Catalyst comprising a mixture of an AFX-structure zeolite and a BEA-structure zeolite and at least one transition metal for selective reduction of NOx

Abstract

The invention relates to a catalyst comprising a mixture of AFX-structure and BEA-structure zeolites and at least one additional transition metal, to the process for preparing same and to the use thereof for the selective catalytic reduction of NOx in the presence of a reducing agent such as NH.sub.3 or H.sub.2.

Claims

1. A process for preparing a catalyst comprising a mixture of AFX-structure and BEA-structure zeolites, and at least one transition metal, comprising at least the following steps: i) mixing, in aqueous medium, a first FAU zeolite having an SiO.sub.2/Al.sub.2O.sub.3 molar ratio of between 30 and 100 and at least one second FAU-structure zeolite having an SiO.sub.2/Al.sub.2O.sub.3 molar ratio of greater than or equal to 2 and less than 30, and wherein the mathematical parameter, P.sub.ze, corresponding to the mass percentage of the FAU zeolite with an SiO.sub.2/Al.sub.2O.sub.3 molar ratio of between 30 and 100, in its anhydrous form (expressed in %) in the mixture of FAU zeolites, multiplied by the SiO.sub.2/Al.sub.2O.sub.3 molar ratio of the same FAU zeolite with an SiO.sub.2/Al.sub.2O.sub.3 molar ratio of between 30 and 100, is such that: 3250<P.sub.ze<7200, of at least one nitrogenous organic compound R, chosen from 1,5-bis(methylpiperidinium)pentane dihydroxide, 1,6-bis(methylpiperidinium)hexane dihydroxide, 1,7-bis(methylpiperidinium)heptane dihydroxide, and mixtures thereof, and of at least one source of at least one alkali metal and/or alkaline-earth metal M of valence n, n being an integer greater than or equal to 1, the reaction mixture having the following molar composition: (SiO.sub.2(FAU))/(Al.sub.2O.sub.3(FAU)) of between 30 and 80, H.sub.2O/(SiO.sub.2(FAU)) of between 1 and 100, R/(SiO.sub.2(FAU)) of between 0.01 and 0.6, M.sub.2/nO/(SiO.sub.2(FAU)) of between 0.005 and 0.45, wherein SiO.sub.2(FAU) is the molar amount of SiO.sub.2 provided by all the FAU-structure zeolites introduced into the mixture, Al.sub.2O.sub.3(FAU) is the molar amount of Al.sub.2O.sub.3 introduced by all the FAU-structure zeolites introduced into the mixture, H.sub.2O the molar amount of water present in the reaction mixture, R the molar amount of the nitrogenous organic compound, M.sub.2/nO being the molar amount of M.sub.2/nO provided by all the FAU zeolites and by the source of alkali metal and/or alkaline-earth metal, until a precursor gel is obtained; ii) hydrothermal treatment of the precursor gel obtained at the end of step i) at a temperature of between 120° C. and 220° C., for a period of between 12 hours and 15 days, to obtain a solid crystalline phase, termed “solid”; iii) at least one ion exchange comprising bringing the solid obtained at the end of the previous step into contact with at least one solution comprising at least one species that is capable of releasing a transition metal, in solution in reactive form, with stirring at ambient temperature for a period of between 1 hour and 2 days; iv) heat treatment by drying the solid obtained at the end of the previous step at a temperature of between 20 and 150° C. for a period of between 2 and 24 hours, followed by calcination under a stream of air at a temperature of between 450 and 700° C. for a period of between 2 and 20 hours.

2. The process as claimed in claim 1, wherein steps iii) and iv) are reversed, and optionally repeated.

3. The process as claimed in claim 1, wherein step iii) is carried out by bringing the solid into contact with a solution comprising at least one species, preferably a single species, capable of releasing a transition metal or by successively bringing the solid into contact with different solutions each comprising at least one, preferably a single, species capable of releasing a transition metal, the different solutions advantageously comprising different species capable of releasing a transition metal.

4. The process as claimed in claim 1, wherein the transition metal released in the solution of step iii) 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.

5. The process as claimed in claim 1, wherein the transition metal content is between 0.5 and 6% by mass relative to the total mass of the final catalyst, in its anhydrous form, the amount of copper in the composite material of the catalyst being taken into account.

6. The process as claimed in claim 1, wherein the catalyst contains only copper as transition metal, in a mass content of between 0.5 and 6%, preferably between 0.5 and 5% relative to the total mass of the final catalyst in its anhydrous form.

7. The process as claimed in claim 1, wherein the catalyst comprises copper and another transition metal, the copper content of the catalyst obtained is between 0.05 and 2% by mass, and that of the other transition metal between 1 and 4% by mass, the transition metal contents being given as percentages by mass relative to the total mass of the final catalyst in its anhydrous form.

8. The process as claimed in claim 1, wherein the catalyst contains only iron as transition metal, in a mass content of between 0.5 and 5 relative to the total mass of the final catalyst in its anhydrous form.

9. The process as claimed in claim 1, wherein the catalyst comprises iron and another transition metal, the iron content of the catalyst obtained is between 0.05 and 2% by mass, and that of the other transition metal is between 1 and 4% by mass, the transition metal contents being given as percentages by mass relative to the total mass of the final catalyst in its anhydrous form.

10. The process as claimed in claim 1, for which the nitrogenous organic compound R is 1,6-bis(methylpiperidinium)hexane dihydroxide.

11. The process as claimed in claim 1, wherein the hydrothermal treatment of step ii) is carried out at a temperature of between 150° C. and 195° C. for a period of between 12 hours and 8 days.

12. The process as claimed in claim 1, for which the heat treatment step iv) comprises drying the solid at a temperature of between 60 and 100° C. for a period of between 2 and 24 hours, followed by calcining, corresponding to a treatment by combustion in air, which is optionally dry, at a temperature of between 500 and 600° C. for a period of between 6 and 9 hours, the flow rate of optionally dry air being between 0.5 and 1.5 l/h/g of solid to be treated.

13. A catalyst comprising a composite material comprising an intimate mixture of a zeolite of AFX type and a zeolite of BEA type, and at least one transition metal, having a zeolite structure comprising: between 30 and 90% by mass by mass of AFX-structure zeolite, relative to the total mass of the catalyst in its anhydrous form; between 10 and 70% by mass by mass of BEA-structure zeolite, relative to the total mass of the catalyst in its anhydrous form; wherein the transition metal 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 or a mixture thereof, wherein the total content of transition metals is between 0.5 and 6% by mass relative to the total mass of the final catalyst, in its anhydrous form, and wherein the catalyst comprises less than 10% by mass of impurities and/or of crystalline or amorphous phase other than AFX and BEA.

14. The catalyst as claimed in claim 13, containing only copper as transition metal and wherein the total copper content by mass is between 0.5 and 6 relative to the total mass of the final catalyst in its anhydrous form.

15. The catalyst as claimed in claim 13, comprises copper and another transition metal, and wherein the copper content is between 0.05 and 2% by mass, while the content of the other transition metal is between 1 and 4% by mass, the transition metal contents being given as percentages by mass relative to the total mass of the final catalyst in its anhydrous form.

16. The catalyst as claimed in claim 13, containing only iron as transition metal and wherein the total iron content by mass is between 0.5 and 5% relative to the total mass of the final catalyst in its anhydrous form.

17. The catalyst as claimed in claim 13, comprises iron and another transition metal, and wherein the iron content is between 0.05 and 2% by mass, while the content of the other transition metal is between 1 and 4% by mass, the transition metal contents being given as percentages by mass relative to the total mass of the final catalyst in its anhydrous form.

18. The use of the catalyst as claimed in claim 13 for the selective reduction of NO.sub.x by a reducing agent such as NH.sub.3 or H.sub.2.

19. The use as claimed in claim 18, for which the catalyst is formed by deposition in the form of a coating on a monolith.

20. The use as claimed in claim 19, for which the honeycomb structure is formed by parallel channels open at both ends or comprises porous filtering walls for which the adjacent parallel channels are alternately blocked at both ends of the channels.

21. The use as claimed in claim 20, for which the amount of catalyst that is deposited on the structure is between 50 and 180 g/I for the filtering structures and between 80 and 200 g/I for the structures with open channels.

22. The use as claimed in claim 18, for which the catalyst is combined with a binder such as ceria, zirconium oxide, alumina, non-zeolitic silica-alumina, titanium oxide, a ceria-zirconia mixed oxide, a tungsten oxide and/or a spinel in order to be formed by deposition in the form of a coating.

23. The use as claimed in claim 18, for which the coating is combined with another coating having the capacity to adsorb pollutants, in particular NOx, to reduce pollutants, in particular NOx, or to promote the oxidation of pollutants.

24. The use as claimed in claim 18, for which the catalyst is in the form of an extrudate containing up to 100% of the catalyst.

25. The use as claimed in claim 18, for which the structure coated with the catalyst or obtained by extrusion of the catalyst is integrated into an exhaust line of an internal combustion engine.

26. A catalyst comprising a composite material comprising an intimate mixture of a zeolite of AFX type and a zeolite of BEA type, and at least one transition metal, having a zeolite structure comprising: between 30 and 90% by mass by mass of AFX-structure zeolite, relative to the total mass of the catalyst in its anhydrous form; between 10 and 70% by mass by mass of BEA-structure zeolite, relative to the total mass of the catalyst in its anhydrous form; wherein the transition metal 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 or a mixture thereof, wherein the total content of transition metals is between 0.5 and 6% by mass relative to the total mass of the final catalyst, in its anhydrous form, wherein the catalyst comprises less than 10% by mass of impurities and/or of crystalline or amorphous phase other than AFX and BEA, and wherein the catalyst is obtained by the process as claimed in claim 1.

27. The use of the catalyst as claimed in claim 26 for the selective reduction of NO.sub.x by a reducing agent such as NH.sub.3 or H.sub.2.

Description

LIST OF FIGURES

(1) FIG. 1 represents the chemical formulae of the nitrogenous organic compounds which may be chosen as the structuring agent used in the synthesis process according to the invention.

(2) FIG. 2 represents the X-ray diffraction pattern of the Cu-AFX-BEA catalyst obtained according to example 2.

(3) The invention is illustrated by the examples that follow, which are not in any way limiting in nature.

EXAMPLES

Example 1: Preparation of 1,6-bis(methylpiperidinium)hexane dihydroxide (Structuring Agent R)

(4) 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 ambient temperature and then filtered. The mixture is poured into 300 ml of cold diethyl ether and the precipitate formed is 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%).

(5) The product has the expected 1H NMR spectrum. 1H NMR (D2O, 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). This 1H NMR spectrum corresponds to that of 1,6-bis(methylpiperidinium)hexane dibromide.

(6) 18.9 g of Ag.sub.2O (0.08 mol, 99%, Aldrich) are placed in a 250 ml Teflon beaker containing 30 g of 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 Catalyst According to the Invention Containing 2% Cu

(7) 0.239 g of an FAU-structure zeolite (CBV712 Zeolyst, SiO.sub.2/Al.sub.2O.sub.3=11.42, LOI=12.81) was mixed with 4.952 g of deionized water. 0.573 g of an FAU-structure zeolite (CBV780 Zeolyst, SiO.sub.2/Al.sub.2O.sub.3=98.22, LOI=8.52, P.sub.ze=7170) is added to the previous mixture, and the preparation obtained is kept stirring for 10 minutes. 2.905 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (20.91% by weight) prepared according to example 1 are added to the above mixture. The mixture is then kept stirring for 10 minutes. 0.330 g of a 20% by weight aqueous solution of sodium hydroxide (solution prepared from 98% by weight sodium hydroxide, Aldrich) is added to the mixture and kept stirring for 10 minutes. The molar composition of the precursor gel is as follows: 60 SiO.sub.2: 1.8 Al.sub.2O.sub.3: 10 R(OH).sub.2: 4.3 Na.sub.2O: 2204 H.sub.2O, i.e. an SiO.sub.2/Al.sub.2O.sub.3 ratio of 33.3.

(8) The precursor gel is then transferred, after homogenization, into an autoclave. The autoclave is closed and then heated for 6 days at 180° C. with stirring at 35 rpm with a rotary spit system. The crystalline product obtained is filtered and washed with deionized water.

(9) Heat Treatment Step (Calcination)

(10) The washed solid is dried overnight at 100° C. The loss on ignition (LOI) of the dried solid, evaluated at 1000° C. for 2 hours, is 10.1%.

(11) The dried solid is then introduced into a muffle furnace where a calcination step is performed under a stream of air: the calcination cycle comprises an increase in temperature of 1.5° C./minute up to 200° C., a stationary phase at 200° C. maintained for 2 hours, an increase in temperature of 1° C./minute up to 550° C., followed by a stationary phase at 550° C. maintained for 8 hours, then a return to ambient temperature.

(12) The calcined solid product was analyzed by X-ray diffraction and identified as being constituted of a mixture of approximately 50% by mass of an AFX-structure zeolite and 50% by mass of a BEA-structure zeolite. The AFX-BEA mixture represents approximately 100% by mass of the product obtained.

(13) NH.sub.4.sup.+ Ion Exchange on the Calcined AFX Zeolite and Heat Treatment

(14) The calcined AFX-BEA composite material is brought into contact with a 3 molar 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, washed with deionized water and dried at 100° C. for 4 hours.

(15) The AFX-BEA composite material 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 loss on ignition (LOI) of the solid obtained, evaluated at 1000° C. for 2 hours, is 4% by weight. The product obtained is an AFX-BEA composite material in protonated form.

(16) Cu Ion Exchange and Heat Treatment

(17) The calcined AFX-BEA composite material is brought into contact with a solution of [Cu(NH.sub.3).sub.4](NO.sub.3).sub.2 for 1 day with stirring at ambient temperature. The final solid is separated, washed with deionized water and dried at 100° C. for 4 hours.

(18) The exchanged and dried solid, obtained after the bringing into contact with the solution of [Cu(NH.sub.3).sub.4](NO.sub.3).sub.2, is calcined under a stream of air at 550° C. for 8 hours.

(19) The calcined solid product is analyzed by X-ray diffraction (cf. FIG. 2) and identified as being constituted of a mixture of approximately 50% by mass of an AFX-structure zeolite and 50% by mass of a BEA-structure zeolite. The AFX-BEA mixture represents 98% by mass of the product obtained. The copper content represents a percentage by mass of 2% as determined by X-ray fluorescence.

(20) The catalyst obtained is denoted Cu-AFX-BEA.

Example 3: NOx Conversion Under Standard-SCR Conditions

(21) A catalytic test of nitrogen oxide (NOx) reduction by ammonia (NH.sub.3) in the presence of oxygen (O.sub.2) under Standard-SCR conditions is carried out at different operating temperatures for the catalyst synthesized according to example 2 (Cu-AFX-BEA).

(22) 200 mg of catalyst in powder form are placed in a quartz reactor. 145 l/h of a representative load of a mixture of exhaust gas from a diesel engine are fed into the reactor. This load 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.

(23) 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 inlet−NOx outlet)/NOx inlet

(24) The results, in particular the catalyst initiation temperatures, are given below for the Standard-SCR conditions:

(25) TABLE-US-00002 T50 T80 T90 T100 Cu-AFX-BEA 196° C. 238° C. 280° C. 400° C.

(26) 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.

(27) It appears that the Cu-AFX-BEA catalyst synthesized according to the invention makes it possible to efficiently convert NOx over the whole of the temperature range tested. A maximum conversion of 100% is reached at 400° C. The initiation temperatures obtained with the Cu-AFX-BEA catalyst according to the invention are satisfactory: they are in fact low, in particular for the degrees of conversion of 50%, 80% and 90%.