PROCESS FOR PREPARING A NANOMETRIC ZEOLITE Y

Abstract

A process for preparing a nanometric zeolite Y of FAU structural type with a crystal size of less than 100 nm and an A/B ratio of greater than 2, by mixing, in aqueous medium, of at least one source AO.sub.2 of at least one tetravalent element A chosen from silicon, germanium and titanium, of at least one source BO.sub.b of at least one trivalent element B chosen from aluminum, boron, iron, indium and gallium, of at least one source C.sub.2/mO of an alkali metal or alkaline-earth metal C chosen from lithium, sodium, potassium, calcium and magnesium, where source C.sub.2/mO also includes at least one source of hydroxide ions, to obtain a gel, maturation and hydrothermal treatment of the gel.

Claims

1. A process for preparing a nanometric zeolite Y of FAU structural type with a crystal size of less than 100 nm and an A/B ratio of greater than 2, said process comprising at least the following steps: i) mixing, in aqueous medium, at least one source AO.sub.2 of at least one tetravalent element A chosen from silicon, germanium and titanium, alone or as a mixture, at least one source BO.sub.b of at least one trivalent element B chosen from aluminum, boron, iron, indium and gallium, alone or as a mixture, at least one source C.sub.2/mO of an alkali metal or alkaline-earth metal C chosen from lithium, sodium, potassium, calcium and magnesium, alone or as a mixture, said source C.sub.2/mO of alkali metal or alkaline-earth metal C also including at least one source of hydroxide ions, to obtain a gel, the reaction mixture having the following molar composition:
vAO.sub.2:wBO.sub.b:xC.sub.2/mO:yH.sub.2O v being between 1 and 40, w being between 0.1 and 5, x being between 1 and 40, y being between 30 and 1000, b being between 1 and 3, b being an integer or rational number, m being equal to 1 or 2, ii) maturing the gel obtained on conclusion of step (i) at a temperature of between 15 C. and 60 C., with or without stirring, for a time of between 10 hours and 60 days, iii) after at least 10 hours and less than 72 hours of maturation, single or repeated addition of at least one source AO.sub.2 of at least one tetravalent element A chosen from silicon, germanium and titanium, alone or as a mixture, to said gel, the molar composition of the gel on conclusion of the addition being as follows:
vAO.sub.2:wBO.sub.b:xC.sub.2/mO:yH.sub.2O v being between 5 and 50, w being between 0.1 and 5, x being between 1 and 40, y being between 200 and 1000, b being between 1 and 3, b being an integer or rational number, m being equal to 1 or 2, iv)hydrothermal treatment of the gel obtained on conclusion of step (iii) at a temperature of between 20 C. and 200 C., at the autogenous reaction pressure, for a time of between 1 hour and 14 days, to obtain crystallization of said nanometric zeolite Y of FAU structural type.

2. The process as claimed in claim 1, in which A is silicon.

3. The process as claimed in claim 1, in which B is aluminum.

4. The process as claimed in claim 1, in which C is sodium.

5. The process as claimed in claim 1, in which the reaction mixture from the mixing step (i) has the following molar composition:
vAO.sub.2:wBO.sub.b:xC.sub.2/mO:yH.sub.2O v being between 15 and 20, w being between 0.2 and 1.5, x being between 1 and 20, y being between 100 and 400, b being between 1 and 3, b being an integer or rational number, m being equal to 1 or 2, in which A, B and C have the same definition as previously.

6. The process as claimed in claim 1, in which seeds comprising crystals of zeolite of FAU structural type are added during the mixing step (i).

7. The process as claimed in claim 1, in which said mixing step (i) is performed in the absence of organic structuring agent.

8. The process as claimed in claim 1, in which said maturation step (ii) takes place at a temperature of between 20 and 40 C., for a time of between 10 hours and 30 days.

9. The process as claimed in claim 1, in which the tetravalent element A added in the maturation step (iii) of the process according to the invention may be identical to or different from the tetravalent element A added in the mixing step (i).

10. The process as claimed in claim 1, in which the single or repeated addition in said step (iii) of at least one source of at least one tetravalent element A is performed after at least 10 hours and strictly less than 72 hours.

11. The process as claimed in claim 10, in which the single or repeated addition of at least one source of at least one tetravalent element A is performed after at least 12 hours and less than 48 hours.

12. The process as claimed in claim 1, in which the molar composition of the gel on conclusion of the addition in step (iii) is as follows:
vAO.sub.2:wBO.sub.b:xC.sub.2/mO:yH.sub.2O v being between 20 and 30, w being between 0.2 and 1.5, x being between 1 and 20, y being between 200 and 500, b being between 1 and 3, b being an integer or rational number, m being equal to 1 or 2, in which A, B and C have the same definition as previously.

13. The process as claimed in claim 1, in which the nanometric zeolite Y of FAU structural type formed on conclusion of step (iv) is filtered off, washed and then dried at a temperature of between 20 C. and 150 C.

14. The process as claimed in claim 13, in which said zeolite obtained on conclusion of the drying step undergoes at least one calcination step and at least one ion-exchange step.

Description

DESCRIPTION OF THE FIGURES

[0076] FIGS. 1 and 2 represent the x-ray diffraction diagrams of the nanometric zeolites X and Y of FAU structural type synthesized, respectively, in examples 1 and 2, in the diffraction angle domain 2=5 to 40.

[0077] FIG. 3 represents the x-ray diffraction diagram of the nanometric zeolite Y of FAU structural type synthesized in example 3 according to the invention, in the diffraction angle domain 2theta=3 to 50.

[0078] FIG. 4 represents the dinitrogen adsorption-desorption isotherm for the zeolite Y of FAU structural type synthesized in example 3 according to the invention.

EXAMPLES

[0079] The invention is illustrated by the examples that follow, which are not in any way limiting in nature.

Example 1

[0080] Preparation of a nanometric zeolite X of FAU structural type and with an Si/Al mole ratio equal to 1.4 according to a process not in accordance with the invention in the sense that no source of silicon or of any other tetravalent element is added during the maturation step.

[0081] A nanometric zeolite X of FAU structural type containing the elements Si and Al, with an Si/Al mole ratio equal to 1.4, is synthesized according to a preparation method known to those skilled in the art. Typically, the source of aluminum (sodium aluminate, Strem Chemicals, 99%) and the mineralizing agent (sodium hydroxide, Fluka, 99%) are dissolved in deionized water, with stirring. The source of silicon (Ludox AS-40, 40%, Sigma-Aldrich) is then added dropwise, so as to obtain a reaction mixture whose molar composition is 15.2 SiO.sub.2: 1 Al.sub.2O.sub.3: 17 Na.sub.2O: 360 H.sub.2O.

[0082] The reaction mixture is maintained under vigorous stirring for 17 days at room temperature. The product is then filtered off and washed, before being oven-dried overnight at 100 C. No source of silicon or of any other tetravalent element is added during the maturation step.

[0083] The x-ray diffraction diagram of the material shown in FIG. 1 is indexable in the cubic system of zeolite of FAU structural type. Analysis of the x-ray diffractogram gives an Si/Al mole ratio equal to 1.4 according to the Fichtner-Schmittler equation. These characteristics correspond to a zeolite X of FAU structural type. The size of the zeolite crystals obtained, measured on eight transmission electron microscopy images, is between 15 and 50 nm.

[0084] The process according to example 1 not comprising any step of adding a source of silicon or of any other tetravalent element during the maturation step thus does not make it possible to obtain a zeolite Y with an Si/Al ratio>2, but a zeolite X with an Si/Al ratio=1.4.

Example 2

[0085] Preparation of the nanometric zeolite Y of FAU structural type and with an Si/Al mole ratio equal to 2.7 according to a process not in accordance with the invention in the sense that the addition of a source of silicon is performed after 7 days of maturation.

[0086] A nanometric zeolite Y of FAU structural type containing the elements Si and Al, with an Si/Al mole ratio equal to 2.7, is synthesized according to a preparation method described in example 1 as regards the mixing step (i). Typically, the source of aluminum (sodium aluminate, Strem Chemicals, 99%) and the mineralizing agent (sodium hydroxide, Fluka, 99%) are dissolved in deionized water, with stirring. The source of silicon (Ludox AS-40, 40%, Sigma-Aldrich) is then added dropwise, so as to obtain a reaction mixture whose molar composition is 15.2 SiO.sub.2: 1 Al.sub.2O.sub.3: 17 Na.sub.2O: 360 H.sub.2O. The gel thus formed is placed under vigorous stirring at room temperature. After seven days of maturation, a source of silicon (Ludox AS-40, 40%, Sigma-Aldrich) is added dropwise. The operation is repeated the following day and the day after that. After the three additions of source of silicon, the gel thus formed has the following composition: 25 SiO.sub.2: 1 Al.sub.2O.sub.3: 18.4 Na.sub.2O: 480 H.sub.2O. The reaction mixture is maintained under vigorous stirring for a further four days at room temperature, and is then transferred into a polypropylene flask. This flask is placed in an oven at 60 C. for 24 hours at the autogenous pressure and without addition of gas.

[0087] After cooling the flask to room temperature, the product is filtered off and washed, and then dried in an oven overnight at 100 C.

[0088] The x-ray diffraction diagram of the material shown in FIG. 2 is indexable in the cubic system of zeolite of FAU structural type. Analysis of the x-ray diffractogram gives an Si/Al mole ratio equal to 2.7 according to the Fichtner-Schmittler equation. These characteristics also correspond to a zeolite Y of FAU structural type. The size of the zeolite crystals obtained, measured on eight transmission electron microscopy images, is between 15 and 50 nm.

[0089] However, the micropore volume of the zeolite Y obtained according to example 2 determined by nitrogen adsorption is equal to 0.24 cm.sup.3/g.

Example 3

[0090] Preparation of the nanometric zeolite Y of FAU structural type and with an Si/Al mole ratio equal to 2.4 according to a process in accordance with the invention in the sense that the addition of a source of silicon is performed after 1 day of maturation.

[0091] A nanometric zeolite Y of FAU structural type containing the elements Si and Al, with an Si/Al mole ratio equal to 2.4, is synthesized according to the following preparation method. The source of aluminum (sodium aluminate, Sigma-Aldrich, 53% Al.sub.2O.sub.3, 43% Na.sub.2O, 4% H.sub.2O) and the mineralizing agent (sodium hydroxide, Carlo Erba, 99%) are dissolved in deionized water, with stirring. The source of silicon (Ludox AS-40, 40%, Sigma-Aldrich) is then added dropwise, so as to obtain a reaction mixture whose molar composition is 15 SiO.sub.2: 1 Al.sub.2O.sub.3: 17 Na.sub.2O: 360 H.sub.2O. The gel thus formed is stirred at room temperature. After one day of maturation, a source of silicon (Aerosil 130V, Evonik, >99.8%) is added. After this addition of source of silicon, the gel thus formed has the following composition: 36 SiO.sub.2: 1 Al.sub.2O.sub.3: 17 Na.sub.2O: 360 H.sub.2O. The reaction mixture is kept stirring for a further six days at room temperature, and is then transferred into a polypropylene flask. This flask is placed in an oven at 70 C. for 16 hours at the autogenous pressure and without addition of gas. After cooling the flask to room temperature, the product is washed by centrifugation, and then dried in an oven overnight at 100 C.

[0092] The x-ray diffractogram of the material shown in FIG. 1 is indexable in the cubic system of zeolite of FAU structural type. Analysis of the x-ray diffractogram gives an Si/Al mole ratio equal to 2.4 according to the Breck-Flanigen equation. The mean size of the zeolite crystals obtained, measured by transmission electron microscopy on a total of 100 particles, is about 90 nm.

[0093] Moreover, the zeolite Y obtained according to example 3 according to the invention has very good crystallinity. Specifically, the dinitrogen adsorption-desorption isotherm shown in FIG. 2 makes it possible to deduce that the micropore volume of the zeolite is equal to 0.34 cm.sup.3/g.

Example 4: Not in Accordance with the Invention

[0094] According to a process not in accordance with the invention, a gel identical in composition to that described in example 3 after the addition of the source of silicon is prepared from the first mixing step: 36 SiO.sub.2: 1 Al.sub.2O.sub.3: 17 Na.sub.2O: 360 H.sub.2O. This gel is stirred at room temperature for 7 days, corresponding to the total maturation time of the gel prepared in example 3. The reaction mixture is then transferred into a polypropylene flask. This flask is placed in an oven at 70 C. for 16 hours at the autogenous pressure and without addition of gas. After cooling the flask to room temperature, the product is washed by centrifugation, and then dried in an oven overnight at 100 C.

[0095] The x-ray diffractogram of the material obtained shows that no crystalline product is formed on conclusion of the crystallization step at 70 C.

[0096] The preparation process described in this example thus does not make it possible to obtain a nanometric zeolite of FAU structural type.