFX zeolite is then brought into contact with a 3 M NH.sub.4NO.sub.3 solution for 1 hour with stirring at 80 C. The ratio between the volume of NH.sub.4NO.sub.3 solution and the mass of solid is 10. The solid obtained is filtered off and washed and the exchange procedure is repeated twice more under the same conditions. The final solid is separated off, washed and dried for 12 hours at 100 C. XRD analysis shows that the product obtained is a pure zeolite of AFX structural type.

[0134] The AFX zeolite in ammoniacal form is treated under a stream of air at 550 C. for 8 hours with a temperature increase gradient of 1 C./min. The product obtained is an AFX zeolite in protonated form.

Cu Ion Exchange

[0135] The calcined AFX zeolite in protonated form is brought into contact with a [Cu(NH.sub.3).sub.4](NO.sub.3).sub.2 solution for 12 hours with stirring at ambient temperature. The final solid is separated off, washed and dried for 12 hours at a temperature of 100 C.

[0136] The exchanged Cu-AFX solid obtained after the contacting with the [Cu(NH.sub.3).sub.4](NO.sub.3).sub.2 solution is calcined under a stream of air at 550 C. for 8 hours.

[0137] The calcined solid product is analyzed by X-ray diffraction and identified as a zeolite of AFX structural type (ICDD file, PDF 04-011-1869). The diffraction pattern created by this solid is given in FIG. 2C.

[0138] The product has a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of 11.42 and a percentage by mass of Cu of 3%, as determined by XRF.

[0139] The catalyst obtained is denoted CuAFX720.

Example 5: Preparation of a Zeolite of AFX Structural Type According to the Invention with 3% Cu

Preparation of the AFX Zeolite

[0140] 28.35 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (18.36% by weight) prepared according to example 1 are mixed with 41.22 g of deionized water with stirring and at ambient temperature. 1.26 g of sodium hydroxide (98% by weight, Aldrich) are dissolved in the above mixture with stirring and at ambient temperature. 5.74 g of Aerosil 380 silica (100% by weight, Degussa) are then poured in in small fractions with stirring. As soon as a homogeneous suspension is obtained, pouring in of 3.43 g of a zeolite of FAU structural type (CBV600 Zeolyst, SiO.sub.2/Al.sub.2O.sub.3=5.48, LOI=12.65%) is commenced and the suspension obtained is kept stirring for 30 minutes at ambient temperature. The reaction mixture then undergoes a step of maturation for 2 hours at ambient temperature with stirring (350 rpm). The precursor gel obtained has the following molar composition: 1 SiO.sub.2: 0.05 Al.sub.2O.sub.3: 0.125 R: 0.12 Na.sub.2O: 27.55 H.sub.2O, i.e. an SiO.sub.2/Al.sub.2O.sub.3 ratio of 20. 0.79 g of seeds of zeolite of AFX structural type (8.7% relative to the mass of anhydrous CBV600 zeolite and of Aerosil 380 silica) are introduced into the precursor gel with stirring. The precursor gel containing the AFX zeolite seeds is then transferred into a 160 mL stainless steel reactor fitted with a stirring system with 4 inclined paddles. The reactor is closed and then heated for 7 hours under autogenous pressure with an increase in temperature of 5 C./min up to 190 C. and with stirring at 200 rpm to allow the crystallization of the zeolite of AFX structural type. 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 12.6%.

[0141] 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./min up to 200 C., a steady stage at 200 C. maintained for 2 hours, an increase of 1 C./min up to 580 C., followed by a steady stage at 580 C. maintained for 10 hours, then a return to ambient temperature.

[0142] The calcined solid product was analysed by X-ray diffraction and identified as consisting of a zeolite of AFX structural type (ICDD file, PDF 04-011-1869) with a purity of greater than 97% by weight. The product has a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of 11.2 as determined by XRF.

[0143] The calcined AFX zeolite is then brought into contact with a 3 M NH.sub.4NO.sub.3 solution for 1 hour with stirring at 80 C. The ratio between the volume of NH.sub.4NO.sub.3 solution and the mass of solid is 10. The solid obtained is filtered off and washed and the exchange procedure is repeated twice more under the same conditions. The final solid is separated off, washed and dried for 12 hours at 100 C. XRD analysis shows that the product obtained is a pure zeolite of AFX structural type.

[0144] The AFX zeolite in ammoniacal form is treated under a stream of air at 550 C. for 8 hours with a temperature increase gradient of 1 C./min. The product obtained is an AFX zeolite in protonated form.

Cu Ion Exchange

[0145] The calcined AFX zeolite in protonated form is brought into contact with a [Cu(NH.sub.3).sub.4](NO.sub.3).sub.2 solution for 12 hours with stirring at ambient temperature. The final solid is separated off, washed and dried for 12 hours at a temperature of 100 C.

[0146] The exchanged Cu-AFX solid obtained after the contacting with the [Cu(NH.sub.3).sub.4](NO.sub.3).sub.2 solution is calcined under a stream of air at 550 C. for 8 hours.

[0147] The calcined solid product is analyzed by X-ray diffraction and identified as a zeolite of AFX structural type (ICDD file, PDF 04-011-1869). The diffraction pattern created by this solid is given in FIG. 2d.

[0148] The product has a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of 11.2 and a percentage by mass of Cu of 3%, as determined by XRF.

[0149] The catalyst obtained is denoted CuAFX600.

Example 6

[0150] In this example, a Cu-exchanged SSZ-16 zeolite is synthesized in accordance with the prior art. In this example, the copper is introduced by ion exchange.

Preparation of the SSZ-16 Zeolite

[0151] 17.32 g of sodium hydroxide are dissolved in 582.30 g of deionized water, with stirring (300 rpm) and at ambient temperature. 197.10 g of sodium silicate are added to this solution and the whole mixture is homogenized with stirring (300 rpm) at ambient temperature. 9.95 g of CBV100 NaY zeolite are then added with stirring (300 rpm), and this is continued until the zeolite has dissolved. 43.67 g of the structuring agent DABCO-C4 are dissolved in the solution obtained and this is then homogenized with stirring (450 rpm) for 30 minutes at ambient temperature.

[0152] The reaction mixture has the following molar composition: 100 SiO.sub.2: 1.67 Al.sub.2O.sub.3: 50 Na.sub.2O: 10 DABCO-C4: 4000 H.sub.2O

[0153] The reaction mixture obtained in the mixing step is maintained at ambient temperature with stirring for 24 hours.

[0154] The gel obtained is introduced into a reactor and heated to a temperature of 150 C. for 6 days with stirring (200 rpm). The crystals obtained are separated off and washed with deionized water until a pH of the washing water of less than 8 is obtained. The washed crystallized solid is dried for 12 hours at 100 C. The loss on ignition (LOI) is 18% by weight.

[0155] An XRD analysis shows that the product obtained is a pure crude synthetic SSZ-16 zeolite of AFX structural type (ICDD file, PDF 04-011-1869).

[0156] The crude synthetic SSZ-16 zeolite is calcined under a stream of dry air at 550 C. for 12 hours. The calcined SSZ-16 zeolite is brought into contact with a 3 M NH.sub.4NO.sub.3 solution for 5 hours with stirring at ambient temperature. The ratio between the volume of NH.sub.4NO.sub.3 solution and the mass of solid is 10. The solid obtained is filtered off and washed and the exchange procedure is repeated once more under the same conditions. The final solid is separated off, washed, and dried for 12 hours at 100 C.

[0157] The SSZ-16 zeolite in ammoniacal form (NH.sub.4-SSZ-16) is treated under a stream of dry air at 550 C. for 8 hours with a temperature increase gradient of 1 C./min. The product obtained is an SSZ-16 zeolite in protonated form (H-SSZ-16).

Cu Ion Exchange on H-SSZ-16

[0158] The H-SSZ-16 zeolite is brought into contact with a [Cu(NH.sub.3).sub.4](NO.sub.3).sub.2 solution for 12 hours with stirring at ambient temperature. The final solid is separated off, washed and dried, and calcined under a stream of dry air at 550 C. for 8 hours. An XRD analysis shows that the product obtained is a pure SSZ-16 zeolite of AFX structural type (ICDD file, PDF 04-011-1869).

[0159] Chemical analysis by X-ray fluorescence (XRF) gives a molar SiO.sub.2/Al.sub.2O.sub.3 ratio of 13 and a percentage by mass of Cu of 3%.

[0160] The catalyst obtained is denoted CuSSZ16.

Example 7: NOx conversion under standard SCR conditions: comparison of the catalysts according to the invention with the prior art

[0161] A catalytic test of the reduction of nitrogen oxides (NOx) by ammonia (NH.sub.3) in the presence of oxygen (O.sub.2) under standard SCR conditions is carried out at various operating temperatures for the catalysts according to example 2 (CuAFX, according to the invention), example 3 (CuAFX780, according to the invention), example 4 (CuAFX720, according to the invention), example 5 (CuAFX600, according to the invention) and the catalyst according to example 6 (CuSSZ16, comparative).

[0162] For testing each sample, 200 mg of catalyst in powder form are placed in a quartz reactor. 145 L/h of a feed representative of a mixture of exhaust gas from a diesel engine are fed into the reactor.

[0163] This feed has the following molar composition: 400 ppm NO, 400 ppm NH.sub.3, 8.5% O.sub.2, 9% CO.sub.2, 10% H.sub.2O, remainder N.sub.2.

[0164] An FTIR analyzer is used to measure the concentration of the species NO, NO.sub.2, NH.sub.3, N.sub.2O, CO, CO.sub.2, H.sub.2O, O.sub.2 at the reactor outlet. The NOx conversions are calculated as follows:

Conversion=(NOx inletNOx outlet)/NOx inlet

[0165] The results of NOx conversion under the standard SCR conditions are presented in FIG. 3, the curves CuAFX, CuAFX780, CuAFX720, CuAFX600 and CuSSZ16 respectively corresponding to the tests performed with the catalysts synthesized in accordance with example 2 (CuAFX, catalyst according to the invention), example 3 (CuAFX780, catalyst according to the invention), example 4 (CuAFX720, catalyst according to the invention), example 5 (CuAFX600, catalyst according to the invention), and example 6 (CuSSZ16, catalyst not in accordance with the invention). It appears that the catalysts according to the invention can be used to convert NOx.

[0166] The catalysts CuAFX, CuAFX780, CuAFX720 and CuAFX600 synthesized in accordance with the invention exhibit a performance which is superior to the catalyst CuSSZ16 synthesized in accordance with the prior art in terms of NOx conversion over the entire temperature range tested. A maximum conversion rate of 100% is achieved between 259 C. and 430 C. for catalyst CuAFX780 in accordance with the invention, whereas catalyst CuSSZ16 synthesized in accordance with the prior art achieves only an 89% conversion rate between 34 and 400 C.

[0167] The light-off temperatures for the catalysts are given below for standard SCR conditions:

TABLE-US-00001 TABLE 1 T50 T80 T90 T100 CuAFX 180 C. 212 C. 228 C. 310 C. CuAFX780 159 C. 190 C. 204 C. 259 C. CuAFX720 173 C. 202 C. 217 C. 272 C. CuAFX600 172 C. 203 C. 222 C. 283 C. CuSSZ16 190 C. 257 C. 350 C.

[0168] T50 corresponds to the temperature at which 50% of the NOx in the gas mixture are converted by the catalyst. T80 corresponds to the temperature at which 80% of the NOx in the gas mixture are converted by the catalyst. T90 corresponds to the temperature at which 90% of the NOx in the gas mixture are converted by the catalyst. T100 corresponds to the temperature at which 100% of the NOx in the gas mixture are converted by the catalyst.

[0169] The catalysts CuAFX, CuAFX780, CuAFX720 and CuAFX600 synthesized in accordance with the invention exhibit a performance which is far superior to the catalyst CuSSZ16 synthesized in accordance with the prior art in terms of light-off temperatures and NOx conversion over the entire temperature range tested under standard SCR conditions. Specifically, at the same conversion rate (50% or 80%), the light-off temperatures obtained with the catalyst according to the invention CuAFX are lower than those obtained with the catalyst CuSSZ16.

Example 8: NOx Conversion in Fast SCR: Comparison of the Catalysts According to the Invention and Comparative Catalyst

[0170] A catalytic test of the reduction of nitrogen oxides (NOx) by ammonia (NH.sub.3) in the presence of oxygen (O.sub.2) under fast SCR conditions is carried out at various operating temperatures for the catalysts synthesized in accordance with the invention (examples 2, 3, 4 and 5) and the sample CuSSZ16 synthesized in accordance with the prior art (example 6). 200 mg of catalyst in powder form are placed in a quartz reactor. 218 l/h of a feed representative of a mixture of exhaust gas from a diesel engine are fed into the reactor. This feed has the following molar composition: 200 ppm NO, 200 ppm NO.sub.2, 400 ppm NH.sub.3, 8.5% O.sub.2, 9% CO.sub.2, 10% H.sub.2O, remainder N.sub.2 for fast SCR conditions.

[0171] An FTIR analyzer is used to measure the concentration of the species NO, NO.sub.2, NH.sub.3, N.sub.2O, CO, CO.sub.2, H.sub.2O, O.sub.2 at the reactor outlet. The NOx conversions are calculated as follows:

[00001]$\mathrm{Conversion}=\left(\mathrm{NOx}\mathrm{inlet}-\mathrm{NOx}\mathrm{outlet}\right)/\mathrm{NOx}\mathrm{inlet}$

[0172] The light-off temperatures for the catalysts are given below for fast SCR conditions:

TABLE-US-00002 TABLE 2 T50 T80 T90 T100 CuAFX 178 C. 210 C. 235 C. 290 C. CuAFX780 162 C. 192 C. 215 C. 270 C. CuAFX720 161 C. 190 C. 214 C. 267 C. CuAFX600 166 C. 200 C. 220 C. 273 C. CuSSZ16 188 C. 233 C. 269 C. 402 C.

[0173] T50 corresponds to the temperature at which 50% of the NOx in the gas mixture are converted by the catalyst. T80 corresponds to the temperature at which 80% of the NOx in the gas mixture are converted by the catalyst. T90 corresponds to the temperature at which 90% of the NOx in the gas mixture are converted by the catalyst. T100 corresponds to the temperature at which 100% of the NOx in the gas mixture are converted by the catalyst.

[0174] The catalysts CuAFX, CuAFX780, CuAFX720 and CuAFX600 synthesized in accordance with the invention exhibit a performance which is superior to the catalyst CuSSZ16 synthesized in accordance with the prior art in terms of light-off temperatures and NOx conversion over the entire temperature range tested under fast SCR conditions. Specifically, at the same conversion rate (50%, 80%, 90% or 100%), the light-off temperatures obtained with the catalyst according to the invention CuAFX are lower than those obtained with the catalyst Cu-SSZ-16.

[0175] In addition, the nitrous oxide (N.sub.2O) emissions, in the case of the catalyst CuAFX according to the invention, remain low over the entire temperature range tested (<20 ppm between 15 and 550 C.).